Trickle-bed reactors pressure drop

Pressure Drop, Mass and Heat Transfer Pressure drop is more important in reactor design than in analysis or simulation. The size of the compressor is dictated by pressure drop across the reactor, especially in the case of gas recycle. Compressor costs can be significant and can influence the aspect ratio of a packed or trickle bed reactor. Pressure drop correlations often may depend on the geometry, the scale, and the fluids used in data generation. Prior to using literature correlations, it often is advisable to validate the correlation with measurements on a similar system at a relevant scale. 

In catalytic applications, monoliths can provide better control of the contact time of reactants and products with the catalyst. This leads to a potential increase in selectivity. Together with the advantages over conventional trickle-bed reactors (pressure-drop surface area, short diffusion lengths), this makes the monohth reactor very suitable for use in consecutive reaction schemes, such as selective oxidation or hydrogenation. Literature dealing with carbon monolith structures is not yet extensive, however, and a limited number of applications have been reported, as shown in Table 11.2. 

Reactors with a packed bed of catalyst are identical to those for gas-liquid reactions filled with inert packing. Trickle-bed reactors are probably the most commonly used reactors with a fixed bed of catalyst. A draft-tube reactor (loop reactor) can contain a catalytic packing (see Fig. 5.4-9) inside the central tube. Stmctured catalysts similar to structural packings in distillation and absorption columns or in static mixers, which are characterized by a low pressure drop, can also be inserted into the draft tube. Recently, a monolithic reactor (Fig. 5.4-11) has been developed, which is an alternative to the trickle-bed reactor. The monolith catalyst has the shape of a block with straight narrow channels on the walls of which catalytic species are deposited. The already extremely low pressure drop by friction is compensated by gravity forces. Consequently, the pressure in the gas phase is constant over the whole height of the reactor. If needed, the gas can be recirculated internally without the necessity of using an external pump. 

For gas-phase reactions, the pressure drop through the reactor is usually less than 10% of the inlet pressure6. The pressure drop through trickle-bed reactors is usually less than 1 bar. A value of 0.5 bar is often a reasonable first estimate for packed and trickle-bed reactors, although pressure drops can be higher. The pressure drop through lluidized-bed reactors is usually between 0.02 and 0.1 bar. 

In terms of shape and size, catalysts are typically presented as extrudates (cylindrical, tri-lobes or four-lobes), in sizes of diameters from 0.8 mm to about 1.7 mm and lengths from 3 mm to about 7 mm. The effect of size and shape on operation concerns the pressure drop control, the fluid flow through the bed, the interparticle and intraparticle flow, the diffusion of the fluids from the external surface to the internal surface. The three and four lobes extrudates facilitates diffusion, but they are usually more fragile than the cylindrical extrudates increasing the risk of pressure drop build up. For operation of a trickle bed reactor with heavy feedstocks, for which the diffusion limitations are important, lobed extrudates are preferred. Meanwhile, for vapor phase reactors large cylindrical extrudates are used. 

In this correlation, CGS units are used. It can be seen that the pressure drop in this reactor is lower than that in the trickle-bed reactor, under the same operating conditions. 

The second section presents a review of studies concerning counter-currently and co-currently down-flow conditions in fixed bed gas-liquid-solid reactors operating at elevated pressures. The various consequences induced by the presence of elevated pressures are detailed for Trickle Bed Reactors (TBR). Hydrodynamic parameters including flow regimes, two-phase pressure drop and liquid hold-up are examined. The scarce mass transfer data such gas-liquid interfacial area, liquid-side and gas-side mass transfer coefficients are reported. 

Such relationships have been inspired from the work of Saez and Carbonell [26] to correlate macroscopically the pressure-drop data corresponding to the low-interaction regime in trickle-bed reactors. 

F. Larachi, A. Laurent, N. Midoux and G. Wild, Experimental study of a trickle-bed reactor operating at high pressure two-phase pressure drop and liquid saturation, Chem. Engng. Science, 46 (1991) 1233-1246. 

M.H. Al-Dahhan and M.P. Dudukovic, Pressure drop and liquid hold-up in high pressure trickle-bed reactors, Chem. Engng. Science, 49 (1994) 5681-5698. 

The increased interfacial area in the microreactor led to an increased pressure drop. The energy dissipation factor, the power unit per reactor volume, of the microreactor process was thus higher (sv = 2-5 kW/m3) than that of the laboratory trickle-bed reactors (sv = 0.01-0.2 kW/m3) [277]. This is, however, outperformed by the still larger gain in mass transfer so that the net performance of the microreactor is better. 

In this paper correlations presented by Sato et al. for liquid holdup and pressure drop in trickle bed reactors were used to examine the characteristics of large-scale columns. The trickling-pulsing transition relationship given by Ng was also employed to determine the flow regime present. Some interesting phenomena were observed, specifically ... 

A correlation more widely used to evaluate the pressure drop in trickle-bed reactors is that of Larkins.et al.48 According to this correlation, the overall two-pfiase energy loss for the gas and liquid passing through the reactor is related to the two individual single-phase energy losses as follows ... 

The pressure drop in a trickle-bed reactor can also be obtained using an Ergun-type equation. This approximate technique is described by Charpentier.12... 

As mentioned earlier, the cocurrent gas-liquid downflow and, in particular, the trickle-flow operation is one of the most widely used three-phase operations in the hydroprocessing industry. The liquid holdup in such a reactor takes on added importance because it is usually low compared to the one for cocurrent upflow under similar flow conditions. Earlier we showed that the pressure drop in a trickle-bed reactor can be related to the liquid holdup. The effective catalyst wetting, as well as the thickness of the liquid film surrounding the catalyst particles, also depends strongly on the liquid holdup. 

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. 

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. 

Advantages of trickle-bed reactors are high catalyst load, no need for catalyst separation, no attrition, and very limited backmixing. Disadvantages are lower catalyst effectiveness and selectivity, flow maldistributions, and large pressure drop. 

Continuous stirred-tank reactors (CSTRs) have been routinely employed for producer gas fermentations. A two-stage reactor system has also been used to maximize ethanol production and minimize the formation of byproducts. Carbon monoxide and hydrogen conversions of 90% and 70%, respectively, were observed in the first reactor, while they were about 70% and 10% in the second reactor. High ethanol-to-acetate ratios were achieved by the use of such a dual reactor system. Bubble colunms are also commonly used for industrial fermentations. A comparative study was performed between a CSTR and a bubble column reactor for CO fermentation using Peptostreptococcus productus. Higher conversion rates of CO were observed with the bubble column without the use of any additional agitation. Producer gas fermentation with packed bubble colunms and trickle bed reactors has also been studied. The trickle bed reactor has a low pressure drop and liquid hold-up, and the conversion rates were the highest compared to CSTRs and bubble columns. 

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