Slurry Reaction

In terms of cost and versatility, the stirred batch reactor is the unit of choice for homogeneous or slurry reactions and even gas/liquid reactions when provision is made for recirculation of the gas. They are especially suited to reactions with half-lives in excess of 10 min. Sam-... 

Topics that acquire special importance on the industrial scale are the quality of mixing in tanks and the residence time distribution in vessels where plug flow may be the goal. The information about agitation in tanks described for gas/liquid and slurry reactions is largely apphcable here. The relation between heat transfer and agitation also is discussed elsewhere in this Handbook. Residence time distribution is covered at length under Reactor Efficiency. A special case is that of laminar and related flow distributions characteristic of non-Newtonian fluids, which often occiu s in polymerization reactors. 

There are several practical scaleup lessons with heterogeneous catalysts in batch slurry reactions. One often uses four times the catalyst concentration in the lab to achieve the same results in the plant reactor. Plant charge is 1 wt. part for 1000 wt. part, where as in the lab one uses 5 grams or more per 1000 grams. A common error is the confusion between wet (gross) weight and dry weight (net)... 

Figure 12 clearly shows the effect of iron sulfide content of the coal on total conversion and liquid product yield during hydrogenation. The conversion increased from about 52 per cent to 70 per cent using the hot-rod reactor with no added catalyst. The yield of toluene soluble product (oil plus asphaltene) increased from about 30 to 44 per cent with total sulfur increase from 1 to 6.5 per cent. Thus it would appear that iron sulfide can act catalytically in the dry hydrogenation reaction as well as in slurried reactions (15). 

In order to rationalize the complex reaction mixtures in these slurry reactions the authors suggested that irradiations of the oxygen CT complexes resulted in simultaneous formation of an epoxide and dioxetane36 (Fig. 34). The epoxide products were isolated only when pyridine was co-included in the zeolite during the reaction. Collapse of the 1,1-diarylethylene radical cation superoxide ion pair provides a reasonable explanation for the formation of the dioxetane, however, epoxide formation is more difficult to rationalize. However, we do point out that photochemical formation of oxygen atoms has previously been observed in other systems.141 All the other products were formed either thermally or photochemically from these two primary photoproducts (Fig. 34). The thermal (acid catalyzed) formation of 1,1-diphenylacetaldehyde from the epoxide during photooxygenation of 30 (Fig. 34) was independently verified by addition of an authentic sample of the epoxide to NaY. The formation of diphenylmethane in the reaction of 30 but not 31 is also consistent with the well-established facile (at 254 nm but not 366 or 420 nm) Norrish Type I... 

The rate of a batch slurry reaction is controlled by diffusion from the bulk liquid to the surface of the catalyst and by a second order reaction on the surface. Equations for the two processes are rd = 0.25(C-Cs)... 

The solids analysis described above can be taken to yet another level by correlating the color measurement to chemical properties. An excellent model system is vanadium pyrophosphate (VPO), which is a well-known catalyst for butane oxidation to maleic anhydride. During the synthesis of the catalyst precursor, solid V2O5 particles are dispersed in a mixture of benzyl alcohol and i-butanol. In this slurry phase, the vanadium is partly reduced. Addition of phosphoric acid leads to a further reduction and the formation of the VPO structure. With a diffuse reflectance (DR) UV-vis probe by Fiberguide Ind., the surface of the suspended solid particles could be monitored during this slurry reaction. Four points can be noted from Figure 4.4 ... 

Batch slurry reactions were carried out in liquid phase in a stirred glass vessel with condenser. Catalyst was added to a preheated solution containing aromatic reactant (35ml, Aldrich) and iso-butyric anhydride (16ml, Aldrich) in a 3 1 molar ratio. Samples of the reaction mixture were removed from the reaction mixture after various reaction times, filtered and analysed by gas chromatography (column DB-5, 30m, He carrier gas, FID detector) to determine reaction progress. Product identification was made by comparison with appropriate reference materials. 

The considerable differences in exponents for Npe and Nsc show that additional work is necessary to arrive at a broadly applicable relationship for k a as a function of system properties and operating parameters in stirred tanks. For recent system-specific studies on industrially important catalytic slurry reaction see Table 5. 

The carrier can be in the form of a powder used for slurry reactions or a particulate such as a sphere, cylinder, or tablet (typically a few mm in diameter) used in fixed bed reactors. The size and shape depend very much on what is anticipated to be the rate-limiting step. For example, for a reaction limited by pore diffusion it is customary to use a smaller particle in the shape of a star, trilobe, or wagon wheel to decrease the diffusion path while increasing the external geometric surface area. Mechanical strength and solubility... 

Model for Three-phase Slurry Reaction (i.e., No Liquid Flow)... 

Normally used for fast gas-liquid reactions, but it can be useful for dilute slurry reactions. 

In a more recent work, Slesser et al.128 showed that small amounts of fine solid particles in an agitated three-phase slurry reaction can change the magnitude of the volumetric mass-transfer coefficient considerably. Chandrasekaran and Sharma14 reported a similar conclusion in the case of the oxidation of sodium sulfide in the presence of activated carbon. They argued that the presence of solids prevents bubble coalescence and thus increases the gas-liquid interfacial area. 

Bubble columns are convenient for catalytic slurry reactions also (67). It is therefore important to know how the hydrodynamic properties of the gas-in-liquid dispersion is influenced by the presence of suspended solid particles. In the slurry reactor absorption enhancement due to chemical reaction cannot be expected. However, if particle sizes are very small, say less than 5 yum, and if, in addition, the catalytic reaction rate is high a small absorption enhancement can occur ( 8). Usually the reaction is in the slow reaction regime of mass transfer theory. Hence, it is sufficient to know the volumetric mass transfer coefficient, kj a, and there is no need to separate k a into the individual values. 

In typical slurry reactions like hydrogenations and oxidations the particle sizes are usually smaller than 200 yum and their concentration is less than 10 wt. percent. Under such conditions, the variations in k] a due to the presence of solids reported (31,69-72) do not commonly exceed 10 to 20 %. If the particles are small ( 50 pm) the suspended solid and the liquid behave as a pseudohomogeneous phase. This can be concluded from a study on the CO conversion reaction on a catalyst suspended in molten paraffin where no significant effect on and kj a could be observed (13,37). ... 

Figure 1. The slurried reaction product is transferred aerobically to the filter frit by applying a slight vacuum.

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