Gas Chromatography - Preparative Gas Chromatography > The Moving Bed Continuous Chromatography System > Page 62

By suitable adjustment of the relative rates of upward carrier gas flow and downward stationary phase flow (contained on the falling support) some components were arranged to move upward with the carrier gas, and others move downwards with the stationary phase. Referring to figure 39, if the ordinary chromatogram of the mixture is that depicted at (A), the relative speed of the carrier gas and the stationary phase defines an imaginary line on the chromatogram. Those components to the left of the line, move up with the carrier gas (B) and those components to the right of the line, move down with the stationary phase (C). The components that move down in the stationary phase are stripped out by arranging a portion of the column to be heated and a second stream of gas elutes them through a second port (D). Scott and Maggs designed a three stage moving bed system to extract pure benzene from coal gas. Coal gas contains a range of saturated aliphatic hydrocarbons, alkenes, naphthenes and aromatics (benzene, toluene and xylenes). The separations they obtained are shown in figure 40.

It is seen that the material stripped from the top section contained the alkanes, alkenes and naphthenes and very little benzene. The material stripped from the center section consisted of almost pure benzene. The residue striped from the lower section contained the toluene, the xylenes and even the thiophene which elutes closely to the benzene. To eliminate the thiophene, however, it was necessary to loose some benzene to the lower stripping section. Nevertheless the separation clearly demonstrates the effective use of the moving bed extraction technique.

Figure 40 The Extraction of Pure Benzene from Coal Gas by continuous Extraction Using a Moving Bed Technique