Models for Ideal Chemical Processes

Any process has the possibility of continual incremental improvement but practically a point will be reached when it is not worth further effort and one’s time and talents are better expended elsewhere. 

 In process development how does one judge the good and the better?  A good process meets its quality and quantity requirements. The better process does this and goes further. The better process must be rugged. In a rugged process, if human error, mechanical failure, or equipment inadequacy creates some small deviations from the prescribed procedure, the result does not suffer seriously either in quality or quantity.

We judge a process by its costs and these include the labor expended, the time utilized in the special equipment, the price of the starting materials, and the price of waste disposal or recycling. A costing not only provides an indication of the efficiency with which inputs are used but it also provides a running assessment on the specific shortcomings that contribute most to the overall expense. A preliminary costing is an important tool in developing any process because it ranks areas where one might invest whatever limited time one has to achieve improvement.  

Trost has philosophized that the ideal process would be a single step and that there should be no co-product. That is, all the atoms in the reacting substances are retained in the products. Nothing then is thrown away. This is an interesting idea to contemplate.  It dramatically highlights that atom economy, as he calls it. It has the benefit of high weight throughput and low waste disposal but it is far from reality in terms of what can be actually practiced.  Every process is indeed ideally only a single transformation but the problem is starting materials for this ideal process are not commercially available- and because of this, the process creator must move retro-synthetically one step at a time until we do reach such commercial precursors.

Another useful idealized conception of a process is a sequence of chemical steps in which the reaction mixture from each step is simply treated over and over again with the reagents for the next transformation until the material which is the synthetic goal is present in the mixture; then, in one isolation operation, this product is separated pure from the complete complex mixture containing all the by-products and co-products of all the prior steps. This model dramatizes that it is most important to eliminate isolations because it is isolations that usually consume the most time and resources in a process.   In practice, of course, there are very valid reasons for performing isolations before the final isolation.

Valid reasons for isolation are:

1. To remove non-productive mass (ballast)
2. To change solvent for reaction optimization
3. To achieve needed purification through phase shifting
4. To correct stoichiometry and so save reagents
5. To provide convenient stopping points for campaign processing
6. To provide rework opportunities for rugged processing