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Hardware Selection Aspects

The 'ideal' hardware configuration is one that offers: maximum catalyst hold-up in the column, good gas-liquid and liquid-solid contact and mass transfer, and low pressure drop. These three requirements cannot all be met by the same packing and most hardware choices represent compromises. Consider, for example, use of the active Raschig rings in the reactive section of an RD column. This packing provides intimate contact between gas, liquid, and catalyst and, as a consequence, the pressure drop is extremely high. In structures such as catalytic bales and Ka-tapak-S, preferential passages ('open' channels) are offered for flow of gas while the liquid is forced through the catalyst-containing envelopes. The vapor only comes into contact with the liquid at the cloth surface and some entrained liquid drops within the open channels. Consequently the pressure drop is low, but the vapor-liquid mass transfer is also low. The gas-liquid contact is much poorer than for Raschig rings: low pressure drop is not compatible with good mass transfer.

To illustrate the quantitative differences, we present the calculations of Baur and Krishna [48] for Raschig rings, of 1 in. size, and catalytic bales for use in the reactive section of an RD column for synthesis of tert-amyl ether (TAME), Fig. 7.19. Correlations for pressure drop and mass transfer were taken from literature sources [22, 23, 49, 50]. From Fig. 7.19a we conclude that with Raschig rings, to avoid danger of flooding, the column diameter has to be much higher than for bales. However, from Fig. 7.19b we note that the mass-transfer performance of bales is about a factor three times lower than that of Raschig rings; therefore the height of the reactive section has to be higher in order to meet the require-

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