Slurry reactors parameters

Hydrogenation of lactose to lactitol on sponge itickel and mtheitium catalysts was studied experimentally in a laboratory-scale slurry reactor to reveal the true reaction paths. Parameter estimation was carried out with rival and the final results suggest that sorbitol and galactitol are primarily formed from lactitol. The conversion of the reactant (lactose), as well as the yields of the main (lactitol) and by-products were described very well by the kinetic model developed. The model includes the effects of concentrations, hydrogen pressure and temperature on reaction rates and product distribution. The model can be used for optinuzation of the process conditions to obtain highest possible yields of lactitol and suppressing the amounts of by-products. 

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. 

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. 

Probably, for most slurry reactor applications, information on the value of the product kLa is sufficient for design purposes. In some cases, however, information on the individual parameters a and/or ki, can be useful. For instance, the reactor capacity will depend on a, rather than on the product k a, if the reaction is so fast that all conversion takes place within the stagnant film (film theory) around the gas bubbles. For first-order conversion kinetics in the porous catalyst particles this will occur for... 

The important design parameters for a stirred slurry reactor are the power input by the stirrer, the pumping flow rate of the stirrer, which affects the... 

In this section, we first evaluate the design of conventional nonaerated and aerated stirred slurry reactors. Since the design of mechanically agitated gas-liquid reactors has already been discussed in Section II, here the main emphasis is placed on the effects of solids on the design parameters. We subsequently illustrate some special-purpose slurry reactors used in the chemical and petrochemical industries. Novel slurry reactors used in biological or polymeric industries are discussed in Sections VI and VII, respectively. 

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. 

Figures 21a, b show the 4-CP, 4-CC, and HQ concentrations derived from inserting the estimated parameters in the kinetic model and a comparison with the experimental data under different operating conditions. Symbols correspond to experimental data and solid lines to model predictions calculated with Equations (64)-(66) and Equations (71)-(74). Eor these experimental runs, the RMSE was less than 14.4%. These experimental 4-CC and HQ concentrations are in agreement with the proposed kinetic mechanism of parallel formafion of fhe intermediate species (Figure 16), and also with the series-parallel kinetic model reported by Salaices et al. (2004) to describe the photocatalytic conversion of phenol in a slurry reactor under various operating conditions. ...

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. 

The fundamental objection to the above relations is that they are derived assuming steady-state flow. In practice, the intensity of turbulence in agitated slurry reactors is time dependent. Also, an accurate estimate of the relative velocity between the liquid and solid is often difficult.45,12S The relative velocity has been related to various system parameters by Kuboi et al.67,68... 

In our discussion of surface reactions in Chapter 11 we assumed that each point in the interior of the entire catalyst surface was accessible to the same reactant concentration. However, where the reactants diffuse into the pores within the catalyst pellet, the concentration at the pore mouth will be higher than that inside the pore, and we see that the entire catalytic surface is not accessible to the same concentration. To account for variations in concentration throughout the pellet, we introduce a parameter known as the effectiveness factor. In this chapter we will develop models for diffusion and reaction in two-phase systems, which include catalyst pellets and CVD reactors. The types of reactors discussed in this chapter will include packed beds, bubbling fluidized beds, slurry reactors, and trickle beds. After studying this chapter you will be able to describe diffusion and reaction in two- and three-phase systems, determine when internal pore diffusion limits the overall rate of reaction, describe how to go about eliminating this limitation, and develop models for systems in which both diffusion and reaction play a role (e.g., CVD). 

Figure 8.11 Design chart for equation (8-112). Semi-batch slurry reactor with reaction first-order in both A and B. Parameter is 0 at r = 0.

Now, our quest for knowledge concerning gas-liquid reactors, if we look at it, began with equation (8-164) so we should feel nearly saturated at this point. In fact, though, there are many other cases considered in the work of Russell et ah, that may be of use in certain applications. We have taken what might be considered the most important, or most frequently encountered for presentation. As in the case for fluid-bed or slurry reactors, we must now determine where the many parameters appearing in the reactor equations for gas-liquid systems originated. But first, an example. 

Addition of modifiers can result in an extremely strong catalyst carrier (lowest line in Fig. 4). In this way it can be possible to strengthen the material independently of some other parameters, notably pore volume and/or diameter. However, still the total pool of alumina properties needs to be harmonized to the slurry reactor operation. 

Three phase systems have been the main focus of activities in chemical reaction engineering, and the many novel aspects of them are too numerous to cover here, hence only a few examples will be referenced. In the case of gas-1iquid-sparingly soluble solid, it has been demonstrated that particles substantially smaller than the diffusion film thickness of film model can enhance the specific rates of mass transfer if the reaction is sufficiently fast (45). Work in this area has been persistently pursued by Sada and coworkers (46,47). Recently Alper et al. (24) has pointed out and demonstrated that in catalytic slurry reactors similar enhancement can be observed if the catalyst particles are sufficiently small. There is however some dispute on the order of magnitude of the enhancement (48,49). Another aspect is complex reactions and in the case of slurry reactors the product distribution may well depend on the degree of diffusional resistance (50). Dynamic methods have been ingeniously employed to obtain physicochemical parameters in slurry reactors (51). 

Selectivity, which is one of the most important characteristics of an industrial process, depends on several parameters temperature control, residence time distribution, gas and liquid hold-ups, catalyst loading, catalyst type, mass transfer rates etc... If homogeneous side reactions are awkward, fixed beds give better results.But if the desired product can react further on the catalyst, small catalyst particles have to be preferred to avoid concentration gradients in the pores and slurry reactors are the best. In this last case, the poor residence time distri-... 

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. ... 

Mass (and heat) transport steps sometimes can be rate limiting in agitated and aerated slurry reactors.Therefore methods have to be provided to determine the relative importance of these steps and furthermore equations have to be at hand for the estimation of the special transport parameters in their dependence on physical properties, operating conditions and reactor geometry. 

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... 

The reaction progress is monitored ofF-Une by HPLC. Flow rates, residence times and initial concentrations of 4-chlorophenol are varied and kinetic parameters are calculated from the data obtained. It can be shown that the photocatalytic reaction is governed by Langmuir-Hinshelwood kinetics. The calculation of Damkohler numbers shows that no mass transfer limitation exists in the microreactor, hence the calculated kinetic data really represent the intrinsic kinetics of the reaction. Photonic efficiencies in the microreactor are still somewhat lower than in batch-type slurry reactors. This finding is indicative of the need to improve the catalytic activity of the deposited photocatalyst in comparison with commercially available catalysts such as Degussa P25 and Sachtleben Hombikat UV 100. The illuminated specific surface area in the microchannel reactor surpasses that of conventional photocatalytic reactors by a factor of 4-400 depending on the particular conventional reactor type. 

In terms of industrial use, the aforementioned three-phase slurry reactors are in themselves amenable for qualitative comparison in terms of their physical attributes and the various operating parameters. While the specifics of these attributes are determined by the process chemistry and detailed design (guidelines to which is discussed later in this chapter), Table 6.4 provides at a glance qualitative comparison of these attributes. 

The weighted sum of squares between measured and estimated concentrations was minimized by a hybrid simplex-Levenberg-Marquardt algorithm implemented in the simulation software Modest (Haario 1994). The model equations were solved in situ during the parameter estimation by the backward difference method. The estimated parameters were the kinetic and adsorption equilibrium constants of the system. The simulation results revealed that the model was able to describe the behaviour of the system. The parameter values were reasonable and comparable with values obtained in previous studies concerning citral hydrogenation in a slurry reactor (Tiainen 1998). 

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