The kraft process is the most prevalent pulping method. Here, heat and chemicals (sodium hydroxide and sodium sulphide, or White Liquor) combine in a large pressurized cooker, or digester, to transform wood chips into pulp by dissolving the wood’s lignin binder. The waste lignin and spent chemicals, or Black Liquor, is routed to a recovery boiler.
Digester level monitoring maintains operational stability, increases throughput and reduces kappa variation (the measure of lignin remaining in the pulp). Process conditions of up to +355 °F (+180 °C), steam, high pressure, and harsh chemicals may challenge many level sensors. Continuous level monitoring and point monitoring for overflow is a common scheme for digesters.