In modern industrial production, the separation and purification of substances are crucial links. As an efficient liquid-liquid mass transfer device, the solvent extraction tower plays a key role in many fields. It uses the difference in solubility or distribution coefficient of different substances in two immiscible solvents to achieve the extraction, separation, enrichment and purification of the target components in the mixture. Whether it is chemical industry, petroleum refining, hydrometallurgy, environmental protection and other industries, solvent extraction towers occupy an indispensable position. ​

1. Analysis of working principle​

(1) Basic principle - distribution law​

The core theoretical basis of solvent extraction is the distribution law. When a soluble substance is added to two immiscible (or slightly soluble) solvents, the substance can be dissolved in the two solvents respectively. At a certain temperature, if the compound does not undergo decomposition, electrolysis, association and solvation with the two solvents, then the ratio of its concentration in the two liquid layers is a constant, which can be expressed by the formula:

K=C_A/C_B, where C_A and C_B are the concentrations of the compound in two immiscible solvents A and B, respectively, and K is the distribution coefficient at a certain temperature. For example, when extracting phenolic substances from phenol-containing wastewater, if a suitable extractant is selected, the concentration of phenols in the extractant and the aqueous phase will follow this law for distribution.

(2) Extraction process​

Construction of two-phase system: The extraction process involves two immiscible liquid phases, most commonly aqueous and organic phases. One phase is the continuous phase (usually the phase with a larger amount), and the other phase is the dispersed phase. For example, when extracting organic acids from fermentation broth, the aqueous fermentation broth may be the continuous phase, while the organic extractant is the dispersed phase. ​

Introduction of feed and solvent: The material to be separated is fed into the extraction tower, and an extraction solvent is added that has high selectivity for the target component and is incompatible or has very low solubility with other components in the raw material. For example, when extracting aromatics from petroleum fractions, a specific aromatic extraction solvent is selected. ​

Contact and distribution occur: The dispersed phase enters the continuous phase through a nozzle or other means to form tiny droplets. These droplets are fully in contact with the continuous phase, and the target component is transferred from the original material to the extraction solvent according to the distribution law. Taking the extraction of lithium from salt lake brine as an example, after the lithium-containing brine contacts the extractant, the lithium element is transferred from the brine to the extractant.​

Mixing and separation promotion: The special structure inside the extraction tower promotes the full mixing of the two phases, and then the two phases are gradually separated by gravity or mechanical devices. The heavier phase settles to the bottom of the tower, and the lighter phase rises to the top of the tower. For example, when extracting impurities in edible oil, the heavier phase where the impurities are located sinks, and the pure oil phase rises. ​

Collection and circulation realization: The extraction phase rich in the target component and the raw material phase depleted in the target component are collected at different positions in the tower. In some cases, the extraction solvent can be recycled. For example, in the pharmaceutical industry, after the extraction of certain drug intermediates, the extraction solvent can be recycled after treatment.

2. Diverse structural types​

(1) Packed extraction tower ​

Structural features: The tower is filled with various types of packing, such as Raschig rings, Pall rings, saddle rings, etc. These packings provide a huge gas-liquid contact area, allowing the two phases to be fully mixed and mass transferred. For example, when treating phenol-containing wastewater, Raschig ring packing can effectively increase the contact between the extractant and the wastewater.​

Working process: The continuous phase flows from top to bottom through the packing layer under the action of gravity, while the dispersed phase enters from the bottom of the tower through the distributor. Under the obstruction and dispersion of the packing, it flows upward through the continuous phase in the form of fine droplets. In this process, the target component is transferred between the two phases. Taking the extraction of heavy metal ions from wastewater as an example, the extractant droplets shuttle between the packing and exchange with the heavy metal ions in the wastewater.

Advantages: Simple structure, low cost, suitable for treating corrosive materials, and relatively high mass transfer efficiency. For example, in fine chemical production, for the separation of some products with small output but high requirements for equipment corrosion resistance, packed extraction towers are widely used.

Limitations: The flux is relatively small, the processing volume is limited, and when the liquid load is low, channeling and other phenomena are prone to occur, affecting the mass transfer effect. In large-scale industrial production, if the processing volume demand is large, it may not be able to meet the production requirements.

(2) Sieve plate extraction tower

Structural features: There are several layers of sieve plates in the tower, and many small holes are evenly distributed on the sieve plates. For example, the diameter of the sieve hole is generally between 3 and 8 mm, and the specific size depends on the properties of the processed materials and the process requirements. ​

Working process: The continuous phase is dispersed into fine droplets through the small holes on the sieve plate and enters the next layer, and contacts with the dispersed phase of the next layer in countercurrent for mass transfer. The dispersed phase flows as a continuous phase in the tower as a whole, and flows to the next layer through the downcomer on the tower plate. For example, when extracting specific components from petroleum products, the petroleum products pass through the sieve holes as the continuous phase, and the extractant flows in the tower in the reverse direction. ​

Advantages: simple structure, low cost, large production capacity, and strong adaptability to changes in liquid flow. In some industrial productions with strict cost control and large processing volume, such as the separation of certain basic chemical raw materials, sieve plate extraction towers are widely used. ​

Limitations: The mass transfer efficiency is relatively low, and problems such as flooding are prone to occur on the tower plate, affecting the stability of the extraction operation. When processing materials with extremely high separation accuracy requirements, it may not meet the process requirements.​

(3) Rotating disc extraction tower​

Structural features: There are multiple discs (rotating discs) of the same size and spacing connected by a rotating shaft in the middle, which rotate at a constant speed as the shaft rotates. The rotating discs are separated by annular discs (fixed discs) of the same size and spacing fixed on the tower wall. For example, the spacing between the rotating disc and the fixed disc is generally between 10 and 50 cm, which is adjusted according to the tower diameter and the characteristics of the processed material. ​

Working process: The solution with lower density enters continuously from the lower part of the tower, flows upward under the action of buoyancy, and is broken up and dispersed into droplets by the centrifugal action of the rotating disc. The solvent with higher density enters continuously from the upper part of the tower, flows downward under the action of gravity and fills the entire tower. The dispersed droplets transfer mass through contact in the continuous solvent. Taking the extraction of free fatty acids from vegetable oil as an example, the light phase vegetable oil is dispersed under the action of the rotating disc and contacts and reacts with the heavy phase extractant. ​

Advantages: High mass transfer efficiency, large production capacity, good adaptability to changes in two-phase flow rates, and can effectively reduce axial backmixing. In the chemical and pharmaceutical industries, for some materials that require efficient separation and large processing volume, the rotary disc extraction tower is widely used.

Limitations: The structure is relatively complex, the energy consumption is high, and the equipment maintenance cost is relatively high. In some production processes with extremely strict energy consumption requirements, its applicability may need to be carefully considered.

(4) Vibrating sieve plate tower

Structural features: There are a series of sieve plates fixed on the central axis in the tower, and the central axis is vibrated up and down by the drive device. The vibration frequency and amplitude of the sieve plate can be adjusted according to the process requirements. The general vibration frequency is between 1-10Hz and the amplitude is in the range of 3-50mm.

Working process: The continuous phase and the dispersed phase pass through the sieve plate in countercurrent. The vibration of the sieve plate causes the liquid to disperse and aggregate continuously, greatly enhancing the mass transfer between the two phases. For example, when extracting rare earth elements from rare earth ore leachate, the vibration of the sieve plate promotes the full mixing and mass transfer of the extractant and the leachate.​

Advantages: high mass transfer efficiency, large processing capacity, good effect on low concentration and high difficulty separation system, and can effectively reduce axial back mixing. In the fields of rare earth extraction, fine chemicals, etc., for the extraction of some difficult-to-separate substances, the vibrating sieve plate tower has unique advantages. ​

Limitations: The equipment structure is relatively complex, and the equipment manufacturing precision and installation requirements are high. The vibrating parts are easy to damage and difficult to maintain. During the operation of the equipment, the vibrating parts need to be inspected and maintained regularly, which increases the operating cost. ​

(5) Multi-stage centrifugal extraction tower ​

Structural features: It consists of multiple centrifugal extraction units connected in series, each unit has a high-speed rotating rotor. The rotor speed is usually between 1000-5000r/min and can be adjusted according to the material properties and separation requirements. ​

Working process: The two-phase liquid is rapidly mixed and separated under the centrifugal force generated by the high-speed rotation of the rotor. The heavier phase is thrown to the outer edge of the rotor, and the lighter phase gathers to the center and is then discharged through different outlets. For example, when extracting antibiotics from biological fermentation broth, centrifugal force is used to achieve rapid and efficient separation.​

Advantages: The extraction efficiency is extremely high, and efficient separation can be achieved in a short time. The equipment occupies a small area and is suitable for processing systems with small density difference between the two phases and easy emulsification. In industries such as biopharmaceuticals and environmental protection that have high space requirements and special material properties, multi-stage centrifugal extraction towers have broad application prospects. ​

Limitations: The equipment investment is large, the energy consumption is high, and the operation and maintenance requirements of the equipment are strict, requiring professional technicians to operate. Due to the high equipment cost and operating cost, it may not be suitable for some small-scale enterprises with limited funds.

Comparison of performance of different types of extraction towers:

3. Wide application areas​

(1) Chemical industry​

Organic synthesis: In the process of organic synthesis, it is often necessary to separate and purify the reaction products. For example, in the process of synthesizing drug intermediates, the target product can be extracted from the reaction mixture by using a solvent extraction tower, impurities can be removed, and product purity can be improved. For example, when preparing acetaminophen intermediates, the target product can be separated by an extraction tower to provide high-purity raw materials for subsequent synthesis steps. ​

Polymer production: In polymer production, solvent extraction towers are used to remove impurities such as residual monomers and catalysts in polymer solutions. Taking polypropylene production as an example, unreacted propylene monomers and catalyst residues can be effectively removed by extraction towers to improve the quality of polypropylene products. ​

(2) Petroleum refining​

Oil refining: In the process of petroleum refining, in order to improve the quality of oil products, it is necessary to remove impurities such as sulfur and nitrogen and undesirable components such as aromatics in the oil products. Solvent extraction towers can use specific extractants to extract these impurities from oil products. For example, in diesel refining, liquid-liquid extraction technology is used to remove impurities such as mercaptans in diesel through an extraction tower, thereby reducing the sulfur content of diesel and improving the quality grade of diesel. ​

Aromatic extraction: Separating and purifying aromatics from petroleum fractions is an important part of petrochemicals. Solvent extraction towers play a key role in the aromatic extraction process, and can efficiently separate aromatics from non-aromatics, providing raw materials for subsequent aromatic processing. For example, when extracting aromatics such as benzene, toluene, and xylene from reformed gasoline, high-purity aromatic products can be obtained by selecting suitable extractants and extraction towers. ​

(3) Hydrometallurgy​

Metal extraction: In the field of hydrometallurgy, solvent extraction towers are used to extract metals from ore leachate. For example, to extract copper from copper ore leachate, an extractant with high selectivity for copper ions is selected, and it is countercurrently contacted with the leachate in the extraction tower to transfer copper ions from the leachate to the extractant. Then, through subsequent operations such as back extraction, copper enrichment and purification are achieved.​

Rare metal separation: For the separation of rare metals, such as separating different rare earth elements from rare earth ore leachate, the solvent extraction tower uses the difference in the distribution coefficients of different rare earth elements in the extractant to achieve the separation of multiple rare earth elements one by one, providing key technical support for the comprehensive utilization of rare earth resources. ​

(4) Environmental protection​

Wastewater treatment: In industrial wastewater treatment, solvent extraction towers can be used to remove harmful substances in wastewater, such as heavy metal ions, phenols, organic acids, etc. For example, when treating phenol-containing wastewater, phenolic substances are extracted from the wastewater through an extraction tower to reduce the phenol content of the wastewater to meet the emission standards. At the same time, phenolic substances can also be recycled to achieve resource recycling. ​

Waste gas treatment: In some cases, solvent extraction towers can also be used to treat certain pollutants in waste gas. By passing the waste gas into an extraction tower containing a specific extractant, the pollutants in the waste gas are dissolved in the extractant, thereby achieving the purpose of purifying the waste gas. For example, when treating organic waste gas, a suitable organic solvent is selected as the extractant to purify the organic waste gas in the extraction tower.

(5) Food and Beverage Industry

Natural product extraction: In the food and beverage industry, solvent extraction towers are used to extract active ingredients from natural raw materials. For example, tea polyphenols can be extracted from tea leaves by using a suitable extractant in an extraction tower to extract tea extracts. High-purity tea polyphenols can be obtained and used in food additives, health products and other fields.

Flavor separation: In beverage production, in order to obtain a unique flavor, it is necessary to separate and extract flavor substances from natural spices or fermentation broth. Solvent extraction towers can utilize the distribution characteristics of flavor substances in different solvents to achieve the separation and enrichment of flavor substances, adding a unique flavor to beverage products.

4. Significant advantages are fully demonstrated

(1) Efficient separation

Through optimized design and selection of appropriate operating conditions, solvent extraction towers can utilize the difference in distribution coefficients between the two phases to achieve efficient separation of target components in a mixture. For some mixture systems that are difficult to separate by other methods, such as substances with similar boiling points and heat-sensitive substances, solvent extraction towers have unique advantages. For example, when separating active ingredients from Chinese herbal medicine extracts, traditional methods such as distillation may cause the active ingredients to decompose due to high temperatures, while solvent extraction towers can achieve efficient separation under mild conditions. ​

(2) Strong adaptability​

Solvent extraction towers are suitable for a variety of different chemical systems and operating conditions. Whether dealing with solvents of different properties (such as polar solvents and non-polar solvents) or in different temperature ranges and pressure environments, good extraction effects can be achieved by adjusting the equipment structure and operating parameters. In chemical production, for some systems with harsh reaction conditions, solvent extraction towers can flexibly adapt to meet production needs. ​

(3) Continuous operation​

Many types of solvent extraction towers support continuous feeding and discharging, which is very suitable for large-scale industrial production processes. Continuous operation can not only improve production efficiency, but also reduce energy consumption and production costs per unit product. Compared with intermittent operation, continuous operation reduces the number of equipment starts and stops, increases the service life of the equipment, and at the same time makes the product quality more stable. For example, in large-scale industrial production such as petroleum refining and chemical raw material production, continuously operated extraction towers are widely used.​

(4) High flexibility​

The design of the solvent extraction tower allows for adjustment of a variety of operating parameters, such as flow rate, solvent ratio, temperature, pressure, etc., to suit different separation tasks. By changing these parameters, the extraction process can be optimized and the extraction rate and purity of the target component can be improved. In addition, the multi-stage extraction configuration can further improve the separation effect and meet the requirements of separation accuracy for different processes. In actual production, the operating parameters and number of stages of the extraction tower can be flexibly adjusted according to the raw material composition and product quality requirements.

(5) Easy maintenance​

The design of modern solvent extraction towers fully considers the ease of cleaning and maintenance of the equipment. For example, the use of removable packing or tower plate structure makes it easy to clean and replace the equipment after it has been running for a period of time, reducing the downtime and maintenance costs of the equipment. At the same time, the equipment is equipped with various monitoring instruments and automatic control systems, which can monitor the operating status of the equipment in real time, detect and solve potential problems in a timely manner, and ensure the stable operation of the equipment.

5. Design and operation considerations​

(1) Key points of design​

Tower size determination: The height and diameter of the tower body need to be accurately calculated based on the processing volume, operating conditions and required separation efficiency. When the processing volume is large, a larger diameter tower body is usually required to meet the flux requirements; while for situations where the separation is difficult and a higher theoretical plate number is required, the tower body height needs to be increased. For example, in large-scale oil refining projects, the size of the extraction tower is accurately designed based on the crude oil processing volume and oil product separation requirements. ​

Internal structure selection: According to the material properties and process requirements, the internal structure is reasonably selected, such as the packing type, sieve plate aperture, turntable size and spacing, etc. For materials that are easy to emulsify, a packing with a simple structure that is not easy to cause clogging may be selected; for systems with large processing volumes and high mass transfer efficiency requirements, a turntable extraction tower structure may be used. In fine chemical production, the internal structure of the extraction tower is carefully designed according to the characteristics of different products. ​

Material selection: Considering factors such as the corrosiveness, temperature and pressure of the material, select appropriate tower body and internal component materials. When dealing with highly corrosive materials, such as acid-containing leaching solutions in hydrometallurgy, corrosion-resistant stainless steel or special alloy materials are usually used to manufacture the extraction tower to ensure the service life and safe operation of the equipment. ​

(2) Optimization of operating parameters​

Flow rate control: Accurately controlling the flow rates of the continuous phase and the dispersed phase is the key to ensuring full contact between the two phases and avoiding abnormal conditions such as flooding. Too fast a flow rate will compress the contact time between the two phases, resulting in a significant decrease in mass transfer efficiency; too slow a flow rate will reduce production efficiency and increase energy consumption costs. In actual industrial operations, it is necessary to dynamically optimize the two-phase flow rate based on the real-time load and separation effect of the extraction tower through an automated control system composed of a flow meter and a regulating valve. For example, in a fine chemical production line, the flow rate is monitored and adjusted in real time through a PLC (programmable logic controller) to ensure efficient and stable mass transfer process. ​

Temperature and pressure regulation: The temperature and pressure parameters of the extraction system directly affect the solubility and distribution coefficient of the substance and are the core variables that determine the extraction efficiency. Temperature changes will not only change the distribution equilibrium of solutes in the two phases, but may also affect the stability of the target product; pressure regulation plays a decisive role in the extraction process of volatile substances. In the extraction process of thermosensitive bioactive substances, low temperature and low pressure operation is usually adopted, and high-precision temperature control equipment and pressure compensation system are equipped to control temperature fluctuations within the range of ±0.5℃ to ensure the activity and yield of the target components.
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Solvent ratio optimization: According to the composition characteristics of the raw materials and the purity requirements of the target products, scientifically adjusting the ratio of extraction solvent to raw materials is an important link in achieving economical and efficient production. If the solvent ratio is too large, it will cause solvent waste and increase subsequent recovery costs; if the solvent ratio is too small, it may lead to incomplete extraction and affect product quality. In modern industrial production, process simulation software such as Aspen is often used, combined with laboratory test data, a dynamic mathematical model is established to optimize the solvent ratio for different batches of raw materials. Taking the pharmaceutical industry as an example, through near-infrared spectroscopy online analysis technology, the changes in raw material composition can be monitored in real time, and the solvent ratio can be dynamically adjusted to increase product purity by 10%-15% while reducing solvent consumption by more than 20%. ​