The trouble with these textbook calculations is that they assume 100% penetration into every corner of tanks and process. It's only 20-20 hindsight but, next time, perhaps a pneumatic pressure test would have been a better choice. Maybe inefficiency is the issue. Allow a few more days. What about drying?
Water can come from two sources: 1) equipment, and 2) the atmosphere. Assuming the mole-sieve dryer is functional and the piping was well drained after hydrotesting, it seems unlikely that water came from there. However, some moisture will be trapped in low spots and gaskets and other soft materials.
Moist air is a strong possibility. At 75°F and 14.7 psia, air at 100% relative humidity has a moisture content of about 18,500 ppm-m. The slightest contamination while taking a sample could lead to hours of fruitless running around on a rainy day.
Purging and drying are different processes. A steady bleed into a tank might work well for O2 purging but be useless for drying. Drying requires that the drains be located in spots where water will flow by gravity. Driving water from a tank should be at a vigorous flow with heating, if possible. I would do the drying first with a high flow and heat and then do the purging. This idea is supported by the pool evaporation equation: http://www.engineeringtoolbox.com/evaporation-water-surface-d_690.html. Maybe the mole-sieve column could be used at the last stage of drying to remove trace water.
Another option might be to purge initially with a heavy gas. For this to be effective, the gas should flow top to bottom allowing gravity to work against the O2. A light purge gas should flow upward from the bottom. Based on price for 99.999% purity (N2 at about $140/unit, CO2 at $157/unit, and argon at $184/unit), C O2 would be the most cost effective choice. A heavy purge gas might be less likely to mingle with the gas in the tank. Purging with heavy gases is quite common in laboratories doing sensitive work.
Dirk Willard, consultant
TRY COPPER CATALYTIC OXIDATION
Use copper fixed on a support, e.g., aluminum oxides, in the form of a bed of granules, with the bed having a high length/diameter ratio. Thereafter, the mole.sieve adsoption, preferable under pressure, also relatively long, should be able to remove water to the necessary concentration.
Walter Schicketanz, Dr.-Ing., Consultant
Readings from two radar level transmitters on our LPG storage tank markedly disagree. The radar elements are installed in stillwells connected to a large pressurized tank currently operating at about 800 kPa (g) and 24°C. The gas feeds burners via underground piping. Right now, demand is so low there's no need to use the vaporizer, evaporation suffices. What could cause these elements to differ so radically? Should we be concerned? Is there anything that can be done to line up the level transmitters?
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