# Calculation Of Distillationcolumn Internal Reflux

In the design of distillation columns, engineers use internal reflux in their calculations rather than external reflux. Important indices for column separation ability are internal reflux/distillate and boilup/bottom-product ratios.

Increasingly we are controlling columns on the basis of internal reflux rather than external reflux. The most common method of determining internal reflux makes use of external reflux flow measurement and the number of degrees of condensate subcooling* (see Figure 11.1 and reference 15):

* This method works fairly well for relatively pure material. For mixtures of components, it is sometimes advisable to take into account the difference between bubble point and dew point.

where

wRi = internal reflux rate, lbm/hr wR = external reflux rate, lbm/hr cp — liquid specific heat, pcu/lbm °C

A. = latent heat of vaporization of vapor in column, pcu/lbm

T0 = vapor temperature, °C

Tr = condensate (external reflux) temperature, °C at point of entry to the column

To implement equation (11.1), we need a summer with adjustable gain and bias, and a multiplier. For the former the Foxboro 136-1 is a suitable devicef while for the latter we would use the Foxboro 556-8 multiplier. The discussion that follows, however, could be readily extended to other pneumatic, electronic, or digital devices. A commonly used hardware arrangement is shown in Figure 11.2.

Summer

The Foxboro 136-1 summer has the equation:

where p = output pressure, psig

A = signal from vapor temperature transmitter, psig

C = signal from external reflux (condensate) temperature transmitter, psig

Kr = summer gain, psi/psi

It is assumed, incidentally, that the two temperature transmitters have the same span, and therefore the same gain:

t There are several satisfactory pneumatic summers on the market, but those that use pressure-dividing networks for gain should be avoided. They are not accurate enough.

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### Responses

• michele
How is internal reflux in a column calculated?
8 years ago
• charles
How to find reflux by material balance?
8 years ago
• Debra
What is internal reflux rate?
8 years ago