Heating Black Liquor by Partial Oxidation with Controlled Cavitation as a Means to Reduce Evaporator Fouling, 2003 Fall Technical Conference

A controlled cavitation device was used to mix oxygen in black liquor for rapid exothermic oxidation. Oxidation of 23 g/L Na2S to 4.7-8.9 g/L was achieved in a single pass through the controlled cavitation reactor; very little oxidation of organic compounds occurred. The partially oxidized liquor was evaluated for fouling tendency in conventional heat exchangers. Several trials were performed using an annular-flow heat transfer fouling test cell (ATC) to simulate the thermal environment in a live-steam-heated black liquor evaporator. Precise control of liquor conditions and sensitive instrumentation in the ATC allow measurement of the rate of fouling due to calcium carbonate deposition. The calculated fouling rates from two baseline tests (no partial oxidation) were 0.33-0.67 mm/hr. Partial oxidation treatment with controlled cavitation reduced the fouling rate to 0.00-0.07 mm/hr. The decrease in fouling rates appears to result from thermal deactivation of the calcium species in the black liquor. Compared with investing in additional evaporator bodies and/or heaters to address a severe scaling problem, this technology offers the advantages of having no heat transfer surface to scale and slightly reducing black liquor heating value, a potential benefit for mills operating recovery boilers at their steaming rate limit.