Simultaneous Rectification and Chemical Reaction

In most rectification systems, chemical reactions among the components to form new stable compounds do not usually occur, but in a few special cases such a condition is involved. In some cases, the combination of rectification and reaction is beneficial as in the preparation of esters while in other cases, it may be detrimental in that it decreases the effectiveness of the separation or the yield of the desired component. 

In the esterification system, if the liquid level on the plates is low and a normal vapor rate is employed, the amount of reaction per unit time will be small relative to the vapor rate and a high reflux ratio will be necessary. This will result in a high heat consumption per gallon of ethyl acetate produced. If a very deep liquid level is employed on the plate to increase the amount of reaction relative to the vapor rate, it may be that the rate at which the ethyl acetate is removed is so low that the liquid on the plates is near equilibrium and the reaction rate 

The large liquid volume required can be obtained by using a deep liquid level on the plates or by having the overflow from a plate pass thro igh a holding tank for the chemical reaction before it is added to the plate below. Deep liquid levels give undesirable action with normal-type bubble plates, but tall caps and risers can be used to obtain satisfactory operation. 

The design calculations for these systems can be made by the usual stepwise procedure making allowance for the chemical reaction on each plate. The calculations frequently require trial-and-error methods, 

Example on Preparation of Cyclopentadiene. Cyclopentadiene is obtained in high-temperature vapor-phase petroleum cracking operations. It is mixed with other hydrocarbons, and its separation is complicated by the fact that it will dimerize and the dimer will depolymerize at about normal distillation conditions. One method of operating is to dimerize the pentadiene and then remove all remaining constituents below C7. The residue is then given a thermal treatment which will depolymerize the dimer, and the mixture is distilled to obtain the cyclopentadiene. A unit is to be designed for this final fractional distillation, and an estimate is to be made of the amount of polymerization that will be obtained. 

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In recent years reactive (or catalytic) distillation (or rectification) has gained some importance in the process industry. Reactive rectification denotes the simultaneous performance of chemical reaction and physical separation within a countercur-rently operated column. The integration of these two unit operations in one column offers advantages for reversible liquid-phase reactions where the reaction products hinder the progress of the reaction (e.g., Sundmacher and Kienle 2003 Frey et al. 2003 Frey and Stichlmair 1998). 

Metallothermic reduction of silicon tetrachloride with zinc. Silicon tetrachloride, SiCl, is preliminarily obtained as a byproduct from the carbochlorination of zircon and, to a lesser extent, from that of pure silica sand with carbon and chlorine. Afterward a high-purity silicon tetrachloride is produced by rectification (i.e., fractional distillation). Simultaneously, zinc is purified by vaporization. Then both chemicals are added to the heated quartz-reaction-heated vessel. Needlelike polycrystalline silicon is grown inside the furnace as a result of the following chemical reaction ... 

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