One question per week; answers win prizes. Question 7 from Chlor-alkali Chemistry (Basic Chemical Engineering Knowledge)

Let’s make a sofa now and refine the manufacturing process before passing it on

Single-stage brine: KAI membrane process； Secondary resin column ； Electrolysis and recycling, vacuum dechlorination, evaporation: double-effect counter-current falling film + rising film

The last edit to this post was made by huawei on 2010-7-31 at 10:02. Liquid chlorine: Chlorine gas produced through the processing of chlorine is liquefied using York refrigerators and then packaged in cylinders or tank trucks for delivery

This post was last edited by Yinozhang on 2010-8-2 20:16. First-stage brine treatment: Membrane process; Second-stage brine purification: Resin tower; Electrolysis: Asahi Kasei; Dechlorination: Vacuum dechlorination; Drying: 3K chlorine compressor

Electrolysis: Process description: The filtered brine passes through the filtered brine heat exchanger E-153, where it is heated to 60°C before entering the filtered brine tank D-150. It is then pumped out by pump P-154 and sent to the chelating resin towers T-160A/B/C, where the levels of Ca2+ and Mg2+ are reduced to below 20 ppb, after which it is sent to the brine sump D-170. Brine is added to the fresh brine from the high-level tank via the brine main pipe, then flows into the branch pipes; after passing through the flow meter FI231, it is mixed with hydrochloric acid and fed into the unit electrolyzer. The brackish water and chlorine generated after electrolysis are separated using a chlorine separator, with the chlorine being sent to the hydrogen chloride treatment unit. The brackish water flows into the brackish water tank D-260; from there, it is sent via the brackish water pump P-164 – one stream to the main saltwater inlet pipe and the other stream to a mixture with hydrochloric acid before entering the vacuum dechlorination tower T-310, where the pH is kept at or above 2. The dechlorinated brackish water is then mixed with alkali, and after that it is pumped to the primary saltwater processing section using the dechlorinated saltwater pump P-314 for desalination (sodium sulfite is added at the pump outlet to further remove free chlorine). Alkali solution circulation process: The alkali solution in the catholyte tank D-270 is pumped out using pump P-274. It passes through the alkali solution cooler E-273, where its temperature is maintained at 80°C, and then is sent to the alkali solution sump D-273. The 32% NaOH solution flows out of this sump; a certain amount of pure water is added to reduce the concentration of the alkali solution to 30%. After that, it flows through the main pipes and branch pipes, as well as the alkali solution flow meter FIT221, before entering the unit electrolyzer. The alkaline solution and hydrogen flowing out of the cathode chamber are separated using a hydrogen separator, with the hydrogen being sent to the hydrogen-chlorine treatment unit. Liquid caustic flows into the circulation caustic tank D-270 and is sent out using pump P-274. One path leads to the alkali high-level tank, while the other path is sent to the evaporation unit after cumulative measurement by the alkali liquid mass flow meter FT274. Or it is cooled to 45°C by the alkali cooler E274 and then sent to the alkali finished product tank.

The HCL synthesis process involves using hydrogen treated by electrolysis, together with raw chlorine gas supplied from the liquefaction unit; the pressure is stabilized through a buffer tank, and water and impurities are separated thereafter. It enters the synthesis furnace where combustion, combined with heat exchange using water at 75–85 degrees Celsius, results in an HCL temperature of around 500–580 degrees. After being cooled by air-cooling tubes to a temperature of 110–180 degrees before passing through the graphite cooler, its purity is between 94–96%, and it is then sent to the conversion unit for use.

Salt conversion: Pretreatment, KAI membrane process, membrane method + cryogenic denitration. Electrolysis: Beihua Machine membrane electrode spacing cell, two-stage brine three-tower process, vacuum dechlorination. Chlorine treatment: Primary washing, secondary titanium cooling, three-filler drying process, Jinhua Machine turbine compression； Process for producing sodium hypochlorite from chlorine-containing waste gas using a secondary absorption tower: Hydrogen treatment and hydrogen chloride handling; pressure stabilization in the hydrogen storage tank at the previous stage; liquid chlorine processing via low-temperature freezing; packaging using magnetic submersible pumps; evaporation through a two-effect counter-current falling-film + rising-film process

Brine: The first-stage brine purification process involves chemically treating, clarifying, and filtering brine, unqualified brine returned from membrane filters, crude brine returned from preprocessors, and solutions obtained after the concentration of dilute brine, in order to produce qualified brine that can be used in the subsequent second-stage brine purification process and electrolysis processes. The brine coming from the brine storage tank is pumped using a brine transfer pump, with the flow rate controlled by a frequency converter (the frequency of the pump is adjusted based on the liquid level in the reaction tank; this frequency is controlled by a DCS system, thereby changing the amount of fluid supplied). The brine is then sent to a brine heat exchanger, where steam is regulated through automatic control valves to maintain a temperature of around 55–60°C. It is subsequently sent to the No. 2 baffled tank, where sodium hypochlorite solution produced through chlorohydrin treatment is added. The NaClO present in the undechlorinated dilute brine is used to eliminate bacteria, algae, organic substances, NH3, and corrosive acids from the brine, by oxidizing and decomposing these substances into smaller molecules that can then be removed. NaOH is added simultaneously in the 2# baffle tank to remove Mg2+. The brine flowing out of the 2# baffle tank enters the front reaction tank, where it mixes with the brine obtained from the concentration of fresh brine; the concentration is maintained at 300±5 g/L. In the front reaction tank, NaClO and NaOH react with this brine, and then the mixture is sent to the gas-water mixer via a pressure pump, where it mixes with the air from the system before entering the pressurized dissolved air tank. There, thorough mixing with the air from the system ensures complete reaction, and the flow rate is controlled by FV101 as it flows into the preprocessor. Before flowing into the preprocessor, the saline solution passes through a venturi mixer, to which 1% FeCl3 prepared from a ferric chloride preparation tank is added. The brine is processed in the pre-treater; sediment, impurities, heavy metals, and the like settle at the bottom of the pre-treater and flow toward the slag tank. FeCl3 acts as a flocculant to cause the Mg(OH)2 formed as a result of the reactions, along with bacteria, algae, organic substances, and small molecules resulting from the oxidation of corrosive acids, to aggregate and float to the surface. These substances are then discharged from the upper part of the pre-treater and flow toward the slag tank. The treated clear liquid flows out through the overflow port and goes into the subsequent reaction tank. Before reaching the inlet of this tank, a 20% Na2CO3 solution prepared from a sodium carbonate solution tank is added to the pipeline to remove calcium, causing it to form CaCO3 precipitates. After sufficient reaction in the subsequent reaction tank, the brine enters the feed tank and is sent to the KAI membrane filter, whose filtration process is controlled by a PLC. The filtered brine coming out from the upper part of the filter flows into Tray No. 3. High-purity hydrochloric acid taken from the hydrochloric acid storage tank is added to Tray No. 3 to adjust the pH to around 8.5–9.5. Additionally, 5% Na2SO3 prepared in the sodium sulfite preparation tank is added to the middle section of Tray No. 3 to remove free chlorine; the presence of free chlorine is detected manually or using an ORP meter. The saltwater free of free chlorine flows by gravity into the filtered saltwater storage tank, and is then sent to the secondary saltwater purification unit using a filtered brine pump. After periodic operation, in order to maintain their filtering capacity and a low filtration pressure, Kay membrane filters need to be regenerated using 15% hydrochloric acid before they can be used again. For the filtration and purification of brine, a acid washing solution tank and an acid washing solution feed pump are provided for the regeneration of Kay membranes. After being recycled several times, the hydrochloric acid in the pickling solution tank is discharged into the wastewater tank, while the backwash fluid from the Kai membrane enters an intermediate tank, from where it is pumped back into the reaction tank using a return pump with variable frequency control. Preprocessor, post-reaction tank); the sludge from the filter is discharged into the slag tank), and then sent to a sludge filter press via a sludge pump for filtration and solidification. The brine filtered by the filter press is recovered to the brine storage tank via a filtrate pump for reuse.

Secondary brine: To ensure the quality of the brine fed into the electrolysis cell, the brine obtained from the primary brine purification process must have its pH adjusted to around 9.5 using hydrochloric acid, after which it is sent to an ion exchange resin tower to further remove trace amounts of polyvalent cations such as Ca2+ and Mg2+ from the brine, ensuring that their concentrations remain below the specified limits. The brine coming from the primary brine filtration tank is pumped to the brine plate heat exchanger by a brine pump. Steam is regulated by the DCS to raise the temperature of the brine to 60–65°C, after which it enters the ion exchange resin towers to remove Ca2+ and Mg2+. Two resin towers are used in series to treat the primary brine: one tower handles the treatment while the other ensures the quality of the brine; the third tower is kept in reserve for regeneration. When the first resin column is being regenerated, the second resin column takes over for production processes; the reserve resin column is put into use to ensure continuity of operations. Once the first resin column has been regenerated, it becomes a reserve unit, and all three resin columns are used in a cyclic manner. The switching and regeneration of the resin column are automatically controlled by DCS. Brine enters at the top of the resin tower and exits at the bottom; the brine that exits passes through a pipeline filter before entering the decarburization system. 3.1.4 Chelating resins that have lost their exchange capacity must be automatically regenerated using acid, base, and pure water every 24 hours. The regeneration process consists of the following steps: adding pure water to discharge saline water, backwashing with pure water, acid regeneration, rinsing with pure water, alkali regeneration, rinsing with pure water, and filtering to replace the saline water. Pure water for regeneration is pumped in by a pure water pump, hydrochloric acid for regeneration is supplied from the high-purity hydrochloric acid process, and alkaline solution for regeneration is delivered by a cathode solution pump. 3.1.5 The brine discharged during the regeneration process, the pure water discharged during backwashing, and the brine discharged during brine replacement are all sent to the recovered brine tank, where they are then pumped by a recovered brine pump to the reaction tank before the primary brine process for reuse. The acidic and alkaline wastewater generated during alkali regeneration is collected in the regeneration wastewater tank; after neutralization, it is pumped by waste water pumps to the company’s sewage treatment plant for centralized treatment. The decarburization system primarily removes carbon dioxide from the brine; after passing through the resin tower, the quality of the brine meets the process control requirements, after which it is sent to the electrolyzer for electrolysis. The exhaust gas removed is drawn through the top of the decarburization tower into the decarburization condenser, where it is cooled to a certain temperature. The gaseous phase is pumped away using a vacuum pump, while the liquid phase flows out from the bottom of the condenser and merges with the brine outlet from the decarburization tower. It then flows into the liquid seal tank and from there into the refined brine storage tank, from where it is pumped by a brine pump for use in electrolysis. The decarburization process uses a vacuum method to reduce the partial pressure at the liquid surface, causing the brine to boil and thereby releasing CO2 gas for disposal.

Electrolysis: The refined brine, coming from the resin tower and containing approximately 310±5 g/L of NaCl at a temperature of around 65°C, flows into the decarbonization tower and then into the brine storage tank, thereby helping to stabilize the liquid level in this tank as well as the flow rate of brine entering the resin tower. The brine is pumped by a brine pump into a brine preheater, where it is preheated by chlorine gas produced in the electrolytic cells. After preheating, it enters a brine heat exchanger to be further heated before being sent to various electrolytic cells for electrolysis. When only one cell is in operation, the brine exits the brine preheater (HE-) and goes to the brine heat exchanger before being sent to that single electrolytic cell for electrolysis； When the electrolyzer is shut down, it is necessary to ensure the circulation of brine and alkali solution; at this time, the brine is supplied to the electrolyzer from the concentrated brine sump (). After electrolysis of the cell using direct current, the NaCl concentration is approximately 210±5 g/L. This brackish water flows out together with chlorine through the hose at the outlet of the electrolyte anode solution, and enters the gas-liquid separator via the main anode solution pipe. The chlorine then goes to the saltwater preheater through the chlorine main pipe, and from there it is sent on to chlorine-hydrogen treatment. The anode solution that comes out of the electrolytic cell and passes through the gas-liquid separator flows into the anode solution storage tank; a pump then extracts the brackish water to remove chlorates and achieve dechlorination. The dechlorinated brine is sent to the denitrification unit, then to brine concentration; after concentration, it is returned as primary brine for reuse. The brine pumped out by the anolyte pump is sent to the anolyte acidification tank, where high-purity hydrochloric acid is added at the anolyte inlet and within the acidification tank; the pH is adjusted to around 2. The mixture is then sent to the chlorate decomposer, where steam is introduced to raise the temperature, and this temperature is controlled in order to increase the intensity of the reaction. The brine resulting from the decomposition of chlorates is sent to the anolyte storage tank, and then to the dechlorination tower for dechlorination. In this tower, the brine undergoes vacuum dechlorination followed by chemical dechlorination; after dechlorination, the brine is pumped by dechlorination pumps to remove nitrate and to be concentrated. The chlorine gas emerging from the dechlorination tower is sent to the main chlorine pipeline via a dechlorination condenser and a dechlorination vacuum pump; the vacuum pump maintains a vacuum level of -0.06 Mpa in the dechlorination tower. It is adjusted by the DCS. NaOH and Na2SO3 are added at the pump inlet for further chemical dechlorination; the pH of the dechlorinated brine is 9-10, with a low free chlorine content. The chlorine gas produced by the anode solution tank, acidification tank, and chlorate decomposer, along with the chlorine gas from the dechlorination vacuum pump, is combined in a main pipe and sent to the chlorine treatment process. Chlorinated water from the vacuum pump, dechlorination condenser, and chlorine seal tank, together with the brackish water from the electrolyzer, enters the anode solution storage tank for dechlorination. Chlorine removal process for chlorinated water in the preheater. Acid is added to the brackish water; in addition to the common-ion effect, which causes the chlorine dissolved in the brackish water to be removed, another effect of this is the decomposition of chlorates. The reaction principle is as follows: ClO3- + 6H+ → 3Cl2 + Cl- + 3H2O. The catholyte is circulated and removed from the system. Purchased ion-exchange membrane NaOH (at a concentration of 30–32%) is used as the base for starting up the process. The base solution circulates from the base storage tank through a pump into the catholyte tank, where it is then pumped into the catholyte cooler. The flow rate of the base solution is regulated by the DCS to ensure it reaches the electrolyzer at full capacity for electrolysis to take place. When a single cell is in use, the base solution flows from the catholyte cooler into the electrolyzer for electrolysis ； During electrolysis shutdown, it is also necessary to ensure the circulation of the alkaline solution; at this time, it is supplied from the alkaline solution reservoir to the electrolytic cell. During normal operation, the high-level tank also continuously receives fresh brine through the flow-limiting orifice plate, and the excess fluid overflows back to the brine storage tank via the overflow port. Pure water is added to the alkali outlet pipeline of the catholyte cooler in a controlled proportion, so as to dilute the alkali to (30±0.5%) before it flows back into the electrolyzer. The catholyte that enters the electrolyzer undergoes electrolysis, resulting in an increased concentration (31–32%); simultaneously, hydrogen is produced in the cathode chamber. The catholyte and hydrogen pass through flexible hoses in the cathode chamber to reach the main catholyte outlet pipe, from where they go into the catholyte gas-liquid separator. After separation, the catholyte flows into the catholyte tank (T-207) ； After being separated, the hydrogen enters the hydrogen main pipe, where it goes together with the hydrogen from the cathode liquid tank to the hydrogen treatment process. The alkali in the catholyte storage tank is pumped partly to the heat exchanger; after heat exchange and dilution with pure water, it enters the electrolyzer for reuse. Part of it is sent to the evaporation process for evaporation, while another part is sent to the secondary brine resin tower for regeneration, dechlorination, and denitration purposes.