Static extractor columns

A. Frank Seibert, Ph.D., P.E. Technical Manager, Separations Research Program, The University of Texas at Austin Member, American Institute of Chemical Engineers (Liquid-Liquid ITispersion Fundamentals, Process Fundamentals and Basic Calculation Methods, Hydrodynamics of Column Extractors, Static Extraction Columns, Process Control Considerations, Membrane-Based Processes)... 

FIG. 15-32 Schematic of common static extractors, (a) Spray coiumn. ib) Packed column, (c) Sieve tray column. 

Choice of Dispersed Phase In general, formation of dispersed drops is preferred over formation of films or rivulets in order to maximize contact area and mass transfer. Static extractors generally are designed with the majority phase dispersed in order to maximizie interfacial area needed for mass transfer i.e., the phase with the greatest flow rate entering the column generally is dispersed. The choice of dispersed phase also depends upon the relative viscosity of the two phases. If one phase is particularly viscous, it may be necessary to disperse that phase. 

Supercritical fluid extraction (SFE) can be coupled to SFC using a series of switching valves and either a loop or an accumulator trap interface [79,172,174,187-192]. The loop interface is used with a closed-loop static extractor, sometimes equipped with a recirculating pump. The fluid from the extraction cell passes continuously through the injection valve loop and back to the extraction cell. Injection of an aliquot of the extract onto a packed column is made by periodically switching the loop so that it is in-line with the flow of mobile phase to the separation column. This approach is used to determine fundamental parameters of the extraction process more so than for analysis. 

Figure 6. Static/Dynamic Selection Valve Setup for Six Vessel Multi-vessel Extractor. Extraction Effluent Received from One Tandem Column Switching Valve.

With the 12-vessel extractor, the 1/8" valve receives the extraction effluent from the vessels in tandem column selectors 1 and 2 (TCS-1 and TCS-2) into two separate ports 1 and 4 as shown in Figure 7. During the static mode, the counter-current valves, i.e. modifier pump valves (MP-3 and MP-4) are closed. Pressure build-up for static extraction then follows. Valves MP-3 and MP-4 are mounted close to the ports so that no accumulation of extract occurs. The valves are connected via a stainless steel tee (T2), to the modifier pump which is also used for flushing the lines after the extractions have been conducted. In the dynamic mode, extract flows from the unblocked ports of 1 and 4 to ports 5 and 6 then through to the delivery nozzles. 

Figure 7.16A depicts a flexible SFE-HPLC coupled assembly developed by Ischi and Haerdi [106] that consists of three main parts [viz. the SFE system (Al), the interface (A2) and the HPLC system (A3)] each furnished with appropriate valves operating as shown in Fig. 7.16B. Thus, valve 5 in Fig. 7.16A is used to provide extraction with or without a modifier, via a tee connector on the other hand, valve 10 allows switching between static and dynamic extraction. The former is done by having the valve close the outlet of the extraction cell after the desired temperature is reached. By switching the valve back, the dynamic state is restored. Valve 13 enables trapping of the extracted analytes, either on a C, silica column placed in an oven for on-line preconcentration and insertion of non-polar or low-polar analytes into the chromatograph after elution or into a liquid phase to implement an off-line operation. When polar ionic analytes are to be preconcentrated, the eluent from the extractor is diverted to valve 18 and retained on the ion-exchange material packed in the column. Preconcentration of both non-polar, low-polar and polar ionic analytes can be accomplished by using both valves (13 and 18) [106],...

Fig. 6.3-4 Typical performance data of ten different extractor designs for the system tolulene/acetone/water. MS mixer-settler, SE static sieve tray column, PC static packed packed column, RDC rotating disc contactor, PSE pulsed sieve tray column, PPC pulsed packed column, RZE agitated cell extractors...

The Graesser contactor takes an extreme position in the diagram. It has the highest separation efficiency (10 stages per meter) but the lowest capacity (1-2 m/h). The other extremum takes the static sieve tray extractor with only one equilibrium stage per meter and up to 50 m/h capacity. The capacity of a pulsed sieve tray column is as high as 30 m/h with a separation efficiency of 5 to 6 stages per meter. 

Static PFE has been coupled to solid-phase extraction (SPE) with both SPE disks placed directly into the extraction chamber jointly with the sample and by using a minicolumn packed with the sorbent material and connected at the end of the extractor by means of a flow injection manifold. The minicolumn can be placed either in the transport tube or in the loop of an auxiliary injection valve of a flow-injection manifold, thus enabling elution in the direction opposite to retention. Analytes can be eluted manually, by disconnecting the column from the extractor, or... 

Dynamic PFE usually requires implementing a concentration step prior to the determinative step, and because the extracted analytes are dissolved in a liquid (usually aqueous) phase, SPE is a highly useful tool for avoiding the dilution effect. For this purpose, SPE cartridges and columns packed with appropriate sorbents and coupled online to the extractor outlet can be employed in the same way as commented on for static PEE. Miniaturized retention has also been developed by using solid-phase microextraction (SPME). 

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