Practical process INTENSIFICATION

'Process Intensification is the key to the survival of the fittest in international competition,' proclaimed Joachim Semel at the 2nd international conference on 'process intensification in practice', held in Antwerp at the end of October. As chairman of the conference's technical advisory committee, Dr Semel might be expected to extol the virtues of PI in such a forthright fashion, but in his 'day job' as head of Hoechst's corporate research and technology division he knows well enough the realities facing the international chemicals industry.

'In global markets with global competition, only superior technologies will win through,' he said in his keynote address to the conference delegates, drawn largely from industry and university research departments.

Part of the problem with PI in the past, of course, has been one of definition. What, exactly, is process intensification? Semel suggests that PI works through a combination of new feedstocks, more selective catalysts, intensified heat and mass transfer, combined reaction and separation steps, new separation processes, even on-line analysis. To most process engineers, it generally implies the use of compact items of plant equipment to reduce process inventories, accelerate heat and mass transfer, and generally produce more from less. Not much exactness there.

More precision comes from PI conference organiser and sponsor, the BHR Group. 'Process intensification is a design philosophy where the reactor fluid dynamics are matched to the physico-chemical requirements of the reaction to achieve high productivity and selectivity.' What this actually means in practice was addressed by many of the conference speakers, including BHR's head of research Andrew Green.

In his view, a problem with the uptake or lack of it of PI is that it has too often been an equipment-driven approach. The novel technologies that do exist, such as the use of compact heat exchangers and static mixers as reactors rather than just as unit operations, are more likely to be considered on the basis of fitting the process to the equipment, rather than choosing the equipment that will allow the process to run at its most efficient rate.

That view may suggest an over-reliance by plant design engineers on the expertise of the equipment manufacturers, but Green traces the problem even further back up the design train.While he acknowledges that chemical engineers are not usually taught about novel technologies in any detail, he points out that much process development is, initially, led by chemists. 'By the time the engineer gets involved,' he says, 'the synthesis may already have been developed to the stage where it can only be done in conventional plant, usually batch or semi-batchwise in stirred tank reactors.'

Co-operation between chemists and engineers is, therefore, a first step in identifying the potential benefits of PI. The chemists have to be asked: 'can the reaction be made faster?', as they may well have assumed that conventional equipment could not cope with some alternative process routes. As part of Green's proposed PI methodology (outlined above right), he says it's important to assume at the start that intensified equipment can cope with any reaction conditions. After further analysis, it might well be concluded that it actually can't, but at least all the possibilities will have been explored.

In his paper for the conference, Green presents a detailed case study of an application of the PI methodology to an existing aldehyde oxidation process. The question posed to BHR was whether the company concerned should build another conventional plant like the existing one, or switch to one based on novel intensified equipment. After an analysis of the business and process drivers which include, among others, improving plant productivity, safety, manufacturing costs, and process problems associated with the traditional gas/liquid tank reactors BHR proposed two PI approaches. These were an integrated chemical reactor/compact heat exchanger design, or one based on a spinning disc reactor.

Compared with the conventional process, which was estimated to have a capital cost of around £745 000, both the PI designs were approximately half as expensive. And offered other benefits such as improved product purity, improved safety (by reducing process inventories by two to three orders of magnitude), and improved energy use.

With results such as these, it might be thought that in this case the last step in the PI methodology shown in the panel 'final choice of equipment' was superfluous. Not so. The methodology, Green emphasised, 'should be complementary and compatible to other process development assessments, such as safety, environmental hazard and economic issues'. Each of these would need to be considered and weighted accordingly before coming to a final decision. Above all, Green says that the methodology is not about forcing process intensification on particular situations, since PI may not after all be the most efficient way overall of achieving the process or business targets.

His conclusion, however, is that there needs to be an increased awareness of PI and that the methodology is one way of introducing the concept where it may normally be overlooked.

Another way, of course, is by events such as the BHR conference itself. Apart from addressing the underlying philosophies of PI, as Green and Semel set out to do, many speakers concentrated on specific examples of how PI can be put into practice in the areas of heat and mass transfer, reactor design and separation processes. Some of these are highlighted in the adjoining panels, although it has to be said that few examples referred to actual, rather than potential, applications.