Why model a process?

“The ability to predict reflects a high degree of process understanding.” – PAT: A Framework for Innovative Pharmaceutical Development, Manufacturing, and Quality Assurance, Food and Drug Administration.

Below are listed some of the areas in which simulation may be beneficial in batch process design and operation.

1. Process modelling develops a better understanding of the process: The act of modelling often provides a better understanding. Mass and heat balances on a chemical reaction require both a scientific understanding (thermodynamics and reaction mechanisms) and an identification and quantitative appreciation of the process design and operating parameters (e.g. heat transfer coefficient and operating temperature). Through simulation the interaction between chemical reactions and chemical plant may become clearer.

2. Process modelling allows design space to be explored very quickly: The effect of varying design parameters, and operating parameters, may be simulated and the effect of 'novel' operating conditions may be examined. Pharmaceutical and fine chemical processing tends to be less flexible in terms of varying reactor design, however reaction design is very important in terms of product yields, by-product formation and batch time. Semi-batch operation involving time varying additions of a number of reactants together with, say, operating temperature sequencing would present a very difficult problem of optimisation if not done by simulation. It must be remembered that the cost of a 'failure' in simulation is the same as that of a 'success'. The same cannot be said for pilot and full-scale studies.

3. Process modelling highlights potential safety problems and other unwelcome surprises: Dynamic simulation can be used to examine the interactions of process chemistry with the reactor and its control system. Insight into this aspect of scale up cannot be gained from safety instruments alone. Instruments can tell you why a reaction is unsafe but not how the unsafe condition may arise. Simulation can reveal that process dynamics are such that a scale up calculation for the steady state system which is apparently safe can behave in an unanticipated or hazardous way. Simulation provides a good way of answering the question 'what if?' Again it costs no more to predict a dangerous operating condition than a safe one. It is a fairly common but dangerous practice to incrementally change the operating conditions of a process in an effort to improve it. Such changes may give no indication that an unsafe operating region is being approached. Trouble shooting The interaction of chemistry and process variables is often very complex and problems discovered in plant operation or upon scale up may be difficult to explain and so correct. Modelling and simulation techniques may offer a means of examining the effect of varying plant and operating parameters on the process chemistry through the batch and so improve the operation of an existing plant.

4. Process modelling can reveal potential emission and waste problems early on: One of the main requirements of the Environmental Protection Act is to show that the level of emissions is below the specified level. Dynamic simulation may be used to examine the effect of scale up on gaseous emissions and by-product formation at the design stage.

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