Level Detection

Level/Interface Detection

Key Benefits Include:

identify boundaries between gas/ liquid; liquid/ liquid extraction; solid / liquid agitation; liquid / foam; emulsion and water / organic phases…
measurements not dependent on hard interfaces, transparency or density differences
non-intrusive probes, able to scan through process fluid
Monitor solid suspension times

Introduction

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.

ITS Solution

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

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.

Right Click to zoom

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.