Adsorption and Microwave Regeneration for Controlling Volatile Organic Compounds Emissions from Automotive Paint Booths

• Author / Creator

  Fayaz, Mohammadreza

• Adsorption is a widely used method for control of organic compounds produced during painting operations. After adsorption, the loaded adsorbent should be regenerated to recover the adsorbates and reuse the adsorbent for subsequent adsorption/regeneration cycles. Accumulation of non-desorbed adsorbates in adsorbent pores (aka heel buildup) is a common challenge associated with adsorbent use, which results in reducing the working capacity and life-time of the adsorbent. The goal of this study is to understand the factors affecting heel buildup and possible ways to mitigate it. In this study, heel buildup associated with volatile organic compounds (VOCs) from painting operations was quantified and analyzed. Then, the performance of microwave heating method during regeneration of adsorbents loaded with high molecular weight VOCs was investigated, in terms of regeneration efficiency and energy consumption, to find proper conditions for preventing or minimizing heel. The effect of regeneration temperature, heating rate, adsorbent porosity, and the dielectric properties of adsorbents and adsorbates on regeneration performance and heel buildup were also investigated. Finally, a non-contact microwave sensor was used to monitor the adsorption progress and the level of heel buildup on beaded activated carbon (BAC) samples. At the early stage of this study, VOCs commonly present in paint solvents were screened for potential heel buildup on BAC. It was observed that compounds with high molecular weight, high boiling point and low vapor pressure tend to provide high heel. Comparing the performance of conductive and microwave heating for regeneration of a heavy VOC (n-dodecane), it was observed that the minimum energy needed to completely regenerate the adsorbent (100% desorption efficiency) using microwave heating was 6% of that needed with conductive heating. Moreover, it was confirmed that neither regeneration method altered the adsorbate composition and adsorbent physical and chemical properties during desorption. Further investigations were completed to study the effect of regeneration temperature, heating rate and adsorbent porosity on heel formation during microwave regeneration of two BACs (88% and 46% microporous) loaded with 1,2,4-trimethylbenzene (TMB). The results showed that for the higher regeneration temperature (400 °C versus 288 °C), increasing heating rate increased heel buildup by as much as 92% and 169% for the mainly microporous and partially microporous BACs, respectively. The elevated heel formation at higher heating rates could be due to adsorbate coking as a result of exposure to high temperature. Conversely, for the lower regeneration temperature (288 °C), increasing the heating rate did not significantly affect the amount of heel buildup; however, it increased the contribution of coke formation in heel. The results from this study indicate that the heating rate, regeneration temperature and adsorbent porosity could be optimized to allow fast desorption with minimal adsorbate decomposition during microwave regeneration. To study the effect of dielectric properties of adsorbent and adsorbate on microwave heating performance, BAC (microwave absorbing) and a polymeric adsorbent (microwave transparent) were partially loaded with TMB or 2-butoxyethanol (BE) and then regenerated with microwave heating. Results indicate that for the polymeric adsorbent, required energy for regeneration was primarily provided by microwave dissipation within the adsorbate. Conversely, for BAC, required energy for regeneration was primarily provided by microwave dissipation within the adsorbent. In the next phase of this study, it was found that during adsorption, the dielectric properties of the adsorbent changed with increased adsorbate loading. Those changes could be measured by a non-contact high resolution microwave sensor in terms of shift in the quality factor and resonant frequency. It was observed that the difference between the 5% breakthrough time and the time that the quality factor was 0.95 of its final value, was less than 5%. Moreover, for all tests, a proportional relationship between adsorption capacity and the final value of the resonant frequency shift was found. Further investigation focused on studying the ability of the non-contact high resolution microwave sensor to determine the degree of exhaustion of BAC spent through many adsorption/regeneration cycles. The resonant frequency shift was linearly correlated (R2= 0.81 – 0.93) with the adsorbent apparent density, Brunauer Emmett Teller (BET) surface area and pore volume. Moreover, the effect of adsorbent porosity on dielectric property variation appeared to be more important than the heel composition. Finally, the ability of the sensor for fast, non-contact measurement, and without sample pretreatment, potentially allows for in-situ measurement and real-time monitoring of continuously operating systems.

• Subjects / Keywords

  • Microwave Heating Regeneration
  • VOCs
  • Adsorption
  • Microwave Sensor

• Graduation date

  Fall 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3VH5CP7K

• 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.