Slurry reactor external

Slurry reactors may take on several physical forms they may be simple stirred autoclaves they may be simple vessels fitted with an external pump to recirculate the liquid and suspended solids through an external heat exchanger or they may resemble a bubble-tray rectifying... 

The reaction r = occurs on the external surface of a sphere of diameter D suspended in a stagnant fluid in which fhe diffusion coefficient of the reactant is Da- Find the total rate of the reaction in terms of these quantities. How does the rate depend on particle diameter How would this influence fhe design of a slurry reactor with this catalyst ... 

A reactant of bulk concentration Cao reacts on the external surface of catalyst spheres of radius 7 in a slurry reactor. The first-order surface reaction rate coefficient is k , and the diffiisivity of A in the solution is Da- Find fhe effective rate coefficient in terms of these quantities, assuming that stirring is sufficiently slow that fhe fluid around particles is stagnant. 

The overall rate of reaction calculated for the three-phase fluidised-bed reactor above is approximately one tenth of the rate calculated for the agitated tank slurry reactor in Example 4.6. The main reasons are the very poor effectiveness factor and the relatively smaller external surface area for mass transfer caused by using the larger particles. Even the gas-liquid transfer resistance is greater for the three-phase fluidised-bed, in spite of the larger particles being able to produce relatively small bubbles these bubbles are not however as small as can be produced... 

Slurry Reactors Slurry reactors are akin to fluidized beds except the fluidizing medium is a liquid. In some cases (e.g., for hydrogenation), a limited amount of hydrogen may be dissolved in the liquid feed. The solid material is maintained in a fluidized state by agitation, internal or external recycle of the liquid using pipe spargers or distributor plates with perforated holes at the bottom of the reactor. Most industrial processes with slurry reactors also use a gas in reactions such as chlorination, hydrogenation, and oxidation, so the discussion will be deferred to the multiphase reactor section of slurry reactors. 

In some cases a stirrer can be avoided and yet the solids can be kept in suspension by a relatively small amount of gas by means of a draft tube placed in the slurry reactor. Another possibility is external slurry recycling. This gives control over the circulation rate and allows for efficient heat transfer in an external heat exchanger placed in the recycle. The Swiss company Buss AG successfully applies the recycle slurry flow to drive a venturi jet tube, thus sucking the gas from the free board above the reactor back into the slurry (Fig. 2). 

Air-water-glass spheres External sampling Slurry reactor 49... 

Glycerine soln- Glass spheres. External sampling of Slurry reactor 47... 

The main advantages of the slurry reactor are the high catalyst utilization, the uniform temperature, and good external mass transfer. Disadvantages are catalyst attrition, the need for catalyst separation, and a high degree of backmixing. 

There are mainly two types of F-T reactors. The vertical fixed tube type has the catalyst in tubes that are cooled externally by pressurized boiling water. The other process uses a slurry reactor in which preheated synthesis gas is fed to the bottom of the reactor and distributed into the slurry consisting of liquid and catalyst particles. As the gas bubbles upwards through the slurry, it is diffused and converted into liquid hydrocarbons by the F-T reaction. The heat generated is removed through the reactor s cooling coils where steam is generated for use in the process. 

Our objective here is to study quantitatively how these external physical processes affect the rate. Such processes are designated as external to signify that they are completely separated from, and in series with, the chemical reaction on the catalyst surface. For porous catalysts both reaction and heat and mass transfer occur at the same internal location within the catalyst pellet. The quantitative analysis in this case requires simultaneous treatment of the physical and chemical steps. The effect of these internal physical processes will be considered in Chap, 11. It should be noted that such internal effects significantly affect the global rate only for comparatively large catalyst pellets. Hence they may be important only for fixed-bed catalytic reactors or gas-solid noncatalytic reactors (see Chap. 14), where large solid particles are employed. In contrast, external physical processes may be important for all types of fluid-solid heterogeneous reactions. In this chapter we shall consider first the gas-solid fixed-bed reactor, then the fluidized-bed case, and finally the slurry reactor. 

The rate constant k is sensitive to temperature and, in principle, should be associated with the temperature of the catalyst particle. However, the high thermal conductivity of liquids (in comparison with that for gases) and the small particle size reduce the temperature difference between liquid and particle. Hence external temperature differences are not normally important in slurry reactors. 

For exothermic reactions, heat transfer is usually also an important factor, for reasons of temperature control and energy costs. In this respect, slurry reactors are superior to fixed-bed reactors. In particular, the jet-loop reactor with its external heat exchanger provides excellent temperature control. 

When the microphase is an externally introduced catalyst, as in a slurry reactor or a physically adsorbing solid such as microfine carbon particles, the solute is consumed within (or on) this phase. Thus there is no accumulation of A in the microphase, resulting in steady-state operation. This enables the use of the simple film theory that is restricted to steady-state transport and is inherently inapplicable to time-dependent situations (see chapter 4). 

Compared to fixed bed multiphase reactors additional modes of operation are available with slurry reactors as also the solid phase can be feeded continuously and eventually recirculated temperature control can be assured by internal cooling coils or/ and external circulation through heat exchangers (for the gas as well as for the liquid phase). Furthermore also partial evaporation and external condensation with recirculation of the liquid phase can support the cooling. 

FIGURE 3.2 (Cont d) (b) Slurry reactor or bubble column with a draft tube cum heat exchanger, (c) Airlift (external downcomer type) slurry or three-phase sparged reactor. (Reprinted from de Deugd et al. (2003) with kind permission from Springer Science + Business Media. 2003.)... 

Effect of support size The size of the catalytic support varies from very small particles (-10 xm in slurry reactors and -100 p,m in fluidized beds) to large ones (-1 cm in packed beds, 10-100 cm in monoliths). When the size of the support is increased, the effect of back-diffusion is reduced, leading to the accumulation of metal at the external surface. This effect is illustrated in Figure 16.11, which shows that the final metal profile changes from egg-shell to nearly uniform when the size of the porous support is decreased. 

The modeling methodology is shown in Figure 6.17.7 for the example of a discontinuous slurry reactor. First, the concentration profiles within the catalyst particles are calculated. This information is then coupled (for each time step) with the change of concentrations in the bulk phase (Cj t)- The link between both procedures, that is, between the bulk phase and the porous catalyst particles, is the concentration gradient of each reactant at the external particle surface. Note that this calculation is also applicable for a continuous plug flow reactor simply by using the residence time t (= x/u) instead of the reaction time, whereby x represents the axial coordinate x in a tubular reactor and u the fluid velocity. 

Though the term "slurry refers to a suspension of fine solid particles in a liquid, the term slurry reactor is often used for a three-phase system, where both gas bubbles and solid particles are suspended in a liquid phase. For a solid/liquid/gas process, slurry reactors have two obvious advantages the possibilities for very large solid/liquid surface areas and for good heat transfer to the reactor wall. Therefore the volumetric capacity of slurry reactors can be relatively large. However, effective separation of the fine catalyst from the liquid phase may offer considerable technical problems. One possibility is an external separation, e.g. with centrifuges or hydrocyclones, and a transport of a concentrated catalyst slurry back into the reactor. More often internal filters are used, usually consisting of porous tubes (sintered stainless steel, or ceramics), that are cleaned every few minutes by a periodic reversal of the flow. 

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