Black liquor evaporator upgrades-life cycle cost analysis

摘要

Black liquor evaporation is generally the most energy intensive unit operation in a pulp and paper manufacturing facility. The black liquor evaporators can represent a third or more of the total mill steam usage,followed by the paper machine and digester. Evaporator steam economy is defined as the unit mass of steam required to evaporate a unit mass of water from black liquor(i.e.,lb/lb or kg/kg.)The economy is determined by the number of effects in an evaporator train and the system configuration. Older systems use four to six effects,most of which are the long tube vertical rising film type. Newer systems may be designed with seven or even eight effects using falling film and forced circulation crystallization technology for high product solids. The median age of all North American evaporator systems is 44 years. Roughly 25% of the current North American operating systems are 54 years or older. Older systems require more periodic maintenance and have a higher risk of unplanned downtime. Also,older systems have chronic issues with persistent liquor and vapor leaks,shell wall thinning,corrosion,and plugged tubes. Often these issues worsen to the point of requiring rebuild or replacement. When considering the age,technology,and lower efficiency of older systems,a major rebuild or new system may be warranted. The intent of this paper is to review the current state of black liquor evaporator systems in North America and present a basic method for determining whether a major rebuild or new installation is warranted using total life cycle cost analysis(LCCA). Application: The LCCA method presented may be used by readers to determine if a major rebuild or a new installation is warranted in their mill. LCCA is a more long-term economic analysis that aligns more with the longer life cycle timeframes of evaporator systems rather than based on short-term analyses,such as return on investment or payback period. The LCCA input data can be customized to fit a given mill's situation to yield accurate results,and follow-on sensitivity analyses can help minimize investment risks due to potential fluctuations in future economic conditions.