serial extraction

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Slide 1 :

Extraction

Slide 2 :

Extraction Process in which two phases come into contact Objective is to transfer a solute or particle from one phase to the other Phases are usually immiscible liquids Solute is in soluble form Special cases Liquid-solid phase Extraction of caffeine from coffee beans

Slide 3 :

Extraction Organic solvents often used as the extracting liquid Typically extract low MW antibiotics Organic solvents not a good choice for proteins Use polymer or polymer/salt mixtures Usually precedes a high resolution step, ie. chromatography Extraction can result in significant volume reduction

Slide 4 :

Extraction Equilibrium Extraction depends on the partitioning of the biomolecules between liquid phases Miscibility of two liquid phases Rate of equilibration of biomolecules between two phases Single-stage extraction: one feed stream contacts one extraction solvent Mixture divides into equilibrium extract and raffinate phases Distribution of solute at equilibrium is defined as the partition coefficient K = y/x

Slide 5 :

Extraction Equilibrium K = y/x y – concentration of solute in extract phase x – concentration of solute in raffinate phase Extract S, y1 Feed F, xf Extraction solvent S, ys Raffinate F, x1

Slide 6 :

Extraction Equilibrium Desirable to have K as large as possible K = 1 require large volumes and many serial extractions K = 0 indicates no extraction at all K depends on many factors Size of molecule being extracted pH Types of solvent Temp. Concentration and MW of polymers or salt in phases

Slide 7 :

Estimating K M – MW of molecule being partitioned K – Boltzmann constant T – absolute temp. ? - constant that includes characteristics of both phase system and partitioning substance

Slide 8 :

Countercurrent Stage Calculations Often more than one equilibrium stage is necessary to achieve the desired separation Feed and solvent usually run countercurrent to each other…Why? Concentration difference is the driving force for separation The solute concentration difference between the raffinate and extraction phases is greatest in countercurrent flow

Slide 9 :

Countercurrent Stage Calculations Can be performed graphically and analytically for each stage For n stages Streams leaving each stage are in equilibrium Streams are numbered according to the stage they are leaving Feed enters at stage 1 and leaves at stage n Extraction solvent flows in the opposite direction Once feed has entered the stage, it is known as raffinate

Slide 10 :

Countercurrent Stage Calculations Assumptions Two solvents are immiscible or already in phase equilibrium Solute concentrations are sufficiently low that the flow rates of raffinate and extract are constant Equilibrium is achieved in each stage F = flow rate of feed or raffinate phase S = flow rate of extract phase

Slide 11 :

Countercurrent Stage Calculations Alternatively yn and xn-1 are concentrations of passing streams on a line of slope F/S – the operating line Determine the number of stages graphically using a plot of y vs. x together with a plot of the operating line

Slide 12 :

Graphical Solution The operating line with slope F/S intersects the x-axis at the point (xn, ys) ys = 0 if the extraction solvent is initially pure (solvent free – not the case if the solvent is recycled) “Step-off stages” beginning on the operating line at (xf, y1) by drawing a horizontal line to the equilibrium curve, followed by a vertical line to the operating line Continue until ys is reached

Slide 13 :

xf, y1 xn, ys x1,y1 xn, yn 5 stages

Slide 14 :

Analytical Solution If K is a constant E is the extraction factor

Slide 15 :

Analytical Solution As n ? ? xn ? xf/En ? 0 For E = 1.0 For E < 1.0 and n ? ? xn ? (1-E)xf

Slide 16 :

Scale-up and Design Mechanically agitated columns Reciprocating plate Centrifugal extractors Overall stage efficiency (# of theoretical stages/ # of actual stages)x100% Height equivalent to a theoretical stage (HETS) Height of extractor/ # of theoretical stages

Slide 17 :

Reciprocating Plate Columns Columns that handle liquids with suspended solids or mixtures that emulsify easily Interdispersion achieved by reciprocating or vibrating plates Size based on # of theoretical stages, diameter of plates, and reciprocation rates

Slide 18 :

Reciprocating Plate Columns For columns of 1, 3, 12, 36” In scale-up, hold S/F, (S+F)/A, plate spacing, and stroke length constant Volumetric efficiency = [(S+F)/CSA]/HETS

Slide 19 :

Centrifugal Extractors Avoid emulsions and separate liquid phases with small density differences Use concept of equivalent time Ratio of feed volume:solvent volume is kept constant Calculate contact time, t, assuming GT is constant See Table 6.3

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