Separation Process PDF

# 9 Separation Process ## Contents - Classification of separation processes - Filtration - Crystallization - Membrane separation - Supercritical fluid extraction (SCFE) - Exercises The various unit operations of food process engineering applied to solve the problem of changing the compositions of...

# 9 Separation Process ## Contents - Classification of separation processes - Filtration - Crystallization - Membrane separation - Supercritical fluid extraction (SCFE) - Exercises The various unit operations of food process engineering applied to solve the problem of changing the compositions of solutions, mixtures and solids. Often these operations must be conducted on a large scale to meet the demands of a large manufacturing plant. In food processes engineering these operations are often referred to as separation / concentration / extraction / purification process. Usually these operations are directed towards separating substances into its component parts or concentrating a particular substance from another. ## 9.1 Classification of Separation Processes Separation processes can broadly be classified into the two categories: 1. Mechanical separation process 2. Contact equilibrium process Separation processes that depend primarily on mechanical/physical forces to accomplish the desired separation of components are referred as mechanical separation processes and include filtration, sedimentation, sieving and centrifugation. The mechanical/physical forces affecting separation may be gravitational and centrifugal, as well as kinetic forces arising from flow. Particles and/or fluid streams are separated because of the difference in the effects produced by these forces on them. Other separation processes can be carried out by the introduction of a new phase to the system, allowing the components of the original raw material to distribute themselves between the phases. These processes normally referred to as contact equilibrium processes and include gas absorption, extraction, distillation, crystallization and membrane separation processes. In this chapter, some common separation methods have been described: ## 9.2 Filtration Filtration involves the separation of solids from a liquid and its effect by using porous medium which retains the solids and allows the liquid to pass through. The variable product may be clear filtrate from filtration (i.e. deep-bed filtration) or the solid cake (i.e. cake filtration). ### 9.2.1 Basic Theory Of Filtration When the fluid passes through the filter medium the solid being deposited in the filter medium. The removal of these solids from fluid results in a build-up of solids in the filter medium resulting in an increase in resistance to flow as the filtration process continues (Figure 9.1). Therefore, higher pressure difference is necessary to maintain the flow rate of filtrate. The rate of filtration can be written as follows Rate of filtration = $\frac{Driving force (\Delta P)}{Resistance}$ Mathematically, $Q = \frac{dV}{dt} = \frac{A\Delta P}{R}$ Where, - Q = Volume rate of flow of filtrate - A = Cross sectional area of filter - V = Volume of filtrate - $\Delta P$ = Pressure difference - t = Filtration time - R = Resistance to flow of filtrate Assuming that the filter cake does not become compressed, R is described by the following expression $R = \mu r (L_c + L)$ Where - $L_c$ = Thickness of filter cake - $\mu$ = Fluid viscosity (Ns m²) - L = Thickness of the filter material or medium (m) - r = Specific resistance to the filter cake (m²) Where driving force is the pressure required to maintain the flow rate of filtrate through the filter medium and resistance depend on several factors. ### 9.2.2 Filtration Equipment #### 9.2.2.1 Classification There are several ways to classify types of filtration equipment. Some of them are listed below: 1. **Based on desired product of filtration process**: - Cake filtration, where solid in the fluid is desired product - Deep bed filtration, where clear filtrate is the desired product 2. **Based on operating cycle**: - Batch operated, where cake is removed after the completion of the process - Continuous operation, where the cake is continuously removed. 3. **Based on force applied in fluid being filtrated**: - Gravity filtration, where the liquid simply flows by hydrostatic forces - Pressure filtration, where pressure is applied to the fluid on the feed side of the filter bed to increase flow rates of fluids - Vacuum filtration, where the vacuum is created to the opposite side of the filter bed - Centrifugal filtration, the application of centrifugal forces to provide the driving force to fluid flow through a filter medium. The basic requirements for filtration equipment are: - Mechanical support for the filter medium - Flow accesses to and from the filter medium - Provisions for removing the filter cake #### 9.2.2.2 Equipments 1. **Plate and Frame Filter Press**: - A Plate and frame filter press consists of a series of ribbed or grooved plates covered on both surfaces by a cloth or paper filters. The plates with their filter cloths may be horizontal, but they are more usually hung vertically with a number of plates operated in parallel to give sufficient area. - Feed liquor is pumped into the press and liquid passes through filter cloths. This is illustrated in Figure 9.3. - It flows down the grooved surfaces of the plates and drained through the outlet channel in the base of each plate. As filtration is continued, filter cake builds up on the upstream side of the cloth i.e. the side away from the plate. - In the early stages of the filtration cycle, the pressure drop across the cloth is small and filtration proceeds at more or less a constant rate. As the cake increases, the process becomes more and more a constant-pressure one and this is the case throughout most of the cycle. - When the available space between successive frames is filled with cake, the press has to be dismantled and the cake is removed and cleaned, after which a further cycle can be initiated. **Advantages**: - The method has relatively low capital costs and high flexibility for different foods. - The equipment is reliable and easily maintained. - It is widely used, particularly for the production of apple juice and cider. **Disadvantages**: - The plate and frame filter press is cheap but it is difficult to mechanize to any great extent. - It is time-consuming and highly labour intensive. 2. **Rotary Filters**: - In rotary filters, the flow passes through a rotating cylindrical cloth from which the filter cake can be continuously scraped. Either pressure or vacuum can provide the driving force, but a particularly useful form is the rotary vacuum filter. - In this, the cloth is supported on the periphery of a horizontal cylindrical drum that dips into a bath of the slurry. Vacuum is drawn in those segments of the drum surface on which the cake is building up. A suitable bearing applies the vacuum at the stage where the actual filtration commences and breaks the vacuum at the stage where the cake is being scraped off after filtration. Filtrate is removed through trunnion bearings. Rotary vacuum filters are expensive, but they do provide a considerable degree of mechanization and convenience. A rotary vacuum filter is illustrated diagrammatically in Figure 9.4. 3. **Centrifugal Filters**: - Centrifugal force is used to provide the driving force in some filters. These machines are really centrifuges fitted with a perforated bowl that may also have a filter cloth on it. ## 9.3 Crystallization - Crystallization is a solid-liquid separation process and therefore a purification process that yields a solid product from a liquid melts, from a solution or from a vapour. - The main feature distinguishing crystallization from other 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 expanded and knowledge available. - This is generally accomplished by lowering the temperature, or by concentration of the solution, in each case to Form a supersaturated solution from which solutes come out of the solution in the form of pure crystals. - The manufacture of sucrose, from sugar cane or sugar beet, is an important example of crystallization in food technology. - Crystallization is also used in the manufacture of other sugars, such as glucose and lactose, in the manufacture of food additives, such as salt, and in the processing of foodstuffs, such as ice cream. - In commercial crystallization, the sizes and shapes of crystals are as important as their yield and purity. Size uniformity of crystals is desirable to minimize caking in the package, for ease of pouring, washing and filtering. ## 9.4 Membrane Separation - Membrane separation involves partially separating a feed containing a mixture of two or more components by use of a semi-permeable barrier (the membrane) through which one or more of the species moves faster than another or other species. - As shown in Figure 9.7, the basic process of the membrane separation involves a feed mixture separated into a retentate (part of the feed that does not pass through the membrane, i.e., is retained) and a permeate (part of the feed that passes through the membrane). - Although the majority of the time the feed, retentate, and permeate are usually liquid or gas, they may also be solid. - The optional sweep is a liquid or gas, used to help remove the permeate There are several types of membrane separation technologies: - **Non-porous membranes**: These membranes are capable of separating molecules of the same size, gases as well as liquids. - **Porous membranes**: Porous membranes are used in microfiltration and ultrafiltration. The dimension of the pores (0.1~10um) mainly determines the separation characteristics. High selectivities can be obtained when the size of the solute is large relative to the pore size in the membrane. Microporous membranes are similar to porous membranes and differ in regards to pore dimension (50~500 angstrom) - **Carrier membranes**: In this type of membrane, separation occurs by carrier molecule transporting the desired component across the barrier. The carrier molecule shows a very specific affinity to one component class of components in the feed, which means that high selectivity can be obtained. Since the separation is completely determined by the carrier solute, interaction of all types of components can be removed: gases or liquid (Figure 9.8c). ### 9.4.1 Membrane Structures Because the membrane must allow certain constituents to pass through, they must have a high permeability to certain types of molecules. Membrane structures consist of the following three basic types: - **Porous membranes:** Porous membranes are used in microfiltration and ultrafiltration. The dimension of the pores (0.1~10um) mainly determines the separation characteristics. High selectivities can be obtained when the size of the solute is large relative to the pore size in the membrane. Microporous membranes are similar to porous membranes and differ in regards to pore dimension (50~500 angstrom). - **Non-porous membranes:** These membranes are capable of separating molecules of the same size, gases as well as liquids. Non-porous membranes do not contain any macroscopic pores. The transport is determined by the diffusion mechanism, which means that components first must dissolve into the membrane and then diffuse through the membrane due to a driving force. Separation is due to differences in diffusivity and/or solubility. These membranes can be found in gas separation (Figure 9.8 b). - **Carrier membranes:** In this type of membrane, separation occurs by carrier molecules transporting the desired component across the barrier. The carrier molecule shows a very specific affinity to one component class of components in the feed, which means that high selectivity can be obtained. Since the separation is completely determined by the carrier solute, interaction of all types of components can be removed: gases or liquid (Figure 9.8c). ### 9.4.2 Membrane Materials The membrane structures are made of various materials of natural or synthetic polymers (macromolecules). The representative membrane materials and applications are listed in Table 9.3. | Materials | Membrane separation process | Pressure Bar | Membrane pore size (um) | |---|---|---|---| | Polypropylene | Microfiltration (MF) | ~30-60 | 10-4-10-3 | | Polysulfone | Ultrafiltration (UF), Gas Separation (GS) | ~20-40 | 10-3-10-2 | | Polyamide | Gas Separation | ~1-10| 10-2-10-1 | | Polyamide | Reverse Osmosis (RO) | ~1-10 | 10-2-10-1 | | Polyacrylonitrile | Ultrafiltration | | | | Cellulose | Microfiltration, Ultrafiltration, Reverse osmosis | | | ### 9.4.3 Classification of Membrane Separation Processes Membrane systems are classified primarily based on their molecular size (i.e. they can separate particles that are about 10 micron to solute molecules that are a few angstroms). Relative pore size of the membranes used in separation systems in decreasing size are: microfiltration, ultrafiltration, nanofiltration and reverse osmosis (Figure 9.9). These systems are discussed in succeeding paragraphs: #### 1. Microfiltration **Applicability range:** - Particles in size range of approximately 0.1 to 10 micrometers (µm) or - In molecular weight greater than 1000 kDa. - The particles being screened are not visible to the naked eye. **Examples of the particle size of this range are:** yeast cells, red blood cells, coal dust, and some bacteria. This pore size is used for sterile filtration, cell harvesting or clarification of fruit juices and in applications where water taste is not as important, like breweries. However, it is the least used because of the availability of finer membrane systems. It retains particles from about 200 to 1000 Å. The least amount of hydrostatic force required (about 1 bar). In general this filtration system falling between ultrafiltration and conventional filtration system. #### 2. Ultrafiltration Ultrafiltration is a process of separating colloidal or molecular particles by filtration, using suction or pressure, by means of a colloidal filter or semi-permeable membrane. #### 3. Nanofiltration Nanofiltration is the newest of the major methods, serving as an intermediate between ultrafiltration and reverse osmosis. **Applicability range:** - Particles in size range of approximately 0.001 to 0.01µm or - In molecular weight between 250 and 400 kDa. Nanofiltration membranes are rated in terms of percent salt rejection and flow. This process allows some salts through the membrane, allowing monovalent ions to pass while rejecting high percentages of divalent cations and multivalent ions. This process is used for sugar concentration, dye desalting, water softening, color removal in water, removing bacteria, some proteins by dairy industry, and meat processors for recovering value added by-products and making water suitable for discharge. #### 4. Reverse Osmosis (RO) RO has the finest membrane size. **Applicability range:** - Particles in the ionic range from about 0.001 micrometers (µm) and below. - In molecular weight less than 125 kDa. In reverse osmosis, the natural process of osmosis is countered by applied external pressure. Normally, pure water would move from a region of higher concentration (such as pure fresh water) into one of lower concentration (such as a solution of water and salt). RO causes the water to move out of the salt solution, opposite of what would naturally occur. The most common force is pressure generated by a pump. The higher the pressure the higher the driving force. Considerably higher pressures are necessary to overcome osmotic pressures. RO allows only pure water through the membrane, filtering out inorganic salts, some forms of non-ionic organic compounds such as fructose (Molecular weight 180) and smaller organics such as ethyl alcohol (Molecular weight 46). RO is used to reduce inorganic salts in water that has demanding specs such as boiler feed water, car wash rinse water, potable water, glass rinsing, pure water for dialyses, beverages, pharmaceutical water and maple syrup concentration. It can also be used to remove bacteria, salts sugars, proteins, particles, dyes, water recycling, concentrating milk solids and removing water from whey. With RO, the charge of the particles (ions) facilitates separation. The larger the charge and the particle, the more likely it will be rejected. ### 9.4.4 Membrane Configuration The membrane and its support usually called a "module". Modules are assembled and can be easily integrated as a filtration system. The important functions of the module design are: 1. To accommodate large membrane areas in small volume. 2. It should withstand pressure required by the filtration and cross flow velocities to maintain a clean membrane surface.