Extraction process requirements

Almost all solvent solutions used in solvent extraction processes require treatment, either before their initial use or after being stripped of the extracted species. After preparation of the solvent by mixing appropriate amounts of extractant, modifier, and diluent, the solution is treated by contacting with an appropriate aqueous solution before entering the extraction stage. Such pretreatment is generally referred to as equilibration of the solvent. 

When freeze-drying is not included, completion of the full extraction process requires 1 /2 day. It is recommended that freeze-drying, which requires at least 12 hr, be performed overnight before the day of extraction. 

Now, depending on the extraction process, and what is begin extracted, the filtered solvent mixture is either treated with chemical reagents, filtered, and then evaporated, or simply evaporated to remove the solvent and leave behind the extracted substance. In some cases, this step can be quite complex, as some extraction process require treatment with multiple reagents, titrations, filtrations, ect., in order to facilitate proper extraction. It should be noted that exact instructions will be given for each extraction process where applicable. 

With the exception of cold pressing, all oil extraction processes require that the oleaginous materials be heated and sometimes further dried before oil extraction. 

For many separation applications, the use of liquid-liquid extraction is an alternative to the various distillation schemes described in Sec. 13, Distillation. In many of these cases, a distillation process is more economical largely because the extraction process requires extra operations to process the extract and raffinate streams, and these operations usually involve the use of distillation anyway. However, in certain cases the use of liquid-liquid extraction is more cost-effective than using distillation alone because it can be implemented with smaller equipment and/or lower energy consumption. In these cases, differences in chemical or molecular interactions between feed components and the solvent provide a more effective means of accomplishing the desired separation compared to differences in component volatilities. 

The economic viability of any process, not just a supercritical fluid extraction process, requires that equipment utilization be efficient because off-line time is a capital investment penalty. In the present activated carbon... 

The qualitative observations which can be drawn from the above criteria are that extraction is Likely to be more energy efficient than distillation in those cases where a very large distillation reflux ratio is required, and a large fraction of the feed must be taken overhead in distillation, On the other hand, distillation will be fiivored if the recoveiy steps of the extraction process require significant reflux, the distillation reflux ratio is small, or if only a relatively small fraction of the feed must be taken overhead in distillation. 

In spite of the laige number of experimental studies of various extraction systems repotted in the literature, the range of variables available in these highly interacting, multicomponent systems leaves many regions unexplored. Therefore, the development of a solvent-extraction process requires extensive experimentation. The general behavior of many available reagents is known, but the effects of interactions in a specific system must be determined experimentally. 

An efficient extraction process requires that the solvent fuUy penetrates into the cells and that the target molecule is soluble in the solvent (e.g., nonpolar solvents for nonpolar lipids). 

Although the process requires the addition of a phosphate donor, such as glycerol-2-phosphate, it may be a valuable tool for cleaning water contaminated with radionuchdes. An alternative mode of uranium precipitation is driven by sulfate-reducing bacteria such as Desulfovibrio desulfuricans which reduce U(VI) to insoluble U(IV). When combined with bicarbonate extraction of contaminated soil, this may provide an effective treatment for removing uranium from contaminated soil (85). 

Solvent Extraction. Extraction processes, used for separating one substance from another, are commonly employed in the pharmaceutical and food processing industries. Oilseed extraction is the most widely used extraction process on the basis of tons processed. Extraction-grade hexane is the solvent used to extract soybeans, cottonseed, com, peanuts, and other oilseeds to produce edible oils and meal used for animal feed supplements. Tight specifications require a narrow distillation range to minimize solvent losses as well as an extremely low benzene content. The specification also has a composition requirement, which is very unusual for a hydrocarbon, where the different components of the solvent must be present within certain ranges (see Exthaction). 

Removing an analyte from a matrix using supercritical fluid extraction (SEE) requires knowledge about the solubiUty of the solute, the rate of transfer of the solute from the soHd to the solvent phase, and interaction of the solvent phase with the matrix (36). These factors collectively control the effectiveness of the SEE process, if not of the extraction process in general. The range of samples for which SEE has been appHed continues to broaden. Apphcations have been in the environment, food, and polymers (37). 

After mixing, the solvent and waste are separated. The solvent with dissolved organics is called the extract. The waste remaining after extraction is called the raffinate. The extract may be sent to a distillation or steam stripping unit to separate the dissolved organics from the solvent and the solvent can be recycled back to the extraction process. The raffinate may require additional treatment or may be disposed or incinerated. 

The value of K is one of the main parameters used to establish the minimum ratio of extraction solvent to feed solvent that can be employed in an extraction process. For exanmle, if the partition ratio K is 4, then a countercurrent extractor woula require 0.25 kg or more of extraction-solvent flow to remove all the solute from 1 kg of feed-solvent flow. 

The main objective for calculating the number of theoretical stages (or mass-transfer units) in the design of a hquid-liquid extraction process is to evaluate the compromise between the size of the equipment, or number of contactors required, and the ratio of extraction solvent to feed flow rates required to achieve the desired transfer of mass from one phase to the other. In any mass-transfer process there can be an infinite number of combinations of flow rates, number of stages, and degrees of solute transfer. The optimum is governed by economic considerations. 

Ice (about 100 g.) and then 100 ml. of cold water are added, the mixture is stirred for 10 minutes, and then the ether is removed by distillation (Note 5). To the residue in the flask is added sufficient water to make the total volume about 300 ml. Steam distillation is then carried out until the distillate comes over clear. This process requires about 1 hour. The aldehyde is extracted from the steam distillate with three equal portions of the ether recovered above, and the ether solution is dried for several hours over anhydrous sodium sulfate. 

---