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

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

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

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

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

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

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

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

2-stage Compression Condensation Evaporation Expansion

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

Interstage Cooling

Gas Turbine-driven Centrifugal Compressor

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

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

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)

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

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

Two-stage Refrigeration

Three-stage refrigeration system

Cascade Refrigeration System

Two-stage Horsepower Requirements