Lecture 55: Leaching and Extraction (Contd.)
Introduction
The instructor introduces the course and briefly discusses the topics that will be covered in this lecture.
Basics of Leaching and Extraction
- The instructor recaps on the basics of leaching and extraction, including equilibrium diagrams.
- They differentiate between solid-liquid extraction (leaching) and liquid-liquid extraction (solvent extraction).
- They discuss the equipment used for liquid-liquid extraction.
Equilibrium Diagrams
- The instructor emphasizes the importance of having a diagram of solvents before designing or assessing any process involving leaching or extraction.
- They explain how to read an equilateral triangle diagram to find a fraction of all components.
- Material balance and component balance are discussed.
Operating Line
- The ideal plot for xA versus yA is explained, with a 45-degree line representing ideal equilibrium conditions.
- Deviation blocks are introduced to show how processes deviate from ideal conditions.
Equipment for Extraction
Different types of equipment used for solvent extraction are discussed.
Mechanical Agitation
- Vessels with mechanical agitation are required for proper mixing during solvent extraction.
- Three types of mechanical agitation vessels are mentioned: agitated columns, mixer settler, and centrifugal extractor.
Flow Mixing
Two types of flow mixing vessels used in solvent extraction are discussed.
Spray Extraction Tower
Packed Extraction Tower
Extraction Towers and Their Operation
This section discusses the different types of extraction towers used in chemical engineering, including Scheible tower, Karr column, mixer settlers, and spray extraction towers. The towers are designed to separate two liquid phases by providing mechanical agitation or perforated trays for proper mixing.
Scheible Tower
- Variable speed drive rotates central vertical shaft at required speed
- Agitators mounted at fixed intervals provide proper mixing
- Heavy phase enters from top and exits from bottom while light phase enters from bottom and exits from upper right section
- Turbine blades and baffles help with mixing
Karr Column
- Reciprocating motion of perforated trays provides well-mixed liquid phases
- Heavy phase enters from top while light phase enters through sparger at bottom
- Liquid phases exit after separation through perforation
Mixer Settlers
- Consists of a mixer for contacting two liquid phases and a settler for their mechanical separation
- Solvent and feed enter into a mixing chamber where they are properly mixed before settling in another chamber called the settling chamber
- Raffinate (heavier liquid) comes down while extract (lighter liquid) flows out
Spray Extraction Towers
- Heavy liquid enters at the top of the spray tower and fills it
Liquid-Liquid Extraction and Supercritical Fluid Extraction
This transcript covers the parameters of liquid-liquid extraction, including solvent selection, operating conditions, mode of operation, extractor type, and design criteria. It also discusses supercritical fluid extraction and its advantages.
Parameters of Liquid-Liquid Extraction
- Solvent selection is crucial for liquid-liquid extraction to ensure that the solid component or flavor compound can come out from the phase.
- Operating conditions such as temperature and mode of operation (batch or continuous; co-current or counter current) affect the efficiency of liquid-liquid extraction.
- The type of extractor used (e.g., spray type or mechanical agitated type) and design criteria are important considerations in liquid-liquid extraction.
Supercritical Fluid Extraction
- Supercritical fluid extraction involves using a solvent held at a pressure and temperature above the critical point of the solvent to achieve specific properties that enhance extraction.
- Carbon dioxide is one gas that easily achieves supercritical fluid condition with low critical temperature around 31.8 degree Celsius.
Overall, this transcript provides an overview of key considerations in liquid-liquid extraction and introduces supercritical fluid extraction as an alternative method with unique advantages.
Supercritical Fluid Extraction
This section discusses the principles of supercritical fluid extraction, including the effects of pressure and temperature on solubility. It also covers the solubility parameter and selectivity of supercritical fluids.
Principles of Supercritical Fluid Extraction
- Supercritical fluid extraction is based on the principle that increasing pressure increases the density and solubility of a gas, making it behave like a liquid.
- The effect of pressure is more pronounced than temperature in supercritical fluid extraction.
- Temperature has little effect on extraction rate at around 10 MPa pressure, but can have a positive effect at very high pressures (around 30 MPa).
- Supercritical fluids act as solvents due to their increased solubility under high pressure.
- Solubility parameter is used to represent the power of a supercritical fluid to dissolve a substance. It is expressed by an equation involving critical pressure and density ratios.
- Compressed gases have higher solubility when compressed, making them useful for solvent extraction methods.
Advantages of Supercritical Fluid Extraction
- Supercritical fluids have broad selectivity and higher solubility due to their strong dependence on temperature and pressure.
- Selective removal of odor-producing volatile components close to the critical point is possible with supercritical fluid extraction.
Operation of Supercritical Fluid Extraction
- Extraction takes place in the supercritical region beyond the gas and liquid region shown in the phase diagram.
Supercritical Fluid Extraction using Carbon Dioxide
This section discusses the process of supercritical fluid extraction using carbon dioxide, including the steps involved in separating components and the advantages and disadvantages of this method.
Separating Components
- The process involves extracting a component from a feed solution using a solvent.
- Pressure is decreased to separate all components, resulting in carbon dioxide in gas form.
- The gas is separated from the product and cooled to become liquid through a heat exchanger.
- Liquid carbon dioxide is then compressed to increase pressure and temperature before being entered into an extraction vessel.
Advantages of Supercritical Fluid Extraction
- Operates at moderate temperatures, making it helpful for preserving heat-sensitive materials.
- Carbon dioxide is non-toxic, non-flammable, and nature-friendly with no ill effects on products or residue left behind.
- Low viscosity allows for good mass transfer due to high density and selective dissolution.
Disadvantages of Supercritical Fluid Extraction
- Limited solvation power can be overcome by adding co-solvent.
Supercritical Fluid Extraction
This section discusses the advantages and disadvantages of supercritical fluid extraction, including its initial high cost and difficulty in running as a continuous process. However, it is an efficient method for batch processing and can be advantageous in the long run due to its low energy requirements for extraction.
Advantages of Supercritical Fluid Extraction
- If run continuously for a long time, supercritical fluid extraction can be very advantageous due to its low energy requirements for extraction.
- Supercritical fluid extraction can reduce costs by eliminating the need for energy-intensive separation techniques.
- The extra costs saved from using supercritical fluid extraction can be utilized for initial setup preparation.
Disadvantages of Supercritical Fluid Extraction
- The initial cost of supercritical fluid extraction is very high.
- It is difficult to run as a continuous process.
Conclusion
This section concludes the discussion on leaching and extraction processes. While detailed mass transfer analysis is required, this section provides an overview of the benefits and drawbacks of these processes.
- For further questions or clarification, refer to other courses on mass transfer or contact the instructor.