Ion Exchange Membranes Integrated with Electro Chemistry in the Kraft Pulping Process

The paper making industry uses sodium hydroxide as a key ingredient to aid the digestion and conversion of cellulose materials from wood-based sources. Sodium hydroxide and sodium sulphide are recovered using the Kraft pulping process. The Kraft pulping process is considered as the global benchmark used to recover the two key chemical compounds that increase the commercial sustainability of paper making. This research investigates an alternative processing technique using ion exchange membranes (IEM) integrated with electrochemistry in the form of a membrane electrolysis cell (MEC) to recover sodium hydroxide. MEC technology may provide the paper making industry improved specific energy, significant reduction in greenhouse gas emissions, reduced chemical contaminants in the recovered sodium hydroxide stream, and higher sodium hydroxide concentration. A synthesised Green Liquor mixed solute solution was used to assess MEC performance and differentiate between membrane characteristics. Research by Mandal et.al., 2022 reported that a small difference was found between a synthesised Green Liquor solution when compared to a sample collected form a paper making facility. The 5% difference reported by Mandal et.al., 2022 suggests that the synthetic Green Liquor solution was a reasonable facsimile for Green Liquor generated in the paper making industry. A MEC cell was designed, constructed, and tested at different current densities, temperatures, and feed and product stream concentrations and flows using one IEM. The optimised operating conditions were then used to compare six different membranes supplied by two membrane manufacturers: Selemion in Japan and DuPont in the USA. The capacity for different membrane design and compositions were assessed and the high ion selectivity membranes were shown to deliver enhanced performance across the range of key metrics established as the benchmark for this specific application. Membranes with the high ion selectivity characteristics produced higher sodium hydroxide concentrations at lower specific energy compared to membranes of higher capacity and lower selectivity. The high ion selectivity membranes provided better operating outcomes than the conventional Kraft pulping process in terms of sodium hydroxide concentration and purity. MEC operating conditions were improved when a metal mesh support was introduced to the apparatus. The use of the spacer in a ‘Zero Gap’ configuration allowed current to directly contact the IEM and improved the specific energy and significantly improved process stability from 2 hours to less than 15 minutes. Adopting the ‘Zero Gap’ configuration proved a step change in MEC performance and improved operating reliability. The conventional Kraft pulping recovery process recovers sodium hydroxide at a specific energy of 320 kJ/mol, whereas the MEC operated at a specific energy of 312kJ/mol. This represents a modest reduction in specific energy of 2%, however, the associated sodium hydroxide purity of greater than 99% contains no ‘deadload’ chemical compounds unlike that conventional recovery methods that contain greater than 20% contamination. These contaminants, known as ‘deadload’, do not add to digestion and reduce the effectiveness of the digestion process. The concentration of sodium hydroxide achieved was 3.2 M at 1.39kA/m2 using the MEC apparatus and was 2.1 times greater than that obtained using the conventional Kraft pulping process, typically 1.5 M. This result represents a significantly better outcome for the pulping process as a similar specific energy provides a much higher mass production of sodium hydroxide and a significant reduction in contaminating chemicals that reduce this crucial operation’s effectiveness. MEC generated 0.09 kg CO2/kg NaOH which compared favourably to the Kraft pulping process that contributes 0.49kg CO2/kg NaOH produced. This substantial reduction of 82% in GHG emissions during caustic recovery represents a significant contribution to the environmental sustainability of the Kraft pulping process. MEC employs electrical energy as DC voltage that can be harvested using renewable energy compared to fossil fuels used in conventional Kraft pulping operations. Renewable energy can be harvested using solar photovoltaic cells, wind turbines or utilising steam turbines to convert surplus energy generated through the paper making process. Renewable energy generation coupled with energy storage devices in the form of large-scale batteries have the potential to increase plant availability. Environmental sustainability through reduced GHG emissions is a key challenge for any industry and the ability to achieve a robust reduction in CO2 emissions coupled with significant improvements in key recovery metrics illustrates the integrated benefits that MEC can provide to the paper making industry. The results of this research clearly identified that using IEM integrated with electrochemistry in the form of a membrane electrolysis cell (MEC) can effectively recover sodium hydroxide using a synthesised Green Liquor solution. Using the MEC apparatus identified several issues that should be considered in any scaled-up production facility and included hydraulic distribution, aspect ratio, hydrogen gas production and extraction, optimum sodium hydroxide extraction, operating temperature and most importantly the ion selectivity of the membrane. Optimising these conditions can improve the already significant benefits that MEC can provide the paper making industry over the currently adopted Kraft pulping process.