Recovery and Enrichment of Organic Acids from Kraft Black Liquor by an Adsorption-Based Process

Kraft black liquor (BL) is a complex multicomponent byproduct from wood pulping processes. After lignin, carbohydrate-derived carboxylic acids are the second most abundant class of organics in kraft BL and comprise a potential biomass-derived feedstock of more than 50 million tons/year globally. These acids can be valorized in biofuels and biobased chemicals production or further fractionated to produce specific high-value carboxylic acids from the mixture. However, both of these routes require a viable process to separate carboxylic acids from BL. In this work, an adsorption-based process was developed to address this challenging issue. The kraft BL stream was first pretreated to remove most of the lignin by membrane nanofiltration (NF) combined with lignin-selective adsorption. To separate organic acids from this pretreated BL, organophilic/hydrophobic granulated activated carbon (GAC) was identified as a promising adsorbent with excellent organic/inorganic selectivity and adsorption capacity (>100 mg/g GAC). We present comprehensive adsorption measurements and analysis to characterize the adsorbent and develop a viable cyclic operation scheme for the adsorption process. The GAC adsorbents show robust separation performance over 20+ cycles. We have successfully obtained concentrated aqueous organic acid mixtures with ∼95% purity from kraft BL. We propose a modified kraft process with energy-efficient BL dewatering by NF and organic acids recovery for valorization.