Separation of weak and strong acids by electro-electrodialysis—Experiment and theory

The experimental results of the separation of acetic acid (HA) from the sulfuric acid by the electro-electrodialysis (EED) method and the modeling of process have been presented. The Neosepta membranes CMX and ACM have been used. It has been found that the efficiency of retention of HA is high (>0.9) when the process is conducted below the limiting current density with respect to HSO₄⁻ or SO₄²⁻ anions. The observed current efficiency of the H₂SO₄ removal was rather low (CE_S = ca. 0.7, when the initial concentration of H₂SO₄ in the mixture was 1 or 2 M) which was caused by the nonideal selectivity of the anion-exchange membrane. The experimental results have been described by the model based on the extended Nernst-Planck equation and the Donnan equilibrium. Since the efficiency of the process depended mainly on the selectivity of anion-exchange membrane (ACM), the concentration of fixed charges of that membrane, , and the ratio of volume fraction of pores to their squared tortuosity, V_p/θ², have been chosen as the main fitting parameters. It has been found that the fitting of the EED data depends mainly on , whereas in the modeling of diffusion experiment (or an EED experiment conducted at low current density) both parameters are important. The best fit has been obtained for , i.e. ca. one order smaller than that determined experimentally. The obtained optimal value of V_p/θ², equal to 0.013, is consistent with those previously obtained for other Neosepta anion-exchange membrane.     

Concentration of HIx solution by electro-electrodialysis using Nafion 117 for thermochemical water-splitting IS process

An experimental study of applying electro-electrodialysis (EED) for improved HI concentration in the HIx solution, a mixture of HI–I₂–H₂O of approximately quasi-azeotropic compositions has been carried out in the conditions of around 90 °C and using Nafion 117 and graphite electrodes. A range of 25–80% increase in initial current efficiency of HI molality in catholyte is measured with the use of EED. In general, the efficiency increases with increasing iodine molality and weight ratio of anolyte solution to catholyte solution. The EED performance degrades in time. In some cases, the HI concentration limits are observed. Electric conductivity of the HIx solution, overvoltage of electrode reaction, and the membrane voltage drop is measured in a temperature range of 20–120 °C. It is found that the EED cell voltage, which is an important cell performance parameter, is governed by the membrane voltage drop.     

Evaluation on the electro-electrodialysis stacks for hydrogen iodide concentrating in iodine–sulphur cycle

In thermochemical water-splitting iodine–sulfur cycle, concentrating hydrogen iodine in the HI–H₂O–I₂ solution is crucial for the efficient hydrogen production. Electro-electrodialysis (EED) is a very promising HI concentrating method. In this paper, EED experiments were carried out using stacked cells, aiming at the scale up of EED equipment. Compared with the single-unit EED cell, the multi-cell EED stacks could concentrate HI in catholyte much more rapidly. During the EED process, the cell voltages increased gradually with the expansion of the concentration difference between catholyte and anolyte. For the stacks with more EED cells, the voltage increased much more steeply. High operating temperature ensured EED process carried out under low cell voltages and avoided voltage swelling. The apparent transport number (t₊) of all the experiments were very close to 1, while the ratio of permeated quantities of water to H⁺ (β) changed in a range of 1.79–3.05, influenced by temperature, I₂ content and current density.     

Effect of sulfuric acid on electro-electrodialysis of HIx solution

The effect of sulfuric acid on the concentration of HIx solution by electro-electrodialysis (EED) was examined for the thermochemical water-splitting iodine–sulfur process. Presence of sulfuric acid in the anolyte HIx solution did not affect the concentration behavior. However, sulfuric acid in the catholyte solution caused side reaction(s) producing whitish precipitates, which indicates that the sulfur compound should be removed prior to the EED operation.     

EIS studies on electro-electrodialysis cell for concentration of hydriodic acid

EIS studies were carried out on an electro-electrodialytic cell used for concentration of hydriodic acid using platinum electrodes and nafion117 membrane. Different impedance spectra were obtained where the concentration of iodine was varied while the concentration of HI was kept fixed at 55 wt%. Equivalent circuit model was used to simulate the experimental data and it was found that the impedance of the cell without membrane can be modeled using a single Warburg element along with ohmic resistance in series. This indicates presence of only diffusion transport resistance at the electrode and absence of any non-electroneutral layer. The impedance spectra for cell with membrane can be modeled using a Warburg element and a CPE with capacitive character along with ohmic resistance in series. This indicates formation of a non-electroneutral (heterogeneous transport) layer at the membrane in addition to a diffusion transport layer. It was found that the ohmic resistance increased with increase in the concentration of iodine while the impedances due Warburg and heterogeneous transport layer decreased with increase in iodine concentration.     

Electro-membrane reactor for separation and in situ ion substitution of glutamic acid from its sodium salt

An electro-membrane reactor with four compartments (EMR-4) (anolyte, catholyte and comp. 1 and 2) based on in-house-prepared cation- and anion-exchange membrane (CEM and AEM, respectively) was developed to achieve separation and recovery of glutamic acid (GAH) from its sodium salt by in situ ion substitution and acidification. The physicochemical and electrochemical properties of CEM and AEM were characterized and its suitability was assessed in operating environment. The separation of GA⁻ from the mixture of nonionic organic compounds and further ion substitution was achieved by EMR-4. But the higher energy consumption (5.75 kWh/kg of GAH produced), low current efficiency (50.5%) and recovery of GAH (57.2%) in this process were main obstacles for the industrial exploration of the process. Latter, electro-membrane reactor with three compartments (EMR-3) (anolyte, catholyte and central compartment) was developed based on CEMs for only in situ ion substitution of GANa to achieve GAH, in which GA⁻ was not allowed for electro-migration from its feed compartment. CE and recovery of GAH were close to 73% and 96% that indicate the suitability of the EMR-3 process for industrial application over the EMR-4. It was concluded that EMR-3 was efficient as compared to EMR-4 for separation and recovery of GAH from fermentation broth by in situ ion substitution in eco-friendly manner.     

HIx concentration by electro-electrodialysis using stacked cells for thermochemical water-splitting IS process

In the thermochemical water-splitting iodine–sulfur process for hydrogen production, efficient concentration/separation of HI from HIx solution, a mixture of HI–H₂O–I₂, is very important. In this paper, an experimental study on concentrating HI in HIx using stacked electro-electrodialysis (EED) cells was carried out under the conditions of 1atm, 80 °C and the current density of 0.10 A/cm². The performance of EED stacks including 1, 2 and 4 EED units was evaluated. The results showed that multi-unit EED cells could concentrate HI in catholyte much faster than single-unit cells. The apparent transport number (t₊) of all the experiments were very close to 1, while the ratio of permeated quantities of water to H⁺ (β) changed in a relatively larger range of 1.98–2.89. Although the current efficiency will degrade faster when using a multi-unit stack than a single-unit cell at the late stage of EED process, at high iodine content multi-unit stack could maintain quite high current efficiency.     

Energy consumption in iodine section of I–S: Effect of operating parameters of electro-electrodialysis on overall efficiency of cycle

Reducing heat demand for increasing concentration of HI in the HI_x solution of the iodine circuit of the Iodine–Sulphur cycle is considered the most effective way of increasing efficiency of the cycle. Electro-electrodialysis has emerged as an energy efficient way of increasing the HI_x concentration above azeotropic value. Simulation of the iodine circuit consisting of an EED, a flash and a decomposer was carried out in Aspen Plus™ simulation platform to study the effect of EED current density and outlet HI concentration on the efficiency of the cycle. Efficiency reduced strongly with increase in current density. For EED current density of 5 A/dm², maximum efficiency was ∼35.9% and the optimal range of EED catholyte's exit HI concentration, iodine-free, mole fraction was 0.19–0.21. Simulation results showed that reducing EED resistance was most effective, among all EED parameters, in increasing the cycle's thermal efficiency and if the EED resistance is completely eliminated the thermal efficiency value would increase to 39.4%.     

A chronopotentiometry based identification of time-varying different transport resistances of electro-electrodialysis cell used for concentration of HIx solution

A simple technique based on the use of under-limiting current chronopotentiometry for determining different transport resistances (and associated energy consumption) of an Electro-electrodialysis (EED) cell has been demonstrated. The technique employed chronopotentiometry studies at different current densities varying from 10 to 60 mA/cm². All the applied current densities were in the underlimiting current region. In the studies, we have identified four different energy consumption modes in EED cell; the ohmic, the mass transfer, the electrodes reactions and open circuit voltage (OCV) which were calculated individually based on the potential drop contribution by each component. The potential drop and corresponding energy consumption, corresponding to the electrode reaction was found to be negligibly low as compared to the other potential drop components. Studies on the variation of different transport resistances with operation time of the EED cell showed that first three energy consumption mechanisms change little with change in HI_x concentration while the OCV increases significantly and becomes the major energy consumption mode especially at low current densities for the desired EED exit HI_x concentration.     

Effect of asymmetric variation of operating parameters on EED cell for HI concentration in I–S cycle for hydrogen production

EED process for HI concentration was studied for the effect of individual operating parameters such as I₂/HI ratio, concentration of HI(xHI_/H₂O)$\text{HI}\left(x_{\text{HI}/{\text{H}}_2\text{O}}\right)$, temperature and pressure. Studies were conducted in an asymmetric system where the effects of operating parameters were varied for anolyte and the catholyte separately. Open circuit voltage (OCV) was found to be a contributor toward the net potential drop across the EED cell. Ohmic resistance was found to decrease with increase in I₂/HI ratio in catholyte and was found to increase with increase in I₂/HI ratio in anolyte. Increase in xHI_/H₂O$x_{\text{HI}/{\text{H}}_2\text{O}}$ decreased the resistance for anolyte section whereas caused an increase in resistance for catholyte section. Increase in temperature reduced the voltage drop and the resistance across the EED cell. A non-zero differential pressure between the two compartments of the cell increased the resistance across the cell without affecting the OCV value. Electrode potential studies at the graphite electrodes showed an increase in the electro potential with increase in the iodine concentration and decrease with the increase in the HI concentration. Energy required for concentrating acid increased linearly with current density favoring operation at low current densities. Energy consumed in overcoming OCV contributed substantial fraction of the total energy consumed in EED process at lower current densities.