The distillation process uses enormous amounts of energy, consuming up to 50 percent of a refinery’s operating costs due to intense heating and cooling cycles. Proper distillation tower operation can reduce energy consumption, but plant personnel need the right information in order to improve operation.
Specifically, operators must have precise measurement and control of numerous variables, including feed and vapor flow rates, tray levels, process pressures, and temperatures. In practice, measurement of all these variables, except for temperature, is often made with pressure transmitters.
Problems can occur when operators and engineers have insufficient information about operating conditions. Failing to properly monitor and control process variables can result in decreased product quality and throughput, increased energy costs, and unsafe operations that put employees and capital equipment at risk.
Using a purpose-built electronic remote sensor system is one way to calculate differential pressure (DP) and to provide additional process information that can be used by plant personnel to increase efficiency, save energy, and boost throughput. Such a system can also cut required maintenance and increase uptime.
DP measurements indicate tower health
Vapor flow rates and feedstock levels are calculated by measuring tower pressures. Flow, pressure, and temperature measurements allow the operator to detect process upsets, such as foaming, entrainment, weeping, and flooding.
A sudden decrease in tower pressure can cause tower feedstock to boil, which in turn drastically increases the vapor flow rate. Entrainment or flooding occurs when vapor flow rates are too high. A rapid increase in pressure can cause immediate vapor condensation, resulting in tray dumping, and ultimately requiring a total tower restart.
DP measurements provide information needed to better control the distillation process. When a distillation column is in an ideal state and operating consistently, the DP within the tower will remain stable. Strategically raising or lowering the pressure will improve product separation and quality. Energy savings can be dramatic, saving up to one-third as compared to operation at a fixed pressure, as heating and cooling cycles can be controlled more efficiently.
At a minimum, a single DP measurement should be made across the entire tower. An even better solution is to additionally measure the DP across the stripping and rectifying sections, as well as individual trays. (Figure 1) Pressure measurements can be implemented as needed across trays to further improve the operator’s process insight.
Daniel P. Lucey is a marketing engineer for Emerson Process Management’s Rosemount Measurement Division. Dan joined Emerson in 2011, and his primary focus is on the 3051S Electronic Remote Sensors (ERS) System. He received a B.S. in Mechanical Engineering from the University of Saint Thomas. Contact: Daniel.Lucey@Emerson.com