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.
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.
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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