Phone:
0086 28 87573647
+86 19150187139
Since Pemea (now AirProducts) launched its Prism hydrogen separation membrane in 1980, the US gas separation membrane market (membranes and membrane modules) has grown from $14 million in 1985 to $150 million in 2000, and has maintained steady growth. In recent years, the US has included membrane research as one of its advanced technology projects, and the European Membrane Association has also submitted documents to the EU requesting that membrane research be included as an important research area.
Based on the separation mechanism, gas separation membranes can be broadly classified into three categories:
"Single" Dissolution-Diffusion Membranes
The mass transfer process of these membranes is as follows: gas molecules in the upstream gas phase first dissolve in the dry membrane, then diffuse through the membrane, and finally desorb in the downstream gas phase. These membranes can be further divided into three types: polymer dissolution-diffusion membranes, molecular sieves, and selective surface flow membranes.
Polymer lattice dissolution-diffusion membranes are the main materials for commercially applied membranes, and are mostly glassy polymers or colloidal polymers. Glassy polymers preferentially permeate small, non-condensable gases such as H2, N2, and CH4; colloidal polymers preferentially permeate large, condensable gases such as propane and butane.
Polymers are more economical than other membrane materials and are the main material for gas separation membranes. Their main problem is that stability is affected by high temperatures, high pressures, and the presence of highly adsorbed components.
Molecular sieves are another option for membrane materials, primarily relying on differences in molecular size for separation. These membranes have very small micropores that repel some molecules while allowing others to pass through. Laboratory studies have shown that these membranes have very attractive permeation performance. However, these membranes are difficult to process, fragile, and expensive to manufacture.
Surface selective flow membranes are used in some cases where it is necessary to facilitate the permeation of larger permeates while retaining smaller components. This separation can be achieved through surface selective flow membranes. These membranes have nanopores on which highly adsorbed components are selectively adsorbed, and then diffuse through the pore surface. Because the adsorbed molecules do not create voids in the membrane pores, they create resistance to the transfer of small, non-adsorbed components. Recently, researchers have been conducting pilot-scale experiments using membrane modules with a surface selective flow mechanism.
“Complex” Dissolution-Diffusion Membranes
These membranes are similar to “single” dissolution-diffusion membranes, but their separation mechanism is more complex. They can be further divided into two categories: enhanced transfer membranes and palladium (alloy) membranes for hydrogen separation.
Enhanced Transfer Membranes: Advantages include high permeability and high selectivity even at low concentrations; disadvantages include poor stability and lack of industrial application to date.
Palladium-Based Membranes: These exhibit high selectivity for hydrogen. Hydrogen molecules adsorb and dissociate on the palladium membrane surface, forming palladium hybrids with partial covalent bonds; then, atomic hydrogen diffuses through the metal and recombines downstream as hydrogen molecules. Since pure palladium membranes experience hydrogen embrittlement after multiple hydrogen adsorption and desorption cycles, palladium alloys are often used instead. A typical application of these membranes is as membrane reactors, combining certain reactions to achieve hydrogen production and separation in a single unit.
Ion Conductor Membranes
These are made of ion-conducting materials, the most important of which are solid oxide membranes and proton exchange membranes.
Solid oxide membranes can be divided into two categories: mixed ion-electron conductors (MIEC) and solid oxides. MIECs can conduct both oxygen ions and electrons, and are used in non-electrochemical processes requiring oxygen or oxygen ions. Solid oxides, on the other hand, only conduct oxygen ions, not electrons. In this case, electrons are conducted through an external circuit to generate electrical energy. The oxygen transfer process involves three steps: electrochemical reactions at two gas-membrane surfaces and the permeation of oxygen ions through the solid oxide membrane. Compared to polymer membranes, these membranes have high selectivity and flux, but require operation at high temperatures (700°C). Before large-scale application, issues such as high-temperature sealing and the membrane's temperature sensitivity need to be addressed.
Proton exchange membranes are, in a sense, analogous to solid oxide membranes, also only conducting protons and not electrons. The membrane material can be polymer or inorganic, with Nafion (a sulfonated polymer) being the most commonly used. These membranes have been applied in fuel cells. Applications
Air Separation
Oxygen and nitrogen rank 3rd and 5th respectively among the world's most produced chemical products, primarily produced from air via cryogenic distillation. Membrane separation offers advantages such as low energy consumption, low investment, and ease of operation, making it competitive in certain applications.
Membrane separation can economically produce nitrogen with a mass fraction of 99.5%. In industrial and commercial applications where ultra-high purity nitrogen is not required, membrane separation nitrogen production is an ideal choice. It is estimated that membrane separation accounts for approximately 30% of total nitrogen production. Polymer membranes are the most advantageous in this field.
Early polymer membranes had an O2/N2 separation coefficient (selectivity) of 4. When using such membranes to produce 99% nitrogen, 75% of the nitrogen in the compressed air was lost in the permeate. Currently used polymer membranes have an O2/N2 separation coefficient of 7-8, and the air compression cost is half of the total production cost. Membrane separation nitrogen production units with a capacity of less than 1200 m³/h are already competitive with cryogenic distillation and pressure swing adsorption. If the O2/N2 separation coefficient is increased to 8-12 at the same permeation rate, and compression costs are reduced by 20%, the production cost of nitrogen can be reduced by 10-15%.
Because nitrogen often permeates with oxygen, it is difficult to separate pure oxygen using polymer membranes. Therefore, it is mainly used to produce oxygen-enriched air, not pure oxygen. The separation process is roughly as follows: under the condition of maintaining a vacuum on the permeate side, oxygen in the air preferentially permeates through the separation membrane. Since the driving force of this method—the pressure difference—is less than 1 atmosphere, a large membrane area is required. Therefore, this separation method requires high-flux membranes and low-cost membrane modules.
Currently, polymer membranes can be used to produce oxygen-enriched air with a mass fraction of 25%-60% for the regeneration of FCC catalysts and for the efficient combustion of methane in high-temperature furnaces or kilns.
Since pure oxygen is required in most cases, a second-stage separation unit can be added to the oxygen-enriched air production process. Since the gas volume sent to the second-stage separation unit is 1/3 to 1/4 of that entering the first stage, and the oxygen purity in the gas is increased, the second-stage separation unit can be relatively small, thus reducing the cost compared to a single method. For plants with a production capacity of less than 6000 m³/h, pressure swing adsorption (PSA) is suitable for the second-stage separation unit, while cryogenic distillation is more suitable for plants with larger production capacities.
Currently, Air Products and Chemicals and Caramatee are developing an oxygen generator under the brand name SEOSIM. It is a small-scale, electrically driven oxygen production unit. This unit benefits from an ion transport membrane made of ceramic materials that can conduct oxygen ions at high temperatures.
Hydrogen Recovery The first large-scale commercial application of gas separation membranes was the separation of hydrogen from ammonia purge gases (H₂, N₂, CH₄, and Ar). Membranes are ideal for this application. Hydrogen permeates more easily in glassy polymer membranes than other gases, thus achieving high selectivity and flux. Furthermore, the purge gas is under high pressure, and the hydrogen-rich permeate can be recycled directly to the ammonia feedstock compressor. Additionally, chlorine permeate membranes are also used in chlorine recovery in refineries, and hundreds of hydrogen separation units are currently in operation.
Removal of Acid Gases from Natural Gas World energy experts believe the 21st century is the age of natural gas. Natural gas is the world's third largest energy source, and its demand, not only clean but also potentially high, is projected to increase from the current 2.1 × 10¹² m³ to 40.2 × 10¹² m³ by 2020.
Natural gas is a complex gas mixture containing hydrocarbons and non-hydrocarbon compounds such as H₂S, CO₂, and H₂O. Since the presence of H₂S and CO₂ corrodes pipelines and lowers the gas's calorific value, removing H₂S and CO₂ from low-molecular-weight hydrocarbons is a crucial process in natural gas processing. Glassy polymer membranes can compete with amine absorption methods.
Steam/Gas Separation High-flux rubbery silicone membranes preferentially permeate condensable gases, making them highly suitable for recovering condensable gases from air or processing exhaust gases.
As early as the 1990s, the United States used steam/gas separation to recover halogenated hydrocarbons from perchlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) emitted from refrigerant manufacturing plants. Around the same time, Europe also had numerous such facilities for recovering hydrocarbons from the air. In recent years, these recovery systems have been used to recover high-value VOCs from petrochemical and refinery effluents. Typical applications include the recovery of vinyl chloride, propylene, or ethylene monomer.
Most steam/gas separation units often include a second process such as condensation or absorption separation. A typical process for separating propylene from nitrogen is as follows: the compressed feed gas is sent to a condenser, where a portion of the propylene is removed as condensate. The retained uncondensed propylene is recovered using membrane separation, along with nitrogen with a mass fraction of 99%. The permeate gas enriched by membrane separation is recycled to the feed gas inlet of the compressor. The mass fraction of propylene in the propylene condensate can be greater than 99.5%.
The first commercial propylene recovery unit (VaporSep), supplied by MTR, was put into operation in Gelean, Netherlands, in October 1996. Due to the recovery of propylene monomer and the reduction in nitrogen consumption, it saves millions of dollars annually, and the investment can be recovered in 1-2 years.
Steam/gas separation has been operating for 10 years, with over 200 units currently in use, and its application has proven the technology's economic viability. Potential Applications
Natural Gas Dehydration and Dew Point Regulation
To prevent water from condensing and freezing or forming hydrates in pipelines, natural gas must be dried. Permea Marifilou Production is one of the leading manufacturers of these membrane modules. To improve dehumidification efficiency, scavenging gas is also introduced into the membrane modules. For moderate dehydration requirements (30°C or removal of 85% H2O), the estimated equipment cost is lower than that of a standard triethylene glycol (TEG) drying process. The first commercial unit has been installed and is operational in Norwegion, North Sea.
Methane Reduction from Associated Gas in Oilfields
The smooth operation of an Otticarbiretor internal combustion engine fueled by natural gas depends on the methane number of the natural gas (similar to the octane number of gasoline). With a pure methane number of 100, the methane number of the fuel gas operating the Carluretor internal combustion engine is 50. The presence of compounds with more than 1 carbon atom in natural gas negatively impacts the methane number. Therefore, high-carbon hydrocarbons need to be removed to bring the methane number of the associated gas to around 50. A 670-hour field test of a composite silicone rubber membrane module revealed its relatively stable performance. A membrane-based associated gas methane number control system can improve the efficiency of internal combustion engines and ensure their stable operation. Compared with cryogenic and adsorption technologies, membrane separation has advantages such as simple operation, low maintenance costs, and low investment costs.
Steam/Steam Separation
Steam/steam separation, especially olefin/alkane separation, is an important processing step in the petrochemical industry. Because these mixtures have similar boiling points, achieving good separation requires high-efficiency distillation columns and large reflux ratios, resulting in significant investment and energy consumption. Recent reports on the application of solid polymer electrolyte membranes in the separation of ethylene/ethane mixtures show that the membrane has good selectivity and stability, with ethylene permeation rates 100% faster than ethane.
Ceramic Membranes
Although ceramic membranes are expensive, they have great potential for application in various fields, and research on their large-scale utilization is underway. Researchers are studying the application of MIEC in methane to syngas production and the direct production of ethylene and propylene via methane oxidative coupling.
Mixed Matrix Membranes
To expand the applications of gas separation membranes, UOPLLC utilizes physical methods to modify polymer membranes to obtain mixed matrix membranes. These membranes come in two types: one is a polymer containing an adsorbent, such as silicaticite-CA, with a CO2/H2 selectivity of 5.15 ± 2.20 (CA membrane selectivity is 0.77). The other is a silicone rubber containing polyvinyl alcohol, which exhibits high selectivity for polar gases such as SO2, NH3, and H2S.
Carbon Membranes
In gas separation, carbon membranes offer 10-20 times higher selectivity than Vycar glass membranes, and their permeation is an order of magnitude greater.
