Liquid-Liquid Extraction PDF

Contents Introduction to Extraction The Equilateral Triangular Diagram Purpose of Extraction Liquid-Liquid Extraction Liquid-Liquid Equilibria(LLE) Classification extraction processes Choice of solvent Advantages of solvent extraction Application of Extractio...

Contents Introduction to Extraction The Equilateral Triangular Diagram Purpose of Extraction Liquid-Liquid Extraction Liquid-Liquid Equilibria(LLE) Classification extraction processes Choice of solvent Advantages of solvent extraction Application of Extraction Distribution law Typical Extraction System In Industry Classification Of Extractor Types Of Extractor Mixer-Settler Decanter Introduction to Extraction Liquid-liquid extraction (also known as solvent extraction) involves the separation of the constituents (solutes) of a liquid solution by contact with another insoluble liquid. Solutes are separated based on their different solubilities in different liquids. Separation is achieved when the substances constituting the original solution is transferred from the original solution to the other liquid solution  The simplest liquid-liquid extraction involves only a ternary (i.e.3 components) system. The solution which is to be extracted is called the feed, and the liquid with which the feed is contacted is the solvent. The feed can be considered as comprising the solute A and the "carrier" liquid C. Solvent S is a pure liquid. During contact, mass transfer of A from the feed to the solvent S occurs, with little transfer of C to S. The solvent (with the solute) is then permitted to separate from the carrier liquid.  The solvent-rich product of the operation is called the extract, and the residual liquid from which solutes has been removed is the raffinate In some operations, the solutes are the desired product, hence the extract stream is the desirable stream. In other applications, the solutes may be the contaminants that need to be removed, and in this instance the raffinate is the desirable product stream.  The Equilateral Triangular Diagram B %S %A P %B A S Figure 8.1 Typical liquid-liquid extraction process. Purpose of Extraction To separate closed-boiling point mixture Mixture that cannot withstand high temperature of distillation Example: - recovery of penicillin from fermentation broth solvent: butyl acetate - recovery of acetic acid from dilute aqueous solutions solvent: ethyl-acetate Liquid-Liquid Extraction Solvent Solution Extract(E) Raffinate(R)  Extraction – is the process by which a solute is transferred from one phase to a new phase. Liquid-liquid extraction, also known as solvent extraction and partitioning, is a method to separate compounds based on their relative solubility's in two different immiscible liquids, usually water and an organic solvent. Process of dissolved substance transferring from one phase to another phase, which are immiscible or restrictedly miscible, is named liquid- liquid –partition or partition between two phase of liquids. Liquid-Liquid Equilibria(LLE) A liquid-liquid extraction system contains at least three components A. Carrier liquid in feed B. The extracting solvent C. The Solute For example, in the process of extraction of penicillin from the fermentation broth. Where penicillin is the solute (c),water is the carrier (a) and butyl acetate is the solvent (b). The equilibrium data for a liquid-liquid system are of vital importance in the selection and design of an extraction equipment. Classification extraction processes: Periodical extraction - is the process in which separatory funnel (which contain substance which extragent) is shaked Continuous extraction Countercurrent extraction Choice of solvent Factors to be considered: Selectivity Distribution coefficient Insolubility of solvent Recoverability of solute from solvent Density difference between liquid phases Interfacial tension Chemical reactivity Cost Viscosity, vapour pressure Flammability, toxicity Advantages of solvent extraction Selectivity of extraction directly from fermentation broths or from reaction medium in the case of biotransformations wherein whole cells or enzymes are used for conversion of a substrate into a desired product. Reduction in product loss due to hydrolytic or metabolic/microbial degradation as the product is transferred to a second phase with different physical and chemical properties. Suitability over a wide range of scales operation.  2. Packed extraction towers 1. Mixer-settlers 3. Perforated-plate towers EQUIPMENT FOR EXTRACTION 6. Centrifugal 4. Agitated extractors tower extractors 7. Auxilary equipment [stills, evaporators, heaters and 5. Pulse columns condenser] Application of Extraction Extraction processes are well suited to the petroleum industry because of the need to separate heat-sensitive liquid feeds according to chemical type (e.g. aliphatic, aromatic, naphthenic) rather than by molecular weight or vapour pressure. Other major applications exist in the biochemical or pharmaceutical industry, where emphasis is on the separation of antibiotics and protein recovery. In the inorganic chemical industry, they are used to recover high- boiling components such as phosphoric acid, boric acid, and sodium hydroxide from aqueous solutions. Distribution law At equilibrium, the ratio of the concentrations of the solute in the two phases is given by CE/CR = K The distribution constant K, must be greater than 1 if the desired product is extract stream. The distribution constant K, must be lesser than 1 if the desired product is raffinate stream. Typical Extraction System In Industry An extraction system always includes at least 1 distillation column (or other separation process) to recover solvent from the extract phase. If the solvent exhibits some degree of miscibility in the feed, then a second separation process (normally distillation) is required to recover solvent from raffinate. Classification Of Extractor Types Of Extractor Single Stage Multi Stage Types of Extractor Equipment Single Stage Multi Stage 1. Mixer-Settler 1. Mixer-settler Battery 2. Decanter (MSB) 2. Mechanically Agitated Extractors i. Scheibel Extractor ii. Rotating Disk Contactor (RDC) iii.Pulse Extractors Mixer-Settler Mixer-settlers are still widely used because of their reliability, operating flexibility, and high capacity. They can handle difficult-to- disperse systems, such as those having high interfacial tension and/or large phase density difference. They can also cope with highly viscous liquids and solid-liquid slurries. The main disadvantages are their size and the inventory of material held up in the equipment. For multiple unit operations, considerable capital costs may be needed for pumps and pipings. Mixer-Settler The mixer and settler can be integral or separate. The operation may be continuous or batchwise.  Decanter Settlers (or sometimes known as decanter), can be as simple as involving simple dispersion, where the entering liquid is dispersed into droplets. The droplets are then allowed to settle by gravity in the main part of the vessel. Most settlers consist of a horizontal vessel, because the separating efficiency is proportional to the area of the phase interface. To increase the size of the droplets and hence their settling rate, the dispersion may be passed through a coalescer (packing or wire mesh) Decanter