Water vapor exchange capacity

Provided that the magnitude of cuticular transpiration is insignificant (which is true in the majority of CAM plants), the transpiration rate is a convenient indicator of the diffusive capacity of stomata. Hence, it is reasonable to consider the properties of transpiration as observed in CAM plants in terms of stomatal movements, and vice versa. In this chapter, we will therefore mainly detail the current knowledge of stomata in CAM plants. The ecological implications of water vapor exchange of CAM plants will be discussed in detail in Chapter 6. 

Purification of Air Prior to Liquefaction. Separation of air by cryogenic fractionation processes requires removal of water vapor and carbon dioxide to avoid heat exchanger freeze-up. Many plants today are using a 13X (Na-X) molecular sieve adsorbent to remove both water vapor and carbon dioxide from air in one adsorption step. Since there is no necessity for size selective adsorption, 13X molecular sieves are generally preferred over type A molecular sieves. The 13X molecular sieves have not only higher adsorptive capacities but also faster rates of C02 adsorption than type A molecular sieves. The rate of C02 adsorption in a commercial 13X molecular sieve seems to be controlled by macropore diffusion 37). The optimum operating temperature for C02 removal by 13X molecular sieve is reported as 160-190°K 38). 

Clay minerals, natural and synthetic zeolites, silica and aluminum oxide forms generally are a mineral phase in mineral-carbon adsorbents. Natural aluminosilicates, particularly zeolites, due to the existence in their structure of ultramicropores and micropores (with pore diameter below 2 nm) with hydrophilic properties, exhibit high sorption capacity for particles of water vapor as well as sieve properties. They also demonstrate very good ion exchange properties. For instance, the ion exchange capacity of zeolite NaA is about 700 mval/100 g. 

Almost linear decrease of adsorption capacity has been observed with thallium—sodium zeolites of type A (n = 1.98) for water vapor with an increase of the degree of exchange (Figure 1). This is a result of a decrease of the cation electric field which causes a decrease of the adsorption layer density. 

Using thallium—sodium zeolites of type X (n = 2.30), we observed an increase of adsorption capacity for water vapor and benzene at 38% replacement of sodium ions by thallium ions, and then its decrease with an increase in the degree of exchange. As thallium ions replace sodium ions in the position Sm (25) we may assume redistribution of cations at dehydration under the conditions of high vacuum and thermal treatment at 350°C, and stronger chemical bonds of thallium atoms in screened positions in comparison with sodium atoms. 

The first natural microporous aluminosilicate, i.e., natural zeolite, was discovered more than 200 years ago, and after long-term practical applications, the intrinsic properties of natural zeolites such as reversible water-adsorption capacity were fully recognized.13 41 By the end of the 19th century, during exploitation of ion-exchange capacity of some soils, it was found that natural zeolites exhibited similar properties some cations in natural zeolites could be ion-exchanged by other metal cations. Meanwhile, natural chabazite could adsorb water, methanol, ethanol, and formic acid vapor, but could hardly adsorb acetone, diethyl ether, or benzene. Soon afterwards, scientists began to realize the importance of such features, and use these materials as adsorbents and desiccants. Later, natural zeolites were also used widely in the field of separation and purification of air. 

Structural Effects of Water Vapor on Na-ZSM-5. The catalytic activity of Cu-ZSM-5 and Mg/Cu-ZSM-5 catalysts was permanently lost after a gas mixture of 20% H20-4%02 He had flowed through the catalyst bed at 750°C for 20 hours (72). The loss of catalytic activity may be attributed to either dealumination of the ZSM-5 material or deactivation of copper, or both. To check for dealumination, the parent Na-ZSM-5 zeolite was pretreated for 20 hours in a gas mixture containing 20% H2O- 4% 02-He at 5(X), 600, and 750°C. It was found that the micropore volume decreased from 0.11 cm /g for the as-received Na-ZSM-5 to practically zero after treatment at 600 C, while the subsequent Cu " " ion exchange capacity was reduced from 141% to 20% (or 15% for the sample steamed at 750°C). These Cu-ZSM-5 catalysts had very low NO decomposition activity. These results are shown in Table 2. 

By more intense oxidation of the carbon, the amount of water vapor adsorbed at low relative pressures (<4 Torr at 25 °C) can be drastically increased. For example. Walker and co-workers showed a 100-fold increase in water vapor adsorption by activated carbon after strong surface oxidation by HNO3 (Mahajan et al 1982). Exchange of the surface H-ions by cations (Li, Na, K, Ca) on the oxidized carbon further increased the moisture capacity at low vapor pressures to amounts comparable with that on zeolites. The ion-exchanged carbon was fully regenerated at 140 °C, in contrast to temperatures >350 °C that are required for zeolite regeneration (Mahajan et al., 1982). 

Near-Ambient Temperature. There has been a long search for sorbents for NOx at near-ambient temperature. Table 10.21 is a summary of the equilibrium capacities for these sorbents. The equilibrium capacity can be deceiving, because of the following problems. The NOx capacities are decreased by the presence of water vapor. The most severe case is with the ion-exchanged zeolites, such as MFl (or ZSM-5) exposure to water vapor destroys the sorbent quickly and completely. SO2 and CO2 iso have some effects on some sorbents, as to be discussed. 

Temperature control is essential and is achieved by boiling a hydrocarbon heat transfer fluid on the outside of the reactor tubes. This vaporized fluid is then condensed in a heat exchanger by transferring heat to a stream of saturated liquid water at 100 bar to produce saturated stream at 100 bar. The reactor feed is 11% C2H4, 13% O2 and the rest N2 at 360°C at 10 bar. The reactor product stream is at 375°C and 10 bar and the conversion of C2H4 is 22% with a 83% selectivity for C2H4O. At the operating pressure used in this system, the heat transfer fluid boils at 350°C with a heat of vaporization of 500 Btu/lb and has a liquid heat capacity of 0.8 Btu/lb °C and a vapor heat capacity of 0.4 Btu/lb °C. The flow of heat transfer fluid is adjusted so that it enters the reactor as liquid at 340°C and leaves as a two-phase mixture with vapor fraction of 21% (see Fig. P2.31). 

---