Condensate segregation is the process whereby volatile organic compounds (VOC) and TRS are disproportionately concentrated into a smaller fraction of condensate. The most effective process for this is two stage condensing, while a second process is flash distillation of an existing condensate.

Two stage condensing requires that a vapor be condensed in series on two distinct surfaces. These surfaces may be in one body or may be in two separate vessels. The efficiency of segregation is dependent upon the amount of vapor that passes from the first surface to the second, as well as truly being series flow without bypassing.

The second method, flash distillation, flashes the saturated condensate from a higher pressure to a lower pressure. The more volatile VOC and TRS components distill more readily that water and are thereby preferentially contained in the vapor stream.

Condensate segregation may be utilized in virtually all foul vapor condensing applications. One of the earliest applications was two stage condensing for turpentine recovery. Two separate condensers are used, with hot process condensate from the first condenser containing little methanol and no TRS or turpentine. The cooler secondary condensate will contain the bulk of the turpentine, TRS and VOC. The condensate from the indirect secondary condenser of the blow heat system contains a high portion of the turpentine found in the blow vapors as well as a diproportionaly large volume of the TRS and methanol.

Every new black liquor evaporation system will utilize condensate segregation. In the evaporation process, the vapor from the feed liquor evaporation effect will be highly enriched in VOC and TRS. To a lesser extent the vapor of the second evaporation stage after the feed will also be enriched. Thus, in a typical six effect system with liquor feeding effect five, the vapor entering the sixth effect, the surface condenser and the vacuum system will contain the bulk of the TRS and methanol. In a new evaporator system both the surface condenser and sixth effect can be internally baffled so that condensing can occur on two surfaces in series. The fraction of separation is dependent upon the split in the body.

In an existing system, segregation may be achieved by the addition of external liquor heaters and an auxiliary surface condenser. Liquor from the sixth effect is reheated in external heaters installed on effect six and at times effect five, using vapor blown through the respective body. The amount of blow through is limited by the preheat possible. Thus, this method of segregation is not highly effective. Since the amount of condensing in each of the surface condensers can be controlled by the cooling water flow, the segregation here is more effective.