Intensification of evaporative precipitation of lignin in a spinning disc evaporator

Abstract

A continuous process for the evaporative precipitation of lignin derived from Organosolv black liquor was successfully developed and implemented in a spinning disc evaporator (SDE). A maximum of 95 % of the lignin was recovered at a disc speed of 1200 rpm, black liquor feed flow rate of 1 ml s−1 on a smooth disc surface heated to 100 ⁰C. These optimal flow rate and disc speed values corresponded to conditions which maximised residence time to <1 s in 4 disc passes without compromising on the rate of evaporative heat transfer. The lignin precipitate did not have any detectable impurities present after being washed and dried. The particle size for the lignin precipitate was in the region of 3 – 9 µm with minor agglomeration occurring, providing good filterability. A scaled up version of the process was modelled for benchmarking purposes and it was found that the SDE had a better theoretical performance than a falling film evaporator (FFE) system developed for the same process, with the energy consumption reduced by 23% and a higher lignin recovery rate per volume of black liquid processed once adjusted for processing time (38.1 g L−1 h−1 for the SDE vs. 3.23 g L−1 h−1 for the FFE).