Measuring softwood yield gain at kraft pulp mills

Several yield measurement methods are available which use carbohydrate analysis of mill-made brownstock samples to predict softwood pulp yield with good accuracy. We have reviewed these methods and have determined their effectiveness for measuring the softwood yield gains provided by changes in process chemistry involving the addition of anthraquinone (AQ), polysulphide (PS) or both additives. The Marcoccia Stromberg Prough Equation is the most complex; it requires detailed viscosity and carbohydrate profiling of brownstock pulps. The carbohydrate-lignin method requires only lignin and carbohydrate determinations. The mannose method simply uses the mannose concentration in pulp to estimate softwood pulp yield. For a given species or species mix, the mannose method predicted yield well, and was not affected by process chemistry changes introduced by using additives. The carbohydrate-lignin method predicted yield fairly well, with some dependency on process chemistry. The Marcoccia Stromberg Prough Equation was a poor predictor of yield gains achieved by adding PS and AQ.