22381091 Liquid Liquid Extraction

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<ul><li><p>Overview</p><p> Introduction</p><p> Important aspects in design and operation of the extraction processes.</p><p> Selectivity and Distribution coefficient</p><p> Extraction techniques</p><p> Extraction apparatus and equipment</p></li><li><p>Definition</p><p>Liquid-Liquid extraction is a mass </p><p>transfer operation in which a liquid </p><p>solution (the feed) is contacted with an </p><p>immiscible or nearly immiscible liquid </p><p>(solvent).</p></li><li><p>.Diluent A and a solute B which are miscible. </p><p>.</p><p>The objective is to recover the solute</p><p>A + B</p><p>S</p><p>A + eB</p><p>S + B</p><p>Raffinate</p><p>Extract</p><p>Two streams result from this contact: the extract, </p><p>which is the solvent rich solution containing the </p><p>desired extracted solute, and the raffinate, the </p><p>residual feed solution containing little solute.</p></li><li><p>Extraction</p><p>Partition between two phases</p><p>Liquid/solid, </p><p> gas/liquid, </p><p> liquid/liquid</p><p>Partition based on relative solubility in the</p><p>two solvents</p><p>Two techniques:</p><p>Macroscale, using a separatory funnel</p><p>Microscale, using pipets and centrifuge tubes</p></li><li><p>Extraction Process</p><p>(in macroscale)</p><p> Mixture dissolved in </p><p>solvent 1 &amp; placed in </p><p>separatory funnel</p><p> Immiscible solvent 2 added, stoppered &amp; </p><p>agitated</p><p> Mixture components</p><p>distribute according to</p><p>solubility</p><p> Bottom layer drained</p><p>through stopcock to </p><p>separate phases</p></li><li><p>Microscale Extraction</p><p> Mixture dissolved in solvent 1 in a</p><p>centrifuge tube</p><p> Immiscible solvent 2 added &amp; agitated</p><p> Bottom layer</p><p>removed with</p><p>Pasteur pipet &amp;</p><p>transferred to clean</p><p>tube</p></li><li><p>The following need to be carefully evaluated</p><p>when optimizing the design and operation of</p><p>the extraction processes. </p><p>Solvent selectionOperating Conditions (pH,T,P &amp; residence time)Mode of Operation Extractor Type Design Criteria </p></li><li><p>Selection of solvent influenced by</p><p> -its physico-chemical properties (allowing an easyrecovery of the solute or of the solvent),</p><p> -a negligible solubility of the solvent in the diluent(post-extraction processes have to be as cheap aspossible),</p><p> -physical characteristics offering acceptabledispersion and separation times of the post-contactphases (viscosity, interfacial tension, densitydifference compared to the feed),</p><p> -favorable properties (Mass transfer kinetics equilibrium after contact less than a few minutes - ;Economy cheap and available solvent - ; Safety ofuse low toxicity, low flammability, low volatility,low corrosion in comparison with usualconstruction materials -),</p></li><li><p>But particularly a property called selectivity</p><p>Selectivity can be defined as the ability of the solvent to pick up the desired component in the feed as compared to other components. The desired properties of solvents are a high distribution coefficient, good selectivity towards solute and little or no miscibility with feed solution. Also, the solvent should be easily recoverable for recycle. </p><p> =/ (mass fraction A in E) &gt; 1 </p><p>/ (mass fraction A in R)</p><p>This ratio is called the As for the second ratio, it is </p><p>distribution coefficient of always greater than 1 (there </p><p>the solute between the is more diluent in the raffinate</p><p>extract and the raffinate than in the extract</p><p>phases, m</p><p>The higher the m, the higher and thus the more selective the solvent. </p><p>Consequently, a sufficient condition for a solvent to be selective is: m greater than 1</p></li><li><p>Distribution ratio</p><p>In solvent extraction, a distribution ratio is often quoted </p><p>as a measure of how well-extracted a species is. </p><p>Partition or Distribution Coefficient</p><p>KD or D = []</p><p>[] = </p><p> = </p><p> / </p><p> ( /) = </p><p>( )</p><p> For typical extractions: solvent 1 = organic, solvent 2 = water </p><p>Example: solubilities of adipic acid @ 15C water (1.5 / 100 ml) ; ether (0.6g / 100 ml) </p><p> KD = .</p><p>. </p><p> = 0.40 </p></li><li><p>Solvent Distribution </p><p>Coefficient @ </p><p>20C </p><p>Miscibility with </p><p>water </p><p>wt% @ 20C </p><p>n-Butanol 1.6 &gt;10 </p><p>Ethyl Acetate 0.9 10 </p><p>MIBK 0.7 2.0 </p><p>Toluene 0.06 0.05 </p><p>n-Hexane 0.01 0.015 </p><p>No solvent offers all possible favorable conditions. Thus, a </p><p>compromise has to be found between all the constraints. </p><p>Solvents for Acetic Acid Extraction </p></li><li><p>Four extraction techniques</p><p>Single</p><p>stage</p><p>Cross-</p><p>current</p><p>Counter-</p><p>current</p><p>Counter-</p><p>current </p><p>with reflux</p><p>To design an extraction apparatus, it is necessary to: </p><p>- determine the number of ideal stages</p><p>- determine the phases flowrates, as well as the </p><p>solute distribution between the phases</p><p>- choose the most adapted apparatus </p><p>- study the hydrodynamics of the apparatus </p><p>- determine the size and the configuration of the </p><p>apparatus</p></li><li><p>EquipmentMixer-Settler</p><p>Centrifugal Extractor</p><p>Static / Agitated Columns</p></li><li><p>References</p><p>1. http://iweb.tntech.edu/chem311dc/</p><p>LabPDF/Extraction.pdf</p><p>2. Paul Ashall 2007. Liquid-liquid extraction</p><p>principle</p><p>3. Nadine LE BOLAY, Gilbert CASAMATTA. </p><p>Liquid extraction</p></li></ul>