In the waters that I swim within, the term “Green Chemistry” is often derided as an environmental extremist codeword used by chemical technology Luddites. I’m sure there are anti-chemical purists who view Green Chemistry as a kind of natural alternative to the use of man-made, unnatural substances. Biodiesel is is commonly held as one example of a green alternative.

The merits of Green vs non-Green, or the inherent Greenness of Green technology is a contraversy too large for this blog. Instead, I would rather turn my attention to the merits of Green thinking in chemical process development.

Specialty chemical manufacture begins with relatively simple raw materials and produces more complex and more valuable compounds.  Limiting the topic to synthesis as opposed to formulation, specialty chemical manufacture relies on the application of functional group manipulation to achieve an end product with the desired features and connectivity. 

In chemical manufacture, there are several general ways to improve process economics. The most general term is process intensification, which describes the overall effect of maximizing the conversion of raw materials into product per volume or per dollar of capital equipment.  This can be done by a) running reactions at more concentrated levels, b) the application of heat and pressure, c) the elimination of solvents altogether, d) the use of more reactive species, or e) telescoping. 

a) Of the many kinds of reaction optimization schemes that we learn in grad school, perhaps the least explored is space yield improvement. Space yield is just the kg of product obtained per liter of reaction mixture used.  In manufacturing, the goal is to maximize the number of kg of product obtained per plant man-hour for a given piece of equipment.  Raw material costs are only slightly adjustable, whereas labor hours can often be lowered with the application of better technology. Labor costs are among the highest costs in a plant, so the goal is to produce the most kg of product per man-hour.  

Intensification by increasing space yield simply means that a solution reaction is run at the highest reasonable concentration. The benefit is that a reactor is run at the highest mass yield per batch.  Since the corresponding increase in labor to run the higher space yield is near zero, the resulting labor costs are spread over a greater quantity of mass- the labor $/kg go down.

b) Most people come out of college with precious little experience or appreciation for the benefits of high temperature and pressure in chemical synthesis. Continuous high temperature short time (HTST) reactions are out there, but require specialized equipment and expertise. Engineers love this kind of processing. For an optimized process, HTST systems have comparitively small reaction zones that are relatively inexpensive and take advantage of the economics of continuous flow chemistry. Good examples are ammonia production or Petroleum refining.

c) The elimination of solvents is a tricky thing. Gas phase chemistry is well known and quite mature technology. It is also amenable to continuous flow processing.  Elimination of solvents in condensed phase chemistry is a bit different problem.  Solvent molecules are like tiny sand bags in their ability to absorb energy.  Neat reactions may be prone to exotherms, so the calorimetry of any given proposed neat reaction needs to be examined carefully.

On the plus side of solventless processing, the space yields are at maximum and reaction rates are not diminished by dilution effects.

d)  The use of more reactive materials in a process can have benefits in time savings if a particular step is slow or does not go to completion. If the reactive reagent affects the rate limiting step, then time and yield savings may be had. On the down side, increased reactivity may offer decreased selectivity. Increased hazards will have to be calculated into the value proposition as well.

e) Telescoping refers to the combination of multiple steps in one reactor.  In it’s best incarnation, telescoping may afford the direct reaction of a product in the reaction mixture of the previous reaction. The benefit is in the savings of man-hours in vessel cleanout and preparation time, minimized hazwaste, and minimum handling.  Another form of telescoping would be that pot residues are left in the vessel and reagents from the next step are charged in without extra transfers and new vessels.  This tends to minimize the number of vessels needed for a process and minimizes the opportunity costs of having empty vessels standing by for use.

The goal of green chemistry as I see it is to minimize environmental insult by the reduction of consumables and the discharge of hazardous waste streams from a plant to the outside world of waste treatment.  The reduction of consumables like solvents, reagents, or electrical power reduces hazards and pollution up and down the value chain. The reduction of hazardous wastes minimizes the mass that has to be consolidated for eventual incineration or burial.

The reduction of consumables is entirely compatible with the economic goals of business.  Process intensification points to the same general direction as the goals of Green Chemistry and should be considered a type of Green activity.  If one draws a picture of the ideal state of any chemical manufacturing process, it surely would include intensification with sustainable raw materials and with maximized throughput and minimized risk.