Water Content of Light Nonhydrocarbon Gases
For determining the saturation (dew point) water content of natural gas systems, the Engineering Data Book of the Gas Processors Association (GPA) recommends Figure 15-14, which is published in their ninth revised edition. The major question posed in our study here is whether this chart might be applicable to light non hydrocarbon gases such as H2, N2, CO, and of particular interest 3:1 H2/N2 (ammonia synthesis gas feed). This application has a direct bearing on the accurate sizing of ammonia plant dryers located upstream of the plant’s cryogenic purification system.
The literature was briefly surveyed to see what data is readily available for the saturation water content of light non hydrocarbon gases. Two major sources were found – Bartlett (3) and Pollitzer/Strebel (4). These data were used to develop a generalized corresponding states correlation.
Analysis and correlation of the data revealed the fact that water content values read from the GPA dew point chart were too high when the chart was applied to light non hydrocarbon gases. This was especially true at higher pressures in excess of 40 atm. For example, in the design of an ammonia syn gas dryer (3:1 H2/N2) at an operating pressure of 200 atm, the GPA chart predicts nearly a 30 percent higher feed gas water content than the corresponding states correlation proposed here. As a result, 30 percent more desiccant packing would be required. This would clearly produce a serious over design for the dryer.
At lower pressures, generally below 40 atm, either the GPA chart or Dalton’s ideal gas law are equally applicable to either natural gas or non hydrocarbon gas mixtures.
(See Figure 1).