Systems interactions analysis for the energy efficiency improvement of a Kraft process

Several techniques are available to improve the energy performance of a process (internal heat recovery, water reutilization, condensates return, energy upgrading and conversion, elimination of non-isothermal mixing). They are applied to specific energy systems on the utility or process side (steam production and distribution, hot or cold water networks, process heat sources and sinks). Since those systems are interconnected, actions taken on one of them may have effects on another. These effects can be positive (synergies) or negative (counter-actions). A systematic, stepwise methodology has been developed to ensure that synergies are exploited and counter-actions avoided, and is presented. It has been validated by application to an existing Kraft pulping mill. Key performance indicators and the evolution of the thermal composite curves were used to monitor progress as the successive steps of the methodology were implemented. It was found that the combined direct and indirect effects of water reutilization constituted the most important source of potential energy savings. Water reutilization also reduced the need for additional purchased heat exchanger area. Overall, the water intake by the mill could be reduced by 33% and steam savings could be 26% of current production. This would liberate sufficient steam production capacity for the installation of a 44.4 MW cogeneration unit.