Figure 31 S

Alternative overhead system for pressure column—vapor product that the condenser runs partially flooded. Common practice, as suggested by Hollander2 and shown in Figure 3.16A, is to bring the condensate into the bottom of the reflux drum (or at least under the liquid surface) and to bring the hot-gas bypass into the top of the drum. Dynamic problems with such a system can be severe. Suppose, for example, that the column pressure has risen, perhaps as a consequence of increased boilup. The pressure controller pinches the bypass valve to force more vapor into the condenser. This results in a temporary increase in pressure since it takes time for the condenser level to drop. Eventually, however, condenser contents drop to a new, lower level, which permits a higher rate of condensation and causes the pressure to be restored. The temporary "wrong-way" pressure response is commonly called "inverse response"; other examples will be discussed later.

Another technique sometimes encountered involves throttling the vapor to the condenser. This suffers from the drawback of requiring a large valve. It also lowers the operating pressure on the condensing side, which limits heat-transfer capabilities.

Another configuration, as shown in Figure 3.16B, has the condenser mounted above the reflux drum. As suggested by Chin3, the hot-gas line around the condenser has no valve in it. A valve on the liquid from the condenser floods the condenser to hold column pressure. The liquid in the reflux drum is subcooled, so there is condensation of vapor at the liquid—gas interface in the drum. A vertical reflux drum is recommended to reduce this interfacial area. The liquid line from the condenser should extend down into the liquid in the drum so that the cold liquid is introduced near the bottom of the drum.

These hot-vapor bypass systems are not recommended for systems with even small amounts of inerts.

Flooded Condenser

A pressure-control technique that is growing in popularity involves partial flooding of the condenser without a hot-gas bypass, as shown in Figure 3.17. If the pressure gets too high, the controller opens either the distillate or reflux valve, thereby dropping the liquid level and increasing the heat-transfer area available for condensation. Maximum cooling-water rate is normally used. A mathematical analysis is presented in Chapter 15.

There are some practical problems that must be taken into account. Consider, for example, the horizontal condenser of Figure 3.1. The vapor enters at the center and uncondensed gas exits at the two ends. At low heat-transfer loads, the liquid level will run high. If there is insufficient clearance between liquid level and the top of the shell, violent surging and hammering may ensue.4 This may be minimized by designing adequate clearance into the condenser, or by injecting inerts into the incoming vapor partially to blanket the tubes, thereby lowering the liquid level.

Another problem was observed by Mueller5 on a partial condenser. At low heat-transfer loads, the liquid inventory in the shell was high. Pressure drop of uncondensed vapor from inlet to the two exits caused a low liquid level in

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