Towards Sustainable Electrochemical Acidification of Kraft Black Liquor for Lignin Extraction: Proof of Concept, Control of Membrane Fouling and Yield Enhancement

Decreasing demand of traditional pulp and paper products, competition from emerging economies and oil price volatility as well as incentives for green products encouraged the pulp and paper industry to look for novel products made from wood components. Transformation of the pulp and paper industry and particularly Kraft pulping mills into integrated forest biorefinery (IFBR) is considered as an effective alternative to increase the revenue of the mills and substantially diversify their product portfolio. Kraft process is the dominant pulp and paper production method worldwide. In most of the conventional Kraft pulping mills around 50% of the wood components (mainly hemicellulose and lignin) are dissolved in a residual stream called black liquor (BL) and combusted in the recovery boiler to produce steam, electricity and re-generate the cooking chemicals. By contrast, in an IFBR plant wood constituents are separated from the pulp stream and transformed into value-added bio-based products. In particular, extracted lignin can be used as biofuels or as a precursor to a vast phenolic platform of chemical pathways. Furthermore, lignin extraction can increase the capacity of the Kraft mill by decreasing the load of its recovery boiler. This PhD project was part of a broader research study which evaluates the possibility of lignin biorefinery implementation in existing Kraft pulping mills to improve their revenue, diversify their portfolio and make them sustainable in the long term. Therefore, the main objective of this thesis was to identify, design and develop an efficient and eco-friendly BL acidification method for lignin extraction which can be an attractive alternative to the chemical acidification technique and eventually integrated into an existing Kraft pulping mill. To this end, electrochemical acidification of the Kraft BL via electrodialysis with bipolar membrane (EDBM) was selected as a promising and sustainable pathway. The main focus of this research was to validate the concept, eliminate the process drawbacks and enhance the performance of the EDBM process in order to make it practically feasible for a large scale implementation. As the first step for conducting this pioneering research, a technical feasibility study was carried out to address the advantages and limitations of the electrochemical acidification method. The results of this feasibility study indicated that the acidification of the Kraft BL via the EDBM technique required a significantly less chemicals versus the chemical acidification approach. However, fouling of the ion exchange membranes (IEM) adversely affected its performance. It was found that the protonation of the lignin phenolic groups resulted in formation of destabilized colloidal lignin and eventually produced lignin clusters on the surface of the IEMs. The focus of the second phase was to eliminate the membrane fouling and enhance the performance of the process by means of screening the most reliable and commercially available IEMs as well as improving the operational conditions of the EDBM process. The experimental results implied that changing the type of the IEMs could not mitigate the fouling phenomenon and a chemical cleaning cycle was inevitable. In addition, it was demonstrated that BL chemical composition as well as operational temperature and hydrodynamics parameters could substantially improve the current efficiency and energy consumption of the EDBM system and postpone the fouling of the IEMs. Yield enhancement by intensifying the electrochemical acidification process was the main intention of the final phase. Implementation of an in-line cleaning step by means of pulsed electric field application could successfully suppress the membrane fouling, intensify the acidification step and enhance the process efficiency up to 80%. On the basis of the promising results presented in this thesis, it was concluded that application of the electrochemical acidification process via the EDBM method substantially reduced the chemical consumption and effluent generation. Furthermore, an in situ production of a valuable side product i.e. caustic soda can make the EDBM process an eco-efficient and profitable operational unit inside the IFBR plant.