Another Example Distillation

One of the most energy-inefficient of widely used industrial processes is distillation, or heat-driven separation processes generally. The traditional distillation column, familiar to students who have had chemistry laboratory courses, has a source of heat at the bottom and a cooling fluid that runs the length of a vertical column, so that there is a temperature gradient, cooling as the 

we follow a later, simpler formulation that illustrates the power of optimal control for finite-time thermodynamic processes [11]. We take as the control variable the set of temperatures at a given number of equally spaced heat-exchange points along the length of the distillation column. The (assumed) binary mixtme comes in as a feed at rate F and is separated into the less volatile bottom at rate B and the distillate, at rate D, that collects at the top of the colmrm. Let x be the mole fraction of the more volatile component in the liquid and y, the corresponding mole fi action in the vapom, and their subscripts, the indications of the respective points of reference. Thus the total flow rates, for steady flow, must satisfy F = D + B, and xpF = x D + xbB. We index the trays from 0 at the top to N at the bottom. Mass balance requires that the rate V +i of vapour coming up from tray n + 1, less the rate of liquid dropping from tray n, L , must equal D for trays above the feed point at which F enters, and must equal —B below the feed point. Likewise the mole fractions must satisfy the condition that Vn+iVn+i —XnLn = xpD above the feed and —xpB below the feed. The heat required at each nth tray is 

The entropy associated with heat conduction in each j/th tray, 

The net saving in entropy is most apparent in a graphic comparison of the entropy change produced in a traditional Adiabatic column and an optimized Diabatic column, that is, one with heat exchangers along the column. The reversible limit is still clearly lower than the finite-time system, but the separation part of that entropy is very similar for the optimized realistic and reversible columns the difference is almost entirely in the heat exchange. This is shown in Fig. 14.5. 

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Let us now consider a few examples for the use of this simple representation. A grand composite curve is shown in Fig. 14.2. The distillation column reboiler and condenser duties are shown separately and are matched against it. Neither of the distillation columns in Fig. 14.2 fits. The column in Fig. 14.2a is clearly across the pinch. The distillation column in Fig. 14.26 does not fit, despite the fact that both reboiler and condenser temperatures are above the pinch. Strictly speaking, it is not appropriately placed, and yet some energy can be saved. By contrast, the distillation shown in Fig. 14.3a fits. The reboiler duty can be supplied by the hot utility. The condenser duty must be integrated with the rest of the process. Another example is shown in Fig. 14.36. This distillation also fits. The reboiler duty must be supplied by integration with the process. Part of the condenser duty must be integrated, but the remainder of the condenser duty can be rejected to the cold utility. 

The use of high or low limits for process variables is another type of selective control, called an override. The feature of anti-reset windup in feedback controllers is a type of override. Another example is a distillation column with lower and upper limits on the heat input to the column reboiler. The minimum level ensures that liquid will remain... 

In another example of enantioselective distillation, it was the enantiomeric mixture to resolve itself which contributed to create a chiral environment. Thus, non-racemic mixtures of a-phenylethylamine were enantiomerically enriched by submitting to distillation different salts of this amine with achiral acids [199]. 

The distillation columns shown in Figure 21.3 both fit. Figure 21.3a shows a case in which the reboiler duty can be supplied by hot utility. The condenser duty must be integrated with the rest of the process. Another example is shown in Figure 21.3b. This distillation column also fits. The reboiler duty must be supplied by integration with the process. Part of the condenser duty in Figure 21.3b must also be integrated, while the remainder of the condenser duty can be rejected to cold utility. 

Another example is sketched in Fig. 8.3i>. A hot oil stream is used to reboil a distillation column. Controlling the flow rate of the hot oil does not guarantee a fixed heat input because the inlet oil temperature can vary and the AT requirements in the reboiler can change. The heat input Q can be computed from the flow rate and the inlet and outlet temperatures, and this Q can then be controlled. 

Apart from these common pretreatments, special pretreatments are necessary in some cases. For example, a small amount of basic impurity (possibly triethyl-amine) in PC is not removed even by repeated distillations. However, if we add p-toluenesulfonic acid to neutralize the basic impurity, we can remove it easily by distillation (Section 10.4). As another example, volatile impurities in PC can be removed only by bubbling inert gases (nitrogen or argon) for many hours. 

Epoxy compounds are prepared by heating halohydrins with strong caustic solutions and, where possible, distilling the product as it is formed. By this procedure, 3-chloro-2-butanol yields a mixture of cis-and frans-2,3-epoxybutane (90%), which can be readily separated by fractional distillation. Another example is the conversion of 2-chloro-cyclohexanol to cyclohexene oxide (73%). The reaction is included in an excellent discussion of the chemistry of ethylene and trimethylene oxides, ... 

A good example of separation on the basis of affinity is the separation of alcohol/ water mixtures using a hydrophobic, silicalite membrane. Pervaporation of an ethanol/ water mixture through such a membrane resulted the removal of the alcohol from the mixture [16]. The separation selectivities achieved are between 10 and 60, depending on temperature and the alcohol content in the feed. In this way azeotropes can be broken. The reason for this is that the principle of separation, namely, differences in adsorptive behavior, is different from separation based on vapor pressure differences, used in distillation. Another example of such a separation is the pervaporation of an acetic acid/water mixture through a silicalite membrane, resulting in the removal of acetic acid [17]. 

Another example in which a Grignard base generates a reactive intermediate by elimination is depicted in Eq. (32) [39]. a-Cyanoeneamines are deprotonated by MeMgl, giving the reactive trialkyl ketenimine. The ketenimine subsequently reacts with primary amines, yielding amidines. The critical step is cyanide elimination from the deprotonated eneamine. The keteneimines could be isolated either by careful distillation or could be directly reacted with amines. 

In another example bromine is added dropwise to a mixture of isobutyric acid and red phosphorus, and the mixture is warmed to 100° in the course of 6 hrs. Excess bromine and hydrogen bromide are removed by distillation at 30 mm., the a-bromo acid bromide is decanted from H3PO3 and distilled, and treated with zinc turnings... 

Another example for the production of natural beer flavour is the extraction of yeast deposits formed during the ripening period in the fermentation tanks. Yeast deposits contain about 10-15% living yeast cells and residual beer with a high content of the desired yeasty flavour substances. A typical natural beer flavour in accordance with the German purity law is obtained [25 [by distillation and rectification of such yeast suspensions. 

Another example of potential three-phase distillation is provided by the methyl ethyl ketone (MEK)-water binary. This system forms two miscible regions and a two-liquid-phase region. Figure 10.8 is a Y-X plot of this binary at 100 kPa. Below 0.051 mole fraction MEK, a single, water-rich liquid phase exists, and above 0.652 mole fraction MEK, a single, MEK-rich liquid phase exists. Between these two concentrations, two liquid phases coexist. 

Another example is a procedure by Bare and House10b for the synthesis of methyl cyclohexyl ketone from cyclohexanecarboxylic acid. A suspension of lithium hydride in 1,2-dimethoxyethane (freshly distilled from LiAlH4) is stirred during dropwise addition of a solution of cyclohexanecarboxylic acid in 1,2-dimethoxyethane, and the mixture is refluxed with stirring to complete the formation of lithium... 

A complex system is one containing so many components that they cannot be separated into discreet pure components by the distillation process. An example of such a system is naturally occurring petroleum, which contains hundreds of chemical constituents. Crude tall oil from paper pulping is another example of a complex system. 

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