Purification of HCl-containing hydrogen streams by adsorption using ion-exchanged zeolites and the ZIF-8 Metal Organic Frame-work

Student thesis: Master's Thesis

Abstract

With depleting fossil fuels and large emissions of CO2, Hydrogen, as carbon-free fuel, is consid ered as fuel of the future due to its high conversion efficiency, recyclability and non-pollutingnature. However, for massive demand of H2, needed for heavy oil upgrading, desulfurizationand upgrading of conventional petroleum and for ammonia production, chemical industry stilldepends on conventional production methods i.e. reformation of hydrocarbons for increasedhydrogen production.H2 produced via this method, requires removal of various impurities such as CO, H2S, hydro carbons (HC), NH3, chlorides, and oil vapor, as their presence cause performance degradation,poisoning of catalysts, and corrosion of conveying equipment such as pipes, valves. Further more, HCl due to its reactive nature, in some cases lead to fouling and plugging of variouscomponents by forming "green oil" and ammonium chloride. Therefore, to prevent equipmentfrom damage and avoid environmental problems, removal of HCl below 1 ppm is required.In the literature, studies regarding removal of impurities such as CO, H2S, hydrocarbons(HC), NH3, siloxane and oil vapor are present, however very few studies are done so far forremoval of HCl from diluted hydrogen stream. Therefore, in this study, purification of HCl containing hydrogen streams via adsorption using ion-exchanged zeolites and ZIF-8, metalorganic framework was investigated through breakthrough experiments using a fixed-bed col umn. SEM-EDS, XRD and TGA analysis were performed to analyze property of adsorbentsbefore and after the adsorption.From the initial screening of available adsorbents, which include commercially available inindustry and new generation adsorbents, zeolite 13X and ZIF-8 gave the best performances.Furthermore, results revealed that adsorption of HCl on these adsorbents was primarily chem ical. To improve the adsorption capacity of zeolite 13X, cation-exchange with alkali, alkalineand transition metal cation was performed. Results showed that cation exchange in 13Xwas governed by two main factors: 1) Hydration energy of the cations in the external aque ous solution, dominant in the initial stage and 2) Cation-cation and cation and frameworkinteractions, governing the final stage.From the modified 13X, meanwhile, 13X-Zn-Na(7-3) showed the best adsorption capacity,Cu2+-13X presented the least HCl removal capacity. For 13X-Zn-Na(7-3), it was hypothesizedthat it might be cation-cation interaction between Na+ and Zn2+ in the external aqueoussolution, which may lead these cations to sites in zeolitic cage with best accessibility ofHCl. Apart from composition (cation), other factors were also studied and for favorable HCladsorption, presence of water, pore diameter higher than kinetic diameter of HCl (4.42 Ã…),and least binder were ought to be needed.Finally, as regeneration of an adsorbent is an important aspect on an industrial level, an intentwas made to recrystallize (regenerate) ZIF-8 using "used" ZIF-8 as zinc source. But, due tothe presence of imidazolate chloride salt, precipitation of an unknown crystalline compoundwas obtained. In case of 13X, regeneration with basic solution improved the sorption capacitysignificantly, however, with higher pH, framework structure seemed severely affected.In summary, this study revealed two notable solutions for HCl removal from diluted H2 streams:1) Zeolite 13X, easily modified, cheap and industrially available and 2) ZIF-8, expensive butwith remarkable adsorption capacity of 1.0 (g of HCl / g of adsorbent).
Date of Award2018
Original languageEnglish
SupervisorJoeri Denayer (Promotor)

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