Catalytic cracking dry gas recovery process and system

Abstract

The application discloses a catalytic cracking dry gas recovery process, comprising the following steps: step one, using a one-stage pressure swing adsorption method to separate catalytic cracking dry gas to obtain hydrogen-rich gas and carbon two concentration gas; step two, sequentially performing deoxygenation treatment, decarburization treatment, drying treatment, arsenic removal treatment and mercury removal treatment on the carbon two concentration gas to obtain ethylene raw gas; and step three, feeding the ethylene raw gas into an ethylene device for treatment. The application has the beneficial effects of significantly shortening a recovery process flow, reducing equipment and investment, and reducing operation energy consumption and operation cost. The application also discloses a catalytic cracking dry gas recovery system, comprising a plurality of pressure swing adsorption towers, a compressor, a steam heater, at least two deoxygenation reactors, a heat exchanger, a decarburization unit, at least two dryers, a dearsenic and demercuration reactor, an ethylene device and a hydrogen production device. The system has the beneficial effects of less equipment investment, small land occupation and low energy consumption.

Description

Catalytic cracking dry gas recovery process and system Technical Field

[0001] This invention relates to the field of refinery dry gas treatment technology. More specifically, this invention relates to a catalytic cracking dry gas recovery process and system. Background Technology

[0002] Oil refineries produce large quantities of dry gas as a byproduct of catalytic cracking and other similar units. The main components are H2, CH4, C2H4, and C2H6, with small amounts of C3 and higher-weight components. Currently, most refineries use this dry gas as fuel for various combustion furnaces, resulting in low utilization value. If ethane and ethylene are extracted from the dry gas and used as feedstock for olefin production, the load on steam cracking ethylene plants (referred to as ethylene plants) can be increased.

[0003] The main processes for dry gas recovery in refineries are as follows:

[0004] 1. Cryogenic Separation Method. Utilizing the differences in relative volatility and boiling point of the components in the dry gas feedstock, the high-pressure gas is first expanded and cooled by a turbine. Then, at a low temperature of -90 to -120°C, the components in the combined gas are condensed into liquid phases according to process requirements. Finally, the components in the condensate are separated sequentially according to their different boiling points using a distillation method.

[0005] This method is technically mature and has a well-developed process, resulting in high product purity and ethylene recovery rate. However, it generally requires gas separation at temperatures around -100℃, resulting in a large cooling load and a complex refrigeration process. It also presents challenges due to the materials used in the equipment and the presence of CO2, H2S, and NO in the dry gas. X The requirements for O2 and impurities such as arsenic and mercury are stringent, requiring deep pretreatment of raw materials, which involves a large investment.

[0006] 2. Absorption Separation Method. This method utilizes the differences in solubility of various components in a liquid absorbent to separate the gas components. Currently, the shallow-cooled oil absorption process of the Beijing Research Institute of Chemical Industry, Sinopec, is widely used, and its flow chart is shown in Appendix 1 of the instruction manual.

[0007] Refinery dry gas is compressed to 4.0 MPaG using a two-stage centrifugal compressor, then cooled to 15°C before being fed into a C4 absorber. The C4 absorber employs shallow-cooled oil absorption technology, using n-butane as the C4 absorbent to absorb C2 and higher components from the dry gas. Unabsorbed light components such as methane and hydrogen collected from the top of the absorber enter the gasoline absorber. The absorbent-rich gas from the bottom of the C4 absorber is fed into a C4 desorption tower. After desorption, the C2-rich gas from the top of the tower passes through a deoxygenation reactor to remove oxygen before entering the decarbonization section. After removing carbon dioxide and hydrogen sulfide, the C2-rich gas passes through desulfurization, arsenic and mercury removal, and a dryer before entering a pre-separation tower. In the pre-separation tower, the C2-rich gas is separated, and the ethylene-rich gas collected from the top is sent to the demethanizer of the ethylene unit. The ethane-rich gas collected from the bottom of the pre-separation tower is sent to the cracking furnace of the ethylene unit. The lean C4 absorbent obtained from the bottom of the C4 desorption tower is cooled and then recycled back to the C4 absorption tower. Simultaneously, a portion of the material is extracted and sent out as a light hydrocarbon product. The fuel gas from the top of the C4 absorption tower, carrying small amounts of C3 and C4, enters the bottom of the gasoline absorption tower. The gasoline absorbent is introduced from the top of the tower and comes into counter-current contact with the exhaust gas, absorbing the C4 and higher-weight components entrained in the gas. The gas at the top of the gasoline absorption tower, rich in methane and hydrogen, is then sent out. The gasoline-rich absorbent from the bottom of the gasoline absorption tower is sent to the gasoline stabilizer. The gaseous product at the top of the gasoline stabilizer, fuel gas, is sent to the fuel gas pipeline. The liquid product at the top of the stabilizer, C4, is recycled as part of the C4 absorbent. The liquid product at the bottom of the stabilizer, gasoline, is recycled as absorbent in the gasoline absorption tower.

[0008] This process boasts a high ethylene recovery rate and readily available absorbent feedstock. Except for the expander-cold box system, the minimum operating temperature of the unit is approximately 5°C, allowing for the use of ordinary carbon steel for the corresponding equipment and piping, significantly reducing investment. However, the process is lengthy, involves numerous pieces of equipment, and requires substantial investment. Furthermore, the dry feedstock gas needs to be compressed and pressurized to 3.5–4.0 MPa, resulting in high energy consumption.

[0009] 3. Adsorption separation method. Utilizing the differences in adsorption capacity of solid adsorbents for various components in a mixed gas, pressure swing adsorption is performed on refinery dry gas. Components with C2 and above are adsorbed onto the solid adsorbent, while components with weak adsorption capacity, such as H2, O2, N2, and CH4, pass directly through the adsorbent bed. Then, the adsorption bed is depressurized for desorption and collection to obtain ethylene-rich products.

[0010] Pressure swing adsorption (PSA) can be operated at room temperature, is highly automated, simple to operate, has low energy consumption and is environmentally friendly. However, the control system of this method is relatively complex, the purity of the ethylene obtained is low and the recovery rate is not high. Generally, the simple PSA process is suitable for enterprises with subsequent gas separation devices.

[0011] 4. Solvent extraction, chemical absorption, hydrate separation, and membrane separation are all methods, but for various reasons, they are rarely used or promoted in industry. Other methods include combined processes such as shallow cold oil absorption + membrane separation, but these involve long processes, numerous towers, and high investment and energy consumption. Summary of the Invention

[0012] One object of the present invention is to solve at least the above-mentioned problems and to provide at least the advantages that will be described later.

[0013] To achieve these and other advantages according to the present invention, a catalytic cracking dry gas recovery process is provided, comprising the following steps:

[0014] Step 1: Use a single-stage pressure swing adsorption method to separate the catalytic cracking dry gas into hydrogen-rich gas and C2 enriched gas;

[0015] Step 2: The C2 enriched gas is subjected to deoxygenation, decarbonization, drying, arsenic removal and mercury removal in sequence to obtain ethylene feed gas;

[0016] Step 3: Send the ethylene feedstock gas into the ethylene unit for processing.

[0017] Preferably, the method of pressure swing adsorption in step one is as follows:

[0018] To perform adsorption, the pressure inside the adsorption tower is increased to the preset adsorption pressure. Then, the catalytic cracking dry gas enters from the bottom of the adsorption tower and is absorbed by the adsorbent inside the adsorption tower for a preset time. The C2 and above gases in the catalytic cracking dry gas are adsorbed by the adsorbent.

[0019] At least one pressure equalization and depressurization operation is performed. After adsorption is completed, the pressure inside the adsorption tower is reduced along the adsorption direction and maintained for a certain period of time. The remaining unadsorbed gases, i.e., hydrogen-rich gases, are sent out from the top of the adsorption tower for later use.

[0020] After performing reverse release and completing pressure equalization and reduction, the pressure inside the adsorption tower is reduced to near atmospheric pressure in the opposite direction of adsorption, and the gas adsorbed by the adsorbent, namely the C2 enriched gas, is desorbed and sent out from the top of the adsorption tower.

[0021] After vacuuming and reverse release, the pressure inside the adsorption tower is further reduced to completely desorb the gas adsorbed by the adsorbent, i.e., the C2 enriched gas, which is sent out from the top of the adsorption tower.

[0022] At least one pressure equalization and pressurization operation is performed. After vacuuming is completed, hydrogen-rich gas separated from other adsorption towers is used to equalize and pressurize the current adsorption tower. The number of pressure equalization and pressurization operations corresponds one-to-one with the number of pressure equalization and depressurization operations.

[0023] After the final pressurization is completed, hydrogen-rich gas is used to pressurize the current pressure inside the adsorption tower to the preset adsorption pressure in preparation for the adsorption of dry gas from the next catalytic cracking.

[0024] Preferably, the deoxygenation treatment method in step two specifically includes:

[0025] The enriched C2 gas is compressed and pressurized to 2.3–2.5 MPaG, and adjusted to a temperature suitable for the deoxygenation reactor. It is then fed into the deoxygenation reactor, where it undergoes a catalytic reaction with a deoxygenation catalyst at 145–235°C to remove O2 and NO from the enriched C2 gas. X .

[0026] Preferably, the decarbonization treatment method in step two specifically includes:

[0027] The deoxidized C2 gas is sent to the decarbonization tower and comes into countercurrent contact with the decarbonizing agent solution flowing down from the top of the tower for heat and mass transfer. The CO2 in the C2 gas is absorbed. The decarbonized C2 gas is sent out from the top of the decarbonization tower and enters the separator to separate the entrained liquid. The decarbonized C2 gas is sent out from the top of the separator.

[0028] Preferably, the top pressure of the decarbonization tower is set to 1.9–2.0 MPa, the top temperature of the decarbonization tower is set to 44–48°C, the bottom temperature of the decarbonization tower is set to 46–58°C, and the decarbonizing agent solution is a compound amine solution.

[0029] Preferably, the drying process in step two specifically includes: sending the decarbonized C2 enriched gas into a dryer and removing moisture using a 3A molecular sieve bed.

[0030] Preferably, the operating pressure during the drying process in the dryer is 1.85–1.95 MPa, and the temperature is 12–30°C.

[0031] The pressure during regeneration of the 3A molecular sieve bed in the dryer is 0.25–0.35 MPa, and the temperature is 180–230 °C.

[0032] Preferably, the method for arsenic removal and mercury removal in step two specifically includes: passing the dried C2 enriched gas through an arsenic removal and de-pumping reactor to remove arsenic and mercury.

[0033] Preferably, the method also includes step four: sending the remaining hydrogen-rich gas into a hydrogen production unit for processing.

[0034] A catalytic cracking dry gas recovery system is provided, comprising:

[0035] Multiple pressure swing adsorption (PSA) towers are provided. The bottom feed pipes of each PSA tower are connected to the dry gas supply end of the catalytic cracking. At least one branch pipe is provided on the hydrogen-rich gas outlet pipe at the top of each PSA tower. The branch pipe is connected to the gas inlet pipe of one of the remaining PSA towers. Each PSA tower is provided with a C2 enriched gas outlet pipe. Control valves are provided on the branch pipe, the hydrogen-rich gas outlet pipe, the gas inlet pipe, and the C2 enriched gas outlet pipe.

[0036] The compressor has its inlet end connected to the C2 enriched gas outlet pipe, and the compressor outlet end is provided with two branch pipes, each of which is equipped with a control valve.

[0037] A steam heater, the cold air inlet of which is connected to one of the branch pipes of the compressor;

[0038] At least two deoxygenation reactors, the feed end of the deoxygenation reactor is connected to the outlet end of the steam heater and another branch pipe, and the two deoxygenation reactors alternately process the operation and regeneration states.

[0039] The heat exchanger has its cold air inlet end connected to the air outlet end of the deoxygenation reactor;

[0040] Decarbonization unit, comprising:

[0041] The decarbonization tower has its bottom air inlet connected to the air outlet of the heat exchanger.

[0042] A solvent storage tank is equipped with an amine circulation pump on its outlet pipe, and the outlet end of the amine circulation pump is connected to the top inlet end of the decarbonization tower.

[0043] The separator has a feed end connected to the discharge end at the top of the decarbonization tower. At least two branch pipes are provided on the gas outlet end at the top of the separator, and each branch pipe is equipped with a control valve.

[0044] The solvent regeneration tower has its inlet end connected to the outlet end at the bottom of the decarbonization tower, and the outlet end of the solvent regeneration tower is connected to the inlet end at the top of the decarbonization tower.

[0045] At least two dryers, and the top air inlets of multiple dryers are respectively connected to multiple branch pipes on the top of the liquid separator, and at least one dryer is used for standby regeneration;

[0046] The arsenic and mercury removal reactor has its top air inlet pipe connected to the air outlet of the dryer.

[0047] An ethylene unit connected to the bottom outlet of the arsenic and mercury removal reactor;

[0048] The hydrogen production unit has its feed end connected to the hydrogen-rich gas outlet pipe at the top of the tower.

[0049] The present invention has at least the following beneficial effects:

[0050] First, compared with the shallow-cooled oil absorption process, the recovery process of this application does not require C4 absorption and desorption, or gasoline reabsorption, reducing the number of three towers (C4 absorption tower, C4 desorption tower, and gasoline absorption tower) and the corresponding circulation system, resulting in a significant reduction in investment, land area, and energy consumption. The recovery process of this application only requires compression and pressurization of the C2 enrichment gas, which reduces the load rate by about 75% and the pressure by about 37% compared with the shallow-cooled oil absorption process, significantly reducing the investment and energy consumption of the compressor unit. The decarbonization system uses a sampling decarbonization agent recycling process to replace the original alkaline washing process, which significantly reduces operating costs and avoids the problem of subsequent waste alkali treatment.

[0051] Secondly, compared with the multi-stage PSA adsorption process, the recovery process of this application adopts a single-stage PSA adsorption method, which eliminates the need for a pressure swing adsorption device. Therefore, the single-stage method requires less land, has lower initial construction investment, and lower energy consumption in later operation.

[0052] Third, the recovery process of this application sends the hydrogen-rich gas, which is rich in methane and hydrogen, to a hydrogen production unit as a raw material to produce hydrogen, thereby maximizing the value utilization of CH4 and H2. After deoxygenation, decarbonization, drying, arsenic removal, and mercury removal, the concentrated C2 gas, which is rich in ethane and ethane, is refined and sent to a light hydrocarbon cracking / / ethylene unit, so that ethylene and ethane can be recovered and utilized.

[0053] Fourth, the recycling process in this application adopts a deoxygenation followed by decarbonization process. Firstly, the catalyst in the deoxygenation reactor is a nickel-based catalyst. During the reaction, DMDS needs to be injected into the nickel-based catalyst, and the excess H2S generated can be removed in the decarbonization unit, protecting the downstream catalyst. Secondly, the plant has deoxygenated water available for the decarbonization unit, ensuring that no oxygen is introduced into the process system during the deoxygenation followed by decarbonization process. Thirdly, the decarbonization system uses a compounded amine solution for decarbonization. If decarbonization is performed first followed by deoxygenation, the amine solution may enter the deoxygenation reactor and damage the catalyst during unstable operation.

[0054] Fifth, the recycling process described in this application is short, requires few equipment, occupies little space, requires little investment, and has low energy consumption.

[0055] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Attached Figure Description

[0056] Figure 1 is a schematic diagram of the absorption separation method in the prior art;

[0057] Figure 2 is a schematic flowchart of the recycling process according to one of the technical solutions of the present invention. Detailed Implementation

[0058] The present invention will now be described in further detail with reference to the accompanying drawings, so that those skilled in the art can implement it based on the description.

[0059] It should be noted that, unless otherwise specified, the experimental methods described in the following embodiments are all conventional methods, and the reagents and materials described are all commercially available unless otherwise specified. In the description of this invention, the orientation or positional relationship indicated by the terms is based on the orientation or positional relationship shown in the accompanying drawings, and is only for the convenience of describing this invention and simplifying the description. It does not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention.

[0060] As shown in Figure 2, the present invention provides a catalytic cracking dry gas recovery process, comprising the following steps:

[0061] 1. Detailed process and implementation method of pressure swing adsorption (PSA):

[0062] First, the raw gas (catalytic cracking dry gas) enters the adsorption tower from the bottom. The adsorption tower is filled with adsorbent. The easily adsorbed C2+ gas is adsorbed by the adsorbent and remains in the adsorption tower, while the hydrogen-rich gas that is not easily adsorbed by the adsorbent is sent out from the top of the adsorption tower. Each adsorption tower goes through at least the following steps in sequence: adsorption, pressure equalization and depressurization, reverse release, vacuuming, pressure equalization and pressurization, and final pressurization.

[0063] The specific processes of each step in the above pressure swing adsorption method are as follows:

[0064] 1.1 Adsorption, denoted by A:

[0065] The feed gas enters the adsorption tower from the bottom. The C2+ gas in the feed gas is adsorbed and retained in the adsorption tower by the adsorbent packed in the adsorption tower. The hydrogen-rich gas, which is not easily adsorbed, is sent out from the top of the adsorption tower. The hydrogen-rich gas has two destinations: one is sent to the hydrogen production unit for processing, and the other is sent to other adsorption towers that need to perform pressure equalization and pressurization operations. The two destinations are regulated by control valves and branch pipes.

[0066] 1.2 Equalizing pressure drop, denoted by ED:

[0067] After the adsorption process is complete, the higher-pressure hydrogen-rich gas in the adsorption tower is transferred to a lower-pressure adsorption tower in a pressure equalization and pressure-boosting state (i.e., one of the destinations mentioned above). This process is not only a depressurization process but also a process of recovering the gas in the adsorption tower bed. The number of pressure equalization and depressurization cycles is set according to the adsorption pressure, but at least once.

[0068] 1.3 Reverse playback, denoted by D:

[0069] After the pressure equalization and depressurization steps are completed, the pressure inside the adsorption tower is reduced to near atmospheric pressure in the opposite direction of adsorption. At this time, the adsorbed C2+ gas begins to desorb in large quantities from the adsorbent. The desorbed gas is then smoothly introduced into the desorbed gas buffer tank after being regulated by the adaptive adjustment system, thus obtaining the concentrated C2 gas.

[0070] 1.4 Vacuuming, denoted by V:

[0071] A vacuum pump is used to evacuate the adsorption tower after reverse release, further reducing the pressure inside the adsorption tower and causing the C2+ gas adsorbed in the adsorbent to be completely desorbed. The vacuum desorbed gas also enters the desorbed gas mixing tank, mixes with the reverse-released desorbed gas, and is then sent out of the boundary as product gas (C2 enrichment gas).

[0072] 1.5 equalization boost, denoted by ER:

[0073] After vacuuming is completed, the adsorption tower is sequentially pressurized with higher-pressure gas from other adsorption towers that are in a pressure equalization and depressurization state. The pressure equalization and pressurization process corresponds one-to-one with the pressure equalization and depressurization process. It is not only a pressurization process, but also a process of recovering gas from the beds of other adsorption towers. The number of pressure equalization and pressurization processes is the same as the number of pressure equalization and depressurization processes.

[0074] 1.6 Final Upgrade FR

[0075] The final pressurization step involves using hydrogen-rich gas from the top of the adsorption tower to pressurize the tower to reach the preset adsorption pressure. Once the pressure inside the tower reaches the preset adsorption pressure, the tower enters the next cycle of adsorption.

[0076] In the above-described pressure swing adsorption (PSA) process, parameters are adjusted based on the PSA equipment and the catalytic cracking dry gas. Taking one example (dragon oil dry gas) as an example, the PSA process is described in detail below, with the parameters set as follows:

[0077] A total of 10 pressure swing adsorption (PSA) towers were set up, namely tower A, tower B, tower C, tower D, tower E, tower F, tower G, tower H, tower I, and tower J. The time, pressure, and temperature parameters for each step of each PSA tower are shown in Table 1 and Table 2, respectively.

[0078] Table 1. Time parameters for each specific step of each pressure swing adsorption tower.

[0079] Step sequence 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 T1 T2 T1 T2 T1 T2 T1 T2 T1 T2 T1 T2 T1 T2 T1 T2 T1 T2 T1 T2 T1 T2 T1 T2 T1 T2 T1 T2 T1 T2 T1 T2 T1 T2 Time 30S ... RAAAAE1DE2DE3DE4DE5DDVVVVE Tower VVE5RE4RE3RE2RE1RFRAAAAE1DE2DE3DE4DE5DDVVF Tower VVVVE5RE4RE3RE2RE1RFRAAAAE1DE2DE3DE4DE5DDG Tower E5DDVVVVE5RE4RE3RE2RE1RFRAAA AE1DE2DE3DE4DH Tower E3DE4DE5DDVVVVE5RE4RE3RE2RE1RFRAAAAE1DE2DI Tower E1DE2DE3DE4DE5DDVVVVE5RE4RE3RE2RE1RFRAAAAJ Tower AAE1DE2DE3DE4DE5DDVVVVE5RE4RE3RE2RE1RFRAA surface

[0080] Note: A: Adsorption; E1D~E5D: Pressure equalization and pressure drop from one to five; D: Reverse release; V: Vacuum;

[0081] E1R~E5R: Leveling boost from one to five; FR: Final boost for product;

[0082] Both T1 and T2 represent adjustable time parameters;

[0083] The time parameters are set as follows:

[0084] Adsorption time: 120s

[0085] Pressure equalization and reduction time: 150s

[0086] Equalization and boost time: 150s

[0087] Rewind time: 30s

[0088] Vacuuming time: 120s

[0089] Final rise time: 30s

[0090] Total process: 600 seconds;

[0091] Table 2 shows the pressure and temperature parameters for each specific step in each pressure swing adsorption tower.

[0092] Step Number | Operating Pressure (MPa) | Temperature 1. Adsorption (A) | 0.85 | Ambient Temperature 2. Equalization and Pressure Reduction (ED) | 0.85 → 0.07 | Ambient Temperature 3. Reverse Expansion (D) | 0.07 → 0.02 | Ambient Temperature 4. Vacuum (V) | 0.02 → -0.08 | Ambient Temperature 5. Equalization and Pressure Increase (ER) | -0.08 → 0.67 | Ambient Temperature 6. Final Pressure Increase (FR) | 0.67 → 0.85 | Ambient Temperature surface

[0093] The existing two-stage pressure swing adsorption (PSA) method recovers C2+ components by using two stages of PSA. The feed gas first enters the first stage PSA unit, where it undergoes adsorption separation, and the product gas is produced from the bottom of the PSA tower. To improve the recovery rate, the hydrogen-rich gas produced at the top of the tower is then fed into the second stage PSA unit for further recovery of the C2+ components. The two-stage method requires two PSA units, resulting in significant land occupation, initial investment, and ongoing operating costs.

[0094] Compared to two-stage pressure swing adsorption (PSA), single-stage PSA requires one less PSA unit, resulting in a smaller footprint, lower initial investment, and lower energy consumption during operation. In the above embodiments, the recovery rate of component C2 using single-stage PSA can reach over 95%.

[0095] 2. The concentrated C2 gas obtained after pressure swing adsorption is then subjected to deoxygenation, decarbonization, drying, arsenic removal, and mercury removal treatments in sequence. Detailed process and implementation methods are as follows:

[0096] 2.1 Deoxygenation treatment is carried out using a deoxygenation reactor:

[0097] The purpose of deoxygenation is to remove O2 and NO from the C2 refined gas. x This is to prevent it from causing deterioration of the decarbonizing agent and amine solution in the post-processing.

[0098] Implementation method: After 75% of methane has been removed by PSA (Pressure Swing Adsorption), the enriched C2 gas (C2+ gas) is compressed to 2.3–2.5 MPaG by a compressor. The temperature of the enriched C2 gas is adjusted to 145–235℃ and then enters the deoxygenation reactor. The deoxygenation reactor uses a Mo-Ni-S system BC-TOS-15 catalyst for catalytic reaction. First, the metal oxides in the catalyst react with the sulfur in the sulfide gas at a certain temperature to form metal sulfides, O2, and NO. x It reacts with H2 in the presence of metal sulfides to produce H2O and N2, thereby removing O2 and NO. xThe reaction equation is shown below:

[0099] H₂S + NiO → NiS + H₂O

[0100] O2 + H2 → H2O

[0101] 2NO + 2H₂ → N₂ + 2H₂O

[0102] The operating parameters for one embodiment (dragon oil dry gas) deoxygenation reactor are shown in Table 3:

[0103] Table 3 Operating parameters of the deoxygenation reactor

[0104]

[0105] 2.2 Decarburization is performed using a decarburization unit:

[0106] The purpose of decarbonization is to remove acidic gases such as CO2 from the C2 concentrate, preventing them from causing excessive load on the alkaline scrubbing tower in the ethylene plant and affecting the quality of ethylene products. The decarbonization unit adopts an absorption-regeneration decarbonization method.

[0107] Implementation method: The C2 enriched gas flows from bottom to top in the decarbonization tower and comes into countercurrent contact with the decarbonizing agent solution (compound amine solution) flowing down from the top of the decarbonization tower. Heat and mass transfer are carried out, and CO2 in the C2 enriched gas is absorbed. The decarbonized C2 enriched gas comes out from the top of the decarbonization tower and enters the separatory tank. The liquid entrained in the C2 enriched gas is separated here. The CO2 content in the C2 enriched gas at the top of the separatory tank reaches no more than 100 ppm.

[0108] The operating parameters for the decarbonization tower in one embodiment (dragon oil dry gas) are shown in Table 4:

[0109] Table 4 Operating parameters of the decarbonization tower

[0110] Decarbonization tower top pressure PIC-98101 (pressure control) MPa 1.9~2.0; Decarbonization tower top temperature TI-98102 (temperature display) °C 44~48; Decarbonization tower bottom temperature TI-98106 (temperature display) °C 46~58 surface

[0111] 2.3 Drying treatment is carried out using a dryer:

[0112] The purpose of drying is to remove the original moisture in the C2 extraction gas, as well as the moisture generated during the deoxygenation reaction and the moisture carried in during the decarbonization process, so as to prevent the downstream light hydrocarbon cracking or ethylene unit separation system from being overloaded with drying load and the risk of freezing and blockage of the separation system due to excessive water content.

[0113] Implementation method: The decarbonized C2 enriched gas enters the feed quench cooler of the dryer for cooling, and then removes moisture through the 3A molecular sieve bed.

[0114] 3A molecular sieves are renewable adsorbents that utilize the principle of physical adsorption to adsorb impurities, relying on the forces (van der Waals forces) of the molecular sieve during adsorption. Type A molecular sieves primarily adsorb highly polar molecules such as water, ammonia, and alcohols. Another characteristic of molecular sieves as adsorbents is shape-selective adsorption. The relatively uniform crystalline structure of the molecular sieve results in uniform pore sizes. Only molecules with diameters smaller than the pore size of a specific molecular sieve can pass through the surface and be adsorbed. Molecules with diameters larger than the pore size are repelled and cannot be adsorbed. Therefore, the pore size of 3A molecular sieves is approximately 3 angstroms. Theoretically, only four molecules in nature have a diameter smaller than 3 angstroms: water, ammonia, hydrogen, and helium. Therefore, 3A molecular sieves can be used for dehydration and drying of certain polar molecular materials.

[0115] The operating parameters of one embodiment (dragon oil dry air) dryer are shown in Table 5:

[0116] Table 5 Operating parameters of the dryer

[0117] State pressure (MPa) Temperature (°C) Operating 1.85~1.95 12~30 Regeneration 0.25~0.35 180~230 surface

[0118] 2.4 Arsenic and mercury removal reactors were used for arsenic and mercury removal treatments:

[0119] The purpose of arsenic removal and mercury removal treatments is to remove heavy impurities such as arsenic and mercury.

[0120] Implementation method: The enriched C2 gas enters from the top of the arsenic and mercury removal reactor and exits from the bottom. The arsenic removal agent TAS-03 in the reactor is a non-renewable adsorbent. A chemical reaction occurs during use. This arsenic removal agent is composed mainly of lead and its oxides, with added auxiliary components. It exhibits good activity at room temperature, high arsenic removal efficiency, long service life, and no hydrogenation side reaction. It is used to remove trace arsenides from petroleum gas and inert gases, and is particularly suitable for the removal of arsenides from petroleum cracking gases containing alkynes (acetylene, propyne, etc.).

[0121] The chemical equation for the reaction is: 3PbO + 2AsH3 → Pb3As + As + 3H2O and...

[0122] 3PbO + 2AsH3 → 3Pb + 2As + 3H2O.

[0123] The mercury removal agent HBT-108 used in the arsenic and mercury removal reactor is also a non-renewable adsorbent. A chemical reaction occurs during its use. HBT-108 is made from high-quality activated carbon with added special activators and additives, processed using a special process. It is widely applicable to the treatment of natural gas, light hydrocarbons, and other waste gases containing mercury [Hg]. It is particularly suitable for treating low-concentration mercury-containing gases that cannot be removed by other methods, resulting in a mercury content of less than 0.01 ug / Nm³ after treatment. 3 .

[0124] The chemical equation for the reaction is: S + Hg → HgS.

[0125] The operating parameters for one embodiment (dragon oil dry gas) arsenic and mercury removal reactor are shown in Table 6:

[0126] Table 6 Operating parameters of the arsenic and mercury removal reactor

[0127]

[0128] 3. Use an ethylene unit to treat C2 refined gas.

[0129] C2 refined gas is used as a feedstock for ethylene plants, and after processing, it yields high-value products such as ethylene.

[0130] In summary, the above process involves the following steps: the feed gas (catalytic cracking dry gas) comes from the catalytic cracking unit → feed gas separator → gas phase to PSA adsorption tower (pressure swing adsorption tower) → unadsorbed CH4, CO, O2, N2, and H2 enter the hydrogen production unit as feed; the adsorbed C2 concentrate is desorbed and then pressurized by a compressor → deoxygenation reactor → decarbonization tower → dryer → arsenic and mercury removal reactor, and then sent to the alkaline washing tower of the light hydrocarbon cracking unit or ethylene unit.

[0131] <Example>

[0132] A 280kt / a dry gas recovery system was constructed and put into operation at Heilongjiang Longyou Petrochemical Co., Ltd. in Daqing City, Heilongjiang Province. The recovery system includes:

[0133] 1. PSA adsorption system

[0134] The PSA adsorption system consists of 10 pressure swing adsorption towers, with 2 towers adsorbing simultaneously and 2 buffer tanks. It is equipped with 5 continuous pressure equalization and depressurization cycles to ensure continuous vacuum regeneration, hence the name 10-2-5PSA.

[0135] The process employs a 10-2-5PSA flow rate, meaning that two of the ten adsorption towers are always simultaneously fed and adsorbing. The adsorption and regeneration process consists of adsorption, five consecutive pressure equalization and depressurization cycles, reverse release, vacuuming, five consecutive pressure equalization and pressurization cycles, and a final pressurization. The ten adsorption towers are designated as tower A, tower B, tower C, tower D, tower E, tower F, tower G, tower H, tower I, and tower J.

[0136] The specific process is briefly described below:

[0137] Catalytic cracking dry gas (long oil dry gas) is sent to adsorption towers C-951A~J. The unadsorbed CH4, CO, O2, N2, and H2 at the top of the adsorption tower enter the hydrogen production unit. The adsorbed C2 components are then separated through multiple pressure equalization and depressurization, reverse release, and vacuuming steps. The resulting concentrated C2 gas enters the reciprocating compressor K-951A / B.

[0138] After being processed by a PSA adsorption system, the catalytic cracking dry gas is sent to downstream hydrogen production units to produce hydrogen. The C2 enriched gas is then purified by a refining system (deoxygenation reactor, decarbonization tower, regeneration tower, dryer, and arsenic and mercury removal reactor) to remove O2 and NO. x After treatment (containing impurities such as CO2, water, sulfur, arsenic, and mercury), it is sent to the alkaline washing tower of the light hydrocarbon cracking unit.

[0139] 2. Refining System

[0140] The refining system includes a deoxygenation reactor, a decarbonization tower, a regeneration tower, a dryer, and an arsenic and mercury removal reactor.

[0141] 2.1 Deoxygenation process:

[0142] After 75% of the methane is removed by the adsorption system, the C2 enriched gas is pressurized to 2.3–2.5 MPaG by a compressor, and then the temperature of the C2 enriched gas is adjusted to 145–235°C before entering the deoxygenation reactor to remove O2 and NO. X .

[0143] The enriched C2 gas from the top of the deoxygenation feed tank (1053-D-975) exchanges heat with the purified enriched C2 gas in the purified gas preheater (1053-E-972A / B). Part of the enriched C2 gas is mixed with DMDS (dimethyl disulfide) from the DMDS injection pump (as needed) and then enters the purified gas steam heater (1053-E-973) to be heated to 205℃~220℃. The other enriched C2 gas is not heated by the steam heater; the inlet temperature of the deoxygenation reactor is controlled by adjusting the amount of cold enriched C2 gas. The two enriched C2 gas streams are mixed and enter the upper part of the deoxygenation reactor (1053-R-971A / B). Inside the deoxygenation reactor, O2 and NO... XIt reacts with hydrogen to produce water and nitrogen. The deoxygenated C2 enriched gas comes out from the bottom of the deoxygenation reactor, and after heat exchange in the purified gas preheater (1053-E-972A / B), it is cooled to 45°C in the deoxygenated mixed gas cooler (1053-E-971) and sent to the decarbonization tower feed tank (1053-D-982).

[0144] The deoxygenation reactor is an adiabatic bed reactor, designed in pairs, one operating normally and the other regenerated and on standby. Each deoxygenation reactor contains approximately 10.8 tons of catalyst (approximately 18 m³). 3 ).

[0145] In deoxygenation reactions, the sulfidated catalyst exhibits catalytic activity. Therefore, before use and after regeneration, the catalyst should be sulfidated by introducing a sulfidating gas containing H2S or capable of producing H2S. After a period of use, as carbon deposits form, the catalyst activity decreases. To maintain catalyst activity, the reaction temperature needs to be gradually increased. When the reaction temperature reaches 220–230°C and the outlet oxygen content is still unacceptable, the deoxygenation catalyst should be regenerated immediately.

[0146] 2.2 Decarbonization process:

[0147] The decarbonization unit consists of a decarbonization tower (1053-C-981), a solvent regeneration tower (1053-C-982), a filter (1053-SR-982), a solvent storage tank (1053-D-984), and an amine circulation pump (1053-P-981A / B). It uses a compounded decarbonization agent solution as an absorbent to absorb acidic gases such as CO2 from the C2 concentrate.

[0148] The C2 enriched gas from the top of the decarbonization tower feed tank (1053-D-982) enters the bottom of the decarbonization tower (1053-C-981); the lean amine solution delivered by the amine circulation pump (1053-P-981A / B) enters the top of the decarbonization tower. Inside the decarbonization tower, the C2 enriched gas flows upwards, countercurrently contacting the decarbonizing agent solution coming down from the top of the tower, undergoing heat and mass transfer. CO2 in the gas is absorbed, and the decarbonized C2 enriched gas exits from the top of the decarbonization tower and enters the separator (1053-D-981), where the entrained liquid is separated. The C2 enriched gas (CO2 content ≤100ppm) exiting from the top of the separator is sent to the dryer feed quencher (1053-E-991).

[0149] The liquid separated by the separator (1053-D-981) and the carbon dioxide-absorbed decarbonizing agent solution flowing from the bottom of the decarbonization tower (1053-C-981) are preheated by the solvent heat exchanger (1053-E-982A / B) and then sent to the top of the solvent regeneration tower (1053-C-982). The regeneration tower condenser (1053-E-983) is located at the top of the tower, and circulating water is used to cool the gas at the top of the tower to 45°C. The condensate is directly returned to the solvent regeneration tower. Acidic gas is discharged from the boundary area through a regulating valve to the acidic gas flare main.

[0150] The lean decarbonizing agent solution collected from the solvent regeneration tower is cooled by the solvent heat exchanger (1053-E-982A / B) and then cooled to 45°C by the solvent cooler (1053-E-981) before being discharged into the solvent storage tank (1053-D-984); the lean amine solution is pressurized by the pump and then enters the upper part of the decarbonizing tower for recycling.

[0151] To maintain the cleanliness of the system solution, a solution filtration device is installed. When the solution is contaminated or degraded, a partial lean solution filtration method is adopted: first, activated carbon is used to adsorb degradation products and larger impurities, and then a post-solvent filter (1053-SR-981A / B) is used to filter out impurities larger than 5μm. Thermo-stable salts in the system are separated by sedimentation in a solvent storage tank (1053-D-984) and are finally sent out of the boundary along with the waste amine solution when the unit is shut down.

[0152] To prevent problems such as decreased processing capacity and increased solvent loss caused by foaming of the decarbonizing agent solvent, it is necessary to add defoamer to the decarbonization system when necessary. The defoamer is injected intermittently, with each injection being approximately 50 ml. The injection point is set at the inlet of the amine liquid circulation pump (1053-P-981A / B), injected into the decarbonization tower, and then carried to the solvent regeneration tower along with the rich amine liquid.

[0153] The solvent storage tank (1053-D-984) is used to store decarbonizing agent solvent. The tank is sealed with nitrogen, and the solution in the tank is circulated by an amine circulation pump.

[0154] The decarbonizing agent solvent discharged during the discharge or maintenance of the decarbonization unit's filtration system is collected in an underground tank (1053-D-986). After being pressurized by the waste amine liquid pump (1053-P-982) and purified by the filtration system, it is then pumped to the solvent storage tank (1053-D-984) for storage or sent out of the boundary area.

[0155] 2.3 Drying process:

[0156] The dryer is filled with 3A molecular sieve desiccant and desulfurizing agent, which removes trace amounts of organic sulfur from the material while dehydrating it. The enriched gas after deoxygenation is cooled to 15°C in the dryer feed quencher (1053-E-991) and then enters the dryer feed separator (1053-D-991) to separate saturated water. The gas at the top of the separator enters the dryer (1053-DR-991A / B) for dehydration. The enriched gas enters from the top of the dryer and exits from the bottom. Two dryers are designed, one operating normally and the other regenerating on standby, with a designed regeneration cycle of 7 days. Cold regeneration gas is introduced from outside the boundary, and the regenerated waste gas is then sent to the fuel gas pipeline network. The dried enriched gas is then sent to the arsenic / mercury removal reactor (1053-R-991).

[0157] 2.4 Arsenic and Mercury Removal Process:

[0158] The dried C2 enriched gas is sequentially passed through an arsenic / mercury removal reactor (1053-R-991) to remove heavy impurities such as arsenic and mercury. The mercury and arsenic removal catalysts adsorb mercury and arsenic from the enriched gas. The reactor is a single-pass type and requires no regeneration. The enriched gas enters from the top of the reactor and exits from the bottom. The enriched gas, after impurity removal, is then sent to the ethylene unit.

[0159] Advantages of the recycling system in this embodiment:

[0160] (1) Achieving continuous pressure equalization five times while realizing continuous vacuum regeneration of two towers can accurately control the regeneration flow rate and solve the problem of secondary pollution in the traditional vacuum process, thus improving the regeneration effect of the adsorbent.

[0161] (2) The 10-2-5PSA process has a longer reverse release time and vacuum time, resulting in better mass transfer during adsorbent regeneration, more thorough regeneration, higher hydrogen recovery rate, and reduced reverse release noise.

[0162] (3) When a fault occurs in an adsorption tower, the faulty tower can be automatically and undisturbed, and the system can switch to operating towers 9, 8, 7, 6, and 5. While a series of towers are being shut down, the towers to be shut down can be repaired at will, while the remaining towers continue to operate. This greatly improves the reliability of the system operation.

[0163] (4) The PSA programmable valve, a key piece of equipment, is a high-performance programmable valve, a patented product of Chengdu Huaxi Chemical Technology Co., Ltd. It has the advantages of small size, light weight, fast action (less than 2s), good sealing performance (ANSI level 6), long service life (more than 1 million cycles), independently adjustable switching speed (1-30s), as well as a closing buffer function and reliable valve position display, which ensures the reliability of the long-term operation of the equipment.

[0164] (5) The adsorbent developed by Chengdu Yizhi is used for C2+ recovery. The adsorbent has a large dynamic adsorption capacity and high precision in removing impurities, which can more effectively ensure product quality and recovery rate.

[0165] (6) The PSA unit program control system is recommended to adopt a hydraulic program control system, which has the characteristics of high operational stability, good rigidity, smooth operation, safe and reliable operation, and long service life. Its valve opening speed adjustment function can control the pressure equalization speed, reduce the scouring of the adsorbent by the airflow, greatly extend the service life of the adsorbent, and reduce the noise of the device.

[0166] (7) The advanced and mature control software package of the PSA unit can automatically optimize the adsorption time and adaptively adjust the adsorption pressure to ensure the product qualification and the highest yield.

[0167] (8) This recovery system uses a deoxygenation catalyst independently developed by Beijing Institute of Chemical Technology to remove O2 and NO from the C2 concentrate. X Impurities. Non-precious metal catalysts can be further divided into imported non-precious metal catalysts and domestically produced non-precious metal catalysts, represented by the deoxygenation catalyst from the Beijing Institute of Chemical Technology (BICT). Imported non-precious metal catalysts are expensive and have long delivery cycles. BICT's deoxygenation catalyst is inexpensive, has a short delivery cycle, and has no restrictions on the content of impurities such as sulfur and arsenic in the raw materials. It is a catalyst capable of simultaneously removing O2 and NO. x This is a bifunctional catalyst. Units using this deoxygenation catalyst are easy to operate, run stably, are highly reliable, can achieve long-term continuous operation, and can be maintained synchronously with other units throughout the plant.

[0168] (9) This recovery system adopts a process flow of deoxygenation followed by decarbonization. First, the catalyst in the deoxygenation reactor is a nickel-based catalyst. During the reaction, DMDS needs to be injected into the nickel-based catalyst. The excess H2S generated can be removed in the decarbonization unit, protecting the downstream catalyst. Second, the plant has deoxygenated water for the decarbonization unit, which can ensure that no oxygen is introduced into the process system during the process of deoxygenation followed by decarbonization. Third, the decarbonization system uses a compound amine solution for decarbonization. If decarbonization is performed first and then deoxygenation is performed, the amine solution may enter the deoxygenation reactor and damage the catalyst when the operation is unstable.

[0169] The following are the raw material details and specifications for this embodiment:

[0170] The system feedstock comes from purified catalytic cracking dry gas produced by a 2.3 million tons / year heavy oil catalytic thermal cracking unit, with a processing capacity of 280,000 tons / year. The composition of the purified catalytic cracking dry gas is shown in Table 7.

[0171] Table 7 Composition of Purified Catalytic Cracking Dry Gas Feedstock

[0172]

[0173]

[0174] The composition and properties of the C2 enriched gas after treatment by the recovery system are shown in Table 8:

[0175] Table 8 Composition and Properties of C2 Extraction Concentrate

[0176]

[0177]

[0178] The composition and properties of the hydrogen-rich gas after treatment by the recovery system are shown in Table 9:

[0179] Table 9 Composition and properties of hydrogen-rich gas

[0180]

[0181]

[0182] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and illustrations shown and described herein.

Claims

1. A catalytic cracking dry gas recovery process, characterized in that, Includes the following steps: Step 1: Separate hydrogen-rich gas and C2 enriched gas from catalytic cracking dry gas using a single-stage pressure swing adsorption (PSA) method. Step 2: Separate the C2 enriched gas through deoxygenation, decarbonization, drying, arsenic removal, and mercury removal treatments to obtain ethylene feedstock gas. Step 3: Send the ethylene feedstock gas to the ethylene plant for further processing. Specifically, the deoxygenation treatment in Step 2 includes: compressing and increasing the pressure of the C2 enriched gas to 2.3~2.5 MPaG, adjusting it to a temperature suitable for the deoxygenation reactor, and then feeding it into the deoxygenation reactor. The gas undergoes a catalytic reaction with a deoxygenation catalyst at 145~235℃ to remove O2 and NO from the C2 enriched gas. X The decarbonization process in step two specifically includes: the deoxidized C2 concentrate gas is sent to the decarbonization tower and comes into countercurrent contact with the decarbonizing agent solution flowing down from the top of the tower for heat and mass transfer. CO2 in the C2 concentrate gas is absorbed. The decarbonized C2 concentrate gas is then discharged from the top of the tower and enters a separatory tank to separate the entrained liquid. The decarbonized C2 concentrate gas is then discharged from the top of the separatory tank. The top pressure of the decarbonization tower is set to 1.9~2.0 MPa, the top temperature is set to 44~48℃, and the bottom temperature is set to 46~58℃. The decarbonizing agent solution is a compound amine solution. The drying process in step two specifically includes: the decarbonized C2 concentrate gas is sent to a dryer and moisture is removed using a 3A molecular sieve bed. The operating pressure during drying in the dryer is 1.85~1.95 MPa. MPa, temperature 12~30℃; the pressure during regeneration of the 3A molecular sieve bed in the dryer is 0.25~0.35 MPa, temperature 180~230℃; the specific methods for arsenic removal and mercury removal in step two include: the dried C2 enriched gas is passed through an arsenic removal and pump removal reactor to remove arsenic and mercury; it also includes step four, sending the remaining hydrogen-rich gas to a hydrogen production unit for processing; the specific method of the first-stage pressure swing adsorption method in step one is as follows: adsorption is performed, the pressure inside the adsorption tower is increased to the preset adsorption pressure, and then the catalytic cracking dry gas enters from the bottom of the adsorption tower. After the adsorbent in the adsorption tower absorbs for a preset time, the C2 and above gases in the catalytic cracking dry gas are adsorbed by the adsorbent; at least one pressure equalization and depressurization is performed. After the adsorption is completed, the pressure inside the adsorption tower is reduced along the adsorption direction and maintained for a certain time, and the other unadsorbed gases, i.e., the hydrogen-rich gas, are sent out from the top of the adsorption tower for preparation. The process involves: 1) Performing reverse release: After pressure equalization and reduction, the pressure inside the adsorption tower is reduced to near atmospheric pressure in the opposite direction of adsorption, desorbing the gas adsorbed by the adsorbent (i.e., C2 enriched gas), which is then discharged from the top of the adsorption tower; 2) Performing vacuuming: After reverse release, the pressure inside the adsorption tower is further reduced to completely desorb the gas adsorbed by the adsorbent (i.e., C2 enriched gas), which is then discharged from the top of the adsorption tower; 3) Performing at least one pressure equalization and pressure increase: After vacuuming, hydrogen-rich gas separated from other adsorption towers is used to equalize and increase the pressure of the current adsorption tower, with the number of pressure equalization and pressure increase cycles corresponding to the number of pressure equalization and pressure reduction cycles; 4) Performing final pressure increase: After pressure equalization and pressure increase, hydrogen-rich gas is used to increase the pressure inside the current adsorption tower to the preset adsorption pressure, in preparation for the next adsorption of catalytic cracking dry gas; the adsorption pressure is 0.

85. MPaG, the equalization pressure drop decreased from 0.85 MPaG to 0.07 MPaG, the reverse discharge pressure decreased from 0.07 MPaG to 0.02 MPaG, and the vacuum pressure decreased to -0.08 MPaG.

2. A catalytic cracking dry gas recovery system based on the process described in claim 1, characterized in that, include: Multiple pressure swing adsorption (PSA) towers, each with its bottom feed pipe connected to the catalytic cracking dry gas supply end, and at least one branch pipe on the hydrogen-rich gas outlet pipe at the top of each PSA tower, the branch pipe connecting to the inlet pipe of one of the remaining PSA towers. Each PSA tower has a C2 enriched gas outlet pipe, and control valves are installed on the branch pipe, the hydrogen-rich gas outlet pipe, the inlet pipe, and the C2 enriched gas outlet pipe. A compressor, its inlet connected to the C2 enriched gas outlet pipe, has two branch pipes at its outlet, each equipped with a control valve. A steam heater, its cold flow inlet connected to one of the compressor's branch pipes. At least two deoxygenation reactors, each with its feed end connected to both the steam heater's outlet and another branch pipe, alternately operating and regenerating. A heat exchanger, the cold air inlet of which is connected to the air outlet of the deoxygenation reactor; a decarbonization unit, comprising: a decarbonization tower, the bottom air inlet of which is connected to the air outlet of the heat exchanger; a solvent storage tank, the outlet pipe of which is equipped with an amine circulation pump, the outlet of which is connected to the top air inlet of the decarbonization tower; a separator, the feed end of which is connected to the top discharge end of the decarbonization tower, the top air outlet of which is equipped with at least two branch pipes, each branch pipe being equipped with a control valve; and a solvent regeneration tower, the inlet of which is... The solvent regeneration tower has its outlet connected to the bottom of the decarbonization tower, and its outlet connected to the top of the decarbonization tower. It also includes at least two dryers, with the top inlets of multiple dryers corresponding to multiple branch pipes at the top of the separator, and at least one dryer for standby regeneration. A dearsenic and mercury removal reactor has its top inlet connected to the outlet of the dryer. An ethylene unit is connected to the bottom outlet of the dearsenic and mercury removal reactor. A hydrogen production unit has its feed end connected to the hydrogen-rich outlet pipe at the top of the tower.

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