Extraction equipment, selection

The solvent, microwave energy applied, and extraction time selected are the main parameters controlled in MAE. The user should use proper extraction vessels and equipment in MAE because very high pressures can be achieved and explosions may result if appropriate precautions are not taken. 

Selected examples of the main categories of extractors are represented in Figures 14.11-14.15. Their capacities and performance will be described in general terms insofar as possible, but sizing of liquid-liquid extraction equipment always requires some pilot plant data or acquaintance with analogous cases. Little detailed information about such analogous situations appears in the open literature. Engineers familiar with particular kinds of equipment, such as their manufacturers, usually can predict performance with a minimum amount of pilot plant data. 

Reactive extraction uses liquid ion exchangers that promote a selective reaction or separation. The solutes are very often ionic species (metal ions or organic/inorganic acids) or intermediates (furfural phenols, etc.), and the extraction chemistry is discussed elsewhere (11-13). Reactive extraction can be used for separation/ purification or enrichment or conversion of salts (14). A 2001 review on reactive phase equilibria, kinetics, and mass transfer and apparative techniques is given in Ref. 8. Reactive extraction equipment is discussed in detail in Ref. 15, and recent advances are given in Ref. 16. 

Solvent extraction has long been established as a basic unit operation for chemical separations. Chapter 7 summarizes the effects of temperature, pH, ion pairs, and solvent selection on solvent extraction for biomolecules. Solvent extraction of fermentation products such as alcohols, aliphatic carboxylic acids, amino acids, and antibiotics are discussed. Enhanced solvent extraction using reversed micelles and electrical fields are also discussed. Solvent-extraction equipment and operational considerations are adequately covered in this chapter. 

Pratt, H. R. C., and Stevens, G. W. (1992). Selection, design, pilot-testing, and scale-up of extraction equipment. In Science and Practice of Liquid-Liquid Extraction (J. D. Thornton, ed.), Vol. 1, pp. 492-589. Oxford University Press, New York. 

At the heart of a leaching plant design at any level—conceptual, preliminary, firm engineering, or whatever—is unit-operations and process design of the extraction unit or line. The major aspects that are particular for the leaching operation are the selection of process and operating conditions and the sizing of the extraction equipment. 

Timothy C. Frank, Ph.D. Research Scientist and Sr. Technical Leader, The Dow Chemical Company Member, American Institute of Chemical Engineers (Section Editor, Introduction and Overview, Thermodynamic Basis for Liquid-Liquid Extraction, Solvent Screening Methods, Liquid-Liquid Diversion Fundamentals, Process Fundamentals and Basic Calculation Methods, Dual-Solvent Fractional Extraction, Extractor Selection, Packed Columns, Agitated Extraction Columns, Mixer-Settler Equipment, Centrifugal Extractors, Process Control Considerations, Liquid-Liquid Phase Separation Equipment, Emerging Developments)... 

Identify useful equipment options for liquid-liquid contacting and liquid-liquid phase separation, estimate approximate equipment size, and outline preliminary design specifications. (See Extractor Selection under Liquid-Liquid Extraction Equipment. ) Where appropriate, consult with equipment vendors. Using small-scale experiments, determine whether sludgelike materials are likely to accumulate at the liquid-liquid interface (called formation of a rag layer). If so, it will be important to identify equipment options that can tolerate accumulation of a rag layer and allow the rag to be drained or otherwise purged periodically. 

A number of equipment selection guides have been published. Pratt and Hanson [Chap. 16 in Handbook of Solvent Extraction, Lo, Baird, and Hanson, eds. (Wiley, 1983 Krieger, 1991)] provide a detailed comparison chart for 20 equipment types considering 14 characteristics. Pratt and Stevens [Chap. 8 in Science and Practice of Liquid-Liquid Extraction, vol. 1, Thornton, ed. (Oxford, 1992)] modified the Pratt and Hanson selection guide to include solvent volatility and flammability design parameters. Stichlmair [Chem. Ing. Tech., 52(3) pp. 253-255 (1980)] and Holmes, Karr, and Cusack (AIChE... 

Many phytochemicals and nutraceutical ingredients are derived from botanicals. In the manufacture of many of these nutraceuticals, processes begin with the extraction of plant materials using a suitable solvent. Many technologies and types of equipment exist to achieve this solid-liquid extraction. To successfully choose and operate the proper equipment for producing the desired product in an economic manner, the fundamentals of equilibrium and mass transfer must be understood. Once these fundamentals are understood, they can be applied to the botanical raw material of interest and the chemical properties of the desired phytochemical to select and operate the most cost-effective extraction equipment. 

In selecting extraction equipment and designing a commercial extraction operation, many items need to be taken into account, including raw material issues, the extraction parameters necessary to achieve economic recovery, the size of the extraction operation, and the product mix. These are discussed below. 

Relationships between three extraction parameters—solvent composition, temperature, and particle size—can be determined from simple beaker tests. Raw material ground to the desired particle size is contacted with the chosen solvent at the chosen temperature in a beaker and mixed over time. Thief samples are removed at various time intervals and analyzed for the marker. At the completion of the test, the marc is separated from the extract, dried, and analyzed. (In instances when the phytochemical is heat sensitive, the marc can be analyzed wet and the assay calculated on a dry basis). Based on this information, the amount of marker remaining in the dry marc can be determined after correcting for the marker in the absorbed extract in the wet marc. From these data, the equilibrium relationship defined in Equation 11.1 can be determined, as well as the time it takes to reach maximum extraction. Conditions are changed to give a high equilibrium constant and a short extraction time to reach equilibrium. Based on this information, one can select the appropriate extraction equipment as well as the operating conditions to operate the equipment. 

One further consideration in selecting extraction equipment is determining how the solvent is separated from the marc. Unless the solvent is water, the marc will need to be essentially solvent-free for disposal. Also, economics will dictate that solvents... 

The selection of the proper extraction equipment type is governed by economics, the chemistry of the phytochemical of interest, and the physical properties of the botanical to be extracted. Each situation is different, and a careful evaluation of extraction needs should be made. Areas to consider include the following ... 

Desired throughput—The discussion above gives a range of capacities for each type of equipment. Equipment selection will be governed by the desired extraction capacity. 

All chapters, especially the last two, contain information that has not been published previously. The chapter on solid-liquid extraction technologies, for example, presents and discusses both theoretical and practical aspects of these technologies, from fundamental concepts of equilibrium and mass transfer to equipment selection and design. Similarly, the chapter on safety of botanicals reviews safety issues of botanicals associated with misidentification of plant species, misuse of products, product adulteration and botanical/drug interactions. 

Development is the preparation the facilities, equipment, and infrastructure required for extraction of the valuable mineral material. It includes land acquisition, equipment selection and specification, inlraslruclure and surface facilities design and construction, environmental planning and permitting, and initial mine planning. 

Extractor equipment considerations are discussed here in the context of their effect on the process performance and not for the purpose of describing detailed design. The main parameter of interest at this point is the number of equilibrium stages that represent the process. Liquid-liquid extraction requires thorough mixing of two liquid phases to achieve thermodynamic equilibrium, followed by complete separation of the phases. The particular equipment selected for a given process is determined, in part, by the mixing and separation characteristics of the phases. 

Small drops lead to more transfer area and better extraction, but to slower settling and less capacity. Thus, selection of extraction equipment frequently involves a compromise choice balancing efficiency against capacity. 

Many pharmaceutical extractions do not lend themselves to simple straightforward analytical solutions. Rarely is there a case of simple extraction of a single solute from a clean feed with pure solvent. There may well be solids present which can stabilize emulsions and cause excessive entrainment. Usually, more than one solute is present, so selectivity as well as extent of extraction becomes important. Also, the solvent may contain residual solute from the solvent recovery section. Again, suppliers of extraction equipment should be contacted for their help in solving real industrial extraction problems. 

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