The existence of multiple hydrodynamic states (MHS) in trickle bed operation has been proved by hysteresis observed in flow loops, as well as variation between different prewetting modes. The most common theory presented as explanation for the existence of MHS, is the film vs. rivulet concept. Based on this concept, it was suspected that in-situ upsets might promote the formations of films, thereby providing a method through which the hydrodynamic states of the Dry and Levec modes can be manipulated to perform like the Kan Liquid and Super modes. Large performance enhancements can be obtained by altering the prewetting procedure, even for systems with a low surface tension. For the water system, the gas liquid mass transfer coefficient of the Kan Liquid and Super modes could be as much as 6 times greater than that of the Dry mode. For the low surface tension system, the gas liquid mass transfer of the Kan Liquid and Super modes could be up to 8 times greater than that of the Dry mode. Through a thorough investigation of various types of transient upsets and manipulation strategies, it was confirmed that prewetting is indeed the only way by which drastic variation in hydrodynamic states may be obtained. None of the investigated upsets (hysteresis, periodic operation or surface tension doping) resulted in changes in the liquid morphology that could compare to the significant variation that was observed by varying the prewetting mode. Two methods were identified by which the hydrodynamic gaps between the less uniform (Dry and Levec) modes and the more uniform modes (Kan Liquid and Super) could be bridged. The first is to reduce the Levec draining time, while the second method may be seen as an in-situ type of Kan Liquid prewetting. This type of prewetting was obtained during doping with a low surface tension liquid, at a flow rate associated with the high interaction regime for the low surface tension system. Though the hysteresis cycles did not drastically alter the predominant flow type, interesting trends were observed, some of which raised doubt about the application of the films vs. rivulet concept. One mode in particular displayed behaviour which contributed to this doubt, namely the Kan Gas mode; • Gas liquid mass transfer on this mode decreased with an increase in liquid flow rate • Relatively low pressure drops on this mode corresponded to relatively high liquid holdup • It was the only mode that exhibited no hysteresis with gas flow variation, on any of the hydrodynamic parameters The various trends and variations observed with the different types of upsets, leads to the conclusion that the concept of films vs. rivulets simply does not provide adequate explanation of the observed results. In general, two flow types may be distinguished. That which is caused by an initial increase in liquid flow rate as opposed to that which is caused by an initial increase in gas flow rate An investigation to determine the behaviour of each of the investigated parameters near the transition boundaries on all the modes, as well as a repetition of this study with non-intrusive visual techniques is recommended. / Dissertation (MEng (Chemical Engineering))--University of Pretoria, 2008. / Chemical Engineering / unrestricted

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No / One of the more difficult tasks in the petroleum refining industries that have not been considered largely in the literature is hydrotreating (HDT) of crude oil. The accurate calculations of kinetic models of the relevant reaction scheme are required for obtaining helpful models for HDT reactions, which can be confidently used for reactor design, operating and control. In this work, an optimization technique is employed to evaluate the best kinetic models of a trickle bed reactor (TBR) process utilized for hydrodenitrogenation (HDN) and hydrodemetallization (HDM) that includes hydrodevanadization (HDV) and hydrodenickelation (HDNi) of crude oil based on pilot plant experiments. The minimization of the sum of the squared errors (SSE) between the experimental and estimated concentrations of nitrogen (N), vanadium (V) and nickel (Ni) compounds in the products is used as an objective function in the optimization problem to determine the kinetic parameters. A series of experimental work was conducted in a continuous flow isothermal trickle bed reactor, using crude oil as a feedstock and the commercial cobalt¿molybdenum on alumina (Co¿Mo/¿-Al2O3) as a catalyst. A three-phase heterogeneous model based on two¿film theory is developed to describe the behaviour of crude oil hydroprocessing in a pilot¿plant trickle bed reactor (TBR) system. The hydroprocessing reactions have been modelled by power law kinetics with respect to nitrogen, vanadium and nickel compounds, and with respect to hydrogen. In this work, the gPROMS (general PROcess Modelling System) package has been used for modelling, simulation and parameter estimation via optimization. The model simulations results were found to agree well with the experiments carried out in a wide range of the studied operating conditions. The model is employed to predict the concentration profiles of hydrogen, nitrogen, vanadium and nickel along the catalyst bed length in three phases.

No / Fossil fuel is still a predominant source of the global energy requirement. Hydrotreating of whole crude oil has the ability to increase the productivity of middle distillate fractions and improve the fuel quality by simultaneously reducing contaminants such as sulfur, nitrogen, vanadium, nickel and asphaltene to the levels required by the regulatory bodies. Hydrotreating is usually carried out in a trickle bed reactor (TBR) where hydrodesulfurization (HDS), hydrodenitrogenation (HDN), hydrodemetallization (HDM) and hydrodeasphaltenization (HDAs) reactions take place simultaneously. To develop a detailed and a validated TBR process model which can be used for design and optimization of the hydrotreating process, it is essential to develop kinetic models for each of these reactions. Most recently, the authors have developed kinetic models for all of these chemical reactions except that of HDAs. In this work, a kinetic model (in terms of kinetic parameters) for the HDAs reaction in the TBR is developed. A three phase TBR process model incorporating the HDAs reactions with unknown kinetic parameters is developed. Also, a series of experiments has been conducted in an isothermal TBR under different operating conditions affecting the removal of asphaltene. The unknown kinetic parameters are then obtained by applying a parameter estimation technique based on minimization of the sum of square errors (SSEs) between the experimental and predicted concentrations of asphaltene compound in the crude oil. The full model with the estimated kinetic parameters is then applied to evaluate the removal of asphaltene (thus affecting fuel quality) under different operating conditions (than those used in experiments).

Yes / The present study explored a novel oxidative desulfurization (ODS) method of light gas oil fuel, which combines a catalytic oxidation step of the dibenzothiophene compound directly in the presence of molecular air as oxidant to obtain high quality fuel for light gas oil. In chemical industries and industrial research, catalysis play a significant role. Heightened concerns for cleaner air together with stricter environmental legislations on sulphur content in addition to fulfill economic have created a driving force for the improvement of more efficient technologies and motivating an intensive research on new oxidative catalysts. As the lower quality fuel becomes more abundant, additional challenges arise such as more severe operation conditions leading to higher corrosion of the refinery installations, catalyst deactivation and poisoning. Therefore, among the technologies to face these challenges is to develop catalysts that can be applied economically under moderate conditions. The objective of this work is to design a suitable synthetic catalyst for oxidative desulfurization (ODS) of light gas oil (LGO) containing model sulphur compound (dibenzothiophene (DBT)) using air as oxidant and operating under different but moderate operating conditions. The impregnation method is used to characterize two homemade catalysts, cobalt oxide (Co3O4/γ-Al2O3) and manganese oxide (MnO2/γ-Al2O3). The prepared catalysts showed that the manganese oxide has a good impregnation (MnO2=13%), good pore size distribution and larger surface area. A set of experiments related to ODS of dibenzothiophene has been carried out in a continuous flow isothermal trickle bed reactor using light gas oil as a feedstock utilizing both catalysts prepared in-house. At constant pressure of 2 bar and with different initial concentration of sulphur within dibenzothiophene, the temperature of the process was varied from 403K to 473K and the liquid hourly space velocity from(LHSV) was varied from 1 to 3 hr-1. The results showed that an increase in reaction temperature and decreasing in LHSV, higher conversion was obtained. Although both catalysts showed excellent catalytic performance on the removal of molecule sulphur compound from light gas oil, the catalyst MnO2 catalyst exhibited higher conversion than Co3O4 catalyst at the same process operating conditions.

Yes / Among several treatment methods Catalytic Wet Air Oxidation (CWAO) treatment is considered as a useful and powerful method for removing phenol from waste waters. In this work, mathematical model of a trickle bed reactor (TBR) undergoing CWAO of phenol is developed and the best kinetic parameters of the relevant reaction are estimated based on experimental data (from the literature) using parameter estimation technique. The validated model is then utilized for further simulation and optimization of the process. Finally, the TBR is scaled up to predict the behavior of CWAO of phenol in industrial reactors. The optimal operating conditions based on maximum conversion and minimum cost in addition to the optimal distribution of the catalyst bed is considered in scaling up and the optimal ratio of the reactor length to reactor diameter is calculated with taking into account the hydrodynamic factors (radial and axial concentration and temperature distribution).