| As to N-methyl diethanolamine (MDEA) aqueous solution contaminated by a lot of chloride ions in desulfurization units in Sinopec Puguang natural gas purification plant, a technology for regenerating MDEA solution had been investigated in this present work. A suitable anion exchange resin adsorbent for removing Cl- from the contaminated MDEA solution had been selected, and the operation conditions had been optimized, the process mechanism had been discussed. In Puguang natural gas purification plant, offline purification for the contaminated MDEA solution using anion exchange resins had been performed, and the properties and desulfurization performance of the purified MDEA solution had been evaluated.The static adsorption results show that SA17 anion exchange resin is a suitable adsorbent for Cl- removal considering its advantages in adsorption ability, working exchange capacity and ion exchange rate. The adsorption isotherms were well fitted to Freundlich model, and the calculated thermodynamic parameters including △H, AS and AG were all negative, indicating the adsorption of chloride ion on SA17 is an exothermic and spontaneous process. The adsorption kinetic results show that the adsorption rate was controlled by the liquid film diffusion, and was coincided with the pseudo-first order model. The calculated activation energy of the adsorption process (Ea) was 29.81 kJ/mol. Infrared spectrum and energy spectrum analysis show that OH groups of N-OH in resin was replaced by Cl- after adsorption, and the resin matrix and functional groups did not change.As to MDEA solution with Cl- content of 1248 mg/L, the experimental results obtained from the dynamic adsorption in SA17 fixed bed show that the removal for chloride ion could be more than 99% under the optimal operating conditions (a speed velocity of 8.6 h"1 and height-to-diameter ratio of 5.0). Meanwhile, the penetration adsorption capacity was 24.88 mg/g in 100 min, and the bed was saturated in 185 min with a saturated adsorption capacity of 40.69 mg/g. The adsorption breakthrough curve could be well explained by Thomas model, and the height of the corresponding mass transfer zone is 2.52 cm. The bed can be penetrated fast under a small space velocity and a large height-to-diameter ratio.pH of the solution increased to 13.15 after first-treatment using SA17, then decreased to 11.91 after second-treatment using SK8 cation exchange resin bed, which is close to the level (pH=11.97) of fresh MDEA solution. Through SA17 and SK8 adsorption bed series, Cl-content in regenerated MDEA solution was below 5 mg/L, and the conductivity was 38.8 μS/cm. Laboratory regenerated MDEA solution is similar to fresh MDEA solution in properties, and their desulfurization performance for natural gas was in the same level. MDEA loss could be reduced by using water wash the fixed bed, and after washing, regenerated MDEA concentration was 42.65%, dissipation rate was 0.73%.Based on the results of laboratory experiments, Beijing Shibo ion exchange method and Zhejiang Hainiu electrodialysis method had been used in offline purification and regeneration experiment to treat the 1400 t high chlorine contaminated MDEA solution in Puguang natural gas purification plant scene. The results show that after treated by ion exchange method, Cl-content, main impurity ions content and heat stable salts in regenerated MDEA solution were all lower than that treated by electrodialysis method. After adding anti-foaming agent, regenerated MDEA solution is similar to fresh MDEA solution in desulfurization performance for natural gas. Compared with electrodialysis method, ion exchange method has a comparative advantage in purification and regeneration of high content chloride ion in MDEA solution. |
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