Extraction equipment centrifugal extractors

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)... 

Acrylic Acid Recovery. The process flow sheet (Fig. 3) shows equipment and conditions for the separations step. The acrylic acid is extracted from the absorber effluent with a solvent, such as butyl acetate, xylene, diisobutyl ketone, or mixtures, chosen for high selectivity for acrylic acid and low solubility for water and by-products. The extraction is performed using 5—10 theoretical stages in a tower or centrifugal extractor (46,61—65). 

The simplest type of extraction equipment is the stirred tank, coupled with a solid-liquid separation step such as a screen, filter, or centrifuge to separate the marc from the extract. This equipment is readily available in a wide variety of sizes and can be operated as a single-batch extractor or run in series in a countercurrent mode as shown in Figure 11.6. 

Extraction is a process that utilizes the difference in solubility of the product of interest between two immiscible phases. Liquid-liquid extraction is commonly used in classical biotechnological downstream processing for production of antibiotics and other small molecules. The second liquid phase is usually a water immiscible organic solvent. Reciprocating plate, Podbielniak centrifugal extractors, Delaval contactors, and Westfalia extraction-decanters are examples of some of the equipment available for carrying out extraction. 

Proprietary Extractors. Manufacturers or proprietary design extraction equipment (such as the Podbielniak Centrifugal Extractor or the RTL (raining bucket) Contactor) provide catalogs listing the relative capacities of the various sizes of equipment which are offered. Pilot equipment is usually available for determining extraction performance, and the manufacturer utilizes both the pilot data and experience with similar systems to provide assured commercial designs. 

Most large-scale equipment for liquid-liquid extraction is available for two-phase extraction and usually multistage systems have to be used for obtaining high yields. Ktihni type columns, Podbielniak centrifugal extractors and disk and bowl centrifuges have been used (Kula et al. 1981 Veide et al. 1983 Cunha and Aires-Barros 2002). 

Apart from the extraction equipment itself, columns, mixer-settlers, or centrifugal extractors, the remainder of the plant is usually fairly conventional. It is usually necessary to provide vessels for conditioning the feed solution to the correct acidity or for correcting the concentrations of the principal solute or other salts. This may be carried out continuously or in special batch conditioning vessels. In the latter case, batch feed vessels will also no doubt be required. 

During recent years pilot scale equipment, smaller than the prototype pilot plants described and capable of operating with exceedingly high efficiency, has been designed. Such equipment as the York-Scheibel solvent extraction tower and the Podbielniak countercurrent centrifugal mixer and extractor are typical. Data from this equipment may be correlated with commercial performance. 

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