Superficial velocity slurry reactors

Furthermore, the superficial gas velocity usG in slurry reactors is equivalent to the superficial gas velocity in a fixed bed ... 

Here, it has to be noted that for calculating the Peclet number in fixed beds, the actual velocity has to be used, i.e. the interstitial velocity, which influences the degree of mixing. In slurry bubble column reactors, the real velocity of the fluid is the bubble velocity, which is much higher than the gas superficial velocity. The mean bubble rise velocity for a batch liquid is (eq (3.201))... 

The gas holdup in a slurry reactor depends upon superficial gas velocity, power consumption, the surface tension and viscosity of the liquids, and the solids concentration. For the first three parameters, the relationship cg oc yO.36-o.75pO.26-o.470.o.36-o.65 holds. For low solids concentration and waterlike liquids, the relationship eg = f(P/V, ug) is useful, although the nature of such a relationship depends upon the foaming characteristics of the liquids. An increase in solids concentration decreases gas holdup, whereas an increase in viscosity first increases and then decreases the gas holdup. A decrease in surface tension and an increase in stirrer speed increases the gas holdup. 

In slurry reactors, the liquid phase is completely backmixed, whereas backmixing in the gas and solid phases may not be complete. The gas-phase mixing depends on the design of the impeller and the nature of the bubbles, as well as the superficial gas velocity. The presence of gas reduces liquid-phase mixing however, an increase in gas flow increases the mixing. The mixing is also dependent upon the coalescence rate of the bubbles. 

The scale-up of monolith reactors is expected to be much simpler. This is due to the fact that the only difference between the laboratory and industrial monolith reactors is the number of monolith channels, provided that the inlet flow distribution is satisfactory. In slurry reactors, scale-up problems might appear. These are connected with reactor geometry, low gas superficial velocity, nonuniform catalyst concentration in the liquid, and a significant back-mixing of the gas phase. 

P14-8b The degree of backmixing in a tall slurry reactor was analyzed by injecting a pulse of methyl orange into the column (presented at the AIChE Los Angeles meeting, November 1982). For a superficial gas velocity of 10 cm/s and a liquid velocity of 3 cm/s ... 

The gas hold-up of the HyperCat system was studied using an air-water system. The gas superficial velocities were varied from 0.1-1.0 m/s. The results are shown in Figure 2. The HyperCat gas hold-up is slightly lower than that in a conventional bubble column reactor ( 30% reduction) and comparable to that in a slurry bubble column with 30-35% solids concentration (4). 

Hydrodynamics of slurry reactors includes the study of minimum gas velocity or power input to just suspend the particles (or to fully homogeneously suspend the particles), bubble dynamics and the holdup fractions of gas, solids and liquid phases. A complicating problem is the large number of slurry reactor types in use (see fig. 1) and the fact that most correlations available are at least partially of an empirical nature. We will therefore restrict ourselves to sparged slurry columns and slurries in stirred vessels. A second problem is the difference with three phase fluidization. To avoid too much overlap we will only consider those cases where superficial liquid velocities are so low that its contribution to suspension of the particles is relatively unimportant. 

Mills, et al [il8] tested this model with the hydrogenation of a-methylstyrene dissolved in n-hexane. A reactor 1.9 cm in diameter by 21 cm long was used packed with 1.6 mm cylindrical catalyst particles. The catalyst consisted of a 5 palladium on alumina catalyst. The effectiveness factor of the catalyst was obtained in a stirred tank slurry reactor and in a stirred basket reactor. Liquid superficial mass velocities of 0.1 1 to 2.H kg/m s... 

In this type of reactor, gas superficial velocity is of the order of 0.1-0.3 m/s which is low enough to avoid mechanical interactions between gas and liquid. The velocity of the liquid, in the range 1 - 8.10 m/s is still low, but sufficient to guarantee satisfactory external wetting of the catalyst particles. Table 1 shows advantages and disadvantages of Fixed Bed Multiphase Reactors. Table 2 shows the characteristic parameters of TBRs compared to the two other important multiphase reactors Stirred Slurry and Flooded Fixed Bed Reactor. 

For the sake of developing commercial reactors with high performance for direct synthesis of DME process, a novel circulating slurry bed reactor was developed. The reactor consists of a riser, down-comer, gas-liquid separator, gas distributor and specially designed internals for mass transfer and heat removal intensification [3], Due to density difference between the riser and down-comer, the slurry phase is eirculated in the reactor. A fairly good flow structure can be obtained and the heat and mass transfer can be intensified even at a relatively low superficial gas velocity. 

Ideally, the axial velocity through the cross-flow unit should be greater than about 4-6 m/s to minimize the boundary layer of particles near the membrane surface. The wax permeate flow from the filter is limited by a control valve actuated by a reactor-level controller. Hence, a constant inventory of slurry is maintained within the SBCR system as long as the superficial gas velocity remains constant. Changes in the gas holdup due to a variable gas velocity are calculated... 

Gas holdup is an important hydrodynamic parameter in stirred reactors, because it determines the gas-liquid interfacial area and hence the mass transfer rate. Several studies on gas holdup in agitated gas-liquid systems have been reported, and a number of correlations have been proposed. These are summarized in Table VIII. For a slurry system, only a few studies have been reported (Kurten and Zehner, 1979 Wiedmann et al, 1980). In general, the gas holdup depends on superficial gas velocity, power consumption, surface tension and viscosity of liquids, and the solid concentration. The dependence of gas holdup on gas velocity, power consumption, and surface tension of the liquid can be described as... 

Based upon the analysis of the previous SBCR runs (in 1995-96), several more design changes were carried out to the SBCR system to increase the conversion stability. An automatic level controller was added to the overhead slurry/gas separation tank. This insured a constant inventory of catalyst particles was being maintained in the reactor vessel if the superficial gas velocity within the column was constant. 

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. 

---