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Spectroscopic Characterization of Cooking Liquor in Kraft Pulping Process by Near-infrared Optical Sensor

Publicerad 2017-10-03

Om uppdragsgivaren
BillerudKorsnäs AB is a leading supplier of renewable packaging materials and solutions. BillerudKorsnäs AB is a Swedish company that operates in the pulp and forest industry. The company was established in 2012 as a fusion of competing companies i.e. Billerud AB and Korsnäs AB.

Beskrivning av examensarbetet
Kraft pulping process is strongly affected by the quality of biomass-based feedstock material. Wood in the form of chips is used as a feedstock material and together with chemicals i.e. cooking white liquor – NaOH and Na2S is brought to a pulp digester i.e. process reactor where the wood is digested depicted in the figure below. The substance generated is a wood pulp, which is further processed resulting in paper as a final product. Most of the pulp digesters nowadays are continuous process reactors with significant residence time 5-7 hours. The quality of the resulting pulp is affected mostly by cooking conditions, type of the wood, physical and chemical properties of the wood, age of the trees, among others. One of the biggest challenges is to maintain stable operation with the limited possibility to measure and control physical and chemical parameters inside.

To optimize kraft pulping process, operators also need to understand and control the green and black liquors of the kraft liquor cycle, which directly impact on the quality and yield of the pulp extracted from the digester. An important consideration in this control is to reduce a negative variation in one part of the cycle that results in downstream unit operations disturbances which in turn can create further variations, causing a downward spiral effect. Traditional manual testing procedures performed by the operators associated with the Kraft liquor cycle do not provide a full spectrum of the needed properties of the black, green and white liquors to optimize the cycle. The infrequency of the test accompanied by the time lags in the process do not allow for corrective actions to mitigate process upsets. In many cases operators respond to upset conditions without ever knowing the originating cause or possibly even induce upsets when acting on erroneous information.

Hence, there is an urgent demand for a method that continuously determines the characteristics of the biomass feedstock and black liquor that affect the conversion into pulp.

Accurate on-line measurements can be realized by introducing the near-infrared spectroscopy NIRS technique. This inexpensive, contactless and rapid optical method enables qualitative and quantitative characterization of solid and liquid materials. The method is based on vibrational spectroscopy and measures interactions between electromagnetic radiation and the chemical bonds of the matter in the near-infrared NIR spectral region. In biomass, electromagnetic radiation interacts with structural molecular groups. The extraction of the information from spectra is done by applying chemometrics which is a discipline that uses mathematical and statistical methods to provide maximum information by analyzing NIR spectra. It is usually done by applying spectral pre-processing techniques which enhance the interesting information included in the spectrum. The quantitative analysis e.g. multivariate or univariate regression method or machine learning techniques are then applied in order to determine quantifiable properties of the material.

According to previous studies NIR has a potential for online determination of reduction efficiencies, effective alkali, sodium carbonate, sodium sulphate, sodium sulphide, and sodium thiosulphate in the green liquor and white liquor and residual effective alkali, lignin content and an organic/inorganic content determination of the black liquor.

Thesis tasks
-Literature review: Identification of what are the most important black liquor properties in kraft pulping process which can be potentially measurable by NIRS
-Black liquor sampling at different extraction points from the kraft pulp digester
-Sample preparation of NIR spectra acquisition
-Determination of the reference properties of interest e.g. soap content, residual affective alkali etc.
-NIR model development signal pre-processing, calibration, cross-validation, independent external validation
-Evaluation of the potential

Expected results
-Possible conference or journal paper
-Production of robust and reliable NIR models predicting various properties of interest of the green/black liquor.

Practical information
Group size: 1-4 students
Credits: 15hp or 30hp
Duration: 10 or 20 weeks

Anders Avelin
021-15 17 98
Mälardalens högskola
Box 883
721 23 Västerås


Mälardalens University, Future Energy Center and BillerudKorsnas AB


Sista ansökningdag

2017, 2018

PDF version
Reference 1

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