Enhanced Membranes for Dewatering of Black Liquor

Compact Membrane Systems (CMS) proposes to employ membrane systems for pressure-driven separation of water from black liquor to replace the first two stages of existing multiple-effect evaporators. This is a promising approach to considerably reduce the steam energy required for concentration. Membrane materials must withstand high temperature, high salinity and high pH conditions. Fouling of the membranes must be minimized to maintain throughput and energy savings. The unique characteristics of CMS' fluorochemistry will ameliorate fouling while at the same time withstand the harsh chemical and thermal environment.Black liquor is formed when pulp is washed to separate spent cooking chemicals and dissolved wood solids from pulp fibers. Weak black liquor is about 15 percent solids, consisting of inorganic salts (30-45%), lignin (30-45%), wood acids and polysaccharides (30-45%), and resins, fatty acids, and methanol (3-5%). Each air-dried ton of unbleached pulp delivers 2000-3500 pounds of black liquor solids, depending on pulp grade. Removing water from black liquor is an energy-intensive step in kraft pulp mills necessary to recover pulping chemicals and generate high-pressure steam from dissolved wood solids. The energy-saving potential varies considerably from mill to mill depending on the configuration of mill steam and power systems, ranging from 0.1 to 1.1 MMBtu per ton of pulp. Assuming an average reduction of 0.5 MMBtu per ton applied to all 47 million tons per year of kraft pulp produced in the U.S., concentration with advanced membrane systems could reduce energy consumption nationally by 23 trillion Btu per year. The economic value of this energy saving would be $70 to $120 million per year, based on energy costs of $3 to $5 per MMBtu.In the Phase I program, CMS will demonstrate the feasibility of developing chemically, thermally, and fouling resistant membranes for partial dewatering of black liquor. Specifically, the membrane process is expected to concentrate the black liquor from 15 to 30% solids via a low pressure filtration process that is projected to be 30% less expensive than existing evaporator systems. These projections should be based on long term performance of membrane system taking into account any fouling.The United States has 99 kraft pulp mills in 24 states, producing 47 million tons of pulp a year. In pulping, dissolved wood components and spent pulping chemicals are washed from pulp and become weak black liquor, a stream with approximately 15 percent solids. To remove water from black liquor, U.S. kraft pulp mills used 164 trillion Btu per year, or 3.5 MMBtu per ton of pulp, in 2010 based on data from the Department of Energy's 2010 Manufacturing Energy Consumption Study. Black liquor is concentrated to 50 to 85 percent solids with steam energy in evaporators. The best technology in use today is falling-film multiple-effect evaporators with six or seven stages, having steam economies near 5.8 pounds of water evaporated per pound of steam used.