(635d) Graphene Oxide Membranes in Extreme Operating Environments: Concentration of Kraft Black Liquor

Kraft weak black liquor (WBL) is a hot (80-95°C), corrosive (pH ~12), high total solids (15-18 wt%), and high-volume (~500 gal/min in a typical pulp and paper mill) by-product of biomass pretreatment in papermaking and other processes. WBL contains lignin, hemicellulose, organic acids, inorganic salts, and water. Dewatering of WBL is currently performed by a six-stage evaporator train to produce strong black liquor (SBL, up to 80 wt% solids). More than 500 million tons/year of WBL are generated worldwide, and the concentration of WBL to SBL by evaporation is highly energy-intensive (about 0.2 Quads/year in the US alone). Membrane-based concentration of black liquor is attractive as an energy-efficient alternative, but challenging due to the harsh operating conditions and high fouling potential of BL.¹

This talk will discuss our pursuit of chemically robust, low-cost, and scalable membranes for black liquor concentration, focusing in particular on the development of graphene oxide (GO) nanofiltration membranes for lignin rejection. Thin (< 250 nm), uniform, and defect-free GO membranes were fabricated on macroporous poly(ethersulfone) (PES) supports and shown to possess excellent stability in real mill-derived BL during prolonged operation (1 month) at realistic temperature and feed conditions. Furthermore, high lignin rejection (~99%) and flux (~20 kg/m²/hr) was obtained when concentrating BL feeds at transmembrane pressures (TMPs) up to 50 bar and temperatures up to 85°C. We discuss in the detail the characterization of the GO membrane structure, morphology, and permeation properties.² A preliminary discussion of the economics and techniques for scale-up of the present membranes will be included.

1. Kevlich, N. S.; Shofner, M. L.; Nair, S., Membranes for Kraft Black Liquor Concentration and Chemical Recovery: Current Progress, Challenges, and Opportunities. Separation Science and Technology 2017, 52 (6), 1070-1094.

2. Rashidi, F.; Kevlich, N. S.; Sinquefield, S. A.; Shofner, M. L.; Nair, S., Graphene Oxide Membranes in Extreme Operating Environments: Concentration of Kraft Black Liquor by Lignin Retention. ACS Sustainable Chemistry & Engineering 2017, 5 (1), 1002-1009.