“They mix like oil and water,” a figure of speech meaning not at all. If you work with immiscible liquids, you know the difficulty of measuring multiple layers during processing and storage.
Trying to determine how much of a tank’s content is one liquid floating on top of another (e.g., oil on water), is a challenge, but not so difficult if you have the right level instrument. That’s the point of my article in the April 2020 issue of Chemical Processing, Accurately Measure Interfaces Between Immiscible Liquids. It considers two approaches, one for point level measurements, and one for continuous.
Let’s first consider when a plant only needs to ascertain what’s at a given level. Various detection technologies could do the job but one of the most economical and practical is a vibrating-fork level switch. It typically determines if the contents of a tank or vessel have reached or passed the instrument’s sensor insertion point. In many situations, just indicating when the fork is immersed is enough. However, some more-sophisticated devices can characterize what the fork is immersed in by the degree of change in frequency. This is because the sensor behaves differently when immersed, e.g., in water versus oil; so, it can indicate if it’s above or below the water/oil interface point.
A perfect product for just this situation is Emerson’s Rosemount 2140 Level Detector – Vibrating Fork. It’s one of the newest and most sophisticated units available today. Still, there are applications requiring more information. Some may be solved by using multiple level switches, but where a more detailed measurement is needed, including the total level and an accurate measurement of the interface position, a different approach is necessary. The answer: guided-wave radar (GWR). It sends a radar pulse down a probe and measures the reflected signal’s return time, sometimes reading multiple reflections to produce the required data.
The pulse is reflected directly off the surface of the liquids — the main reflection from the top and a secondary reflection from the interface layer. This works provided the liquid on top has a lower dielectric constant (DK) or relative permittivity than the liquid underneath. GWR suits sensing oil on water well because most oil products have a very low DK, <5 typically, while water’s is >50.
That’s the key: GWR can measure more than one reflection. The question becomes how close can the reflections be, meaning how thin a layer can it measure.
Newer, highly sophisticated GWR instrument technologies have improved the ability to resolve very short pulse-duration differences, allowing some of the latest models to measure well-defined thin interfaces down to 25 mm. This significant improvement results from improved software able to detect signal peaks that are closer together without having to decrease signal bandwidth.
This kind of performance calls for Emerson’s Rosemount 5300 Level Transmitter - Guided Wave Radar. It is ideal for challenging measurements on liquids, slurries, and solids, offering state of the art reliability and safety features in level and interface applications. The Rosemount 5300 offers several advantages such as easy installation, no need for calibration, and ability to handle ultra-thin layers through Peak-in-Peak technology.
The article goes into more detail on applications, so give it a full read. You’ll find information on additional topics, such as working with emulsions and indistinct layers. It will stimulate your thinking about how best to answer questions like those below:
Let’s hear about your experiences. Have you solved the problems, or are you still trying to figure out solutions? You can find more information like this and meet with other people looking at the same kinds of situations in the Emerson Exchange365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the Chemical Processing and Oil & Gas Groups and other specialty areas for suggestions and answers.