On the Local Filtration Properties of LignoBoost lignin: Studies of the influence of xylan and ionic strength
Doktorsavhandling, 2017

Kraft pulping technology is currently the most commonly-used method for producing paper pulp from wood. A new and promising opportunity for Kraft pulp mills is to take a step towards becoming biorefineries by implementing technologies able to extract and convert the organic by-products, such as lignin, into a wide range of value-added products and chemicals. The LignoBoost process, which is a new technique that has recently been implemented on an industrial scale, is designed to extract lignin from the Kraft process with a high degree of purity, making it potentially suitable for the manufacture of e.g. carbon fibres. Following a lignin precipitation stage, filtration, performed by dead-end filtration, is one of the key steps of the LignoBoost process. The aim of this work is to improve the efficiency of the filtration stage of the LignoBoost process further. The local and average filtration properties of the cake formed from softwood lignins extracted using the LignoBoost process were investigated through the use of model liquors and by varying the condition parameters and the preparation procedures. The influence of the hemicellulose xylan on the filtration and precipitation of LignoBoost lignin was studied. LignoBoost lignin was (i) suspended in acid water with xylan added and (ii) dissolved together with xylan and then re-precipitated. The effects of ionic strength, applied pressure, slurry concentration, pH, precipitation temperature and rate of acidification on the resulting material and its filtration properties were all investigated. Moreover, the evolution of the size of the particles agglomerates during the course of precipitation was monitored in situ using the Focus Beam Reflectance Measurement (FBRM) technique.   The lignin-xylan mixtures were more difficult to filter than the original LignoBoost lignin. The latter was found to be a material that was relatively easy to filter (2 to 6 ·1011 m/kg in filtration resistance), forming weakly compressible filter cakes over the filtration pressure range studied (2-28 bar). The slurry concentration (8.8-21.6 wt%) was not found to affect the filtration behaviour. Chemical analysis of different layers of the filter cakes formed showed that xylan was distributed evenly on the solid lignin when both solids were precipitated together. It is thus likely that xylan is sorbed onto the surface of the lignin particles-agglomerates, opening their structure and increasing the contact area between solid and liquid during filtration: the flow resistance is increased. Furthermore, it was found that increasing the ionic strength of the slurries made the solid/liquid separation process easier. A plausible explanation for this is a decrease in the electrostatic repulsive interactions between the solids and the subsequent formation of a denser solid structure. Similarly, lowering the pH below the pKa values of the carboxylic acid groups of xylan made the lignin-xylan mixtures significantly easier to filter. Finally, the onset of precipitation (particle sizes ≥ 1µm) was not found to be affected by either the precipitation temperature (45-77 °C) or the addition of xylan; a broader particle size distribution was obtained when acidification was rapid compared to that performed slowly and stepwise.

acid precipitation.

LignoBoost process

compressible filter cake

particle interactions


softwood lignin

constitutive relationships

local filtration properties

dead-end filtration

KC-salen, Kemigården 4, Chalmers.
Opponent: Professor Eugene Vorobiev, Université de Technologie de Compiègne (UTC), France


Julie Durruty

Chalmers, Kemi och kemiteknik, Kemiteknik

Wood is one of the best candidates for replacing fossil oil as a raw material in the production
of consumer material commodities: it is an abundant and renewable resource that does not
compete directly with the production of food. Moreover, the current dominant wood-based
industries, i.e. mills that produce paper pulp via the Kraft process, generate an energy surplus
that is not generally utilized to its full potential. Only about half of the wood is actually used
for the production of the paper pulp, while the main organic by-product, lignin (an aromatic
polymer), is currently used almost exclusively as an on-site biofuel. Yet the total amount of
lignin generated in modern pulp mills contains more energy than is required internally in the
process. A promising opportunity thereby presents itself to modern Kraft pulp mills: the
implementation of technologies that enable the surplus lignin to be extracted and converted into
a wide range of value-added products and chemicals. The conversion of lignin into carbon fibres
is, for example, the focus of a large number of on-going investigations.
The LignoBoost process is a new, commercially-implemented, technology designed to extract
lignin during the Kraft process with a high degree of purity. Filtration is one of the main steps
of the LignoBoost process; the aim of this work is to improve the efficiency of the filtration
stage of the LignoBoost process further. The influence several parameters have on the filtration
of lignin was therefore investigated, and included the content of xylan (a sugar polymer which
is also present in the by-product stream of Kraft pulp mills) and the concentration of salt.
Furthermore, the filtration apparatus used in this work allows for the determination of local
filtration data, something that is not very common in the field of filtration: it is usually average
filtration data, which is easier to determine, that is considered. Local data provides nevertheless
a basis for a much better modelling of the filtration process and thereby also allows for a better
design of the filtration unit.





Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4337



KC-salen, Kemigården 4, Chalmers.

Opponent: Professor Eugene Vorobiev, Université de Technologie de Compiègne (UTC), France

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