Reactors with a Fixed Bed of Catalyst

Batch reactor system for liquid-liquid reaction 

An alternative to the trickle-bed reactor, developed only recently, is the three-phase monolith reactor [3,4].The monolith catalyst has the shape of a block with straight narrow channels with the catalytic species deposited on the walls of these channels. Advantages of the monolith reactor compared with the trickle-bed reactor are its low pressure drop and the much smaller diffusion distance, because of the thin catalyst layer. They also are claimed to be intrinsically safe. 

Although the monolith reactor has already found a number of applications in gas-phase reactions (e. g., the catalytic purification of exhaust gases from automobiles), it is not currently applied in commercial gas-liquid reactions in fine chemicals production. 

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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. 

A finishing reactor with a fixed bed of catalyst completes the catalytic hydrogenation of any residual, unreacted benzene. The effluent from this reactor is then cooled and flashed to remove most of the hydrogen and then fractionated to produce high purity cyclohexane. 

Applied on an industrial scale [4], this reaction, the Phillips triolefin process, is carried out in a tubular reactor with a fixed bed of catalyst containing cobalt molybdate. Separation of the reaction products is effected by fractionation and subsequent distillation (Fig. 1). 

Catalytic studies were performed in a continuous flow reactor with a fixed bed of catalyst. The catalyst was packed in the reactor and was treated at... 

So far, no reference has been made to the presence of more than one phase in the reactor. Many important chemicals are manufactured by processes in which gases react on the surface of solid catalysts. Examples include ammonia synthesis, the oxidation of sulphur dioxide to sulphur trioxide, the oxidation of naphthalene to phthalic anhydride and the manufacture of methanol from carbon monoxide and hydrogen. These reactions, and many others, are carried out in tubular reactors containing a fixed bed of catalyst which may be either a single deep bed or a number of parallel tubes packed with catalyst pellets. The latter arrangement is used, for exjimple, in the oxidation of ethene to oxiran (ethylene oxide)... 

Catalyst samples were prepared by extiTusion with 2058 Plow isothermic reactor with a fixed bed of a catalyst was employed for catalyst testings. Liquid reaction products were analyzed by GC on a column contained Inerton N Super with 558 Carbo-wax 20M. 

Internal recycle reactors are designed so that the relative velocity between the catalyst and the fluid phase is increased without increasing the overall feed and outlet flow rates. This facilitates the interphase heat and mass transfer rates. A typical internal flow recycle stirred reactor design proposed by Berty (1974, 1979) is shown in Fig. 18. This type of reactor is ideally suited for laboratory kinetic studies. The reactor, however, works better at higher pressure than at lower pressure. The other types of internal recycle reactors that can be effectively used for gas-liquid-solid reactions are those with a fixed bed of catalyst in a basket placed at the wall or at the center. Brown (1969) showed that imperfect mixing and heat and mass transfer effects are absent above a stirrer speed of about 2,000 rpm. Some important features of internal recycle reactors are listed in Table XII. The information on gas-liquid and liquid-solid mass transfer coefficients in these reactors is rather limited, and more work in this area is necessary. 

The reaction equipment is operated by means of a data acquisition and control program. The reactor is of stainless steel 316, with 9 mm internal diameter. It is provided with a fixed bed of catalyst diluted with alumina as inert and operates in isothermal regime. The reaction products are analysed by gas chromatography (Hewlett Packard 6890) by means of detectors based on thermal conductivity (TCD) and flame ionization (FID). The separation of products is carried out by means of a system made up of three eolumns 1) HP-1 semicapillary column for splitting the sample into two fi actions a) volatile hydrocarbon components (C4.) and polar components (ethanol, water and diethyl ether) b) remaining products (C5+). 2) SUPEL-Q Plot semicapillary column for individually separating out both volatile components and polar components, which will be subsequently analysed by TCD and FID. 3) PONA capillary column for separation of Cs+ hydrocarbons, which will be analysed by FID. 

One of the most common reactors employed to treat continuously three phase systems (gas-liquid-solid) is the Trickle Bed Reactor. It consists of a column with a fixed bed of catalyst particles through which liquid flows in the form of films, droplets and rivulets. Gas moves cocurrently sometimes counter-current flows are also used. Usually one reactant is introduced in the liquid phase and the other in the gas phase. The cocurrent type of TBR is schematically shown in Fig. 1. 

The most reliable recycle reactors are those with a centrifugal pump, a fixed bed of catalyst, and a well-defined and forced flow path through the catalyst bed. Some of those shown on the two bottom rows in Jankowski s papers are of this type. From these, large diameter and/or high speed blowers are needed to generate high pressure increase and only small gaps can be tolerated between catalyst basket and blower, to minimize internal back flow. 

Fixed-bed reactors Trickle-flow reactor (TFR) This is a tubular flow reactor with a concurrent down-flow of gas and liquid over a fixed-bed of catalyst (Figure 3.10). Liquid trickles down whereas the gas phase is continuous. This reactor is mainly used in catalytic applications. Typical application examples of this reactor type are the following HDS of heavy oil fractions and catalytic hydrogenation of aqueous nitrate solutions. 

Trickle-bed reactors usually consist of a fixed bed of catalyst particles, contacted by a gas liquid two-phase flow, with co-current downflow as the most common mode of operation. Such reactors are particularly important in the petroleum industry, where they are used primarily for hydrocracking, hydrodesulfurization, and hydrodenitrogenation other commercial applications are found in the petrochemical industry, involving mainly hydrogenation and oxidation of organic compounds. Two important quantities used to characterize a trickle-bed reactor are... 

The disadvantages of conventional microbalanccs when used as catalytic reactors (vide supra) can apparently be overcome with a recently developed oscillating microbalancc reactor [40]. It provides a fixed-bed of catalyst particles through which the complete gas stream is forced to flow. Changes in the mass of the catalyst, which is located at the tip of an oscillating... 

The coupling of a permselective membrane with a packed bed of catalyst pellets (Fig. 5b) has been one of the most widely studied membrane reactor setups. Generally, the catalyst fixed bed is enclosed on the tube side of a porous membrane, although several cases can be found in the literature in which permselective tubular membranes have been inserted at regularly spaced intervals into the packed bed of catalyst pellets (e.g.. Ref. 25). The most interesting property of this membrane reactor type is that the amount of catalyst and the membrane surface area can be varied almost independently within wide ranges, so as to optimize the coupling of reaction and separation. 

One of the most common catalytic reactors is the fixed-bed type, in which the reaction mixture flows continuously through a tube filled with a stationary bed of catalyst pellets. Because of its importance, and because considerable information is available on its performance, most attention will be given to this reactor type. Fluidized-bed and slurry reactors are also considered later in the chapter. Some of the design methods given are applicable also to fluid-solid noncatalytic reactions. The global rate and integrated conversion-time relationships for noncatalytic gas-solid reactions will be considered in Chap. 14. 

The kinetic measurements were performed by monitoring the gas phase composition along the length of a fixed bed of catalyst. The reactor was treated as an isothermal plug flow system. The reaction kinetics can be described with a simple triangle network consisting of the main reaction (aldehyde to carboxylic acid), a consecutive reaction (carboxylic acid to byproducts) and a parallel reaction (aldehyde to by-products). 

The HPC feedstock is mixed with recycle gas containing hydrogen and is preheated to reactor temperature in a feed/product heat exchanger and a furnace. In the reactor the charge mixture is contacted with a fixed bed of hydroconversion catalyst under mild conditions. 

Transient tests have been performed with a fixed bed of 0.125 g catalyst in a 1 mm id, quartz reactor exposed to a series of concentration steps CjHs-He, Fie, Oa+FFe, Fie, CsHg-Fle, etc. This was obtained by distributing the three constant gas flows between the reactor and two parallel ballasts with two couples of three way eletrovalves [6] The concentration of products was determined using a Balzers QMS 420 system based on quadripole mass spectroscopy. The reactor conception was taken from the works of Floffmann and Muller [7]. 

NSSTK and SSITK experiments were performed with an atmospheric flow system using either a tubular quartz microreactor (70 mg of catalyst) or a catalytic DRIFT cell from Spectratech, allowing the gases to flow through a fixed bed of catalyst pellets (about 30 mg) and able to be heated up to 1173 K. The gas composition was continuously monitored at the reactor outlet by online mass spectrometer and the surface composition was investigated by a FT-BR. spectrometer (Nicolet 550) with one spectrum recorded per second In all cases, the catalyst was pre-treated with He at 1013 K for 40 min. The reacting feed was composed of 10 vol.% methane ( CHj, CHj or CD4) and 90 vol.% He with a total flow rate of 24 ml/min. The reaction was carried out at 1 atm and 1013 K. Ar was used as an inert tracer. 

The solid-catalyzed reaction of a gas with a liquid can be carried out in a slurry reactor, where fine catalyst particles are suspended in the liquid, or in a fixed bed of catalyst pellets, where gas and liquid flow continuously through the bed. For both types, there are several mass transfer steps to consider in modeling the reactor, since the gas dissolves and diffuses into the liquid and then both reactants diffuse to the catalyst and into the pores. The models are therefore more complex than those given in Chapter 7 for gas absorption plus reaction in a liquid. 

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