The extraction operation

Rather than an in-depth technical description of the mechanics of extraction, this section presents briefly a typical infusion process, focusing on the factors that make extracts different from single-chemical components. Extracts by then nature are complex mixtures of (often) diverse active compounds contained within a plant matrix which are brought into solution by the extraction process. The aim of the extractor is to produce, over a period of time, batches of an extract meeting a customer s individual specification with as little variation as possible. There are parameters over which the extractor has some control, and these can be used to help achieve product consistency and also to fine-tune an extract to a particular customer s needs. 

The maturity of the plant at harvest can also have a bearing on the actives content of the raw material. If it is possible to control the supply line, then it may be possible to control the harvesting of the crop at a specified maturity otherwise 

Some control can be built in by specifying minimum contents of actives, but this standard is prone to downward revision for reasons of commercial expediency in times when the harvest yields levels of actives that are mainly below the specification. At such times, it is likely that parcels of the herb meeting a particular specification will carry a price premium. 

If the herb selected for a given development project is one for which market demand is insufficient to warrant a commercial growing operation, it will probably come from wild sources and will be a spot market purchase subject to variability of price and availability. Probably the best that can be done here is to source from a single wholesaler. 

3 Summary. It is obvious from the foregoing paragraphs that there is inherent variability in the raw material supply chain before the extraction process itself is even stalled. This will introduce variability into the subsequent process and influence product costs. 

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A recent and extremely important development lies in the application of the technique of liquid extraction to metallurgical processes. The successful development of methods for the purification of uranium fuel and for the recovery of spent fuel elements in the nuclear power industry by extraction methods, mainly based on packed, including pulsed, columns as discussed in Section 13.5 has led to their application to other metallurgical processes. Of these, the recovery of copper from acid leach liquors and subsequent electro-winning from these liquors is the most extensive, although further applications to nickel and other metals are being developed. In many of these processes, some form of chemical complex is formed between the solute and the solvent so that the kinetics of the process become important. The extraction operation may be either a physical operation, as discussed previously, or a chemical operation. Chemical operations have been classified by Hanson(1) as follows ... 

In commercial practice, when a volatfle component must be extracted from a polymeric solution of high viscosity, say 500 P or greater, the extraction operation is almost always conducted in geometries which mechanically generate a wiped film. And here, screw extruders have played a major role. 

Liquid-liquid extraction, using one or two solvents respectively, is widely used when distillation is impractical, especially when the mixture to be separated is temperature-sensitive and/or more than 100 distillation stages would be required. When one solvent is used, it selectively dissolves only one or a fraction of the components in the feed mixture. In a two-solvent extraction system, each solvent has its own specific selectivity for dissolving the components of the feed mixture. Additional separation operations are generally required to recover, for recycling, solvent from streams leaving the extraction operation. 

FIGURE 14.10. Extraction behaviour of N-cytc and G-cytc into the organic phase containing DOLPA. The amount of depleted proteins from aqueous solution (open symbols) and extracted proteins in the surfactant solution (closed symbols) were estimated by the decrease and increase in the protein concentration during the extraction operation, respectively [19]. 

A liquid-liquid extraction process produces a solvent-rich stream called the extract that contains a portion of the feed and an extracted-feed stream called the raffinate. A commercial process almost always includes two or more auxiliary operations in adcfition to the extraction operation itself. These extra operations are needed to treat the extract and raffinate streams for the purposes of isolating a desired product, recovering the solvent for recycle to the extractor, and purging unwanted components from the process. A typical process includes two or more distillation operations in addition to extraction. 

For dilute feeds, consider options for preconcentrating the feed to reduce the volumes of feed and solvent that must be handled by the extraction operation. Consider evaporation or distillation of a high-volatility feed solvent or the use of reverse osmosis membranes to concentrate aqueous feeds. (See Hybrid Extraction Processes under Commercial Process Schemes. )... 

Chemical and thermal stability—The solvent must be stable at the operating conditions of the extraction operation as well as in downstream processing. 

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. 

Another process (demonstrated by Bitmin Resources) also attempts to minimizes fines dispersion in the extraction operation by minimizing... 

The extraction operator can, within limits, control bed depth, solvent input and to some degree its (solvent/miscella) distribution within the extractor as well as extractor temperatures. However, the operator must accept the quality and quantity of press cake produced in the preparation plant. The quality of extracted product is highly dependent on a good press cake. Poor quality cake commits the extraction plant to poor oil recovery with high solvent loss and even high levels of solvent in the meal, both of which are beyond the extraction operator s power to effectively control. 

Solvent Recovery. Although not strictly part of the extraction operation, all such separations are ordinarily followed by removal of solvent from the extract and raffinate solutions to give the finished products. Figure 6.5 shows a typical extraction solvent recovery scheme. Distillation is... 

Extension of a simple countercurrent operation to one involving the use of reflux provides a process which is analogous in its essentials to the rectification type of distillation. Whereas in the extraction operations described previously the richest extract leaving the plant was nearly in equilibrium with the feed solution, by proper use of reflux it is possible not only to separate systems showing unfavorable distribution coeflScients but also to enrich the extract beyond that which is merely in equilibrium with the feed (7, 14, 18). 

Stages / + 1 through n constitute the raffinate stripping section of the cascade, where the raffinate solution is exhausted of its C content. The primary raffinate product Rn is divided into two streams Pr, the saturated raffinate product, may be further treated to remove solvent and to provide the finished raffinate and operation not concerned with the extraction operation and which when mixed with the extracting solvent S, provides the raflSnate reflux Pn, Bn, and Pr are of the same composi-... 

The extent of extraction of mercaptans further depends upon the extent of their removal from the solvent in the stripper, but a general expression relating the extraction and stripping steps can be readily derived (65). Provided extracting solutions are sufficiently strong and mercaptan concentration sufficiently dilute that the value of rriT can be considered constant, Eq. (6.134) can be written for the extraction operation for each mercaptan - imrS/G) ... 

Figure 4.10, where any slight fall in the equilibrium Iine,i e.g. as a result of a change of acidity, causes the extraction operating line to move to the right and release additional solute into the rafiSnate. 

There are numerous analytical tests that are used to assess the extraction operation and to help optimize the treater tower. A few of the most important tests are listed below in Table 7. 

Sxs = solvent added to system at top of column In many cases, the mixture to be separated is a binary, since it is usually desirable to separate all but two components by regular distillation and then subject them to the extractive operation. Generally, the amount of solvent added at the top of the tower is varied with the reflux ratio in order to maintain a constant mol fraction of solvent in the total liquid returned to the top region of the column. An alternative method of operation is to employ a given solvent rate independent... 

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