Co-production of methanol, ammonia, and urea

Abstract

The present invention relates to a continuous, once-through process for co-producing methanol and ammonia by reacting the ammonia with carbon dioxide recovered from the primary reformer exhaust gas and converting at least a portion of the ammonia, along with carbon dioxide separated from the reformed gas in a carbon dioxide removal step, to urea.

Description

[Technical Field]

[0001] The present invention relates to a process for the co-production of methanol, ammonia, and urea from a hydrocarbon feed, while reducing atmospheric carbon dioxide emissions and providing flexible control over the amounts of methanol, ammonia, and urea produced from the feed. More particularly, the present invention relates to a continuous, once-through (single-pass) process for the co-production of methanol and ammonia by the reaction of ammonia with carbon dioxide recovered from the primary reformer tail gas, along with carbon dioxide separated from the reformed gas in a carbon dioxide removal step, and for the conversion of at least a portion of the ammonia to urea. [Background technology]

[0002] Current methods for the co-production of methanol and ammonia generally involve parallel processes, using a common reformer to produce synthesis gas that is then split in parallel, one for methanol synthesis and the other for ammonia synthesis. Alternatively, the co-production of methanol and ammonia can be carried out sequentially or in series, where the synthesis gas produced in the reformer is first converted to methanol and then the unreacted gases containing nitrogen and hydrogen are used for ammonia synthesis.

[0003] In a first aspect, the present invention provides a process for the in-line co-production of methanol, ammonia, and urea, which allows for flexible control of the amounts of methanol, ammonia, and urea produced from a given amount of hydrocarbon, while minimizing carbon dioxide emissions to the atmosphere. Summary of the Invention [Problem to be solved by the invention]

[0004] The co-production process produces methanol and ammonia, which can be used, along with CO2, to further produce urea. (Since methanol is produced from carbon oxides and hydrogen,) CO2 can be extracted from the co-production process, reducing the amount of methanol produced. It was found that additional CO2 capture on the exhaust gas side, in order to match production needs, can meet CO2 requirements and reduce CO2 emissions. This allows the process to be controlled to match the demand for methanol and urea (ammonia). [Means for solving the problem]

[0005] Accordingly, the present invention provides a process for the co-production of methanol, ammonia and urea from a hydrocarbon feedstock, comprising: a) primary and secondary steam reforming of a hydrocarbon feedstock to obtain a steam reforming effluent comprising hydrogen, nitrogen, carbon monoxide and carbon dioxide; b) passing a portion of the steam reforming effluent from step (a) through a carbon dioxide removal stage to produce an effluent having a reduced carbon dioxide content; c) bypassing a remaining portion of the steam reforming effluent to a carbon dioxide removal stage and combining the effluent removed from step (b) with the bypassed portion of the steam reforming effluent to provide a methanol synthesis gas comprising hydrogen, nitrogen, carbon monoxide and carbon dioxide; d) adding hydrogen recovered from a downstream ammonia synthesis stage to the methanol synthesis gas obtained in step (c); e) catalytically converting the methanol synthesis gas in a once-through methanol synthesis step to obtain a liquid effluent containing methanol and a gas effluent containing nitrogen and hydrogen; and, e) catalytically converting the gas effluent removed in step (e) into ammonia in an ammonia synthesis stage; and f) converting at least a portion of the ammonia from step (e) into urea by reaction of the carbon dioxide removed in step (b) with carbon dioxide contained in the tail gas recovered from the primary steam reforming of step (a). DETAILED DESCRIPTION OF THE INVENTION

[0006] As used herein, the term "primary reforming" refers to reforming carried out in a conventional steam methane reformer (SMR), i.e., a tubular reformer, with the heat required for endothermic reforming coming from radiant heat from burners, e.g., burners located along the wall of the tubular reformer.

[0007] As used herein, the term "secondary reforming" means reforming carried out in an autothermal reformer or catalytic partial oxidation reactor using air or oxygen-enriched air.

[0008] In the process of the present invention, the amount of methanol produced is controlled by the amount of carbon dioxide bypassed from the carbon dioxide removal stage. Increasing the amount of carbon dioxide in the methanol synthesis gas containing bypassed carbon dioxide increases methanol production, and vice versa.

[0009] To provide the amount of hydrogen required for adding carbon dioxide to the methanol synthesis gas, hydrogen recovered from the ammonia synthesis stage must be added to the synthesis gas, preferably in an amount such that the module M=(H2-CO2) / (CO+CO2) is at least 2.5, e.g., 2.5-10.

[0010] Recovering hydrogen from ammonia synthesis minimizes the size of the primary reformer, with the added benefit of improved utilization of the carbon dioxide in the exhaust gas from the reformer burner because less heat is required in the minimized reformer.

[0011] In one embodiment, the amount of hydrogen in the reformed effluent can be further adjusted by the water gas shift reaction.

[0012] Preferably, the amount of hydrogen added to the methanol synthesis gas in step (d) is adjusted so that the module M is at least 2.5, for example between 2.5 and 10.

[0013] In the present invention, the carbon dioxide generated in the burner is advantageously utilized in the preparation of urea, reducing the carbon dioxide footprint of the process.

[0014] The amount of carbon dioxide recovered from the burner exhaust gas and carbon dioxide removal stage is adjusted to the desired urea production.

[0015] The above measures allow for flexible production of methanol, ammonia and urea according to the actual demand of producers.

[0016] The process of the present invention directly utilizes the reactions responsible for reforming, methanol synthesis, and ammonia synthesis, and therefore can co-produce methanol and ammonia without emitting large amounts of carbon dioxide captured from synthesis gas. Carbon oxides emitted from the process can be fully utilized to produce methanol and urea.

[0017] Removal of some of the carbon dioxide contained in the steam reforming effluent is typically achieved by a very expensive carbon dioxide removal step in the form of acid gas scrubbing, for example, conventional MDEA and carbonate scrubbing processes.

[0018] Thus, a further advantage of the present invention is that it reduces the amount of carbon dioxide that must be removed when bypassing a portion of the steam reforming effluent to the removal stage.

[0019] The process can include further parallel methanol processes, i.e., one or more additional methanol processes can operate in parallel in the methanol synthesis step of the process of the invention. One, two, three or more parallel methanol processes can also be interconnected by one or more synthesis gas lines.

[0020] Thus, in an embodiment of the present invention, the once-through methanol synthesis step is carried out in parallel methanol production lines.

[0021] As used herein, the term "once-through methanol synthesis step" means that methanol is produced in at least one catalytic reactor operating in a single-pass configuration, i.e., without significant recycle (not more than 5%, i.e., less than, often 0%) of any volumetric stream of gas produced in the methanol synthesis back to at least one methanol reactor of the methanol synthesis stage, particularly of gas effluents containing hydrogen and unconverted carbon oxides.

[0022] The method of the present invention is environmentally friendly because the CO2 recovered from the methanol and ammonia synthesis gas is not discharged into the environment. Substantially all of the carbon monoxide (and carbon dioxide) produced by this method is used in the synthesis of methanol and urea.

[0023] The methanol synthesis step is carried out by conventional means by passing the synthesis gas through at least one methanol reactor containing at least one fixed bed of methanol catalyst at elevated pressure and temperature, preferably between 60 and 150 bar, preferably between 120 bar and 150 and 300° C. A particularly preferred methanol reactor is a fixed bed reactor, such as a boiling water reactor (BWR), cooled by a suitable coolant, e.g., boiling water.

[0024] In a specific embodiment, the methanol synthesis stage of step (e) is carried out by passing the synthesis gas through one boiling water reactor followed by an adiabatic fixed bed reactor, or by passing the synthesis gas through a series of boiling water reactors followed by an adiabatic fixed bed reactor.

[0025] Because the methanol synthesis stage is once-through, there is no need to recycle a portion of the overhead fraction from the separator of the adiabatic fixed bed reactor back to the first methanol reactor of methanol synthesis stage 7.

[0026] If the amount of carbon monoxide in the gas effluent from the methanol synthesis stage in step (e) exceeds the amount acceptable for use in the ammonia synthesis stage, the effluent is passed through a methanation stage to remove the carbon monoxide by reaction to methane.

[0027] Thus, in an embodiment of the invention, the method further comprises the step of subjecting the gas effluent from step (d) to a methanation reaction upstream of step (e).

[0028] In step (e), the ammonia synthesis gas from the methanation step, containing an appropriate ratio of hydrogen and nitrogen (preferably a 3:1 H:N molar ratio), is optionally passed through a compressor to obtain the required ammonia synthesis pressure, e.g., 120-200 bar, preferably about 130 bar. Ammonia is then produced in a conventional manner in an ammonia synthesis loop. The ammonia-containing effluent also contains hydrogen, nitrogen, and inerts, such as methane and argon. Ammonia can be recovered from the ammonia-containing effluent as liquid ammonia by condensation and subsequent separation. Preferably, an off-gas stream containing hydrogen, nitrogen, and methane is removed from the ammonia synthesis stage, as well as a hydrogen-rich stream (>90 vol% H). These streams originate, for example, from a purge gas recovery unit. This hydrogen stream is added to the methanol synthesis stage, for example, by combining it with the methanol synthesis gas. By recycling this hydrogen-rich stream, valuable hydrogen can be utilized for methanol synthesis and subsequent ammonia synthesis, rather than simply being used as fuel, thereby increasing the efficiency of the process. The present invention includes the following items. [Item 1] 1. A process for co-producing methanol, ammonia and urea from a hydrocarbon feedstock, comprising: a) primary and secondary steam reforming of a hydrocarbon feedstock to obtain a steam reforming effluent comprising hydrogen, nitrogen, carbon monoxide and carbon dioxide; b) passing a portion of the steam reforming effluent from step (a) through a carbon dioxide removal stage to produce an effluent having a reduced carbon dioxide content; c) bypassing a remaining portion of the steam reforming effluent to a carbon dioxide removal stage and combining the effluent removed from step (b) with the bypassed portion of the steam reforming effluent to provide a methanol synthesis gas comprising hydrogen, nitrogen, carbon monoxide and carbon dioxide; d) adding hydrogen recovered from a downstream ammonia synthesis stage to the methanol synthesis gas obtained in step (c); e) catalytically converting the methanol synthesis gas in a once-through methanol synthesis step to obtain a liquid effluent containing methanol and a gas effluent containing nitrogen and hydrogen; and f) catalytically converting the gas effluent removed in step (e) into ammonia in an ammonia synthesis stage; and g) converting at least a portion of the ammonia from step (e) into urea by reaction of the carbon dioxide removed in step (b) with carbon dioxide contained in the exhaust gas recovered from the primary steam reforming of step (a). [Item 2] 2. The method of claim 1, further comprising subjecting the steam reforming effluent of step (a) to a water gas shift reaction. [Item 3] 3. The method according to item 1 or 2, further comprising a step of subjecting the gas effluent from step (d) to a methanation reaction upstream of step (e). [Item 4] 4. The method according to any one of items 1 to 3, wherein the amount of hydrogen added to the methanol synthesis gas in step (d) is adjusted to provide a module M of at least 2.5, for example, from 2.5 to 10. [Item 5] 5. The method according to any one of items 1 to 4, wherein the once-through methanol synthesis step is carried out in a parallel methanol production line.

Claims

1. 1. A process for co-producing methanol, ammonia and urea from a hydrocarbon feedstock, comprising: a) primary and secondary steam reforming of a hydrocarbon feedstock to obtain a steam reforming effluent comprising hydrogen, nitrogen, carbon monoxide and carbon dioxide; b) passing a portion of the steam reforming effluent from step (a) through a carbon dioxide removal stage to produce an effluent having a reduced carbon dioxide content; c) bypassing a remaining portion of the steam reforming effluent to a carbon dioxide removal stage and combining the effluent removed from step (b) with the bypassed portion of the steam reforming effluent to provide a methanol synthesis gas comprising hydrogen, nitrogen, carbon monoxide and carbon dioxide; d) adding hydrogen recovered from a downstream ammonia synthesis stage to the methanol synthesis gas obtained in step (c); e) catalytically converting the methanol synthesis gas in a once-through methanol synthesis step to obtain a liquid effluent containing methanol and a gas effluent containing nitrogen and hydrogen; and subjecting the gas effluent from step (e) to a methanation reaction upstream of step (f); f) catalytically converting the gas effluent removed in step (e) into ammonia in an ammonia synthesis stage; and g) converting at least a portion of the ammonia from step (f) into urea by reaction of the carbon dioxide removed in step (b) with carbon dioxide contained in the tail gas recovered from the primary steam reforming of step (a); wherein the ammonia synthesis gas from the methanation reaction step in step (e) is passed through a compressor to obtain the required ammonia synthesis pressure of 120-200 bar; The method.

2. The method of claim 1 further comprising subjecting the steam reforming effluent of step (a) to a water gas shift reaction.

3. 3. The method of claim 1 or 2, wherein the amount of hydrogen added to the methanol synthesis gas in step (d) is adjusted to provide a module M of at least 2.

5.

4. 4. The method according to claim 1, wherein the once-through methanol synthesis step is carried out in a parallel methanol production line.