Distillation control and optimization, Part 3

Reboilers can be inserted into the column or can be external (see Figure 1 below). The process fluid circulation in them can be natural or forced. The kettle reboiler is the most common for external forced circulation applications. Thermosyphon reboilers (vertical or horizontal) are operated by natural circulation, induced by the hydrostatic pressure imbalance between the heavier liquid inside the tower and the lighter (two-phase) mixture in the reboiler tubes. 

A newer development is the use of self-cleaning shell-and-tube heat exchangers for applications where heat exchange surfaces are prone to fouling. Common heat sources include hot oil, steam, fuel gas (fired reboilers) and the condensation of compressed vapors in vapor recompression systems.

FIGURE 1: REBOILER DESIGN VARIATIONS

Reboiler design variations are distinguished by the type of circulation—natural or forced (pumped)—and the point from which they take their liquid. Most reboilers take their inlet from the column bottoms, while horizontal thermosyphon reboilers take it from one of the bottom trays.

Vapor recompression is a means of improving energy efficiency. The overhead vapor from the distillation column is compressed to a pressure at which the condensation temperature is greater than the boiling point of the process liquid in the bottom of the tower (see Figure 2 below). This way the heat of condensation, which was removed from the overhead vapors, is reused as the heat source for reboiling the bottoms.

This type of reboiler operation is often used when the boiling points of the top and bottom products are similar. Examples of such processes are the cryogenic de-methanization processes, where the column pressure is controlled by throttling the speed of the recompression compressors, or the propylene fractionation process. The reboiler heat input of propylene columns is often controlled by a temperature cascade loop to maintain the bottoms’ composition (bottom of Figure 2), while the column pressure is often controlled by throttling a bypass valve around the vapor recompression heat pump.

FIGURE 2: VAPOR RECOMPRESSION SYSTEM

The vapor recompression system uses recovered heat (top). The pressure of such a distillation process can be controlled by modulating the speed of the compressor or the bypass around it (bottom).

Feed Controls
Distillation columns are stabilized by holding the feed flow and temperature constant, because constant feed conditions simplify the operation of the control system. Feed composition is seldom subject to adjustment, while feed flow, if under level control, can be allowed to swing over a wide range (wide proportional setting) without causing much change in flow.

Surge tanks can be used to minimize flow fluctuations by cascading the column’s feed flow controller to a nonlinear level master on a surge tank (A in Figure 3). Such a level recorder controller (LRC) won’t change the set point of the flow recorder controller (FRC) as long as the level is between 25% and 75%. Naturally, if the level drops below 25% or rises above 75%, the FRC set point is changed to protect the surge tank from being drained or flooded.

Feedback controls give inferior performance compared to feed-forward controls because they can only compensate for upsets after they’ve occurred and been detected. On the other hand, feed-forward control can maintain the column material and heat balance while feed properties vary. Feed-forward is a simplified form of model-based process control. It can predict the consequences of changes in the inputs before they have time to evolve, and enables corrective action before the tower is significantly affected. This correction can consider the dead times and time constants of the process, and the non-linearities between separation efficiency and column loading, loop interactions and process measurements.

In general, feed-forward based product composition control can provide 5% to 15% energy savings.
For example, if feed-flow variations are unavoidable, the impact of these disturbances can be reduced by feed-forward correction of the material balance (B in Figure 3). In this configuration, as the feed flow changes, the dynamic element (FY) serves to change the distillate flow in the right proportion (m) and at the right time, because it can be tuned to reflect the time constant and dead time of the process. However, if the feed composition changes the value of m also must be changed.

FIGURE 3: FEED CONTROL SYSTEMS

Feed control systems: A – Surge tank with nonlinear level controller as cascade master; B – Feed-forward adjustment of distillate product flow; C – Maximizing throughput by fully loading the condenser; D – Maximizing throughput against a reboiler constraint; E – Feed pre-heater controls; F – Maximum recovery of the heat content of bottom product by economizer.