In Eqs. (14-167) through (14-170), Csult is the system limit C-factor based on the tower superficial area [see Eq. (14-77) for C-factor definition]; LS is the liquid superficial velocity, m/s; c is the surface tension, mN/m; Ap is the difference between the liquid and gas densities, kg/m3; and pG is the gas density, kg/m3.

Stupin and Kister (loc. cit.) relate the flattening of the curve in Fig. 14-76 at low liquid loads to the formation of more, smaller, easier-to-entrain liquid drops when the liquid load is lowered beyond the limiting liquid load. It follows that devices that can restrict the formation of smaller drops may be able to approach the system limit capacity predicted by Stupin's original equation [Eq. (14-167)] even at low liquid loads.

The only devices capable of debottlenecking a tray system-limit device are those that introduce a new force that helps disentrain the vapor space. Devices that use centrifugal force (see "Centrifugal Force Deentrainment") are beginning to make inroads into commercial distillation and have achieved capacities as high as 25 percent above the system limit. Even the horizontal vapor push (see "Truncated Downcomers/Forward-Push Trays") can help settle the entrained drops, but to a much lesser extent. It is unknown whether the horizontal push alone can achieve capacities exceeding the system limit.

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