Removal Of Cyanogens From Acrylonitrile Wastewater By Using Membrane Technology

In this paper, the application of membrane technology in separating cyanogens from wastewater of the acrylonitrile industry was investigated. And the successful implementation of this technology in acrylonitrile apparatus which production capacity is 80,000t/a was introduced in detail. Hence, such a new method or technology that was using to remove cyanogens from effluents of the acrylonitrile apparatus was put forward. Thereby the difficult problem that the serious environmental pollution was brought by the process of flaming this kind of wastewater containing cyanogens was resolved and the benefit of economic and society was extremely made. Nowadays membrane process has been the important step of gas separation, solution separation, fractionation and purification of chemical and biochemical products in the industry and widely used in the area of food industry, beverage process, wastewater treatment, large-scale air separation, wet metallurgy industry, producing gas and liquid fuels and petrochemical industry. The rapid development of membrane technology was resulted in the characteristics of membrane as follows: 1) high efficiency. The limit size of separation technology such as gravity separation is micrometer, but the membrane technology can separate the matter which molecular weight is several thousand even hundred. 2) low energy costing. In the process of membrane, the matter separated can hardly exist phase change. Comparing with this, other separation processes such as evaporation, distillation and adsorption all go with the phase change which is from liquid phase or adsorbed phase to gaseous phase. In addition the membrane process is often operated in ambient temperature and the energy of heating or cooling materials need less. 3) Little maintaining, high reliability, easy operation and small device volume. 4) The scale and capacity of membrane process may change in wide range but its efficiency, unit price and operation cost all rarely change. According to the function the membrane process can be classified as follows: 1) microfiltration(MF): removing the particle of 50 nm size; 2) ultrafiltration(UF): removing the particle of 5-100 nm size; 3) reverse osmosis(RO): removing soluble salts from aqueous solution; 4) dialysis(D): removing inorganic acids and salts from aqueous solution; 5) electrodialysis(ED): removing acids, alkali and salts from aqueous solution; 6) gas separating(GP): separating mixed gases; 7) pervaporization (PV): separating the mixture of water and organic; 8) liquid membrane(L): separating salts and biological activity materials. There are 10 sets of acrylonitrile apparatus in our country at present, which capacity exceeds 1,000,000 t/a, but as one ton acrylonitrile is prepared, one ton wastewater containing cyanogens will be born and flaming the wastewater is the only treatment method now. However, flaming the wastewater not only needs high cost, which is about 300 RMB/t, but also makes a large amount of carbon dioxide and oxynitride, which seriously pollute the environment. Thus, people are searching one new cheaper and more environmental treatment method. We decided to use two kinds of membrane processes, that is UF and RO, to remove the cyanogens from acrylonitrile wastewater according to the characteristic of membrane processes combined with the feature of wastewater containing cyanogens. There is such one device used for the capacity of 80,000 t/y acrylonitrile apparatus in Daqing Petrifaction Corporation. In the wastewater containing cyanogens, the concentration of cyanogens is generally between 300 and 400 ppm, and the COD value is from 4000 to 5000, but the standard of discharge water is that the concentration of cyanogens is less than 5 ppm and the COD is less than 1500. According to the experiments, we successfully made it through one stage UF and two stage RO and the concentration of cyanogens and COD of discharge water completely reached the standard after identification. Moreover the operation cost of processing wastewater by using membrane technology is less than 100 RMB/t and saving costs are about 2,000,000 RMB/y for one acrylonitrile apparatus which capacity is 80,000 t/y, and the pollution of waste gas is solved at the same time. So the benefit of economic and society is enormous. The success of using membrane technology to remove the cyanogens from wastewater provides one completely new treatment method for the acrylonitrile industry of our country. This technology of processing the wastewater containing cyanogens not only solved the problem of environmental pollution of such industry, but also effectively reduced the cost of acrylonitrile products due to the low operation cost and increased the international competitive power of our countries'acrylonitrile products, at the same time improved the technology level of our countries'acrylonitrile industry, therefore this technology is extremely worth extending.