Classification Summary of Various Membranes

Classification Summary of Various Membranes

(I) Definition of Membranes

A membrane refers to a thin condensed phase within one fluid phase or between two fluid phases. It separates the fluid phases into two non-communicating parts while enabling mass transfer between them.

In recent years, membrane separation processes have progressively become crucial separation techniques in chemical engineering, food processing, wastewater treatment, and pharmaceutical technology. Industrialized methods include microfiltration, ultrafiltration, reverse osmosis, electrodialysis, and gas separation. Permeation vaporization has also rapidly scaled up to industrial-scale units in recent years. Processes integrating membrane separation with reactions, along with research and application of various membrane reactors, have also advanced significantly. Other non-separation membrane processes, such as controlled-release technology, medical membranes, and membrane sensors, are also numerous. Some of these are developing at a pace that may surpass membrane separation processes.

(II) Characteristics of Membranes

◆ Regardless of thickness, a membrane always possesses two interfaces. These interfaces are in contact with the fluids on either side.

◆ Membrane mass transfer is selective. It allows one or several substances in a fluid phase to pass through while preventing others.

(III) Classification Methods for Membranes

Membranes vary widely in type and function, with multiple classification approaches. They can be broadly categorized by material, structure, shape, separation mechanism, separation process, or pore size.

3.1 Classification by Material: Inorganic Membranes and Organic Membranes

(1) Organic Membranes
Organic membranes are made from polymeric materials such as cellulose acetate, aromatic polyamide, polyethersulfone, polyfluoropolymers, etc. They achieve oil-water separation by effectively retaining oil droplets and suspended particles. Advantages include high-quality effluent, convenient operation, compact footprint, and no generation of new sludge.

(2) Inorganic Membranes
Inorganic membranes are a type of solid-state membrane. They are semipermeable membranes made from inorganic materials such as metals, metal oxides, ceramics, zeolites, and inorganic polymeric materials. Inorganic separation membranes can be classified into two categories: dense membranes and porous membranes.

Pore size > 50 nm: Coarse-pore membrane

Pore size 2–50 nm: Filtration membrane

Pore size < 2 nm: Microporous membrane

According to IUPAC standards, porous inorganic membranes are classified into three major categories based on pore size range. Currently industrialized inorganic membranes are exclusively coarse-pore and filtration membranes.

Currently, practical organic polymer membrane materials include: cellulose esters, polysulfones, polyamides, and other materials. In terms of variety, over a hundred types of membranes have been developed, with approximately 40+ types already applied in industrial and laboratory settings. Taking Japan as an example: cellulose ester membranes account for 53%, polysulfone membranes for 33.3%, polyamide membranes for 11.7%, and membranes made from other materials for 2%. This demonstrates the dominant position of cellulose ester materials in membrane materials.

3.2 Classification by Structure: Symmetrical Membranes (Microporous Membranes, Homogeneous Membranes), Asymmetrical Membranes, Composite Membranes

(1) Symmetrical Membranes
The chemical and physical structures of the membrane are consistent in all directions, with similar porosity in every orientation. These are also known as isotropic membranes. Although symmetric membranes are isotropic, the presence of symmetrical elements in their structure can also result in anisotropy. Examples include radially anisotropic membranes in hollow fibers, laterally anisotropic membranes in other configurations, and double-layer hollow fiber membranes—all classified as symmetric membranes.

(2) Asymmetric Membranes
The most widely used membranes today feature precision asymmetric structures. These membranes possess the two fundamental properties essential for material separation: high mass transfer rates and excellent mechanical strength. They consist of a very thin active layer (0.1–1 μm) and a porous support layer (100–200 μm).
This extremely thin active layer determines the separation characteristics through its pore size and surface properties, while the thickness primarily governs the mass transfer rate. The porous support layer serves only a structural function, exerting minimal influence on separation characteristics or mass transfer rates. Beyond high permeation rates, asymmetric membranes offer another advantage: the majority of removed substances accumulate on their surface, facilitating easy removal.

(3) Composite Membranes
◆ Composite membranes, also termed “thin-film composite membranes,” represent the fastest-developing and most extensively researched membrane type today. Initially employed in reverse osmosis processes, and are now applied in membrane separation processes such as gas separation and permeation vaporization. The selective membrane layer (or active layer) of this membrane is deposited onto the surface of a microporous base membrane (support layer), resembling the continuous skin of an asymmetric membrane, except that the surface layer and base layer are made of different materials, whereas the asymmetric membrane is composed of the same material.
◆ The performance of composite membranes depends not only on the selective surface layer but also on the microporous support material, structure, pore size, pore distribution, and porosity. Higher porosity in the porous membrane structure is preferable, minimizing contact between the membrane surface layer and the support layer while facilitating mass transfer.

3.3 Classification by Shape

(1) Flat Sheet Membranes.
Flat sheet membranes are primarily used in flat sheet membrane modules

. Features of flat sheet membrane modules: Significantly larger specific surface area than tubular modules, easy membrane replacement, suitable for microfiltration and ultrafiltration

Flat sheet membrane sheets

(2) Tubular membranes
Tubular membranes are primarily used in tubular membrane modules. These modules feature simple structure, strong adaptability, low pressure drop, high permeate flux, easy cleaning and installation, high pressure resistance, and suitability for treating high-viscosity and viscous liquids. However, they have a smaller specific surface area and are suitable for microfiltration and ultrafiltration.

(3) Hollow Fiber Membranes
These self-supporting membranes resemble fibers in appearance. As a type of asymmetric membrane, their dense layer can be located on the outer surface of the fiber (e.g., reverse osmosis membranes) or on the inner surface (e.g., microfiltration and ultrafiltration membranes). For gas separation membranes, the dense layer can be positioned on either the inner or outer surface.

4. Classification by Separation Mechanism

4.1 Diffusion Membranes
Diffusion membranes, also termed “separation membranes,” are metallic diaphragms with microporous structures. These pores restrict conventional gas flow while permitting diffusion flow, enabling isotope separation based on mass differences. Their development was a key technological breakthrough for gas diffusion plants.

4.2 Ion Exchange Membranes
Ion exchange membranes are primarily used for separating charged substances (typically electrolytes). The fundamental principle involves utilizing the selective permeability of anion and cation exchange membranes to separate or concentrate electrolytes in solutions.

Classified by ion selectivity:

Cation exchange membrane: R-SO₃H, which becomes negatively charged after ionization in water

Anion exchange membrane: R-CH₂N(CH₃)₃OH, which becomes positively charged after ionization

4.3 Selective Membranes
Selective permeable membranes are active biological membranes that combine the physical properties of semipermeable membranes with active selectivity, such as cell membranes. Therefore, a membrane with selective permeability must possess semipermeability, but a semipermeable membrane does not necessarily exhibit selective permeability. Only active biological membranes possess selective permeability.

5. Classification by Separation Process
5.1 Reverse Osmosis Membrane
Definition: A semipermeable membrane used in reverse osmosis that permits only water molecules to pass through while blocking salt molecules. Reverse osmosis technology operates by applying pressure exceeding the solution's osmotic pressure, separating other substances from water based on their inability to permeate the semipermeable membrane. The membrane's extremely small pore size effectively removes dissolved salts, colloids, microorganisms, and organic matter from water. The system offers advantages such as high water quality, low energy consumption, pollution-free operation, simple process, and easy handling.

6. Classification by Pore Size: Microfiltration Membranes, Ultrafiltration Membranes, Nanofiltration Membranes, and Reverse Osmosis Membranes
6.1 Nanofiltration Membranes
Nanofiltration (NF) is a membrane separation technology positioned between ultrafiltration and reverse osmosis, featuring pore sizes of several nanometers. Due to its superior separation characteristics, NF demonstrates broad application prospects in pharmaceuticals, biotechnology, food processing, and other industries. Nanofiltration technology was initially applied in desalination of seawater and brackish water. In the food industry, nanofiltration membranes can be used in juice production, significantly saving energy; in the pharmaceutical industry, they can be used in amino acid production, antibiotic recovery, and other applications.

6.2 Ultrafiltration Membranes
Ultrafiltration (UF) is a membrane process between microfiltration and nanofiltration, with a membrane pore size between 0.05 μm and 1000 molecular weight units. Ultrafiltration is a membrane separation technology capable of purifying, separating, and concentrating solutions. The ultrafiltration process can generally be understood as a sieving process related to the size of the membrane pores. Driven by the pressure difference across the membrane, when water flows over the membrane surface, only water and small molecules smaller than the membrane pores are allowed to pass through, achieving the purification, separation, and concentration of the solution.

Ultrafiltration typically retains molecules with molecular weights ranging from 1000 to 300,000. Consequently, ultrafiltration membranes can separate macromolecular organic substances (such as proteins and bacteria), colloids, and suspended solids. They are widely used for clarifying feed solutions, separating and purifying macromolecular organic substances, and removing pyrogens.

6.3 Microfiltration Membranes
Generally, microfiltration membranes refer to a highly uniform porous solid continuous medium with a pore size of 0.1–10 μm, characterized by sieving filtration. Based on pore morphology, microfiltration membranes are categorized into two types: curved-pore membranes and cylindrical-pore membranes. Curved-pore membranes feature an interlaced network of tortuous channels, while cylindrical-pore membranes exhibit nearly parallel cylindrical capillary pores penetrating the membrane wall.

6.4 Reverse Osmosis Membranes
Reverse osmosis membranes can be classified by driving force into high-pressure, low-pressure, and ultralow-pressure membranes; by membrane shape into flat sheet membranes, hollow fiber membranes, and tubular membranes; and by fabrication method into phase inversion membranes and composite membranes. Additional classifications exist based on membrane materials and application targets.
Structurally, reverse osmosis membranes feature a thin dense layer (0.1–1.0 μm) on the surface—the desalination layer or activation layer—overlying a porous support layer 100–200 μm thick. The activation layer fundamentally determines the membrane's separation performance, while the support layer primarily serves as its carrier with negligible impact on separation efficiency.

IV. Summary
◆ Separation membranes are manufactured from materials such as polymers, metals, and ceramics, with polymeric materials being predominant. Based on their physical state, they can be categorized into solid membranes, liquid membranes, and gas membranes. Gas membrane separation remains in the experimental research phase, while liquid membranes have achieved pilot-scale industrial applications, primarily in wastewater treatment.

◆ Currently, large-scale industrial applications predominantly utilize solid membranes, primarily synthetic polymer membranes. Polymer membranes can be fabricated as dense or porous, symmetric or asymmetric structures.

◆In recent years, inorganic ceramic membrane materials have experienced rapid development and entered industrial applications. Particularly in microfiltration, ultrafiltration, membrane-catalyzed reactions, and high-temperature gas separation, they fully demonstrate advantages such as chemical stability, high-temperature resistance, and high mechanical strength. Ceramic membranes and metal membranes can also be symmetric or asymmetric, but their preparation methods are entirely different.