(445e) Systematic Design of Chemical Conversion Processes - Applied on the Methanol Synthesis

A process design is to a large extent a consequence of developments on catalyst, reaction routes, fluid type etc. Developments at this primary level of development determine the structure of the chemical system and the kinetics. Much research is focused on these topics because even incremental improvements may have large economical consequences. If the process economics looks feasible, the next step is to find a suitable reactor and a complete process in which to deploy the system on a larger scale, such as a pilot plant or a full scale production plant. The selection of a process structure and type of equipment are often based on comparison to similar systems. Repeated simulations are performed and additional laboratory tests are made. All this is useful, but it is likely that the best possible conceptual process design is missed.

Most chemical plants can be partitioned into a feed pre-treatment section, a chemical conversion section, post-treatment of raw products and process utilities. The post-treatment of raw product may be a sequence of distillation columns to isolate the desired products. For polyolefin plants the post-treatment is the dry section where the polymer is isolated and additives are added and the polymer is extruded. Unconverted reactants will often be recycled from the post-treatment section to the chemical conversion section. The chemical conversion process is normally more than a single reactor. It will often include heat exchangers, separators, compressors and recycle. A method of systematic process design is discussed. The method is applied on the chemical conversion process, which is focused here. The idea is to simplify the design problem by stating that the chemical conversion process proceeds along a path or a line of production. The reactants go through a series of basis operations and end up as products or intermediate products. However, recycle of unconverted reactants and the possibility of integrating two or more paths are possible. Functional basis operations such as reactions, phase change, separation, heat exchange, fluid mixing, extra feedings and pressure change take place on the path in order to convert raw material into desired products. A set of design functions representing the basis operations are constructed. Furthermore, a central part of the method is a design model, describing how the process states change along the path as function of the design functions. Here, a consistent model formulation is presented, handling transformation between different species where reactions and phase change are treated equivalently. The critical phenomena should be part of the design model, such as reaction kinetics (activity and stability), phase transition, mass and heat transfer, heat of reaction, chemical equilibrium etc.

A design objective such as space-time-yield is maximized with respect to the design functions. Alternative objectives taking into account energy efficiency may in some cases be important. The result of the optimization is a sequence of multi-functional process units on the path. For practical and economical reasons it may be desirable to exclude or set some basis operations to fixed values in certain units. Different optimization strategies such as control vector iteration and control vector parameterization are tested.

Optimization of the design functions (path optimization) provides only a part of the whole process design solution. Energy and mass integration within the path, with the rest of the plant or with other plants are important and concurrent activities of the path optimization. In addition, synthesis of the separation section and the design of the feed preparation section are activities that go along with the path optimization. These are more mature and established techniques and fit well into this framework. Moreover, process intensification involves detail equipment design. However, a conceptual design with multi-functional units is indeed a good starting point for doing process intensification.

Low pressure methanol synthesis on a Cu/ZnO/Al2O3 catalyst is studied. The reactants and products are gas phase, but methanol and water are condensed at lower temperatures. The main reactions are exothermic. Equilibrium is favoured by low temperatures, whereas catalyst activity is favoured by high temperature. The catalyst activity is crucial for the optimal temperature profile to be so low that the condensation of products can take place in the reaction zone.