Trickle bed operation

Trickle-bed operation is the oldest and the most commonly used its development is described in a recent publication (VI). Cobalt-molybdenum catalysts may be used at a temperature of 360°C and a pressure of 57 atm for the hydrogenation of straight-run gas oils. 

The absorption of reactants (or desorption of products) in trickle-bed operation is a process step identical to that occurring in a packed-bed absorption process unaccompanied by chemical reaction in the liquid phase. The information on mass-transfer rates in such systems that is available in standard texts (N2, S6) is applicable to calculations regarding trickle beds. This information will not be reviewed in this paper, but it should be noted that it has been obtained almost exclusively for the more efficient types of packing material usually employed in absorption columns, such as rings, saddles, and spirals, and that there is an apparent lack of similar information for the particles of the shapes normally used in gas-liquid-particle operations, such as spheres and cylinders. 

It may be noted here that the expression trickle-bed operation covers a number of quite distinct flow patterns some studies of these that have been published in recent papers will be reviewed at the end of the section. 

In this section, four other papers may be finally referred to, which, in addition to liquid holdup, deal with various other aspects of liquid flow in trickle-bed operation. 

These filled tubes or capillaries hence are mini fixed beds with internal micro flow regions. Such approach is a link between real micro structured units and conventional equipment. The type of processing may be analogous to conventional processing, e.g. fixed-bed or trickle-bed operation. 

However, in contrast to the two classes of dispersive mixers mentioned before, the attached flow-through channel contains a packed bed of particles which may carry a catalyst. This chamber is much larger than the typical dimensions of the inlet channels (e.g. compare with Section 5.1.2). The packed bed and its interstices influence the gas/liquid flow patterns, e.g. a trickle-bed operation may be established. 

In advance, comparative fixed-bed measurements were undertaken. It was ensured that the performance of a plug-flow operation with both flows having the same direction is superior to trickle-bed operation, using counter-flow instead. The plug flow was assumed to model the slug-flow behavior in the micro reactor. 

Fig. 22. Thermocouple temperature readings in an experimental trickle bed operating under periodic flow interruption with t = 60 min and a time-average u = 1.65 mm/s (a) r = 0.5, (b) s = 0.25, (b) s = 0.1. (Figure from Haure et al., 1990, with permission, 1990 Elsevier Science Publishers.)...

Several parameters that tell what is going on in trickle bed operation have been correlated. Apply some of those correlations from the summary of Ramachandran Chaudhari (Three-Phase Chemical Reactors, 1983). 

This result indicates the need for an efficient distributor design for this trickle-bed operation at low flow rates (GL< 12). 

F, Larachi, A. Laurent, N. Midoux and G. Wild, Liquid saturation data in trickle beds operating under elevated pressure, AIChE Journal, 37 (1991) 1109-1112. 

Fig. 4.17. Flow regimes in three-phase fixed-bed reactors, (a) Gas and liquid in co-current downwards flow (trickle-bed operation). (b) Gas and liquid in co-current upwards flow (liquid floods bed), (c) Gas and liquid in countercurrent flow (not often used for catalytic reactors)...

In principle, any catalyst bed used for reactive distillation or trickle bed operation can also be applied in reactive stripping. The performance will depend mainly on the optimal ratio between catalyst hold-up, the gas-liquid and the liquid-solid interface. However, recycling of the strip gas flow makes a low pressure drop (and therefore a high voidage) especially beneficial. In countercurrent operation, flooding - a well-known problem - must be avoided. The present studies have focused on structured catalyst supports, developed for either reactive distillation or reactive stripping, with a particular emphasis being placed on the use of so-called film-flow monoliths. 

In monolithic catalyst carriers with wider channels, the hquid forms a film on the channel walls, whereas in the core of the channel a continuous gas phase exists. As shown by Lebens [10], countercurrent gas-liquid operation is now possible, and shows certain advantages over the countercurrent trickle bed operation. Typical channel diameters are 3-5 mm, and the geometric surface areas are between 550 and 1000 m2 m 3. Below the flooding point, almost no hydrodynamic interaction between the gas and hquid can be observed for example, the RTD is the same for both co-current and countercurrent operation. Apart from some surface waves, the film flow is completely laminar. 

Sulzer katapak-S were developed for use in reactive distillation, though their use for trickle bed operations [18, 19] and as catalyst carrier in bubble flow columns [20] has also been investigated. Like DX, these packings consist of corrugated sheets made from wire gauze. Every second channel between the corrugated sheets is closed to form a tea bag which is filled with catalyst particles (Fig. 8.13). Therefore, voidage (ca. 40 %) and surface area (440 m2 m-3) are approximately half of those for the DX. 

In the once-through studies reported in the literature, a downflow reactor scheme was used for catalytic hydrocracking (9) in contrast to an upflow reactor scheme used in this study. It has been reported in the literature that an upflow reactor scheme is superior to the usual trickle-bed operation for residual feedstocks (18,19). Desulfurization, denitrogena-tion, and demetallization conversions were better in an upflow reactor. 

Deactivation owing to metal poisoning was not determined. The Wyoming coal contained significant amounts of iron, calcium, magnesium, sodium, and titanium. Some of these metals would exist in the coal tar and be deposited on the catalyst. However, concentrations were too low to be significant compared with the aged catalyst from a trickle-bed operation. 

Since dissolved gas concentrations in the liquid phase are more difficult to measure experimentally than the liquid reactant concentration, Equation 8 evaluated at the reactor exit 5=1 represents the key equation for practical applications involving this model. Nevertheless, the resulting expression still contains a significant number of parameters, most of which cannot be calculated from first principles. This gives the model a complex form and makes it difficult to use with certainty for predictive purposes. Reaction rate parameters can be determined in a slurry and basket-type reactor with completely wetted catalyst particles of the same kind that are used in trickle flow operation so that DaQ, r 9 and A2 can be calculated for trickle-bed operation. This leaves four parameters (riCE> St gj, Biw, Bid) to be determined from the available contacting efficiency and mass transfer correlations. It was shown that the model in this form does not have good predictive ability (29,34). 

At low flow rates upflow behaves like a bubble column, i.e gas as a dispersed phase, liquid as a continuous phase. In downflow trickle-bed operation, gas is a continuous phase and liquid flows as a film. 

The pressure-drop correlations outlined above assume a constant value of e, the bed void fraction. In industrial-hydroprocessing trickle-bed operations, such as in a hydrodcsulfurization reactor, the pressure drop has been found to increase with time. A typical behavior of the pressure drop across an industrial HDS reactor as a function of time is shown in Fig. 6-4. The pressure drop remains essentially constant over a long initial period, where the correlations given above should be useful. After a while, however, as shown in Fig. 6-4, the pressure drop increases very rapidly with time until the operation requires termination due to an excessive pressure drop across the bed. 

The trickle-bed operation is characterized by comparatively poor heat-transfer properties. Schoenemann,88 for example, indicates difficulties in controlling temperatures in a trickle-bed reactor. 

SO removal. Regeneration can be accomplished in two ways (a) One can continuously or cyclically flush the activated carbon with water to remove the sulfuric acid as a by-product. Either trickle bed operation [45,46] or a series of cyclically loaded and purged beds may be employed, (b) One can heat the activated carbon in an inert atmosphere to decompose the sulfuric acid into SO and water and obtain a concentrated stream of gas phase SO . Thermal regeneration may lead to the loss of carbon as CO and CO . 

Trickle bed reactors have grown rapidly in importance in recent years because of their application in hydrodesulfurization of naphtha, kerosene, gasoil, and heavier petroleum fractions hydrocracking of heavy gasoil and atmospheric residues hydrotreating of lube oils and hydrogenation processes. In trickle bed operation the flow rates are much lower than those in absorbers. To avoid too low effectiveness factors in the reaction, the catalyst size is much smaller than that of the packing used in absorbers, which also means that the overall void fraction is much smaller. 

The Larkins and Sweeney equations were developed for downward bubble flow. In trickle bed operation, the liquid and gas flow rate are not as high as in packed absorbers, so that there is much less interaction. Single-phase flow pressure drop equations could be used as a first approximation, with the void fraction reduced to... 

There are no effective interfacial area correlations in the literature for the specific cases discussed here. The correlation that conies closest to that required for trickle bed operation is that of Puranik and Vogelpohl [29], which is for a continuous gas phase and a dispersed liquid phase, but in a countercurrent packed column, well below the loading point. They derived the following correlation (for piLfQ, = 1.5 kg/m s) ... 

Deviations from plug flow in the gas phase are not ordinarily of concern in trickle bed operation. 

As previously mentioned, the mass transfer from the gas to the active sites of the catalyst involves several steps. For trickle bed operation, in which the interaction between gas and liquid is limited, the values for kg and are of the same... 

The primary nses of trickle bed reactors are for hydrodesulfurization, hydrocracking, and hydrotreating of various high-boiUng petroleum fractions. The direct and capital costs are significantly less for trickle bed operation than for an equivalent hydrodesulfurization unit operating... 

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