Gas Separation Membrane Working Principle

The working principle of a membrane separation system is to use a polymer (usually polyimide) thin film to selectively "filter" feed gas to achieve separation. When a mixture of two or more gases passes through a polymer film, differences in the solubility and diffusion coefficients of each gas component in the polymer lead to different rates of permeation through the membrane wall. This allows gases to be divided into "fast gases" (e.g., H ₂ O, H ₂ , He, etc.) and "slow gases" (e.g., N ₂ , CH ₄ and other hydrocarbons). When the mixed gas is subjected to the driving force—the partial pressure difference of the corresponding components on both sides of the membrane—faster permeating gases preferentially pass through the membrane wall and are enriched on the low-pressure permeate side, while slower permeating gases are enriched on the high-pressure retentate side

A typical membrane separator contains tens of thousands of fibers, sealed at both ends with epoxy resin. The ends of the fiber bundles are cut to expose the fiber pores at both ends, allowing gas to move from one end to the other. The fiber bundle is enclosed in a suitable casing. This casing protects the fibers and guides the gas flow correctly

Gas molecules permeate through the thin wall of the membrane fiber driven by different partial pressures. Factors affecting permeability include solubility, diffusion rate, gas-polymer coordination status, and the permeation rates of different gas components. The greater the difference in permeation rates, the better the separation efficiency

Description of Gas Separation Membrane Performance

The core technology for all types of membranes used in gas separation involves modified polymer materials with different components added. The principle of gas separation is the same, and membrane performance depends on each manufacturer's membrane material formula and production process. Therefore, different membranes have different separation efficiencies, pressure resistances, and service lives

Generally, membrane performance can be described by gas production rate, recovery rate, and membrane lifespan:

1) Gas production rate: The amount of product gas that a single membrane can produce;

2) Recovery rate: The percentage of the target product gas yield to the content of the same component in the raw gas. A high recovery rate means less raw gas is needed to obtain the same amount of product gas, leading to more economical operation;

3) Membrane lifespan: The durability of the membrane's use is related to various factors, including the membrane material itself, the rationality of the separation system design, and operation and maintenance

Membrane performance is also related to various factors such as the purity of the produced gas, operating temperature, and operating pressure. For membranes used in air separation to produce nitrogen, nitrogen recovery decreases with increasing operating temperature and slightly increases with increasing operating pressure, while membrane production increases with increasing temperature and pressure. Therefore, membrane operation must be at a specific temperature and pressure to achieve optimal performance

Gas Separation Membrane Structure - Hollow Fiber Membrane

The core component of a membrane separation system is a membrane separator, similar to a shell-and-tube heat exchanger, where tens of thousands of fine hollow fibers are cast into a tube bundle and placed inside a pressure-bearing shell. After the mixed gas enters the separator, it flows axially along one side of the fibers. "Fast gases" continuously permeate through the membrane wall and are enriched on the other side of the fibers, exiting through the permeate gas outlet, while the retentate gas exits from the non-permeate gas outlet at the opposite end to the gas inlet

Gas Separation Membrane Structure - Membrane Separator Design and Arrangement

Gas Separation Membrane Structure - Hollow Fiber Membrane

1. Externally Pressurized Type: Process gas treatment membranes----PO, P2, and P3 membranes, with over 30 specifications from 2 to 8 inches

　　　　Natural gas treatment membranes------Material same as P2, with dozens of specifications from 1 to 8 inches

2. Internally Pressurized Type: Air separation oxygen-enriched nitrogen production membranes------P1, N1, N2, N3, and P3 membranes, with various specifications from 1 to 8 inches

Features of Gas Separation Membrane System

Flexible: When unexpected changes occur in plans or processes, the membrane system offers operational flexibility. To meet increased production, simply add more membrane separators. If production needs to be reduced, closing the separator's control valve can maintain the system's recovery rate and purity. Various integration methods can yield different purities and flow rates from the permeate side

Compact: Suitable for small or crowded plants with minimal on-site installation time, cost, and potential construction errors. On-site preparation time is short, requiring only simple concrete support pads and utilities. The membrane system is skid-mounted for easy relocation

Effective and Economical: In most applications, membrane systems have high recovery rates, with efficiencies of 80-95% for hydrogen and hydrocarbons. The operating pressure of membrane systems is essentially the same as the pressure during refining, requiring no additional compression energy for the separation process. It consumes very little steam (for temperature control), instrument air, and purge nitrogen. This system is easy to start and stop, and the product gas does not require cooling or pretreatment

Low Maintenance: Membrane separator groups have no moving parts to inspect, maintain, or replace. They are maintenance-free under proper design, installation, and operating conditions. However, attention should be paid to various process conditions during operation and some tolerable pollutants, such as liquid water, ammonia, hydrogen sulfide, hydrocarbons, and aromatics.

Long lifespan: Sound design and structure ensure a long lifespan in industrial applications.

Advantages

Gas separation membrane applicable industries and scenarios

Nitrogen purification/enrichment membrane engineering

Membrane separators are used to separate and enrich high-purity nitrogen from compressed air. These robust separators use hollow fiber membrane technology to separate nitrogen molecules from other components in compressed air. The resulting nitrogen stream is clean and dry, suitable for direct use in most industrial applications.

The main function of membrane separators is to separate nitrogen from compressed air. The vast majority of applications involve producing nitrogen as an inert gas to eliminate flammable atmospheres or protect items that oxidize easily in air. Membrane separators are assembled using N1, N2, or P3 membrane fibers. PA membrane separators can produce nitrogen with a purity of up to 99.9%.

N1 fibers are suitable for systems that require a large supply of nitrogen. Application examples include: inerting product warehouses and ship holds, inerting oil and gas transport ships, low-oxygen air, fire prevention, and many other applications using nitrogen.

N2 fibers are similar to N1 but are used in situations where frequent membrane start-stop is required. Applications of N2 fibers include: beverage dispensing and tire inflation.

P3 fiber is a high-selectivity membrane. Compared to N1 fiber, this membrane has a lower gas production rate, but more nitrogen can be produced per unit volume of compressed air supply. P3 fiber is the most selective type on the market. This fiber is suitable when compressed air is limited or energy saving is required. Application examples include offshore oil platforms and ships.

Nitrogen membrane separators use asymmetric hollow fiber composite membrane technology to separate and recover nitrogen from compressed air. The membrane uses the principle of selective permeation to produce high-purity nitrogen. Each gas has a specific permeation rate, which depends on the gas's solubility and diffusion ability in the membrane. In the membrane separator, compressed air flows along the inside of the hollow fiber. Fast-permeating gases (oxygen, carbon dioxide, and water vapor) and a small amount of slow-permeating gases pass through the membrane wall to the outside of the fiber. They are then collected as permeate gas at atmospheric pressure. Most of the slow-permeating gas and a very small amount of fast-permeating gas continue to flow along the fiber until they reach the end of the membrane separator, and the resulting nitrogen product is transported to the point of use via pipeline.