| Description |
Three phase ebullieted bed reactors are appropriate for processing petroleum residues, bitumen and bitumen derived liquids. The length of a three phase ebullieted bed reactor, which is long at the commercial scale, can be reduced, for process research and development studies in the laboratory, by decreasing the superficial liquid velocity and solid particle size. It is necessary to maintain same phase holdups of the commercial reactor, to simulate the process kinetics in a laboratory reactor. Similarity criteria that ensured identical phase holdups in commercial and laboratory units were identified, through extensive similitude studies. These criteria required the equality o f six dimensionless numbers. It was impractical to establish all the parameters in the set of dimensionless numbers at the desired values for reacting systems. Therefore, a procedure was developed to achieve similarity by varying a minimum number of parameters, such as liquid and gas superficial velocities and particle size. This resulted in two conditions, which when satisfied yielded essentially equal holdups in the two reactors. These criteria and procedure were validated using the generalized wake model and experimental data for three phase systems. The similitude studies identified the importance of the bubble rise velocity for scale down. Two different approaches were developed to predict the bubble rise velocity in three phase ebullieted beds. In the first approach, a mathematical model was developed to predict the volume of a single bubble generated at an orifice in a gas-liquid system at a constant gas flow rate. The model was based on a rigorous bubble closure mechanism and incorporated the interaction between the primary bubble and subsequent bubbles formed at the orifice at high gas flow rates. The model also calculated the distance traveled by the bubble from the orifice before it detached. The model is applicable for both viscous and nonviscous liquids and for systems over wide ranges of hydrodynamic properties. The model was validated by comparison with the available experimental data and it was found that this model represented an improvement over previous models. This model was used to approximate the value of the bubble size in a high pressure three phase ebullieted bed with small solid particles. In the second approach the concept of effective bubble rise velocity was introduced. The generalized wake model equations were manipulated to give correlations for the effective bubble rise velocity at atmospheric pressure. The parameters for the correlations were liquid and gas superficial velocities, liquid viscosity, surface tension and solid particle size. These correlations were categorized as per the type of three phase system, solid particle size and liquid and gas superficial velocities. Flow transition liquid velocities for various three phase systems were identified. Forms of the correlations were explained by addressing various hydrodynamic phenomena for three phase ebullieted beds such as flow regimes and their transitions, flow transition liquid velocity, solid wettability, bubble behavior, apparent bed viscosity and the effect of solid particles. The performance of the correlations was tested with experimental phase holdup data. The influence of pressure on bubble behavior and bubble rise velocity in a three phase ebullieted bed was considered. This led to the introduction of a pressure factor in the bubble rise velocity correlations. The modified correlations were used to predict the bubble rise velocity in three phase ebullieted bed operating at high temperature and high pressure. The predictions of the modified bubble rise velocity correlations were evaluated, using the concept of drift flux, against experimental plots available from the literature. The trends of drift flux vs. gas holdup in the plots were found satisfactory. Values of the gas and liquid densities, liquid viscosities and surface tensions at high temperature and high pressure were required for reactor scale down. A plot for temperature versus weight fraction distilled up to 813 K was obtained by simulated distillation for the native bitumen. A method was then developed to extrapolate the low temperature (813 K -) SIMDIS curve to high temperature (813 K + ) region by matching the measured value of specific gravity of the native bitumen with the specific gravity calculated from the extrapolated curve. The extrapolated SIMDIS curve was used to develop a predictive correlative procedure for estimating the viscosity and surface tension of bitumen fractions and bitumen at high temperature and high pressure. The predictive method identified a new mixing rule for fractions of heavy feeds, where the viscosities of the individual fractions vary over a few orders of magnitude. An overall procedure for scaling down a commercial three phase ebullieted bed reactor to a laboratory scale was then developed. The procedure ensured reduction in reactor length and maintained identical phase holdups and bubble rise velocity in both the reactors. The space velocity in the laboratory reactor was adjusted to achieve similar intraparticle mass transfer as the commercial reactor. Using the methods mentioned above for calculating the bubble rise velocity and physical properties of the feed and the overall scale down procedure, a detailed design of a laboratory scale three phase ebullieted bed reactor was carried out. This reactor can be used to carry out process development studies for hydrotreating/hydrocracking of bitumens and bitumen derived liquids in the laboratory, under conditions similar to the commercial reactor. |
