Viscosity Of Black Liquor

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Table 2.6 shows the effect of pulping conditions on black liquor viscosity. The viscosity may vary significantly from mill to mill. Viscosity increases with increased dry solids content. An increase in temperature lowers the viscosity. The practical limit for handling the liquor is the pumping limit of 300-500 cp. The viscosity must always be below this level, and is usually much lower in the evaporation plant. If the liquor at final concentration is stored at atmospheric pressure, the limit of the final concentration is 70%-75% dry solids at the maximum temperature of 115°C. If the final product liquor is stored in a pressurized storage tank, the final con¬centration can be 75% - 85% dry solids and even higher at a storage temperature of 125°C-150°C. These extremely high concentrations might require using medium pres¬sure steam in the evaporation plant. A heat treatment process can also reduce the viscosity. Certain flow additives are available (Llamas, 2007; Gagnon et al., 1996) which reduce the viscosity of the liquor in such a way that high solids firing is possible in the conventional recovery furnace. High solids firing is environmentally attractive as it reduces hydrogen sulphide and sulphur dioxide emissions as well …show more content…

Any increase in dry solids will decrease specific heat capacity and an increase in temperature will decrease it. The specific heat capacity represents the heat necessary to raise the temperature of 1 kg of a material by 1 °C. Enthalpy data for black liquor are essential for estimating energy balances of kraft recovery boilers. The heat capacity of the black liquor decreases along with the increase in dry solids content. It can be approximated by a linear addition of the specific enthalpy contributions of water and black liquor solids. Moreover, an excess heat capacity function is incorporated to account for changes in black liquor heat

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