Reactors trickle-bed

Selection of fluidised-bed operating conditions for partial catalytic oxidation. 6.3 TRICKLE-BED REACTOR 

Research with pilot scale units has shown that the major resistances to mass transfer of reactant to catalyst are within the liquid film surrounding the wetted catalyst particles and also intraparticle diffusion. A description of these resistances is afforded by Fig. 14. Equating the rate of mass transfer across the liquid film to the reaction rate, first order in hydrogen concentration 

It is possible that the pores of wetted catalyst particles eire filled with liquid. Hence, by virtue of the low values of liquid diffusivities (ca. 10 cm s ), the effectiveness factor will almost certainly be less than unity. A criterion for assessing the importance of mass transfer in the trickling liquid film has been suggested by Satterfield [40] who argued that if liquid film mass transport were important, the rate of reaction could be equated to the rate of mass transfer across the liquid film. For a spherical catalyst particle with diameter dp, the volume of the enveloping liquid fim is 7rdp /6 and the corresponding interfacial area for mass transfer is TTdn. Hence 

If the criteria adopted is that when Cj 0.95c, liquid film mass transport is rate-limiting, then eqn. (69) reduces to the inequality 

Now reaction limits the rate so the mass-balance equation becomes 

The residence times of gas and liquid in a completely stirred reactor are equal, 

As with the falling film reactor, the rate of mass transfer to the catalyst goes as R, while the size of the reactor goes as R, so this reactor becomes very inefficient except for very small-diameter tubes. However, we can overcome this problem, not by using many small tubes in parallel, but by allowing the gas and liquid to flow (trickle) over porous catalyst pellets in a trickle bed reactor rather than down a vertical wall, as in the catalytic wall reactor. 

The petroleum hydrotreating reactor is the most important example of this reactor. As discussed in Chapter 2, the hydrogenation and cracking of the heavy fraction of crude oil to produce lighter and volatile products occurs in an overall reaction 

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Gas-liquid mixtures are sometimes reacted in packed beds. The gas and the liquid usually flow cocurrently. Such trickle-bed reactors have the advantage that residence times of the liquid are shorter than in countercurrent operation. This can be useful in avoiding unwanted side reactions. 

Conversions of ca 75% are obtained for propylene hydration over cation-exchange resins in a trickle-bed reactor (102). Excess Hquid water and gaseous propylene are fed concurrentiy into a downflow, fixed-bed reactor at 400 K and 3.0—10.0 MPa (30—100 atm). Selectivity to isopropanol is ca 92%, and the product alcohol is recovered by azeotropic distillation with benzene. 

R. A. Holub, Hydrodynamics of Trickle Bed Reactors, D.Sc. Thesis, Washington University, St. Louis, Mo., May 1990. 

The overhead of the depropanizer is sent to the propylene fractionator. The methylacetylene (MA) and propadiene (PD) are usually hydrogenated before entering the tower. An MAPD converter is similar to an acetylene converter, but operates at a lower temperature and in the Hquid phase. Due to recent advances in catalysis, the hydrogenation is performed at low temperatures (50—90°C) in trickle bed reactors (69). Ordy rarely are methylacetylene and propadiene recovered. 

R Liquid film flowing over solid particles with air present, trickle bed reactors, fixed bed... 

Toluene hydrodealkylation to benzene and methane Phthalic anhydride by air oxidation of naphthalene Trickle bed reactor for hydrodesulfurizatiou... 

Some contrasting characteristics of the main lands of three-phase reactors are summarized in Table 23-15. In trickle bed reactors both phases usually flow down, the liquid as a film over the packing. In flooded reactors, the gas and hquid flow upward through a fixed oed. Slurry reactors keep the solids in suspension mechanically the overflow may be a clear liquid or a slurry, and the gas disengages from the... 

Interfacial area 20-50% of geometrical Like trickle bed reactor 100-1,500 mVm 100-400 mVm Less than for entrained solids... 

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... 

The effect of physical processes on reactor performance is more complex than for two-phase systems because both gas-liquid and liquid-solid interphase transport effects may be coupled with the intrinsic rate. The most common types of three-phase reactors are the slurry and trickle-bed reactors. These have found wide applications in the petroleum industry. A slurry reactor is a multi-phase flow reactor in which the reactant gas is bubbled through a solution containing solid catalyst particles. The reactor may operate continuously as a steady flow system with respect to both gas and liquid phases. Alternatively, a fixed charge of liquid is initially added to the stirred vessel, and the gas is continuously added such that the reactor is batch with respect to the liquid phase. This method is used in some hydrogenation reactions such as hydrogenation of oils in a slurry of nickel catalyst particles. Figure 4-15 shows a slurry-type reactor used for polymerization of ethylene in a sluiTy of solid catalyst particles in a solvent of cyclohexane. 

The calcium bisulfite acid used in the manufacture of sulfite cellulose is the product of reaction between gaseous sulfur dioxide, liquid water, and limestone. The reaction is normally carried out in trickle-bed reactors by the so-called Jenssen tower operation (E3). The use of gas-liquid fluidized beds has been suggested for this purpose (V7). The process is an example of a noncatalytic process involving three phases. 

It is well known that trickle-flow operation is characterized by comparatively poor heat-transfer properties, this being one of the disadvantages of this type of operation. Schoenemann (S4), for example, refers to the difficulties of controlling temperature in trickle-bed reactors. 

Comparatively few kinetic experiments in trickle-bed reactors have been described in the literature. 

Compare Equation (11.42) with Equation (9.1). The standard model for a two-phase, packed-bed reactor is a PDE that allows for radial dispersion. Most trickle-bed reactors have large diameters and operate adiabaticaUy so that radial gradients do not arise. They are thus governed by ODEs. If a mixing term is required, the axial dispersion model can be used for one or both of the phases. See Equations (11.33) and (11.34). 

Gallezot, P., Nicolaus, N., Fleche, G., Fuertes, P., and Perrard, A. (1998) Glucose hydrogenation on ruthenium catalysts in a trickle-bed reactor. J. 

Two basically different reactor technologies are currently in operation low temperature and high temperature. The former operates at -220 °C and 25-45 bar, employing either a multitubular, fixed bed (i.e. trickle bed) reactor or a slurry bubble column reactor with the catalyst suspended in the liquid hydrocarbon wax product. 

Figure 1. Continuous/trickle-bed reactor circulation of fluids.

Cas/Liquid Micro Flow Packed-bed or Trickle-bed Reactors... 

The cyclohexene hydrogenation is a well-studied process especially in conventional trickle-bed reactors (see original citations in [11,12]) and thus serves well as a model reaction. In particular, flow-pattern maps were derived and kinetics were determined. In addition, mass transfer can be analysed quantitatively for new reactor concepts and processing conditions, as overall mass transfer coefficients were determined and energy dissipations are known. In lieu of benchmarking micro-reactor performance to that of conventional equipment such as trickle-bed reactors, such a knowledge base facilitates proper, reliable and detailed comparison. 

The reaction is carried out using a Pt/Al203 catalyst [11,12]. Information on this reaction when conducted in trickle-bed reactors is available, comprising flow-pattern maps, kinetic data, mass transfer data and energy dissipation data (see original citations in [11]). 

GL 16] ]R 12] ]P 15] Using a simple thin-film model for mass transfer, values for the overall mass transfer coefficient were determined for both micro-channel processing and laboratory trickle-bed reactors [11]. The value for micro-reactor processing (fCL = 5-15 s ) exceeds the performance of the laboratory tool Ki a = 0.01-0.08 s ) [11, 12], However, more energy has to be spent for that purpose (see the next section). 

The oxidation of benzyl alcohol to benzaldehyde is carried out for elucidating mass transfer effects in a mini trickle-bed reactor [58]. 

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. 

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. 

Column reactors are the second most popular reactors in the fine chemistry sector. They are mainly dedicated reactors adjusted for a particular process although in many cases column reactors can easily be adapted for another process. Cocurrently operated bubble (possibly packed) columns with upflow of both phases and trickle-bed reactors with downflow are widely used. The diameter of column reactors varies from tens of centimetres to metres, while their height ranges from two metres up to twenty metres. Larger column reactors also have been designed and operated in bulk chemicals plants. The typical catalyst particle size ranges from 1.5 mm (in trickle-bed reactors) to 10 mm (in countercurrent columns) depending on the particular application. The temperature and pressure are limited only by the material of construction and corrosivity of the reaction mixture. 

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