By Fan Yiqun, Nanjing Tech University,
Cai Yuanyuan, Nanjing Membrane Materials Industry Technology Research Institute
Due to their high temperature resistance, solvent resistance, high mechanical strength and high separation precision, ceramic membrane materials are especially suitable for solving many separation problems faced by the chemical industry in harsh environments, promoting green development, transformation and upgrading of the chemical industry, and improving production safety.
Research and development status of Chinese ceramic membrane technology
The ceramic membrane material is a separation membrane material having an asymmetric structure formed by metal oxides through special processes, mainly including Al2O3, TiO2, ZrO2, SiO2, SiC, and inorganic composite materials. The research on ceramic membranes in China began in the 1980s. The number of institutions engaged in ceramic membrane research increased from less than 20 before 2000 to more than 100 in 2018. The number of SCI papers published in this field also increased from less than 50 articles before 2000 to about 900 in 2018, with an average growth rate of over 15%. China has realized large-scale production of tubular, multi-channel and flat ceramic ultra-microfiltration membranes of various materials and specifications, and these products have been promoted and applied in many fields such as water treatment, food, medicine, petrochemical, etc. In 2018, the total output value of the ceramic membrane industry exceeded RMB1 billion. The development of ceramic membranes has also promoted technological advances in related industries.
The development of Chinese ceramic membrane materials has evolved from the initial microfiltration membrane and ultrafiltration membrane to the nanofiltration membrane. Ceramic membrane modification technology also provides the single-surface ceramic membrane with new characteristics, achieving the regulation from pore diameter screening to surface properties. However, there is still huge room for development in its industrial applications. Focusing on the problems of high cost and limited variety of ceramic membrane applications, development will be directed towards high performance ceramic nanofiltration membrane design and preparation technology, ceramic membrane surface function precision control technology, and high loading density ceramic membrane design and production technology.
Application of ceramic membrane technology in China's chemical industry
Ceramic membrane technology has been used in many fields of China's chemical industry, such as caprolactam production, the chlor-alkali industry, the pharmaceutical industry, etc., and plays an increasingly important role.
Caprolactam is one of the important organic chemical raw materials in China, with an annual production capacity of 3.5 million tons. Cyclohexanone oxime is an intermediate product during the production of caprolactam. Sinopec has developed a new liquid phase cyclohexanone ammoximation process and hence has significantly reduced the production cost of cyclohexanone oxime, whereas the separation problem of the ultrafine catalyst TS-1 molecular sieve has become one of the key difficulties of the process. The ceramic membrane can be coupled with the production process to construct a suspended bed ceramic membrane reactor, thereby realizing the combination of reaction and separation, as well as the in-situ separation of the ultrafine catalyst and the continuous reaction process, thus reducing the catalyst loss and improving the product quality. In this process, the conversion and selectivity rate of cyclohexanone oxime are both over 99.5%, the catalyst content in the permeate is less than 1 ppm, the quality of the caprolactam in the final product reaches the superior grade, and the energy consumption per ton of product is reduced by 30%. The suspended bed ceramic membrane reactor technology has been applied in the cyclohexanone ammoximation reaction by companies such as Sinopec Baling PC and has been expanding at a pace of 800 000 tons per year. At present, the technology has been further popularized and applied in production processes such as benzenediol and p-aminophenol.
The main production process of the chlor-alkali industry is ion-exchange membrane electrolysis, and the quality of saturated brine is the main factor affecting its efficiency. Therefore, brine refining is the core process of ion-exchange membrane electrolysis. In view of the long process flow, complicated separation process, and unstable operation of the brine refining process, China has developed a ceramic membrane brine refining process with independent intellectual property rights. The core of the process is to couple the precipitation reaction process and the ceramic membrane separation process, forming a precipitation ceramic membrane reactor. The purified brine filtered by a ceramic membrane has high quality and stability, with Ca2+ and Mg2+ content both less than 0.5mg/L and SS at ≤0.1mg/L, which fully meets the requirements of the ionized membrane method for refined brine (Ca2+ and Mg2+ content less than 1mg/L). Compared with the conventional process, the equipment investment cost of the ceramic membrane brine refining process is reduced by 45%, and the consumption per ton of the medicament is reduced by 55%. At present, the ceramic membrane brine refining process has been promoted and adopted by China National Salt Industry Group and other companies, and the scale has exceeded 10 million tons.
In the process of bio-fermentation pharmaceuticals (such as antibiotics, vitamins, organic acids, amino acids, etc.), there are a large number of mycelium, cells, biomacromolecules and other substances in the fermentation broth system, and conventional clarification and impurity removal methods (such as centrifugation, plate and frame filtration, precipitation, etc.) have problems such as complicated processes and long wait times. The use of the ceramic membrane process for clarification and impurity removal of the fermentation broth can completely separate the impurity components such as mycelium and biomacromolecules and improve the quality of the filtrate and the adsorption efficiency of the subsequent resin. In the cephalosporin C production process, the investment is reduced by 70% when the ceramic membrane process is adopted, compared to the centrifugal process, and the operating energy consumption is reduced by 20%. The "ceramic ultrafiltration membrane + organic nanofiltration membrane" combination process has been widely adopted by China’s large and medium-sized pharmaceutical companies. The traditional Chinese medicine production uses ceramic membrane instead of water extraction and alcohol precipitation, which can effectively improve the clarity and the content of traditional Chinese medicine, shorten the production cycle and reduce ethanol consumption.
The ceramic membrane technology has also been industrialized in fields like nano-powder production, chemical wastewater treatment (such as petrochemical industry oilfield production water, natural gas sulfur wastewater, etc.), and so on.
Development prospect
As a green separation technology, with the rich variety of ceramic membrane materials, the improvement of separation precision and the development of low-cost production technology, it has shown bright prospects for application in the purification and regeneration of waste lubricating oil, the washing production of graphene nano-new materials, the separation and purification of functional oligosaccharides, and coal chemical wastewater treatment and reuse. Ceramic membrane technology is also gradually expanding to the field of gas separation (such as high-temperature gas dedusting, flue gas desulfurization, carbon dioxide captures, and oxygen separation in coal chemical industry).