Internal vs. external reflux
by Pamela Tokerud
“External reflux” is the measured liquid flow to the top of a distillation column. “Internal reflux” is the unmeasured liquid flow leaving the top equilibrium stage. The purity of the top product is more a function of the internal, rather than the external, reflux rate.
Differences between the external and internal reflux rates are caused by temperature and/or pressure differences between the external reflux and the fluids in the top of the tower. For example:
- Small internal reflux - In a propylene vapor recompression unit, the external reflux stream usually enters the column at a very high pressure. When it enters the column, there is approximately a 15% flash-off (by weight or by volume). The resultant internal reflux stream is far smaller than the external stream.
- Large internal reflux - More common is the case where the reflux of a non-heat-pumped column is subcooled. In this case, a portion of the upflowing vapor is condensed soon after it encounters the cold reflux. The resultant internal reflux stream is larger than the external stream.
To understand this situation, and ascertain the mass transfer equipment is properly designed, distillation designers often use terms such as the following:
- IR = Internal Reflux
- IRR = Internal Reflux Ratio
- ER = External Reflux
- ERR = External Reflux Ratio
It is the Internal Reflux (IR) and the Internal Reflux Ratio (IRR) that determine the distillate purity. Many of the unintentional over-purifications that occur industrially are the result of unintentional reflux subcooling and higher-than-design IRRs.