(444a) Cross-Flow Separation Characteristics and Piloting of Graphene Oxide Nanofiltration Membrane Sheets and Tubes for Kraft Black Liquor Concentration

Black liquor (BL) is a complex alkaline byproduct stream generated from the kraft process for biomass pulping at ~1 billion tons/yr globally. “Weak” BL from kraft pulping contains about 15 wt% dissolved solids including lignin, hemicellulose fragments, and inorganic salts. In kraft mills, BL is first concentrated/dewatered to 75-80 wt% solids and then combusted to generate steam and electricity while recovering the inorganic components as smelt for recycle. BL concentration by multi-effect evaporation is a highly energy-intensive operation. Membrane-based BL concentration up to 30 wt% solids is attractive for significantly reducing energy consumption, but conventional nanofiltration membranes are precluded by low solute rejections and poor stability in BL, which combines alkaline pH (~13) and high temperature (70-85°C). Our recent works [1,2] have demonstrated the engineering of graphene oxide (GO) membranes, their stability and promising performance in BL conditions, and preliminary scale-up into sheets and tubes. In this talk, we discuss in detail the separation characteristics of GO membrane sheets and tubes under real BL conditions and crossflow operation. The separation of lignin and hemicellulose fragments is characterized. Furthermore, detailed characterization of the permeation properties of the multicomponent inorganics (sulfate, sulfite, sulfide, thiosulfite, carbonate, hydroxide) is presented. We also present summary results of a slipstream pilot in a kraft mill. The results have strong implications for the industrial application of GO membranes in BL concentration and other related applications.

References:

1. Z. Wang, C. Ma, S. A. Sinquefield, M. L. Shofner, S. Nair, “High-Performance Graphene Oxide Nanofiltration Membranes for Black Liquor Concentration”, ACS Sustainable Chemistry & Engineering 7 (17), 14915-14923 (2019).

2. Z. Wang, C. Ma, C. Xu, S. A. Sinquefield, M. L. Shofner, S. Nair, “Graphene oxide nanofiltration membranes for desalination under realistic conditions.”, Nature Sustainability (2021). https://doi.org/10.1038/s41893-020-00674-3