Column distillation entrainment

The fatty acids that emerge from the top of the column contain entrained water, partially hydroly2ed fat, and the Zn—soap catalyst. This product stream is passed into a vacuum dryer stage where the water is removed through vapori2ation and the fatty acid cooled as a result of this vapori2ation process. The dried product stream is then passed to a distillation system. 

Wahnschafft OM and Westerberg AW (1993) The Product Composition Regions of Azeotropic Distillation Columns n. Separability in Two-Feed Columns and Entrainer Selection, Ind Eng Chem Res, 32 1108. 

Figure 2 shows a continuous azeotropic column using a fixed amount of entrainer which remains in the unit. Since reflux is largely supplied by feed of the emulsion near the top of the column, the entrainer from the decanter passes to a reboiler and is fed back to the tower as vapors. This gives a more nearly counter-current action of the azeotropic distilling operation, and a lesser heat input required into the viscous oil at the base of the column, usually with more or less silt in suspension while... 

In the conventional reactive distillation combined with azeotropic distillation an azeotropic distillation takes place in the second column. An entrainer is fed in order to obtain pure isopropanol at the bottom and a water/entrainer mixture at the top. The entrainer is chosen such that it forms an... 

Purifying ethanol from ethanol/water solution by simple distillation is limited by the formation of a minimum-boiling azeotrope containing 90.37 mole% ethanol at 78.14°C, 760 mmHg. Raising the ethanol concentration at 100 kPa above 90.37 mole%, or dehydrating it, can be accomplished by adding to the distillation column an entrainer to form a ternary azeotrope. 

Extractive distillation is the most used separation method of azeotropes by homogeneous distillation with important applications in industry (see Table 7.31). Extractive distillation consists of using a high boiling component as mass separation agent (entrainer or solvent). The separation is of type direct-sequence (Fig. 9.13). The main product is obtained from the first column as top distillate, while the other component (co-product) leaves in the bottoms with the solvent. The second column distillate the co-product and recover the entrainer. Note that the product is not always the most volatile species. In fact, the role of entrainer is to change significantly the relative volatility of the components to be separated. 

Figure 2.14. (a) A sequence with recycle for extractive distillation (first column, extractive column second column, column of entrainer recovery) (b) the distillation trajectory of extractive column for separation of acetone(l)-water (2)-methanol(3) mixture (water-entrainer). xp, initial feed xp+p, total feed into first column. 

We understand by distillation complex a countercurrent cascade with branching of flows, with recycles or bypasses of flows. Columns with side stripping or side rectifier and columns with completely connected thermal flows (the so-called Petlyuk columns ) are examples of distillation complexes with branching of flows. A column of extractive distillation, together with a column of entrainer regeneration, make an example of a complex with recycle of flows. Columns of this complex work independently of each other therefore, we do not examine it in this chapter, and the questions of its usage in separation of azeotropic mixtures and questions of determination of entrainer optimal flow rate are discussed in the following chapters. 

Wahnschafft, O. M. (1997). Advanced Distillation Synthesis Techniques for Nonideal Mixtures Are Making Headway in Industrial Applications. Presented at the Distillation and Absorption Conference, Maastricht, pp. 613-23. Wahnschafft, O. M., Kohler, X, Westerberg, A. W. (1994). Homogeneous Azeotropic Distillation Analysis of Separation Feasibility and Consequences for Entrainer Selection and Column Design. Comput. Chem. Eng., 18, S31-S35. Wahnschafft, O. M., Westerberg, A. W. (1993). Tie Product Composition Regions of Azeotropic Distillation Columns. 2. Separability in Two-Feed Columns and Entrainer Selection. Ind. Eng. Chem. Res, 32,1108-20. 

The economic optimal process flowsheet was obtained by minimization of the total annual cost (TAG) with five design variables IPA distillate composition (XD2) in the recovery column, total number of stages for the heterogeneous azeotropic column and the recovery column (Ni and N2), and the two feed stages (Api and Apa). The product specification of IPA is set to be ultrapure (99.9999 mol%) to be used in the semi-conductor industry. The product specification of water is set to be 99.9 mol%. In each simulation run, the IPA product specification is achieved by varying the reboiler duty of the heterogeneous azeotropic column, and the water product specification is achieved by varying the reboiler duty of the recovery column. The entrainer makeup flowrate will be very small to balance the entrainer loss from the two bottom streams. 

An example of heterogeneous azeotropic distillation is the system ethanol and water with benzene as entrainer (Figure 3.3.20). In a first column (without entrainer, column I in Figure 3.3.20b), the binary ethanol-water mixture is separated by normal distillation. An azeotrope with about 90 mol.% ethanol (96wt%) leaves the column on top (A) while water forms the bottom product. The azeotrope is fed to a second column where benzene (recycle of a phase rich in benzene from the separator of the top products of column II and III) is added as entrainer. A new low-boiling heterogeneous azeotrope (B) leaves column II as distillate, and pure ethanol remains as bottom product. After condensation, the heterogeneous azeotrope separates into two phases rich in either benzene (C) or water (D). The phase rich in benzene is recycled back into column II while the phase rich in water is reconditioned in a third column by distillation. The small amount of benzene is separated as top product (azeotrope B), and a mixture of ethanol and water (E) is recycled into column I. 

In the first class, azeotropic distillation, the extraneous mass-separating agent is relatively volatile and is known as an entrainer. This entrainer forms either a low-boiling binary azeotrope with one of the keys or, more often, a ternary azeotrope containing both keys. The latter kind of operation is feasible only if condensation of the overhead vapor results in two liquid phases, one of which contains the bulk of one of the key components and the other contains the bulk of the entrainer. A t3q)ical scheme is shown in Fig. 3.10. The mixture (A -I- B) is fed to the column, and relatively pure A is taken from the column bottoms. A ternary azeotrope distilled overhead is condensed and separated into two liquid layers in the decanter. One layer contains a mixture of A -I- entrainer which is returned as reflux. The other layer contains relatively pure B. If the B layer contains a significant amount of entrainer, then this layer may need to be fed to an additional column to separate and recycle the entrainer and produce pure B. 

There are two serious problems associated with continuous tar distillation. Coal tar contains two types of components highly corrosive to ferrous metals. The ammonium salts, mainly ammonium chloride, associated with the entrained Hquor remain in the tar after dehydration, tend to dissociate with the production of hydrochloric acid and cause rapid deterioration of any part of the plant in which these vapors and steam are present above 240°C. Condensers on the dehydration column and fractionation columns are also attacked. This form of corrosion is controlled by the addition of alkaU (10% sodium carbonate solution or 40% caustic soda) to the cmde tar in an amount equivalent to the fixed ammonia content. 

The second Hquefaction process is carried out at temperatures from 261 to 296 K, with Hquefaction pressures of about 1600—2400 kPa (16—24 atm). The compressed gas is precooled to 277 to 300 K, water and entrained oil are separated, and the gas is then dehydrated ia an activated alumina, bauxite, or siHca gel drier, and flows to a refrigerant-cooled condenser (see Drying agents). The Hquid is then distilled ia a stripper column to remove noncombustible impurities. Liquid carbon dioxide is stored and transported at ambient temperature ia cylinders containing up to 22.7 kg. Larger quantities are stored ia refrigerated iasulated tanks maintained at 255 K and 2070 kPa (20 atm), and transported ia iasulated tank tmcks and tank rail cars. 

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). 

Fig. 18. Separation of ethanol from an ethanol—water—benzene mixture using benzene as the entrainer. (a) Schematic representation of the azeo-column (b) material balance lines where I denotes the homogeneous and the heterogeneous azeotropes D, the end points of the Hquid tie-line and A, the overhead vapor leaving the top of the column. The distillate regions, I, II, and III, and the boundaries are marked. Other terms are defined in text.

Fig. 19. Separation of ethanol and water from an ethanol—water—benzene mixture. Bottoms and are water, B is ethanol, (a) Kubierschky three-column sequence where columns 1, 2, and 3 represent the preconcentration, azeotropic, and entrainer recovery columns, respectively, (b) Material balance lines from the azeotropic and the entrainer recovery columns, A and E, respectively, where represents the overall vapor composition from the azeo-column, 2 1SP Hquid in equiUbrium with overhead vapor composition from the azeo-column, Xj, distillate composition from entrainer...

The entrainer recovery column takes the distillate stream, from the azeo-column and separates it into a bottoms stream of pure water, and a ternary distillate stream for recycle to column 2. The overall material balance line for column 3 is shown in Figure 19b. This sequence was one of two original continuous processes disclosed in 1915 (106). More recendy, it has been appHed to other azeotropic separations (38,107,108). 

Extensive design and optimization studies have been carried out for this sequence (108). The principal optimization variables, ie, the design variables that have the largest impact on the economics of the process, are the redux ratio in the azeo-column the position of the tie-line for the mixture in the decanter, determined by the temperature and overall composition of the mixture in the decanter the position of the decanter composition on the decanter tie-line (see Reference 104 for a discussion of the importance of these variables) and the distillate composition from the entrainer recovery column. 

Fig. 20. Three sets of material balance lines for the Kubierschky three-column sequence where design 1 corresponds to the upper tie-line having Tmin = 8.78 design 2, to the subcooled upper tie-line having = 12.23 and design 3, to the lower tie-line having = 17.31 represents overall decanter composition , the overall feed composition to the azeo-column , the distillate composition from the entrainer recovery column and O, the...

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