Purge from synthesis loop

Using only steam reforming, the synthesis gas will contain a surplus of hydrogen in relation to the MeOH stoichiometry in the synthesis gas. This surplus hydrogen has to be purged from the loop. 

The raw methanol (7) is sent to the distillation section (8), comprising two or three columns, where byproducts and contained water are separated out to obtain the desired purity for the methanol product (9). The inerts contained in the synthesis gas are purged from the loop (10) and recycled as fuel to the primary reformer (2). 

The compressed syngas reaches the synthesis loop where it is converted into methanol via the Casale plate-cooled converter (12), characterized by the highest conversion per pass and mechanical robustness. The heat of reaction is used to generate directly medium-pressure steam. The gas is cooled (13), and the raw methanol is condensed and separated (14), while the unreacted syngas is circulated back to the converter. The inerts (15) contained in the synthesis gas are purged from the loop, and the hydrogen contained is recovered in a hydrogen... 

Basically two different layouts of the ammonia synthesis loop exist, depending upon the quality of the make-up synthesis gas feeding the loop. In most cases the make-up gas also contains (apart from H2 and N2 in proper ratios) some inerts (CH4, Ar and minor traces of rare gases), which have to be purged from the loop to avoid build-up of inerts. (Inerts containing loop layout). This layout is primarily used in plants where the synthesis gas is generated from reformer based frond-ends with primary and secondary reforming. 

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. 

This excess hydrogen is normally carried forward to be compressed into the synthesis loop, from which it is ultimately purged as fuel. Addition of by-product CO2 where available may be advantageous in that it serves to adjust the reformed gas to a more stoichiometric composition gas for methanol production, which results in a decrease in natural gas consumption (8). Carbon-rich off-gases from other sources, such as acetylene units, can also be used to provide supplemental synthesis gas. Alternatively, the hydrogen-rich purge gas can be an attractive feedstock for ammonia production (9). 

PSA would be used as a front-end purifier for the synthesis of ammonia. The PSA system would produce, in a single purification step, an amnonia synthesis gas, free of the usual argon and methane inerts and, as a further advantage, would eliminate the need for the purge stream from the ammonia synthesis loop. 

Application To produce ammonia from natural gas, LNG, LPG or naphtha. Other hydrocarbons—coal, oil, residues or methanol purge gas— are possible feedstocks with an adapted front-end. The process uses conventional steam reforming synthesis gas generation (front-end) and a medium-pressure (MP) ammonia synthesis loop. It is optimized with respect to low energy consumption and maximum reliability. The largest single-train plant built by Uhde with a conventional synthesis has a nameplate capacity of 2,000 metric tons per day (mtpd). For higher capacities refer to Uhde Dual Pressure Process. 

A purge is taken from the synthesis loop to remove inerts (nitrogen, methane), as well as surplus hydrogen associated with non-stoichiometric operation. The purge is used as fuel for the reformer. Crude methanol from the separator contains water, as well as traces of ethanol and other... 

Inerts and excess nitrogen from the ammonia synthesis loop are removed by a purge from the circulator delivery and treated in a hydrogen recovery unit. Recovered hydrogen is recycled to the circulator suction. 

The synthesis loop consists of a recycle compressor, feed/effluent exchanger, methanol reactor, final cooler and crude methanol separator. Krupp Uhde s methanol reactor is an isothermal tubular reactor with a copper catalyst contained in vertical tubes and boiling water on the shell side. The heat of methanol reaction is removed by partial evaporation of the boiler feedwater, thus generating 1—1.4 tons of MP steam per ton of methanol. Advantages of this reactor type are low byproduct formation due to almost isothermal reaction conditions, high heat of reaction recovery, and easy temperature control by regulating steam pressure. Tb avoid inert buildup in the loop, a purge is withdrawn from the recycle gas and is used as fuel for the reformer. 

In all commercial plants ammonia is recovered from the synthesis loop by cooling the synthesis gas to condense the ammonia under synthesis pressure. The liquid ammonia product is separated from the gas, which is recycled. Arrangement and location of the ammonia separator(s), recirculation compression, addition of makeup gas and extraction of purge gas are discussed in Section 4.5.1 (see also Figure 77). 

Synthesis operates at 82 bar in a proprietary tubular converter loaded with a cobalt-enhanced formulation of the classical iron catalyst. Purge gas is recycled to the PSA unit, and pure COz is recovered from the PSA waste gas by an aMDEA wash. Very little steam (60 bar) is generated in the synthesis loop and from waste gases and some natural gas in the utility boiler in the utility section, and all drivers are electric. 

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