Forward osmosis: A new hotspot in energy-saving and emission-reduction technologies for the petrochemical industry

Thread Content

　Osmotic permeation is a new type of membrane separation technology. Compared with common membrane separation techniques such as ultrafiltration, microfiltration, and reverse osmosis, its most notable feature is that it does not require external equipment such as high-pressure pumps as a driving force for separation; instead, the osmotic pressure difference of the solution itself drives the osmotic permeation process. This technology causes relatively little fouling of the membrane, allowing the permeation membrane to operate for extended periods without the need for cleaning. Countries and regions such as the United States, Singapore, and Europe have long conducted extensive research on forward osmosis membrane separation technology, and it has demonstrated great practical value in areas such as seawater desalination, brine treatment, and the separation of chemical products. 　　Technical research is gaining momentum quietly. Osmotic pressure, a technology that has become a focus of international research and is increasingly playing an important role in the petrochemical industry, is seeing steady progress in China as well. 　　“Osmotic desalination technology provides a low-energy, high-efficiency solution to water resource and environmental problems; it is a highly practical and environmentally friendly technology. ”Gao Congjie, an academician of the Chinese Academy of Engineering and director of the Membranes and Membrane Processes Laboratory at the Hangzhou Water Treatment Technology Development Center under the State Oceanic Administration, said in an interview with a reporter from China Chemical Industry News recently. “Osmotic separation technology is not only energy-saving, efficient, and environmentally friendly, but it also allows for the utilization of osmotic energy in various production processes to achieve energy savings. Soda ash plants and chlor-alkali plants require large amounts of salt to be dissolved, as well as large volumes of highly saline brine that need to be concentrated. By using osmotic separation technology, these two processes can be combined, enabling full utilization of osmotic energy – this approach could become a new way to reduce energy consumption and emissions in the inorganic chemicals industry. ”He Tao, a researcher at the Shanghai Advanced Institute of the Chinese Academy of Sciences, further explained to a reporter from China Chemical Industry News the role of osmotic pressure technology in the chemical industry. 　　It is understood that in the United States, projects aimed at concentrating wastewater generated from shale gas production in the Permian Basin have utilized forward osmosis membrane technology for testing purposes, in order to treat wastewater from the petrochemical industry and achieve zero emissions; the quality of the treated water can reach that of drinking water. The application of this technology is highly cost-effective; it not only represents a historic breakthrough in enabling the treatment of petrochemical wastewater to drinking water standards at low costs for reuse in industrial processes, but also eliminates the damage and pollution caused by the oil and gas industry to the natural environment. It solidifies various pollutants present in the wastewater, thereby achieving true \"zero emissions\". 　　Osmotic reverse osmosis also demonstrates its potential application value in areas such as seawater desalination, brine treatment, and the separation of chemical products. Taking seawater desalination as an example, forward osmosis can increase the water production rate from 40% with current technologies to 85%, while raising the concentrations of bromine, magnesium, and potassium in the concentrated brine – which facilitates the extraction and utilization of these resources. This approach significantly reduces the operating costs of seawater desalination, lowers environmental pollution caused by highly saline water, and its energy consumption is 70% of that of current water treatment technologies. It is reported that British Modern Water Company also uses forward osmosis membranes for seawater desalination. 　　At present, wastewater treatment in our country generally faces problems such as high energy consumption, low efficiency, and difficulties in treating concentrated brine. The concentrated brine discharged from industries such as coal chemical industry in China is directly released into evaporation ponds. When the volume of water is large or during winter when the evaporation rate is slow, these ponds even become wastewater discharge pools. By using forward osmosis treatment technology, the total salt content in the concentrated water can be increased to 20%, which significantly reduces emissions; the investment cost can be reduced by at least 30%, while operating costs can be cut by up to 70%. 　　Breaking through two key core technologies: “Osmotic reverse osmosis membranes are currently under pilot and scale-up testing both domestically and internationally; there are no large-scale industrial applications of this technology yet. However, we have noticed that the United States is investing in venture capital in the field of osmotic reverse osmosis technology, and it is already at the forefront internationally in terms of membrane materials and applications.” Currently, there is a need for investment from the capital market, as well as continuous experimentation and research, to identify the most promising applications of forward osmosis technology and to promote the optimization of membrane materials. ”He Tao said so. Overall, if forward osmosis technology is to make rapid progress, it can start with the two key core technologies in forward osmosis water treatment: the forward osmosis membrane and the draw solution. The focus of research and development both domestically and internationally is on finding more suitable drive fluids and osmosis membranes. 　　Extractant – Improves stability and reduces energy consumption for regeneration. The extractant is a key factor that provides the driving force for the forward osmosis process. The draw solution should have a higher osmotic pressure than the feed solution, so as to create an osmotic pressure difference that provides the driving force for the forward osmosis process. The driving force for forward osmosis is the transmembrane osmotic pressure difference, and the choice of extractant plays a crucial role in the performance of forward osmosis membranes. Therefore, the extractant determines the performance and efficiency of the forward osmosis process. As one of the key technologies for forward osmosis, an ideal draw solution should have a high solubility in water and a low relative molecular mass in order to generate a high osmotic pressure ； Non-toxic, stable in water; harmless to the human body when only small amounts remain in the produced water ； It does not react with the forward osmosis membrane, nor does it degrade its performance ； It can be easily and economically separated from water, and can be reused. 　　When different solutes are used as the extraction liquid, different osmotic pressures will be generated; simultaneously, they will exhibit different behaviors during the mass transfer process, and the degree of concentration polarization will also vary. How to design, develop, and select extraction fluids with high osmotic pressure, low energy consumption, low cost, ease of recovery, and low or no toxicity is considered by some experts to be a key issue in the development of forward osmosis technology. However, it is no easy task to develop an ideal absorbent that can meet all the above requirements simultaneously. 　　The researchers have studied different types of extraction liquids, including volatile gases, inorganic salt solutes, organic solutes, and synthetic materials. Among them, highly soluble gases with acidic or alkaline properties such as SO2 and NH3 result in extraction solutions that exhibit high osmotic pressure and high water flux. However, the regeneration of such extraction solutions requires the addition of a large amount of heat, leading to high energy consumption. Moreover, gases like SO2 and NH3 are corrosive, posing certain safety risks ； Water-soluble organic substances such as methylimidazole compounds and polyelectrolytes have relatively high molecular weights; they are not easily reverse-osmosed and are thus also chosen as draw solutions ； Some new synthetic materials, such as magnetically nanoparticles encapsulated in temperature-sensitive polymers and intelligent gels made from copolymers of sodium acrylate and N-isopropylacrylamide, have also been developed; they offer certain advantages in terms of reducing energy consumption. However, agglomeration occurs after repeated use, which reduces the water flux. 　　He Tao told reporters that the biggest problem with the extraction solution is its stability; during operation, the solutes in this solution can undergo reverse osmosis and thus leak into the feed solution, increasing costs and potentially causing membrane contamination. Therefore, it is recommended to use simple, low-cost, and readily available salts as driving solutions. He Tao believes that the osmotic draw solution comes from the raw materials used in the process; these raw materials drive the osmotic process to produce water for dissolution, thereby simplifying the process and enabling the reuse of wastewater generated during it. This can reduce environmental pollution on the one hand, and lower energy consumption and investment in fixed equipment on the other. 　　Li Yuping, a researcher at the Institute of Process Engineering, Chinese Academy of Sciences, said that although various types of extraction fluids have been developed for forward osmosis, most of this research remains confined to laboratory settings, with limited practical experience at an international level. The evaluation of the performance of the extractant should be based on factors such as water flux and the amount of solutes reversed osmosised; stability and operability during the operation of large-scale, long-term forward osmosis systems are also important factors to consider. 　　Li Yuping told reporters that the drawback of forward osmosis technology is the relatively high energy consumption required for the regeneration of the extraction fluid. Polymer extraction agents such as sodium polyacrylate have the advantage of a low reverse osmotic permeability; however, due to their high molecular weight and low solubility, the osmotic pressure they can generate is likely to be much lower than that of strong electrolytes or small molecules. Moreover, the energy required for regenerating such polymer extraction agents is also high. Ammonium bicarbonate extraction solution has a high solubility, and the osmotic pressure generated by its ionization in water is high. Its key advantage is that it can be decomposed into gases through heating and then recombined, thereby enabling easy regeneration of the extraction solution; the technical risks are low, making it suitable for industrial application. However, the energy consumption for regenerating ammonium bicarbonate is high. Moreover, reverse osmosis across the membrane causes the ammonia nitrogen level on the feedwater side to exceed acceptable limits (this is due to the extremely high concentration difference of ammonium bicarbonate on both sides of the membrane). In order to recover ammonia nitrogen without causing its level in the concentrated wastewater to become too high, it is necessary to recover ammonia nitrogen from the concentrated wastewater, which further increases energy consumption. Therefore, a simple forward osmosis ammonium bicarbonate draw solution regeneration process has no cost advantage. Other small-molecule strong electrolytes such as sodium chloride can be regenerated via membrane distillation (MD) when forward osmosis is used alone. However, wastewater often contains various organic substances, which cause hydrophilic changes in the distillation membrane, leading to liquid water leakage; thus, industrial application remains difficult. 　　Osmotic membranes – increasing water production capacity and resistance to fouling. Zong Tongqiang, a senior engineer at Guangzhou Zhongke Testing Technology Services Co., Ltd., said that an ideal osmotic membrane should possess the following characteristics: a membrane material with good hydrophilicity, in order to enhance the membrane’s capacity to produce pure water and its resistance to fouling ； The selectively permeable layer of the membrane is relatively dense to achieve high salt retention ； The membrane thickness should be as thin as possible to reduce the resistance to water flow and, at the same time, minimize concentration polarization ； It possesses good mechanical strength and can withstand certain pressures and shear forces ； The membrane material has certain corrosion resistance, can be used within a wide pH range, is stable in contact with the draw solution, exhibits no chemical reactions or physical adsorption between them, and can withstand the impact of acidic and alkaline cleaning solutions. 　　According to investigations by journalists, the osmotic membranes under research at home and abroad currently include cellulose triacetate (CTA) membranes, cellulose acetate (CA) membranes, polybenzimidazole (PBI) membranes, polyamide (PA) composite membranes, as well as modified composite membranes and nanofiltration membranes. He Tao told reporters that cellulose acetate membranes are currently the most widely used in applied research. Cellulose acetate osmotic membranes have a low scaling rate, but they also present many problems; the most common issue is that the actual water production capacity of these membranes is far lower than expected. The main reason is concentration polarization. Concentration polarization results from an increase in the concentration of the feed solution due to the accumulation of solutes on the feed side, as well as a decrease in the concentration of the extract solution caused by the dilution of this solution by the pure water that permeates through the membrane on the extract side. The fundamental cause of concentration polarization is that the pore sizes on the surface of the membrane are larger than those of the solute molecules in the extract solution; as a result, the solute concentration inside the pores is much lower than the concentration in the main mass of the solution, which leads to a reduction in the effective driving force. The issue of concentration polarization during forward osmosis is also a key topic that requires research. “These problems know no boundaries between domestic and foreign; they are industry-wide issues. ”He Tao emphasized this. 　　He Tao believes that interface-polymerized osmotic membrane materials could be the key to commercial success in developing osmotic membranes with high water flux, a wide pH range, high desalination efficiency, and good stability, by using different membrane materials and altering the membrane fabrication process. The main reason is that this technology is relatively mature, with many techniques available for borrowing from reverse osmosis membrane materials. However, osmotic membrane materials exhibit a special phenomenon of internal concentration polarization, which requires higher standards for the support material compared to reverse osmosis. Furthermore, the water treated by reverse osmosis membranes is water that has undergone multiple stages of complex pre-treatment; however, there is a desire for water that has only undergone simple pre-treatment to be able to proceed directly to the reverse osmosis stage. For this reason, forward osmosis membranes must be more resistant to contamination by colloids, organic substances, and even organic solvents, and the design of both the membrane material and the membrane components is a key focus. 　　Chi Lina, a lecturer at Shanghai Jiao Tong University, said that in the future, when synthesizing reverse osmosis membranes, more suitable materials should be selected to produce membranes with better resistance to corrosion caused by acids, alkalis, and salts ； Synthesize membranes with higher clarity and lower impedance ； Synthesize membranes with higher strength and thinner support layers to reduce internal concentration polarization ； Synthesize symmetric membranes with high mechanical strength. 　　Industrialization: One must take a long-term perspective. Technical development and industrial application are at an impasse. ”He Tao told the reporters, “It’s not that there’s no technology; it’s a lack of drive.” He Tao, who has been involved in the research of the major basic research project “973” on forward osmosis since 2009, believes that the environment for industrializing technological innovations in China is not as well-developed as that in developed countries, and the capital market is also immature ； Domestic enterprises are reluctant to take risks in investing in forward-looking technologies. As a result, industries often lag behind scientific research; they tend to wait until a particular technology has matured before making substantial investments to commercialize it. Furthermore, due to information asymmetry, the industrialization of forward osmosis also finds it difficult to attract investment interest. 　　Li Yuping, who has been engaged in osmotic process research for many years, believes that osmotic process has certain advantages in the treatment of high-salinity water. The future of forward osmosis lies in making full use of its advantages and integrating it with other processes such as membrane distillation. As long as the extraction fluid and its regeneration process are appropriate, the technical risks associated with osmotic pressure reversal are low, even in the absence of membrane products available on a commercial scale at present. For example, China has better fundamental theories and industrialization foundations in the decomposition and synthesis of ammonium bicarbonate than foreign countries. 　　It is understood that He Tao’s research team established the first pilot-scale osmotic process project in China, using sodium chloride to concentrate calcium chloride brine from 10% to 15%–16% ； Tests were conducted on the concentration of high-salinity wastewater from coal chemical industry and gray water using hollow fiber membrane modules ； Analysis and testing of wastewater from Sinopec’s shale gas production sites in Sichuan were carried out to obtain pure water from the shale gas wastewater. 　　He Tao told reporters that investment involves risks, but forward osmosis technology has its advantages; it can reach a new level in 3 years, and results will be evident in 5 years. If the industries involved in forward osmosis as well as the investment community take a longer-term perspective and be more proactive, focusing more on cutting-edge technologies like forward osmosis and being willing to invest in such technologies while taking on risks, then it will definitely be possible to overcome the challenges associated with bringing these technologies to commercial use. 　　It is encouraging to note that a small number of companies in China have already taken action; Guozhong Water Services Co., Ltd., together with Boten Environmental Group Co., Ltd. and the Danish company AQP, signed a formal cooperation agreement. The three parties will jointly invest in establishing a joint venture to set up production lines for aquaporin membranes in China. Zhu Yongjun, chairman of Guozhong Water Services, believes that aquaporin membrane products have broad application prospects in the fields of forward osmosis and reverse osmosis treatment. He said that the joint venture has made preparations in terms of capital, technology, market, and other aspects, laying the foundation for industrialization. It is understood that this technology is currently in the early stages of industrialization. An investment of 20 million US dollars will be made in the initial phase for research, development, and production, with total investments expected to reach 200 million yuan in the future to build production lines capable of producing 5 to 6 million square meters per year. 　　Last October, China’s first laboratory for zero discharge of industrial wastewater using forward osmosis technology, as well as a laboratory for seawater desalination and comprehensive utilization, was inaugurated in Beijing. Established jointly by Beijing Water Technology Co., Ltd., a subsidiary of China National Engineering Corporation, and the Fushun Research Institute of Petrochemical Technology under Sinopec, this laboratory is capable of achieving zero discharge of industrial wastewater, as well as conducting full-process simulations for seawater desalination and comprehensive utilization, along with independent experiments at each stage. In 2013, Wotter Co., Ltd. invested in the American company Oasys to introduce internationally leading forward osmosis technology. Currently, the first pilot plant for seawater concentration using this technology, established by the company in Huanghua, Hebei, is in operation, providing reliable operational data for the research activities carried out in the laboratories focused on seawater desalination and comprehensive utilization.