Process Description
The Propane Refrigeration System uses propane as the working fluid and provides chilling at three temperatures: -35.9 DEG F, 7.5 DEG F, and 43.2 DEG F. Propane refrigerant liquid is used to partially condense a stream of mixed light gases which are representative of a typical process stream found in industrial processes such as liquefied natural gas (LNG) plants (in this case, a Mixed Refrigerant (MR) Supply).
The Propane Refrigeration System consists of:
- Propane Compressor, K-601
- LP Propane Compressor Suction Drum, V-601
- MP Propane Compressor Suction Drum, V-602
- HP Propane Compressor Suction Drum, V-603
- Propane Desuperheater, E-601
- Propane Condenser, E-602
- Propane Accumulator, V-604
- Propane Subcooler, E-603
- HP Evaporator, E-704
- MP Evaporator, E-705
- LP Evaporator, E-706
The purpose of this process is to compress propane vapors from the three propane evaporators so that they can be condensed using cooling water. There are three operating pressures in the circulating propane system:
- Low (LP) which operates at 3.92 PSIG and -35.9 F
- Medium (MP) which operates at 30.4 PSIG and 7.5 DEG F
- High (HP) which operates at 68.6 PSIG and 43.2 DEG F
After compression and condensing of the propane vapors, the liquid propane refrigerant is accumulated and distributed to the propane evaporators in the unit. The liquid from the accumulator is distributed to the HP Evaporator, E-704, first. The shell side of the HP Evaporator also acts as reservoir of liquid refrigerant for the MP Evaporator, E-705. As a result, refrigerant is supplied to the MP Evaporator at a lower temperature. In the same fashion, the MP Evaporator acts as a reservoir of liquid refrigerant for the LP Evaporator, E-706.
The vaporized propane from each evaporator is returned to the appropriate propane compressor suction drum. The propane pressure in an evaporator determines the minimum achievable outlet temperature of the evaporator. By using multiple pressure levels, the overall efficiency of the refrigeration system is maximized.
V-601 LP Propane Compressor Suction Drum
The purpose of LP Propane Compressor Suction Drum, V-601, is to remove any liquid that may be contained in the vapor from the LP Evaporator and the compressor anti-surge line. Any liquid entering the Propane Compressor, K-601, would lead to mechanical damage of the compressor internals and likely would require a shutdown to repair the compressor.
Accumulated liquids can be drained to the LP Evaporator, E-706, by gravity. V-601 can be depressured to flare, if needed, using HIC-661. The vapor from V-601 flows into the LP stage of the Propane Compressor.
V-602 MP Propane Compressor Suction Drum
The purpose of MP Propane Compressor Suction Drum, V-602, is to remove any liquid that may be contained in the vapor from the MP Evaporator, E-705, and the compressor anti-surge line.
Accumulated liquids can be drained to the MP Evaporator by gravity. The vapor from V-602 flows into the MP stage of the Propane Compressor, K-601.
V-603 HP Propane Compressor Suction Drum
The purpose of HP Propane Compressor Suction Drum, V-603, is to remove any liquid that may be contained in the vapor from the HP Evaporator, E-704, and the compressor anti-surge line.
Accumulated liquids can be drained to the HP Evaporator by gravity. The vapor from V-603 flows into the HP stage of the Propane Compressor, K-601.
K-601 Propane Compressor
Propane Compressor, K-601, is a single-body compressor with multiple stages for compression of propane vapors at three pressures. It is driven by a gas turbine under speed control by SIC-661. Vapor from the LP stage discharge mixes with MP vapor internally before compression by the MP stage. Likewise, vapor from the MP stage discharge mixes with HP vapor internally before compression by the HP stage. There is no cooling between compressor stages because the refrigerant vapors from the MP and HP suction drums are cold and accomplish the needed cooling.
The propane normally discharges from the HP stage of the compressor at 228.9 PSIG and 146.8 DEG F.
Each stage of K-601 is provided with an anti-surge valve that recycles desuperheated vapor from the outlet of the Propane Desuperheater, E-601, to the three suction drums. During normal operation, the anti-surge controllers, XIC-661 and XIC-662, close the anti-surge valves since there is a lot of vapor generated in the propane evaporators. However, when the compressor is initially started and there is no evaporation of propane these valves will be fully opened by the anti-surge controllers. Note that XIC-662 controls both the MP and the HP anti-surge valves.
E-601 Propane Desuperheater
Propane Desuperheater, E-601, reduces the temperature of the Propane Compressor, K-601, outlet using fresh cooling water. The purpose is of this unit is to provide partial cooling of the propane such that it mainly remains as vapor because it is near the condensation point. This is important in the event of opening the Propane Compressor’s anti-surge valves opening so that liquid propane does not flow to the compressor suction drums. To ensure only vapor flows to the suction drums the anti-surge line is taken from the top of the shell of E-601.
E-602 Propane Condenser
Propane Condenser, E-602, condenses the desuperheated propane from Propane Desuperheater, E-601, using fresh cooling water. The outlet temperature of E-602 is 115.2 DEG F. Most of the heat is used to condense the propane. A small amount of subcooling of the condensed propane normally occurs. Condensed propane flows into the Propane Accumulator, V-604.
A small line connects the top of E-602 with V-604 to allow equalization of the pressures between the two. Without this line, the pressure in the accumulator would drop during times when the propane becomes significantly subcooled. This drop of pressure induces a surging of propane liquid from E-602 into V-604 and causes the pressure in the system to fluctuate. The small vapor line ensures that the pressure in V-604 is the same as in E-602 and avoids liquid surging.
V-604 Propane Accumulator
Propane Accumulator, V-604, is the reservoir of propane liquid for the Propane Refrigeration System. Liquid from Propane Condenser, E-602, is collected and distributed to the propane evaporators. V-604 normally operates at 213.2 PSIG which is the equilibrium pressure of propane at 115.2 DEG F.
Any non-condensables (e.g. nitrogen, methane) present in the system will get trapped in the vapor space of the accumulator. Non-condensables are vented from the top of V-604. A rise in the pressure indicated at PIC-665 (top of V-604) above the equilibrium pressure of propane (determined by the temperature of propane in V-604) indicates that non-condensables are accumulating in V-604.
E-603 Propane Subcooler
Condensed propane from V-604 is subcooled to 103.1 DEG F using cooling water. Subcooling the propane increases the efficiency of the Propane Refrigeration System.
E-704 HP Evaporator
HP Evaporator, E-704, uses HP propane liquid from the Propane Subcooler, E-603, to chill Mixed refrigerant (MR) down to 45.1 DEG F. The MR partially condenses at this temperature. E-704 also acts as a reservoir for propane that is used in the downstream MP Evaporator, E-705. The level of propane liquid is controlled by LIC-671. Propane vapor is returned to the HP Propane Compressor Suction Drum, V 603.
E-705 MP Evaporator
MP Evaporator, E-705, uses propane liquid from HP Evaporator, E-704, to chill Mixed refrigerant (MR) from E-704 down to 13.5 DEG F. Additional condensation of the MR occurs. E-705 also acts as a reservoir for propane that is used in the downstream LP Evaporator, E-706. The level of propane liquid is controlled by LIC-672. Propane vapor is returned to the MP Propane Compressor Suction Drum, V-602.
E-706 LP Evaporator
LP Evaporator, E-706, uses propane liquid from E-705 to chill Mixed refrigerant (MR) from E-705 down to -29.2 DEG F. Additional condensation of the MR occurs. The level of propane liquid is controlled by LIC-673. Propane vapor is returned to the LP Propane Compressor Suction Drum, V-601.
Instrumentation
The speed of the Propane Compressor Turbine, KT-601, is controlled by SIC-661 which controls the flow of fuel gas to the gas turbine. The flow of fuel gas is indicated on FI-665.
The Propane Compressor, K-601, has minimum flow controllers to protect the LP, MP and HP compressor stages from operating in the surge region. XIC-661 protects the LP stage and XIC-662 protects both the MP and HP stages. These controllers must always be in automatic mode. The output of XIC-661 opens the minimum flow valve of the LP stage while XIC-662 opens the minimum flow valves of the MP and HP stages.
All three compressor stages have pressure, temperature and flow indicators to monitor performance. Each suction drum has a level indicator. Normally there are no liquids in the drums. It is assumed that any liquid reaching the suction drums is drained to its corresponding propane evaporator.
HIC-661 permits venting the LP Suction Drum, V-701, to flare during upsets.
The outlet temperature from the Propane Condenser, E-602, is indicated on TI-665.
The level of the Propane Accumulator, V-604, is indicated on LI-664. Makeup propane is supplied to the system using FIC-664. Any non-condensable vapors that accumulate in V-604 will cause the pressure to increase as indicated on PIC-665. This controller will automatically vent any non-condensables.
The temperature of propane refrigerant leaving the Propane Subcooler, E-603, is indicated on TI-667.
Mixed refrigerant (MR) vapor from battery limits is chilled in Propane Evaporators E-704, E-705 and E-706. There are level controllers to control the propane refrigerant levels in these three evaporators. The MR outlet temperatures from each of these evaporators are also provided.