English 
Views: 0 Author: Site Editor Publish Time: 2026-09-28 Origin: Site
Extraction is a method of separating compounds by exploiting their relative solubility in two immiscible liquids (typically water and a suitable organic solvent). Put simply, it is a separation method that uses the property that the target substance prefers to reside in another liquid (medium) to "lure" it out of the original mixture.
Extraction technology enters the scene as a "heavy weapon" in the initial separation stage. With its extremely high processing throughput, fast phase-change mass transfer rate, large mass transfer area, and the ability to achieve large-fold volume concentration, it has become an indispensable core unit operation in the purification of everything from small-molecule antibiotics and organic acids to recombinant proteins and natural products. Relatively speaking, however, the use of extraction technology in bioprocessing downstream operations exhibits strong industry differentiation: in small-molecule fermentation and natural product extraction, its application is extensive, but in modern biopharmaceuticals involving biological macromolecules (antibodies, vaccines), it is rarely applied directly.
Based on differences in the state of the two phases, extraction mechanism, and driving force, extraction technology can be mainly divided into six categories, including: liquid-liquid organic solvent extraction, reactive extraction, aqueous two-phase extraction, solid-liquid leaching, supercritical extraction, and cloud point extraction. In bioprocessing downstream operations, organic solvent extraction, reactive extraction, and supercritical extraction occupy relatively dominant positions. Let us briefly introduce them.
Liquid-Liquid Extraction
Liquid-liquid extraction utilizes the difference in solubility between the aqueous phase and the organic solvent phase for separation. It has the following characteristics: fast mass transfer rate and high throughput, but organic solvents are flammable and explosive, and proteins are prone to denaturation and inactivation. Common types of liquid-liquid extraction equipment include: extraction centrifuges, extraction columns, and mixer-settlers.
Extraction centrifuges come in various types, including vertical annular gap type, horizontal disc type, and horizontal screw type. The centrifuge completes phase separation during high-speed rotation, with the light and heavy phases discharged through separate outlets, which is somewhat similar to the principle of disc centrifuges. Extraction centrifuges have advantages such as short residence time (from a few seconds to a dozen seconds), strong emulsion-breaking capability, and small liquid holdup. Compared with other forms, this may also be the reason why they are relatively widely used in the field of biopurification.
Extraction columns also come in several types, including static extraction columns such as sieve-plate and packed types, dynamic extraction columns with stirring capability, as well as rotating disc contactors (RDC) and pulsed extraction columns. Taking the RDC rotating disc contactor as an example for analysis: the heavy phase enters from the upper part, and the light phase enters from the lower part. Due to the density difference, the heavy phase flows out from the bottom of the column under gravity, while the light phase is discharged from the top. In the intermediate process, not only does stirring facilitate the shearing and breakup of liquid droplets, but the countercurrent exchange between the upper and lower phases also promotes mass transfer, thereby achieving mixing.
The mixer-settler is relatively simpler. It mainly consists of three parts: first, stirring and mixing; in the middle, a natural settling zone; and finally, separation of the light and heavy phases. From a statistical and trend perspective, it is also simpler and easier to implement, and certainly lower in price. However, in terms of efficiency or degree of automation, the previous two types, especially the centrifugal type, are certainly superior.
Reactive Extraction
Reactive extraction enhances the transfer of target solutes from the aqueous phase to the organic phase through reversible chemical reactions with an extractant. This type of extractant is highly specific. The extractant can undergo reversible chemical complexation (ion-pair association) with polar small molecules, enabling it to break through the limits of traditional physical partitioning. Later, during back-extraction, the pH or temperature needs to be adjusted to break the complex and carry out the reverse reaction
Supercritical Fluid Extraction
It utilizes a fluid maintained above its critical temperature and pressure to extract target components from solids or liquids. CO₂ is the most widely used supercritical fluid, with a critical temperature and pressure of 31°C and 74 bar, respectively. Supercritical fluids have a liquid-like density, giving them high dissolving power, as well as a gas-like diffusion coefficient. They leave no solvent residue and can protect heat-sensitive components at low temperatures. The disadvantages are the high investment in high-pressure equipment and the cost of sealed pressure control.
Solid-Liquid Leaching
Solid-liquid leaching, also known as solid-liquid extraction or percolation, uses a liquid solvent to extract soluble components from a solid matrix. During the process, the solvent penetrates the cell wall, dissolves the intracellular solutes, and then diffuses into the bulk liquid phase. It is a commonly used process in the industrial extraction of Chinese herbal medicines and natural products, but it is generally limited by solid-phase mass transfer resistance, resulting in relatively long cycle times.
The development of an extraction process follows a rigorous closed-loop engineering decision-making methodology: first, understanding the physical properties; then, selecting the extraction mode; next, scientifically screening the solvent; and finally, back-extraction and engineering closed-loop verification.
In plain terms, the considerations include: product tolerance (to solvent, temperature, and time), partitioning feasibility (determining whether to use ordinary physical extraction or reactive complexation), interfacial physical properties for phase separation (does the fermentation broth contain large amounts of proteins and polysaccharide colloids? Can it be thoroughly clarified in a short time? This determines whether upstream ultrafiltration for protein removal is needed, and whether to choose a column or a centrifuge), and back-extraction and economic viability.
Molecular Physical Properties
Whether an extraction process can be used is the main guiding direction, and generally several factors are considered:
Antibiotics, organic acids, vitamins, etc. with a molecular weight < 1500 Da and rigid structures are generally well suited for extraction. However, for biological macromolecules such as monoclonal antibodies, enzymes, and nucleic acid vaccines, they are highly prone to denaturation and precipitation upon contact with organic solvents, so membrane filtration or chromatography would be adopted instead.
The oil-water partition coefficient refers to the concentration ratio of a compound when it reaches equilibrium in a mixture of two immiscible solvents, expressed as log P or log D. log P = 0: half in oil and half in water; log P = 2: the concentration in oil is 10² = 100 times that in water, very lipophilic; log P = −2: the concentration in water is 100 times that in oil, extremely hydrophilic. Therefore, if log P > 1.5, the compound is highly lipophilic, such as microalgal lipids, vitamin E, steroids, etc., and conventional physical extraction can be easily achieved. When −1.0 < log P < 1.5, the compound is amphiphilic, such as penicillin, cephalosporins, etc., and extraction requires adjusting the pH to suppress dissociation. When log P < −1.5, the compound is extremely hydrophilic, such as lactic acid, amino acids, polyols, etc., and ordinary solvents simply cannot pull it out, so reactive extraction must be forcibly introduced.
The dissociation constant pKa is the molecule's "charge-switching switch." Determining the pH range in which the product is in its molecular form (neutral) and the pH at which it is in its dissociated form (ionic) helps in considering extraction and back-extraction.
Extraction Mode
Based on the molecular physical properties above, a preliminary determination can be made. If the compound is strongly lipophilic / an oil or fat (log P > 1.5), conventional physical solvent extraction (n-hexane / supercritical CO₂) can be selected. If it is a dissociable weak electrolyte (moderately polar / has a pKa), pH-regulated physical extraction can be selected (neutralization at low pH + esters/alcohols). If it is super-hydrophilic / highly polar (log P < −1.5), reactive extraction can be selected (tertiary amines / phosphorus-oxygen complexing agents).
Solvent Screening
First, Hansen Solubility Parameters (HSP) are used for preliminary solvent screening, calculating the "distance Ra" between the solvent and the target solute in the three-dimensional coordinate space of dispersion forces (δd), polar forces (δp), and hydrogen bonding forces (δh). The smaller the Ra, the higher the theoretical thermodynamic compatibility for dissolution. Currently, there are tools such as COSMO-RS and HSPiP that can be used for rapid assistance.
In industry, pure extractants are rarely used alone. Instead, a ternary blend of "extractant + diluent + modifier" is typically employed (especially in reactive extraction). The diluent (accounting for 60%–90%) serves to reduce the viscosity of the system, increase the density difference between the oil and water phases, and accelerate settling and phase separation. The modifier (accounting for about 5%–15%) serves to adjust the microscopic polarity and prevent the formation of a third phase (interfacial gel) that cannot be separated between the two phases.
From an operational standpoint, the boiling point difference must also be considered. The boiling point difference between the solvent and the product should be > 30–50°C to facilitate low-energy desolventization and recovery in subsequent distillation. The solubility of the solvent in the aqueous phase must be very low to avoid environmental and safety issues and wasteful losses. Reference should also be made to solvent classification for selection. In pharmaceutical engineering, Class 1 solvents (carcinogens such as benzene and carbon tetrachloride) are strictly prohibited, and Class 3 low-toxicity solvents (such as ethyl acetate, butyl acetate, and ethanol) should be preferentially selected.
Back-Extraction and Engineering Closed-Loop Verification
Back-extraction verification and closed-loop design are the watershed between industrial success and failure. The target must go in, and it must also come out. For a qualified extraction process, a back-extraction plan must be developed simultaneously on the very first day the extractant is selected. Several common back-extraction methods include: temperature-swing back-extraction (e.g., extraction at 25°C and back-extraction at 85°C), which is the cleanest; pH-swing back-extraction, suitable for polar ionic molecules (e.g., extracted into the organic phase in molecular form at low pH, and forced back into the aqueous phase by ionization at high pH); and evaporative desolventization, suitable for physical extraction with solvents of extremely low boiling point, where the solvent is evaporated off by vacuum thin-film evaporation and the pure product is obtained directly from the bottom stream.
Penicillin G is mainly produced industrially through fermentation by filamentous fungi. After fermentation is completed, penicillin is extracted from the fermentation broth into an organic solvent by adjusting the pH, then transferred back into the aqueous phase and subjected to multi-step purification.
Pre-Extraction Pretreatment
Adjusting pH to change the molecular dissociation state: For weak acid/weak base products, the pH must be adjusted to below or above their pKa so that they exist in the non-dissociated molecular form and dissolve in the organic phase. For penicillin G (pKa ≈ 2.75), the pH is rapidly adjusted to 2.0–2.5 after harvesting.
Rapid cooling to prevent degradation: Penicillin is prone to hydrolysis and ring-opening inactivation under acidic conditions at pH 2.0. Therefore, it is rapidly chilled to 0–4°C through a heat exchanger.
Demulsification and flocculation: A 0.1% demulsifier (such as long-chain polyether) or cationic flocculant (PEI) is added to precipitate extracellular polysaccharides and dissolved proteins, remove interfacially active substances, and prevent stubborn emulsification during extraction.
Multi-Stage Countercurrent Extraction
The clarified fermentation broth and butyl acetate are fed into a centrifugal extractor at a flow ratio of S/F ≈ 0.2–0.25. Under an extremely strong centrifugal force field, the two phases undergo highly efficient contact and mass transfer within seconds and instantaneously separate under the gravitational field. The penicillin molecules rapidly transfer into the butyl acetate phase, achieving a 4–5 fold initial concentration.
Back-Extraction
The target-laden organic phase rich in the product enters the back-extraction unit, where an appropriate amount of phosphate buffer at pH 6.8–7.2 is added. At neutral pH, the carboxyl group of penicillin dissociates into a strongly polar ionic form, the partition coefficient is reversed, and the penicillin is instantly extracted back from the organic phase into the aqueous phase and further concentrated, while non-polar impurities remain in the organic phase.
Solvent Recovery
The raffinate aqueous phase contains 1–2% dissolved solvent and must be sent to a stripping column for steam purging and recovery, reducing the solvent content in the wastewater to the ppm level. This not only reduces the environmental COD burden but also saves on the cost of expensive solvents. The lean organic phase after back-extraction is washed with dilute alkali, decolorized with activated carbon, and dehydrated by distillation before being recycled back to the first-stage extraction end, forming a closed loop.
In industrial extraction applications, the two most common problems are emulsification and flooding. These can be improved through the following optimization measures:
Shear force control: The input power per unit volume (P/V ratio) of agitation must not be too high, to avoid shear force breaking dispersed-phase droplets into a stable emulsion.
Pre-removal of surface-active proteins: Before the fermentation broth enters the extraction column, dissolved proteins and extracellular polysaccharides must be removed through heat-shock coagulation, flocculation, or microfiltration to maintain interfacial tension at a relatively high level, ensuring clean phase separation.
Interface position control: Adjust the boundary layer between the two immiscible liquids in the separation vessel or extraction column to avoid entrainment of the heavy phase by the light phase, etc. This can be achieved by adjusting the heavy-phase overflow weir size or the outlet back-pressure valve to lock the physical interface between the two phases at the center of the clarification zone.
Prevention of flooding: There is an upper limit to the total throughput of an extraction column. When the flow rate exceeds the critical flooding velocity, the continuous phase cannot be discharged smoothly. A differential pressure transmitter can be installed to monitor the pressure difference. Once the pressure difference rises abnormally, the feed rate should be automatically reduced immediately.
There are multiple technical routes for plant extraction processes, and the principles of different extraction methods vary greatly. Equipment selection requires comprehensive evaluation of raw material characteristics, target extracts, production capacity, cleanliness requirements and operating costs. As a manufacturer of complete plant extraction production lines, we continuously optimize equipment structure and system processes to address production pain points based on numerous implemented project cases. We provide integrated complete plant extraction solutions ranging from preliminary scheme design and customized equipment manufacturing to production line commissioning and application optimization, helping extract manufacturers improve yield, reduce energy consumption and achieve stable, compliant large-scale production.
