Enhancing the Energy Efficiency of a Black Liquor Evaporation Plant by Mechanical Vapor Recompression Integration

Abstract

Black-liquor-thickening in an integrated multi-effect evaporation plant consumes substantial amounts of steam produced in pulp and paper mills, and its efficient operation is, thus, crucial. Industrial applications of heat pumps in the pulp and paper industry, especially in black liquor evaporation, show promise in terms of cutting energy consumption and in decarbonizing this industrial branch. Modelling of such a plant includes momentum, heat and mass transfer issues, enriched with black liquor material specification. An existing black liquor evaporation plant which thickens inlet black liquor from 17% to 75% wt. dry solids with a dry solid flow of 2500 tonnes per day is considered. It already includes a MVR (mechanical vapor recompression) pre-evaporator as well as water-condensate stripping columns. A mathematical model of this plant is created in a Matlab environment and, after verification of obtained results, it serves for analyses of possible plant modifications. Among the modification options, installation of a second MVR is modeled and its impact on the whole plant is examined. As a result, a differential (marginal) change in steam and electricity consumed in the plant is obtained. Model results indicate the possibility of a reduction of process steam consumption of around 10 tonnes per hour and an increase in electricity consumption of 600 kW. A favorable simple payback period of 2.5 years can be expected for the considered investment.

Keywords

• evaporator
• black liquor
• mathematical modelling
• heat pump