Scaling up of trickle-bed reactors

A perennial problem in multiphase reactors is scale-up, that is, how to achieve the desired results in a large-scale reactor based on the observations made in the laboratory unit, which remains elusive due to complexities associated with transport-kinetic coupling [14]. The success of scale-up of trickle-bed reactors is based on the ability to understand and quantify the transport-kinetic interactions at the particle scale level (or single eddy scale), the interphase transport at the particle and reactor scales, and the flow pattern of each phase and phase contacting pattern and their changes with the changes in reactor scale and operating conditions [1]. 

The risks of using space velocity for scale-up of fixed-bed (gas-solid) catalytic reactors are well known [12]. Trickle-bed 

The recommended procedure for scale-up of trickle-bed reactors is, first, to establish in the laboratory the rate of reaction for single catalyst pellets which will include the effect of wetting efficiency [12]. If there is a soluble gaseous reactant, the rate should account for mass transfer from both the gas- and liquid-covered surfaces of the pellet. This basic rate data then can be used with intrareactor mass and, if necessary, energy conservation expressions to design the large-scale reactor. This second step should include the liquid distribution. The required mass and energy transport rates will limit application of this approach because the majority of the literature is concerned exclusively with nearly atmospheric conditions. 

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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). 

The design and the scale-up of trickle-bed reactors are still rather difficult problems despite of the high research activity in this area for many years. As a matter of fact an accurate modelling of these reactors should basically involve the knowledge of the fluid flow hydrodynamics as well as of the various heat and mass transport resistances between the three phases. The various attempts in modelling these processes and in predicting... 

Table 1.3 Success and Failure in Scale-Up of Trickle Bed Reactors with LHSV=Const...

Micro and macro thermal gradients are generated within or outside the pellets by the heat of reaction. This suggests that scaling up of Trickle Bed Reactors for exothermic reactions must be done with caution to avoid temperature excursions which could cause excessive vaporization of the liquid outside and inside the pellets and give rise to local hot spots, an increase in the heat release rate and a decrease of external heat transfer coefficients. 

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. 

But in literature still pseudo-homogeneous models are in use for scale-up in trickle-bed reactors, especially if hydrotreating processes of the petroleum industry are concerned, where the whole reaction dynamics is lumped totally either into the gas or the liquid phase,... 

The scale-up and design configurations of fluid-bed chemical reactors have evolved rapidly and empirically. An example is fluid catalytic cracking (FCC) [13]. The general fluid-bed concepts developed early. However, the correlations describing the various rate processes and other operational phenomena developed slowly because they could not easily be related back to already established data bases developed for other systems in the case of trickle-bed reactors, data developed for packed-bed absorption towers were utilized. 

A pure phenomenological model of such an intricate process, taking into account all possible reaction steps, is therefore a powerful tool for the scale up and the prediction of performances of trickle-bed reactors. Such a model (20 has proved to be able to correctly reproduce experimental data using only two adjustable parameters. It has been checked in several cases (hydrogenation of alphamethylstyrene (3J, hydrogenation of 2-butanone (, hydrorefining (J5) ), with more or less volatile liquid reactants and it appeared to be also useful to calculate a posteriori the extent of the different types of wetted catalyst area and their different effectiveness factor. 

The main features of monolith reactors (MR) combine the advantages of conventional slurry reactors (SR) and of trickle-bed reactors (TBR), avoiding their disadvantages, such as high pressure drop, mass transfer limitations, filtration of the catalyst, and mechanical stirring. Again, care must be taken to produce a uniform distribution of the flow at the reactor inlet. Scale-up can be expected to be straightforward in most other respects since the conditions within the individual channels are scale invariant. 

Fixed-bed catalytic reactors are widely applied to reaction systems in which the reactants are present in a single vapor phase. The scale-up and performance of commercial reactors can be predicted from experiments in small-scale reactors. On the other hand, the mixed-phase trickle bed reactor is considerably more complex to analyze and scale up. The performance of trickle bed reactors is influenced by many factors associated with mixed-phase (gas-liquid-solid) processing. Some of... 

In general, it can be concluded that substantial progresses have been made in the experimental and theoretical analysis of trickle-bed reactors under unsteady-state conditions. But until now these results are not sufficient for a priori design and scale-up of a periodically operated trickle-bed reactor. The mathematical reactor models, which are now available are not detailed enough to simulate all of the main transient behavior observed. For solving this problem specific correlations for specific model parameters (e.g. Hquid holdup, mass transfer gas-solid and liquid-solid, intrinsic chemical kinetic, etc.) determined under dynamic conditions are required. The available correlations for important hydrodynamic, mass-and heat-transfer parameters for periodically operated trickle-bed reactors leave a lot to be desired. Indeed, work for unsteady-state conditions on a larger scale may also be necessary. 

This review deals with the engineering aspects of trickle-bed reactors as it applies to general design, scale-up, pilot plant design and operation, and reactor troubleshooting. Also, research needs in trickle-bed technology are discussed. 

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. 

A comparison between monolith and trickle-bed reactors is shown in Table 1. The superiority of the monolith reactor over the conventional packed-bed reactor is mainly due to much lower pressure drop, the case of scaling up, and higher mass transfer rates in the former option. 

These trickle bed reactors normally operate in the downflow configuration and have a number of operational problems, including poor distribution of liquid and pulsing operation at high liquid and gas loading. Scale-up of these liquid-gas-solid reactors is much more difficult than for a gas-solid or gas-liquid reactor. Nevertheless, the downflow system is convenient when the bed is filled with small catalyst particles. And, because the catalyst particles are free from diffu-sional limitations, these reactors are quite effective as filters of the incoming feed. Any suspended fine solids, such as fine clays from production operations, accumulate at the front end of the bed. Eventually, this will lead to high-pressure differentials between the inlet and outlet end of the reactor. 

Three-phase fluidized beds and slurry reactors (see Figs. 30g-l) in which the solid catalyst is suspended in the liquid usually operate under conditions of homogeneous bubbly flow or chum turbulent flow (see regime map in Fig. 33). The presence of solids alters the bubble hydrodynamics to a significant extent. In recent years there has been considerable research effort on the study of the hydrodynamics of such systems (see, e.g., Fan, 1989). However, the scale-up aspects of such reactors are still a mater of some uncertainty, especially for systems with high solids concentration and operations at increased pressures it is for this reason that the Shell Middle Distillate Synthesis process adopts the multi-tubular trickle bed reactor concept (cf. Fig. 30e). The even distribution of liquid to thousands of tubes packed with catalyst, however poses problems of a different engineering nature. 

The hydrodynamics of trickle beds are complex, to say the least, and although an enormous amount of time and effort has been expended on research in this area, it is probably true to say that design and scale-up procedures are somewhat more tenuous than for fixed- or even fluid-bed reactors. Fortunately, there are relevant reviews that give some insight [C.N. Satterfield, Amer. Inst. Chem. Eng. Jl., 21, 209... 

What system parameters are important in the scale-up of a trickle-bed reactor to be used for a gas-liquid-solid catalytic reaction if the reaction occurs in the liquid phase only and is controlled by... 

A trickle bed reactor (TBR) consists of a fixed bed of catalyst particles, where liquid and gas phases flow cocurrently downward through the bed. Although its wide application in chemical and petrochemical industry it is one of the most complicated type of reactor in its design and scale-up. Essencially, the overall rate can be controlled by one or a combination of the following processes mass transfer between interphases, intraparticle diffusion, adsorption and surface reaction. The hydrodynamics, solid-liquid contacting efficiency and axial mixing can also affect the performance of TBR. 

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