Influences of incondensable gas on Robert evaporator condensates

摘要

With the development of cogeneration at Racecourse Mill and other Australian mills, more evaporation is being undertaken in the front end of the evaporator stage to achieve better steam economies. Longer residence times for juice at higher temperatures at the front of the evaporator set comprising Robert type evaporator vessels leads to increased rates of sucrose degradation and to increased corrosion rates of calandria, condensate pipework and condensate pumps. The higher corrosion rates require premature replacement of these items and associated increases in production costs. In addition, lower revenues result from increased sucrose degradation. This study investigates at full scale the influence on condensate pH from passing incondensable (NOX) gases through either of two pathways in an evaporator set comprising Robert type evaporator vessels: path (A) from calandria to subsequent calandria in the set, or path (B) direct from each calandria to the evaporator condenser. The effect of feeding caustic soda into primary juice to mitigate pH changes of condensate and juice in the evaporator stage is also studied at full scale. The pHs of the condensates for both incondensable gas pathways at Racecourse Mill show a steady decline from the first effect to the final effect, with little difference between each pathway of the incondensable gas. The initial expectation that ammonia formed as degradation product is better channeled with incondensable gases to exert an influence on pH in path (A) than path (B) was found to be barely noticeable for the Racecourse evaporators. In tests with caustic soda added to primary juice, the pH of both juice and condensate decreased slower from first effect to final effect. The condensate pHs increased during caustic addition compared to tests with no addition. The low-level addition of caustic soda to juice mitigates the effect of low pH condensates and should decrease the propensity for corrosion in the condensate circuits. Both Farleigh and Marian Mills (non-cogeneration factories) were investigated at the same time. Farleigh's evaporators are set to path (A) and Marian's to path (B). In these factories the incondensable gases/condensates are more alkaline at the end of the set compared to Racecourse and support the hypothesis of increased corrosion rates and higher levels of sucrose degradation for factories configured for cogeneration.