Liquid-Liquid Extraction: A Game-Changer in Biomass Processing

By Kyle Parkinson, Donald Glatz, and Brendan Cross, Koch Modular
Special to The Digest

The biochemical market is projected to experience significant growth in the coming years. According to the 2024 Biochemical Global Market Report, it is projected to reach $117 billion in 2028, with a compound annual growth rate of 8.6%. A growing global awareness of the need for sustainable practices and products fuels this demand (1). While the production of bio-derived products is on the rise, challenges remain in developing efficient recovery and purification processes.

Liquid-liquid extraction (LLE) is an ideal unit operation for recovering valuable chemicals from biomass directly or after pre-treatment to remove less desirable components such as cell mass and lignin (2). It works by making use of the relative solubility of components in two immiscible liquid phases. Unlike distillation, which has drawbacks in energy consumption and chemical degradation (due to heat), LLE offers an economically viable and sustainable alternative. This article highlights the essential steps in designing an LLE system for biomass processing, covering everything from solvent selection and equilibrium testing to pilot testing and complete system design.

Advantages of Liquid-liquid Extraction

LLE has several key advantages over other separation techniques, such as distillation:

• Lower Operating Costs: When LLE is determined to be the optimal operation unit, it offers cost advantages over conventional distillation, especially in processes requiring one or more energy-intensive distillation steps. In such scenarios, less energy is required, reducing operational expenses and enhancing cost efficiency.
• Recovery of Higher Boiling Compounds: LLE efficiently recovers compounds with higher boiling points than water. This makes it ideal for processes where traditional distillation methods are impractical due to the need to boil off large amounts of water, which has an extremely high heat of vaporization.
• Extraction of Non-Volatile Components: LLE effectively recovers non-volatile components like hormones, nutraceuticals, and metals, which cannot be separated through vaporization. It provides a reliable solution for isolating these components, facilitating their utilization in various industrial processes.
• Separation of Heat-Sensitive Materials: Liquid-liquid extraction is essential for separating heat-sensitive materials such as antibiotics. Unlike other methods, such as distillation, which involves high temperatures, LLE enables separation at ambient or lower temperatures, preserving the integrity and efficacy of heat-sensitive compounds.
• Efficient Separation of Close-Boiling Mixtures: Liquid-liquid extraction excels in separating close-boiling mixtures that challenge traditional distillation techniques. This is because LLE relies on differences in solubilities in different liquids instead of distillation, which employs differences in relative volatility to achieve a separation. Furthermore, by selecting an optimal solvent, the LLE process can be designed to be selective, resulting in more efficient downstream purification steps, if required.

Designing an Efficient LLE System

When considering LLE for your process, three critical steps are essential for a successful design: solvent selection and laboratory testing, pilot testing, and complete system design.

Solvent selection is critical in LLE process development (3), and once chosen, laboratory testing is essential for generating liquid-liquid equilibrium data and assessing hydraulic behavior. Various laboratory equipment, such as round-bottom flasks with agitation, can be used. A series of mix-decant runs with feed solution and fresh solvent (commonly referred to as shake tests) to generate equilibrium data from the feed concentration to the desired raffinate concentration. This data aids in determining the solvent-to-feed ratio and the required number of theoretical stages.

Hydraulic behavior evaluation is vital for selecting the appropriate type of extraction column. Columns with rotating internals can be used for systems that mix and separate quickly without emulsifying. However, the reciprocating agitation of a KARR^® column is superior for emulsifying systems (4), commonly observed in biomass systems from fermentation or algae ponds (Figure 1).

Figure 1: Up-close view of an emulsion band observed during shake testing

After laboratory data generation and column type selection, the next step involves pilot testing of the extraction column to optimize its performance. This optimization includes capacity, height, S/F ratio, agitation speed, and temperature. If VLE data are available, downstream distillation columns can be designed using process simulation tools. However, when information is lacking, VLE testing and/or testing of distillation steps may be necessary.

Downstream distillation is often needed to regenerate solvents and purify the desired chemicals (Figure 2). Understanding the flow rates and compositions of the extract and raffinate phases leaving the extraction column is essential for designing the distillation columns effectively.

Figure 2: 3D Model of a commercial modular system incorporating downstream distillation columns to recover solvent and generate a pure product stream.

Three Industry Applications

When we consider the production and recovery of valuable organic chemicals from biomass, three typical industry applications are currently in use: fermentation broth, woody biomass, and Algae broth.

Application 1: Fermentation Broth

This application involved extracting carboxylic acids from an aqueous fermentation broth generated in a cellulosic ethanol process. Ethyl acetate was identified as an effective solvent. Initial attempts using a rotating disc contactor (RDC) column were unsuccessful due to emulsification, poor product recovery (<90%), and a high solvent-to-feed ratio (3). These observations led to the selection of the KARR column, which has reciprocating internals and is ideally suited for systems that tend to emulsify. Testing in a 1” diameter KARR column demonstrated excellent operability, resulting in an improved solvent-to-feed ratio (1.5) and carboxylic acid recovery (98-99%). A capacity (or the feed + solvent volumetric flux) of 650 GPH/ft2 was chosen for scale-up. Subsequently, a production-scale KARR column was designed and installed as part of a modular system, incorporating downstream distillation columns for further recovery and purification (Figure 3).

Figure 3: Modular process system designed to recover carboxylic acids from fermentation broth

Application 2: Woody Biomass

Woody biomass, with its complex composition, poses challenges for chemical extraction. LLE proved to be an effective method in extracting two different valuable acids from a woody biomass liquor generated in a pulp and paper plant. Initial shake tests with the selected organic solvent proved effective and demonstrated that this system mixed and separated easily without any tendency to emulsify. These observations led to the selection of the SCHEIBEL^® column, which has rotating internals (Figure 4). The column demonstrated excellent operability, generating fine dispersion of the solvent phase at moderate to high agitation speed.

Figure 4: Internals of a SCHEIBEL column with rotating impellers

Initial pilot testing indicated that one of the two acids was much more challenging to extract. After testing several variations of operating conditions, the required recovery of both acids was achieved with a 1.5 solvent-to-feed ratio and a capacity of 600 GPH/ft².

The pilot test results informed the design of a complete extraction/distillation system for recovering and purifying the desired chemicals from the woody biomass feed.

Application 3: Algae Broth

Algae biomass, emerging as a sustainable feedstock for chemical production, can be processed using LLE to extract lipids, nutraceuticals, pigments, and other valuable compounds from aqueous broths generated in algae ponds. In this application, shake tests showed sensitivity to emulsification and poor phase separation, prompting a recommendation for pilot testing and scale-up using a 1” diameter KARR column.

Figure 5: Internals of a KARR Column with Reciprocating Plates

Pilot testing confirmed the effectiveness of the KARR column for this application. Its reciprocating perforated plates facilitated sufficient solvent dispersion within the column while avoiding emulsification (Figure 5). A product recovery of 98% was achieved with a solvent-to-feed ratio of 1 and a capacity of 500 GPH/ft².

In Closing

LLE proves instrumental in addressing the complexities of biomass processing, offering economic viability, sustainability, and breakthroughs in recovering valuable chemicals. To achieve high product recovery while minimizing solvent use, agitated columns are necessary to generate sufficient theoretical stages. In systems prone to emulsification, reciprocating agitation emerges as the optimal choice.

LITERATURE CITED

1. The Business Research Company. (March 2024). Biochemical Global Market Report 2024. Retrieved from https://www.thebusinessresearchcompany.com/report/biochemical-global-market-report#faqs  Accessed July 2024.
2. Cusack, R., Fremeaux, P., and Glatz, D., “A Fresh Look at Liquid-Liquid Extraction, Extractor Design and Specification,” Chemical Engineering, February 1991.
3.  Cusack, R., Glatz, D., “Apply Liquid-Liquid Extraction to Today’s Problems,” Chemical Engineering, July 1996.
4.  Cusack, R., Fremeaux, P., and Glatz, D., “A Fresh Look at Liquid-Liquid Extraction,” Chemical Engineering, March 1991.

Authors

Donald Glatz is the Manager of Extraction Technology at Koch Modular, where he focuses on evaluating and optimizing extraction processes, as well as scaling up and designing extraction systems. With 25 years of experience in this field, Don has authored several papers and articles on the subject. He holds a Bachelor of Science in Chemical Engineering from Rensselaer Polytechnic Institute and an MBA from Fairleigh Dickinson University.

Brendan Cross is a Principal Extraction Engineer at Koch Modular, where he is responsible for evaluating liquid-liquid extraction applications, developing extraction processes, designing pilot tests, and commissioning and starting up extraction columns. With 15 years of experience at Koch Modular, Brendan also has significant expertise in distillation and process design. He holds a Bachelor of Science in Chemical Engineering from Columbia University.

Kyle Parkinson is a Process Engineer at Koch Modular, bringing over 11 years of experience in the chemical and petrochemical industry, with a recent focus on liquid-liquid extraction. He holds a Bachelor of Science in Chemical Engineering from Bucknell University.