A Brief Overview of Commonly Used Membrane Modules – EcoLan
Table of Contents
After over half a century of development, membrane separation has moved from the laboratory to large-scale industrial applications. It has become an efficient and energy-saving new separation technology and has made significant progress in water resource utilization and environmental protection. Membrane modules, which are the key to this technology, are discussed below according to their level of development and application.
What is a membrane module?
Different separation membranes can only be assembled into membrane modules and installed with pumps, filters, valves, instruments, and piping to complete the separation task. A membrane module is a device in which membranes are assembled in some form in a basic unit of equipment that can separate the components of a mixture under specific driving forces. The larger the membrane area, the greater the permeability per unit time. Therefore, when using membrane separation technology in practice, it is necessary to develop membrane modules with the largest membrane area per unit volume.
In industrial membrane separation processes, several or even hundreds of membrane modules can be installed in a membrane separation plant, depending on the production requirements. In addition to selecting suitable membranes, the type, design, and fabrication of the membrane modules directly impact the final separation results.
What are the types of membrane modules?
The design of separating membrane modules starts from the simulation of plate filters. Currently, there are five primary forms of membrane modules commonly used in industry: plate and frame module, tubular module, capillary module, hollow fiber module, and spiral wound module.
These five modules have advantages and disadvantages, compared in the table below. In general, the plate and frame module and the tubular module have low throughput and are particularly suitable for high-viscosity objects with highly suspended impurities. On the other hand, hollow fiber modules and spiral wound modules should be used for larger volumes.
| Key Features of Different Types of Membrane Modules | |||||
| Membrane module type | Hollow fiber module | Capillary module | Spiral wound module | Plate and frame module | Tubular module |
|---|---|---|---|---|---|
| Production cost | Low | Lower | Moderate | High | Moderate |
| Filling density | High | Moderate | Moderate | Low | Low |
| Pollution resistance | Moderate | Good | Moderate | Good | Very good |
| Pressure drop generation | High | Moderate | Moderate | Moderate | Low |
| High-pressure operation | Yes | No | Yes | Hard | Hard |
| Cleaning difficulty | Difficult | Easy | Moderate | Easy | Easy |
What should you think of when designing membrane modules?
In general, a good-performing membrane module should meet the following conditions. The selection of membrane modules should consider the cost, filling density, application, system process, membrane fouling and cleaning, lifetime, etc.
- Adequate mechanical support for the membrane, minimal dead space, good flow channel, and strict separation of feed and permeate sides.
- Under minimum energy consumption, the flow state of raw water on the membrane surface is uniform and reasonable to reduce the concentration polarization and improve the separation effect.
- Have the highest possible filling density, i.e., the unit volume of the membrane module is filled with a more effective membrane area, and make the installation and replacement of the membrane easy.
- The unit is robust, safe, reliable, inexpensive, and easy to maintain.
What are the essential elements of a membrane module?
The essential elements of a membrane module include the membrane sheet, the membrane support or connector, the flow channels related to fluid distribution in the membrane module, the membrane sealing, the housing or jacket, and the external interface.
Membrane sheet
The membrane is the core element of the membrane module, membrane separation system, and membrane separation process. According to the membrane definition, the membrane is a medium with selective permeability, i.e., it allows some substances to pass through while retaining others, which can be molecules, ions, or particles. Membranes have two characteristics: (i) they must have two interfaces that are in contact with the fluid phase on either side; and (ii) the membrane must be selectively permeable, allowing one or more substances in the fluid phase to pass through while preventing other substances from passing through.
The membrane itself can be homogeneous or a composite of more than two phases of cohesive substances; according to different membrane preparation processes, it can be divided into the symmetric membrane (also called homogeneous membrane), asymmetric membrane, and composite membrane; according to the separation process, it can be divided into reverse osmosis (RO) membrane, ultrafiltration (UF) membrane, microfiltration (MF) membrane, nanofiltration (NF) membrane, electrodialysis (ED) membrane, and so on. Currently, most of the separation membranes are solid membranes in terms of output, production value, variety, function, or application object. Generally speaking, membranes pursue high separation performance, high stability, low cost, and long lifetime to meet the requirements of technical applications.
Supports or connectors
Membranes require a support body to assist in the assembly process into modules to hold their shape and achieve the necessary strength. The shape and structure of the support body vary for different membrane forms. For example, flat membranes require a smooth, porous backing for practical use due to their low mechanical strength and tendency to break. The support body of the spiral wound membrane is a spacer fabric sandwiched between two membrane sheets and wound with the membrane, sealed, and loaded into the housing. The support body of the tubular or hollow fiber type membrane is the tube or hollow fiber itself, i.e., the membrane can be coated on the inner wall of the tube, the outer wall of the tube, or both the inside and outside of the tube.
Since the support body plays the role of support as well as the function of insulation and flow conduction, there are specific requirements for the chemical properties, structural shape, and pollution resistance of the support body in various membrane module designs, which in turn are closely related to the flow channel design of the module.
Flow channels
In the membrane separation process, the space in which the raw water enters the membrane module for separation and the product and residue flow out of the module after separation is called the flow channel. The flow channel is achieved in most membrane modules using a support body, a guide plate, or an isolation layer between the membranes.
The hollow and turbine guide plates are mainly used for flat membrane modules with a 0.5 ~ 1.0 mm height. The flow channel in the spiral membrane module is controlled by the mesh spacer material in the raw water flow channel, and its thickness is 0.76 mm or 1.1 mm; the flow channel in the tubular membrane module depends on the pipe diameter, internal pressure type or external pressure type, and the turbulence promoter placed in the pipe. The flow channel of the hollow fiber membrane module depends on the fiber distribution in the shell, the internal or external pressure type, and the location of the feed.
Sealing
All membrane separation processes require the external application of various forms of energy to operate, such as pressure difference, potential difference, concentration difference, and temperature difference. Therefore, in the production of different membrane modules, to make the raw water and permeate in the module go their way to achieve the separation purpose, certain sealing measures are required, including the sealing between membrane and membrane, the sealing between membrane and support body, the sealing between module and module, and the sealing with external interfaces.
Different membrane modules have different sealing requirements due to their different construction. For example, the spiral wound membrane module is mainly the seal between the membrane and the three sides of the support material, and the series seal between the center tubes of multiple components; the seal of the hollow fiber membrane module is mainly the seal of the epoxy tube sheet at one end of the fiber and the seal of the epoxy head at the other end; other membrane modules can be sealed by the usual methods such as rubber gaskets, which need to be considered separately in the design of the membrane module.
Housing
Most membrane modules have housings, and the requirements for the shape and structural materials of the housing vary depending on the purpose of the application. The most commonly used membrane filter housings can be divided into two types.
- Sanitary filtration housings. These filters are mainly designed for cleaning or sterilizing filtration and usually have to meet the sanitary requirements of the pharmaceutical and food and beverage industries. For example, all 300 series stainless steel should be used, the fluid should not come in contact with the nut connection, and all inlets and outlets should be connected with a sanitary flange. In addition, the welding area should be smoothed and polished, and the surface processing accuracy of all wetted parts must be consistent with reducing the roughness of the surface and the possibility of contaminants sticking to the surface, improving the degree of easy cleaning and corrosion resistance, etc.
- Industrial filtration housings. Industrial filter housings produced by general manufacturers are standardized with construction materials ranging from high-quality plastics to application-specific 316L stainless steel. For high strength and corrosion resistance requirements, composite designs are available in which the interior of the stainless steel body is coated with PEA or PVDF. However, fiberglass epoxy housings, including rolled reverse osmosis housings for seawater desalination, are commonly used today. Less expensive are PP and PVC housings.
External interface and connection
The interface between the membrane module and the application project’s process piping, equipment, and valves is called the external interface. Most membrane modules have three primary external interfaces, including raw water inlet, permeate outlet, and residual outlet, and the connection method depends on the specific situation. Generally, they can be divided into removable connections (e.g., threaded and flanged) and non-removable connections (e.g., welded).
In the case of sanitary filtration modules, the connection between the housing and the filter and the interface connection requires a special O-ring seal connection. For small industrial filters, the external connections are typically nuts or the same convex flange as large industrial housings.
Conclusion
Membrane technology is an efficient and environmentally friendly new separation technology formed by the cross-fertilization of material, chemical, textile, and environmental disciplines, which has become one of the standard technologies to solve significant problems in water resources, energy, and the environment. It is crucial in promoting economic development, industrial-technological progress, and international competitiveness. At the same time, membrane separation technology is still a developing and comprehensive discipline in an upward development stage. In the future, membrane science and technology will continue to improve and perfect existing membrane processes and continuously explore and develop new techniques and materials to play a more significant role in various application fields.
