Robust Membranes For Black Liquor Concentration

Executive Summary

Black liquor (BL), also known as “spent pulping liquor”, is a high-volume byproduct of lignocellulosic biomass pretreatment (i.e., wood pulping by the kraft process). BL is a corrosive, toxic, and complex mixture. About 500 million tons/yr of BL are produced in more than 200 kraft process units worldwide (including 99 in the US, with about 0.2 quads/yr energy spent for BL concentration by multi-effect evaporation). Currently, BL concentration is performed by multi-effect evaporators and is one of the most energy-intensive industrial separation processes. Development of a more efficient BL concentration technology is a high priority for the forest products industry, with membrane technology being a particularly feasible alternative. However, membrane technology has been elusive because of the lack of a long-lived/stable, low-cost, high-performance membrane. This project will develop and demonstrate a bench-scale modular graphene oxide (GO)-based membrane system that substantially improves the energy efficiency of concentrating kraft black liquor (BL) from 15 wt% solids (lignin, organic molecules, and inorganic salts) to 30 wt% solids by removing water (maximum 0.1 wt% solids). The key innovation is the development of BL-stable, scaled-up, GO-based NF and RO membranes supported on macroporous polymeric (polyethersulfone, PES) supports for dewatering black liquor. The challenge is in developing and scaling up low-cost membranes that are long-lived in the corrosive conditions. The recent work on this project promises successful development and scale-up of BL-stable graphene oxide (GO) membranes. It would allow this technology to be quickly integrated into existing Kraft processing facilities to leverage existing assets.

Technical Challenge

Developing membranes with sufficiently long life in harsh conditions (pH 12-13, 85 °C)
Scale-up of GO membranes into large-area sheets and tubes while maintaining performance
Fabrication and validation of spiral wound and tubular elements as scaled-up configurations

Potential Impact

If successful, this project will make a critical leap in the path to industrializing a membrane-based process intensification technology for a key process in the renewable bioproducts industry. The capability to inexpensively purify components such as lignin and smaller organic molecules is also critical for valorizing lignocellulosic biomass. The work on BL-stable GO-based membranes will create a body of new knowledge on how these membranes can be engineered to deliver performance and stability in challenging applications once considered “out-of-bounds” for membranes. This approach uses a combination of low-cost macroporous supports and thin GO membrane coatings deposited by benign and low-cost vacuum filtration processes; and has substantive potential as a design approach allowing fabrication of customized modules for different applications and problems. The ultimate impact is to develop a membrane technology that can deliver high performance in much harsher environments than typical applications.