Preparative Chromatography - Alternative Preparative Techniques > The Moving Bed Continuous Chromatography System > The Continuous Moving Bed Process for the Isolation of Pure Benzene from Coal Gas > Page 54
By adjusting the magnitudes of (QG(3)) and (T(3)), the conditions defined by equation (8) can be established. As would be expected, to prevent cross-flow, the pressures at the second stripping flow inlet and outlet for solute (C) must also be made equal, or close to equal. Scott and Maggs (12) designed a three stage moving bed system, physically very similar to that described above, to extract pure benzene from coal gas. Coal gas contains a range of saturated aliphatic hydrocarbons, alkenes, naphthenes and aromatics. In the above discussion the saturated aliphatic hydrocarbons, alkenes and naphthenes are represented by solute (A). The main product, benzene, is represented by solute (B), and the high boiling aromatics are represented by solute (C) (toluene and xylenes). An analysis of the products that were obtained are shown in figure 27. The material stripped form the top (section (1)) is seen to contain the alkanes, alkenes and naphthenes and very little benzene. The product stripped from the center section appears to be virtually pure benzene. The product from section (3) contained toluene, the xylenes and thiophen which elutes close to benzene. The thiophen, however, was only eliminated at the expense of some loss of benzene to the lower stripping section. Although the system worked well, Scott and Maggs found it experimentally difficult to set up and maintain production under constant operating conditions. The problems arose largely from the need to adjust the pressures that must prevent cross-flow. Without doubt, today, the system would be computer controlled. The simple system as described, although satisfactory for GC operation, would be virtually impossible to operate with a liquid mobile phase.