By increasing the solids content in the black-liquor the need to evaporate water in the recovery boiler can be reduced. This in turn reduces the energy demand. Two primary approaches used for solids concentration in black liquor include submerged tube concentrators and falling film evaporators.

In a submerged tube concentrator, black liquor is circulated in submerged tubes where it is heated but not evaporated; the liquor is then flashed to the concentrator vapor space, causing evaporation.

A tube type falling film evaporator effect operates almost exactly the same way as a more traditional rising film effect, except that the black liquor flow is reversed. The falling film effect is more resistant to fouling because the liquor is flowing faster and the bubbles flow in the opposite direction of the liquor. This resistance to fouling allows the evaporator to produce black liquor with considerably higher solids content (up to 70% solids rather than the traditional 50%), thus eliminating the need for a final concentrator (Kramer et al., 2009. p.94).