Fluid catalysts flow properties

Currently available data for the flow properties of the fluidized catalyst bed are fragmentary, since the local motion of the emulsion phase is diflicult to measure experimentally. Therefore, it is useful to clarify the flow properties of the bed in terms of our knowledge of bubble columns. First, the fluid-dynamic properties of the bubble columns will be explained then, the available data will be adapted to apply to fluid catalyst beds. The reader will be able to picture an emulsion phase of carefully prepared catalyst particles operating in intense turbulence for fluidized beds under conditions of practical interest. This turbulence distinguishes the flow properties of fluid catalyst beds from those of widely studied teeter beds. 

Based on the interpretation developed here, the assumption of constant Pt and its empirical expression by Eq. (3-32), seem to be sound physically for bubble columns. As shown in later sections, the flow properties of a fluid catalyst bed of good fluidity (e.g., cracking catalyst) are in many respects similar hence the same interpretation can be applied to the fluid catalyst bed (FCB). 

Longitudinal dispersion in the continuous phase (the liquid phase for a bubble column, and the emulsion phase for a fluidized catalyst bed) is closely related to flow properties of the equipment. Here, we wish to describe the longitudinal dispersion phenomena in terms of the fluid-dynamic properties of the equipment. The prime purpose is to test whether the fluid-dynamic analysis developed earlier is sound, but lon-... 

Good heat transfer on the outside of the reactor tube is essential but not sufficient because the heat transfer is limited at low flow rates at the inside film coefficient in the reacting stream. The same holds between catalyst particles and the streaming fluid, as in the case between the fluid and inside tube wall. This is why these reactors frequently exhibit ignition-extinction phenomena and non-reproducibility of results. Laboratory research workers untrained in the field of reactor thermal stability usually observe that the rate is not a continuous function of the temperature, as the Arrhenius relationship predicts, but that a definite minimum temperature is required to start the reaction. This is not a property of the reaction but a characteristic of the given system consisting of a reaction and a particular reactor. 

Introduce the above value in the simulation model in which the reaction rate is expressed per mass of catalyst Determine the total number of stages needed to achieve the desired conversion. Pay attention to the profiles of temperatures, concentrations, and reaction rate. Extract liquid and gas flows, as well as fluid properties. 

Three different zeolites (USY-zeolite, H-ZSM-5 and H-mordenite) were investigated in a computer controlled experimental equipment under supercritical conditions using the disproportionation of ethylbenzene as test reaction and butane or pentane as an inert gas. Experiments were carried out at a pressure of 50 bar, a flow rate of 450 ml/min (at standard temperature and pressure), a range of temperatures (573 - 673 K) and 0.8 as molar fraction of ethylbenzene (EB) in the feed. The results showed that an extraction of coke deposited on the catalysts strongly depends on the physico-chemical properties of the catalysts. Coke deposited on Lewis centres can be more easily dissolved by supercritical fluid than that on Brnsted centres. 

In this text, the conversion rate is used in relevant equations to avoid difficulties in applying the correct sign to the reaction rate in material balances. Note that the chemical conversion rate is not identical to the chemical reaction rate. The chemical reaction rate only reflects the chemical kinetics of the system, that is, the conversion rate measured under such conditions that it is not influenced by physical transport (diffusion and convective mass transfer) of reactants toward the reaction site or of product away from it. The reaction rate generally depends only on the composition of the reaction mixture, its temperature and pressure, and the properties of the catalyst. The conversion rate, in addition, can be influenced by the conditions of flow, mixing, and mass and heat transfer in the reaction system. For homogeneous reactions that proceed slowly with respect to potential physical transport, the conversion rate approximates the reaction rate. In contrast, for homogeneous reactions in poorly mixed fluids and for relatively rapid heterogeneous reactions, physical transport phenomena may reduce the conversion rate. In this case, the conversion rate is lower than the reaction rate. 

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