Understanding and Improving Gas Phase Capture of Organic Vapors by Carbonaceous Adsorbents

• Author / Creator

  Jahandar Lashaki, Masoud

• Vehicle painting booths are among the major sources of volatile organic compounds (VOCs) emissions in the auto manufacturing sector. A challenge with controlling VOCs by adsorption onto activated carbon (AC) is the occurrence of strongly, or even permanently, adsorbed species. Irreversible adsorption (aka heel formation) prevents complete regeneration of the sorbent, decreasing its capacity and lifetime. A better understanding of the factors enhancing the irreversible adsorption is instructive in finding ways to decrease heel buildup and eventually increasing the lifetime of the adsorbent. The adsorption capacity of AC for VOCs is also of concern for the design of a reliable adsorption system. Experimentally determining the adsorption capacity of AC needs considerable effort, time, and cost. Hence, there is interest in developing reliable models to predict the adsorption capacity of adsorbents. The objective of this dissertation, therefore, is to understand and improve gas phase capture of VOCs by carbon-based adsorbents. This research was conducted in two parts, 1) enhancing the prediction of the adsorption capacity of VOCs on carbonaceous adsorbents, and 2) studying the reason and mechanism for irreversible adsorption of VOCs on AC adsorbents, and subsequent use of these results to improve process know-how. In the first part, the effect of the kinetic diameter (KD) of the reference adsorbate on the accuracy of the Dubinin-Radushkevich (D-R) equation for predicting the adsorption isotherms of organic vapors on microporous AC was investigated (Chapter 3). Adsorption isotherms for 13 VOCs on microporous beaded activated carbon (BAC) were experimentally measured, and predicted using the D-R model and affinity coefficients. Choosing a reference adsorbate with a KD similar to that of the test adsorbate resulted in better prediction of the adsorption isotherm. The proposed hypothesis was also used to explain reports of inconsistent findings among published articles. In the second part, the effect of operational parameters such as adsorption and regeneration temperature (Chapter 4), BAC’s surface oxygen groups (Chapter 5), desorption purge gas oxygen impurity (Chapter 6), and BAC’s pore size distribution (PSD; Chapter 7) on the irreversible adsorption of a mixture of organic compounds typically emitted from automobile painting operations was explored. Results indicated that increasing the adsorption temperature from 25 to 45 C increased heel buildup on BAC by about 30% irrespective of the regeneration temperature possibly due to chemisorption. Conversely, increasing the regeneration temperature from 288 to 400 oC resulted in 61% reduction in the heel at all adsorption temperatures, possibly due to more effective desorption of chemicals from narrow micropores. PSD and pore volume reduction confirmed that the heel was mainly formed in narrow micropores, which can be occupied or blocked by some of the adsorbates. Microporous BAC was treated with hydrogen to remove oxygen groups or treated with nitric acid to add oxygen groups. Derivative thermogravimetric (DTG) results showed heel formation due to physisorption for heat-treated and hydrogen-treated BACs, and weakened physisorption combined with chemisorption for nitric acid-treated BAC. Microporous BAC was also regenerated using different concentrations of oxygen (≤ 5 – 10,000 ppmv) in the nitrogen desorption purge gas. With increasing O2 concentration, mass balance cumulative heel increased by up to 35% and the fifth cycle adsorption capacity decreased by up to 55% relative to baseline scenario (≤ 5 ppmv O2 in N2). DTG analysis showed heel formation due to physisorption for ≤ 5 ppmv O2 and a combination of physisorption and chemisorption for other samples. Finally, five BAC samples with varying PSDs but similar elemental compositions were tested. Heel formation was linearly correlated with BAC micropore volume. Meanwhile, first cycle adsorption capacities and breakthrough times correlated linearly with BAC total pore volume. Overall, results showed that highly microporous adsorbents turn a higher portion of adsorbed species into heel due to higher share of high energy adsorption sites in their structure. In summary, it was found that high adsorption temperature, low regeneration temperature, high levels of surface oxygen groups on adsorbents, oxygen impurities in desorption purge gas, and high adsorbent microporosity may increase heel formation. Some of the outcomes of this research have been implemented in full scale adsorbers controlling VOC emissions from automobile painting operations, resulting in longer adsorbent lifetime and lower solid waste generation.

• Subjects / Keywords

  • Irreversible Adsorption
  • Adsorption Capacity
  • Activated Carbon
  • Regeneration
  • Surface Oxygen Groups
  • Physisorption
  • Kinetic Diameter
  • Organic Vapors
  • Pore Size Distribution
  • Vehicle Painting Operations
  • Purge Gas
  • Thermogravimetric Analysis
  • Volatile Organic Compounds
  • Chemisorption
  • Adsorption

• Graduation date

  Spring 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3V980303

• 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.