10.1. TYPES OF SEPARATION PROCESSES AND METHODS
10.1A. Introduction
Many chemical process materials and biological substances occur as mixtures of different components in the gas, liquid, or solid phase. In order to separate or remove one or more of the components from its original mixture, it must be contacted with another phase. The two phases are brought into more or less intimate contact with each other so that a solute or solutes can diffuse from one to the other. The two bulk phases are usually only somewhat miscible in each other. The two-phase pair can be gas–liquid, gas–solid, liquid–liquid, or liquid–solid. During the contact of the two phases the components of the original mixture redistribute themselves between the two phases. The phases are then separated by simple physical methods. By choosing the proper conditions and phases, one phase is enriched while the other is depleted in one or more components.
10.1B. Types of Separation Processes
1. Absorption
When the two contacting phases are a gas and a liquid, this operation is called absorption. A solute A or several solutes are absorbed from the gas phase into the liquid phase in absorption. This process involves molecular and turbulent diffusion or mass transfer of solute A through a stagnant, nondiffusing gas B into a stagnant liquid C. An example is absorption of ammonia A from air B by the liquid water C. Usually, the exit ammonia–water solution is distilled to recover relatively pure ammonia.
Another example is absorbing SO2 from the flue gases by absorption in alkaline solutions. In the hydrogenation of edible oils in the food industry, hydrogen gas is bubbled into oil and absorbed. The hydrogen in solution then reacts with the oil in the presence of a catalyst. The reverse of absorption is called stripping or desorption, and the same theories and basic principles hold. An example is the steam stripping of nonvolatile oils, in which the steam contacts the oil and small amounts of volatile components of the oil pass out with the steam.
When the gas is pure air and the liquid is pure water, the process is called humidification. Dehumidification involves removal of water vapor from air.
2. Distillation
In the distillation process, a volatile vapor phase and a liquid phase that vaporizes are involved. An example is distillation of an ethanol–water solution, where the vapor contains a concentration of ethanol greater than in the liquid. Another example is distillation of an ammonia–water solution to produce a vapor richer in ammonia. In the distillation of crude petroleum, various fractions, such as gasoline, kerosene, and heating oils, are distilled off.
3. Liquid–liquid extraction
When the two phases are liquids, where a solute or solutes are removed from one liquid phase to another liquid phase, the process is called liquid–liquid extraction. One example is extraction of acetic acid from a water solution by isopropyl ether. In the pharmaceutical industry, antibiotics in an aqueous fermentation solution are sometimes removed by extraction with an organic solvent.
4. Leaching
If a fluid is being used to extract a solute from a solid, the process is called leaching. Sometimes this process is also called extraction. Examples are leaching copper from solid ores by sulfuric acid and leaching vegetable oils from solid soybeans by organic solvents such as hexane. Vegetable oils are also leached from other biological products, such as peanuts, rape seeds, and sunflower seeds. Soluble sucrose is leached by water extraction from sugar cane and beets.
5. Membrane processing
Separation of molecules by the use of membranes is a relatively new separation process and is becoming more important. The relatively thin, solid membrane controls the rate of movement of molecules between two phases. It is used to remove salt from water, to purify gases, in food processing, and so on.
6. Crystallization
Solute components soluble in a solution can be removed from a solution by adjusting the conditions, such as temperature or concentration, so that the solubility of one or more of the components is exceeded and they crystallize out as a solid phase. Examples of this separation process are crystallization of sugar from solution and crystallization of metal salts in the processing of metal ore solutions.
7. Adsorption
In an adsorption process, one or more components of a liquid or gas stream are adsorbed on the surface or in the pores of a solid adsorbent and a separation is obtained. Examples include removal of organic compounds from polluted water, separation of paraffins from aromatics, and removal of solvents from air.
8. Ion exchange
In an ion-exchange process, certain ions are removed by an ion-exchange solid. This separation process closely resembles adsorption.
10.1C. Processing Methods
Several methods of processing are used in the separations discussed above. The two phases, such as gas and liquid, or liquid and liquid, can be mixed together in a vessel and then separated. This is a single-stage process. Often the phases are mixed in one stage, separated, and then brought into contact again in a multiple-stage process. These two methods can be carried out batchwise or continuously. In still another general method, the two phases can be contacted continuously in a packed tower.
In this chapter, humidification and absorption will be considered; in Chapter 11, distillation; in Chapter 12, adsorption, ion exchange, liquid–liquid extraction, leaching, and crystallization; in Chapter 13, membrane processes; and in Chapter 14, mechanical–physical separations. In these processes the equilibrium relations between the two phases being considered must be known. This is discussed for gas–liquid systems in Section 10.2 and for the other systems in Chapters 11, 12, and 13.