Process Technology: An Introduction - Haan A.B. 2015

2 The structure of chemical and biochemical process systems
2.4 Process synthesis

The development of a commercial product from a laboratory chemical involves the talents and efforts of many people. At the laboratory stage, the market potential of the product is estimated. The physical and chemical properties of the chemicals involved in the process are then determined, and the conditions under which the product can be produced in the laboratory are explored. However, a long and complex effort still remains.

There are times when an engineer is called upon to devise a plausible process long before all of the development effort has taken place. Perhaps a decision whether to invest in the development effort is required. If a tentative process can be suggested, the cost and difficulty of the development of the process can be estimated. Also, a rough estimate of the cost of the product can be calculated, and the size of the market can be forecast. Like most economic forecasts, these “guesstimates” are subject to an enormous amount of uncertainty. Accordingly, many technically and economically sound processes and products have been shipwrecked on the rocks of poor marketing forecasts.

The development of a process scheme involves coming up with that configuration of processing steps which efficiently and safely produces the desired product. An enormous amount of art, skill, intuition, and innovation goes into developing the processing scheme. Literally millions of designs are possible. One of the tasks of a process engineer is to choose from these possibilities, taking into account the many conditions set by product markets, geographical location of the plant, the social situation, legal regulations, etc. This is not only important in choosing among existing processes but also in developing new processes. This process synthesis step is where vast amounts of money can be made or lost and where a good, innovative chemical engineer can be worth his or her weight in gold.

These activities are not merely a straightforward application of the scientific disciplines on which chemical technology is based (chemistry, physical transport processes, unit operations, reactor design). It is necessary to select relevant knowledge from these fields, combine different aspects, and interpret these quantitatively. This means integration of knowledge from various fields of science. With such complicated systems as chemical processes some of the questions will inevitably be answered in a semiquantitative or even qualitative way. Typical questions that must be answered as the engineer conceives and develops a process to make a marketable product are:

· (1) What reaction steps are required to get the product?

· (2) What is the best type and size of reactor to use, and what are the optimum reactor operating conditions (temperature, pressure, agitation, catalyst concentration, etc.)?

· (3) What is the optimum reactor conversion, and how does it affect the design and operation of the downstream separation steps? The optimization must incorporate the entire plant.

· (4) Will the catalyst degrade or be poisoned?

· (5) What side reactions occur? What will the likely by-products be? What effect will they have on yield and performance?

· (6) What are the best raw materials? Air, water, petroleum, natural gas, coal, minerals, and agricultural products are the basic raw materials. However, a host of semiprocessed intermediates may also be used.

· (7) How can the raw materials be brought to conditions suitable for the reaction? Usually several purification steps are needed, as well as heating and compressing to the appropriate conditions.

· (8) How can the products and by-products be separated and purified to meet market specifications?

· (9) Should cooling water be used, or should air cooling be considered?

· (10) Are any of the materials used, produced, or present in the process toxic or carcinogenic? Are there other health or safety hazards in the process? Is there a potential for hot spots or explosions in the reactor? Should the reactor be shielded and remotely operated?

All of these questions have several possible answers. Each choice the engineer makes has technical, economic, social, and political repercussions. The engineer is expected to make prudent and wise choices, and the impact on the environment must be considered. In many cases experience is a necessary asset to assist sound intuition and judgement.