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What is extraction? Liquid-liquid extraction is one of the many techniques available to the chemical, pharmaceutical and allied industries to allow separation of products from one another. Liquid-liquid extraction is a technique closely-related to distillation — and is often combined with it in commercial processes — but liquid-liquid extraction does not use volatility differences to drive the separation. Instead, it uses the difference in solubility of one of the components in the feed mixture between the feed solvent and a second, immiscible solvent. The key to successful extraction is, firstly to choose a solvent with an affinity for the product to be separated, and secondly to provide equipment for the extraction which promotes an intimate contact between the feed liquor and solvent. The efficiency of extraction is then determined by the difference in solubility of the product of interest between the two solvents and the efficiency of the contacting equipment used.

Why use liquid-liquid extraction? Liquid-liquid extraction is a very widely used technique for: • Difficult separations • Effluent treatment problems • Recycling from waste streams Its main uses can be categorised as follows: 1 Separating products with small relative volatility differences.

In mixtures of this kind, distillation will often prove uneconomic either because it requires a column with a very large number of theoretical plates or because it means the operation of a column at high reflux ratio — which would need an un-acceptably high energy input.

QVF Process Systems Ltd

Liquid/liquid extraction systems from QVF By correct choice of solvent, the extraction can be made very simple giving a good separation with low energy input and small equipment. The component of interest would then be separated from the second solvent by distillation, although again, correct choice of solvent will allow this stage to be very straight-forward.

Examples include:

• Lube oil refining • Separation of aliphatic and

aromatic compounds • Acetic acid/water 2 Separating products which

would require high energy input in distillation. As well as the type of mixtures re-ferred to in section (1) above, where high energy input is required because of high reflux ratios, high energy input is also required when recovering components from very dilute aqueous streams.

Since extraction can be used to give a high concentration stream in a second solvent, the energy input to a subsequent distillation can be substantially reduced.

Examples of this include:

• Effluent treatment • Solutions of phenols and cresols • Solutions of aniline or other

aromatic a mines 3 Separating products which form

azeotropes. This type of system cannot be separated by conventional distillation because of the formation of a constant-boiling, constant-composition mixture. They can sometimes be separated by a more complicated distillation employing a third agent to alter the vapourliquid equilibrium conditions — a process which is called Extra ct/ye orAzeotropic distillation. However, this type of process is not always possible and even when it is, liquidliquid extraction can very often be shown to be more economic

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In aqueous systems, typical ex-amples include separating the following materials from water:

• Tetrahydrofuran • Pyridine • Formic acid

In organic systems, typical ex-amples include the following separations:

• Dichloromethane/methanol • Ethyl acetate/ethanol • Alkyl chlorides/alcohols 4 Separations in aqueous mixtures

Containing inorganic salts. Here, extraction is used to selec-tively remove, for example, an organic product from a reaction mixture where inorganic salts have also been formed during reaction. Distillation would be difficult be-cause both the water and the product may both be volatile or the product of interest may not be very volatile and will remain with the salts in the distillation bottoms stream.

By correct choice of solvent and equipment, a separation which po-tentially could have been very difficult by other means can be successfully carried out.

Example processes include:

• The oxidation of organic products • The production of caprolactam • The synthesis of organic acids 5 Separation of heat sensitive

materials. In this type of system, distillation is not acceptable since the temp-eratures required in the distillation process may cause, breakdown, polymerization or loss of biological activity in the product of interest.

Liquid-liquid extraction can solve the problem. Choice of solvent will be very important, particularly if a subsequent distillation is needed to separate the product from the extraction solvent. In this type of problem a solvent

must be chosen that either has a very low boiling point or can be separated by techniques such as thin film evaporation from the product.

In the food and pharmaceutical industries, examples include ex-traction of:

• Glycerol • Caffeine • Vitamins • Penicillin • Flavours and fragrances

and in the chemical indust,y, the extraction of:

• Aldehydes • Certain organic acids 6 Separation of non-volatile or

high-boiling compounds These will either be impossible, or very difficult to separate by distilla-tion (due to high temperatures, low pressures or high energy input). However, liquid-liquid extraction can be used to substantially reduce energy input by operating with much higher concentrations of the product of interest in the solvent than in the original feed material.

Examples include inorganic products such as:

• Hydrogen peroxide • Mineral acids

Organic acids such as: • Lactic, maleic, salicylic and citric

acids • Cyanoacetic acid • Thioglycollic acid

High boiling organics such as: • Phenols, aniline, aromatic amjnes

and aldehydes 7 Hydrometallurgy

Liquid extraction is a very well-established technique in this area

where solvents are selected to chelate or solvate metal ions. Reversal of the process yields an aqueous phase from which pure metal compounds or metals may be recovered.

For example:

• Copper • Uranium • Precious metals • Rare earths • Zinc • Nickel/Cobalt

Chromium/Vanadium separations

8 Other difficult separations

In general, liquid-liquid extraction should be considered as a possible solution to a wide range of sep-aration problems which are difficult by other means.

For example, removing Dimethyl Formamide or Dimethyl Acetic Acid from water. These products tend to hydrolyse in water at the temperatures that would be re-quired for distillation. This means that they become contaminated with the free acids (formic or acetic in this case) causing a need for extra separation and loss of product yield. By extracting at normal temperatures into a non-aqueous solvent, these difficulties can be overcome.

Summary of the main benefits of liquid-liquid extraction • Low energy consumption • Low running costs • Low capital costs • Ability to produce very pure

products • Ability to operate successfully with

dilute feed solutions • Ability to perform separations

which would be very difficult by other unit operations

Product Information

The QVF approach

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Liquid liquid extraction is a specialised technology requiring understanding of the fundamentals of the technique, a range of pilot equipment and an understanding of the factors determining the choice of the best equipment to perform a given extrac-tion at commercial scale. This combination of expertise, pilot plant, experience and a full range of equipment designs is the key to solving liquid-liquid extraction problems cost-effectively and right the first time. QVF offers all four in a unique unrivalled approach.

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QVF experience and expertise QVF offers the benefit of many years practical experience in solving extraction problems combined with know-how in the fundamental techniques which underlie this technology. In addition, close links are maintained with UK and other international centres of academic expertise in the field, such as the University of Bradford. Over the last 10 years QVF has developed considerable experience in a number of liquid-liquid extraction systems. Many case histories are available and a selection have been included with this brochure. However, liquid-liquid extraction is rarely used as a stand-alone unit operation. Frequently distillation is required to separate the product from the extraction solvent. QVF Process Systems is not only able to design and supply the extraction stage, but also any subsequent distillation units, pumps, vessels and pipework to form a complete package. With installation and commissioning also available, a complete turnkey service is offered.

Materials of construction QVF Process Systems, best known as the market leader in the supply of borosilicate glass process equipment, is by no means restricted to glass as a material for extraction equipment. Extraction columns have been supplied, for example, in stainless steel, titanium or other materials where these best suit the process.

Commercial equipment QVF offers an unrivalled range of equipment designs to provide the best answer to Customers’ problems. Each of the most important types of equipment is described in more detail on the following pages:

• The mixer-settler

• The pulsed sieve plate column

• The Karr column

• The rotating disc contactor

• The QVF rotary-agitated cell extractor (RZE)

Each type of contactor has its own specific advantages and dis-advantages which QVF is able to assess in a totally impartial way to ensure that the final plant provides the optimum solution. If you have a difficult separation problem, liquid-liquid extraction may be the answer — let QVF use experience and expertise to evaluate and then to solve your problem.

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Product Information QVF offers a full range of laboratory and pilot extraction equipment ready for use in the trials Area at our Mainz.factory in Germany. Coupled with expertise in the field, this equipment allows evaluation of equipment performance to produce optimised full-scale designs in a highly professional and cost-effective manner. The importance of pilot work Pilot scale work will still be necessary to ensure the correct design of full-scale equipment in spite of having a good understanding of the fundamental concepts underlying mass transfer in liquid-liquid extraction.This is because of the uncertain effects of non-ideal flow patterns, difficulty in predicting drop sizes and the effects of trace impurities on the equilibrium conditions in the equipment. Of course, the use of glass as a material of construction for pilot and even full scale equipment is of great help because it allows visual moni-toring of operating phenomena such as drop size, drop breakage etc. The understanding of these parameters is vital to the correct performance of fullscale equipment.

Pilot work must be carried out in a highly logical, structured, and scientific manner to enable the problem to be solved in the most cost-effective way. 1 Laboratory and small-scale

work Firstly, the physical properties are measured at a number of equili-brium conditions. From this data an estimate can be made of the maximum drop size that is likely to occur at these conditions. From this information a selection of equipment for further investigation is made.

On the basis of these results, single drops are then studied in a small scale short column in order to examine phenomena such as drop velocities, drop breakage and mass transfer coefficients which together allow estimation of the number of stages/transfer units and of the column diameter required to achieve the desired separation.

QVF pilot plant facility This preliminary investigation re-duces the amount of pilot scale experiments required and minimises the requirement for the process fluids — which may be expensive or in extremely short supply. Pilot work Having established the nature of the mass transfer processes involved from the laboratory work, the process is then taken onto pilot scale equipment to put the predic-tions to the test. Pilot plant trials can be performed using many types of contactor to ensure that the equip-ment proposed will be the best for the duty and right the first time. The process is piloted on the pro-posed equipment to achieve the actual desired separations, thus ensuring that the height and dia-meter of the column are correct.

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Equipment available Ready for use in The Customer Trials Area at QVF Process Systems’ UK based headquarters in Stone, Staffordshire are the following items of liquid-liquid extraction pilot plant, together with the necessary ancillary and laboratory equipment: Mixer settler A two stage, DN100 mixer settler complete with feed pumps, vessels etc, mounted on a wheeled structure. This unit may be easily trans-ported to customers’ own premises, if preferred. Pulsed sieve-plate column A DN5O glass pulsed sieve plate column with stainless steel sieve plates (alternative fractional free areas available) and a PTFE piston type pulsing unit. Karr column A DN5O glass reciprocating plate column with PTFE or stainless steel plates. Rotating disc contactor A DN8O glass RDC with stainless steel internals Rotary-agitated cell extractor (RZE) A DN100 glass bodied RZE with stainless steel internals. In addition to the above items, pilot plants are available for the con-centration of sulphuric and nitric acids, various distillation and reaction equipment, and a climbing film evaporator. A purpose-built pilot plant unit to meet customers’ own specific requirements can easily be installed — please contact QVF Process Systems for further details.

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The mixer-settler The design of a typical mixer-settler is shown in the diagram below, It is basically a two-piece item consisting of a mixing zone and a settling zone. In the mixing zone the two phases are brought into intimate contact by the action of a high-speed mixer. This is used to create large interfacial areas to allow a high rate of mass transfer between the phases. Hence, in this section a single equilibrium stage is approached where mass transfer takes place. In the settling zone, the two liquid phases are allowed to separate and form two distinct and discrete layers. These layers are then discharged either as products, if sufficient mass transfer has taken place, or pass to the next mixer-settler if a further contact stage is required. In multi-stage systems, it would be usual to operate in a counter-current fashion i.e. the last mixer-settler stage would be fed with the feed solution, depleted in the previous stages along with clean solvent. Certain features are very important to ensure good performance in the mixer-settler. These have, of course, already been incorporated in the design of all QVF mixer-settlers.

1 Mixing system

The QVF mixer consists of a special glass mixing zone, a PTFE turbine agitator and a variable speed drive. As well as providing an intimate dispersion the stirrer provides the necessary pumping action. For this reason the position of the stirrer is of great importance — as low as possible in the mixing zone. This eliminates the need for external pumps and ensures a large inter-facial area for mass transfer.

The energy required to produce this dispersion will depend very much on the characteristics of the material and the system used. (Interfacial tension, viscosity, flows etc).

Between the mixing and separation zones, a weir has to be incorporated to prevent back mixing taking place. For units up to 300mm nominal bore, this weir is made of glass and from 450mm from PTFE.

2 Settling system

The settler comprises a horizontal cylindrical pipe which, depending upon size may be one or more pipe sections. If special separation aids (porous media, mesh, plates etc) are required, these may be installed in an additional pipe section at one end of the settling zone.

From the settler, the outlet of the heavy phase is controlled either by a simple overflow (fixed or variable height) or a control valve.

Product Information

Additional energy input is introduced by pulsing the liquid.

The pulsed sieve plate column

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In a pulsed sieve plate column the two phases flow countercurrently under gravity — the heavy phase down the column and the light phase up the column. The internals are sieve plates — similar to those used in distillation but without downcomers. The plates break up the two phases into distinct droplets in order to increase the area available for mass transfer.

This increases the time available for mass transfer and further breaks up the droplets to increase the area available for mass transfer. The active section of the column contains packs of sieve plates which have been pre-assembled into modules before being fitted into the column sections. These modules are clamped between the glass buttress ends of each column section. The diameter, length and free area of the plates will depend on the throughputs, physical properties and the desired separation. At both ends of the extraction zone, the diameter may be increased to provide a larger cross sectional area for coalescence and separation of the two phases. Pulsed sieve plate columns are available in glass from 50 to 600mm with fractional free area of the plates between 7% and 50%. Small aperture ratios are best for low interfacial tension systems and larger holes for high interfacial tension systems. These contactors are very suitable for systems that emulsify since agitation frequency and amplitude can be controlled to suit

Materials of construction The standard QVF pulsed sieve plate column has a body of borosilicate glass containing a stainless steel sieve plate assembly. The pulse is supplied by means of a PTFE bellows type puls-ing unit. Sieve plates can be supplied in other materials if required, and the column itself may be fabricated from stainless steel or other materials e.g. coated steel. In addition to the excellent corrosion resistant properties of borosilicate glass, visibility is a further advantage, particularly with liquid extraction systems where it is desirable to be able to observe the position of the interfaces, the dispersion band and the droplet sizes.

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The Karr column It is the intention of the Karr column to provide as large an exchange surface as possible. The Karr column is conceptually similar to the pulsed sieve plate column except that instead of the fluids pulsing, the plates reciprocate. The participating phases are in countercurrent flow through a series of perforated plates which vibrate axially in the column at a specified amplitude and frequency. The shear stress created by this movement disperses the discontinuous phase. The separation effect of a Karr column corresponds to that of a pulsed sieve plate column of similarly designed internals if they are used under the same operating conditions. Generally, however, the holes in the plates tend to be larger and thus there is a certain preference for using the Karr column if there are also solids present during the extraction process.

Construction The basic construction of the Karr column is very similar to the other gravity columns but with the addition of mechanical energy reciprocating the plates. The plate pack used, which normally has a free area of about 50%, is constructed on a modular basis. A single module essentially consists of perforated plates at a spacing of 50mm. The plates may be fabricated from a wide variety of materials, selected for their wetting or corrosion resistance properties e.g. stainless steels, titanium, PTFE or other plastic materials. The installed plate pack is moved up and down by means of a continuously variable, direct-coupled geared motor and crank mechanism. The stroke and frequency can be altered within the predefined range of the drive unit.

Materials of Construction Karr columns can be supplied up to 600mm nominal bore with aglass body and metal or plastic internals. Plates can be made fromstainless steel, hastelloy, titanium, tantalum or plastic materialssuch as PTFE,PVDF etc. Borosilicate glass is an ideal material of construction for liquid –liquid extraction systems. Its transparency allows the drops to beobserved and the process controlled based upon these visualobservations.

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The rotating disc contactor is a useful, widely applicable contactor, suitable for many liquid-liquid operations. It can be used for a wide variety of separations and under a wide variety of process conditions. The RDC is an agitated extractor in which the dispersion is achieved by means of external mechanical energy, namely rotation. It comprises a ver-tical shell — similar in construction to all the other gravity column contactors produced by QVF Process Systems — in which horizontal annular stator rings are installed. In the middle of the compartments formed by the stators, a series of discs attached to a central rotating shaft is fitted. The diameter of the rotors is less than the diameter of the stators, thus allowing the whole assembly to be installed and removed in one piece. The rotor serves as a means to convey the mixing energy to the dispersed phase. The actual flow patterns achieved are rather complex but it is generally assumed that the cells are well mixed. Additionally, there will be a rotation of the fluid around the shaft. Except in the case of the larger diameter units, or at very high throughputs, the amount of axial mixing (a reverse flow between the cells) will not be signifi-cant. The countercurrent flow of the two phases will be superimposed on this cyclonic flow. In the extraction zone, the flow pattern described exerts con-siderable shear forces on the two phases, breaking up one of the phases into small droplets. The size of these droplets will be a function of rotor speed and position in the column, although after a number of stages which may be estimated from bench data the drop size will be approximately constant with no breakage occurring. At each end of the column separating vessels of larger diameter may be provided to facilitate coalescence. A gear motor with stepless speed control is mounted at the top of the unit driving the rotor shaft. This drop breakage leads to a con-tinuous provision of new exchange surface and excellent mass transfer performance.

The rotating disc contactor

Materials of construction The standard RDC comprises a borosilicate glass shell and separating vessels with stainless steel internals. The only other wetted components are of PTFE. In cases where glass cannot be used, due to large diameters or higher operating pressures, rotating disc contactors are available with all metal column components. Also, the internals can be fabricated from other materials (titanium, hastelloys, tantalum,plastics etc) on request.

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The stirred cell extractor is a special development by QVF. The basic construction is similar to that of the family of rotary column contactors. The diameter and active height of the column are selected depending upon the throughput and desired separa-tion. At both ends of the column the diameter may be increased to form separating vessels to aid coalescence of the dispersed phase. The internals of the column are modular in construction — individual modules being joined together to give the required active length. The inter-nals comprise annular stators, similar to the RDC, but with less free area and a notched weir around the inner edge. Between the stators, on the central rotating shaft is a series of agitators to provide the mixing energy in each cell. The agitators disperse one of the phases into droplets in each cell. The weir encourages the phases to settle out at each stator — the light phase underneath the stators and the heavy phase above — forming several classical mixer-settler zones within the column. The maximum throughput of the QVF rotating cell extractor is dependent mainly on the free area of the stator plates, the rotational speed of the agitators and the properties of the materials used. As the speed of the rotors is increased, the size of the droplets decreases and the maximum specific load of the column is also decreased. A further feature of the QVF RZE is the larger turndown compared with other column types. This feature is due to the notched weirs which permit an ordered flow of the two phases from cell to cell, even under low throughput conditions Materials of construction Whilst the body of the standard RZE would usually be constructed from borosilicate glass, columns can be fabricated from other materials on request. The standard internals are stainless steel but again can be fabricated from other materials if required.

The QVF rotary-agitated cell extractor (RZE)

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Other liquid-liquid extraction equipment from QVF In addition to the above range of contactors, the following may be supplied.

Packed columns It may sometimes be possible to use a simple packed column for the separation ,either with or without pulsation. Packed columns can be supplied with either glass Raschig rings or any other ceramic , metal or plastic packing as appropriate. However unlike in distillation, in liquid-liquid extraction it is desirable to use a smaller size of packing. Unpulsed packed columns do no often achieve high efficiency except with very low interfacial tension systems for which packed column are particularly suitable. To improve this situation, it is possible to add pulsation. This improves the dispersion ir the column and leads to an increase ir mass transfer and throughput by providing a fresh mass transfer surface on each pulse. Advantages • Simple • Low cost if small diameter • Good throughput • Flexible in turndown Limitations • Difficult to clean • Wetting characteristics may

change • The drop size cannot be

changed unless pulsation is used

Interface detector and controller In some cases automatic control of a liquid extraction column will be required, for which we supply an interface detector and controller comprising a probe and a signal converter/controller. The probe works by capacitance, and the controller produces a 24V signal to operate a magnetic isolation valve in the heavy phase offtake. Alternatively, a pneumatic isolation valve may be used in the process fluid and 24V solenoid used to control the air supply.

Product Information Tesef •••

•• TroaS

Solvent suitability and s

Selection of extraction unit

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he selection of an optimal xtraction unit is dependent upon everal conditions relating to any xtraction process. The major

actors under consideration are:

Space availability Efficiency of extraction Solvent suitability and recovery

facilities Physical properties of the system Capital cost/operational costs

he chart below illustrates the elative advantages of various types f extraction systems which are vailable from QVF Process ystems.

Space availability If the headroom is restricted then a horizontal type unit such as the Graesser contactor or mixer-settler is a good choice, whereas if floor space is at a premium a vertical unit would be a better choice. Efficiency of extraction For simple systems with a small number of stages and low throughputs, packed or plate columns are the most desirable choice. However, as throughput becomes of greater importance and more stages of separation are required, mechanicallyagitated equipment should be specified.

recovery facilities Solvents should be selected with respect to their relative partition or distribution coefficients for the sy-stem under consideration Physical properties Perhaps the most important para-meters in the selection of extraction equipment are the actual physical properties of the solutions concerned; densities, viscosities, and the inter-facial tension. QVF Process Systems are able to select the most appropriate contactor to meet all the criteria and to provide the best equip-ment to meet the required duty. Capital cost/operational costs Frequently the selection of the cheapest plant leads to the highest unit operational cost and the choice between capital cost and operational cost is not easy. This chart is an aid to the initial choice of an extractor. It is advisable, except on very simple systems, to execute pilot plant trials in order to determine the design of the full scale plant and QVF Process Systems would be pleased to advise you. QVF Process systems pursues a policy of continuous product improvement . we therefore reserve the right to alter any product or process as described and illustrated. © 2005 QVF Process Systems Ltd

Tollgate Industrial Estate Stafford ST16 3HS

England Tel +44 1785 609900 Fax +44 1785 609899 Items shown in bold type are available

from QVF Process Systems Ltd E-mail qvf@qvf.co.uk