The latest pressure swing adsorption hydrogen production process

We are interested in using pressure swing adsorption for hydrogen production; could experts please introduce us to some new processes? 1# Zhang from Suzhou

Pressure swing adsorption is one of the chemical separation technologies that has developed fastest in recent decades. Pressure swing adsorption separation technology was first proposed by Skarstrom and Guerin in their patent. Thereafter, with further research and development of adsorbent materials, pressure swing adsorption separation technology has developed rapidly. Due to its advantages such as low energy consumption, simple process, high purity of the product gas, ease of operation, and high degree of automation, pressure swing adsorption has been rapidly adopted and applied in industries such as petroleum, chemicals, metallurgy, electronics, pharmaceuticals, and food. Today, pressure swing adsorption separation technology has become the main technique for air drying, hydrogen purification, the removal of n-alkanes, and small-to-medium scale air separation. I. Basic Principles of Pressure Swing Adsorption Pressure swing adsorption (PSA) is based on the physical adsorption equilibrium of gases on solid adsorbents. It is based on the difference in the amount of adsorption of various components in the mixture by the adsorbent at different pressures; adsorption occurs at (relative) high pressures, while desorption takes place at (relative) low pressures, thereby achieving the separation of the mixture, as shown in Figure 4-1. Figure 4-1 Principle of pressure swing adsorption separation. Once the adsorbent is specified, the amount of each gas component absorbed is a function of temperature and pressure, which is typically represented by the adsorption isotherms shown in Figure 4-1. The graph shows the adsorption isotherms of gases A and B at the same temperature; clearly, at the same pressure, A is more easily adsorbed than B. If a mixture of A and B is passed through a bed filled with an adsorbent at a (relative) high pressure P1, component A will be adsorbed preferentially because its equilibrium adsorption amount qA1 is much higher than that of component B, qB1. By controlling the inlet flow rate to the bed, component A can be enriched in the bed, while component B is enriched in the permeating gas stream. To regenerate the adsorbent, the bed pressure is reduced to a (relative) low pressure P2, at which the equilibrium adsorption amounts of the two components are qA2 and qB2, respectively. During the process of reaching a new adsorption equilibrium, the amounts of A and B desorbed are respectively qA1–qA2 and qB1–qB2. By periodically varying the bed pressure in this way, separation can be achieved. II. Common adsorbents used in pressure swing adsorption separation Adsorbents are an important technical component in pressure swing adsorption processes, and the proper selection of adsorbents has a direct impact on the efficiency of the adsorption separation process. In pressure swing adsorption, the selection of the adsorbent is determined by its separation coefficient for the components in the feed gas and its capacity to adsorb the more readily adsorbed components. The separation coefficient a between components A and B is defined as: a = xA yB / yA xB, where z and y represent the concentrations in the adsorbed phase and the gas phase, respectively. Generally, the separation factor between other components in the feed gas and the product gas should be no less than 3. The following introduces several common adsorbents. 1. Activated carbon: Activated carbon is carbon that has been physically or chemically activated, resulting in a high specific surface area and a complex pore structure; it is an excellent adsorbent. It possesses a unique pore structure and surface functional groups, as well as sufficient chemical stability, mechanical strength, and resistance to acids, alkalis, and heat. Its most notable feature is its very high specific surface area. For example, the specific surface area of molecular sieves is only around 40 m2•g‑1, whereas that of activated carbon is as high as 1000–4000 m2•g‑1. Its adsorption capacity depends mainly on the specific surface area and porosity of the activated carbon. The microstructure of activated carbon is an amorphous structure based on graphite microcrystals, and this structure enables it to possess a high specific surface area and pore volume. 2. Zeolite molecular sieves Zeolite molecular sieves are crystals of a three-dimensional silico-aluminate metal structure formed by silicon-aluminum tetrahedra; they are highly polar adsorbents with uniform pore sizes. Zeolite molecular sieves modified by the exchange of different metal cations or other methods possess high selective adsorption and separation properties. The polarity of zeolite molecular sieves decreases as the Si/Al ratio increases. Zeolites with a low Si/Al ratio are capable of deeply drying and dehydrating gases or liquids, and they also exhibit high adsorption capacity at higher temperatures and relative humidities. The disadvantages of zeolite molecular sieves are their poor heat stability, resistance to acids and bases, chemical stability, mechanical strength, and wear resistance; in addition, they have a low adsorption capacity and require high regeneration temperatures. The synthetic zeolite molecular sieves commonly used in industry include Type A, Type X, Type Y, Type L, mordenite, and ZSM series zeolites, among which Type A, Type X, and Type Y are more frequently used in gas separation. 3. Carbon molecular sieve: Similar to zeolite molecular sieves, carbon molecular sieves possess uniform ultramicro-pores of size close to that of molecules. The properties of carbon molecular sieves are the same as those of activated carbon; their surface is hydrophobic, and the shape of their pores is completely different from that of zeolite molecular sieves. Carbon molecular sieves have good acid and heat resistance as well as chemical stability, but they are not resistant to combustion. 4. Silica gel: Silica gel is a type of hard, amorphous, chain-like and network-like silica polymer particle; it is a hydrophilic, polar adsorbent. The molecular formula of silica gel is SiO2•nH2O, and its pore size ranges from 2 to 20 nm. Due to the polarity of the hydroxyl groups on the surface of silica gel, it exhibits significant selectivity for polar molecules and unsaturated hydrocarbons, as well as strong selectivity for aromatic double bonds. Silica gel can absorb a large amount of water, which is why it is often used for the drying and dehydration of gases. 5. Activated alumina: Activated alumina refers to crystalline and amorphous hydrated alumina that has been dehydrated or partially dehydrated, and it has a strong affinity for water. Activated alumina is stable against most gases; it does not soften, swell, or crack when immersed in liquids, and it possesses good impact resistance and wear resistance. In industry, activated alumina is mainly used for the drying of gases and liquids.

Both the original Skarstrom cycle and the Guerin-Domine cycle are two-tower processes, as shown in Figure 4-2. One adsorption tower is used for adsorption, while another is used for depressurized desorption and washing. The disadvantage of these two simple cycles is that the product flow rate fluctuates during the desorption phase as it transitions into the adsorption phase, and it only gradually stabilizes once the pressure rises to the operating pressure. Since then, the pressure swing adsorption process has undergone many significant improvements building on these foundations, such as the introduction of a co-current depressurization step and additional pressure equalization steps between the various adsorption towers, resulting in the four-tower process that is widely used in industry today. Although pressure swing adsorption processes vary depending on the conditions and applications, some basic operational steps remain the same. Figure 4-2 Two-tower process (1) Pressurization: Gas at a certain pressure is introduced into the adsorption tower, raising the pressure inside it to the desired adsorption pressure. The gas used in the pressurization process can be the feed gas, the product gas, or other gases released during the depressurization stage. (2) Adsorption: The feed gas enters the adsorption bed at a predetermined adsorption pressure, and the adsorption process begins. Since the easily adsorbed components are adsorbed as early as at the inlet of the tower, the gas composition at the outlet of the adsorption tower is close to that of a pure gas consisting of the poorly adsorbed components. The gas phase composition changes significantly along the axial distance of the adsorption tower; after adsorption has proceeded for a certain period of time, it is stopped and the pressure reduction stage begins. (3) Pressure reduction: During the adsorption phase, some of the adsorbents become saturated with the easily adsorbed components. To enable the adsorbents to be reused, it is necessary to reduce the pressure in the bed in order to desorb those easily adsorbed components that have been absorbed during the adsorption phase. (4) Cleaning: The purpose is to remove the impurities remaining after pressure reduction from the adsorption tower, thereby allowing the adsorbent to be regenerated. IV. Main factors affecting pressure swing adsorption separation operations 1. Types of adsorbent components and impure components When using pressure swing adsorption to treat raw gas, it is necessary to first determine whether separation or purification is the goal. Main separation is the process of obtaining a product with a concentration of 20% to 80% from the raw material mixture. For example, oxygen- or nitrogen-enriched gases are obtained in air separation, and minor gaseous components at a concentration of 5% to 20% are removed in gas purification. When removing only small amounts of impurities, such as in air drying, the bed can be regenerated to allow the adsorption phase to continue for several hours or even days. However, in typical separation processes, the adsorption time is relatively short, generally ranging from 30 seconds to 5 minutes; the bed needs to be regenerated before the adsorbent becomes fully saturated, that is, before the impurities have passed through the adsorption bed. 2. Types of adsorbents: When selecting an adsorbent, it is necessary to determine the appropriate one based on the properties of the components in the feed gas that are to be adsorbed. For example, in air separation, when 5A molecular sieve is used as the adsorbent, oxygen is a component that is not easily adsorbed; it passes through the bed to become an oxygen-enriched product, while nitrogen remains adsorbed in the bed. The purity of the nitrogen obtained through desorption can reach 98%–99%. Since argon does not adsorb, the purity of the oxygen-enriched gas can only reach 95%. If carbon molecular sieves are used as the adsorbent, nitrogen becomes the product that passes through the bed, and the purity of oxygen in the oxygen-enriched gas obtained through separation can also be increased. Zeolite molecular sieves and carbon molecular sieves can also be used together to improve the purity of both nitrogen and oxygen. 3. Bed operating pressure and cleaning: The pressure selected for pressure swing adsorption separation operations depends on the pressure of the feed gas and the properties of the product components. If the feed gas can be separated at low pressure, it is best to operate at low pressure. When regenerating the bed layer by reducing pressure or evacuating it, the bed layer can first be cleaned with cleaning gas, or cleaning gas can be used alone without evacuating. The types of purge gas include the exhaust gas from the adsorption tower, product gas, inert gas that is not part of the feed, and some low-purity product gas. 4. Bed temperature and thermal efficiency: The exothermic adsorption and endothermic desorption cause fluctuations in the local temperature of the bed. An increase in local temperature during adsorption and a decrease in local temperature during desorption are both detrimental to the separation process; the best separation results can only be achieved when the adsorption bed is operated under constant temperature conditions. YangL et al. proposed using inert (non-sorbent) additives with high heat capacity (such as iron) to increase the heat capacity of the bed, thereby maintaining a stable bed temperature during circulation. 5. Number of adsorption towers The number of adsorption towers can be determined based on the product specifications (purity, recovery rate, etc.) or the allowable range of fluctuations in the flow rate and composition of the feed gas at constant pressure. Generally speaking, multi-tower pressure swing adsorption units can make better use of low-purity feed gas; they yield gas with high purity and high recovery rates, consume less energy per unit of product, and offer greater operational flexibility. However, the more towers there are, the more complex the operation becomes, and the higher the costs associated with equipment investment and maintenance. V. Hydrogen production by pressure swing adsorption: The off-gases generated in industrial processes often contain large amounts of hydrogen. For example, the off-gases from hydrocracking in refineries, the by-products from catalytic reforming, the dry gas from catalytic cracking, the off-gases from ammonia synthesis, the off-gases from acetylene black production, and the butadiene off-gases all contain hydrogen. These exhaust gases contain a high amount of hydrogen, and burning them directly would be a great waste. Since the adsorption selectivity of H2 compared to other components such as CO, CH4, and CO2 on adsorbents like molecular sieves and activated carbon varies significantly (see Table 4-1), and the pressure of these hydrogen-containing sources is generally above 1.5 MPa, it is feasible from a technical standpoint to use pressure swing adsorption to separate and purify the hydrogen from them. Taking the pressure swing adsorption hydrogen extraction process using the off-gases from ammonia plants as an example, the hydrogen content in the off-gases from ammonia synthesis is over 60%; under standard conditions, the volume of these off-gases is approximately 230–280 m3•t‑1. Recovering the hydrogen from this portion of the off-gas and returning it to the synthesis system for reuse can increase the production capacity of the ammonia plant. Table 4-2 lists the composition of the off-gases from some ammonia synthesis plants. After being separated and purified by pressure swing adsorption, this portion of the off-gas has its inert components removed, allowing it to be returned to the ammonia synthesis system to increase production. At the same time, the return of this amount of hydrogen also **reduces** the content of the inert gas CH4 in the circulating gas, which has a significant effect on reducing compressor energy consumption and improving compressor efficiency. Furthermore, in the hydrogen extraction unit for ammonia synthesis off-gases, the feed gas does not need to be compressed; the off-gases enter the pressure swing adsorption unit directly after being depressurized, resulting in low energy consumption. Table 4-1 Separation coefficients of common components for adsorbents. Separation coefficient a: 5A sieve – 13X sieve – Activated carbon: N2/H2, CO/H2, CH4/H2, CO2/H2, CH4/N2, CO/N2, Ar/H2 – 6.9, 17.2, 9.65, 54.18, 1.4, 2.5, 2.5, 5.25, 12.6, 8.0, 59.2, 1.52, 2.4, 5.1, 6.97, 14.4, 30.1, 2.84, 1.3. Table 4-2 Composition of some exhaust gases from ammonia synthesis plants. Ammonia synthesis plants; Raw materials; Gas flow rate at standard conditions/(m3•h⁻¹); Composition/%: H2, N2, CH4, Ar, NH3. Luotianhua, Sichuan Chemical, Guangxi Petrochemical: Natural gas, Natural gas, Naphtha – 9890, 7823, 8788; 60.29, 58.67, 58.16; 20.12, 19.56, 19.37; 13.23, 13.47, 12.09; 3.79, 4.33, 3.68; 2.60, 3.92, 6.70. Pressure swing adsorption for hydrogen extraction from ammonia plant exhaust gases generally employs a four-tower process; Figure 4-3 shows the process flow for hydrogen extraction via pressure swing adsorption of ammonia synthesis plant exhaust gases. In this process, the exhaust gas is first washed with high-pressure water to remove NH3; after being dried with silica gel to remove moisture and trace amounts of NH3, it enters the pressure swing adsorption system. The operating pressure of a pressure swing adsorption system is generally 1.6~2.4 Mpa. Figure 4-3 Process flow for hydrogen extraction by pressure swing adsorption of ammonia synthesis off-gas 1, 2 – Drying towers ； 3 – Product tank ； 4, 5, 6, 7 – Adsorption tower: Methanol off-gas is the waste gas emitted during the methanol production process. In this process, there are certain requirements regarding the content of inert components (such as CH4, N2, Ar, etc.) in the recycle gas; therefore, a certain amount of this recycle gas must be vented. The gas that is vented is the methanol off-gas, and its composition is shown in Table 4-2. Methanol off-gas has a high hydrogen content; the separation coefficients between the impurity components and hydrogen, as well as among those impurity components themselves, are all greater than 2 (see Table 4-1), and the discharge pressure of methanol exhaust gas is usually greater than 3.9 MPa. Using pressure swing adsorption to separate and recover the hydrogen from it offers good economic benefits. The figure shows a schematic diagram of the process for separating and recovering hydrogen from methanol off-gases using pressure swing adsorption. Table 4-3 Composition of Methanol Off-gas Components: CO, CH4, CO2, CH3OH, Ar, N2, H2; Percentage of components: 5–15, 3–5, 5–15, 0.5–1.0, 4–5, 1–3; 50–70. Figure 4-4 Schematic diagram of the process for separating and recovering hydrogen from methanol off-gas using pressure swing adsorption; 1 – Separator ； 2 – Preprocessor ； 3 – Exhaust gas storage tank ； 4 – Hydrogen storage tank ； 5－Heater ； 6 – Pressure swing adsorption unit: The methanol off-gases pass through a separator at a relief pressure to have the liquid components removed, and then enter a pretreater to have fusel oils removed. The pretreated methanol exhaust gas is pressurized to a certain level before being fed into the pressure swing adsorption unit. In this process, impurity components such as Ar, N2, CO, CO2, and CH3OH are removed, and the hydrogen product is delivered to the hydrogen storage tank at a pressure slightly lower than that used during the adsorption operation. The pressure swing adsorption unit can adopt a four-tower, two-stage pressure equalization process or an eight-tower, four-stage pressure equalization process, depending on the production scale and product requirements. Coke oven gas (COG) is a by-product of the coking process. Apart from containing large amounts of hydrogen and methane, the other components are quite complex and vary significantly depending on the raw coal used; they are also related to the operating conditions of the coke oven. Taking Baosteel’s coke oven gas as an example, its main components are detailed in Table 4-4. The typical process for hydrogen extraction from coke oven gas is shown in Figure 4-5. Coke oven gas is fed into a pretreatment unit to remove high-boiling-point impurity components such as hydrocarbons with C5 content or higher and aromatics, after which it proceeds to the pressure swing adsorption process. In this process, all gases in the coke oven gas except H2 and a small amount of O2 are adsorbed. The pure hydrogen produced contains only 0.1%–0.5% O2, with the remaining impurities present at levels of ug/g. Table 4-4 Main Components of Baosteel’s Coke Oven Gas
Component: H2, CH4, CO, N2, CO2, CnHm, O2, H2O
Volume %: 52.3–55.6, 27.1–30.4, **4.9, 2.0, 2.8, 0.1
Saturation: …
Figure 4-5 Process Flow for Separating and Purifying Coke Oven Gas to Produce Hydrogen via Pressure Swing Adsorption
VI. Advances in Pressure Swing Adsorption Technology for Hydrogen Production
Since the 1970s, pressure swing adsorption (PSA) separation technology has developed rapidly, becoming the primary technique for tasks such as gas drying, gas purification, removal of n-paraffins, and small-scale air separation. From the initial Skarstrom cycle to the processes currently used in industry, pressure swing adsorption technology has undergone many improvements. These include the introduction of multi-bed pressure swing adsorption processes, the use of steps such as sequential pressure reduction and pressure equalization, as well as the combined use of multiple adsorbents within the same adsorption tower – all of which have contributed to the development of pressure swing adsorption separation technology. The multi-bed pressure swing adsorption process is conducive to achieving high product purity and recovery rates, but an increase in the number of adsorption beds also raises the equipment investment cost. Therefore, the four-tower pressure swing adsorption process is often widely used. Currently, the adsorption pressure in pressure swing adsorption processes for hydrogen extraction from various hydrogen-containing waste gases is generally above 1.5 MPa. However, when the pressure of the hydrogen-containing waste gases is low and the hydrogen content in them is small, it becomes very uneconomical to use conventional pressure swing adsorption technology for the separation and purification of hydrogen. Separating and purifying hydrogen from low-pressure waste gas not only provides the factory with the required hydrogen but also reduces its production costs and enhances the economic efficiency of the enterprise. Therefore, whether from the perspective of hydrogen recovery or environmental protection, it is of great significance to study how to use pressure swing adsorption to directly separate and purify hydrogen from these low-pressure off-gases. To address the issue of hydrogen production from low-pressure gas sources, Zhou Li proposed a pressure swing adsorption process with an intermediate pressure equalizing tank as shown in Figures 4-6, along with a new sequence for the pressure swing adsorption cycle. This approach overcomes the interdependence among the various adsorption towers in traditional pressure swing adsorption processes and reduces the adsorption pressure to ≤1.0 OMPa. Experimental results show that, while ensuring a hydrogen purity of not less than 99.99%, the new process maintains a product recovery rate of over 60% even when the operating pressure is reduced to 0.4–0.6 MPa; at an operating pressure of 0.8–1.0 MPa, the product hydrogen recovery rate exceeds 80%. Moreover, its resilience to accidents other than those occurring in a particular adsorption tower is enhanced. Figures 4–6 New four-tower pressure swing adsorption process with intermediate pressure equalization tanks; A, B, C, D – adsorption towers ； HPB – High-voltage voltage equalizing tank ； LPB – Low-voltage voltage equalizing tank ； FB – Raw gas tank ； PB – Product gas cylinder ； SP – Take sample ； MFC – Mass flow controller ； MFM – Mass flow meter. After thirty years of development, pressure swing adsorption for hydrogen production has now become a relatively mature application technology. The future development direction of pressure swing adsorption for hydrogen production is the recovery of hydrogen from hydrogen-poor or low-pressure hydrogen mixtures. Recently, variable vacuum adsorption (VSA) for hydrogen recovery from hydrogen-poor gas sources has also been reported. In industrial production, it has been found that combining pressure swing adsorption with traditional separation techniques such as low-temperature condensation or distillation can yield excellent results. The combined process is superior to the single process, offering significant energy savings and lower costs. Currently, the processes associated with pressure swing adsorption mainly include deep cryogenic separation – pressure swing adsorption, membrane separation – pressure swing adsorption, membrane separation – pressure swing adsorption – catalytic reaction, etc. The Special Gases Plant of Jiangsu Xinyuan Group Company uses a pressure swing adsorption–cryogenic separation combined method to produce hydrogen, by combining pressure swing adsorption units with cryogenic separation equipment to utilize ammonia synthesis off-gas as raw material for hydrogen production. The raw gas, after ammonia removal via high-pressure water washing and freeze-drying, enters a pressure swing adsorption unit and a low-temperature separation unit at a pressure of 2.0–3.0 Mpa; the pressure swing adsorption process yields product hydrogen with a purity of over 99.9%. At the same time, the purge gas and regeneration wash gas released periodically by the pressure swing adsorption unit are recovered as recycled gas, which is then pressurized, cooled, and pre-treated

Many manufacturers, including Beida Xianfeng and West China Hospital in Chengdu, produce it

Is there a process diagram, something that’s more straightforward? Thank you

Have studied*.......................

Regarding the new PSA hydrogen production processes, no significant technological innovations or breakthroughs have been reported yet!

Our company has advanced technology in hydrogen production, and we can arrange visits for manufacturers to our facilities

Please leave your contact information. . . . . . .

QQ: 156441529