Separation of Close Boilers

The terms entrainer and solvent are commonly used interchangeably to refer to the separating agent used to enhance the separation of close boilers or azeotropes by azeotropic or extractive distillation. For consistency, the term entrainer will be used to designate the azeotropic distillation agent and solvent, the extractive distillation agent. 

Phase separation is controlled by phase equilibrium relations or rate-based mass and heat transfer mechanisms. Chemical reactions are controlled by chemical equilibrium relations or by reaction kinetics. For reactive distillation to have practical applications, both these operations must have favorable rates at the column conditions of temperature and pressure. If, for instance, the chemical reaction is irreversible, it may be advantageous to carry out the reaction and the separation of products in two distinct operations a reactor followed by a distillation column. Situations in which reactive distillation is feasible can result in savings in energy and equipment cost. Examples of such processes include the separation of close-boilers, shifting of equilibrium reactions toward higher yields, and removal of impurities by reactive absorption or stripping. 

Process synthesis and design of these non-conventional distillation processes proceed in two steps. The first step—process synthesis—is the selection of one or more candidate entrainers along with the computation of thermodynamic properties like residue curve maps that help assess many column features such as the adequate column configuration and the corresponding product cuts sequence. The second step—process design—involves the search for optimal values of batch distillation parameters such as the entrainer amount, reflux ratio, boiler duty and number of stages. The complexity of the second step depends on the solutions obtained at the previous level, because efficiency in azeotropic and extractive distillation is largely determined by the mixture thermodynamic properties that are closely linked to the nature of the entrainer. Hence, we have established a complete set of rules for the selection of feasible entrainers for the separation of non ideal mixtures... 

Isobutane and 1-butene are close boilers and, although they do not form an azeotrope, are difficult to separate by conventional distillation. Using a single stage, a feed stream containing 40% mole isobutane and 60% mole 1-butene is flashed at 520 kPa so that 50% of this stream is vaporized. With no solvent added, the vapor and liquid product compositions are about the same. The ratio of 1-butene to isobutane in the liquid product is about 1.6 (Figure 2.10)... 

Extractive distillation is another process used for separating azeotropes or close boilers. In this process, a solvent is added that alters the relative volatilities of the feed components through its preferential affinity for one or more of the components over the others. 

For instance, the separation of benzene and cyclohexane, two close boilers that... 

The feed consists of two components to be separated, A and B, that are close boilers or that form an azeotrope with each other. An entrainer E that forms an azeotrope with one of the components is added to the feed. If the component forming the azeotrope is A, the resulting azeotrope AE is separated from B by distillation. The problem is shifted from separating azeotrope AB to separating azeotrope AE. Eor the process to be viable, there should be an economical means for separating AE. If AE has a pressure-sensitive azeotropic composition or is a heterogeneous azeotrope, the methods described in Section 10.1.1 or 10.1.2 could be used to round out the process. 

The separation process depends on the nature of the vapor-liquid equilibrium relationships of the system, which can be represented on a ternary diagram. Figure 10.3a shows a ternary diagram at some fixed system pressure. Components A and B are close boilers, and A forms an azeotrope with the entrainer E. The curves in the triangle represent liquid isotherms. A corresponding vapor isotherm (not shown) could be drawn to represent the vapor at equilibrium with each liquid curve with tie lines joining vapor and liquid compositions at equilibrium. The temperature of the isotherms reaches a minimum at point Z that corresponds to the composition of the azeotrope formed between A and E. 

Figure 10.6 illustrates a typical extractive distillation process consisting of the extractive distillation column and the solvent recovery column. Fresh feed containing the binary AB is introduced around the middle of the extractive distillation column, and the solvent S is introduced near the top. Components A and B are close boilers and/or potentially azeotrope formers that are difficult or impossible to be separated by ordinary distillation. Whether individual component A is more volatile than B or vice versa, in the presence of the solvent, B becomes less volatile due to its higher affinity to the solvent. As a result, essentially pure A is distilled as the overhead of the extractive distillation column. Component B is entrained with the solvent in the bottoms stream, which is sent to the solvent recovery column. The solvent is substantially less volatile than component B, allowing easy separation by ordinary distillation. Practically pure B is recovered in the overhead, and pure solvent in the bottoms. The solvent is recycled to the extractive distillation column with makeup that might be required to compensate for losses. 

It has already been shown in Sections 10.1.2 and 10.1.5 how the formation of two liquid phases plays a part in the separation of azeotropes or close boilers. The two phases, having distinct compositions, are separated in a single stage by simple decantation, followed by further processing of each phase. These may be considered as special cases of three-phase distillation since the formation of two liquid phases is confined to a single stage the condenser or the liquid-liquid separator. 

Inen-puige cycles, given the recently demonstrated success in drying of azeotropes,1 would seem to be poised for use in several new separations. Prime candidates include those systems now separated by azeotropic and extractive distillation, many of which contain water as one constituent. The use of inert-purge cycles for isomer and other close-boiler sepurations sheuld also grow for those systems whose components can be easily separated from the purge gas,... 

The absence of an interphase compositional differential makes the separation of azeotropes into their constituent components impossible by conventional vapor-hquid separation processes. The azeotropic pattern may be altered by adding an external component— an endainer—that breaks the original azeotrope while forming new ones with the feed components. The process may be designed so that the new azeotropes may be separated from individual compouents or from other azeotropes by what is known as azeotropic distillation. Azeotropic distillation is not limited to the separation of azeotropes it is also used for separating close boilers that are difBcult to separate by conventional distillation. 

Hydrogen bonding is one contributor to the preferential attraction between the solvent and certain components. Similarity of the chemical structure is another. For instance, the separation of benzene and cyclohexane, two close boilers that also form an azeotrope, can be achieved by extractive distillation using phenol as the solvent Benzene is more strongly attracted to phenol because both contain an aromatic ring, whereas cyclohexane does not. Therefore, in this process, benzene is taken in the column bottoms with the phenol, and cyclohexane is recovered in the colunm overhead. 

The feed consists of two components to be separated, A and B, that are close boilers or that form an azeotrope with each other. An entrainer E that forms an azeotrope with one of the... 

The output from the turbine might be superheated or partially condensed, as is the case in Fig. 6.32. If the exhaust steam is to be used for process heating, ideally it should be close to saturated conditions. If the exhaust steam is significantly superheated, it can be desuperheated by direct injection of boiler feedwater, which vaporizes and cools the steam. However, if saturated steam is fed to a steam main, with significant potential for heat losses from the main, then it is desirable to retain some superheat rather than desuperheat the steam to saturated conditions. If saturated steam is fed to the main, then heat losses will cause excessive condensation in the main, which is not desirable. On the other hand, if the exhaust steam from the turbine is partially condensed, the condensate is separated and the steam used for heating. 

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