COMPRESSOR BYPASS FOR EXCHANGER PROTECTION
An increase in relative volatility
facilitates the separation. Therefore,
a greater separation can be achieved
with the same energy input.
A lower tower pressure would
also benefit the heat integration of
side and bottom reboilers with process cooling, as a lower temperature
profile with decreasing tower pressure increases temperature approach.
However, it would add load to the
• Option 2—Raise feed pressure. In the 2012 operation, the feed
pressure to the NGL section was 680
psig. This option considered increasing the feed pressure to 850 psig by
using an existing standby compressor or by installing a new compressor.
A review of the equipment design
pressure was needed to confirm that
raising pressure was practically feasible. All the major equipment from the NGL section inlet to
the expander suction vessel have design pressures of 1,000
psig. The expander outlet vessel has a design pressure of 550
psig. The demethanizer and the stabilizer both have a design
pressures of 450 psig.
Therefore, the existing piping and equipment should be
able to handle the elevated feed pressure.
Based on process simulation, ethane recovery could increase 17% by an elevation of the NGL section’s inlet pressure to 850 psig.
• Option 3–Lower the cold separator’s operating temperature. In the 2012 operation, feed gas to the cold separator
drum was chilled by propane refrigerant in the propane gas
chiller to -23º F. This option considered lowering the feed-gas temperature, leaving the propane gas chiller to -30º F.
This can be achieved by reducing the propane refrigerant’s
pressure on the shell-side of the gas chiller (E- 5).
Pioneer was concerned that the propane compressor had
experienced liquid carryover from the low-stage suction
drum. A further reduction in the low-stage suction pressure
may increase the risk of liquid carryover.
Pioneer said, however, that the feed gas leaving the propane gas chiller had been operated successfully at -28º F.
Therefore, this option was refined to achieve feed-gas temperature leaving the propane gas chiller to -28º F. by lowering the low-stage propane refrigerant pressure. Ethane recovery would increase by 6.3% in this option.
• Option 4–Incorporate gas subcooled process. The gas
subcooled process (GSP) consists of taking a portion of cold
feed gas from the cold separator (suction drum to turboex-
pander). It is condensed and further subcooled with refrigeration from cold residue gas leaving the demethanizer.
The subcooled liquid then becomes the reflux to the demethanizer. GSP usually achieves a higher ethane recovery
compared with the case in which all the gas from the cold
separator flows through the turboexpander.
Fig. 2 shows the simulation with GSP to improve ethane
Repiping V- 5 and V- 7 overheads would be required for
this option. A new heat exchanger to subcool the reflux was
also required. A 4% improvement in NGL recovery could be
expected with this retrofit.
Because the turboexpander was already operated in turndown condition, however, taking away a portion of gas to
GSP from the turboexpander might worsen the turndown
situation and cause the inlet guide vane to close further.
Therefore, the marginal recovery improvement would not
justify the cost and complexity of new equipment.
• Option 5–Upgrade turboexpander. Based on process
simulation of 2012 operation, the turboexpander was estimated to operate at 65% adiabatic efficiency and the associated compressor at 60% efficiency. The low efficiency was
largely caused by the turboexpander running at turndown
Rewheeling the existing turboexpander would allow the
turboexpander to operate at 80% adiabatic efficiency and the
associated compressor to operate at 70% adiabatic efficiency.
Ethane recovery was estimated to increase by 4% with
the turboexpander upgrade. This option had a drawback of
high capital cost and long lead time.
Option 6–Residue-gas recycle as demethanizer reflux.
In this option, a portion of residue gas is compressed, con-