Chemical reactor waste minimisation

The waste minimisation reaction related examples in this text are represented mainly by combinations of consecutive and parallel type reactions. Although the major details of such problems are dealt with in conventional textbooks, it may be useful to consider the main aspects of such problems from the viewpoint of solution by digital simulation. 

Consider the following first-order consecutive reaction sequence 

Solving the kinetic equations clearly demonstrates that the concentration of B passes through a maximum in respect to reaction time. If B is the desired product and C is waste, an optimal time t opt can be defined for the maximum concentration of B and where both the optimal yield and optimum reaction time are functions of the kinetic rate constants kj and k2- In waste minimisation terms, however, the quantity of B obtained both in relation to the unreacted A and waste product C is important, since these may represent quite distinct separation problems and may also have quite distinct associated environmental loadings. In general, one wants the rate of decomposition of A to B to be high relative to the rate of decomposition of B to C. Since these rates are also temperature dependent, a favourable product distribution can also be effected by varying the reaction temperature. 

Now consider the following parallel reaction where A and B are reactants, P is useful product and Q is byproduct waste. 

It is obviously important to achieve complete reaction for A and B and high selectivity for the formation of P with respect to Q. 

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Smith, R., E. A. Petela, 1991-1992, Waste minimisation in process industries. The Chemical Engineer (UK), ibid. 1. The problem, 24-25, Oct. 1991, ibid. 2. Reactors. 17-23, Dec. 1991, ibid 3. Separation and recycle systems, 24-28 Febr. 1992, ibid 4. Process operations, 21-23, April 1992, ibid 5. Utility waste, 32-35, July, 1992 Towler, G. P., R. Mann, A. J. Serriere, C. M. D. Gabaude, 1996, Refinery hydrogen management cost analysis of chemically-integrated facilities, Ind. Eng. Chem. Res., 35 (78), 2378-2388... 

Process wastes arise from a variety of sources which are associated with the chemistry and plant. They may arise from the preparation of feedstocks the reaction/reactor itself from product separation and from the utility system, particularly combustion emissions associated with provision of heat and electrical power [13]. Plant operations also contribute and smart operation of a process which is certainly not inherently clean can sometimes do a good deal to minimise the production of wastes. This book is, however, concerned with the chemistry of waste minimisation, rather than chemical engineering and operations, both of which are of great importance. 

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