Restoring Reactor Stoichiometric Control

Engineering Situations → Solving a Known Problem → Example

A Control Problem Hidden Behind Apparently Satisfactory Loops

In one MTBE plant, operators experienced persistent variability in reactor operation even though the individual control loops appeared to perform satisfactorily.

The process used C4's hydrocarbons, methanol and reactor recycle as its principal feed streams. Although the individual flow controllers were operating, the required relationship among these streams were not being maintained in the way required for stable reactor operation.

The problem was therefore not simply the performance of an individual controller.

It was the way the controlled variables interacted during normal plant operation.

What the Historical Data Revealed

A process engineer used DCS data from a historical database to review the MTBE plant performance.

The review identified several important observations:

1. C4's feed flow exhibited variations. Because this flow came from a surge drum, the variations were attributed to operator actions on the C4's flow controller to maintain the surge-drum level within its operating limits.
2. Methanol and reactor-recycle flows were maintained at constant values by their respective flow controllers.
3. The operating relationship among the C4's, methanol and reactor-recycle flows were not being maintained dynamically according to the licensor requirements.
4. Frequent deficits and excesses of methanol flow were observed relative to the current C4's and reactor-recycle flows.

These observations changed the engineering understanding of the problem.

The individual flow controllers could appear to operate satisfactorily while the required relationship among the process flows were not being maintained.

The Required Control Relationships

The next step was to establish what the process actually required.

After reviewing the design data and the licensor documentation, the relevant plant controls and reactor stoichiometric relationships were identified in terms of:

• Isobutylene composition in the C4's flow.
• Isobutylene-to-total-reactor-inlet ratio control.
• Methanol-to-pure-isobutylene molar ratio.

Review of the licensor documentation showed that the required control relationships had been defined, but the operating strategy was no longer maintaining those relationships dynamically during normal operation.

The engineering problem could therefore be expressed more precisely:

The issue was not that the individual flow controllers were unable to control their respective variables. The issue was that the overall control strategy did not maintain the required process relationships dynamically.

From Engineering Understanding to Control Strategy

With the required relationships established, a control strategy was developed according to the licensor requirements.

However, implementation had to address an important operating constraint.

One of the customer's main concerns was how to maintain stable MTBE plant operation if the C4's feed flow had to be manually adjusted to maintain the surge-drum level within its operating limits.

The operating practice was to maintain fixed flow for the reactor feed streams. Therefore, manually adjusting the C4's flow to maintain the surge-drum level introduced a process disturbance, as the fixed-flow control strategy did not automatically adjust the other feed flows to preserve the required stoichiometric relationships when the C4's feed flow changed.

The engineering challenge was therefore to implement the required control of the stoichiometric relationships without creating an operating conflict between surge-drum level control and reactor-feed control.

Eliminating the Manual Disturbance

To eliminate the manual disturbance, the surge-drum level controller was configured to automatically regulate the C4's feed flow.

The control objective was to maintain the surge-drum level within its operating limits while allowing the level to vary appropriately, rather than requiring operators to manually adjust the C4's flow.

This changed the way the required operating relationships were maintained.

Instead of relying on manual adjustment of the C4's flow to maintain the surge-drum level, the level-control function automatically regulated the C4's feed flow.

With the new control strategy implemented and the controllers properly tuned, the automatic operation could maintain the required process stoichiometric relationships while keeping the surge-drum level within its operating limits.

Implementation and Result

Following implementation and tuning of the new control strategy, the MTBE unit operated smoothly.

The required feed-flow relationships were maintained automatically.

The surge-drum level remained within its operating limits without the previous oscillatory behavior.

Manual adjustment of the C4's flow for surge-drum level control was no longer required.

The identified engineering problem was therefore successfully resolved through appropriate control-strategy implementation, rather than by improving the performance of individual flow controllers in isolation.