due compressor KR- 104 and operate with a lower reflux rate
until the new compressors could be installed.
The only other major piece of equipment required was a
heat exchanger to condense the reflux to the demethanizer.
Pioneer decided to reuse equipment from the idled Fain II
gas plant because there is no plan to restart that plant in the
near future. Several multipass brazed aluminum exchangers from Fain II were evaluated for this service. A three-pass
exchanger that fit the service was relocated and put into its
The difficulty with using this brazed aluminum exchanger in its new service was the possibility of drastic temperature changes during operation, especially if the demethanizer reflux compressors shut down. In order to protect this
exchanger, a bypass with check valve around the compressor was installed to ensure that there would always be flow
to the reflux pass of the exchanger (Fig. 4).
If the compression fails, this feature should protect the
exchanger from changing temperature too quickly. A control
scheme was installed in the pressure control valve downstream of the new demethanizer reflux compressors. The
valve will open if the flow rate to the new reflux condenser
falls below a required value, allowing the gas to bypass the
exchangers and enter the demethanizer.
Two new electric, motor-driven, 600-hp compressors
were chosen for the residue reflux compressors. Electric motors were chosen because of the shorter lead time compared
with gas-driven compressors and they help minimize fuel
use and because of the availability of purchased power at
the plant. The new compressors would compress residue gas
from about 380 psig to 1,000 psig.
These compressors were the longest lead item in the project; piping was therefore also installed to use the existing
KR- 104 in the interim for reflux compression. There would
not be 15 MMscfd available for reflux, but the compression
would pay for itself while Pioneer waited for the new compression to be installed.
Since the residue gas would reach temperatures well below the freezing point of lubricating oil, it was necessary to
install a filter coalescer downstream of the new and existing
Pioneer enlisted outside help for the detailed engineering
and to generate isometric drawings for the new and some
existing piping. A brief shutdown was
needed to install several critical tie-in points and also to complete some
insulation work. These tie-in points
were spooled and flanged to minimize
downtime (Fig. 5).
Construction began on the tie-in
points after the engineering kickoff
meeting in mid-February 2012. The
materials arrived in late March. The
plant was shut down for 2 days to in-
stall the tie-ins in mid-May. After the tie-ins were installed,
the remaining piping was installed and insulated without a
Construction was completed and the plant started up
with the existing residue compressor KR- 104 in the middle
of August. The new compressors were installed and started
up in early December.
The project was completed for about $4 million. Fig. 6
describes the cost breakdown.
During start-up it was important to maintain sales specifications of all plant products. Among these was the increase in
methane content in the NGL product due to the change in
demethanizer operating conditions. As the project began to
recover more ethane and propane, the methane in the NGL
Changes were made to ensure that the NGL product
would meet the spec and that maximum NGLs were recovered. The ethane recovery was estimated at about 80% and
the propane recovery was as high as 99%. These recoveries
are based on a reflux rate of 9. 3 MMscfd.
Another key aspect of the start-up process was that, as
more ethane and propane were recovered from the feed, the
heating value of the residue gas decreased. The inlet gas to
the plant is high in nitrogen.
To raise the heating value, higher flow needs to go
through the nitrogen rejection unit.
Proper procedure was developed to start up the new
ETHANE-RECOVERY IMPROVEMENT OPTIONS
––– increase, –––
Option 1 5
Option 2 17
Option 3 6. 3
Option 4 4
Option 5 4
Option 6 30
Optimize demethanizer pressure
Raise feed pressure
Lower cold separator temperature
Residue-gas recycle as
Feed compressor, aftercooler
Rewheel of existing turboexpander
Residue-gas recycle stream
boost compressor and chiller