Recovery and Purification of High-Value Organic Acids from Kraft Black Liquor

Biorefining – the conversion of different types of biomass into valuable chemicals and materials – has an increasingly important role in a sustainable economy. For example, forest biorefineries convert trees into chemical/material feedstocks (cellulose) and energy (by combustion of byproducts such as lignin and depolymerized hemicellulose). The initial treatment of biomass feedstock in biorefineries usually leads to multicomponent byproduct streams. In the kraft treatment process widely used in forest biorefineries, black liquor (containing lignin, carboxylic acids, inorganic chemicals, and water) is generated as a high-volume byproduct (about 1 billion tons/year globally). At present, kraft black liquor is concentrated in multiple-effect evaporators and then combusted in recovery boilers. Thus, steam, and in some cases electricity, are produced from the organic portion, and the inorganics are recovered as solids for recycling and reuse in the process. However, the non-lignin organic molecules (mostly C1-C¬6 aliphatic acids) dissolved in black liquor have market value if they could be economically separated from lignin, inorganics, and water. This organic acid fraction makes up about 3-5 wt% of black liquor (i.e., 30-50 million tons/yr globally). They could be valorized either as a bioderived feedstock for chemical production, or further fractionated to produce specific high-value hydroxy acids from the mixture. The main aims of this thesis research are to develop the fundamental aspects of organic acid adsorption in nanoporous materials, and to use this information for the design of viable organic acid recovery and separation processes from kraft black liquor streams. Adsorption is an energy-efficient and versatile separation technology for large-scale recovery and fractionation of organic molecules from multicomponent feeds. However, very little is currently known about organic acid recovery and fractionation using adsorbent materials and adsorption processes. In this context, several key issues of interest will be studied in this work: (1) demonstration of an adsorbent capable of selective recovery of organic acids from the multicomponent black liquor stream, (2) adsorbents capable of high-resolution separations of different classes of the recovered organic acids, (3) stability of adsorbent materials in black liquor-derived streams, and (4) development and analysis of an integrated separation process based upon the fundamental information obtained. This thesis is organized as follows: Chapter 1 provides a literature review of organic molecules recovery from biorefinery mixtures, and in-depth perspective of organic acid recovery from aqueous mixture by adsorptive separation. In Chapter 2, a proof-of-concept study was conducted in acid recovery and enrichment from black liquor. A nanoporous granulated activated carbon was identified as a promising adsorbent to recover organic acids with excellent organic/water selectivity. Adsorption-desorption cycling of the activated carbon column shows stable and promising performance over more than 20 cycles with the pretreated kraft black liquor stream. A concentrated aqueous carboxylic acid mixture high purity and recovery (both well above 90%) was achieved successfully with a single column. With these encouraging results, the scale-up demonstration of adsorptive recovery of organic acids from black liquor was performed in Chapter 3. A model-guided simulated moving bed chromatography design approach was applied. An enriched acid mixture with a higher quality, higher productivity, and lower material consumption was produced from black liquor stream on a continuous simulated moving bed unit. A preliminary work on the fractionation of the recovered acid mixture to produce specific hydroxy acids was then conducted in Chapter 4. The adsorption isotherms of multiple C1-C6 organic acids on activated carbon materials were developed. The results show that activated carbon adsorbents have potential to separate different classes of recovered acids from kraft black liquor streams. In summary, my Ph. D. thesis work unlocks an economical and sustainable route to produce a valuable bioderived chemical mixture (and potentially finished products) from a low-cost byproduct/waste stream. This work demonstrates integrated strategies to separate and purify molecular components present in large-scale biorefinery streams using materials-based separation mechanisms such as adsorption.

Date

2023-12-06

Resource Type

Text

Resource Subtype

Dissertation

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