Gas separation membranes are a new technology that has developed rapidly in recent years. Different polymer membranes have different transmittances and selectivities for different kinds of gas molecules, so that a certain gas can be selectively separated from the gas mixture. For example, oxygen is collected from the air, hydrogen is recovered from the ammonia tail gas, and hydrogen, carbon monoxide, and the like are separated from the petroleum cracked mixture. A chemist at the University of California, Los Angeles, in the United States, produces a thin film of a conductive organic material called polyaniline. This polymer can be incorporated into charged atoms, using the amount of dopant to alter the permeability of the film. When passing through this film, oxygen is faster than nitrogen, carbon dioxide is faster than methane, and hydrogen is faster than nitrogen, so the cost of oxygen and nitrogen produced by this film is low. They may also be used to eliminate pollutants in exhaust gases from automobiles and industries. At present, the research of gas separation membranes mainly focuses on oxygen-rich membranes. As a polymer of an oxygen-rich film, it is required to have both high permeability and high selectivity. General Electric Company uses a copolymer of polycarbonate and silicone as a separation membrane to obtain 40% oxygen-enriched air after primary separation. If the oxygen-enriched air is used instead of the ordinary air, the efficiency of various combustion devices will be greatly improved, and the pollution will be reduced. A submersible is also being developed abroad, which is a diving device that extracts dissolved oxygen directly from seawater. It is used by immersing the hemoglobin of a person capable of carrying oxygen in a polyurethane sponge. When the hemoglobin absorbs oxygen in the seawater, the oxygen is released by a weak current to be used for breathing in the water supply. If you carry a diving device containing 900 grams of hemoglobin, you can live in the sea for a long time.
Gas separation membrane related knowledge
1. According to the separation mechanism, gas separation membranes can be roughly classified into three categories:
1. "Single" dissolution-diffusion film test - the country's largest education website ()
The membrane mass transfer process is such that the gas molecules in the upstream gas phase first dissolve in the membrane, then diffuse through the membrane, and finally desorb in the downstream gas phase. Such membranes can be further divided into three types: polymer dissolution-diffusion membranes, molecular sieves, and surface selective flow membranes.
Polymer dissolving-diffusion films are the main materials for commercial applications, mostly glassy polymers and colloidal polymers. The glassy polymer preferentially passes through small non-condensable gases such as H2, N2 and CH4; like colloidal polymers preferentially permeate large condensable gases such as propane and butane.
Polymer dissolving-diffusion membranes are more economical than other membrane materials and are the main materials for membranes for gas separation. The main problems are high temperature, high pressure and high adsorption components, the stability will be affected.
Zeolite membrane materials are another option that relies primarily on molecular size differences for separation. Such membranes have very small ultramicropores that can repel certain molecules while allowing other molecules to pass. Laboratory studies have shown that the permeability of such membranes is very attractive. However, such films are difficult to process, fragile, and expensive to manufacture.
The surface selective flow membrane facilitates the passage of larger permeate through the membrane while trapping smaller components. This type of separation can be achieved by surface selective flow membranes. Such membranes have nanopores, which are selectively adsorbed on the surface of the pores, and then adsorbed components are diffused through the pore surface. Since the adsorbed molecules do not generate voids in the pores of the membrane, resistance to the transfer of small non-adsorbed components occurs. Recently, researchers are using a membrane module with a surface-selective flow mechanism for intermediate amplification experiments.
2. "Complex" dissolution-diffusion membranes Source: Examination membranes are similar to "single" dissolution-diffusion membranes, but the separation mechanism is more complex than the "single" dissolution-diffusion membrane. It can be further classified into two types: a palladium (alloy) film for promoting transfer film and hydrogen separation.
Promoting the transfer film The advantage is that high permeability can be achieved with low concentration driving force, and the selectivity is high; the disadvantage is poor stability, so far no industrial application.
Palladium-based membranes have a high selectivity to hydrogen. The hydrogen molecules are adsorbed and dissociated on the surface of the palladium membrane to form a palladium hybrid having a partial covalent bond; then the atomic hydrogen diffuses through the membrane inside the metal and recombines as hydrogen molecules downstream of the membrane. Since the pure palladium membrane undergoes hydrogen embrittlement after a plurality of hydrogen adsorption and desorption cycles, a palladium alloy is usually used instead. A typical use of such membranes is as a membrane reactor that combines certain reactions to complete hydrogen production and separation in one unit.
3. The ion conductor membrane is made of an ionic conductor material, the most important of which is a solid oxide membrane and a proton exchange membrane.
Solid oxide films can be classified into two categories: mixed ion electronic conductors (MIEC) and solid oxides. MIEC is capable of conducting oxygen ions and electrons for non-electrochemical processes that require oxygen or oxygen ions. Solid oxides only conduct oxygen ions and do not conduct electrons. In this case, electrons are conducted through an external circuit to generate electrical energy. The oxygen transfer process includes three steps of electrochemical reaction of two gas-film surfaces and oxygen ions permeating through the solid oxide film. Compared with polymer membranes, such membranes have high selectivity and flux, but require high temperature (700 ° C) operation, high temperature sealing, and temperature sensitivity of the membrane before large-scale application.
Proton exchange membranes are, in a sense, analogs of solid oxides that also conduct only protons and do not conduct electrons. The membrane material can be a polymer or an inorganic material, the most commonly used being Nafion (a sulfonated polymer). Such membranes have found application in fuel cells.
Second, gas separation membrane application 1 air separation In the world's mass production of chemical products, oxygen and nitrogen are mostly obtained by separation membrane gas separation technology, mainly by air by cryogenic rectification. Membrane separation has the advantages of low energy consumption, low investment, easy operation, and has certain competitiveness in some application fields.
With a separation membrane, it is economical to produce nitrogen with a purity of 99.5%. In industrial and commercial applications where ultra-high purity nitrogen is not required, membrane separation and nitrogen production is an ideal choice. It is estimated that membrane separation nitrogen production accounts for about 30% of the total production. Polymer membranes are the most advantageous in this field.
The O2/N2 separation factor (selectivity) of the early polymer film was 4, and when the film was prepared to produce 99% by mass of nitrogen, 75% of the nitrogen loss in the compressed air was in the infiltration process. The current polymer film has an O2/N2 separation factor of 7-8, even 8-12, which can greatly increase the productivity at the same permeation rate. .
Since nitrogen is often infiltrated with oxygen, it is difficult to separate pure oxygen production from a polymer membrane, so it is mainly used to produce oxygen-enriched air instead of pure oxygen. The separation process is roughly as follows: in the case of maintaining a permeate side vacuum, oxygen in the air preferentially passes through the separation membrane. Because of the driving force of this method - the pressure difference is less than 1 atmosphere, a larger membrane area is required. This separation therefore requires flux membranes and low cost membrane modules. Source:
At present, polymer membranes can be used to produce oxygen-rich air with a purity of 25% to 60% for regeneration of FCC catalysts and efficient combustion of methane in high temperature furnaces or kiln.
Since pure oxygen is required in most cases, a second stage separation unit can be added to the production of oxygen-enriched air. Since the volume of gas sent to the second-stage separation unit is 1/3 to 1/4 of the first stage, and the purity of oxygen in the gas is increased, the second-stage separation unit can be relatively small, and the cost is lower than the single method. . For plants with a small production capacity of 6000m3/h, the second-stage separation unit is more suitable for variable pressure suction, and for deep-process distillation, it is more suitable for deep-process distillation.
Currently, Air Products and Chemicals and Caramatee are developing an oxygen generator under the trademark SEOSIM. It is an electric-powered small-scale oxygen plant. The device benefits from an ion transport membrane made of a ceramic material that conducts oxygen ions at high temperatures.
2. The first commercial application of the hydrogen recovery gas separation membrane is the separation of hydrogen from the ammonia gas (H2, N2, CH4 and Ar). The membrane is ideal for this application. Hydrogen is more easily penetrated in the glassy polymer film than other gases, so high selectivity and throughput can be obtained. In addition, the purge gas is in a high pressure state, and the hydrogen rich permeate gas can be recycled to the ammonia feedstock compressor directly. In addition, the chlorine permeation membrane is also used in the chlorine recovery of refineries, and there are now hundreds of hydrogen separation devices.
3. Removal of acid gases from natural gas The world's energy experts believe that the 21st century is the natural gas era. Natural gas is the third largest energy source in the world, not only is the demand for one type of cleaning increasing from the current 2.1×1012m3 to 40.2×1012m3 in 2020.
Natural gas is a complex gas mixture containing hydrocarbons and non-hydrocarbons such as H2S, CO2 and H2O. Since the presence of H2S and CO2 corrodes the pipeline and reduces the calorific value of the gas, the removal of H2S and CO2 from low molecular weight hydrocarbons is an important process for natural gas processing. The glassy polymer separation membrane can compete with the amine absorption method.
4. Steam/gas separation source:
The high-throughput rubber-based silicone rubber membrane preferentially penetrates the condensable gas and is highly suitable for recovering condensable gases from air or process vent gas.
Steam/gas separation was used in the United States as early as the 1990s to recover halogenated hydrocarbons from perchlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) emitted from cryogen manufacturers. At the same time, there are a large number of such devices in Europe for the recovery of hydrocarbons from the air. In recent years, such recycling systems have been used to recover high value VOCs from petrol and refinery vents. Typical applications are the recovery of vinyl chloride, propylene or ethylene monomers.
Most steam/gas separation units often have a second process such as condensation or absorption separation. A typical process for separating propylene from nitrogen is that the compressed feed gas is sent to a condensing separator, some of the propylene is removed as a condensate, and the trapped uncondensed propylene is separated and recovered by a membrane with a mass fraction of 99% nitrogen. The membrane separates the permeate venting enriched in propylene 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 propylene recycling commercial unit (VaporSep) was supplied by MTR and was put into operation in Gelean, the Netherlands, in October 1996. Due to the recovery of propylene monomer and the reduction of nitrogen consumption, the cost can be saved in millions of dollars in one year, and the investment can be recovered in one to two years.
The steam/gas separation has a 10-year operating history and there are more than 200 sets of equipment. The application proves that the technology is economical and practical.
Potential Applications 1. Natural Gas Dehydration and Dew Point Adjustment Sources: The test is designed to prevent water from freezing or freezing in the pipeline or forming hydrates. Natural gas must be dried. Permea Marifilou Production is one of the leading producers of such membrane modules. In order to improve the dehumidification efficiency, a kiss scavenging gas is also introduced into the membrane module. For medium dewatering requirements (30 ° C or removal of 85% H 2 O), the estimated equipment price is lower than the triethylene glycol (TEG) standard drying process. The first commercial unit was installed and put into operation at Norwegion in the North Sea.
2. Controlling the sputum in the associated gas in the oil field The operation of the zither of the Otticarbiretor internal combustion engine fueled by natural gas depends on the methane value of the natural gas. (similar to gasoline octane number). With a pure methane value of 100, the methane value of the fuel gas operating the Carlurre internal combustion engine is 50. The presence of a compound having a carbon number greater than 1 in natural gas has a negative effect on the methane value. Therefore, it is necessary to remove high carbon hydrocarbons so that the methane value of the associated gas is about 50. A 670h field test of the composite silicone rubber membrane module found that the performance of the membrane was relatively stable. The membrane-based associated gas methane value control system can improve the efficiency of the internal combustion engine and provide a guarantee for its smooth operation. Compared with low temperature, sucking and other techniques, the membrane separation method has the advantages of simple operation, low maintenance cost, and low investment cost.
3. Steam/steam separation Steam/steam separation, especially olefin/paraffin separation, is an important process in the petrochemical industry. Since such mixtures have similar boiling points, in order to achieve a better separation effect, a high rectification column and a large reflux ratio are required, and investment and energy consumption are very large. The recently reported application of solid polymer electrolyte membranes in the separation of ethylene/ethane mixtures shows that the membranes have good selectivity and stability, and the penetration rate of ethylene is 100% faster than ethane.
New materials and application sources:
1. Ceramic membranes Although ceramic membranes are expensive, they have great potential for application in all aspects, and people are engaged in research on their large-scale utilization. Researchers are investigating the use of MIEC to produce ethylene and propylene directly from methane production synthesis gas and methane oxidative coupling.
2. Mixed matrix membranes In order to increase the application of gas separation membranes, UOPLLC has a physical method of modifying the polymer membrane to obtain a mixed matrix membrane. The membrane is divided into two types: one is an adsorbent-containing polymer such as silicalite-CA, and its CO2/H2 selectivity is 5.15±2.20 (CA membrane selectivity is 0.77). The other is a silicone rubber containing polyvinyl alcohol, which has high selectivity to polar gases such as SO2, NH3, and H2S.
3. The carbon film is 10 to 20 times more selective than Vycar glass in gas separation, and the permeation is an order of magnitude larger.
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