Rectification reactive distillation

Figure 2 illustrates the three-step MIBK process employed by Hibernia Scholven (83). This process is designed to permit the intermediate recovery of refined diacetone alcohol and mesityl oxide. In the first step acetone and dilute sodium hydroxide are fed continuously to a reactor at low temperature and with a reactor residence time of approximately one hour. The product is then stabilized with phosphoric acid and stripped of unreacted acetone to yield a cmde diacetone alcohol stream. More phosphoric acid is then added, and the diacetone alcohol dehydrated to mesityl oxide in a distillation column. Mesityl oxide is recovered overhead in this column and fed to a further distillation column where residual acetone is removed and recycled to yield a tails stream containing 98—99% mesityl oxide. The mesityl oxide is then hydrogenated to MIBK in a reactive distillation conducted at atmospheric pressure and 110°C. Simultaneous hydrogenation and rectification are achieved in a column fitted with a palladium catalyst bed, and yields of mesityl oxide to MIBK exceeding 96% are obtained. 

After the chlorination, hydrogen chloride is removed from the reactive mixture by nitrogen flow the reactive mixture is sent to the rectification section (into tower tank 32). Carbon tetrachloride and intermediate distillate are the first to be distilled in the tower, at 120 °C in the tank and at 30 °C and 0.1 MPa on top. The vapours of carbon tetrachloride which escape the tower are condensed and collected in receptacle 34 the condensate is sent back to chlorinate. 

Methylchlorosilanes are difficult to separate due to the closeness of some of their boiling points. It is especially difficult to separate pure dimethyldichlorosilane (the boiling point is 70.2 °C) devoid of methyltrichlorosilane (the boiling point is 66.1 °C), because the difference of their boiling points is only 4.1 °C. It is known that the efficiency of separating reactive mixtures depends on the number of theoretical plates in the rectification towers moreover, in distillation there is a certain dependence between the number of theoretical plates and the difference in the boiling points of the components. For precise distillation and compete separation of methyltrichlorosilane from dimethyldichlorosilane, one needs a rectification tower with the efficiency of 60-80 theoretical plates. 

The catalytic distillation process of Smith [15], by providing for the fixing of the catalyst in a reactive section of a column between nonreactive stripping and rectification sections, and thereby for the continuous removal of MTBE from the reactants, boosts the conversion of isobutene to well in excess of 99%. The concept is still more economically attractive when OCFS are employed to secure the catalyst in the reactive section— DeGarmo et al. [16]—due to their significantly higher mass transfer efficiency. 

Reactive rectifications without a separate reactor, in which the reaction takes place in the distillation column, are increasingly being used in industry. Typical classes of reactions are esterification, transesterification, acetal formation and cleavage, etherification, oxidation, and hydrogenation. With regard to the design of such processes, three cases can be distinguished ... 

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). 

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