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).
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.
