The production of acids and bases using bipolar membranes enables clean production of organic acids and bases with zero emissions

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Research on a One-Step Process for Producing Acids and Bases from Salts Using Bipolar Membrane Electrodialysis – Guo Chunyu, Liu Fen, Zhang Lili (Beijing Tingrun Membrane Technology Development Co., Ltd.) Abstract: A three-compartment bipolar membrane electrodialysis (BMED) configuration was employed, with domestic bipolar membranes, homogeneous anion and cation exchange membranes arranged alternately. Through experimental tests on a small scale, the key performance parameters for process design—current density, current efficiency, and optimal feed and product concentrations—were determined. A preliminary design for a production facility capable of producing 10,000 tons (100% NaOH) per year using this one-step bipolar membrane electrodialysis process was developed, along with a preliminary economic analysis. Keywords: bipolar membrane ； electrodialysis ； sodium hydroxide ； Hydrochloric acid: One-step method for preparing acids and bases through NaCl splitting using bipolar membrane electrodialysis. GUO Chunyu, LIU Fen, ZHANG Lili (BEIJING TING RUN MEMBRANE TECHNOLOGY DEVELOPMENT CO.,LTD). Abstract: Domestic bipolar membranes, as well as homogeneous anion and cation exchange membranes arranged alternately, constitute the three compartments of a bipolar membrane electrodialysis (BMED) system. Through experimental tests, the main performance parameters such as feed and discharge current density, current efficiency, and optimal concentration were determined. A preliminary design was developed for a 10,000 tons/year (100% NaOH) bipolar membrane electrodialysis-based one-step acid and alkali salt production system, along with a preliminary economic analysis. Keywords: bipolar membrane; electrodialysis; sodium hydroxide; hydrochloric acid; One-step acid and alkali salt system. Introduction: In traditional processes for producing acids and bases from salts, electrolysis is commonly used, including diaphragm electrolysis, mercury electrolysis, and the most energy-efficient and environmentally friendly method, which is ion membrane electrolysis. The ion-exchange membrane electrolysis method involves using an anion-exchange membrane electrolyzer to electrolyze aqueous solutions of table salt or potassium chloride in order to produce chlorine, hydrogen, and high-purity caustic soda (sodium hydroxide) or potassium hydroxide. With the breakthroughs in high-performance bipolar membrane materials technology and the successful industrialization of such technologies, the one-step salt-based process for producing acids and alkalis using bipolar membrane electrodialysis has also entered the industrial stage, representing a revolutionary improvement over traditional salt-based methods for alkali production. A bipolar membrane (BPM for short) is a new type of composite ion exchange membrane, which is composed of a cation exchange layer, an anion exchange layer, and a hydrophilic interfacial layer in between. When a reverse voltage is applied across the BPM, charged ions migrate from the transition zone between the two ion exchange layers toward the host solutions on either side. The interface layer between the anode and cathode membranes develops a high electric potential gradient due to ion depletion, causing water molecules to dissociate into H+ and OH-. BMED is an efficient membrane reaction process developed on the basis of the unique hydrolysis function of BPM. Compared to traditional ion-exchange membrane electrolysis, it features ① lower operating energy consumption: under a current density of 4000 A/m2, the direct current energy consumption for producing one ton of caustic soda via ion-exchange membrane electrolysis is 2100–2200 kWh ； Under a current density of 200 A/m2, the DC power consumption per ton of caustic soda in the BPED salt-based acid-base method is 1500–2000 kWh ； ②Low equipment investment: Bipolar membranes are made from polyethylene and propylene as raw materials, rather than the expensive polytetrafluoroethylene material. In BPED membrane stacks, a pair of electrode plates is shared among multiple membrane layers, whereas in the ion-exchange membrane electrolysis method, each membrane layer requires its own pair of expensive electrode plates. For 10,000 tons (100% NaOH) per year, only the ion exchange membrane stack requires an investment of 5.5 million to 6 million yuan; in addition, related equipment for the recovery and evaporation of hydrogen chloride gas is needed, bringing the total cost to 15 million to 20 million yuan. For the same production capacity, BPED requires an investment of 12–14 million yuan in membrane stacks, with a total equipment investment of 14.5–15 million yuan. If it is high-concentration acids or bases, only evaporation equipment is required. ③Salt-based acids and bases can be produced in a single step; unlike the electrolytic method, it does not produce chlorine and hydrogen gases as by-products. At the same time, since BPED can be applied directly on-site, it reduces salt discharge issues. It also allows for the direct production of acids and bases, thereby minimizing the costs associated with the transportation of these substances, as well as the energy losses resulting from phase changes during their production, curing, and re-dissolution. Therefore, the one-step process of acid and base production using bipolar membrane electrodialysis offers advantages such as low energy consumption, minimal pollution, and low costs, and it holds significant practical value in a wide range of applications including the clean production of acids and bases, the separation of organic acids, and the removal of inorganic salts. This paper uses self-developed bipolar membranes and a self-designed BPED membrane stack to directly produce NaOH and HCl from the monovalent salt NaCl as the raw material. Through experimental scale-up tests, the key performance parameters for process design—current density, current efficiency, and optimal feed and product concentrations—were determined. A preliminary design was developed for a bipolar membrane electrodialysis one-step process for producing acids and bases at a capacity of 10,000 tons (100% NaOH) per year, along with a preliminary economic analysis. 1 Experimental Section 1.1 Materials, Instruments, and Reagents BPM-I type bipolar membrane, JAM-II type homogeneous cathode membrane, JCM-II type homogeneous anode membrane; the experimental membrane stack is TRPB3010 ； All the experimental machines TRPB3010-Ⅰ-3 were manufactured by Beijing Tingrun Membrane Technology Development Co., Ltd ； Anhydrous sodium chloride, anhydrous sodium sulfate: analytical grade (Tianjin Tianta Chemical Experiment Factory) ； Hydrochloric acid standard solution ； Pure water: (conductivity 3.4 µs×cm-1), prepared in the laboratory. file:///C:/Users/Guo Chunyu/AppData/Local/Temp/msohtmlclip1/01/clip_image002.gif1.2 BMED membrane stack configuration: As shown in Figure 1, the BMED device has a 3-compartment configuration, consisting of 2 bipolar membranes, 1 cation exchange membrane, and 1 anion exchange membrane arranged alternately. The bipolar membrane on the side close to the cathode and the anion membrane form the alkali chamber, while the bipolar membrane on the side close to the anode and the cathion membrane form the acid chamber; the middle chamber serves as the feed chamber. Under the influence of an electric field, hydrolysis occurs at the interface layers between the cathode and anode membranes of the bipolar membrane; the resulting H+ and OH- ions migrate toward the cathode and anode respectively. The cations and anions in the feed solution migrate in a directed manner, passing through the cathode membrane and the anode membrane respectively, where they combine with the H+ and OH- ions produced by the hydrolysis of the bipolar membrane to form acids and bases. In this way, the BMED device converts salt solutions into the corresponding acids and bases without introducing any new components. The anode chamber is connected to the cathode chamber, and the H+ and OH- ions produced by the dissociation of the bipolar membrane combine to form H2O, thereby preventing the formation of other harmful gases. 1.3 Experimental procedure file:///C:/Users/Guo Chunyu/AppData/Local/Temp/msohtmlclip1/01/clip_image004.gif The experimental procedure for BMED is shown in Figure 2. The raw material solution NaCl is placed in the salt chamber circulation tank 9; it is pumped into the salt chamber by a peristaltic pump and then flows back to the circulation tank for a closed-loop circulation ； The initial solution in both the acid chamber circulation tank 10 and the alkali chamber circulation tank 8 is deionized water, which enters each chamber under the action of peristaltic pumps and then undergoes closed-loop circulation ； The polar chamber uses a 2% sodium sulfate solution, which is passed sequentially into the anode and cathode chambers before undergoing a closed-loop circulation. The volume of the initial solution in each of the acid chamber, alkali chamber, salt chamber, and electrode chamber circulation tanks is 1 L, and the flow rate of each of the four water streams is 8 L×h-1. The membrane stack is powered by a DC voltage and current stabilizing power supply. 1.4 Analysis Methods and Data Processing: The conductivity and pH of the acid chamber, alkali chamber, and salt chamber are monitored respectively using an online conductivity meter and an online pH meter ； The concentration of the NaOH solution product is determined by titration with hydrochloric acid standard solution using phenolphthalein as an indicator. During the operation, 10 ml of NaOH is taken every 20 minutes for titration to determine its concentration; at the same time, the voltage and current values of the membrane stack at those moments, as well as the conductivity and pH values of the acid chamber, alkali chamber, and salt chamber, are recorded, along with the volume of solution in each circulation tank. Taking the yield of NaOH (R), current efficiency file:///C:/Users/Guo Chunyu/AppData/Local/Temp/msohtmlclip1/01/clip_image006.gif, and energy consumption (E) as evaluation indicators, the calculation methods are as follows: file:///C:/Users/Guo Chunyu/AppData/Local/Temp/msohtmlclip1/01/clip_image008.gif (1) file:///C:/Users/Guo Chunyu/AppData/Local/Temp/msohtmlclip1/01/clip_image010.gif (2) file:///C:/Users/Guo Chunyu/AppData/Local/Temp/msohtmlclip1/01/clip_image012.gif (3) In these equations, Ct represents the concentration of NaOH at time t, in mol×L-1 ； Vt is the volume of NaOH at time t, in liters ； file:///C:/Users/Guo Chunyu/AppData/Local/Temp/msohtmlclip1/01/clip_image014.gif represents the initial concentration of the NaCl solution in mol×L-1 ； V is the volume of the material chamber at the initial moment, L ； F is the Faraday constant, 96500 C×mol-1 ； n is the membrane logarithm (in this experiment, the membrane logarithm is 5) ； U is the membrane stack voltage V ； I is the membrane stack current A ； M is the molar mass of NaOH, 40 g×mol-1. 2 Results and Discussion 2.1 Effect of current density on the operating performance of the membrane stack file:///C:/Users/Guo Chunyu/AppData/Local/Temp/msohtmlclip1/01/clip_image016.gif file:///C:/Users/Guo Chunyu/AppData/Local/Temp/msohtmlclip1/01/clip_image018.gif Using NaCl with an initial concentration of 1.5 mol×L-1 as the feed solution, operation was carried out at constant current at current densities of 10, 20, 30, and 40 mA×cm-2. The membrane stack voltage, NaOH yield, current efficiency, and energy consumption for the BMED system obtained under these conditions are shown in Figure 3. file:///C:/Users/Guo Chunyu/AppData/Local/Temp/msohtmlclip1/01/clip_image021.gif file:///C:/Users/Guo Chunyu/AppData/Local/Temp/msohtmlclip1/01/clip_image022.gif Fig. 3 Effect of current densities on the operating performance of the stack file:///C:/Users/Guo Chunyu/AppData/Local/Temp/msohtmlclip1/01/clip_image023.gif 2.2 Effect of feed solution concentration on the operating performance of the stack Under a current density of 20 mA×cm⁻², NaCl feed solutions with concentrations of 0.7 mol×L⁻¹, 1.0 mol×L⁻¹, 1.5 mol×L⁻¹, and 2.0 mol×L⁻¹ were used respectively to investigate the effects of feed solution concentration on the stack voltage, NaOH yield, process current efficiency, and energy consumption. The experimental results are shown in Fig. 4. file:///C:/Users/Guo Chunyu/AppData/Local/Temp/msohtmlclip1/01/clip_image028.gif 3 Process Plan 3.1 Design Requirements The design is based on a production capacity of 10,000 tons (100% NaOH) per year ； The annual working hours are 6,600 hours ； Then a processing capacity of 1.25 t (100% NaOH)/h. The sodium chloride concentration in the feed water is 8%, while the concentration of NaOH in the output is 4% ； Hydrochloric acid is produced at a concentration of 4%. 3.2 Parameter selection and calculation: Flow rate of 4% NaOH produced: 1.25/4% = 31.25 (t/h); Flow rate of 4% HCl produced: 1.25/40 * 36.5/4% = 28.51 (t/h); Flow rate of 8% NaCl fed in: 1.25/40 * 58.5/8% = 22.85 (t/h). Based on the experimental data, the base flux is set at 0.5 kg (100% NaOH)/(m2·h), with a voltage of 2 V per pair and a current efficiency of 80% ； Therefore, the following parameters are calculated as follows: Membrane area: 1.25*1000/0.5=2500 m2. Number of membrane stacks (TRPB12060-500): 2500/250=10 units. 3.3 Equipment list: file:///C:/Users/Guo Chunyu/AppData/Local/Temp/msohtmlclip1/01/clip_image030.gif. 3.4 Total cost: Sequence number, Symbol, Name, Value, Unit: 1, i, Current density, 500 A·m2; 2, ξtot, Current efficiency (determined through testing), 0.7; 3, η, Pump efficiency, 0.7; 4, T, Temperature, 298 K; 5, F, Faraday constant, 96485 A·s·mol-1; 6, Vam, Cost of cation exchange membrane, 500 RMB per m2; 7, Vcm, Cost of anion exchange membrane, 600 RMB per m2; 8, Vbm, Cost of bipolar membrane, 2900 RMB per m2. Nam, Service life of cation exchange membrane: 5 years; Ncm, Service life of anion exchange membrane: 5 years; 9, NBm, Service life of bipolar membrane: 2 years; 10, No, Amortization period for peripheral equipment: 10 years; 11, Vp, Maintenance cost, 0.1, as a percentage of investment; 12, Ve, Energy consumption, 0.5 RMB per (kW·h); 13, Trun, Annual operating time, 6600 h per year; 14, a1, Cost of membrane stacks, 1.5 times the cost of membranes; 15, a2, Cost of peripheral equipment, 1.5 times the cost of membrane stacks. 1) Operating energy consumption: The installed power of each ED module is 228 kW, so the total operating power is 10×240=2280 kW. The energy consumption per ton of pure NaOH is 2280/1.25 t/h=1824 kWh per ton of alkali. The cost of electricity for producing one ton of alkali is 1824 kWh/ton × 0.6 RMB/kWh = 1094 RMB per ton of alkali. 2) Membrane replacement cost: The service life of the cation and anion membranes in homogeneous electrodialysis is assumed to be 5 years, with 20% of the membranes needing to be replaced each year on average. The lifespan of the bipolar membrane is assumed to be 2 years, with 50% replaced on average each year. The total cost of the membranes accounts for 60% of the cost of the membrane stack; therefore, the annual cost of replacing the homogeneous membranes is equal to (10 units × 1 million yuan/unit × 60% × 0.38) / 5 + (10 units × 1 million yuan/unit × 60% × 0.62) / 2 = 45.6 + 108 = 153.6 million yuan. The average cost per ton of caustic soda for membrane replacement is equal to 153.6 million yuan/year ÷ 6600 hours/year ÷ 1.25 tons/hour = 186.18 yuan per ton of caustic soda ； 3) Equipment depreciation: The lifespan of other ED equipment that does not include membranes is 10 years. The total investment in other equipment amounts to 13.379 million yuan – (10 units × 1 million yuan × 60%) = 7.379 million yuan. The average annual equipment depreciation is 7.379 / 10 = 737,900 yuan per year ； Average depreciation per ton of caustic soda = 737,900 yuan/year ÷ 6,600 hours/year ÷ 1.25 tons/hour = 89.44 yuan/ton of caustic soda. The total cost is: 1,094 + 186.18 + 89.44 = 1,369.62 yuan/ton of caustic soda. The amount of acid produced is calculated as 31%, i.e., 1.25/40 * 36.5/31% = 1.14; divided by 31% gives 3.68 tons/hour. If the average price per ton of acid is 100 yuan/ton of 31% HCl, then… ； The average price per ton of alkali is: 1369.62 yuan/ton – 100*3.68 yuan/ton = 1001.62 yuan per ton of 100% NaOH. The average price per ton of acid is: 300 yuan per ton of 31% HCl. 4 Conclusions: (1) By using a self-designed bipolar membrane electrodialysis (BMED) membrane stack, NaOH and HCl can be directly produced from NaCl as the raw material. At current densities of 10–40 mA×cm-2, for a NaCl solution with a concentration of 1.5 mol×L-1, the yield of NaOH can reach 80%, with energy consumption ranging from 1.5 kWh×kg-1 to 5.5 kWh×kg-1 ； (2) The higher the operating current density of BMED, the greater the yield of NaOH and the higher the energy consumption of the process ； For a constant operating current density, increasing the concentration of the feed solution can significantly reduce the resistance of the BMED membrane stack, thereby lowering the energy consumption of the process ； The BMED design was carried out with a production capacity of 10,000 tons (100% NaOH) per year, and the total investment cost was 15.65 million yuan ； Output: 4% alkali, 31.25 t/h ； Operating cost including depreciation: 1001.62 yuan/ton of 100% NaOH ； The flow rate of 4% HCl produced is 28.51 t/h, with operating costs including depreciation amounting to 100 yuan per ton of 31% HCl. References: Xu Tongwen, Wang Zhiwu. 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