Interface detection

Level detection is one of the most common measurements (alongside temperature, mass and flow). 

However the detection of an interface below the surface of a liquid is more challenging for traditional level detection techniques such as bubblers, microwave, ultrasound, vibration.  Such interfaces include:

•    Liquid-liquid levels, e.g. in solvent extraction and phase separation
•    Solid-liquid levels, e.g. in management of suspensions and slurries
•    Liquid-foam levels, e.g. in flotation separators.

Interfaces exist between two immiscible fluids and also between a liquid and foam or the solids component of a slurry; gel or soft solid and a liquid supernatant.

Examples are found in many different industries, for example:

•  Pharmaceuticals where an immiscible organic and aqueous solutions are mixed so that impurities can be preferentially absorbed in one phase and purified product more easily isolated
•  FMCG where foams can form in brewing; rehydration of milk powders
•  Petrochems where oil / water mixtures are left in separating tanks until the two phases separate – often leaving an emulsion layer between the two components
•  Nuclear waste management where many slurries are kept in artificial lagoons.  Where these slurries are agitated the solids are mobilised up to a level described as “cloud height”, after which there is clear supernatant

In each case, withdrawing too much fluid causes some of the second fluid to be decanted with the first and spoiling the separation.  This can effect yield, product quality and / or process efficiency.

Many interfaces are easily observed for example using a sight glass or dipstick.  However if a visual indicator is not feasible interfaces can often be very difficult to identify, particularly as single point measurements are usually at fixed positions and hence cannot cope with variable levels.

For example, without accurate measurement of the interface the tendency is to stop decanting a safe distance above the interface, with consequential loss of yield and a need to repeat the process.  In other cases, the interface is missed entirely leading to the purified phase being lost to waste.

Process tomography probe can be used to measure the depth of different components in a vessel in real time. 

ITS linear probes have often been used to observe these types of interfaces. 

Through recent developments in sensing techniques and software processing algorithms, ITS has been able to significantly improve the accuracy and reliability of these sensors.

The sensors operate on the principle of electrical resistance tomography.  Sensors are constructed using an array of electrodes (usually 316 steel) supported on a PTFE matrix.  Other materials may be selected (hastelloy, ceramics etc.) where necessary for compatibility with particular processes.

ITS sensors can detect the level of a non-conducting substrate (such as solids, organic solvents, gases or foams) in a conducting (usually aqueous) medium.  Additional data on emulsions, colloids or suspensions can also be made available.

Tomography level sensors can be deployed in hazardous environments (such as explosive gases, high temperatures and pressures, chemically aggressive materials or high radiation fields), providing data to within 0.5% of full scale at update rates of better than 1x second.   Despite the sophistication of the measurements, set-up and calibration are quick and straightforward, following which data is available on line.
 

Data outputs can be processed and linked to selected process devices that can control mixing conditions and other parameters.

The p2+ instrument using a linear probe is the suitable system for monitoring interfaces in aqueous systems.

This shows an interface detection user interface developed by ITS for use with the linear ERT sensor. A sand-water mixture was agitated and then allowed to settle. The accumulation of sand at the bottom of the vessel is clearly detected.

Key benefits include:

The benefits of a tomography probe are that it can be left in the process fluid without disturbing process conditions and is then able to scan across a wide range of depths.  In addition electrical tomography probes are extremely robust, able to withstand challenging temperatures, pressures, chemicals and (in the case of nuclear applications) high radiation fields.

• identify boundaries between gas/ liquid; liquid/ liquid extraction; solid / liquid agitation; liquid  / foam; emulsion and water / organic phases, etc

• measurements not dependent on hard interfaces, transparency or density differences

• non-intrusive probes, able to scan through process fluid

• Monitor solid suspension times

Publications:

G.T. Bolton and S.J. Stanley (2009) Measurement of solid-liquid mixtures using electrical tomographic measurement techniques, Proceedings of the 12th International Conference on Environmental Remediation and Radioactive Waste Management (ICEM09), October 11-15, 2009, Liverpool, UK

Bennett, M.A. and Williams, R.A. (2004) Monitoring the operation of an oil/water separator using impedance tomography, Minerals Engineering, Vol. 17, No. 5, pp 605-614

For more information about this paper, please contact ITS.

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