Water can be intentionally or unintentionally introduced into the hydrocarbon process at many points in a refinery or petrochemical facility. In all cases, the two substances must ultimately be separated. The hydrocarbons, free of water, can then move along in the process; the water, free of hydrocarbons and other environmentally unfriendly contents, can be reintroduced into the environment.
There are environmental and economic consequences if the separation of water and hydrocarbons is less than perfect. Environmental regulations have been progressively tightening in recent years, and penalties for noncompliance can be stiff and nonnegotiable. Likewise, water entering the process, if not removed prior to reaching specific pieces of equipment, can increase maintenance cost and/or reduce catalyst life.
Water and hydrocarbons separate readily in vessels if given ample time to settle. Once separated, it is easy to determine the location of the interface layer between the two. By determining the interface layer in the vessel, it is possible to open a valve at the bottom of the vessel and let out (decant) the water until the interface level reaches a specified point, as shown in Figure 1.
Why they might not work
There are numerous existing level measurement devices that can detect hydrocarbon/water interface. Lyondell Chemical Company (www.lyondell.com) has tried most, if not all of them, in an attempt to find the most reliable, accurate, and cost-effective method for its Channelview Chemical Complex in Channelview, Texas. There are approximately 17 instances where the various technologies — capacitance, float, conductivity, thermal, microwave, optical, ultrasonic, and radiation-based detection — have been employed within Lyondell’s wastewater systems, and none have satisfied requirements.
With a typical accuracy of 0.5 to 2.0 percent of full scale, the previous level measurement installations did not provided adequate control within the boundaries of modern environmental regulations. Many of them had difficulty determining the interface when a rag layer (an area of mixed constituents) was present. And some of them were unreliable and required frequent maintenance or repair due to plugging or coating, resulting in added cost and potential environmental incidents or chemical exposure to maintenance personnel.
In some of the prior systems, an auxiliary level switch was installed as a failsafe near the bottom of the vessel to prevent hydrocarbon dumping in the event of primary measurement failure. If the main level system erroneously read the interface level and failed to close the water outlet valve, the switch closed the valve when the level reached the vessel’s bottom, dumping air to the valve (Figure 2).
A Different Approach:
Coriolis meter offers alternative
Coriolis flowmeters are typically associated with mass flow measurement, but they also measure density directly. Because of the measurable difference in density between water and hydrocarbons, a Coriolis flowmeter can measure the density difference between plain water and water with hydrocarbons. The sensitivity of a Coriolis flowmeter can be as high as 0.0005 g/cc. This can discern a slight change in density even with a rag layer present or at the beginning of the interface layer. It can then initiate a valve closure and/or sound an alarm to other devices, such as a distributed control system (DCS).
To take advantage of the Coriolis flowmeter’s extremely sensitive density measurement capability, Lyondell installed one along the pipe leading from the bottom of the vessel, between the controlled valve and the vessel (Figure 3). Engineers configured the flowmeter to measure fluid and initiate an output whenever there is a downward shift in the density. The DCS then receives the alarm signal and initiates the downstream valve to close. In some instances, the output from the meter closes the valve and the DCS triggers an alarm.
In the case of Lyondell’s first two installations, the valve is always opened manually, either locally at the valve or remotely via the DCS. When operators see that water has accumulated in the vessel, they open the valve and then permit the decanting cycle to stop automatically when the level interface device signals the valve to close. A board operator is able to see the alarm that signals the stoppage. The delay between opening and closing of the valve could be between a few seconds to several minutes, depending on vessel size and the amount of water to be decanted.
Because the water and hydrocarbons are separated by gravity, piping configuration must be considered. Ideally the piping between the flowmeter and the bottom of the vessel should slope downward slightly, as shown in figures 3 and 4. At a minimum, the piping must be level and not have any traps. Gravity pulls the denser water to the bottom of the vessel. This forces the hydrocarbons to the top. Therefore, the water will drain out of the waste pipe during the decanting process. Piping with traps or with any upward sloping from the tank to the flowmeter undermines the system. This is because the hydrocarbons will ultimately be captured by the piping configuration and will prevent water from displacing the hydrocarbons back into the vessel.
High accuracy, low cost
The result of using Coriolis flowmeters instead of traditional level technology was very positive. In fact, the meters were so accurate and responsive that they closed the valves quickly enough to lead operators to believe they were malfunctioning. In some cases, the operators would press the button to initiate the valve to open, and the valve would almost instantly slam closed again. To remedy this problem, a guard valve downstream of the dump valve was partially blocked-in to allow the system to drain gradually.
An added benefit of using Coriolis flowmeters in some applications is the ability to measure and keep track of the amount of water and/or hydrocarbon mixture passing through the meter. In particular, in one application where the next stop was the methanol/water strippers, knowing how much water was being sent to them was useful in operations planning, as any water carryover from the methanol/water strippers degrades the catalyst life in the flex unit.
Using a Coriolis mass flowmeter to control the water outlet valve at the bottom of a hydrocarbon/water separation vessel delivers a higher degree of accuracy and reliability. This helps to reduce or eliminate emissions and associated penalties. In addition, less water is permitted into the process, reducing cost and improving the overall quality of the product. Finally, operating and maintenance costs are reduced by the reliability and simplicity of the control systems.
About the Authors
Donald Dunn is principal IEA & controls engineer at Lyondell”s Channelview Chemical Complex. He has co-authored numerous papers for a variety of industry technical organizations. In addition, Mr. Dunn has been a presenter at various IEEE conferences and other industry forums. He is IEEE Region 5 South Area Chairman and serves as a member of several other committees within IEEE and ISA. Mr. Dunn can be reached at [email protected] or 281 862-5533. Michael Klein is a senior sales representative for Micro Motion, a division of Emerson Process Management. He is a member of the ISA with over 10 years of experience in the instrumentation business. Mr. Klein can be reached at [email protected] or 281 274-0543.
For More Information: www.lyondell.com; www.micromotion.com