Liquid extraction INTRODUCTION   INTRODUCTION SUMMARY PAGES Introduction General principles 2 3 RETURN TO Nadine LE BOLAY Gilbert CASAMATTA GENERAL SUMMARY

S E P S E P Reactor … INTRODUCTION Reagents Products + Reagents Energy 2 Retour INTRODUCTION Introduction sommaire INTRODUCTION A process is often made up of a reaction step followed by separations. When analyzing the process in details, one can see that it consists of a reactor fed with reagents and energy. The products of the reactions (main reaction, but also secondary reactions) flow out of the reactor, as well as the reagents that have not reacted. The purpose is then to separate the different products leaving the reactor. In that way, separation equipment (generally several items) are implemented. Reagents S E P S E P Products + Reagents Reactor … Energy In most cases, distillation is carried out. However, in certain conditions, it is not suitable. Then, one may choose other separation techniques, such as absorption, adsorption or liquid extraction, also called solvent extraction. It is particularly the case when: Liquids have boiling points with are close to each other Their boiling points have a high value The mixture forms an azeotrope Distillation must be performed under high vacuum conditions The products are heat-sensitive (recovery of antibiotics or vitamins) The mixture to be processed is a very diluted aqueous solution (the latent heat of vaporization of water is very high) … We will study the case of liquid extraction which is a physico-chemical separation of constituent mixtures during a homogeneous phase

S S + B A + B A + eB Extract Raffinate GENERAL PRINCIPLES 3 Retour GENERAL PRINCIPLES GENERAL PRINCIPLES Généralités sommaire Let us consider a homogeneous liquid mixture constituted by a diluent A and a solute B which are miscible. The objective is to recover the solute. Let us add a liquid solvent phase, non miscible with A. Under the effect of agitation, the solvent phase is dispersed as droplets in the diluent phase. A part of the solute is transferred from the diluent phase to the solvent phase. When agitation is stopped, the two phases are separated by decantation, the lighter phase (it will be supposed here that the solvent is lighter than the diluent) being concentrated in the upper part of the vessel. The solvent phase is called the extract , the diluent phase is called the raffinate. The two phases are at equilibrium. This equilibrium is defined by an ideal stage. S A + eB S + B Extract A + B Raffinate A stage is considered as ideal when the contact between the phases is long enough in order to reach the solute distribution equilibrium between the extract and the raffinate

S + B + eA A + eB + eS S E P Raffinate Extract S E P 4 In practice, the diluent and the solvent are often partially miscible. A small volume of the diluent may thus be contained in the extract, while a small volume of the solvent may be present in the raffinate. The extraction will thus be followed by as many separations as necessary in order to recover the different constituents. At that stage, separations may eventually be distillations if the limitation imposing extraction use no longer exists (for example, the solvent and the solute do not form an azeotrope which existed between the diluent and the solute). The phases containing mainly the solvent can be recycled in the extraction apparatus. S E P S + eB (recycling) B + eS eA eS + eA (recycling) A + eS eB A + eB + eS S + B + eA Raffinate Extract S E P Numerous solvents are used in liquid extraction. However all of them are not suitable. The feed phase being imposed, the solvent choice must be optimized. This choice is influenced by: -its physico-chemical properties (allowing an easy recovery of the solute or of the solvent), -a negligible solubility of the solvent in the diluent (post-extraction processes have to be as cheap as possible), -physical characteristics offering acceptable dispersion and separation times of the post-contact phases (viscosity, interfacial tension, density difference compared to the feed), Retour sommaire

[ ] b = b = Solute B concentration Diluent A concentration 5 -favorable properties (Mass transfer kinetics – equilibrium after contact less than a few minutes - ; Economy – cheap and available solvent - ; Safety of use – low toxicity, low flammability, low volatility, low corrosion in comparison with usual construction materials -), but particularly a property called selectivity. The selectivity b is defined as the ratio: b = [ Solute B concentration Diluent A concentration ] Raffinate Extract The necessary and sufficient condition for an extraction to be possible is that b is greater than 1. The higher the value of b compared to 1, the easier the extraction. The selectivity can be written in a different form: b = Concentration of diluent A/R Concentration of diluent A/E Concentration of solute B/E Concentration of solute B/R x This ratio is called the distribution coefficient of the solute between the extract and the raffinate phases, m As for the second ratio, it is always greater than 1 (there is more diluent in the raffinate than in the extract) Consequently, a sufficient condition for a solvent to be selective is: m greater than 1. It should be noted that the higher the value of m, the higher the value of b and thus the more selective the solvent. No solvent offers all possible favorable conditions. Thus, a compromise has to be found between all the constraints. Retour sommaire

Single stage Cross- current Counter- current Counter- current 6 Once the solvent has been chosen, the extraction process has to be defined and designed. Four extraction techniques will be studied in this course: Single stage Cross- current Counter- current Counter- current with reflux To design an extraction apparatus, it is necessary to: - determine the number of ideal stages - determine the phases flowrates, as well as the solute distribution between the phases - choose the most adapted apparatus - study the hydrodynamics of the apparatus - determine the size and the configuration of the apparatus In the following chapters, we will learn how to determine the number of ideal stages, the transfer of the solute in the extract, and the flowrates of the phases with respect to the feed flow rate. The other concepts will be studied in another course. Retour sommaire

END OF THE INTRODUCTION 7 The course will be divided into 6 chapters: - CHARACTERISTICS AND PROPERTIES OF SOLVENTS - LIQUID EQUILIBRIUM - SINGLE STAGE EXTRACTION - CROSS-CURRENT EXTRACTION - COUNTER-CURRENT EXTRACTION - COUNTER-CURRENT EXTRACTION WITH REFLUX END OF THE INTRODUCTION Retour sommaire