Extractor static

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)... 

A number of semiempirical drop size data correlations have been developed for different types of extractors (static and agitated) including a term for holdup. See Kumar and Hartland, Ind. Eri2. Chem. Res., 35(8), pp. 2682-2695 (1996) and Kumar and Hartland, Chap. 17 in Liquid-Liquid Extraction Equipment, Godfrey and Slater, eds. (Wiley, 1994). These equations predict a characteristic drop size. They do not provide information about the drop size distribution or the minimum drop size. For discussion of minimum drop size, see Zhou and Kresta, Chem. Eng. Sci., 53(11), pp. 2063-2079 (1998). 

The sohd can be contacted with the solvent in a number of different ways but traditionally that part of the solvent retained by the sohd is referred to as the underflow or holdup, whereas the sohd-free solute-laden solvent separated from the sohd after extraction is called the overflow. The holdup of bound hquor plays a vital role in the estimation of separation performance. In practice both static and dynamic holdup are measured in a process study, other parameters of importance being the relationship of holdup to drainage time and percolation rate. The results of such studies permit conclusions to be drawn about the feasibihty of extraction by percolation, the holdup of different bed heights of material prepared for extraction, and the relationship between solute content of the hquor and holdup. If the percolation rate is very low (in the case of oilseeds a minimum percolation rate of 3 x 10 m/s is normally required), extraction by immersion may be more effective. Percolation rate measurements and the methods of utilizing the data have been reported (8,9) these indicate that the effect of solute concentration on holdup plays an important part in determining the solute concentration in the hquor leaving the extractor. 

Static mixers Vibratmg-plate extractors Ultrasonic extractors Parametric pumping extractors... 

We have developed a multi-vessel extractor in which six or twelve samples can be analyzed in a segmented parallel fashion in three hours or less. Timing has been set up so that periods of static and dynamic extraction overlap within the individual vessels. This time overlap eliminates deadtime which would occur if the samples were extracted serially. 

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. 

Samples of sand spiked with 36 nitroaromatic compounds, 19 haloethers, and 42 organochlorine pesticides, and a standard reference soil (certified for 13 polynuclear aromatic hydrocarbons, dibenzofuran, and pentachlorophenol) were extracted with supercritical carbon dioxide in a two- or four-vessel supercritical fluid extractor to establish the efficiency of the extraction and the degree of agreement of the parallel extraction recoveries. Furthermore, the many variables that influence the extraction process (e.g., flowrate, pressure, temperature, moisture content, cell volume, sample size, extraction time, modifier type, modifier volume, static versus dynamic extraction, volume of solvent in the collection vessel, and the use of glass beads to fill the void volume) were investigated. 

Static pressurized hot solvent extraction (SPHSE), which shall henceforward be referred to as accelerated solvent extraction (ASE) for the reasons stated above, is the less flexible PHSE mode in terms of alteration or coupling to other techniques but is so far the more widely used — in fact, it accounts for over 65% of the PHSE publications reported since 1994. This is mainly the result of the sole commercially available extractor (the Dionex 200 model) implementing the static mode alone and also of the large number of studies conducted by different or even the same authors on the same analytes in the same matrices, which have therefore contributed little or nothing new in this area [58-63]. 

Accelerated solvent extraction as implemented in commercial equipment is basically discrete in nature, so it is rarely coupled to other operations of the analytical process. In fact, only in two reported applications was the static mode coupled on-line to other operations such as chromatographic separation, preconcentration and detection. Both used custom extractors as the compact design of the commercial models precluded their adaptation. 

In many cases, the extraction process includes an additional, static extraction step. To this end, the outlet valve (OV in Fig. 6.10) is supplemented by an inlet valve (IV in Fig. 6.10) between the high-pressure pump and the extractor. Once the system has been pressurized, the inlet valve is closed, the high-pressure pump stopped and the oven temperature raised. After the desired temperature is reached, the system is maintained under a static regime with both valves closed, and then the valves are opened and the pump restarted to allow the solvent to flow over the dynamic extraction period. Several studies [147,150,153] have shown that a combination of both extraction modes can result in substantially improved extraction and shorter extraction times. This is commented on in greater detail in discussing specific applications below. 

The DPHSE technique has also been used for the determination of organic pollutants and metals in fly ash and coal, respectively. The extraction of dioxins [48,179] and PAHs [180] from fly ash was accomplished with toluene [48,180] or a toluene-methanol mixture [179], with results as good as or even better than those provided by Soxhiet extraction for 24 h. On the other hand, the extraction of major ash-forming elements (Fe, AI, Ca, Mg, Na and K) [148] and minor inorganic pollutants (As, Se and Hg) [46] from coal was done with acidified water. In the latter case, a combination of static and dynamic extraction was found to provide quantitative recoveries within a shorter time and with less dilution of the extracts than dynamic extraction alone. Acidified water is more corrosive than pure water, so the high temperatures required for extraction (150-200°C) call for the use of an extractor made of a material more corrosion-resistant than steel hastel-loid. However, in proportions above 4%, nitric acid — the acidulant most frequently added to the water — has been found to result in clogging of the system and the restrictor, so the recommended acid concentration is much lower than that. 

Many SFE-SFC applications use a second pump to pressurize the extraction chamber, the first pump being solely employed to effect the chromatographic separation. These systems pose few technical problems and are highly flexible by virtue of the extractor and chromatograph operating independently (in a static or dynamic manner). 

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],...

In nonagitated (static) extractors, drops are formed by flow through small holes in sieve plates or inlet distributor pipes. The maximum size of drops issuing from the holes is determined not by the hole size but primarily by the balance between buoyancy and interfacial tension forces acting on the stream or jet emerging from the hole. Neglecting any viscosity effects (i.e., assuming low dispersed-phase viscosity), the maximum drop size is proportional to the square root of interfacial tension a divided by density difference Ap ... 

The proportionality constant typically is close to unity [Seibert and Fair, Ind. Eng. Chem. Res., 27(3), pp. 470-481 (1988)]. Note that Eq. (15-42) indicates the maximum stable drop diameter and not the Sauter mean diameter (although the two are proportionally related and may be close in value). Smaller drops may be formed at the distributor due to jetting of the inlet liquid through the distributor holes or by mechanical pulsation of the liquid inside the distributor [Koch and Vogelpohl, Chem. Eng. Technol., 24(12), pp. 1245-1248 (2001)]. In static extractors, hydrodynamic stresses within the main body of the... 

Common Features and Design Concepts Static extractors... 

---