Lars Olausson

The effect of temperature on the catalytic conversion of Kraft lignin using near-critical water

The catalytic conversion of suspended LignoBoost Kraft lignin was performed in near-critical water using ZrO2/K2CO3 as the catalytic system and phenol as the co-solvent and char suppressing agent. The reaction temperature was varied from 290 to 370°C and its effect on the process was investigated in a continuous flow (1kg/h). The yields of water-soluble organics (WSO), bio-oil and char (dry lignin basis) were in the ranges of 5-11%, 69-87% and 16-22%, respectively. The bio-oil, being partially deoxygenated, exhibited higher carbon content and heat value, but lower sulphur content than lignin. The main 1-ring aromatics (in WSO and diethylether-soluble bio-oil) were anisoles, alkylphenols, catechols and guaiacols. The results show that increasing temperature increases the yield of 1-ring aromatics remarkably, while it increases the formation of char moderately. An increase in the yields of anisoles, alkylphenols and catechols, together with a decrease in the yield of guaiacols, was also observed.

Scale formation and growth when evaporating black liquor with high carbonate to sulphate ratio

Scaling in the recovery cycle of kraft pulp mills has long been recognised as a problem, especially in black liquor evaporators. Most problems are connected with the precipitation of sodium salts. In the black liquor evaporation, as the black liquor becomes concentrated, sodium carbonate and sodium sulphate will saturate and precipitate from the solution. When precipitation occurs the salts may crystallise on the heat transfer surface or adhere to the surface as well as they can crystallise in the bulk. If crystals remain on the heat transfer surface an insulating layer of scales can quickly build up, causing the heat flux to drop and eventually forcing the evaporator to be shut down for cleaning. In this research the scaling rate, or the reduction in heat transfer rate over time, has been analysed. For the research a pilot evaporator has been used, built in cooperation with Metso Power (former Kvaerner Power). The falling film type evaporator is a 4.5 m tube with the black liquor on the outside. One of the objectives of the thesis was to investigate whether there are local scaling variations at different positions on the heat transfer area by monitoring the fouling growth rate at different local positions in the pilot evaporator. The scaling was found to depend on the vertical position along the heat transfer surface. The scales start to form on the bottom of the evaporator tube where the concentration is highest, and then expand upwards. A clean surface starts to foul only when the area beneath it has fouled. Furthermore the research in this thesis focused primarily on scaling for black liquors with high ratio of carbonate to sulphate, a condition shown to be important in industrial black liquor evaporators (Frederick et al., 2004). The aim was to find favourable operating conditions decreasing or eliminating scaling. The examined parameters were different heat flux, circulation flow rate, internal residence time and black liquor feed concentration. For the investigated operating conditions, the main parameters affecting scaling were the circulation flow rate and heat flux, whereas the scaling rate was seen to be independent of the internal residence time.

Crystallization in a pilot evaporator - Comparison between black liquor and black liquor model substances

Crystallization measurements during semi-batch operation of a pilot evaporator have been performed. The experimental analysis is based on changes in heat transfer coefficients and crystal masses. With this method of operation, the total crystal mass as well as the distribution of the crystal mass between the circulating solution and on surfaces can be measured continuously during the evaporation. Experiments with black liquor and different black liquor model substances, all with equal carbonate-to-sulfate ratio, crystallizing burkeite have been evaluated. The results are compared to each other and to two aqueous reference solutions crystallizing dicarbonate and sodium carbonate. The results presented include solubilities and metastable limits as well as different crystallization rates and fouling rates. Generally, the supersaturation at the metastable limit observed for the different liquors was high (14-25%). All the different liquors crystallizing burkeite were seen to have significantly lower fouling rates than the reference solutions crystallizing dicarbonate and sodium carbonate. The burkeite liquors showed no specific attraction to crystallize on the heat transfer surface as the reference solutions did.

Non-evaporating black liquor falling film hydrodynamics

Knowledge of wave behaviour is a key factor for understanding heat transfer in falling film evaporators. Here the industrially important fluid black liquor has been examined, together with water which has been used as a reference. Notable results are that there are large local fluctuations in the film thickness; for instance at a Re number of 4100 in the black liquor falling film, the thickness varies between 0.2 and 8.0 mm. For the water falling film, both correlations (Brotz and Brauer) are mostly within the 95% confidence interval of the measured average film thickness. The black liquor film tends to be thicker than the film thickness calculated with the correlations, especially for higher Re numbers.

Turbulence models for falling film hydrodynamics and heat transfer compared with experimental values for black liquor

Increased knowledge about the hydrodynamics and the heat transfer in falling film evaporation of black liquor is an important support for design of energyand cost-effective evaporation plants. Two comprehensive turbulence models for falling film hydrodynamics are compared with experimental results, gained in this study, for heat transfer in falling film evaporation of black liquor. The turbulence models provide increased understanding about the velocity, temperature and heat transfer profile in the evaporating black liquor falling film. The largest temperature gradients are, according to the turbulence model, in the region close to the wall and the region close to the film surface. The heat transfer coefficients calculated with the turbulence models were in agreement with the experimental water heat transfer data. However, they did not agree well with the experimental heat transfer coefficients for black liquor. Several possible reasons for the deviation are discussed.