Methanol synthesis purge

The small amount of hydrogen required for the HGT can be readily obtained from a portion of the methanol synthesis purge gas. This purge stream is normally used as fuel the fuel value of the hydrogen can be replaced by the small amount of gas produced from the HGT step. 

In methanol synthesis purge recovery, a water scrubber is also used with a similar purpose, and it too pays for itself in recovered methanol. The meth-anol/water mixture is simply sent to the existing crude methanol distillation column. Hydrogen recovered from this purge can result in energy savings or if additional carbon oxide is available, it can be used to obtain increased methanol production. PRISM separators have operated on stoichiometric as well as non-... 

There has been an increasing interest in utilising off-gas technology to produce ammonia. A number of ammonia plants have been built that use methanol plant purge gas, which consists typically of 80% hydrogen. A 1250 t/d methanol plant can supply a sufficient amount of purge gas to produce 544 t/d of ammonia. The purge gas is first subjected to a number of purification steps prior to the ammonia synthesis. 

The purge gas from the methanol synthesis unit contains mainly methane. Depending upon the size of the plant, this stream may be reformed to supplement the synthesis gas produced from coal gasification. 

JM proprietary methanol synthesis catalyst is packed in the shell side of the reactor. Reaction heat is recovered and used to efficiently generate steam in the tube side. Reactor effluent gas is cooled to condense the crude methanol. The crude methanol is separated in a separator (10). The unreacted gas is circulated for further conversion. A purge is taken from the recycling gas used as fuels in the reformer (3). 

The preconverted gas is routed to the shell side of the gas-cooled methanol reactor, which is filled with catalyst. The final conversion to methanol is achieved at reduced temperatures along the optimum reaction route. The reactor outlet gas is cooled to about 40°C to separate methanol and water from the gases by preheating BFW and recycle gas. Condensed raw methanol is separated from the unreacted gas and routed to the distillation unit. The major portion of the gas is recycled back to the synthesis reactors to achieve a high overall conversion. The excellent performance of the Lurgi combined converter (LCC) methanol synthesis reduces the recycle ratio to about 2. A small portion of the recycle gas is withdrawn as purge gas to lessen inerts accumulation in the loop. 

Unconverted methane present in the refonmng effluent behaves in the successive operations like an inert diluent To prevent its buiid-up in the recycle, which constitutes the methanol synthesis loop", a purge is necessary. 

Sulphur is detrimental to the sjmthesis and trace amounts of sulphur are removed using zine oxide prior to synthesis. After the production of synthesis gas, the methanol sjmthesis requires compression to about lOObar. The methanol synthesis loop con rises a reactor, a separator and recompression of the reeyele gas. A purge gas can be used to produce power supplemented by steam raised in the methanol reactor and the coal gasifier. The crude methanol produced can be upgraded to chemical grade product by distillation. The intermediate methanol is passed into storage. The reaction stoichiometry is ... 

The quantity of coal needed to generate the process steam for the production of the CO + H2 required in the methanol synthesis unit. This figure does not only allow for tiie steam that is fed to the gasifier and/or required for the partial oxidation of hydrocarbons and for purge gas reforming, but includes also the quantities needed for CO shift conversion to arrive at an H2/CO ratio of 2 which is desirable for methanol synthesis, i.e. to produce hydrogen and reduce the CO content by the water gas reaction CO + H2O CO2 + H2. 

In the above section, the importance of carbon monoxide and carbon dioxide conversion and the technically attainable approach to the equilibrium has been described. However, these two parameters alone do not decide upon the optima-tion for the production of methanol from a specific synthesis gas. The methanol yield from the synthesis gas is of quite decisive importance for economically producing methanol on a commercial scale. Its this yield on which depend the quantity of synthesis gas which must be produced horn coal, cleaned, conditioned and compressed and the quantity of CO2, CO and H2 which must be removed from the methanol synthesis as purge gas and thus is lost to methanol production by the direct route. 

The steam boiler plants are preferably fired with sulfur-fiee or low-sulfur fuels from the plant itself, for instance purge gas from methanol synthesis or flash gas from coal gasification and gas scrubbers. Wherever this heat supply is insufficient, it may be supplemented by firing natural gas, if available, or coal. The latter often requires complex desulfurization systems so that in certain cases even the production of low-sulfur fuel gas by coal gasification with air and subsequent desulfurization of the gas may be considered as an alternative to coal-fired systems. 

The diagram of a plant that produces substitute natural gas (SNG) in addition to methanol is shown in Fig. 7.2. The entire purge gas stream from methanol synthesis is delivered to a methanation unit where the hydrogen and carbon oxides are reacted at a nickel catalyst to yield methane and water according to the following formulae... 

Hydrogen and helium have relatively small molecular sizes compared to other gases and exhibit high selectivity ratios in glassy polymers. Applications can be found in the recovery of H2 from purge gas streams in ammonia synthesis, petroleum refineries and methanol synthesis. 

Because carbon is the limiting factor, the carbon conversion to methanol, also referred to as carbon efficiency, is an important operating parameter for overall ener efficiency. Carbon efficiency is a measure of how much carbon in the feed is converted to methanol product. There are two commonly used carbon efficiencies, one for the overall plant and one for the methanol synthesis loop. For the overall plant all the carbon-containing components in the process feedstock from the battery limits and the methanol product from the refining column are considered. For a typical plant and natural gas feedstock, an overall carbon efficiency is about 75%. The methanol synthesis loop carbon efficiency for the same plant is about 93%. The synthesis loop carbon efficiency is calculated using only the carbon in the reactive components in the makeup gas (CO and C02). Carbon in the form of methane is not considered because it is inert in the methanol synthesis reaction and is ultimately purged from the loop and burned. The carbon in the product for this calculation is that in the form of methanol in the crude leaving the methanol synthesis loop. 

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