Ultrafiltration ( UF ) are membrane filtration in which forces such as pressure or concentration gradients lead to separation through semipermeable membranes. Suspended solids and high molecular weight solutes are maintained in so-called retentates, while water and low molecular weight solutes pass through the membrane in the permeate (filtrate). This separation process is used in industry and research to purify and concentrate macromolecule solutions (10 3 - 10 6 Da), especially protein solutions. Ultrafiltration is essentially no different from microfiltration. They are separated by size exclusion or particle capture. This is fundamentally different from separation of the membrane gas, separated by different amounts of absorption and different diffusion rates. The ultrafiltration membrane is defined by the cut-off molecular weight (MWCO) of the membrane used. Ultrafiltration is applied in cross-flow or dead-end mode.
Video Ultrafiltration
Apps
Industries such as chemical and pharmaceutical manufacturing, food and beverage processing, and wastewater treatment, use ultrafiltration to recycle the flow or add value to the product later. Blood dialysis also uses ultrafiltration.
Drinking water
Ultrafiltration can be used to remove particulates and macromolecules from raw water to produce drinking water. It has been used both to replace secondary systems (coagulation, flocculation, sedimentation) and tertiary filtration (sand filtration and chlorination) used in water treatment plants or as stand-alone systems in remote areas with an ever-increasing population. When treating water with high suspended solids, UF is often integrated into the process, utilizing primers (screening, flotation, filtration) and some secondary treatment as a pre-treatment stage. The UF process is currently preferred over traditional treatment methods for the following reasons:
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- No chemicals are needed (other than cleaning)
- The product quality is constant regardless of the quality of the feed
- Compact plant size
- Be able to exceed water quality regulatory standards, achieve 90-100% removal of pathogens
The UF process is currently limited by the high costs incurred due to membrane wetting and replacement. Additional pretreatment of feed water is required to prevent excessive damage to the membrane unit.
In many cases, UF is used for pre-filtration in reverse osmosis plants (RO) to protect RO membranes.
protein concentration
UF is widely used in the dairy industry; particularly in the processing of whey cheese to obtain whey protein concentrate (WPC) and lactose-rich infiltration. In one stage, the UF process is able to concentrate whey 10-30 times the bait.
The original alternative for whey membrane filtration is to use steam heating followed by drum drying or spray drying. The product of this method has limited application because of its insoluble and insoluble texture. The existing methods also have inconsistent product compositions, high capital and operating costs and because excessive heat used in drying will often alter some of the protein.
Compared to traditional methods, UF processes are used for this application:
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- More energy efficient
- Having consistent product quality, 35-80% of protein products depends on operating conditions
- Do not change the properties of proteins because they use moderate operating conditions
The potential for fouling is widely discussed, identified as a significant contributor to the decrease in productivity. Whey cheese contains a high concentration of calcium phosphate which has the potential to cause crust deposits on the membrane surface. Consequently substantial pretreatment should be performed to balance the pH and feed temperature to maintain the solubility of the calcium salt.
Other apps
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- Effluent filtration from the pulp factory
- Making cheese, see ultrafiltered milk
- Removes pathogens from milk
- Wastewater treatment and processing
- Enzyme recovery
- Fruit juice concentration and clarification
- Dialysis and other blood care
- Desalting and protein-solvent exchange (via diafiltration)
- Laboratory level manufacturing
- Radiocarbon dating from bone collagen
Maps Ultrafiltration
Principles
Prinsip operasi dasar dari ultrafiltrasi menggunakan suatu tekanan yang diinduksi pemisahan zat terlarut dari suatu pelarut melalui suatu membran semi permeabel. Hubungan antara tekanan yang diterapkan pada larutan untuk dipisahkan dan fluks melalui membran paling sering digambarkan oleh persamaan Darcy:
where J is the flux (flow rate per membrane region), TMP is the transmembrane pressure (pressure difference between the feed and the permeate flow) ,? is the solvent viscosity, R t is the total resistance (number of membrane resistance and fouling).
Membrane fouling
Polarization concentration
When filtration occurs the local concentration of the rejected material on the surface of the membrane increases and may become saturated. In UF, increased ion concentrations may develop osmotic pressure on the feed side of the membrane. This reduces the effective TMP of the system, thereby reducing the permeation rate. Increased concentrated layers on the membrane wall lower the permeate flux, due to increased resistance which reduces the driving force for the solvent to transport through the membrane surface. CP affects almost all membrane separation processes available. In RO, the solute held on the membrane layer produces a higher osmotic pressure than the mass flow concentration. So a higher pressure is needed to overcome this osmotic pressure. Polarization concentration plays a dominant role in ultrafiltration compared to microfiltration because of the small pore size membrane. It should be noted that the concentration polarization differs from the fouling because it has no lasting effect on the membrane itself and can be reversed by removing the TMP. However it has a significant effect on many types of fouling.
Fouling type
Particulate deposits
The following models illustrate the mechanism of particulate deposition on the membrane surface and in the pores:
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- Standard Blocking : macromolecules are uniformly deposited on the pore wall
- Full blocking : pores of closed membranes by macromolecules
- Cake formation : the accumulation of particles or macromolecules forms a fouling layer on the membrane surface, in UF it is also known as the gel layer
- Intermediate Blocking : when macromolecules enter the pores or into clogged pores, contribute to cake formation
Scaling
As a result of the polarization of concentrations on the membrane surface, increased ion concentrations may exceed the solubility and precipitation thresholds on the membrane surface. These inorganic salt deposits can block the pores that cause decreased flux, membrane degradation and loss of production. Scale formation is highly dependent on factors affecting solubility and polarization of concentrations including pH, temperature, flow rate and permeation rate.
Biofouling
Microorganisms will stick to the surface of the membrane that forms the gel layer - known as biofilm. The film increases resistance to flow, acting as an additional barrier to permeation. In spiral-wound modules, the blockages formed by biofilms can cause uneven flow distribution and thus increase the effect of concentration polarization.
Membrane settings
Depending on the shape and the material of the membrane, different modules can be used for ultrafiltration process. The commercially available designs in ultrafiltration modules vary according to the hydrodynamic and economic constraints required as well as the mechanical stability of the system under certain operating pressures. The main modules used in industry include:
Tubular Module
The design of tubular modules utilizes polymer membranes thrown on the inside components of plastic or porous paper with a diameter typically in the range 5-25 mm in length from 0.6 to 6.4 m. Some tubes are placed in a PVC shell or steel. The module feed is passed through the tube, accommodating the radial transfer from seeping to the shell side. This design allows for easy cleaning but its main weakness is low permeability, high volume buildup inside membrane and low packing density.
Hollow fiber
This design is conceptually similar to a tubular module with shell and tube settings. A single module can consist of 50 to thousands of hollow fibers and therefore stands alone unlike tubular designs. The diameters of each fiber range from 0.2 to 3 mm with the feed flowing in the tube and the product absorbing radially on the outside. The advantages of having a self-support membrane such as ease that can be cleaned because of its ability to re-flow. But the replacement cost is high, because one wrong fiber will require the entire bundle to be replaced. Taking into account the small diameter tubes, using this design also makes the system vulnerable to clogging.
Spiral-wound module
It consists of a combination of flat membrane sheets separated by a thin spacer material that serves as a porous plastic screen support. These sheets are rolled around a central hollow tube and fitted into a tubular steel tubular pressure vessel. The feed solution passes through membrane surfaces and a translucent spiral to the collection center tube. Spiral-wound modules are a compact and inexpensive alternative to ultrafiltration design, offering high volumetric throughput and can also be easily cleaned. However, it is limited by a thin channel in which the bait solution with suspended solids may cause partial blockage of the membrane pores.
Plates and frames
It uses a membrane placed on a flat plate separated by a material such as a net. The feed is passed through the system from which the seeps are separated and collected from the edge of the plate. The channel length can range from 10 - 60 cm and the channel height from 0.5 to 1 mm. This module provides low volume retention, relatively easy membrane replacement and the ability to feed a condensed solution due to low channel height, unique to this particular design.
Process characteristics
The process characteristics of the UF system depend largely on the type of membrane used and its application. The manufacturers' specifications of membranes tend to limit the process to the following typical specifications:
Process design considerations
When designing a new membrane separation facility or considering integration into an existing plant, there are many factors to consider. For most applications a heuristic approach can be applied to determine many of these characteristics to simplify the design process. Some areas of design include:
Pre-care
Pre-membrane feed treatment is essential to prevent damage to the membrane and minimize the fouling effect which greatly reduces separation efficiency. The type of pre-treatment often depends on the type of feed and its quality. For example in wastewater treatment, household wastes and other particulates are screened. Other types of pre-treatments that are common to many UF processes include pH balancing and coagulation. The exact sequence of each pre-treatment phase is critical in preventing damage at a later stage. Pre-treatment can even be used only by using dosing points.
Membrane specifications
Materials
Most UF membranes use polymeric materials (polysulfone, polypropylene, cellulose acetate, polylactic acid) but ceramic membranes are used for high temperature applications.
Pore size
The general rule for the choice of pore size in a UF system is to use a membrane with a pore size of one tenth of the size of the particle to be separated. This limits the number of smaller particles entering the pores and absorbs to the pore surface. Instead they block the entrance to the pores allowing a simple adjustment of cross-flow speed to drive them away.
Operation strategy
Flowtype
UF system can operate with cross flow or dead end. In the dead-end filtration the flow of the feed solution is perpendicular to the surface of the membrane. On the other hand, in the flow system, the flow flow is parallel to the surface of the membrane. Dead-end configurations are more suitable for batch processes with low suspended solids as solids accumulate on membrane surfaces requiring frequent backflushes and cleaning to maintain high flux. Cross-flow configuration is preferable in continuous operation because the solids continuously flush from the membrane surface resulting in a thinner cake layer and lower resistance to permeation.
Flow rate
Flow velocity is very important for hard water or suspension-containing fluid in preventing excessive fouling. Higher cross-flow speeds can be used to enhance the effect of sweeping across membrane surfaces thereby preventing macromolecule deposition and colloidal material and reducing the effects of concentration polarization. However, expensive pumps are required to achieve this condition.
Flow temperature
To avoid excessive damage to the membrane, it is recommended to operate a plant at the temperature specified by the membrane manufacturer. However in some cases temperature outside the recommended area is needed to minimize the effects of fouling. Economic analysis of the process is needed to find a compromise between the increased cost of membrane replacement and separation productivity.
Pressure
Pressure drops during multi-stage separation can result in a drastic reduction in flux performance in the final stages of the process. This can be improved by using a booster pump to increase TMP in the final stages. This will lead to greater capital and energy costs that will be offset by increased process productivity. With multi-stage operations, the retentate streams of each stage are recycled through the previous stage to improve their separation efficiency.
Multi-stage, multi-module
Several stages in the series can be applied to achieve higher permeation flow purity. Due to the modular nature of the membrane process, some modules can be arranged in parallel to handle larger volumes.
Post-care
Post-treatment of the product flow depends on the composition of the permeate and retentate and its final use or government regulation. In cases such as milk separation, both streams (milk and whey) can be collected and made into useful products. Additional drying of the retentate will result in whey powder. In the paper mill industry, retentat (non-biodegradable organic material) is burned to recover energy and seep (pure water) is discharged into waterways. It is important for the water to absorb into a balanced and cooled pH to avoid thermal waterway pollution and change its pH.
Clean
Membrane cleansing is done regularly to prevent accumulation of foulants and reverses the degradation effects of fouling on permeability and selectivity.
Regular backwashing is often done every 10 minutes for some process to remove the cake layer formed on the membrane surface. By suppressing the permeate flow and forcing it back through the membrane, the accumulated particles may dislodge, increasing the flux of the process. Backwashing is limited in its ability to eliminate more complex forms of impurities such as biofouling, scaling or adsorption into the pore wall.
These types of foulants require chemical cleansing to be removed. Types of chemicals commonly used for cleaning are:
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- Acid solution to control inorganic scale deposits
- Alkali solution to remove organic compounds
- Biocides or disinfection such as Chlorine or peroxide when bio-fouling is proven
When designing cleaning protocols, it is important to consider: Cleaning time - Adequate time should be allowed for chemicals to interact with foulants and seep into membrane pores. However, if the process is extended beyond its optimum duration, it can lead to membrane denaturation and deposition of foulants being removed. A complete cleaning cycle including a rinse between stages may take 2 hours to complete.
<<> Chemical aggressiveness - With high levels of fouling it may be necessary to use aggressive cleaning solutions to remove impurities. However in some applications this may not be suitable if the membrane material is sensitive, causing improved membrane aging.
Disposal of cleaning fluid - The release of some chemicals into the wastewater system may be prohibited or regulated therefore this should be considered. For example the use of phosphoric acid can cause high levels of phosphate into the water and must be monitored and controlled to prevent eutrophication.
Summary of common types of fouling and chemical treatments respectively
New developments
To improve the life cycle of membrane filtration systems, energy-efficient membranes are being developed in membrane bioreactor systems. Technology has been introduced that allows the power required to aerate the membranes for clearance to be reduced while retaining high flux levels. The mechanical cleaning process has also been adopted using granulates as an alternative to conventional cleaning forms; this reduces energy consumption and also reduces the area required for filtering tanks
Membrane properties have also been enhanced to reduce fouling tendencies by modifying surface properties. This can be noted in the biotechnology industry in which the membrane surface has been altered to reduce the amount of binding protein. The Ultrafiltration module has also been upgraded to allow for more membranes for specific areas without increasing the risk of fouling by designing more efficient internal modules.
The initial processing of seawater desulphonation uses an ultrafiltration module that has been designed to withstand high temperature and pressure while occupying a smaller footprint. Each module vessel is self-supported and corrosion resistant and accommodates easy removal and replacement of the module without the vessel replacement cost itself.
References
External links
Media related to Ultrafiltration on Wikimedia Commons
Source of the article : Wikipedia
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