Slurry reactors holdup

Various methods may be used for the determination of gas holdup—for example, displacement measurements and tracer experiments. Farley and Ray (F2) have described the use of gamma-radiation absorption measurement for the determination of gas holdup in a slurry reactor for the Fischer-Tropsch synthesis. 

Consider a bubble-free slurry. It is known that solid concentrations up to 10% can be handled in slurry reactors (Perry and Green, 1999). Thus, for particle densities of 1-3 g/cm3 and water as liquid phase, the maximum values of VS/VL are 0.1-0.03, which means that hs is 3.2-9.1%. However, for low ms, e.g. 2% (w/v) (g/100 cm3), these values drop to 0.6-1.9%, which is fairly low. Considering that on introducing the gas-phase, the total volume of the reactor will be even higher, the solid-phase holdup is decreased even more and becomes minimal in many practical applications. ... 

Note that the parameter VL/VR in a slurry reactor is the fractional liquid holdup. In the packed bubble bed reactor and the trickle-bed reactor, under complete recycling of the liquid stream, VL/VR is the ratio of total volume of the liquid that is processed (recycled) to the volume of reactor, and is always greater than 1. By recycling, it is possible to process a larger volume of liquid than the reactor volume by having a surge tank in the recycle line. 

Hydrodynamics of slurry reactors include the 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 variety in reactor types (sec Fig. I) and the fact that most correlations are of an empirical nature. We will therefore focus on sparged slurry columns and slurries in stirred vessels. 

No systematic research has been done yet on the influence of pressure and gas density on the holdup in three-phase slurry reactors. The only data [45-51] for three-phase bubble columns under pressure suggest rather high values of eq under pressure. Qualitatively, this is in line with the effect of a high density as predicted by Wilkinson [44], The effect might decrease with increased solids concentration [52]. Clearly, additional research is necessary here. 

Power or energy dissipated in the aerated suspension has to be large enough (a) to suspend all solid particles and (b) to disperse the gas phase into small enough bubbles. It is essential to determine the power consumption of the stirrer in agitated slurry reactors, as this quantity is required in the prediction of parameters such as gas holdup, gas-liquid interfacial area, and mass- and heat-transfer coefficients. In the absence of gas bubbling, the power number Po, is defined as... 

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 this chapter, we review the reported studies on the hydrodynamics, holdups, and RTD of the various phases (or axial dispersion in various phases), as well as the mass-transfer (gas-liquid, liquid-solid, and slurry-wall), and heat-transfer characteristics of these types of reactors. It should be noted that the three-phase slurry reactor is presently a subject of considerable research investigation. In some cases, the work performed in two-phase (either gas-liquid or liquid-solid) reactors is applicable to three-phase reactors however, this type of extrapolation is kept to a minimum. Details of the equivalent two-phase reactors are considered to be outside the scope of this chapter. 

A three-phase slurry reactor is characterized by the holdup of the three phases, satisfying the equation... 

A review of earlier studies on gas, liquid, and solid holdups in a three-phase slurry reactor is given by Ostergaard.97 Kato54 studied the effects of gas velocity, particle size, the amount of solids and liquid in the bed, and the density of the solids on the gas holdup. The gas holdup [defined as volume of gas/(volume of gas + volume of liquid)] decreased with increasing particle size and amount of solids in the bed, and with the decreasing nominal gas velocity. 

When the heat transfer is considered, the slurry reactors are more efficient, due to large liquid holdup and a relatively high flow velocity of the reaction mixture at the heat exchange surface. Also, it is relatively easy to arrange heat-exchanging devices in the slurry reactors as compared to monolith reactors. 

In slurry systems, similar to fluidized beds, the overall rate of chemical transformation is governed by a series of reaction and mass-transfer steps that proceed simultaneously. Thus, we have mass transfer from the bubble phase to the gas-liquid interface, transport of the reactant into the bulk liquid and then to the catalyst, possible diffusion within the catalyst pore structure, adsorption and finally reaction. Then all of this goes the other way for product. Similar steps are to be considered for heat transfer, but because of small particle sizes and the heat capacity of the liquid phase, significant temperature gradients are not often encountered in slurry reactors. The most important factors in analysis and design are fluid holdups, interfacial area, bubble and catalyst particle sizes and size distribution, and the state of mixing of the liquid phase. ... 

The most important fluiddynamic parameters of aerated stirred slurry reactors are energy dissipation and pumping efficiency of agitator and gas throughput, gas-holdup and mean bubble diameter produced (i.e. interfacial area resulting) and flooding characteristics. ... 

Energy dissipation and gas holdup. Gas holdup in aerated stirred slurry reactors is usually defined as fraction Eq of the total volume of the aerated suspension. 

In this chapter it was shown that the major engineering parameters which might affect the performance of a FT slurry reactor can be estimated from rather reliable correlations. There are, however, some controversial results in the literature which concern gas holdup and interfacial area (bubble diameter). Additional studies would be valuable for further clarification of this point. However, one can state, at least, that gas holdup and interfacial area are surprisingly large in the... 

Minimum velocity for complete solids suspension Gas holdup Loop slurry reactors... 

In many solid/liquid/gas systems the liquid phase is the continuous one (section 4.7.1). The other two phases are often dispers, which means that the liquid phase is the "intermediate one that separates the other two. In that case we have either a slurry reactor or a three phase-packed bed reactor. The relative merits of these have been discussed in sections 4,722 and 4,7,2.3, The final choice may be determined by the desired selectivity. When the reaction product tends to undergo a consecutive reaction in the liquid phase, the liquid holdup has to be low (section... 

Bubble columns. Tracers are used in bubble columns and gas-sparged slurry reactors mainly to determine the backmixing parameters of the liquid phase and/or gas-liquid or liquid-solid mass transfer parameters. They can be used for evaluation of holdup along the lines reviewed in the previous Section 6.2.1. However, there are simpler means of evaluating holdup in bubble columns, e.g. monitoring the difference in liquid level with gas and without gas flow. Numerous liquid phase tracer studies of backmixing have been conducted (132-149). Steady-state or continuous tracer inputs (132,134,140,142) as well as transient studies with pulse inputs (136,141,142,146) were used. Salts such as KC Jl or NaCil, sulfuric acid and dyes were employed as tracers. Electroconductivity detectors and spectrophotometers were used for tracer detection. The interpretation of results relied on the axial dispersion model. Various correlations for the dispersion... 

BCR are particularly well suited to carry out reactions in the slow reaction regime of absorption. Due to the high liquid holdup BCR provide for a large liquid volume where the reaction can take place. Also, in slurry reactors where the reaction takes place at the surface of the solid catalyst particles belong to the slow reaction regime. Only a few exceptions are known where absorption enhancement due to the slurry phase reaction has been observed [6, 20 - 22]. Strictly speaking, enhancement and hence transition to the fast reaction regime can only be expected if the diameters of the particle fines are considerably less than the liquid film thickness at the gas/liquid interface. 

Stirred-slurry operation, 120-123 holdup, axial dispersion, 122-123 mass transfer, 120-122 reactors, 80 Subcooling, 236-238 inlet, 261 Subreactors, 363 Sulfite-oxidation, 300-301 Summerfield, combustion equation, 44-43 Surface-active agents, 327-333 experiment, 327-329 theory, 329-333... 

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... 

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