Steam stripping of foul process condensates is recognized as an efficient method for the removal of methanol and Total Reduced Sulfur (TRS) compounds. Methanol is formed during the pulping process through a reaction with the lignin in the wood. TRS compounds, principally hydrogen sulfide and methyl mercaptan along with other compounds are also formed during pulping.

Foul process condensates are formed by condensation of the foul vapors generated in the pulping process. These vapors fall into three general categories: 1) Digester relief and flash vapors, 2) Batch blow heat recovery systems, and 3) Vapors generated during evaporation of spent pulping liquors.

To minimize equipment size, cost, and energy use, various methods may be employed to concentrate both TRS and methanol into a smaller fraction of condensate through condensate segregation.

Every A. H. Lundberg Associates, Inc. stripping system contains the same basic components including a feed tank, fiber filter, condensate preheater, stripper column with rectification section and a reflux condenser. A steam reboiler may be added at the stripping steam inlet to recover live steam condensate or permit use of a foul heat source.

The stripper feed tank smoothes out the condensate flow to the stripper while the automatic back wash filter removes fiber from the condensate to minimize the fouling of the condensate preheaters. The preheaters are either shell and tube or spiral units that preheat the feed condensate using stripped condensate. The stripper column is made up of a number of valve trays. The condensate enters the column several trays down from the top and passes counter current to the stripping steam entering at the bottom of the column. The section above the feed tray is the rectification section which is used to enrich the overhead vapors in methanol for disposal.

Since the stripping process is energy intensive, maximizing energy recovery at its highest state is essential. The heat is recovered in the reflux condenser. The energy may be recovered by heating boiler feed water, heating process water, heating black liquor, evaporation of black liquor or by generation of clean steam at a reduced pressure. Furthermore, the stripping system can be integrated into the black liquor evaporator system. The preferential method for heat recovery is a matter of study for each mill.