Air stripping of kraft foul condensates to remove volatile impurities including methanol
Creators: name: Blackwell, Brian Robin
Publisher: name: University of British Columbia
Year: 2010

description: From an environmental point of view, foul condensates generated in kraft pulping are low-volume/high-strength wastes. Therefore, foul condensates are obvious candidates for selective treatment and reuse. Accordingly, twenty-five experiments were conducted on a four-inch-diameter column to determine the feasibility of air stripping foul condensates. The column was packed to a height of thirty feet with 1/4" ceramic Intalox saddles. Theoretical considerations and practical evidence suggest that of the main impurities in foul condensates, methanol is the most difficult to strip. Therefore, the research focused on methanol removal. Twenty-two of the experiments involved a dilute solution of methanol in water while three experiments used foul condensate from a kraft mill. The foul-condensate experiments demonstrated that from the point of view of mass transfer, methanol behavior in foul condensates is the same as in dilute solutions of methanol in water. Removal of methanol during ambient-air stripping was low, even when the ratio of stripping-gas flow to liquid flow was high. This poor methanol removal was partially due to the adverse effects of evaporative cooling. Evaporative cooling was counteracted by increasing the enthalpy of the stripping-gas by adding steam to the ambient air. The highest-enthalpy stripping-gas used in the experiments was air saturated at 160°F. Using about 0.5 moles of this stripping-gas per mole of liquid-feed, 90% methanol removal was achieved. The experimental results on methanol removal are consistent with published information (experimental data and industrial experience) concerning air stripping and steam stripping of foul condensates. The liquid-temperature profiles measured in some of the experiments were unusual, resulting from situations that involved humidification in the bottom region of the tower and dehumidification in the upper region of the tower. Methanol transfer was not always via stripping; in some experiments methanol was absorbed in the upper portion of the tower and stripped in the lower portion. Theory involving simultaneous heat transfer and mass transfer was used to analyse the experimental results. This theory was developed by expanding and modifying an algorithm presented by R.E. Treybal. Three of the main expansions were provisions for the change in total pressure with packing height, for heat loss from the column to the surroundings and for formation of fog in the gas phase. Theoretically calculated values of various parameters are in good agreement with experimental measurements. Heat-transfer and mass-transfer capacity-coefficients estimated via the theoretical analysis are consistent with similar coefficients determined by other investigators in related situations. Due to problems with sensitivity of the calculational procedure, the theory here is not recommended as the primary basis for design; experimental results are preferred. However, the theory should be used as support, especially when the design situation involves conditions different from those in experiments that have been conducted. An example is given to illustrate the use of the theoretical calculations for design. Any industrial implementation of the research results will likely involve the use of hot humid waste gases to strip foul condensates. The most attractive waste gases for this purpose are recovery furnace flue-gas and off-gas from strong-black-liquor oxidation. Two stripping-process arrangements are proposed, the most promising being a two-stage process using ambient air in the first-stage stripper and recovery furnace flue-gas in the second-stage stripper. The gas/liquid chromatography methods used were well suited to analysing dilute concentrations of methanol in water, foul condensate or air. Also, as a consequence of the research, it is possible to recommend a reliable expression for the value of the activity coefficient for methanol (at infinite dilution in water) as a function of temperature.