Slurry reactors flow operation

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. 

Liquid residence-time distributions in mechanically stirred gas-liquid-solid operations have apparently not been studied as such. It seems a safe assumption that these systems under normal operating conditions may be considered as perfectly mixed vessels. Van de Vusse (V3) have discussed some aspects of liquid flow in stirred slurry reactors. 

The expression gas-liquid fluidization, as defined in Section III,B,3, is used for operations in which momentum is transferred to suspended solid particles by cocurrent gas and liquid flow. It may be noted that the expression gas-liquid-solid fluidization has been used for bubble-column slurry reactors (K3) with zero net liquid flow (of the type described in Sections III,B,1 and 1II,V,C). The expression gas-liquid fluidization has also been used for dispersed gas-liquid systems with no solid particles present. 

Bubble slurry column reactors (BSCR) and mechanically stirred slurry reactors (MSSR) are particular types of slurry catalytic reactors (Fig. 5.3-1), where the fine particles of solid catalyst are suspended in the liquid phase by a gas dispersed in the form of bubbles or by the agitator. The mixing of the slurry phase (solid and liquid) is also due to the gas flow. BSCR may be operated in batch or continuous modes. In contrast, MSSR are operated batchwise with gas recirculation. 

Slurry reactors (bubble towers) are fluidized with continuous flow of gas. The particles are smaller (less than 0.1 mm) than in the liquid fluidized systems (0.2-1 mm). In some operations the liquid and solid phases are stationary, but in others they circulate through the vessel. Such equipment has been used in Frscher-Tropsch plants and for hydrogenation of fatty esters to alcohols, furfural to furfuryl alcohol, and of glucose to sorbitol. Hydrogenation of benzene to cyclohexane is done at 50 bar and 220-225°C with Raney nickel of 0.01-0.1 mm dia. The relations between gas velocities, solids... 

Slurry reactors achieve a similar intimate contacting of oil and catalyst and yet may operate with a lower degree of backmixing than the ebullated or expanded bed. In the slurry design, heavy oil is mixed with finely divided catalyst particles and fed upward, with hydrogen, through an empty reactor vessel. Oil and catalyst flow concurrently and may approach plug-flow behavior. 

In the process (Figure 9-37), the residue feed is slurried with a small amount of finely powdered additive and mixed with hydrogen and recycle gas prior to preheating. The feed mixture is routed to the liquid phase reactors. The reactors are operated in an up-flow mode and arranged in series. In a once through operation conversion rates of >95% are achieved. Typically the reaction takes place at temperatures between 440 and 480°C and pressures between 150 and 250 bar. Substantial conversion of asphaltenes, desulfurization and denitrogenation takes place at high levels of residue conversion. Temperature is controlled by a recycle gas quench system. 

Particularly in co-current down-flow operation, very high gas-slurry interfacial areas per cubic meter of reactor volume can be realized [6]. To improve the solids suspension and/or to improve mass or heat transfer, in many cases a stirrer is added to the system. Particularly where pure gases have to be absorbed in the slurry and no gases are produced, the gas may be sparged into the liquid via a hollow shaft stirrer, sucking the gas from the free board above the slurry. 

In spite of these advantages of slurry reactors, some technical difficulties are involved in the operation of these reactors. For example, separation of the catalyst and handling of the slurry is difficult the solids can produce erosion of the equipment and significant backmixing of the liquid phase does not allow operation in a plug-flow manner. 

Figure 19.21 shows a schematic of the H-Coal process, which employs a single catalytic stage to produce a synthetic crude oil.37,38 Coal is crushed, dried, and mixed with recycle oil and hydrogen before being preheated to approximately 850°F (454°C). The preheater effluent is fed to the bottom of an ebullated bed reactor. During operation, fresh catalyst (a cobalt-molybdenum extrudate) is fed to the top of the reactor, while spent catalyst is removed from the bottom to maintain constant reactivity and inventory. The upward flow of the coal slurry and hydrogen causes... 

Since dissolved gas concentrations in the liquid phase are more difficult to measure experimentally than the liquid reactant concentration, Equation 8 evaluated at the reactor exit 5=1 represents the key equation for practical applications involving this model. Nevertheless, the resulting expression still contains a significant number of parameters, most of which cannot be calculated from first principles. This gives the model a complex form and makes it difficult to use with certainty for predictive purposes. Reaction rate parameters can be determined in a slurry and basket-type reactor with completely wetted catalyst particles of the same kind that are used in trickle flow operation so that DaQ, r 9 and A2 can be calculated for trickle-bed operation. This leaves four parameters (riCE> St gj, Biw, Bid) to be determined from the available contacting efficiency and mass transfer correlations. It was shown that the model in this form does not have good predictive ability (29,34). 

Any form of convection, of course, increases the value of Ks. In slurry operation with no liquid flow, gas flow induces convection. In an agitated slurry reactor, stirring causes convection. In a pulsating slurry reactor, pulsation of the slurry induces convection and in a three-phase fluidized bed, the movements of both gas and liquid phases cause convection. Any one or more modes of convection will increase the value of the solid-liquid mass-transfer coefficient. In broad terms, the convective liquid-solid mass-transfer coefficient is correlated by-two steady state theories. Here we briefly review and compare them. 

Polymerizations that use supported chromium (Phillips) catalysts are conducted predominantly in slurry processes (though a small portion employs the gas phase process, see below). The historical development of the Phillips process has been expertly reviewed by Hogan (5, 6) and McDaniel (7-9). The slurry process originally developed by Phillips Petroleum (now Chevron Phillips) has been called the "particle form loop slurry process" and the "slurry loop reactor process" for production of HDPE and LLDPE (10). Hexene-1 is most often used as comonomer for LLDPE in the Phillips process. A simplified process flow diagram for the Phillips loop-slurry reactor process is shown in Figure 7.3 and key operating features are summarized in Table 7.4. 

As can be seen in the NEDOL process in Figure 12.4, pulverized coal and 2% 4% by weight iron-based catalysts are first slurried together with recycled solvents. The slurry is mixed with H2 and preheated before it is delivered to the liquefaction reactor. The tubular up-flow reactor typically operates at temperatures of... 

Intraparticle diffusion resistance may become important when the particles are larger than the powders used in slurry reactors, such as for catalytic packed beds operating in trickle flow mode (gas and liquid downflow), in upflow gas-liquid mode, or countercurrent gas-liquid mode. For these the effectiveness factor concept for intraparticle diffusion resistance has to be considered in addition to the other resistances present. See more details in Sec. 19. 

Finally, the chemical stability of the catalysts employed in this study was tested by means of XRD and EDXS analyses. The examination of fresh and used catalysts shows that during the reaction course metal ions are slowly leached into the aqueous solution, which can be attributed either to the temperature of operation or the presence of complexing carboxylic acids and benzoquinones in the liquid-phase. Contrary to the results obtained in continuous-flow fixed-bed reactors [8, 9], the extent of catalyst dissolution in the slurry reactor was considerable. This is probably due to the higher accumulation of benzoquinones which are known to form stable complexes with metal ions. Examination of the X-ray powder diffraction patterns of the molecular sieves before and after the liquid-phase phenol oxidation... 

Studies also showed higher methanol production rates in trickle-bed reactors as compared to slurry reactors at identical operating conditions. This is attributed to better gas/liquid/solid contact in trickle beds combined with its close proximity to plug flow conditions as opposed to high extent of back-mixing in the slurry reactors [16]. 

The BEA-coated monolith exhibits an activity of 0.034 1 x g-i x h in the acylation of anisole with octanoic acid (Scheme 4.18). The activity is lower than in a slurry reactor (0.055 1 x g-i x h ) in which the catalyst surface is optimally used. Both the slurry and monolithic approaches show good selectivities (78% for monolith and 86% for slurry). The coated monolith is tested in a larger scale (2 m reactor length). The reactor can be utilized for two runs, giving similar conversions (80% in 8 h at 155 C). The use of monolithic reactors allows easy removal of the produced water by a countercurrent stripping operation using a gas flow through the reactor. 

Three-phase fluidized beds and slurry reactors (see Figs. 30g-l) in which the solid catalyst is suspended in the liquid usually operate under conditions of homogeneous bubbly flow or chum turbulent flow (see regime map in Fig. 33). The presence of solids alters the bubble hydrodynamics to a significant extent. In recent years there has been considerable research effort on the study of the hydrodynamics of such systems (see, e.g., Fan, 1989). However, the scale-up aspects of such reactors are still a mater of some uncertainty, especially for systems with high solids concentration and operations at increased pressures it is for this reason that the Shell Middle Distillate Synthesis process adopts the multi-tubular trickle bed reactor concept (cf. Fig. 30e). The even distribution of liquid to thousands of tubes packed with catalyst, however poses problems of a different engineering nature. 

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