Vapor Pressure and Boiling Point Elevation of Slash Pine Black Liquors: Predictive Models with Statistical Approach
A. A. Zaman,*,†,‡ T. W. McNally, and A. L. Fricke†
Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611, and Engineering Research Center for Particle Science and Technology, University of Florida,Gainesville, Florida 32611
Vapor-liquid equilibria and boiling point elevation of slash pine kraft black liquors over a wide range of solid concentrations (up to 85% solids) has been studied. The liquors are from a statistically designed pulping experiment for pulping slash pine in a pilot scale digester with four cooking variables of effective alkali, sulfidity, cooking time, and cookingtemperature. It was found that boiling point elevation of black liquors is pressure dependent, and this dependency is more significant at higher solids concentrations. The boiling point elevation data at different solids contents (at a fixed pressure) were correlated to the dissolved solids (S/(1 - S)) in black liquor. Due to the solubility limit of some of the salts in black liquor, a change in theslope of the boiling point elevation as a function of the dissolved solids was observed at a concentration of around 65% solids. An empirical method was developed to describe the boiling point elevation of each liquor as a function of pressure and solids mass fraction. The boiling point elevation of slash pine black liquors was correlated quantitatively to the pulping variables, using differentstatistical procedures. These predictive models can be applied to determine the boiling point rise (and boiling point) of slash pine black liquors at processing conditions from the knowledge of pulping variables. The results are presented, and their utility is discussed.
1. Introduction Knowledge of black liquor equilibrium vapor pressure and boiling point rise over wide ranges of temperature andnonvolatile solids concentration is of particular importance for the development and design of kraft recovery components. These properties vary considerably with the solids concentrations (and solids composition) and, therefore, become more critical at higher solids concentrations as more evaporation stages are required to achieve higher solids concentrations and to increase performance characteristicsof the evaporation units. Accurate vapor pressure data and boiling point rise data are necessary for design and operation of evaporators for black liquor concentration to high solids. The present knowledge of the vapor pressure data and boiling point elevation of black liquors is incomplete due to the variation with pulping conditions, black liquor composition, solids concentration, and type ofthe wood species. Past research has not established methods that can be used to estimate vapor pressure or boiling point rise at processing conditions from knowledge of pulping conditions and/or black liquor composition. In this paper, we report results on equilibrium vapor pressure and boiling point rise-concentration-temperature relations for softwood slash pine black liquors based on dataobtained for liquors derived from statistically designed pulping experiments that were conducted in a pilot scale digester with liquor circulation. The cooking variables were effective alkali, sulfidity, cooking temperature, and cooking time, and the cooks were
* To whom correspondence should be addressed. Phone: (352) 392-6509. Fax: (352) 392-9513. E-mail: zaman@ eng.ufl.edu. † Department of ChemicalEngineering. ‡ Engineering Research Center for Particle Science and Technology.
made at a liquor-to-wood ratio of 4/1. In all cases, the white liquor was adjusted to a causticizing efficiency of 85% and a reduction of 93% with Na2CO3 and Na2SO4. The details of the pulping procedure and experimental results have been reported in several earlier publications (Fricke 1987, 1990, 1993; Zaman et...