Pulse chromatographic system

OSC and OSCC measurements can be carried out in a pulse chromatographic system which was described earlier [2]. The experimental setup is schematized below on Fig. 7.1. 

The technique just described requires the porous medium to be sealed in a cell, so It cannot be used with pellets of irregular shape or granular material. For such materials an alternative technique Introduced by Eberly [64] is attractive. In Eberly s method the porous pellets or granules are packed into a tube through which the carrier gas flows steadily. A sharp pulse of tracer gas is then injected at the entry to the tube, and Its transit time through the tube and spreading at the exit are observed. A "chromatographic" system of this sort is very attractive to the experimenter,... 

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)]. 

Ion chromatography (see Section 7.4). Conductivity cells can be coupled to ion chromatographic systems to provide a sensitive method for measuring ionic concentrations in the eluate. To achieve this end, special micro-conductivity cells have been developed of a flow-through pattern and placed in a thermostatted enclosure a typical cell may contain a volume of about 1.5 /iL and have a cell constant of approximately 15 cm-1. It is claimed15 that sensitivity is improved by use of a bipolar square-wave pulsed current which reduces polarisation and capacitance effects, and the changes in conductivity caused by the heating effect of the current (see Refs 16, 17). 

The pulse chromatographic technique was used to study low coverage adsorption of linear and monobranched alkanes in the CS-C8 range on two different 10-membered ring zeolites, ZSM-22 and ZSM-23. Henry adsorption constants, enthalpies of adsorption, preexponential Actors of the van t Hoff equation and separation fectors were determined. A detailed interpretation of the experimental data confirmed the previously proposed pore-mouth adsorption mode for branched alkanes on zeolite ZSM-22, where these molecules do not have access into the depth of the pores, but rather point their linear part herein. Zeolite ZSM-23 shows adsorption properties between zeolite ZSM-22 and zeolite ZSM-S, which is a shape selective zeolite where normal and monobranched molecules have both access into its pore system. 

A pulse reaction technique was used to study (1) the behavior of the catalysts in the first step of the reaction, (2) the initial carbon deposition and, (3) the effects of carbon deposition on conversion and selectivity. Experiments were carried out at 853 K by injecting pulses of pure propane (pulse volume 0.50 cm 8TP) to the catalytic bed, which was maintained under flowing He (30 ml min ) between two successive pulses. The samples were previously reduced for 3h at 853 K under flowing H2. The reaction products were analyzed in a FID chromatographic system with a packed column (Porapack Q). 

After selecting the chromatographic system the operation mode of the batch reactor has to be chosen. High productivities require a high throughput. Therefore, pulsed operation is used (Fig. 8.8). Reactants are supposed to be injected as a rectangle pulse of period tcic le and duration tinj. These parameters are strongly affected by the reaction kinetics, reaction stoichiometry and adsorption isotherm. 

A pump delivers the mobile phase through the chromatographic system. In general, either single-piston or dual-piston pumps are employed. A pulse-free flow of the eluent is necessary for the sensitive UV/Vis and amperometric detectors. Therefore, pulse dampeners are used with single-piston pumps and electronic circuitry with dual-piston pumps. 

The fundamentals of the phenomena involved in the pulse methods are discussed elsewhere in detail (Chapter 13). Suffice to say here that when a small sample of an n-component mixture is injected into a chromatographic system the mobile phase of which contains p additives, n + p peaks are formed and eluted, as illustrated in Figure 4.27. Although in pulse methods, tracers have the same isotherms as the components studied, their concentrations are different and, accordingly, the migration rates of the tracer peaks are different from the velocities of the component perturbations at the plateau concentrations. Thus, upon injection of one tracer (n = 1) in a the binary solution (p = 2) used as the mobile phase, we expect to see three peaks on the plateau of each component, as shown in Figure 4.27 in the case of a binary mixture, assuming that the two components... 

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. 

The kinetic characteristics are determined in GC methods both directly and indirectly. In direct pulse chromatographic methods the reaction rate can be established by the direct determination of the amount (concentration) of the reacting component, whereas in indirect methods this is done on the basis of the variations with time of the chromatographic properties of the reacting system, which are usually determined from the relationship between and the retention times of the non-reacting components and the composition of the reaction mixture used as the stationary phase [58]. Pulse chromatography... 

After desorption target compounds are transferred directly onto the chromatographic system cryofocused or preconcentrated in cryotraps in order to eliminate interferences and to inject a sharp pulse of sample onto the separation column. 

The only possibility to deliver a pulse-free mobile-phase flow through the chromatographic system is the syringe pump design (Fig. 7). Frequently used in the past for micro-LC applications (flow rates below 10 p.l/min), the syringe pump suffers from limitation of the delivered... 

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