Moisture regulation 177 nominal pore size of 4 Angstroms. Other common grades are 3A and 5A.A peciality grade is type 13X with a nominal pore size of -10 Angstroms Figures 9.4 and 9.5 illustrate adsorption of types 4A and 13X at selected relative 9.4.3 Selective and preferential adsorption Retention and selectivity among various adsorbed compounds is proportional to their polarity and effective molecular size. This is the rationale for the name molecular sieve. The ability of a particular type of molecular sieve selectively to adsorb a particular molecular species depends on what other compounds are present, e.g., more polar will displace less polar compounds in the same size range 9.4.4 Moisture adsorption Thus in a moist environment, water will displace nearly every other compound The adsorption capacity of a molecular sieve in a moist environment will therefore be mostly taken up by water. As a result, desiccant applications most often utilise a type 4A molecular sieve since its pore size is suitable for water and its polarity favours water. These properties combine to give type 4A a significant affinity for moisture even at low ERH. As can be seen in Figs 9.4 and 9.5, a type 4A molecular sieve can adsorb 10% of its weight in moisture at less than 10% relative humidity. This permits the moisture of a closed environment to be maintained at less than a fraction of a percent with a suitable quantity of molecular sieve molecular sieve Ly pe4∧ watcr vapour adsorption at 25"C 086420 lInc MrI ( a 80: RH MTe4n保RH MTT G 20 RH Fig 9.4 Adsorption isotherms of common types of molecular sieve.nominal pore size of 4 Angstroms. Other common grades are 3A and 5A. A speciality grade is type 13X with a nominal pore size of ~10 Angstroms. Figures 9.4 and 9.5 illustrate adsorption of types 4A and 13X at selected relative humidity conditions. 9.4.3 Selective and preferential adsorption Retention and selectivity among various adsorbed compounds is proportional to their polarity and effective molecular size. This is the rationale for the name ‘molecular sieve’. The ability of a particular type of molecular sieve selectively to adsorb a particular molecular species depends on what other compounds are present, e.g., more polar will displace less polar compounds in the same size range. 9.4.4 Moisture adsorption Thus in a moist environment, water will displace nearly every other compound. The adsorption capacity of a molecular sieve in a moist environment will therefore be mostly taken up by water. As a result, desiccant applications most often utilise a type 4A molecular sieve since its pore size is suitable for water and its polarity favours water. These properties combine to give type 4A a significant affinity for moisture even at low ERH. As can be seen in Figs 9.4 and 9.5, a type 4A molecular sieve can adsorb 10% of its weight in moisture at less than 10% relative humidity. This permits the moisture of a closed environment to be maintained at less than a fraction of a percent with a suitable quantity of molecular sieve. Fig. 9.4 Adsorption isotherms of common types of molecular sieve. Moisture regulation 177