Extractive distillation columns, azeotropic

Because a number of Industrially important liquid mixtures are azeotropic systems, means had to be found whereby they may be separated by distillation. Two approaches, extractive distillation and azeotropic distillation, are used. In either case, a separating agent is added to the column so as to alter favorably the relative volatilities of the... 

The results of calculations for the two separation methods are summarized in Table VIII. Fewer trays are required in the azeotropic distillation column than the extractive distillation column. The heat loads are also smaller. The quality of the ethanol product is also slightly better for the azeotropic distillation method. Including the stripper for processing the aqueous phase, the total heat load for reboilers for the azeotropic distillation method is less than half that for the extractive distillation method. The total condenser load is roughly two-thirds that for the extractive distillation method. 

The design of azeotropic or extractive distillation columns, as with con-A ventional columns, demands a knowledge of the vapor-liquid equilibrium properties of the system to be distilled. Such knowledge is obtained experimentally or calculated from other properties of the components of the system. Since the systems in azeotropic or extractive distillation processes have at least three components, direct measurement of the equilibrium properties is laborious and, therefore, expensive, so methods of calculation of these data are desirable. 

The bottom product from column (G) passes to the hydro extractive distillation column (H). The water feed rate to column (H) is five times that of the bottom product flow from column (G). It may be assumed that the acetonitrile and other byproducts are discharged as bottom product from column (H) and discarded. The overhead product from column (H), consisting of the acrylonitrile water azeotrope, is condensed and passed to a separator. The lower aqueous layer is returned to column (H). 

The high boiling reactant is fed as feed 1 and the low boiling reactant as feed 2. Between the two feeds, there is the reaction zone. As a special application, feed 1 can serve as an extractive agent, e.g. in the case of the production of methyl acetate, acetic acid serves as an entrainer for the binary azeotropic mixture methanol and methylacetate. The ensemble is then a reactive extractive distillation column. 

Extractive distillation is a suitable distillation process for the separation of azeotropic systems or systems with separation factors tti2 close to unity. A typical extractive distillation process for the separation of aliphatics firom aromatics is shown in Figure 1. In extractive distillation processes, the high boiling selective solvent (entrainer), introduced not far from the top of the extractive distillation column, has to alter the volatilities in such a way that the separation factor attains a value very different from unity. Typical entrainers for the separation of aliphatics from aromatics are Ai-Methyl-pyrrolidone (NMP) or //-Formylmorpholine (NFM). In the presence of NMP or NFM,... 

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. 

The use of a polar and a nonpolar solvent to separate acetone and methanol from a mixture of tetramethylene oxide and other oxides has been described by Hopkins and Fritsch.17 A schematic drawing of this purification process is shown in Fig. 6-1. The ternary azeotrope of acetone, methanol, and tetramethylene, a cyclic ether, may be broken by an extractive distillation using the highly polar solvent, water. The volatility of the methanol is lowered by the water to such an extent that the azeotrope of acetone and tetramethylene oxide may be distilled overhead in the extractive distillation column, and the methanol is withdrawn with the water from the bottom of the column. A second column is used to separate the azeotropic mixture of acetone and tetramethylene oxides by use of the relative nonpolar solvent, pentane. An azeotrope of pentane and acetone boiling at 32°C, is removed from the top of the column. The azeotrope is broken by adding water which results in the formation of two phases, a pentane phase and an acetone-water phase. 

Prior to the treatment of systems of azeotropic and extractive distillation columns, two other topics are considered, namely, the quantitative description of the behavior of solvents and the solution of problems involving columns in which three phases exist in the accumulator. 

FORMULATION OF THE CAPITAL 0 METHOD FOR SYSTEMS OF AZEOTROPIC AND EXTRACTIVE DISTILLATION COLUMNS... 

In Chaps. 2 through 5, the theta methods and variations of the Newton-Raphson method are applied to all types of single columns and systems of columns in the service of separating both ideal and nonideal solutions. Applications of the techniques presented in Chaps. 2 through 5 to systems of azeotropic and extractive distillation columns are presented in Chap. 6. An extension of these same techniques as required for the solution of problems involving energy exchange between recycle streams is presented in Chap. 7. Special types of separations wherein the distillation process is accompanied by chemical reactions are treated in Chap. 8. 

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