Removal and recovery of residual chlorine from the liquid material using membrane technology
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Recently, we conducted an on-site pilot test for gas-phase membrane dechlorination of saline solutions containing 6% sodium chloride, 0.2% cyanuric acid, 0.10–0.55% available chlorine, and a pH of 0.6–1.8, using polytetrafluoroethylene hollow fiber microporous hydrophobic membranes. The results show that the gas-phase membrane process can easily reduce residual chlorine from its initial value to around 10 mg/L; if further dechlorination is required, adding some sodium sulfite will suffice. The so-called gas-phase membrane dechlorination process involves using a microporous hydrophobic membrane: on one side of the membrane is an aqueous solution containing free chlorine, while on the other side is a reactive absorbent such as sodium hydroxide (or sodium sulfite). This arrangement ensures that the concentration of free chlorine in the absorbent solution remains strictly zero at all times and in all locations, thereby providing the greatest driving force for mass transfer and enabling the maximum removal and recovery of free chlorine. Theoretically, as long as the pH value of the liquid feed is low enough, the concentration of free chlorine in it can be reduced to very low levels, for example below 1 mg/L. Theoretically, the relationship between the free chlorine concentration in the fluid entering and leaving a membrane module, the membrane area S, the mass transfer coefficient K, and the flow rate of the fluid is given by: ln(C0/C1) = KS/Q, where C0 is the free chlorine concentration of the fluid when it enters the membrane module, and C1 is the free chlorine concentration of the fluid when it exits the membrane module. Therefore, the membrane area required to remove free chlorine from the feed solution from 100 mg/L to 10 mg/L is the same as that required to remove it from 1000 mg/L to 100 mg/L. Therefore, to reduce it from 10 mg/L to 1.0 mg/L, it is better to simply add a small amount of sodium sulfite. Since the equipment is made of perfluoroplastics, there is no issue with corrosion. Let’s see if this gas-phase membrane dechlorination method is useful in the chlor-alkali industry, as it requires less electricity for the process.
Your method is likely not applicable to the dechlorination step in the chlor-alkali electrolysis process, as the so-called dechlorination actually involves the use of chemical agents for absorption, and the cost of these absorbents is quite high. If possible, could dissolved chlorine be removed from the solution in gaseous form and recovered into the system? Only in this way would it be useful in chlor-alkali production. However, this new concept is good and could be applied in other industrial productions.
I’ve experienced it! Then change the operation mode, switching from the gaseous membrane process to a membrane desorption process. That is, in the same way, chlorine-containing brackish water is passed over one side of the microporous hydrophobic membrane, while vacuum is applied to the other side of the membrane. The characteristic is that the mass transfer area per unit of equipment is relatively large; in addition, the liquid forms a very thin film. For example, by using a hollow fiber microporous hydrophobic membrane with an inner diameter of 0.50 mm, the liquid flows through the tube side of this membrane, resulting in an equivalent thickness of the liquid of only 0.55/3.66 = 0.15 mm. Even the best packed towers cannot achieve such a thin layer of liquid. The dechlorination effect is definitely good, but I’m not sure how its cost-performance compares to conventional dechlorination equipment. Furthermore, if there is only a small amount of residual chlorine in the lightly salted water, and adding sodium sulfite directly would increase the denitration costs, it is also possible to use gas-phase membrane equipment to indirectly react with sodium sulfite on the other side of the membrane in order to absorb the chlorine gas on that side. At this time, the amount of absorbent consumed is not large.
Wrong; the saltwater parameters after dechlorination using the current process are <10PPm, which is exactly the same as the parameters obtained after dechlorination using the gas membrane method you mentioned. All of this free chlorine can only be removed by chemical methods, and the cost of denitration remains the same. If vacuum equipment is added on the removal side, it is similar to the current process, which is equivalent to replacing the dechlorination tower with a gas-phase membrane device. My understanding is that it’s like this; is that correct?
As an additional note, filler towers are no longer used in current dechlorination towers either.
More or less. I’ve been thinking about your issue with dechlorination using ‘dilute’ sulfuric acid. It’s quite thought-provoking.
Based on the composition of the waste liquid you described, it seems to be a mother liquor of cyanuric acid chloride derivatives. The most ideal way to dispose of this waste liquid is to sell it directly as a disinfectant and bleaching agent. The advantage is that no wastewater treatment equipment or costs are required, and it can be sold directly as a product; the disadvantage is that it is not easy to find suitable customers. The most reasonable approach is to remove free chlorine, active chlorine, and cyanuric acid from the waste liquid. Cyanuric acid is converted into its sodium salt and then returned to the chlor-alkali production process for further use, while the remaining brackish water is sent back to the brine preparation process. After being turned into qualified brine, it is electrolyzed again to produce chlorine gas and caustic soda, which are then used in the production of cyanuric acid chloride derivatives, thus creating a closed-loop production system. The advantages are closed-loop recycling of wastewater with no emissions and high element utilization rates. The disadvantages include complex equipment for wastewater treatment, high operating costs, which may make it uneconomical. Additionally, cyanuric acid is difficult to remove completely; when present as a substance with high nitrogen content in the electrolysis system, it can pose a threat to normal production and may also trigger explosions involving nitric trichloride.