Dynamic pumping modes

In the dynamic extraction mode, the extractant is pumped through the sample into the collection system or interface when the extractor is connected on-line to a chromatograph or detector. In this way, the supercritical fluid is passed through the sample once before it is driven to the restrictor. 

The results have been recently obtained by Olsen et al. [38], and they show that even for almost vanishingly small e, which is inversely proportional to the initial mean value of the number of the pump mode photons, usually very large, the quantum fluctuations have huge macroscopic effect on the system dynamics. It is evident that the quantum noise, which is always present, is responsible for the oscillations between the two regimes of second-harmonic generation and downconversion. The period of oscillation is becoming infinite as e vanishes. 

An Sl E system can be operated in one of two ways. In the dynamic extraction mode, the valve between the extraction cell and the restrictor remains open so that the sample is continually supplied with fresh supercritical fluid and the extracted material flows into the collection vessel where depressuriz-ation occurs. In the static extraction mode, the valve between the extraction cell and the restrictor is closed and the extraction cell is pressurized under static conditions. After a suitable period, (he exit valve is opened and the cell contents arc iransferred through the rcsiricior by a dynamic flow of fluid from the pump. The dynamic mode is more widciv used than the static mode. 

These above-mentioned dynamic operating mode plants have, besides pumps, no moving parts and produce the product in liquid form. The scale-up of the plants is quite easy, just by adding tubes or a new apparatus of tubes. AcryUc acid, benzoic acid, bisphenol A, and napthalene are few examples of materials which are purified with such a dynamic solid layer operating mode (Sulzer falling film process). 

The dynamics of fast processes such as electron and energy transfers and vibrational and electronic deexcitations can be probed by using short-pulsed lasers. The experimental developments that have made possible the direct probing of molecular dissociation steps and other ultrafast processes in real time (in the femtosecond time range) have, in a few cases, been extended to the study of surface phenomena. For instance, two-photon photoemission has been used to study the dynamics of electrons at interfaces [ ]. Vibrational relaxation times have also been measured for a number of modes such as the 0-Fl stretching m silica and the C-0 stretching in carbon monoxide adsorbed on transition metals [ ]. Pump-probe laser experiments such as these are difficult, but the field is still in its infancy, and much is expected in this direction m the near fiitiire. 

Pump-probe absorption experiments on the femtosecond time scale generally fall into two effective types, depending on the duration and spectral width of the pump pulse. If tlie pump spectrum is significantly narrower in width than the electronic absorption line shape, transient hole-burning spectroscopy [101. 102. 103. 104. 105. 106. 107. 108. 109. 110. 111. 112 and 113] can be perfomied. The second type of experiment, dynamic absorption spectroscopy [57, 114. 115. 116. 117. 118. 119. 120. 121 and 122], can be perfomied if the pump and probe pulses are short compared to tlie period of the vibrational modes that are coupled to the electronic transition. 

Although the concern is primarily for the response of the piping. system, the possibiliis of dynamic coupling with the containment structure should not be neglected. A concern is whether or not the secondary shield wall will withstand the dynamic interaction between the walls and the pump. This is answered by examining the mode shapes if there were no coupling between the walls and the pump. 

The natural frequency, co associated with the mode shape that exhibits a large displacement of the pump is compared with the fundamental frequency, of the wall. If co is much less than ru, then the dynamic interaction between the wall and the loop may be neglected, but the kinematic constraint on the pump imposed by the lateral bracing is retained. If nearly equals nr , the wall and steam supply systems are dynamically coupled. In which case it may be sufficient to model the wall as a one-mass system such that the fundamental frequency, Wo is retained. The mathematical model of the piping systems should be capable of revealing the response to the anticipated ground motion (dominantly translational). The mathematics necessary to analyze the damped spring mass. system become quite formidable, and the reader is referred to Berkowitz (1969),... 

The double proton transfer of [2,2 -Bipyridyl]-3,3 -diol is investigated by UV-visible pump-probe spectroscopy with 30 fs time resolution. We find characteristic wavepacket motions for both the concerted double proton transfer and the sequential proton transfer that occur in parallel. The coherent excitation of an optically inactive, antisymmetric bending vibration is observed demonstrating that the reactive process itself and not only the optical excitation drives the vibrational motions. We show by the absence of a deuterium isotope effect that the ESIPT dynamics is entirely determined by the skeletal modes and that it should not be described by tunneling of the proton. 

Once the extraction is complete, the static/dynamic selection valve is repositioned to the dynamic mode to allow flow. Subsequently, pressure and density are rapidly reduced to prevent significant losses of the supercritical fluid from the syringe pump tank and the extraction effluent, which is being transferred for collection. With a non-re-stricted transfer, the flow of supercritical fluid effluent is rapid. This desire for rapid depressurization led to the development of a delivery nozzle which would ensure collection of the extracted solutes without losses. Details of this delivery system can be found in the next section. 

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. 

In-situ chemical derivatizations were performed by adding 0.5 to 2 mL of the derivatization reagent, trimethylphenyl ammonium hydroxide (TMPA) in methanol (Eastman Kodak Company, Rochester, NY) directly to the sample cell of an ISCO model SFX extraction unit. The sample and reagent were pressurized with C02 (typically 400 to 500 atm), and heated to an appropriate temperature (typically 80°C). The derivatization was performed in the static SFE mode for 5 to 15 minutes, then the derivatized analytes were recovered by dynamic SFE using a typical flow rate of 0.6 to 1.0 mL/min (measured as liquid C02 at the pump). No other sample preparation of the derivatized extracts was performed prior to GC analysis. All GC analyses were performed using Hewlett-Packard 5890 GCs with appropriate detectors and HP-5 columns (25 m X 250 pm i.d., 0.17 pm film thickness). 

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