Condensation The Kelvin Equation

TABLE 2.14. Comparison of BET Area of NH] Synthesis Catalyst Determined with Different Sorbates  

In a porous adsorbent there is a continuous progression from multilayer adsorption to capillary condensation in which the smaller pores become completely filled with liquid sorbate. This occurs because the saturation vapor pressure in a small pore is reduced, in accordance with the Kelvin equation, by the effect of surface tension. 

If the restriction to cylindrical pore geometry is relaxed, the same argument leads to the more general relationship  

FIGURE 2.11. Experimental isotherms for sorption of water on 4A molecular sieve pellets showing capillary condensation at 293 K. (Experimental data of Kytc for Laporte 4A. sieve.) 

Taking benzene at 20°C as an example (o 29 dyne/cm, F ,sti89 cmVn iole, 0) we note that in cylindrical pores of 50 A capillary condensation will occur at a relative pressure of p/p, 0.67 whereas in pores of 500 A the relative pressure will be 0.96. It is evident that the capillary effect is significant only in quite small pores. 

---