Why is Pressure-drop Important and Disproportionally so for Downstream and Fine-Chemical Distillations

In sizing calculations, we process engineers are often obsessed with pressure drops (DPs). And for good reasons. In most cases, pressure drops represent system resistance to be overcome and are directly related to energy consumption and equipment cost; while in others, pressure drops might be more critical. They are carefully evaluated, for example, to avoid flashing, to check critical flows, or to ensure the pressure relief sensing is not affected by the pressure loss in its inlet piping. But in distillation column design, a C2 splitters’ DP allowance of 180 kPa in a petrochemical unit is not as critical as that of a column operating in a fungicide plant which specifies a DP of less than 2.0 kPa but truly wants only 1.0 kPa.

The tighter DP requirement is not hard to understand. The downstream industries often produce more complicated molecules that tend to have higher molecular weights and therefore, have higher boiling points. If the chemistry of thermal degradation, polymerization, or fouling concurs with the higher boiling points, the only way to make distillation viable is to lower the operating pressure. In this way, the boiling point of the key component may also be lowered to less than the degradation temperature. This is why the column’s pressure drop in this case is critical, as a high DP through the column would cause a significant increase in sump pressure and consequently in sump temperature, thus nullifying the effort for a low operating pressure and temperature. A previous Bernoulli Chem-tech (2026) article has explained one of the thermodynamic reasons behind this DP criticality. This is also why, a column operating in a pharmaceutical plant absolutely requires it when it specifies a DP of less than 2.0 kPa. Exceeding that value could mean that the sump temperature will become so high that the products are degraded. In contrast, relaxing a C2 splitters’ DP specification operated under an elevated pressure of 1.7 MPa in a petrochemical unit is often more an increased-energy penalty than an absolute dead cut.

Kister (1992) mentions that the almost sole reasons for setting a column at vacuum are:

• The distillation column would otherwise have a bottom temperature that increases chemical degradation, polymerization, and fouling; or
• The distillation column would otherwise have a reboiler temperature too high to select an economical heating medium like steam.

For downstream or fine-chemical distillation columns especially in the separation of end products, the author rarely sees a distillation temperature above 200 °C. Therefore, reason 2) is almost moot. If a downstream or fine-chemical distillation column is designed as a vacuum tower, then the reason most likely is due to the concern for product side reactions like degradations. In this case, satisfying the DP requirement is mandatory, and the lower the pressure drop the better, as the vacuum is maintained by a vacuum pump or blower. A lower DP means lower the vacuum pump power consumption.

Note that the overall distillation industry is becoming more accepting to trying vacuum distillation for otherwise feasible atmospheric or pressurized distillations. As a published case illustrates, favorable utility costs may be achieved by revamping a positive-pressure column to vacuum (Luyben, 2016). However, the decision to set a pressurized distillation to vacuum requires complicated and comprehensive studies, including balancing equipment costs and utility savings on cooling and heating media. It is more like an optimization effort then an initial design anchor. Regardless, a reduced DP can sometimes change the math in the vacuum-or-pressurized verdict.

In some of the future essays, the author will discuss the factors that affect a column’s DP especially for the downstream and fine-chemical distillation scenarios.

References

[1] Bernoulli Chem-tech. (2026). Clausius-Clapeyron tells you that your column’s pressure drop truly matters. 苏州伯尔努利化工科技有限公司. https://bernoulli-chem.com/en/clausius-clapeyron-tells-you-that-your-columns-pressure-drop-truly-matters/.

[2] Luyben, W. L. (2016). Distillation column pressure selection. Separation and Purification Technology, 168, 62–67. https://doi.org/10.1016/j.seppur.2016.05.015

[3] Kister, H. Z. (1992). Distillation design. McGraw-Hill.