Study On Circuit Characteristics And Thermal Effect Mechanism Of Bipolar Membrane Electrodialysis Device

In this thesis,the problem of membrane stack heating in the industrial application of bipolar membrane electrodialysis technology was investigated.The process variation,characteristics of membrane reactor circuit variation,thermal effectvariation and distribution characteristics of the 5-cell BP-A-C bipolar membrane electrodialysis desalination experiment were investigated.A circuit network diagram is established for the bipolar membrane electrodialysis device.The circuit network diagram is simplified according to the symmetrical circuit law,and a1×N-order resistor network diagram is obtained.According to the Multisim 14.0simulation circuit,the voltages at the two compartments of the main circuit of the membrane stack of the bipolar membrane electrodialysis system and the connections and channels of each compartment are obtained.Further calculation and derivation is carried out under the condition of known resistance of each compartment.The current and thermal effect values of the main circuit and each compartment connections and channels are obtained.（1）Treating simulates industrial wastewater by 5-unit BP-AC bipolar membrane electrodialysis with a constant current density of 30 m A/cm²,a membrane flow velocity of 5 cm/s in the acid-base salt compartment.Solution temperature increased from initial19.3°C to 26.6°C,and the removal rate of ammonium chloride in the salt compartment reached 91.2%after 350 minutes of operation,the current efficiency decreased from the initial 65.9%to the final 41.7%,the average energy consumption was 1.75 k Wh/kg;The acid room hydrochloric acid concentration reached 0.61 mol/L,and the current efficiency decreased from the initial 55.4%to the final 31.1%,the energy consumption is 3.13 k Wh/kg;The alkali room ammonia concentration reaches 0.68 mol/L,the current efficiency is reduced from the initial 45.1%to the final 33.4%,and the average energy consumption is 3.65 k Wh/kg.（2）The resistance of each part of the membrane acid-base compartment gradually decreases with time,and the resistance of each part of the salt compartment increases with time;the voltage shared by the acid-base compartment of the main circuit of the membrane stack gradually decreases,and the voltage shared by the salt compartment of the main circuit of the membrane stack gradually increases.The change trend of voltage characteristics at the connections and channels is the same as the change trend of the total resistance of the membrane stack and the total voltage of the membrane stack,and the overall decrease first and then increase.At the same time,the voltages of the connections and channels of the acid-base and salt compartments are equal,and the voltages of membrane units on both sides of the membrane stack are symmetrically distributed,the voltage values are equal in magnitude,and the direction is opposite,and the voltage value of each compartment in the intermediate membrane unit is zero.（3）At any time,the current in each compartment of each unit of the membrane main circuit is consistent with the input current at both ends of the membrane stack.The thermal effects of the main cells in the same compartment of the membrane stack are the same;the thermal effects of the acid and alkali compartments are gradually reduced with time while the salt compartment gradually increases with time;the thermal effect of the alkali compartment is much larger than that of the acid and salt compartments.In the acid,alkali and salt compartments,the leakage current and the thermal effect of the connections and channels are“symmetric”in the membrane stack,and the closer to the membrane unit on both sides of the membrane stack,the greater the current and thermal effects of the connections,the leakage current and thermal effect values of the intermediate membrane unit are almost negligible.The closer to the membrane unit on both sides of the membrane stack,the smaller the leakage current and thermal effect of the channels,the greater the leakage current and thermal effect in the middle part.The thermal effect of the main circuit of the membrane stack is much larger than that of the connections and channels,and he thermal effect of the connections is far greater than that of the channels.The thermal effect of the membrane stack is mainly generated in the main circuit and the connections of each compartment,and the local heating of the main circuit is not obvious due to the water circuit circulation.The heating problem of the membrane stack is mainly caused by the thermal effect of the connections.（4）The heat effect of the membrane stack is mainly affected by the leakage current of the connections in the compartment.Improve the partition structure and increase the the ratio of r₀/r₁ and r₂/r₁ reasonably,inject Air bubbles to the salt compartment and control the operating conditions as much as possible to avoid operation under hyperpolarized conditions can be useful to reduce the leakage current and thermal effect.