Summary
Efficiency and economics have motivated RasGas Trains 3 through 5 to
implement a novel compressor-driver arrangement comprising both propane and
mixed-refrigerant (MR) compressors on a single shaft. One Frame 7E gas
turbine drives the low-pressure (LP) and medium-pressure (MP) MR compressors,
while a second Frame 7E drives the propane and high-pressure (HP) MR machines.
With minor control system adjustments after startup, the refrigerant
compressors in RasGas Train 3 are operating reliably.
Introduction
The propane precooled mixed refrigerant (C3MR) process
developed by Air Products has been the dominant process technology employed in
the liquefied natural gas (LNG) industry for more than 30 years. This
process uses a propane refrigerant system to chill the feed gas from ambient
temperature to approximately –33°C and an MR to chill the gas to LNG
temperatures. Because pure propane is used in the precooling refrigeration
cycle, the minimum temperature of this cycle is limited to the boiling
temperature of propane near atmospheric pressure. The total duty that can
be transferred to the propane refrigerant is thereby naturally limited to heat
that can be transferred within this range.
Because of the propane’s limited temperature range, one of the
characteristics of the C3MR process arrangement is that
the MR refrigeration system requires nearly double the compression power of the
propane refrigeration system. The actual difference depends on the number
of propane stages, the ambient conditions, the environmental cooling medium,
and other process arrangements. To drive these refrigeration compressors,
early LNG plants used steam turbines that could be sized to fit the required
service (e.g., Marsa el Brega). Later projects determined that gas-turbine
drivers were a more efficient and less costly option for driving the
compressors.
For procurement, maintainability, and sparing purposes, an operating
facility would prefer to have each of the compression services driven by the
same type and model of gas-turbine driver. For the C3MR process, such an arrangement is typically inconvenient
because of the inherent power consumption mismatch between the propane and MR
services.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
12 December 2005
- Manuscript approved:
20 March 2006
- Version of record:
20 June 2006
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