III Component Separation Fundamental Outline Heterogeneous Separation: 1. Gas-liquid (or vapor liquid) 2. Gas solid (or vapor solid) 3. Liquid liquid (immiscible) 4. Liquid solid 5. Solid solid. Homogeneous Separation 1. Creation of another phase 2. Addition of a mass separation agent Page 1 of 32
III.1. HETEROGENEOUS SEPARATION Heterogeneous Separation (Smith, R., 2005) If a heterogeneous (multiphase mixture), separation can be done physically by exploiting the differences in density between the phases. Separation of the different phases of a heterogeneous mixture should be carried out before homogeneous separation Phase separation tends to be easier and should be done first. The phase separations likely to be carried out are: Gas liquid (or vapor liquid) Gas solid (or vapor solid) Liquid liquid (immiscible) Liquid solid Solid solid Page 2 of 32
The principal methods for the separation of heterogeneous mixtures are: 1. 2. 3. 4. 5. 6. 7. Settling and sedimentation Inertial and centrifugal separation Electrostatic precipitation Filtration Scrubbing Flotation Drying. III.1.1. Settling and Sedimentation Particles are separated from a fluid by gravitational forces acting on the particles. The particles can be liquid drops or solid particles. The fluid can be a gas, vapor or liquid. Page 3 of 32
Gravity settler for the separation of gas liquid and vapor liquid mixtures The velocity of the gas or vapor through the vessel must be less than the settling velocity of the liquid drops. It is normally not practical to separate droplets less than 100 µm diameter in such a simple device. Thus, the design basis for simple settling devise is usually taken to be a vessel in which the velocity of the gas (or vapor) is the terminal settling velocity for droplets of 100 µm diameter. Gravity settler (Decanter) for the separation of liquid liquid mixtures The horizontal velocity must be low enough to allow the lowdensity droplets to rise from the bottom of the vessel to the interface and coalesce and for the high density droplets to settle down to the interface and coalesce. The decanter is sized on the basis that the velocity of the continuous phase should be less than the terminal settling velocity of the droplets of the dispersed phase. The velocity of the continuous phase can be estimated from the area of the interface between the settled phases Page 4 of 32
Gravity settler for the separation of fluid solid mixtures A mixture of gas, vapor or liquid and solid particles enters at one end of a large chamber. Particles settle toward the base. Again the device is specified on the basis of the terminal settling velocity of the particles. A thickener for liquid solid separation. When separating a mixture of water and fine solid particles in a gravity settling device, it is common in such operations to add a flocculating agent to the mixture to assist the settling process. This agent has the effect of neutralizing electric charges on the particles that cause them to repel each other and remain dispersed. The effect is to form aggregates or flocs, which, because they are larger in size, settle more rapidly. Page 5 of 32
Simple gravity settling classifier The larger particles, faster-settling particles settle to the bottom close to the entrance The smaller particles, the slower-settling particles settle to the bottom close to the exit III.1.2. Inertial and Centrifugal Separation Sometimes gravity separation (discussed earlier) may be too slow because of the closeness of the densities of the particles and the fluid, because of small particle size leading to low settling velocity or, in the case of liquid liquid separations, because of the formation of a stable emulsion. Inertial or momentum separators improve the efficiency of gas solid settling devices by giving the particles downward momentum, in addition to the gravitational force. Centrifugal separators take the idea of an inertial separator a step further and make use of the principle that an object whirled about an axis at a constant radial distance from the point is acted on by a force. Use of centrifugal forces increases the force acting on the particles. Particles that do not settle readily in gravity settlers often can be separated from fluids by centrifugal force. Page 6 of 32
Inertial separators increase the efficiency of separation by giving the particles downward momentum. A cyclone generates centrifugal force by the fluid motion. The simplest type of centrifugal device is the cyclone that consists of a vertical cylinder with a conical bottom. Centrifugal force is generated by the motion of the fluid. The mixture enters through a tangential inlet near the top, and the rotating motion so created develops centrifugal force that throws the dense particles radially toward the wall. The entering fluid flows downward in a spiral adjacent to the wall. The particles of dense material are thrown toward the wall and fall downward, leaving the bottom of the cone. Page 7 of 32
A centrifuge uses rotating cylindrical bowl to produce centrifugal force. In centrifuges, a cylindrical bowl is rotated to produce the centrifugal force. The cylindrical bowl is shown rotating with a feed consisting of a liquid solid mixture fed at the center. The feed is thrown outward to the walls of the container. The particles outward. settle horizontally Different arrangements are possible to remove the solids from the bowl. A centrifuge uses rotating cylindrical bowl to produce centrifugal force. two liquids having different densities are separated by the centrifuge. The more dense fluid occupies the outer periphery, since the centrifugal force is greater on the more dense fluid. Page 8 of 32
III.1.3. Electrostatic Precipitation Electrostatic precipitators are generally used to separate particulate matter that is easily ionized from a gas stream. Particles collect on the plates and are removed by vibrating the collection plates mechanically, thereby dislodging particles that drop to the bottom of the device. Electrostatic precipitation is most effective when separating particles with a high resistivity. The operating voltage typically varies between 25 and 45 kv or more, depending on the design and the operating temperature. The application of electrostatic precipitators is normally restricted to the separation of fine particles of solid or liquid from a large volume of gas. corona III.1.4. Filtration Suspended solid particles in a gas, vapor or liquid are removed by passing the mixture through a porous medium that retains the particles and passes the fluid (filtrate). The solid can be retained on the surface of the filter medium, which is cake filtration, or captured within the filter medium, which is depth filtration. The filter medium can be arranged in many ways: 1. Plate and Frame Filter (separation of solid-liquid) 2. Bag Filter (separation of solid-gas) 3. Belt Vacuum Filter (separation of solid-liquid) 4. Rotary Vacuum Filter (separation of solid-liquid) When separating solid particles from a liquid filtrate: 1. Filtrate is a product (cake as a waste) 2. The cake is a product (filtrate as a waste): it is usual to wash the cake to remove the residual filtrate from the filter cake. Page 9 of 32
Filtration can be arranged in many ways III.1.5. Srubbing Scrubbing with liquid (usually water) can enhance the collection of particles when separating gas solid mixtures. Three of the many possible designs for scrubbers: 1. Packed-bed Scrubber 2. Spray Scrubber 3. Venturi Scrubber Page 10 of 32
Packed-bed Scrubber a packed tower is similar to an absorption tower. Whilst this can be effective, it suffers from the problem that the packing can become clogged with solid particles. Towers using perforated plates similar to a distillation or absorption column can also be used. As with packed columns, plate columns can also encounter problems of clogging. Spray Scrubber Spray Scrubber uses a spray system that will be less prone to fouling. The design of spray scrubber uses a tangential inlet to create a swirl to enhance the separation. Page 11 of 32
Venturi Scrubber Liquid is injected into the throat of the venturi, where the velocity of the gas is highest. The gas accelerates the injected water to the gas velocity, and breaks up the liquid droplets into a relatively fine spray. The particles are then captured by the fine droplets. Very high collection efficiencies are possible with venturi scrubbers. The main problem with venturi scrubbers is the high pressure loss across the device. III.1.6. Flotation Flotation is a gravity separation process that exploits the differences in the surface properties of particles. Gas bubbles are generated in a liquid and become attached to solid particles or immiscible liquid droplets, causing the particles or droplets to rise to the surface. This is used to separate mixtures of solid solid particles after dispersion in a liquid, or solid particles already dispersed in a liquid or liquid liquid mixtures of finely divided immiscible droplets. The liquid used is normally water and the particles of solid or immiscible liquid will attach themselves to the gas bubbles if they are hydrophobic (e.g. oil droplets dispersed in water). The bubles of gas can be generated by three methods: 1. dispersion, in which the bubbles are injected directly by some form of sparging system 2. dissolution in the liquid under pressure and then liberation in the flotation cell by reducing the pressure 3. electrolysis of the liquid. Page 12 of 32
A typical flotation cell for solid separation The mixture is fed to a flotation cell, and gas is also fed to the cell where gas bubbles become attached to the solid particles, thereby allowing them to float to the surface of the liquid. The solid particles are collected from the surface by an overflow weir or mechanical scraper. The separation of the solid particles depends on the different species having different surface properties such that one species is preferentially attached to the bubbles. A number of chemicals can be added to the flotation medium to meet the various requirements of the flotation process: a. Modifiers are added to control the ph of the separation. These could be acids, lime, sodium hydroxide, and so on. b. Collectors are water-repellent reagents that are added to preferentially adsorb onto the surface of one of the solids. Coating or partially coating the surface of one of the solids renders the solid to be more hydrophobic and increases its tendency to attach to the gas bubbles. c. Activators are used to activate the mineral surface for the collector. d. Depressants are used to preferentially attach to one of the solids to make it less hydrophobic and decrease its tendency to attach to the gas bubbles. e. Frothers are surface-active agents added to the flotation medium to create a stable froth and assist the separation. Page 13 of 32
Dissolved air Flotation (DAF) When separating lowdensity solid particles or oil droplets from water, the most common method used is dissolved-air flotation. DAF shows some of the effluent water from the unit being recycled, and air being dissolved in the recycle under pressure. The pressure of the recycle is then reduced, releasing the air from solution as a mist of fine bubbles. This is then mixed with the incoming feed that enters the cell. Low-density material floats to the surface with the assistance of the air bubbles and is removed. III.1.7. Drying Drying refers to the removal of water from a substance through a whole range of processes, including distillation, evaporation and even physical separations such as centrifuges. Here, consideration is restricted to the removal of moisture from solids into a gas stream (usually air) by heat, namely, thermal drying. Some of the types of equipment for removal of water also can be used for removal of organic liquids from solids. Four of the more common types of thermal dryers used in the process industries are: 1. Tunnel Dryer 2. Rotary Dryer 3. Drum Dryer 4. Spray Dryer Page 14 of 32
Tunnel Dryer Wet material on trays or a conveyor belt is passed through a tunnel, and drying takes place by hot air. The air-flow can be counter-current, co-current or a mixture of both. This method is usually used when the product is not free flowing Rotary Dryer A cylindrical shell mounted at a small angle to the horizontal is rotated at low speed. Wet material is fed at the higher end and flows under gravity. Drying takes place from a flow of air, which can be counter-current or co-current. The heating may be direct to the dryer gas or indirect through the dryer shell. This method is usually used when the material is free flowing. Rotary dryers are not well suited to materials that are particularly heat sensitive because of the long residence time in the dryer. Page 15 of 32
Drum Dryer Drum dryer consists of a heated metal roll. As the roll rotates, a layer of liquid or slurry is dried. The final dry solid is scraped off the roll. The product comes off in flaked form. Drum dryers are suitable for handling slurries or pastes of solids in fine suspension and are limited to low and moderate throughput. Spray Dryer In spray dryer, a liquid or slurry solution is sprayed as fine droplets into a hot gas stream. The feed to the dryer must be pumpable to obtain the high pressures required by the atomizer. The product tends to be light, porous particles. An important advantage of the spray dryer is that the product is exposed to the hot gas for a short period. Also, the evaporation of the liquid from the spray keeps the product temperature low, even in the presence of hot gases. Spray dryers are thus particularly suited to products that are sensitive to thermal decomposition, such as food products. Page 16 of 32
Spray Dryer for Milk Powder Manufacture Page 17 of 32
III.2. HOMOGENEOUS SEPARATION Homogeneous Separation (Smith, R., 2005) If the mixture is homogeneous, separation can only be performed by the creation of another phase within the system and the addition of a mass separation agent. For example, if a vapor mixture is leaving a reactor, another phase could be created by partial condensation. Alternatively, a liquid solvent could be contacted with the vapor mixture to act as a mass separation agent to preferentially dissolve one or more of the components from the mixture. Further separation is required to separate the solvent from the process materials so as to recycle the solvent, and so on. Page 18 of 32
Partial Condensation of Reactor Product Partial condensation for separating H2 from others The principal methods for the separation of homogeneous mixtures are: 1. 2. 3. 4. 5. 6. 7. Distillation Absorption and Stripping Liquid-Liquid Extraction Adsorption Membrane Crystallization Evaporation Page 19 of 32
III.2.1. Distillation The separation of a homogeneous fluid mixture requires the creation of another phase If this liquid mixture is partially vaporized, then another phase is created, and the vapor becomes richer in the more volatile components (i.e. those with the lower boiling points) than the liquid phase. The liquid becomes richer in the less volatile components (i.e. those with the higher boiling points). If the system is allowed to come to equilibrium conditions, then the distribution of the components between the vapor and liquid phases is dictated by vapor liquid equilibrium considerations. All components can appear in both phases. A cascade of equilibrium stages with refluxing and reboiling It is assumed in the cascade that liquid and vapor streams leaving each stage are in equilibrium. Using a cascade of stages in this way allows the more volatile components to be transferred to the vapor phase and the less-volatile components to be transferred to the liquid phase. In principle, by creating a large enough cascade, an almost complete separation can be carried out. At the top of the cascade, liquid is needed to feed the cascade (by condensing the top product, as reflux). total condenser or partial condenser At the bottom of the column, vapor is also needed to feed the cascade (by vaporizing vaporizing some of the liquid leaving the bottom stage) The feed to the process is introduced at an intermediate stage, and products are removed from the condenser and the reboiler. Page 20 of 32
Distillation Tray and Packing Distillation Tray Distillation Packing Distillation Column Page 21 of 32
Choice of operating conditions of distillation Specified conditions: 1. Feed composition and flowrate 2. Product specifications: product purities of recoveries of certain component The operating parameters to be selected by the designer include: 1. operating pressure 2. reflux ratio 3. feed condition 4. type of condenser Operating pressure of distillation column As pressure is raised: separation becomes more difficult (relative volatility decreases), that is, more stages or reflux are required; latent heat of vaporization decreases, that is, reboiler and condenser duties become lower; vapor density increases, giving a smaller column diameter; reboiler temperature increases with a limit often set by thermal decomposition of the material being vaporized, causing excessive fouling; condenser temperature increases. As pressure is lowered: The lower limit is often set by the desire to avoid: 1. vacuum operation 2. refrigeration in the condenser Both vacuum operation and the use of refrigeration incur capital and operating cost penalties and increase the complexity of the design. They should be avoided if possible. Page 22 of 32
Reflux ratio For a stand-alone distillation column (i.e. utility used for both reboiling and condensing), there is a capital energy trade-off. As the reflux ratio is increased from its minimum, the capital cost decreases initially as the number of plates reduces from infinity, but the utility costs increase as more reboiling and condensation are required The optimal ratio of actual to minimum reflux is often less than 1.1. However, most designers are reluctant to design columns closer to minimum reflux than 1.1, except in special circumstances, since a small error in design data or a small change in operating conditions might lead to an infeasible design. Feed condition Heating the feed most often: increases trays in the rectifying section but decreases trays in the stripping section requires less heat in the reboiler but more cooling in the condenser. As the condition of the feed is changed from saturated liquid feed (q = 1) to saturated vapor feed (q = 0), the minimum reflux ratio tends to increase. Thus the ratio of heat added to preheat the feed divided by the heat saved in the reboiler depends on the change in q, the relative volatility between the key components, feed concentration and ratio of actual to minimum reflux. Page 23 of 32
Type of condenser (Either a total or partial condenser can be chosen.) Total Condenser: Most designs condenser. Partial Condenser: use a total A total condenser is necessary if the top product needs to be sent to intermediate or final product storage. Also, a total condenser is best if the top product is to be fed to another distillation at a higher pressure as the liquid pressure can readily be increased using a pump. A partial condenser reduces the condenser duty, which is important if the cooling service to the condenser is expensive, such as low-temperature refrigeration. It is often necessary to use a partial condenser when distilling mixtures with lowboiling components that would require very low-temperature (and expensive) refrigeration for a total condenser. III.2.2. Absorption and Stripping In absorption, a gas mixture is contacted with a liquid solvent that preferentially dissolves one or more components of the gas. Absorption processes often require an extraneous material to be introduced into the process to act as liquid solvent. Liquid flowrate, temperature and pressure are important variables to be set. Page 24 of 32
PFD of CO2 Removal Addition of DEA PFD of Dehydration Unit Addition of TEG Page 25 of 32
III.2.3. Liquid-Liquid Extraction Liquid liquid extraction carries out separation by contacting a liquid feed with another immiscible liquid. The separation occurs as a result of components in the feed distributing themselves differently between the two liquid phases. The liquid with which the feed is contacted is known as the solvent. The solvent extracts solute from the feed. The solvent-rich stream obtained from the separation is known as the extract and the residual feed from which the solute has been extracted is known as the raffinate. Liquid-Liquid Extraction Page 26 of 32
III.2.4. Adsorption Adsorption is a process in which molecules of adsorbate become attached to the surface of a solid adsorbent. Adsorption processes can be divided into two broad classes: 1. Physical adsorption, in which physical bonds form between the adsorbent and the adsorbate. 2. Chemical adsorption, in which chemical bonds form between the adsorbent and the adsorbate. An example of chemical adsorption is the reaction between hydrogen sulfide and ferric oxide: The ferric oxide adsorbent, once it has been transformed chemically, can be regenerated in an oxidation step: adsorbent Activated Carbon Ferric Oxide Page 27 of 32
Physical and Chemical Adsortion Physical Adsorption Chemical Adsorption Heat of adsorption Small, same order as heat of Large, many times greater Vaporization (condensation) than the heat of vaporization (condensation) Rate of adsorption Controlled by resistance to mass transfer; Rapid rate at low Temperatures Controlled by resistance to surface reaction; Low rate at low temperatures Specificity Low, entire surface availability for physical adsorption High, chemical adsorption limited to active sites on the surface Surface coverage Complete and extendable to Multiple molecular layers Incomplete and limited to a layer, one molecule thick Activation energy Low High, corresponding to a chemical reaction Quantity adsorbed per unit mass High Low Types of physical adsorbent: 1. Activated carbon: a form of carbon that has been processed to develop a solid with high internal porosity. The most commonly used methods are the separation of organic vapors from gases, and a liquid-phase application, e.g. decolorizing or deodorizing aqueous solutions. 2. Silica gel (SiO2) : its surface has an affinity for water and organic material. It is primarily used to dehydrate gases and liquids 3. Activated aluminas: a porous form of aluminum oxide (Al2O3) with high surface area, manufactured by heating hydrated aluminum oxide to around 400 C in air. Activated aluminas are mainly used to dry gases and liquids, but can be used to adsorb gases and liquids other than water. Page 28 of 32
Types of physical adsorbent: 4. Molecular sieve zeolites: Zeolites are crystalline alumi-nosilicates. They differ from the other three major adsorbents in that they are crystalline and the adsorption takes place inside the crystals. This results in a pore structure different from other adsorbents in that the pore sizes are more uniform. Access to the adsorption sites inside the crystalline structure is limited by the pore size, and hence zeolites can be used to absorb small molecules and separate them from larger molecules, as molecular sieves. Typical applications are the removal of hydrogen sulfide from natural gas, separation of hydrogen from other gases, removal of carbon dioxide from air before cryogenic processing, separation of p-xylene from mixed aromatic streams, separation of fructose from sugar mixtures, and so on III.2.5. Membrane Membranes act as a semipermeable barrier between two phases to create a separation by controlling the rate of movement of species across the membrane. The separation can involve two gas (vapor) phases, two liquid phases or a vapor and a liquid phase. The feed mixture is separated into a retentate, which is the part of the feed that does not pass through the membrane, and a permeate, which is that part of the feed that passes through the membrane. The driving force for separation using a membrane is partial pressure in the case of a gas or vapor and concentration in the case of a liquid. Differences in partial pressure and concentration across the membrane are usually created by the imposition of a pressure differential across the membrane. However, driving force for liquid separations can be also created by the use of a solvent on the permeate side of the membrane to create a concentration difference, or an electrical field when the solute is ionic. Page 29 of 32
Idealized flow patterns in membrane separation Schematic of Separation using membrane based on pressure Page 30 of 32
III.2.6. Crystallization Crystallization involves formation of a solid product from a homogeneous liquid mixture. Often, crystallization is required as the product is in solid form. The reverse process of crystallization is dispersion of a solid in a solvent, termeddissolution. The dispersed solid that goes into solution is the solute. As dissolution proceeds, the concentration of the solute increases. Given enough time at fixed conditions, the solute will eventually dissolve up to a maximum solubility where the rate of dissolution equals the rate of crystallization. Under these conditions, the solution is saturated with solute and is incapable of dissolving further solute under equilibrium conditions. III.2.7. Evaporation Evaporation separates a volatile solvent from a solid. Single-stage evaporators tend to be used only when the capacity needed is small. For larger capacity, it is more usual to employ multistage systems that recover and reuse the latent heat of the vaporized material. Three different arrangements for a three-stage evaporator are: 1. Forward feed: The boiling temperature decreases from stage to stage, and this arrangement is thus used when the concentrated product is subject to decomposition at higher temperatures. It also has the advantage that it is possible to design the system without pumps to transfer the solutions from one stage to the next 2. Backward feed: is used when the concentrated product is highly viscous. The high temperatures in the early stages reduce viscosity and give higher heat transfer coefficients. Because the solutions flow against the pressure gradient between stages, pumps must be used to transfer solutions between stages. 3. Parallel feed: The vapor from each stage is still used to heat the next stage. This arrangement is used mainly when the feed is almost saturated, particularly when solid crystals are the product Page 31 of 32
Three possible arrangements for a threestage evaporator: (a) forward feed operation (b) Backward feed operation (c) Parallel feed operation Page 32 of 32