Process Selection – Considerations

FOLLOWING THE feasibility study, the next stage in the design project is the evaluation and comparison of the alternative process routes for manufacture of the chemical. The selection of an appropriate process is an important decision, all the subsequent work depends upon this choice. Although the selection can be changed or modified at a later stage, at least before the plant is built, such a decision results in a serious waste of time and money. However, probably not such a waste as building an uneconomic or unsafe plant! Obtain a process flow diagram (flowsheet) or Process and Instrumentation Diagram (P & ID) for each process route under consideration. These diagrams are usually quite complex and include all the ancilliary equipment, instrumentation and control details.

To obtain an initial appreciation of each process, and to make an initial comparison, draw a simple block diagram from each process flowsheet showing only the main chemical engineering plant items, e.g. reactor, absorption column, purification stages, etc. This simple diagram will not provide sufficient detail for final process selection, but it will provide an initial comparison of different processes and a quick familiarisation with the process stages comprising each route. The reactor is the heart of most processes, developments in process technology often centre around improvements in the design and operation of the reactor. This is often the basis of ‘new and improved’ processes.

Subsequent stages in the chemical process are usually concerned with the separation of various chemicals from the desired product, followed by the final purification stages. Previously in the feasibility study (Chapter 2) the appropriate purity of the final product should have been established. The product purity must be dictated by the customer (market) requirements. In some situations it may be possible to remove a low-purity product from the process for a particular application, and then purify the remaining chemical for another customer, The possibility of further purification should be determined in anticipation of new applications, environmental regulations, etc.

Processes often differ in terms of the process conditions, e.g. highpressure and low-pressure processes, or the type of reactor that is used, e.g. gas-phase catalytic reactor or liquid-phase CSTR. These differences should be clearly marked on both the simple block diagrams for each process and the detailed process flowsheets, they often determine which process route is ultimately selected. Many older, established processes were operated under conditions of high pressure or high temperature (mainly in the reactor), whereas the newer processes have often been ‘improved’ by operation under less severe conditions. For student design projects. the older and less efficient processes are often the only ones for which detailed process data (suitable for the detailed equipment design) are available.

Information concerning the ‘new’ technology and relevant experimental design data may still be kept secret. Hence, the requirement sometimes for students to select a process route and perform a design study for a less efficient process than they would ideally choose. However, the training and experience in plant and process design is no less diminished, and the real thing is awaiting their talents in industry! Many older processes (developed before the 1970s oil price crisis) were less energy efficient than those developed more recently. This should be apparent by consideration of the energy conservation features included in new plants, and the subsequent increase in complexity of the associated P & ID. Energy conservation is discussed in Section 8.2.1. The selection of a process route for production of a chemical will depend upon the following factors/considerations:

(a) Will the process produce what the customer requires?

(b) Is it possible to design, build and operate this plant economically (and safely)?

(c) The necessary design data, technology, fabrication methods and materials, raw materials, etc., must be available.

(d) The plant must operate in a safe manner, providing an acceptable hazard risk (see Section 8.2.3) to the plant employees and the public.

As with many aspects of design work, the final choice will usually depend upon a compromise between various features of different processes. It is unlikely that one process will possess all the advantages and no disadvantages. Sometimes there will be one overriding factor that influences the selection of a particular process, e.g. availability of a particular raw material, minimum cooling water requirements, etc. However, whichever process route is finally selected, it must fulfill all the criteria established in the project brief (Section 1.1). The aim of process design is to build the ‘best’ overall plant, all units within the designed plant may not operate at maximum efficiency or full potential. It is necessary to achieve a balance between the conflicting requirements of the individual units to produce the ‘best’ plant possible. Plant design is an exercise in compromise and optimization.

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