Co-production of methanol, ammonia and urea

Abstract

A process and plant for the co-production of methanol and ammonia together with urea production from a hydrocarbon feed without venting to the atmosphere carbon dioxide captured from the methanol or ammonia synthesis gas and without using expensive air separation units and water gas shift. Carbon dioxide is removed from flue gas from reforming section and used to convert partially or fully all ammonia into urea.

Claims

1. A plant for co-producing methanol, ammonia and urea from a hydrocarbon feedstock, comprising: a first methanol process line for carrying out a first reforming step and a first methanol conversion step to obtain a first effluent comprising methanol and a first gas effluent comprising hydrogen, nitrogen and unconverted carbon oxides; a second methanol process line for carrying out a second reforming step and a second methanol conversion step to obtain a second effluent comprising methanol and a second gas effluent comprising hydrogen, nitrogen and unconverted carbon oxides; an interconnecting synthesis line allowing fluid communication between the first and second methanol process lines, allowing a methanol synthesis gas from each of the two first and second reforming steps to be distributed to each of the first and second methanol conversion steps; a catalytic methanation stage for producing ammonia synthesis gas from the first gas effluent and the second gas effluent, the ammonia synthesis gas having a H.sub.2:N.sub.2 molar ratio of approximately 3:1; an ammonia synthesis stage for catalytically converting the nitrogen and hydrogen of the ammonia synthesis gas to produce an effluent comprising ammonia and a purge-gas stream comprising hydrogen, nitrogen and/or methane; and a urea plant for reacting at least part of the effluent comprising ammonia with at least part of a flue gas comprising CO.sub.2 from at least one of the first and second reforming steps to produce urea; wherein the first reforming step of the first methanol process line and the second reforming step of the second methanol process line are each carried out in a respective primary reforming stage and a respective secondary reforming stage, wherein the hydrocarbon feedstock is steam reformed in the respective primary reforming stage to obtain a partially reformed gas and the partially reformed gas is then further reformed under addition of air in the respective secondary reforming stage to obtain the methanol synthesis gas.

2. The plant according to claim 1, wherein the first methanol process line and the second methanol process line each further comprise a respective methanol synthesis stage for catalytically converting carbon oxides and hydrogen of the methanol synthesis gas to produce the first and second effluents comprising, respectively, methanol and a gas effluent comprising nitrogen, hydrogen and unconverted carbon oxides.

3. The plant according to claim 1, wherein the secondary reforming stage is an air-blown secondary reforming stage.

4. The plant according to claim 2, wherein the methanol synthesis stage of each of the first and second methanol process lines comprises one or more boiling water reactors for cooling the methanol synthesis gas produced by the first reforming step and the second methanol process line.

5. The plant according to claim 4, wherein the methanol synthesis stage of each of the first and second methanol process lines comprises one or more adiabatic fixed bed reactors or one or more gas cooled reactors, wherein the methanol synthesis gas is passed through the one or more boiling water reactors and subsequently through the one or more adiabatic fixed bed reactors or the one or more gas cooled reactors.

6. The plant according to claim 4, wherein the one or more boiling water reactors are each a single condensing-methanol reactor which comprises within a common shell a fixed bed of methanol catalyst particles and cooling means adapted to indirectly cooling the methanol synthesis gas with a cooling agent.

7. The plant according to claim 1, further comprising a formaldehyde production section.

8. The plant according to claim 1, further comprising a section for producing a concentrate comprising 85% formaldehyde and urea, and 15% water.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 shows a simplified block diagram of the process according to a specific embodiment of the invention including reforming, methanol synthesis stage, methanation stage and ammonia synthesis stage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(2) In first reforming section, natural gas 1 is added to primary reforming stage 20 (steam methane reformer) under addition of steam 2. The partly reformed gas is then further reformed in air-blown secondary reforming stage 21 (autothermal reformer) under addition of air 3. The methanol synthesis gas 4 containing hydrogen, carbon oxides and nitrogen is cooled in waste heat boiler(s) under the production of steam and then compressed to methanol synthesis pressure (not shown). In first methanol synthesis stage 22, the methanol synthesis gas 4 is converted in once-through operation (single-pass operation, no recirculation) under the production of a liquid effluent 5 containing methanol and a gas effluent 6 containing nitrogen, hydrogen and unconverted carbon oxides. Approximately 80 wt % of the total plant capacity goes to the production of methanol of effluent 5.

(3) In second reforming section, natural gas 1B is added to primary reforming stage 20B (steam methane reformer) under addition of steam 2B. The partly reformed gas is then further reformed in air-blown secondary reforming stage 21B (autothermal reformer) under addition of air 3B. The methanol synthesis gas 4B containing hydrogen, carbon oxides and nitrogen is cooled in waste heat boiler(s) under the production of steam and then compressed to methanol synthesis pressure (not shown). In second methanol synthesis stage 22B the methanol synthesis gas 4B is converted in once-through operation (single-pass operation, no recirculation) under the production of a liquid effluent 5B containing methanol and a gas effluent 6B containing nitrogen, hydrogen and unconverted carbon oxides. Approximately 80 wt % of the total plant capacity goes to the production of methanol of effluent 5B.

(4) The carbon oxides in gas effluents 6 and 6B are hydrogenated to methane in the common methanation stage 23 thereby generating an ammonia synthesis gas 7 having a H.sub.2:N.sub.2 molar ratio of 3:1. The ammonia synthesis gas 7 is then passed through ammonia synthesis stage 24 under the production of an effluent 8 containing ammonia and an effluent stream 9 containing hydrogen, methane and nitrogen which is treated to give two effluent streams.

(5) First effluent stream 11 is returned as off-gas fuel to the primary reforming stage 20. Second effluent stream 10, a hydrogen-rich stream (>90 vol % H.sub.2) being returned to the methanol synthesis stage 22 by combining with the methanol synthesis stream 4. Approximately 20 wt % of the total plant capacity goes to the production of ammonia in effluent 8. The plant obviates the use of Air Separation Units (ASU) as well as water gas shift and CO.sub.2-removal stages.

(6) The following table shows the temperatures, pressures and flow rates of the different streams for a process according to FIG. 1 where we are able to produce approximately 3000 MTPD methanol and 750 MTPD ammonia despite the use of a difficult feedstock. The feedstock used is heavy natural gas (85 vol % methane):

(7) TABLE-US-00001 TABLE Pres- Posi- Temp. sure Flow rate/kmol/h tion ° C. Bar g H.sub.2O H.sub.2 N.sub.2 CH.sub.4 CO CO.sub.2 Ar 4, 4B 947 30.1 5890 12023 1414 419 3147 1043 16 6, 6B 35 120.3 2.7 4574 1457 463 17 38 20 7 35 119.3 8742 2914 1036 40 10, 10B 35 32 1463 66 61 4 11, 11B 35 12 167 477 450 16

(8) The process air compressor 25 can be one common or one for each reforming section.

(9) Stream 4/4B can be distributed to the methanol processes 22/22B depending on individual catalyst activity in order to optimize OPEX via the line 12.

(10) Hydrogen rich stream 10/10B and off-gas 11/11B can be distributed to optimize the process requirements in reforming sections and methanol synthesis sections.

(11) By the nature of the co-production of methanol and ammonia from natural gas, there will not be significant amount of CO.sub.2 available in the process gas to be recovered for urea production. If it is desired to use part or all the ammonia production in the presented process, then CO.sub.2 from the flue gas from one or more reforming sections can be recovered in a CO.sub.2 removal section 27.

(12) There would typically be sufficient of CO.sub.2 in the flue gas from one reforming section to convert all the ammonia from the combined methanol section effluent gas into urea. Thereby an economy of scale can be achieved for the urea plant, section 29.

(13) A widely used coating material in urea production is urea-formaldehyde-concentrate (UFC85) containing up to 85% urea+formaldehyde and balance is water, in order to avoid caking of urea product. The synergy for production of the coating material in this invention is clear since a small stream of methanol 16 is used to produce formaldehyde and urea solution is required for the absorption of formaldehyde to produce UFC85 or more diluted UFC products in section 28.