The methanol production process generally involves directing a compressed synthesis gas comprising hydrogen, carbon monoxide and carbon dioxide at an elevated temperature and pressure to a methanol converter reactor containing one or more beds of a methanol synthesis catalyst such as a copper and zinc oxide catalyst. The carbon monoxide and carbon dioxide in the synthesis gas react with the hydrogen to form methanol across the catalyst. The methanol synthesis process is usually operated in a loop where a portion of the compressed synthesis gas is converted to methanol each pass through the methanol converter reactor. Most of the unconverted gas is recycled to the methanol converter. A small portion is purged to prevent the buildup of inerts such as nitrogen, argon and methane. Methanol product is recovered by cooling the methanol product gas stream to a temperature below the dew point of the methanol such that a product composition comprising crude methanol and water condenses out, with the remaining gas being recycled through the methanol converter reactor. The crude methanol and water produced in the methanol converter reactor are typically reduced in pressure in a let-down or “flash” vessel. Since most crude methanol contains a range of impurities, including higher alcohols, the crude methanol must be purified so as to remove such impurities to produce methanol of chemical grade quality. The preferred technique used for methanol purification is a distillation process.
Synthesis gas used for methanol synthesis is typically characterized by the stoichiometric ratio (H2−CO2)/(CO+CO2), often referred to as the module or stoichiometric number, wherein H2, CO2 and CO denote the mole fractions of hydrogen, carbon dioxide and carbon monoxide, respectively, in the synthesis gas. A module of about 2.0 defines the desired stoichiometric ratio of synthesis gas for the production of methanol. Other important properties of the synthesis gas in methanol production include the carbon monoxide to carbon dioxide ratio and the concentration of inerts in the synthesis gas. A high carbon monoxide to carbon dioxide ratio typically increases the reaction rate of the formation of methanol and the achievable per pass conversion while it concurrently decreases the formation of water thereby reducing the catalyst deactivation rate. A high concentration of inerts in the synthesis gas, such as methane, argon, nitrogen, etc. typically lowers the partial pressure of the active reactants. Since the methanol conversion reaction is exothermic, lower temperatures favor conversion of the synthesis gas to methanol. Pressure will also affect the methanol conversion reaction, with increasing pressure also favoring methanol formation.
In many methanol production facilities, the incoming compressed synthesis gas is often mixed with recycled unreacted gas stream to form the synthesis gas stream that is supplied to the methanol converter reactor. A portion of the unreacted gas stream may be purged to prevent the buildup of inerts in the methanol converter reactor. The amount of purge flow typically varies anywhere from 1% to 10% of the total unreacted gas stream and often depends on the amount of inerts in the incoming synthesis gas, with higher level of inerts generally requiring higher purge flows and lower level of inerts generally requiring lower purge flows.
Some of the prior art uses of the purge stream include use of the hydrogen and/or methane slip in the purge stream as a feed or source of fuel to be used in the front-end steam methane reforming (SMR), partial oxidation (POx), autothermal reforming (ATR) processes. Other prior art has suggested the recovery of hydrogen from the purge stream and mixing the recovered hydrogen with the synthesis gas to improve the module of synthesis gas for methanol production.
As used herein, steam methane reforming (SMR) is a catalytic conversion of natural gas, including methane and light hydrocarbons, to synthesis gas containing hydrogen and carbon monoxide by reaction with steam. The reactions are endothermic, requiring significant amount of energy input. The steam methane reforming process is carried out at high temperatures within catalyst filled tubes inside a fired furnace. The amount of steam used is in excess of the reaction stoichiometry requirements, as required to prevent the catalyst from coking. No oxygen is used in steam methane reforming.
Partial oxidation, on the other hand, is a non-catalytic process where a sub-stoichiometric amount of oxygen is allowed to react with the natural gas creating steam and carbon dioxide at high temperatures. The residual methane is reformed through reactions with the high temperature steam and carbon dioxide to produce synthesis gas. In principle, the partial oxidation reaction can be carried out without any steam addition. Autothermal reforming is a variant of the partial oxidation process, but which uses a catalyst to permit reforming to occur at lower temperatures than the partial oxidation process. Moderate amounts of steam are typically required to prevent the catalyst from coking.
Many synthesis gas generation methods also employ pre-reforming and secondary reforming. When the feedstock contains significant amounts of heavy hydrocarbons, SMR and ATR processes are typically preceded by a pre-reforming step. As generally known in the art, pre-reforming is a catalyst based process for converting higher hydrocarbons to methane, hydrogen, carbon monoxide and carbon dioxide. The reactions involved in pre-reforming are typically endothermic. Most pre-reformers operate adiabatically, and thus the pre-reformed feedstock typically leaves at a lower temperature than the feedstock entering the pre-reformer. A secondary reforming process conventionally refers to an autothermal reforming process that is fed product from a SMR process. Thus, the teed to a secondary reforming process is primarily synthesis gas from the SMR. Depending on the end application, some natural gas may bypass the SMR process and be directly introduced into the secondary reforming process. Also, when a SMR process is followed by a secondary reforming process, the SMR may operate at a lower temperature, e.g. 650° C. to 800° C. versus 850° C. to 950° C.