| The continuous advances in technologies of SO2 and H2S removal have greatly reduced the emissions of the sulfur-containing pollutants in the coal utilization.However,requirements in sustainability and energy efficiency seek not only the removal technologies but the reutilization technologies of sulfur-containing pollutions.The concept of the H2S splitting cycle,which consists of the following reactions:H2S+H2SO4?2H2O+SO2+S,S+O2?SO2,SO2+I2+2H2O?2HI+H2SO4,and 2HI?H2+I2,potentially provides a large-scale hydrogen and sulfuric acid production route from the sulfur-containing pollutants,SO2 and H2S,in processes of using coal and other fossil fuels.Moreover,incorporated with nuclear or renewable energies,the hydrogen and sulfuric acid production from the sulfur-containing pollutants via the H2S splitting cycle can be carbon-neutral and more environmentally friendly.The H2S splitting cycle was derived from the sulfur-iodine?S-I?water splitting cycle for hydrogen production.The Bunsen reaction?SO2+I2+2H2O?2HI+H2SO4?is the center step of both the H2S splitting cycle and the S-I cycle.Great challenges including the excessive use of reactants,separation of the products?HI/H2SO4 mixed acid?,side reactions,iodine volatilization and deposition,and severe corrosion hazard exist when engineering the Bunsen reaction section in both cycles.Using iodine-toluene solution to provide fluid iodine can have the Bunsen reaction occur at ambient temperature and thus minimize most challenges such as side reactions,iodine vaporization,and corrosion hazard.Therefore,this thesis studies the remaining challenge,the separation of the Bunsen reaction products,i.e.,the mixed sulfuric acid and hydroiodic acid.Instead of the direct physical or chemical separations where many difficulties such as azeotrope must be overcome,this thesis investigates the electrolysis of the hydroiodic acid from the mixed acid solution in order to achieve hydrogen production and acid separation.After electrolysis,the remaining sulfuric acid is sent for upgrading and reuse?or sale?.The iodine is extracted into toluene for the Bunsen reaction.The direct electrolysis of Bunsen reaction products is promising to avoid the challenging separation of HI from H2SO4 and reduce the excessive use of reactants.As well,the purification,concentration,distillation and decomposition of HI which are complex and energy-intensive are avoided.The basic mechanism of electrode reactions,the mass transfer of iodide in the electrolyte,the feasibility and optimization of mixed acid electrolysis under the continuous flow mode have been investigated.The main discoveries and conclusions are as follows:?1?The electrode reactions and electrochemical mechanism of the Pt electrode in the direct electrolysis of Bunsen reaction products was characterized by using linear sweep voltammetry?LSV?and cyclic voltammetry?CV?in a batch,Nafion membrane separated electrolysis cell.The effects of the toluene added to the anolyte and stirring applied to the anolyte chamber on the anodic reaction were also investigated.In the 1.0 mol L-1 HI+0.5 mol L-1 H2SO4 mixed solution,the oxidation current begins to rise and the oxidation of I-to I2 occurs when the anodic potential reaches 0.35 V?against the saturated calomel electrode or SCE?.The produced iodine forms a film on the anode surface and increases the resistance,resulting a peak at?0.5 V?against SCE?and then a dramatical declines to low balanced value.When the potential reaches 1.7 V?against SCE?,the water electrolysis begins,the current raises again,and O2 is found to form on the anode.The oxidation of I-shows poor reversibility.The addition of toluene to the anolyte slows down the reaction rate.However,it can extract I2 from the aqueous phase,promotes the conversion of I-to I2,mitigates the accumulation of I2 on the anode surface.The stirring significantly increases the anodic reaction rate,enhances the diffusion of I-,and accelerates the consumption of I-.Stirring can also prevent the accumulation of I2 on the anode surface.In the 2 mol L-1 H2SO4 solution,both the reduction of H+to H2 and the electrolysis of water are irreversible.?2?In situ synchrotron X-ray radiography at the Canadian Light Source in conjunction with the electrochemical technique and the color photography has first been used to investigate the anodic reaction and the mass transfer in the electrolyte in the direct electrolysis of Bunsen reaction products,HI/H2SO4 mixed solution.The variation of gray values in radiographs represents the concentration change of the iodine species during the run of electrolysis,which enables to measure the iodine or iodide concentrations at any location in the solution relative to the anode and at any time during the electrolysis.In other words,the concentration profile at any location in the anolyte vs.time and that at any time vs.location relative to the anode can be drawn based on the gray value data.The results confirm that the iodine film is formed on the surface of the Pt anode and the I-concentration significantly decreases in the certain zone around the anode when the current reaches the oxidation peak at the cell voltage of 1.02 V.The semi-quantitative gray value analysis shows that this mass transfer limiting zone of I-is about 240?m around the anode.The images also show that the iodine film is“peeled”off from the anode surface and diffuses into the solution as the cell voltage is high enough to trigger water electrolysis to produce oxygen gas on anode.?3?The customer-designed electrolysis apparatus based on the zero-gap electrolysis cell was used for the direct membrane electrolysis of Bunsen reaction products,HI/H2SO4 mixed acid with or without toluene,under a continuous flow mode.The anolyte was 1 mol L-11 HI+0.5 mol L-11 H2SO4 mixed acid and the catholyte was 2 mol L-11 H2SO4 solution.The setup was tested for 6 hours.The cell voltage dramatically increases after 1.5 h and stabilizes at about 3.33 V.The cell voltage and the electrode reaction rate reach the steady state after 3 h operation.The results show that the hydrogen production rate is 35.34 Ncm3 min-11 and the current efficiency for hydrogen production is nearly 100%.The specific energy consumption is 7.89 kWh per Nm-3-H2 at steady state.The outlet concentration of I-in the anolyte is 0.56 mol L-11 after 6 hours,resulting in 44%conversion.The backward diffusion of I-from the anolyte to the catholyte through the proton exchange membrane exists.The addition of toluene into the anolyte?the volume ratio of toluene to solution is 1:3?decreases the outlet concentration of I-to 0.51mol L-1 and lessens the backward diffusion of I-.Three-time repeating experiments show that the relative standard deviation is lower than 14.31%,indicating that the experimental results are reproducible and the continuous-flow electrolysis apparatus is reliable.?4?In order to reduce the energy consumption,improve the reaction rate,decrease the I-outlet concentration in the anolyte,increase the I2 concentration in the toluene,and inhibit the backward diffusion of I-,the effects of the operating variables of the continuous-flow electrolysis of Bunsen reaction products were studied and the optimal operating conditions were determined.These operating variables included the current density,the volume ratio of the toluene to the solution in anolyte,the stirring speed in the anodic chamber,the inlet concentration of HI/H2SO4 in anolyte,the initial concentration of H2SO4 in catholyte,the flow rate of anolyte,and the circulation rate of catholyte on the electrolysis.High current density can improve the electrode reaction rate,promote the conversion of I-to I2 and mitigate the backward diffusion of I-.But further increasing the current density to 10 or 15 A dm-22 results in a significant increase in the cell voltage and energy consumption.Raising the volume ratio of toluene to solution in the anolyte enhances the conversion of I-and decreases the I-outlet concentration in the anolyte,but leads to an increase in the resistance of the electrolytic cell and thus the cell voltage.The lowest energy consumption and the highest I2 concentration in the toluene are obtained at the volume ratio of toluene to solution of 1:1.Increasing the stirring speed in the anodic chamber is in favor of decreasing the I-outlet concentration in the anolyte,increasing the I2concentration in the toluene,and inhibiting the backward diffusion of I-.The decrease in the anolyte concentration from 2 mol L-1 HI+1 mol L-1 H2SO4 to 1mol L-1 HI+0.5 mol L-1 H2SO4 leads to a decrease in the I-outlet concentration in the anolyte from 0.91 to 0.35 mol L-1 but an increase in the cell voltage from1.5 V to 3.4 V after 6 h electrolysis.Further decreasing the anolyte concentration to 0.5 mol L-1 HI+0.25 mol L-1 H2SO4 can decrease the I-outlet concentration in the anolyte to 0.08 mol L-1.However,its I2 concentration in the toluene is quite low and H+concentration in the catholyte declines markedly during the 6 h electrolysis.2 mol L-1of the H2SO4 concentration in the catholyte is suitable for the continuous-flow electrolysis.Increasing or decreasing the catholyte concentration leads to a significant increase in the cell voltage and energy consumption.The very high H+concentration in the catholyte of 4 mol L-1 H2SO4leads to the dehydration of the Nafion membrane and thus inhibits the transport of H+and I-.Lowering the flow rate of the anolyte is in favor of decreasing the I-outlet concentration in the anolyte,increasing the I2 concentration in the toluene,inhibiting the backward diffusion of I-,and improving the current efficiency of hydrogen production.Decreasing the flow rate of the catholyte to 12 mL min-1benefits inhibiting the backward diffusion of I-.For different flow rates of the catholyte,the lowest I-outlet concentration in the anolyte and the highest I2concentration in the toluene are obtained at 48 mL min-1.The optimal conditions for the continuous-flow electrolysis of Bunsen reaction products are identified as5 A dm-2 for the current density,1 mol L-1 HI+0.5 mol L-1 H2SO4 solution and toluene(Vtoluene:Vsolution=1:1)for the anolyte,700 rpm for the stirring speed in the anodic chamber,2 mol L-1 H2SO4 solution for the catholyte,4 mL min-1 for the flow rate of anolyte,and 48 mL min-1 for the flow rate of catholyte. |
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