Common separation processes

PT catalysts are often difficult to separate from the product, while it is also desirable that the catalyst should be reusable or recyclable. Distillation and extraction are the most common separation processes. The main disadvantage of lipophilic quats is their tendency to remain in the organic phase and consequently contaminate the product. Therefore, extraction in water often is not satisfactory. Furthermore, products in the fine chemicals industry often have high boiling points and/or are heat sensitive, which makes separation of the catalyst by distillation impossible. Often the only means to remove the catalyst in these cases is to adsorb it using a high surface area sorbent such as silica, Florisil or active carbon (Sasson, 1997). After filtration, the catalyst can then be recovered by elution. 

As stated earlier, distillation and related vapor-liquid operations are by far the most common separation processes In the organic-chemical, petroleum and allied Industries It Is unlikely that adsorption will ever rival distillation In frequency of use, but adsorption s share of the separation task should grow substantially The purpose of this section Is to indicate the likely areas of growth in existing and new applications, as well as the technological Innovations which would foster this growth ... 

In most common separation processes, the main mass transfer is across an interface between a gas and a liquid or between two liquid phases. At fluid-fluid interfaces, turbulence may persist to the interface. A simple theoretical model for turbulent mass transfer to or from a fluid-phase boundary was suggested in 1904 by Nernst, who postulated that the entire resistance to mass transfer in a given turbulent phase lies in a thin, stagnant region of that phase at the interface, called a him, hence the name film theory.2 4,5 Other, more detailed, theories for describing the mass transfer through a fluid-fluid interface exist, such as the penetration theory.1,4... 

Table 2.2 is a listing of several common separation processes, their primary separation mechanisms, and the separating agent used. The separating agent concept will be explained in some detail in a later section of this chapter. 

At the risk of some entries being obeolete by the time of printing, a list of single-line upper limits for the various types of common separation process is given in Table 22.2-1, An entry of "no limit" in the table implies that some other equipment, either upstream or downstream of the separation, is almost certain to detemiine the single-line bottleneck. 

An examination of extraction with reaction processes reveals that it is an area which exploits chemistry to a greater extent than, for instance, other common separation processes. A variety of liquid-liquid reactions are encountered in practice and some illustrative examples have been presented. Further challenging examples are frequently presented in many publications such as the special section on Journal of Separation Science, Hydrometallurgy etc. as well as the more common journals, e.g. Chemistry and Industry etc. However, the real highlights are documented mainly at the tri-annual International Solvent Extraction Conferences ISEC s, as well as many more specialised meetings e.g. hydrometallurgy etc. 

Distillation/rectification is by far the most common separation process in the chemical industry. Here, the difference in vapour pressure/fugacity of the different components of a reaction mixture is used as the driving force for separation. Extractive distillation, extraction and absorption processes gain importance, if distillation proves unfeasible, for exan5)le, due... 

Some frequently quoted correlations for fluid-solid interfaces are given in Table 8.3-3. These correlations are rarely important in common separation processes like absorption and extraction. They can be important in leaching, in membrane separations, and in adsorption. However, the chief reason that these correlations are quoted in undergraduate and graduate courses is that they are close analogues to heat transfer. Heat transfer is an older subject, with a strong theoretical basis and more familiar nuances. This analogy lets lazy lecturers merely mumble, Mass transfer is just like heat transfer and quickly compare the correlations in Table 8.3-3 with the heat transfer parallels. 

Extraction is a common separation process used where distillation and gas absorption fail. Most obviously, extraction can be used for nonvolatile components like metal ions. It is effective for valuable solutes like flavors, which can be unstable at distillation temperatures. Less obviously, extraction is useful for volatile solutes that have nearly equal boiling points or that show azeotropes. 

Separation Processes. Separation of the catalyst from the products is a significant expense the process flow diagram and the processing cost are often dominated by the separations. Many soluble catalysts are expensive, eg, rhodium complexes, and must be recovered and recycled with high efficiency. The most common separation devices are distiUation columns extraction is also appHed. 

Distillation Columns. Distillation is by far the most common separation technique in the chemical process industries. Tray and packed columns are employed as strippers, absorbers, and their combinations in a wide range of diverse appHcations. Although the components to be separated and distillation equipment may be different, the mathematical model of the material and energy balances and of the vapor—Hquid equiUbria are similar and equally appHcable to all distillation operations. Computation of multicomponent systems are extremely complex. Computers, right from their eadiest avadabihties, have been used for making plate-to-plate calculations. 

Deviations from Raonlt s law in solution behavior have been attributed to many charac teristics such as molecular size and shape, but the strongest deviations appear to be due to hydrogen bonding and electron donor-acceptor interac tions. Robbins [Chem. Eng. Prog., 76(10), 58 (1980)] presented a table of these interactions. Table 15-4, that provides a qualitative guide to solvent selection for hqnid-hqnid extraction, extractive distillation, azeotropic distillation, or even solvent crystallization. The ac tivity coefficient in the liquid phase is common to all these separation processes. 

OH- ions combine with ions of some metals to form insoluble metal hydroxides (precipitation). Precipitated metals settle out and thus are removed from the water adsorption, using activated carbon, improves this separation process. Iron is one of many metals which is commonly removed in this way. 

All refining operations may be classed as either conversion processes or separation processes. In the former, the feed undergoes a chemical reaction such as cracking, polymerization, or desulfurization. Separation processes take advantage of differences in physical properties to split the feed into two or more different products. Distillation, the most common of all refinery separation processes, uses differences in boiling points to separate hydrocarbon mixtures. 

Precipitation involves the alteration of the ionic equilibrium to produce insoluble precipitates. To remove the sediment, chemical precipitation is allied with solids separation processes such as filtration. Undesirable metal ions and anions are commonly removed from waste streams by converting them to an insoluble form. The process is sometimes preceded by chemical reduction of the metal ions to a form that can be precipitated more easily. Chemical equilibrium can be affected by a variety of means to change the solubility of certain compounds. For e.xample, precipitation can be induced by alkaline agents, sulfides, sulfates, and carbonates. Precipitation with chemicals is a common waste stream treatment process and is effective and reliable. The treatment of sludges is covered next. 

Centrifugation is a well-established liquid-solid separation process popular in commercial and municipal waste treatment facilities. It is usually used to reduce slurry and sludge volumes and to increase the solids concentration in these waste streams. It is a technically and economically competitive process and is commonly used on waste sludges produced from water pollution control systems and on biological sludges produced in industry and municipal treatment facilities. 

Membranes used for the pressure driven separation processes, microfiltration (MF), ultrafiltration (UF) and reverse osmosis (RO), as well as those used for dialysis, are most commonly made of polymeric materials. Initially most such membranes were cellulosic in nature. These ate now being replaced by polyamide, polysulphone, polycarbonate and several other advanced polymers. These synthetic polymers have improved chemical stability and better resistance to microbial degradation. Membranes have most commonly been produced by a form of phase inversion known as immersion precipitation.11 This process has four main steps ... 

Table 16.2. Module designs most commonly used in major separation processes...

A limitation to the more widespread use of membrane separation processes is membrane fouling, as would be expected in the industrial application of such finely porous materials. Fouling results in a continuous decline in membrane penneation rate, an increased rejection of low molecular weight solutes and eventually blocking of flow channels. On start-up of a process, a reduction in membrane permeation rate to 30-10% of the pure water permeation rate after a few minutes of operation is common for ultrafiltration. Such a rapid decrease may be even more extreme for microfiltration. This is often followed by a more gradual... 

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