ADIGRAT UNIVERSITY COLLEGE OF ENGINEERING AND TECHNOLOGY DEPARTEMENT OF CHEMICAL ENGINEERING PROJECT ON MASS TRANSFER UNIT OPERATION
Course code: (ChEng3114) PREPARED BY NETWORK -4-(four) SECTION ONE (1) Year 3rd (third) NETWORK MEMBER
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1. G/georgs Berhe……………………………………………………….RET 0569/06 2. Jemal Seid…………………………………………………………….RET 0834/06 3. Rigbey Kidey………………………………………………………….RET 1242/06 4. Te-ame G/Tsadik…………………………….....................................RET 1406/06 5. T/haimanot Abraha………………………….....................................RET 1443/06 6. Tesfaye Bayle…………………………………………………………RET 1499/06
Submitted to instructor H/Mikael Tsegay Email:
[email protected]
Submission Date 01/09/2008 E.C
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Acknowledgement……………………………………………………………………….....І 1) Drying of solids………………………………………………………………………….1 1.1) Fundamental principles of drying…………………………………………………1 1.2) Field of application……………………………………………………………….2 1.3) Basic principles of drying…………………………………………………………2 1.4) Thermodynamic properties of air-water mixtures and moist solids………………2 1.4.1) Psychrometry…………...…………………………………………….2 1.4.2) Psychrometric terms….........................................................................3 1.5) Mechanism of drying……………………………………………………………...4 1.6) Types of dryers……………………………………………………………………..6 1.7) Dryer efficiency…………………………………………………………………...10 2) Crystallization……………………………………………………………………………11 2.1) Definition Crystallization…………………………………………………………11 2.2) Types of Crystallizers……………………………………………………………..12 2.3) Solid-Liquid-Equilibria…………………………………………………………..13 2.3.1) Solubilities and Phase Diagrams……………………………………….13 3) Extraction………………………………………………………………………………...15 3.1) Solid-Liquid Extraction………………………………………………....................16 3.1.2) General principles……………………………………………………….16 3.2 Liquid-liquid extraction ………………………………………………………………...17 3.2.1) Extraction Processes………………………………………………….....18 3.2.2) General principles………………………………………………….........18 3.2.3) Equilibrium Data…………………………………………………….......19 3.2.4) Factors influencing extraction performance……………………………..19 3.3 Liquid-Gas Extraction (absorption)……………………………………………………...20 4) References…………………………………………………………………………………21
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1) Figure1. Segment AB of the graph represents the constant-rate drying period, while segment BC is the falling-rate period………………………………………………………4 2) Figure2 spray dryer…………………………………………………………….9 3) Figure3; Simplified representation of liquid-liquid extraction………………………….18
Acknowledgement First and for most we would like to say great thank you for our God, and We would like to say thank you to all support for us during doing this project and our teacher in case of giving or guiding us how to write our project.
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CHAPTER ONE DRYING OF SOLIDS 1.1) FUNDAMENTAL PRINCIPLES OF DRYING Introduction The separation operation of drying converts a solid, semi-solid or liquid feedstock into a solid product by evaporation of the liquid into a vapour phase via application of heat. Drying is an essential operation in the chemical, agricultural, biotechnology, food, polymer, ceramics, pharmaceutical, pulp and paper, mineral processing, and wood processing industries. Drying is perhaps the oldest, most common and most diverse of chemical engineering unit operations. Over four hundred types of dryers have been reported in the literature while over one hundred distinct types are commonly available. Drying of various feed stocks is needed for one or several of the following reasons: need for easy-to-handle free-flowing solids, preservation and storage, reduction in cost of transportation, achieving desired quality of product, etc. In many processes, improper drying may lead to irreversible damage to product quality and hence a non-saleable product.
OBJECTIVE Drying is defined as the application of heat under controlled conditions, to remove the water present in liquid foods by evaporation to yield solid product. It differs from evaporation, which yields concentrated liquid products. The main purpose of drying is to extend the shelf life of foods by reducing their water activity.
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1.2) FIELD OF APPLICATION Typical applications for drying techniques include dairy products (milk, whey, creamers), coffee, coffee surrogates, tea, flavours, powdered drinks, and processed cereal based foods.
1.3) BASIC PRINCIPLES OF DRYING Drying is a complex operation involving transient transfer of heat and mass along with several rate processes, such as physical or chemical transformations, which, in turn, may cause changes in product quality as well as the mechanisms of heat and mass transfer. Physical changes that may occur include: shrinkage, puffing, crystallization, glass transitions. In the manufacture of catalysts, for example, drying conditions can yield significant differences in the activity of the catalyst by changing the internal surface area. Drying occurs by effecting vaporization of the liquid by supplying heat to the wet feedstock. As noted earlier, heat may be supplied by convection (direct dryers), by conduction (contact or indirect dryers), radiation or volumetrically by placing the wet material in a microwave or radio frequency electromagnetic field. Over 85% of industrial dryers are of the convective type with hot air or direct combustion gases as the drying medium. Over 99% of the applications involve removal of water. All modes except the dielectric (microwave and radio frequency) supply heat at the boundaries of the drying object so that the heat must diffuse into the solid primarily by conduction. Transport of moisture within the solid may occur by any one or more of the following mechanisms of mass transfer: Liquid diffusion, if the wet solid is at a temperature below the boiling point of the liquid Vapor diffusion, if the liquid vaporizes within material Knudsen diffusion, if drying takes place at very low temperatures and pressures, e.g., in freeze drying. Surface diffusion (possible although not proven) Hydrostatic pressure differences, when internal vaporization rates exceed the rate of vapour transport through the solid to the surroundings. Combinations of the above mechanisms Note that since the physical structure of the drying solid is subject to change during drying the mechanisms of moisture transfer may also change with elapsed time of drying. 1.4. Thermodynamic Properties of Air-Water Mixtures and Moist Solids
1.4.1. Psychrometry Psychrometry is the science of various properties of air, method of controlling its temperature and moisture content or humidity and its effect on various material and human beings. The term air conditioning means treating of air or conditioning the air to change its temperature or the moisture as per the requirements of various applications. They are the devices or machine that condition or alter the state of the air by changing its temperature and the humidity level.
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As noted earlier, a majority of dryers are of direct (or convective) type. In other words, hot air is used both to supply the heat for evaporation and to carry away the evaporated moisture from the product. Notable exceptions are freeze and vacuum dryers, which are used almost exclusively for drying heat-sensitive products because they tend to be significantly more expensive than dryers operate near to atmospheric pressure.
1.4.2. Psychrometric terms Dry and wet bulb temperature, specific volume, humidity, enthalpy and more…. Psychrometry is the science of studying thermodynamic properties of moist air and the use of those to analyse moist air conditions and processes. Air conditioning processes can be determined with psychrometric charts or Moller Diagrams. Common properties in the charts includes Dry-bulb temperature wet-bulb temperature relative humidity (RH) humidity ratio specific volume dew point temperature enthalpy With at least two known properties it is possible to characterize the air in the intersection of the property lines – the state – point. With the intersection point located on the chart or diagram other properties can be read directly.
1.4.3. Equilibrium Moisture Content The moisture content of a wet solid in equilibrium with air of given humidity and temperature is termed the equilibrium moisture content (EMC). A plot of EMC at a given temperature versus the relative humidity is termed sorption isotherm. An isotherm obtained by exposing the solid to air of increasing humidity gives the adsorption isotherm. Water Activity In drying of some materials, which requires careful hygienic attention, e.g., food, the availability of water for growth of microorganisms, germination of spores, and participation in several types of chemical reaction becomes an important issue. This availability, which depends on relative pressure, or water activity, aw, is defined as the ratio of the partial pressure, p, of water over the wet solid system to the equilibrium vapour pressure, pw, of water at the same temperature. Thus, aw, which is also equal to the relative humidity of the surrounding humid air, is defined as: aw=
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Different shapes of the X versus aw curves are observed, depending on the type of material (e.g., high, medium or low hygroscopicity solids). 3
1.5) MECHANISM OF DRYING Drying may be defined as the vaporization and removal of water or other liquids from a solution, suspension, or other solid-liquid mixture to form a dry solid. It is a complicated process that involves simultaneous heat and mass transfer, accompanied by physicochemical transformations. Drying occurs as a result of the vaporization of liquid by supplying heat to wet feedstock, granules, and filter cakes and so on. Based on the mechanism of heat transfer that is employed, drying is categorized into direct (convection), indirect or contact (conduction), radiant (radiation) and dielectric or microwave (radio frequency) drying. Heat transfer and mass transfer are critical aspects in drying processes. Heat is transferred to the product to evaporate liquid, and mass is transferred as a vapour into the surrounding gas. The drying rate is determined by the set of factors that affect heat and mass transfer. Solids’ drying is generally understood to follow two distinct drying zones, known as the constant-rate period and the falling-rate period. The two zones are demarcated by a break point called the critical moisture content. In a typical graph of moisture content versus drying rate and moisture content versus time (Figure 1), section AB represents the constant-rate period. In that zone, moisture is considered to be evaporating from a saturated surface at a rate governed by diffusion from the surface through the stationary air film that is in contact with it. This period depends on the air temperature, humidity and speed of moisture to the surface, which in turn determine the temperature of the saturated surface. During the constant rate period, liquid must be transported to the surface at a rate sufficient to maintain saturation.
Figure1. Segment AB of the graph represents the constant-rate drying period, while segment BC is the falling-rate period.
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At the end of the constant rate period, (point B, Figure 1), a break in the drying curve occurs. This point is called the critical moisture content, and a linear fall in the drying rate occurs with further drying. This section, segment BC, is called the first falling-rate period. As drying proceeds, moisture reaches the surface at a decreasing rate and the mechanism that controls its transfer will influence the rate of drying. Since the surface is no longer saturated, it will tend to rise above the wet bulb temperature. This section, represented by segment CD in Figure 1 is called the second falling-rate period, and is controlled by vapor diffusion. Movement of liquid may occur by diffusion under the concentration gradient created by the depletion of water at the surface. The gradient can be caused by evaporation, or as a result of capillary forces, or through a cycle of vaporization and condensation, or by osmotic effects. The capacity of the air (gas) stream to absorb and carry away moisture determines the drying rate and establishes the duration of the drying cycle. The two elements essential to this process are inlet air temperature and air flow rate. The higher the temperature of the drying air, the greater its vapour holding capacity. Since the temperature of the wet granules in a hot gas depends on the rate of evaporation, the key to analysing the drying process is psychometric, defined as the study of the relationships between the material and energy balances of water vapour and air mixture.
1.5.1) DRYING END POINT There are a number of approaches to determine the end of the drying process. The most common one is to construct a drying curve by taking samples during different stages of drying cycle against the drying time and establish a drying curve. When the drying is complete, the product temperature will start to increase, indicating the completion of drying at specific, desired product-moisture content. Karl Fischer titration and loss on drying (LOD) moisture analysers are also routinely used in batch processes. The water vapor absorption isotherms are measured using a gravimetric moisture-sorption apparatus with vacuum-drying capability. For measuring moisture content in grain, wood, food, textiles, pulp, paper, chemicals, mortar, soil, coffee, jute, tobacco, rice and concrete, electrical-resistance-type meters are used. This type of instrument operates on the principle of electrical resistance, which varies minutely in accordance with the moisture content of the item measured. Dielectric moisture meters are also used. They rely on surface contact with a flat plate electrode that does not penetrate the product.
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1.6) TYPES OF DRYERS Adiabatic dryers are the type where the solids are dried by direct contact with gases, usually forced air. With these dryers, moisture is on the surface of the solid. Non-adiabatic dryers involve situation where a dryer does not use heated air or other gases to provide the energy required for the drying process. Dryer classification can also be based on the mechanisms of heat transfer as follows: Direct (convection) Indirect or contact (conduction) Radiant (radiation) Dielectric or microwave (radio frequency) drying.
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With adiabatic dryers, solid materials can be exposed to the heated gases via various methods, including the following: Gases can be blown across the surface (cross circulation) Gases can be blown through a bed of solids (through-circulation); used when solids are stationary, such as wood, corn and others Solids can be dropped slowly through a slow-moving gas stream, as in a rotary dryer Gases can be blown through a bed of solids that fluidize the particles. In this case, the solids are moving, as in a fluidized-bed dryer. Solids can enter a high-velocity hot gas stream and can be conveyed pneumatically to a collector (flash dryer). Direct, or adiabatic, units use the sensible heat of the fluid that contacts the solid to provide the heat of vaporization of the liquid. Non-adiabatic dryers (contact dryers) involve an indirect method of removal of a liquid phase from the solid material through the application of heat, such that the heat-transfer medium is separated from the product to be dried by a metal wall. Heat transfer to the product is predominantly by conduction through the metal wall and the impeller. Therefore, these units are also called conductive dryers.
Although more than 85% of the industrial dryers are of the convective type, contact dryers offer higher thermal efficiency and have economic and environmental advantages over convective dryers. A. BATCH DRYERS The following are descriptions of various types of batch dryers.
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a) Tray dryers. This dryer type operates by passing hot air over the surface of a wet solid that is spread over trays arranged in racks. Tray dryers are the simplest and least-expensive dryer type. This type is most widely used in the food and pharmaceutical industries. The chief advantage of tray dryers, apart from their low initial cost, is their versatility. With the exception of dusty solids, materials of almost any other physical form may be dried. Drying times are typically long (usually 12 to 48 h). b) Vacuum dryers. Vacuum dryers offer low-temperature drying of thermo labile materials or the recovery of solvents from a bed. Heat is usually supplied by passing steam or hot water through hollow shelves. Drying temperatures can be carefully controlled and, for the major part of the drying cycle, the solid material remains at the boiling point of the wetting substance. Drying times are typically long (usually 12 to 48 h). c) Fluidized-bed dryers. A gas-fluidized bed may have the appearance of a boiling liquid. It has bubbles, which rise and appear to burst. The bubbles result in vigorous mixing. A preheated stream of air enters from the bottom of the product container holding the product to be dried and fluidizes it. The resultant mixture of solids and gas behave like a liquid, and thus the solids are said to be fluidized. The solid particles are continually caught up in eddies and fall back in a random boiling motion so that each fluidized particle is surrounded by the gas stream for efficient drying, granulation or coating purposes. In the process of fluidization, intense mixing occurs between the solids and air, resulting in uniform conditions of temperature, composition and particle size distribution throughout the bed. d) Freeze dryers. Freeze-drying is an extreme form of vacuum drying in which the water or other solvent is frozen and drying takes place by subliming the solid phase. Freeze-drying is extensively used in two situations: (1) When high rates of decomposition occur during normal drying; and (2) With substances that can be dried at higher temperatures, and that are thereby changed in some way. Microwave vacuum dryers, High-frequency radio waves with frequencies from 300 to 30,000 MHz are utilized in microwave drying (2,450 MHz is used in batch Microwave processes). Combined microwave-convective drying has been used for a range of applications at both laboratory and industrial scales. The bulk heating effect of microwave radiation causes the solvent to vaporize in the pores of the material. Mass transfer is predominantly due to a pressure gradient established within the sample. The temperature of the solvent component is elevated above the air temperature by the microwave heat input, but at a low level, such that convective and evaporative cooling effects keep the equilibrium temperature below saturation. Such a drying regime is of particular interest for drying temperature-sensitive materials. Microwave-convective processing typically facilitates a 50% reduction in drying time, compared to vacuum drying.
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B. CONTINUOUS DRYERS Continuous dryers are mainly used in chemical and food industries, due to the large volume of product that needs to be processed. Most common are continuous fluid-bed dryers and spray dryers. There are other dryers, depending on the product, that can be used in certain industries — for example, rotary dryers, drum dryers, kiln dryers, flash dryers, tunnel dryers and so on. Spray dryers are the most widely used in chemical, dairy, agrochemical, ceramic and pharmaceutical industries.
a) Spray dryer. The spray-drying process can be divided into four sections: atomization of the fluid, mixing of the droplets, drying, and, removal and collection of the dry particles. The flow of the drying gas may be concurrent or counter current with respect to the movement of droplets. Good mixing of droplets and gas occurs, and the heat- and mass-transfer rates are high. In conjunction with the large interfacial area conferred by atomization, these factors give arise to very high evaporation rates. The residence time of a droplet in the dryer is only a few seconds (5–30 s). Since the material is at wet-bulb temperature for much of this time, high gas temperatures of 1,508 to 2,008C may be used, even with thermo labile materials. For these reasons, it is possible to dry complex vegetable extracts, such as coffee or digitalis, milk products, and other labile materials without significant loss of potency or flavour. The capital and running costs of spray dryers are high, but if the scale is sufficiently large, they may provide the cheapest method.
Figure2 spray dryer
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1.7) DRYER EFFICIENCY With increasing concern about environmental degradation, it is desirable to decrease energy consumption in all sectors. Drying has been reported to account for anywhere from 12 to 20% of the energy consumption in the industrial sector. Drying processes are one of the most energy-intensive unit operations in the CPI. One measure of efficiency is the ratio of the minimum quantity of heat that will remove the required water to the energy actually provided for the process. Sensible heat can also be added to the minimum, as this added heat in the material often cannot be economically recovered. Other newer technologies have been developed, such as sonic drying, superheated steam, heat-pump-assisted drying and others. 1.8) Summary Drying is an essential unit operation used in various process industries. The mechanism of drying is well understood as a two-stage process and depends on the drying medium and the moisture content of the product being dried. Batch dryers are common in chemical and pharmaceutical industries, while continuous dryers are routinely used where large production is required. Since the cost of drying is a significant portion of the cost of manufacturing a product, improving efficiency or finding alternative drying routes is essential.
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CHAPTER TWO CRYSTALLISATION Introduction Crystallization is a thermal separation, and therefore a purification process that yields a solid product from a melt, from a solution or from a vapour. As for all thermal separations, nonequilibrium conditions are required as a driving force for the process. Here, evaporation of solvent or temperature reduction (cooling) is the most frequent. Means employed to establish the required non-equilibrium conditions. Pressure can, in principle, also be used to enforce the non-equilibrium state necessary for a phase change. However, in industrial applications this parameter is most frequently kept constant. The main feature distinguishing crystallization from other thermal separation processes is the fact that it leads to a solid product. This is one of the key reasons why it lags behind separation techniques involving liquid-liquid or liquid-gaseous phase change processes in terms of research effort expended and knowledge available. Crystallization is a highly selective process and operates at lower temperatures when compared to a separation by distillation for the same material. Melt crystallization has the additional advantage of not requiring a solvent, although it is not a method suitable for all materials. Manifold reasons exist for the growing importance of crystallization as an industrial separation process. Two processes are important in crystallization, both with their own characteristic kinetics. One of these processes is nucleation [Kashehiev], the other is crystal growth. Both of these phenomena are dependent on a large number of variables that in some cases may be illustrated.
Objective Crystallisation is the formation of solid crystals from a solution. Crystals solidify in a definite geometric form. The objective of crystallisation is to separate a solute from a solvent. Any impurities in the liquid are usually not incorporated in to the lattice structure of the desired crystal. Accordingly crystallisation is also purification processes. Finally crystals possess an internal structure, an external shape and consequently a finite size (or size distribution in the case of a quantity of crystals). These parameters determine many bulk properties of a given crystalline material, such as dissolution rate, bio-availability, color, flow properties etc. A general overview of these properties will also be provided.
2.1. Definition Crystallization Crystallization is a process which consists of mixing the massecuite for certain time after dropping from the pans, and before passing to the centrifugals. The process is with the aim of completing the formation of crystals and forcing further exhaustion of the mother liquor. The massecuite when discharged from the pan is at a high super saturation. If it is allowed to stand, the sugar still contained in the mother liquor will continue to be deposited as a crystal, but this masscuite is very viscous. Therefore the masscuite must be kept in motion in order to change constantly the relative positions of the particles of mother liquor and crystals until the mother liquor becomes a substantially exhausted molasses 11
2.2) Types of Crystallizers •There are crystallizer of batch and continuous operation. Depending on the system of cooling there are1. Air Cooled Crystallizers 2. Water Cooled Crystallizers
1. Air-Cooled Crystallizers •The massecuite is cooled by the circulation of air from the walls of crystallizers and from the surface of the massecuite. •The cooling effect is slow. Nowadays the air cooled crystallizers are commonly used for storing high grade massecuite and their use for cooling low grade massecuite is not favored
2. Water Cooled Crystallizers These crystallizers have three arrangements: Water jacketed Cooling coil of rotating type •Water jacketed crystallizers are not found very satisfactory, recently they are not used. •The cooling coil of stationary type is less efficient due to poor contact of the massecuite with the cooling surface. They are only used because of lower cost.
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2.3. Solid-Liquid-Equilibria 2.3.1. Solubilities and Phase Diagrams A solution is a homogeneous mixture of two or more chemical species. For a liquid solution saturation is reached when the liquid phase, in contact with the solid phase, no longer changes its composition. A saturated solution therefore has a constant composition that is not changed by the addition of further amount of the dissolved material. For a two component system, the solubility of one component in the other is dependent on temperature and pressure. For three and more component systems the solubility of one component also depends on the relative amounts of the other components present. For liquid solutions the pressure dependence is negligible if the pressure difference is small (which is the case in most applications) and will not be considered here. It is common terminology for solutions of solids in liquids to denote the liquid component as the solvent and the solid component as the solute, even if the amount of solute in the solution exceeds the amount of solvent. Various measures of composition are in use to report solubilities and the use of a particular set of units should be carefully selected to suit the purpose the data are required for. The most common measures are Mass of solute per unit mass of solvent (kg/kg) Mass of solute per unit mass of solution (kg/kg) Mass of solute per unit volume of solvent (kg/m3 or g/L) Molar amount of solute per unit volume of solvent (kmol/m3 or mol/L) From the above list it is clear that, for the first two cases, it has to be stated explicitly whether composition reported refers to the solvent or the solute in order to avoid confusion. Moreover, it is often necessary to mention the initial state of the solute, as many substances can exist as solvated or un solvated solids (vide infra). For example, two solutions of sodium carbonate in water will have different compositions if in one case the decahydrate is used as starting material and in the other the same mass of monohydrate is employed. Ambiguity arises if this is not accounted for. The solubility of most materials increases with temperature. However, a number of examples exist, where the solubility shows the reverse trend (sodium sulfate, calcium carbonate, iron sulfate dehydrate, to name but a few). The dependence of the solubility upon temperature is best represented graphically in the form of a solubility curve, which maps the composition of a solution at the solubility limit onto the temperature. Solubilities are determined experimentally and any convenient analytical technique that provides a quantitative measure of composition can be employed. Not every technique. The driving force for crystallization (both nucleation and crystal growth, see section on kinetics below) increases with increasing super saturation. Super saturation can be achieved in a number of ways, most commonly by cooling a saturated solution (for solutes where solubility increases with temperature, solutes with reverse temperature dependence must, of course, be heated) or by evaporation of solvent. As is the case for solubility, several measures exist to express super saturation.
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These are the difference between the equilibrium composition at the temperature of the supersaturated solution C ⃰ and the actual composition in the supersaturated solution C. ΔC=C-C ⃰……………………………………………… (1) The ratio of these two concentration S=C/C ⃰. .......................................................................... (2) σ=ΔC/C ⃰ =S-1 ………………………………………... (3) For melts, the driving force is usually expressed in terms of the super cooling Δθ=θ ⃰-θ, ……………………………………………… (4) Which represents the difference between the equilibrium temperature of the melt θ* and the actual temperature θ. occasionally supercooling also finds use in solution crystallization. Quantifying the super saturation is important for two reasons. As mentioned above the super saturation is a measure of the driving force for the crystallization process. The second reason concerns the theoretical yield of the process.
Summary Crystallization as a separation process yields a solid product from a solution or a melt. The process itself, the product characteristics and the temporal behaviour are determined by thermodynamics, i.e. solubility of the materials, and crystallization kinetics, respectively. The thermodynamics of solutions and melts are dependent on a range of physical parameters as discussed within. Crystallization kinetics can be divided into two separate processes, nucleation and crystal growth. Both play a significant role in the design of equipment for a given process and are treated in detail below. Additional factors such as the effect of crystallization on product properties as well as the effect of solid-state properties on the crystallization process are discussed in section 4. Section 5 provides details on commonly used crystallizer designs and highlights the distinction between solution and melt crystallization.
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CHAPTER THREE EXTRACTION Introduction Extraction is a widely used method for the separation of a substance from a mixture.it involves the removal of a component of a mixture by contact with second phase. Solid-liquid and liquid-liquid extraction are commonly performed by batch and continuous process. The removal of caffeine from coffee beans with dichloromethane is an example of solid-liquid extraction. In numerous applications, extraction is a more efficient, selective and cost-effective alternative to competing separating methods such as distillation, evaporation and diaphragm technology. Applications of this method include obtaining oil from oil seeds or leaching of metal salts from ores
Objective The objective of extraction is to recover valuable soluble components from raw materials by primarily dissolving them in a liquid solvent, so that the components can be separated and recovered later from the liquid.
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3.1 Solid-Liquid Extraction The Solid-Liquid Extraction experimentation stand separates solid mixtures using solidliquid extraction (leaching). Solid-liquid extraction allows soluble components to be removed from solids using a solvent. The range of experiments covers the following areas: Familiarisation with the fundamental principles of solid-liquid extraction Demonstration of solid-liquid extraction as a continuous and discontinuous process Investigation of a single, two and three stage process Influence of solvent flow rate and temperature on the extraction process Influence of extraction material mass flow and extractor speed on the extraction process. Function The unit works on the counter flow principle, I.e. fresh solvent is fed to leached extraction material. With this operating method, the concentration gradient is the driving force for the mass transfer.
3.1.2 General principles Solid-liquid extraction involves dissolving soluble components out of solid mixtures using an solvent. In the simplest form of this method, the extraction material and the solvent are mixed well. The solvent and the dissolved usable substance are then removed and processed. Processing of the solvent with dissolved usable substance normally involves evaporation. The solvent is evaporated and the usable substance remains as a product. The solvent is condensed and can then be reused. A day-to-day example of solid-liquid extraction is the preparation of coffee. Here, water (solvent) dissolves the colours and flavourings (usable substance) out of the coffee powder (extraction material, consisting of the solid carrier material and the soluble components). Ideally, drinkable coffee is obtained (extract) and the leached coffee powder (extraction residue) remains in the coffee filter. Solid-liquid extraction is mainly carried out as percolation extraction and immersion extraction discontinuously or, preferably, continuously. Percolation extraction: The crushed and solubilised solid is passed through the extraction apparatus and sprayed with solvent in stages. The solvent must flow effectively through the solid.
Solvent requirements There is no universal solvent. The required solvent must be identified specifically for the relevant extraction task. The solvent can either be identified experimentally using solution experiments or from the results of extraction tasks already investigated.
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There are particular requirements of the solvent: Selectivity It should only dissolve the usable substance, otherwise a subsequent separating method is required to separate the usable substance from the extract. Solubility It should dissolve the usable substance as quickly as possible and dissolve the maximum possible amount of usable substance Chemical reaction properties It should not react chemically with the components of the extraction material Boiling properties The boiling point of the solvent should not be too high and the evaporation heat should be as low as possible, to ensure efficient recovery of the solvent.
3.2 Liquid-liquid extraction Introduction The separation of the components of a liquid mixture by treatment with a solvent in which one or more of the desired components is preferentially soluble. Three stages are involved: (a) Bringing the feed mixture and the solvent in to intimate contact, (b) Separation of the resulting two phases, (c) Removal and recovery of the solvent from each phase. It is complementary to distillation and is preferable in the following cases: (a) Distillation requires excessive amounts of heat. (b)The formation of azeotropes limits the degree of separation obtainable. (c) Heating must be avoided. (d)The components to be separated are quite different in nature. Liquid-liquid extraction refers to the dissolving out of one or more components of a liquid mixture using a solvent. Examples of liquid-liquid extraction are Separation of aromatic compounds from crude oil fractions. Separation of vitamins from aqueous solutions. Removal of lecithin from vegetable oil. Therefore liquid-liquid extraction involves at least three liquids. Names such as solution, solvents, etc. often lead.
Important applications include The separation of aromatics from kerosene-based fuel oils to improve their burning qualities. The separation of aromatics from paraffin and naphthenic compounds to improve the temperature-viscosity characteristics of lubricating oils. To obtain pure compounds such as benzene, toluene, and xylene from catalytically produced reformates in the oil industry. In the production of un hydrous acetic acid. In the extraction of phenol from coal tar liquors. In the metallurgical and biotechnology industries. The extraction operation may be either a physical or a chemical operation. Chemical operations have been classified as follows: 17
(a) Those involving cat ion exchange such as, for example, the extraction of metals by carboxylic acids; (b) Those involving anion exchange, such as the extraction of anions involving a metal with amines (c) Those involving the formation of an additive compound, for example, extraction with neutral oregano-phosphorus compounds. A new technology -the use of supercritical or near supercritical fluid as solvent. In biotechnology, many of the usual organic solvents will degrade a sensitive product.
3.2.1. Extraction Processes It can be carried out either as a batch or as a continuous process. See the single-stage batch process illustrated in the fig below. Mixing and separation occur in the same vessel.U-LIQUID EXTRACTION
3.2.2. Basic principles
To misunderstandings, especially in comparison to different sources. these experiment instructions use the following designations for the liquids involved. Transition component Carrier liquid Solvent Liquid-liquid extraction in a beaker, with the following simplifications: Only three liquids are involved. The transition component transfers completely from the carrier liquid into the solvent. On the left is the beaker with two liquid phases. Carrier liquid and solvent together form the solvent system. The carrier liquid and solvent together form a phase boundary. They are insoluble in one another. This is the condition so that separation into two phases can occur after the actual extraction. Also required is a clear density difference between the carrier liquid and solvent.
Before Mixing Settling Figure3; Simplified representation of liquid-liquid extraction.
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After
In this example, the solvent is specifically heavier than the carrier liquid. Therefore the carrier liquid floats on the solvent. The carrier liquid "carries" the transition component; the transition component is dissolved in the carrier liquid. The next step shows is the mixing of the two liquid phases. Droplets are formed; the specific surface area increases and the three liquids are brought into intensive contact. In doing so, the transition component transfers from the carrier liquid into the solvent. Another condition for extraction is that the transition component is soluble in the solvent. After mixing, two further liquid phases form.
3.2.3 Equilibrium Data The equilibrium condition for the distribution of one solute between two liquid phases is conveniently considered in terms of the distribution law. The ratio of the concentrations of the solute in the two phases is given by CE/CR =K’. Applicable only if both solvents are immiscible and if there is no association or dissociation of the solute. The addition of a new solvent to a binary mixture may lead to several mixtures: For low concentration, the distribution law holds for non- reactive components. (a)A homogeneous solution may be formed. (b)The solvent may be completely immiscible with the initial solvent. (c)The solvent may be partially miscible with the original solvent resulting in the formation of one pair of partially miscible liquids. (d)The new solvent may lead to the formation of two or three partially miscible liquids.
3.2.4 Factors influencing extraction performance Extraction performance is the amount of usable substance extracted per unit of time. It can be influenced by the following factors:
Differences in concentration The greater the difference in concentration of the usable substance in the extraction material and in the solvent, the greater the driving force when extracting. The difference in concentration is increased by rapidly removing the dissolved extract from the surface of the extraction material and frequently replacing the loaded solvent with fresh solvent.
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Extraction surface area The surface of the extraction material is proportional to the extracted quantity of material. The extraction performance increases as the extraction surface area rises. In practice, this is achieved by crushing the extraction material.
Diffusion resistance The diffusion resistance depends on the size of the particles, the porosity and the penetrability of the extraction material for the solvent. The diffusion resistance countering the dissolving of the extraction material should be as low as possible.
Temperature The extraction performance is increased by higher temperatures. Higher temperatures increase the thermal agitation, reducing the viscosity and thus accelerating the dissolving of the usable substance.
3.3 Liquid-Gas Extraction (absorption) Absorption is a basic procedure in thermal process engineering. Absorption is used to separate one or more gas components from a gas flow using a solvent (detergent). The Absorption unit provides a clear method of separating a gas mixture containing air and CO2 by absorption in water.
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REFERENCES Brockmann, M.C., 1973, Intermediate Moisture Foods, in W.B. van Arsdel, M.J. Copley, A.I. Morgan (Eds.) Food Dehydration, The AVI Publishing Co., Westport. Bruin, S., Luyben, K.Ch.A.M., 1980, Drying of Food Materials: A Review of Recent Developments, pp. 155-216, in A.S. Mujumdar (Ed.) Advances in Drying, Vol. 1, Hemisphere, Washington.
Book: principles of Refrigeration boy Roy J. Dossat www.google.com Process ІІ text book etc….
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