Petrochemical industry (Chlor-alkali denitrification)

Application description

Petrochemical Industry (Chlor-alkali denitrification)

Application details

Ion membrane caustic soda system requires high brine precision. Too high sulfate content in brine will shorten the service life of the anode, reduce the current efficiency of the electrolytic cell, and make the membrane effective. The area is reduced, which affects the current conduction and increases the power consumption. Generally, the sulfate content in the light brine entering the electrolytic cell is controlled below 6-7g/h. However, the amount of sulfate radicals brought out during the production of ion-exchange membrane caustic soda is almost negligible. This requires chemical or physical methods to remove excess sulfate in the brine circulation system to ensure normal production.

Membrane Denitrification Technology

Membrane method is a physical separation method, which utilizes the selective separation function of membrane. The excess sulfate in the brine circulation system is separated from the brine system in the form of sodium sulfate, and the qualified brine is returned to the brine circulation system. During the whole separation process, no phase change and no addition of any chemicals are required, the brine water quality is not affected, the retention rate of sulfate ions is stable, and the operation and maintenance are simple. Thereby, the production cycle is shortened, the production efficiency is improved, the investment and operating costs are reduced, and there is no pollution. is the future development direction.



Nanofiltration membrane denitrification process principle

Nanofiltration, like microfiltration, ultrafiltration and reverse osmosis, is driven by pressure difference, but its mass transfer mechanism is different. The comparison is shown in Table 1. Due to the large pore size of microfiltration and ultrafiltration membranes, the mass transfer process is mainly in the form of pore flow, that is, the sieving effect; the reverse osmosis membrane is a non-porous membrane, and its mass transfer process is the dissolution and diffusion process, that is, the electrostatic effect. Nanofiltration membranes have nano-scale pores. And most of the negative charges, the separation behavior of inorganic salts is not only controlled by the chemical potential, but also affected by the potential gradient. For a pure electrolyte solution, due to donnan equilibrium, the ions of the same sex will be repelled by the charged active layer of the membrane. If the ions of the same sex are multivalent, the retention rate will be higher, and at the same time, in order to maintain the charge balance, the counter ions will also be retained, resulting in electron migration. Flow is opposite to convection. However, the rejection rate of co-ions with polyvalent counter ions is lower than that of co-ions with monovalent counter ions, which may be due to the adsorption and shielding effect of polyvalent counter ions on the membrane charge.

      Since the separation range of nanofiltration membrane is between ultrafiltration and reverse osmosis, it can retain sulfate ions and have high flux for sodium ions and chloride ions. Figure 1 below is a schematic diagram of the separation principle of the nanofiltration membrane.



For two solutions of homoionic mixtures, according to Donnan's theory, multivalent co-ions are more likely to be trapped than monovalent co-ions compared to their respective pure salt solutions. Mixtures of two common ions, due to their different mobilities, progressively reduce the retention of low-mobility counter ions. The concentration of highly mobile counter ions increases, causing "offset" of convection and electromigration. The selective retention of polar small organics by nanofiltration membranes is based on the size and charge of the solute. For polar (or charged) solutes, the rejection rate when passing through the nanofiltration membrane is determined by both electrostatic and steric hindrance effects, while for non-polar solutes, it is mainly determined by steric hindrance effects.
      The nanofiltration membrane for desalination of salt water in the chlor-alkali industry is a special type of membrane with a surface pore size of 0.51 nm and a certain charge on the surface of the membrane. Ions have a high and stable rejection rate, while monovalent ions have a high transmission rate. Its material structure is stable, and it operates stably for a long time in salt water with high sodium chloride content. See Figure 2.



3 Membrane denitrification production equipment

Membrane denitrification process has been well used in actual industrial production. The following takes a chlor-alkali company in Yantai to use the nanofiltration membrane method to treat 50m3/h salt water denitrification process as an example for analysis. The process is mainly composed of a pretreatment unit and a membrane treatment unit. The pretreatment unit is shown in Figure 3: add naso3 to the raw raw brine, remove free chlorine to zero, cool it to the process requirements through a heat exchanger, adjust the pH value to the process requirements with acid, remove impurities through a filter device, and enter the brine Tank ready for use. Under normal circumstances, the online detection instrument ensures that the raw materials of the membrane filtration unit are controlled within the range of process requirements.



Through the economic analysis of the equipment investment and operating costs of the above-mentioned denitrification 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 mainly consumes electricity. The price of barium chloride varies greatly with market fluctuations;
(3) Avoid toxic operations, do not need to add any other chemicals in the separation process, and the brine water quality is not polluted; (4) No secondary pollutants are generated, no need to deal with waste residue, environmental protection; (5) Operation and maintenance are simple, reducing labor intensity; (6) The retention rate of sulfate ions is stable and the yield is high.

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