Page 288 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

290 Chapter 11 Distillation

oversized offering only a small pressure drop. The condenser pressure drop is also small. Column top
pressure is controlled by manipulating the noncondensable venting rate from the reflux drum.
The distillate product and the reflux streams are drawn out either by common or by individual
pumps. The distillate flow is manipulated to control and maintain the liquid level in the reflux drum as
shown in Fig. 11.4. The pressure developed by the product pump has to overcome the pressure drop in
piping, fittings, heat exchanger(s) and the liquid head of the product storage tank. Flow rate of reflux is
regulated as per the required purity of the top product and hence the line is fitted with a control valve.
The column top temperature is the dew point of the overhead product at the column pressure. Since the
column top temperature directly relates to the quality of the distillate and is easily monitored, the
overhead temperature is controlled by manipulating the reflux flow. This in turn fixes the distillate
quality purity).
Partial condensers are used when the top product stream is withdrawn as vapour and it is considered
as one vapoureliquid contacting stage. In such cases, the column may or may not have any liquid
distillate product stream drawn from the accumulator and the entire liquid from the condenser may be
refluxed to the column. The vapour withdrawal from the top of the reflux drum is similar to that for
venting noncondensable. In industry parlance, when no distillate product stream is drawn as liquid
from the accumulator, it is referred to as a ‘total reflux column’. Only a vapour overhead product is
drawn in this case and the entire liquid is refluxed back to the column. Strictly speaking, this is a
misnomer as under ‘total reflux’ condition, no net top product (either as vapour or as liquid) is drawn
from the column.
In most plants, the specified battery limit temperature is 40 C for liquids that are not very viscous

and in case of viscous streams, the temperature may be 90 C or higher.

The overhead condenser system may have variations, some of which are
• Condenser-subcooler: This is used to reduce the reflux flow requirement by subcooling the same.
A subcooled reflux achieves the same column top temperature with a lower reflux flow rate. For
this, (horizontal) condensers are designed with partial submergence of the tubes that achieve
subcooling of the condensed liquid.
Operating reflux for total condenser is often subcooled at column pressure particularly if the
condenser is overdesigned or there is significant heat loss in condenser. When the bubble temperature
at condenser pressure is much higher than the cooling water inlet temperature, the condenser outlet
temperature may fall below the bubble point due to the ‘cold’ cooling water. If the condenser is partial
(liquid reflux, vapour distillate/top product) or mixed (liquid reflux, both liquid and vapour as top
product), reflux stream is a saturated liquid unless heat losses cause it to sub-cool. With the condenser
outlet pressure being less than the column top pressure, reflux is subcooled for all condenser types.
This needs to be considered for an accurate prediction of the number of equilibrium stages required,
or else the number of equilibrium stages is slightly overestimated. The procedure to incorporate the
correction in the McCabeeThiele construction for binary mixtures is elaborated in Section 11.4.2
(Eq. 11.9).
• Hot reflux configuration: In this case (Fig. 11.6) the reflux provided is not subcooled. A subcooled
reflux may condense some components that form a separate liquid phase (on condensation) on the
top tray. In crude distillation columns, this is done to avoid moisture condensation in contact with