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Lecture 16

The document discusses liquid-liquid extraction systems with extract and raffinate reflux. Extract reflux involves sending the extract stream to a solvent recovery step, removing most of the solvent and returning a portion as extract reflux. Raffinate reflux involves withdrawing a portion of the raffinate and adding it with fresh solvent. Optimal reflux ratios can be determined. Triangular diagrams can show composition limitations due to plait points. Industrial extractors like mixer settlers and column extractors are selected based on design considerations like stages required and costs.

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55 views9 pages

Lecture 16

The document discusses liquid-liquid extraction systems with extract and raffinate reflux. Extract reflux involves sending the extract stream to a solvent recovery step, removing most of the solvent and returning a portion as extract reflux. Raffinate reflux involves withdrawing a portion of the raffinate and adding it with fresh solvent. Optimal reflux ratios can be determined. Triangular diagrams can show composition limitations due to plait points. Industrial extractors like mixer settlers and column extractors are selected based on design considerations like stages required and costs.

Uploaded by

Anas Nasir offical
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Separation Processes-I

(ChE-206)
Lecture No. 16
Liquid-Liquid Extraction systems
Extract and Raffinate Reflux
• Reflux:
• The reflux is the part of the condensed vapor
from distillation which is returned to the
process.
• Minimum reflux ratio: Infinite number of stages
• Total Reflux: Minimum number of stages
• Optimum reflux ratio
• A single-section extraction cascade can be
refluxed to resemble distillation.
Liquid-liquid extraction system with Extract
and Raffinate Reflux
• L is used for raffinate flows
• V for extract flows
• Stages are numbered from the solvent end of
the process.
• Extract reflux LR is provided by sending the
extract, VN, to a solvent-recovery step, which
removes most of the solvent and gives a solute
rich solution, LR + D, divided into extract reflux
LR, which is returned to stage N, and product D.
• At the other end of the cascade, a portion, B, of the raffinate, L1, is withdrawn
in a stream divider and added as raffinate reflux, VB, to fresh solvent, S.
• The remaining raffinate, B, is sent to a solvent removal step (not shown) to
produce a carrier-rich raffinate product.
• When using extract reflux, minimum and total reflux conditions,
corresponding to infinite and minimum stages, bracket the optimal extract
reflux ratio.
• Raffinate reflux is not processed through the solvent-removal unit because
fresh solvent is added at this end of the cascade. It is necessary, however, to
remove solvent from extract reflux.
Liquid-liquid extraction system with Extract
Reflux only
• The use of raffinate reflux has been judged to be of
little.
• The amount of raffinate reflux does not affect the
number of stages required.
• Accordingly, only a two-section cascade that
includes extract reflux will be considered.
Triangular Diagram
• For binary distillations, product purity may be limited by
formation of azeotropes.
• A similar limitation can occur for a Type I system when
using a two-section cascade with extract reflux, because
of the plait point.
• Consider the equilibrium data for the A–C–S system
where A is the solute and S is the solvent.
• The maximum solvent-free solute in the extract, achieved
by a countercurrent cascade with extract reflux, is
determined by the intersection of line SE’ , drawn
tangent to the binodal curve, from pure solvent point S
to solvent free composition line AC, giving 83 wt% solute.
Home work
• Example 8.2
Theory and Scale-up of extractor
Performance
• Industrial extraction equipment can be selected using the scheme as discussed in
previous lecture.
• Often in the chemical industry, the choice is between a cascade of mixer-settlers
and a multi-compartment, column-type extractor with mechanical agitation
• The main considerations being:
• Stages required
• Floor space and headroom available
• Capital and operating costs
• For biochemical applications, the choice may be between a Karr column and a POD.
• Different methods are used for estimating size and power requirements of these
four extractor types: mixer-settlers, column-type extractors, Karr column and POD.
Book
• Seader, J. D.; Henley, E. J.; Roper, D. K., Separation Process Principles:
Chemical and Biochemical Operations. 3rd Ed.; John Wiley & Sons,
Inc.: 2011.
• Chapter 8: Liquid–Liquid Extraction with Ternary Systems

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