Deposition of Fines Entrained in Bitumen-derived Light Gas Oil on Hydrotreating Catalyst: Impact of Process Parameters
In this work, the impact of hydrotreating process parameters on the deposition of fines that are present in bitumen-derived light gas oil was studied. NiMo/γAl2O3 catalyst was synthesized in the laboratory to run the hydrotreating experiments in a batch system. The process parameters studied were temperature (355, 365 and 375 °C) and pressure (1200, 1300 and 1400 psig). Additionally, different loadings of the fines (1, 1.25 and 1.5 g) in 200 ml of light gas oil were taken to understand the impact of variation in particle loading on their deposition. The study was conducted in two phases; the first phase involved kaolin as model fines and in the second phase asphaltene coated kaolin was synthesized and used as model fines. Most of the experiments reported were designed using statistical technique, ie. central composite design and hence the results account for reproducibility. Other experiments testing the individual parameters were repeated. In all cases the error for bed deposition was ±5% and for sulfur conversion was ±2%. For kaolin suspended feed, temperature and particle loading had the most significant impact on particle deposition as individual parameters. High temperature (375 °C) led to more bed deposition and the trend was similar for particle loading in the feed. However, statistical analysis suggested that there was a combined effect of temperature and pressure on bed deposition. Bed deposition and sulfur conversion were optimized for a feed with varying particle loading (0.8-1.6 g). It was found that for a feed with high particle loading (1.6 g) the hydrotreating temperature should be 360 °C and the pressure should be 1450 psig. When the particle loading in 200 ml of feed was reduced to 0.8 g the optimized temperature for hydrotreating was 364 °C and the pressure was 1380 psig. In both optimizations the target bed deposition on the catalyst bed was taken close to the minimum (0.1 g) and the sulfur conversion was taken close to the maximum (60 wt%). When asphaltene coated kaolin was taken as model fines, two different sized particles ranges (4-6 and 9-11 μm) were synthesized to study the impact of particle size on bed deposition. Maximum bed deposition and sulfur conversions were attained at 375 °C; however pressure did not show a clear impact for bed deposition. It was also found that smaller particles (4-6 μm) deposited more on the catalyst bed as compared to the large size particles. When asphaltene coated kaolin is used as model fines, there is a preferential deposition of the fines on the reactor assembly as opposed to kaolin where the deposition is more on the catalyst bed. X-ray fluorescence (XRF) results suggest that in case of asphaltene coated kaolin, the fines settle at the bottom of the catalyst bed causing deep-bed filtration. The optimization results show that higher pressure (1300 psig), lower temperature (365 °C) would lead to less bed deposition without affecting the sulfur conversion of the feed. Additionally, the physical and chemical properties of the spent catalyst with deposited fines were studied. Brunauer Emmett Teller (BET) analysis showed that the deposition of fines on the catalyst did not affect the textural properties of the catalyst significantly. The Pore size and pore volume of the spent catalyst with and without fines did not differ; however, there was a slight reduction in the surface area and this was due to the agglomeration of fines at certain regions on the catalyst. Scanning electron microscope images of the spent catalyst showed larger agglomerates formed at higher temperature during hydrotreating as compared to small deposits at low temperature. Fourier transform infrared spectra of the spent catalyst suggested chemical interaction between the catalyst and the alumina silicates (kaolin). Thus, it could be inferred that the deposition of the fines on the catalyst is due to both physical adsorption of the fines as well as their chemical interaction with the hydrotreating catalyst. The regeneration experiment showed no improvement in the catalyst activity after the fines were removed from the catalyst surface which further supports that there was a chemical interaction between the catalyst and the fines. It was found that there is a chemical as well as physical interaction between the hydrotreating catalyst and the model fines.
Hydrotreatment, process optimization, fine particles deposition, kaolin, catalyst fouling, pressure drop, statistical modeling.
Master of Science (M.Sc.)
Chemical and Biological Engineering