liquid liquid extraction--_basic_principles

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Process Engineering Guide: GBHE-PEG-MAS-613

Liquid-Liquid Extraction: Basic Principles Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the information for its own particular purpose. GBHE gives no warranty as to the fitness of this information for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.



Process Engineering Guide: Liquid-Liquid Extraction: Basic Principles

CONTENTS SECTION 0 INTRODUCTION/PURPOSE 2 1 SCOPE 2 2 FIELD OF APPLICATION 2 3 DEFINITIONS 2 4 LIQUID-LIQUID EXTRACTION PROCESS 3 4.1 General 3 4.2 Choice of Solvent 3 4.3 Principles of Extraction 4 4.4 Liquid-Liquid Extraction Equipment 5 4.5 Operating Modes 6 4.6 Operating And Design Pitfalls 6 5 BIBLIOGRAPHY 9 FIGURES 1 SCHEMATIC OUTLINE OF LIQUID-LIQUID EXTRACTION

PROCESS: COUNTERCURRENT FLOW IN ROTATING DISC CONTACTOR 8

2 COUNTERCURRENT EXTRACTION WITH REFLUX 8 DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE 10



0 INTRODUCTION/PURPOSE Current trends indicate that the process industry will need to meet tighter standards in the use of energy and in the control of effluents in order to remain viable. Liquid-Liquid Extraction may have an increasingly important role to play in providing an economically acceptable solution to these demands. It is also a unit operation which, unlike distillation, does not subject process material to high temperature and is therefore sometimes a more appropriate method of separation when complex molecules are involved. This guide is one in a series of Process Engineering Guides concerning Liquid-Liquid Extraction and has been prepared for GBH Enterprises. 1 SCOPE This Guide describes the basic features and the underlying fundamental principles of Liquid-Liquid Extraction. It does not give detailed advice on the design and operation of Liquid-Liquid Extraction equipment. 2 FIELD OF APPLICATION This Guide is applicable to the Process Engineering community in GBH Enterprises worldwide. 3 DEFINITIONS For the purposes of this Process Engineering Guide the following definitions apply:- Extract This is the exit stream from the process being substantially

Solvent material into which the Solute has transferred. Feed This is the inlet stream to the unit in which the substance to

be extracted is originally dissolved. Liquid-Liquid This is the unit operation by which a substance or Extraction substances may be substantially passed from solution in one

liquid to solution in another by the contacting of the liquids. This process is also known as Solvent Extraction.



Raffinate This is the exit stream from the process being substantially Feed material from which the Solute has been transferred.

Solute This is the substance or substances which are to be

transferred from the Feed. Solvent This is the second liquid phase fed to the process into which

the Solute is transferred. The Solvent should be substantially immiscible with the Feed.

With the exception of terms used as proper nouns or titles, those terms with initial capital letters which appear in this document and are not defined above are defined in the Glossary of Engineering Terms. 4 LIQUID-LIQUID EXTRACTION PROCESS 4.1 General Liquid-Liquid Extraction is the process of extracting a Solute from a Feed by use of a Solvent to produce an Extract and a Raffinate. In its simplest form, it may take the guise of a single stage mixing and separation unit analogous to a single stage flash in distillation. The choice of Solvent is critical in effecting a Liquid-Liquid Extraction. Factors affecting the choice are summarized later; it is usually necessary to compromise in one area or another. As in distillation it is frequently impossible to achieve the separation required by use of a single stage unit, and a multistage unit is required. These units are summarized later and modes of operation further considered. A number of pitfalls in operation and design of Liquid-Liquid Extraction units are briefly mentioned. GBHE reports covering Liquid-Liquid Extraction include “An Illustrated Brochure on Liquid-Liquid Extraction". Other useful reference books are listed in Clause 5, Bibliography.



4.2 Choice of Solvent The choice of Solvent is influenced by many factors some of which are listed below: (a) High Selectivity:

The ability of a Solvent to extract a component or class of components in preference to others. This factor will determine the number of extraction stages required.

(b) Distribution or Partition Coefficient:

The ratio of the solubility of the Solute in the Solvent compared to the Feed. This factor will affect the selectivity and the amount of Solvent phase required.

(c) Density:

The greater the density difference between the Feed and the Solvent the easier it will be to obtain phase separation.

(d) Viscosity:

A high viscosity will inhibit both mass transfer and separation of the phases. A low viscosity (say less than 10 cP) is desirable.

(e) Interfacial Tension:

This affects the settling, coalescence and mass transfer coefficient of a system. Coalescence and settling are generally aided by high interfacial tension whilst mass transfer is hindered.

(f) Volatility:

The Solvent is likely to need to be separated from the Solute and/or the Feed. If this is to be done by distillation the volatility should, where possible, be chosen to allow this separation to be easily effected.



(g) Stability:

The Solvent should be stable at process conditions in order to minimize losses by degradation and generation of further impurities.

(h) Corrosivity:

If possible, there is a strong incentive to use a component that is already in the process, such as a reactant feed stream, as the Solvent. This may avoid additional materials handling, environmental and corrosion penalties later in the process.

(j) Toxicity:

The advantages of a non-toxic Solvent are self evident in considering inherent process safety and capital cost. Some solvents now appear on the "Environmental Red List" and should be avoided.

(k) Cost:

The extraction process may only be a small part in the overall process and solvent losses should not greatly affect process economics.

No Solvent is likely to meet all the above criteria and the list is not claimed to be exhaustive. A compromise will be necessary based on overall process economics. A common approach to Solvent selection is to carry out a literature survey of solvents used in similar processes. It may be necessary to consult with Physical Property specialists or with a Liquid-Liquid Extraction expert.



4.3 Principles of Extraction Solvent Extraction depends on a favorable distribution of the Solute between the Solvent and Feed streams. The two important parameters which fix the number of extraction stages and the Solvent usage are the distribution coefficient and the selectivity factor. The distribution or partition coefficient, K, is defined as:-

where C refers to the composition of a component in any consistent convenient units and the subscripts E and R refer to the Extract and Raffinate phase respectively. A high value of K is required for the component to be extracted, this determines the quantity of solvent required to affect the recovery. Ideally K is independent of the concentration of the Solute and the ratio of Extract to Raffinate phase. The separation or selectivity factor, S, is defined as:-

where the subscript!1 refers to the component to be preferentially extracted and subscript 2 refers to the component to be preferentially retained. The separability of component 1 from component 2 increases with increasing separation factor. A high value of S indicates the potential for a high degree of separation in a small number of extraction stages. A paper by Souders and Mott [Ref 13] indicated the relative uses of fractional distillation, extractive distillation and Liquid-Liquid Extraction. The design of a separations facility to carry out a particular separation was said to be the same in a case when fractional distillation was used with a relative volatility of 1.5, extractive distillation with a relative volatility of 2.0 and Liquid-Liquid Extraction with a Selectivity factor of 6.0.



It is possible sometimes to tailor a particular solvent to give the desired combination of selectivity and partition coefficient. In a mixed hydrocarbon solvent this might be achieved by varying the proportion of aromatic to paraffinic material for example or in a chemical solvent pH might be varied to influence solvent activity. A solvent may be tailored to "pick up" a given solute from a particular Feed then "drop it" into a further solvent by addition of acid or alkali to alter the pH or some chemical equilibrium constant. Choice as to whether to make the raffinate or the extract phase continuous will be based on a number of considerations. These include the stability of the mixture interface which may improve mass transfer but inhibit settling, the relative mass transfer coefficients which may be influenced by surface tension and more mundane considerations such as inventory of flammable or toxic materials. 4.4 Liquid-Liquid Extraction Equipment It has already been indicated that it may require more than one stage of Liquid-Liquid Extraction in order to achieve the degree of separation required. It is possible to achieve this by removing the Extract and making the Raffinate the Feed to another Liquid-Liquid Extraction unit using fresh Solvent. This requires a considerable amount of Solvent to be used and as in distillation it is more usual to employ equipment where a countercurrent flow of one phase against the other occurs. Equipment to achieve this is summarized in 4.4.1 to 4.4.4 but covered more extensively in a subsequent Process Engineering Guide. 4.4.1 Mixer Settlers These units consist of a number of mixing and settling units connected alternately in series. They are normally used when only a small number of stages are required. Mechanical agitation is required in the mixing zone, flow of fluid from one zone to another may either be effected by gravity or by use of pumps. Traditionally, the mixing chamber will have been a container with an agitator followed by a settling chamber all known as a "Box Mixer Settler". The mixing function may be performed by pumping the fluids through a static mixer or by mixing the fluids in an appropriate pump followed by a settling device. Scale up is usually good in these units.



4.4.2 Packed and Plate Column Contactors These devices are used widely for simple separations requiring less than 3 stages. The interface is controlled to be either above or below the internals depending on whether the light or the heavy phase is to be continuous. They have the advantage of there being no moving parts but are difficult to scale up with confidence due to the possibility of back-mixing and bypassing of either phase. 4.4.3 Mechanically Agitated Columns As throughput and numbers of stages required increases, mechanically agitated columns become the preferred unit. There are many types of unit offered by manufacturers including the Rotating Disc Column and the Kuhni Column. Agitation may either be by movement of the internals in the column, e.g. by rotation, or by pulsing the flow of the feeds to and/or products from the column. Mechanically agitated columns are more usually used in the 4.4.4 Sundry Devices There are a number of other sundry devices for achieving multistage contacting to effect Liquid-Liquid Extraction including the rotating Podbielniak Extractor and RTZ (Graesser) falling bucket contractor. 4.5 Operating Modes The extension of a single stage Liquid-Liquid Extraction operation to a multi-staged countercurrent operation is most simply achieved by making the Feed and the Solvent inputs to the extreme stages in the unit. This is shown in Figure 1 where a dense Feed enters at the top of the extractor and the less dense Solvent enters near the bottom. In this case the Feed is taken to be the continuous phase with an interface control near the top of the unit, but this is a matter of choice. A Liquid-Liquid Extraction unit is not usually an isolated piece of equipment in a flowsheet but will exist in conjunction with other items such as distillation units to further process the product streams.



At the end of the column where fresh Solvent is introduced, at the bottom in Figure 1, there will be a loss of Solvent due to any solubility in the Raffinate stream. This would normally be recovered by distillation where the Solvent might be expected to behave as a light component due to a high activity coefficient in the Raffinate stream. In the case where Aromatics are recovered from a mixed hydrocarbon stream the Solvent is polar and this may be recovered in a separate Liquid-Liquid Extraction unit by contacting with water. At the top of the column in Figure 1 the best separation between the desired product and any impurity will depend on the Selectivity factor and the concentration in the Feed. It may be possible to introduce the Feed midway down the column, such as in Figure 2, and return Solute until it becomes insoluble in the Solvent acting as Reflux such as in distillation. An alternative to this may be to recover a portion of the Raffinate which is either insoluble in the Solvent or easily separable from both Solvent and Solute and to reflux that. 4.6 Operating And Design Pitfalls Distillation is a unit operation than depends primarily on the bulk properties of the liquid and vapor phases. These bulk properties are so different that surface properties tend not to have a great affect on the design and operation of distillation units unless the columns are known to be operating with systems that support stable foams. Hence, distillation is a unit operation that is readily amenable to simulation for both theoretical plate analysis and for mass transfer analysis. In Liquid-Liquid Extraction the phases have more similar bulk properties therefore surface effects, that are generally less easily measured, have greater affect. Impurities, particularly surfactants, may concentrate at boundaries modifying the surface properties and therefore mass transfer, separation and coalescence. Hence, few Liquid-Liquid Extraction designs can be considered complete without a laboratory simulation as discussed in GBHE-PEG-MAS-602.

4.6.1 Emulsions

Stable Emulsions which may be formed when surface active contaminants are present, are an extreme example of how surface properties of a system can make a Liquid-Liquid Extraction unit inoperable. Laboratory simulations should be done wherever possible using actual plant streams and the style of equipment that is operating



4.6.2 Crud Layer

Crud, as related to Liquid-Liquid Extraction, is material that collects at the interface of the two liquid phases. A crud layer will occur in most processes due to the concentration of finely divided particulate matter, but in the majority of cases it is not troublesome. Where biological crud is occurring due to the occurrence of bacteria or fungi, or where difficulties are experienced, provision should be made for periodically removing material from the interface for elimination and treatment of the crud.

4.6.3 Phase Inversion

Two types of phase inversion are possible as indicated below:

(a) It is possible that the dense phase at one end of the extractor, for example the Feed, may be the less dense phase at the other end, the Raffinate. This is only likely to occur when the Solute forms a significant proportion of either the Feed, the Extract or both. Under these circumstances, operation of a column contactor is impossible. A series of mixer settlers may be possible but control will be difficult as the location of the inversion may move down the units. It is better to avoid having to operate this type of system if at all possible.

(b) Phase inversion may occur by which the phase that was intended

to be continuous becomes dispersed and vice versa. This may have a number of consequences, mass transfer may decrease, coalescence may take longer and in a column contactor the implication is that control has been lost as the interface has moved to the opposite end of the column to that intended.



FIGURE 1 SCHEMATIC OUTLINE OF LIQUID-LIQUID EXTRACTION PROCESS: COUNTERCURRENT FLOW IN ROTATING DISC CONTACTOR



FIGURE 2 COUNTERCURRENT EXTRACTION WITH REFLUX



5 BIBLIOGRAPHY



DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE This Process Engineering Guide makes reference to the following documents: GBHE ENGINEERING GUIDES GBH Enterprises Glossary of Engineering Terms (referred to in Clause

3). GBH Enterprises Guide for laboratory extraction (referred to in 4.6). GBHE REPORTS An illustrated brochure on liquid-liquid extraction (referred to in 4.1).