Explaining Process Intensification to Process Chemists

Process Intensification is a concept well known to engineers and essentially unknown to process chemists. This blog will try to narrow that gulf.

In the 1980s, Colin Ramshaw at ICI coined the term “process intensification” to describe his engineer rethink about gas/liquid mass transfer. That resulted in aiming for much smaller chemical plants that would be markedly cheaper and safer than existing ones.

Ramshaw’s thinking assumed no pre-existent equipment. That is to say, he did not devise a process to fit a particular plant’s physical assets. He started thinking afresh.  The most widely publicized outgrowth of such thinking was the high g centrifugal distillation. Distillation he saw as fundamentally a gas-liquid mass transfer for which the key cost drivers for a given system were well established:

·   Well mixed liquid and gas phases

·   Lots of interfacial surface area

·   Thin liquid film

·   Counter-current operation

In general, gases mix well in all conditions as do low viscosity liquids in thin films. Simple geometry teaches us that smaller, finer, packing gives us more surface area so that would be the obvious way to go - a column with very fine packing with counter-current gas flow.

However, a liquid film running through a bed of fine material floods when the film thickness becomes approximately equal to the clearance between the bits of packing.  The limiting factor is the thickness of the liquid film and most of the factors determining film thickness are physical properties of the fluid and are not open to modification.  Only gravity was independent. The higher the applied gravity the thinner the film and the smaller the packing could be. If gravity could be varied that would give a lot of mass transfer surface area for volume i.e. an intensified plant. To increase virtual gravity the centripetal effect of rotating the packing in a “high-g” machine was demonstrated to deliver an order of magnitude reduction in size. The idea was a major announcement at the time. An article appeared in Chemistry & Engineering News, “Novel Separation Technology May Supplant Distillation Towers’March 7, 1983.

Even though the high-g machine never became widely adopted this zero-based engineering that starts afresh from first principles exemplified the essential process of science and had appeal as a creative process. Understanding a process (a reaction, a crystallization etc.) with sufficient depth so that the key rate-controlling steps are understood and then matching that process to the right processor was seen as potentially breakthrough methodology.

Heat exchangers are another example. Obviously one of the keys to performance is heat transfer area so it is surprising that many heat exchangers are based on pipes that have a minimum surface area! It has been proposed that this reflects mechanical engineering considerations rather than process ones. Clearly the plate heat exchanger is a much more effective way of providing area, albeit with some mechanical downsides.

This is diametrically opposed to the normal approach in the chemical and pharmaceutical process industry, which creates a process to match standard equipment. Although there are good economic reasons for this in a batch process industry, there was a desire not to lose sight at the design stage of the possibility that intransigent difficulties operating in the standard way may become trivial with different equipment. For example, the ubiquitous batch reactor might be used to carry out a polymerization in the laboratory but the recipe used on plant scale will be adjusted to match the relatively poor heat transfer performance of a larger reactor. Here, the process has been tuned to match a characteristic of the processor. 

Perchance in some particular instances, the rationale for this matching process may even be lost in corporate history. Perhaps a batch takes a certain length of time to complete because many years ago it was matched to a particular reactor or type of reactor.

Just as important is the corollary that the process that has been matched to a particular processor cannot be simply transferred to a different processor without adjustments. For example, for exothermic reactions rate is proportional to temperature. A reaction temperature is selected so that  the heat can be removed and the reaction condition kept under control. One can make an order of magnitude change in the rate and still dissipate heat by going to a plate reactor. Thus a higher operating temperature can be held in control and a much shorter reaction time becomes practicable. The reaction time may become so short that continuous processing becomes possible. In fact, the new reactor will not “work” unless the process conditions are changed to harmonize with its new character. 

In the above example of an exothermic reaction, the matching of process temperature is key. Other characteristics that might need adjustment are mass transfer, mixing, diffusion, etc. Often the controlling step is obvious, sometimes it is completely unknown and sometimes there are different rate controlling steps during the course of a reaction. What the critical variables are constitutes fundamental understanding.

Batch reactors or in their continuous form continuous stirred reactors (CSTR) will match a process that inherently needs long times (perhaps diffusion controlled with real maximum temperature limitations). Oscillating columns offer moderate residence times with better than batch heat transfer. Plate heat exchanger type reactors (HEX reactors) are a good match for clean high heat transfer duties.  Spinning disc reactors offer good heat and mass transfer as well as good mixing. It is erroneous to claim that one is inherently “better” than another, any more than to claim a Posidrive screwdriver is better than a crosshead. What is required is to match the process and the processor! All the benefits of process matching, precision processing or process intensification are not always obvious. Clearly capital cost saving is the classic rationale with smaller reactors, less civil costs, less safety systems but improvements in yield, higher conversions, less or no solvent use are also important along with energy reduction. Improvements to product properties and even novel products that competitors find difficult to match are other potential major benefits.

As discussed above, the matching of the process to the processor’s capital equipment is key to precision processing. It is also important to recognize that the way a business is run often reflects the physical assets the processor has access to. For example, there is an essential connection between the fact that multiple products are usually synthesized in a batch reactor and such processing is normally conducted in some form of campaign operation because there is a need to clean scrupulously between batches. Again, consequently, a warehouse is usually needed to meet customer delivery demands.

The way the business works is matched to the characteristic of the processor. Change the processor to a low inventory continuous reactor and it might be possible to move to just-in-time (JIT) manufacturing with all those benefits. The business operation has been properly matched to the new processor characteristics.

Process Intensification does not mean the same thing to different authors. For example Aman A. Desai, Erich J. Molitor, and John E. Anderson in Process Intensification via Reaction Telescoping and a Preliminary Cost Model to Rapidly Establish Value, Org. Process Res. & Dev. 2012, 16, 160-165 define 'process intensification' as simply measures which significantly increase the productivity of a chemical process. Since ‘significantly’ is not defined this confuses a useful definition and replaces it with something indistinct. The idea of starting afresh and selecting appropriate engineering and equipment based on the needs of the chemistry without limitations is lost in their redefinition and this concept is worth retaining with its own lexicography.