Columns pulsing methods

In a typical pulse experiment, a pulse of known size, shape and composition is introduced to a reactor, preferably one with a simple flow pattern, either plug flow or well mixed. The response to the perturbation is then measured behind the reactor. A thermal conductivity detector can be used to compare the shape of the peaks before and after the reactor. This is usually done in the case of non-reacting systems, and moment analysis of the response curve can give information on diffusivities, mass transfer coefficients and adsorption constants. The typical pulse experiment in a reacting system traditionally uses GC analysis by leading the effluent from the reactor directly into a gas chromatographic column. This method yields conversions and selectivities for the total pulse, the time coordinate is lost. 

The dependence of HETP upon solution flow rate when exchanging the H -Na pair of ions between 0.2 M chloride solution and cation exchanger, KU-2 x 8, with beads 0.25-0.50 mm in diameter, has been compared in the literature for columns of different types [76,84]. The pulse method which has been used possesses the advantage that with the pulsed injection of a small amount of substance all physicochemical and hydrodynamic characteristics of the system remain invariably determined by the primary substance, Na" " ion in this case. Dynamic parameters obtained characterize the process when the concentration range of impurity is low. 

Jandera et al. [179] showed excellent agreement between the adsorption isotherm data obtained by the pulse method for benzophenone on a Cig-bonded silica using two columns of different diameters, 3.2 and 0.32 mm, made with exactly the same packing material. They showed that the pulse method might be far easier to use for accurate measurements than the FA method. 

Concentration pulse chromatography (also called elution on a plateau, step and pulse method, system peak method, or perturbation chromatography) is experimentally much simpler [100,103,104]. The same experimental procedure is used as for the determination of smgle-component isotherms. First, the column is equilibrated with a solution of the multicomponent mixture of interest in the mobile phase. When the eluent has reached the composition of the feed to the column, a small pulse of the pure mobile phase (vacancy) or of a solution having a composition different from that of the plateau concentration (see end of this section) is... 

In most cases, chromatography is performed with a simple initial condition, C(f = 0,z) = q t = 0,z) = 0. TTie column is empty of solute and the stationary and mobile phases are under equilibrium. There are some cases, however, in which pulses of solute are injected on top of a concentration plateau (see Chapter 3, Section 3.5.4). The behavior of positive concentration pulses injected xmder such conditions is similar to that of the same pulses injected in a column empty of solute and they exhibit similar profiles. Even imder nonlinear conditions (high plateau concentration), a pulse that is sufficiently small can exhibit a quasi-linear behavior and give a Gaussian elution profile. Its retention time is linearly related to the slope of the isotherm at the plateau concentration. Measuring this slope is the purpose of the pulse method of measurement of isotherm data. Large pulses may also be injected and they will give overloaded elution profiles similar to those obtained with a column empty of solute. 

Some investigators have developed methods in which the reaction proceeds directly in the chromatographic system. The most important of these are the so-called pulse chromatographic methods. In such methods of studying the kinetics of chemical transformations, a pulse of a volatile compound whose transformation provides information on the reaction taking place in the reactor column is fed to the inlet of the reactor column in a flow of carrier gas. In the pulse methods the chemical reaction and separation (analysis) are integrated into a single procedure. 

Note also the possibility of using in kinetic studies the reacting pulse method when the reaction proceeds in a reactor column between two pulses of volatile compounds which are introduced into the column successively and move along the reactor at different speeds (the second pulse overtakes the first one) [41]. The pulse methods have been developed and are applied primarily for studying catalytic processes. Kokes et al. [42] were the first to propose this application. Important contributions to the development of pulse methods were made by Yanovsky and co-workers [43,44] Rosental [45], Langer and Patton [46], and Keller and Giddings [47]. 

The pulse chromatographic method was also used to study the kinetics of the etherification of alcohols of various structures with acetic anhydride [74]. The reaction kinetics were studied for high-boiling alcohols the volatile reagent (acetic anhydride) was fed into the rdactor column in the form of a pulse, and the involatile one (alcohol) was present in the column reactor as the stationary phase. Unlike the pulse method used in studying the reactions involved in diene synthesis, in the etherification of an alcohol with acetic anhydride one of the reaction products (acetic acid) is eluted from the column reactor after the starting component (acetic anhydride). The reaction of alcohol etherification was examined at 80—130 C. The mixture of acetic anhydride with the standard (benzene) was pulsed into the reactor column in which the alcohol under study served as the stationary phase. Various extents of reaction were achieved by varying the carrier gas flow-rate. Table 2.6 summarizes the kinetic characteristics of the etherification of alcohols with acetic anhydrides [74]. The rate constants decrease in the order primary > secondary > tertiary. 

A characteristic feature of ESI is that the sample can be pumped into the mass analyser continuously. MALDI, on the other hand, is a pulsed method which requires a dry sample. Thus, ESI-MS can be coupled directly to liquid separation methods such as RP-HPLC (section 2.3.1) and CE (section 3.3). As the sample emerges from the separation column it is directly pumped into the electrospray chamber. As outlined earlier, MALDI-TOE is capable of separating... 

Here p is the bed porosity. The dead time % is the time reqrrired for the passage of a sample pulse through the empty volume of the connecting tube from the injection point to the detector plus the void space in the packed column This method can be applied to determine binary adsorption isotherms (Harlick et al. 2003, 2004). 

L. Burkhart, A. Survey of Simulated Methods for Modeling Pulsed Sieve-Plate Extraction Columns, UCRL-15101, Ames Laboratory, Iowa State University, Ames, Iowa, 1979. 

Method of Moments The first step in the analysis of chromatographic systems is often a characterization of the column response to sm l pulse injections of a solute under trace conditions in the Henry s law limit. For such conditions, the statistical moments of the response peak are used to characterize the chromatographic behavior. Such an approach is generally preferable to other descriptions of peak properties which are specific to Gaussian behavior, since the statisfical moments are directly correlated to eqmlibrium and dispersion parameters. Useful references are Schneider and Smith [AJChP J., 14, 762 (1968)], Suzuki and Smith [Chem. Eng. ScL, 26, 221 (1971)], and Carbonell et al. [Chem. Eng. Sci., 9, 115 (1975) 16, 221 (1978)]. 

Elution with salt pulses A multiple step elution is performed by the introduction of, for example, 5%, 10%, 25%, 50%, and 100% of 1.5 M sodium chloride in 19 mM phosphate buffer (pH 2.5) containing 5% methanol. Each step is for 10 min and run at 0.5 mL/min. This elution method compromises analytical system dimensionality, as the peak capacity of the ion-exchange chromatography (IEX) step is equal at most to the number of salt steps. However, in the second dimension only one or two columns are needed and there is no particular limitation in the second dimension separation time as peptides are eluted in portions in a controlled manner. However, the number of salt steps is limited by the total analysis time. In this case the multidimensional system is relatively simple. 

In the commonly used pulse technique introduced by Kokes et al. C2), a spike of reagents is introduced into a carrier gas flowing over the catalyst and after that over a gas chromatography column which is used to determine the conversion of reagents to products. In this method, the concentrations of reagents in the catalyst bed are not defined, and kinetic measurements relating reaction rates to concentrations are impractical. 

A recent and extremely important development lies in the application of the technique of liquid extraction to metallurgical processes. The successful development of methods for the purification of uranium fuel and for the recovery of spent fuel elements in the nuclear power industry by extraction methods, mainly based on packed, including pulsed, columns as discussed in Section 13.5 has led to their application to other metallurgical processes. Of these, the recovery of copper from acid leach liquors and subsequent electro-winning from these liquors is the most extensive, although further applications to nickel and other metals are being developed. In many of these processes, some form of chemical complex is formed between the solute and the solvent so that the kinetics of the process become important. The extraction operation may be either a physical operation, as discussed previously, or a chemical operation. Chemical operations have been classified by Hanson(1) as follows ... 

The method of calculation introduced in this chapter not only allows an exact determination of the column diameter for nonpulsed sieve tray columns, but also allows a good estimation of the diameters of pulsed and stirred extractors. For the latter, however, more exact specific equations exist for the flooding point, see for example [1,4]. 

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