Page 246 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

246 Chapter 9 Phase equilibria

GSE and LSE: adsorption
The amount of substance adsorbed at equilibrium depends on:
• Temperature e Since adsorption is an exothermic process, higher temperature reduces adsorbate
loading at constant solute partial pressure for gas adsorption. The same is also true for liquid
adsorption, but the effect is much less. The temperature effect is neglected in water treatment and
ambient vapor phase applications.
• Solute partial pressure e for gas adsorption, higher pressure increases loading at a constant
temperature
• Specific surface area/porosity of adsorbent - Higher porosity ensures higher specific surface and
larger adsorption capacity per unit adsorbent weight.
• Nature of solute - vapors and gases with higher molecular weight and lower critical temperature
are more readily adsorbed. Chemical differences as the extent of unsaturation also influence the
extent of adsorption. Permanent gases are usually adsorbed to a relatively small extent. Molecules
with higher polarity are adsorbed more readily than nonpolar molecules due to which water is
more readily adsorbed than hydrocarbons.
For gas adsorption, equilibria can be expressed as e fðq; p; TÞ¼ 0; or q ¼ fðp; TÞ where “q”
represents the concentration of an adsorbed component in the solid.
Adsorption equilibria are usually represented by keeping one of the aforementioned parameters
constant, i.e.

• Adsorption isotherms: q ¼ fðpÞ; at constant T
• Adsorption isobars: q ¼ fðTÞ; at constant p
• Adsorption isostere: p ¼ fðTÞ; at constant q
Isotherms are the most common form of reporting equilibrium data for adsorption and are plotted
or tabulated as capacity or loading (equilibrium concentration of the adsorbed component on the solid)
versus the equilibrium concentration in the fluid phase. The solid is generally referred to as adsorbent
or substrate and the adsorbed component as adsorbate/solute. Loading in the solid phase is usually
expressed as adsorbed mass per unit mass of (solute free) adsorbent. It can also be represented as the
amount adsorbed or the number of molecules adsorbed per unit area.
For commercial adsorbents used in air driers, this is usually specified as static adsorbent capacity at
10% and 60% relative humidity, often denoted as E 0.1 and E 0.6 .
At room temperature, when gas pressure does not exceed atmospheric, adsorption isotherms for
most gases are linear. The nonlinearity arises due to concentration dependence of activity coefficient of
adsorbate in the fluid and the solid phase.
Fluid phase adsorbate concentration is expressed as partial pressure (p) or relative humidity in the
3 3
vapor phase and mass (or mole) fraction per volume (mol/m , kg/m or ppm, etc) for the liquid phase.
Adsorption isotherms for vapor with partial pressure as ordinate can obviously extend only up to
the saturation vapor pressure at the isotherm temperature. Beyond this pressure, the vapor liquefies.
This characteristic is not shown for gases above their critical temperature. Adsorption isotherms of
vapors often exhibit hysteresis at least over a part of an isotherm. This phenomenon is discussed in
greater detail in Chapter 12.
It is interesting to note that unlike solubility curves, adsorption isotherms are not always concave to
the pressure axis.