Technical Information 3KDE010003R3001
Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
Instrumentation Solutions
Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
3KDE010003R3001
Table of contents Page
2
1
Sugar production from sugar beets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2
Juice production – Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1 2.2 2.3
Juice purification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Evaporator station . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Lime kiln. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3
Process control in the sugar house . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.1 3.2 3.3
Control of discontinuous juice boilers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Continuous evaporating crystallization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Cooling crystallization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4
Pulp drying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
3KDE010003R3001
1 Sugar production from sugar beets This paper describes the plant stations in a beet sugar factory. It is intended to give you an overview of the individual processes and interrelations in such a factory to allow for a better understanding of the sugar production process and facilitate the selection of the appropriate process measurement and control equipment. The nominal diameters of the measuring instruments and final control elements described here have to be adapted accordingly to the actual capacity requirements of the respective sugar factory. The examples detailed here are based on the assumption that the described factory has an average daily throughput of 8000 t of sugar beets. Sugar beets are usually sowed in March or April and reach their maximum weight in September or October. The sugar is a result of photosynthesis in the leaves of the plants. The sugar content of a beet usually amounts to 16 to 18 % of its total weight. As sugar beets are perishable agricultural products, sugar factories normally work in 4 shifts 24 hours a day to avoid considerable sugar loss. The processing season ("campaign") ends before Christmas every year, depending on the sugar beet harvest in the catchment area of the respective sugar factory. In Germany, sugar beets are transported from the farm to the factory by truck or farm vehicle. The trucks/ vehicles first pass the weighing station, where the beets are weighed and tested automatically. The samples are taken to determine the sugar content, degree of dirtiness, etc. Subsequently, the beets are unloaded through stationary or mobile tipping units or by washing them off. The unloaded - or intermediately piled - sugar beets are dumped into a wet hopper where water is used to transport them to the beet washer. Here the beets undergo a separation from dirt and rocks sticking to them. The portion of dirt and rocks varies between 10 % and 30 %, depending on the weather conditions. After washing, the beets are transported on a belt conveyor into a hopper above the slicers. The slicers cut the beets into slices called “cossettes” that are fed to the extraction tower and extracted with hot water of about 70 °C. The extraction tower uses the reverse direction flow principle, i.e. the cossettes enter the tower at the bottom and are transported to the top by mechanical elements on a rotating shaft. Hot water is added at the top of the tower and slowly flows down through the cossettes, carrying the sucrose and other watersoluble organic and inorganic substances washed out of the sugar beets. The wet pulp (extracted cossettes) is transported to presses where some of the remaining water is removed. The water extracted by the presses is added to the fresh water in a ratio control loop. This procedure allows to extract approximately 98 % of the beets' sugar content. After pressing the pulp is conveyed to a pulp dryer and dried to a final moisture content of approximately 10 %. The dried pulp is processed to pellets and sold to farmers as a high-quality nutritious animal feed rich in carbohydrates and proteins. The raw juice from the extraction station is purified in order to remove as much of the non-sugar content as possible. This process requires unhydrated lime (calcium oxide) produced in a separate lime kiln in the factory. Milk of lime is added to the raw juice in a ratio control loop in the preliming station. The raw juice is alkalized from a pH of approx. 6.3 to a pH of approx. 11.4. The raw juice must slowly pass this pH stage to permit the flocculation of colloids (pectins, proteins). A calcium oxide quantity of approximately 0.3 % of the beets weight is required for this. The next step in the process is cold main liming. Here another specific quantity of calcium oxide amounting to approximately 1.5 % of the total beets weight is added to the raw juice. Then the raw juice is heated to 90 °C. At this stage called hot main liming most of the glucose and fructose contained in the juice is either converted to lactic acid or precipitated. Additionally, existing amides are saponified, forming the respective acids or salts. Then the juice is pumped to the first carbonation tank. Here, the limed juice is treated with carbon dioxide (CO2) gas resulting from the lime rock production in the lime kiln. Excess calcium is segregated as calcium carbonate, which is used as a purification agent for covering the precipitated colloids. This carbonation juice (turbid juice) flows through thickening filters where it concentrated to an extent allowing for continuous filtering through rotary filters. The mud extracted by these filters is put out as slurry and is often used for filling sinks, etc., together with the rocks and sand washed off the beets. The filtered juice is again mixed with approx. 0.25 % calcium oxid of beets weight and then subject to a second carbonation. At this stage of purification and filtration the CO2 gas added to the juice is exactly dosed to ensure optimum precipitation of the calcium carbonate. The clarified juice continues its journey through the subsequent filters (e.g. cartridge filters) and a multi-stage evaporator station. The sugar juice now has a sugar concentration between 12 and 15 %.
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
3KDE010003R3001
In the sugar production season (“sugar campaign”) the factory's demand for energy and heat is very high. The required power is generated by steam-driven turbo generators. The steam is produced in high-pressure vessels. The waste steam from the turbines is used to heat the first evaporator stage where the juice thickening process commences. The exhaust steam (steam 1) heats the subsequent evaporator stage 2. Again, the exhaust steam (steam 2) is re-used to heat evaporator stage 3, and so on, up to evaporator stage 4 or 5. Steam 4 or 5, respectively, is used for the sugar house work. Steam 2 and steam 3 also heat the juice in the individual parts of the plant. The sugar juice is concentrated in the evaporator stages to approx. 68-71 % dry substances. This "thick juice" reaches the sugar house where it is crystallized to become sugar. All stations of the sugar production plant described so far are continuously working units. On the contrary, the classic sugar house work in the juice boilers (vacuum pans) is discontinuous. The following description details a 3-product sugar house which is commonly used in the sugar industry. The thick juice from evaporation is boiled in the A-vacuum pans (juice boilers for second-class white sugar (A-sugar)), where the juice is subject to a vacuum and to steam 4 and steam 5 to thicken it until it is super-saturated. The crystallization process is started by adding seed crystals. Adding more juice while evaporation is being continued lets the crystals grow to maturity. The supplied juice quantity and the rheological quantity have a specific ratio to each other. The juice level in the A-vacuum pan is the external setpoint. When reaching the max. level, the full crystal size is reached. The mass is then discharged from the A-vacuum pans into the discharge mingler. In the subsequent centrifugals the sugar crystals are separated from the remaining juice. The run- off (green syrup) is boiled again in the pans for second class raw sugar (B-sugar). Some of the green syrup is also treated in the crystallizer station (affination mingler) for low grade raw sugar (after product sugar). The wash run-off (purge syrup) from the centrifugals is mixed with the incoming thick juice from the evaporator station and boiled again in the A-vacuum pans. The procedure for second class raw sugar is the same as for secondclass white sugar. The sugar produced in the centrifugals is dissolved in water, yielding a remelt syrup of approx. 78 % dry substances. The remelt syrup is boiled again in vacuum pans for first product white sugar (white refined sugar). The process is the same as for A-class vacuum pans. The green syrup from the subsequent centrifugals is mixed with the incoming thick juice from the evaporator station and boiled again in the A-class vacuum pans for the second-class white sugar. The purge syrup from the centrifugals for first product white sugar is mixed with the remelt syrup and is boiled again in the first product white sugar pans. Note that the seeding points and the boiling temperatures are different for second class raw sugar and first class white sugar. The green syrup from the centrifugals of the juice boilers for second class raw sugar is boiled in the low raw sugar (C-product) vacuum pans. The process in these vacuum pans is the same as in the vacuum pans of the other sugar products, whereas the seeding points, the boiling curve, and the boiling temperatures are different. The sugar crystals are separated from the remaining juice in subsequent centrifugals. The runoff (molasses) of this last crystallization stage practically do not contain any sucrose that may crystallize. The molasses are added to the pellets (dried and pressed pulp) in a specific ratio of mixture to be used as high-nutritious cattle food. After the boiled liquid has been discharged from the vacuum pan to the affination mingler, it is cooled down. The super-saturation increases, and the crystals continue growing. The cooling process must be controlled exactly; super-saturation must be monitored to avoid the formation of fine grain in the massecuite. When the crystals reach the desired size, the massecuite is spinned off in the affination centrifugals. The sugar is dissolved in water, and this syrup is boiled in vacuum pans for seed magma white sugar. The run-off is boiled again in the pans for after product sugar and second class raw sugar. The crystal sugar discharged from the centrifugals is dried in drum-driers using warm air until a residual moisture of approx. 0.03 % is reached.
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
3KDE010003R3001
Subsequently, the sugar is cooled down to 25 °C and carried via belt conveyors to big sugar silos made of concrete. The stored sugar is removed from the silos and conveyed to a screen station. When a customer orders sugar of a specific granular size, the crystals of the ordered size are screened out. Depending on the order, the sugar is then packed in 1 kg family packs or 50 kg sacks or loaded directly on to special trucks.
Superheated steam
Boiler Steam converter 2
Steam converter 1
Feed water tank
Feed water
Deionized water tank
Exhaust steam cooler
Summer boiler Evaporator
Pellet mills Slicers Pulp dryers Tanks, etc. Condensate cigar
Feed water
Fig. 1-1: Schematic diagram of process measurement and control in the power station
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
3KDE010003R3001
2 Juice production – Extraction In the extraction station most of the sugar is washed out of the sliced beets. The produced raw juice should be as pure, concentrated and processable as possible. The following section describes a juice extractor with a cossette scalder using the reverse direction flow principle. After washing, the beets are transported to the beet hopper above the slicers. This buffer provides for a constant supply of cossettes to the juice extractor, even if the quantity of delivered beets varies. The beet hopper is hence an important interface in the sugar process. The "beet hopper volume" is a decisive process variable for the beet conveyors and washers as well as for all subsequent stations like the juice extractor station, juice purification station, evaporator, and sugar house. The most reliable and precise method for determining the "beet hopper volume" of a new plant is a measurement using load cells. In an already existing plant the beet hopper volume can also be measured with capacitive gauges. The cossette quantity is measured by a belt weigher in the cossette conveyor and input to the slicer speed control unit (2) as the reference value. This control unit ensures that the specified cossette quantity required for constant juice extraction is always met. A conveyor belt transports the cossettes to the filler neck of the cossette scalder. After being heated and denaturated in the scalder, they continue their travel to the extraction tower as a cossettes-juice mixture. The prerequisites for an efficient heat exchange between the cossettes and the raw juice in the reverse direction flow are a high filling ratio and a high juice level. In order to allow for immediate compensation of disturbances caused by output variations of the cossette pumps, level control of the extraction tower is realized as a cascade control loop (3). This control loop ensures constant feed to the extraction tower. The high filling ratio required for the extraction tower (4) can be achieved by feeding in a controlled juice quantity in the reverse direction flow section of the cossette scalder. The added juice quantity is controlled in dependence on the power consumption of the cossette scalder drive. The drive shaft speed (5) has to be changed only slightly if the basic quantity of the circulating raw juice is adjustable and, thus, a wider control range of the additional juice is ensured. As a result, a constantly good raw juice cooling and constant processing are achieved, even with varying cossette quantities and qualities. The foam from the ventilation grille of the cossette scalder is fed to the defoamer tank and precipitated through steam. The level control (6) keeps the filling level at a constant value and controls the return flow via a flow control loop (7). Before the defoamed juice is pumped back into the cossette scalder, it is heated to approx. 80 °C in a temperature control loop. This heating also has a sterilizing effect. The raw juice is then discharged from the cossette scalder through a front screen at the cossette input side. The juice volume drawn off is controlled by a flow control loop (9). The extraction tower juice or circulation juice is discharged through screens at the side or bottom of the tower. The discharge is flow-controlled (10) as well. Similar to the optimization measures taken for the cossette scalder, special efforts are spent on improving and optimizing the extraction tower output. This is mainly achieved by conveying the cossettes inside the extraction tower at a constant speed and with a high filling level. If unhindered discharge of the juice is ensured, the mechanical behavior of the cossettes during the transport is the only process variable that might be influenced. The optimal filling ratio is maintained by levelling the liquid column and changing the tower shaft speed. In manual mode, the extraction tower operator will not use the full shaft drive power, for safety reasons. Additionally, variations in the extraction process may not be recognized immediately and are difficult to control in an optimal way over a longer time period. Filling ratio control (11) permits to increase and decrease the level in dependence on the extraction tower's power consumption. A higher filling ratio leads to a higher power consumption and - through meshed control - to a higher level. Due to the higher level the cossette guidance is reduced, resulting in an optimal filling ratio again. If the level exceeds the maximum limit defined, the speed is increased. If the level falls below the defined minimum limit, the tower drive speed is reduced accordingly. In order to avoid that variations of the press water flow - one of the main disturbances - have to be compensated by level or filling ratio control, they are compensated by an overall water flow control which adds more or less fresh water. This means the total water flow as a process variable is made up of the fresh water flow and the press water flow and is controlled only by controlling the fresh water supply. The press
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
3KDE010003R3001
water is continuously drained off. With this one of the main disturbances that might impair constant and optimal extraction work of the tower is eliminated in advance. The press water pump feed level control is a good example for the different level control loops in the press water circuit. Depending on the type of press water discharge and sterilization, various additional level and temperature controls may be required. The process measurement and control equipment for fresh water treatment depends on various technical factors. In the simplified schematic diagram the fresh water tank is used to compensate variations of the fresh water flow. The volume and type as well as the circulation have an important influence on the quality of pH and temperature control. One way to improve pH control is to connect the fresh water flow as the main disturbance variable with the output value of the pH controller (13). As a result, a specific acid quantity is added to a specific volume of water, to achieve a defined ratio. The pH controller then influences the process by only changing the acid quantity. The continuously measured values (pH and Brix degree in the middle of the extraction tower, Brix degree of press water) allow for continuous monitoring of the extraction value and early recognition of disturbances. Additionally, the following measurements were made for general process monitoring: Pressure measurement: – Mingler, upstream and downstream of the screen – Tower, upstream and downstream of the screen Temperature: – Raw juice – Magma – Cross flow upstream and downstream of the pre-heater – Extraction juice downstream of sand separator – Tower - top - middle - bottom – Press water downstream and upstream of the press water tank Process measurement and control equipment used Temperature: – Pt 100 resistance thermometer with dual measuring inset – Temperature transmitter, 2-wire, temperature linear, output: 4-20 mA, power supply: 24 VPressure: – Pressure transmitter Measuring range: adjustable as required, 2-wire, output: 4-20 mA, power supply: 24 V Process connection: flush diaphragm seal, flange DN80 or DN100 Level: – Transmitter for level, flush diaphragm seal DN 80 or DN100; PN 10 Measuring range: adjustable as required, 2-wire, output: 4-20 mA, power supply: 24V Flow: Electromagnetic flowmeter, remote design – Flowmeter primary Nominal diameter: DN...; PN 10, liner: PTFE, electrodes: Hastelloy C – Flowmeter converter, wall-mounting Flow velocity: adjustable as required, output: 4-20 mA, power supply: 230V/ 50 Hz
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
3KDE010003R3001
Condenser water Beet hopper and slicers
Condensate
Steam Fresh water tank
Defoamer tank Cossette scalder
Steam
Extraction tower Cossette presses Press water tank
Raw juice Tower juice sand separator sand separator Raw juice out to purification
Fig. 2-1: Schematic diagram of process measurement and control in the juice extraction station
2.1 Juice purification In the purification process (Figure 2-4) the non-sugars are to be removed from the raw juice to the greatest possible extent. The result of this process considerably influences the total sugar yield and the sugar quality. The means and methods used for juice purification must be as gentle as possible to avoid unwanted destruction of the sugar. Usually, burned lime is used as an auxiliary agent for purifying the raw juice. A by-product of lime production is carbon dioxide (CO2). Adding carbon dioxide to the juice in the purification process removes excess lime and, thus, improves the juice quality. Various level and temperature control loops are required for continuous juice purification. They contribute to the required process control by ensuring the appropriate response time, i.e. they provide for a constant work process even with different beet material and a varying raw juice flowrate. To ensure gentle treatment of the juices, the raw juice flowrate is usually adapted to the process requirements via speed-controlled pumps. All level controls are realized as discharge controls. This kind of flow control avoids longer dwell times in the individual process sections. Milk of lime is added to the juice in the following process sections: – Intermediate liming – Cold main liming – 1st. carbonation – 2nd. carbonation The added quantity depends on the juice flowrate. As a result, optimal process control can be achieved at any time by adapting the supply of milk of lime in the individual process sections to the requirements.
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
3KDE010003R3001
Appropriate pH control in the first and second carbonation phase is essential for optimal results of process control and juice purification. pH control avoids e.g. turbid juice over-carbonation and, thus, prevents a degradation of the juice quality. Known disturbances in the carbonation process are the juice properties, the juice quantity, the chemical reactions in the carbonation tank, the CO2 gas pressure, CO2 content of the gas and the CO2 gas quantity. The CO2 gas pressure is held at a constant value through pressure control of the distributing system. All other disturbances are either fully compensated in advance through a complex circuit or detected via the pH control and then compensated accordingly. Both approaches have been investigated, and the pH control described in the following section has proved to be quite useful in practical tests. Ideally, pH measurement should be performed at process temperature and pressure and with short dead time. This can be achieved by installing the pH sensor in line i.e. directly in the tank or pipe, using the appropriate process fitting. So far the harsh operating conditions to which the pH electrodes were exposed, i.e. the aggressive juice-lime mixture and temperatures of nearly 100 °C in the 2nd. carbonation were obstructions in the way to efficient in-line measurements with acceptable maintenance requirements and sufficient durability. The pH sensor is installed in a separate glass or stainless steel cell through which the measured medium flows. A sample is continuously taken in via a hose pipe that can be shut off. After measurement the sample is returned to the process in the intermediate liming section. The maintenance and cleaning requirements of the method described here are extremely high, especially after adding milk of lime and CO2 (carbonation) for precipitating non-sugars. In many cases, the electrodes have to be cleaned every eight hours and have an extremely short service life (i.e. they must be replaced several times during one sugar campaign). The precipitated lime compounds plug the hose pipes, causing extensive disassembly and cleaning works during the 4-month production period. The most problematic measuring point is the 2nd. carbonation tank, where the precipitation processes take place at temperatures of approximately 95 °C and at a pH of 8.8 - 9.2, leading to extremely hard deposits on all process-wetted parts. Although similar precipitation processes take place in the intermediate liming tank and the first carbonation tank, the resulting deposits are not that hard. Sensors and process fittings Modern pH sensors are designed as single rod electrode assemblies, i.e. the measuring electrode and the reference electrode are accommodated in one compact body. The sensor diameter of 12 mm commonly used in Germany only allows for few design variants regarding the reference electrode and especially the junction, so that the adaptation to the process requirements is quite difficult. Another approach well-proven in process measurement throughout the world uses sensors with a diameter of approximately 25 mm, allowing for other reference electrode and junction designs (Figure 2-2).
Fig. 2-2: ABB pH sensor, sectional drawing
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
3KDE010003R3001
The solid state reference electrodes used for the pH sensors described here stands out for its high resistance to poisoning and pressure. The wood or Teflon junction with an effective surface of more than 2 cm is resistant to coating with all kinds of particles and deposits. These features together with the high-temperature proof glass element with integrated temperature compensator make this pH sensor the perfect tool for the juice purification process. A hot-tap retractable sensor with stainless steel sheath is used for in-line measurement applications (Figure 2-3).
Fig. 2-3: TB557 Hot-tap retractable sensor
The sensor is protected by a stainless steel sheath of approx. 600 mm with anti-blowout lip which allows for a variable insertion depth. The sheath is fastened by a massive compression fitting. The rinsing chamber attached to the ball valve enables sensor cleaning, e.g. with hot water or dilute hydrochloric acid, prior to retracting. Additionally, it permits to verify the electrode data by loading the appropriate calibration standards via the two ¼“ rinsing connectors. When using the appropriate pH sensors with solid state reference electrode it is also possible to realize in-line pH measurements even at difficult measuring points, e.g. in the second carbonation tank. The advantages over the extractive method are: – – – – – –
considerably reduced maintenance and calibration efforts improved handling convenience improved sensor durability no return of sample liquid (i.e. increase in efficiency) no sampling hose pipes or sampling cells required taking of lab samples directly at the fitting.
A well-proven solution is the use of 2 sets of measuring equipment at each measuring point. One can be connected with the control loop while the other is being maintained or calibrated or used as a second measured value source for comparison. A change-over switch unit provides for maximum operational safety and flexibility. This ensures optimal process guidance and pH control at any time. The control loop is designed such that a specific quantity of CO2 is added to the juice in the first step. This ratio is adjustable and can be changed any time. The output signal E1 of the ratio controller is logically linked with the output signal E2 of the pH controller by a summation element and then supplied to the CO2 gas flow controller as the reference variable A. The weight and thus the influence of the two input signals is determined by the two adjustable factors K1 and K2 in the summation element. The following mathematical function is valid: A = K1 · E1 + K2 · E2. This allows to consider the main disturbance, i.e. fluctuations in the juice flow, in advance. pH control then only has to control the end of carbonation and correct the process is required. The given quantity of CO2 gas is measured with a primary element and controlled via final control elements.
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
3KDE010003R3001
In order to improve the control performance, the constant load is adjusted via a control valve according to the rated power of the plant. Control deviations are compensated through the CO2 flow controller and a small control valve. This setup considerably improves the work capacity of the final control elements and optimizes the control and control dynamics and, thus, the control performance. All flows of juice, milk of lime and soda water are measured through electromagnetic flowmeters. Additionally, the following measurements are made for general monitoring and control: – Pressure: upstream of thin juice 2 filter upstream of thickening filter – Flow: juice upstream of thickening filter juice downstream of pump tank juice downstream of carbonation slurry concentrate 1 tank – Level: carbonation slurry concentrate 1 tank – Flow: filter returned juice sweet water
Process measurement and control equipment used Temperature: – Pt 100 resistance thermometer with dual measuring inset – Temperature transmitter, 2-wire, temperature linear, output: 4-20 mA, power supply: 24 VPressure: – Pressure transmitter, process connection G1/2“ external thread Measuring range: adjustable as required, 2-wire, output: 4-20 mA, power supply: 24 V Level: – Transmitter for level DN80 or DN100; PN 10 with flush diaphragm seal Measuring range: adjustable as required, 2-wire, output: 4-20 mA, power supply: 24 V Flow: Electromagnetic flowmeter, remote design – Flowmeter primary Nominal diameter: DN...; PN 10, liner: PTFE, electrodes: Hastelloy – Flowmeter converter, wall-mounting Flow velocity: adjustable as required Output: 4-20 mA, power supply: 230 V/ 50 Hz – CO2total quantity flowmeter Segment orifice for in-line mounting between flanges in accordance with DIN Nominal diameter: DN 300, PN10, material: stainless steel 1.4571 – Transmitter for differential pressure, 2-wire Measuring range: adjustable Process connections: 1/2" NPT, output: 4-20 mA, power supply: 24 V-
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
3KDE010003R3001
pH: – pH sensor TB557 25 mm with solid state reference, with stainless steel retractable sensor and rinsing chamber, ball valve 11/2", insertion length 600 or 900 mm, pressure-resistant up to 6.9 bar at 140 °C max. temperature 140 °C – 2-wire pH transmitter 4-20 mA, with multifunctional LCD, for pH/redox/ion-sensitive electrodes, sensor diagnostics in conjunction with TBX sensors, Pt100 resistance thermometer and 3 kΩ Balco, connectable, electrically isolated analog output, 4-20 mA optionally with HART or PROFIBUS DP communication interface, power supply 13...53 V DC, PU-coated aluminum housing 144 x 144 mm, with mounting kit for wall or pipe mounting
Milk of lime
Mixed juice tank Intermed. liming
Milk of lime
Cold main liming
Warm main liming
Pump 1st carbonation tank
Raw juice
Steam
Steam
Steam
Milk of lime
Thickening filter
Thin juice 1 tank
Soda 2nd carbonation
Pump tank
Reaction tank Steam Carbonation slurry concentrate 1 tank
Thin juice 2 filter and safety filter Thin juice
To first carbonation
Filter presses Carbonation slurry, to mud ponds
Carbonation slurry concentrate 1 tank
Fig. 2-4: Schematic diagram of process measurement and control in juice purification
2.2 Evaporator station The evaporator station forms an important link between the juice production and crystallization parts of the sugar production plant. Basically, it has the following main tasks: – Evaporation of the water content to thicken the purified thin juice with approx. 15 % dry substance to a thick juice with 68 % to 71 % dry substance. – Supply of heating steam to the individual plant parts like juice production, purification, sugar house, etc. – Supply of condensate 1 to the boiler house for feed water
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
3KDE010003R3001
The 5-stage evaporator station seen in Figure 2-5 is provided with the basic and necessary process measurement and control equipment. Further optimization of the evaporator station automation is possible, depending on possible additional requirements. Usually, the evaporator station comprises 4 to 5 evaporator stages, of which the first ones consist of several evaporators. As the pressure in the evaporators is controlled in such a way that it decreases from stage to stage, the juice passes the entire evaporator station. Once the required dry substance (density) is reached, the thick juice is discharged from the last stage of the evaporator station. Basically, the following variables are controlled in the evaporator station: – Level of the individual evaporator stages – Dry substance (density) of the thick juice It is mandatory for optimal evaporation in each evaporator that a fixed juice level above the calandria is always maintained. Ideally, the calandria should be only just covered with boiling juice. An excessive juice level reduces the flow rate of the juice-steam-mixture in the calandria tubes and, thus, the heat transition. A too low juice level causes poor evaporator performance due to bad heat transmission and, thus, increases the energy demand. Seen from the measuring point of view the "apparent" juice level is determined by level transmitters using the differential pressure method. Transmitters with flange remote seal with extended diaphragm on the + and - side have proved to be especially suitable for this application. The +side of the transmitter is connected at the bottom of the juice chamber; the -side is connected on top right underneath the juice catcher at the evaporator steam chamber. The lenghts of the transmitters’ capillary tubes for the + side and the -side must be adapted to the existing evaporators. The evaporator level control can be realized as supply or discharge control in conjunction with the evaporator station flow control. In case of supply control the juice supply is increased or decreased, depending on the juice level in the evaporator. In case of discharge control the juice discharge is controlled as a function of the juice level in the evaporator. The main disturbance of level control and thick juice density control in the last evaporator stage is the varying steam consumption of the boiling house at the fourth evaporator stage. It causes a considerable juice concentration and, thus, decreases the juice level in this stage. When using a discharge control, this temporary disturbance is compensated by reducing the discharge. On the contrary, a supply control system would increase the supplied juice quantity. As a result, this disturbance would have an effect up to the evaporator station input. Due to the time response of each evaporator stage dynamic control problems for the level controls and, thus, a decrease in the evaporator station efficiency would result from permanent fluctuations in the juice flow. In order to reduce the influence of this disturbance, the pressure in the evaporator steam chamber is additionally limited by pressure control. In the sugar house work described in the following section every discontinuous juice boiler is provided with a steam pressure control. Discharge level controls at the thin juice intake tank provide for integration and smoothing of the individual thin juice flows from the purification station, which are then supplied to the evaporator station with minor gain from the ratio controller. In case of a minimum filling level in the thin juice intake tank "emergency feed water" is supplied to ensure that sufficient liquid is always available to the evaporator station. If the evaporator station, especially its last stage, is dimensioned accordingly, the thick juice density can be controlled via the temperature difference in the last evaporator stage, under consideration of the condensate quantity from the boiler house. It is an advantage if the last evaporator stage is equipped with a downdraft evaporator. A shorter dwell time of the juice in the evaporator results from a reduced juice volume, leading to a better transient behavior in the controlled system. In order to avoid that the downdraft evaporator runs dry, the circulating juice volume is measured by an electromagnetic flowmeter. If the circulating juice quantity should be insufficient, condensate is added via a special circuit. In some sugar factories the thick juice volume in the last stage is additionally measured with a mass flowmeter to allow for sugar house balances. In the so-called "condensate cigar" the condensate from the evaporator stages and the sugar house is collected and expanded step by step. The condensate from evaporator stage 1 is supplied as feed water or injection water to the power station, via a level discharge control from compartment 1. Prior to supply to the boiler house the condensate is checked for residual sugar.
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
3KDE010003R3001
This is achieved by using a continuous flame photometer. The sugar content of the condensate is determined by measuring the potassium content. When sugar is found in the condensate, the 3-way-valve is triggered via alarm signalling. The condensate is then diverted to compartment 2 of the "condensate cigar" and reaches the hot water tank after being expanded in several steps. Level control of the individual "condensate cigar" compartments ensures undisturbed operation of the evaporator station. Process measurement and control equipment used Temperature: – Pt 100 resistance thermometer with dual measuring inset – Temperature transmitter, 2-wire, temperature linear, output: 4-20 mA, power supply: 24 VPressure: – Pressure transmitter, connection G1/2“ external thread Measuring range: adjustable as required, 2-wire, output: 4-20 mA, power supply: 24 V Level: – Transmitter for level, extended diaphragm DN 80 or DN100; PN 10 Measuring range: adjustable as required, 2-wire, output: 4-20 mA, power supply: 24 V Flow: Electromagnetic flowmeter, remote design – Flowmeter primary Nominal diameter: DN...; PN 10, liner: PTFE, electrodes: Hastelloy – Flowmeter converter, wall-mounting Flow velocity: adjustable as required Output: 4-20 mA, power supply: 230 V/ 50 Hz
To boiling house Thin juice from purification
Thick juice tank
To central condenser
Residual condensate
Heating steam
Water
To sugar house
Thin juice tank
Exp. steam
Condensate, to power station
Condensate tank, sugar house
Condensate cigar
Fig. 2-5: Schematic diagram of process measurement and control in the evaporator station
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To hot water tank
Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
3KDE010003R3001
2.3 Lime kiln The burned lime for the milk of lime and the carbon dioxide for purification are produced in the lime kiln (Figure 2-6). Process automation shall adapt the lime kiln performance to the production process and provide for constant supply of carbon dioxide and milk of lime. To achieve high control performance in the purification process possible disturbances have to be compensated or eliminated in advance. A control system ensures that the lime kiln is charged with the appropriate quantity of lime rock and coke. It is important to monitor and verify the lime kiln products i.e. the burned lime and lime kiln gas in order to be able to evaluate whether or not the ratio of mixture is correct. Level measurement is for the most part realized by using radiometric measuring equipment. This type of measurement ensures smooth and uninterrupted charging and discharging of the lime kiln. Among other equipment a temperature monitoring system installed in the combustion zone contributes to optimal process control. The combustion zone is continuously monitored by using thermocouples connected to subsequent electrical temperature transmitters. Three measurements on three levels are made on the circumference of the kiln. The fire is kept at a constant height through kiln gas flow control in dependence of the lime discharge. The flow measurement takes place at the suction side of the CO2 compressors and is realized via a standardized venturi flow meter. Flow control is achieved via gas feedback to the suction side of the CO2 compressors. The gas temperature and pressure are measured at the kiln outlet and downstream the CO2 washer for general process monitoring. The level control system of the water separator continuously discharges the water produced. The CO2 content of the lime kiln gas is permanently determined by a heatconductive gas analyzer, to allow for automatic process monitoring and for production in compliance with the latest limit values. The CO2 gas pressure is held at a constant value through pressure control in the distributing system. This pressure control system provides for a constant pre-pressure for all consumers. The pressure controller output signal is supplied to two control valves connected in parallel. They are used to drain off excess gas via the roof. As variations occur in the gas consumption of various consumers whereas the produced total gas quantity is constant, the range of a single control valve is not sufficient to meet the requirements. Therefore, two control valves with different operating ranges are used. – Control valve 1: 4-12 mA = 0-100 % stroke – Control valve 2: 12-20 mA = 0-100 % stroke This improves the control performance and dynamics of pressure control and pH control in the carbonation process. The burned lime is output to the lime slaking drum where sweet water or condensate is added to slake it. The burned lime is discharged from a storage bin via a vibrating conveyor. The discharge is controlled by a timer in dependence of the slaking water flow. This ratio roughly defines the density of the milk of lime. It is slightly higher than the wanted value of approx. 20 Beaumé. The milk of lime then continues its travel through a sand classifier to the raw lime milk tank. A stirrer in this tank accelerates the slaking process and provides for homogenization. In the flow fed back from the raw lime milk tank to the sand classifier the lime milk density is measured by radiometric equipment. The desired lime milk density is achieved by adding more slaking water in the sand classifier. The subsequent operating lime milk tank and the ring pipeline for lime milk distribution offer sufficient volume to provide a buffer which can be used to compensate for density variations of the lime milk. As a result, in most cases no additional density measurement of the operating lime milk with cascade control is required. The level control system in the operating lime milk tank also influences the quantity of the slaking water. With correct optimization this suffices to adapt continuous lime milk production to the operating conditions. Process measurement and control equipment used Temperature: – Temperature sensor, thermocouple with dual measuring inset – Temperature transmitter, 2-wire, temperature linear, output: 4-20 mA, power supply: 24 VPressure: – Pressure transmitter, process connection G1/2“ external thread Measuring range: adjustable as required, 2-wire, output: 4-20 mA, power supply: 24 V -
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
3KDE010003R3001
Level: – Transmitter for level, flush diaphragm seal DN 80 or DN100; PN 10 Measuring range: adjustable as required, 2-wire, output: 4-20 mA, power supply: 24 V Slaking water flow: Electromagnetic flowmeter, remote design – Flowmeter primary Nominal diameter: DN...; PN 10, liner: PTFE, electrodes: Hastelloy C – Flowmeter converter, wall-mounting Flow velocity: adjustable as required Output: 4-20 mA, power supply: 230 V/ 50 Hz Radiometric density measuring equipment for lime kiln filling level measurement (recognition of lime rock/coke/empty at the charging bin) Radiometric density measurement LB 444 – Evaluating computer, single-channel, 2-wire, CS137 with shield Measuring range: can be calibrated as required, output: 4-20 mA, power supply: 230 V/50 Hz or 24 V AC/DC
Fresh water Via roof
To juice purification
Lime kiln
CO2 washer
Level 1 Level 2 Level 3 Sweet water or condensate To juice purification
CO2 distributor Ring pipeline
Water separator Lime slaking drum
Sand classifier
Water Raw lime milk tank
Operating lime milk tank
Fig. 2-6: Schematic diagram of process measurement and control in the lime station
Equipment for lime milk density measurement Microwave measuring system Micro-Polar Brix LB 565 – Gauge head DN 65/50 (in-line measurement), evaluation computer, single-channel, Measuring range: can be calibrated as required, output: 4-20 mA, power supply: 90-260 V/50 Hz or 24 V AC/DC
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
3KDE010003R3001
3 Process control in the sugar house Introduction Since the end of the sixties various papers and patents dealing with a new crystallization technique that is exclusively based on the magma's flow properties have been published. Major advances have been achieved in the automation of the sugar house processes by developing transmitters that are capable of measuring all important rheological state variables during the entire boiling process in discontinuous juice boilers (vacuum pans). Early automation approaches for sugar house processes still used pneumatic or electrical transmitters and standard controllers. A fully automatic boiling process controlled by a distributed process control system was invented in Germany in the year 1978 sugar campaign. Since these early days the use of process control systems for this sugar industry application has proved to be a very good investment. Special software modules like controller modules, calculating modules, memory modules and sequence modules with the respective safety modules and step modules have contributed to a considerable improvement in the automation of sugar house boiling processes. Important research results from the German "Zuckerinstitut" (institute for sugar research) in Braunschweig like – production of separate seed magma through cooling crystallization, – installation of stirring devices in juice boilers, – continuous evaporating crystallization, etc. allowed for further progress in sugar house automation, especially regarding the sugar crystallization. The following section describes the individual production stations in the sugar house and details the state of the art of sugar house work. In the example described here a process control system from ABB is used for process automation.
3.1 Control of discontinuous juice boilers In the last few years, more and more micro-wave-based density measuring systems have been installed in the juice boilers for measuring the rheological magma properties. Primary elements of this kind measure the content of dry substances in the entire automated section of the boiling system. Fully automatic control of the boiling process requires the measurement and evaluation of the following physical variables. Dry substance content – Micro-Polar Brix LB 565, immersion probe, DN 65, for in-line mounting in tanks/vessels Evaluation computer, single-channel, measuring range: can be calibrated as required, Output: 4-20 mA, power supply: 90-260 V/50 Hz or 24 V AC/DC Level An electrical transmitter for differential pressure is installed for measuring the filling level Design: – 2-wire transmitter, flange design, vacuum-resistant – Plus side: flange DN80, PN10 with extended diaphragm, 100 mm – Minus side: flange DN80, PN10, flush diaphragm, with 6 m capillary tube Measuring range: 0-600 mbar, output: 4-20 mA, power supply: 24 VVacuum Electrical pressure transmitter, 2-wire Type: for absolute pressure Connection: 1/2" NPT, internal Measuring range: 0-1 bar abs., output: 4-20 mA, power supply: 24 V-
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
3KDE010003R3001
Pressure of heating steam Electrical pressure transmitter, 2-wire Type: for absolute pressure Connection: 1/2" NPT, internal Measuring range: 0-1.5 bars abs., output: 4-20 mA, power supply: 24 VTemperature: Dual resistance thermometer with head-mounted transmitter Insertion length: 360 mm Connection: 1" external thread Measuring range: 50-150 °C, output: 4-20 mA, power supply: 24 VSeed magma volume Electromagnetic flowmeter, remote mounting, with AC magnetic field excitation and absolute zero stability Liner: thick PTFE (vacuum-proof) Electrodes: Hastelloy C, output: 4-20 mA, power supply: 220 V/50 Hz The listed physical variables are linked with each other in a useful way in the process control system of the boiling system. A fully automatic boiling system for raw sugar and white sugar comprises the individual phases detailed below: START phase The prerequisite for controlling a discontinuous juice boiler is that all shutting valves (OPEN-CLOSE) on the boiler are closed. The valves listed below for juice, water, boiling steam and heating steam are provided with electro-pneumatic positioners and are also closed. A sequence of stored step modules is used to poll the states of all mechanical limit switches or proximity switches in 3-wire circuits to check if they are all in the “0” state (i.e. all valves are closed). If one of the shutt-off valves should be open, the "CLOSE all valves" command is output to close it. If one of the control valves should be open, the sequence sets all software controllers to manual mode and the controller outputs to “0”. Then it returns to the start of the logic and checks if all valves and final control elements have the logic state "0". If the requested conditions are met, the sequence passes on to the "MAX vacuum" phase. MAX vacuum phase Within the last few year it has become more and more established to evacuate the vacuum pan only when new liquor is to be boiled, for the following reasons: In case of leakages evacuating the vacuum pan although it is currently not in use would mean an unnecessary load of the overall vacuum system. The preevacuation is realized via a valve of DN80, PN10. In case of a partial evacuation of approx. 0.3 bars absolute (limit value 1 of the vacuum indicator), the valve remains open, and the sequence sets the vacuum controller to automatic mode. The setpoint is set to a fixed value of 0.2 bars and remains unchanged for the entire boiling process. Limit value 2 (0.03 bars) in the vacuum controller is set in a certain proportion to the setpoint, i.e. when the setpoint increases or decreases the limit value increases/decreases proportionally. A ramp function is provided for the controller output by the software, i.e. the controller output continuously changes within a defined time and in accordance with the control parameter settings. When limit value 2 (0.23 bars) is reached, the sequence passes on to the Feed phase. FEED phase Before the sequence starts this phase, the feed liquor tank is polled to check whether or not sufficient liquor is available for control. In order to reduce the liquor feed time, a feed valve is installed in parallel with the liquor control valve in most of the vacuum pans.
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
3KDE010003R3001
In this phase, the sequence executes the following important "steps": – Open the parallel liquor feed valve – Set the "Level density" controller to automatic mode – Fetch the setpoint for the "Level density" controller (located approx. 10 cm above the calandria) from a storage location in the parameter module and write it to the "Level density" controller. As the "Level density" controller has a double function in the boiling automatic, the following section details the functional principle of the controller in this phase: The "Level density" controller is a pure proportional controller with maual reset. With increasing level the controller output decreases in dependence of the control deviation (process value - setpoint), in order to maintain the "calandria covered" level. The controller action "increase of controlled variable causes decrease of output variable" is set by the sequence in this phase. A calculation module as a selection relay is "switched into the circuit" before the "level density controller" from the software side. This module has 2 inputs and 1 output. Assignment: Input 1: Level; physical variable = 0-100 % Input 2: Density; physical variable = 0-100 % TS (dry substance) In this phase the level is connected through by the sequence as the controlled variable of the "Level density" controller. When reaching level limit 1 (approx. 15 %) the sequence initiates the following steps: – Switch stirring device ON (quick mode) – Set heating steam pressure controller to automatic mode The setpoint (approx. 0.9 bar) is fetched from a storage location of a parameter module and then written to the software controller. The controller output is changed over a given time and in accordance with the control deviation (process value - setpoint) by a ramp function. This ensures a "slower" change of the control behavior. A level limit value of -2 % is set in proportion to the setpoint (calandria covered). When this limit value is reached, the parallel liquor feed valve is closed. The level density controller then automatically controls the level until reaching the setpoint, in dependence of the evaporation through the heating steam. The continuous evaporation thickens the magma, and the magma is super-saturated until reaching the desired seed point. During the boiling process the sugar crystallization is a result of water evaporation.
yü
Super-saturation coefficient
yü =
q(Z/W)Lös. q(Z/W)sat
ϑ [°C]
Solution temperature
yü =
wTS.Lös. · qLös. (1 - wTS.Lös.) · q(Z/W)sat
qLös.
Purity of the solution
q(Z/W)sat =
q(Z/W)sat
Sugar/waterratio in the state of saturation
wTS.sat
Dry substance of the solution in the state of saturation
wTS.sat · qLös. 1 - wTS.sat
for pure solutions: wTS.sat = 0.64397 + 7.251 · 10-4 · ϑ + 2.0569 · ϑ2 - 9.035 · 10 - 8 · ϑ3 Fig. 3-1: Formulas
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
3KDE010003R3001
Although the evaporation and crystallization processes are closely related to each other, they must be considered independently. The evaporation process provides for a concentration of the sugar and nonsugars in the solution fed to the juice boiler (vacuum pan). Additionally, the evaporation lets the magma circulate and, thus, transports the substances. The crystallization process, on the contrary, is basically determined by the super-saturation as the moving force, the existing infant crystals, the size of the crystal surfaces, and the transport intensity of the dissolved sugar to the crystal surface. As there is a direct relation between the super-saturation and the rheological properties of the magma, the seed point of the liquor is only given by super-saturation. In "Seed magma" mode the super-saturation seed point has a value of 1.0, as compared to a value of 1.1 in "Slurry" mode. Super-saturation is governed by a third degree polynomial. The variables: The dry substance content (75-95 %) of the radiometric density measuring system and the liquor temperature (depending on the vacuum) are available as real measuring variables. The purity of the magma is a variable defined in the lab and may change within one sugar campaign. Prior to starting the process, the operator has to select the mode i.e. whether the liquor is to be seeded with slurry or seed magma. Depending on the selected mode the sequence then fetches the corresponding seed point from the storage location of a parameter module and writes it as the limit value to the calculating module which calculates the super-saturation. Via the sequence a fixed value (e.g. 0.02) is automatically subtracted from the selected limit value and then set as the "pre-seeding contact" in the calculating module. When reaching the "pre-seeding contact" the sequence fetches a fixed value (e.g. 0.78 bars) from the storage location of a parameter module and writes it as the setpoint to the heating steam controller. This step is very important from the process control point of view, as the evaporation speed is reduced right before the seed point. The liquor is further thickened, and when reaching the seed point (super-saturation) the sequence passes on to the Seeding phase.
Vacuum Steam out Air in
Steam
Density
Thick juice Remelt syrup Water
Slurry Seed magma
Outlet
Fig. 3-2: Discontinuous juice boiler
SEEDING phase (Slurry mode) The sequence performs the following steps: – OPEN the seed valve – Supply density value to level density controller as the controlled variable (0-100 % value) – Start time function element "Seeding" When reaching the given nominal time (approx. 5 sec.) the seed valve is closed. The sequence passes on to the Crystallization phase.
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
3KDE010003R3001
SEEDING phase (Seed magma mode) The sequence performs the following steps – Polls the position of the seed magma feed valves (must be closed) and the filling level of the crystallization liquor feed mingler (must be sufficient). If yes, there are two possible ways to feed the seed magma: First solution (very exact): An electromagnetic flowmeter is mounted in the ring pipeline in the feed flow and in the return flow, each. The feed quantity difference is determined by a calculating module. When opening the feed valve, a software integrator is set to automatic mode. The flow difference is integrated and compared with a limit value read from a parameter module (module for recipe storage). When reaching the limit value, the feed valve is closed, and the sequence passes on to the Crystallization phase. Second solution The fixed increase in volume by adding the seed magma is defined as the level limit value 3, in relation with the level setpoint (calandria covered). The sequence performs the following steps – Polls whether or not all feed valves of the ring pipeline are closed and sufficient seed magma is available in the feed liquor mingler. If yes, the following steps are performed: – OPEN feed valve – When reaching limit level 3, the feed valve is closed. – The density value is supplied to the "Level density" controller as the controlled variable. The sequence passes on to the Crystallization phase. CRYSTALLIZATION phase In this phase the crystals shall distribute all over the vacuum pan. The appropriate nominal time for crystallization is set in the software time function element "Crystallization", depending on whether "Slurry mode" or "Seed magma mode" has been selected. The different lines can be found in different storage locations of a parameter module. The following steps are performed: – Set the "Level density" controller to manual mode. – Set the controller output to the value -5 % , i.e. the liquor control valve remains closed in this phase. The nominal time from the storage location of a parameter module is set as the limit value in the software timer "Crystallization", depending on whether "Slurry mode" or "Seed crystal mode" has been selected. For seeding with slurry: Nominal time: approx. 1 min. For seeding with seed magma: Nominal time: approx. 6-7 min. – Start timer – Poll if nominal time has been reached. If yes, the software time function element is set to "0", and the control passes on to the Control phase. Annotation: The long nominal time granted for crystallization (due to the big volume percent of seed magma) is required to enable the radiometric density measuring equipment to find a representative rheological value for the subsequent Control phase. In the Seeding phase and in the Crystallization phase the vacuum control and the heating steam pressure control stay in automatic mode and hold the given setpoint of the physical variable.
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
3KDE010003R3001
CONTROL phase In this phase, the sequence performs the following steps: – Write the density value to “Level density” controller as the controlled variable (0-100% TS) – Fetch reset time TR from the storage location of a parameter module and write it to the "Level density" controller. – Change over the software bit "Direct/reverse mode/ to "Direct mode", i.e. when the controlled variable increases as compared to the setpoint, the controller output will increase in accordance with the set PI parameters. – Set the "Level density" controller to manual mode. – Set the "PVT" (Process variable tracking) software bit to state "1", i.e. this software bit causes that the setpoint of the "Level density" controller is set to the value of the currently controlled variable. Seen from the process control point of view this ensures that, at the start of the Control phase, the setpoint is equal to the process value and no controller bumps or overshoots occur. – Set heating steam pressure controller to external setpoint. The level of the vacuum pan is connected as the reference variable, i.e. with increasing level the setpoint of the heating steam pressure controller increases as well. – Set the "Level density" controller to automatic mode – Set "Level density" controller to external setpoint. The "Level density" controller automatically controls the rheological value of the magma in accordance with the boiling curve stored in a calculating module. The constellation: The rheological value of the level can be stored in seven segments. 4 fixed points are sufficient for controlling the boiling liquor. B0 = K0 = B1 = K1 =
"Calandria covered" level Initial value of the dry substance content (percentage of dry substance at seeding point). "Vacuum pan filled" level Final value of the desired dry substance of the magma with full vacuum pan (value as a percentage of the dry substance)
Value table: B0= 33 % Level K0= 23.5 % Value TS B1= 96 % Level K1= 93 % Value TS
Level %
Fig. 3-3: Boiling curve for Slurry mode
In order to avoid possible overshooting of the 'Level density" controller with increasing level a new reset time value is fetched from the storage location of a parameter module and written to the "Level density" controller at a level of approx. 55 % (limit value 4). Additionally, the heating steam pressure controller is changed over to the internal setpoint when this limit value is reached. A new setpoint (approx. 0.95 bars) is fetched from the storage location of a parameter module and then set in the heating steam pressure
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
3KDE010003R3001
controller as the setpoint. When reaching limit value 5 (MAX level) the sequence asks whether or not the operator has selected the Hold phase consciously. If the Hold bit is set, the sequence passes on to the Hold phase. If the Hold bit is not set, the sequence passes on to the Boiling phase. HOLD phase There are two ways to select the Hold phase: 1.) User-selected. This way is used e.g. if not enough thick juice is available in the feed mingler to allow for proper pre-boiling. 2.) The sequence automatically selects the hold phase if the level in the discharge mingler is not "good" when the filling level in the vacuum pan reaches its maximum value, i.e. there is not enough space in the discharge mingler to empty the vacuum pan. A separate software density controller controls the magma density in dependence of the control deviation (density - setpoint) via the water control valve. When the controller is in manual mode, the setpoint is tracked to the density as the controlled variable. The following steps are performed from the process control point of view: – Set the density controller to automatic mode The dry substance content of the magma is held at its value via the water control valve, in dependence of the control deviation. – Fetch a new setpoint (approx. 0.78 bars) from a storage location of a parameter module and then write it to the heating steam controller. The sequence remains in this phase until the user set the hold bit to 0 or the level in the discharge mingler reaches the "good" range. If one of the listed conditions is fulfilled, the sequence performs the following steps: – Set the density controller to manual mode – Fetch the -5 % value from the storage location of a parameter module and set the density controller output to this value. The water control valve moves to the Close position. – The sequence returns to the Control phase.
Seed point
Fig. 3-4: Control curves for discontinuous juice boiler
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
3KDE010003R3001
BOILING phase In this phase, the sequence performs the following steps: – Switch the stirring device from "quick motion" to "slow motion". – Set the "Level density" controller to manual mode. – Fetch the -5% value from the storage location of a parameter module and set the controller output to this value. The liquor control valve moves to the Close position. – Set the heating steam controller to manual mode – Set the vacuum controller to manual mode – Fetch the 100 % value from the storage location of a parameter module and set the outputs of both controllers to this value. This measure allows to reach the boiling point in the “shortest possible” time. – The sequence polls the following limit values (logically ORed) Has the maximum density limit (limit value 3) been reached? Has the maximum stirring device current limit (limit value 1) been reached? When one of theses two limit values is reached, the liquor has boiled sufficiently, and the sequence passes on to the End of Boiling phase. END OF BOILING phase In this phase, the sequence performs the following steps: – Set the heating steam controller to manual mode – Fetch the -5 % value from the storage location of a parameter module and set the controller output to this value. – Poll the state of the heating steam valve (must be closed) If the heating steam valve is closed, the sequence passes on to the MIN Vacuum phase. MIN VACUUM phase In this phase, the sequence performs the following steps: – CLOSE the pre-vacuum valve – Set the vacuum controller to manual mode – Fetch the -5% value from the storage location of a parameter module and set the controller output to this value. – Poll the state of the pre-vacuum valve and the main vacuum valve (must be closed). If both valves are closed, the sequence passes on to the Aeration phase. AERATION phase In order to save time, the vacuum pan under vacuum is exclusively aerated via the steam outlet valve. The sequence performs the following steps – OPEN steam outlet valve – Check whether or not vacuum limit value 3 has been reached. (set value = 1 bar atmospheric pressure) When the limit value is reached, the sequence passes on to the Discharge phase. DISCHARGE phase In this phase, the sequence performs the following steps: – Fetch the 100% value from the storage location of a parameter module and set a controlling device output to this value. (The discharge valve is a pneumatic valve with an electro-pneumatic positioner). The level decreases until reaching level limit 1 (15 %). – When reaching the limit value, the sequence starts a software timer for the discharge. – When reaching the given nominal time (approx. 5 min.), the software timer is set to 0. The sequence passes on to the Steam Out phase.
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
3KDE010003R3001
STEAM OUT phase In this phase, the sequence performs the following steps: – Fetch the 30 % value from the storage location of a parameter module and set the output of a controlling device for the outlet valve to this value. The sequence can terminate the Steam out phase of the vacuum pan in different ways: 1.) The sequence polls the temperature limit value (approx. 100 °C). When reaching this temperature, the sequence starts the Steam Out software timer. (Nominal time: approx. 7 min.). When this nominal time is over, the Steam Out timer is set to 0 and the sequence passes on to the Start phase. 2.) The temperature limit value and the steam outlet time are logically ORed. When reaching the temperature limit or the defined outlet time, the Steam Out timer is set to 0. 3.) The sequence starts the Steam Out timer. When this nominal time is over, the Steam Out timer is set to 0 and the sequence passes on to the Start phase. This terminates the boiling cycle, and the sequence is now ready to start with new liquor. Vacuum Steam
Magma
Feed
Fig. 3-5: R/I diagram of a VKH factory
3.2 Continuous evaporating crystallization Continuous evaporating crystallization has been successfully used in the sugar industry since the early 80ies. As compared to discontinuous evaporating crystallization it has the following advantages: – The improve in heat transmission resulting from the constantly low magma filling level (approx. 30 cm) above the calandria means a reduced driving temperature drop (difference between heating steam and magma) for the operation side. – Continuous operation results in a constant consumption of magma and heating steam and, thus, in constant operating conditions, which have a positive effect on both the product quality and the energy consumption. – In the case of new investments made in the sugar house part of the sugar factory continuous evaporating crystallization reduces the building volume. The prerequisite for continuous evaporating crystallization, however, is the automation and high-quality production of seed magma, with reproducible crystal size distribution. Seed magma production has been an important quality standard criterion and is further detailed in the following section.
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
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There are four different types of apparatuses which have already been used for years and have proved to be reliable: – continuously working evaporating crystallization tower from BMA Braunschweig, with forced circulation of raw sugar, white sugar and after product sugar. – continuously working evaporating crystallizer with natural circulation from Fives-Cail-Babcock (FCB apparatus) for raw sugar and after product sugar. – continuously working evaporating crystallizer with natural circulation from Torgaat-Hulett (TH apparatus) for after product sugar. – continuously working horizontal evaporating crystallizer (VKH) for after product sugar. Basically, this system consists of the following units: – Cooling crystallization system – discontinuous stirring device crystallizer with seed magma feed mingler and seed magma collector tank. – VKH apparatus with four crystallization chambers and one feed tank for the feed magma.
The following section describes examples of the most important process measurement and control loops. The four continuously working crystallization chambers have the following identical equipment: Level measurement Electrical transmitter for differential pressure, 2-wire, flange design, vacuum-proof – Plus side: DN80, PN10 with extended diaphragm, 100 mm – Minus side: flange DN80, PN10 with flush diaphragm seal and 4 m capillary tube Measuring range: 0-250 mbars, output: 4-20 mA, power supply: 24 VVacuum Electrical pressure transmitter, 2-wire Type: for absolute pressure Connection: 1/2" NPT, internal Measuring range: 0-1 bars abs. adjusted to: 0-0.5 bars abs., output: 4-20 mA, power supply: 24 VPressure of heating steam Electrical pressure transmitter, 2-wire Type: for absolute pressure Connection: 1/2" NPT, internal Measuring range: 0-1.6 bars abs., output: 4-20 mA, power supply: 24 VTemperature: Dual resistance thermometer, quick response, Class A to DIN Insertion length: 250 or 360 mm, with separate transmitter, 2-wire Measuring range: 0-100 °C, Output: 4-20 mA, Power supply: 24 VVolume measurement of seed magma Volume measurement of green run-off syrup from chambers 1-4 Volume measurement of condensate Electromagnetic flowmeter FSM4000 with AC magnetic field excitation and zero stability. Liner: thick PTFE (vacuum-proof) for seed magma and green syrup Liner: PTFE for condensate Electrodes: Hastelloy C, output: 4-20 mA, power supply: 220 V/50 Hz Annotation: Seen from the physical point of view seed magma is a 2-phase mixture. In the continuous evaporating crystallization process the listed physical variables are combined with each other in a useful way. The most important control loops for the continuous crystallization in the VKH have the following setup:
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
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Vacuum control The four vacuum pans are connected to a vacuum main. The chambers 1-4 are provided with a separate vacuum control, each. Fixed value control through a standard software controller with proportional integral action is used here. It is important for this control that the absolute pressure transmitters are set to a measuring range of 0-0.5 bars abs. to achieve a high sensitivity. The measuring range should have a resolution of 3 digits to the right of the decimal point. The listed setpoints for the individual control loops for temperature (measurement of the increase in the boiling point and, thus, assignment to the dry substances in chambers 1-4) and the heating steam pressure controls (according to the simplified R/I diagram) are calculated for a vacuum setpoint of 0.2 bars abs. The vacuum is held at the given setpoint by the control valves (with electro-pneumatic positioners) installed in the steam exhaust pipes of chambers 1-4. Heating steam pressure control Chambers 1-4 are provided with a separate heating steam pressure control, each. Fixed value control with a standard software controller with proportional integral action is used here. The individual setpoints of chambers 1-4 an be seen in the R/I diagram. Annotation: As the steam pressures in chambers 1 to 4 always have a specific ratio to each other, a common external setpoint generator is connected to the four heating steam pressure controllers to adjust all setpoints. To adapt every external setpoint individually, a multiplication factor (K1) with which the given setpoint is multiplied is assigned to each external setpoint in the software controller. The controllers are set to the external setpoint.
Boiling steam Superheated steam
Fig. 3-6: VKH data
Temperature control (cascade) Chambers 1 to 4 are equipped with a cascade control system for temperature/flow control, each. Master controller (temperature controller) High-precision sensors must be used for temperature measurement. The transmitters are set to a common measuring range of 0-100 °C. Refer to the simplified R/I diagram for the setpoints. The controller used is a pure proportional controller with operating point setting. Due to its special structure the proportional controller has a "permanent control deviation". However, the advantage of such a controller is that the controller output cannot "open" further, due to the missing integral action. The controller output is input to the slave flow controller as the external setpoint. Flow controller Chambers 1 to 4 have a separate feed pipe for green syrup, each. An electromagnetic flowmeter in remote design is used for flow measurement. Specified flow volumes per hour shall be fed/taken in, according to the specifications for continuous crystallization (VKH). The specified values are used as the
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
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setpoint for the software flow controller. The software controller is a standard controller with proportional/ integral action. The controller is set to the external setpoint. The setpoint is adjusted "gradually", according to the temperature deviation in the master controller. Annotation: It is strongly recommended to use control valves with electro-pneumatic positioners or ball valves with control characteristic as the final control elements.
Volume SM
Volume GS 1
Volume GS 2
Volume GS 3
Volume GS 4
Fig. 3-7: Software modules – Ratio of SM and RS flow
Seed magma flow control in chamber 1 The flow control is realized as a ratio control system in dependence of the green syrup feed to chambers 1-4. The green syrup flows are added up in 2 software calculating modules. In a subsequent software calculating module the ratio (relation between all feed flows and the required ratio) is calculated. The ratio has been defined as follows: 20 % of the total green syrup flow to chambers 1 to 4 shall be feed to chamber 1 as seed magma flow. The required ratio is set in a software controller. The flow controller is a standard software controller with proportional action. The controller is set to the external setpoint. For this controller a control zone of 0.1 m3/h of the setpoint is given, i.e. within this zone the controller's proportional action is not active. The final control element for the seed magma is a pump with a frequency converter. The 4...20 mA controller output continuously readjusts the pump speed in dependence of the control deviation.
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
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Level control in chamber 4 Chambers 1 to 4 are provided with one level transmitter, each. As the chambers 1 to 3 are linked with each other, the magma level is the same in all of them. The level in chamber 4 is a fixed control and is realized as a sequence control. The software controller is a standard controller with proportional/integral action. The final control element for the magma outflow from chamber 4 is a pump with a frequency converter. The setpoint is fixed. The 4...20 mA controller output continuously readjusts the pump speed in dependence on the control deviation. The equipment listed below like – Level measurement equipment in chamber 4 – Sampling valves in chambers 1 to 4 – Intake pipes for RS green syrup feed to chambers 1 to 4 is provided with special rinsing or steam output devices. The process control system contains a sequence which provides for continuous rinsing of this equipment in an active and passive time ratio.
SM apparatus
Fresh water
RS 1 apparatus
RS 2 apparatus
RS 2 apparatus
Thick juice evaporator station
Fig. 3-8: R/I diagram for cooling crystallization
3.3 Cooling crystallization High-quality sugar house work is achieved, among other reasons, by producing for – raw sugar – white sugar and – after product sugar a high-quality seed magma containing only a small amount of agglomerates. The cooling crystallization method has been developed by the German “Zuckerinstitut” (institute for sugar research) in Braunschweig and has become a standard in the German and European sugar industries for years. The cooling crystallization for the products – raw sugar and after product sugar and – white sugar has been realized with a process control system in the Lage factory of Pfeiffer & Langen.
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
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The following descriptions related to process automation refer to cooling crystallization for producing seed magma with a low agglomerate content for the crystallization of raw sugar. The basic system has the following setup: In the actual cooling crystallizer (apparatus 1) the thick juice from the last evaporation stage is directly taken in the cooling vessel, with an initial dry substance of 74 %. A temperature control system cools down the juice to a temperature of 50 °C. When a super-saturation of 1.1 is reached, the liquid is seeded with slurry. After this the juice is continuously cooled down to approx. 25 °C by a temperature control system with a fixed setpoint delta T = 6-8 Kelvin. After reaching the magma temperature of 25 °C the seed magma in the cooling vessel is held at a fixed (setpoint) temperature of 25 °C. In the discontinuous vacuum pan of 80 t that is equipped with a considerably shearing stirring device the liquor is normally thickened until the desired super-saturation is reached. The seed magma in the cooling vessel is automatically fed to the 80 t vacuum pan (apparatus 2) via the created software sequence. After this a normal controlled boiling phase with the appropriate subsequent control phases follows, which is detailed in section 3.1. The final crystal content wk is approximately 0.2. Since the cooling crystallization takes place in the low-temperature range, the agglomerate content of the magma is very low. This is achieved by selecting a low crystallization speed resulting in a high shear rate. The crystals grow from an average grain size dk 0.1 mm to dk 0.35 mm. The final crystal content amounts to approx. wk 0.5. After the boiling phase has been terminated, the seed crystal liquor is discharged to the mingler (apparatus 3).
Seed point
Suspension
Cooling water
Fig. 3-9: Schematic diagram of the cooling curve
Cooling curve The magma is pumped to a higher mingler (apparatus 4) via a ring pipeline, passing the 3 raw sugar vacuum pans. The mingler is provided with an overflow, i.e. the seed magma is continuously circulated. The 3 raw sugar vacuum pans can 'order' the required seed magma via the respective sequence of the 3 raw sugar vacuum pans. The control processes in the cooling crystallizer run automatically. The required software controls and sequences are performed in the process control system. The temperature control for juice cooling in the cooling tank is realized as follows: The surface required for heat transmission is realized in the cooling crystallizer in the form of a double helix. In accordance with the control diagram the fresh water or well water with a temperature of approx. 12 °C is pumped via a heat exchanger and through the double helix in the cooling crystallizer.
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
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The following temperatures are measured by Pt100 resistance thermometers: 1. Well water temperature 2. Juice temperature in the cooling crystallizer The differential temperature is calculated in a software calculating module. After the intake of the thick juice the sequence passes on to Cooling phase 1. COOLING phase 1 A bypass valve is installed in parallel with the cooling water control valve. The sequence performs the following steps – Set the temperature controller to manual mode – Fetch the 100% value from the storage location of a parameter module and set the controller output to this value. – OPEN the parallel cooling water valve. When reaching temperature limit 1 of 67 °C, the bypass valve is closed. – Fetch setpoint of approx. 6 Kelvin from the storage location of a parameter module and then write it to the temperature controller as the setpoint . – Set the temperature controller to automatic mode. The software temperature controller is a standard controller with proportional/integral action. The control valve (with electro-pneumatic positioner) is adjusted according to the control deviation. When reaching the temperature limit 2 (55 °C, close to the seed point) the sequence polls the limit value set for the dry substance. When reaching this limit value, the sequence passes on to the Seeding phase. Annotation: A process refractometer is used for measuring the dry substance. Seed phase The sequence performs the following steps – OPEN the seed valve – Set the temperature controller to manual mode – Fetch the -5% value from the storage location of a parameter module and set the controller output to this value. The control valve is closed. – When reaching the limit value for MIN level in the slurry tank, the seed valve is closed. The sequence passes on to the Cooling 2 phase. COOLING phase 2 The sequence performs the following steps – Set the temperature controller to automatic mode The temperature controller with a fixed setpoint of 6 Kelvin holds the temperature until the seed magma has cooled down to a temperature of 25 °C. – When reaching the temperature limit 3 (25 °C) the setpoint 25 °C is fetched from the storage location of a parameter module and then written to the temperature controller as the setpoint. The temperature of 25 °C is held until the seed magma is filled in the raw sugar seed magma vacuum pan. After this, the sequence performs the following steps: – Set the temperature controller to manual mode – Fetch the -5 % value from the storage location of a parameter module and set the controller output to this value. The control valve is closed. – The stirring device is switched off. The sequence passes on to the Cleaning phase.
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
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CLEANING phase The sequence performs the following steps – OPEN the valve for condensate intake – The cooling crystallizer is filled with condensate. When reaching the MAX level, the condensate valve is closed. – Switch stirring device ON The cooling crystallizer is rinsed via a software timer. – When reaching the nominal time, the cleaning valve is opened. – When reaching the MIN level, the cleaning valve is closed, and the sequence returns to the Feed phase. The sequence then waits for a new start command from the operator.
Process measurement and control equipment used Temperature: – Pt 100 resistance thermometer with dual measuring inset – Temperature transmitter, 2-wire, temperature linear, output: 4-20 mA, power supply: 24 VPressure of heating steam – Electrical transmitter for absolute pressure, process connection G1/2“ external thread Measuring range: adjustable as required, 2-wire, output: 4-20 mA, power supply: 24 V Vacuum – Electrical transmitter for absolute pressure, process connection G1/2“ external thread Measuring range: 0-1 bars absolute 2-wire, output: 4-20 mA, power supply: 24 V Level: – Transmitter for level, flange design, vacuum-proof – Plus side: DN80, PN10 with extended diaphragm, 100 mm – Minus side: flange DN80, PN10 with flush diaphragm seal, with 4-6 m capillary tube Measuring range: adjustable as required, 2-wire, output: 4-20 mA, power supply: 24 V Flow: Electromagnetic flowmeter, remote design – Flowmeter primary Nominal diameter: DN...; PN 10, Liner: PTFE, electrodes: Hastelloy – Flowmeter converter, wall-mounting Flow velocity: adjustable as required Output: 4-20 mA, power supply: 230 V/ 50 Hz Dry substance content – Micro-Polar Brix LB 565, immersion probe, DN 65, with rinsing unit for in-line mounting in tanks/vessels Evaluation computer, single channel, measuring range: can be calibrated as required, Output: 4-20 mA, Power supply: 90-260 V/50 Hz or 24 V AC/DC
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
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4 Pulp drying As can be seen in the schematic diagram of the process control system (Figure 4-1) the pulp resulting from juice production is first pressed, then dried to a residual moisture and finally processed to pellets. In the wet pulp press station the wet pulp is pressed in order to achieve the highest possible dry substance content. A successful pressing process depends not only on the type and setting of the presses used and the nature of the pulp, but also on a steady and constant pulp feed. If a distributing screw is used to supply the presses arranged in a line with pulp, only the last press will have to process varying quantities. A radiometric level measuring system is used to measure the filling height of the filler neck for the last press. The power control system using the filling level as the controlled variable and the press speed as the output level allows for optimal control of the pressing process, even with different pulp quantities. This ensures a constant dry substance content of the total quantity of pressed pulp and also eliminates one of the disturbances of the subsequent drying processes. A specific quantity of molasses is added to the pulp via a flow control system, in dependence of the quantity of pressed pulp which is measured with a conveyor type weigher. The pulp-molasses mixture is mixed as required while the pulp is transported to the wet pulp silo. Due to the long dwell time of the pulp in the drum dryer and the dead times resulting from this exact control is not possible at an acceptable cost/performance ratio. Therefore, the control system described below only controls the residual moisture of the dry pulp. Drum dryer as controlled system In the pulp drying process the pulp is dried until the desired residual moisture is reached. The wet pulp has a water content of approx. 77-80 % and – depending on the pressing process and the molasses added – should have a residual moisture of 5.6 % after pressing and of 9-11 % after pelleting. In order to ensure economical heating and good storability of the dried pulp, the residual moisture is given as a fixed setpoint value. The task of residual moisture control is to reach this fixed value and hold it – even in case of disturbances. Drum dryer operation as a function of the wet pulp flowrate The operating mode of the pulp dryer is essential for designing and configuring the control system. In normal mode the wet pulp flowrate is influenced by load variations. Additionally, the nature of the pulp – e.g. the molasses or water content, apparent weight, etc. – changes permanently. This makes manual operation by the operator very difficult, and the operator must be very attentive to control the big drying drum and achieve constant drying results. The pulp drying control tasks demands to set the residual moisture setpoint to a specific, fixed value. As a result, the setpoint for residual moisture control must be set so as to ensure that extreme load variations and disturbances resulting from variations in the wet pulp nature are considered. Control performance requirements In the steady state condition the deviation between the residual moisture content and the adjusted setpoint should not be more than ± 0,25 %. In case of extreme load variations or changes of the disturbances due to variations of the wet pulp nature the highest temporary control deviation should be ± 1,5 % at the most.
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
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Control loop: Control of residual moisture A good and successful process control loop is characterized by a useful signal combination with disturbances and auxiliary variables. The schematic diagram of the control system below shows a control loop that has proved to be very successful in practice. The third part temperature of the dryer is used as the auxiliary variable here. This kind of temperature control ensures that more or less fuel is supplied to the pulp drum dryer. The temperature controller which is arranged in a cascade control system receives its reference variable (external setpoint) from the controller for the residual moisture of the dry pulp. To allow for disturbance feedforward, this reference variable is additionally processed in a calculating device to become the common reference variable for the temperature controller. The weight of both signals can be adjusted in the calculating device as required. The temperature controller has a proportional/integral control action. The residual moisture controller with proportional control action receives its reference variable via an integrator, so that it becomes, more or less, a PI controller. Wet pulp flowrate Optimal signal processing in the control loop would require an exact wet pulp flowrate measurement. In order to make life easier, the speed of the dosing screw is used as the measured variable. It is assumed that the screw is always filled evenly and, thus, the speed is proportional to the wet pulp flowrate. The speed is converted to a standard 4...20 mA signal by using a rotational speed transmitter. Moisture content of the dry pulp The moisture content of the dry pulp is the actually controlled variable. Due to the dead time behavior of the drum dryer it is carefully connected to the temperature controller as the reference variable via a cascade control loop with an integral controller. Third part temperature The third part temperature is the most important auxiliary variable for signal processing. It is measured at the first third of the rotating drying drum by using four Pt100 resistance thermometers in a series-parallel connection. The measured temperature signal is connected to a transmitter for Pt100 signal input. The transmitter is attached to the rotating drum. A 2-wire circuit ensures that a 4...20 mA output current proportional with the measured variable flows in the common supply and signal circuit. Resistance variations at the transmitter output resulting from the carbon slip ring carrier are not considered for the measurement, provided that the maximum permissible load is not exceeded. Measuring probes with a nominal length of approximately 500 to 800 mm are used. Fuel flowrate The temperature controller output changes the fuel flowrate. Various fuel burner types use a principle where the oil flow and, at the same time, the combustion air flow are controlled via a compound rod system. In this case a common actuator for oil and combustion air suffices. In some cases the traditional fuel control is used as it is known from steam generators. Combustion chamber pressure control This control system is absolutely independent of the meshed control loops described above and is realized as a fixed value control system. The pressure curve over the entire drum is a straight line which starts at the burner with an overpressure and ends at the induced draught ventilator with an underpressure. The zero point of this pressure curve should in any case be in the combustion chamber. The pressure controller compares the combustion chamber pressure with the setpoint. In case of a control deviation (caused by a load change), the induced draught valve is readjusted. This changes the pressure curve in the drum in such a way that the zero point position of the pressure curve is maintained.
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Process Automation Examples from Various Plant Stations in a Beet Sugar Factory
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Process measurement and control equipment used 1/3 Temperature: – Pt 100 resistance thermometer with dual measuring inset and mounting thread Nominal length: 500 or 800 mm – Temperature transmitter, 2-wire, temperature linear, output: 4-20 mA, power supply: 24 VCombustion chamber pressure – Pressure transmitter, process connection G1/2“ external thread Measuring range: adjustable as required, 2-wire, output: 4-20 mA, power supply: 24 V Filling level at filler neck and silo – Radiometric level meter LB 440 Evaluation computer, 2-wire, stick probe CS 137 with shield, Measuring range: can be calibrated as required, output 4-20 mA for level, Digital output: relay, power supply: 230 /50 Hz or 24 V AC/DC Moisture measurement (pressed and dry pulp) – Micro-Moist LB 456 transmission measurement at the belt conveyor or in the filler neck Evaluation computer, single channel, horn antennas – Radiometric basis weight compensation CS 137 with shield and scintillation detector Measuring range: can be calibrated as required, output: 4-20 mA, power supply: 230 V/50 Hz Molasses flowrate: Electromagnetic flowmeter, remote design – Flowmeter primary Nominal diameter: DN...; PN 10, Liner: PTFE, Electrodes: Hastelloy – Flowmeter converter, wall-mounting Flow velocity: adjustable as required, output: 4-20 mA, power supply: 230 V/ 50 Hz Molasses Pulp
to chimney
Wet pulp presses
Fuel oil Combustion air Pulp drum dryer Flue gas from boiler house Steam from steam converter 1
Pellet silo
Pellet mills
Fig. 4-1: Schematic diagram of a process control system for pulp drying
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