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 topt can be defined for the maximum concentration of B, and where both the optimal yield and optimum reaction time 

Now consider the following parallel reaction where A and B are reactants, P is useful product and Q is by-product 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 and Petela EA (1992) Waste Minimisation in the Process Industries Part 2 - Reactors, Client Eng, 12, 509-510 12. 

Hence, at the level Reactor-Separators-Recycles the material balance can be brought in a narrow optimal region. On this basis can be started the process integration steps regarding the optimal management of energy, mass separation agents, process water, waste minimisation, etc. 

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. 

The batch process reactor system adds more flexibility but is probably the biggest contributor to waste generation. A detailed breakdown of the unit operations will give clear indications to the areas of manipulation which give most impact on waste minimisation. A step by step breakdown, as used in a Hazard and Operability (HAZOP) study, is applicable to optimising batch processes with respect to waste minimisation. 

Fast neutron reactors with a closed fuel cycle to achieve a durable production of electricity while minimising needs of uranium and the burden of long-lived radioactive waste. 

Some AGM participants pointed out that small reactor concepts which are currently developed are based on downsizing today s large reactor technology. They consider that die infrastructure necessary to support conventional nuclear power development is very expensive, and beyond the resources of most developing countries. A new innovative approach to system design is to be used to reduce the need for such an infrastructure. The efforts should be focused on the development of the reactor systems with joint consideration of the overall fuel cycle, waste issues proliferation, simplified operation, simplified and minimised system maintenance. Several organizations presented their preliminary studies on innovative small reactor concepts. 

In parallel with the work done in collaboration with the European partners BNFL has conducted studies of the potential role of fast reactors in the UK and elsewhere. It is important to consider the fuel cycle as a whole and to make use of fast reactors in the optimum way to maximise safety and economic advantage while minimising environmental impact and proliferation risks. To this end accelerator-based systems as alternatives to critical reactors, and the thorium cycle as an alternative to the uranium-plutonium cycle, have been examined with particular reference to the implications for fuel fabrication, reprocessing and waste disposal. This work continues but the initial conclusion is that the critical Pu-fuelled fast reactor, properly integrated with reactors of other types, and with optimised arrangements for Pu recycling, has many attractive advantages. 

Wastes also arise from the product separation or work up parts of a process. Solvents may be used to extract or wash the product, acid or base to neutralise reaction products, catalysts, particularly homogeneous ones may escape, or be unsuitable for reuse. Careful selection and operation of plant, from reactors to filtration equipment can help to minimise the amounts of wastes which are produced. 

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