Adaptable calculation method for segregation of contaminants in vapours formed from black liquor

A calculation method for prediction of methanol concentrations in evaporati on condensates is presented, in which the equilibrium relations of electrol yte/water/methanol systems are established. Data needed in the calculations are pressure and amount of evaporated water in each effect together with t emperature, flow and methanol concentration of the feed liquor. The methano l concentration in the feed liquor may be obtained experimentally or calcul ated from analyses of the different outgoing condensate streams. All vapour -liquid equilibrium calculations were performed using the NRTL (Non Random Two Liquid) activity coefficient model. It was shown that this model is sui table for mixtures of water and methanol of all concentrations. Furthermore , it may be extended to incorporate thermodynamic calculations with electro lytes in the liquid phase. Results from measurements at two Swedish kraft pulp mills demonstrated that calculated methanol flows corresponded well with measured methanol flows. This shows that the calculation method used is an appropriate tool for pred iction of methanol concentrations in evaporator condensates. The influence of sodium carbonate, sodium sulphate and sodium hydroxide on the volatility of methanol was found to be negligible in practice. The presented vapour-liquid equilibrium calculation method is very flexible in that it may easily be adapted to any electrolyte/water/methanol system. Concentrations of methanol in the condensates generated in an evaporator b ody with or without internal and/or external condensate segregation can eas ily be calculated. Concentrations of other contaminants can also be calcula ted if needed data is available. The calculation method may be used to pred ict the concentrations of contaminants not only in condensates formed in ev aporator effects, but also in vapours and condensates from surface condense rs, liquor preheaters, flash tanks, black liquor cyclones and integrated st rippers. Consequently, the method can be used to optimise the condensate ha ndling in any type of evaporation plant.