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Process technology equipment and systems chapter 16&17, Extraction & Other Separation Systems, Plastic Systems
399 Extraction and Other Separation Systems O BJECTIVES After studying this chapter, the student will be able to: Describe the scientific principles associated with adsorption. • Describe the extraction process. • Describe the stripping process. • Identify the basic equipment associated with extraction. • Describe the crystallization process. • Explain how a scrubber works. • Identify the basic equipment associated with the adsorption process. • Describe the solvent dewaxing process. • Explain the basic components and operation of a sidestream-stripping column. • 399 400 Chapter 16 ● Extraction and Other Separation Systems Key Terms Absorber—a device used to remove selected components from a gas stream by contacting it with a gas or liquid. Adsorber—a device (such as a reactor or a dryer) filled with a porous solid designed to remove gases and liquids from a mixture. Extract—the second solution that is formed when a solvent dissolves a solute. Extraction—a process for separating two materials in a mixture or solution by introducing a third material that will dissolve one of the first two materials but not the other. Feed—the original solution to be separated in liquid-liquid extraction. Raffinate—in liquid-liquid extraction, material that is left after a solvent has removed solute. Scrubber—a device used to remove chemicals and solids from process gases. Solute—the material that is dissolved in liquid-liquid extraction. Solution—a uniform mixture of particles that are not tied together by any chemical bond and can be separated by purely physical change. Solvent—a chemical that will dissolve another chemical. Extraction One of the most frequently encountered problems in chemical process o perations is that of separating two materials from a mixture or a s olution. Distillation is one way of making such a separation, and it is perhaps the most frequently used method. Another useful method is extraction. E xtraction is a process for separating two materials in a mixture by introducing a third material that will dissolve one of the first two materials but not the other. There are three basic types of extraction: leaching, washing, and liquid- liquid. In leaching, a material is removed from a solid mass by contacting it with a liquid. Metals are removed from their ores by leaching. In wash- ing, material sticking to the surface of a solid is removed by dissolving it in a l iquid and flushing it away. You use this process every time you take a shower. In liquid-liquid extraction, all three materials are liquids, and the mixture is separated by allowing them to layer out by weight or density. This chapter focuses on the liquid-liquid extraction process. Reasons for Extraction Each method of separation has its own advantages and disadvantages, and several situations call for the use of liquid-liquid extraction rather than distillation or some other method. 401 Extraction In many cases, it is impractical to separate two chemicals by distillation because the boiling points of materials are too close together. In such a case, it is frequently possible to find a third chemical that will dissolve one of the two chemicals. In this situation, extraction would be a better method of making the separation than distillation. Many chemicals are highly sensitive to heat and will degrade or decom- pose if raised to a temperature high enough for distillation. In this case extraction, which can usually be carried out at normal temperatures, would be a practical alternative. Often, one of the materials to be separated is present in very small amounts. It might be possible to recover such a material by distillation, but it is u sually much easier and more economical to do so by extraction. Finally, the key requirement of any commercial process is that it be eco- nomical. In situations in which several alternative means of separating two chemicals could be used, the one that is the most economical is chosen. Because many relatively inexpensive solvents are available, and because the equipment required for an extraction operation is relatively simple, eco- nomic considerations often favor liquid-liquid extraction. Liquid-Liquid Extraction Process There are basically three steps in the liquid-liquid extraction process: (1) contact the solvent with the feed solution; (2) separate the raffinate from the extract; (3) separate the solvent and the solute. Step 3, recovery of the solvent and solute, is left to be done by some other process such as distil- lation. In liquid-liquid extraction, the feed is the original solution. The feed solution, containing the solute (the material that will be dissolved), is fed to the lower portion of the extraction column (Figure 16.1). The solvent (the material that dissolves the solute) is added near the top. Because of density differences, the lighter feed solution tends to rise to the top while the heavier solvent sinks to the bottom. As the two streams mix, the solvent Solvent Feed Extract Raffinate Mixer Figure 16.1 Liquid-Liquid Extraction Chapter 16 ● Extraction and Other Separation Systems 402 dissolves the solute. Thus, the solute, which was originally rising with the feed solution, actually reverses its direction of flow and goes out with the solvent through the bottom of the column. This new solution, consisting of solvent and solute, is called the extract. The other chemical in the feed stream, now free of the solute, goes out the top as the raffinate. The raf- finate and extract streams are not soluble in each other and will layer out. Properties of a Good Solvent The solvent must be able to dissolve the solute, but it should not be a substance that will dissolve the raffinate or contaminate it. It also must be insoluble so that it will layer out. The density of the solvent should vary sufficiently from the density of the raffinate so that they can layer out by the effects of gravity. The solvent must be a substance that can be sepa- rated from the solute. It should be inexpensive and readily available, and it should not be hazardous or corrosive. Equipment Basically, the equipment for commercial operations is designed to ensure contact between the solvent and the feed and to separate the extract from the raffinate. The simplest extraction apparatus is the single-stage batch unit. In such an operation, the feed and solvent are added to a tank or some other suitable container (Figure 16.2). They are then thoroughly mixed by a mixer in the tank or by circulation in the tank. After the materials have been thoroughly mixed, the mixing is stopped, and the materials are allowed to layer out. The extract and raffinate layers can be removed. Such a process could be converted to continuous operations by continu- ously adding the feed and solvent and continuously withdrawing the raf- finate and extract. For such a process to be successful, some means of mixing or contacting the materials must be provided while still allowing ample space for the raffinate and extract to layer out. Many simple yet inge- nious means have been employed for this purpose. Most frequently, a single-stage device as described will not provide a perfect separation, and the raffinate must be contacted again with more Figure 16.2 Single-Stage Extraction Solvent Feed Mixer 403 Extraction solvent to complete the removal of all solute. This problem leads to the concept of the multistage operation. In its simplest form, this could consist merely of a series of single-stage units, coupled close together, as shown in Figure 16.3. In this case, three stages are used, but obviously any num- ber could be used. Notice that the solvent and feed both enter the system on the left and the raffinate and extract are removed at the right. The raf- finate from each stage is the feed for the next stage, and the solvent for each stage is the extract from the preceding stage. Such a flow pattern is called concurrent; that is, the flows are in the same direction. In effect, such an arrangement is an attempt to use several stages to a ccomplish the separation that could be accomplished in a single stage with perfect mixing by remixing the materials again and again. A more e fficient approach would be to use a countercurrent flow arrangement, i ntroducing the solvent at the opposite end of the chain of stages from the feed (Figure 16.4). In such a system, the feed to each subsequent stage is contacted with fresher solvent than was in the preceding stage, thus providing a more efficient operation. Such countercurrent flows are almost always used in commercial equipment to provide greater efficiency. Extraction Columns From the previous section, we have seen that an efficient extraction col- umn should provide for continuous countercurrent flows. It must provide some means of mixing the solvent with the feed and yet allow the raffinate Solvent Feed Raffinate Extract Figure 16.3 Concurrent Extraction Feed Raffinate Extract Solvent Figure 16.4 Countercurrent Extraction Chapter 16 ● Extraction and Other Separation Systems 404 and extract to settle out. There are three main classifications of extraction columns, which are designed for this purpose: packed columns, tray col- umns, and mechanical columns. A packed column is the simplest and most commonly used type of extrac- tion column. Basically, it is a hollow shell that has been filled with small objects packed closely together. As a liquid stream flows through this pack- ing, it is divided into many small streams winding their way through the dense packing. The stream flowing upward competes with the stream flow- ing downward for the same passageways through the packing, resulting in enhanced surface contact. The types of packing commonly used in this process are rings and saddles. Tray columns can also be used for liquid-liquid extraction. The tray de- signs are similar to those employed in distillation operations. Some common types are sieve, bubble-cap, and baffle trays (Figure 16.5). Figure 16.5 Extraction Columns Raffinate Out Solvent In Feed In Extract Out Baffle Tray Column Mechanical Column Sieve or Bubble-cap Tray Column Note: Packed columns are used in extraction. 405 Absorption Columns The principle of operation is similar to that for distillation. In the case of the sieve tray, one stream is made to flow across the trays while the other flows through the sieve holes. Contact is achieved as the tiny droplets of the rising l iquid pass through the flow of the falling liquid across the top of the tray. A bubble-cap tray should perform similarly. A baffle tray is a device for breaking up the countercurrent flows to provide mixing. Baffle trays could take the form of disc and donut trays or crossflow trays. Obvi- ously, contacting is less e fficient with such an arrangement, but it is sim- pler and less susceptible to plugging than are sieve and bubble-cap trays. Finally, the newest and most complicated extraction columns are those with mechanical mixing. These employ some sort of rotating shaft with paddlewheels or other types of mixers affixed to the shaft. Such columns are used primarily when a difficult separation requiring a great deal of mix- ing must be made. Other m echanical equipment involves the use of ultra- sonic vibrations for pneumatic pulsation. The vibrations thus established promote mixing of materials. Extraction Column Terms and Principles The contact area between the extract (heavy phase) and the raffinate (light phase) is called the interface. The term dispersed phase is used to d escribe the “one that bubbles through.” The higher the feed rate, the more solvent required. The higher the concentration of solute in the feed, the more sol- vent required. The product customer specifies partial or total extraction o perations. Temperature is not as important in the extraction process as in distillation unless it affects density or solubility or approaches the boiling points. Intimate contact with feed and solvent is required, so good distribu- tion inside the column is needed. Absorption Columns An absorption column is a device used to remove selected components from a gas stream by contacting it with a gas or liquid. Absorption can roughly be compared to fractionation. A typical gas absorber is a plate or packed distillation column that provides intimate contact between raw nat- ural gas and an absorption medium. Absorption columns work differently than typical fractionators because during the process the vapor and liquid do not vaporize to any degree. Figure 16.6 illustrates the scientific princi- ples involved in absorption. Product exchange takes place in one direction, vapor phase to liquid phase. The absorption oil gently tugs the pentanes, butanes, and so on out of the vapor. In an absorber, the gas is brought into the bottom of the column while lean oil is pumped into the top of the col- umn. As the lean oil moves down the column it absorbs elements from the rich gas. As the raw, rich gas moves up the column, it is robbed of specific hydrocarbons and exits as lean gas. Chapter 16 ● Extraction and Other Separation Systems 406 Stripping Columns Stripping columns are used with absorption columns (see Figure 16.6) to remove liquid hydrocarbons from the absorption oil. To the untrained eye, a stripping column and an absorption column are identical. As rich oil leaves the bottom of the absorber, it is pumped into the midsection of a stripping column. Figure 16.6 illustrates how steam is injected directly into the bot- tom of the stripper, allowing for 100% conversion of Btus. As the hydrocar- bons break free from the absorption oil, they move up the column while the lean oil is recycled back to the absorber. Adsorption Adsorption is the process in which an impurity is removed from a process stream by making it adhere to the surfaces of a solid. It should not be con- fused with absorption. During the adsorption process (Figure 16.7), a column is filled with a p orous solid designed to remove gases or liquids from a mixture. Typically, the process is run in parallel with a primary and secondary vessel. The adsorption material can be activated alumina or charcoal or a variety of other adsorption materials. The adsorption material has selective proper- ties that will remove specific components of the mixture as it passes over it. Absorption Column Stripping Column One direction component removal. The liquid phase removes lighter components from the vapor phase. Reverses absorption process Strips out hydrocarbons from absorption oil. Lean Oil Lean Gas Product Steam Rich Gas Rich Oil Figure 16.6 Absorption and Stripping 407 Adsorption A stripping gas is used to remove the stripped components from the a dsorption material. During the adsorption process, the mixture to be separated is passed over the fixed bed medium (adsorbent) in the primary device. At the conclu- sion of the cycle, the process flow is transferred to the secondary device. A stripping gas is admitted into the primary device. The stripping gas is designed to remove or separate the selected chemical from the adsorp- tion material. At the conclusion of this cycle, the stripping gas stops as the p rocess switches back and repeats the process. Adsorption processes exist in a variety of forms. Ion exchange, molecu- lar sieves, silica gel, and activated carbon are all examples of adsorption. These processes are used in such widely varying applications as water softening and as cigarette filters. We will look at two of these processes, ion exchange and molecular sieves, in more detail. Ion Exchange Ion exchange resins are very small, beadlike particles that contain charged ions on their surfaces. You will recall from your chemistry course that ions are atoms that have gained or lost electrons in their outer orbits, thereby obtaining either a positive or a negative charge. A positively charged ion is called a cation because it would be attracted to the negative electrode, Figure 16.7 Adsorption Process Packed Tower Activated Alumina or Charcoal Stripping GasStripping Gas Chapter 16 ● Extraction and Other Separation Systems 408 the cathode, in an electromagnetic field. Similarly, negatively charged ions are called anions because they are attracted to the positive electrode, or a node. Ion exchange resins are classified as cationic if they remove c ations and anionic if they remove anions. In an ion exchange process, the process stream containing the impurities, which are in an ionized form, is passed through a bed of the ion exchange resins. Water treatment is a good example. Hard water contains salts of metals such as calcium, magnesium, or iron. Water-treating resins have sodium ions active on their surfaces. The sodium ion replaces the “hard” ion; the latter remains attached to the surface of the resin bead. Depending upon the use for which it is intended, an ion exchange resin may have any one of a number of different ions active on its surface. H ydrogen ions are commonly used in chemical processes. Obviously, over a period of time, all of the ions available on the surface of the resin will have been exchanged, and no further exchange will be possible. At this point, the bed is said to be saturated, and it must be regenerated before it can be useful again. Regeneration is the restoring of the original ion to the surface of the resin beads. In the case of the water treatment resins, the bed could be soaked in a concentrated solution of sodium chloride, replacing the hard metallic ions with fresh sodium ions. Molecular Sieves Molecular sieves are an example of a different type of adsorption process. The sieves are small, porous solids containing submicroscopic holes. These holes are actually about the size of an individual molecule. Some molecules will fit inside these holes; other molecules, because of their size or shape, will not. One application is the removal of traces of water from an organic chemical stream; the relatively small water molecule will fit i nside the pores while the bulkier organic molecule will not. In the isosieve pro- cess, kerosene, containing both normal—or straight-chained—paraffins and isoparaffins with branched chains, is fed to a molecular sieve bed. The normal paraffins fit inside the pores, but the branches on the isoparaffin molecule prevent it from doing so. In this manner, a mixture of normal paraf- fins and isoparaffins can be separated. This separation cannot be made by more conventional means because the physical properties of the p araffins are too similar. As in the case of the ion exchange resins, a point is reached when all the pores are filled and the bed is saturated. At this point it must be regener- ated. Regeneration can be done by another, smaller molecule. Equipment Equipment for adsorption operation is relatively simple. It consists primarily of a tank or vessel containing a bed of adsorbent, be it ion exchange resins, molecular sieve, or whatever (Figure 16.8). In most cases the bed is fixed; . extraction process. • Describe the stripping process. • Identify the basic equipment associated with extraction. • Describe the crystallization process. . Identify the basic equipment associated with the adsorption process. • Describe the solvent dewaxing process. • Explain the basic components and operation of