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Utilization of methanol from pulp mill foul condensate as chlorine dioxide generator chemical feed
University of New Brunswick
Northern Pulp bleaches its pulp with chlorine dioxide (ClO2), generated on site by the ERCO R-8 process using methanol as a reducing agent. To save $900,000/year in purchasing costs, Northern Pulp would like to separate methanol from its foul condensate waste stream. Present at 0.5 wt%, methanol is currently concentrated by a steam stripper, which also removes sulfides and many other impurities. This is presently burnt in a lime kiln. The feed to the ClO2 generator is a 20 vol% solution of methanol in water; all impurities must be separated. The proposed solution is to add a second identical steam stripper and run the two units in parallel. This would improve the operation of the current unit, and produce a more concentrated stripper off-gas product of 48 wt% methanol. Pervaporation is the chosen technology to remove the remaining sulfur and terpene compounds from the methanol and water solution. Methanol purity is unchanged by this step. The total capital investment required for this design is $2.27 million with an operating cost of $752,500/year. The ClO2 generator consumes 1,500 tonnes/year of pure methanol; approximately 2,000 tonnes/year is recoverable in the foul condensate stream. Therefore, once methanol storage is at capacity, SOG can be burnt in the lime kiln without further treatment. Accounting for $250,000/year in fuel savings, this will save Northern Pulp $1.15 million/year. The payback period for this design is 1.8 years, and the investor’s rate of return is 29%.