Process Description
Feed Chilling
Feed Chiller, E-801
Raw natural gas feed from battery limits at 68.0 DEG F passes through feed flow control valve FV-801 and is cooled in the tubes of Feed Chiller E-801 by evaporating (boiling) propane liquid refrigerant on the shell side of E-801. The design flow rate of feed gas is 2.311 MMSCF/H. As the feed mixture chills, heavier hydrocarbons in the natural gas feed condense on the tube-side. The chilled process stream exits E-801 at -14.8 DEG F and flows to battery limits for downstream processing.
Propane liquid (refrigerant) from the Propane Refrigeration System flows through refrigerant level control valve LV-801 and into the shell side of E-801. The refrigerant depressures from about 120 PSIG to 13.4 PSIG while going through LV-801. Depressurization of the refrigerant causes a portion of the liquid to vaporize which causes the flashed stream’s temperature to decrease from 76.0 DEG F to -16.6 DEG F. The design flow of propane liquid to E-801 is 275 GPM.
Normally, the tubes of E-801 are fully immersed in cold refrigerant liquid which draws heat from the process gas flowing through the tubes. The absorbed heat will evaporate (boil) some of this liquid. The evaporated refrigerant is collected on the top of the shell side of E-801 and returned to the Propane Refrigeration System through refrigerant pressure control valve PV-801 for recompression and liquefaction.
Propane Refrigeration System
Compressor Suction Drum, D-101
Cold, low pressure propane vapor from Feed Chiller E-801 passes through vapor isolation valve XV-101 and is combined with any recycled warm propane vapor from the vapor space of the propane accumulator D-102 that passes through recycle valve PV-101. In the design condition, PV-101 is fully closed. The combined vapor enters Compressor Suction Drum D-101 to remove any liquid contained in the combined vapor prior to entering Propane Compressor K-101. Normally there is no liquid present in the combined vapor. D-101 normally operates at 14.5 PSIA and the vapor temperature is -16.6 DEG F.
Propane Compressor, K-101
The vapor from D-101 passes through a check valve at the inlet of K-101 which is a rotary screw compressor driven by Compressor Motor KM-101 which has a variable frequency drive (VFD) electric supply. The rotary screw compressor is a dry type which means oil is not injected into the inlet of the compressor to ensure low-friction rubbing of the two helical screws to achieve efficient compression of the propane vapor while minimizing wear of the screws. To achieve precise alignment of the two helical screws without oil injection, a timing gear is used instead.
The compression of propane vapor occurs in one single stage. This is possible with the high-pressure ratio across the compressor since propane vapor has a lower temperature rise than most other refrigerants. The operating discharge temperature, therefore, is well under the limit at which mechanical damage to the compressor will occur, even under total recycle conditions at which the suction temperature will be at a maximum.
The two helical screws capture incoming propane vapor as they rotate and push the vapor through the space between the screws. The volume of the space between the screws continuously decreases up to the outlet resulting in a pressure increase of the vapor at the discharge. Because of the tight clearances between the screws, very little of the propane vapor back-flows through the screws. The gas leaves K-101, passes through another check valve and then enters Propane Condenser E-101.
K-101 normally operates at 2184 RPM. The discharge pressure of K-101 is normally 127.0 PSIG8.76 BARG and the discharge temperature is normally 177.6 DEG F80.9 DEG C.
Propane Condenser, E-101
Propane Condenser E-101 uses ambient air to cool and condense the hot, high pressure propane vapor from K-101. Air is supplied using a motor-driven fan. The outlet temperature of E-101 is normally 76.0 DEG F24.4 DEG C.
Propane Accumulator, D-102
Condensed propane from E-101 is collected in Propane Accumulator D-102 which supplies a reserve capacity for the propane refrigerant in the system during normal operation and at shutdown. The vapor above the liquid propane in D-102 is in equilibrium with the liquid and attains a pressure that is dependent on the temperature of the liquid propane.
The normal pressure of D-102 is 121.6 PSIG8.38 BARG. A recycle line through PV-101 to D-101 allows recycling vapor through K-101 when it is at minimum speed (30%) per the VFD. Liquid in D-102 is used to supply Feed Chiller E-801 through liquid isolation valve XV-103.
Instrumentation
Feed Chiller Controls and Instruments
E-801
FIC-801 controls the flow of feed gas through flow control valve FV-801. The feed gas temperature is indicated on TI-801.
TIC-802 controls the gas outlet temperature from Feed Chiller E-801 by adjusting the setpoint of PIC-801 which controls the pressure of the vapor space of the refrigerant side of E-801. In turn, PIC-801 adjusts the position of control valve PV-801 which will affect the flow rate of vaporized refrigerant returned to the Propane Refrigeration System. Changing the pressure of the refrigerant side of E-801 will directly change the temperature at which the propane refrigerant boils in E-801.
LIC-801 controls the refrigerant level in E-801 by adjusting the position of the refrigerant supply valve LV-801. The flow rate of refrigerant through LV-801 is indicated by FI-802.
Propane Refrigeration System Controls and Instruments
D-101
Switch XV-101 controls the position of vapor isolation valve XV-101. Switch XV-101 is locked in the CLSD state in the event of a trip of the Propane Refrigeration System (see Interlock I-101 below).
The level of liquid in Compressor Suction Drum D-101 is indicated on LAH-101 which is provided as a trip to the refrigeration system interlock (see Interlock I-101 below). The temperature of vapor leaving D-101 is indicated on TI-101.
The pressure of D-101 is controlled by PIC-101. The output of PIC-101 controls both the position of recycle control valve PV-101 and the frequency of the VFD of Compressor Motor KM-101 using a split-range calibration. Under normal operation, the output of PIC-101 directly controls the speed of KM-101 between 30% and 100% while PV-101 remains closed. When the output of PIC-101 falls below 30%, the frequency of the VFD will be held at 30% of power supply frequency and the position of PV-101 will increase to 100% opening at 0% controller output. The opening of PV-101 will occur at very low or no load operation of the Propane Refrigeration System.
K-101/KM-101
The power to the VFD (variable frequency drive) of KM-101 is controlled by switch HS-101. The speed of KM-101 is indicated on SI-101. The power used by motor KM-101 is indicated on JI-101.
TI-102 indicates the discharge temperature of K-101. TAH-102 is an independent indication of the discharge temperature and is a trip signal to the refrigeration system interlock (see Interlock I-101 below). PAH-102 indicates the discharge pressure of K-101 which is provided as a trip to the refrigeration system interlock (see Interlock I-101 below).
E-101
The motor of the fan for Propane Condenser E-101 is controlled by switch HS-102. The outlet temperature of E-102 is indicated on TI-103.
D-102
The pressure of Propane Accumulator D-102 is indicated on PI-103. The level of propane liquid in D-102 is indicated on LI-103. LI-103 will vary as conditions of pressure change and as the refrigerant level changes in Feed Chiller E-802. Switch XV-103 controls the position of liquid isolation valve XV-103. Switch XV-103 is locked in the CLSD state in the event of a trip of the Propane Refrigeration System (see Interlock I-101 below). TI-104 indicates the temperature of refrigerant from D-102 going to the Feed Chiller E-801.
Interlock I-101
This interlock protects the Propane Refrigeration Compressor from mechanical damage due to excessive process conditions. It is a non-latched interlock and is activated by any of the following:
- Compressor discharge temperature TAH-102 exceeds 347 DEG F
- Compressor discharge pressure PAH-102 exceeds 290 PSIG
This interlock has the following effects:
- Stops power to KM-101
- Locks HS-101 (KM-101 motor switch) in the STOP state
- Locks switch XV-101 in the CLSD state
- Locks switch XV-103 in the CLSD state
- Causes an alarm on XA-101
The interlock automatically resets when all trip conditions are cleared. However, KM-101 must be manually restarted with HS-101 and the isolation valves XV-101 and XV-103 must be manually reopened.