Patent Application
20170197894
In known methanol production plants and processes carbon oxides (CO and CO2) are removed from the synthesis section together with other gaseous components in a purge stream in order to obtain/maintain preferred reaction conditions in the plant.
In known plants and processes this step may be relatively expensive due to the means needed in order to separate H2 from the purge for recycling to the synthesis section.
Producing methanol theoretically requires a synthesis gas (syngas) with a module M equal to 2. The module M defined as
M=(H2−CO2)/(CO+CO2)
As syngas typically also contains inert compounds, the optimum module may become slightly higher than 2, typically 2.05, allowing purge of the inerts which inevitable also will result in purge of reactants, H2, CO and CO2.
For a syngas with a module less than the optimum module as defined above, surplus carbon oxides are present, and the module must be adjusted to the required level, e.g. by recovery of H2 from the purge stream and recycle of the recovered H2 the synthesis section. In known processes this is done by recovering H2 from the purge in a separation unit, e.g. a PSA unit or a membrane unit, which produces a H2-enriched gas for recycle and a H2-depleted waste gas.
Typically, part of the H2 and the main part of the carbon oxides are lost with the waste gas.
In some known processes, the purge requirements are dictated by the requirement to control the content of inert compounds (mainly CH4, N2 and Ar) in the reaction gas in the synthesis section. In other known processes, the purge requirements are dictated by the need to provide sufficient H2 for adjustment of the module in the synthesis section. In the latter case, it is desirous to increase the content of H2 available for recycle by increasing the amount of H2 obtained by the separation process, thus reducing the required purge and thereby increasing the production of methanol in the synthesis section.
In a first aspect of the present invention is provided a plant and process which reduce the purge, thereby increasing the production of e.g. methanol and reducing the requirements for the H2 separation means.
In a second aspect of the present invention is provided a plant and process which enable the use of a less costly H2 separation means using less energy.
In a third aspect of the present invention is provided a plant and process which increase the efficiency of converting a syngas to methanol as less reactants are lost.
In a fourth aspect of the present invention is provided a plant and process which allows use of a syngas feed with a module M lower than 2.
These and other advantages are achieved by a process for recovering H2 from the purge gas for recycle to the synthesis section with minimum loss of reactants from a production section, said process comprising the steps of
The H2 depleted gas can be let out of the separation step via a waste gas line and may for example be used as fuel in fired heaters in the process plant or e.g. for power production.
Thus the present invention provides a process which ensures an increased yield of H2 in the separation step/unit, thereby decreasing the required purge. In other words, the present process increases the production of methanol by reducing the loss of reactants. The applicant has shown that even though the introduction of a shift conversion section traditionally may be considered an expensive and thus undesirable step unlikely to be used in relation to e.g. purge gases, the result in form of an increased production in the present setup is significant and thus the overall result is desirable.
In preferred embodiments the purge stream is obtained from one or more processes including a methanol process, where the one or more processes are carried out based on a process stream such as syngas.
The purge stream may comprise CO, CO2, H2 and inerts. Said inerts can comprise but are not limited to CH4, Ar, N2, He.
If the synthesis section produces methanol the reactions preferably are:
CO+2H2->CH3OH and/or
CO2+2H2->CH3OH+H2O
In some embodiments the purge stream is passed to the shift conversion step essentially without pressure drop, i.e. at a pressure P1 between 30-200 atm, between 50-110 e.g. between 30-60, between 60-100, between 100-200 atm.
The stream is preheated preferably to a temperature between 100-400 C., 150-300, between 175-250 C., between 180-220C., steam can be added in an amount corresponding to a steam to dry gas ratio of 0.1-5, 0,3-2, 0,5-1.
In other embodiments, the pressure of the purge gas is reduced before the shift conversion to a pressure less than 100 atm, less than 50 atm, between 15 and 40 atm. The resulting stream is preheated and steam is added in an amount corresponding to a steam to dry gas ratio of 0.1-5, 0,3-2, 0,5-0.1. The mixed gas is passed to the shift conversion step at a temperature 100-400 C., 150-300, between 175 - 250 C such as between 180-220 C.
After the shift conversion step, the gas may be cooled to below its dew point where after condensate is separated, and the gaseous stream passed to a H2 separation unit such as a membrane unit or a PSA unit.
The H2 separation unit such as a membrane unit or a PSA unit produces a H2-enriched stream, which is returned to the synthesis process, and a H2-depleted waste gas.
The temperature in the shift conversion step is preferably adjusted to the preheat temperature before the addition of steam.
When H2 is returned to the feed mixing point the module M for the process, such a methanol production process, may be controlled thereby allowing an optimization of one or more of the processes in the production step/section.
Preferably the process stream is mixed with H2 to achieve a module around M=2. In case of a module larger than 2 the reaction section contains excess H2 and in case of a module lower than 2 the reaction section contains excess CO2. Known methanol processes requires a syngas feed with a module between 1.0 and 100, typically between 1.9 and 3.0.
The present invention may allow processing of feed, such as a synthesis gas, with a module of less than that of the module desired for the production such as a module less than 2 for a methanol production. Some feeds and reforming processes may provide a syngas with a low module M. As an example auto thermal reforming ATR, may result in such a low module syngas. As the present process enhance the amount of H2 recovered from the purge compared to traditional setups it may advantageously be applied in relation to such low module syngas. Also due to the enhanced H2 recovery less purge needs to be taken out which may result in an increase in production in the synthesis section. Especially in the case of methanol production the decreased purge and increased H2 recovery may be advantageous.
Thus the present process allows efficient methanol production from low module syngas such as syngas provided by ATR.
The invention further relates to a plant comprising
Thus the present invention provides a plant which by simple means allows for reduction of the loss of reactants carbon oxides and H2.
The condensate separation unit is arranged downstream the synthesis section and upstream the gas separation unit.
The condensate separation unit provides a condensate stream and a vapor stream, which vapor stream comprises H2.
The gas separation unit provides a H2 enriched stream and a H2 depleted stream. The H2 enriched stream is brought back to the synthesis section via a H2 line which H2 line in some setups may comprise a compressor. The at least part of the H2 enriched stream is added in the synthesis section at one or more mixing points.
The gas separation unit may for example comprise a membrane or be a PSA.
Preferably the plant is of the kind comprising units arranged to process a syngas stream in form of CO, CO2, H2 and inerts comprising but not limited to CH4, Ar, N2, He. Preferably the plant is arranged to process a syngas stream with a module M between 1.0 and 2.1. By the present method and plant it is possible to process a feed with a lower module than if known processes are used.
The synthesis section may comprise different units e.g. a syngas feed unit mixing syngas feed and recycled H2 containing gas, heating means, one or more reaction units converting syngas to Methanol as well as a cooling unit wherein the reaction product is condensed to liquid Methanol.
The plant comprises a H2 line arranged to bring H2 from the purge gas treatment section to one or more syngas feed mixing points in the synthesis section and/or upstream the synthesis section (e.g. a methanol production section). The H2 recovered from the purge gas by the present method can be used to adjust the module M of the process gas to a desired value.
The present process and plant thus provides a treated purge gas with increased H2 content compared to the untreated purge and reduced CO content (i.e. the H2 enriched stream). This means that the requirements to the means for isolating at least H2 from gas outlet/the purge section may be less strict when compared to known plants while still obtaining an increased H2 concentration in the H2 line.
The process as described herein is preferably carried out on a plant as also described. Arguments and described advantages of common features and corresponding steps/units in the process/plant respectively may apply to both step/unit where appropriate even if only described in relation to one of step/unit or plant part.
The present method and plant may be particularly useful where the feed provided to the synthesis section is a synthesis gas made by auto thermic reforming, ATR at a low Steam/Carbon ratio.
Thus by the present invention is provided a method and a plant to increase the yield of H2 recovered from the purge stream, thus reducing the required purge. The reduced purge results in an increased production of methanol in the synthesis section, reduced loss of valuable reactants, H2 and carbon oxides, and reduced cost and energy consumption of the means to separate H2 from the purge.
In the following the invention will be further described with reference to the accompanying drawing. The drawing is exemplary and is not to be construed as limiting for the invention.
In a plant designed and operating according to the invention as described above and exemplified the accompanying
When a similar unit designed without the steps of treating the purge gas is fed with the same synthesis gas as the unit designed and operating according to the invention, a total production of 987.4 MTPD of methanol is obtained. I.e. the present process of treating the purge gas including heating, adding steam, converting CO and steam to CO2 and H2, cooling and separating condensate, all before admitting the gas to the gas separation unit, leads to an increased methanol production.
| Number | Date | Country | Kind |
|---|---|---|---|
| PA 2014 00236 | Apr 2014 | DK | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2015/058936 | 4/24/2015 | WO | 00 |
