The environmental performance of chemical plants has become increasingly important over the past decade, and is now possibly the most important constraint on plant design.
The way that engineers cope with environmental factors has changed considerably in recent years, however. When environmental issues first came to prominence, the emphasis was on reducing emissions from existing plants by installing ‘end-of-pipe’ solutions, such as equipment to filter out particulates or absorb harmful vapours. Now, however, as those plants become outdated, the focus has shifted to designing processes which are inherently more environmentally-friendly, consuming fewer raw materials, using less energy, and avoiding wherever possible the need to use hazardous substances.
One important concept for achieving these goals is process intensification (PI). Although the term implies simply doing things faster, it has a strict definition for chemical engineers. Chemical plants are designed by combining various options from a library of processes, such as heating, cooling, grinding, mixing, distilling and evaporating. Known as unit processes, these are carried out in items of equipment which can be used in virtually any process – a heat exchanger in an oil refinery will be identical, apart from its size, to one used in water treatment plant, for example.
In PI, engineers design new equipment which can carry out two or more unit processes simultaneously. This not only saves space, but can make the processes far more efficient – for example, introducing the action of a mixer into a chemical reactor ensures that the reacting chemicals mix more thoroughly, which in turn means that more chemicals react.
The concepts of PI have been around for many years, but they have only started to be used in industry relatively recently. For example, oscillatory baffled reactors (OBRs), which use a method of mixing new to the process industry, are now used at James Robinson, a producer of speciality chemicals based in Huddersfield.
Most process mixers work in one of two ways. They are either based around a tank with a stirrer, or they force the process liquids through a tortuous flow-path to induce enough turbulence to mix the fluids together.
OBRs also work by inducing turbulence, but the mechanism is quite different. The reactor consists of a cylindrical tube, divided into sections by circular orifice plates with holes, which are free to move backwards and forwards inside the tube and are linked at equal intervals by rods.
When fluids flow through an orifice plate, the edges of the plate create turbulence. The design and position of the plates ensure that when they are vibrated back and forth at the correct frequency they create a very ordered type of turbulence that effectively transforms each section of the tube between two plates into a small stirred tank. As the sections are linked by the holes in the orifice plates, successive sections improve the mixing characteristics of the entire apparatus. The result is a mixer that also works as a reactor, whose behaviour is very predictable, very controllable, and very efficient.
The OBRs used at James Robinson were designed by Xiong-Wei Ni of Heriot-Watt University in Edinburgh. They have allowed the company to change the way it manufactures a particular dye, from a batch to a continuous process.
As well as making more efficient use of raw materials and energy, continuous processes produce products of more consistent quality. They also increase production volumes since there is no need to stop the reactor between each batch to wash it out. Moreover, Ni said, it saves space; a reactor occupying 45m3 of space has replaced one which took up 1,200m3.
Ni is continuing his development of OBRs in a spin-off company, NiTech. Other engineers are looking at ways to carry out reactions under milder conditions. London-based HEL Group and a research group at the University of Nottingham is looking at hydrogenation reactions. These are extremely important in the pharmaceutical and fine chemical industries – replacing an active group of a chemical with a hydrogen atom adjusts the properties of the molecule.
Hydrogenations are difficult reactions because hydrogen is barely soluble in the solvents used in the processes. One way of improving the solubility is to replace the organic solvent with carbon dioxide at high pressure, which endows it with ‘supercritical’ properties, so that it behaves partly like a gas and partly like a liquid. Hydrogen dissolves completely in supercritical carbon dioxide, but pressures of around 100bar are required, and handling hydrogen at this pressure is an extremely dangerous business.
The Nottingham team, led by Martyn Poliakoff, has devised a new type of reactor that, instead of using compressors to raise the gas to the necessary pressures, generates the high-pressure gas at the inlet of the reactor itself, using a simple liquid, formic acid, as a starting material. There’s no need to store, meter or control any gases, so the equipment is simpler than a conventional supercritical CO2 rig, and the possibility of gas leaks is also reduced.
The system works by passing formic acid over a catalyst containing 5 per cent platinum, which causes it to decompose into carbon dioxide and hydrogen. Another liquid, ethyl formate, is also passed over the catalyst. This decomposes to form carbon dioxide and ethane, which both become supercritical under the same conditions.
Altering the flow-rate of the formic acid and formate allows the researchers to change the proportion of hydrogen in the gas stream to that required for the hydrogenation reaction. ‘Thus, the creation of a supercritical mixture of H2/CO2 is reduced to the pumping of two common liquids into small pieces of heated tubing,’ explained Jasbir Singh of HEL Group.
Supercritical reactors already have an important environmental advantage over conventional solvent-based reactions – to recover the product, operators merely have to reduce the pressure. This changes the supercritical CO2 back into a normal gas, which evaporates, leaving the product behind. By contrast, conventional reactions require many process steps to separate the product from the solvent and then the solvent itself must be treated or disposed of.
Removing the need for separations is also the focus of a new innovation from the University of Reading. Edman Tsang and colleagues are working with precious metal specialist Johnson Matthey and the life scientists at AstraZeneca to develop a type of catalyst that can be removed from reaction mixtures without the need for complex chemical processes. The team has developed nano-scale catalyst particles which are magnetic, making them easy to remove from the mixture by simply applying a magnetic field.
The size of catalyst particles is important in process engineering. Small particles increase surface areas exposed to the reaction mixture and this generally leads to faster reactions. However, very small particles, and nanoparticles in particular, are very difficult to separate from liquid mixtures by conventional processes, and tend to clog filters and valves.
The particles being tested by the researchers have a core of iron oxide or an iron-nickel alloy, which have magnetic properties, coated with an inert material to which active catalytic compounds can be attached. Johnson Matthey is helping the team to develop a carbon coating, with a fullerene shell grown over the crystalline iron-nickel particles. AstraZeneca is looking at coating iron oxide with silica, which can be bonded to biological enzyme catalysts.
But perhaps some of the most ambitious environmental innovations are coming from Japan, and have their origins in the completely different culture of that country’s process industries. Japan has centuries of expertise in fermentation reactions, gathered from the complex processes needed to make sake and soy sauce. And the country’s chemical engineers are putting this expertise to good use.
Fermentation reactions and their related processes, enzyme-mediated biotransformations, have an environmental edge over conventional processes. In general, they don’t require high temperatures or pressure; they take place in aqueous solutions, rather than using organic solvents; and they leave only water as a by-product. Moreover, Japan’s vast range of different climates gives it access to one of the widest diversities of micro-organisms in the world.
Japanese scientists are now combining this diversity with genetic modification techniques to create biological catalysts which can be used to generate different compounds, ranging from complex pharmaceutically-active molecules to bulk chemicals such as acrylamide, which is used to make acrylic resin.