Figure 16: The effect of initial concentration and

hence the carbon loading on the rate of

supercritical desorption in caprolactam / GACF400

system, at 250 bars and 40ºC.

In the micropores, the pore diameters would be of comparable sizes to the adsorbate molecular diameter, hence the adsorption mechanism is based on multidimentional interactions forming bonds that are much harder to break, the factor that is responsible for irreversible adsorption no matter how powerful the regenerating solvent. Table 4 below shows the initial carbon loadings based on the caprolactam solutions that they were prepared from:

Table 4: Carbon loadings and mass of caprolactam desorbed, which are correspondent to various initial

solution concentrations.

To conclude, the study led to the following conclusions:

· Supercritical CO 2 can be successfully used to regenerate GAC-F400, originally used to remove caprolactam from aqueous waste streams, by recovering up to 70% of the caprolactam. The latter can be recovered if required, to 100% purity.

· Both the rate of desorption and maximum amount desorbed favoured higher operating pressures (= 200 bars).

· The optimum desorption temperature is 25ºC for pressures = 200 bars, and 40ºC for pressures > 200 bars.

· The rate of desorption increases with the solvent (regenerant) flowrate and initial solute concentration in the effluent stream.