Why distillation column is difficult to control

The type and magnitude of disturbances affecting a distillation column have a direct effect on the resulting product variability. An analysis of the major types of disturbances encountered in distillation columns follows

Feed composition upsets
Feed flow rate upsets
Feed enthalpy upsets
Subcooled reflux changes
Loss of reboiler steam pressure
Column pressure upsets

Feed composition upsets

Changes in the feed composition represent the most significant upsets with which a distillation control system must deal on a continuous basis. A feed composition change shifts the composition profile through the column resulting in a large upset in the product compositions. Most industrial columns do not have a feed composition analyzer; therefore, feed composition upsets usually appear as unmeasured disturbances. When a feed composition analyzer is available, a feedforward controller can be applied using the on-line measurements of the feed composition. Because feed composition changes represent a major disturbance for distillation control, the sensitivity of potential control configurations to feed composition upsets is a major issue for configuration selection.

Feed flow rate upsets

The flow rates in a steady-state model of a column with constant tray efficiencies scale directly with column feed rate. Therefore, ratio control (using L/F, D/F, V/F or B/F as composition controller output) is an effective means of handling feed flow rate upsets. Dynamic compensation is normally required to account for the dynamic mismatch between the response of the product compositions to feed flow rate changes and the response to changes in the MVs. When certain ratios (e.g., L/D, V/B) are used as MVs, these ratios, combined with the level control, automatically compensate for feed flow rate changes.

Feed enthalpy upsets

For columns that use a low reflux ratio, feed enthalpy changes can significantly alter the vapor/liquid rates inside the column, causing a major shift in the internal composition profile and, therefore, a significant upset in the product compositions. This upset can be difficult to identify because (1) most industrial columns do not have feed temperature measurements and (2) even if a feed temperature measurement is available, it does not detect feed enthalpy changes for a two-phase feed. This disturbance may be difficult to distinguish from feed composition upsets without a more detailed analysis. It may be necessary to install a feed preheater or cooler to maintain a constant feed enthalpy to a column.

Subcooled reflux changes

When a thundershower passes over a plant, the reflux temperatures for the columns can drop sharply. Columns that use finned-fan coolers as overhead condensers are particularly susceptible to rapid changes in ambient conditions. If internal reflux control is not applied, severe upsets in the operation of the columns result because of major shifts in the composition profiles of the columns. When internal reflux control is correctly applied, the impact of a thunderstorm on column operations can be effectively eliminated.

Loss of reboiler steam pressure

When a steep drop in steam header pressure occurs, certain columns (those operating with control valves on the reboiler steam that are nearly fully open) experience a sharp drop in reboiler duty. This results in a sharp increase in the impurity levels in the products. When the steam header pressure returns to its normal level, the composition control system for the column attempts to return to the normal product purities. Because of the severity of this upset, if the composition controllers are not properly tuned, the upset can be amplified by the composition controllers, requiring the operators to take these controllers off-line to stabilize the column, greatly extending the duration of the period of production of off-specification products. This disturbance is, in general, the most severe disturbance that a control system on a distillation column must handle and may require invoking overrides that gradually bring the operation of the column to its normal operating window instead of expecting the composition controllers to handle this severe upset by themselves.

Column pressure upsets

Column pressure has a direct effect on the relative volatility of the key components in the column. Thus, changes in the column pressure can significantly affect product compositions. A properly implemented pressure control scheme maintains column pressure close to its set point, with only short-term and low-amplitude departures. A large class of columns (e.g., refinery columns) is operated at maximum condenser duty to maximize column separation, which minimizes steam usage. For these cases, the column pressure increases during the day, when the cooling water or ambient air temperature is the greatest, and decreases at night, but the resulting pressure changes are usually slow enough that the composition controller can efficiently reject this disturbance.