Vapour-liquid partition of volatile organic compounds in kraft black liquors

The vapour-liquid partition of various VOC (Volatile Organic Compounds) in a number of industrial black liquors was investigated. The information can be used to determine the distribution of VOC in different condensate and gas streams in a kraft mill. Batchwise evaporation of weak black liquors was performed on a lab scale. The evaporation was carried out at atmospheric pressure up to over 90% dry solids content. During evaporation, condensate samples were collected and analysed with respect to 15 compounds, total organic carbon (TOC), chemical oxygen demand (COD) and pH. The study resulted in concentration profiles of various VOC in the vapour (condensate) during evaporation. An increase in concentrations of total reduced sulphur compounds (TRS), ammonia, methanol and guaiacol at high dry solids content indicated a chemical degradation of the liquor. The condensates with the lowest TOC and COD levels were obtained for a dry solid content of 30-40 wt-% regardless of the original dry solids content in the black liquor.