Optimal packing density

Hummel showed that the Fuller parabola (n = 0.5) does not yet furnish the optimal packing densities 14). In the case of spherical aggregates n = 0.4 ought to be chosen as the greatest density in the case of crushed aggregates n = 0.3 if the density maximum is to be reached for the aggregate (Figure 4). 

A catalyst manufactured using a shaped support assumes the same general size and shape of the support, and this is an important consideration in the process design, since these properties determine packing density and the pressure drop across the reactor. Depending on the nature of the main reaction and any side reactions, the contact time of the reactants and products with the catalyst must be optimized for maximum overall efficiency. Since this is frequendy accompHshed by altering dow rates, described in terms of space velocity, the size and shape of the catalyst must be selected carehiUy to allow operation within the capabiUties of the hardware. 

Fig. 3.—The structure of trisilver cobalticyanide. There are three mutually interpenetrating rhombohedral frameworks, distorted from the cubic frameworks of Fig. 2 in such a way as to achieve increased density and closer approximation to the optimal packing of the bulky cobalticyanide groups.

Although efficient columns have reportedly been obtained by packing at presrares not exceeding 2000 psi (136 atm), the use of higher pressures appeaf to be necessary to optimize not only the efficiency bu also the stability of the column. It is not unusual to pack columns t 10,000 to 12,000 psi (680 to 816 atm) final inlet pressure. With silica rased bonded phases the packing density is about 0.39 ml of dry column material per milliliter of column volume. 

As discussed in Section 7.3, the optimization in chromatography proceeds as follows. From the discovery experiments the mobile-phase composition, particle size (apparent or nominal), the thermodynamic parameters and the Knox parameters are known. From the mobile-phase composition, the feed composition (simplify to a binary mixture, one impurity and the product), the viscosity can be estimated from data in the literature. The void fraction can be measured from the retention of unretained component (Eq. (7.24)), or estimated based on vendor information. The packing density is typically known by... 

An optimal contact of the apphcation system is obtained if the lower surface of the liquid in the capillary comes into contact with the mountain tops of the layer, whereupon it begins to flow out. To achieve this contact, gentle pressure is necessary so that the surface of the layer close to the circular outlet of the capillary becomes slightly deformed. This pressure depends on the packing density of the phase and on the binder material used. It must be determined experimentally before the analysis. Too great a pressure (>10 g/mm of the surface) leads to damage to the layer [30]. 

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