Process Intensification in the Pulp and Paper Industry

Process intensification techniques can improve the yield and overall efficiency of the kraft chemical pulping process.

The pulp and paper industry annually produces over 300 million tons of paper products used across society in print media, packaging, tissue, and hygiene. Transforming wood into useful products involves energy- and resource-intense processing to separate cellulose fibers from wood components, including hemicellulose, lignin, and extractives. Pulp mills, which can operate independently or be integrated with papermaking mills, liberate high-value cellulose fiber from wood through chemical, mechanical, or coupled chemical-mechanical means.

Kraft chemical pulping is by far the predominant process used to produce high-strength, purified fiber. This process is based on the digestion of wood using sodium sulfide (Na₂S) and sodium hydroxide (NaOH), which degrade lignin and hemicellulose through alkaline hydrolysis. Challenges of the kraft process that present process intensification (PI) opportunities include its high water and energy intensity, capital expense, and limited yield. For example, the pulping process consumes up to 20 GJ per ton of pulp in heat energy and up to 1,000 kW-hr per ton of pulp in electricity (1). Furthermore, the overall pulping and papermaking process combined can consume between 30 m³ to 70 m³ of water to produce 1 kg of paper product. Through a unique and integrated recovery loop that burns lignin as a biofuel, an unbleached kraft mill can generate nearly all of its energy internally, with the exception of the 2 GJ per ton of fossil fuel required for the lime kiln.

Final paper products (e.g., paper, tissue, hygienic fiber for personal care, corrugated containers, and liner-board) are produced by casting a dilute fiber slurry onto a fast-moving wire bed, followed by dewatering and thermally driven drying steps. A paper mill producing 1,000 tons of paper each day may consume 5–6 GJ of thermal energy per ton of paper to remove water via evaporation (1).

As a result of the energy and water consumption, the pulp and paper industry is responsible for nearly 2% of global CO₂ emissions. Although a considerable fraction is biogenic CO₂ from lignin consumption, opportunities exist to more efficiently use lignin, and to improve energy efficiency where high temperatures or lack of pulp-mill integration require some fossil fuel consumption. PI opportunities in pulp and paper production are driven by the overall scale of the industry, the limitations in yield (product loss), high energy and water use, and correspondingly high CO₂ emissions...