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Adsorption and Mechanism of Low-concentration HCN in Closed Calcium Carbide Furnace Off-gas

Author: JiangMing
Tutor: NingPing
School: Kunming University of Science and Technology
Course: Environmental Engineering
Keywords: hydrogen cyanide calcium carbide furnace off-gas molecular sieve activatedcarbon fiber adsorption
CLC: X781
Type: PhD thesis
Year: 2013
Downloads: 6
Quote: 0
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Abstract


Hydrogen cyanide (HCN) is an unconventional, toxic and hazardous gaseous pollutant and generated in multitudinous sources. Deeply removed HCN from exhaust gas is needed to solve technical problems in China’s chemical industry, metallurgy, carbon fiber, chemical intermediates and other important industries for meeting environmental requirements. Closed calcium carbide furnace off-gas is a kind of typical exhaust gas which contained a large amount of CO (70-90%). The off-gas can not be recycled due to the existence of HCN, H2S, PH3, COS and other impurities. Nowadays, there is a less purification technology to remove HCN from off-gas. Especially, at low temperatures, HCN removal from industrial off-gas, which contains high concentration of CO, has not been reported. For low-concentration HCN in model off-gas low temperature adsorption (adsorption temperature<100℃), some typical adsorbents can resolve HCN efficient removing from calcium carbide furnace off-gas while meeting the needs of deep purification and recycle, but also bring energy savings. Therefore, the low temperature adsorption technical is possessing broad application prospect.In view of this, the paper took removal of low-concentration HCN in calcium carbide furnace off-gas as the research object, and developed series of adsorbents, such as molecular sieve adsorbent and activated carbon fiber (ACF) adsorbent, studied its adsorption properties and the purification mechanism of HCN at low temperature. Molecular sieve adsorbent and ACF adsorbent were prepared through selecting experiment of supports and active components, and discussed the influence of the preparation conditions and operation conditions on the properties of the adsorbents, and optimized and regenerated the adsorbents. The adsorption isotherms is also obtained by isothermal adsorption experiment of adsorbents. and the adsorption isosteric heat was got by calculation, thus the type of adsorption of HCN on the adsorbent was determined. Then adsorbents before and after adsorbing HCN were characterized by TG, SEM/EDS, N2-BET, XRD, XPS. The structure of adsorbents in the preparation process, adsorption process, and deactivation mechanism were discussed. Finally, the real calcium carbide furnace off-gas was used to test the properties of the adsorbent for removing HCN at low temperature.The results show that, Zn (NO3)2can be selected as relatively optimal modifier for preparation of molecular sieve adsorbent used for removing HCN from the tail gas which contains high concentration of CO. The highest performance of Zn/13X adsorbent is observed at the Zn loading on13X is21.34%(w/w), the optimal conditions are as follows: space velocity is15000h’’, oxygen content is3%, adsorption temperature is20℃. The adsorption isotherm of the adsorbent is conformed to the Freundlich isotherm equation, and isosteric heat is29.71-62.71kJ/mol. ZnO is the active component in Zn/13X adsorbent for removing HCN. CO is more likely to disproportionate and decompose produce carbon(C) and carbon dioxide(CO2) in the adsorbent surface with higher adsorption temperature in gas flow. And CO2can compete with HCN in reacting and adsorbing with ZnO, the adsorption products ZnCO3, Zn (CN)2and C jointly block and fill the mesopore with pore diameter ranging from59.5A to133.8A in the adsorbent, thus Zn/13X adsorbent deactivate.ACF type of adsorbent can reach the highest adsorption efficiency for adsorbing HCN through modifying with ACF as support, Zn (NO3)2and Cu (NO3)2as modifier. Zn/Cu/ACF composite adsorbent reaches the optimal performance, when the preparation conditions are as follows:both the concentration of Zn (NOs)2and Cu (NO3)2are0.2mol/L, Zn/Cu=5:l,0.5h of calcination time, calcination temperature of300℃. The optimal operating conditions are that adsorption temperature is20℃and oxygen content is2%. The Freundlich isotherm equation was suitable for describing the adsorption process of HCN on the adsorbent, and the isosteric heat is between43.99kJ/mol and67.54kJ/mol. There is a strong interaction between the modified solution and ACF support, it decrease the heat stability of the modified ACF adsorbent. ZnO and CuO are the main active component of Zn/Cu/ACF adsorbent for removing HCN, the chemical adsorption products are Zn (CN)2and Cu (CN)2which block and fill the active adsorption sites on the mesopore with pore diameter ranging from24.2A to120A.After regeneration of deactivated adsorbents, ACF type adsorbent has a high regeneration rate reaching80.2%. And both the two adsorbents can be reutilized many times.The field small and pilot-plant experiment of the calcium carbide furnace off-gas have been studied. The results show that activated carbon (AC) adsorbent gained from previous studies and Zn/13X type adsorbent obtained at the present stage can be used for removal of low-concentration HCN in calcium carbide furnace off-gas at low temperature. Zn/13X adsorbent has a ideal purification efficiency. When AC adsorbent was used to purify calcium carbide furnace off-gas, the adsorbent has a long lifetime and high stability. After the purification, the outlet concentration of all impurities in calcium carbide furnace off-gas was less than0.5mg/m3.

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CLC: > Environmental science, safety science > Processing and comprehensive utilization of waste > Chemical industry,waste disposal and comprehensive utilization > Inorganic Chemical Industry
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