High-Performance Graphene Oxide Nanofiltration Membranes for Black Liquor Concentration

Abstract

Black liquor (BL) concentration by multieffect evaporation is an extremely energy-intensive operation in the kraft pulping cycle. Membranes can significantly save energy in this process, but conventional membranes are strongly challenged by low solute rejections and poor stability in BL, which is a complex mixture containing dissolved lignin, other nonlignin organics, multiple inorganic salts at highly alkaline pH, and process temperatures of 70–85 °C. Here we describe in detail the fabrication, modification, and characterization of robust and high-performance graphene oxide (GO) nanofiltration membranes for BL concentration. We show that poly­(ether sulfone) (PES)-supported GO membranes prepared from chemically reduced GO, and then subjected to high-pressure hydraulic compaction, show excellent chemical and mechanical stability under real BL conditions in comparison to conventional GO membranes. These membranes (referred to as “GO-3” in this work) show near-perfect (>99%) lignin rejection, high total organic carbon (TOC) rejection (up to 93%), and greatly improved inorganic rejections especially for divalent anions that are predominant in BL. Finally, the GO-3 membranes are scaled up on larger PES sheets (∼660 cm2 in size) and are operated under realistic cross-flow conditions with real BL feed flow rates as high as 10 L/min at 70 °C. The GO-3 membranes show robust performance over more than 1500 h (60 days) of continuous operation in multiple cycles of 10–50 bar transmembrane pressures, attaining stable and sustained permeate fluxes as high as 25 LMH and excellent rejection performance equal to that obtained at smaller scale. The main results of this work have strong implications on the development of membrane processes for BL dewatering and more generally for processing of complex biorefinery feed streams.

Kraft black liquor concentration with graphene oxide membranes: Process simulations and technoeconomic analysis

“…The composition of the 15 wt% BL is reported in our previous work. [11] Roughly 5 wt% TS consists of lignin, 5 wt% of other organics (such as hemicellulose fragments The wide range of types and molecular weights (from ~100 to ~20 000) in BL, combined with the high pH and temperature, make membrane-based dewatering infeasible for conventional NF and RO membranes. However, we recently showed that GO NF membranes are remarkably functional and stable in BL conditions (pH ~13, temperature >70 C), and have reported detailed data on their performance.…”

“…However, we recently showed that GO NF membranes are remarkably functional and stable in BL conditions (pH ~13, temperature >70 C), and have reported detailed data on their performance. [10,11] Particularly, "GO-3" NF membranes are able to reject nearly all lignin, much of the other organics, and some of the inorganics (Table 2). They have an overall rejection of ~60% for divalent salts, ~70% for non-lignin organics, and little/no rejection of monovalent salts.…”

“…[5] Motivated by the need for robust and functional BL concentration membranes, we have recently developed a series of graphene oxide (GO) nanofiltration (NF) membranes on porous polyethersulfone (PES) supports, that have showed excellent BL stability and promising performance. [10,11] With a 15 wt% solids BL feed, these membranes can achieve 98%-99.5% lignin rejection, high permeate flux around 10-20 L m À2 h À1 under 50 bar transmembrane pressure, sustained performance over 2 months of operation, and have good potential for largescale fabrication. [10,11] Some of these GO NF membranes also showed up to 80% rejection for small organic molecules such as glucose (MW = 180 Da) and higher rejections for divalent salts.…”

“…[10,11] With a 15 wt% solids BL feed, these membranes can achieve 98%-99.5% lignin rejection, high permeate flux around 10-20 L m À2 h À1 under 50 bar transmembrane pressure, sustained performance over 2 months of operation, and have good potential for largescale fabrication. [10,11] Some of these GO NF membranes also showed up to 80% rejection for small organic molecules such as glucose (MW = 180 Da) and higher rejections for divalent salts. [12] In this paper, we develop process designs and technoeconomic analyses for GO membrane-based BL concentration.…”

Low-Molecular-Weight Lignin Recovery with Nanofiltration in the Kraft Pulping Process

“…One of the developed membranes had a promising permeate flux of 24 L/m 2 ·h at 21.5 bar, and a total organic content rejection of 92%. Wang et al went in another direction and explored the possibility of enhancing polyether sulphone-supported membranes with graphene oxide [ 25 ]. With their membrane modification, they obtained a 99% rate of lignin rejection.…”