Critical factor

With environmental issues higher on the agenda than ever before, chemicals producers are anxious to find cleaner methods to make and process their products.

One way of improving the environmental performance of a process is to reduce - or eliminate altogether - the use of organic solvents, a feat which can be achieved through the use of supercritical fluids as solvents.

Thomas Swan, a fine chemicals company based in Consett, Co Durham, has recently opened the world's first full-scale chemical synthesis plant using supercritical carbon dioxide as a solvent.

All materials have a 'critical point' - the combination of temperature and pressure which allow its solid, liquid and gas phases to exist together in a stable equilibrium. If the temperature and/or pressure are increased above this point, the material becomes a supercritical fluid, with the solvent power of a liquid, but able to flow and diffuse like a gas. Supercritical carbon dioxide (scCO2) acts as a replacement for many organic solvents, with the added advantage that it can be removed simply by dropping the pressure.

Supercritical CO2 has been used on a large scale for extracting natural products such as caffeine from tea and coffee. However, it is not widely used as a solvent in organic synthesis. This has been the focus of research at the University of Nottingham's Clean Technology Research Group, under Professor Martyn Poliakoff. The group has found that because scCO2 combines the transport properties of a gas with the density of a liquid, it accelerate reaction kinetics, which boosts the output from the reactor. The promise of large throughputs from a relatively small reactor attracted Thomas Swan's interest, and the company began to support Poliakoff's research.Swan's support helped the Nottingham team develop their small lab-scale rig into a full-sized pilot plant, based around a continuous fixed-bed catalytic reactor.

Continuous flow allows the reaction to be controlled kinetically, rather than thermodynamically, which gives access to a wider range of products than would be achievable under normal conditions. For example, the system is extremely efficient at hydrogenation reactions, where the scCO2 is used as a solvent for hydrogen. One such reaction would be the production of 3-ethylcyclobenzene from its corresponding diene. Under normal conditions, the best result that Thomas Swan's engineers could achieve was a 4:1 mixture of the desired product and an unwanted fully-saturated by-product, even at just 80 per cent conversion. Using scCO2, they can achieve complete conversion with 100 per cent selectivity to the desired olefin.

Send for the specialists

Having researched and scaled-up the process to pilot scale, Swan's next task was to translate the technology to a full-scale plant. For this, it turned to an expert in supercritical process technologies, Chematur of Sweden. Based in Karlskoga, the company builds plants for supercritical oxidation of sludges and other wastes, for natural product extractions, and for applications such as particle formation, textile drying and chromatography.

The Swan-SCF (supercritical fluid) plant incorporates a work tank to store the fluid, where it is sub-cooled before being fed into the pump unit. This boosts the fluid pressure and sends it into a heat exchanger which heats it to the desired supercritical conditions. These conditions vary, depending on the type of reaction that is being performed, so the plant allows the temperature and pressure to be adjusted accordingly.

The reactants are then fed into the solvent via two feed lines, then into a mixer, and finally into the reactor itself. The reaction solution passes over a heterogeneous catalyst and out of the reactor vessel via a pressure control valve, which knocks the pressure down to subcritical levels. This creates a two-phase system. Once the mixture is inside the separator vessel, the temperature is increased to evaporate the solvent. Plant operators can then draw off the liquid product, while the gaseous phase passes back into the work tank via a condensor.

The plant has a capacity of up to 2000tpa. Completed in October 2003, it is currently being used to synthesise trimethyl-cyclohexanone, an acetone used in styrene products, as a trial product for calibration. Swan plans to use it for a range of reactions, including Friedel-Crafts acylations and alkylations, where the scCO2 allows the reaction to proceed without the hazardous catalysts, such as aluminium trichloride or hydrofluoric acid, which are necessary under conventional conditions; and for synthesising linear ethers with unreacted hydroxyl groups.

Hydroformylations, where carbon monoxide and hydrogen are added to an olefin across a double bond, are also possible, with a high degree of control over the ratio of isomers.

Automation for the plant was provided by Emerson Process Management. The system is based around a DeltaV digital system, with Rosemount temperature and pressure transmitters to monitor these parameters. As the plant is used for process development and proving, it has a higher level of instrumentation than a more conventional plant. For example, Micro Motion Coriolis meters measure the density of the CO2 in the pressurisation stages.

The system uses two display screens, with one showing the overall process and the other focusing in on an area of interest such as the reactor or the utility services. 'Our operators use on-screen alarms to react to any process problems,' says Richardson. 'As we develop the process, we add modules and control loops: this we can do ourselves, easily.'

The data from these instruments is fed into a DeltaV Historian, which records the information. This 'has been invaluable for tracking all events throughout the run, particularly any process upsets,' says project engineer Ian Richardson. 'Analysis of the final product shows how well the process conditions were tuned to the requirement, and allows us to identify any differences run to run. In this way, we can tune our process to achieve 100 per cent selectivity and 100 per cent conversion in a single pass.'