Efficient Distillation Columns Require Accurate DP Measurements

The distillation process is used in many industries to separate mixtures into components. It is defined as:

…a process in which a liquid or vapour mixture of two or more substances is separated into its component fractions of desired purity, by the application and removal of heat.

Emerson’s Daniel Lucey has written an Intech article, Improving distillation tower operation. In it, he discusses the importance and challenges in measuring differential pressure (DP) across the distillation column, and sections within the column.

The application and removal of heat makes the distillation process energy intensive:

…consuming up to 50 percent of a refinery’s operating costs due to intense heating and cooling cycles.

Having accurate measurements to feed the control system is critical for energy efficient, safe, and reliable operation. Dan highlights some of the important measurements [hyperlink added to quote]:

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.

Source: ISA Interchange blog (http://jimc.me/15Ri1nd) – Conditions at the bottom of the distillation tower are different from those at the top of the tower, so the more DP measurements made, the better the operator’s process insight.

Effective control requires accurate DP measurement:

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…

Traditional DP measurement requires impulse piping to feed the high- and low-pressure sides of the process from taps along the column, to the DP measurement device. The low pressure or reference leg side of the impulse line can be filled with a liquid (wet leg) or non-reactive gas (dry leg). We explained these methods in detail an earlier post, Eliminating Temperature Effects in DP Level Measurement.

Each of these methods has maintenance issues. Wet legs:

…require a constant level of liquid within the tubing for a reliable measurement. If the liquid evaporates, the DP measurement will drift.

They can also freeze in cold weather. Dry legs:

…need to be kept free of condensation. If process vapors condense into the dry leg tubing, the DP measurement will drift.

To avoid these issues, remote seal and capillary systems were developed. A remote seals and capillary system:

…consists of external sensing diaphragms mounted to the process and connected to the DP transmitter with oil-filled capillaries…

While this technology can eliminate issues associated with impulse piping, tall distillation towers can pose issues. Dan notes:

As the tap-to-tap distance grows, the sensitivity of the system to ambient temperature changes increases. In addition, tall towers are almost always located outdoors, where temperatures can vary widely.

A solution to these extended distances and ambient temperature swings is electronic remote sensor systems, which electronically connect the pressure readings from two pressure transmitters together. While this can be done with traditional pressure transmitters back at the control system, scan rates and timing issues can cause swings in the differential pressure calculations. If the devices are Foundation fieldbus-based, the communications are synchronized and accurate calculations can be performed by the control system.

Dan describes the process for HART-based pressure transmitters in an electronic remote sensor system configuration:

One of the two transmitters calculates the DP using internal software and transmits the DP measurement back to the control system using a standard 4–20mA HART signal.

Unlike impulse piping and oil-filled capillary tubing, the electronic connection between transmitters:

…does not require heat tracing, never plugs, and is immune to temperature-induced drifting.

Having accurate DP measurements provided back to the control system allow the regulatory and advanced process controls to run the distillation column at optimum performance. Dan closes [hyperlink added to quote]:

The more DP measurement points on a distillation column, the greater the insight the operator has into the distillation tower and process, allowing for improved operation. Because of this, electronic remote sensors are becoming a new best practice on distillation towers. It is an even better practice to install independent systems across the stripping sections and the rectification sections, as well as the entire tower.

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