Azeotropes breaking

Exploitation of boundaiy curvature for breaking azeotropes is veiy similar to exploiting pressure sensitivity from a mass-balance point of view, and suffers from the same disadvantages. Separation schemes have large recycle flows, and in the case of minimum-boiling azeotropes, the recycle streams are distillates. However, in the case of maximum-boihng azeotropes, these recycles are underflows and... 

T.C. Frank, Break azeotropes with pressure sensitive distillation, Chem. Engng. Progress, April, (1997) 52-63. 

A novel type of membrane reactor, emerging presently, is the pervaporation reactor. Conventional pervaporation processes only involve separation and most pervaporation set-ups are used in combination with distillation to break azeotropes or to remove trace impurities from product streams, but using membranes also products can be removed selectively from the reaction zone. Next to the polymer membranes, microporous silica membranes are currently under investigation, because they are more resistant to chemicals like Methyl Tertair Butyl Ether (MTBE) [23-24], Another application is the use of pervaporation with microporous silica membranes to remove water from polycondensation reactions [25], A general representation of such a reaction is ... 

The feasibility of separations of nonideal mixtures, as well as the screening of mass-separation agents for breaking azeotropes can be rationalized by means of thermodynamic methods based on residue curve maps. The treatment was extended processes with simultaneous chemical reaction. Two comprehensive books have been published recently by Stichlmair and Frey [10], as well as by Doherty and Malone [11]. 

The combinabon with L-L extraction is appropriate for breaking azeotropes, such as for example cyclohexane/benzene by acetone, in turn extracted selectively with water, distilled and recycled. 

Distillation systems for the separation of nonazeotropic mixtures are discussed in this subsection. Many of the results extend also to azeotropic mixtures when the desired splits do not attempt to break azeotropes or cross a distillation boundary. 

Liquid separation Liquid drying Trace impurity removal Xylene, cresol, cymene isomer separation Fructose-glucose separation Fatty chemicals separation Breaking azeotropes Carbohydrate separation... 

Solvent dehydration Breaking azeotropes Batch and continuous pervaporation, vapor permeation—often coupled with distillation Hydrophilic, e.g., PVA polymer composite, ceramic Well-established... 

Debottlenecking distillations Breaking azeotropes Continuous pervaporation and vapor permeation Various Growing... 

Catalytic distillation could be used to break azeotropes. 

Pervaporation is a membrane process in which a liquid is maintained on the feed side of a membrane and permeate is removed as a vapor on the downstream side of the membrane. Pervaporation is used, because of its low energy consumption and low cost, to separate dissolved organics from water, purify waste water or volatile chemicals, and break azeotropes. Pervaporation plants range from processing a few grams per hour up to thousands of tons per year. For waste water treatment flow of less than 76 L min pervaporation is more cost-effective than other treatment options, such as chemical oxidation, ultraviolet destruction, air stripping followed by carbon adsorption, steam stripping, or distillation/incineration [262]. 

The above sequencing methods valid for zeotropic systems cannot be applied in the case of mixture with strong non-ideal character and displaying distillation boundaries, as those in the case of breaking azeotropes. Fortunately, the sequencing problem in this case has a different character. Most of the separations of multi-component non-ideal mixtures can be reduced by appropriate splits to the treatment of ternary mixtures, for which two or three columns are normally sufficient. The separation sequence follows direct or indirect sequence. The energetic consumption due to the recycle of entrainer dominates the economics. From this viewpoint preferred is that sequence in which the entrainer is recycled as bottoms. Hence, in azeotropic distillation the main problem is the solvent selection and not columns sequencing. 

Are there any azeotropes in the solvent system that would prevent the required specifications being met by simple means There are techniques to break azeotropes, so that it is not necessary to abandon hope of using some type of fractionation if there is an azeotrope preventing the achievement of the purity required. Azeotropes are sufficiently frequent among the commonly used solvents to allow the problem they present to be ignored at the early stage of an assessment. 

The process shown in Fig. 11.3-1 utilizes the specific advantages of distillation and decantatioa The advantage of distillation is the ability to produce pure fractions. However, distillation cannot break azeotropes. On the other hand, decantation can break heteroazeotropes but cannot produce pure products. Thus, the combination of distillation and decantation is a very effective process for fractionating mixtures with heteroazeotropes. Such processes are extensively used in industiy. Table 11.3-1 lists some important binary mixtures fiactioiated by the process shown in Fig. 11.3-1. 

Hybrid processes are combirrations of distillation with alternative separation processes. These processes rrrake rrse of the advantages of distillation (i.e., fractionation irrto prrre srrbstances) as well as of alternative processes (i.e., breaking azeotropes). In prirrciple, all alternative separation techrriques can be combined with distillation (Schweitzer 1997). Especially well suited are decantation, absorptiorr, extractiorr, stripping, adsorptiort, and merrrbrane permeation. In most cases hybrid processes corrsist of two distillation colttrrrrts and one alternative separation unit. 

In creating your design, give special consideration to processes that reduce the energy expenditure of the plant. In one such process, pervaporation membranes are used to dehydrate ethanol. Pervaporation is a membrane separation process in which the feed and residue streams are liquid, but the permeate is a vapor. The combination of permeation and evaporation in the membrane gives rise to separation factors much greater than can be accomplished by distillation and can be used to break azeotropes. 

Figure 8-1. Breaking azeotropes (A) separator uncoupled with distillation, (B) recycle from...

Before the relatively recent development of detailed and accurate VLE correlations, most VLE data were only available at one atmosphere thus, many azeotropic systems have probably not been explored as candidates for two-pressure distillation. Fortunately, it is fairly easy to simulate two-pressure distillation with a process simulator (see Lab 7, part A in the appendix to Chapter 81. Methods for estimating VLE and rapidly screening possible systems are available fFrank. 19971. Because two-pressure distillation does not require a mass separating agent, it is a prefered method when it works. If two-pressure distillation were routinely considered as an option for breaking azeotropes, we would undoubtedly discover additional systems where this method is economical. 

Frank, T. C., Break Azeotropes with Pressure-Sensitive Distillation, Chem. Engr. Prog., 93 (4) 52 (April 1997). 

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