Fig. 2.18 Principles of the choice of equipment (heterogeneous catalysis)

Fig. 2.18 Principles of the choice of equipment (heterogeneous catalysis)

ol rectification column with catalytid^^^l packings ol rectification column with catalyti^^^^l internals in the downcomersl^^^^^l ol rectification column with side stream^M reactors^^^^^^^^^^^^^^^^H

ol column and reactor with pumparound^B ol evaporator and reactor with pumparound ol

Some Remarks on the Role of Catalysis

Catalysis can be autocatalytic, homogeneous, or heterogeneous. This is of course also true for RD.

In the case of autocatalytic reactions the reaction velocity can only be influenced by the reaction temperature, in other words for RD by the pressure of the equipment.

Homogeneous catalysis allows the reaction velocity to be influenced by changing the catalyst concentration. Thus the reaction velocity can be adapted over a wide range to the needs of the distillation equipment.

Heterogeneous catalysis requires a structure to fix the catalytic particles in the reaction zone. This may cause construction and operation problems and is also factor that limits the catalyst concentration that can be achieved. The reaction velocity can be enhanced only to the limit set by the attainable concentration range. Furthermore, the possibility of enhancing the reaction velocity by higher temperature or pressure is limited, because in general the catalyst consists of ion-exchange particles, whose temperature range is limited.

So, homogeneous catalysis is much more flexible but has its price in an additional separation step necessary for catalyst recycling and by demands for expensive materials in the case of mineral acids. Heterogeneous catalysis is simpler in principle, but technical problems have to be solved. In general the equipment will need more volume, for example the columns must have a bigger diameter. It should be clear from these considerations that decisions have to be made in each case.

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