Successfully Applying Non-Contacting Radar in Black Liquor Mixing Tanks

In the Kraft pulping process, wood fibers are separated, and resinous materials removed, using an alkaline (typically sodium hydroxide and sodium sulfide) cooking liquor, also known as white liquor. This process converts wood into cellulosic pulp used in the papermaking process.

This digesting process turns the white liquor into black liquor, which contains:

…lignin fragments, carbohydrates from the breakdown of hemicellulose, sodium carbonate, sodium sulfate and other inorganic salts.

This black liquor:

…has about 20–30% solids. At this concentration the rosin soap rises to the surface and is skimmed off. The collected soap is further processed to tall oil. Removal of the soap improves the evaporation operation of the later effects.

Once this black liquor is concentrated, it’s sent to the recovery boiler to provide energy for the pulp making process.

I share all this as background for this short, 2:20 YouTube case study video, Rosemount 5408 Solves Measuring Issue in Black Liquor Mixing Tanks. These 18-meter mixing tanks contain a mix of lye and soap with heavy condensation and a sticky media.

The mix tank is used to separate the soap from the black liquor. A Rosemount 5408 non-contacting radar level transmitter was used to more accurately identify the interface between the black liquor and soap/lye interface. A pressure transmitter was originally used to measure level, but it could not determine the soap level as well as the radar level transmitter.

They highlighted the key challenge being the foam from the soap and condensation from the steam, so a process seal antenna for the radar was used since it was very resistant to the condensation and the foam which builds up in the tank.

This solution has been in place for over a year and works much better than the prior measurement methods in contending with the steam and foam.

Visit the Rosemount 5408 page on for more about the technology and tools to determine if it is the right fit for your level measurement application. You can also connect and interact with other level measurement experts in the Measurement Instrumentation group in the Emerson Exchange 365 community.

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