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Optimize Liquid-Liquid Extraction
Liquid-Liquid extractors are often a neglected part of process plants because they are not well understood and generally form only a small part of the overall process scheme. Often, significant savings in operating costs can be achieved by fine-tuning extraction systems. This article describes important parameters that should be considered when optimizing extraction systems. Liquid-Liquid extraction is a mass transfer operation in which a liquid solution (the feed) is contacted with an immiscible or nearly immiscible liquid
SOLVENT SELECTION Solvents differ in their extraction capabilities depending on their own and the solute’s chemical structure. Robbins (1) presents a table showing Organic-Group interactions from which one can identify the desired functional group(s) in the solvent for any given solute. Once the functional group is identified, possible solvents can be screened in the laboratory. The distribution coefficient and selectivity are the most important parameters that govern solvent selection. The distribution coefficient (m) or partition coefficient for a component (A) is defined as the ratio of concentration of a A in extract phase to that in raffinate phase. 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. Designing an extractor is usually a fine balance between capital and operating costs. Usually, good solvents also exhibit some miscibility with feed solution (see Table 1). Consequently, while extracting larger quantities of solute, the solvent could also extract significant amount of feed solution.
Other factors affecting solvent selection are boiling point, density, interfacial tension, viscosity, corrosiveness, flammability, toxicity, stability, compatibility with product, availability and cost. For an existing process, replacing the solvent is usually a last resort because this this would call for going back to laboratory screening of the solvent and process optimization. However, changes in environmental regulations and economic considerations often induce the need to improve the processes in terms of solute recovery. Also the availability of specialized and proprietary solvents that score over conventional solvents in terms of performance and economics for several extraction processes can provide additional incentives for a solvent change. SELECTION OF EXTRACTION CONDITIONS Depending on the nature of the extraction process, the temperature, pH and residence time could have an effect on the yield and selectivity. Operating pressure has a negligible affect on extraction performance and therefore most extractions take place at atmospheric pressure unless governed by vapor pressure considerations. Temperature can also be used as a variable to alter selectivity. Elevated temperatures are sometimes used in order to keep viscosity low and thereby minimizing mass-transfer resistance. Other parameters to be considered are selectivity, mutual solubility, precipitation of solids and vapor pressure. The pH becomes significant in metal and bio-extractions. In bio-extractions (e.g., Penicillin) and some agrochemicals (e.g. Orthene), pH is maintained to improve distribution coefficient and minimize degradation of product. In metal extractions, kinetic considerations govern the pH. In dissociation-based extraction of organic molecules, pH can play a significant role (e.g., cresols separation). Sometimes, the solvent itself may participate in undesirable reactions under certain pH conditions (e.g., ethyl acetate may undergo hydrolysis in presence of mineral acids to acetic acid and ethanol). Residence time is an important parameter in reactive extraction processes (e.g., metals separations, formaldehyde extraction from aqueous streams) and in processes involving short-life components (e.g., antibiotics & vitamins) SELECTION OF MODE OF OPERATION Extractors can be operated in crosscurrent or counter-current mode. The following section compares these configurations. CROSS-CURRENT OPERATION Crosscurrent mode is mostly used in batch operation. Batch extractors have traditionally been used in low capacity multi-product plants such as are typical in the pharmaceutical and agrochemical industries. For washing and neutralization operations that require very few stages, crosscurrent operation is particularly practical and economical and offers a great deal of flexibility. The extraction equipment is usually an agitated tank that may also be used for the reaction steps. In these tanks, solvent is first added to the feed, the contents are mixed, settled and then separated. Single stage extraction is used when the extraction is fairly simple and can be achieved without a high amount of solvent. If more than one stage is required, multiple solvent-washes are given. Though operation in crosscurrent mode offers more flexibility, it is not very desirable due to the high solvent requirements and low extraction yields. The following illustration gives a quick method to calculate solvent requirements for crosscurrent mode of extraction.
DIMINISHING RETURNS The following chart shows solvent
requirements for a typical reduction ratio (X f /Xr) of 10
using crosscurrent extraction. With one stage, 18,000 kg of solvent is required for 1,000 kg
of feed (m = 1 and Xf / Xr = 10). With two stages, solvent
requirement reduces to 8,650 kg, and with three stages, it reduces further to 6,930
kg. However, as can be seen from the chart, using more than three stages has minimal
effect on solvent usage. This fact combined with practical limitations of solvent
handling and increased batch time confines the number of solvent washes to three. COUNTERCURRENT OPERATION As described above, the crosscurrent operation is mostly used in low capacity multi-product batch plants. For larger volume operation and more efficient use of solvent, countercurrent mixer-settlers or columns are employed. Countercurrent operation conserves the mass transfer driving force and hence gives optimal performance.
The dimensionless term mS/F, included in all the above equations, is called the extraction factor (E), and is an important parameter in the design of extraction processes. For a given number of stages, the higher the E factor, the higher is the reduction ratio and easier is the extraction. Systems with E of less than 1.3 are not likely to be commercially feasible. CROSS vs. COUNTERCURRENT OPERATION The following graph compares the reduction ratios (Xf / Xr) of the crosscurrent and countercurrent modes of operation.
The graph shows that for a given extraction factor (E), and number of stager (n), the countercurrent mode of operation outperforms the crosscurrent mode. This is demonstrated in a case study presented at the end of this paper. Koch-Glitsch has demonstrated these benefits on their pilot and commercial scale extraction columns for several systems. The equations given above can be used to compare solvent requirements for various modes of operation and can serve as a starting point for identifying scope for optimizing solvent quantity. However, these equations should be used with caution as the assumptions of immiscibility, constancy of partition coefficient over desired range and solute-free fresh solvent are not valid in all practical applications. As the solvent quantity is reduced, the solute concentration in the extract increases. This usually affects the physical properties and the selectivity. Therefore optimization exercise should be backed up by laboratory extraction data. SELECTION OF EXTRACTOR TYPE Commercially important extractors can be classified into the following broad categories.
In batch mode of operation, these mixer-settlers could be simple batch vessels where feed and solvent are mixed and settled. This operation is repeated with fresh solvent washes as described earlier. Centrifugal extractors are high-speed rotary machines that offer advantages of very low residence time. The number of stages in a centrifugal device is usually limited to one, but currently devices with multiple numbers of stages are common. These extractors are mainly used in pharmaceutical industry. Countercurrent column contactors are most popular in the chemical industry. These could be static or agitated. Several types of extractors are available (see table) and each has its own advantages. FACTORS AFFECTING SELECTION OF EXTRACTORS Important factors to consider when selecting extractor types are the stage requirements, fluid properties and operational considerations. The following table outlines the capabilities and characteristics of different extractor-types:
The Karr reciprocating plate extractor can effectively handle low interfacial tension systems. Other factors governing extractor selection are presence of solids, safety and maintenance requirements. DESIGN CRITERIA The basic function of extraction equipment is to mix two phases, form and maintain droplets of dispersed phase and subsequently separate the phases. The following section outlines some of the factors that need to be considered while designing and optimizing extraction equipment. 1. MIXING The
amount of mixing required is determined by physical
properties such as viscosity, interfacial tension and density differences between the
two phases. It is important to provide just the right amount of mixing. Less
mixing causes the formation of large droplets and decreases
interfacial area (interfacial area varies with the square of the droplet diameter).
This reduces mass transfer and decreases stage efficiency. Higher agitation (more
mixing) minimizes mass transfer resistance during reactions and extraction but contributes
to the formation of small and difficult-to-settle droplets or emulsions. In agitated batch extractors, the agitator design is often optimized for reaction and heat transfer, not extraction, as these are generally multi-purpose vessels. The agitator imparts maximum energy at the tip where the velocity is highest and minimum energy at the center. This creates non-uniform droplet sizes, with the smallest being formed at the agitator tip. Reaching extraction equilibrium is controlled by the largest droplet size and the smallest droplet controls settling time. Therefore, over-agitation sometimes takes its toll by causing difficulties in phase separation. Usually a redesign in terms of configuration or change in agitation speed helps in optimizing batch time.
Static extraction columns rely completely on the packing/internals and fluid flow
velocities past the internals to create turbulence and droplets. Therefore these are
restricted by minimum flow requirement of at least one of the phases. Agitated columns
have more operating flexibility as the specific energy input can be varied in most
designs.
Axial mixing (along column length) in column contactors reduces stage efficiency.
Baffles or similar arrangements are used to minimize axial mixing in static as well as
agitated columns. It is also important to avoid temperature gradients in columns to
prevent thermal currents contributing to axial mixing. 2. SETTLING The settling characteristics depend on the fluid properties (density difference, interfacial tension, and continuous phase viscosity) and the amount of mixing. Settling in agitated batch vessels is carried out by stopping the agitator. In continuous columns, a settling section is provided either as a part of the extractor or as a separate piece of equipment downstream of the extractor. Emulsions are usually formed due to over agitation and in such cases, settling needs to be carried out over an extended period. Emulsions can also form due to the inherent nature of the chemical compounds involved or due to contaminants that substantially lower the interfacial tension. Sometimes coagulants are added to prevent or minimize emulsification. Passing the emulsion layer through a coalescer can break some of these emulsions. In continuous extractors, the creation of emulsions is less severe as good droplet size distribution can be attained at lower agitation speeds in a lesser diameter column. Also, columns such as the Karr reciprocating plate extractor impart uniform energy throughout the radius as a result of the reciprocating motion and this creates a much narrower droplet distribution.
A similar phenomenon to emulsions is the formation of a ‘rag layer’. This
is a layer containing loose solid substances that float at the interface. These
solid substances are generally foreign impurities that exist in the feed streams or those
that precipitate from the system during extraction. In continuous extraction the
liquid interface containing the rag layer can be continuously withdrawn, filtered and sent
back to extractor.
Selection of continuous and dispersed phases can have an effect on formation of emulsion
and rag layer. Reversing continuous and dispersed phases sometimes drastically
reduces or eliminates emulsion formation. Changing extraction temperature could also
help in reducing emulsion and rag layer. 3. SELECTION OF CONTINUOUS AND DISPERSED PHASE In column extractors, the phase with the lower viscosity
(lower flow resistance) is generally chosen as the continuous phase. Also note that
the phase with the higher flow rate can be dispersed to create more interfacial area and
turbulence. This is accomplished by selecting an appropriate material of
construction with the desired wetting characteristics. In general, aqueous phases
wet metal surfaces and organic phases wet non-metallic surfaces. Change in flows and
physical properties along the length of extractor should also be considered. Choosing a continuous phase is generally not available in
batch processes, as the larger liquid phase usually becomes the continuous phase. CONCLUSION As we have seen in the previous sections, there are a number
of factors affecting extraction performance. Laboratory and pilot plant testing
using actual feed and solvent help immeasurably in optimization. The study could
often be an iterative cycle involving laboratory testing followed by process simulation
and design. In most industrial extractors, there is usually a good scope for
optimizing solvent usage and energy consumption. REFERENCES 1.
Robbins, Chem. Eng. Prog., 76(10),
58-61 (1980). 2.
Cusack, R.W., & Glatz, D., et al, “A Fresh Look at Liquid-Liquid
Extraction”, Chemical Engineering, February, March & April 1991.
By: R. Madhavan, Third Content Manager (read
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