Wash Oil Control:Inferred Versus Measured Rates

Crude oil distillation towers use a wash section to remove entrainment from the feed. This section requires wash (wetting) liquid to maximize performance and prevent the wash section from coking. This is a difficult control problem. The liquid rate has a sharp optimum. Too low a rate, and the wash section cokes, shutting down the unit. Too high a rate and significant yield or capacity losses can result. Being wrong in either direction has a high cost.

The liquid rate can be controlled by either direct measurement and control or by inferring a rate based on energy and mass balance around the unit and controlling a secondary stream. The direct measurement system has higher capital and operating costs. The indirect method has lower capital and operating costs, but has a higher risk of upsets and larger uncertainties.

This presentation reviews the results of Monte Carlo modeling of a crude unit to evaluate the economic benefits of the direct approach versus the uncertainties in the indirect approach.