Sample Essay On Types Of Membrane Separation Processes And Industrial Application

Industrial Applications of Membrane Separation Processes

Abstract
Membrane separation processes are widely used in various industrial operations to separate the desired components from the mixture. The membrane that is used depends on the type of process employed in the system. The membrane also varies in size and materials. This paper focused on the industrial application of ultrafiltration, microfiltration, and pervaporation. Ultrafiltration is effectively used in recovery of milk proteins in whey and reduction of waste. The process of microfiltration is successfully use in solids removal and disinfection of wastewaster. Pervaporation shows good result in the removal of volatile organic compounds in contaminated water.

Introduction

Membrane separation processes are used to perform physical separation of the desired component from its source. Most of the membrane separation processes are water based but there are also separation processes wherein gas-liquid and gas-gas systems are involved (Judd & Jefferson, 2003). These processes make use of semi-permeable membranes and are pressure-driven processes. The membranes come in different sizes of pores depending on its application. Usually, the membranes with larger pores are used for microfiltration. Such type of membrane employs lower operating pressure. Meanwhile, membranes with smallest pores are usually aimed for reverse osmosis and nanofiltration use. This type of membrane requires the highest operating pressures (Tamime, 2012). The materials use for the membrane can be dense or porous. The physicochemical interactions between the material of the membrane and the component being separated are important to achieved effective separation. On the other hand, components are separated in porous membrane by size exclusion. In this process, the undesired material is suspended or dissolved relative to the size of the pores. Membranes can also be organic, like those made of polymers, or inorganic which can be metallic or ceramic (Judd & Jefferson, 2003).
Membrane separation is different with filtration as the size of the particulates that are separated varies. In filtration, the filters separate the particles with size ranging from 1 to 10 micrometer. In contrast to filtration, membrane separation makes use of semi-permeable membrane to separate smaller particles (Tamime, 2012).
The processes involving separation using membranes have a wide range of applications. These processes are essential in various industries such as food and beverages, petroleum, pharmaceutical, and other applications that are used in protecting the environment. There are also different types of membrane separation processes that cater to the need of a specific industry.

There are different types of membrane separation processes. Electrodialysis, reverse osmosis, ultrafiltration, and microfiltration are some of the early developed membrane separation processes. These processes are well developed and are widely used in different industries (Baker, 2012). Also, these processes differ in sizes and working pressures that are used upon its operation. The working pressure range for some of the membrane separation processes are as follows: 0.07 to 0.69 MPa for microfiltration, 0.21 to 1.03 MPa for ultrafiltration, 1.38 to 8.28 MPa for reverse osmosis, and 1.03 to 2.76 MPa for nanofiltration (Tamime, 2012).
One of the most common membrane separation techniques is microfiltration. This process is used to separate colloidal particles and bacteria having sizes ranging from 0.1 to 10 μm in diameter while ultrafiltration is used for the separation of macromolecules from solutions (Baker, 2012). Additionally, gas separation and pervaporation are two of the developing membrane separation processes. The process of pervaporation uses selective membrane to separate the desired material from the mixture. In pervaporation mechanism, permeate is removed as a vapor from one side of the membrane while the liquid mixture is in contact with the other side of the membrane. Pervaporation can be used to separate components of the mixture which are difficult to separate using other separation processes, such as distillation (Baker, 2012). In this paper, the industrial application of microfiltration, ultrafiltration, and pervaporation will be discussed as well as a brief background of the mechanism involved in these processes.

Ultrafiltration and its importance in the Dairy Processing Industry

Dairy processing is one of the first to employ the process of ultrafiltration in a large-scale commercial operation. In 1971, a whey processing facility in New Zealand use ultrafiltration to concentrate the proteins content of the whey. Upon the application of ultrafiltration process to whey processing, this method was further developed for dairy products processing (Zeman, 1996). Large membranes were used to facilitate the large-scale processes. In 1980, over 20,000 m2 membrane area was used for the ultrafiltration in dairy plants (Fondation de technologielaitière du Québec, 1985). Nowadays, ultrafiltration is mainly use in production of skim milk and different types of cheese (Zeman, 1996).
In the cheese making process, milk proteins are precipitated to initiate the coagulation or curdling of milk. The milk solids called curds are process in the cheese fermentation plant. The supernatant liquid that is separated from the curd is called whey. The disposal of whey is one of the major concerns in cheese manufacturing since the whey does not have any other industrial use. Also, the whey still contains 25 percent of the milk protein. In order to maximize the extraction of milk protein, ultrafiltration is used to recover the milk proteins in the whey. By using the method of ultrafiltration, the maximization of protein recovery became feasible and the waste generated was reduced (Baker, 2012). Figure 1 shows the ultrafiltration process in recovering milk proteins from the whey.
Figure 1.Ultrafiltration process in recovering milk proteins from the whey (Baker 267).
Since carbohydrates, water, organic acids, and inorganic salts have lower size relative to the pore size of the membrane; these molecules can pass through the membrane. The components with high molecular weight such as peptides cannot pass through the membrane and can be found on the permeate. Ultrafiltration membranes have filtration threshold ranging from 10,000 to 50,000 daltons. When milk is filtered through the membrane fat globules are retained (Fondation de technologielaitière du Québec, 1985). This mechanism is illustrated in Figure 2.
Figure 2. Diagram of the mechanism in ultrafiltration process.
The ultrafiltered milk that is concentrated up to two times is used in production of different types of cheese. Concentrated milk that contains 3.7 to 4.7 g per 100 of protein per 100 grams of ultrafiltered milk is used in cheese making. With this method, the yield of cheese was increased due to higher fat and protein recovery with only little whey remaining (Tamime, 2012). During the fabrication of the membrane, its selectivity and permeability can be controlled. This characteristic makes the separation effective since only the desired substance is filtered. Another advantage of ultrafiltration in dairy process is that the amount of permeate that is removed can be varied and obtaining up to 60 percent of the protein. Higher amounts of protein can be recovered by adding fresh water and further concentrating the milk by ultrafiltration (Chandan et al., 2012).
Ultrafiltration is not only used to recover milk proteins but also to minimize the cost of evaporator drying and for lactose removal. However, it is difficult to remove lactose since the flux is needed to be reduced at high volume to achieve higher removal efficiency. To solve this problem, another ultrafiltration step is used to reduce the volume up to 5 to 10 times and to achieve higher lactose removal (Baker, 2012). The purification process is improved when ultrafiltration is used with diafiltration. With this method, the purity of the product can be maximized and result to high yield since lactose is removed from the milk at the preferred degree (Pabby et al., 2008).
The membrane used for the ultrafiltration of the whey must be in complement with the working pressure of the operation. Ceramic membranes are commonly used for dairy processing because this type of membrane can withstand the high temperature during operation. Additionally, it is chemically stable. However, these membranes are more expensive than other membranes (Zeman, 1996).Other characteristic of the membrane that should be considered is its porosity. Temperature of the product is an important factor as well and it must be considered upon choosing the most suitable membrane. As the temperature of the product increases, the viscosity, as well as the expansion of the pores, increases. Temperature must be monitored and controlled because this is one of the important parameters in the process. The working pressure is also important since it is the driving force in the separation process (Chandan et al, 2012).Spiral-wound ultrafiltration modules in multistage feed-and-bleed systems are widely used in whey processing plants. The design of the modules helps to remove stagnant areas in the housing of the module and sterilized the plant with high and low pH solutions. However, this cleaning method can make the membrane easily worn-out (Baker, 2012).
Presently, the use of ultrafiltration in the fractionation and concentration of proteins in whey is regarded as one of the successful application of this membrane separation method in industrial-scale processes (Pabby et al., 2008).Still, there are challenges on this process that are yet to be overcome such as membrane fouling, cleaning, and efficiency parameters including purity, specificity, and yield. Further studies and development of membrane have been made to address these problems (Tamime, 2012).

Microfiltration and its application to wastewater treatment

Membrane separation processes are used in wastewater treatment to replace gravitational sedimentation biological processes and removal of residual solids in the secondary treatment of wastewater (Ahn & Son, 1999). In order to increase the efficiency of eliminating waste, it is immediately treated after being generated rather than controlling the source of waste. In this case, membrane separation processes is beneficial in wastewater treatment because these processes helps to minimize waste and can help in recycling water. Many industries are using membrane separation processes for wastewater treatment (Caetano et al., 2012). The use of microfiltration is one of the established methods in treatment of wastewater. Microfiltration allows the separation of larger materials from the solution. This process uses membranes with pore size ranging from 0.1 to 10 micrometers. It allows disinfections which even allow the smallest bacteria such as Pseudomonas diminuta, with a diameter that is smaller than 0.3 micrometers, to be filtered. Uniform pore size, the thinness of the active layer, and the density of the pores are the important factors to ensure efficient filtration (Noyes, 1994).Microfiltration is widely used in wastewater treatment because the membranes in this process have high retention for microorganisms. It is also effectively used in treatment of wastewater generated in electroplating industries, metal finishing, and circuit board manufacturing. During the process the heavy metals are removed by filtering in the membranes (Caetano et al., 2012).
The membranes use in microfiltration physically separates the solid particles in the wastewater by preventing them to pass through the membrane. This occurs since membranes have smaller pore size than the solid particles. Since the membrane also blocks the pathogens, this process also allows the disinfection of the wastewater (Kolarik & Priestly, 1996). Microfiltration membranes can be capillary-pore which is characterized as straight cylindrical capillaries and are made of polycarbonate or polyester. The membrane can also be tortuous-pore that looks like sponge with interconnecting tortuous pores which are cellulosic or polymeric. Membranes made of polypropylene are most commonly used in microfiltration (Noyes, 1994). Figure 3 shows the mechanism of the separation process using microfiltration. The cartride contains two or more membranes. The pump forces the movement of the liquid through the filter. The prefilter separates large particles while the final filter separates smaller particles. The use of prefilter prevents the blinding of the membranes. Thus, it extends the life of the microfiltration membranes (Baker, 2012).
Figure 3. Microfiltration process using cartridge filter in series.
In the Marulan project in Australia, microfiltration is the process employed in wastewater treatment. The membrane modules that were used were submerged in open tank from a series of removable manifolds. There were about seven manifold racks which contain a total of 56 membrane modules (Hillis, 2000). The operation requires minimum supervision. Alarms were also installed on the control system to ensure the maximum removal of desired components (Hillis, 2000).Results of the application of this process in wastewater treatment show that the cost is efficient at larger capacities greater than 100 ML/ day. Operating cost is also lower due to the simple control and less power requirement. The simplicity of the process also makes the capital cost in installation of this system lower (Hillis, 2000).

Pervaporationand the Water Purification process

Pervaporation is one of the recent types of membrane separation processes. In pervaporation, the phase of the permeate changes from liquid to gas. The separation happens when the liquid feed mixture that is in contact with one side of the membrane is maintained at atmospheric pressure while low pressure on the permeate side is obtained. The low pressure on the permeate side can be obtained by using a vacuum pump or carrier gas (Caetano et al., 2012). This process is illustrated in Figure 4.
Figure 4.Pervaporation process with downstream vacuum or an inert carrier-gas (Caetano et al., 2012).
The first to study pervaporation scientifically were Heisler in 1956 and Binning and James in 1958. Different lab-scale researches were made in 1960. Moreover, a pilot-plant scale of the pervaporation process was developed. One of the industrial applications of pervaporation is the removal of organic solvents from contaminated water (Caetano et al., 2012).
Groundwater can be contaminated with volatile organic compounds. This situation gives problem to industrial sites as well as government sites. The contamination of soil brought by volatile organic compounds also leads to the contamination of groundwater. Petroleum hydrocarbons, chlorinated hydrocarbons, carbon tetrachloride, and methyl t-butyl ether are some of the volatile organic compounds that can contaminate the water stream. These volatile organic compounds must be removed because it can cause health hazards such as cancer and to avoid further contamination that can affect the environment. Pervaporation shows high efficiency in removing volatile organic compounds in water. Other methods such as carbon adsorption, air stripping, incineration, steam stripping, and biological treatment can also be used. However, these processes generate a secondary waste which is not the desired end result of the water treatment process. In using pervaporation for water purification, the rate of permeation of the components through the membrane is determined first. The relative volatility of the components is defined as well. Generally, volatile organic compounds pass through the membrane faster than water (Kujawski, 2000).
The characteristics of the membranes use in pervaporation are important to ensure the efficiency of the separation process. The type of membrane to be use depends on the type of application. Additionally, the nature of the component that is to be separated must also be considered. The component having the smallest fraction in the mixture is generally separated from the mixture through the membrane. For water purification, membranes that are made of hydrophobic polymers are used to remove the organic liquid from water since these materials do not possess affinity towards water. Membranes of this type can be made of polyvinylidenefluoride (PVFD), polydimethylsiloxane (PDMS), polyethylene (PE), polytetrafluoroethylene (PTFE), and polypropylene (PP) (Kujawski, 2000). The membranes can have tubular or flat configuration. They must also be incorporated in modules so that in can be used in the process. The module can be a plate-and-frame system. In this kind of module, plates are made of stainless steel and are used to form the feed channels and compartments. These plates also support the membranes. The flow of the feed mixture in the membrane is uniform. Hollow fiber modules, on the other hand, are made of small fibers. However, its application is limited due to the polarization in the fibers (Kujawski, 2000).
Pervaporation has developed from lab-scale application to industrial-scale. Despite being a recent membrane separation process relative to others, pervaporation has shown high efficiency in different applications. Most importantly, this process is important in environmental protection. This process also shows various advantages compared to other membrane separation processes since it is easy and simple to control. This process also gives high purity of the product and does not cause environmental pollution. The compact design makes the equipment to be less space-consuming and allows short retention time (Kujawski, 2000).

Conclusion

Several separation processes are available which are mainly used for separation of components such as solid, liquid, and gas. One of the categories in separation processes involved the use of membrane. The membrane separation processes, which makes use of semi-permeable membranes and pressure-driven processes, are employed in the physical separation of the desired component from its source. This process is different from filtration since the target particulate size of this process is different. Different membrane separation techniques can be used in different industries such as water treatment and processes involving production of materials.
The processes that were discussed in this paper are ultrafiltration, microfiltration, and pervaporation. Ultrafiltration is widely used in different industry but is majorly used in dairy processing. This process is important to reduce the waste that is generated during the process and to achieve maximum recovery of the desired product. Porosity of the membrane, temperature of the product, and working pressure are some of the factors that must be considered. The second process discussed is microfiltration which is commonly used in wastewater treatment. This process is used to remove solid particles in the wastewater. It also helps in disinfection of the water as it removes pathogens. Lastly, pervaporation is effectively used in reducing volatile contaminants in the water. This process shows various advantages compared to other membrane separation processes such as simple control, high purity of the product, and no environmental pollution, less space-consuming, and allows short erection time. With a wide range of application, membrane separation is widely used in different industries.

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