GAS-ADSORPTION ISOTHERMS FROM COMPOSITION AND FLOW-RATE TRANSIENT TIMES IN CHROMATOGRAPHIC COLUMNS .3. EFFECT OF GAS VISCOSITY CHANGES

A method of determining binary adsorption isotherms from the flow-rate and composition responses of a packed column to a step change in inpu t composition has been described previously by Mason & Buffham (1996 P roc. R. Soc. Lond. A452, 1263-1285). The method involves adding a smal l perturbation stream to a carrier gas mixture passing through a chrom atographic column. The bulk of the observed effects arise from the sma ll change in composition of the gas in the column caused by adding the perturbation gas. However, the increase in flow caused by adding the perturbation stream raises the pressure in the column and, under certa in circumstances, this increase of pressure can cause significant adso rption (Mason & Buffham 1996 Proc. R. Soc. Lond. A452, 1287-1300). Apa rt from changing the amount of adsorption, the change in composition a lso alters the viscosity of the gas in the column and this too can aff ect the column behaviour. The change in viscosity is small but can hav e two consequences. The first is minor and is a small shift in the mea n pressure in the column. The second effect is peculiar to the apparat us used in the previous experiments. A capillary flowmeter was used to monitor flow and capillary flowmeters respond to viscosity as well as flow. The effect of the change in viscosity in the capillary flowmete r is significant, even for gas mixtures with almost linear variation o f viscosity with composition. Both effects can be corrected for by the ory. The second effect might be removed by the addition of a delay lin e between the column and the flowmeter. A delay line is simply an empt y tube which delays the arrival of gas of different viscosity and enab les the capillary flowmeter to act as an ideal flowmeter for the limit ed period that the composition front is passing through, and leaving, the column. Precise isotherms for nitrogen-argon mixtures on 5A molecu lar sieve at 50 degrees C have been obtained and they are compared wit h binary Langmuir isotherms. The comparison is in terms of the gradien ts of the isotherms of each of the two components. Partial differentia ls of the amounts adsorbed in terms of the concentrations of nitrogen and argon are also obtained.