Trickle hydrogenation

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. 

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. 

All these gas-liquid-particle operations are of industrial interest. For example, desulfurization of liquid petroleum fractions by catalytic hydrogenation is carried out, on the industrial scale, in trickle-flow reactors, in bubble-column slurry reactors, and in gas-liquid fluidized reactors. 

A gas-liquid-particle process termed cold hydrogenation has been developed for this purpose. The hydrogenation is carried out in fixed-bed operation, the liquefied hydrocarbon feed trickling downwards in a hydrogen atmosphere over the solid catalyst, which may be a noble metal catalyst on an inert carrier. Typical process conditions are a temperature of 10°-20°C and a pressure of 2.5-7 atm gauge. The hourly throughput is as high as 20-kg hydrocarbon feed per liter of catalyst volume. 

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. 

Trickle-bed hydrocracking Refinery residues, C30+ Hydrogen Stationary catalyst particles... 

Catalytic hydrogenation is typically carried out in slurry reactors, where finely dispersed catalyst particles (<100 (tm) are immersed in a dispersion of gas and liquid. It has, however, been demonstrated that continuous operation is possible, either by using trickle bed [24] or monoHth technologies [37]. Elevated pressures and temperatures are needed to have a high enough reaction rate. On the other hand, too high a temperature impairs the selectivity of the desired product, as has been demonstrated by Kuusisto et al. [23]. An overview of some feasible processes and catalysts is shown in Table 8.1. 

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

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. 

When hydrogenation is carried out in a continuous process often so-called trickle-ttow reactors are used. Mass-tran.sfer limitations often occur. An elegant improvement is the application of extrudates with a noncircular cross section, which increa.ses the external surface without increasing the pressure drop. Trilohe and Quadrilohe shapes are generally used in oil-refinery processes and they might also be useful in fine chemicals production. 

P. Gallezot, N. Nicolaus, G. Fleche, P. Fuertes and A. Perrard, Glucose Hydrogenation on Ruthenium Catalysts in a Trickle-Bed Reactor, Journal of Catalysis 180 (1998) 51. 

N. Dechamp, A. Gamez, A. Perrard and P. Gallezot, Kinetics of glucose hydrogenation in a trickle-bed reactor. Catalysis Today 24 (1995) 29. 

In continuous flow experiments, catalyst was packed into a downflow trickle-bed reactor of 30 cc bed volume. Hydrogen was passed slowly over the catalyst at atmospheric pressme and the temperature was slowly raised to the desired reduction/activation temperature and held for at least four hours. After activation, the reactor was cooled to the desired reaction temperature, the pressure was raised, and flow of an aqueous feed of glycerol and sodium hydroxide initiated along with a corresponding amonnt of hydrogen. A large set of reaction conditions was tested. 

Table 1 illustrates the effect of hydrogen pressure on the selectivity to MIBK based on the initial rate of MIBK production and the rate of IPA production. Palladium gives very high selectivity to MIBK, typically in excess of 93% with the selectivity improving significantly with decreasing pressure. This result is of particular importance since the CD process for MIBK production is carried out at relatively low pressure (< IMPa). In contrast, alternative one-step processes for MIBK production are carried out in the liquid phase in trickle-bed reactors at pressures as high as 10 MPa. 

Trickle Hydrodesulfurization A process for removing sulfur-, nitrogen-, and heavy-metal-compounds from petroleum distillates before catalytic cracking. The preheated feed is hydrogenated, without a catalyst, in an adiabatic reactor at 315 to 430°C. Developed by Shell Development Company. As of 1978, 91 units had been installed. 

Trickle-bed reactors are used in catalytic hydrotreating (reaction with H2) of petroleum fractions to remove sulfur (hydrodesulfurization), nitrogen (hydrodenitrogena-tion), and metals (hydrodemetallization), as well as in catalytic hydrocracking of petroleum fractions, and other catalytic hydrogenation and oxidation processes. An example of the first is the reaction in which a sulfur compound is represented by diben-zothiophene (Ring and Missen, 1989), and a molybdate catalyst, based, for example, on cobalt molybdate, is used ... 

Aniline present as an impurity in a hydrocarbon stream is to be hydrogenated to cyclo-hexylamine in a trickle bed catalytic reactor operating at 403 K (130°C). 

Crotonaldehyde is hydrogenated at 1 atm and 51 C in a trickle bed reactor using palladium on porous alumina as catalyst (Kenney Sedricks, Chem Eng Sci 27 2029, 1972). The reaction is first order. These data are known,... 

Possible hazards introduced by variations in experimental techniques in Kjeldahl nitrogen determination were discussed [1]. Modem variations involving use of improved catalysts and hydrogen peroxide to increase reaction rates, and of automated methods, have considerably improved safety aspects [2], An anecdote is given of the classic technique when sodium hydroxide was to be added to the sulphuric acid digestion and was allowed to trickle down the wall of the flask. It layered over the sulphuric acid. Gentle mixing then provoked rapid reaction and a steam explosion [3],... 

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