“…In the back-mixing controlling region, both the actual reaction rate constant k rA and the mass transfer capacitance coefficient k ob a b can be estimated by comparing the reaction data with the PME model. In the case of the results shown in Figure , k ob a b is estimated to be about 0.3 s -1 , and this value is in good agreement with the data reported by Nozaki et al, Furusaki et al, and Kai et al 9 Effect of contact time on apparent reaction constant in an FCB hydrodealkylation and comparison with the models. …”
Section: Assessment Of Selectivity In Fcb Reactionssupporting
confidence: 89%
“…The values of the mass transfer coefficient and the amount of directly contacting particles are derived from the experimental data from ethylene hydrogenation, which are shown in Figure . The experimental analysis of the mass transfer coefficient and the amount of direct contact catalyst were given by Furusaki et al According to our studies, − the value of k ob a b increased with U G and varied from 0.2 to 0.8 s -1 when U G was changed from 0.1 to 0.6 m/s. The fraction of direct contacting particles, ν, was below 0.05 and was not affected by gas veolocity .…”
Section: Successive Contact Mechanismmentioning
confidence: 68%
“…In the free board region, the contact efficiency is high, but the heat capacity is low due to the small particle density. Therefore, a temperature rise can be observed in the case of fast and exothermic reactions, e.g., ethylene hydrogenation . In highly exothermic reactions such as the partial oxidation of hydrocarbons, this may cause drastic instability.…”
Fluidized catalyst beds have been used in chemical and refinery processes and studied extensively for more than 50 years. However, the features of this reactor are still not understood perfectly. There is still much more to learn about the fluidized catalyst beds. It is important to elucidate the bubble behavior and the contact between catalyst particles and reactant components in the reactors. Understanding the features of the reactors contributes to the proper catalyst design together with the appropriate design and operation of the reactors. In this paper, the influence of gas properties on the bubble behavior is described, and pseudo-3-dimensional images of bubbles are shown. The important effects of direct and successive contact in the bed are discussed regarding conversion and selectivity. The reaction analysis showed that the selectivity could be improved by the effective use of the direct contact catalyst particles. The synergy effect of the three factors, i.e., catalyst activity, contacting feature between gas and particles, and transport/ mixing properties in the bed, plays a salient effect in raising selectivity of consecutive reactions.
“…In the back-mixing controlling region, both the actual reaction rate constant k rA and the mass transfer capacitance coefficient k ob a b can be estimated by comparing the reaction data with the PME model. In the case of the results shown in Figure , k ob a b is estimated to be about 0.3 s -1 , and this value is in good agreement with the data reported by Nozaki et al, Furusaki et al, and Kai et al 9 Effect of contact time on apparent reaction constant in an FCB hydrodealkylation and comparison with the models. …”
Section: Assessment Of Selectivity In Fcb Reactionssupporting
confidence: 89%
“…The values of the mass transfer coefficient and the amount of directly contacting particles are derived from the experimental data from ethylene hydrogenation, which are shown in Figure . The experimental analysis of the mass transfer coefficient and the amount of direct contact catalyst were given by Furusaki et al According to our studies, − the value of k ob a b increased with U G and varied from 0.2 to 0.8 s -1 when U G was changed from 0.1 to 0.6 m/s. The fraction of direct contacting particles, ν, was below 0.05 and was not affected by gas veolocity .…”
Section: Successive Contact Mechanismmentioning
confidence: 68%
“…In the free board region, the contact efficiency is high, but the heat capacity is low due to the small particle density. Therefore, a temperature rise can be observed in the case of fast and exothermic reactions, e.g., ethylene hydrogenation . In highly exothermic reactions such as the partial oxidation of hydrocarbons, this may cause drastic instability.…”
Fluidized catalyst beds have been used in chemical and refinery processes and studied extensively for more than 50 years. However, the features of this reactor are still not understood perfectly. There is still much more to learn about the fluidized catalyst beds. It is important to elucidate the bubble behavior and the contact between catalyst particles and reactant components in the reactors. Understanding the features of the reactors contributes to the proper catalyst design together with the appropriate design and operation of the reactors. In this paper, the influence of gas properties on the bubble behavior is described, and pseudo-3-dimensional images of bubbles are shown. The important effects of direct and successive contact in the bed are discussed regarding conversion and selectivity. The reaction analysis showed that the selectivity could be improved by the effective use of the direct contact catalyst particles. The synergy effect of the three factors, i.e., catalyst activity, contacting feature between gas and particles, and transport/ mixing properties in the bed, plays a salient effect in raising selectivity of consecutive reactions.
“…For the other type of model, the empirical correlations are derived from fluidization experiments. Numerous correlations for determining inter‐phase mass transfer have been developed for bubbling beds or turbulent beds 9, 19–21. In these models, the inter‐phase mass transfer efficiency in the fluidized bed is usually expressed by a volumetric mass transfer coefficient ( k g a i ).…”
“…Numerous correlations for determining inter-phase mass transfer have been developed for bubbling beds or turbulent beds. 9,[19][20][21] In these models, the inter-phase mass transfer efficiency in the fluidized bed is usually expressed by a volumetric mass transfer coefficient (k g a i ). Miyauchi-Masao 9 presented an empirical correlation to predict the volumetric mass transfer coefficient in bubbling and turbulent beds of FCC catalyst for a superficial gas velocity range of 0.1-0.5 m/s, which was expressed as…”
This article presents an experimental and computational study of mass transfer characteristics and stripping efficiency of bubbling fluidized beds within FCCU strippers. The hydrodynamic and mass transfer characteristics are investigated experimentally. Based on the granular kinetics theory, a two-fluid CFD model coupled with a modified Gidaspow drag model and an empirical mass transfer model has been developed and verified against the experimental results. The overall trend of the time-averaged bed density, concentration of tracer and stripping efficiency at various superficial gas velocities were in reasonable agreement with the experimental data. It was found that stripping efficiency depends strongly on the operating conditions as well as on the internal configuration of the strippers. Increasing superficial gas velocity increases stripping efficiency. Internal baffles improve break-up and redistribution of bubbles and enhance the gas-solids contacting remarkably, showing a benefit for the mass transfer in the V-baffled stripper. AIChE J, 54: 1164AIChE J, 54: -1177AIChE J, 54: , 2008 Keywords: fluidization, FCCU stripper, stripping efficiency, Eulerian simulation, FCC catalysts
American Institute of Chemical Engineers
IntroductionFluid catalytic cracking (FCC) is the main heavy oil conversion process in most petroleum refineries.1 Circulating fluidized bed technology is used in modern FCC units where cracking reactions take place as the vaporized gas oil feed and catalyst flow up the riser. Having been disengaged from the product gas, the catalyst needs to be regenerated because of deactivation due to coke deposition. However, hydrocarbon products are entrained, adsorbed, or present on the spent catalyst. It is important to strip these hydrocarbons off quickly and efficiently prior to regeneration. Incomplete stripping leads to loss of valuable product due to their combustion in the regenerator. The increase in hydrocarbon burned also increases the regenerator temperature, and subsequently, affects the FCC unit heat balance, while promoting faster catalyst degradation and mechanical wear of components. 2,3 Stripping is generally accomplished in a bubbling or turbulent fluidized bed by adding steam which bubbles upwards, counter-currently to the down flowing catalyst particles. 4 Understanding the hydrodynamics, mass and heat transfer in the stripper is essential for improving its performance and optimizing the design process. However, research on the stripper published in the open literature has been scarce until recently. Senior et al. 5 have described in detail the flow probCorrespondence concerning this article should be addressed to Chunming Xu at xcm@cup.edu.cn.
American Institute of Chemical EngineersAIChE Journal May 2008 Vol. 54, No. 5 1164 lems encountered in commercial FCCU stripper operation. Cui et al. 6,7 investigated experimentally the gas-solids mixing and the effect of jet configuration on fluidized bed stripping in a dynamically scaled fluid coker stripper.With the increase of computational...
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