Clever contrivance could cut crucial chemical's capital costs
15 Jan 2000
Methanol is one of the largest volume chemical products in the world, and its production could grow even further if its use as a motor fuel catches on. However, the processes in current use - developed by ICI, Kellogg and Lurgi - are inefficient, converting only 60-65 per cent of the methane feedstock into methanol. The large amount of energy needed to recycle unconverted synthesis gas, which has to be cooled, recompressed and heated, pushes up costs.
Researchers led by Roel Westertep of the Twente University of Technology in the Netherlands have devised a new process which converts all the methane to methanol, doing away with the expensive recycle stream completely. The process works by tipping the process equilibrium (between the syngas feed and the methanol) towards the product by continuously removing the methanol.
Speaking at a recent conference organised by the European Chemical Industry Council in Brussels, Westertep explained that the process is based on an elegant reactor known as the gas-solid-solid trickle flow reactor. This trickles a powdery adsorbent down through a catalyst bed, while the feedstock is pumped in from the bottom of the reactor. The methanol generated is adsorbed immediately, and the loaded adsorbent is recovered from the base of the reactor. This can't work in industry, however, because there's no `solids pump' to circulate the adsorbent.
Instead, Westertep's team used a principle known as a reactor system with interstage product removal (RSIPR), which uses a sequence of reactors. The solid adsorbent is replaced here by a liquid, tetraethyleneglycol-dimethylether. Instead of trickling through the reactor, this is held in absorption tanks which follow each reactor (see diagram).
The reactors - which are Lurgi, ICI or Kellogg models - achieve their normal 60 per cent conversion, but the removal of the methanol by the absorbers means that the total conversion by four reactor/absorber stages approaches 97 per cent. Westertep estimates that the absence of a recycle loop means that RSIPR requires 40-60 per cent less capital investment than the standard process.
