Regeneration Of Amine Desulfurizing Agents By Using Electrodialysis With Bipolar Membranes

Wet absorption using amines is a predominant technique employed to desulfurize flue-gas and fuel-gas since it can regenerate amines and recover SO₂ by heating heat-unstable amine sulfites.Nonetheless,heat stable salts form and accumulate in the recirculation because of the oxidation of sulfur dioxide or amine sulfites during absorption and regeneration.These heat stable salts cannot be regenerated by heating and will lead to a loss of amines,a decrease in desulfurizing efficiency,and an increase in chemical cost.If discharged without treatment,these salts will give rise to a second pollution and waste of resources. On the other hand,heat stale salts,if accumulated in the desulfurizing solution, will result in an increase in fluid viscosity and then a chain reaction:bubbling, flooding,and even shutdown.Therefore,whether from the viewpoint of process cost or environmental protection,it is necessary to couple current desulfurizing processes with green chemical techniques and achieve an air pollution control without second pollution.Electrodialysis with bipolar membranes(EDBM)is such green chemical technique because these heat stale salts can be converted to amines and sulfuric acid in EDBM.If coupled with current desulfurizing processes in situ,EDBM can regenerate amines timely and ensure a long-term,efficient,stale desulfurization. In addition,second pollution is eliminated since EDBM adopts water and heat stable salts as reactants and the supporting electrolyte used in the EDBM stack can be circulated in the corresponding cycle.Therefore,the application of EDBM to regeneration of amine desulfurizing agents becomes the focus of this research.The results and conclusions are as follows.A highly-efficient amine flue-gas desulfurizing agent—piperazine is chosen to test the feasibility of applying EDBM to regeneration of amines since it forms piperazine sulfate—a heat stale salt—in the process of SO₂ absorption.The factors considered include electrolyte concentration,piperazine sulfate concentration,current density,and stack configuration.The results indicate that the low energy consumption and high current efficiency are achieved when applying electrolyte solutions of middle concentration(c₀(Na₂SO₄)=0.3～0.4 mol/L),piperazine sulfate solution of middle concentration (c₀(Pz.H₂SO₄)=0.08～0.13mol/L),and EDBM stack of BP-C-C(BP,bipolar membrane;C,cation-exchange membrane)configuration.The results also indicate that when applying a high current density to the EDBM stack,it has a high current efficiency and energy consumption.The process cost is estimated to be 0.96$/kg Pz for regeneration of piperazine with the laboratory-scale experimental equipment.The co-ion flux is considered as an unfavorable factor for high current efficiency.However,a certain amount of H⁺ flux can improve the current efficiency in the case of the EDBM stack of BP-C-A(A,anion-exchange membrane)configuration because it increases the amount of PzH_nⁿ⁺ions in the salt compartment without an increase in H⁺/PzH_nⁿ⁺ratio in the buffering range of piperazine sulfate.In that sense,the ion selectivity of anion-selective membrane does not need to be too high when applied to regenerate piperazine by using a EDBM stack of BP-C-A configuration.Under identical conditions,a systematic comparison is made on the regeneration of piperazine by using EDBM and conventional electrodialysis(ED): the latter is a competitive technique as compared with EDBM.ED has higher piperazine yield and current efficiency,and much lower voltage drop and energy consumption.However,its process cost is higher than that of EDBM due to an extra expenditure for the base and its tank and pumps.The process cost is estimated to be 0.96$/kg Pz for EDBM and 1.14$/kg Pz for ED.The feasibility is also tested on the application of EDBM to regeneration of three fuel-gas desulfurizing and decarbonizing agents:monoethanolamine(MEA), diethanolamine(DEA),and N,N'-dimethylethanolamine(DMEA),which still form heat-stable salts—alkanolamine sulfates.The effects of operation parameters (electrolyte concentration,alkanolamine sulfate concentration,and current density) on regeneration are analyzed on the basis of ion dimensions and intrinsic transport velocities,ion concentration,Donnan dialysis,ion orientation,and the interaction between alkanolamines and membranes.The process cost is estimated to be 0.48, 0.32,and 0.30$/kg for MEA,DEA,and DMEA,respectively.To push EDBM to industrialization,a win-win process coupling is investigated in EDBM to simultaneously regenerate piperazine and produce gluconic acid.The feasibility of such coupling is verified after investigating the operation parameters(such as feed concentration,current density,and operation mode).The process cost is estimated to be $0.80 kg^-1Pz and $0.17 kg^-1HGlu in the coupled operation,which are less than those in separate operations—$0.96 kg^-1Pz and $0.24 kg^-1HGlu.The purpose of this coupling is to achieve a win-win collaboration between enterprises via allocation of investment and economies of scale.As proven,EDBM is not only scientifically novel but also economically feasible and attractive besides its environmental benefits.In view of those properties characteristic of industrial ecology,EDBM will become one of the mainstays for the development of green and environmentally-friendly chemical industries.