Packed advantages

Figure 1.7 (a) Molecular model of coronene (b) hexagonal close packing at the van der Waals diameter parts (c) and (d) illustrate the packing advantage, which can be obtained by a concerted rotation, counterclockwise or clockwise, of all molecules to allow... 

Gas-liquid mixtures are sometimes reacted in packed beds. The gas and the liquid usually flow cocurrently. Such trickle-bed reactors have the advantage that residence times of the liquid are shorter than in countercurrent operation. This can be useful in avoiding unwanted side reactions. 

The most common mobile phases for GC are He, Ar, and N2, which have the advantage of being chemically inert toward both the sample and the stationary phase. The choice of which carrier gas to use is often determined by the instrument s detector. With packed columns the mobile-phase velocity is usually within the range of 25-150 mF/min, whereas flow rates for capillary columns are 1-25 mF/min. Actual flow rates are determined with a flow meter placed at the column outlet. 

One report (13) describes the procedure for spinning dry asymmetric ceUulose acetate fiber with a bore skin. Such fibers are spun in a modified dry-spinning process in which a volatile Uquid (methyl formate) is used as the ceUulose acetate solvent. The bore coagulating Uquid is isopropyl alcohol, which is subsequentiy removed. The advantages of these dry fibers over most ceUulose acetate membranes are that they can be stored dry, they are wet-dry reversible, they can be sterilized and packed dry, and they are ready for use without removal of preservatives. 

An advantage of pack carburizing is that the charcoal pack supports the workload which minimizes distortion. However, the components must be removed from the pack for quenching, or the mass cooled to room temperature and then the components reaustenitized and quenched. 

There are notable cases where plate columns have been converted to packed columns to gain advantage of the low pressure drop exacted from the vapor stream. More recently the packings have been largely of the stmctured type. Illustrative of this is the trend toward the use of stmctured packing in ethylbenzene—styrene fractionators, some of which have diameters of 10 m or higher. 

A significant advantage of immobilized enzymes is the total absence of catalytic activity in the product. Moreover, the degree of substrate-to-product conversion can be controlled during processing, eg, by adjusting the flow rate through a packed-bed column reactor of immobilized enzyme. 

Use of the peUetted converter, developed and used by General Motors starting in 1975, has declined since 1980. The advantage of the peUetted converter, which consists of a packed bed of small spherical beads about 3 mm in diameter, is that the pellets were less cosdy to manufacture than the monolithic honeycomb. Disadvantages were the peUetted converter had 2 to 3 times more weight and volume, took longer to heat up, and was more susceptible to attrition and loss of catalyst in use. The monolithic honeycomb can be mounted in any orientation, whereas the peUetted converter had to be downflow. AdditionaUy, the pressure drop of the monolithic honeycomb is one-half to one-quarter that of a similar function peUetted converter. 

The chief advantage of packing over other types of seals is the ease with which it can be adjusted or replaced. Most equipment is designed so that disassembly of major components is not required to... 

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