Other distillation systems

In batch distillation the mixture to be distilled is charged as a batch to the still and the distillation carried out until a satisfactory top or bottom product is achieved. The still usually consists of a vessel surmounted by a packed or plate column. The heater may be incorporated in the vessel or a separate reboiler used. Batch distillation should be considered under the following circumstances  

Where the feed composition varies over a wide range. 

Where the choice between batch and continuous is uncertain, an economic evaluation of both systems should be made. 

Batch distillation is an unsteady-state process, the composition in the still (bottoms) varying as the batch is distilled. 

Fixed reflux, where the reflux rate is kept constant. The compositions will vary as the more volatile component is distilled off, and the distillation stopped when the average composition of the distillate collected, or the bottoms left, meet the specification required. 

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In the example, the minimum reflux ratio and minimum number of theoretical plates decreased 14- to 33-fold, respectively, when the relative volatiHty increased from 1.1 to 4. Other distillation systems would have different specific reflux ratios and numbers of theoretical plates, but the trend would be the same. As the relative volatiHty approaches unity, distillation separations rapidly become more cosdy in terms of both capital and operating costs. The relative volatiHty can sometimes be improved through the use of an extraneous solvent that modifies the VLE. Binary azeotropic systems are impossible to separate into pure components in a single column, but the azeotrope can often be broken by an extraneous entrainer (see Distillation, A7EOTROPTC AND EXTRACTIVE). 

Wiped film evaporators are very effective for processing sweetwater crudes. Yields of glycerine are very good and the quality of finished product is equal to glycerine from other distillation systems discussed previously. Salt crude s can be distilled in these systems, but special modifications are required to handle the salt that precipitates out during distillation. 

F. Generalize. The method illustrated here can obviously be used in other distillation systems. Since the curve for Oq can be very nonlinear, it is a good idea to plot the curve as shown in Figures 16-... 

Towler, Gavin P., and R. K Sinnott. ChemicalEngneering Design Principles, Practice, and Economics of Plant and Process Design. Boston Elsevier/Butterworth-Heine-mann, 2008. Chapter 11 examines continuous and multicomponent distillation, looking at the principles involved and design variations. Also discusses other distillation systems and components of a system. 

Dialkylphenols are also produced in specialized plants. These plants combine complex batch reactors with vacuum distillation trains or other recovery systems. Alkenes with carbon numbers between 4 and 9 react with phenol to make an unrefined alkylphenol mixture, which is fed into the recovery section where very high purity product is isolated. The product is stored, handled, and shipped just as are the monoalkylphenols. 

Several other catalyst systems have been suggested, including boron fluoride and both crystalline and noncrystalline siUcas and alurninosihcates. Although no commercial faciUty exists, the concept of using a crystalline siUca or alurninosihcate catalyst in an integral reaction and distillation apparatus has been proposed (9). 

Most distillation systems ia commercial columns have Murphree plate efficiencies of 70% or higher. Lower efficiencies are found under system conditions of a high slope of the equiHbrium curve (Fig. lb), of high Hquid viscosity, and of large molecules having characteristically low diffusion coefficients. FiaaHy, most experimental efficiencies have been for biaary systems where by definition the efficiency of one component is equal to that of the other component. For multicomponent systems it is possible for each component to have a different efficiency. Practice has been to use a pseudo-biaary approach involving the two key components. However, a theory for multicomponent efficiency prediction has been developed (66,67) and is amenable to computational analysis. 

Hquid—Hquid-phase spHt the compositions of these two feed streams He oa either side of the azeotrope. Therefore, column 1 produces pure A as a bottoms product and the azeotrope as distillate, whereas column 2 produces pure B as a bottoms product and the azeotrope as distillate. The two distillate streams are fed to the decanter along with the process feed to give an overall decanter composition partway between the azeotropic composition and the process feed composition according to the lever rule. This arrangement is weU suited to purifying water—hydrocarbon mixtures, such as a C —C q hydrocarbon, benzene, toluene, xylene, etc water—alcohol mixtures, such as butanol, pentanol, etc as weU as other immiscible systems. 

Open or direct injection of steam into a distillation system at the bottom may be used to heat the mixture as well as to reduce the effective partial pressure of the other materials. In general, if steam is used to replace a reboiler, one tray is added to replace the reboiler stage, and from one-third to one or more trays may be needed to offset the... 

This type of evaluation of a distillation system involves a material and heat balance around each tray. It is extremely tedious to do by conventional means, and is now handled with computers. But even with this untiring worker, the volume of calculations is large and requires a relatively long time. Only those special systems that defy a reasonable and apparently economical solution by other approaches are even considered for this type of solution. 

Pressure-relieving systems are unique compared with other systems within a chemical plant hopefully they will never need to operate, but when they do, they must do so flawlessly. Other systems, such as extraction and distillation systems, usually evolve to their optimum performance and reliability. This evolution requires creativity, practical knowledge, hard work, time, and the cooperative efforts of the plant, design, and process engineers. This same effort and creativity is essential when developing relief systems however, in this case the relief system development must be optimally designed and demonstrated within a research environment before the plant start-up. 

The collection of the total CO2 effluent as a solid or liquid and subsequent isolation of the entrained organic compounds. Such a trapping system would minimize losses of organic substances due to the effects of poor thermal transfer possible in the other trap systems. In addition, if the effluent could be collected as liquid CO2 (e.g., at 900 lb/in.2 and 20 °C), several additional benefits were deemed possible. For example, minimal loss of the organic compounds should occur because their concentration in the gas distilled off is... 

Bourbon Distillation. The basic distillation system for the production of bourbon and other straight whiskeys consists of a beer still and a beer heater, thumper, or doubler (Fig. 4). The whiskey still consists of between 14 and 21 stripping trays. The upper portion of the still is fitted with either a bubble cap section or a section packed with copper rings to enhance the removal of unwanted flavors and ethyl carbamate precursors. The reduction of carbamate precursors requires strict adherence to a cleaning protocol with a 5% caustic solution as often as twice a week. 

Measurements of binary vapor-liquid equilibria can be expressed in terms of activity coefficients, and then correlated by the Wilson or other suitable equation. Data on all possible pairs of components can be combined to represent the vapor-liquid behavior of the complete mixture. For exploratory purposes, several rapid experimental techniques are applicable. For example, differential ebulliometry can obtain data for several systems in one laboratory day, from which infinite dilution activity coefficients can be calculated and then used to evaluate the parameters of correlating equations. Chromatography also is a well-developed rapid technique for vapor-liquid equilibrium measurement of extractive distillation systems. The low-boiling solvent is deposited on an inert carrier to serve as the adsorbent. The mathematics is known from which the relative volatility of a pair of substances can be calculated from the effluent trace of the elutriated stream. Some of the literature of these two techniques is cited by Walas (1985, pp. 216-217). 

Experience Factors These are tabulations of efficiencies previously measured for various systems. Tray efficiency is insensitive to tray geometry (above), so in the absence of hydraulic anomalies and issues with VLE data, efficiencies measured in one tower are extensible to others distilling the same system. A small allowance to variations in tray geometry as discussed above is in order. Caution is required with mixed aqueous-organic systems, where concentration may have a marked effect on physical properties, relative volatility, and efficiency. Table 14-12 shows typical tray efficiencies reported in the literature. 

Estimation of column costs for preliminary process evaluations requires consideration not only of the basic type of internals but also of their effect on overall system cost. For a distillation system, for example, the overall system can include the vessel (column), attendant structures, supports, and foundations auxiliaries such as reboiler, condenser, feed neater, and control instruments and connecting piping. The choice of internals influences all these costs, but other factors influence them as well. A complete optimization of the system requires a full-process simulation model that can cover all pertinent variables influencing economics. 

On the other hand, a pervaporation membrane can be coupled with a conventional distillation column, resulting in a hybrid membrane/distillation process (228,229). Some of the investigated applications of such hybrid pervaporation membrane/distillation systems are shown in Table 9. In hybrid pervaporation/ distillation systems, the membrane units can be installed on the overhead vapor of the distillation column, as shown in Figure 13a for the case of propylene/ propane splitting (234), or they can be installed on the feed to the distillation column,... 

From a thermodynamic viewpoint, a typical thermodynamic efficiency of a distillation system is about 10 percent (4). This can be enhanced if intercondensers and interreboilers are used, In fact, it has been shown that conceptually, a distillation system can be devised which requires only the minimum work of separation. Although a thermodynamic efficiency of 10 percent appears low, not many other processes are more efficient (4). 

Safety factors have not been discussed inasmuch as these factors should reflect the engineer s confidence in the data he or she has, the distillation system performance deduced in manipulating these data, the reliability the equipment must demonstrate etc., in short, engineering judgment. There is never anything better than reliable experimental data on the system which is to be distilled, of course. In other, less-well-defined circumstances, conservatism is advised (9). Batch stills are usually (at least relatively) low-cost items, and increased capabilities represent small incremental costs. 

The use of readily available raw materials and absence of co-products reduces production costs and investment needed for distillation and other separation systems. Such simplification results in a very attractive process in an industry where the principally accepted measure of business quality is retum-on-investment. 

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