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Heel Build-up During Thermal Desorption of VOCs in Presence of Oxygen
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- Author / Creator
- Feizbakhshan, Mohammad
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Oxygen presence during thermal desorption of volatile organic compounds (VOCs) from adsorbent can promote heel build-up and loss of adsorption capacity, which reduces the performance of adsorbent in a cyclic adsorption/desorption process. The heel build-up, in some cases, could be attributed to the chemical reactions between VOCs and the purge gas oxygen impurities that shortens adsorbent’s life span, and leads to a more frequent adsorbent replacement, repeated reactivations, and disposal, and imposes high operation and maintenance cost on the industries. Therefore, this research aims to understand the effect of desorption conditions and adsorbent physical properties and material on oxygen-induced reactions and corresponding heel build-up. Several commercially available adsorbents (activated carbon, zeolite, and polymer) were tested and different characterization methods were used to identify the possible mechanisms for heel build-up. Knowing the nature of the oxygen-induced reactions is important as it could help in 1) finding optimal operational parameters, 2) selecting proper material, 3) minimizing heel build-up in the presence of oxygen, 4) investigating potential regeneration methods of spent adsorbents, 5) increasing adsorbent’s lifetime, and 6) savings in capital and operational costs associated with adsorbent replacement.
The combined effects of desorption temperature and oxygen impurity on heel build-up in cyclic adsorption/desorption of 1,2,4-trimethylbenzene (TMB) on beaded activated carbon (BAC) was studied (Chapter 2). Three impurity levels of oxygen in the purge gas (≤5 ppm, 10,000 ppm, 21% O2 in N2) at two desorption temperatures (200°C and 288°C) were investigated and a new method was proposed to identify heel nature using thermogravimetric analysis. The effect of adsorbent’s physical properties (pore size distribution and surface area) on heel build-up was studied using three commercial activated carbons (Chapter 3). Micropore surface analysis, XPS, Boehm titration, and differential thermogravitric (DTG) analysis were used to assess differences in the physical and chemical properties of these adsorbents. To understand the effect of adsorbent material on heel build-up, activated carbon, zeolite, and polymer adsorbents were tested under similar conditions and in the presence of oxygen (Chapter 4). It was observed that desorption temperature and oxygen concentration thresholds for heel build-up depended on the adsorbent material. Finally, the mechanism of heel build-up was investigated using three activated carbons (ACs) and one zeolite adsorbent at a severe condition of high oxygen concentration and at temperatures of 150, 200, and 250 ºC to analyze and characterize the products of oxygen-induced reactions. In addition to the previously used characterization methods, gas chromatography-mass spectrometry was used to understand the mechanism of heel build-up.
In summary, it was found that 1) combination of desorption temperature and purge gas oxygen increases heel build-up, 2) polymers have limited use in thermal desorption at high concentration of oxygen due to oxidizing reactions, 3) the highly microporous structure of activated carbon promotes heel build-up, while a hierarchy of pores eases the desorption with less heel, 4) confined pore opening of zeolite restrict reactions between VOCs and oxygen in the porous network and therefore, molecular sieve adsorbents are advantageous in the cyclic process of VOC adsorption when high oxygen impurity exists. -
- Graduation date
- Spring 2021
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- Type of Item
- Thesis
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- Degree
- Doctor of Philosophy
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- License
- This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.