Delayed reaction

Process Intensification (PI), the design of equipment which combine two or more of the conventional process engineering unit operations within one piece of equipment, has great potential for operators in many process sectors.

Several different types of PI technologies have been, or are being, developed, including the spinning disc reactor (SDR), the oscillatory baffled reactor (OBR), printed circuit heat exchanger (PCHE), and in-line mixers.

They claim to help processes to become safer, cleaner, smaller and cheaper. They are all intellectually satisfying and make us wonder why we did not think of the idea before. However, the ideas have all been around for many years — solutions in search of a problem — and yet the uptake has been small.

So why have we not replaced our stirred tank reactors (STRs) by spinning discs or OBRs? The simple answer is that we wrote off the capital of the STRs long ago; we understand them; and will not spend valuable money to replace them just for the intellectual stimulation of running a new piece of kit. There is rarely any incentive to stray outside our comfort zone in order to have a plant that is smaller and cheaper — even safer and cleaner.

A question of proportions

In fact, in the balance of risk and reward, the risk is perceived to be higher with PI technology since we do not know how to design it. For example, for a spinning disc, what size should the disc be, and how fast should it rotate? Are there any guidelines about what can or cannot be done, or do we have to try every application on a pilot rig?

In a spinning disc reactor (SDR), liquid is fed onto the centre of a rotating disc, which can be heated or cooled by an internally-circulating fluid. Intense interfering waves are formed in the liquid under the influence of the centrifugal force as the liquid moves towards the edge of the disc. This enables very high heat transfer coefficients to be realised between the disc and liquid, as well as very high mass transfer between the liquid and the gas above the liquid. The waves formed also produce intense local mixing. The liquid flow involves very little back mixing and is therefore almost pure plug flow. The residence time is short, typically seconds.

This knowledge is held by a limited number of companies that, understandably, need to protect their intellectual property, but it is the lack of that knowledge that makes the rest of us cautious. Nevertheless, the intellectual argument in favour of PI technology is very convincing.

Residence times within a continuous stirred tank reactor follow a symmetrical Gaussian distribution, as they do in a spinning disc reactor, but the standard deviation in the SDR is very much narrower. Because of this, the SDR’s developers claim that every molecule passing through the reactor has the same experience. What’s more, the heat and mass transfer are considerably enhanced in the SDR.

This represents a chemical engineering ‘Utopia’: every molecule experiences identical and intense processing — surely the route to the ultimate green plant: small and almost effluent-free.

It is not only the SDR that has excellent green credentials. Microreactors are used already in high throughput screening in the development of products such as drugs and catalysts. Miniaturisation technologies mean that screening tests may be undertaken using less material, and in a shorter time.

The rewards for companies that screen most molecules are enormous, while the financial risk of investing in PI screening technology is relatively low, so companies will take the risk. But ‘greenness’ alone is still not good enough. To tempt us to invest in these technologies, they need to deliver a product or effect not available by conventional means.

For example, in the manufacture of an intermediate for a photographic chemical, Huddersfield-based specialities manufacturer James Robinson replaced traditional batch technology with an OBR, operating continuously. This brought three process stages together in one step and resulted in a greatly increased production rate and a reduction in byproducts. The floor area occupied by the plant was also greatly reduced.

The oscillatory baffled reactor (OBR) offers uniform fluid mixing, excellent particle suspension, and better heat and mass transfer than conventional reactor systems. In a tube with diameter D, orifice-plate internal baffles are placed, spaced 1.5D apart, and with orifices of diameter 0.5D. The oscillating fluid motion interacts with each baffle to form vortices, and the resulting fluid motion gives efficient and uniform mixing in the space between two baffles. This results in significant enhancement of heat and mass transfer, and control of residence time distribution. The mixing is controlled entirely by the oscillations and not by the throughput. The enhancements are effective at low bulk flow rates (Re < 300) and long residence times require only short tubes and small L/D.

As a result, the university which developed the OBR, Heriot Watt University in Edinburgh, spun out a company to develop the technology, Nitech Solutions. The company aims to deliver similar improvements into reactions such as hydrogenation, oxidation and fermentation.

Printed circuit heat exchangers (PCHEs) offer genuine savings in space and weight, and hence cost, to the superlarge equipment used in gas treatment plants, for example. Here, a shell-and tube heat exchanger requiring three shells and weighing 100 tonnes can be replaced by a PCHE weighing only 15 tonnes.

Small solutions, big promise

Heatric, which specialises in PCHEs, has devised a reactor based on the technology. Known as a multiple adiabatic bed printed circuit reactor (MAB-PCR), the reactor routes the process stream through a preheating section and into an alternating series of heterogeneous catalyst beds and PCHE ‘cores’.

The multiple small reaction stages and heat exchange steps allow the temperature to be controlled precisely and eliminates ‘hot spots’. Reactants can be added — or products removed — at each reactor stage. Also, the reactor stages can use very small, and therefore highly efficient, catalyst particles. MAB-PCRs can be for applications ranging from fuel processing to the production of fine and bulk chemicals.

Printed circuit heat exchangers (PCHEs) are constructed by a process of chemically etching fluid flow channels into flat metal plates which are then diffusion-bonded together. This entails pressing the metal surfaces together at high temperature, producing a bond of strength equivalent to that of the parent metal, and resulting in a solid metal block containing precisely engineered fluid flow channels. A PCHE can be four to six times smaller than a conventional shell and tube heat exchanger of the equivalent duty. In addition, many different fluid contact regimes are possible. PCHEs can withstand pressures in excess of 600 bar and can cope with temperatures ranging from cryogenic to 900°C. They can achieve a thermal effectiveness of over 98% and can incorporate more than two process streams into a single unit.

PI also opens the door to having a chemical plant at the point of need. Its small inventory means greater inherent safety, so sophisticated chemical engineering skills are perhaps not required to operate it. This paves the way for small biodiesel plants to be sited close to the crop used as feedstock, and producing diesel direct for the local community, or for miniature plants converting the NOx from lorry exhausts into ammonia.

PI technology can greatly increase heat and mass transfer and enable reactions to be carried out very rapidly. This is very useful for medical instruments and can enable results to be obtained very rapidly. It was announced recently that the test time for detection of the hospital ‘superbug’ MRSA had been reduced from 1-2 days to 1-2 hours. Perhaps PI technology will soon enable the test result to be obtained instantaneously.

PI technologies make processes safer, cleaner, smaller and cheaper. Laudable as this is, there is little perceived need for such benefits since, by and large, chemical plants are safe and fully depreciated, and meet the requirements of current environmental legislation.

However, to remain competitive, industry needs technology that will deliver something innovative, such as a product with properties that cannot be made with conventional technology, or to enable a rethink of plant location and supply chain. PI technologies have the potential to do these things and may be able to deliver a ‘Green Utopia’ as well. If every molecule undergoes the same intense experience then conversion and yield could approach 100%. This is the sort of performance that should provide the impetus for companies to reconsider the benefits of PI.

Clive Whitbourn is a technology translator for the Crystal Faraday Partnership.