Three process requirements must be met for gas to be dried in a standard glycol dehydration unit:
1. The gas velocity through the contactor tower must not be great enough to entrain glycol into the dried gas. Theoretically, the entrainment of glycol does not interfere with drying. In practice, the continuous loss of glycol will knock a drying plant off-line as the unit’s inventory of glycol disappears. Incidentally, it is not possible to measure the water content of gas containing a glycol mist.
2. The glycol pump must have the capacity to circulate enough glycol to absorb the water vapor contained in the natural gas. Of course, hotter gas can contain more water vapor. Increasing the gas temperature from 80°F to 100°F may double its water content.
3. The glycol reboiler must have a sufficient heat-duty capacity to regenerate the glycol at a high enough temperature to adequately dehydrate the gas.
As the temperature of the gas flowing through a dehydration contactor tower rises, its capacity will decrease as follows:
If a tower temperature increases from 80°F to 120CF, a tower’s capacity will decrease by barely 3V2%.
On the other hand, the amount of glycol circulation may or may not greatly increase as the gas inlet temperature rises. Figure 6-3 clarifies this point. A large booster compressor is serving a concentrated gas field. The gas produced from the wells enters the compressor’s suction scrubber at a temperature independent of seasonal fluctuations. However, the aerial cooler on the compressor’s discharge cools the gas to 80° in the winter versus 120°F in the summer. Question: How much more glycol circulation is required to dry the gas? The requisite data to perform the calculation are given in Figure 6-4.
At first glance, it would appear that three or four times as much glycol circulation is required. But remember that the 120°F compressed gas is not saturated with water vapor; it is really superheated. The compressed gas will have the same water content until it is cooled by the aerial cooler to below its dewpoint, in this case 79°F. If a contactor tower with 10—15 trays were employed, there would likely be no effect at all on glycol circulation requirement. For the typical 6-tray contactor, industry correlations indicate that an additional 10-30% of glycol circulation is needed; that is, far less that the 300—400% required if the gas were saturated with water at the compressor discharge temperature.
Suppose, however, that the gas coming out of the ground is hot, perhaps 110°F. This gas, after compression and cooling to 1,000 psig and 120°F, would be saturated with moisture. Then, during winter operation, when the gas is cooled to 80°F, only one-third as much glycol circulation would be required as in the summertime. The condensed water corresponding to the difference in water content of 110°F, 700 psig gas vs 80°F, 1,000 psig gas would drop out in the bottom section of the contactor tower.