Improving distillation tower operation
Measuring differential pressure across long sections of distillation columns has always been challenging, but purpose-built sensor systems provide a solution
By Daniel P. Lucey
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.
Problems associated with traditional measurement techniques
Many distillation processes use impulse piping to measure DP across sections of the tower or column. In an impulse piping configuration, the reference leg (low pressure side) is filled with either a column of liquid (wet leg), or a suitable non-reactive gas (dry leg). While relatively simple in concept, these installations can be difficult to maintain. Evaporation often occurs in wet legs, and condensation can occur in dry legs. Both conditions will cause errors in the DP reading due to drift in the low-pressure side of the measurement.
Impulse piping can be problematic when used on distillation towers. When impulse piping plugs, or when a wet leg freezes, the pressure measurement is lost. Many process plants have installed complicated flushing systems onto impulse piping systems to clear plugged lines. These flushing systems can be expensive and often require a control system to operate correctly. Expensive heat tracing systems are often used to prevent impulse lines from freezing in cold climates. Both flushing and heat tracing systems can fail, resulting in loss of measurement and required repairs.
Additionally, wet leg and dry leg installations must be inspected and maintained frequently. Wet leg installations require a constant level of liquid within the tubing for a reliable measurement. If the liquid evaporates, the DP measurement will drift. Dry reference legs need to be kept free of condensation. If process vapors condense into the dry leg tubing, the DP measurement will drift. Maintenance engineers can spend numerous hours simply verifying that wet legs are at the proper level and that the dry legs are free of condensation.
A drifting measurement reduces confidence that the process is operating at capacity and safely. For example, measurement drift caused by evaporation or condensation of a wet leg/dry leg could lead the operator to believe that a distillation tower is weeping or has vapor entrainment, leading to overcorrection. The cost of maintaining proper readings adds up quickly as frequent trips must be made in order to verify whether the change registered is caused by process changes or measurement error.
Remote seal and capillary systems eliminate many of the issues with impulse tubing installations, such as plugging caused by viscous processes. A remote seal system consists of external sensing diaphragms mounted to the process and connected to the DP transmitter with oil-filled capillaries (Figure 2).
Changes in pressure cause the diaphragm to deflect, and the pressure is propagated through the oil-filled capillary and on to the transmitter sensor, resulting in a measurement output.
Capillary systems eliminate the plugging and maintenance issues associated with impulse piping installations, although capillary systems remain susceptible to temperature-induced error when installed outdoors and on tall tanks and vessels.
DP level capillary technology challenged in tall towers
Tall towers have always posed a significant measurement challenge. In particular, long vertical tap-to-tap distances require extended lengths of capillary. 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.
For example, an installation that has a 50-foot (15 meters) tap distance can experience as much as 2.5 percent measurement drift for every 50°F (28°C) change in ambient temperature. This is a common temperature variation from summer to winter in many locations. As the tap distance increases, the length of capillary increases, and it becomes difficult to get an accurate measurement during large ambient temperature shifts.
The low side pressure is directly affected by the tap distance and fill-fluid density. When ambient temperatures decrease, fill-fluid density will increase, and thus cause a low side pressure increase. This shift likewise changes the DP measurement.
These shifts in DP are entirely temperature-induced but are often mistaken as a process DP shift, when in fact the process has remained unchanged and only the ambient temperature has changed.
Time response will also slow down in installations with long lengths of capillary. The more capillary required, the longer it takes for a change in process conditions to travel through the capillary to the DP transmitter. As shown in Figure 3, the time response can become a critical issue with long lengths of capillary and impulse lines.
Digital measurement system improves DP measurement performance
Electronic remote sensor technology solves many of the problems in making a DP measurement on tall vessels or towers. Rather than using a single DP transmitter with mechanical impulse piping or capillary, an electronic remote sensor system uses two direct-mount gage or absolute pressure transmitters that are connected with a single electrical wire (Figure 4).
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. Alternately, two transmitters can communicate independently to the control system using the FOUNDATION Fieldbus digital protocol. In this implementation, the DP is calculated within the control system. In either case, the sensor system provides many benefits on distillation towers that traditional systems cannot realize.
The electronic remote sensor system does not require heat tracing, never plugs, and is immune to temperature-induced drifting. This means that plant personnel will be able to get accurate pressure measurements over a large range of ambient temperatures. Additionally, electronic remote sensor technology substantially improves time response by eliminating slower mechanical components. The electronic remote sensor system is able to almost immediately sense changes in process pressures.
Electronic remote sensor systems improve operations
An electronic remote sensor system provides additional process information that can be used for optimized control. In addition to the DP calculation, the electronic remote sensor system provides pressure measurements from each pressure sensor – as well as a scaled variable output that can be used to calculate level or volume.
This highly accurate measurement data allows plant personnel to know the exact location of the distillate on the vapor-pressure curve, as well as feed stock levels. Armed with this information, the tower parameters can be fine-tuned to increase throughput and quality while cutting energy consumption. Costs are also decreased because the need for additional instrumentation to read the top and bottom pressures is eliminated.
Installation time is reduced because difficult-to-install impulse tubing and long lengths of capillary are replaced with a flexible electrical wire. The electrical wire can be fed through floor grates and wound around plant obstacles, further simplifying installation. Further, because fluid-filled mechanical parts are replaced with an electrical wire, no heat tracing or insulation is required to adjust for swings in ambient temperature.
An electronic remote sensor system eliminates maintenance rounds in which technicians verify measurements or check impulse lines for leaks or clogging. Additionally, substantial storeroom space may be necessary to stock all the necessary repair parts for traditional systems, whereas electronic remote sensors reduce inventory to a standardized transmitter and a spool of wire.
Do-it-yourself solution not as accurate
An electronic remote sensor system is purpose-built for digital communication within the HART communication protocol. The overall column DP is fully synchronized within the system software and is calculated ten times per second. Additional process information, such as scaled variable and additional pressure measurements, can be sent to the control system digitally via HART or as separate 4–20mA signals with a HART-to-analog converter.
It is also possible to perform the DP measurement with a do-it-yourself (DIY) solution. A DIY solution consists of two independent pressure transmitters sending two HART 4–20mA signals to the control system for calculation. Because the control system’s polling rate is not synchronized between the two transmitters, there can be a time-lapse error between the two signals (Figure 5). When this occurs, an erroneous DP will be calculated within the control system. Additionally, many control systems are programmed to scan/update at rates slower than once per second.
An electronic remote sensor system solves the polling rate error by synchronizing the measurements and by calculating the DP within the transmitter and at the measurement point prior to sending the signal to the control system. A further advantage is that only one I/O connection is required, whereas the DIY version requires two I/O points and two independent 4–20mA loops, which doubles the cost of wiring.
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.
Employing electronic remote sensor technology will allow operators to increase efficiency, save energy, and boost throughput. Electronic remote sensors provide the key performance indicator for the overall health of the operation.
ABOUT THE AUTHOR
Daniel P. Lucey (Daniel.Lucey@Emerson.com) 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.
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