GAMMA SCANNING TECHNIQUE

MODULE 1: GAMMA SCANNING TECHNIQUE 1. 1. BACKGROUND OF GAMMA SCANNING METHOD FOR TROUBLESHOOTING AND OPTIMIZING PROCESSING COLUMNS, VESSELS AND PIPES Gamma Scanning is the best technique to carry out an internal inspection of any process equipment, without interrupting production. A collimated beam of penetrating gamma rays is allowed to pass through the shell of a vessel gets modified by the vessel internals and then comes out of the other side. By measuring the intensity of the transmitted radiation, valuable information can be obtained about the densities of the materials present inside the vessel. The higher the density of the material, the less radiation gets through; so significantly more gamma rays are transmitted through a vapor compared to a liquid phase. Density scanning of distillation columns is the most commonly used application of this technique. Without affecting processing unit, this reliable and accurate technique can be used to determine:    

The liquid level on trays The presence or absence of internals, such as trays, demister pads, packing and distributors The extent and position of jet and liquid stack flooding The position, and the density characteristics of foaming

Gamma ray source and radiation detectors are moved simultaneously down opposite sides of the column. The intensity is recorded at appropriate intervals and a profile of the instantaneous operating state is obtained by plotting the detector response against the column elevation. The tray structure and the liquid on the trays give high absorption, while the presence of foam and entrainment slightly moderates the expected vapor profile. Studies of the degree of foaming can be carried out by generating density profiles at different concentrations of antifoam additive. The scanning of pipelines for blockages or build-up is another excellent use of Gamma Scanning because it is faster and uses lower radiation levels than conventional X-ray techniques. 1.2. GAMMA-RAY SCAN METHODOLOGY 1.2.1.

General principles

Gamma scan method uses the transmission or absorption principle (Fig.1.1). Transmission of a mono-energetic beam of collimated -ray photons through a simple absorption medium can be described by Lambert-Beer’s equation

FIG. 1. 1. Principle of gamma absorbtion method

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The quantity of gamma radiation absorbed into the material between the radioactive source and detector is described by transmission law: 𝐼𝑥 = 𝐼0 𝑒

−𝜇𝑥

Where:  is the linear absorption coefficient with dimension cm-1, x the sample thickness in cm. Or 𝐼𝑥 = 𝐼0 𝑒

−𝜇𝜌𝑥

where:     

Ix is the intensity of radiation going through the material Io is the intensity of incident radiation µ is the absorption coefficient of the inspected material ρ is the density of material. x is the thickness of material (radiation path length in cm2/g)

A quantity more commonly found tabulated is the mass absorption coefficient / with dimension cm2/g. In a composite sample (with w, o and g materials) the attenuation is additive according to

𝐼𝑥 = 𝐼0 𝑒

𝜇 𝑔 𝑥 𝑚 ,𝑔 𝜇 𝑥 𝜇 𝑥 − 𝑤 𝑚 ,𝑤 − 0 𝑚 ,0 − 𝜌𝑤 𝜌0 𝜌𝑔

𝑚

This is the basic equation used for experimental design, measuring, data processing and interpretation of gamma-ray absorption scanning technique in column, vessels and pipe inspection. The gamma scan technique is used to determine what is happening inside a vessel. A small gamma ray source and a detector are positioned on opposite sides of a vessel. As the source and detector move along the exterior of the vessels a density profile is generated of the interior contents. Gamma rays emitted from the source travel through the vessel, are moderated by the contents, and are then counted by the detector. If the source and detector spacing is held constant the count rate at the detector is directly related to the density of the material the gamma rays pass through. Vapors allow more gamma rays through than either liquid or vessel internals creating the density plot. TABLE 1.1. MAJOR GAMMA SEALED SOURCES USED IN INDUSTRIAL SCANNING

Radioisotope

T1/2

Energy (MeV)

Cs-137

30,2 y

Co-60

5,27 y

0,662 (89,9%) 1,173 (100%) 1,332 (100%)

Gamma constant rem m2 / h Ci 0,399 1,31

Baseline gamma scans are performed to obtain the current “normal” operating profile of a vessel. This profile is then used as a reference point for future scans. If problems are suspected, bottlenecks occur or an expansion is planned; a new gamma scan is compared to the baseline to determine differences in the operation conditions. The experimental design of gamma scanning presented in the figure 1.2 and figure 1.3 shows the laboratory experimental design to test and validate the method for various columns. Radioactive source is normally collimated, while the detector could be collimated or not.

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FIG. 1.2. Source-detector configuration in performing gamma scanning of a column

FIG.1.3. Laboratory set-ups for testing of gamma scanning designs The radioactive source is moving inserted into a panoramic collimator (360 degree) (Fig.1.4). The panoramic (or all sides open) collimator is important for ensuring the same detection geometry in various conditions of source torching or slight movement under wind conditions. Typical gamma-ray scans can be carried out within 60 minutes after arrival at a plant. Scanning rate of approximately 12 m/h can be maintained on tray columns. It is therefore possible within a few hours, to discuss the results with process engineers so that they can take immediate action, such as shut down or keep on running the plant.

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FIG. 1.4. Source panoramic collimator Gamma scanning is the only diagnostic tool available that can be applied with confidence on any distillation process to obtain the true hydraulic behavior of that system (Fig.1.5)

FIG. 1.5. Scaning responses of typical malfunctions

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The principle of gamma scanning profile is illustrated by Fig. 1.6:

FIG. 1.6. Principle of gamma scanning profile When scanning a distillation column or a similar vessel, a small suitably sealed gamma radiation source and a detector (NaI/Tl) are moved simultaneously in small increments on opposite sides, along the exterior length of the unit. A relative density profile of the contents of the column is thus obtained; areas containing relatively high density material (such as liquid and/or metal) provide a relatively low intensity of transmitted radiation, while areas of relatively low density (vapor spaces between trays) result in a high intensity level. Deductions can be made regarding possible mechanical damage of trays inside the unit, as well as with regard to certain operational conditions in the unit, such as flooding, blockages, weeping and other process anomalies. Comparing mechanical drawings with relative density profile (gamma absorption profile) of a unit, deductions can be made with regard to:            

Presence or absence of trays and other internals inside a column, Presence and formation of coke, Location and extent of flooding, Blockages caused by downcomer obstruction tray fouling, dirt or high liquid loading on trays. Location and severity of entertainment, Presence of liquid weeping, Top and bottom positions of packed beds, Maldistribution of packing material in packed beds, Liquid levels on trays.

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Repeating scans under different operating conditions such as, temperature, pressure, feed flow rate and reflux ratios can obtain additional information on degree of entertainment (carry over of liquid), tray and “jet” flooding and foaming or “weeping”. The following principles are important:   

When radiation from a radioactive source passes through a medium containing a tray with aerated liquid, much of the radiation is absorbed and the amount of radiation reaching the detector is relatively small. If a radiation beam passes through unaerated liquid, most of the radiation is absorbed by the medium, and the intensity is low. When radiation beam passes through vapour, there is little mass present to absorb the radiation and therefore high radiation intensities are transmitted to the detector.

Figure 1.7 shows on-stream gamma scanning in petrochemical plant in progress. There are two ways of execution of a gamma scanning of a column:  

Classical top-dawn performance staying on the top of the column platform and releasing simultaneously step by step (every 5 cm) the source and detector, and recording the data in each position (Fig. 1.7) Automatic gamma scanning performance using winches and engines to move continuously the source and detector. In this case, the operators and equipment stay on the ground (Fig. 1.8).

Both techniques are used; they have their advantages and disadvantages. Classical scan from top down is simple, has a short preparatory work but is difficult in operation and needs 2.3 operators. Automatic gamma scan is easy and faster in operation, gives continuous records but has a rather long preparatory work.

FIG. 1.7. Performing gamma scanning of distillation column in petroleum refinery

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FIG. 1.8. Automatic gamma scanning operation in a refinery A gamma scan of a vessel can detect and locate liquid and vapour regions within a column. It can discriminate between aeration of liquid and detect foam or spray heights in vapour regions. By measuring and analyzing density changes, many parameters indicating column performance can be obtained. Each tray and the vapor space above it “tells the story” of its operating status. A properly operating tray has a reasonable level of aerated liquid showing a rapidly decreasing density gradient until it reaches a clear vapor space just under the next tray (Fig. 1.9 and Fig. 1.10). To distinguish the above symptoms, experience is required.

FIG. 1.9. Scanning profile of a normal distillation column

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FIG. 1.10. Normal column scan without distillation column problems Figures 1.11 and 1.12 present typical gamma profiles of columns partly with collapsed tray.

FIG. 1.11. Gamma profile of column part with collapsed tray Figure 1.12 shows a typical gamma profile of part of column with flooding.

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FIG. 1.12. Gamma profile of part of column with flooding

FIG. 1.13. Gamma profile of column part with entrainment (dragging)

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FIG. 1.14. Gamma scanning of a column sector with foaming

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FIG. 1.15. Gamma profile of a column part with weeping (shower Figure 1.16 shows the major causes of column malfunctions.

FIG. 1.16. Causes of column malfunctions

  

Gamma Scan limitations are: Large diameter vessels Very large wall thicknesses High packing density

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1.2.2.

Selection of source and planning 1.2.2.1. Radioisotope activity calculation

A typical radioactive sealed source used for gamma-ray scan of a distillation column is only approximately 1 % of the strength needed to investigate X-ray welds. Co-60 and Cs-137 are main sources used for gamma scanning. The activity depends of the column diameter, going from 5-10 mCi for 1-2 m up to 100 mCi for 9-10 m. An estimation of the suitable source strength required can be calculated as follows: 𝐷 𝑑2 2𝑊𝑡 ℎ𝑙 𝐴𝑐𝑡𝑖𝑣𝑖𝑡𝑦 𝐴 = 𝑇     

Where D =dose rate required d =diameter of column Wt =total wall thickness of column (mm) + wall thickness of scan container. hl =half layer thickness value of material (25 mm for steel for 60Co) T gamma-ray constant for a specific source (1.31 R/h on a distance of 1 meter for a 1 Ci radioactive source).

60

Co

When using the above equation it is suggested that 200 mm be added to the diameter of the column to make provision for the source and detector container on the outside. The above equation is an approximation, and build-up factors of the material are not taken into account. Shielding calculation software can be used to greater effect. It is further recommended to work with a maximum count rate of approximately 7 000 to 9 000 cps through the vapour space (gas line) of the column. The count rate decreases to approximately 1000 cps at the position of the trays and liquids (liquid line). This count rate range ensures good statistics. Figure 1.17 presents the relation: Column diameter versus activity for two gamma sources Co60 and Cs-137.

FIG. 1.17. Column diameter versus activity for two gamma sources Co-60 and Cs-137

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The relationship between Co-60 and Cs-137 activities is approximately 1:4 that is for example if for a 2 m diameter column a sealed source of Co-60 of 10 mCi is required, for Cs-137 the source should have the strength of 40 mCi to provide a clear scan picture. Example of activity calculation A Stripper distillation column needs to be scanned. The internal column diameter is 2.9 m. and wall thickness 15 mm. The sensitivity of a NaI/Tl detector can be measured placing a 60Co or a 137Cs radioactive source at the distance from the detector, where the dose rate is 1mR/h. The sensitivity of a normal scan detector (2" x 2" NaI/Tl) was found to be 7500 cps/mR/h. This means that 1 mR/h is necessary for good statistics. It fact, dose rates of approximately 0.5-1 mR/h at the position of the panoramic collimated detector provide clear gamma profiles. Let’s calculate the activity of a Co-60 source that is needed to obtain a clear gamma profile of this column. The gamma-constant for Co-60 is 1.31 R/h. Calculations are based on a typical panoramic scan pot. 𝐷 𝑑2 2𝑊𝑡 ℎ𝑙 1 × 2.9 + 2 2 × 2 30 25 𝐴𝑐𝑡𝑖𝑣𝑖𝑡𝑦 𝐴 = = = 16.4 𝑚𝐶𝑖 60 𝐶𝑜 𝑇 1.332 For 2-3 m. diameter columns Co-60 source activity of 15-20 mCi is quite enough to provide a good picture of the internal structure. 1.2.2.2. Planning of a gamma-ray scan investigation     

The following data are needed to be known before a scan can be carried out: inside diameter and wall thickness of the column (mm) bulk density and type of packing material, for packed beds downcomer orientation and type of trays present (single, double pass trays) operating problems experienced, e.g. low or high-pressure problems across the column, or temperature differences along the length of the column detailed mechanical drawings of the unit showing internal structure, such as elevations, tray or packing assemblies, nozzle and pipework locations as well as other special features.

Such information is critical for interpreting data from column scan profiles obtained and for identifying and visualizing possible mechanical problems. It is also suggested that the following be obtained:  gamma scan profiles of an “empty” column (with all the internals but not in operation)  a scan profile before a maintenance shutdown  a scan profile after a maintenance shutdown when the column is under normal operating condition. There is a clear difference between an empty (dry -black) and operated (wet -red) scans (Fig. 1.18).

FIG. 1.18. Example of empty (dry) and normal (wet) scan profiles

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It is very important for comparison that the scan is carried out along the same scan lines (lines between radioactive source and detector). Measurements must be taken out at small intervals (50 mm). Some of main factors that can influence gamma scanning are presented in Fig. 19. :

FIG. 1.19. Some of main factors that can influence gamma scanning 1.2.2.3. Equipment/Tools. Preparation of the check-list         

The service provider should make his own check list. Items may include Detector Computer (laptop or palmtop) Source holder/source container – with collimator Measuring tapes (2) Cable connectors (> 4) Coaxial cables Computer cables Guide steel wires or cables (2) Portable scaler-ratemeter

 

Gamma source: Co-60 or Cs-137 or Ir-192

  

Radiological safety equipment: Survey meters Personnel monitors

 

Others: Detail as required

The service provider should carry out all necessary pre–job checks prior to mobilization. These should include checks to confirm compliance with statutory legislation. The attached photograph shows typical gamma-ray scanning equipment. The service provider may prefer different equipment of similar type. Figure 1.20 below shows the typical equipment for gamma scanning. The equipment is portable.

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FIG. 1.20. Typical gamma ray scanning equipment

1.2.3.

An expert system for the design of radioisotope sealed source applications based on beta, gamma or x-rays transmission (janu) 1.2.3.1. Background

A knowledge based software for radioisotope sealed source applications design, mainly density, thickness, mass per unit area, level and two-phase flows gauges, is presented. Its aim is to optimize the different components of a transmission gauge (radioactive source, detector, electronic device, collimators and shielding), taking into account parameters and constraints linked to the configuration (nature and composition of materials, presence of shields and walls, etc.), as well as users requirements (accuracy, counting time, beam collimation, duration of tests, etc.). The database includes characteristics of radionuclides and industrial sources, photon cross sections, build-up factors, specific dose constants, physical properties of elements, usual scintillation detectors, and shielding, materials. It allows the determination of the most suited emitter, as well as a precise characterization of a given emitter, including required source activity, expected counting rates, dose rates, etc. It has been extended to X rays generators, voltage and current intensity replacing in this case the energy and activity of the source. Information supplied by JANU has been validated by applications developed during the past 30 years. Its choices have always revealed most judicious and, in general, numerical results in good agreement with experiments. Thus, it has become an essential and reliable tool for radioisotope practitioners. 1.2.3.2. Software of assistance to design and interpretation of measurements based on the radiation detection beta, gamma or X Figure 1.21 represents the principle of the JANU software or expert system:

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FIG. 1.21. Measurements by transmission: optimised parameters (JANU software) JANU expert software provides data for selection of radioisotope sources, detectors and collimators for optimal assembling of source-detector configuration in transmission method for measuring of:  level,  density,  thickness, The main objective of this software is to provide radiation detection parameters for reducing as much as possible the error (standard deviation σ) in measurements. The major parameters are: Photonic cross sections for atomic numbers going from Z = 1 to 94 Energies: from E = 1 keV till 10 MeV, Symbol, density, number and atomic mass of the elements, Characteristics of main radioisotopes β, γ and X (energy, intensity, period, radiotoxicity), Mains characteristics of the sources available (dimensions, incorporated activity, selfabsorption, manufacturer), • Mains characteristics of detectors with scintillation (composition, density, resolution, maximum rate counting), • • • • •

The JANU software is formulated in DOS operation system. 1.2.3.3. Gamma Scanning Columns with Trays Gamma scanning provides a fast, efficient mean of determining:     

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the location of damaged and/or missing trays the severity and extent of flooding, entrainment, foaming, or weeping the location of process bottlenecks the depth and relative densities of the aerated liquid on the trays, and the liquid level in the base of the column and preventing unnecessary shutdowns

Column scans, a well established troubleshooting tool, are quick, safe, and easy to set up. They require no support from process personnel other than drawings of the column, a work permit, and access to the top of the column. The effectiveness of gamma scanning is always increased when the opportunity arises to compare two or more scans, whether they are at different operating conditions, or a current scan vs. a baseline scan taken after a turnaround. These situations offer great process optimization opportunities. A gamma scan is performed by placing a small radioactive source and a sensitive radiation detector on opposite sides of an operating distillation column. As seen in Fig. 1.22, the source and detector are aligned so that the path between them is across the tray active area, but avoids intersecting the tray downcomers. Maintaining a constant geometry, any variation in the signal is due to density differences within the column, as the scan proceeds down the tower.

FIG. 1.22. Scan orientation in columns In Fig. 1.23, overlying a baseline scan (red), is a scan profile (blue) of a column that had experienced damage to tray 6, the debris from which has caused flooding from tray 9 upwards. The second scan (green), on the lower half of the chart, shows the profile typically seen when a column experiences entrainment problems.

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FIG. 1.23 Typical Gamma scan results of a column with trays 1.2.3.4. Gamma Scanning Columns with Packed Beds A single TowerScan on a packed bed column will determine:  whether all the packed beds are present in the column  if the beds have experienced any damage or settling of the packing  the liquid level on chimney and collector trays  if any of the beds have experienced flooding or fouling  whether the demister pads and distributors are at their proper elevations By performing and analyzing a full 2 x 2 grid scan, it can be evaluated:  the extent and location of liquid maldistributions within the packed beds  if tower internals such as demister pads, distributors, trays, and the top of beds are level In addition to using gamma scans as a troubleshooting tool, some companies make excellent use of the information to schedule shutdowns. In processes where they know the packing gradually becomes fouled, periodic scans monitor the progress of the fouling. They can then accurately schedule their turnarounds, as opposed to having to incur sudden, unexpected, and costly shutdowns. Examples of “before” and “after” scans performed on a tower are shown in Fig. 1.24.

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FIG. 1.24. Typical Gamma scan results of a column with packed beds The initial scan (red) showed that the demister pad, distributor, packed bed, and chimney tray were at the proper elevations. The bed however had experienced severe fouling, especially in the bottom two thirds of the packing. The column was shut down and the packing replaced. The scan taken after the turnaround (blue) exhibited a far more uniform bed density. Some higher densities were seen at the top and bottom of the bed due to the hold down plate and bed support, and part way down the bed due to external mechanical interference. 1.2.4.

Types of columns and problems to be diagnosed 1.2.4.1. Distillation columns in oil refineries

An oil refinery is an industrial process plant where crude oil is processed and refined into more useful petroleum products, such as gasoline, diesel fuel, asphalt base, heating oil, kerosene, and liquefied petroleum gas. Crude oil is separated into fractions by fractional distillation (Fig. 1.25). Fractional distillation is useful for separating a mixture of substances with narrow differences in boiling points, and is the most important step in the refining process. The various components of crude oil have different sizes, weights and boiling temperatures; so, the first step is to separate these components. Because they have different boiling temperatures, they can be separated easily by a process called fractional distillation. The steps of fractional distillation are as follows:

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        

You heat the mixture of two or more substances (liquids) with different boiling points to a high temperature. Heating is usually done with high pressure steam to temperatures of about 600 degrees Celsius. The mixture boils, forming vapor (gases); most substances go into the vapor phase. The vapor enters the bottom of a long column (fractional distillation column) that is filled with trays or plates. The trays have many holes or bubble caps (like a loosened cap on a soda bottle) in them to allow the vapor to pass through. The trays increase the contact time between the vapor and the liquids in the column. The trays help to collect liquids that form at various heights in the column. There is a temperature difference across the column (hot at the bottom, cool at the top). The vapor rises in the column. As the vapor rises through the trays in the column, it cools.

FIG. 1.25. Fractional distillation column for crude oil separation into fractions 1.2.4.2. Disturbances of processing columns The type and magnitude of disturbances affecting a distillation column have a direct effect on the resulting product variability. An analysis of the major types of disturbances encountered in distillation columns follows. Feed composition upsets Changes in the feed composition represent the most significant upsets with which a distillation control system must deal on a continuous basis. A feed composition change shifts the composition profile through the column resulting in a large upset in the product compositions. Most industrial columns do not have a feed composition analyzer; therefore, feed composition upsets usually appear as unmeasured disturbances. When a feed composition analyzer is available, a feed forward controller can be applied using the on-line measurements of the feed composition. Because feed composition changes represent a major

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disturbance for distillation control, the sensitivity of potential control configurations to feed composition upsets is a major issue for configuration selection. Feed flow rate upsets The flow rates in a steady-state model of a column with constant tray efficiencies scale directly with column feed rate. Therefore, ratio control (using L/F, D/F, V/F or B/F as composition controller output) is an effective means of handling feed flow rate upsets. Dynamic compensation is normally required to account for the dynamic mismatch between the response of the product compositions to feed flow rate changes and the response to changes in the MVs. When certain ratios (e.g., L/D, V/B) are used as MVs, these ratios, combined with the level control, automatically compensate for feed flow rate changes. Feed enthalpy upsets For columns that use a low reflux ratio, feed enthalpy changes can significantly alter the vapor/liquid rates inside the column, causing a major shift in the internal composition profile and, therefore, a significant upset in the product compositions. This upset can be difficult to identify because (1) most industrial columns do not have feed temperature measurements and (2) even if a feed temperature measurement is available, it does not detect feed enthalpy changes for a two-phase feed. This disturbance may be difficult to distinguish from feed composition upsets without a more detailed analysis. It may be necessary to install a feed preheater or cooler to maintain a constant feed enthalpy to a column. Subcooled reflux changes When a thundershower passes over a plant, the reflux temperatures for the columns can drop sharply. Columns that use finned-fan coolers as overhead condensers are particularly susceptible to rapid changes in ambient conditions. If internal reflux control is not applied, severe upsets in the operation of the columns result because of major shifts in the composition profiles of the columns. When internal reflux control is correctly applied, the impact of a thunderstorm on column operations can be effectively eliminated. Loss of reboiler steam pressure When a steep drop in steam header pressure occurs, certain columns (those operating with control valves on the reboiler steam that are nearly fully open) experience a sharp drop in reboiler duty. This results in a sharp increase in the impurity levels in the products. When the steam header pressure returns to its normal level, the composition control system for the column attempts to return to the normal product purities. Because of the severity of this upset, if the composition controllers are not properly tuned, the upset can be amplified by the composition controllers, requiring the operators to take these controllers off-line to stabilize the column, greatly extending the duration of the period of production of off-specification products. This disturbance is, in general, the most severe disturbance that a control system on a distillation column must handle and may require invoking overrides that gradually bring the operation of the column to its normal operating window instead of expecting the composition controllers to handle this severe upset by themselves. Column pressure upsets Column pressure has a direct effect on the relative volatility of the key components in the column. Thus, changes in the column pressure can significantly affect product compositions. A properly implemented pressure control scheme maintains column pressure close to its set point, with only short-term and low-amplitude departures. A large class of columns (e.g., refinery columns) is operated at maximum condenser duty to maximize column separation, which minimizes steam usage. For these cases, the column pressure increases during the day, when the cooling water or ambient air temperature is the greatest, and decreases at night, but the resulting pressure changes are usually slow enough that the composition controller can efficiently reject this disturbance.

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Regulatory Controls Improperly functioning flow, level or pressure controllers can undermine the effectiveness of the product composition controllers. Flow controllers Flow controllers are used to control the flow rates of the products, the reflux and the heating medium used in the reboiler and their set points are determined by the various level and composition controllers. To assess the performance of a flow control loop, you can applying block sine waves and comparing these results for the dead band and time constant with the expected performance levels. Level controllers Level controllers are used to maintain the level in the accumulator, the reboiler and the intermediate accumulator of a stacked column (i.e., a distillation column composed of two separate columns when there are too many trays for one column). Loose level control on the accumulator and reboiler has been shown to worsen the composition control problem for material balance control configurations (when either D or B is used as a MV for composition control). When D or B is adjusted, the internal vapor/liquid traffic changes only after the corresponding level controller acts as a result of the change in D or B. On the other hand, if a level controller is tuned too aggressively, it can result in oscillations passed back to the column and contribute to erratic operation. When the reboiler duty is set by the level controller on the reboiler, a level controller that causes oscillation in the reboiler can also cause cycling in the column pressure. Column pressure controllers The column overhead pressure acts as an integrator and is determined by the net accumulation of material in the vapor phase. Column pressure is controlled by directly changing the amount of material in the vapor phase of the overhead or by changing the rate of condensation of the overhead, which converts low-density vapor to high-density liquid. A variety of approaches can be used to control column pressure. 1.2.5.

Experimental design of gamma scanning 1.2.5.1. Scanning procedure The following procedures are recommended for gamma scanning set-up:

    

Obtain detailed mechanical drawings of the unit Request assistance from the process or chemical engineer for process details. Obtain operational data before, during and after scanning Decide upon scan line orientation and number of scans. Check for source and detector alignment every 50 cm. of scanning

To conduct a tray-column scan, it is advisable to execute a scan across the trays and to avoid scanning through the downcomers of the trays. Typical and recommended scan line orientations for trayed columns are shown in the following figure 1.26.

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FIG. 1.26. Typical scan line orientations Tray flow arrangement for single pass trays and two pass or double pass trays are given in the following pictures (Fig. 1.27).

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FIG. 1.27. Tray flow arrangement for single pass trays and two pass or double pass trays 1.2.5.2. Grid Scanning Grid scanning is recommended for packed bed columns (Fig. 1.28).

FIG. 1.28. Outlay of a typical pack bed column (vacuum column) 24

A typical orientation of grid scan lines is shown in the next figure 1.29. At least four scans are recommended to examine a packed bed column. Grid scans may be conducted to investigate process-related conditions such as:   

flooding or blockages entrainment or carry over of liquid or maldistribution of liquid flow through packed beds.

Grid scans can also be used to investigate mechanical construction problems such as:  

collapsed packed beds or maldistribution of packing material.

An important factor to take into account is that as far as is possible the operating conditions (such as feed rate, temperature and other process parameters) must remain constant especially during the time of the scan investigation. It is very important to record any process changes during the time of the scan. This will facilitate the interpretation of the scan profile if anomalies are indicated. Grid scanning is recommended on packed columns with diameters up to approximately 3m. Larger diameter columns must be approached in a different way, since too large an area (especially in the centre) is not covered.

Radioactive source

D

D

D D

D

Radiation Detector

FIG. 1.29. Orientations of grid scan lines Grid scanning is also useful for investigating the correct installation of distributors as well as the correct distribution of incoming liquid feed. An irregular distributor can undermine the performance of the entire packed bed and column. Liquid distributors must spread liquid uniformly on top of a bed, resist plugging and fouling, and also provide free space for gas flow. A not correctly water level installed distributor that is a tilted distributor, could cause liquid to flow preferentially on one side of the column (Fig. 1.30).

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FIG. 1.30. Typical packed column grid scan lines

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