GB2039893A - Thermally integrated ammonia/urea process - Google Patents
Thermally integrated ammonia/urea process Download PDFInfo
- Publication number
- GB2039893A GB2039893A GB7941931A GB7941931A GB2039893A GB 2039893 A GB2039893 A GB 2039893A GB 7941931 A GB7941931 A GB 7941931A GB 7941931 A GB7941931 A GB 7941931A GB 2039893 A GB2039893 A GB 2039893A
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- GB
- United Kingdom
- Prior art keywords
- ammonia
- urea
- synthesis gas
- crude
- ammonia synthesis
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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- QGZKDVFQNNGYKY-UHFFFAOYSA-N Ammonia Chemical compound N QGZKDVFQNNGYKY-UHFFFAOYSA-N 0.000 title claims abstract description 265
- 229910021529 ammonia Inorganic materials 0.000 title claims abstract description 125
- 239000004202 carbamide Substances 0.000 title claims abstract description 105
- XSQUKJJJFZCRTK-UHFFFAOYSA-N Urea Chemical compound NC(N)=O XSQUKJJJFZCRTK-UHFFFAOYSA-N 0.000 title claims abstract description 104
- 238000000034 method Methods 0.000 title claims abstract description 55
- 230000008569 process Effects 0.000 title abstract description 37
- 238000003786 synthesis reaction Methods 0.000 claims abstract description 110
- 230000015572 biosynthetic process Effects 0.000 claims abstract description 107
- 238000004519 manufacturing process Methods 0.000 claims abstract description 73
- 239000002699 waste material Substances 0.000 claims abstract description 9
- 239000007789 gas Substances 0.000 claims description 83
- CURLTUGMZLYLDI-UHFFFAOYSA-N Carbon dioxide Chemical compound O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 claims description 82
- 229910002092 carbon dioxide Inorganic materials 0.000 claims description 41
- 239000002918 waste heat Substances 0.000 claims description 32
- 238000006243 chemical reaction Methods 0.000 claims description 28
- BVCZEBOGSOYJJT-UHFFFAOYSA-N ammonium carbamate Chemical compound [NH4+].NC([O-])=O BVCZEBOGSOYJJT-UHFFFAOYSA-N 0.000 claims description 19
- KXDHJXZQYSOELW-UHFFFAOYSA-N carbonic acid monoamide Natural products NC(O)=O KXDHJXZQYSOELW-UHFFFAOYSA-N 0.000 claims description 19
- 238000010521 absorption reaction Methods 0.000 claims description 16
- 239000001569 carbon dioxide Substances 0.000 claims description 12
- 238000001035 drying Methods 0.000 claims description 12
- 238000000746 purification Methods 0.000 claims description 12
- LCGLNKUTAGEVQW-UHFFFAOYSA-N Dimethyl ether Chemical compound COC LCGLNKUTAGEVQW-UHFFFAOYSA-N 0.000 claims description 10
- 238000010438 heat treatment Methods 0.000 claims description 9
- 238000000629 steam reforming Methods 0.000 claims description 8
- 239000000463 material Substances 0.000 claims description 7
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims description 7
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims description 6
- 229910052799 carbon Inorganic materials 0.000 claims description 6
- 238000001816 cooling Methods 0.000 claims description 6
- 230000003647 oxidation Effects 0.000 claims description 6
- 238000007254 oxidation reaction Methods 0.000 claims description 6
- 238000011084 recovery Methods 0.000 claims description 6
- UGFAIRIUMAVXCW-UHFFFAOYSA-N Carbon monoxide Chemical compound [O+]#[C-] UGFAIRIUMAVXCW-UHFFFAOYSA-N 0.000 claims description 5
- 239000002202 Polyethylene glycol Substances 0.000 claims description 5
- 229910002091 carbon monoxide Inorganic materials 0.000 claims description 5
- 229920001223 polyethylene glycol Polymers 0.000 claims description 5
- 230000002194 synthesizing effect Effects 0.000 claims description 5
- 238000012546 transfer Methods 0.000 claims description 5
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 claims description 4
- 238000000354 decomposition reaction Methods 0.000 claims description 4
- 229930195733 hydrocarbon Natural products 0.000 claims description 4
- 150000002430 hydrocarbons Chemical class 0.000 claims description 4
- 239000004215 Carbon black (E152) Substances 0.000 claims description 3
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 claims description 3
- 238000001704 evaporation Methods 0.000 claims description 3
- 230000008020 evaporation Effects 0.000 claims description 3
- 239000001257 hydrogen Substances 0.000 claims description 3
- 229910052739 hydrogen Inorganic materials 0.000 claims description 3
- 239000002904 solvent Substances 0.000 claims description 3
- 229910001873 dinitrogen Inorganic materials 0.000 claims description 2
- 230000004927 fusion Effects 0.000 claims description 2
- 150000002431 hydrogen Chemical class 0.000 claims description 2
- 238000002844 melting Methods 0.000 claims description 2
- 230000008018 melting Effects 0.000 claims description 2
- 239000012535 impurity Substances 0.000 claims 2
- 238000005406 washing Methods 0.000 description 19
- 239000007788 liquid Substances 0.000 description 13
- OKKJLVBELUTLKV-UHFFFAOYSA-N Methanol Chemical compound OC OKKJLVBELUTLKV-UHFFFAOYSA-N 0.000 description 9
- 238000011282 treatment Methods 0.000 description 7
- 230000010354 integration Effects 0.000 description 6
- 239000003054 catalyst Substances 0.000 description 5
- 230000009467 reduction Effects 0.000 description 5
- 238000009833 condensation Methods 0.000 description 3
- 230000005494 condensation Effects 0.000 description 3
- 230000000694 effects Effects 0.000 description 3
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 description 3
- XTHFKEDIFFGKHM-UHFFFAOYSA-N Dimethoxyethane Chemical compound COCCOC XTHFKEDIFFGKHM-UHFFFAOYSA-N 0.000 description 2
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 2
- 238000011161 development Methods 0.000 description 2
- 238000007599 discharging Methods 0.000 description 2
- 238000002309 gasification Methods 0.000 description 2
- 229910000069 nitrogen hydride Inorganic materials 0.000 description 2
- 238000002407 reforming Methods 0.000 description 2
- 230000008439 repair process Effects 0.000 description 2
- 239000000126 substance Substances 0.000 description 2
- 239000000725 suspension Substances 0.000 description 2
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 description 1
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 description 1
- 230000002745 absorbent Effects 0.000 description 1
- 239000002250 absorbent Substances 0.000 description 1
- PPBAJDRXASKAGH-UHFFFAOYSA-N azane;urea Chemical compound N.NC(N)=O PPBAJDRXASKAGH-UHFFFAOYSA-N 0.000 description 1
- 230000008859 change Effects 0.000 description 1
- 239000012295 chemical reaction liquid Substances 0.000 description 1
- UPHIPHFJVNKLMR-UHFFFAOYSA-N chromium iron Chemical compound [Cr].[Fe] UPHIPHFJVNKLMR-UHFFFAOYSA-N 0.000 description 1
- 238000007906 compression Methods 0.000 description 1
- 230000006835 compression Effects 0.000 description 1
- 238000010276 construction Methods 0.000 description 1
- 239000002826 coolant Substances 0.000 description 1
- 229910052802 copper Inorganic materials 0.000 description 1
- 239000010949 copper Substances 0.000 description 1
- 238000002425 crystallisation Methods 0.000 description 1
- 230000008025 crystallization Effects 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 238000004090 dissolution Methods 0.000 description 1
- 238000002474 experimental method Methods 0.000 description 1
- 239000008187 granular material Substances 0.000 description 1
- 238000005469 granulation Methods 0.000 description 1
- 230000003179 granulation Effects 0.000 description 1
- 238000011835 investigation Methods 0.000 description 1
- 229910052742 iron Inorganic materials 0.000 description 1
- 238000012423 maintenance Methods 0.000 description 1
- 239000000203 mixture Substances 0.000 description 1
- 239000003345 natural gas Substances 0.000 description 1
- 229910052757 nitrogen Inorganic materials 0.000 description 1
- 238000011017 operating method Methods 0.000 description 1
- 230000001172 regenerating effect Effects 0.000 description 1
- 239000007787 solid Substances 0.000 description 1
- 239000007858 starting material Substances 0.000 description 1
Classifications
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B3/00—Hydrogen; Gaseous mixtures containing hydrogen; Separation of hydrogen from mixtures containing it; Purification of hydrogen
- C01B3/02—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen
- C01B3/025—Preparation or purification of gas mixtures for ammonia synthesis
-
- C—CHEMISTRY; METALLURGY
- C07—ORGANIC CHEMISTRY
- C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
- C07C273/00—Preparation of urea or its derivatives, i.e. compounds containing any of the groups, the nitrogen atoms not being part of nitro or nitroso groups
- C07C273/02—Preparation of urea or its derivatives, i.e. compounds containing any of the groups, the nitrogen atoms not being part of nitro or nitroso groups of urea, its salts, complexes or addition compounds
- C07C273/10—Preparation of urea or its derivatives, i.e. compounds containing any of the groups, the nitrogen atoms not being part of nitro or nitroso groups of urea, its salts, complexes or addition compounds combined with the synthesis of ammonia
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P20/00—Technologies relating to chemical industry
- Y02P20/10—Process efficiency
- Y02P20/129—Energy recovery, e.g. by cogeneration, H2recovery or pressure recovery turbines
Landscapes
- Chemical & Material Sciences (AREA)
- Organic Chemistry (AREA)
- Engineering & Computer Science (AREA)
- Combustion & Propulsion (AREA)
- Inorganic Chemistry (AREA)
- Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)
Abstract
A thermally integrated ammonia production plant and urea production plant in which the waste process heat in the crude ammonia synthesis gas discharged from the low temperature CO shift converter is fed to heat-using apparatus portions of the urea synthesis plant when the urea synthesis plant is operating under steady state operating conditions, and when the urea synthesis plant is shutdown or is operating under non-steady state operating conditions said crude ammonia synthesis gas is fed to a different heat- using apparatus. Thereafter, the crude ammonia synthesis gas is fed from whichever heat-using apparatus is employed into the CO2-removing apparatus in the ammonia production plant.
Description
SPECIFICATION
Thermally integrated ammonia/urea process
The present invention relates to a thermally integrated method of producing ammonia and urea.
As a means for reducing the amount of energy required for the operation of various production plants, it has been proposed to integrate the process steps carried out in different production plants. The integration of the process steps between the different plants sometimes provides more economical utilization of waste process heat than the case wherein waste process heat is recovered and utilized within each production plant separately.
Atypical process involving integration of an ammonia production plant and a urea production plant is disclosed in U.S. Patent No. 3303215. This process for synthesizing urea is characterized by the features that the carbon dioxide-containing ammonia synthesis gas, to be used for ammonia synthesis in the ammonia synthesis step, is contacted with liquid ammonia mixed with a medium for assisting the dissolution of ammonium carbamate, under a pressure at least as high as the urea synthesis pressure, whereby to transfer substantially all the carbon dioxide in the ammonia synthesis gas into the liquid. The thus-obtained liquid is maintained at the urea synthesis temperature. This process comprises a skillful integration of both production plants for producing ammonia and urea, respectively, and the effect thereof of reducing the heat energy required is great.However, this process has a serious disadvantage that when the operation of one production plants is suspended, the operation of the other production plant must also be suspended, thereby reducing the rates of operation of both production plants, because there is a high degree of integration of both plants, not only thermally but also in terms of material flow.
According to the process of the present invention, the above-described disadvantage can be overcome by integrating both plants only thermally so that a simultaneous reduction in the rates of operation of both plants can be avoided completely. The value of the process of U.S. Patent No. 3303215 with respect to the saving of heat energy is maintained in the present invention by an effective untilization of the heat generated in the ammonia production plant (which heat is caused to become present in excess in the plant by eliminating the chemical absorption step for separating carbon dioxide from the crude ammonia synthesis gas, which step is necessary in other conventional processes) as a heat source for a step of decomposing ammonium carbamate, which is formed in the urea production plant but is not converted to urea yet, and also by reducing the compression power required for pressurizing the carbon dioxide.
According to experiments carried out in a pilot plant for the purpose of scaling up the process of U.S.
Patent No. 3303215 for industrial utilization, it has been found that the temperature at the bottom of the crude ammonia synthesis gas-washing column, in which liquid ammonia is used as the carbon dioxide-absorbing liquid, cannot be elevated to higher than about 1600C because of vapor-liquid equilibrium in the presence of high pressure hydrogen and nitrogen. A considerable part of the heat of formation of ammonium carbamate generated in the washing column is removed through the top of the washing column by the evaporation of the liquid ammonia, thereby making it impossible to introduce a high temperature liquid containing ammonium carbamate formed by the absorption of the carbon dioxide into the urea synthesis step and, therefore, considerable heat must be added in the urea synthesis step.Although the prior process has the above-mentioned disadvantage in heat economy, the process is still superior to other conventional processes with respect to heat economy. However, this process has not been employed in commercial practice yet in spite of the above-mentioned superiority. The reasons why this prior integrated process has not been adopted yet are that the ammonia and urea plants now being constructed are generally of extremely large size and, therefore, the adoption of such an extremely large size plant, which has not demonstrated its improved results in actual practice yet, is quite risky, and also that the ammonia production plant is not connected in series with the urea production plant, but rather, there is one treatment step in common between them and, therefore, the combination of the two plants is excessively integrated.Namely, the CO2-removing step in the ammonia production plant corresponds to the ammonium carbamate production step in the urea production plant. Accordingly, if the operation of the urea production plant is suspended, for example, due to a machine problem, the operation of the ammonia production plant is also necessarily suspended, thereby seriously reducing the productivity. Further, if the operation of the ammonia production plant is started, the operation of the urea production plant must also be started so as to remove
CO2 from the crude ammonia synthesis gas. Therefore, it is required to store a quite large amount of ammonia required in the urea production plant until the operation of the ammonia production plant reaches steady-state conditions and until the supply of ammonia used as a starting material in the urea production plant becomes stable after starting the operation.If it is desired to make the storage of ammonia unnecessary, then it is required to provide a conventional CO2 removal step in the ammonia production plant, thereby increasing construction costs, maintenance costs and other losses.
The potential for a great reduction in the productivity of the prior art integrated process will be shown with reference to the results of three years of operation of existing plants. The following data is derived from the operations of an existing ammonia production plant and an existing urea production plant which were not integrated, but rather, they were operated independently from each other.
Ammonia pro- Urea pro
duction plant duction plant
Number of times of suspension of operation due to trouble which 5 29 caused decrease in production
Average time loss per trouble 45 hours 7 hours
The average time loss is the average of the values obtained by dividing the decrease in the amount of production (unit:ton) that occurred for each trouble by the capacity of the plant (unit:ton/hr.). It indicates an average time loss per trouble. The above results suggest that if the operation is once suspended due to a trouble in the ammonia production plant, a long time is required for completing the repair of reconstruction before the operation can be started again in the ammonia production plant, even though the occurrence of suspensions of operation due to troubles is not so frequent.On the other hand, the frequency of the troubles in the urea production plant is high, even though a long time is not required to effect repairs so that the operation can be started again in the urea production plant. Thus, the respective operational characteristics (down time) of the two plants are shown clearly by the above results. If these two production plants are combined together in a completely integrated system, as suggested in U.S. Patent No.3303215, the considerably frequent shut-downs of the urea production plant make it necessary also to suspend the operation of the ammonia production plant, whereby the total decrease in the production ammonia is about 7 times higher than in a non-integrated ammonia production plant.According to the above results, the loss in the production of ammonia due to troubles in the ammonia production plant in the past three years is calculated as follows (operation time: 8,000 hrs./yr): 5(times) x 45(hrs). (per trouble) x 100% = 0.96% 3(yrs.) x 8,000(hrs.) (per x O year) If both plants are completely integrated, as taught in U.S. Patent No. 3303215, the loss in production of ammonia is increased to be as follows: [5(times) + 29(times)] x 45(hrs.) (per trouble) x 100 = 6.4% 3(yrs.) x 8,000(hrs.) (per year) As is clearly demonstrated by this calculation, the loss of ammonia production due to reductions in the rate of operation is quite large.Such a serious loss is not acceptable, particularly because the capacities of plants are now generally extremely large. This disadvantage inhibits the industrial practice of the integrated process, i.e. the integration of the two plants, even through the integrated process is otherwise superior with respect to heat economy.
The inventors made intensive investigations on the compatibility of both heat economization and productivity in an integrated system of an ammonia production plant and a urea production plant. The inventors searched for a practical means of eliminating the disadvantages in the operation of the combined system so as to make it possible to combine both plants for rationalizing the heat balance. Forthis purpose, the unit steps of each plant were analyzed in detail. High temperature waste heat can be recovered and utilized easily in an advantageous manner. However, it is difficult to recover and advantageously utilize low temperature waste heat which is generated in a large quantity in general. Therefore, the low temperature waste heat has been discharged to the atmosphere in many cases.Also in the ammonia production plant, waste heat is produced in a large quantity, which necessitates a special consideration for its recovery and utilization. In the ammonia production plant, the low temperature waste heat is generated in the purification step after shift conversion of the crude ammonia synthesis gas. A large quantity of waste heat must be removed from the crude synthesis gas after the shift conversion of the crude ammonia synthesis gas. CO2 is removed from the crude ammonia synthesis gas after the shift conversion by either physical absorption or chemical absorption. Physical absorption is recommended because of the ease of regenerating the wash solution.The conventional processes for CO2 removal by physical absorption (a typical example of which is washing with water) have the disadvantages that the absorption capacity of the washing liquid is poor and, therefore, a large quantity of the washing liquid must be circulated, a large apparatus is required and, in addition, a large amount of power is required for circulating the washing liquid. Another well-known process comprises using methanol as the washing solution. However, this process has the disadvantages that a source of very low temperature coolant is required, because the gas is washed at a temperature as low as from -30 C to -70 C and that the loss of the washing liquid is comparatively large, because the vapor pressure of the washing liquid is high, even though the washing liquid has a quite high absorption capacity and the quantity of the washing liquid that is circulated is very small. Those disadvantages of the conventional absorption processes become more remarkable as the pressure in the washing column is reduced. At present, the technique of washing with water is not employed on an industrial scale and the technique of washing with methanol is employed only for washing a crude ammonia synthesis gas prepared by high pressure partial oxidation gasification processes.
In the gasification by a steam reforming process, the pressure employed in the primary reforming step has been elevated recently because of the development of improved materials for the reformer tubes and, therefore, the utilization of the technique of washing with methanol has become possible. Further, the dimethyl ether of polyethylene glycol (Selexol system) exhibits a relatively high absorbing capacity even at a temperature of around 0 C and its vapor pressure is quite low. Accordingly, if this absorbent is used as the washing liquid, the above-described disadvantages of the physical absorption process can be completely overcome.The inventors have found that if this development in the gas washing technique is utilized, there can be provided a process for producing ammonia and urea by integrated plants, with high heat economy, without making it impossible to operate the ammonia plant and the urea plant independently when the need arises.
The present invention provides a combined process for the production of ammonia and urea comprising the steps of preparing a crude synthesis gas containing hydrogen, carbon monoxide, carbon dioxide, etc. by steam reforming or partial oxidation of a carbon-containing material (hydrocarbon), synthesizing ammonia from the hydrogen gas obtained by purification of the crude synthesis gas and nitrogen gas, and synthesizing urea from the carbon dioxide removed during the purification of said crude synthesis gas and the ammonia synthesized as set forth above, chracterized by the features that the carbon dioxide removal is effected by physical absorption, the crude ammonia synthesis gas is subjected to indirect heat exchange without using any heat transfer medium between the carbon monoxide conversion step (shift conversion) and the carbon dioxide-removing step, the resulting waste heat is utilized as a heat source in the urea synthesis step, or when the urea synthesis step is stopped or is operating in a non-steady state, the synthesis gas is allowed to bypass the indirect heat exchange step and it is sent into another waste heat recovering or cooling step.
According to the present invention, the waste process heat of the crude synthesis gas having a relatively low temperature of 1 00-250"C fed into the indirect heat exchange step is utilized as a heat source for the urea production, for instance, for preheating the drying air, preheating the ammonia, decomposition of unreacted ammonium carbamate, evaporation of excess ammonia, purification, concentration and drying of the urea solution and melting of the urea. Other recovered waste heat is utilized for preheating water to be fed into a boiler, or the waste heat is discharged by a cooling step.
The present invention will now be described in more detail. According to the present invention, a carbon-containing material is subjected to steam reforming and/or partial oxidation to form a crude ammonia synthesis gas comprising H2, CO, CO2, etc. The CO in the crude synthesis gas is converted to H2 and CO2 by reaction with steam. Then, the crude synthesis gas is subjected to a CO2-removing treatment by physical absorption, followed by a treatment for removing the remaining CO. The resulting H2 is catalytically reacted with N2 to synthesize NH3. A part of or the entirety of the NH3 thus obtained is reacted with the CO2 obtained by the CO2-removing treatment, at a high temperature, under a high pressure, to synthesize urea.
The resulting crude urea solution is subjected to one or more treatments selected from heating, expelling and pressure reduction, thereby separating the ammonium carbamate that was not converted to urea and the excess ammonia therefrom. The ammonium carbamate and excess ammonia thus separated out are returned into the urea synthesis reaction zone. The resulting urea solution is concentrated, dehydrated and, if desired, shaped into granules to obtain a solid urea product. According to the present invention, ammonia and urea are thus produced. The flow of the crude synthesis gas leaving the low temperature CO conversion step is introduced into and is passed through indirect heat exchangers.A relatively low waste heat in the range of 100-250"C of the crude synthesis gas is utilized as a heat source for at least one of the steps of purification, concentration and drying of the urea solution and fusion of urea, without using any heat transfer medium. When operation for the urea synthesis is stopped or is in a non-steady operating state, the flow of the crude synthesis gas is bypassed around the above-mentioned indirect heat exchangers and it is sent directly to another waste heat-recovering device placed downstream from the above-mentioned indirect heat exchangers and waste heat is recovered therefrom and is used for a purpose other than the above-described purposes, and/or the gas is fed to a cooling device to cool the gas, whereby the waste heat is recovered and/or is discharged.Then, the gas is subjected to a CO2-removing treatment and to a residual
CO-removing treatment. The gas thus treated is then introduced into the ammonia synthesis zone.
As described above in detail, the present invention comprises the combination of an ammonia production plant and a urea production plant, wherein relatively low process waste heat generated in the ammonia production step is recovered for further utilization or is discharged.
In the accompanying drawings,
Figure 1 is a graph illustrating the cooling curve of a crude synthesis gas released from a low temperature carbon monoxide conversion step which explains the utilization of the heat.
Figure 2 is a schematic block flow diagram showing an embodiment of the present invention.
An embodiment of the present invention will be described with reference to Figure 2.
Figure 2 shows a typical embodiment of the present invention. Desulfurized hydrocarbons are subjected to steam reforming in a steam reforming reactor 2 to obtain a crude synthesis gas. The crude synthesis gas is subjected to a CO conversion reaction in a high temperature CO shift conversion reactor 4, in the presence of an iron-chromium catalyst, at a high temperature of 350-450"C. The resulting crude synthesis gas is further subjected to a CO conversion reaction in a low temperature CO shift conversion reactor 5 in the presence of a copper catalyst at a low temperature of 200-250"C. The crude ammonia synthesis gas discharged from the low temperature shift conversion reactor (LTS) is introduced into a reboiler 7 in a high pressure ammonium carbamate decomposing column, an ammonia preheater 8 and a preheater 9 for drying air, all of which are elements of the urea production plant, so that the waste heat of crude ammonia synthesis gas is transferred to the urea production system by indirect heat exchange. In Figure 2, numerals 26, 27 and 28 indicate indirect heat exchangers. Thereafter, the discharged gas is cooled to ambient temperature in a preheater 10 for the water to be fed into a boiler and/or in a cooler 11. The gas is then introduced into a CO2-removing step 12 (i.e., the Selexol method wherein the dimethyl ether of polyethylene glycol is used) to effect decarbonation by physical absorption.
The thus obtained synthesis gas is subjected to a methanation reaction in a methanation reactor 13 in the presence of a nickel catalyst, whereby the residual very small amounts of CO and CO2 are methanized. After the final gas purification in the methanator, the pressure of the gas is elevated to the ammonia synthesis pressure. The gas is subjected to the ammonia synthesis reaction in the synthesis loop 14 in the presence of an iron catalyst to obtain liquid ammonia.
The thus-obtained liquid ammonia and ammonia recovered from a urea-recovering step and pressurized by a pump 22 and CO2 from the CO2-removing step 12 is pressurized with a compressor to the urea synthesis pressure. The urea synthesis reaction is carried out in a reactor 18. A reaction liquid discharged from the urea synthesis reactor is subjected to pressure reduction and then is heated in the high pressure decomposing column reboiler 7, whereby unreacted ammonium carbamate is decomposed and excess ammonia is expelled. The bottoms in the highzpressure decomposing column are further reduced in pressure and heated to expel unreacted ammonium carbamate and excess ammonia. The resulting aqueous urea solution is converted to a granular urea product by, for example, crystallization, drying and granulation steps.On the other hand, the expelled gas containing ammonia and CO2 is recovered as liquid ammonia and as recovered ammonium carbamate solution in the recovery step and returned into the urea synthesis reactor by means of a pump.
When the urea production operation is stopped or it is in non-steady state operational condition, such as during the initial stage of the operation, the crude ammonia synthesis gas discharged from the converter 5 is sent directly to the boiler water preheater 10 and/or cooler 11 by opening valve 21 and closing valves 23 and 25 so that the crude ammonia synthesis gas bypasses the indirect heat exchangers 26, 27 and 28, whereby the waste heat of the gas discharged from the LTS converter 5 is recovered and/or discharged in the devices 10 and/or 11, and the waste heat is not transferred to the urea production plant.
Air is introducted into the preheater 9. The air preheated therein is sent into a urea dryer 24.
Example
A gas (147,200 dry Nm3/hr., containing 51.7 t/hr. of steam) is obtained, in a system in which the ammonia production rate is 1,000 metric tons/day and urea production rate is 1,000 metric tons/day, by subjecting natural gas to steam reforming, effecting CO conversion in a high temperature CO conversion step (HTS) and further effecting a CO conversion reaction in a low temperature CO conversion step using conventional catalysts and operating procedures for these steps. The crude ammonia synthesis gas discharged from the
LTS converter 5 has the following composition:
N2 20.0 vol. % (dry)
H2 61.1
CH4 0.2
A 0.3
CO2 18.2
CO 0.2
Total: 100%(dry) The temperature and pressure of the gas are 223"C and 34 Kg/cm2.G, respectively. The gas from the LTS
has a cooling curve as shown in Figure 1.The waste heat is utilized in the high pressure decomposition column, ammonia preheater and preheater for drying air in the urea plant as shown in Figure 2. The quantity
of external steam required to be supplied to the urea production plant under these conditions is 13.3 t./hr.
(0.3 t./t.-urea).
When the urea plant is operating under non-steady state conditions, such as in the starting-up stage, the
crude ammonia synthesis gas is bypassed around the heat exchangers 26, 27 and 28 and the waste heat is
recovered or discharged in the waste heat recovering device, (boiler feed water preheater 10) and/or cooler
11, arranged downstream from the heat exchangers 26, 27 and 28.
The gas is introduced into CO2-removing step 12 wherein the dimethyl ether of polyethylene glycol is used
as solvent. After the CO2 removal, the gas is passed through a methanator 13, subjected to the final gas
purification and then is fed into the ammonia-synthesis loop 14.
The process of the present invention has the following advantages:
The waste heat having a relatively low temperature (100-2S00C), which becomes superfluous by effecting the CO2-removing operation with an absorbing medium according to the physical absorption method in the step 12, can be transferred efficiently to the urea production plant.
This will be illustrated with reference to Figure 1.
Figure 1 shows an example of the change in the heat content of the crude ammonia synthesis gas discharged from the LTS converter 5, in an ammonia production plant that produces 1,000 metric tons of liquid ammonia/day. 13.9 t./hr. of steam at about 7 Kg/cm2.G pressure (condensation temperature 1700C) can be formed by said waste heat, and 35.9 t./hr. of steam at about 3.5 Kg/cm2.G pressure (condensation temperature 147"C) can be obtained from the same.
On the other hand, the quantity of steam required in a conventional urea plant of a capacity of 1,000 metric tons/day is as follows:
Steam of higher than for high pressure
7 Kg/cm2.G: decomposing column
(condensation temperature: 28.2 t./hr.
above 1700C) for melter 7.7
Steam for higher than for low pressure
3.5 Kg/cm2.G decomposing column 1.2
for gas separator 1.3
for others 3.1
The amount of steam at a pressure of 7 Kg/cm2.G which can be recovered from the waste heat of the crude ammonia synthesis gas from the LTS converter 5 is insufficient for the urea production plant but the amount of 3.5 Kg/cm2.G steam is excessive.
If the waste heat of the crude ammonia synthesis gas is used for the urea production system, without using steam as the heat transfer medium, the total heat needed in the high pressure decomposing column can be supplied by the waste heat of the crude ammonia synthesis gas.
By providing the bypass line 30 around the indirect heat exchangers 26, 27 and 28 for transferring the waste heat of the crude ammonia synthesis gas to the urea production system, the ammonia-producing plant can be continuously operated in a stable condition independently of the occurrence of any trouble in the urea-producing system. Consequently, a high operation rate can be maintained in the combined ammonia-urea synthesis plant
Workers skilled in the art will appreciate that in the ammonia synthesis plant, the individual steps of reforming, shift conversion, CO2 removal, methanation and ammonia synthesis can be performed using well-known conventional procedures including reaction temperatures, pressures, catalyst, space velocities, etc., that are appropriate for those respective steps.Similarly, workers skilled in the art will appreciate that in the urea synthesis plant the individual steps of urea synthesis, decomposition of ammonium carbamate, purification of urea solution and prilling of urea are performed using well-known conventional procedures appropriate for those respective steps.As set forth above, the invention is concerned with integrating the ammonia synthesis plant and the urea synthesis plant only thermally by utilizing waste process heat in the crude ammonia synthesis gas discharged from the low temperature CO shift converter 5 to supply heat to one or more heat-using devices in the urea production plant when the plant is operating under steady state conditions, and recovering and/or discharging that waste process heat in different heat-using or heat-discharging devices when the urea production plant is shut down or is operating under non-steady state conditions. There is no integration of the two processes in terms of material flow between intermediate portions of the two processes so that the ammonia production plant can operate whether or not the urea plant is operating. However, the urea plant is connected in series with the ammonia plant, in terms of material flow, so that the ammonia and carbon dioxide produced in the ammonia plant can be fed to the urea plant. The invention requires the use of a physical absorption system, known per se, for effecting CO2 removal after the waste process heat has been removed from the crude ammonia synthesis gas discharged from the LTS converter 5.
Claims (8)
1. Athermally integrated method for preparing ammonia and urea comprising the steps of preparing crude ammonia synthesis gas containing hydrogen, carbon monoxide and carbon dioxide, by steam forming or partial oxidation of a carbon-containing material, synthesizing ammonia from the hydrogen gas obtained by purification from the crude synthesis gas and nitrogen gas, and synthesizing urea from carbon dioxide removed in the purification of said crude synthesis gas and ammonia synthesized as set forth above, wherein the removal of carbon dioxide from the crude ammonia synthesis gas is effected by physical absorption; between a carbon monoxide conversion step in the purification and the carbon dioxideremoving step the crude ammonia synthesis gas is subjected to indirect heat exchange without using any heat transfer medium, the resulting waste heat is utilized as a heat source in the urea production plant, or when the urea production plant is shut shown or is operating in a non-steady state, the crude ammonia synthesis gas is bypassed around the said indirect heat exchange step and is sent into another waste heat-recovering or cooling step.
2. A method as claimed in claim 1 in which the relatively low waste heat of the crude ammonia synthesis gas having a temperature of 1000 to 250"C is utilized as a heat source for the preheating of drying air, preheating of ammonia, decomposition of unreacted ammonium carbamate, evaporation of excess ammonia, purification, concentration and drying of urea solution and/or melting fusion of urea in the urea production process.
3. A method as claimed in claim 1 in which when the low waste heat is not transferred to the urea production process, it is utilized for preheating water to be fed into a boiler.
4. A thermally integrated method for preparing ammonia and urea, which comprises the steps of: preparing a crude synthesis gas containing H2, CO and CO2 by steam reforming or partial oxidation of a desulphurized hydrocarbon feedstock; effecting shift conversion of CO in said crude synthesis gas to CO2 in a high temperature shift converter and then in a low temperature shift converter to produce a crude ammonia synthesis gas; then removing CO2 from said crude ammonia synthesis gas by physically absorbing said CO2 in the dimethyl ether of polyethylene glycol; then methanating said crude ammonia synthesis gas and removing impurities therefrom to obtain a purified ammonia synthesis gas; then compressing the purified ammonia synthesis gas and feeding same into ammonia synthesis reactor and forming liquid ammonia therein; separating said CO2 from said dimethyl ether of polyethylene glycol and feeding same into a urea synthesis reactor and feeding at least a portion of said liquid ammonia through an ammonia preheater and thence into said urea synthesis reactor and effecting a urea-forming reaction in said urea synthesis reactor; then heating the reaction solution from said urea synthesis reactor to decompose ammonium carbamate present therein; separating ammonia and CO2 and recovering urea from said reaction solution using drying air, wherein: when the urea synthesis reactor is operating under steady state conditions, the crude ammonia synthesis gas discharged from said low temperature shift converter is fed through one or more indirect heat exchangers for, respectively, heating said reaction solution to decompose ammonium carbamate, heating said ammonia before feeding same into said urea synthesis reactor and heating said drying air, and then said crude ammonia synthesis gas is fed into the step for removing CO2, and, alternatively, when said urea synthesis reactor is not operating or is operating under non-steady state conditions, said crude ammonia synthesis gas discharged from said low temperature shift converter is bypassed around said indirect heat exchangers and is fed through a different waste heat recovery device before it is fed into said step for removing CO2.
5. In an apparatus for carrying out a thermally integrated method for preparing ammonia and urea, which comprises: a crude synthesis gas reactor for preparing a crude synthesis gas containing H2, CO and
CO2 by steam reforming or partial oxidation of a desulphurized hydrocarbon feedstock; a high temperature shift converter and a low temperature shift converter connected in series for effecting shift conversion of CO in said crude synthesis gas to CO2 to produce a crude ammonia synthesis gas; a CO2-removing apparatus for removing CO2 from said crude ammonia synthesis gas by physically absorbing said CO2 in a solvent; a methanator for methanating said crude ammonia synthesis gas and removing impurities therefrom to obtain a purified ammonia synthesis gas; an ammonia synthesis reactorforforming liquid ammonia; means for separating said CO2 from said solvent; an ammonia preheater for preheating liquid ammonia produced in said ammonia synthesis reactor; a urea synthesis reactor connected for receiving said CO2 and the preheated liquid ammonia for effecting a urea-forming reaction in said urea synthesis reactor; a high pressure ammonium carbamate decomposing column for heating the reaction solution from said urea synthesis reactor to decompose ammonium carbamate present therein and to separate ammonia and CO2 therefrom; a urea recovery system for recovering urea from said reaction solution using drying air; one or more indirect heat exchangers for, respectively, heating said reaction solution to decompose ammonium carbamate, heating said ammonia before feeding same into said urea synthesis reactor and heating said drying air; conduit means having valve means therein and connecting said indirect heat exchanger or heat exchangers in series between said low temperature shift converter and said CO2-removing apparatus; a different waste heat recovery apparatus; and a bypass conduit having valve means therein and connected between said low temperature shift converter and said different waste heat recovery apparatus.
6. A method as claimed in claim 1 and substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.
7. A method as claimed in claim 1 and substantially as hereinbefore described with reference to the
Example.
8. Apparatus as claimed in claim 5 and substantially as herein before described with reference to Figure 2 of the accompanying drawings.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP15143778A JPS5575914A (en) | 1978-12-06 | 1978-12-06 | Ammonia and urea production combined process |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB2039893A true GB2039893A (en) | 1980-08-20 |
| GB2039893B GB2039893B (en) | 1983-03-02 |
Family
ID=15518582
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB7941931A Expired GB2039893B (en) | 1978-12-06 | 1979-12-05 | Thermally integrated ammonia/urea process |
Country Status (7)
| Country | Link |
|---|---|
| JP (1) | JPS5575914A (en) |
| BR (1) | BR7907942A (en) |
| FR (1) | FR2443456A1 (en) |
| GB (1) | GB2039893B (en) |
| IN (1) | IN153372B (en) |
| IT (1) | IT1194597B (en) |
| NL (1) | NL7908808A (en) |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR1494555A (en) * | 1965-09-27 | 1967-09-08 | Toyo Koatsu Ind Inc | New urea synthesis process |
-
1978
- 1978-12-06 JP JP15143778A patent/JPS5575914A/en active Pending
-
1979
- 1979-11-27 IN IN850/DEL/79A patent/IN153372B/en unknown
- 1979-12-05 BR BR7907942A patent/BR7907942A/en unknown
- 1979-12-05 GB GB7941931A patent/GB2039893B/en not_active Expired
- 1979-12-06 NL NL7908808A patent/NL7908808A/en not_active Application Discontinuation
- 1979-12-06 FR FR7929987A patent/FR2443456A1/en active Granted
- 1979-12-07 IT IT27936/79A patent/IT1194597B/en active
Also Published As
| Publication number | Publication date |
|---|---|
| FR2443456B1 (en) | 1982-07-02 |
| NL7908808A (en) | 1980-06-10 |
| IN153372B (en) | 1984-07-14 |
| JPS5575914A (en) | 1980-06-07 |
| BR7907942A (en) | 1980-07-08 |
| IT1194597B (en) | 1988-09-22 |
| GB2039893B (en) | 1983-03-02 |
| IT7927936A0 (en) | 1979-12-07 |
| FR2443456A1 (en) | 1980-07-04 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| PCNP | Patent ceased through non-payment of renewal fee |