Development Of An Online Near-Infrared Spectroscopy Analyzer For The Determination Of Kraft Black Liquor Properties

Abstract
Black liquor, a by-product of the cooking process in a kraft mill, contains used white liquor chemicals, partially
removed lignin, hemicelluloses, and other extractives from the wood chips. Accurate and continuous online
measurements of black liquor’s properties and composition would have a variety of benefits to mills, for example:
adjusting evaporator targets to maximize steam economy and minimize fouling; managing the location and rate of
recycled or side streams; providing an operational baseline for process troubleshooting; and avoiding unscheduled
downtime. Near-infrared spectroscopy (NIR) is a tool with the potential for black liquor measurements as the NIR
region of the electromagnetic spectrum contains information on water, organic bonds, and particle size. This work
will present a Fourier transform NIR demo unit that has been integrated into an operating kraft mill for the
determination of black liquor properties up to 55% dry solids content. While NIR is commercially available for the
analysis of low solids liquor streams in kraft mills, challenges arise at solids contents higher than 20% as darker
black liquor limits the amount of light passing through the sample. Validation of the demo unit was carried out and
the resulting standard error of prediction (SEP) for solids was 0.32% for liquors between 15-20% solids (WBL) and
0.73% for liquors between 45-55% solids (SBL). For residual effective alkali, the SEP was 0.62 g as Na2O/L for
WBL and 1.11 g as Na2O/L for SBL. A longer-term evaluation is ongoing where additional properties will be
examined, including HHV, lignin concentration and inorganic content. 

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