Membrane-type CO2 separation process

Abstract

Described herein is a membrane process for separating CO2 from flue gas. An exemplary process includes passing a fluid stream comprising flue gas across a membrane permeable to CO2 and H2O, recovering a treated gas from the feed side of the membrane having less CO2 than the flue gas, and removing a permeate from the permeate side of the membrane consisting of CO2 and H2O. Preferably, the permeate is recovered at a fresh air pressure below atmospheric pressure. The permeate is then cooled and at least a portion of the H2O is removed from the permeate, and a smaller amount of H2O is removed to form a CO2-rich permeate.

Description

[Technical Field]

[0001] This invention relates to the separation of carbon dioxide (CO2) from exhaust gases produced by the combustion of carbon-containing fuels. A novel membrane process design is disclosed that enhances CO2 separation efficiency by utilizing the co-permeation of water vapor, which is also present in the flue gas. [Background technology]

[0002] The capture of carbon dioxide from industrial gas emissions has become a matter of great interest in recent years. These gaseous flows are generated not only by coal and natural gas power plants, but also by steel mills, cement and petrochemical plants, and oil refineries. These gaseous flows are often collectively referred to as flue gas. The CO2 concentration in flue gas is approximately 4 to 25%. Many government agencies around the world are sponsoring research aimed at separating and concentrating this CO2 to produce CO2 with a concentration of over 95%, which can then be compressed and sequestered deep underground.

[0003] Membranes are being considered for these separation processes. A multi-stage membrane process is required for separation, and combinations of membrane processes with final processing steps (e.g., cryogenic concentration) are often proposed. The key step in this process is the initial membrane separation step, in which 50-90% of the CO2 content in the exhaust gas stream is separated into a concentrated stream, and the CO2 is concentrated to 3-5 times the concentration of the initial flue gas. The initial step is usually the largest and most costly operation in the entire process. While the present invention will be explained using this initial step as an example, the process can also be applied to the second step or other separation steps.

[0004] The gas separation membrane process is shown in a simplified form in Figure 1(a). As the supply gas flows across the supply side membrane, a portion of the gas permeates the membrane. The flow from the supply side to the permeate side of the membrane is proportional to the difference in the partial pressure driving force of each component crossing the membrane. The partial pressure on the supply side of the membrane is n io p o (molar concentration n) io pressure p o ) and the partial pressure on the permeation side of the membrane is nil p l (molar concentration n il , pressure p l ). Therefore, the partial pressure driving force is given by the following equation.

[0005]

Equation

[0006] And the permeability of the component (J i ) is linked to the partial pressure driving force by the following equation.

[0007]

Equation

[0008] Here, Pi is a proportionality constant called the membrane permeability of component i, and l is the thickness of the membrane. The performance of the membrane that distinguishes between two components i and j is a function of several factors including the membrane selectivity (α i / j ), i.e., the ratio of the permeabilities of components i and j,

[0009]

Equation

[0010] Similar to the pressure ratio across the membrane as follows:

[0011]

Equation

[0012] The molar concentration n of the more permeable component on the feed side of the membrane io .

[0013] As an example for explaining these effects, the separation shown in Fig. 1(b) can be considered. In this example, when the feed crosses the membrane from left to right, only a very small amount of the permeate gas CO2 is removed from the feed. That is, the CO2 concentration on the permeate side of the membrane is almost the same everywhere. As described above, membrane permeation occurs only when the partial pressure on the permeate side of the membrane is smaller than that of the feed.

[0014]

Number

[0015] This inequality can be transformed as follows:

[0016]

Number

[0017]

Number

[0018] In the example of Fig. 1(b), this means that, regardless of the selectivity of the membrane, n il does not exceed 50% CO2 (n io = 10% CO2 and

[0019]

Number

[0020] ). This result has several implications. First, at least half of the permeate must be the low-speed component (N2), and it is the permeation of the low-speed component that determines the membrane area required to process a specific amount of feed. Also, as the selectivity of the membrane increases, the membrane area required to permeate the same amount of CO2 increases. In the case of infinite selectivity, the low-speed component does not permeate, so the required membrane area becomes infinite.

[0021] The maximum transmission concentration given by equation (5)

[0022]

number

[0023] If the ratio is less than 100%, the membrane process is considered to be well within the pressure ratio limit. In this region, the effect of the pressure ratio is generally significant. Furthermore, if the membrane selectivity is greater than the pressure ratio, the difference becomes even larger, and if the membrane selectivity is greater than 2 to 3 times the pressure ratio, it becomes even larger. The effect of the pressure ratio on membrane separation is described in detail in Huang, et al., Journal of Membrane Science, 463, 33 (2014).

[0024] Returning to Figure 1(b), let's consider a specific example where the CO2 / N2 selectivity of the membrane is 25. In this case, the membrane selectivity is 5 times the pressure ratio, and the membrane is limited by the pressure ratio. In this example, the compositions of the feed (1), residue (2), and permeate (3) are as follows.

[0025] [Table 1]

[0026] Let's consider a very similar separation using the same membrane, but this time the feed contains 10% water. As before, the membrane has a CO2 / N2 selectivity of 25 and an H2O / CO2 selectivity of 3. Water is the most permeable component, followed by CO2 and N2. The separation performed by this membrane has the following composition.

[0027] [Table 2]

[0028] Since water in the feed permeates even faster than CO2, allowing water to permeate the permeate side of the membrane dilutes the CO2 on that side, increasing the driving force for CO2 transport through the membrane. The membrane area required to permeate the same amount of CO2 is reduced to 60% of the dry feed amount. The 0.2 bar permeate (3) has a slightly lower CO2 concentration and a much lower N2 concentration, but a higher water concentration. After removing the water in the dehydration process, the CO2 concentration in the remaining permeate becomes 52.9%, resulting in much better separation than the results in Table 1. By adding water to the feed gas and removing water from the permeate, CO2 can evade the limitation expressed in Equation 5.

[0029] This patent demonstrates our application of these results to a novel type of membrane separation process for separating CO2 from flue gas. [Overview of the project] [Means for solving the problem]

[0030] This specification describes a membrane process for separating CO2 from flue gas. An exemplary process involves passing a fluid flow containing flue gas through a membrane permeable to CO2 and H2O, recovering the processed gas from the feed side of the membrane which has less CO2 than the flue gas, and recovering the permeate from the permeate side of the membrane which contains CO2 and H2O. The permeate is cooled, and at least some of the H2O is removed from the permeate, forming a less H2O-depleted, CO2-enriched permeate.

[0031] In one embodiment, a membrane process for separating CO2 from flue gas involves passing a fluid stream containing flue gas through a membrane permeable to CO2 and H2O. The treated gas is recovered from the feed side of the membrane. The treated gas contains less CO2 than the flue gas. The permeate is recovered from the permeate side of the membrane at sub-atmospheric pressure of 0.1 to 0.4 bar. The permeate contains CO2 and H2O. The permeate is cooled to remove at least some of the H2O from it, forming an H2O-depleted, CO2-enriched permeate. A vacuum pump is used to raise the gas pressure to at least approximately atmospheric pressure.

[0032] In certain embodiments, the fluid flow passing through the membrane contains at least 70% of its saturation concentration of water.

[0033] In one or more embodiments, the temperature difference between the fluid flow passing through the membrane and the cooled H2O-depleted CO2-enriched permeate is at least 40°C.

[0034] In exemplary embodiments, H2O is added to the flue gas before the fluid flow passes through the membrane, so that the fluid flow passing through the membrane contains flue gas and water. In some embodiments, adding H2O to the flue gas involves adding H2O to the flue gas using a direct contact cooler before the fluid flow containing the flue gas passes through the membrane. In some cases, the direct contact cooler regulates the temperature of the flue gas.

[0035] In certain embodiments, the process further includes pressurizing a fluid flow containing flue gas to a pressure of 0.8 to 1.5 bar before passing the fluid flow through a membrane.

[0036] In one or more embodiments, the process further includes heating the fluid stream, which contains flue gas, to a temperature above 50°C before the fluid stream passes through the membrane. An exemplary embodiment further includes heating the fluid stream, which contains flue gas, to a temperature in the range of about 50°C to about 80°C before the fluid stream passes across the membrane.

[0037] In some embodiments, the process includes making the fluid flow containing the flue gas have more than 10 mol% water vapor.

[0038] In one or more embodiments, the process includes bringing a fluid stream containing flue gas to a concentration greater than 70% of its saturated water concentration.

[0039] In an exemplary embodiment of the process, the membrane has an H2O / CO2 selectivity greater than 2 as measured under the process operating conditions, for example, a CO2 / N2 selectivity greater than 10 as measured under the process operating conditions.

[0040] In one or more embodiments, the membrane process removes at least 50% of the CO2 in the flue gas, for example, about 50% to about 80% of the CO2 in the flue gas.

[0041] In certain embodiments, the cooling of the permeate includes cooling the permeate to a temperature in the range of about 5°C or higher and about 30°C or lower.

[0042] In an exemplary embodiment, the water depletion permeate has a CO2 concentration of more than 35%.

[0043] In another embodiment, a system for separating CO2 from flue gas comprises a membrane configured to be fluidly connected to a flue gas source so that a fluid flow containing flue gas can pass through the membrane. The membrane is configured to separate CO2 and H2O from the fluid flow passing through it, forming separate flows of the treated fluid and permeate. A concentrator is fluidly connected to the membrane to receive the permeate. The concentrator is configured to concentrate the H2O in the permeate, forming separate flows of concentrated H2O and H2O depletion.

[0044] In an exemplary embodiment, the system includes a pretreatment unit configured to add H2O to the flue gas before it passes through the membrane. The pretreatment unit may include a direct contact cooler.

[0045] In certain embodiments, the system further comprises a vacuum pump configured to draw H2O depletion permeate from the concentrator.

[0046] In another embodiment, a membrane process for separating CO2 from a flue gas exhaust stream comprises: (i) a step of pressurizing the flue gas to 0.8 to 1.5 bar and raising its temperature above 50°C, and containing more than 10 mol% water vapor; (ii) a step of passing the flue gas from step (i) across a membrane permeable to water and CO2, wherein the membrane has an H2O / CO2 selectivity greater than 2 and a CO2 / N2 selectivity greater than 10 as measured under the operating conditions of the process; and (iii) a step of passing at least 5 of the CO2 content in the gas from the feed side of the membrane in step (ii). The process includes: (iv) recovering the depleted flue gas stream from which 0% has been removed; (iv) removing CO2 and H2O-rich permeate gas from the permeate side of the membrane in step (ii) at a pressure of 0.1 to 0.4 bar; (v) cooling the permeate gas from step (iv) to a temperature of 5 to 30°C to condense a portion of the H2O content in the gas, thereby reducing the H2O concentration of the permeate gas and generating water-depleted permeate gas; (vi) separating concentrated water from the water-depleted permeate gas stream; and (vii) using a vacuum pump to raise the water-depleted permeate gas from step (v) to atmospheric pressure or higher.

[0047] In certain exemplary embodiments, the membrane process removes 50-80% of the CO2 content from the membrane feed stream.

[0048] In one or more embodiments, the gas supplied to the membrane unit has a temperature between 50 and 80°C.

[0049] In some embodiments of the membrane process, the water vapor content of the membrane feed gas is 70-100% of the water saturation concentration of the gas.

[0050] In certain embodiments, a direct contact cooler is used in step (i) to adjust the temperature and water concentration of the film supply gas flow.

[0051] In an exemplary embodiment of a membrane process, the flue gas exhaust flow is generated by a coal-fired power plant, a natural gas power plant, a natural gas boiler, a cement plant, a steel mill, or an oil refinery.

[0052] In one or more embodiments, the water concentrations of the supply gas (i) and the CO2 and H2O-rich permeate gas (iv) differ by at least a coefficient of 2.

[0053] In one embodiment, the temperature difference between the flue gas passing through the membrane in (ii) and the permeate gas cooled in (iv) exceeds 30°C.

[0054] Other aspects and features will be revealed below. [Brief explanation of the drawing]

[0055] [Figure 1A] Figure 1A is a general block diagram of a prior art membrane separation process. [Figure 1B] Figure 1B is a block diagram similar to Figure 1A, showing a prior art membrane separation process used for a feed gas containing 10% CO2 and 90% N2 at 1.0 bar. [Figure 2] Figure 2 is a simple block diagram of the unit operation used in the process of the present invention. [Figure 3] Figure 3 shows a specific example of the process of the present invention. [Figure 4] Figure 4 is a plot showing the advantages of increasing the water concentration in the membrane feed gas before separation. [Figure 5] Figure 5 is a plot showing the impact of CO2 recovery rate on system performance. [Modes for carrying out the invention]

[0056] In the following text, the concentrations of components in the gas are expressed as molar concentrations unless otherwise specified. Also, all process pressures are expressed in absolute bar.

[0057] The separation process of the present invention, applied to the separation of CO2 from flue gas, is shown in a simplified form in the block diagram of Figure 2. This diagram shows the process of the present invention as four steps.

[0058] Pre-treatment process

[0059] Pretreatment: The gas flowing into process (201) is CO2-containing flue gas containing 4-25% CO2. The gas is usually discharged through the chimney at near atmospheric pressure, but the gas pressure can be increased to 1-1.1 bar using a gas blower. In many cases, the gas already contains a relatively high concentration of water, and it is possible to process the gas directly using the process of the present invention. However, it may be necessary to bring the gas to a controlled temperature and water vapor content by feeding the gas through a direct-contact water spray tower that sprays water at the appropriate temperature onto the gas. Such equipment has the additional advantage of removing particulate matter and other contaminants that may be present in the flue gas. Other devices, including heat exchangers and blowers, can also be used to bring the gas to the required temperature, pressure, and humidity.

[0060] In this step (203), the CO2, N2-containing flue gas is brought to a composition, temperature, and pressure suitable for the subsequent separation step. The flue gas mixture may already contain some water, but additional water (202) can be added to increase the required water vapor, CO2, and N2 composition. The gas produced from this operation is typically between 70% and 100% of its saturated water content.

[0061] In exemplary embodiments, the treated flue gas (204) satisfies several requirements. First, the gas temperature is at least 30°C, preferably 40°C, higher than that of the subsequent concentration step (208) so that a useful fraction of water vapor in the membrane permeate (207) can be removed by cooling. In principle, the concentration step (208) can be carried out at any low temperature, but for economic viability, the separation of CO2 from the flue gas must be a low-cost process. Available cooling is typically provided by an evaporative cooling plant, and the resulting cooling water is usually not below 15°C. This means that if the pretreatment gas to the membrane unit (204) is 30-40°C higher than the cooled permeate gas (210), the minimum temperature of the pretreatment gas (~20°C) is about 50°C to 60°C.

[0062] The upper limit temperature of the pre-treated gas is determined by the cost of supplying energy to heat and humidify the gas, and the stability of the usable membrane (205) at high temperatures. Generally, the upper limit of the treated flue gas temperature is 90°C, more preferably 70-80°C.

[0063] The water content of the pretreated flue gas must be high in order to produce a high concentration in the water vapor-rich gas supplied to the permeate concentration process. The pretreated gas (204) should contain at least 10% water vapor, more preferably at least 15% water vapor, and most preferably at least 25% by weight of water. Achieving these water vapor concentrations in a gas flow between 50 and 90°C means that the pressure of the pretreated flue gas cannot exceed 2 bar, and is usually less than 1.5 bar. It is also preferable that the gas has a saturation value of at least 70% of its saturation value, preferably close to 80% or 90%.

[0064] Membrane separation process

[0065] The pre-treated flue gas (204) is passed through a membrane separation process (205) fitted with a membrane that is permeable to water vapor and CO2, and relatively impermeable to N2, O2, and Ar. While many polymer membranes possess these properties, the most suitable membranes are made of polar rubber materials, such as the family of polyamide polyether block copolymers sold under the trade name Pebax®. Polaris membranes from Membrane Technology and Research can also be used. Most membranes currently used in this type of application are multilayer composite membranes made from these types of polymers. By making the selective layer of the membrane very thin, on the order of 0.1 to 0.5 μm, a density of 1000 to 2000 gpu (1 gpu = 1 × 10⁻¹⁶) can be achieved at 30°C. -6 cm 3 (STP) / cm 2It is possible to manufacture membranes with a CO2 permeability of (· seconds· cmHg). The permeability of these membranes more than doubles at temperatures of 50-80°C. At temperatures of 25-30°C, a good quality CO2 separation membrane operated with flue gas has a CO2 / N2 selectivity in the range of 25-50. When operated at higher temperatures, the permeability increases, but the CO2 / N2 selectivity may decrease to the range of 20-30. Because water is a small, easily condensable molecule, the permeability of water through almost all membranes is high and significantly higher than that of CO2. Typical water / CO2 selectivity is in the range of 2-10 under the high water content and high feed gas temperature required for the process of the present invention.

[0066] To achieve effective separation, a membrane with high permeability and high selectivity is required, but for the reasons mentioned above, it is also necessary that the pressure ratio across the membrane be at least 5. Since the maximum pressure is 1.5 to 2.0 bar, to create a pressure difference across the membrane in our process, the pressure on the permeate side of the membrane must be less than 0.3 to 0.4 bar. The minimum practical pressure in large industrial plants is in the range of 0.1 to 0.2 bar. Therefore, the preferred operating range on the permeate side of the membrane is 0.1 to 0.4 bar.

[0067] Concentration and separation process

[0068] The simplest way to generate low pressure on the permeate side of our process is to use a vacuum pump. However, such pumps are expensive and consume a lot of energy. This problem is overcome by our invention, which uses a cooling and concentration step (208) before the gas is sent to the vacuum pump (211). By cooling and concentrating much of the water content of the gas (207), the volume of gas (210) sent to the vacuum pump is significantly reduced.

[0069] If the membrane feed gas meets the above composition and temperature requirements, the membrane system produces a permeate gas at a temperature of 50-90°C containing 40-70% water. When this gas is cooled to approximately 20°C, most of the water vapor in the gas is concentrated and removed as liquid water, even under reduced pressure of 0.1 to 0.4 bar. At this point, the amount of CO2 and N2 in the residual water sent to the vacuum pump becomes very small, thus reducing the size of the required vacuum pump (211). More importantly, the concentration and removal of water vapor concentrates the CO2 in the permeate gas. The CO2 content of the gas is concentrated once in the membrane separation step (205) and again in the water vapor concentration step (208).

[0070] vacuum process

[0071] The final step of the process in Figure 2 is the vacuum step (212), in which CO2, N2, and residual water vapor are compressed to above atmospheric pressure and discharged, or sent to another process. If necessary, a final cooling step can be used after the vacuum pump to remove residual water. [Examples]

[0072] The calculation examples used to illustrate our invention below utilize the permeability characteristics shown in Table 3. However, these permeability and selectivity values ​​do not mean to limit the scope of the invention. The invention requires a CO2 / N2 selectivity of at least 10, and it is recognized that a selectivity of up to 50 or more is possible. It also requires an H2O / CO2 selectivity of about 2 or more, but it is recognized that a selectivity of up to 10 or more is possible.

[0073] [Table 3]

[0074] Example 1. In the process shown in Figure 3, the temperature of the supply gas is varied.

[0075] This embodiment illustrates the advantages of our invention by operating the membrane separation system shown in Figure 3 at different temperatures. In this example, the flue gas feed (301) entering the membrane unit (309) and being processed is in the range of 50–80°C. In all reported calculations, the gas is saturated with water vapor at the feed temperature and pressure of 1.0 bar. However, in industrial plants, the gas may not be completely saturated and may have a relative humidity of 70–100%. For the sake of simplicity in calculations, the effect of temperature on transmittance and selectivity is ignored, and the transmittance numbers shown in Table 3 are used in all calculations.

[0076] Table 4 shows the results at a membrane feed gas temperature of 80°C, and Table 5 shows the results at a feed temperature of 40°C. In both examples, the temperature of the concentration process is set to 20°C. Therefore, the temperature difference between the membrane feed (301) and the concentration process (304) is 60°C in Table 4 and 20°C in Table 5. In both examples, the dry-based flue gas feed gas (312) has a composition of 10% CO2 and 90% N2, and is 5,100 (standard) m 3 The gas has a flow rate of 1 ton / h. This gas contains 1 ton / h of CO2. Before being sent to the membrane module, the gas is brought to the required temperature and saturated with water in a pretreatment unit (313). At 80°C, gas (301) contains 47.4% water, and at 40°C, gas (301) contains 7.4% water. In both examples, the membrane unit (309) has the membrane area necessary to remove 80% of the CO2 content of this gas into the permeate flow (303). The key flow characteristics of this process are shown in Tables 4 and 5.

[0077] [Table 4]

[0078] [Table 5]

[0079] Comparing these two tables, it is clear that even with the same membrane properties and pressure across the membrane, operating the process with water-saturated gas at 80°C yields far better results than operating the process at 40°C. At 80°C, the membrane area required to remove 80% of the CO2 is 40% less than at 40°C. This advantage is a result of the dilution effect on the permeate side by the co-permeate water. The water that permeates through the membrane dilutes the CO2 in the permeate gas. This dilution increases the driving force of the CO2 partial pressure passing through the membrane, and thus increases the CO2 flow rate. As a result, the membrane area (310) required to remove 80% of the CO2 from the membrane feed gas (301) is reduced. In addition, power consumption is reduced by 34% in this process, and most of the co-permeate water is removed after cooling and concentration. This advantage is caused by the increase in CO2 concentration and the decrease in the volume of gas leaving the permeate concentrator (304) and heading toward the vacuum pump (311). Finally, the gas exiting process (306) has a significantly higher CO2 concentration of 39% compared to 26.3%.

[0080] Figure 4 shows additional calculation results in graph form, demonstrating the benefits of these important processes over the membrane feed gas temperature range of 30–80°C, where 80% of the total CO2 is captured in the flow (306). At operating temperatures below approximately 50°C, the benefit of adding water to the flue gas flow is small because the temperature difference between the feed (301) and the concentrated gas (304) is only 30°C. This is because the water content in the gas is less than 10%, so the water concentration in the permeate (303) is not very high, and concentrating to 20°C to remove water is not very effective. At high temperatures such as 80°C, the temperature difference is 60°C, so the water content of the feed gas is significantly higher, in the range of approximately 20–50%, and the permeate-side dilution effect of water becomes greater. Typically, the temperature difference needs to be at least 40°C. In this process, the feed gas to the membrane can achieve a sufficiently large benefit to enhance the value of the process by containing at least 15% water, most preferably at least 25% water. This is the most preferred operating range.

[0081] The need for a relatively high concentration of water on the membrane feed side to bring about a significant improvement in process performance also explains why the process is limited to low pressure on the feed side and vacuum operation on the permeate side. At a feed pressure of 1 bar and 80°C, the maximum water concentration in the feed gas is 47%, at 2 bar and 80°C the water content is only 24%, and at 3 bar it is only about 16%.

[0082] Example 2: The process shown in Figure 3 is carried out with a saturated supply gas at 80°C, and the CO2 recovery rate of the process is varied.

[0083] The recovery rate in the process shown in Figure 3 can be changed by varying the membrane area. Table 6 shows the effect of this on an 80°C flue gas supply with CO2 recovery rates ranging from 30% to 90%. Figure 5 is a graph of these same results. These results show that as the recovery rate decreases, both the power consumption and membrane area used per ton of CO2 decrease. Also, the dry-based CO2 concentration in the final permeate (306) increases as the recovery rate decreases. This makes downstream processing of the permeate easier to achieve CO2 concentrations above 95%. However, since users of this technology need to reduce the impact of CO2 on the environment, plant builders seek high CO2 recovery rates. Also, since the cost of implementing the process is almost the same regardless of the recovery rate, a high recovery rate is desirable. In other words, there is a trade-off between recovery rate and process cost. The optimal range for this process is a recovery rate of 50-80%. Above 80%, the area and power consumption increase sharply, so our process is possible but undesirable.

[0084] [Table 6]

[0085] Example 3. The process of the present invention was applied to flue gas in a coal-fired power plant.

[0086] Flue gas from modern coal-fired power plants typically contains about 12% CO2, 18% H2O, and 70% N2, O2, and Ar. This gas is usually treated by flue gas desulfurization units and discharged at a temperature several degrees above their dew point, generally 56–58°C. Table 7 shows the calculations for treating this gas using the process in Figure 3. Since gas (301) is at 58°C, the operating temperature is the lower limit of the temperature preferred for our process. However, if used without pretreatment, the feed gas contains 18% H2O, so the permeate will contain more than 40% water. When the permeate (303) is cooled to 20°C, more than 80% of the water content condenses, the size of the required vacuum pump is significantly reduced, and the CO2 concentration of the cooled gas (304) is concentrated to 42.1% CO2. Finally, when gas (306) is produced using any (20°C) concentrator (308), the CO2 concentration rises to 46.6%.

[0087] [Table 7]

[0088] Some power plants may have access to low-grade heat in the 70-100°C range. If this heat is available, it can be used to increase the temperature and water saturation concentration of the gas being processed by the membrane. For example, by increasing the water concentration in the feed gas to 25% H2O, the temperature can be raised by just a few degrees from 58 to 65°C, improving the process as shown in the data in Table 8.

[0089] [Table 8]

[0090] The residue from process (302) can be discharged into a chimney. The final permeate (306) contains 50% CO2 and can find applications such as various algae or CO2 treatment applications in cement plants. More commonly, the gas can be sent for further concentration by absorption, membrane, or cryogenic processes, producing more than 98% CO2, which can then be used in sequestration or enhanced petroleum recovery processes.

[0091] Example 4. The process of the present invention is applied to a natural gas power plant that recycles natural gas boiler exhaust or a portion of the exhaust gas.

[0092] The exhaust gas from boilers in natural gas plants used to generate high-temperature steam is typically very hot, often around 150°C, and has a typical composition of approximately 8% CO2, 16% H2O, 4% O2, and 72% N2. The dew point of the gas is approximately 56°C, but when cooled in contact with H2O using a direct-contact cooler, the gas saturates at 62°C when cooled from 150°C. At this temperature, the gas contains 20.9% H2O. The performance of our membrane process using the design shown in Figure 3 is shown in Table 9.

[0093] [Table 9]

[0094] Example 5. Process shown in Figure 3 using a continuous concentration system

[0095] In the calculation examples reported in Tables 7 to 9 to illustrate the process of the present invention, a single-stage concentrator, represented as unit (314), is shown in Figure 3. While such a simple system can be used, in larger systems, multiple concentrators can be used in series to reduce the amount of cooling water required. For example, a first concentrator using 25°C cooling water can be used to bring the permeate gas to 30°C, a second concentrator using 20°C cooling water can bring the gas to 25°C, then a final concentrator using 15°C chilled water can bring the gas to 20°C, and chilled water can be used as needed to bring the gas to an even lower temperature of 5-10°C. Using these systems reduces the amount of gas passing through the vacuum pump, thus reducing the power consumption of unit (311). However, this benefit must be offset by the cost of supplying the chilled water. Using a continuous cooling process reduces the cost of supplying the required chilled water.

[0096] When describing elements of this disclosure or its preferred embodiments, the articles “a,” “an,” “the,” and “said” are intended to indicate that there are one or more elements. The terms “comprising,” “including,” and “having” are intended to indicate comprehensiveness and that there may be additional elements other than those listed.

[0097] Considering the above, it will be evident that several objectives of this disclosure have been achieved and other favorable results have been obtained.

[0098] Because various modifications can be made to the above products and methods without departing from the scope of this disclosure, all statements contained herein are intended to be illustrative and not restrictive.

Claims

1. CO2 from flue gas exhaust ２ A membrane process for separating, (i) A step of increasing the pressure of the flue gas from 0.8 to 1.5 bar and raising the temperature to above 50°C, so that the gas contains more than 10 mol% water vapor at a concentration of 70 to 100% of the water saturation concentration; (ii) The flue gas from step (i) contains water and CO ２ A step of crossing a permeable film to H, wherein the film is measured under the operating conditions of the process. ２ O / CO ２ Selectivity is greater than 2, and CO ２ / N ２ Selectivity is greater than 10; (iii) From the supply side of the film in step (ii), the CO in the gas ２ A process of removing depleted flue gas stream from which at least 50% of the content has been removed; (iv) A step of removing a permeated gas rich in CO ２ and H ２ O from the permeation side of the membrane in step (ii) at a pressure of 0.1 to 0.4 bar;​​​​ (v) The permeate gas from step (iv) is cooled to a temperature of 5 to 30°C, and the H in the gas is removed. ２ A portion of the O content is condensed, and the H of the permeate gas ２ A step of reducing the O concentration to generate a water-depleted permeate gas, wherein the temperature difference between the flue gas crossing the membrane in step (ii) and the permeate gas cooled in step (iv) exceeds 30°C; (vi) a step of separating the concentrated water from the water depletion permeate gas flow; and (vii) A membrane process comprising the step of using a vacuum pump to raise the water-depleted permeate gas from step (v) to atmospheric pressure or higher.

2. The aforementioned membrane process uses the CO2 of the membrane supply flow. ２ The membrane process according to claim 1, wherein 50 to 80% of the content is removed.

3. The membrane process according to claim 1, wherein the supply gas to the membrane unit has a temperature between 50 and 80°C.

4. The membrane process according to claim 1, wherein the amount of water vapor in the membrane supply gas is 80 to 100% of the water saturation concentration of the gas.

5. The membrane process according to any one of claims 1 to 4, wherein in step (i), a direct contact cooler is used to adjust the temperature and water concentration of the membrane supply gas flow.

6. The membrane process according to any one of claims 1 to 4, wherein the flue gas exhaust flow is generated by a coal-fired power plant, a natural gas power plant, a natural gas boiler, a cement plant, a steel mill, or an oil refinery.

7. The supply gas of step (i) and the CO of step (iv) ２ and H ２ The membrane process according to any one of claims 1 to 4, wherein the difference in water concentration of the oxygen-rich permeate gas is at least two times.

8. The membrane process according to any one of claims 1 to 4, wherein the temperature difference between the flue gas that crossed the membrane in step (ii) and the permeate gas that was cooled in step (iv) is greater than 40°C.

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