Component Separation Conventional Distillation

Chapter 7 Multi-Component Separation Conventional Distillation... 

Separation and Purification of C4 Isomers. 1-Butene and isobutylene cannot be economically separated into pure components by conventional distillation because they are close boiling isomers (see Table 1 and Fig. 1). 2-Butene can be separated from the other two isomers by simple distillation. There are four types of separation methods available (/) selective removal of isobutylene by polymerization and separation of 1-butene 2) use of addition reactions with alcohol, acids, or water to selectively produce pure isobutylene and 1-butene (3) selective extraction of isobutylene with a liquid solvent, usually an acid and (4) physical separation of isobutylene from 1-butene by absorbents. The first two methods take advantage of the reactivity of isobutylene. For example, isobutylene reacts about 1000 times faster than 1-butene. Some 1-butene also reacts and gets separated with isobutylene, but recovery of high purity is possible. The choice of a particular method depends on the product slate requirements of the manufacturer. In any case, 2-butene is first separated from the other two isomers by simple distillation. 

Fiaal purification of propylene oxide is accompHshed by a series of conventional and extractive distillations. Impurities ia the cmde product iaclude water, methyl formate, acetone, methanol, formaldehyde, acetaldehyde, propionaldehyde, and some heavier hydrocarbons. Conventional distillation ia one or two columns separates some of the lower boiling components overhead, while taking some of the higher boilers out the bottom of the column. The reduced level of impurities are then extractively distilled ia one or more columns to provide a purified propylene oxide product. The solvent used for extractive distillation is distilled ia a conventional column to remove the impurities and then recycled (155,156). A variety of extractive solvents have been demonstrated to be effective ia purifyiag propylene oxide, as shown ia Table 4. 

These are azeotropic points where the azeotropes occur. In other words, azeotropic systems give rise to VLE plots where the equilibrium curves crosses the diagonals. Both plots are however, obtained from homogenous azeotropic systems. An azeotrope that contains one liquid phase in contact with vapor is called a homogenous azeotrope. A homogenous azeotrope carmot be separated by conventional distillation. However, vacuum distillation may be used as the lower pressures can shift the azeotropic point. Alternatively, an additional substance may added to shift the azeotropic point to a more favorable position. When this additional component appears in appreciable amounts at the top of the column, the operation is referred to as an azeotropic distillation. When the additional component appears mostly at the bottom of the column, the operation is called extractive distillation. 

As mentioned earlier the ease or difficulty of separating two products depends on the difference in their vapor pressures or volatilities. There are situations in the refining industry in which it is desirable to recover a single valuable compound in high purity from a mixture with other hydrocarbons which have boiling points so close to the more valuable product that separation by conventional distillation is a practical impossibility. Two techniques which may be applied to these situations are azeotropic distillation and extractive distillation. Both methods depend upon the addition to the system of a third component which increases the relative volatility of the constituents to be separated. 

Azeotropic Systems. An azeotropic system is one wherein two or more components have a constanl boiling point at a particular composition. Such mixtures cannot be separated by conventional distillation methods. If rhe constant boiling point is a minimum, the system is said lo exhibit negomv azeotropy if it is a maximum, positive azeotropy. Consider a mixture of water and alcohol in the presence of the vapor. This system of two phases and two components is divarianl. Now choose some fixed pressure and study the composition of the system at equilibrium us a function of temperature. The experimental results arc shown schematically in Fig. 5. 

Conventional distillation tends to be difficult and uneconomical because of the large number of stages required when the relative volatility between the components to be separated is very low. In the extreme case, in which an unwanted azeotrope is formed, distillation past the azeotrope becomes impossible. Extractive or azeotropic distillation can sometimes be used to overcome these difficulties. 

An azeotrope is a liquid mixture that has the same composition both in the liquid and in the vapor phase. This means that the components cannot be separated by conventional distillation. In such cases an entrainer or a third component is added to make the compositions of the liquid and the gas phases different. In the case of liquid-phase butane autoxidation (Section 8.4), 2-bu-tanone is separated as a pure component by adding entrainment agents such as ether to break the azeotrope the ketone forms with water. 

An ideal mixture of n components requires a sequence of n - 1 conventional distillation columns (two product streams) to separate the components completely. The columns can be arranged sequentially without recycle between them. This picture changes when mixtures forming azeotropes must be separated. Nonideal systems sometimes require complex distillation arrangements involving more than n - 1 columns with recycle of material between the columns. For the analysis of such systems, we recommend the use of residue curve maps. We base the following summary on the excellent book by Doherty and Malone (1998), who pioneered the use of these techniques. 

Isobutane and 1-butene are close-boilers and are difficult to separate by conventional distillation. Using a single stage, a 100 kmol/h stream containing 50% mole isobutane and 50% mole 1-butene is flashed at 480 kPa, producing a 50 kmol/h vapor stream. With no solvent added, the vapor composition is about the same as the feed. Furfural, which is less volatile than both components, is now added as a solvent to alter the isobutane-l-butene relative volatility by depressing the 1-butene A -value relative to that of isobutane. 

Various processes are used for separating components that are difficult or impossible to be separated by conventional distillation. Whether the difficulty of separation arises from the components close boiling points or their tendency to form azeotropes, the separation processes must take into account the complex vapor-liquid equilibrium relationships of the system. The system to be considered involves both the components to be separated and the separating agent that, in one way or another, enhances the desired separation. The vapor-liquid equilibria of such mixtures is highly nonideal, and it is precisely this nonideality that is capitalized on to bring about the separation. 

A fresh feed with a composition in region LI (single liquid that is predominantly component 1) may be distilled in a conventional column to produce essentially pure component 1 in the bottoms and an overhead at or close to the azeotropic composition X. If the feed composition is in region L2 (single liquid that is predominantly component 2), it may be separated by conventional distillation into a bottoms product that is relatively pure component 2 and an overhead that is around the azeotropic composition The overhead from either column may be cooled... 

Figure 10.3c is a schematic of a possible process for the separation of such a system. The fresh feed is mixed with the azeotrope-forming entrainer and is then fed to the azeotropic column, where pure component B is taken as column bottoms and the azeotrope AE is taken as overhead. The next step is to separate AE, an azeotrope that cannot be separated by simple distillation. A workable process involves liquid-liquid extraction where a solvent S is used to extract the entrainer, leaving a product that is virtually pure A. The solvent and entrainer are then separated by conventional distillation, the entrainer is recycled and mixed with fresh feed and makeup entrainer, and the solvent is recycled to the liquid-liquid extractor where some makeup solvent may be added. 

The boiling point of DMH at 100 kPa is 109.3°C, and that of toluene is 110.7°C, making it difficult to separate the two by conventional distillation. Each one of these components forms an azeotrope with methanol as an entrainer ... 

A feed stream at the rate of 100 kmol/h contains 50% mole acetone and 50% mole chloroform. The two components form a maximum boiling azeotrope which prevents their separation by conventional distillation. It is proposed to separate them by extractive distillation using benzene as a solvent, at a rate of 800 kmol/h. Both the main feed and the solvent are at 75 C and 110 kPa, and the column pressure is assumed uniform, also at 110 kPa. A total condenser is used, with a reflux ratio of 4. The distillate composition is specified at 95% mole acetone and the bottoms at 5% mole acetone on a solvent-free basis. Using the pseudo-binary... 

A stream of 100 kmol/h contains 70% mole benzene (1) and 30% mole cyclohexane (2). These components form a minimum-boiling azeotrope, which prevents their separation by conventional distillation. They can be separated by azeotropic distillation using acetone (3) as the entrainer. Acetone forms a minimum-boiling azeotrope with cyclohexane at the column pressure of 100 kPa, where the azeotropic composition is 73.9% mole acetone and 26.1% mole cyclohexane. The entrainer, at the rate of 75 kmol/h, is mixed with the feed and sent to the column. The distillate contains 99% mole acetone-cyclohexane azeotrope and 1% mole benzene, and the bottoms product is mostly benzene with a small percentage of cyclohexane. Use material balances to calculate the flow rates and compositions of the products. 

A distillation column, for instance, would be modeled with a column section for the stripping section, a column section for the rectifying section, and single equilibrium stages for the feed tray, the condenser, and the reboiler. In order to solve the distillation column separation equations as one unit, two sets of Equation 12.33, each with the appropriate stripping factors for the corresponding section, would have to be solved simultaneously along with a component balance around the feed tray, the condenser, and reboiler equations. Such a solution does exist for conventional distillation and for certain extraction problems (Smith and Brinkley, 1960). 

Addition of chlorine to the residua ethylene contained in the effluent resulting from the absorption of vinyl chloride, by passage through a solution of ferric chloride in ethylene dichloride. The temperature is maintained at between 50 and 70°C, under pressure between 0.4 and 0.5.10 Pa absolute. The chlorine is injected in sub-stoichiometric quantities in relation to ethylene (96 to 98 per cent) to eliminate the risks of explosive reactions due to the presence of hydrogen. The ethylene dichloride produced is rid of the catalyst by extraction with water. Added to that employed as a solvent in the previous stage, it is neutralized with caustic soda, filtered, washed with water, separated frojn the light components (especially water)hnd heavy components by azeotropic or conventional distillation. 

The tetm fractional distillation (which may be contracted to "fractionation ) originally was applied to the collection of separate fractions of condensed vapor, each fraction being segregated. Currently, the term is applied to distillation separations in general, where an effort is made to separate an original mixture into several components by means of distillation. When the vapors ate enriched by contact with connterflowing liquid reflux, the process often is called rectification. When operated with a contianous feed of Liquid mixture and continuous removal of product fractions, the process is continuous distillation. When steam Is added to die vapors to reduce the partial pressures of the components to be separated, the term steum distillation is used if such a process is altered to eliminate, the steam, dry distillation ("conventional distillation ) results. 

Many separations which would be difficult to achieve by conventional distillation processes may be effected by a distillation process in which a solvent is introduced which reacts chemically with one or more of the components to be separated. Three methods are presented for solving problems of this type. In Sec. 8-1, the 0 method of convergence is applied to conventional and complex distillation columns. In Sec. 8-2, the 2N Newton-Raphson method is applied to absorbers and distillation columns in which one or more chemical reactions occur per stage. The first two methods are recommended for mixtures which do not deviate too widely from ideal solutions. For mixtures which form highly nonideal solutions and one or more chemical reactions occur per stage, a formulation of the Almost Band Algorithm such as the one presented in Sec. 8-3 is recommended. 

This formulation of the Newton-Raphson method for columns with infinitely many stages is analogous to the 2N Newton-Raphson method for a column with a finite number of stages. First the procedure is developed for a conventional distillation column with infinitely many stages for which the condenser duty Qc (or the reflux ratio Lx/D) and the reboiler duty QR (or the boilup ratio VN/B) are specified and it is required to find the product distribution. Then the procedure is modified as required to find the minimum reflux ratio required to effect the specified separation of two key components. 

Consider a binary mixture of components A and B, to be separated into two product streams using conventional distillation. The mixture is fed in the column as a saturated liquid (i.e., at its bubble point), onto the feed tray / (Figure 4.10), with a molar flow rate (mol/min) F/ and a molar fraction of component A, overhead vapor stream is cooled and completely condensed, and then it flows into the reflux drum. The cooling of the overhead vapor is accomplished with cooling water. The liquid from the reflux drum is partly pumped back in the column (top tray, N) with a molar flow rate FR (reflux stream) and is partly removed as the distillate product with a molar flow rate FD. Let us call Mrd the liquid holdup in the reflux drum and xD the molar fraction of component A in the liquid of the reflux drum. It is clear that xD is the composition for both the reflux and distillate streams. 

Owing to the non-ideality of binary or multicomponent mixtures, the liquid phase composition is often identical with the vapor phase composition. This point is called an azeotrope and the corresponding composition is called the azeotropic composition. An azeotrope can not be circumvented by conventional distillation since no enrichment of the low-boiHrig component can be achieved in the vapor phase. Separating azeotropic mixtures therefore requires special processes, e.g. azeotropic or extractive distillation or pressure swing distillation. Azeotropic information is available in literature (Gmehling et al., 2004). 

Many separations are favored by lower temperatures, so conventional distillation wisdom recommends operating at the lowest pressure permitted by the use of cooling water in the condenser. Therefore, many columns are designed for 120 °F reflux-drum temperatures. If the components going overhead in the distillate are fairly high boiling, the column could operate under vacuum conditions. 

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