| The tail gas derived from phosphorus-coal-based chemical industry contained high content carbon monoxide (CO), which could be used as a potential raw material gas for C1 chemical industry. However, the reuse of tail gas was restricted strictly because the tail gas contains phosphine (PH3) that was a potent catalyst poison in CO synthesizing chemistry. An attractive purification technology would be gas-solid dry adsorption, but there was a disadvantage about regeneration and subsequent treatment of exhausted adsorbent. Therefore, the key and difficult points in gas-solid dry adsorption lay in developing the excellent adsorbent, which had the well property of PH3 adsorption capacity, adsorption selectivity, regeneration performance, and adsorption stability. Moreover, in the process of PH3 gas-solid dry adsorption, it was not thorough enough for adsorption thermodynamics and adsorption mechanism. If a lot of experimental research, theoretical calculation of adsorption thermodynamics, together with analysis and measurement characterization of adsorbent samples had been investigated, adsorption thermodynamics and adsorption mechanism were accurately proposed. The results of this thesis were of great theoretical significance for PH3 adsorption removal from tail gas of phosphorus-coal-based chemical industry.In order to resolve these above problems, there were four parts of research contents: preparation and PH3 adsorption performance of modified coal-based activated carbon adsorbents (MCACs), preparation and PH3 adsorption performance of modified walnut-shell activated carbon adsorbents (MWSACs), PH3 adsorption thermodynamics, and PH3 adsorption mechanism.Preparation and PH3 adsorption performance of MCACs. A series of MCAC adsorbents were prepared by wet impregnation method and used for PH3 adsorption removal from tail gas. The effects of preparation conditions (kinds of active component, different precursors of copper, copper loading amounts, and calcination temperature) on the property of MCAC adsorbents for phosphine adsorption removal were investigated. Moreover, copper-modified activated carbon adsorbents were prepared in order to investigate the effect of Zn, Ce, and La addition on Cu-modified CAC adsorbent for PH3 adsorption removal. Base on the optimal adsorbent preparation conditions, the effects of adsorption operation condition (adsorption temperature, oxygen content, and PH3 inlet concentration) were carried out. Regeneration of exhausted MCACs was preliminarily studied. The regeneration result showed that there was a great adsorption capacity difference between fresh MCACs and regenerated MCACs. The reason might be that high content impurities existed in the MCACs were disadvantageous for regeneration of exhausted MCACs. Therefore, a novel wooden activated carbon, modified walnut-shell activated carbon adsorbents, was prepared and used for PH3 adsorption removal.Preparation and PH3 adsorption performance of MWSACs. Walnut-shell activated carbons (WSACs) were prepared by KOH chemical activation and MWSACs were used for PH3 adsorption removal from tail gas. The effects of carbonization temperature, activation temperature, and ratio of KOH to chars on physical and chemical characteristic of WSACs were studied. The effects of carbonization temperature, activation temperature, and ratio of KOH to chars on PH3 adsorption performance of MWSACs were investigated. Criteria for determining the optimum preparation conditions were PH3 removal efficiency and PH3 breakthrough adsorption capacity of MWSACs. The effects of microwave heating activation and conventional heating activation on PH3 adsorption performance of MWSACs were preliminarily studied. Base on the optimal WSACs preparation conditions, the effects of adsorption operation condition (gas hourly space velocity based on the actual adsorbent volume (GHSV), adsorption temperature, oxygen content, and PH3 inlet concentration) were investigated. Adsorption selectivity, regeneration performance, and adsorption stability of MWSAC adsorbent were also studied. The effect of three different adsorbent supports (walnut-shell activated carbon, coconut-shell activated carbon, and coal-based activated carbon) on PH3 adsorption removal was studied when active components and preparation conditions had been fixed.PH3 adsorption thermodynamics. PH3 adsorption isotherms over MWSAC adsorbent were measured by dynamic adsorption breakthrough curve at 60℃～90℃. PH3 adsorption equilibrium data at various temperatures were fitted to Toth Euqation and their isosteric heats of adsorption were determined by Clausius-Clapeyron Equation. Adsorbed species under the thermodynamic equilibrium state and energy function changes (enthalpy, entropy and Gibbs free energy) were calculated by Factsage 6.0. Based on these results, adsorption species and adsorption mechanism over the modified activated carbon were preliminarily analyzed.PH3 adsorption mechanism. A lot of experimental researches about PH3 adsorption were carried out. Theoretical calculation of adsorption thermodynamics were calculated by Factsage 6.0. Analysis and measurement characterization of adsorbent samples were investigated. According to these above results, PH3 adsorption mechanism over the modified activated carbon was accurately proposed. The main original conclusions of this thesis are as follows:(1) The optimal preparation conditions of MCACs are active component precursors of nitrates (copper nitrate, zinc nitrate, and lanthanum nitrate), an incipient wetness method of 30℃for 40min with the help of ultrasonic, a dried temperature of 110℃for 12h, a calcination temperature of 350℃for 6h. And copper, zinc, and lanthanum loading amounts are 2.5%,0.167%, and 0.0833%, respectively. The optimal adsorption operation conditions are an adsorption temperature of 70℃, a GHSV of 5000h-1, a PH3 inlet concentration of 1300ppm, and an oxygen content of 1%. Under this condition, PH3 breakthrough adsorption capacity over the optimal MCAC adsorbent is 95.74mg/g.(2) The result of WSACs preparation process shows that the suitable carbonization temperature of WSACs is 700℃. When activation temperature ranges from 700℃to 900℃, the pore development of WSACs almost keep unchanged. When the ratio of KOH to chars ranges from 3 to 4, WSACs with high surface area and well-developed pore structure is obtained and results in an amorphous structure. The result shows the optimum preparation conditions of WSACs are a carbonization temperature of 700℃, an activation temperature of 700℃, and a mass ratio of 3. The BET surface area, the micropore volume, and the micropore volume percentage of the optimal WASC are 1636m2/g,0.641cm3/g, and 81.97%, respectively. Both MWSACs activated by microwave heating activation and conventional heating activation are beneficial for PH3 adsorption removal. The optimal operation conditions are an adsorption temperature of 70℃, a GHSV of 21000h-1, a PH3 inlet concentration of 1040ppm, and an oxygen content of 0.5%. Under this condition, the PH3 breakthrough adsorption capacity over the optimal MWSAC adsorbent is 219.44mg/g. The result shows that PH3 breakthrough adsorption amounts over the MWSAC adsorbent, the MCAC adsorbent, and the modified coconut-shell activated carbon adsorbent are 284.12mg/g, 193.49mg/g, and 176.38mg/g, respectively. WSAC adsorbent owns the biggest PH3 breakthrough adsorption amount due to the biggest specific surface area, total pore volume and micropore volume. MWSAC adsorbent has the well property of PH3 breakthrough adsorption capacity, adsorption selectivity, regeneration performance, and adsorption stability. And this excellent MWSAC adsorbent has been identified.(3) According to PH3 adsorption equilibrium data, when the adsorption temperature ranges from 60℃to 90℃, the optimal adsorption temperature is 70℃and the equilibrium adsorption amount is 595.56mg/g at 70℃. It is found Toth Equation is suitable for description of phosphine adsorption process. The isosteric heat of adsorption decreases with an increase of the surface loading on the MWSAC adsorbent, which means that MWSAC adsorbent has an energetically heterogeneous surface. The isosteric heat of adsorption ranges from 43kJ/mol to 90 kJ/mol, which indicates adsorptive phosphine removal performance may be a dominant of chemical adsorption. According to the thermodynamic equilibrium calculation, the PH3 adsorption process over the modified activated carbon adsorbent is a spontaneous exothermic process of decreased entropy. When the molar ratio of O2 to PH3 is not less than 2, PH3 adsorbed will act with oxygen derived from the active component and the reaction products are H3PO4 and only a few (P2O5)2. Oxygen contained in the active component is added by pre-adsorbed O2 derived from tail gas. The active component plays an oxygen transmission role on PH3 adsorption process.(4) PH3 adsorption mechanism over modified activated carbon adsorbent has been proposed. PH3 and O2 contained in the tail gas adsorb firstly onto modified activated carbon adsorbent by physical adsorption. PH3 adsorbed act with lattice oxygen derived from the active component and the oxidation reaction products are H3PO4 and (P2O5)2. According to the reduction-oxidation (redox) mode, lattice oxygen contained in the active component is added by pre-adsorbed O2 derived from tail gas. The active component plays an oxygen transmission role on PH3 adsorption process mechanism. The reason of PH3 removal efficiency decreasing with increasing the adsorption time contains two points. On the one hand, the exhausted adsorbent can not adsorb H3PO4 and (P2O5)2 with the increasing of adsorption time. On the other hand, the active component is coated by adsorbed species of H3PO4 and (P2O5)2, resulting in interruption of oxygen transmission. The exhausted adsorbent can be regenerated by water washing together with heated air drying. Concentration of H3PO4 in regeneration liquid achieved 20%. MWSAC adsorbent would be beneficial for PH3 adsorption removal from tail gas of phosphorus-coal-based chemical industry and recycling of adsorption species.The authors would like to acknowledge financial support from the Key Program of National High Technology Research and Development Program of China (863 Program) (2008AA062602), the Young and Middle-aged Academic and Technical Back-up Personnel Program of Yunnan Province (2007PY01-10). |
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