Some aspects of process synthesis with emphasis on reactors and reactions
Two major, largely unknown, areas of process synthesis are explored in this dissertation, and procedures are proposed for the solving of problems within them. In each area, case studies are reported. The work on the first area results in the development of a general procedure for the efficient screening of alternative process routes. The procedure is applied to two examples of commodity chemicals: methyl methacrylate (MMA) and vinyl chloride monomer (VCM). By using suitable knowledge of organic chemistry, thermodynamics, kinetics, and costing, gradual screening of the process alternatives can be achieved from the earliest stages of conceptual design, with minimum calculation effort. The MMA example is concentrated on the selection of raw materials and of chemical routes; by following a number of heuristic and of rigorous rules, a list of prototype raw materials is rapidly developed. From this list it proves possible to create a reaction network connecting the selected raw materials with the target molecule, and 54 routes to MMA (including the two current commercial routes) were found to be of potential interest. Compared with the MMA case study, the VCM example moves further through the procedure to include pre-design capital costing; 14 reactions, resulting in 63 routes to VCM, are rapidly selected, and equilibrium and kinetics calculations reduce the number of promising VCM processes to 24. The operating conditions of the process reactors are also specified, and only three processes are eventually admitted to the final costing stage. 'Elementary plant sections', effecting only one reaction each, are costed separately for the three remaining processes, and it is shown that the three most promising processes can be analysed by joining together the 'sections' relevant to each process. The costing estimates are found to agree well with current practice. Furthermore, a graphical approach is devised to give quick estimates of product selling prices for a wide range of plant capacities and raw materials costs. The second major area of process synthesis considered is the synthesis of reactor networks integrated with the rest of the flowsheet. The complex reaction scheme by Van de Vusse (1964) was employed as an example in a pioneering study. A new procedural approach, which uses a hierarchy of three heuristics, is implemented. In this approach, the designer first devises a simple base case, and then methodically increases its complexity. A powerful analytical tool used is to target for maximum reactor system selectivity, in contrast with the target of maximum reactor yield proposed by previous workers. The dissertation ends with an example illustrating the scope for heat and power integration techniques in the later stages of the design of a flowsheet. A computerised model of a pressurised sulphuric acid plant is developed, and a number of suggestions are made for the improvement of a commercial process, leading to an increase of 15% in steam production.