Ammonia collection and recycling in ammonia crackers

Abstract

A method is provided for separating and reusing unconverted ammonia from cracked ammonia gas produced by an ammonia cracking unit. This method includes the steps of introducing a cracked ammonia gas stream into a water washing column to generate a clean gas stream and a watery effluent stream, and introducing the watery effluent stream into a stripping column to generate a cleaned washing water stream and a recovered ammonia stream. Here, the cracked ammonia gas stream has an ammonia concentration of 0.003 mol% to 10 mol%. Here, the clean gas stream has an ammonia concentration of 1 ppm to 2500 ppm. Here, at least a portion of the recovered ammonia stream is used as fuel in the ammonia cracking unit.

Description

Background Art

[0005] ,

[0004] , ,

[0001] The separation of ammonia from a gas or liquid stream is typically carried out in various fields. For example, it is possible to strip the wastewater containing ammonia with air. In order to enhance the transfer of ammonia from the wastewater to the air, it is possible to increase the pH value of the wastewater. Then, the air containing ammonia can be post-washed with dilute sulfuric acid to form ammonium sulfate that can be used as a liquid fertilizer. Another option is to strip the alkalized wastewater with steam instead of air. In a suitable process, a concentrated ammonia solution that can be used in a DeNOx unit can be obtained.

[0002] The removal of ammonia from a gas stream typically plays an important role in ammonia synthesis. Typically, ammonia is separated as a product through partial condensation at a temperature of -20°C to 30°C and a pressure of 100 bar to 1000 bar. This technology is very mature.

[0003] Regarding ammonia cracking applications, it is known in the technical field to use a washing column. Such cracked gas typically contains 8% ammonia and can be reduced to about 0.06% in the washing column. Then, ammonia can be recovered through evaporation, cooled, compressed, and recycled to the cracker feed.

[0004] In an industrial-scale ammonia cracker, the desired products are typically hydrogen and / or nitrogen. Focusing on these products, the formation of potentially undesirable by-products should be avoided. Depending on the process conditions, a significant amount of unreacted ammonia, typically 10% or less, can be present in the cracked gas. Therefore, washing with sulfuric acid to form ammonium sulfate is not desirable because the ammonia for forming ammonium sulfate is lost for hydrogen production.

[0005] The separation of ammonia via partial condensation requires cooling to below freezing point and preferably high pressure. This is typically used in ammonia synthesis. However, because the pressure of the cracked gas is lower than that in ammonia synthesis and requires significant effort to cool, this is not a preferred method for ammonia separation. In addition, the residual ammonia concentration in the off-gas can be up to 5 mol%, which may be too high for downstream hydrogen purification steps such as thermal swing adsorption or pressure swing adsorption (TSA / PSA).

[0006] The recovery of unconverted ammonia in ammonia crackers equipped with water washing columns and recovery columns is known in the art. Absorption of ammonia in the water washing column is advantageous at high pressure. In contrast, its desorption is advantageous at low pressure. When ammonia is recovered at low pressure, a compression step is required for recycling into the feed, resulting in additional capital expenditures (CAPEX) and operating expenses (OPEX). [Overview of the project] [Means for solving the problem]

[0007] A method is provided for separating and reusing unconverted ammonia from cracked ammonia gas produced by an ammonia cracking unit. This method includes the steps of introducing a cracked ammonia gas stream into a water washing column to generate a clean gas stream and a watery effluent stream, and introducing the watery effluent stream into a stripping column to generate a cleaned washing water stream and a recovered ammonia stream. Here, the cracked ammonia gas stream has an ammonia concentration of 0.003 mol% to 10 mol%. Here, the clean gas stream has an ammonia concentration of 1 ppm to 2500 ppm. Here, at least a portion of the recovered ammonia stream is used as fuel in the ammonia cracking unit.

[0008] For a further understanding of the nature and purpose of the present invention, the following detailed description should be referred to in conjunction with the accompanying drawings, in which similar elements are denoted by the same or similar reference numerals. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 is a schematic diagram of a method for separating cracking ammonia gas according to one embodiment of the present invention. [Figure 2] Figure 2 is a schematic diagram of a method for separating cracking ammonia gas, showing additional possible details according to one embodiment of the present invention. [Modes for carrying out the invention]

[0010] Exemplary embodiments of the present invention are described below. While the present invention is readily adaptable to various modifications and alternative forms, specific embodiments are illustrated in the drawings and described in detail herein. However, it should be understood that the description of specific embodiments herein is not intended to limit the invention to any particular form disclosed, but rather to encompass all modifications, equivalents, and alternatives that fall within the spirit and scope of the invention as defined by the appended claims.

[0011] Naturally, in developing any such actual embodiment, it will be recognized that numerous implementation-specific decisions must be made to achieve the developer's particular goals, including adaptations to system-related and business-related constraints that will differ from implementation to implementation. Moreover, while such development efforts may be complex and time-consuming, it will be recognized that they are routine tasks for those skilled in the art who have an interest in the present disclosure.

[0012] To overcome the above-mentioned problems regarding the separation of ammonia from a gaseous flow, an innovative novel process has been proposed. The proposed process comprises an ammonia aqueous washing column and a stripper for purifying the cracked gas in an ammonia cracker, where the energy for ammonia cracking is provided by the combustion of an ammonia-containing fuel mixture. The fuel contains ammonia, and / or hydrogen and / or nitrogen derived from the cracked ammonia. Typical processes known in the art differ in that the stripped gaseous ammonia is compressed and recycled to the feed. In contrast, the presented process avoids the use of a compressor and recycles the gaseous ammonia into the fuel system. This makes it possible to reduce the amount of external ammonia used for ammonia cracking. The process utilizes a high-pressure (typically 20-35 bar) cracking gas for ammonia absorption and a low-pressure (typically 1-2 bar(a)) in the fuel system for ammonia desorption. The fuel flow can be preheated through thermal integration related to the washing-stripping cycle. In addition, it is possible to extract ammonia-containing streams for use in selective catalytic reduction (SCR, DeNox) units for removing nitrous oxide. During the combustion of ammonia-containing fuels, a considerable amount of NOx is formed and needs to be removed from the flue gas before being discharged into the atmosphere.

[0013] Referring to Figure 2, a basic ammonia cracking unit 135 is illustrated. An ammonia feed stream 136 and optionally a warm ammonia feed stream 124 are introduced into an ammonia storage tank 137. If necessary, ammonia is withdrawn from the ammonia storage tank 137 and introduced into an ammonia heat exchanger 138, where it indirectly exchanges heat with a high-temperature cracking ammonia stream 141, thereby producing a warm ammonia stream 139 and a cold cracking ammonia stream 142. The ammonia heat exchanger 138 may be thermally integrated with a reboiler 113, thereby providing at least some of the required heat (not shown). An ammonia reactor fuel stream 143 and optionally ammonia fuel stream 111 are introduced into an ammonia cracking reactor 140 together with the warm ammonia stream 139, thereby producing a high-temperature cracking ammonia stream 141 and an ammonia reactor untreated flue gas stream 144. Next, the untreated ammonia furnace flue gas stream 144 is introduced into the selective catalytic reformer 145 along with the ammonia stream 146 for SCR and optionally one or both of the first SCR stream 106 and / or the second SCR stream 112, thereby generating the treated flue gas stream 147.

[0014] Referring here to Figures 1 and 2, the inlet flow 101 is introduced into the washing column 102, thereby generating at least a clean gas flow 103 and a washing column effluent flow 104. At least a portion of the inlet flow 101 may be a cold cracking ammonia flow 142 from an upstream ammonia cracking unit 135. At least a portion of the inlet to the washing column 102 may be a cold recycle flow 129. The inlet flow 101 may contain unconverted ammonia in the range of 0.003 mol% to 10 mol%. The remainder consists of 25 mol% nitrogen and 75 mol% hydrogen.

[0015] After leaving the ammonia heat exchanger 138, the inlet stream 101 may enter the washing column 102 at a temperature of approximately 35°C. The cold washing water stream 122 may enter the washing column 102 at a temperature of 5°C to 50°C. The coldness of the ammonia feed stream 123, which can be approximately -33°C, may cool the cold washing water stream 122 in the washing water cooler 121, thus warming the ammonia feed stream 123, and the resulting warm ammonia feed stream 124 is sent to the ammonia cracking unit 135.

[0016] Within the washing column 102, ammonia present in the inlet vapor 101 is absorbed by the countercurrent of water and exits as a clean gas stream 103 with a residual ammonia concentration of 50 ppm to 2500 ppm. Here, the ammonia concentration in the clean gas stream 103 is low enough to achieve the separation of hydrogen and nitrogen downstream. The clean gas stream 103 may optionally be introduced into a hydrogen-nitrogen separation unit 148, thereby generating a hydrogen-rich stream 149 and / or a nitrogen-rich stream 150. The hydrogen-nitrogen separation unit 148 may utilize an absorption or cryogenic process.

[0017] The ammonia concentration in the wash column effluent flow 104 may be in the range of 1 mol% to 20 mol%, which is sufficiently high to allow the optional use of a first SCR flow 106 in the selective catalytic reforming (SCR) unit 145. Optionally, to increase the ammonia concentration in the wash column effluent flow 104, a recycle flow 125 may enter the recycle pump 126, thereby generating a pressurized recycle flow 127. The pressurized recycle flow 127 enters the recycle cooler 128, thereby generating a cold recycle flow 129. The cold recycle flow 129 may then be introduced into the wash column 102.

[0018] Next, the wash column effluent flow 104 indirectly exchanges heat with the pressurized wash water flow 119 in the heat exchanger 107, thereby generating a cold effluent flow 108 and a hot wash water flow 120. The cold effluent flow 108 contains ammonia and is then processed in the stripper / recovery column 109. The hot condenser flow 131 exits the stripper column 109 and enters the condenser 130, where it is condensed (and cooled) by the cold condenser inlet flow 133, thereby generating a cold condenser flow 132. When the cold condenser flow 132 re-enters the stripper column 109, the fluid inside is cooled. The temperature and water content of the recovered ammonia flow 110 can be adjusted by controlling the reflux flow rate through the condenser 130, as well as the flow rate and temperature of the cold condenser inlet flow 133. The reboiler liquid flow 114 exits the stripper column 109 and enters the reboiler 113, where it is heated and evaporated by the reboiler heat input flow 116, thereby generating a reboiler steam flow 115. When the reboiler steam flow 115 re-enters the stripper column 109, the ammonia inside evaporates. The reboiler heat input flow 116 can be steam. As an alternative to the steam flow 116, the reboiler 113 may be directly thermally integrated with a waste heat recovery system (not shown) for the cracking process.

[0019] The recovered ammonia stream 110 is a gas containing recovered ammonia in the range of 20 mol% to 100 mol%, with the remainder being mainly water. Since the pressure is approximately 2 bar, at least a portion, 111, can be recycled to the fuel stream 143 of the ammonia cracking unit 135, or a portion (the second SCR stream 112) can be sent to the SCR unit 145 without compression. In one embodiment, at least a portion of the recovered ammonia stream 110 (the recovered ammonia product stream 134) is removed from the system as a product stream. Because the temperature range of the recovered ammonia stream 110 is 30°C to 120°C, it can be used to thermally integrate other plant flows, such as combustion air or fuel preheating (not shown).

[0020] The cleaned washing water stream 117 is pressurized by a washing water pump 118, thereby generating a pressurized washing water stream 119. Next, the pressurized washing water stream 119 exchanges heat with the washing column effluent stream 104, thereby generating a warm washing water stream 120. The warm washing water stream 120 is cooled by a washing water cooler 121, thereby generating a cold washing water stream 122. The cold washing water stream 122 is reintroduced into the washing column 102 and is thus continuously circulated. The water lost in the recovered ammonia stream 110 is replenished by a makeup water stream 105.

[0021] The SCR process is exothermic, exhibits a temperature range for optimal operation, and requires a supply of ammonia. The temperature in SCR can be affected by the choice of ammonia stream. Stream 104 is liquid and stream 110 is gaseous. By selecting one of these streams or a mixture thereof for ammonia supply, the temperature in the flue gas duct can be adjusted and the addition of external ammonia to the SCR unit can be reduced or avoided.

[0022] [Table 1]

[0023] <000008�>It will be understood that many additional changes in the details, materials, steps, and arrangement of parts described herein in order to explain the nature of the present invention may be made by those skilled in the art within the principles and scope of the present invention as set forth in the appended claims. Therefore, it is not intended that the present invention be limited to the specific embodiments in the above examples. [Description of Reference Numerals]

[0024] 101 Inlet stream 102 Washing column 103 Clean gas stream 104 Washing column effluent stream 105 Makeup water 106 First SCR stream (optional) 107 Heat exchanger 108 Cold effluent flow 109 Stripping Column 110 Recovered ammonia flow 111 Ammonia fuel flow 112 Second SCR flow (optional) 113 Reboiler 114 Reboiler Liquid Flow 115 Reboiler Steam Flow 116 Reboiler heat input flow 117 Cleaned wash water stream 118 Washing water pump 119 Pressurized cleaning water flow 120 Hot water wash stream 121 Wash water cooler 122 Cold wash water flow 123 Ammonia feed flow 124 Warm ammonia feed flow 125 Recycling Flow 126 Recycling pump 127 Pressurized Recycling Flow 128 Recycled Cooler 129 Refrigerated Recycling Flow 130 Condenser 131 Hot condenser flow 132 Cold condenser flow 133 Cold condenser inlet flow 134 Recovered ammonia product flow 135 Ammonia Cracking Unit 136 Ammonia feed flow 137 Ammonia storage tank 138 Ammonia heat exchanger 139 Hot ammonia flow 140 Ammonia Cracking Reactor 141 High-temperature cracking ammonia flow 142 Cold cracking ammonia flow 143 Ammonia reactor fuel flow 144 Ammonia reactor untreated flue gas flow 145 Selective catalytic reformer 146 Ammonia flow for SCR 147 Processed flue gas flow 148 Hydrogen-Nitrogen Separation Unit 149. Rich Hydrogen Flow 150 Nitrogen-rich flow

Claims

1. A method for separating and reusing unconverted ammonia from cracked ammonia gas produced by an ammonia cracking unit: - A step of introducing a cracking ammonia gas stream into a water washing column, thereby generating a clean gas stream and a water-containing effluent stream. - The water-containing effluent stream is introduced into a stripping column, thereby generating a cleaned wash water stream and a recovered ammonia stream. The cracking ammonia gas stream contains an ammonia concentration of 0.003 mol% to 10 mol%, The clean gas stream has an ammonia concentration of 1 ppm to 2500 ppm, and A method wherein at least a portion of the recovered ammonia stream is used as fuel in the ammonia cracking unit.

2. The method according to claim 1, wherein a portion of the recovered ammonia stream is used in the recovered ammonia stream of the selective catalytic reforming unit within the ammonia cracking unit.

3. The method according to claim 1, wherein at least a portion of the required ammonia flow is supplied to the selective catalytic reforming unit in the ammonia cracking unit from the water-containing effluent flow, the recovered ammonia flow, or both.

4. The method according to claim 1, wherein the stripping column comprises a reboiler, and the reboiler uses flow as at least a partial heat source.

5. The method according to claim 1, wherein the stripping column comprises a reboiler, the ammonia cracking unit comprises a waste heat recovery heat exchanger, and the reboiler is thermally integrated with the waste heat recovery heat exchanger as at least a partial heat source.

6. The method according to claim 1, wherein the washing column is operated at a pressure of 20 bar to 35 bar and at a temperature of 10°C to 50°C.

7. The method according to claim 1, wherein the stripping column is operated at a pressure of 1 bar to 3 bar.

8. The method according to claim 1, wherein the water-containing effluent has an ammonia concentration, and at least a portion of the water-containing effluent is recirculated and introduced into the water washing column, thereby changing the ammonia concentration.

9. The method according to claim 1, wherein the water-containing effluent stream is heated by indirect heat exchange with at least a portion of the cleaned wash water stream and then introduced into the stripping column.

10. The method further includes the steps of returning the cleaned wash water stream to the wash column and adding a supply water stream to the wash column, The cleaned washing water stream has a first flow rate, The aforementioned supply water flow has a second flow rate, and The second flow rate is 0.3% to 21% of the first flow rate. The method according to claim 1.

11. The method according to claim 1, wherein the cleaned wash water stream is cooled by indirect heat exchange with an ammonia feed stream, thereby generating a cold wash water stream, and the cold wash water stream has a temperature of 5°C to 50°C.

12. The method according to claim 1, wherein at least a portion of the recovered ammonia stream is discharged as a product stream.

13. The stripping column further comprises an overhead condenser including a cold condenser inlet flow, The cold condenser inlet flow has a temperature and a flow rate. The recovered ammonia stream has a temperature and water content, The temperature and / or water content of the recovered ammonia stream can be adjusted by controlling the temperature and / or flow rate of the cold condenser inlet stream. The method according to claim 1.

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