Understanding the Kraft Liquor Cycle: A need for online measurement and control

摘要

The Kraft digester has two products - pulp and black liquor. Approximately 50% of the log that enters a Kraft mill goes to the Kraft recovery boiler as black liquor. The value of this black liquor varies with the value of oil and natural gas. At times of high oil prices the energy value of black liquor can rival that of pulp. Even at low oil prices however black liquor is a valuable "green" energy source that when optimized can determine the economic viability of the mill. Every dollar of energy savings associated with the efficient combustion of black liquor goes to the bottom line. There is a need for a more complete understanding of the black liquor going to the recovery boiler to allow for this optimization.To optimize the Kraft pulping process, mills also need to more fully understand and control the green and white liquors of the Kraft liquor cycle, which directly impact on the quality and volume of the pulp exported from the mill. An important consideration in this control is to reduce a negative variation in one part of the cycle that results in downstream unit operations disturbances which in turn can create further variations, causing a downward spiral effect. Traditional manual testing procedures performed by the operators associated with the Kraft liquor cycle do not provide a full spectrum of the needed properties of the black, green and white liquors to optimize the cycle. The infrequency of the tests accompanied by time lags in the process do not allow for corrective actions to mitigate process upsets. In many cases operators respond to upset conditions without ever knowing the originating cause or possibly even induce upsets when acting on erroneous information. They simply state that "the liquor has changed."The FT-NIR (Fourier Transform-Near Infrared), a vibrational spectroscopy technique, is an optical measurement based on absorption of infrared light by chemical constituents in a sample. Its application as a process analyzer in the pulp and paper industry was developed by FPInnovations - Paprican in the early 90s and has been implemented in Kraft mills as fully automated and self calibrated process analysers since 1996. It has been proven to be accurate and precise with measurement frequency that far exceeds traditional testing. The maintenance required is minimal. It has allowed for the online determination of reduction efficiencies, effective alkali, sodium carbonate, sodium sulphate, sodium sulphide, and sodium thiosulphate in the green liquor and white liquor. It has allowed for on line residual effective alkali, lignin content and an organic/ inorganic content determination of the black liquor.This paper will present actual online data from FT-NIR installations along with thoughts as to how this new knowledge can lead to new understandings which eventually will improve the control of the Kraft liquor cycle. Present participants in this exercise include: FPInnovations, the Institute of Paper Science and Technology and the University of Toronto.