Types of membranes: What are they and how are they classified?
Membrane synthesis allows us to offer solutions aligned with the circular economy. Thanks to this technology, we achieve more sustainable processes in order to reduce greenhouse gas emissions, capture microplastics, generate new energy storage methods or produce different compounds.
What is a membrane?
A membrane is a semi-permeable physical interface or barrier that separates two phases and acts as a selective barrier regulating the transport of matter. Membrane technology has been widely developed to carry out separation and concentration processes of liquid or gaseous mixtures. However, membranes have other important applications such as biomaterials, catalysts (including fuel cell systems), energy storage, and CO2 separation, among others.
Classification of the membranes
Different types of criteria exist for the classification of membranes, according to their characteristics and properties. The most common is according to their nature and structure.
1. According to its nature
The membranes are classified as natural and synthetic.
1.1. Natural Membranes: Biological or non-biological
The membranes of original nature are more regular due to their low cost; however, they have little industrial use. These can be classified by either biological or non-biological membranes.
- Biological membranes. They have more selective permeable barriers that are presented as being alive. They are bilayers of self-sealing and flexible lipid molecules that incorporate proteins and sugars in their structure. The phospholipids of the membranes make up the insulation function of the membranes (barrier of permeability), whilst the proteins fulfil the different functions of transport of substances, cellular communication, cellular adhesion, and enzyme activity, among other functions.
- No-biological. These selectively permeable barriers are not presented as being alive.
Synthetic membranes are more useful at an industrial level in separation processes in pharmaceutical, food, chemical or automotive sectors. They can be classified according to their source material.
- They possess great mechanical, chemical and thermal stability. They can be classified according to their origin as metallic, ceramics, zeolites, or glass. Among the most highlighted uses, we find the treatment of water, separation of leachates, catalyst of gas reactions, elimination of bacteria and the purification of effluents. The porous ceramic supports should be chemically inert, non-biodegradable and not contain organic carbon that can generate microorganisms.
- They are obtained from organic polymers such as cellulose acetate, polyamide, polysulfide, polypropylene, and polyvinylidene fluoride, among others. The development of polymers has been investigated due to their high selectivity and permeability compared to currently available commercial membrane materials. The methods of preparation include phase inversion (most used commercially), interfacial reaction, coating, stretching and evaporation etc. One of the critical parameters in all applications is the membrane’s permeability. Polyurethane membranes are moisture-proof, flexible and tough microporous materials that can be used in the fabric manufacturing process in combination with other polymers such as polytetrafluoroethylene (PTFE).
- These membranes are bulk membranes, either supported or immobilised (SLM or ILM) and emulsion membranes. Liquid membrane transport involves liquid-liquid extraction (LLX) processes, and the membrane operates continuously, where two homogeneous and completely miscible liquids are separated by a third liquid that is immiscible and practically insoluble to the two liquids. Liquid membranes have two considerable advantages over solid membranes: molecular diffusion in liquids (except in super viscous fluids) is several orders of magnitude faster than in solids and, on the other hand, in some cases, molecular diffusion in the liquid membrane is replaced by turbulent diffusion, which intensifies the transfer process. Therefore, solid membranes, even those of submicron thickness, cannot compete with liquid membranes with respect to transfer intensity.
An example would be water or acrylic-based polyurethane membranes capable of being applied as a UV-resistant, waterproof, highly elastic coating with high adhesion to concrete.
- These are those with chemically or structurally different layers. They may be composed of layers, inclusions or polymer blends. These membranes have better selective properties and permeabilities, are prepared by multi-step processes and usually use polysulphone and its derivatives as they combine good resistance to compaction with high surface porosity.
2. According to its structure
It can be considered a microscopic or macroscopic structure.
2.1. Microscopic Structure
The microscopic structure of membranes allows them to be classified according to their porosity and configuration.
According to their porosity:
- Porous or microporous. They are structures with a small pore size distribution, very similar in structure and function to a conventional filter. They are characterised by a rigid and very light structure, a random distribution, and interconnected pores. Separation occurs mainly as a function of molecular size and pore size distribution. The driving force responsible for the permeate flux (amount of material passing through the membrane per unit area of the membrane per unit time) is a pressure differential. This type of membrane is usually used in microfiltration processes (pores of 0.1 to 10 µm) and ultrafiltration (pores of 0.001 to 0.1 µm) preventing by the exclusion of size the passage of certain substances.
- Dense/ not porous. Structures without pores consisting of a dense film through which permeant substances are transported by diffusion under a pressure, concentration, or electrical gradient. Processes using this type of membranes are reverse osmosis and nanofiltration. These membranes work with the concept of equivalent pore diameter (the size of the largest molecule that is able to pass through). An example of this type of membrane would be ceramics because they are considered impermeable membranes with an increased application in the separation of gases due to the size of the pore that prevents the passage of water vapour, including other gases, making it more selective.
Depending on their configuration:
- Symmetric or isotropic. Symmetrical membranes are characterised by uniform pores and equal flow resistance throughout the membrane. They can be porous, dense, or electrically charged (electrodialysis). Their morphological properties (pore diameter, porosity, etc.) and functional properties (permeability, retention, etc.) do not depend on which side of the membrane is chosen for the analysis.
- Asymmetric or anisotropic. These are built for laminate or tubular structures with varying pore sizes. This type of membrane allows the ability of larger flows. They have morphological properties and/or different functions for both faces of the membrane.
2.2. Macroscopic structure
The macroscopic structure refers to the geometry of the membranes and their position in space in relation to the flow of the feed fluid and permeate. Therefore, they can be classified by function in the following configurations:
Polymeric hollow fibre membranes are prepared by extruding a polymer solution through an annular spinneret and a perforating fluid flowing in the annular centre, the size of the central channel is less than 1 mm. This synthesis process is complex as it involves many spinning parameters, the thermodynamics of the polymer solution and the phase inversion process, the rheologies of the polymer solution within the spinneret and in the air gap, and other spinning conditions. These membranes are very sensitive to fouling and therefore require very controlled handling of the fibres, but they have a high surface area/volume filtering ratio of up to 30,000 m2/m3. In this type of membrane, the fluid to be treated can circulate inside the hollow fibres or perpendicularly to the fibres.
Laminar or flat sheet
Laminar membranes are prepared by casting or moulding a polymer solution (10-30% by weight). This solution is then subjected to evaporation or inversion in another solvent, usually water, to obtain the laminar membrane. The manufacture of flat sheet membranes is also a complicated process, involving the preparation of the solution, the rheology of the casting solution, the air space and the immersion precipitation in the coagulant bath. These membranes present a low surface/volume relation of filtration (100-400 m2/m3). However, as they can form a laminar membrane module, they can be arranged in series or parallel, increasing the filtering surface area, and they are compact and easy to clean.
Flat sheet membrane synthesized in AIMPLAS
Tubular modules consist of a set of filter elements of tubular or multi-channel geometry. In this type of configuration, the membranes are arranged inside cylindrical housings that act as supports. These modules can be regenerated chemically, mechanically or with pressurised water, are highly resistant and able to accept almost any fluid without pre-treatment. However, they present a low surface/volume ratio of around 400 m2/m3.
The spiral module consists of the winding of various flat sheets or membranes separated from each other by layers of fabrics of different types that function as transporters and generators of turbulence of the feed and permeate solutions, using a central perforated tube. This type of configuration considerably improves the surface/volume ratio, which can reach between 300 and 1000 m2/m3 and reduces energy costs. However, they are easily fouled and difficult to clean.
AIMPLAS experience in membranes synthesis
Due to the great challenges in terms of the circular economy that society has been facing in recent years, the need has arisen to research new methods to reduce greenhouse gas emissions, capture microplastics present in water, generate new methods of energy storage or produce different compounds in a more sustainable way.
AIMPLAS develop projects focused on the synthesis of different types of membranes to achieve these objectives, offering a wide range of possibilities and advantages in the development of more sustainable processes, both in the capture and separation of pollutants in liquid effluents and gases and in the production and storage of hydrogen-type energy.
In these projects we are pursuing different objectives:
- CO2 Capture in order to later convert it into compounds of high added value for the chemical industry such as ethylene, polycarbonates, and carboxylic acids.
- Development of innovative processes that imply the combination of membrane technology where it can reduce the effects of greenhouse gas emissions.
- Generation of synthetic membranes for energy storage applicable to fuel cells or electrochemical cells.
- Capturing of micro and nano plastics of residual waters by the means of the ultrafiltration of membranes in combination with anaerobic digestion processes.
Daniela Andrea Ramírez Espinosa Técnico investigación del grupo de Descarbonización en AIMPLAS
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