Industrial application of backwashing filter and nanofiltration membrane denitration process
Industrial application of backwashing filter and nanofiltration membrane denitration process
The accuracy of the brine required by the ion-exchange membrane caustic soda system is relatively high. The high sulfate content in the brine will shorten the service life of the anode and reduce the current efficiency of the electrolytic cell. The effective area of ​​the membrane will be reduced, which will affect the current conduction and increase the power consumption. Generally, the sulfate content in the brackish water entering the electrolytic cell is controlled to be 6 to 7 g/h or less. However, the amount of sulfate in the production of ionic membrane caustic soda is almost negligible. This requires chemical or physical removal of excess sulfate in the brine circulatory system to ensure proper production. Membrane Denitration Membrane is a physical separation method that utilizes the selective separation function of the membrane. The excess sulfate in the brine circulatory system is separated from the brine system as sodium sulphate and the qualified brine is returned to the brine circulatory system. During the whole separation process, no phase change and no need to add any chemicals, no impact on the water quality of the brine, stable retention of sulfate ions, and simple operation and maintenance. This shortens the production cycle, increases production efficiency, reduces investment and operating costs, and is non-polluting. It is the future development direction.
The principle of nanofiltration membrane denitration process nanofiltration is the same as microfiltration, ultrafiltration and reverse osmosis. The pressure difference is the driving force, but the mass transfer mechanism is different. Due to the large pore size of the microfiltration and ultrafiltration membranes, the mass transfer process is mainly the pore flow form, ie the sieve effect; the reverse osmosis membrane belongs to the non-porous membrane, and the mass transfer process is the dissolution diffusion process, ie the electrostatic effect. There are nano-scale micropores in the nanofiltration membrane. And most of the negative charge, the separation behavior of the inorganic salt is not only controlled by the chemical potential, but also by the potential gradient. For pure electrolyte solutions, the isotropic ions are repelled by the charged membrane active layer due to Donnan equilibrium. If the isotropic ions are multivalent, the rejection will be higher, and in order to maintain charge balance, the counter ions will also be trapped, resulting in electron migration. The flow is opposite to the direction of convection. However, the co-ion with a multivalent counterion has a lower rejection than the co-ion with a monovalent counterion, which may be due to the adsorption and shielding of the membrane charge by the multivalent counterion. Since the separation interval of the nanofiltration membrane is between ultrafiltration and reverse osmosis, the sulfate ion can be trapped with high flux for sodium and chloride ions.
For the two isotropic ion mixture solutions, according to Donnan theory, the multivalent common ions are more easily trapped than the monovalent common ions compared to their respective simple salt solutions. The mixture of two common ions, due to their different mobility, gradually reduces the interception of low-migration counterions. The concentration of highly transported counterions increases, causing "offset" of convection and electromigration. The selective retention of polar small molecule organics by nanofiltration membranes is based on the size and charge of the solute. For polar (or charged) solutes, the rejection of the nanofiltration membrane is determined by both electrostatic and steric effects, while for non-polar solutes it is primarily dependent on the steric effect. The chlor-alkali industry's desalination nanofiltration membrane is a special membrane type with a surface pore size of 0.51 nm. The surface of the membrane has a certain charge and is highly stable and stable to divalent ions or high-valent ions such as sulfate ions. The rejection rate is higher for monovalent ions. The material structure is stable and stable in long-term stable operation in brine with high sodium chloride content.
Membrane denitration production equipment Membrane denitration process has been well used in actual industrial production. The following is an example of a chlor-alkali company using a nanofiltration membrane process to treat a 50m3/h light brine denitration process. The process consists essentially of a pretreatment unit and a membrane processing unit. Pretreatment unit: adding NaSO3 to the raw material light brine, removing free chlorine to zero, cooling through the heat exchanger to the process requirements, then adjusting the pH value to the process requirements with acid, removing impurities through the filter device, and entering the fresh brine storage tank. use. Under normal circumstances, the online inspection instrument ensures that the raw material indexes of the membrane filtration unit are controlled within the scope of the process requirements.
Conclusion Through the economic analysis of equipment investment and operating cost of the above several denitration processes, it can be concluded that the nanofiltration membrane denitration process has the following advantages:
(1) The investment cost is low, only 1/3 of the freezing method;
(2) The operating cost is low, about 1/4 of the precipitation method. The cost is stable, and the nanofiltration membrane method consumes electricity mainly. The price of barium chloride varies greatly with market fluctuations;
(3) Avoid toxic operations, no other chemicals need to be added during the separation process, and the salt water quality is not polluted;
(4) No secondary pollutants are produced, no need to deal with waste residue, and environmental protection;
(5) Simple operation and maintenance, reducing labor intensity;
(6) The retention rate of sulfate ion is stable and the yield is high.