INFLUENCE OF CATALYST ACIDITY ON DEPOSITION OF FINE PARTICLES IN HYDROTREATERS

Abstract

Dealing with quantities of foulants higher than that commonly found in conventional crudes is a major attendant problem of processing unconventional crudes. In the case of oilsands derived bitumen, one of the foulants of concern is very small sized mineral solids commonly referred to as fines. These fines elude capture and persist in bitumen after separation of oilsands into its components and can even be found in derivative fractions of the bitumen such as heavy gas oil. Deposition of these fines in the catalyst bed of reactors while hydrotreating the bitumen or its fractions reduces the available bed voidage leading to an increase in pressure drop build-up. Pressure drop build-up in hydrotreating reactors has been widely studied owing to the negative economic impact it has on refining operations because of the premature reactor shutdown it eventually necessitates. Several parameters suspected to influence this problem have been investigated including catalyst properties and reaction parameters. Considering the importance of catalyst acidity in enhancing hydrotreatment, it is a glaring omission that a possible contribution of this catalyst property to fines deposition has not been confirmed or ruled out. In the first phase of this work, a possible relationship between catalyst acidity and the deposition of fine particles added to bitumen-derived heavy gas oil was investigated during hydrotreating in a batch reactor. NiMo/Ti-Al2O3 catalysts of varying acidities used in the hydrotreating reactions were synthesized by varying the Ti/Al ratio in their supports. Characterization of the catalysts showed that increasing the Ti/Al ratio in the support produced an increase in catalyst acidity. Kaolin and asphaltene coated kaolin were used as the model fines in the HGO feed. Experimental design based on a 3-factor optimal (custom) response surface methodology was performed and analysed considering temperature (360 and 380 °C), catalyst acidity (Ti/Al = 0, 0.025 and 0.05), and asphaltene coating concentration (0, 389 and 1084 ppm). Results of the study showed that increasing catalyst acidity corresponded to a reduction in the quantity of fines deposited during hydrotreating. The fine particles with asphaltene coating concentrations of 0 and 1084 ppm showed similar depositions which were the highest while the 389 ppm fines showed the least deposition. The results of the experiments indicated that fines deposition was independent of the reactor temperature in the range of 360–380 °C studied. Analysis of the 3 types of prepared catalysts showed an absence of acidic sites with intermediate strength. Between the acidic sites with weak and strong acidic strength, the former was most impacted by fine particle deposition during hydrotreating. NH3 temperature programmed desorption results of the spent catalysts revealed a decrease in the quantity of acidic sites of weak strength. Finally, fine particles upon deposition were shown to foul the catalysts causing a reduction in the hydrotreating efficiency of the catalysts. The effects of 3 factors; temperature (370, 385, 400 °C), pressure (1000, 1200, 1400 psi), and LHSV (0.5, 1, 2 h-1), on fine particles deposition were studied during hydrotreating in the second phase. This was to gain insight on how reaction parameters in addition to the established effects of acidity affected fines deposition and as such a packed bed reactor was used purposely for its similarity with industrial scale hydrotreaters. The Ti/Al = 0.05 catalyst was used in all reactions together with new model fines of asphaltene coating concentration of 542 ppm. With an experimental design based on temperature, pressure and LHSV, a series of reactions were carried out and the deposition of fines as well as the efficiency of hydrodesulfurization and hydrodenitrogenation were measured. The reaction results indicated that increasing LHSV from 0.5 to 2 h-1 resulted in a decrease in fines deposition. Temperature and pressure showed a similar trend on how they impacted fine particles deposition. A convex curve was obtained from plotting deposition against temperature or pressure with the minimum deposition occurring around the midpoint values of 385 °C and 1200 psi respectively. Furthermore, fines deposition was shown to impact hydrodesulfurisation with the combination of temperature and pressure which gave low fines deposition being the same conditions for high sulfur removal. Finally, a study of the surface charge of the fine particles as well as the fresh and spent catalysts used in both the batch and packed bed reactor hydrotreatment reactions was carried out. Zeta potential charge measurements revealed that the surface of fresh catalysts were positively charged in the acidic pH range of 2 to 10 of the analysis medium whereas both uncoated and asphaltene coated fines were negatively charged irrespective of pH. This suggested a possible interaction between the fine particles and the hydrotreating catalysts via electrostatic attractive forces leading to their deposition and adhesion on the catalyst surface. This was confirmed by a reduction in the positive surface charge of the catalyst after deposition of the negatively charged fine particles. A deposition mechanism incorporating findings from both phase 1 and 2 was thus proposed.