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The spent liquor from the cotton linter pulping process is unique. The organic compounds stripped from the linters are primarily fats which have a high tendency to foam. The spend liquor from the pulping process has a dissolved solids (i.e., organic and inorganic material) concentration of around 8%. A large amount of the water must be removed from the liquor before it can be combusted. A multiple-effect evaporator is typically used to remove the water, however, special care must be taken in the design of the evaporator system to minimize the generation of foam. The Siloxy Evaporator System The novel Siloxy evaporator system utilizes the forced-circulation evaporation principle and a specially designed steam/liquor separator to minimize foaming. An evaporator system based on these principles is in operation at a cotton linter mill in Spain. The overall design of the Siloxy process, (see Siloxy Oxidation Technology) minimizes the size of the evaporator system. In the Siloxy process, high-purity oxygen is used for combustion, and the water in the combustion gases is condensed and the latent heat in the water can be recovered. Therefore, the thermal efficiency of the Siloxy system is not a function of the black liquor concentration from the evaporator system. Since oxygen is the combustion gas, the black liquor firing solids can be much lower. As a result, the solids concentration from the evaporator set is lower, enabling a reduction in the size of the evaporator system. Typical System Configuration (4-Effect) Primary steam at 4-bar (a) pressure from the Siloxy oxidizer is fed to the first evaporator effect. Secondary steam from the evaporated liquor is led through the effects in the order 2–3–4–condenser. Atmospheric steam generated in the Siloxy oxidizer is added to the secondary steam entering the third evaporation effect.
Primary steam condensate is extracted at about 120°C and piped to the mill's feed-water tank, where the excess heat is utilized to heat the make-up feed-water. Secondary condensate (i.e., the water separated from the black liquor during evaporation) is collected in a condensate tank operated under vacuum. The secondary condensate is pumped from the condensate tank at a temperature of about 60°C. This hot, only slightly contaminated water can be used for pulp washing. |
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© 2007 Siloxy Limited | Legal Notice |
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