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 50
The procedure was used to isolate pure benzene from coal gas. Domestic gas at that time was derived from the pyrolysis of coal during the production of coke and contained a number of aromatic hydrocarbons including significant quantities of benzene and toluene and some xylenes. The stationary phase coated on a support is arranged to free-fall down a tower. This, in effect, provides the moving stationary phase. A gas feed containing the solute vapors enters the tower at the center. By the careful adjustment of pressures the gas is arranged to pass up the tower and out through an exit at the top. If the band velocity of a solute is greater than the velocity of stationary phase descent, then the solute will pass up the tower. If the solute velocity is less than the velocity of stationary phase descent, then it will be carried down the tower in the moving stationary phase into the lower part of the tower.
The stationary phase in the lower part of the tower meets a second upward stream of gas and a portion of the lower part of the tower is heated. This results in the distribution coefficients of the solutes being reduced and their velocity up the tower increased so that they are now greater than the velocity of stationary phase descent. The stripping gas exits at another port at the top of the lower section of the tower. The packing passes out of the tower at the base, the amount of fall being controlled by the rate of rotation of a feed Table. The packing is then gas-lifted back to the top hopper for recycling. This procedure will split the solutes into two fractions and if the lower part of the tower contains a similar splitting system, then a specific solute can be selected from the mixture and obtained pure.
The practical system used by Scott for the isolation of pure benzene from coal gas is shown diagramatically in figure 26. Let the gas flow carrying the sample through the upper half of the tower be (QG(1)). Let the volume of mobile phase passing down the tower per unit time be (QL) and the distribution of solute (A) between the stationary phase and the mobile phase at temperature (T(1)) be (KA(T)).