Slurry reactors features

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. 

Usually, the typology of batch reactors also includes the semi-batch gas-liquid reactors, in which a gaseous phase is fed continuously in order to provide one of the reactants. A typical example is given by the reactors used both in different oxidative industrial processes and in the active sludge processes for the treatment of wastewater. It is possible to distinguish between the bubble columns (Fig. 7.1(c)), in which the gas rises undisturbed in the liquid phase, and the bubble stirred reactor, in which a mechanical mixer is added. Finally, the slurry reactors can be considered, in which the liquid phase contains a finely dispersed solid phase as well, which can act as a reactant or as a heterogeneous catalyst these reactors assume in general the features of Fig. 7.1(d). 

The main features of monolith reactors (MR) combine the advantages of conventional slurry reactors (SR) and of trickle-bed reactors (TBR), avoiding their disadvantages, such as high pressure drop, mass transfer limitations, filtration of the catalyst, and mechanical stirring. Again, care must be taken to produce a uniform distribution of the flow at the reactor inlet. Scale-up can be expected to be straightforward in most other respects since the conditions within the individual channels are scale invariant. 

Liquid-phase hydrogenations are common processes in both large-and small-scale industry. Here, a great emphasis is generally placed upon the reaction selectivity and conversion. Two conventional reactors used in hydrogenation processes are slurry reactors and trickle-bed reactors. The main features of the monolith reactors are a combination of... 

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. 

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. 

Experimental studies have demonstrated that conventional methanol-synthesis catalysts deactivate slowly in a slurry reactor, even with a concentrated, CO rich feedstream. The catalyst activity correlates with the BET surface area and the rate of deactivation increases rapidly with temperature. This limits the utility of temperature programming as a means for maintaining a constant methanol production rate as the catalyst ages. Continuous catalyst addition and withdrawal is the preferred means to maintain constant methanol production. The key mechanical and process features of this technique were demonstrated In the pilot plant. 

In contrast to the gas phase reactions, surprisingly, zeolitic catalysts show a superior catalytic performance in the liquid phase in a slurry reactor. Good conversions and selectivities are obtained over H-US-Y (70) and H-US-Y (70) treated with HCl whose catalytic features are described elsewhere (Table 5). ... 

In slurry reactors, the catalyst particles are freely dispersed in the fluid phase (water) and consequently, the photocatalyst is fully integrated in the liquid mobile phase. The immobilized catalyst reactor design features a catalyst anchored to a fixed support, dispersed on the stationary phase (the catalyst-support system). 

Of the reactors listed in Table 17.1, the four most important are the mechanically agitated slurry reactor (MASR), the bubble-column slurry reactor (BCSR), the loop-recycle slurry reactor (LRSR), and the trickle-bed reactor (TBR). The first three, sketched in Figure 17.5, are slurry reactors, whereas the fourth is a fixed-bed reactor. The features most relevant to a preliminary design of these reactors in organic synthesis and technology are briefly described here. 

The second type of non-catalytic reactor is the continuous-flow, stirred-tank reactor (Figure 18.7), which has the notable feature of encouraging complete mixing of aU of the ingredients, and if there is added catalyst (suspended in the fluid phase) the reactor may be referred to as a slurry reactor. 

The salient features of the slurry reactors used in three coal refining and conversion processes and the models developed for each case have been discussed. Considerable pilot plant... 

The H-Cocd Process, based on H-Oil technology, was developed by Hydrocarbon Research, Inc. (HRI). The heart of the process was a three-phase, ebullated-bed reactor in which catalyst pellets were fluidized by the upward flow of slurry and gas through the reactor. The reactor contained an internal tube for recirculating the reaction mixture to the bottom of the catalyst bed. Catalyst activity in the reactor was maintained by the withdrawal of small quantities of spent catalyst and the addition of fresh catalyst. The addition of a catalyst to the reactor is the main feature which distinguishes the H-Coal Process from the typical process. 

Table V shows the salient features of several Fischer-Tropsch processes. Two of these—the powdered catalyst-oil slurry and the granular catalyst-hot gas recycle—have not been developed to a satisfactory level of operability. They are included to indicate the progress that has been made in process development. Such progress has been quite marked in increase of space-time yield (kilograms of C3+ per cubic meter of reaction space per hour) and concomitant simplification of reactor design. The increase in specific yield (grams of C3+ per cubic meter of inert-free synthesis gas) has been less striking, as only one operable process—the granular catalyst-internally cooled (by oil circulation) process—has exceeded the best specific yield of the Ruhrchemie cobalt catalyst, end-gas recycle process. The importance of a high specific yield when coal is used as raw material for synthesis-gas production is shown by the estimate that 60 to 70% of the total cost of the product is the cost of purified synthesis gas.

Herri and coworkers (Fidel-Dufour et al., 2005) have developed a flow loop reactor (in Saint Etienne, France) operating at pressures of 1-10 MPa at 0-10°C. The flow section is 36.1 m long, 1.0 cm internal diameter, and has a constant slope of 4°. The unique feature of this flow loop is that the exit of the flow section is connected to a gas lift riser (10.6 m long and 1.7 cm internal diameter) in which gas coming from a separator located at the top of the column is re-injected. The gas lift is thereby able to move an emulsion or suspension slurry without any pump or mechanical system. 

The design of conventional biological reactors is very similar to those of gas-liquid, slurry, and polymerization reactors outlined in other chapters. As a matter of fact, biological reactors are the most versatile of all reactors, since such a reactor can carry two or three phases, the liquid can be Newtonian or non-Newtonian, the solids can be heavy or light, and the reaction mixture can be simple or complex. A biological reactor, however, carries certain distinct features ... 

The unique feature of each process is the reactor system configuration. The reactor or reactors normally provide from 2 to 6 hr detention for the gypsum slurry. This is about 0.8-2.5 m3 of reactor volume per ton of P205 per day, meaning relatively large vessels totaling 1500 m3 to over 2000 m3 in size for large plants. The individual reactor systems are described briefly below. 

An SBC is a vertical, tubular column in which a three-phase (gas-solid-liquid) mixture is used. The slurry phase consists of FT catalysts and FT wax. The syngas flows though the slurry phase in the form of bubbles, as shown in Figure 12.12. The effective heat and mass transfer, low intraparticle diffusion, low pressure drop, and design simplicity are important advantages of this type of reactor. However, considerable problems arise in separating the liquid-phase synthesis products from the catalyst. With their attractive features, the SBC reactors are receiving extensive investment in both R D and commercialization. The concept of SBC is not new. 

Another unique feature of the LPMEOH process is the ability to periodically withdraw spent catalyst slurry and add fresh catalyst online. This facilitates uninterrupted operation and also allows perpetuation of high production rate of methanol from the reactor. Furthermore, choice of catalyst replacement rate permits the optimization of methanol production rate versus catalyst replacement cost. 

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