Total site heat integration of multi-effect evaporators with vapour recompression for older kraft mills

Abstract

This paper aims to apply Total Site Heat Integration (TSHI) to appropriately integrate Mechanical and Thermal Vapour Recompression with multi-effect evaporators at older Kraft Mills, to cause a step reduction in fossil fuel use and its associated emissions. Heat and power demands for older Kraft Mills are chiefly satisfied from Recovery Boilers (RB), heavily supplemented by biomass/fossil fuel boilers, and integrated with steam turbines. Prior to firing, black liquor – the RB fuel – is concentrated from about 18 % to 67 % in a multi-effect evaporator, which demands 20 % of site-wide thermal energy. With access to renewable electricity, this study finds that vapour recompression can be economically integrated into a multi-effect evaporator at older Kraft Mills. The vapour recompression configuration with the greatest economic potential used 2-stages of mechanical vapour recompression and 1-stage of thermal vapour recompression. This system achieved a levelised profit of NZD 8.56 M/y, a payback period of 1.0 y and an internal rate of return of 103 %. An optimum integrated set-up needs to account for site-specific heat demand and utility supply profiles through TSHI.