Effective control of process variables is critical to the production of high-quality petroleum products. In the crude oil refining process, distillation is impacted by two factors; reflux ratio and temperature gradient through the distillation column.
Control of Reflux Ratio
Reflux is condensed top product that is returned to the upper part of the tower to be reprocessed. Reflux ratio is defined as the flow rate of reflux returned to the tower, divided by the flow rate of overhead product removed from the tower.
As the reflux liquid falls down the tower, it cools and condenses a portion of the upflowing vapor, thereby increasing the effectiveness of the distillation column. Generally, the higher the reflux ratio, the fewer the trays needed for a desired separation of lower boiling point from higher boiling point products. Conversely, in an existing distillation tower, an increase in reflux ratio will result in improved separation.
Too much reflux, however, results in flooding of the tower, with reduced efficiency of separation. Too little reflux will result in inefficiency through liquid starvation across the trays. The reflux ratio is normally regulated by a temperature controller in the vapor outlet linked to a control valve in the reflux pump discharge line.
Temperature Gradient Control
The temperature gradient through the distillation column determines the purity of products. Effective control of temperatures in the feed, reboiler and at the top of the tower will produce the desired temperature profile.
The top temperature is controlled by reflux. High feed and reboiler temperatures will produce too much heavy vapor, compromising the purity of side-stream products. Low temperatures will result in a reduction in the amount of vapor rising through the tower, which is essential for product distillation.
Pressure also has a role to play in distillation efficiency because it affects the boiling points of crude oil components. Today, many distillation columns operate under high vacuum. This decreases the boiling points of components, thereby reducing the amount of heat needed for vaporization and making the process more energy efficient.