Process intensification: smaller plants, same output

Process intensification makes a plant smaller by removing whatever limits it, usually mixing, heat transfer or an equilibrium, and by doing more than one job in one vessel. It is not a drawing exercise. It pays where the limitation is real and the plant is being built, and it is a poor fit for large, long-optimised production.

What the term means

Stankiewicz, Van Gerven and Stefanidis open their 2019 book on the subject with the history. Process intensification as chemical engineers now use it dates from Colin Ramshaw’s 1983 paper on applying centrifugal fields, HiGee, to distillation at ICI, work triggered by a NASA project on obtaining high transfer rates in zero gravity. For its first years the field was dominated by rotating equipment; heat exchanger reactors, intensive mixers and microchannel reactors came later.

The same chapter gives four generic principles: maximise the effectiveness of intramolecular and intermolecular events; give each molecule the same processing experience; optimise the driving forces and resistances at every scale, and maximise the specific surface areas to which they apply; and maximise the synergistic effects from partial processes. Those principles are realised, the authors say, in four elementary domains, spatial, thermodynamic, functional and temporal.

The European Roadmap on Process Intensification puts the same idea in commercial terms. Its working definition calls intensification a set of radically innovative principles, a paradigm shift in process and equipment design, which can benefit, often by more than a factor of two, process and chain efficiency, capital and operating expenses, quality, wastes and process safety. The roadmap also makes the safety argument directly: smaller is safer, because smaller inventories would have avoided many of the industry’s worst accidents, and it names Bhopal.

What it looks like as equipment

The United States Department of Energy’s 2015 technology assessment on process intensification describes the mechanism plainly. Separate unit operations such as reaction and separation are combined into one piece of equipment; mixing is enhanced, which improves mass and heat transfer, kinetics, yield and selectivity; and the result is fewer items of equipment, less footprint and less process complexity.

Its worked example is the one the field keeps returning to. Eastman Chemical Company’s tower reactor integrated five processing steps in the production of methyl acetate from methanol, achieving an 80 per cent reduction in energy and a large reduction in capital cost.

That is not a laboratory result. The same assessment tabulates commercial application in the petrochemical industry: reactive distillation with capital cost reduction of 20 to 80 per cent, energy reduction of 20 to 80 per cent and more than 150 installations; divided wall column distillation at 10 to 30 per cent on both counts with more than 100; and reverse flow reactors with capital reduction above 20 per cent, low energy reduction and more than 100 installations. Reactive distillation works by separating products from reactants inside the column, so an equilibrium-limited reaction is driven forward while the heat of reaction goes straight into the separation.

An Indian case the regulator has already measured

The Central Pollution Control Board’s charter for zero liquid discharge in molasses based distilleries contains a clean comparison of two distillation configurations. Against atmospheric distillation, its Table 7 gives multipressure distillation a steam consumption of 2.2 against 3.5 kilograms per litre of rectified spirit, distillation efficiency of 98.5 against 97 to 98 per cent, impure spirit at 5 to 6 against 10 to 15 per cent, spent wash at 9 to 10 against 12 to 15 litres per litre of alcohol, and 80 per cent of the steam condensate recoverable as boiler feed water where atmospheric distillation recovers none.

That is heat integration, which we would place in the thermodynamic domain: columns operated at different pressures so that heat leaving one drives another, the same heat used twice rather than a bigger boiler.

The charter’s Table 8 pairs the front end with it. Batch fermentation followed by atmospheric distillation runs at 20 kL of fresh water and 15 of spent wash per kL of alcohol; Biostill continuous fermentation with multipressure distillation runs at 12 and 8.0. The charter also states the honest caveat, that process technology is difficult to benchmark against spent wash generation because the figure depends on the alcohol concentration reached in fermentation, which in turn depends on the molasses and the yeast strain.

Where it does not apply

The Department of Energy assessment is candid about the barriers: risk of failure, scale-up unknowns, unreliability of equipment, and uncertain safety, health and environmental impacts. For large-volume production whose processes have been incrementally optimised over decades, it says the risk and development investment may outweigh the energy and environmental benefit.

It also reports three conditions industry representatives named at a workshop sponsored by the National Science Foundation as favourable: applications where traditional economies of scale do not apply, such as remote or distributed sites; applications where conventional technology does not work at all; and applications involving new construction or expansion rather than retrofitting. Read the third carefully. Intensification is not naturally a retrofit.

Control is the other thing to test early. Integrated systems have fewer degrees of freedom than the steps they replace, narrower acceptable operating windows, and faster dynamics because the spatial and temporal scales are smaller. The penalties are specific: reactive distillation brings complex design, expensive catalytic packing, and the risk that the best temperature and pressure for the reaction are not the best for the separation; divided wall columns bring greater height, higher pressure drop and a single operating pressure for both separations.

The European roadmap is equally careful with its own numbers: its sector benefits, such as 20 per cent higher overall energy efficiency in petrochemicals over 30 to 40 years, are planning targets rather than an industry commitment.

What this means for your plant

• Start from the limitation, not the catalogue. If the unit is limited by intrinsic kinetics rather than by mixing or heat transfer, the intensified hardware you can buy today does little; the field’s first principle does aim at kinetics, but by molecular-scale methods such as selective bond excitation, not on a supplier’s list.
• Look first at equilibrium-limited reactions, at column pairs, and at any recycle that exists only because a separation is incomplete. Those are where the published savings are.
• Ask whether this is a retrofit or a new line. The favourable conditions in the DOE assessment point at new build, expansion, and duties conventional equipment cannot do.
• Check controllability before capital. Fewer degrees of freedom and a narrower operating window are a real operating cost.
• Count the safety credit. Smaller inventories are the roadmap’s own first argument, and on a hazardous duty they can matter more than the energy.
• Prove it at a scale you can afford to be wrong about. Scale-up is named as a barrier by the people who deploy these technologies.

Deciding which of these applies to an operating plant is an assessment before it is a design, and that is water and process consulting work.

If you have been asked to add capacity without adding plot, tell us what the unit is doing and we will say what we would measure first.

Sources

1. Stankiewicz, Van Gerven and Stefanidis, The Fundamentals of Process Intensification, chapter 1, Short History of Process Intensification, Wiley-VCH 2019, sample PDF
2. United States Department of Energy, Quadrennial Technology Review 2015, Technology Assessment 6J, Process Intensification, PDF
3. Central Pollution Control Board, Charter for Zero Liquid Discharge in Molasses Based Distilleries, PDF
4. European Roadmap on Process Intensification, PDF