Plant InSites: Cast a cold eye on columns

Subcooling can complicate and compromise tower performance

By Andrew Sloley, contributing editor

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Contacting equipment such as trays can handle both mass transfer and heat transfer services. Some processes will simultaneously combine the two. However, using a tray for a dual purpose requires care, as examination of a common combined application — subcooled reflux going to the top tray of a tower — will show.

Many plants will allow the tower overhead temperature to float with the temperature of the utility cooling water or air. Often, the overhead duty removed isn’t a major issue because the tower effectively operates with a total condenser overhead. Figure 1 depicts a complex column with this configuration.

If the pressure control system can handle varying overhead drum temperature, the direct effect of subcooling variation is small. The subcooled reflux returns to the tower. In heating up to the bubble point, the subcooled reflux condenses some of the rising vapor. The internal reflux rate inside the tower is higher than the measured reflux rate from the pump to the tower.

The effect of subcooled reflux can be estimated by:

 LF = F [1 + (h*- h)/(Heq – h*)]

where LF is the change in liquid flow at the feed stage, F is the total moles of feed (reflux), h is the molar enthalpy of liquid feed at actual column conditions, h* is the molar enthalpy of liquid feed at the column pressure boiling point, and Heq is the molar enthalpy of vapor that would exist in equilibrium with the feed if the liquid feed were at the column pressure boiling point.

As long as the tower internals are designed to cope with the required liquid rate after subcooling, the tower won’t flood.

Subcooling also introduces a second issue. Some part of the tray’s vapor/liquid contacting area is being used only for heat transfer and condensing the vapor; no equivalent mass transfer of material from the liquid to the vapor occurs. So, in general, if a tray (or height of packing) is required for subcooling, don’t take any credit for the tray as a mass transfer device. This isn’t always exactly true — a single tray may accomplish all the heating required and then go on to have mass transfer between the liquid and vapor phases. However, it’s good practice never to count on trays that are heating subcooled liquids to accomplish any mass transfer.

Returning to Figure 1, we see that a liquid product is drawn from either the sump on Tray 1 or the sump on Tray 2. Due to complex product blending requirements, a liquid side-stream also is taken from the top of the column (Tray 1 or Tray 2) and added to the bottoms product. A vapor side-stream is drawn from below the feed, condensed and added to the overhead product.

Usually, liquid side-draws are some distance down from the reflux return — so changes in reflux subcooling don’t have significant impact. Here, however, the correct draw point was only one tray below the reflux and actually varied depending upon the amount of subcooling. Taking liquid from the wrong location could result in significant product quality swings. The solution was to provide an alternative draw point on Tray 2 that can be switched to as needed.

Subcooled liquid feeds (and refluxes) can create both hydraulic and efficiency problems in towers. So, take the safe approach and don’t count the necessary trays for heating the liquid as mass transfer trays.

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