Rate measurement extraction

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. 

Flow rate and extraction time. Dynamic techniques for the extraction of carotenoids with SC-CO2 use flow rates that vary from 0.5 to 15 mL/min (measured at extraction temperature and pressure) with different effects depending on the matrix (Rozzi and others 2002 Subra and others 1998 Saldana and others 2006). Subra and others (1998) extracted (3-carotene from 1 to 2.5 g freeze-dried carrots and studied the effect of flow rates (0.4 and 1.2 liter/min) they obtained higher yields of (3-carotene at a flow rate of 1.2 liter/min. Sun and Temelli (2006) also evaluated the effect of flow rate (0.5 and 1.0 liter/min) on the extraction of (3-carotene with SC-CO2 + canola oil. The total carotenoids yield increased with flow rate, ranging from 934.8 to 1,973.6 pg/g dry carrot at C02 flow rates from 0.5 to 2 liter/min (measured at STP), respectively (Sun and Temelli, 2006). However, the lycopene yield decreased from 38.8% to 8% as flow rate was increased from 2.5 to 15 mL/min (measured at extraction temperature and pressure) (Rozzi and others 2002). 

Tests have been carried out on the rate of extraction of benzoic acid from a dilute solution in benzene to water, in which the benzene phase was bubbled into the base of a 25 mm diameter column and the water fed to the top of the column. The rate of mass transfer was measured during the formation of the bubbles in the water phase and during the rise of the bubbles up the column. For conditions where the drop volume was 0.12 cm3 and the velocity of rise 12.5 cm/s, the value of Kw for the period of drop formation was 0.000075 kmol/s m2 (kmol/m3), and for the period of rise 0.000046 kmol/s m2 (kmol/s m3). 

The apparent values of the rate constants of the solvent extraction reaction are nsnally evaluated by measuring the rate of extraction of as function of [BH] (at [H ] constant), of [H ] (at [BH] constant), and of [MB] (at [H ] and [BH] constant). The experimental conditions are usually chosen in such a way that the reaction can be assumed pseudo-first-order for [M ]. The apparent rate constants are evaluated from the slope of the straight lines obtained by plotting... 

To derive a rate of extraction in terms of easily measurable quantities, we express the concentration of the reagent adsorbed at the liquid-liquid interface as function of its bulk organic concentration. This can be done by utilizing the Langmuir s adsorption law that is,... 

Equations (5.34) and (5.47) have the same dependence on the concentration variables. The only difference is that for the interfacial reaction the apparent rate constants are directly proportional to the ratio S = QN. This dependency can be experimentally verified by measuring the apparent rate of extraction as a function of the interfacial area and the volume of the phases. A plot of the apparent rate constant of the forward rate of extraction vs. S must yield a straight line through the origin of the axes when case 2 holds. 

In the moving drop technique (also described in Chapters 7 and 9), a drop of the organic or aqueous phase is produced at the end of a vertical column filled with the other phase. The drop travels along the tube, during which extraction occurs across the drop surface. By measuring the time of traveling, the drop size, and from the volume of collected drops, it is possible to evaluate the rate of extraction (see Chapter 9 for a detailed discussion of drop behavior and mass transfer). 

Clearly the concept of a stage has no meaning in such a tower. Instead, we deal with differential transfer units, which are a measure of the change in concentration per unit of difference in concentration (recall that the rate of extraction is determined largely by the difference between the actual and the equilibrium concentration of a solute, or driving force ). 

The most commercially important mechanism of all is the kinetics of solute transfer from an aqueous to a reverse micelle phase. The kinetics of extraction of metal ions have not received the same research attention as the extraction capacity of W/O microemulsions. As the mechanism of extraction of metal ions is chemical, the effect of creating a microemulsion in an organic phase that contains the reactant can be measured experimentally. Results indicate that, as in the case of extraction equilibrium, the rate of extraction may increase substantially by the presence of the microemulsion as compared with the conventional system [20,38,44] or decrease it to... 

In 1985, Ruzicka and Hansen established the principles behind flow injection optosensing [13-15], which has subsequently been used for making reaction-rate measurements [16], pH measurements by means of immobilized indicators [17,18], enzyme assays [19], solid-phase analyte preconcentration by sorbent extraction [20] and even anion determinations by catalysed reduction of a solid phase [21] —all these applications are discussed in Chapters 3 and 4. Incorporation of a gas-diffusion membrane in this type of sensor results in substantially improved sensitivity (through preconcentration) and selectivity (through removal of non-volatile interferents). The first model sensor of this type was developed for the determination of ammonium [13] and later refined by Hansen et al. [22,23] for successful application to clinical samples. 

Fig. 5. Plot showing the effect of radiation field intensity on U02 dissolution rate (data extracted from Christensen Sunder 1996). Dissolution rates were obtained by electrochemical measurements. A significant enhancement in the reaction rate is observed with dose and in the presence of oxygen.

It seems reasonable to assume that if the same flow regime can be produced, it should be possible to predict the rates of extraction for a large vessel from data measured in a small one. Rushton (R4) has explained in detail how this may be done under conditions where dynamic and geometric similarity of the two systems is maintained. The success... 

Because dopachrome is subject to autooxidation and polymerization reactions, the rate measurements should not be continued past 3 min. Similarly, the enzyme extract should be... 

In supercritical extraction, the dissolution step is diffusion-controlled and the transport properties of the supercritical phase govern the rate of extraction. Due to difficulties in measuring time-dependent phenomena at high... 

A major difficulty with this analysis, however, is that the assumption AS t % 0 requires that the solvation environment of the transition state is unaffected by its proximity to the electrode surface (Sect. 3.4). Stated equivalently, it is often expected that the temperature-dependent work terms required to extract kscorr from k ob contain large components from short-range solvation and other factors in addition to the usual "electrostatic doublelayer effects (Sect. 2.4 and 4.6). As noted in Sect. 2.3, the situation is somewhat more straightforward for surface-attached reactants since then the effects of work terms at least partly disappear. This question underscores the inevitable difficulties involved in extracting quantitative information on electron-transfer barriers from rate measurements. 

GAP-DEPENDENT APPARENT SHEAR RATE. Indirect evidence of slip, as well as a measurement of its magnitude, can be extracted from the flow curve (shear stress versus shear rate) measured at different rheometer gaps (Mooney 1931). If slip occurs, one expects the slip velocity V (a) to depend on the shear stress a, but not on the gap h. Thus, if a fluid is sheared in a plane Couette device with one plate moving and one stationary, and the gap h is varied with the shear stress a held fixed, there will be a velocity jump of magnitude Vs(ct) at the interfaces between the fluid and each of the two plates. There will also be a velocity gradient >(a) in the bulk of the fluid thus the velocity of the moving surface will be y = 2V,(a) + y (a)/i. The apparent shear rate V/h will therefore be... 

Note that Eq. (n) applies only to one-electron reactions. Whereas the theoretical treatment can be extended formally to multielectron processes, such reactions commonly occur in microscopically separate, one-electron steps. If the first step is rate controlling, n can be set at unity in Eq. (b) in 12.3.7.1, regardless of the number of electrons transferred in the overall reaction. A general difficulty for such multistep processes is that AG, cannot be extracted from rate measurements unless the standard potential for the redox couple comprising the elementary reaction is known. For multielectron reactions, only the formal potential for the overall reaction normally can be obtained. Similar remarks apply to other multistep electrode reactions, such as those involving phase transfer (e.g., metal deposition or gas evolution). 

A pragmatic approach to extraction kinetics is to measure extraction rates under mass transfer conditions that are similar to those expected in processing equipment and that are characterized well enough io allow estimation of interfacial concentrations. With such measurements ons can dalarmine whether the kinetics of a particular system are fast or slow computed to mass transfer, and if they are slow, the rate can be correlaled with interfacial conditions. Several methods are used to measure extraction kinetics ihass include measurements with (l) well-stirred vessels, (2) single drops or jets, and (3) the Lewis cell. 

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