1. Natural Gas Production Chapter 5 Dehydration of Natural Gas
PTRT 2323
Natural Gas Production
Chapter 5
Dehydration of Natural Gas

2. Dehydration Removal of water vapor
Adsorption
Water is trapped on the surface of pores in a solid (think of connate water)
Absorption
Water is absorbed by a liquid with a higher affinity for water than the natural gas
Removal takes place inside an absorber (contactor or tower)
Key factor is controlling dew point – dew point depression.
Dew point is the temperature at any given pressure for which the gas is fully saturated with water vapor
Dew point must be decreased below the hydrate formation temperature for a given set of conditions
Two primary systems in use today use one of these two techniques
Liquid desiccant dehydration
Solid desiccant dehydration

3. Liquid Desiccant
Dry Desiccant

4. Dew Point Depression
Hydrates do not form unless the gas is saturated AND contains additional free water
Warm gas holds more water than cold gas and thus takes more water vapor to reach saturation
Heating cold gas allows it to absorb the free water and if temp is high enough will become undersaturated
Undersaturated gas cannot form hydrates

5. Dew Point - example Natural gas at 500 psia and 60 ⁰F
At saturation this gas will hold 30 lbs water vapor per MMcf
The dew point is 60 ⁰F
Suppose this gas is to be transported to a location where the temp is only 20 ⁰F
The dew point at 20 ⁰F is only 7 lb/MMcf
Thus there would now be 23 lbs of free water in the gas.
The dew point must be depressed below 20 ⁰F
Real world is a bit more complex

6. $ $$ TEG $$$ $$$$ Superior desiccant Easier to regenerate
Higher decomposition temperature

7. Glycol
Wet Gas
Dry Gas

8. Glycol must be reused
Too expensive to throw away
> $4.00 / gal

9. Definitions Wet gas – Gas containing water vapor prior to desiccation
Dry gas – gas leaving the absorber
Desiccant – dehydrating (or absorbing) medium (in most cases TEG)
Lean glycol – 95-99% TEG by weight
Rich glycol - <95% TEG by weight (remainder is water plus impurities)

10. Flow diagram of liquid desiccant system

11. Absorber Tower Wet gas enters at bottom
Intimate contact with gas via bubble caps
Water is absorbed by the TEG
Mist extractor at top
Gas processing is now complete
Remainder of system is for reprocessing the glycol

12. One Tray from an Absorber Tower

13. Bubble Caps

14. Mist Extractor Surface area on which glycol mist can condense
Reduce carryover and hence lost of glycol from system
Cost $$$

16. Operating Conditions Glycol Boiling Temp = 549 ⁰F
Water Boiling Temp – 212 ⁰F
Packed column in stripper tower condenses glycol vapors to reduce losses
Top of stripper is essentially pure water vapor (steam)

17. Operational Targets
At constant temp – lower P yields higher Sw inlet gas
At constant pressure – higher T yields higher Sw inlet gas
See Figure 4.1
For P up to 1200 psig circulate 2 gal of glycol per 1 lb water to be removed (for 55 ⁰F dew point depression)
Above 7 gal per lb improvement is minimal
95-96% glycol yields 55 ⁰F dew point depression
99% required for 65 ⁰F dew point depression
Reboiler Operating temp (at 1 atm)
350 ⁰F yields 95-96% glycol
375 ⁰F yields 99% glycol
Most operating problems are the result of contamination in the glycol or decomposition due to high operating temps

18. Solid Desiccant Dehydrators
Highest possible Dew-point Depression
½ lb / MMcf possible
Corresponds to Dew Point = - 40 ⁰F
Packaged systems
3 – 500 MMscf/day
psig

19. Essential Features Inlet gas separator Two (or more) adsorption towers
High-T heater (to produce hot regeneration gas)
Regeneration gas cooler (condense water out of gas)
Gas separator (to remove water)
Piping and controls

20. Regeneration
Dehydration
Adsorber Tower in Regeneration Process
Adsorber Tower in Adsorption Process

21. Terms Wet-gas (inlet gas stream) Dry Gas (Output gas stream)
Regeneration gas
Wet gas heated to F
Passed through saturated tower to remove moisture
Desiccant
Solid granules
Large surface area (4 MM sq.ft/lb)

22. Terms
Adsorption – physical trapping of moisture in the desiccant. No chemical reactions involved
Activated alumina
Silica gel
Adsorbants are often chemical specific
Regeneration – drive off the water by raising temperature or reducing pressure
Inlet separator critical to avoid fouling

23. Regeneration Cycle 4-12 hours adsorbing
8 hour typical regeneration cycle
6 hours regenerating
2 hours cooling
Multiple towers to reduce down time
Regeneration Cycle

24. Adsorption Process
Water adsorbed first in the top layers of the bed – higher affinity
Then lighter hydrocarbons followed by heavier ones
Water displaces light hydrocarbons
Finally heavier hydrocarbons as adsorption proceeds
For each component in the inlet gas stream there will be:
a layer in the bed that is saturated by the particular component (top of layer)
And where the bed is just seeing that component (bottom of the layer)
Depth of the layer = mass transfer zone

25. Effects of Contaminants
Useful life 1-4 yrs
Contaminants decrease life of desiccant
H2S with O2 can rapidly foul the desiccant bed by creating free sulfur
CO2 + H2S can cause corrosion
Desiccant attrition caused by excess flow velocity

26. Hydrocarbon Recovery Units
Based upon dry-desiccant systems
Short-cycle the bed (15-20 min)
Allows heavier hydrocarbons to be adsorbed but NOT replaced by water
During regeneration water + hydrocarbons are driven off and water condenses and is separated

27. Hydrocarbon Recovery Unit

28. Three-tower System Separate dehydration not needed
Little moving machinery – reduced down time
High Pressure not necessary
Stage separation required to reduce evaporation losses