Circulating gas or solid reactions

Column reactors for gas-liquid-solid reactions are essentially the same as those for gas-liquid reactions. The solid catalyst can be fixed or moving within the reaction zone. A reactor with both the gas and the liquid flowing upward and the solid circulating inside the reaction zone is called a slurry column reactor (Fig. 5.4-10). The catalyst is suspended by the momentum of the flowing gas. If the motion of the liquid is the driving force for solid movement, the reactor is called an ebullated- or fluidized-bed column reactor (Fig. 5.4-10). When a catalyst is deactivating relatively fast, part of it can be periodically withdrawn and a fresh portion introduced. 

Figure 11 allows us to draw some important conclusions about the true nature of the CFB fluid mechanics. From it we can infer that the overall flow structures are as shown in Figure 13. A core-annular flow structure dominates, with particles carried up in the central core and travelling down at the column walls. Along the height of the unit there is a net particle transfer from core to annulus which creates the decrease in overall bulk density with height. Superimposed upon the internal structure is a net flux through the unit which, depending upon the particles, gas velocity, solids flux, and exit employed, may be large or small compared to the net internal circulation. Typically, it is desirable that it be small to assure temperature uniformity. However, in reactions where plug flow of solids is desirable, this may not be the case.

Most of the generated vapour is condensed in spray condensers which are equipped with circulation pumps and an EG cooler. The vapour that is still uncondensed is withdrawn from the gas phase with the help of a vapour jet which is located down-stream behind the spray condenser and generates the necessary vacuum in the reaction zone. The most critical part of the spray condenser system is the end of the pipe leading the vapour from the prepolycondensation reactors and the finishers into the spray condenser. The transition from a hot to a cold environment causes deposition of solid material onto the cold walls which has to be removed manually or by means of a mechanical scraper. 

The best per pass yield to C2 + C3 products (aldehydes plus acids with two and three C atoms) with the said catalyst was obtained at a propene conversion of 61.3% (selectivity to acrolein 83.7%), at the reaction temperature of 355 °C, with the following feed composition C3H6/H20/N2 11.6 10.0 78.4 (mol.%), with a gas contact time of 2.4 s. A decrease in solids circulation rate, while keeping gas residence time constant, led to a considerable decrease in propene conversion, while selectivity to C2 + C3 oxygenated products was not much affected by circulation rate. With a less concentrated feed, the amount of solid to be circulated for a defined olefin conversion is lower, but productivity also becomes lower. Other catalysts based on Bi/Mo/O or on V/Mo/W/Cu/O [72c] afforded conversions >70% and selectivity >90% industrial... 

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