TYPES OF EXTRACTION EQUIPMENT

Extraction equipment can be classified by function as providing discrete stages or continuous differential contact. Separation may be by gravity alone or by centrifugal force. Additional energy may be applied to control drop size, either by mechanical agitation or pulsation. This classification is shown in Table 1, along with major examples of available equipment. 

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The fields of application of the various types of extraction equipment are also well summarised in Volume 2, Chapter 13. The basic principles of liquid-liquid extraction are covered in several specialist texts Treybal (1980), Robbins (1997), and Humphrey and Keller (1997). 

Mass transfer to or from droplet dispersions is employed in nearly all types of extraction equipment, so it is important to be able to estimate the two values of k for the droplet phase (dispersed) and the surrounding liquid phase (continuous). 

Viscosity—Most types of extraction equipment have liquid viscosity limitations. As the dissolved solids content in the extract increases, higher viscosity results. Higher temperatures will signihcantly decrease the extract viscosity. 

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. 

The dispersion is normally conducted in a stirred tank producing globule sizes of around 1 fiin. So far mostly batch operations in stirred tanks have been investigated. Other types of extraction equipment, e.g. spray columns, could also be employed. 

The following heuristics are useful for making an initial decision on the type of extraction equipment to use ... 

Godfrey and Slater (1994) have detailed chapters on all the major types of extraction equipment with considerable detail on mass transfer rates. Hunphrey and Keller (19971 discuss equipment selection in considerable detail and have height equivalent to a theoretical plate (HETP) and capacity data. Frank et al. (2Q08) discusses equipment selection, holdup, flooding, and mass transfer. 

Al. What is the designer trying to do in the extraction equipment shown in Figure 13-2 and listed in Table 13-1 Why are there so many types of extraction equipment and only two major types of equipment for vapor-liquid contact ... 

The processing of citrus fruit calls for special extraction equipment designed to minimize the content of peel oil in the resulting expressed juice. Two main types of extraction equipment are widely used throughout the industry ... 

Fig. 14.2-1. Four important types of extraction equipment. Types (a) - (c) are differential contactors, described in the same manner as gas absorption. Type (d), a three-stage mixer-setder, depends on stages, as does distillation.

The heights of a transfer unit ia each phase thus contribute to the overall heights of a transfer unit. Data on values of HTU for various types of countercurrent equipment have been reviewed (1,10). In normal operating practice, the extraction factor is chosen to be not greatiy different from unity, within the range of 0.5—2. 

Process Variations. The conventional techniques for tea manufacture have been replaced in part by newer processing methods adopted for a greater degree of automation and control. These newer methods include withering modification (78), different types of maceration equipment (79), closed systems for fermentation (80), and fluid-bed dryers (81). A thermal process has been described which utilizes decreased time periods for enzymatic reactions but depends on heat treatment at 50—65°C to develop black tea character (82). It is claimed that tannin—protein complex formation is decreased and, therefore, greater tannin extractability is achieved. Tea value is believed to be increased through use of this process. 

Two types of SFE equipment are used. One is a large scale flow type apparatus(1.5Q which is used to obtain the total extracts. The SFE condition in this equipment was 50 °C and 30 MPa. The other is a microscale extractor(60 ml) which is used to establish the optimal SFE conditions. In the SFE, carbon dioxide is mainly used and the experiment is performed in the ranges of 35-55 °C and 10-30 MPa. 

In the design of extraction equipment with complex flows, mass-transfer coefficients are determined by experiment and then correlated as a function of molecular diffusivity and system properties. The available theories provide an approximate framework for the data. The correlation constants vary depending upon the type of equipment and operating conditions. In most cases, the dominant mass-transfer resistance resides in the feed (raffinate) phase, since... 

The use of supercritical fluid extraction (SEE) as an extraction technique is related to the unique properties of the supercritical fluid. These fluids have a low viscosity, high diffusion coefficients, low toxicity, and low flammability, all clearly superior to the organic solvents used in SPE extraction. The most common fluid used is carbon dioxide. SEE extractions of sediment samples have shown recoveries of >95% for all the individual PCBs. The separation of PCDDs from PCBs and chlorinated benzenes is difficult because of their similar solubility. An interesting development is the use of fat retainers. Samples, mixed in different weight ratios with, e.g., silica/silver nitrate 10% or basic alumina, can be placed in 7 ml extraction cells. The analytes are recovered by elution with 1.5-1.8 ml of hexane. With the correct fat-silica ratios and SEE conditions, no additional cleanup procedure is necessary for GC with an electron-capture detector (ECD). One drawback of SEE may be that the methods developed are valid for a specific matrix, but as soon as, e.g., the fat content of a biota sample or the type of lipids changes, the method has to be adapted. SEE is relatively complicated compared to other extraction techniques. In addition, the cell volumes are small, which limits the sample intake, and, with that, the detection limits. Einally, some reliable types of SEE equipment have recently been withdrawn from the market. This will have a substantial negative effect on the use of SEE in the near future. 

In the past, use has been made of Amberlite XAD-4 resin columns in the field for sample extraction of fenitrothion and aminocarb in rain water. The rain water was collected and extracted with the aid of a special sampling (collection) device equipped with Amberlite XAD-4 columns. It is possible that these types of extraction columns could have some field application in the extraction of large volumes of surface water for the determination of specific organic contaminants. However, one drawback associated with resin columns is the requirement of exhaustive cleaning with various solvents to remove all trace contaminants. Preparation of blanks, spiked blanks, spiked samples (in replicate), and sequential replicate sampling should be included as part of the specific QA/QCPs that are needed, if these Amberlite XAD-4 resin columns (or others) are used more extensively in the future. 

The crux of the SX process is the transfer of a particular species across an aqueous-organic interface. If mass transfer conditions are not adequate, then extraction and/or stripping efficiencies will be lower than expected. Optimized operation of the mass transfer process is crucial, whatever type of contacting equipment is used. For... 

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