Hydrocarbon cascade recovery system and method for methanol off-gas and multi-source tail gas

Abstract

This invention relates to the field of coal chemical industry, specifically disclosing a hydrocarbon cascade recovery system and method for methanol off-gas and multi-source tail gas. The system includes structural units such as off-gas pretreatment, primary membrane separation, primary pressure swing adsorption (PSA) hydrogen extraction, CO selective conversion, tail gas integration, deep conversion, secondary PSA hydrogen extraction, and secondary membrane separation. It aims to achieve graded utilization of carbon resources through a two-stage conversion process of "CO selective conversion + deep conversion," and to recover hydrogen step-by-step using a "membrane + PSA + membrane" cascade separation. This system moderately converts high-grade CO for reuse, deeply converts low-grade CO into hydrogen, and reduces energy consumption through thermal integration. The total hydrogen recovery rate is consistently above 99%, the effective gas recovery rate is significantly improved, and the system is flexible and adaptable.

Description

Technical Field

[0001] This invention relates to the field of coal chemical industry, specifically to a hydrocarbon cascade recovery system and method for methanol off-gas and multi-source tail gas. Background Technology

[0002] In the methanol synthesis process, to prevent the accumulation of inert gases, purge gases must be continuously released. Furthermore, processes such as crude methanol flash evaporation and pre-distillation also generate tail gases containing hydrogen and carbon monoxide (CO). These gases are rich in valuable H2 and CO, which can be used as syngas feedstock. Directly using them as fuel or venting them results in resource waste and increased carbon emissions.

[0003] Currently, industrial applications primarily utilize membrane separation and pressure swing adsorption (PSA) technologies to recover hydrogen, with polyimide or polysulfone membranes being widely adopted. For example, patent CN105460891B discloses a method where methanol purge gas is separated via membrane, the permeate is then processed through PSA to produce hydrogen, and the non-permeate gas is directly returned to the methanol system. This method does not utilize CO in the non-permeate gas, resulting in low carbon resource utilization. Patent CN115845573A discloses a system integrating multiple gas sources, which converts all CO to H2 through membrane separation, PSA, and a deep conversion reaction. While this method improves hydrogen yield, it completely consumes CO, negating its value as a direct syngas feedstock for reuse. Furthermore, the deep conversion process is energy-intensive and complex.

[0004] Based on research and industry analysis, the common shortcomings of existing technologies are: 1) The utilization mode of CO resources is singular, either directly discarded or completely converted, failing to achieve "making the most of resources"; 2) When dealing with multiple tail gases with different pressures and compositions, the system integration and energy efficiency optimization are insufficient; 3) There is a lack of flexible adjustment capabilities in interaction with the main process (methanol synthesis).

[0005] Therefore, there is an urgent need to develop an integrated solution that can simultaneously achieve efficient hydrogen recovery and targeted utilization of carbon resources, while also being energy efficient and flexible in operation. Summary of the Invention

[0006] This invention aims to overcome the shortcomings of existing technologies and provide an integrated, energy-efficient methanol off-gas and multi-source tail gas hydrocarbon cascade recovery system and method. The core concept of this invention is "graded conversion, cascade recovery, and thermodynamic coupling".

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A hydrocarbon cascade recovery system for methanol off-gas and multi-source tail gas, comprising: The purge gas pretreatment unit is used to remove methanol and preheat the purge gas; The first-stage membrane separator assembly has its inlet connected to the outlet of the purge gas pretreatment unit, and is used to separate hydrogen-rich permeate gas and carbon-rich non-permeate gas. The first-stage pressure swing adsorption unit has its inlet connected to the permeate outlet of the first-stage membrane separator assembly and is used to produce high-purity hydrogen. A CO selective conversion unit is used to controllably convert a portion of CO to adjust the hydrogen-to-carbon ratio of the syngas; it includes a preheater and a CO conversion furnace, wherein the non-permeable gas outlet of the first-stage membrane separator group is connected to the inlet of the CO conversion furnace via the preheater, and the outlet of the CO conversion furnace is connected to the methanol synthesis system. The exhaust gas integration unit is used to collect and mix the desorbed gas, methanol flash vapor and methanol pre-distillation exhaust gas from the first-stage pressure swing adsorption unit; it includes a mixer, a mixer buffer tank and a mixer compressor connected in sequence, and the desorbed gas outlet of the first-stage pressure swing adsorption unit is connected to the inlet of the mixer; A deep conversion reactor, whose inlet is connected to the outlet of the mixer compressor, is used to deeply convert CO in the mixed exhaust gas into hydrogen. A steam drum, connected to the deep conversion reactor, is used to remove the heat of reaction and generate steam; The first cooler has its inlet connected to the outlet of the depth conversion reactor; The second-stage pressure swing adsorption unit has its inlet connected to the outlet of the first cooler and is used to produce industrial hydrogen. A gas purification unit, the air inlet of which is connected to the desorption gas outlet of the secondary pressure swing adsorption unit; The secondary membrane separator assembly has its inlet connected to the outlet of the gas purification unit for recovering residual hydrogen and returning it to the methanol synthesis system, and its non-permeable gas outlet connected to the fuel system.

[0008] As one implementation, the operating conditions of the CO selective conversion unit are: reaction temperature 200-250℃, using a Co-Mo-based sulfur-resistant conversion catalyst, controlling the CO conversion rate to 30%-60%, and adjusting the effective hydrogen-carbon molar ratio M of the outlet gas to 2.05-2.15, where M=(H2-CO2) / (CO+CO2).

[0009] In one embodiment, the purge gas pretreatment unit includes a water washing tower (1), a primary separator, and a heater connected in sequence.

[0010] In one embodiment, the steam or heat medium generated by the steam drum is used to heat the process gas entering at least one of the heater and the preheater.

[0011] In one embodiment, the residence time of the mixed gas in the mixer buffer tank of the exhaust gas integration unit is 5-20 minutes.

[0012] In one embodiment, the gas purification unit includes a second cooler, a secondary separator, and a dryer connected in sequence.

[0013] In one embodiment, the primary membrane separator group uses hollow fiber membranes made of polyimide or polysulfone; the secondary membrane separator group uses polyimide membranes.

[0014] In one embodiment, the fuel system includes at least one of an internal combustion engine generator set and a waste heat boiler, for generating electricity or producing steam using the low-calorific-value non-permeable gas discharged from the secondary membrane separator set.

[0015] This invention provides a method for hydrocarbon gradient recovery of methanol off-gas and multi-source tail gas using a system as described in any of the above-described contents, comprising the following steps: S1. Pretreatment and preliminary separation of purge gas: After washing and heating, methanol purge gas is subjected to primary membrane separation to obtain hydrogen-rich permeate gas and carbon-rich non-permeate gas. S2. High-purity hydrogen extraction and syngas conditioning: High-purity hydrogen is produced from hydrogen-rich permeate gas through a first-stage pressure swing adsorption; carbon-rich non-permeate gas is sent to a CO selective conversion unit for appropriate conversion, controlling the CO conversion rate at 30%-60%, and then returned to the methanol synthesis system after adjusting the hydrogen-carbon ratio. S3. Multi-source tail gas collection and mixing: Collect the first-stage pressure swing adsorption desorption gas, methanol flash vapor and methanol pre-distillation tail gas, and mix and buffer them; S4. Deep Conversion and Industrial Hydrogen Production: After pressurizing the mixed tail gas, a deep conversion reaction is carried out to reduce the CO content to below 1%; the resulting hydrogen-rich gas is then used to produce industrial hydrogen through a two-stage pressure swing adsorption process. S5. Residual Hydrogen Recovery and Energy Utilization: After purification and dehydration of the desorbed gas from the secondary pressure swing adsorption, secondary membrane separation is performed to recover hydrogen and return it to the methanol synthesis system, while the non-permeable gas is sent to the fuel system for utilization.

[0016] In one implementation, in step S2, the operating pressure of the CO selective conversion unit is matched with the upstream purge gas pressure and controlled at 5.0-9.0 MPaG, and the water-gas molar ratio is controlled at 0.1-0.4.

[0017] In one implementation, in step S5, the non-permeable gas has a calorific value of 4-8 MJ / Nm³ and is directly used to drive a dedicated low-calorific-value internal combustion engine generator set, or enters a waste heat boiler to produce steam.

[0018] Compared with the prior art, the present invention has the following advantages: 1. This invention innovatively employs a two-stage conversion strategy combining "CO selective conversion" and "deep conversion" to achieve optimal carbon resource allocation based on the gas source grade. For carbon-rich components in high-pressure, high-CO-concentration purge gas, moderate conversion (30%-60% conversion rate) is performed in the CO selective conversion unit, and the hydrogen-to-carbon ratio is adjusted before direct return to the methanol synthesis system, preserving the chemical feedstock value of CO. For low-pressure, dispersed, low-concentration multi-source tail gas, the deep conversion unit completely converts CO into high-value-added hydrogen. This staged utilization model overcomes the drawbacks of traditional processes where carbon resources are either completely vented or completely converted, achieving efficient and targeted recovery of carbon atoms and significantly improving the overall carbon utilization rate of the system.

[0019] 2. This invention achieves efficient, step-by-step recovery of hydrogen of different grades through a cascade separation process of "membrane separation + pressure swing adsorption + membrane separation". First, high-purity hydrogen (≥99.999%) is extracted from the high-pressure purge gas; second, industrial-grade hydrogen (≥99%) is recovered from the tail gas after deep conversion; finally, residual hydrogen in the desorbed gas is recovered and returned to the system via secondary membrane separation. This system boasts a high overall hydrogen recovery rate and can simultaneously produce hydrogen products of different purity levels, meeting diverse market demands and ensuring full resource utilization.

[0020] 3. This invention utilizes a systematic thermodynamic integration design to preheat the feed gas of the upstream CO selective conversion unit by using the high-temperature heat released from the deep conversion reaction, achieving efficient internal recovery of the reaction heat. Simultaneously, the resulting low-calorific-value non-permeable gas is used for power generation or steam production, realizing cascaded energy utilization. Simulation calculations show that, compared to the full deep conversion process, this invention avoids high water-to-gas ratio conditions and unnecessary compression power consumption, reducing overall energy consumption by approximately 15-20% and significantly improving operational economics.

[0021] 4. The CO conversion rate of the CO selective conversion unit can be flexibly adjusted within the range of 30%-60%, thereby quickly responding to the real-time demand of the methanol synthesis system for the hydrogen-to-carbon ratio (H / C) of the feed gas and ensuring the stable operation of the main process. The system adopts a modular design, which is easy to adapt to different scales of methanol production units and changes in gas source composition, enhancing the overall process adaptability and operational flexibility. Attached Figure Description

[0022] The accompanying drawings are provided to further understand the technical solutions of the present invention and constitute a part of the specification. They are used together with the embodiments of this application to explain the technical solutions of the present invention and do not constitute a limitation on the technical solutions of the present invention.

[0023] Figure 1 This is a structural diagram of the hydrocarbon cascade recovery system for methanol off-gas and multi-source tail gas described in this invention. intention.

[0024] Figure 2 yes Figure 1 A schematic diagram of the composition of a primary or secondary pressure swing adsorption unit. Figure 3 The process flow of the hydrocarbon cascade recovery method for methanol off-gas and multi-source tail gas described in this invention is as follows: Process diagram.

[0025] Marked in the image: 1-Water washing tower; 2-First-stage separator; 3-Heater; 4-First-stage membrane separator group; 5-Preheater; 6-CO 7-Shift converter; 8-Mixer; 9-Mixer buffer tank; 10-Mixer compressor; 11-Deep shift reactor; 12-Steam drum; 13-First cooler; 14-Second-stage pressure swing adsorption unit; 15-Second cooler; 16-Second-stage separator; 17-Dryer; 18-Second-stage membrane separator group; 19-Fuel system; 101 - Variable voltage unit cooler; 102, 103, 105, 106 - Adsorption tower; 104 - Pressure equalization tank; 107 - Hydrogen buffer tank; 108 - Hydrogen compressor; 109 - Desorbed gas buffer tank; 110 - Vacuum pump; 111 - Desorbed gas mixing tank. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0027] In this invention, unless otherwise specified, the materials or instruments used are conventional methods in the art, and those skilled in the art can choose accordingly based on the actual situation, for example: The membrane modules used in both the primary membrane separator group 4 and the secondary membrane separator group 18 are conventional commercial products used in the field for hydrogen / hydrogen separation. Those skilled in the art can determine the specific membrane material type and number of modules based on the composition of the feed gas (e.g., H2 concentration, CO content), operating pressure (5.0-9.0 MPaG), and target separation requirements (e.g., permeate H2 concentration > 90%), through conventional membrane module selection or performance curves provided by the supplier. For example, for membranes made of polyimide or polysulfone, their H2 / CH4 separation factor is typically within a range known in the art (e.g., 50-150), sufficient to meet the separation requirements of this invention. There is no need to limit specific separation factors or permeate rate values; their selection is a conventional experimental method used by those skilled in the art.

[0028] The adsorbent loading schemes in the primary pressure swing adsorption (PSA) unit 7 and the secondary pressure swing adsorption (PSA) unit 14 are configured according to the composition of the gas to be separated and the purity of the target product gas, following conventional PSA process design principles in the art. Taking the processing of feed gas rich in H2 and small amounts of CO, CO2, and CH4 as an example, those skilled in the art typically employ a layered loading method: the bottom layer is filled with activated alumina to remove trace amounts of moisture, the middle layer with activated carbon to adsorb CO2 and CH4, and the top layer with molecular sieves (such as 5A or 13X) to ultimately purify hydrogen. The specific loading ratio of each adsorbent (such as the volume ratio of activated carbon to molecular sieve) can be optimized through conventional breakthrough curve experiments or based on the recommended parameters from the adsorbent supplier, and can be determined without creative effort. Generally, as long as the feed gas conditions and product requirements are clearly defined, those skilled in the art can implement the PSA separation process of this invention; the specific adsorbent model and precise ratio are not necessary disclosures for implementing this invention.

[0029] like Figure 1 As shown, the main component of the methanol off-gas and multi-source tail gas hydrocarbon cascade recovery system of the present invention is... It should include a purge gas pretreatment unit, a primary membrane separation unit, a primary pressure swing adsorption unit, a CO selective conversion unit, a tail gas integration unit, a deep conversion and hydrogen extraction unit, and a gas purification and secondary membrane separation unit.

[0030] As a specific example, the specific composition of each of the above structural units is as follows: Purge gas pretreatment unit: Used to remove methanol entrained in the purge gas and preheat it. As an example, this unit includes a water washing tower 1, a primary separator 2, and a heater 3 connected in sequence. High-pressure purge gas from the methanol synthesis loop first enters the water washing tower 1, where it is countercurrently washed with demineralized water to reduce the methanol content in the gas phase to below 200 ppm. The washed gas then passes through the primary separator 2 to remove entrained droplets, and then enters the heater 3, where it is heated to a suitable membrane separation temperature of 50-65°C.

[0031] The primary membrane separation unit: This unit, 4, is a primary membrane separator composed of hollow fiber membrane modules made of polyimide or polysulfone (any commercially available hollow fiber membrane module made of polyimide or polysulfone can achieve the desired separation effect). Its inlet is connected to the outlet of the heater 3. The preheated purge gas is separated here, resulting in two gas streams: one is a hydrogen-rich permeate gas, where the hydrogen volume concentration is typically greater than 90%; the other is a carbon-rich non-permeate gas, where the combined concentration of components such as carbon monoxide (CO), carbon dioxide (CO2), and methane (CH4) is typically greater than 85%.

[0032] The first-stage pressure swing adsorption unit 7 has its inlet connected to the permeate outlet of the first-stage membrane separator group 4. Operating at a pressure of 2.5-3.5 MPaG, this unit purifies high-purity hydrogen (≥99.999%) from hydrogen-rich permeate through a multi-tower adsorption-desorption cycle, while simultaneously discharging desorbed gas containing residual hydrogen, CO, CO2, and other components.

[0033] The CO selective conversion unit is used for the controlled conversion of a portion of CO to precisely adjust the hydrogen-to-carbon ratio of the gas returned to the synthesis system. As an example, this unit specifically includes a preheater 5 and a CO converter 6. The non-permeable gas outlet of the primary membrane separator group 4 is connected to the inlet of the preheater 5, and the outlet of the preheater 5 is connected to the inlet of the CO converter 6. The CO converter 6 is filled with a Co-Mo-based sulfur-resistant conversion catalyst (any commercially available model is acceptable. The cobalt oxide content of the activity promoter is generally 2wt% to 6wt%, and the support is usually γ-alumina. In addition, the catalyst may contain a small amount of alkali metal (such as potassium) as an auxiliary agent to adjust its activity and selectivity). The reaction is carried out at 200-250℃ and 5.0-9.0 MPaG (basically matching the upstream purge gas pressure). By controlling the water-to-gas molar ratio at 0.1-0.4, the CO conversion rate is precisely controlled within the range of 30%-60%. The effective hydrogen-carbon molar ratio M of the gas after the reaction is adjusted to the ideal range of 2.05-2.15, and then directly returned to the methanol synthesis system as feed gas, where: M=(H2-CO2) / (CO+CO2). This parameter is a well-known indicator in the field of methanol synthesis that characterizes the stoichiometric balance of the feed gas, and its theoretical optimal value is 2.0.

[0034] Tail Gas Integration Unit: This unit collects and mixes multiple low-pressure tail gases from different processes. It comprises a mixer 8, a mixer buffer tank 9, and a mixer compressor 10 connected in sequence. The desorbed gas outlet of the first-stage pressure swing adsorption unit 7, the flash vapor outlet of the methanol synthesis process, and the top tail gas outlet of the methanol pre-distillation column are all connected to the inlet of the mixer 8. After initial mixing in the mixer 8, the multiple gas streams enter the mixer buffer tank 9 for buffering and homogenization. The designed residence time is 5-20 minutes to smooth out fluctuations in the flow rate and composition of each gas source. The homogenized tail gas is then pressurized to 2-5 MPaG by the mixer compressor 10 to provide the necessary pressure conditions for subsequent deep shift reaction.

[0035] Deep conversion and hydrogen extraction unit: including deep conversion reactor 11, steam drum 12, first cooler 13 and second-stage pressure swing adsorption unit 14.

[0036] The outlet of the mixer compressor 10 is connected to the inlet of the deep conversion reactor 11. The reactor is filled with an iron-chromium or cobalt-molybdenum based conversion catalyst (both are mature catalyst types used in industry to achieve deep CO conversion, and either model is acceptable). A deep water-gas conversion reaction is carried out at 320-380℃ to deeply convert the CO content in the mixed tail gas to below 1%, thereby significantly increasing the hydrogen concentration.

[0037] As an example of commonly used commercial catalysts, the typical composition (based on usable oxides) of iron-chromium catalysts includes: 70 wt% to 90 wt% ferric oxide as the active component, and 8 wt% to 12 wt% chromium trioxide as the structural stabilizer. This catalyst exhibits high activity in the temperature range of 320–450 °C, capable of deep conversion of CO content to below 0.5%. Numerous commercial models are available, such as Topsoe's SK-201 series and BASF's K6-10 series. The typical composition (based on oxides) of cobalt-molybdenum catalysts includes: 10 wt% to 20 wt% molybdenum trioxide, 3 wt% to 8 wt% cobalt oxide, and γ-alumina as the support. It can deeply convert CO in sulfur-containing gases at 280–400 °C. Commercial catalysts include Clariant's ShiftMax® 200 series.

[0038] Regardless of the catalyst used, as long as the CO content in the outlet gas is stably reduced to below 1%, preferably 0.1%-0.5%, it is acceptable.

[0039] The steam drum 12 is connected to the shell side or heat transfer tube bundle of the deep shift reactor 11, and removes the intense heat of reaction by generating medium-pressure steam (such as 0.8 MPaG saturated steam), thus achieving energy recovery. The high-temperature process gas after the reaction is discharged from the outlet of the deep shift reactor 11, enters the first cooler 13 to be cooled to near room temperature, and then is sent to the second-stage pressure swing adsorption unit 14.

[0040] The secondary pressure swing adsorption unit 14 purifies industrial hydrogen products with a purity of ≥99% from the hydrogen-rich gas at an operating pressure of 2.0-4.0 MPaG and discharges the desorbed gas.

[0041] Gas purification and secondary membrane separation unit: This unit processes the desorbed gas discharged from the secondary pressure swing adsorption unit 14 and recovers any residual hydrogen. This section includes a gas purification device and a secondary membrane separator assembly 18.

[0042] The gas purification device specifically includes a second cooler 15, a secondary separator 16, and a dryer 17 connected in sequence.

[0043] The desorbed gas from the secondary pressure swing adsorption unit 14 first enters the second cooler 15 to be cooled to below 40°C, causing most of the water vapor to condense. It then enters the secondary separator 16 to remove the condensate. Finally, it enters the dryer 17 (such as a molecular sieve adsorption tower) for deep dehydration, lowering the gas dew point to below -40°C. The purified and dried gas then enters the secondary membrane separator group 18 (any commercially available polyimide membrane module can achieve the above separation effect). The separated hydrogen-rich permeate (H2 concentration 40-80%) is returned to the methanol synthesis system for further recovery of hydrocarbon resources; the remaining non-permeate is a low-calorific-value fuel gas with a calorific value of approximately 4-8 MJ / Nm³. 3 .

[0044] like Figure 2 As shown, an example of the structure of a primary pressure swing adsorption unit 7 or a secondary pressure swing adsorption unit 14 includes: a cooler 101, multiple adsorption towers connected in parallel, a pressure equalization tank 104, a hydrogen buffer tank 107, a hydrogen compressor 108, a desorbed gas buffer tank 109, a vacuum pump 110, and a desorbed gas mixing tank 111.

[0045] The feed gas from upstream (for the first-stage pressure swing adsorption unit 7, it is the permeate gas from the first-stage membrane separator group 4; for the second-stage pressure swing adsorption unit 14, it is the shift gas from the first cooler 13) first enters the pressure swing group cooler 101 through the inlet pipeline, where it is cooled to a suitable adsorption temperature (usually 10-40℃), and then distributed to multiple parallel adsorption towers via the main pipeline. One of the adsorption towers in the unit is always in a state of simultaneous feed and adsorption. Specifically, at least four adsorption towers can be set up (e.g., Figure 2 The adsorption towers (numbered 102, 103, 105, and 106) are used to achieve cyclic switching between steps such as adsorption, pressure equalization, reverse release (desorption), purging, pressure equalization increase, and final charging, ensuring continuous production. Each adsorption tower is filled from top to bottom with 35%~45% 5A molecular sieve, 40%~50% activated carbon, and 10%~15% activated alumina. The filling ratio is adjusted according to the specific gas composition.

[0046] During the adsorption process, hydrogen, as a weak adsorbent component, penetrates the adsorption bed and becomes product hydrogen. The product gas outlets at the top of each adsorption tower are controlled by valves and sequentially flow into the product hydrogen main pipe according to a set program, entering the hydrogen buffer tank 107 for pressure stabilization. Subsequently, the product hydrogen can be pressurized to the required pressure by the hydrogen compressor 108 and sent out of the system as high-purity hydrogen or industrial hydrogen products.

[0047] To recover hydrogen from the dead space in the adsorption bed and improve the recovery rate, a pressure equalization tank 104 and corresponding connecting pipelines are installed. During the depressurization (pressure drop) and pressurization (pressure rise) steps in the adsorption tower, the gas inside the tower is connected to the pressure equalization tank 104 or other adsorption towers at different pressure stages to achieve pressure energy recovery.

[0048] After the adsorption step is completed, the adsorption tower undergoes reverse venting (pressure reduced to near atmospheric pressure) and possible evacuation steps to desorb the adsorbed impurities (such as CO, CO2, CH4, etc.) and form desorption gas. The desorption gas outlets at the bottom of each adsorption tower are controlled by valves and flow into the desorption gas main pipe sequentially according to a set program. The desorption gas first enters the desorption gas buffer tank 109 for buffering, and then the pressure can be further reduced by the vacuum pump 110 (e.g., for the evacuation step) to enhance desorption.

[0049] Finally, the desorbed gas is collected in the desorbed gas mixing tank 111 and discharged from the system through the outlet of the desorbed gas mixing tank 111. For the first-stage pressure swing adsorption unit 7, this outlet is connected to the mixer 8; for the second-stage pressure swing adsorption unit 14, this outlet is connected to the second cooler 15.

[0050] To reduce system energy consumption, the present invention further incorporates a thermal integration unit. Specifically, the medium-pressure steam or hot water generated by the steam drum 12 can be used as a heat transfer medium to preheat the process gas entering the heater 3 and / or preheater 5, thereby reducing or replacing external steam consumption and achieving efficient cascade utilization of the internal reaction heat of the system.

[0051] Fuel system 19, whose inlet is connected to the non-permeable gas outlet of the secondary membrane separator assembly 18. This system includes a dedicated low-calorific-value gas-fired internal combustion engine generator set and / or a waste heat boiler, for fully utilizing the chemical and thermal energy of the final fuel gas to generate electricity or produce low-pressure steam, achieving end-use energy utilization.

[0052] refer to Figure 1 and Figure 3 The method for hydrocarbon cascade recovery of methanol off-gas and multi-source tail gas described in this invention The specific steps are as follows: S1. Pretreatment and initial separation of purge gas The high-pressure purge gas (approximately 8.0 MPaG) from the methanol synthesis loop first enters water washing tower 1, where it is countercurrently washed with deionized water to remove entrained methanol vapor, resulting in a methanol content of less than 200 ppm in the exit gas. After washing, the gas passes through primary separator 2 for liquid removal and is then heated to 50-65°C in heater 3 before entering primary membrane separator group 4. Here, the gas is separated into hydrogen-rich permeate gas (H2 > 90%) and carbon-rich non-permeate gas (CO + CO2 + CH4, etc. > 85%).

[0053] S2, High-purity hydrogen extraction and syngas conditioning The hydrogen-rich permeate gas obtained in step S1 enters the first-stage pressure swing adsorption unit 7. Under an operating pressure of 2.5-3.5 MPaG, it undergoes adsorption, pressure equalization, reverse release, and evacuation cycles to produce a high-purity hydrogen product with a purity ≥99.999%.

[0054] Simultaneously, carbon-rich non-permeable gas enters the CO selective conversion unit: it is first preheated in preheater 5, and then enters CO converter 6. Inside CO converter 6, the reaction temperature is controlled at 220-240℃, the water-to-gas molar ratio at 0.2-0.3, and the space velocity at 2000-4000 h⁻¹. -1 Under specific conditions, CO undergoes a moderate conversion reaction, with the conversion rate controlled within the range of 40±5%. The effective hydrogen-to-carbon ratio of the post-reaction gas is precisely adjusted to approximately 2.10, and then directly fed back to the methanol synthesis system as a syngas feedstock.

[0055] S3, Multi-source exhaust gas collection and mixing The three gases—low-pressure desorption gas (approximately 0.05 MPaG) emitted from the first-stage pressure swing adsorption unit 7, flash vapor (approximately 0.8 MPaG) from the methanol flash tank, and top tail gas (approximately 0.03 MPaG) from the methanol pre-distillation column—are introduced together into the tail gas integration unit. After initial mixing in mixer 8, they enter mixer buffer tank 9 for buffering and homogenization for approximately 10 minutes, resulting in a mixed tail gas with relatively stable composition and flow rate.

[0056] S4, Deep Conversion and Industrial Hydrogen Production The mixed tail gas obtained in step S3 is pressurized to approximately 3.0 MPaG by mixer compressor 10 and then fed into deep shift reactor 11. A deep shift reaction occurs under the action of a catalyst, with the outlet gas temperature reaching approximately 350°C. The CO content decreases to below 0.5%, and the H2 content increases to 70-75%. The heat generated by the reaction is removed by steam drum 12 connected to deep shift reactor 11, producing medium-pressure steam. The shifted gas is then cooled to room temperature by first cooler 13 and enters second-stage pressure swing adsorption unit 14, producing industrial hydrogen product with a purity ≥99%.

[0057] S5. Residual hydrogen recovery and energy utilization The desorbed gas discharged from the secondary pressure swing adsorption unit 14 first enters the secondary cooler 15 to be cooled to below 40°C to condense the water; then it passes through the secondary separator 16 to remove the condensate; and then enters the dryer 17 for further dehydration to a dew point <-40°C. The purified and dried gas is pressurized to approximately 1.0 MPaG and then enters the secondary membrane separator group 18. The separated permeate (rich in hydrogen) is returned to the methanol synthesis system. The final non-permeate gas is used as a low-calorific-value fuel gas (calorific value approximately 5-7 MJ / Nm³). 3 The fuel is fed into the fuel system 19 to drive a low-calorific-value internal combustion engine to generate electricity or to enter a waste heat boiler to produce steam, thus achieving the final utilization of energy.

[0058] Example 1 uses a 1.8 million tons / year methanol production unit as the application background and employs Aspen HYSYS software to analyze... The process of this invention is based on steady-state simulation. The PR-MHV2 method is selected for property analysis. The main gas source design conditions are shown in Table 1 below.

[0059] The simulation used in this embodiment is based on the following commonly used industrial design parameters and models: Each separator in the primary / secondary membrane separator group uses commercially available polyimide membrane modules, with an ideal separation factor for H2 / CH4 set at α=80-120. Membrane modules of this performance are commercially available in industry, and those skilled in the art can select the appropriate model based on feed gas conditions and product requirements to achieve the same or similar results. The adsorption tower 106 of the primary / secondary pressure swing adsorption unit uses a composite adsorbent of activated carbon and molecular sieves, with a hydrogen recovery rate designed to be 90-95%. The CO conversion reaction is calculated using an equilibrium model based on Langmuir-Hinshelwood kinetics. All the above parameters are standard choices made by those skilled in the art when designing a system under the described process conditions.

[0060]

[0061] Compared to the comparative process that performs deep conversion of all carbon-rich gas, the present invention reduces the CO conversion load by about 40% by using selective conversion, thus saving corresponding steam consumption and converter size.

[0062] Through thermal integration, approximately 0.95 t / h of steam required for preheating selective conversion feed is saved.

[0063] Based on comprehensive calculations, the overall energy consumption (standard coal equivalent) per unit of hydrogen product in this system is reduced by approximately 18% compared to the comparative process. The effective gas returned to the system can be converted into an additional methanol production of approximately 4,800 tons per year.

[0064] Example 2 (Low Energy Optimization Mode) Based on Example 1, the thermal integration network is further optimized: a portion of the 0.8 MPa steam generated by the deep shift reactor 11 is directly used as supplementary steam for the CO selective shift unit, reducing the external supply of low-pressure steam. Simultaneously, the CO selective shift conversion rate is set to a lower limit of 30% to maximize the retention of CO for direct reuse.

[0065] Simulation results show that, under this mode, the yield of high-purity hydrogen decreases slightly to approximately 3,900 Nm³. 3 While the amount of steam purchased from outside the system is reduced by more than 60%, the system's steam self-sufficiency rate reaches 85%. The overall operating economy is more advantageous in areas with high steam prices, demonstrating the system's operational flexibility.

[0066] Example 3 (Adaptability to High CO Exhaust Gas) Simulated treatment of methanol unit tail gas from the coal-water slurry gasification route showed that the CO content in both the flash vapor and distillation tail gas increased to ~25%. In this case, the buffer time of the tail gas integration unit was extended to 15 minutes to ensure uniformity. The deep shift reactor 11 employed segmented water inlet quenching isothermal control to strictly control the bed temperature rise. Simultaneously, the dehydration capacity of the gas purification unit was enhanced.

[0067] Simulations show that the system can still operate stably, with industrial hydrogen production increasing by about 15% and the total hydrogen recovery rate remaining above 99%, demonstrating the strong adaptability of this invention to fluctuations in the composition of the feed gas.

[0068] It should be noted that this invention aims to protect a hydrocarbon cascade recovery system and its integrated process for methanol off-gas and multi-source tail gas. Its core innovation lies in achieving graded recovery and thermal integration of hydrocarbon resources of different grades through a combination of "two-stage conversion (selective conversion + deep conversion)" and "cascade separation (membrane + PSA + membrane)." For the unit operations involved (such as membrane separation, pressure swing adsorption, and shift reaction), the equipment used is all mature industrial equipment in the field. The specific selection of equipment, fine-tuning of operating parameters, and conventional replacement of adsorbents / catalysts are all within the scope of what a person skilled in the art can achieve after reading this specification and combining common knowledge in the field. The technical solution of this invention can be reproduced and the stated technical effects achieved without excessive experimentation. Any person skilled in the art can make any modifications and changes in the form and details of the implementation without departing from the spirit and scope disclosed in this invention; however, the scope of patent protection of this invention shall still be determined by the scope defined in the appended claims.

Claims

1. A hydrocarbon cascade recovery system for methanol off-gas and multi-source tail gas, characterized in that, include: The purge gas pretreatment unit is used to remove methanol and preheat the purge gas; The first-stage membrane separator group (4) has its inlet connected to the outlet of the purge gas pretreatment unit and is used to separate hydrogen-rich permeate gas and carbon-rich non-permeate gas. The first-stage pressure swing adsorption unit (7) has its inlet connected to the permeate outlet of the first-stage membrane separator group (4) for producing high-purity hydrogen. CO selective conversion unit is used to controllably convert part of CO to adjust the hydrogen-carbon ratio of synthesis gas; including a preheater (5) and a CO conversion furnace (6), the non-permeable gas outlet of the first-stage membrane separator group (4) is connected to the gas inlet of the CO conversion furnace (6) via the preheater (5), and the gas outlet of the CO conversion furnace (6) is connected to the methanol synthesis system. The exhaust gas integration unit is used to collect and mix the desorbed gas, methanol flash vapor and methanol pre-distillation exhaust gas from the first-stage pressure swing adsorption unit (7); it includes a mixer (8), a mixer buffer tank (9) and a mixer compressor (10) connected in sequence, and the desorbed gas outlet of the first-stage pressure swing adsorption unit (7) is connected to the inlet of the mixer (8); A deep conversion reactor (11), whose inlet is connected to the outlet of the mixer compressor (10), is used to deeply convert CO in the mixed tail gas into hydrogen. A steam drum (12) is connected to the deep conversion reactor (11) to remove the heat of reaction and generate steam; The first cooler (13) has its inlet connected to the outlet of the depth conversion reactor (11); The secondary pressure swing adsorption unit (14) has its inlet connected to the outlet of the first cooler (13) and is used to produce industrial hydrogen. The gas purification unit has its inlet connected to the desorption gas outlet of the secondary pressure swing adsorption unit (14); The secondary membrane separator assembly (18) has its inlet connected to the outlet of the gas purification unit for recovering residual hydrogen and returning it to the methanol synthesis system, and its non-permeable gas outlet connected to the fuel system (19).

2. The hydrocarbon cascade recovery system for methanol off-gas and multi-source tail gas according to claim 1, characterized in that, The operating conditions of the CO selective conversion unit are as follows: reaction temperature 200-250℃, using Co-Mo based sulfur-resistant conversion catalyst, controlling the CO conversion rate to 30%-60%, and adjusting the effective hydrogen-carbon molar ratio M of the outlet gas to 2.05-2.15, where M=(H2-CO2) / (CO+CO2).

3. The hydrocarbon cascade recovery system for methanol off-gas and multi-source tail gas according to claim 1 or 2, characterized in that, The purge gas pretreatment unit includes a water washing tower (1), a primary separator (2), and a heater (3) connected in sequence; Preferably, the steam or heat medium generated by the steam drum (12) is used to heat the process gas entering at least one of the heater (3) and the preheater (5).

4. The hydrocarbon cascade recovery system for methanol off-gas and multi-source tail gas according to claim 1, characterized in that, In the exhaust gas integration unit, the residence time of the mixed gas in the mixer buffer tank (9) is 5-20 minutes.

5. The hydrocarbon cascade recovery system for methanol off-gas and multi-source tail gas according to claim 1, characterized in that, The gas purification unit includes a second cooler (15), a secondary separator (16), and a dryer (17) connected in sequence.

6. The hydrocarbon cascade recovery system for methanol off-gas and multi-source tail gas according to claim 1, characterized in that, The primary membrane separator group (4) uses hollow fiber membranes made of polyimide or polysulfone; the secondary membrane separator group (18) uses polyimide membranes.

7. The hydrocarbon cascade recovery system for methanol off-gas and multi-source tail gas according to claim 1, characterized in that, The fuel system (19) includes at least one internal combustion engine generator set and waste heat boiler, for generating electricity or producing steam using the low calorific value non-permeable gas discharged from the secondary membrane separator set (18).

8. A method for hydrocarbon gradient recovery of methanol off-gas and multi-source tail gas using the system described in any one of claims 1-7, characterized in that, Includes the following steps: S1. Pretreatment and preliminary separation of purge gas: After washing and heating, methanol purge gas is subjected to primary membrane separation to obtain hydrogen-rich permeate gas and carbon-rich non-permeate gas. S2. High-purity hydrogen extraction and syngas conditioning: High-purity hydrogen is produced from hydrogen-rich permeate gas through a first-stage pressure swing adsorption; carbon-rich non-permeate gas is sent to a CO selective conversion unit for appropriate conversion, controlling the CO conversion rate at 30%-60%, and then returned to the methanol synthesis system after adjusting the hydrogen-carbon ratio. S3. Multi-source tail gas collection and mixing: Collect the first-stage pressure swing adsorption desorption gas, methanol flash vapor and methanol pre-distillation tail gas, and mix and buffer them; S4. Deep Conversion and Industrial Hydrogen Production: After pressurizing the mixed tail gas, a deep conversion reaction is carried out to reduce the CO content to below 1%; the resulting hydrogen-rich gas is then used to produce industrial hydrogen through a two-stage pressure swing adsorption process. S5. Residual hydrogen recovery and energy utilization: After the desorbed gas from the secondary pressure swing adsorption (14) is purified and dehydrated, it is subjected to secondary membrane separation to recover hydrogen and return it to the methanol synthesis system. The non-permeable gas is sent to the fuel system (19) for utilization.

9. The method according to claim 8, characterized in that, In step S2, the operating pressure of the CO selective conversion unit is matched with the upstream purge gas pressure and controlled at 5.0-9.0 MPaG, and the water-gas molar ratio is controlled at 0.1-0.

4.

10. The method according to claim 8, characterized in that, In step S5, the non-permeable gas has a calorific value of 4-8 MJ / Nm³ and is directly used to drive a dedicated low-calorific-value internal combustion engine generator set, or enters a waste heat boiler to produce steam.

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