Back-extraction simultaneous processes

Due to the favorable thermodynamic conditions created at the F/M interface some components are selectively extracted from the F and transported into the membrane liquid. Simultaneously, at the M/R interface, conditions are such that the back extraction is favored. Various factors that could affect the transport of a metal ion through the LM are (a) the (transport) resistance encountered by the metal ion in the F and R phases, (b) the physicochemical properties of the carrier and diluent, and (c) the nature of membrane support such as its pore size, porosity, tortuosity, hydrophilicity, surface tension, and surface area to volume ratio encountered in the transport process. 

The design of any process has to be supported by a proper understanding of the system behavior. Next, the kinetics of the simultaneous extraction and back-extraction processes of two metallic ions by membrane-assisted solvent extraction is analyzed theoretically. 

Membrane-assisted solvent extraction processes have known an increasing number of applications in the last decades. The technique not only overcomes the limitations of conventional liquid extraction, such as flooding, intimate mixing, hmitations on phase flow rate variations, and requirement of density difference but also provides a large surface area of mass transfer per volume of contactor. Simultaneous extraction and stripping of the solute has been developed using two HE modules in series, one for the extraction and the other for the back-extraction processes. 

There is one report of an offline SPME (or perhaps more properly SPE) extraction system for CE analysis in which the offline extraction is automated. The extraction medium is a monolith placed in the loop position of a six-port loop injector. An injector loop upstream contains a large loop with sample. The sample from the loop in the upstream injector is passed through the extraction medium for a controlled time at a controlled flow rate. Following a rinse, the back extraction solvent is then pumped through the monolith into a collection vial. This system has the advantage of the reproducibility of an automated system, and simultaneously avoids the complexity of the interfacing process. 

In the chromatographic liquid adsorptive separation process, the adsorption and desorption processes must occur simultaneously. After the desorption step, both the rejected product (product with lower selectivity, resulting in less adsorption by adsorbent) and the extracted product (product with higher selectivity, resulting in strong adsorption by adsorbent) contain desorbent In general, the desorbent is recovered by fractionation or evaporation and recycled back into the system. 

Integrating liquid-liquid extraction and detection is far from easy, as reflected in the few attempts made so far. Many of the devices developed for this purpose fail to comply with the definition of sensor. Such is the case with continuous liquid-liquid extraction systems without phase separation, where programmed switching of the propulsion system (a peristaltic pump) allows the extracting phase to be passed iteratively by the detection point in a back-and-forth motion that enriches it gradually with the extracted species [9-11]. This type of system is much too Complex to be considered a sensor, though in addition, the extraction process is not completely simultaneous with detection. 

The stripped extractant is now free to diffuse back to the exterior oil-water interface to extract another selenium anion and the extracted selenite or selenate ion is now trapped in the interior aqueous phase. The net result of this simultaneous extraction and stripping process is an aqueous internal phase with significantly higher selenium concentrations than the original process stream. Thus, one has significantly reduced the volume of the waste stream. The overall reactions are ... 

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