Slurry reactor particle suspension

Emulsion Polymerization. Emulsion and suspension reactions are doubly heterogeneous the polymer is insoluble in the monomer and both are insoluble in water. Suspension reactions are similar in behavior to slurry reactors. Oil-soluble initiators are used, so the monomer—polymer droplet is like a small mass reaction. Emulsion polymerizations are more complex. Because the monomer is insoluble in the polymer particle, the simple Smith-Ewart theory does not apply (34). 

Slurry reactors. For three-phase systems the definition of conditions at which (catalyst) particles are in motion is important. Two limiting states with respect to particle behaviour can be distinguished (1) complete suspension, i.e. all particles just move, and (2) uniform suspension, i.e. the particles are evenly distributed over the whole reaction zone. The power required to reach the second state is much higher, while uniform suspension is not often necessary. Circulation of the liquid with the dissolved gas is usually sufficiently fast to provide reactants to the surface of catalyst particles if they are suspended at all. 

In a slurry reactor (Fig 5.4.74), the catalyst is present as finely divided particles, typically in the range 1-200 pm. A mechanical stirrer, or the gas flow itself, provides the agitation power required to keep the catalytic particles in suspension. One advantage is the high catalyst utilization not only is the diffusion distance short, it is al.so possible to obtain high mass-transfer rates by proper mixing. 

Slurry phase (or suspension) process. The uniquedooldng equipment in Figure 23—5 is a loop reactor. This process also takes place in a solvent (in this case, normal hexane, isobutane, or isopentane) so that the mixture can be pumped continuously in a loop while the polymerization is taking place. Feeds (the solvent, comonomer if any, ethylene and Ziegler-Natta catalyst) are pumped into the loop and circulated. Polymerization rakes place continuously at temperatures below the melting point of the polyethylene allowing solid polymer particles to form enough to form slurry. The reaction takes place at 185—212°F and 75—150 psi. A slurry of HOPE in hexane is drawn off continuously or intermittently. 

For maximum utilization of the solid phase in a bubble column, it is essential that all particles be suspended in the reactor (Ramachandran and Chaudhari, 1984). This means that the gas velocity should be sufficiently high to enable suspension of all solids in the liquid. In slurry bubble column reactors, two suspension states exist ... 

A key feature of catalytic slurry reactors is that the particles are small ( 0.1 mm), so it is relatively easy to promote suspension by the mechanical action of the impeller. Moreover, because of their small size they travel together with the liquid, and therefore a significant mass transfer resistance develops at the liquid/solid interface that cannot be removed completely with the standard impellers. Also, because of the liquids large Prandtl number, the catalyst and the liquid are at the same temperature, so hot spots do not occur in multiphase slurry reactors. 

The main difference is the particle size. In three-phase fluidized beds these are so large that a net upward liquid flow is necessary to keep the solids in suspension, whereas in slurry reactors the turbulence of the liquid is sufficient to keep the solids suspended particle sizes in slurry reactors are usually below 200 ftm. Particularly for fast reactions where intraparticlc dif-... 

Power or energy dissipated in the aerated suspension has to be large enough (a) to suspend all solid particles and (b) to disperse the gas phase into small enough bubbles. It is essential to determine the power consumption of the stirrer in agitated slurry reactors, as this quantity is required in the prediction of parameters such as gas holdup, gas-liquid interfacial area, and mass- and heat-transfer coefficients. In the absence of gas bubbling, the power number Po, is defined as... 

Reactions in the liquid phase require small particles, or even powder, because the diffusion rate in liquids is smaller by several orders of magnitude, compared to gaseous diffusion rates. For slurry reactors small particles are also desirable, to keep them in suspension. Unfortunately, fine powders are difficult to remove by settling or filtration. 

Heat transfer in bubble column slurry reactors was studied by Kolbel and coworkers (75-77) and Deckwer et al. (13). The addition of solids increases the wall-to--suspension heat transfer coefficient. However, this increase is only due to changes in the physico-chemical properties and represents no independent contribution of the particles. Therefore, the heat transfer model, i.e. eqn. (17), developed by Deckwer (<53) for two-phase BCR also applies to slurry reactors as was proved for particle sizes up to 120 yum. This confirms that solids and liquid in the slurry can be regarded as a pseudo-homogeneous phase provided the gas velocity is large enough to provide for complete fluidization of the particles. 

The comparison between slurry and monolith reactors is summarized in Table 1. Based on the known features of slurry and monolith reactors, it can be concluded that the slurry reactors are preferable for mass-transfer-limited processes as far as the overall process rates are concerned. However, due to the low concentration of solid catalyst in slurry reactors, the productivity per unit volume in these reactors is not necessarily higher than that of monolith reactors. For processes occurring in kinetic regime, the monolith reactors are preferable due to their easier operation. The productivity of slurry reactors might apparently be increased by increasing the catalyst concentration. However, suspensions with a high concentration of fine catalyst particles behave as non-Newtonian liquids, with all the negative consequences in heat and mass transfer. 

Hydrogenation of carbon monoxide was carried out in a slurry-bed reactor, as previously described (ref. 2). Syngas submitted to the slurry reactor was let to react on the catalyst suspended in a liquid carrier (hexadecane). The product gas was partly recycled, and catalyst particles were agitated in the liquid carrier by the recycled gas. An ultrasonic generator was used to prepare the suspension of UFP prior to reaction (ref. 3). 

The evaluation of absorption photon rates in slurry reactors is a rather challenging task since light can experience a combination of reflection, scattering and absoiption in the Ti02 particle suspension. 

Suspension reactors are frequently operated continuously as so-called bubble column or slurry reactors (Figure 4.10.12c). The liquid represents the continuous phase in which the gas and the solid catalyst are distributed. The particle size is much smaller than in other three-phase reactors (<0.1 mm). The solid concentration is merely 1%, to keep the energy required for suspension low. However, the small particles complicate the situation as the separation of the solid catalyst is essential. Batchwise operated suspension reactors (not shown in Figure 4.10.12) are mostly mechanically stirred to keep the particles in suspension. 

In most two-phase processes, one phase has to be dispersed in the other, preferably in such a way that there can be sufficient mass transfer between the phases, and that afterwards the phases can be separated. When a solid has to be used in a dispersed state, the solid particles are usually brought into the desired shape outside the reactor. If the solid is a raw material that is to be converted, it often has to be broken and ground till a sufficiently small particle size. If, on the other hand, the solid reactant is a catalyst, the particles are carefully built up, so that the size and the structure is best suited to the catalytic process (see section 12.1). In fixed bed reactors, the catalyst particles are resting on a grid and the fluid phases (gas and/or liquid) are passed through them. In fluidized beds and in slurry reactors the particles are suspended in a fluid phase and the relative flow velocity of the particles is determined primarily by gravity. The suspension is maintained by an upward fluid flow in both cases. Examples of reactors with solid/fluid dispersions are given in Chapters 10 - 12. 

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. 

In slurry reactors and in fluidized bed reactors, particles under a certain size may give rise to filtration problems. When there are severe heat transfer requirements, one may prefer a fine catalyst in a suspension that is well agitated a stirred slurry reactor, a bubble column or a fluidized bed. 

Hydrodynamics of slurry reactors includes the study of minimum gas velocity or power input to just suspend the particles (or to fully homogeneously suspend the particles), bubble dynamics and the holdup fractions of gas, solids and liquid phases. A complicating problem is the large number of slurry reactor types in use (see fig. 1) and the fact that most correlations available are at least partially of an empirical nature. We will therefore restrict ourselves to sparged slurry columns and slurries in stirred vessels. A second problem is the difference with three phase fluidization. To avoid too much overlap we will only consider those cases where superficial liquid velocities are so low that its contribution to suspension of the particles is relatively unimportant. 

The agitated slurry reactors are most commonly used on industrial scales in liquid phase esterification processes. In agitated reactors, the catalyst particles are used in a smaller form and kept in suspension by means of mechanical agitation. Due to smaller catalyst particle size, intraparticle diffiisional effects are negligible in these reactors. Also... 

Suspension polymerization of VDE in water are batch processes in autoclaves designed to limit scale formation (91). Most systems operate from 30 to 100°C and are initiated with monomer-soluble organic free-radical initiators such as diisopropyl peroxydicarbonate (92—96), tert-huty peroxypivalate (97), or / fZ-amyl peroxypivalate (98). Usually water-soluble polymers, eg, cellulose derivatives or poly(vinyl alcohol), are used as suspending agents to reduce coalescence of polymer particles. Organic solvents that may act as a reaction accelerator or chain-transfer agent are often employed. The reactor product is a slurry of suspended polymer particles, usually spheres of 30—100 pm in diameter they are separated from the water phase thoroughly washed and dried. Size and internal stmcture of beads, ie, porosity, and dispersant residues affect how the resin performs in appHcations. 

Guichardon etal. (1994) studied the energy dissipation in liquid-solid suspensions and did not observe any effect of the particles on micromixing for solids concentrations up to 5 per cent. Precipitation experiments in research are often carried out at solids concentrations in the range from 0.1 to 5 per cent. Therefore, the stirred tank can then be modelled as a single-phase isothermal system, i.e. only the hydrodynamics of the reactor are simulated. At higher slurry densities, however, the interaction of the solids with the flow must be taken into account. 

Bubble columns in which gas is bubbled through suspensions of solid particles in liquids are known as slurry bubble columns . These are widely used as reactors for a variety of chemical reactions, and also as bioreactors with suspensions of microbial cells or particles of immobilized enzymes. 

Step 3—In a separate step, styrene-acrylonitrile (SAN) resin is prepared by emulsion, suspension, or mass polymerization by free-radical techniques. The operation is carried out in stainless-steel reactors operated at about 75°C (167°F) and moderate pressure for about 7 hours. Tlie final chemical operation is the blending of the ABS graft phase with the SAN resin, plus adding various antioxidants, lubricants, stabilizers, and pigments. Final operations involve preparation of a slurry of fine resin particles (via chemical flocculation), filtering, and drying in a standard fluid-bed dryer at 121-132°C (250-270°F) inlet air temperature. 

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