Process for the co-production of methanol and ammonia

Abstract

A process for the combined preparation of methanol and ammonia based on primary steam reforming a hydrocarbon feed stock and adiabatic secondary reforming with oxygen enriched air from electrolysis of water.

Claims

1. Process for the co-production of methanol and ammonia comprising the steps of: (a) providing a hydrocarbon feed stock; (b) preparing a separate hydrogen stream and a separate oxygen stream by electrolysis of water; (c) primary steam reforming the hydrocarbon feed stock provided in step (a) to a primary steam reformed gas; (d) providing process air for use in a secondary reforming step by enriching atmospheric air with the separate oxygen stream from step (b); (e) secondary reforming the primary steam reformed gas from step (c) with the oxygen enriched air to a process gas stream comprising hydrogen, nitrogen, carbon oxides; (f) introducing at least part of the separate hydrogen stream from step (b) into the process gas stream obtained in step (e), or optionally into the process gas stream after a shift and/or carbon dioxide removal step; (g) catalytically converting the carbon oxides and a part of the hydrogen contained in the process gas stream in a once-through methanol synthesis stage and withdrawing an effluent containing methanol and a gas effluent containing un-converted carbon oxides, hydrogen and nitrogen, wherein the hydrogen prepared by electrolysis of water is introduced into the process gas stream in step (f), prior to the methanol synthesis, by introducing the hydrogen into a suction section of a process gas compressor in an amount to adjust a hydrogen/nitrogen molar ratio in the gas effluent to a stoichiometric ratio of 2.7-3.3 required for production of ammonia; (h) purifying the gas effluent from step (g) and obtaining an ammonia synthesis gas containing hydrogen and nitrogen; and (i) catalytically converting the nitrogen and the hydrogen of the ammonia synthesis gas in an ammonia synthesis stage and withdrawing an effluent containing ammonia.

2. Process of claim 1, wherein at least a part of the process gas stream from step (e) is subjected one or more water gas shift reactions.

3. Process of claim 1, wherein at least a part of the process gas stream from step (e) is subjected to carbon dioxide removal.

4. Process of claim 1, wherein the purifying of the gas effluent in step (h) comprises methanation.

5. Process of claim 1, wherein the purifying of the gas effluent in step (h) comprises a cryogenic process.

6. Process of claim 1, wherein the electrolysis of water in step (b) is powered by renewable energy.

Description

(1) The present invention is based on a combination of primary and secondary steam reforming using oxygen from the electrolysis of water in the partial oxidation of hydrocarbon feed stock in the secondary steam reforming process. Hydrogen from the electrolysis is used to adjust the hydrogen/nitrogen molar ratio in the effluent gas from the methanol synthesis to provide an ammonia synthesis gas approximately to the stoichiometric ratio required for the production of ammonia, as well as additional synthesis gas production.

(2) Compared to prior art methods using electrolysis of water for hydrogen production and air separation for nitrogen production, the oxygen product from electrolysis of water is advantageously used for partial oxidation in the secondary reformer so that the costly and energy intensive ASU is avoided in the method according to the invention.

(3) Thus, this invention is a process for the co-production of methanol and ammonia comprising the steps of

(4) (a) providing a hydrocarbon feed stock;

(5) (b) preparing a separate hydrogen stream and a separate oxygen stream by electrolysis of water;

(6) (c) primary steam reforming the hydrocarbon feed stock provided in step (a) to a primary steam reformed gas;

(7) (d) providing process air for use in a secondary reforming step by enriching atmospheric air with the separate oxygen stream from step (b);

(8) (e) secondary reforming the primary steam reformed gas from step (c) with the oxygen enriched air to a process gas stream comprising hydrogen, nitrogen, carbon oxides;

(9) (f) introducing at least part of the separate hydrogen stream from step (b) into the process gas stream obtained in step (e) or optionally into the process gas stream after a shift and/or carbon dioxide removal step;

(10) (g) catalytically converting the carbon oxides and a part of the hydrogen contained in the process gas stream in a once-through methanol synthesis stage and withdrawing an effluent containing methanol and a gas effluent containing un-converted carbon oxides, hydrogen and nitrogen;
(h) purifying the gas effluent from step (g) and obtaining an ammonia synthesis gas containing hydrogen and nitrogen; and
(i) catalytically converting the nitrogen and the hydrogen of the ammonia synthesis gas in an ammonia synthesis stage and withdrawing an effluent containing ammonia.

(11) The methanol synthesis in the absence of carbon dioxide is governed by the reaction CO+2H.sub.2CH.sub.3OH. In the presence of carbon dioxide, methanol is otherwise also generated according to the reaction CO.sub.2+3 H.sub.2CH.sub.3OH+H.sub.2O. As apparent from the latter methanol synthesis reaction a lower molar ratio of CO/CO.sub.2 in the synthesis gas for the methanol synthesis requires a larger amount of hydrogen.

(12) Thus, in an embodiment of the invention the amount of hydrogen in the process gas is increased by subjecting at least a part of the process gas stream obtained in step (e) to one or more water gas shift reactions, wherein carbon monoxide is reacted to carbon dioxide and hydrogen according the reaction:
CO+H.sub.2O CO.sub.2+H.sub.2

(13) Ideally, the process gas for the synthesis methanol is a gas containing the highest possible molar ratio of CO/CO.sub.2.

(14) Thus, in further an embodiment of the invention at least a part of the carbon dioxide is removed from the process gas stream obtained in step (e) or the water gas shifted process gas stream.

(15) Removal of carbon dioxide can be performed by a physical or chemical method known in the art.

(16) The methanol synthesis stage is preferably conducted by conventional means by passing the process gas at high pressure and temperatures, such as 60-150 bars and 150-300 C. through at least one methanol reactor containing at least one fixed bed of methanol catalyst. A particularly preferred methanol reactor is a fixed bed reactor cooled by a suitable cooling agent such as boiling water, e.g. boiling water reactor (BWR).

(17) To provide the required methanol synthesis pressure, the process gas is compressed by means a compressor arranged in front of the at least one methanol reactor.

(18) Accordingly, the invention enables the operation of the methanol and ammonia synthesis section at similar operating pressures, for instance 130 bars, which implies that the process gas needs only be compressed to synthesis pressure upstream the methanol synthesis step and no further compression is necessary after the methanol synthesis. The hydrogen gas stream from the water electrolysis is introduced into the suction section of a process gas compressor in front of a methanol reactor in an amount to provide a molar ratio of the hydrogen to the nitrogen of 2.7-3.3 in the gaseous effluent from the methanol synthesis.

(19) Prior to the gaseous effluent is passed into the ammonia synthesis loop, the gaseous effluent is preferably purified by removing remaining amounts of carbon monoxide and carbon dioxide, preferably by methanation according to the reactions:
CO+3H.sub.2CH.sub.4+H.sub.2O; and
CO.sub.2+4H.sub.2CH.sub.4+2H.sub.2O

(20) The purifying step can also be based on cryogenic methods, like the so-called coldbox process, which also can be used for adjustment of the N.sub.2/H.sub.2 molar ratio by removing excess of N.sub.2.

(21) The advantages of the process according to the invention are essentially less consumption of hydrocarbon feed stock (natural gas) and process air and less emission of CO2 in flue gas from the firing of the primary steam reformer at same production rate of methanol and higher production rate of ammonia compared with the conventional process without electrolysis as summarized in the Comparison Table below.

(22) TABLE-US-00001 Comparison Table Natural Natural Technology gas feed gas fuel Air CH.sub.3OH NH.sub.3 Power for CO.sub.2 in for syngas consumption consumption consumption production production electrolysis flue gas preparation (Nm.sup.3/h) (Nm.sup.3/h) (Nm.sup.3h) (MTPD) (MTPD) (MW) (Nm.sup.3/h) Prior art 45330 15392 22387 1350 415 0 23104 According to 43607 11531 20328 1350 459 74 17348 the invention