Effect of adsorbate’s polarity, steric hindrance, aromaticity, and boiling point on competitive adsorption in a multi-staged countercurrent fluidized bed reactor using activated carbon and zeolite
Creators: Alizadeh, Saeid
Publisher: name: University of Alberta Library
Year: 2022

<p>Understanding the multi-component adsorption of volatile organic compounds (VOCs) is of essential importance for real-world engineering applications. Various properties of VOCs can affect their competitive adsorption when there is a mixture of these compounds in the process stream. This thesis investigated the effects of polarity, steric hindrance, aromaticity, and boiling point on competitive adsorption of VOCs in a multi-staged countercurrent fluidized bed reactor using beaded activated carbon (Kureha BAC G-70R) and beaded zeolite (ZEOCAT F603). The former is a highly microporous activated carbon, and the latter is a (50:50 wt. %) mixture of ZSM5 &amp; USY.</p>

<p>Adsorption isotherms were obtained for each VOC on each adsorbent for the concentration range of 50 to 1000 ppm. For the multicomponent tests, four pairs of VOCs (methyl isobutyl ketone (MIBK) and heptane for the polarity effect, hexane and cyclohexane for the steric hindrance effect, p-xylene and octane for the aromaticity effect, and 1,2,4-trimethylbenzene (TMB) and cumene for the boiling point effect) were selected based on their physical/chemical properties and each pair of VOCs targeted a specific factor.</p>

<p>In terms of adsorption isotherms, Kureha BAC G-70R exhibited a remarkably higher adsorption capacity as opposed to ZEOCAT F603 for all the VOCs due to its higher surface area and pore volume. Moreover, the results showed that BAC had the highest affinity for TMB followed by cumene with a slight difference, and hexane and octane showed the lowest adsorption capacity within the entire range of concentration. For the zeolite, the lowest affinity in the entire concentration range was observed for cyclohexane. For the lower range (&lt; 200 ppm), MIBK had the highest adsorption capacity.</p>

<p>Regarding the multicomponent tests, the adsorption capacity of the VOCs was similar when BAC was used as the adsorbent, while the zeolite had a higher adsorption capacity for MIBK which indicated its affinity toward the polar compound. Regarding the effect of steric hindrance, the cyclohexane molecular structure and conformations seem to hinder its adsorption into both adsorbents’ pores and as a result, cyclohexane’s adsorption capacity was lower compared to hexane on both adsorbents. Further, the aromatic structure of p-xylene may have caused this VOC to have a stronger affinity toward BAC, which led to its higher removal efficiency in the middle stages of the fluidized bed (2, 3, &amp; 4), but not strong enough to impact its overall removal efficiency in comparison to that of octane. The zeolite was not affected by the aromaticity effect and both p-xylene and octane had a similar removal efficiency in the fluidized bed reactor. The difference of approximately 20℃ between the boiling point of TMB and cumene did not impact their competitive adsorption. TMB and cumene showed a similar removal efficiency in multicomponent adsorption using both adsorbents. The X-ray photoelectron spectroscopy (XPS) results from Kureha BAC G-70R provided additional explanations for the results from the multicomponent test. Oxygen-containing groups (hydrophilic sites) were detected on BAC surface but, it is assumed that due to the low content of hydrophilic sites, they could not facilitate the adsorption of the polar compound. The graphitic carbon on BAC surface may have caused pi electron donor-acceptor (pi-EDA) interactions between the aromatic compound and BAC, and presumably, they promoted p-xylene adsorption in the middle stages of the fluidized bed.</p>