Mathias Gourdon

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.

Effect of Co-flowing Vapor during Vertical Falling-film Evaporation

A large number of industrial processes use falling-film evaporation to concentrate liquid products. This technology allows for small temperature differences during operation and is often significantly more energy efficient than other techniques. When processing dairy products, a reduction in the solvent fraction results in an increased product viscosity and may thus result in non-Newtonian features. The interaction between a co-flowing vapor that is produced during the evaporation process and the falling film is an important feature of the process. Few studies have accurately studied the effect of co-flow on evaporative falling films at high solid contents. In this work, an experimental study of the influence of co-flowing vapor on the heat transfer coefficient for a dairy product is presented as a function of both the solid content (from 10 to 50%) and the mass flow rate of the feed. The experimental set-up, consisting of a unique industrial pilot-scale evaporator, provides the possibility of obtaining results useful for realistic industrial conditions. An analytical approach that enables the simultaneous evaluation of heat transfer in every experimental condition, e.g., for Newtonian or non-Newtonian fluids and with or without co-flowing vapors, is presented.

Black Liquor Scaling Propensity - Experimental Investigation in a Pilot Evaporator

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.