Question

Name pescribe the process of solvent extraction. What are the properties to look for in an extractant? Conduct a mass balance on a single stage of a solvent extraction proce Show how outlet heavy phase concentration is related to other process variables

Answer 1

solvent extraction also known as liquid liquid extraction is a method for the seperation of components of a liquid mixture by contacting with an immiscible or partially miscible solvent. The cause of the separation is the differential distribution of components between two insoluble phases. Suppose, a binary mixture consisting of solute C and carrier liquid A is extracted using a solvent B. The products that are seperated are the solvent rich phase called extract and the carrier rich phase called the raffinate.

the choice of solvent is made on the desirable charecteristics that are listed below:

• selectivity $\small (\beta )$ - the solvent should have good selectivity for the solute to be extracted. it should dissolve maximum amount of solute in minimum amount of solvent. No seperation is possible if selectivity is 1.

$\beta = \tfrac{\frac{X_C_E}{X_A_E}}{\frac{X_C_R}{X_C_A}}$ where $X_C_E$ =weght fraction of C in extract

$X_A_E$ =weght fraction of A in extract

$X_C_R$ =weght fraction of C in raffinate

$X_A_R$ =weght fraction of A in raffinate

• distribution coefficient (m)- it is the ratio of weight fraction of solute in the extract to that in the raffinate .larger the distribution coefficient smaller quantitiy of solvent required for extraction
• capacity - this indicates the quantity of solute that unit weight of solvent can dissolve. hence a good solvent not only gives a large value for distribution coefficient but also large possible concentration of solute in the extract.(i.e high capacity)
• solvent insolubility - solvent should be insoluble in the carrier A
• solvent recoverability - recovery of solvent from the extract is important to permit the reuse of solvent and to prevent comtamination of product
• high density difference between extract and raffinate phases to permit rapid coalescence and disengagement of phases and to achieve a high capacity in the extractor
• high interfacial tension between the phases for easier disengagement of phases
• chemical stability and no reactivity with the componenets in feed and material of construction of equipment.
• non toxic and non flammable
• solvent should be inexpensive and readily available
• non corrosive to common materials of construction
• low vapour pressure so that storage and extraction are possible at atmospheric conditions and loss by vaporisation is minimized.however should not be very low by which recovery of solvent by distillation becomes difficult.
• low viscosity leads to low power requirements for pumping and agitation

     

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