A process synthesis approach to low-pressure methanol/dimethyl ether co-production from syngas over gold-based catalysis
Date
2009-06-10T11:38:25Z
Authors
Mpela, Arthur Nseka
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Abstract
Catalysts are involved in a very large number of processes leading to the
production of industrial chemicals, fuels, pharmaceutical, and to the avoidance,
as well as the clean-up of environmental pollutants. In respect to the latter
aspect, efforts are being made by different stake-holders (governments,
researchers, industrials, etc) in order to prevent or to minimize pollution of our
cities. A notably way to reduce pollution for a friendly environment is to make use
of clean fuels. After years of research work, it is only recently that dimethyl ether
alone or when combined with methanol has been identified as a potential
alternative clean fuel.
Nonetheless, the technology used for the methanol synthesis from syngas
requires high pressure (>120 atm) to reach an acceptable CO conversion. The
dimethyl ether production from methanol in a separate unit makes DME more
expensive than methanol. However, the transformation of syngas directly into
dimethyl ether can be used to relieve the thermodynamic constraints requiring
operation at high pressure. If the synthesis of methanol and dimethyl ether takes
place in the same reactor, the process should, in principle, be able to operate at
a much lower pressure, making it a potentially cheaper process to produce methanol and dimethyl ether. The catalysts that need to be used for this coproduction
have to be catalytically stable, selective and able to catalyze the main
reactions (methanol and dimethyl ether synthesis) involved in this process at the
same temperature. Unfortunately, existing commercial methanol/DME catalysts
are not able to function efficiently in the presence of large concentrations of
water or at high temperature. Thus, it is relevant to have a catalyst satisfying the above criteria. Recently, it has been reported that a supported gold catalyst
could be used for methanol synthesis; accordingly this study has developed
bifunctional gold-based catalysts for the methanol and DME synthesis.
This study utilized process synthesis approach to determine the optimal
operating conditions for methanol/dimethyl ether production that yielded results
used to drive an experimental programme to get the most useful information for
designing a process route. In a comparative way and by using the feed
compressor work load per unit of valuable material generated as objective
function, this study showed that the system where methanol is co-produced with
DME is more efficient than the one involving the production of methanol alone
and this is applicable for the operating reactor temperatures of 500-700K and the
loop pressure ranging from 10 to 100 atm. The catalysts systems chosen in this
study were consisted in the physical mixture of gold-based catalysts
incorporating respectively gamma-alumina and zeolite-Y. The gold-based
catalysts were prepared by a co-precipitation method, then characterized by
XRD, Raman Spectrometry and Transmission Electron Microscopy and,
afterwards tested using a 1/4 inch tubular fixed bed reactor between 573 and
673K at 25 atm.
Amongst the catalysts tested at 673K, and 25 atm, 5%Au/ZnO/γ-Al2O3 produced
both methanol and dimethyl ether with moderate yield, whereas 5%Au/ZnO/LZ
Y-52 gave high dimethyl ether selectivity (75.7%) with a production rate of 252.3
μmol.h-1.g -1
cat . The presence of hydrocarbons detected by the GC-FID in the gas
products requires that further investigations be done to determine the eventual
source and optimize this new catalyst system based on gold for a large scale coproduction
of methanol and dimethyl ether from syngas.
Description
Keywords
process synthesis, methanol/dimethyl ether, syngas, gold-based catalysis