Increased Conversion and Selectivity

In many processes, downstream processing might constitute the major energy requirement of the plant. Separation also frequently dominates the capital costs of the process plant. Increased conversions in the reactor, for example, by better and more selective catalysts, may substantially reduce energy requirements for downstream separations.

The chemical industry has been very successful in identification and implementation of new catalysts as a way of improving its processes. As a result, catalyzed chemical reactions now constitute about 80% of all reactions in the chemical industry ) 10] ) Nevertheless, progress in the development of new catalysts represents a substantial lever in efficient use of materials and energy. Not only catalysts do reduce the energy barrier required for the reaction to take place, but they also reduce the usage of raw materials by increased selectivity.

To demonstrate how important the appropriate usage of catalysts can be, an example of acrylic acid production will be given. Through the continuous improvement of the catalyst over a period of 20 years, the yield of reaction has increased by more then 11%. Through targeted selectivity of the reaction, that favors intermediate reaction of acrolein, it was possible to stifle formation of side reaction and consequently shift the equilibrium towards the desired product. Given the total annual capacity of 800 000 tones of acrylic acid, CO2 savings of 230 000 tones per year were achieved [10].

An additional example is the production of styrene monomer from ethylene and benzene. The stages of the reaction are alkylation to produce ethylbenzene followed by dehydrogenation to produce styrene. The dehydrogenation reaction is not complete. The products are condensed and give a mixture of ethylbenzene, styrene, and other hydrocarbons. Most of the energy used for the entire process is required to separate this mixture by distillation. Improvements in operating conditions and catalysts have increased the conversion to styrene from approx. 40% to over 75% [1].

Considering the operating conditions of reactors, it can be observed that they are frequently operated with an excess of reactant or solvent compared with the required process conditions. This leads to an increased load of the subsequent separation of the product form raw materials, by-products and solvents. Only by operating reactors at their design conditions a large amount of energy can be saved.

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