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Contacting residence time distribution

The CFB catalytic cracking reactor plays an important role in the petroleum industry because of its better gas-solids contact and narrow residence time distribution, but its non-uniform radial flow structure and the extensive backmixing of gas and solids lead to a lower conversion rate and poorer selectivity to desired intermediate products [14]. [Pg.85]

In the compound mill, the cylinder is divided into a number of compartments by vertical perforated plates. The material flows axially along the mill and can pass from one compartment to the next only when its size has been reduced to less than that of the perforations in the plate. Each compartment is supplied with balls of a different size. The large balls are at the entry end and thus operate on the feed material, whilst the small balls come into contact with the material immediately before it is discharged. This results in economical operation and the formation of a uniform product. It also gives an improved residence time distribution for the material, since a single stage ball mill approximates closely to a completely mixed system. [Pg.127]

Analytical solutions for x and y as functions of the bed-length, z, and time, t, are available [45,52], The expressions are a useful extension of two-phase model applied to plug-flow. These two models are appropriate in describing the extraction of crushed or broken seeds to recover the seed oil, either in shallow beds or in plug flow. As shown by Sovova [52], applying the plug-flow model requires corrections for non-ideal residence-time distribution (non-plug flow) of the fluid in contact with the solid. [Pg.131]

The CFD simulations should be linked with the rate-based process simulator, providing important information on the process hydrodynamics in the form of correlations for mass transfer coefficients, specific contact area, liquid holdup, residence time distribution, and pressure drop. An ability to obtain these correlation via the purely theoretical way rather than by the traditional experimental one should be considered a significant advantage, because this brings a principal opportunity to virtually prototyping of new optimized internals for reactive separations. [Pg.339]

The rate-based stage model parameters describing the mass transfer and hydrodynamic behavior comprise mass transfer coefficients, specific contact area, liquid hold-up, residence time distribution characteristics and pressure drop. Usually they have to be determined by extensive and expensive experimental estimation procedures and correlated with process variables and specific internals properties. [Pg.5]

Residence-time distributions for the gas and liquid phases in this type of reactor can be evaluated easily. The reactor is operated under transient conditions if the catalyst decays rapidly. Otherwise, steady-state operation is obtained. Baffles can be installed to obtain better contact. When both homogeneous and heterogeneous reactions occur simultaneously, their rates can be separated by obtaining the results at various stirrer speeds. This type of reactor has several dis-... [Pg.156]

Various methods of contacting fluids with particulate solids are shown in Figure 5.13. These contacting methods include countercurrent, crosscurrent, and cocurrent plug flow as well as mixed solids flow-intermediate gas flow, and semi-batch operations. Consideration of the residence time distribution for each type of fluid-solid contact is necessary to understand its effect on the conversion. As a result of given residence time distribution, E t), the average conversion of B, Xb, is given by... [Pg.162]

A model of a reaction process is a set of data and equations that is believed to represent the performance of a specific vessel configuration (mixed, plug flow, laminar, dispersed, and so on). The equations include the stoichiometric relations, rate equations, heat and material balances, and auxiliary relations such as those of mass transfer, pressure variation, contacting efficiency, residence time distribution, and so on. The data describe physical and thermodynamic properties and, in the ultimate analysis, economic factors. [Pg.1827]

Aside from the requirement of a sharp residence time distribution, the ideal fixed-bed reactor should also allow all parts of the catalyst bed to fully participate in the overall conversion, i.e., all catalyst particles must be contacted by the reactant fluid. With a single fluid phase, this condition is generally met when the plug flow criterion is obeyed since in this case there is a uniform flow through the bed. However, in two... [Pg.19]

Johnson, R, et al. (1998). Determining the optimal theoretical residence time distribution for chlorine contact tanks. Aqua (Otford). 47, 5, 209-214. [Pg.795]

However, there are two possible diflSculties in determining A lO by physical absorption. First, the flow patterns and residence-time distributions of the phases may be undetermined (Cao is not a known function of CX), hence the value of ki a cannot be deduced from . Second, in efiicient contacting devices the gas and liquid may approach equilibrium quite closely. The determination oik a depends on the difference between the actual and the equilibrium extent and necessitates extremely accurate measurement of the flow rates. [Pg.41]

X 10 min and = 3.74 X lO J moD Consequently, the side reaction sets in at temperatures > 25 °C. Accordingly, improving heat removal is essential. Also, contact between product and the KOH feed solution should be avoided. Narrowing the residence time distribution is another means of reducing impurities. [Pg.106]

This assumes that all chemical species have the same residence time distribution, and is very convenient to compute the reaction paths for different contacting patterns. Matsuyama and Miyauchi [16] have also considered some aspects of this. An important conclusion of Wei [15] is that for a reactor with distribution of residence times, all reactions are slowed down in comparison with those in a plug flow reactor, but the faster reactions are slowed down a great deal more than the slower ones. Consequently, the occurrence of distribution of residence times makes all reaction rates of the characteristic species nearly equal. That is, the differences between the various reaction rates are decreased, thereby decreasing the selectivity. This is similar to the diffusion effects considered in Chapter 3. [Pg.609]

Contact FB units are characterized by the residence time distribution of the individual particles inside the unit. A broad residence time distribution is obtained in a back-mixed FB in which the length/width ratio of the bed is relatively small. The narrow residence time distribution is obtained in a plug-flow FB in which the length/width ratio of FB is very large. [Pg.942]

When a three phase system seems to be the best (or the sole) solution for a specific application, there remains the difficult task of selecting the most suitable reactor type among the numerous possibilities of contacting a gas and a liquid in the presence of a solid catalyst. Several papers have been devoted to this problem (see for example references 2,3, and 5) Fundamental characteristics such as residence time distribution are as important as technological aspects such as tightness of pressure vessels. Main features on which can be based a comparison between the two broad classes of three phase reactors - slurry and fixed bed-have been collected in Tables 2 and 3. Of course, such a general comparison is very rough and each mentioned item has to be discussed for every specific case. [Pg.699]

The development of an hydrodynamic model involves the prediction of pressure drop, hold-up, contact areas between phases, phases ratios and residence time distributions for a given column internal. Although various predictive models are available in the open domain (see e.g. Kooijman and Taylor (2000)), the model developed by The University of Texas Separations Research Program (SRP model) (Rocha et al., 2004, 1996 Fair et al., 2000) enjoys a widespread preference within packed columns internals. Lately, the Delft model has been introduced (Fair et al, 2000) and vahdated for the case of zig-zag triangular flow channels. Note that the following descriptions are referred to a VL system. [Pg.29]

The residence time distribution (RTD) is a probability distribution function used to characterize the time of contact and contacting pattern (such as for plug-flow or complete backmixing) within the reactors. Excessive retention of some elements and shortdrcuiting of others due to backmixing and other dispersive phenomena lead to a broad distribution in the residence times of individual molecules in the reactor. This tends to decrease conversion and exerts a negative influence on product selectivity/yield. The RTD depends on the flow regime and is characterized by Reynolds (Re) and Schmidt (Sc) numbers. [Pg.400]

This criterion is important to consider with respect to the kinetics of the desired reaction. Slow reactions require longer residence times in the reactor and not every reactor provides the same contact time of the gaseous educts with the catalyst Packed-bed microreactors offer the longest residence times, since the gaseous molecules have to pass through a region of closely packed catalyst particles. Unfortunately, this also negatively effects the residence time distribution. [Pg.1067]


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See also in sourсe #XX -- [ Pg.583 , Pg.584 ]




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