Variants

Various working modes can be implemented in preparative GC. They include temperature gradients, multidimensional GC, moving bed or simulated moving bed GC. Although these techniques can theoretically be useful, they are not used in practice.

Temperature programming can be used for selectivity enhancement and/or elution time reduction when the feedstock components have a large range of retention factors but, due to the high thermal inertia of large systems, the time required to re-equilibrate the temperature between two injections is too long and overall productivity is affected. In such cases, the separation is made in several isothermal steps.

Multidimensional chromatography uses several columns online, packed with different stationary phases. One of the fractions eluted from the first column is directed to a second column, where it is further fractionated, while the second column is bypassed when the other fractions are eluted. The advantages of this configuration often fail to compensate for the increased complexity of the process and such separations are made in several steps.

Table 2 Examples of production scales

Columndiameter Column length Typical loadingcapacity Carriergasflowrate

Moving bed GC (also named continuous counter-current chromatography) and its practical variant, simulated moving bed GC (SMB-GC) offer some of the theoretical advantages of the related technique SMB-high performance liquid chromatography (HPLC). In SMB-GC, the feed flow and fraction flow are continuous, thus the thermal control is easier in both vaporizer and condensers. The stationary phase is used more efficiently, thanks to the countercurrent process. However, in SMB-HPLC the complexity of the system is counterbalanced by savings on the mobile-phase consumption. This is not the case in SMB-GC because the carrier gas is already recycled.

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