Coupled column sections

First consider thermal coupling of the simple sequences from Fig. 5.1. Figure 5.14a shows a thermally coupled direct sequence. The reboiler of the first column is replaced by a thermal coupling. Liquid from the bottom of the first column is transferred to the second as before, but now the vapor required by the first column is supplied by the second column instead of by a reboiler on the first column. The four column sections are marked as 1, 2, 3, and 4 in Fig. 5.14a. In... 

Figure 4.1 Schematic diagram of a coupled column system. The first column (ID) is connected to the second column (2D) tlirough the interface or valve system. The interface can be a diiect coupling, a live T-union, a complex multiport valve, or a thermal or cryogenic modulation system. The stimulus can be the switching of the valve, abalancing pressure to divert flow towards 2D, an added flow that is used in pressure tuning, or the drive mechanism for the modulator. The line to detector 1 will normally be a non-retaining section of column. In a two-oven system, ID and 2D will be in different ovens the dotted line indicates separately heated zones.

The partitioned thermally coupled prefractionator in Figure 11.14c can be simulated using the arrangement in Figure 11.14b as the basis of the simulation. However, like side-rectifiers and side-strippers, fully thermally coupled columns have some important degrees of freedom for optimization. In the fully thermally coupled column, there are six column sections (above and below the partition, above and below the feed in the prefractionator and above and below the sidestream from the main column side of the partition). The degrees of freedom to be optimized in partitioned columns are ... 

The significant intrinsic limitation of SEC is the dependence of retention volumes of polymer species on their molecular sizes in solution and thus only indirectly on their molar masses. As known (Sections 16.2.2 and 16.3.2), the size of macromolecnles dissolved in certain solvent depends not only on their molar masses but also on their chemical structure and physical architecture. Consequently, the Vr values of polymer species directly reflect their molar masses only for linear homopolymers and this holds only in absence of side effects within SEC column (Sections 16.4.1 and 16.4.2). In other words, macromolecnles of different molar masses, compositions and architectures may co-elute and in that case the molar mass values directly calculated from the SEC chromatograms would be wrong. This is schematically depicted in Figure 16.10. The problem of simultaneous effects of two or more molecular characteristics on the retention volumes of complex polymer systems is further amplifled by the detection problems (Section 16.9.1) the detector response may not reflect the actual sample concentration. This is the reason why the molar masses of complex polymers directly determined by SEC are only semi-quantitative, reflecting the tendencies rather than the absolute values. To obtain the quantitative molar mass data of complex polymer systems, the coupled (Section 16.5) and two (or multi-) dimensional (Section 16.7) polymer HPLC techniques must be engaged. 

Coupled column (multidimensional) systems in chromatography have also been developed to improve resolution. The coupled column procedure, as noted in Section 6.4, requires two or more columns of different kinds having different retention mechanisms. 

Dimensionality can be realized in either 2D or higher dimensional form by using coupled column systems. The resulting multidimensional chromatography provides a powerful extension of ID chromatography (see Section 6.4 and ref. [26]). 

The dynamic coupled-column liquid chromatographic (DCCLC) technique that will be discussed in this section circumvents all the problems stated above. In addition to a description of this technique, this section includes a critical evaluation of the method in terms of the... 

By thermal coupling the heat is transferred by direct contact between vapour and liquid flows that connect sections of different columns. This is a major difference with heat integrated columns , where the heat exchange takes place by condenser/reboilers. Hence, thermal coupled columns have a more complex behaviour. Figure 11.21 illustrates two basic arrangements with side-columns. The first is the side-rectifier, derived irom a direct sequence. The second one is the side-stripper that corresponds to... 

Coupled column (or column switching) techniques encompasses a variety of techniques used for sample separation, cleanup and trace enrichment [76-81]. Applications involving sample preparation are discussed in section 5.3.2. The most important separation applications are multidimensional liquid chromatography (section 5.6.1) and comprehensive two-dimensional liquid chromatography (section 5.6.2). A characteristic feature of these methods is the use of two or more columns for the separation with automatic switching of fractions between columns by a valve interface. These techniques require only minor modification to existing equipment, and of equal importance, enable the sample preparation and separation procedures to be completely automated. 

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