Evaluation of recovery characteristic of acidic and alkaline solutions from NaNO3 using conventional electrodialysis and electrodialysis with bipolar membranes

We compared the electrodialysis performance for HNO₃ and NaOH recovery from NaNO₃ solution by conventional electrodialysis (ED) and electrodialysis with bipolar membranes (EDBM) at constant current and constant voltage. The individual resistances of the components of the electrodialysis systems were also evaluated. The electrodialysis extent for HNO₃ and NaOH recovery from NaNO₃ solution was almost proportional to the total amount of electricity supplied to the system, regardless of the operation mode and the electrodialysis systems. For the same volume of feed solution, the energy consumption and current efficiency differed depending on the operation mode and the electrodialysis system. In both the ED and EDBM systems, the conductivity of the feed solution strongly affected the overall cell resistance after approximately 50% of the ions in the feed solution had migrated.     

Design and study of a novel ammonia-based CO2 capture process with bipolar membrane electrodialysis

Aqueous ammonia is a promising absorbent in the field of post combustion CO₂ capture. However, the high volatilization of NH₃ results in a high energy requirement, as well as solid precipitation during the CO₂ regeneration process. A novel process was designed to reduce energy consumption and solve the problem. The bipolar membrane electrodialysis (EDBM) unit and CO₂ regeneration reactor were taken as the regeneration part. In the novel process, the bubble in the EDBM unit would be eliminated, and the regeneration of CO₂ and aqueous ammonia would be operated separately, which significantly reduced energy consumption and avoided the risk of precipitation during regeneration. According to the simulation and calculation results, the CO₂ regeneration energy consumption of the novel process using H₂SO₄ for CO₂ regeneration is 39.0% lower than that of the conventional ammonia-based process, which shows good energy saving potential. Moreover, the novel process will be more competitive as membrane technology develops.     

Application of bipolar electrodialysis to the recovery of acids and bases from water solutions

The studies of acid purification and base purification via bipolar electrodialysis are presented. The process of acid recovery involved spent solutions of acid (HCl or H₂SO₄) and iron salt obtained in the course of conventional electrodialysis. Bipolar electrodialysis yielded an acid solution concentrated 51-fold (in the case of hydrochloric acid) and 63-fold (in the case of sulphuric acid) as compared to the feeding solution for conventional electrodialysis. The solution of the recovered acid was only slightly contaminated with iron salt (0.12 and 0.13%). The process of base purification yielded an NaOH solution concentrated nine-fold as compared to the feeding solution for the bipolar electrodialysis process. The contamination of the recovered base varied from 1.75 to 2.50%.     

Entwicklung und Erprobung eines Wäschersystems zur CO2‐Abreinigung aus Rauchgasen mithilfe alkalischer Reststoffe

An innovative, technical approach for the reduction of CO₂ emissions is presented that utilizes alkaline wastes to capture CO₂ from flue gases in stable mineral form. Comprehensive pilot‐scale experiments were conducted with the developed flue gas scrubbing system at a power plant site. By optimizing the process parameters gas flux, CO₂ partial pressure, circulation flux and suspension liquid‐to‐solid ratio, a CO₂ binding of 40 – 90 g kg^–1 waste could be reached and up to 25 % of the CO₂ could be captured. The new technique is economically advantageous especially when both alkaline waste and CO₂ are produced on site and when the carbonated products can be used as secondary resources.     

Structural effects of ion‐exchange membrane on the separation of L‐phenylalanine (L‐Phe) from fermentation broth using electrodialysis

The effects of membrane structure on the separation of L ‐phenylalanine (L ‐Phe) by electrodialysis from a fermentation broth and on the fouling tendency were investigated in this study. Two anion‐exchange membranes (Neosepta AFX and AM‐1, Tokuyama, Japan) were selected and characterized using the chronopotentiometry method. For a fresh membrane, AFX showed a lower electrical resistance and a lower permselectivity than AM‐1. After being fouled with humic acid, however, the electrical resistance of AFX was higher than that of AM‐1. The L ‐Phe selectivities for both membranes were lower than those of the fresh membranes. The result may be attributed to the structural difference between AFX and AM‐1 membranes. AFX has a lower repulsion force against the co‐ion and could be more strongly affected by the foulants than AM‐1 because AFX has a more porous structure than AM‐1. Experiments on the separation of L ‐Phe from the fermentation broth were carried out using two different stack configurations, ie desalting electrodialysis and water‐splitting electrodialysis. It was observed that the recovery efficiency of L ‐Phe through electrodialysis for 100 min reached 95% for AFX and 85% for AM‐1. In the desalting configuration of electrodialysis, the solution pH must be adjusted to alkaline conditions to recover the L ‐Phe through the anion‐exchange membrane. On the contrary, it was possible to recover the L ‐Phe without adjustment of the solution pH in the water‐splitting electrodialysis because OH^? generated from the bipolar membrane converted neutral L ‐Phe into an anion. © 2002 Society of Chemical Industry     

Modelling the transport of carbonic acid anions through anion-exchange membranes

Electrodiffusion of carbonate and bicarbonate anions through anion-exchange membranes (AEM) is described on the basis of the Nernst-Planck equations taking into account coupled hydrolysis reactions in the external diffusion boundary layers (DBLs) and internal pore solution. The model supposes local electroneutrality as well as chemical and thermodynamic equilibrium. The transport is considered in three layers being an anion exchange membrane and two adjoining diffusion layers. A mechanism of competitive transport of HCO₃⁻ and CO₃²⁻ anions through the membrane which takes into account Donnan exclusion of H⁺ ions is proposed. It is predicted that the pH of the depleting solution decreases and that of the concentrating solution increases during electrodialysis (ED). Eventual deviations from local electroneutrality and local chemical equilibrium are discussed.     

One-step conversion of sulfate lithium into high-purity lithium hydroxide crystals via bipolar membrane crystallization

To date, bipolar membrane electrodialysis (BMED) is being developed as a competitive technology for waste lithium-ion battery recovery. However, the purity and concentration of lithium hydroxide generated from a BMED plant could not meet the product criteria for ternary lithium batteries, thus requiring additional condensation, purification, evaporation, and crystallization procedures. Herein, bipolar membrane crystallization (BMC) was proposed for the one-step conversion of sulfate lithium into high-purity lithium hydroxide monohydrate crystals. By mediating a continuous saturated feedstock in the salt compartment, it is possible to convert Li₂SO₄ into 5+ mol/L LiOH at a current density higher than 500 A/m². Therefore, this unique design allows the production of 99.9% LiOH∙H₂O by taking the principle of water dissociation in the bipolar membrane and the simultaneous crystallization procedure. This proof-of-concept study proves the feasibility and competitiveness of the BMC for waste lithium recovery by abandoning the condensation and evaporation procedures.     

Separation of CO2 from flue gas using electrochemical cells

Past research with high temperature molten carbonate electrochemical cells has shown that carbon dioxide can be separated from flue gas streams produced by pulverized coal combustion for power generation. However, the presence of trace contaminants, i.e., sulfur dioxide and nitric oxides, will impact the electrolyte within the cell. If a lower temperature cell could be devised that would utilize the benefits of commercially-available, upstream desulfurization and denitrification in the power plant, then this CO₂ separation technique can approach more viability in the carbon sequestration area. Recent work has led to the assembly and successful operation of a low temperature electrochemical cell. In the proof-of-concept testing with this cell, an anion exchange membrane was sandwiched between gas-diffusion electrodes consisting of nickel-based anode electrocatalysts on carbon paper. When a potential was applied across the cell and a mixture of oxygen and carbon dioxide was flowed over the wetted electrolyte on the cathode side, a stream of CO₂ to O₂ was produced on the anode side, suggesting that carbonate/bicarbonate ions are the CO₂ carrier in the membrane. Since a mixture of CO₂ and O₂ is produced, the possibility exists to use this stream in oxy-firing of additional fuel.From this research, a novel concept for efficiently producing a carbon dioxide rich effluent from combustion of a fossil fuel was proposed. Carbon dioxide and oxygen are captured from the flue gas of a fossil-fuel combustor by one or more electrochemical cells or cell stacks. The separated stream is then transferred to an oxy-fired combustor which uses the gas stream for ancillary combustion, ultimately resulting in an effluent rich in carbon dioxide. A portion of the resulting flow produced by the oxy-fired combustor may be continuously recycled back into the oxy-fired combustor for temperature control and an optimal carbon dioxide rich effluent.     

Carbon Dioxide Desorption from Saturated Organic Solvents

Carbon dioxide removal from various gaseous streams and its subsequent recovery are of considerable industrial importance. The absorption technologies represent the most important operation to separate CO₂. In this work, the mass transfer rates during CO₂ absorption and desorption from propylene carbonate dimethyl ether of polyethylene glycol, and N‐methyl‐2‐pyrrolidone solutions were measured over the temperature range of 293.15–323.15 K in a baffled agitated reactor with a flat gas‐liquid interface operated in a batchwise manner. Two distinct mechanisms of desorption were observed: bubbling and diffusive desorption. The volumetric gas‐liquid mass transfer coefficients for the bubbling desorption were determined from the measured overall and diffusive desorption rates, and were correlated by the semi‐empirical equation. The proposed correlation is a power relationship of supersaturation, Reynolds and Weber numbers, and was found to quantitatively explain the observed phenomena with satisfactory agreement.     

Adsorptive cyclic purification process for CO2 mixtures captured from coal power plants

CO₂ capture technology combined with bulk separation and purification processes has become an attractive alternative to reduce capture costs. Furthermore, the required purity in the application for CO₂ conversion and utilization is more stringent than that required from a captured CO₂ mixture for geological storage. In this study, an adsorptive cyclic purification process was developed to upgrade a CO₂/N₂ mixture captured from greenhouse gas emission plants as a feasibility study for a second capture unit or captured CO₂ purifier. To purify 90% CO₂ with balance N₂ as a captured gas mixture, two‐bed pressure swing adsorption and pressure vacuum swing adsorption (PVSA) processes using activated carbon were experimentally and theoretically studied at adsorption pressures of 250–650 kPa and a fixed vacuum pressure of 50 kPa. CO₂ with higher than 95% purity was produced with more than 89% recovery. However, a four‐bed PVSA process could successfully produce CO₂ with greater than 98% purity and 90% recovery. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1051–1063, 2017     

Cycle synthesis and optimization of a VSA process for postcombustion CO2 capture

A systematic analysis of several vacuum swing adsorption (VSA) cycles with Zeochem zeolite 13X as the adsorbent to capture CO₂ from dry, flue gas containing 15% CO₂ in N₂ is reported. Full optimization of the analyzed VSA cycles using genetic algorithm has been performed to obtain purity‐recovery and energy‐productivity Pareto fronts. These cycles are assessed for their ability to produce high‐purity CO₂ at high recovery. Configurations satisfying 90% purity‐recovery constraints are ranked according to their energy‐productivity Pareto fronts. It is shown that a 4‐step VSA cycle with light product pressurization gives the minimum energy penalty of 131 kWh/tonne CO₂ captured at a productivity of 0.57 mol CO₂/m³ adsorbent/s. The minimum energy consumption required to achieve 95 and 97% purities, both at 90% recoveries, are 154 and 186 kWh/tonne CO₂ captured, respectively. For the proposed cycle, it is shown that significant increase in productivity can be achieved with a marginal increase in energy consumption. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4735–4748, 2013     

Technico‐economical assessment of MFI‐type zeolite membranes for CO2 capture from postcombustion flue gases

A detailed survey of the effect of moisture on the CO₂/N₂ permeation and separation performance of Mobile Five (MFI) zeolite membranes in view of downstream postcombustion CO₂ capture applications in power plants and incinerators is presented. The membranes, displaying a nanocomposite architecture, have been prepared on α‐alumina tubes by pore‐plugging hydrothermal synthesis at 443 K for 89 h using a precursor clear solution with molar composition 1 SiO₂:0.45 tetrapropylammonium hydroxide:27.8 H₂O. The synthesized membranes present reasonable permeation and CO₂/N₂ separation properties even in the presence of high water concentrations in the gas stream. A critical discussion is also provided on the technico‐economical feasibility (i.e., CO₂ recovery, CO₂ purity in the permeate, module volume, and energy consumption) of a membrane cascade unit for CO₂ capture and liquefaction/supercritical storage from standard flue gases emitted from an incinerator. Our results suggest that the permeate pressure should be kept under primary vacuum to promote the CO₂ driving force within the membrane. © 2011 American Institute of Chemical Engineers AIChE J, 58: 3183–3194, 2012     

Utilizing Formate as an Energy Carrier by Coupling CO2 Electrolysis with Fuel Cell Devices

Electrochemical reduction of CO₂ to useful chemicals can change the role of CO₂ from harmful waste to a valuable feedstock. Despite a lot of progress in the alkaline electrochemical conversion of CO₂ to formate, there is still a lack of potential applications for the generated aqueous formate solution. Here, the general ability of formate to be used as an energy or hydrogen carrier is discussed and compared to well‐known energy storage chemicals. Concepts to employ formate solution as an energy carrier by combining CO₂ electrolysis with the reconversion of formate into electricity via a direct formate fuel cell or catalytic decomposition to H₂ combined with a proton exchange membrane fuel cell are demonstrated.     

CO2 removal by single and mixed amines in a hollow‐fiber membrane module—investigation of contactor performance

This work investigates CO₂ removal by single and blended amines in a hollow‐fiber membrane contactor (HFMC) under gas‐filled and partially liquid‐filled membrane pores conditions via a two‐scale, nonisothermal, steady‐state model accounting for CO₂ diffusion in gas‐filled pores, CO₂ and amines diffusion/reaction within liquid‐filled pores and CO₂ and amines diffusion/reaction in liquid boundary layer. Model predictions were compared with CO₂ absorption data under various experimental conditions. The model was used to analyze the effects of liquid and gas velocity, CO₂ partial pressure, single (primary, secondary, tertiary, and sterically hindered alkanolamines) and mixed amines solution type, membrane wetting, and cocurrent/countercurrent flow orientation on the HFMC performance. An insignificant difference between the absorption in cocurrent and countercurrent flow was observed in this study. The membrane wetting decreases significantly the performance of hollow‐fiber membrane module. The nonisothermal simulations reveal that the hollow‐fiber membrane module operation can be considered as nearly isothermal. © 2014 American Institute of Chemical Engineers AIChE J, 61: 955–971, 2015     

Influence of Alkaline Additives and Buffers on Mineral Trapping of CO2 under Mild Conditions

Carbon dioxide (CO₂) gas is the main contributor to climate change. CO₂ storage in underground brines and oil‐field brines by mineral trapping has been considered as a promising alternative in order to reduce CO₂ emissions. However, permanent storage of CO₂ in stable carbonate minerals is greatly dependent on brine pH, being favored over an alkaline pH. The effect of alkaline additives (NaOH, KOH, CaO) and buffer solutions (NaHCO₃/NaOH, Na₂HPO₄/NaOH, NH₄Cl/NH₄OH) on the mineral trapping of CO₂ under mild conditions using a synthetic brine is investigated. The results indicate that both NaOH+NH₄Cl/NH₄OH and KOH+NH₄Cl/NH₄OH mixtures promote precipitation mainly of calcium carbonate (CaCO₃).     

Anodic Oxidation of Polyhydric Alcohols on a Pt Electrode in Alkaline Solution

The electrochemical oxidation of various polyhydric alcohols, ethylene glycol, glycerol, meso-erythoritol, and xylitol, on a platinum electrode was investigated systemematically in acidic H₂SO₄, and in alkaline KOH and K₂CO₃ solutions to evaluate the potential of these polyhydric alcohols as fuels in micro-fuel cells for portable electronic devices. All polyhydric alcohols tested in the present study showed high reactivities in both alkaline solutions. Ethylene glycol showed the highest reactivity. Although the reactivity of ethylene glycol was lower in K₂CO₃ than in KOH, the carbonate solution is a potential candidate as an electrolyte solution due to its resistance to solution carbonation. Furthermore, ethylene glycol showed much less significant electrode poisoning by adsorbed CO upon oxidation in alkaline solution.     

液相共存成分对双极膜电解吸氨法脱碳富液的影响

考察了烟气中SO₂、NO_x和CO₂与NH₃·H₂O的反应机理以及双极膜电解吸的机理,对本文自制实验系统的操作工况进行了进一步的调整,探究了氨法脱碳吸收富液双极膜电解吸的一般规律,研究了模拟解吸液解吸过程中主要液相共存成分,如(NH₄)₂SO₃、(NH₄)₂SO₄、NH₄NO₃、NaCl和NH₄Cl对CO₂双极膜电解吸的影响。研究表明,液相共存成分种类、质量分数、pH、表面张力等对CO₂的双极膜电解吸均能产生一定影响。少量NaCl、NH₄Cl和(NH₄ )₂SO₄成分的共存对吸收富液的电解吸是有利的,且对电解吸的影响程度为NaCl> NH₄Cl> (NH₄)₂SO₄;吸收富液中应该极力避免(NH₄)₂SO₃成分的存在;NH₄NO₃成分的共存对脱碳富液电解吸的影响不大。因此,在碳捕集前应对烟气中的杂质成分进行脱除,减少其对解吸液理化特性以及双极膜电解吸的影响。     

Interaction of ammonium bicarbonate with lithium chloride solutions

The possibility of lithium carbonate precipitation in the interaction of a lithium chloride solution with ammonium bicarbonate is studied. It is shown that the theoretically possible degree of Li₂CO₃ precipitation cannot exceed approximately 80% and is determined by the solubility of the forming lithium carbonate in the NH₄Cl solution. In a real system, the degree of Li₂CO₃ precipitation is virtually temperature-independent within the range 20–90°C and does not exceed 70%. A closed process for mother liquor recovery at a significantly higher degree of Li₂CO₃ precipitation from the mother liquor is proposed.     

Facilitated transport of carbon dioxide through an immobilized liquid membrane of K2CO3/KHCO3 aqueous solution

The facilitated transport of CO₂ through a hydrophilic polymeric membrane immobilized with K₂CO₃/ KHCO₃ buffer solution has been investigated. The reactions of dissolved CO₂ in electrolyzed alkaline solution must consider hydration of CO₂ with water, chemical reaction of CO₂ with OH^- and dissociation of HCO ₃ ^2- into CO ₃ ^2- . It is necessary to simplify these reactions as a simple model, which is used to analyze the transport system. From experiments in the liquid membrane with alkaline buffer solution, it is shown that the flux of CO₂ into K₂CO₃KHCO₃ aqueous solution can be enhanced by the presence of CO ₃ ^2- . A diffusion model with an overall reaction based on the film theory is proposed that predicts the experimentally observed facilitation factor with reasonable accuracy. The present model is compared with the rigorous diffusion model involving the complicated conventional chemical reactions.     

Partial oxidation of methane in hollow‐fiber membrane reactors based on alkaline‐earth metal‐free CO2‐tolerant oxide

The U‐shaped alkaline‐earth metal‐free CO₂‐stable oxide hollow‐fiber membranes based on (Pr_0.9La_0.1)₂(Ni_0.74Cu_0.21Ga_0.05)O_4+δ (PLNCG) are prepared by a phase‐inversion spinning process and applied successfully in the partial oxidation of methane (POM) to syngas. The effects of temperature, CH₄ concentration and flow rate of the feed air on CH₄ conversion, CO selectivity, H₂/CO ratio, and oxygen permeation flux through the PLNCG hollow‐fiber membrane are investigated in detail. The oxygen permeation flux arrives at approximately 10.5 mL/min cm² and the CO selectivity is higher than 99.5% with a CH₄ conversion of 97.0% and a H₂/CO ratio of 1.8 during 140 h steady operation. The spent hollow‐fiber membrane still maintains a dense microstructure and the Ruddlesden‐Popper K₂NiF₄‐type structure, which indicates that the U‐shaped alkaline‐earth metal‐free CO₂‐tolerant PLNCG hollow‐fiber membrane reactor can be steadily operated for POM to syngas with good performance. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3587–3595, 2014