Flow pattern slurry reactor

Toseland, B. A., Brown, D. M., Zou, B. S., and Dudukovi..M., Flow Patterns in a Slurry-Bubble-Column Reactor Conditions, Trans. Inst. Chem. Engrs., 73 297 (1995)... 

A reactor model based on solid particles in BMF may be used for situations in which there is deliberate mixing of the reacting system. An example is that of a fluid-solid system in a well-stirred tank (i.e., a CSTR)-usually referred to as a slurry reactor, since the fluid is normally a liquid (but may also include a gas phase) the system may be semibatch with respect to the solid phase, or may be continuous with respect to all phases (as considered here). Another example involves mixing of solid particles by virtue of the flow of fluid through them an important case is that of a fluidized bed, in which upward flow of fluid through the particles brings about a particular type of behavior. The treatment here is a crude approximation to this case the actual flow pattern and resulting performance in a fluidized bed are more complicated, and are dealt with further in Chapter 23. 

All types of contactors—trickle beds, slurry reactors, and fluidized beds—can be treated at the same time. What is important is to recognize the flow patterns of the contacting phases and which component, A or B, is in excess. First consider an excess of B. Here the flow pattern of liquid is not important. We only have to consider the flow pattern of the gas phase. So we have the following cases. 

In addition to using different catalyst flow patterns in packed and slurry reactors, the flow can be varied to attain different catalyst contacting patterns. As shown in Figure 7-27, many flow patterns such as radial flow and fluid recirculation can be used. These allow variation of the flow velocity u for a given reactor size and residence time x. These recirculation flow patterns approach the flow of recycle reactors so the reactor performance approaches that of a CSTR at high recirculation. 

The hydrodynamics of bubble columns and slurry bubble column reactors depend strongly on the flow regime (Figure 3.27). There are three flow patterns that prevail in these reactors (Wallis, 1969 Shall et al., 1982) ... 

Figure 1. Slurry reactors classified by the contacting pattern and mechanical devices (a) slurry (bubble) column (b) countercurrent column (c) co-current upflow (d) co-current downflow (e) stirred vessel (C) draft tube reactor (g) tray column (h) rotating disc or multi-agitated column reactor (i) three-phase spray column — liquid flow —> gas flow.

Pitch-blade turbine (paddle stirrer with pitched blades) and propeller stirrers provide high mixing with an axial flow pattern. Both of these stirrers are normally used for low-viscosity liquids and in vessels with baffles. They are well suited for providing liquid homogenization and suspension of solids in slurry reactors. The stirrers can also be used in viscous fluids and for vessels with H/dT > 1, which are generally encountered in fermentation processes. For these situations, axial flow is increased with the use of multistage stirrers with pitched stirring surfaces. 

Flow patterns in a mechanically agitated reactor with disk turbine, pitched-blade turbine, and propeller types of agitator are schematically illustrated by Joshi et al. (1982). The flow pattern in the presence of gas is described later in the section on slurry reactors. In each of these cases, the dimensionless velocity profile with respect to the impeller tip velocity has been found to be independent of the impeller speed and has shown slight dependence on the impeller diameter. 

In this chapter, first, the existing correlations for three-phase monolith reactors will be reviewed. It should be emphasized that most of these correlations were derived from a limited number of experiments, and care must be taken in applying them outside the ranges studied. Furthermore, most of the theoretical work concerns Taylor flow in cylindrical channels (see Chapter 9). However, for other geometries and flow patterns we have to rely on empirical or semiempirical correlations. Next, the modeling of the monolith reactors will be presented. On this basis, comparisons will be made between three basic types of continuous three-phase reactor monolith reactor (MR), trickle-bed reactor (TBR), and slurry reactor (SR). Finally, for MRs, factors important in the reactor design will be discussed. 

Finally, the chemical stability of the catalysts employed in this study was tested by means of XRD and EDXS analyses. The examination of fresh and used catalysts shows that during the reaction course metal ions are slowly leached into the aqueous solution, which can be attributed either to the temperature of operation or the presence of complexing carboxylic acids and benzoquinones in the liquid-phase. Contrary to the results obtained in continuous-flow fixed-bed reactors [8, 9], the extent of catalyst dissolution in the slurry reactor was considerable. This is probably due to the higher accumulation of benzoquinones which are known to form stable complexes with metal ions. Examination of the X-ray powder diffraction patterns of the molecular sieves before and after the liquid-phase phenol oxidation... 

Fig. 30. Contacting patterns and contactor types for gas-liquid-solid reactors, (a) Co-current downflow trickle bed. (b) Countercurrent flow trickle bed. (c) Co-current downflow of gas, liquid, and catalyst, (d) Downflow of catalyst and co-current upflow of gas and liquid, (e) Multi-tubular trickle bed with co-current flow of gas and liquid down tubes with catalyst packed inside them coolant on shell side, (f) Multi-tubular trickle bed with downflow of gas and liquid coolant inside the tubes, (g) Three-phase fluidized bed of solids with solids-free freeboard, (h) Three-phase slurry reactor with no solids-free freeboard, (i) Three-phase fluidized beds with horizontally disposed internals to achieve staging, (j) Three-phase slurry reactor with horizontally disposed internals to achieve staging, (k) Three-phase fluidized bed in which cooling tubes have been inserted coolant inside the tubes. (1) Three-phase slurry... 

The gas-liquid-sohd reactions are carried out in various types of reactors, such as packed beds, fluidized/slurry, and catalytic wall reactors (Figure 8.1). The advantages and limitations of these reactors are described in Table 8.1. Compared to fluid-sohd systems, an additional phase makes it difficult to predict flow patterns... 

In the case of airlift reactors, the flow pattern may be similar to that in bubble columns or closer to that two-phase flow in pipes (when the internal circulation is good), in which case the use of suitable correlations developed for pipes may be justified [55]. Blakebrough et al. studied the heat transfer characteristics of systems with microorganisms in an external loop airlift reactor and reported an increase in the rate of heat transfer [56], In an analytical study, Kawase and Kumagai [57] invoked the similarity between gas sparged pneumatic bioreactors and turbulent natural convection to develop a semi-theoretical framework for the prediction of Nusselt number in bubble columns and airlift reactors the predictions were in fair agreement with the limited experimental results [7,58] for polymer solutions and particulate slurries. 

What is proposed earlier is also in line with the three levels of reactor engineering discussed by Krishna and Sie [50] and indeed is eminently applicable in the context of three-phase slurry reactors. Naturally, the goal here is to decide on a flow pattern that optimally utilizes the catalyst. In other words, the catalyst has a certain intrinsic activity, and the contacting pattern should try and realize that activity in all parts of the reactor. Thus, level [I] design explained earlier must establish the effective performance metric at the catalyst level, which will be the major topic of discussion in this section. 

Ideally, purely from a reactor efficiency point of view, the ideal three-phase reactor would be a countercurrent one (shown schematically in Figure 6.3a), in which gas and liquid (with species A and B, respectively) enter from opposite directions, and where there is higher concentration of A (near gas inlet), one would have a depleted liquid stream (lower concentration of B) and vice versa at the other end. The catalyst particles would be suspended in slurry phase, and with this countercurrent trick, one would ensure relatively uniform rate on the catalyst particles no matter what their locations in the vessel. The ideal contacting flow pattern involves the countercurrent movement of gas and liquid (slurry) phases in a plug flow manner. 

Figure 6.3 Possible ideal contacting patterns in three phase slurry reactors, (a) Countercurrent (gas and liquid in plug flow).

The internals of the bubble column reactor may have a dramatic impact on the flow patterns of the bubbles and the liquid. Companies have not divulged details about the internals to date. Some details of the US DOE pilot plant (22.5 inch 0.57 m diameter) have been published [ 106]. In this report the dimensions of the cooling tubes, their location, and their number are provided. These cooling coils occupied about 10% of the total volume of their commercial reactors slurry volume. The gas holdup and bubble characteristics as well as their radial profiles were determined in a column that was about the size of the US DOE reactor [107-109]. Dense internals were found to increase the overall gas holdup and to alter the radial gas profile at various superficial gas velocities. The tube bundle in the column increased the liquid recirculation and eliminated the rise of bubbles in the wall region of the column. These results indicate that further studies of bubble column hydrodynamics are directed toward larger scale units equipped with heat exchange tubes. 

Figure 21 Particle flow patterns under various liquid contents in a fluidized bed reactor, (a) Dry particle fluidization, (b) particle agglomeration, (c) agglomerate segregation, (d) bottom channeling, (e) whole bed channeling, (1) paste or slurry bed.

Recently, a novel technology for three-phase processes has been developed the monolith catalyst, sometimes also called the frozen slurry reactor. Similar to catalytic gas-phase processes (Section 4.1), the active catalyst material and the catalyst carrier are flxed to the monolith structure. The gas and liquid flow through the monolith channels. The flow pattern in the vertical channels is iUustrated in Figure 6.14. At low gas velocities, a bubble flow dominates, and the bubble size distribution is even. At higher gas flow rates, larger... 

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