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The first membrane developments were achieved using readily available membranes in nature, such as bladders of pigs or sausage casings made from animal gut Baker, 2004. But later research led to the usage of nitrocellulose to manufacture membranes which were preferred as they could be manufactured in series (Baker, 2004). In the beginning of the XX century, Bechhold, Elford and Bachmann developed a method to manufacture nitrocellulose membranes of specific pore sizes, and by the 1930s microporous nitrocellulose membranes were commercially available (Baker, 2004).
A key discovery that converted membrane separations from a laboratory technique to an industrial application was the development of the Loeb-Sourirajan process to manufacture defect free, high flux, reverse osmosis membranes (Baker, 2004). These membranes consisted of a selective film over a more thick, permeable and porous support that provided high mechanical resistance (Cheryan, 1998). The flux through this membrane resulted larger than any other available in the market at that time and made possible the application of reverse osmosis as a practical method. The work of Loeb and Sourirajan, and high investments of the US government were an important factor in the further development of ultrafiltration, microfiltration and electrodialysis resulting in membranes with selective layers as thin as 0.1 Î¼m (Baker, 2004).
In the subsequent years, packing methods for membrane applications were developed, such as spiral wound, hollow fiber and plate and frame configurations which enabled a broader industrial utilization. By 1980, ultrafiltration, reverse osmosis, microfiltration and electrodyalisis were established processes with broad application in the industry. The principal development during that decade was gas separation membrane technologies. Companies such as Monsanto and Dow introduced the first membranes for hydrogen separation, nitrogen from air separation and carbon dioxide from natural gas (Baker, 2004). Gas separation membrane technologies have been in constant development and are spreading at a high rate.
Types of Membranes
There are different types of synthetic membranes that differ in their chemical and physical composition and in their operation mechanisms. Basically, a membrane is a discrete interface that moderates the penetration of different chemical substances in contact with it. A membrane can be either physically or chemically heterogeneous or it can be uniform in its composition (Baker, 2004). The basic types of membranes are described below and shown in figure 1.
Nonporous, Dense Membranes
Although membranes classified as nonporous or dense might have pores in their structure in the range of 5 to 10 angstroms, the model in which permeation occurs differs from other types of membranes and is better explained by solution-diffusion phenomena (Brüschke, 1995), and the driving force for the separation using these type of membranes can be an applied pressure, a difference in concentration or an electrical potential gradient. Since these types of membranes do not rely on the size of the pores to achieve the separation process, components of similar molecular size can be separated if their solubility in the membrane is different. Dense membranes are widely used in gas separation and reverse osmosis (Baker, 2004).
A microporous membrane has a solid matrix and a random distribution of connected pores.
Figure 1. Membrane types (Baker, 2004)
Separation of components in this case is achieved by a sieving mechanism in which particles larger than the pores are rejected by the membrane, while particles smaller can be partially rejected according to the pore size distribution in the membrane. These types of membranes are used mainly for microfiltration and ultrafiltration (Baker, 2004). Microporous membranes can be either symmetric or asymmetric (anisotropic, as shown in figure 1), where the latter are composed of a thin layer which acts as the selective part of the membrane and a thick support or substructure which provides physical strength and stability.
Electrically Charged Membranes
The ion-exchange membranes used in electrodialysis and diffusion dialysis are essentially sheets of ion-exchange resins. Cation-exchange membranes have negatively charged groups chemically attached to the polymer chains, ions with an opposite charge can permeate through these sites and since their concentration is high they are able to carry the electric current through the membrane. Ions of the same charge are repelled. Attachment of positive fixed charges to the polymer chains forms anion-exchange membranes, which are selectively permeable to negative ions. Electrically charged membranes may be either nonporous or porous and the separation is affected by the ionic strength in the solution (Porter, 1990).
Ceramic, Metal and Liquid Membranes
The interest in membranes made from unconventional materials which can be stronger and withstand severe conditions such as very high or low pH values, broader operation temperatures or strong solvent management have been continuously growing as technological advances allow their fabrication, and microporous ceramic and metallic membranes are being used in ultrafiltration and microfiltration applications where these kinds of conditions are present. Dense metal membranes are also being considered in gas separation processes (Baker, 2004).
The more developed industrial membrane separation processes are microfiltration, ultrafiltration, reverse osmosis, electrodialysis - diffusion dialysis and gas separation. These processes are well established and the market is served by experienced companies, like Millipore and General Electric (Baker, 2004). Different application ranges for the pressure driven separation processes; microfiltration, ultrafiltration and reverse osmosis are shown in figure 2.
Figure 2. Pressure driven membrane separation spectrum. (Suppliers of Liquid Filtration Products, 2011)
Ion Exchange Membrane Processes
The basic principles of electrodialysis and diffusion dialysis processes are very similar to those of ion exchange, in which positive and negative ions diluted in a solution are driven through ion exchange membranes with opposite charged constituents, while ions with the same charge are mostly rejected. The driving force for these separation processes are chemical potential in the case of diffusion dialysis, or an applied electrical potential in the case of electrodialysis. Membranes are usually placed in a stack and alternating between cation or anion, selective in a way that the feed solution is ion depleted throughout the process. A schematic of diffusion dialysis is shown in figure 3.
Figure 3. Diffusion dialysis. (Functional Membranes and Plant Technology, 2012)
Because both positive and negative ions move in opposite directions under the effect of an electrical potential, in the case of electrodialysis the process is often analyzed by the number of electric charges transported through the membrane, and not by the material permeated (Baker, 2004).
This process is used to remove particles in the size range of 0.1 to 10 micrometers from liquids (figure 2, Cheryan, 1998). There are two main types of microfiltration techniques: dead-end and cross flow microfiltration (Figure 4). Dead-end is a common type of microfiltration encountered in the industry, where it finds application in sterile filtration and clarification (Cheryan, 1998). It employs depth or surface membranes. In this type of filtration, retained particles build up in the membrane void spaces by a sieving action on the fibrous materials from which they are fabricated.
DEAD END FILTRATION CROSS FLOW FILTRATIONhttp://www.ridgelea.com.au/crsflw1.jpg
Figure 4. Dead end and Cross flow Microfiltration. (Ridgelea, 2012)
In surface microfiltration, the particles are retained on the upstream surface of the filter by a sieving mechanism (Cheryan, 1998). Build-up of particles during dead-end filtration requires the replacement or cleaning of the filter medium when the flow decreases. For this reason, dead-end filtration is a batch process. The cross flow configuration on the other hand, has the advantage that particles do not build up in the same intensity on the membrane's surface because the feed flows tangentially to the surface of the membrane and they are sloughed off by the high shear imposed by the tangential flow of bulk suspension. For this reason higher flux rates can be maintained for longer periods of time. Nevertheless, fouling of the membrane will occur over time and the flux rate will decline (Baker, 2004).
Appropriate membrane selection is an important factor in microfiltration, as well as all other membranes separation processes, as adsorption can play a fundamental part in fouling. For example, hydrophobic membranes (e.g., PTFE) generally show a greater tendency to be fouled, especially by proteins (Cheryan, 1998).
Ultrafiltration is a membrane separation process in cross-flow operation. In a solution containing low molecular weight and high molecular weight solutes, the latter will be retained by the membrane, while the smaller low molecular weight particles will permeate through. The driving force in order to achieve the separation is a pressure difference applied to a solution on the feed side of a membrane. Ultrafiltration membrane pore sizes are usually classified according to the molecular weight of the species that will be retained by assigning to them a molecular weight cut off (MWCO). A schematic of this process is shown in figure 5. The solvent and low molecular weight species passes through the membrane while solutes with a larger weight than the MWCO are retained.
Figure 5. Ultrafiltration principle of operation. (Functional Membranes and Plant Technology, 2012)
Since micro molecular components have significantly lower molecular weights, it is possible to separate them from other macromolecular compounds in aqueous solution by using ultrafiltration. Membrane pore diameters in this case are typically between 0.1 and 0.005 micrometers and are able to retain proteins, polymers, and chelates of heavy metals (Figure 2) (Cheryan, 1998). Since low-molecular-weight solutes flow through the membrane, osmotic pressure is not an issue. However, since retained large molecules and colloidal particles have low diffusivities in the liquid medium, ultrafiltration membranes are more susceptible to fouling and concentration polarization than reverse osmosis or microfiltration membranes (Cheryan, 1998).
Usually, not all the particles larger than the molecular weight cut off of the membrane are rejected, and some particles smaller than this parameter may be partially rejected also (Paterson, 1993). In order to estimate the separation degree attained by the process, a mathematical model has been developed for the rejection of the solutes (Cheryan, 1998):
Where R is the rejection coefficient
CP is the concentration in the permeate
CR is the concentration in the retentate
During this process, the total volume of a solution will be reduced as the solvent and low molecular weight components are being removed resulting in the concentration of the macromolecular species, since their quantity remain unchanged. The concentration and volume relationship in ultrafiltration systems are characterized by the following equation (Cheryan, 1998):
Where Cf is the final concentration of the feed
C0 is the initial concentration of the feed
V0 is the initial feed volume
Vf is the final feed volume
CF is the concentration factor
R is the rejection coefficient
These mathematical models can also be applied in the same way to the microfiltration process (Cheryan, 1998).
Ultrafiltration membranes can be either polymeric of ceramic. Polymeric membranes are asymmetric and are available in different configurations, such as tubular, plate and frame, hollow fiber or spiral wound (Cheryan, 1998). Some ultrafiltration membranes are illustrated in figure 6.
Figure 6. Ultrafiltration membranes
Reverse osmosis can be defined as the movement of solvent molecules through a semipermeable membrane into a region of higher solvent concentration, or lower solute concentration. The driving force for osmosis is the difference in the chemical potential of the solutions at both sides of the membrane, where molecules will tend to move from a higher chemical potential zone (pure solvent) to a lower chemical potential one (solution). This difference will generate an osmotic pressure that depends on the concentration of the solute, its molecular weight, the number of ions for ionized solutes and the temperature of the system (Cheryan, 1998). As other membrane separation processes, in reverse osmosis the solvent moves from a high solute concentration zone to a low concentration one, overcoming the osmotic pressure of the solution by means of an applied external pressure (Figure 7). The basic relationship between the applied pressure by a pump, the osmotic pressure, and the flow of solvent through a membrane is expressed in terms of the rate of solvent transport per unit area per unit time, also called flux, and also the driving force and resistances, described by the following equation (Baker, 2004):
Where J is the flux through the membrane
A is the water transport coefficient
ï„p is the pressure differential across the membrane
ï„ï° is the osmotic pressure differential across the membrane
Osmotic pressure increases as concentration increases and the molecular weight of the solute decreases. Because the typical particle sizes involved in microfiltration and ultrafiltration processes, the osmotic pressure due to their presence is usually low enough to be negligible. In reverse osmosis, on the other hand, osmotic pressure effects are likely to be the dominant resistance (Cheryan, 1998).
Reverse osmosis membranes are non-porous and asymmetric, as described in the introduction section of this paper and consist of a thin skin, which is supported by a porous substructure. The membranes can be made of a single polymer such as cellulose acetate, non-cellulosic polymer or of thin-film composites (Baker, 2004). Due to the small pore size, reverse osmosis membranes are susceptible to plugging and it is necessary to pretreat the feed. In addition, there are limitations on the allowable pH and temperature of feed due to physical instability of the membrane materials in harsh environments (Baker, 2004).
Figure 7. Reverse osmosis principle of operation and reverse osmosis in cross flow configuration (Aquatruewater, 2008)
Gas Membrane Separation
Membranes can be used for gas and vapor separation in a variety of applications, including VOC removal and/or recovery. The driving force for the separation of a gas mixture by a membrane process is a concentration difference between the two sides of the membrane, where the permeable species will move from the high pressure side to the low pressure side. Membranes for gas separation can be either polymeric, including materials such as polyethersulfone, polyamides and other cellulosic derivatives, or ceramic and even metallic. Membranes used for gas separation can be of two different kinds; porous and nonporous (Figure 8).
In the case of porous membranes, depending on the size of the pores, the mathematical models that govern the separation and hence the separation itself will be affected, and a molecular sieving separation can be achieved with pore diameters in the order of 5 to 20 angstroms (Baker, 2004).
With non-porous membranes gases are separated due to their different diffusivity and solubility values in the membrane (Porter, 1990). Gases dissolve into the material, diffuse through, and desorb on the other side. Both the molecular size and the chemical nature of the gas will influence the separation process. As polymer science has developed during the past years, many have been tested and some have very good selectivity (Porter, 1990).
Figure 8. Gas separation membranes. (CO2CRC, 2011)
The most important elements that will determine the economic feasibility of a gas membrane separation process are the permeability, selectivity and membrane life (Baker, 2004)
Membrane Technology Limitations
The main limitations for membrane separation processes are the concentration polarization and membrane fouling. Concentration polarization controls the performance of electrodialysis, diffusion dialysis, microfiltration, ultrafiltration, and to a lower extent reverse osmosis and gas separation processes, because of the high diffusion coefficient of gases (Baker, 2004). It is an effect where particles rejected by the membrane tend to form a layer near the surface causing further resistance to the flow of the permeate. The flux decrease is usually explained by two mechanisms: The first one is an increase in the osmotic pressure due to the increased solute concentration near the surface of the membrane in comparison to the bulk concentration in the feed, and the second one is the hydrodynamic resistance of the boundary layer (Cheryan, 1998). To reduce the effect of concentration polarization several factors such as pressure, feed concentration, temperature and turbulence in the feed channel must be optimized.
Membrane fouling on the other hand is characterized by an irreversible decline in the flux that cannot be counteracted with fluid management techniques. It is due to the accumulation of feed components on the membrane surface or within the pores of the membrane and is influenced by the chemical natures of both the membrane and the solutes and membrane-solute and solute-solute interactions (Cheryan, 1998). Usually the only way of restoring the flux of a fouled membrane is through cleaning. Fouled membranes and auxiliary equipment are generally cleaned by clean-in-place procedures (Lindau and Jönson, 1993) which are usually based on various chemical or enzymatic treatments to restore the membrane to its original state. Many appropriate cleaning agents are available. Acids, such as nitric acid or ethylenediaminetetra-acetic acid (EDTA), are used to remove salt deposits (Cheryan, 1998). Caustic-based detergents are used to remove proteinaceous deposits. Enzyme cleaning agents containing hydrolytic enzymes, such as amylases, proteases, or glucogenases, are sometimes used for specific applications, and are used at the optimal pH for the respective enzyme. Rinsing with water at high circulation rates and reduced pressure, or back-flushing from the permeate side of the membrane are also used to clean membranes (Baker, 2004).
Ion Exchange Membrane Applications
Electrodialysis is the most used ion-exchange membrane separation process today and its most common application is brackish water desalination to obtain potable water and sea salt (Baker, 2004). Other uses for electrodialysis are found in the food industry for whey desalination, fruit juice demineralization, control of the cation balance in milk and the replacement of strontium by calcium to reduce the radioactive elements in milk or related products (Cheryan, 1998). In the pulp and paper industry, for the treatment of bleaching waste water solutions, in the glass manufacturing industry for the processing of a waste stream of ammonium fluoride solution, and similarly in effluents containing hydrogen fluoride solutions in the quartz tube manufacturing process (Leitz, 1976).
The degree of water recovery in each case is limited by precipitation of insoluble salts in the feed. There are additional applications for microfiltration in wastewater treatment including regeneration of waste acid streams used in metal pickling processes and the removal of heavy metals from other waste waters (Gering and Scamehorn, 1988), where electrodialysis membranes separate electrolytes and can also separate multivalent ions. The arrangement of membranes in these systems depends on the application.
Regarding electrodialysis application in the production of table salt by concentration of seawater, several processes have been developed along with electrodialysis such as reverse osmosis - electrodialysis (Tanaka, Ehara, Itoi and Goto, 2003) and reverse electrodialysis (Turek, 2002). This process is mainly practiced in Japan, which rely on the sea as the only salt supplier (Baker, 2004).
Additional applications for electrodialysis can be found in the preparation of ultrapure water for the electronics industry (Yang, 2004) where salt concentrations must be reduced to the ppb range. A problem with electrodialysis in this case is that the feed streams are diluted and separation becomes inefficient, in these cases the addition of ion exchange beads in the stacks can further aid the separation to the objective values.
The use of microfiltration technology has many practical applications. Most of them are based on the properties of semi permeable microfiltration membranes that allow separation and/or concentration of ultrafine particles, large molecules (0.1 to 10 micrometers) and microorganisms (Cheryan, 1998). The process is widely used in dairy and beverage industry as well as pharmaceutical industry to produce sterile water (Porter, 1990).
Considering environmental pollution prevention, microfiltration helps to reduce the amounts of wastewater and concentrate pollutants generated by industries like: landfill leachate treatment, metal finishing industry and laundry industry (Cheryan, 1998). Wastewater treatment is one of the major applications of microfiltration technology. Landfill leachate - is a by-product generated by precipitation and degradation at solid waste disposal facilities. Managing leachate is considered one of the most important problems with designing and maintaining a landfill. Many different organic and inorganic compounds dissolved or suspended in leachate pose a potential pollution problem for local ground and surface water. Current leachate treatment options include on-site treatment, recycling and re-injection, biological treatment, discharge to a municipal water treatment facility or a combination of these processes. Typical systems used for treatment of leachate are: activated sludge, fixed film and constructed wetlands. Modern on-site treatment of relatively dilute landfill leachate includes the use of microfiltration process to concentrate leachate after chemical precipitation of toxic metals. The use of cross flow filtration allows high level of solids (2-4%) to be processed (Zenon Environmental, 1994). Microfiltration is usually followed by reverse osmosis of the permeate which concentrates remaining inorganic and organic contaminants. The cost of application of membrane filtretion technology to treat landfill leachate varies depending on the composition of the leachate. In the end a treatment process which incorporates precipitation, microfiltration and reverse osmosis estimates to be more cost-effective, compared to biological and other treatments, that allows to meet new standards of released wastewater (Zenon Environmental, 1994). In the metal finishing industry microfiltration found its application in electroplating rinse bathe maintenance. This is a relatively new area of application of microfiltration. The main reason the technology was not used before is the lack of membranes that could tolerate hostile conditions of electroplating process (Cushnie, 2009). Polymeric membranes deteriorate at high temperatures and corrosive nature of washing solutions. Ceramic membranes, on the other hand being chemically inert, are capable of working under these conditions (Baker, 2004).
Prior to the application of microfiltration technology, the contents of an aqueous degreasing bath supposed to be discarded after 80 hours of constant use (Cushine, 2009). The process allowed removal of fine oil emulsion and colloidal particles from degreasing baths, thus making the contents reusable for longer (Porter, 1990). Microfiltration application in metal finishing industry also has some limitations. Some of the cleaning formulations used in the process contain colloidal silicic acid, which has a tendency to plug the pores of the ceramic membrane. Also aluminum cleaning solutions cannot be used together with microfiltration, as dissolved aluminum concentration will build up because it is unaffected by filtration process. Examples of microfiltration process use in electroplating industry estimate around 2.1 years of return on investment with initial investment of around 27 000$ and operating cost of 6250$ (Cushine, 2009). Laundry industry is a major generator of wastewater. Wastewater from laundry sources accounts for 10% of municipal sewer release (Porter,1990). Laundry wastewater contains large amount of suspended solids, a high BOD load, oil, grease, heavy metals, and other organic compounds which in sum largely exceed municipal discharge standards. A common method for such wastewater treatment consists of lime coagulation and flocculation followed by clarification by dissolved air (Porter,1990). Application of cross-flow microfiltration allows the recycle of permeate back to the plant, thus reducing the amounts of discharged water. Furthermore, the process allows reusing of up to 90% of the wastewater with good washing results by use of a modular washing system (Hoinkis, Panten, 2008).
As with microfiltration process - applications of ultrafiltration are based on ability of membranes to separate the retained material because of small pores on their surface.
The largest area of application of the ultrafiltration technology is in electrocoat painting. Ultrafiltration helps to recover more than 90% of the paint drag-out, and substantially reduces the load on wastewater treatment (Nath, 2008). It is widely used in the automotive and appliance fields (Porter, 1990). In electrocoating process the paint is applied to metal parts in a tank containing 15-20% of the paint emulsion (Baker, 2004). After coating, the part is removed and rinsed to remove the excess of paint. Ultrafiltration system removes ion impurities from the paint tank carried over from earlier steps of the process and recovers clean rinse water for countercurrent rinse operation. The retentate containing paint emulsion is returned back to the tank (Baker, 2004). The savings in recovered paint alone cover the cost of process operation. The estimated payback period of ultrafiltration system installation is less than one year not to mention the savings in sewage treatment and deionized water cost (Cheryan, 1998).
Another application of ultrafiltration technology is the use of membrane bio-reactors. The use of membrane bio-reactors (MBR) in wastewater treatment becomes more common, due to lower space requirements, lower operation involvement, modular expansion capabilities and consistent quality of output water. The technology allows to treat high strength waste with poor biodegradability and old sludges. MBR technology combines common activated sludge treatment with low-pressure membrane filtration (AMTA, 2007). The ultrafiltration process creates a barrier to contain microorganisms and makes possible to treat raw sewage and wastewater. The process ensures an effluent free of solids, due to a membrane barrier and helps to overcome the problems associated with poor sludge setting in common activated sludge processes (AMTA, 2007). The high quality permeate produced by MBRs is suitable for variety of applications for industrial and municipal purposes. The operation of MBR also has some limitations. Those include the need of fine screening to remove abrasive, stringy and fiborous material as it can damage the membrane or can increase fouling. Other pretreatement of industrial wastewater may vary depending on factors like COD, temperature, TDS or high content of inorganic solids. Because of the variable parameters of operation, the cost of implementing a MBR technology also varies. For smaller facilities lesser than 1 MGD general guidelines estimate expected equipment cost of 2-6$ peer gallon of plant capasity and plant construction cost of 12-20$ per gallon of plant capasity (AMTA, 2007). Estimated operation costs range from 350$ to 550$ per million gallons treated (AMTA, 2007). Facilities larger than 1 MGD can expect equipment cost of 0.75-1.50$ peer gallon of plant capasity and plant construction cost of 5-12$ per gallon of plant capasity (AMTA, 2007). Estimated operation costs range from 300$ to 500$ per million gallons treted (AMTA, 2007).
Reverse Osmosis Applications
Approximately one-half of the reverse osmosis systems currently installed are used for desalination of brackish or seawater. The remaining half is used in the production of ultrapure water for the power generation, pharmaceutical, and electronics industries and for applications such as pollution control and food processing (Baker, 2004). Since we aim to discuss applications related to pollution prevention, desalination will not be covered in this paper.
An established and growing application for reverse osmosis is the production of ultrapure water for the electronics and pharmaceutic industries. In this case, the feed is usually municipal water which contains less than 200 ppm of dissolved solids (Baker, 2004). Reverse osmosis typically removes more than 98% of the salts and other dissolved particles, additional processing with carbon absorption and ion exchange will remove the remaining impurities (Ganzi, 1989).
Apparently, pollution control should be a major application for reverse osmosis but in practice, membrane fouling, one of the limitations of membrane processing, can cause low plant reliability. This has inhibited its widespread use in this area. On the other hand, reverse osmosis has several advantages that make it attractive such as simplicity in design and operation, modern units require very low maintenance if used properly, inorganic and organic pollutants can be removed at the same time, the process do not affect the nature of the material being recovered, and depending on the application waste streams can be considerably reduced and can be further treated in a more efficient and cost effective way if needed (Williams, 2003).
One of the successful uses of reverse osmosis is in the recovery of nickel from nickel-plating rinse tanks, where a stream used to rinse the material after nickel-plating ends up containing around 3000 ppm of nickel, which represent a pollution problem, as it cannot be directly wasted, and a valuable material lost for the industry, the application of reverse osmosis allows to produce a permeate stream with only around 50 ppm of nickel that can be reused in the process and a concentrate that is sent to the plating tank (Baker, 2004). The same principle can be applied for the recovery of copper, zinc, copper cyanide, chromium, aluminum and gold and in general the metal finishing industry, allowing recoveries between 75 up to 95% (Benito and Ruis, 2001).
One of the areas of research for the reverse osmosis membranes is its use in the recovery and tertiary treatment of water to produce drinking water from sewage (Abel-Jawad, 2002). Although the process is economically feasible, particularly in water limited regions, psychological barriers are still the biggest obstacle for its implementation. Attempts have been made in the US to introduce this operation, injecting treated water into the aquifer and mixing it with natural groundwater which somewhat has helped to its acceptance (Baker, 2004).
Because of high rejection of inorganic compounds, reverse osmosis membranes have also been studied for treatment of radioactive effluents (Arnal, Sancho, Verdu, 2003) and the removal of other toxic componds (Ning, 2002) and have been used for the treatment ofÂ uraniumÂ conversion process effluents containing corrosive, toxic and radioactive compounds.
Gas Membrane Separation Applications
The principal established and developed gas separation processes at industrial level are used for Hydrogen and Nitrogen separation, carbon dioxide and methane separation, nitrogen from air and water from air. After the first gas membrane separation units proved to work successfully for hydrogen separation, further development lead to a process to separate carbon dioxide from natural gas during extraction, after which it is reinjected into the ground (Baker, 2004). This application is an example in the mitigation of greenhouse gases emissions to the atmosphere and is widely spread in wells that use carbon dioxide as a pressurization medium. The largest application for membrane separation is the production of Nitrogen from air, process that uses polysulfone and ethyl cellulose membranes.
A growing application for these membrane systems is the removal of volatile organic compounds from air and other streams. In this case, rubbery membranes are used, which are more permeable to organic compounds. Most of the plants of this type installed aim to recover gasoline vapors from air vented during transfer operations, although this technology is also applied for the recovery of fluorinated hydrocarbons from refrigeration streams (Freeman, 1995).
Conclusions and Recomendations
Since the appearance and industrial application of membrane separation processes, several decades ago, there has been a period of very rapid growth (Nath, 2008). In the areas of microfiltration, ultrafiltration, reverse osmosis, electrodialysis and diffusion dialysis we can say that the technology is relatively mature in terms of their utilization. However, significant advances have been made as membranes continue to displace conventional separation techniques. The most rapidly expanding area is the use and development of gas separation membrane techniques; although its market share is still very small in comparison to the other technologies, it is projected to grow further as development of more selective and high flux membranes allow its economic use in the petrochemical and natural gas processing areas. In terms of market development and applications, gas separation processes can be divided in two groups; the first one includes established applications, such as nitrogen-air separation and hydrogen recovery, which represent up to 80% of the current market and have undergone significant improvements in membrane selectivity and flux, increasing efficiency and decreasing costs (Baker, 2004). Another group is comprised by developing processes, which include carbon dioxide separation from natural gas, volatile organic component separation from air and recovery of hydrocarbons from petrochemical plant purge gases, all these are already used on a commercial scale and their application is directly related for pollution prevention in a very important and relevant area; control of greenhouse gas emissions. Significant expansion in these applications and process designs is occurring. The combination of a gas separation process with others, such like distillation of organic vapor mixtures, for example, is other of the developing areas.
A 2001 market analysis for membrane separation technologies confirms that the expanding use of membranes mainly in water and wastewater treatment and gas separation technologies has made possible important advances in the area. Also, increasingly strict environmental regulations and awareness, applied during the past decades have increased the adoption of membrane separation processes, influenced also by the reduction in waste disposal costs and the increased opportunity of materials recovery and recycling (Atkinson, 2002). Table 1 shows the summary of membrane materials demand and their growth.
Table 1. Summary of membrane materials demand in US$ million (Atkinson, 2002)
% Annual growth
Gross domestic product (bil US$)
Membrane demand (mil sq ft)
According to the data, microfiltration membranes account for the largest share of the market, as it is a very popular and low-cost alternative in applications that do not require high levels of purity. Its use is common, many times as pretreatment for other more specific separation processes. There is still a good opportunity for the growth of the industry in the bacterial control of drinking water and other beverages and treatment of sewage (Baker, 2004). So we can conclude that municipal water treatment is likely to develop into a major future application of this technology.
The reverse osmosis industry is one of the better established when considering membrane separation processes. It has the second largest share of the US market. Demand for reverse osmosis membranes have advanced rapidly because this process can deliver a high level of purity, demanded in wastewater treatment systems and other applications in the industry. Two of the main industries served are the electronics and pharmaceutical, but the desalination market to produce fresh water has been growing over the past years. Recent developments have also lowered water desalination costs and increased membrane unit fluxes, as well as improved resistance (Elimelech, 2011).
Ultrafiltration accounts for the third largest share of the membrane market, the expansion of this technology is limited due to the high cost per liter of permeate produced in most wastewater and industrial process stream applications. Since membrane fluxes are not high, and large amounts of energy are used for the feed recirculation in order to control fouling and concentration polarization, costs are usually high (Baker, 2004). Research and development of fouling resistant membranes is now the preferred approach, changing the membrane surface absorption characteristics. Although ceramic membranes do not present these disadvantages, costs are still very high in comparison to polymeric membranes and should be reduced by an order of magnitude to be competitive (Cheryan, 1998).
Electrodialysis is by far the largest used of ion exchange membranes, although it accounts for a very small share of the market. Both desalting brackish water and salt production are well established processes and major technical innovations that will change their competitive position of the industry do not appear likely. And the total market is small.
In figure 9, the membrane demand by market is presented.
Figure 9. Membrane demand by market, 2001 (Atkinson, 2002)
As shown in the figure, water and wastewater treatment accounted for 55% of the membrane demand in the year 2001, this is due to the emphasis on reducing contaminants in water feed streams and reclaiming process components and recycling water.
It is evident, by the data provided by this market study that the most used membrane technology in wastewater and water treatment is microfiltration, followed by reverse osmosis. Both processes have found broad and successful applications in pollution prevention. The fact that the membrane market forecast is to keep growing during the next years and that applications such as gas separation have still a long way to go in terms of research and development, tells us that they will play a fundamental role in pollution prevention and even in pollution remediation. But in addition to new improved membranes and membrane processes, there is also the need for application "know-how", which often requires the cooperation of various scientific disciplines. Also it appears to be a lack of education in membrane science technology, while other unit operations are included in technical schools and university programs, membrane science and technology seldom is.