Page 96 - Gas Adsorption Equilibria
P. 96

82                                                          Chapter 2



























          Figure 2.1. Experimental setup for (static) volumetric measurements of pure gas adsorption
                   equilibria.

             To measure adsorption a certain amount of gas of mass (m*) is prepared in
          the storage vessel and the adsorption chamber is evacuated. Upon opening the
          expansion valve, the gas expands to the adsorption chamber where it is partly
          adsorbed on the  (external  and internal) surface of the  sorbent material.  This
          process may last milliseconds, minutes, hours or even several days – as in
          case of helium on activated carbon (Norit R1) [2.8]. After thermodynamic
          equilibrium, i. e.  constancy of pressure (p) and temperature  (T)  inside  the
          vessels has been realized,  these  data can be  taken as  a basis to  calculate the
          mass of the gas  adsorbed on the  sorbent   That is, volumetric adsorption
          experiments  mainly  result in  pressure measurements. Hence  the  name
          “Manometry” for this method should be used [2.2].


          2.2      Theory

             To determine  the mass      adsorbed in  a  volumetric  experiment we
          consider the mass balance of the sorptive gas:





             Here                       is the  mass of gas  initially  prepared in  the
          storage  vessel,   is the mass of the sorptive gas in the adsorption equilibrium
          state  considered. It  also  can be  calculated  from  its thermal  equation of state
          (EOS)
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