Study On The Properties Of Carbonate Buffer Solution And The Effect In Tannin Extract Desulfurization Technology

Tannin extract desulfurization technology (TEDT) belongs to the wet desulfurization technology, which was used to remove H2S. TEDT is currently one of the most widely used desulfurization method in coal gas purification in China. For the desulfurization process, pH value of the desulfurization solution is the most important factor. In tannin extract desulfurization process, carbonate buffer solution was used to absorb H2S gas and stabilize pH value of desulfurization liquid. The properties of carbonate buffer solution and the effect on tannin extract desulfurization were studied in this paper. The main conclusions were as follows.The reaction of NaHS and NaVO3 was researched by 314A-type sulfide ion selective electrode, the effect of pH value and total alkalinity of the Na2CO3-NaHCO3 buffer solution on the reaction was studied. The results were as following:(1) The reaction of NaHS and NaVO3 was related to pH value of buffer solution. The conversion and reaction rate of HS- decrease with increase of pH value of the buffer solution. In the experimental condition, the pH value of buffer solution should not be more than 9.40.(2)In the experimental conditions, the total alkalinity of buffer solution had little influence on the reaction of NaHS and NaVO3. But when total alkalinity of the buffer solution was too small, adding Na2S to HS- reserve solution will lead to the change of the pH value.The effect of Na2CO3 and NaHCO3 content changes on pH value in the Na2CO3-NaHCO3 buffer solution was researched by PHS-3C-type precision pH meter. The results were as follows:(1) When the concentration of Na2CO3 increase or the concentration of NaHCO3 decrease, the pH value of the buffer solution increase gradually. After the concentration of NaHCO3 is more than 10.0 g/L, pH value of the buffer solution become stability. In order to meet the requirement that the pH value of desulphurization solution is not more than 9.50 in the desulfurization process, the largest concentration of Na2CO3 shall not be more than 10.0 g/L. Therefore, the Na2CO3-NaHCO3 buffer solutions which meet the requirement of the tannin extract desulfurization technology were that the concentration of Na2CO3 was 1.0-10.0g/L and the concentration of NaHCO3 was 10.0-30.0g/L(2) The buffer capacity of the buffer solution enhance with increase of the concentration of Na2CO3 and NaHCO3. In order to make the carbonate buffer had better buffer capacity and the pH value stabilized at around 9.00, the best concentration of buffer solution could be selected:the concentration of Na2CO3 was 5.0g/L, the concentration of NaHCO3 was 30.0g/L, the mass ratio of Na2CO3 to NaHCO3 was 1:6.(3) Based on formula in the existed literature and experimental data in the paper, a new calculation formula of pH value of Na2CO3-NaHCO3 buffer solution was given. The formula showed good linear relationship. The formula is as follows:pH=1g (cNa2CO3/cNaHCO3)-0.1136Ln(mNa2CO3)+10.143Effect on pH value of buffer solution with different pH value and total alkalinity of hydrogen sulfide absorption was studied. The results were as follows:(1) When the concentration of Na2CO3 in buffer solution is low (cNa2CO3≤3.0g/L), pH value of solution decrease rapidly with the increase of hydrogen sulfide absorption time. Then the buffer capacity of buffer solution is small, can't maintain the stability of pH value.(2) When the initial pH value of buffer solution is fixed, with the increase of hydrogen sulfide absorption. time, the more concentration of Na2CO3, the slower pH value decrease. When the concentration of Na2CO3 maintained at 4.0g/L～6.0g/L, the buffer solution can fully absorb hydrogen sulfide, and the pH value can stably maintain stability the range of technical requirements.(3) The most appropriate initial pH value of buffer solution was between 9.00 and 9.20. For the pH value, the best initial concentration of Na2CO3 was selected as 4.0g/L～6.0g/L, then a stable and suitable pH value can be maintained in the hydrogen sulfide absorption process. It will be good for the reaction of HS- and V(V).The influences of carbon dioxide on pH value of buffer solution and by-products on pH value of desulfurization solution were studied in this paper, the results were as follows:(1) Carbon dioxide had great effect on pH value of buffer solution. The results show that the pH value decreased rapidly with carbon dioxide absorption process. After absorbing carbon dioxide between 20 and 30 minutes, the pH value of solution drops below 7.50, the solution has lost buffer capacity, and the pH value is too low to meet the requirements of hydrogen sulfide absorption.(2) This experiment also further verified the experimental results of the previous chapter, when the concentration of Na2CO3 maintained at 4.0g/L-6.0g/L, the buffer solution can fully absorb hydrogen sulfide, and have bigger buffer capacity. After absorbing carbon dioxide, solution have longer buffer period.(3) The influences of carbon dioxide on pH value of different buffer solutions were studied by experiment, these buffer solutions have the same concentration of Na2CO3 but different initial pH value includes 8.60,8.80,9.00, 9.20,9.50. The results show that with the absorption of carbon dioxide, the higher initial pH value is, the shorter the time to reach equilibrium of pH value is, the shorter the buffer period of solution is. After the careful comparison, the most appropriate initial pH value of buffer solution was between 9.00 and 9.20.(4) The influence on pH value of desulfurization solution of by-products NaSCN, Na2S2O3, Na2SO4 and NaHCO3 was studied. The results show that pH value of desulfurization solution reduces with the increase of by-products. NaHCO3 causes the strongest effect on pH value, next is NaSCN and Na2S2O3 is smallest.