M. Appl, consultant from Dannstadt-Schauemheim, Germany, highlighted the developments in industrial methanol production technology since 1923 at the 1998 World Methanol Conference held in Frankfurt, Germany, last December.Methanol History in the Wood Age
Up to 1923 "wood alcohol" was the only source of methanol which was needed in increasing quantities in the chemical industry. In 1924 about 3 million tons of wood were processed worldwide, which would have corresponded to about 30,000 tons of methanol. Taking all products into account by their calorific value, 33 percent charcoal, 6 percent acetic acid, 1.6 percent "wood spirit" and around 10 percent tar, the whole process has a thermal efficiency of roughly 65 to 70 percent, which is on the same order of modern methanol plants based on natural gas.Synthetic Methanol and Coal Age
With the industrial revolution in the early 19th century wood was replaced largely by the fossil energy coal which supplied the increasing demand for steel fabrication and the mechanical energy for industrial productions. Along with this, town gas for city illumination became increasingly important and was received as a byproduct of coke production for the steel works. The emerging organic chemical industry found a great raw-material source in the coal tar, which was another byproduct. And when the gas from the cokeries was no longer sufficient to supply the needs, gasification processes for coke were developed by which with steam and heat, hydrogen and carbon monoxide containing gases could be obtained. The development of the technical ammonia synthesis by F. Haber and C. Bosch was a breakthrough which subsequently led to a number of chemical processes including methanol synthesis.
The successful hydrogenation of molecular nitrogen to ammonia suggested looking at the possibilities of hydrogenating other molecules. A. Mittasch and C. Schneider in 1913 succeeded in obtaining oxygen-containing products which, depending on the catalysts used, contained various amounts of methanol. Poor selectivity and yield provided not promising prospects for a technical methanol synthesis.
After the First World War BASF resumed the search for a methanol process, after M. Pier had joined the company. In February 1923 Pier successfully produced methanol with good selectivity at a pressure of 1,000 bar using zinc chromate as catalyst. On September 26, 1923, the first tank-car with crude methanol left a newly installed methanol plant in Leuna, Germany.
United States production of synthetic methanol was started after 1926 by DuPont. Wood methanol production decreased rapidly, for example its share was reduced to 50 percent in 1930, and to 20 percent in 1935. One reason for the quick technical realization of the methanol process was that synthesis gas production technology had already been developed for the ammonia synthesis. With only minor adaptations it was possible to draw a suitable CO/H2 mixture from the syngas train and so quite often methanol was a side production of ammonia.
In the 1940s high pressure methanol synthesis based on this technology had a total energy consumption of around 70 gigajoules per ton (GJ/t) methanol, which is more than 2 times the value of a modern natural gas-based methanol plant.Low-Pressure Methanol Synthesis and the Hydrocarbon Age
The advent of hydrocarbon feedstocks and the development of new gasification and purification processes was a turning point for all chemical production based on synthesis gas. Steam reforming of natural gas began in the 1940s in the United States, based on developments of BASF in the 1930s. ICI improved technology and catalysis for this process considerably and extended it to naphtha feedstock. Alternatively the partial oxidation of heavy oil processes developed by Shell and Texaco became an option. Naphtha-based high pressure methanol plants came down to a total energy consumption of around 42 GJ/t methanol.
The steam reforming process, because of the extreme purity of the synthesis gas, was the precursor for the technical realization of the low-pressure methanol process. ICI is credited for the development of low-pressure methanol technology and its introduction into commercial methanol production. Successful research efforts and pilot plant operation led to the decision to build the first low-pressure methanol plant in Billingham in 1966. The process proved to be much more robust than expected and the catalyst for which a minimum lifetime of 6 months was required, lasted 2 years with the low-pressure process. The design capacity, originally 300 metric tonnes per day (mtpd), was greatly exceeded and reached over 600 mtpd. The rapid acceptance of the new process by the industry was remarkable. Only one other high pressure process was built after 1966.
The first ICI low-pressure plant had an energy consumption of 36 GJ/t MeOH. Today low-pressure methanol plants with steam reforming of natural gas can reach 29 GJ/t methanol.
Not much later in 1969 Lurgi successfully launched their own low-pressure process with a 10 mtpd demonstration plant followed by a 220,000-metric tonne per year unit in 1970, which used purified synthesis gas produced by the Shell Partial Oxidation Process. Different from ICI which used for its first plants quench converters with adiabatic reaction in the individual catalyst beds, Lurgi applied a quasi-isothermal reactor with catalyst in tubes cooled by circulation of boiling water.State-Of-The-Art and Further Development of Production Technology
In the last 30 years, says Appl, methanol production has become a mature technology and considerable progress has been made in the synthesis section especially with respect to catalyst performance and converter design. Synthesis gas production technology has profited enormously from the developments in the ammonia field, with respect to catalysts, materials and designing skills. The single-train concept, integrated-steam and energy systems and the introduction of the centrifugal compressors are mentionable.
Reliable and innovative catalyst producers and experienced contractors are essential for the progress in methanol technology development. Today there are four catalyst suppliers and complete proprietary processes for methanol synthesis are available from six companies: ICI Lurgi, Topsøe, Mitsubishi, M.W. Kellogg and Uhde.
Design figures for converters can be today as high as 25 tonnes per day methanol per cubic meter catalyst. A good catalyst in a natural gas-based plant may over its lifetime of about 4 years produce 20,000 to 25,000 tonnes or more methanol.
Energy consuming steps, like synthesis gas and recycle gas compression, and also the distillation achieved considerable improvements. Energy recovery in the gasification and synthesis loop is another key area. In modern methanol plants, especially in natural gas-based steam reforming plants, a surplus of energy which either has to be exported as steam or by inclusion of a generator as electric power is available. Figure 1 shows how the total energy consumption has been improved over the years.
The synthesis of methanol from carbon oxides and hydrogen produced from fossil feedstock has reached the status of a rather mature technology with not much improvement potential left, says Appl. But in principle the synthesis gas route (even with an energy efficiency of 74 percent) is still energetically inferior to the direct oxidation of methane to methanol.
For the established technology (the low-pressure methanol process) the selectivity is high, in excess of 99.8 percent. Conversion per pass depends on process conditions, but can be as high as 40 percent. The recycle of the non-reacted gas in the synthesis loop raises the overall conversion of carbon in the feedstock to 88 to 94 percent. These figures are challenging targets for a direct oxidation process.
Appl makes the following broad predictions: