Slurry reactor liquid-solid mass transfer

Mass transfer across the liquid-solid interface in mechanically agitated liquids containing suspended solid particles has been the subject of much research, and the data obtained for these systems are probably to some extent applicable to systems containing, in addition, a dispersed gas phase. Liquid-solid mass transfer in such systems has apparently not been studied separately. Recently published studies include papers by Calderbank and Jones (C3), Barker and Treybal (B5), Harriott (H4), and Marangozis and Johnson (M3, M4). Satterfield and Sherwood (S2) have reviewed this subject with specific reference to applications in slurry-reactor analysis and design. 

The effectiveness factor is very low, indicating that intraparticle mass transfer resistance is very significant. The gas-liquid mass transfer resistance is also important, as expected. On the other hand, the liquid-solid mass transfer resistance is negligible. As a result, the rate of reaction in the slurry reactor is about 50 times higher than that in the trickle-bed. Therefore, in cases of such high rates of reaction, the slurry reactor is a better choice, although the gas-liquid mass transfer and the filtration of the catalyst may be a problem. 

Figure 13. Sherwood number for liquid-solid mass transfer in sparged stirred tank slurry reactors (adapted from Asai et al. [119]).

In a batch slurry reactor, the liquid-solid mass-transfer coefficient can be measured by dissolving a sparingly soluble solid in liquid. The concentration of dissolved solid in liquid (Bt) can be measured as a function of time, preferably by a continuous analytical device. Systems such as the dissolution of benzoic acid, jS-naphthol, naphthalene, or KMn04 in water can be used. A plot of B( as a function of time and the slope of such plot at time t = 0 can give ks as... 

If a transport parameter rc — CS/CL is defined, where Cs is the concentration of C at the catalyst surface, then Peterson134 showed that for gas-solid reactions t)c < rc, where c is the catalyst effectiveness factor for C. For three-phase slurry reactors, Reuther and Puri145 showed that rc could be less than t)C if the reaction order with respect to C is less than unity, the reaction occurs in the liquid phase, and the catalyst is finely divided. The effective diffusivity in the pores of the catalyst particle is considerably less if the pores are filled with liquid than if they are filled with gas. For finely divided catalyst, the Sherwood number for the liquid-solid mass-transfer coefficient based on catalyst particle diameter is two. 

Any form of convection, of course, increases the value of Ks. In slurry operation with no liquid flow, gas flow induces convection. In an agitated slurry reactor, stirring causes convection. In a pulsating slurry reactor, pulsation of the slurry induces convection and in a three-phase fluidized bed, the movements of both gas and liquid phases cause convection. Any one or more modes of convection will increase the value of the solid-liquid mass-transfer coefficient. In broad terms, the convective liquid-solid mass-transfer coefficient is correlated by-two steady state theories. Here we briefly review and compare them. 

In catalytic slurry reactors the locale of the reaction is the catalyst surface. Hence, in addition to the mass transfer resistance at the gas-liquid interface a further transport resistance may occur at the boundary layer around the catalyst particle. This is characterized by the solid-liquid mass transfer coefficient, kg, which has been the subject of many theoretical and experimental studies. Brief reviews are given by Shah (82). In general, the liquid-solid mass transfer coefficient is correlated by expressions like... 

Slurry reactor Effective utilization of catal t Good liquid—solid mass transfer Good heat transfer Moderate gpas—liquid mass transfer Catalyst separation is difficult and a filtration step is required Low conversion and selectivity in continuous mode following backmixing... 

On the basis of extensive computations which were performed with a three-phase dispersion model for bubble column slurry reactors accounting for all relevant jiie-nomena, i.e., dispersion in all phases, catalyst settling, nonlinear reaction kinetics, gas-liquid and liquid-solid mass transfer resistances, intraparticle diffusion and variable gas flow rate, the following conclusions can be stated ... 

In may be noted that a level [I] reactor selection can be done even with the effective reaction rate expressions (Equations 6.3 and 6.4). For instance, one should always attempt to select a reactor that helps to quicken the otherwise slowest step in the effective rate. For instance, if internal diffusion within catalyst particles is the limiting step, then one has to use fine particles in a slurry bubble column. If liquid-solid mass transfer is... 

When the reaction in the porous catalyst is very rapid, the conversion rate will be determined by gas/liquid or liquid/solid mass transfer. Particularly volumetric gas/liquid mass transfer coefficients (liquid side) are not very much different in slurry-reactors and in three phase packed beds (aU under optimum conditions). 

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... 

This gives a uniform description for the models <11> to <13>. The space-time-yield (specific absorption rate) can be calculated from the expressions given in Table 2 whereby the appropriate equations for Q are found from eq. (16) (model <11>), eq. (25) (model <12>), and eq. (13) (model <13>). The entire treatment can be extended to catalytic slurry reactors (BCSR) by considering the liquid/solid mass transfer resistance and pore diffusional effects. In this case the value of Q in the absorption rate expressions of Table 2 has to be replaced by Q which is defined by... 

Slurry phase and three-phase fluidized-bed reactor — high external mass transfer (gas-hquid, liquid—solid) — Low intraparticle resistance — Ease of catalyst addition/regeneration — Ease of thermal management — Catalyst separation — high axial mixing — Low catalyst load — High liquid-to-solid ratio... 

Only one publication on gas-liquid mass transfer in bubble-column slurry reactors has come to the author s attention. However, a relatively large volume of information regarding mass transfer between single bubbles or bubble swarms and pure liquid containing no suspended solids is available, and this information is probably of some relevance to the analysis of systems... 

Slurry Reactors. Slurry reactors are commonly used in situations where it is necessary to contact a liquid reactant or a solution containing the reactant with a solid catalyst. To facilitate mass transfer and effective catalyst utilization, the catalyst is usually suspended in powdered or in granular form. This type of reactor has been used where one of the reactants is normally a gas at the reaction conditions and the second reactant is a liquid, e.g., in the hydrogenation of various oils. The reactant gas is bubbled through the liquid, dissolves, and then diffuses to the catalyst surface. Obviously mass transfer limitations can be quite significant in those instances where three phases (the solid catalyst, and the liquid and gaseous reactants) are present and necessary to proceed rapidly from reactants to products. 

In high pressure work, slurry reactors are used when a solid catalyst is suspended in a liquid or supercritical fluid (either reactant or inert) and the second reactant is a high pressure gas or also a supercritical fluid. The slurry catalytic reactor will be used in the laboratory to try different catalyst batches or alternatives. Or to measure the reaction rate under high rotational speeds for assessing intrinsic kinetics. Or even it can be used at different catalyst loadings to assess mass transfer resistances. It can also be used in the laboratory to check the deactivating behaviour. 

A key feature of catalytic slurry reactors is that the particles are small ( 0.1 mm), so it is relatively easy to promote suspension by the mechanical action of the impeller. Moreover, because of their small size they travel together with the liquid, and therefore a significant mass transfer resistance develops at the liquid/solid interface that cannot be removed completely with the standard impellers. Also, because of the liquids large Prandtl number, the catalyst and the liquid are at the same temperature, so hot spots do not occur in multiphase slurry reactors. 

Gas-liquid bubble columns and gas-liquid-solid slurry bubble columns are widely used in the chemical and petrochemical industries for processes such as methanol synthesis, coal liquefaction, Fischer-Tropsch synthesis and separation methods such as solvent extraction and particle/gas flotation. The hydrodynamic behavior of gas-liquid bubble columns and gas-liquid-solid slurry bubble columns are of great importance for the design and scale-up of reactors. Although the hydrodynamics of the bubble and slurry bubble columns has been a subject of intensive research through experiments and computations, the flow structure quantification of complex multi-phase flows are still not well understood, especially in the three-dimensional region. In bubble and slurry bubble columns, the presence of gas bubbles plays an important role to induce appreciable liquid/solids mixing as well as mass transfer. The flows within these systems are divided into two... 

With a finely divided solid catalyst as typically used In the Flscher-Tropsch synthesis In slurry reactors It Is generally agreed that the major mass-transfer resistance, If It occurs, does so at the gas-liquid Interface. There are considerable disagreements about the magnitude of this resistance that stem from uncertainties about certain physical parameters, notably interfacial area, but also the solubility and mass transfer coefficients for H2 and CO that apply to this system. However when this resistance Is significant, the concentrations of Hg and CO in the liquid in contact with the solid catalyst become less than they would be otherwise, which not only reduces the observed rate of reaction but can also affect the product selectivity and the rate of formation of free carbon. 

Due to the consumption of reactants and the production or consumption of heat, concentration and temperature profiles can develop in the stagnant zone around and in the particle itself (Fig. 11). In the following paragraphs, criteria are derived to ensure that the effect of these gradients on the observed reaction rate is negligible [4, 27, 28]. In gas/liquid/solid slurry reactors, the mass transfer between the gas and liquid phase has to be considered, too (see Refs 9 and 29). 

E will be different from 1 only if R4 is small relative to / 2, resulting in a bulk concentration of c — 0 and in a real parallel mechanism of the enhancement. The advantage of the concept of the enhancement factor as defined by eq 33 is the separation of the influence of hydrodynamic effects on gas-liquid mass transfer (incorporated in Al) and of the effects induced by the presence of a solid surface (incorporated in E ), indeed in a similar way as is common in mass transfer with homogeneous reactions. The above analysis shows that an adequate description of mass transfer with chemical reaction in slurry reactors needs reliable data on ... 

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... 

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... 

The pulsating three-phase reactor has been examined only at the laboratory level. The pulsation gives good mixing and l)eat- and mass-transfer characteristics in the column. The first three types of gas-liquid-suspended-solid reactor are the most commonly used in practice. Schematic diagrams for these reactors are shown in Fig. l-3fn), (b), and (c), respectively. The agitated and nonagitated slurry... 

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