Condenser and Pressure Control—Two-Phase Products

When it is desired to produce a liquid overhead product but total condensation is impractical because of the presence of inerts, the column produces an overheads inerts product stream in addition to the normal liquid product. The most suitable control method depends on the nature, quantity, and purity requirements of the products. The following situations can be distinguished. 

The quantity of inerts is small losses of vaporized liquid in the gas are of little economic consequence This case coincides with venting small quantities of inerts from the condenser and the reflux drum. In this case, the control methods shown in Figs. 17.5a-d and f 17.6 or l.la, c are suitable. The inerts as vented either manually or on flow control (except 17.7c). 

The vapor contains components that can condense out and are undesirable in the liquid in excessive quantities vaporized liquid losses in the gas are of little economic consequence This case is identical to case 2 above, except that the condensation is not maximized but is set (usually manually) at a rate that will ensure that the undesirable components remain in the vapor. This case occurs when the product value of the gas is much the same as the liquid, or if the vaporized liquid is recovered from the gas in a downstream facility. As in case 2, the control systems in Figs. 17.4a- and 17.7c are suitable. 

The vapor contains components that can condense out and are undesirable in the liquid in excessive quantities vaporized liquid losses in the gas incur a significant economic penalty Here excessive condensation will render the liquid product off-spec on lights insufficient condensation will cause too much liquid product to escape in the vapor stream, incurring an economic penalty. In this case, in addition to column pressure control, the rate of condensation must be controlled to obtain the desired vapor-liquid split. This case is perhaps the most common in the chemicals industry and is discussed in detail below. 

Flooded condenser schemes shown in Fig. 17.5a to c and f can be used instead of coolant flow variations. In such cases, the inerts normally leave finm the top of the condenser instead of the reflux drum (Fig. 17.8e). If the reflux drum is not flooded, a pressure balance line must be included otherwise, a stable pressure will be impossible to keep in the reflux drum. An overflow line should also be included in this arrangement. 

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Figure 17.8 Condenser and pressure control, two-phase products, (o) PC on inerts stream, superatmospheric (6) PC on inerts stream, vacuum (c) PC on coolant, superatmospheric id) PC on coolant, vacuum (e) flooded partial condenser arrangement.

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