Slurry reactors analysis

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. 

P12C-2 Use the references given in Ind. Eng. Chem. Prod. Res. Lev. /4, 226 (1975) to define the iodine value, saponification number, acid number, and experimental setup. Use the slurry reactor analysis to evaluate the effects of mass transfer and determine if there are any mass transfer limitations. 

The slurry reactor analysis given above employed the concept of an overall effectiveness factor. It is informative to break down the problem into analysis of individual phase effectiveness factors assembled together as was done for the three-phase fluid-bed model. Equating the rates of the individual steps in a manner similar to... 

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... 

Kinetic experiments were carried out isothermaUy in autoclave reactors of sizes 300 and 600 ml. The stirring rate was typically 1800 rpm. In most cases, the reactors were operated as slurry reactors with small catalyst particles (45-90 tm), but comparative experiments were carried out with a static basket using large catalyst pellets. HPLC analysis was appHed for product analysis [22, 23]. 

Attempts have been made to expand the technique to include the analysis of soil biotransformations f23.29V While the hydrodynamic nature and physical structure of soil systems vary widely and are difficult to establish with certainty, two limiting conditions may be specified. The first is where the soil particles are suspended and all phases are well-mixed. This case is not typically found in nature, but is found in various types of engineered soil-slurry reactors. The reactors currently used in our systems experiments include continuous stirred tank reactors (CSTRs) operated to minimize soil washout. 

This latter interpretation would mean that with the approach depicted in Fig. 10, the catalyst itself could be monitored. The authors reported that the silica-supported Nafion could not be observed in the beginning of their experiments and appeared in the spectra only after the catalyst interacted with octanol. This observation may indicate that the octyl groups promote the sticking of the catalyst particles onto the ATR probe, within the evanescent field. However, the example also shows that this approach may not be without problems, because it depends on the adsorption of the particles from the slurry reactor onto the ATR element. This process is accompanied by the adsorption of molecules on the catalyst surface and complicates the analysis. More important, as also indicated by the work of Mul et al. (74). this adsorption depends on the surface properties of the catalyst particles and the ATR element. These properties are prone to change as a function of conversion in a batch process and are therefore hardly predictable. 

On each of these, random and structured reactors behave quite differently. In terms of costs and catalyst loading, random packed-bed reactors usually are most favorable. So why would one use structured reactors As will become clear, in many of the concerns listed, structured reactors are to be preferred. Precision in catalytic processes is the basis for process improvement. It does not make sense to develop the best possible catalyst and to use it in an unsatisfactory reactor. Both the catalyst and the reactor should be close to perfect. Random packed beds do not fulfill this requirement. They are not homogeneous, because maldistributions always occur at the reactor wall these are unavoidable, originating form the looser packing there. These maldistributions lead to nonuniform flow and concentration profiles, and even hot spots can arise (1). A similar analysis holds for slurry reactors. For instance, in a mechanically stirred tank reactor the mixing intensity is highly non-uniform and conditions exist where only a relatively small annulus around the tip of the stirrer is an effective reaction space. 

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 ... 

KolmogorofTs theory Brian et al.,9 Elenkov et al.,26 and Middleman84 used Kolmogoroff s theory of local isotropic turbulence in an attempt to correlate the effective relative velocity with some macroscopic variables, such as stirrer speed and particle diameter. From the dimensionless analysis of agitated slurry reactors,45,84 they suggested a correlation... 

P7C-3 In J. Catal, 79, 132 (1983), a mechanism was proposed for the catalyzed hydrogenation of pyridine in slurry reactors. Reexamine the data and model using an Eley-Rideai adsorption mechanism and comment on the appropriateness of this new analysis. 

One of the things we want to achieve in our analysis of slurry reactors is to learn how to detect which resistance is the largest (i.e., slowest step) and how we might operate the reactor to decrease the resistance of this step and thereby increase the efficiency of the reactor. 

Fundamentals The basic reaction and transport steps in trickle bed reactors are similar to those in slurry reactors. The main differences are the correlations used to determine the mass transfer coefficients. In addition, if there is more than one component in the gas phase (e.g., liquid has a high vapor pressure or one of the entering gases is inert), there is one additional transport step in the gas phase. Figure 12-17shows the various transport steps in trickle bed reactors. Following our analysis for slurry reactors we develop the equations for the rate of transport of each step. The steps involving reactant A in the gas phase are... 

Cybulski, A., Stankiewicz, A., Edvinsson Albers, R.K. and Moulijn, J.A. (1999), Monolithic reactors for fine chemicals industries A comparative analysis of a monolithic reactor and a mechanically agitated slurry reactor, Chem. Eng. Sci., 54, 2351-2358. 

Our objective here is to study quantitatively how these external physical processes affect the rate. Such processes are designated as external to signify that they are completely separated from, and in series with, the chemical reaction on the catalyst surface. For porous catalysts both reaction and heat and mass transfer occur at the same internal location within the catalyst pellet. The quantitative analysis in this case requires simultaneous treatment of the physical and chemical steps. The effect of these internal physical processes will be considered in Chap, 11. It should be noted that such internal effects significantly affect the global rate only for comparatively large catalyst pellets. Hence they may be important only for fixed-bed catalytic reactors or gas-solid noncatalytic reactors (see Chap. 14), where large solid particles are employed. In contrast, external physical processes may be important for all types of fluid-solid heterogeneous reactions. In this chapter we shall consider first the gas-solid fixed-bed reactor, then the fluidized-bed case, and finally the slurry reactor. 

Calderbank et al. studied the hydrogenation of ethylene using a large concentration of Raney nickel particles in a slurry reactor in order to approach these conditions. Analysis of the data indicated that the controlling step was the mass transfer of hydrogen from gas bubble to bulk liquid. 

In the laboratory either integral or differential (see Sec. 4-3) tubular units or stirred-tank reactors may be used. There are advantages in using stirred-tank reactors for kinetic studies. Steady-state operation with well-defined residence-time conditions and uniform concentrations in the fluid and on the solid catalyst are achieved. Isothermal behavior in the fluid phase is attainable. Stirred tanks have long been used for homogeneous liquid-phase reactors and slurry reactors, and recently reactors of this type have been developed for large catalyst pellets. Some of these are described in Sec. 12-3. When either a stirred-tank or a differential reactor is employed, the global rate is obtained directly, and the analysis procedure described above can be initiated immediately. 

The activity tests of liquid-phase oxidation of aqueous phenol solution were conducted in a semibatch slurry reactor at operating conditions given in the caption of Figure 1. The experimental apparatus, the procedure of these measurements and the analysis of the reaction samples are described in detail in a preceding paper [6]. Additional kinetic and mechanistic investigations were carried out in an isothermal, differentially operated "liquid-saturated" fixed-bed reactor [8, 9] which was packed with a pretreated EX-1144.3 catalyst (Sfld-Chemie... 

The analysis of the effects of catalyst deactivation on CSTR performance is straightforward and there is really not too much to write about however, this can be of considerable importance in the design of slurry reactors, which will be discussed in Chapter 8. We can start with the familiar relationship for a first-order reaction given in equation (4-68)... 

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. ... 

Slurry reactors are also sometimes used in the semi-batch or batch-filling mode, and we should look at an analysis of this type of operation. These are most often used in situations where a gas phase is passed through an agitated slurry phase with... 

At first glance gas-liquid reactors might appear to be easier to analyze than slurry reactors since they both involve gas and liquid phases, but the solid phase is not present in the former. On the other hand, the fluid mechanics and transport behavior have been investigated in more detail in gas-liquid systems than in gas-liquid-solid systems, so it is possible to include a little more detail in analysis if desired. The analysis and design equations can also be applied to liquid-liquid systems, as described below. 

Kinetic Analysis. A stirred tank slurry reactor was constructed, using a 500 ml polyethylene flask equipped with gas dispersing stirrer and aeration was performed with pressurized air. 

Rgure 17.4 The variables and dimensionless groups used in the analysis of continuous three-phase slurry reactors. 

Complex reactions with consecutive or parallel steps are commonly encountered in catalytic slurry reactors, e.g. in hydration of propylene oxide, ethynylation of formaldehyde to bu-tinediol or hydrogenation of unsatturated oils [35, 36, 37, 38, 39]. If one can assume in a simplified analysis of these reactions that the concentrations of the liquid reactants are in excess, compared to the gaseous one (Aj ) the rate of reaction of the gaseous reactant (in the absence of intraparticle diffusion) can be expressed as... 

Chaudhari and Ramachandran (8-9) give a detailed analysis of slurry reactors including all mass transfer resistances and different kinds of kinetic expressions but they do not account for catalyst settling, dispersion and conversion induced volume change in the gas phase. 

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