Slurry reactors hydrogenation

Example 10-4 Coenan has reported rates of hydrogenation of sesame seed oil with a nickel-on-silica catalyst in a slurry reactor. Hydrogen was added at the bottom of a small cylindrical vessel equipped with stator and stirrer blades. Initial rates of reaction were measured as function of catalyst concentration at 180°C, a stirrer speed of 75G rpm, atmospheric pressure, and a hydrogen rate of 60 liters/hr. The data, converted to global rates in terms of g moles/(min)(cm oil), are given in Table 10-6 (based on an oil density of 0.9 g/cm ). Estimate kiaJH from these data. Comment on the importance of the resistance of hydrogen to solution in the oil and estimate what the reaction rate would be if this resistance could be eliminated for a catalyst concentration of 0.07% Ni in oil. 

Survey of the patent Hterature reveals companies with processes for 1,4-butanediol from maleic anhydride include BASF (94), British Petroleum (95,96), Davy McKee (93,97), Hoechst (98), Huels (99), and Tonen (100,101). Processes for the production of y-butyrolactone have been described for operation in both the gas (102—104) and Hquid (105—108) phases. In the gas phase, direct hydrogenation of maleic anhydride in hydrogen at 245°C and 1.03 MPa gives an 88% yield of y-butyrolactone (104). Du Pont has developed a process for the production of tetrahydrofuran back-integrated to a butane feedstock (109). Slurry reactor catalysts containing palladium and rhenium are used to hydrogenate aqueous maleic acid to tetrahydrofuran (110,111). 

Hydrogenations can be carried out in batch reactors, in continuous slurry reactors, or in fixed-bed reactors. The material of constmetion is usually 316 L stainless steel because of its better corrosion resistance to fatty acids. The hydrogenation reaction is exothermic and provisions must be made for the effective removal or control of the heat a reduction of one IV per g of C g fatty acid releases 7.1 J (1.7 cal), which raises the temperature 1.58°C. This heat of hydrogenation is used to raise the temperature of the fatty acid to the desired reaction temperature and is maintained with cooling water to control the reaction. 

Continuous slurry reactors are generally either of one of two designs. One type uses a reactor loop, generally known as a Buss loop design the other is a co-current hydrogen/fatty acid/catalyst system mainly marketed by Lurgi. Continuous slurry reactors are more popular in Europe, Asia, and South America than in the United States. 

Most industrial processes with slurry reactors are usea for gases with liquids, such as chlorination, hydrogenation, and oxidation. 

Substances that have been hydrogenated in slurry reactors include nitrobenzene with Pd-C, butynediol with Pd-CaCO,3, chlorobenzene with Pt-C, toluene with Raney Ni, and acetone with Raney Ni. 

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 catalytic hydrogenation of fatty oils, the desulfurization of liquid petroleum fractions by catalytic hydrogenation, Fischer-Tropsch-type synthesis in slurry reactors, and the manufacture of calcium bisulfite acid are familiar examples of this type of process, for which the term gas-liquid-particle process will be used in the following. 

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. 

In stirred-slurry reactors, momentum is transferred to the liquid phase by mechanical stirring as well as by the movement of gas bubbles. Small particles are used in most cases, and the operation is usually carried out in tank reactors with low height-to-diameter ratios. The operation is in widespread use for processes involving liquid reactants, either batchwise or continuous— for example, for the batchwise hydrogenation of fats as referred to in Section II. 

Calderbank et al. (C6) studied the Fischer-Tropsch reaction in slurry reactors of 2- and 10-in. diameters, at pressures of 11 and 22 atm, and at a temperature of 265°C. It was assumed that the liquid-film diffusion of hydrogen from the gas-liquid interface is a rate-determining step, whereas the mass transfer of hydrogen from the bulk liquid to the catalyst was believed to be rapid because of the high ratio between catalyst exterior surface area and bubble surface area. The experimental data were not in complete agreement with a theoretical model based on these assumptions. 

Stirred-slurry reactors are of considerable industrial importance in batch-wise processing. The catalytic hydrogenation of fats and fatty acids is an example of a process that is carried out almost exclusively in mechanically stirred slurry reactors. The operation is of less significance with respect to continuous processing. 

Among the earlier studies of reaction kinetics in mechanically stirred slurry reactors may be noted the papers of Davis et al. (D3), Price and Schiewitz (P5), and Littman and Bliss (L6). The latter investigated the hydrogenation of toluene catalyzed by Raney-nickel with a view to establishing the mechanism of the reaction and reaction orders, the study being a typical example of the application of mechanically stirred reactors for investigations of chemical kinetics in the absence of mass-transfer effects. 

In some applications such as catalytic hydrogenation of vegetable oils, slurry reactors, froth flotation, evaporative crystallisation, and so on, the success and efficiency of the process is directly influenced by the extent of mixing between the three phases. Despite its great industrial importance, this topic has received only limited attention. 

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. 

For small catalyst particles used in sugar hydrogenation (slurry reactors), one would intuitively conclude that the system is safely within the kinetic regime. 

Warna, J., Flores Geant, M., Salmi, T., Hamunen, A., Orte, J., Hartonen, R., and Murzin, D. (2006) Modelling and scale-up of sitosterol hydrogenation process from laboratory slurry reactor to plant scale. Ind. Eng. Chem. Res., 45, 7067-7076. 

Hydrogenation of lactose to lactitol on sponge itickel and mtheitium catalysts was studied experimentally in a laboratory-scale slurry reactor to reveal the true reaction paths. Parameter estimation was carried out with rival and the final results suggest that sorbitol and galactitol are primarily formed from lactitol. The conversion of the reactant (lactose), as well as the yields of the main (lactitol) and by-products were described very well by the kinetic model developed. The model includes the effects of concentrations, hydrogen pressure and temperature on reaction rates and product distribution. The model can be used for optinuzation of the process conditions to obtain highest possible yields of lactitol and suppressing the amounts of by-products. 

Three-phase slurry reactors are commonly used in fine-chemical industries for the catalytic hydrogenation of organic substrates to a variety of products and intermediates (1-2). The most common types of catalysts are precious metals such as Pt and Pd supported on powdered carbon supports (3). The behavior of the gas-liquid-sluny reactors is affected by a complex interplay of multiple variables including the temperature, pressure, stirring rates, feed composition, etc. (1-2,4). Often these types of reactors are operated away from the optimal conditions due to the difficulty in identifying and optimizing the critical variables involved in the process. This not only leads to lost productivity but also increases the cost of down stream processing (purification), and pollution control (undesired by-products). 

Slurry Reactors. Slurry reactors are commonly used in situations where it is necessary to contact a liquid reactant or a solution containing the reactant with a solid catalyst. To facilitate mass transfer and effective catalyst utilization, the catalyst is usually suspended in powdered or in granular form. This type of reactor has been used where one of the reactants is normally a gas at the reaction conditions and the second reactant is a liquid, e.g., in the hydrogenation of various oils. The reactant gas is bubbled through the liquid, dissolves, and then diffuses to the catalyst surface. Obviously mass transfer limitations can be quite significant in those instances where three phases (the solid catalyst, and the liquid and gaseous reactants) are present and necessary to proceed rapidly from reactants to products. 

The hydrogenation of acetone to isopropanol with a Raney nickel catalyst in a slurry reactor at 14 C and 10 atm was found to be of half order with respect to hydrogen (Lemcoff Jameson, AJChE Journal 21 730, 1975). These... 

Skeletal catalysts are usually employed in slurry-phase reactors or fixed-bed reactors. Hydrogenation of cottonseed oil, oxidative dehydrogenation of alcohols, and several other reactions are performed in sluny phase, where the catalysts are charged into the liquid and optionally stirred (often by action of the gases involved) to achieve intimate mixing. Fixed-bed designs suit methanol synthesis from syngas and catalysis of the water gas shift reaction, and are usually preferred because they obviate the need to separate product from catalyst and are simple in terms of a continuous process. 

Slurry tank hydrogenation. Predict the conversion of glucose to sorbitol in a stirred slurry reactor using pure hydrogen gas at 200 atm and 150°C. The catalyst used is porous Raney nickel, and under these conditions Brahme and Doraiswamy, lEC/PDD, 15,130 (1976) report that the reaction proceeds as follows ... 

Hydrogenation in a slurry reactor. The batch hydrogenation of Example 22.2 takes just about an hour to run. Let us suppose that in practical operations we can run eight batches of fluid per day in this unit. Thus, in the long run a batch of fluid is processed every three hours. 

Kinetic experiments were carried out isothermally in autoclave reactors of sizes 500 ml and 600 ml. The stirring rate was typically 1500 rpm. In most cases, the reactors operated as slurry reactors with small catalyst particles (45-90 micrometer), but comparative experiments were carried out with a static basket using large trilobic catalyst pellets (citral hydrogenation). Samples were withdrawn for analysis (GC for citral hydrogenation and HPLC for lactose hydrogenation). The experimental details as well as qualitative kinetics are reported in previous papers of our group Kuusisto et al. (17), Aumo et al. (5). 

Of the technological modifications, Fischer-Tropsch synthesis in the liquid phase (slurry process) may be used to produce either gasoline or light alkenes under appropriate conditions249,251 in a very efficient and economical way.267 The slurry reactor conditions appear to establish appropriate redox (reduction-oxidation) conditions throughout the catalyst sample. The favorable surface composition of the catalyst (oxide and carbide phases) suppresses secondary transformations (alkene hydrogenation, isomerization), thus ensuring selective a-olefin formation.268... 

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