1. PTRT 1391 Natural Gas Processing II Chapter 3
Dew Point Control and Refrigeration Systems

2. Process Description
Prevents liquid condensation in the pipeline grid under various temp and pressure conditions
Water dew-point control
Glycol dehydration (Chapter 6)
Silica gel (adsorption)
Molecular sieve
Hydrocarbon dew-point control
LTS (Low-temperature separation)
Expander process
J-T (Joule-Thomson)
Silica gel
Refrigeration system
Removes heat in heat exchanger
Evaporators
Chillers
Refrigerant called “working fluid”
Selected based upon temperature requirements and availability
Ethane and Propane are readily available
Propane normally the preferred refrigerant

3. Dew-point Depression
Difference between the feed gas temperature and the dew point of the same gas at the desired pressure and temperature
Economic evaluation is performed to compare installation costs and operating costs for various means of accomplishing the desired dew-point depression (80 ⁰F typical)
Silica gel
Glycol/propane refrigeration
Glycol/J-T valve cooling process

4. Silica Gel Process
Used for both water and hydrocarbon dew point control
Removal takes place on multiple packed beds of silica gel ADSORBANT
Examine flow diagram to understand regeneration process for adsorber beds
4 regeneration beds recommended
2 in adsorption mode
1 in regeneration mode
1 in cooling mode
Export compressor “train” includes 100% spare (standby) unit

6. Glycol/Propane System
Glycol used for water dew-point control
Methyl ethylene glycol (MEG)
Ethylene glycol (EG)
Diethylene glycol (DEG)
Triethylene glycol (TEG)
Selection based upon economics and efficiency
Refrigeration used for hydrocarbon dew-point control

7. Process Description
Feed gas water content lowered by treating with a circulating stream of glycol
Gas/gas exchangers
Sprayed with 80% glycol/water mixture
Hydrocarbon liquids condensed by refrigeration
Second exchanger uses propane refrigeration-cooled gas to condense heavier hydrocarbon liquids
All liquids pass through separation for recovery
Glycol is re-concentrated (using reboiler) and reused in the process

9. Glycol/J-T Valve Process
Glycol injection control water dew-point
Joule-Thomson effect provides cooling to control the hydrocarbon dew-point
J-T effect is cooling caused by the pressure drop across a valve
Small amount of glycol is injected ahead of the J-T valve to prevent hydrate formation
Liquid streams are separated as before
Export compressor train to pipeline

11. Refrigeration System Continuous recirculation of a refrigerant
Expansion
Evaporation
Compression
Condensation
Similar to home air conditioner but using different refrigerant

12. 2-stage
Compression
Condensation
Evaporation
Expansion

13. Economizers
Propane in the surge tank of the refrigeration system is similar in properties to the liquids coming from a high-pressure oil well
To increase the recovery of these liquids we want to minimize the evaporation of these liquids
At the oil well this process is called staged separation and serves to maximize the amount of recovered liquids
In the gas plant a similar system, called an economizer, serves to maximize the amount of liquid propane reaching the chiller
Cooled gas also used to reduce heating during gas compression

15. Interstage Cooling

16. Gas Turbine-driven Centrifugal Compressor

17. Chillers
Maximum refrigeration (or heating) occurs when tubes are completely covered by refrigerant (heating medium)
Level controller set to keep level just above tubes to allow adequate space for liquid/vapor separation to occur

18. Possible Operating Problems
Propane Quality
Excess ethane increases compressor discharge pressures
Excess butane increases the temperature in the chiller
and are used along with physical property graphs for propane/ethane or propane/butane mixtures to determine quality of the propane. TI PI

19. Record T and P in the propane storage tank Compare to graph
Stay below 2% ethane by venting gas
Record T and P in the chiller
Compare to graph
Drain excess butanes (liquid) from the tank (density)

20. Additional Problems
Air-cooled condensers must run efficiently and be properly maintained
Fans
Drivers
Drive assembly (bearings, gear reduction)
Cooling water condensers
Maintain circulation
Record temps
Refrigerant output temp
Cooling water output temp
When scaling occurs these two temps become farther apart (poor heat transfer) at the same refrigerant flow rate

21. Multi-stage refrigeration
Common to have two-, three- or four-stages in a refrigeration system
Note: cooling demand for condensing refrigerant into liquid at the surge tank goes up from two-stage to three stage
Air cooling for two stage
Cooling water for three stage
Propane cooling used for ethane system (note: the very low evaporation temperatures that can be achieved)
Power consumption varies with evaporation and refrigerant-cooling requirements

22. Two-stage Refrigeration

23. Three-stage refrigeration system

24. Cascade Refrigeration System

25. Two-stage Horsepower Requirements