Scale trickle bed reactors

The present investigation has the overall objective of testing the predictive ability for a trickle-bed reactor model in which the gaseous reactant is limiting. First, a model is presented which contains particle-scale incomplete contacting as one of the key parameters. Second, model predictions for various limiting cases are compared to experimental results obtained in a laboratory scale trickle-bed reactor using independently measured model parameters and available literature mass transfer correlations. 

The holdups can play an important role in the reactor performance. For example, in a pilot-scale trickle-bed reactor, the liquid holdup can play an important role in changing the nature of the apparent kinetics of the reaction. When homogeneous and catalytic reactions occur simultaneously, the liquid holdup plays an important role in determining the relative rates of homogeneous and catalytic reactions. In a three-phase fluidized-bed reactor, the holdup of the solid phase plays an important role in the reaction rate, particularly when the solid phase is a reactant. The gas holdup, of course, always plays an important role in reactor performance when a gaseous reactant takes part in the reaction. 

It is difficult to ascertain whether the poor performance observed in pilot-scale trickle-bed reactors is due either to ineffective catalyst wetting or to the axial dispersion effects, because both these effects are physically realistic and both occur under similar operating conditions (i.e., low liquid flow, large catalyst size, and shorter beds). It should be noted, however, that the criterion for removing the axial dispersion effect is available. A similar criterion for removing ineffective catalyst wetting is, however, presently not available. 

When both gas and liquid flow downward over a string of spheres, it can simulate a trickle-bed reactor if the liquid flows downward in the form of a thin film. The hydrodynamics for this type of reactor are reasonably well known. Both the hydrodynamics of the liquid flow over a single sphere and the phenomena taking place at the junctions of two spheres have been extensively studied. Flow maldistribution encountered in the pilot-scale trickle-bed reactor is eliminated in this type of reactor. Furthermore, good estimations of the various mass-transfer resistances can be ascertained. The reactor is successfully used by Satterfield et al.st) for the catalytic hydrogenation of a-methyl styrene. Their experimental setup is shown in Fig. 5-9. 

Several other reports have also shown the importance of effective catalyst wetting on the performance of a bench-scale trickle-bed reactor. Hartman and Coughlin37 concluded that for sulfur dioxide oxidation in qojjntercurrejQt trickle-bed reactor packed with carbon particles, the catalyst was not completely wet at low liquid flow rates (of the order of 5 x 10 4 cm s-1). Sedricks and Kenney86 found that, during catalytic hydrogenation of crotonaldehyde in a cocurrent trickle-bed reactor, liquid seeped. into dry palladium-on-alumina... 

The effect of catalyst wetting on the performance of a bench-scale trickle-bed reactor was also theoretically evaluated by Sylvester and Pitayagulsarn.94,9S Using the method of moments of Suzuki and Smith,93 they developed a procedure to show the combined effects of axial dispersion, external diffusion, intraparticle diffusion, and surface reaction on the conversion for a first-order irreversible reaction in an isothermal trickle-bed reactor and evaluated the effect of catalyst wetting on these combined effects. 

Al-Dahhan, M.H. Wu, Y. Dudukovic, M.P. Reproducible technique for packing laboratory scale trickle-bed reactors with a mixture of catalyst and fines. Ind. Eng. Chem. Res. 1995, 34, 741. 

T-K Ranatakyla, S Toppinen, T Salmi, J Aittamaa. Investigation of the hydrogenation of some substituted alkylbenzenes in a laboratory scale trickle-bed reactor. J Chem Tech Biotechnol 67 265-275,1997. 

It seems that application of this type of force to multiphase catalytic reactors is still poorly explored. In the proposed work, we will illustrate the application of strong inhomogeneous magnetic fields to a small-scale trickle-bed reactor that consists of two-phase gas-liquid cocurrent flow through a fixed bed of catalyst... 

Metaxas, K., and Papayannakos, N. (2008), Gas-liquid mass transfer in a bench-scale trickle bed reactor used for benzene hydrogenation, Chemical Engineering Technology, 31(10) 1410-1417. 

Tsamatsoulis D, Papayannakos N. Axial dispersion and hold-up in a bench-scale trickle-bed reactor at operating conditions. Chem. Eng. Sci. 1994 49 523. 

Compared to the respective experiment with the two-phase reactor, the reaction rate in the trickle-bed is by a factor of about two lower. Although the operation of the laboratory-scale trickle bed reactor could possibly be improved by adjusting for optimal conditions with respect to the velocity of the liquid and the gas phase, a higher reaction rate than the hmiting intrinsic rate reached with the two-phase reactor is in any case not possible [details of the two-phase concept are given in Schmitz (2003) Schmitz, Datsevich, and Jess (2004) Wache et al. (2006) and Wache (2006)]. 

FIGURE 7.15 Sulfur profiles along the catalyst bed length in the bench-scale trickle-bed reactor at different times. (— -) 60s, ( ) 400s, (-X-) 900s, (-1700s, and (o) experimental value. 

Trickle Bed Hydrodesulfurization The first large-scale apph-cation of trickle bed reactors was to the hydrodesulfurization of petroleum oils in 1955. The temperature is elevated to enhance the specific-rate and the pressure is elevated to improve the solubihty of the... 

GL 23] [R 12] [P 16] Conversions near 70% were determined for a mini trickle-bed reactor (flow rate 20 mg min ) [36]. The corresponding reaction rate was 10 times larger than in typical batch operation on a laboratory-scale, which is restricted to milder conditions. 

The scale-up of trickle beds presents many difficulties mainly due to maldistribution of fluids, which leads to different routes for the liquid and gas, stagnant zones, and hot spots. In trickle-bed reactors, the particle diameter and residence time are the same for all scales. The consequence is that in different scales we have different Reynolds numbers and velocities. 

Failing to identify the limiting reactant can lead to failure in the scale-up of trickle-bed reactors (Dudukovic, 1999). Gas-limited reactions occur when the gaseous reactant is slightly soluble in the liquid and at moderate operating pressures. For liquid-limited reactions, concurrent upflow is preferred (packed bubble columns) as it provides for complete catalyst wetting and thus enhances the mass transfer from the liquid phase to the catalyst. On the other hand, for gas reactions, concurrent downflow operation (trickle-bed reactors), especially at partially wetted conditions, is preferred as it facilitates the mass transfer from the gas phase to the catalyst. The differences between upflow and downflow conditions disappear by the addition of fines (see Section 3.7.3, Wetting efficiency in trickle-bed reactors). 

Trickle-bed reactors are widely used in the oil industry because of reliability of their operation and for the predictability of their large-scale performance from tests on a pilot-plant scale. Further advantages of trickle-bed reactors are as follows The flow pattern is close to plug flow and relatively high reaction conversions may be achieved in a single reactor. If warranted, departures from ideal plug flow can be treated by a dispersed plug-flow model with a dispersion coefficient for each of the liquid and gas phases. 

Scale-up is in principle straightforward. Larger channel geometries (e.g., in the internally finned monolith channels) allow countercurrent operation of gas and liquid. Monolith reactors are intrinsically safer. The monolith channels have no radial communication in terms of mass transport, and the development of runaway by local hot spots in a trickle-bed reactor cannot occur. Moreover, when the feed of liquid or gas is stopped, the channels are quickly emptied. 

UOP then carried out pilot-scale tests at still higher pressures in a fully automated explosion cell to reproduce vendor work and to study conditions and kinetics. Design was based on direct hydrogen peroxide synthesis using a mini-trickle bed reactor with a micro mixer. 

This reactor also allows for easy laboratory scale operation for determining rate data, since the flow rate is low. Experimental-scale trickle beds can be on the order of 0.5 in. in diameter. Trickle bed reactors are used for the hydrodesulfurization of liquid petroleum fractions. 

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