Revolutionary Membranes to Slash Energy Consumption in Kraft Pulping Processes

A membrane separation process is considered to replace two thermally-driven units, which consume 50% of evaporation energy, in the Kraft pulping process. In particular, pressure-driven nanofiltration followed by reverse osmosis has the potential to reduce this energy consumption by 8-10 fold. Nanofiltration can remove larger organic and inorganic species while additional reverse osmosis processing can produce a purified water permeate and an inorganic-rich retentate for recycle. The first stage of this project is focused on developing an economically viable nanofiltration membrane that exhibits chemical resistance to harsh weak black liquor streams. A nanofiltration membrane comprising an ultra-thin NafionŽ layer on a robust, relatively inexpensive microporous fiber support is being pursued. NafionŽ is a unique material that can withstand conditions of high alkalinity and elevated temperatures while maintaining high permeability to water. If NafionŽ shows adequate rejection of organics and larger ions, it would provide the basis for this nanofiltration unit resulting ultimately in significant energy savings. Currently, the forest products industry consumes 2.74 quadrillion BTU annually, which is equal to 12% of the total U.S. manufacturing energy demand. Implementation of hollow fiber modules containing defect-free NafionŽ sheath layers of 0.3-1 micron thicknesses has the potential to save the U.S. pulp and paper industry about 100 trillion BTU annually. UV-Vis spectroscopy is used to monitor the breakthrough and rejection of various ionic species as a function of time in order to probe the nanofiltration capabilities of NafionŽ. Preliminary experiments using commercially available NafionŽ (EW = 1100) membranes have shown reasonable water flux and rejection properties.