American Oil and Gas Reporter - March 2017 - 61

SpecialReport: Gas Processing & Treating
TABLE 2

still have significant propane losses at 30
percent ethane recovery.

Ethane Rejection versus Recovery Performance
GSP

Preferred Process Designs

ETHANE REJECTION MODE
SCORE
SRC

RSV

Recovery
C2
C3

0.7%
90.2%

0.7%
99.5%

0.8%
99.6%

0.8%
99.4%

Compression HP
Residue
Refrigeration
Reflux
TOTAL

16,000
4,250
-
20,250

11,810
6,410
-
18,220

11,450
6,920
850
19,220

21,000
4,490
-
25,490

GSP

ETHANE RECOVERY MODE
SRC

RSV

Recovery
C2
C3

95.4%
99.7%

99.0%
100%

98.9%
100%

Compression HP
Residue
Refrigeration
Reflux
TOTAL

16,000
6,580
-
22,580

15,990
6,850
510
23,350

16,330
7,100
-
23,430

recovery level is usually slightly more
than 30 percent ethane recovery. However,
it is possible to keep the propane recovery
very high, even when approaching the
methane content limit. In addition, this
very high propane recovery can be
achieved at much lower compression
power than a GSP design, which will
FIGURE 4

Propane Recovery versus Ethane Recovery
100
99
98
97
Propane Recovery

covery level required 16,000 horsepower
of residue compression and 6,580 HP of
propane refrigeration, for a total of 22,580
HP for each 300 MMcf/d train. This became the base line for comparing process
design options.
An RSV design could achieve 98.9
percent ethane recovery and 100 percent
propane recovery using 23,430 HP. The
SRC process could achieve 99.0 percent
ethane recovery and 100 percent propane
recovery at 23,350 total (residue, refrigeration and SRC compression) HP.
The RSV and SRC designs add a
packed bed above the GSP reflux feed to
the column. Below the GSP reflux feed
nozzle, the columns are very similar for
all three designs. If the RSV or SRC top
feeds are reduced to zero flow for any
reason, the column operates as a GSP
column and the top bed is passive.
Because of the way the reflux system
operates, SCORE is best suited for full
ethane rejection operation. However, a
SCORE design also can be operated in
an "incidental ethane recovery mode" in
which the reboiler heat is significantly
reduced from what is required to meet
the normal ethane/propane ratio at full
rejection, allowing some ethane recovery
in the bottoms product. The heat can be
reduced until the amount of methane in
the bottoms product approaches the
pipeline product limit.
For most SCORE designs the incidental

Figure 4 shows how propane recovery
changes as a function of the ethane recovery level over the full range of possible
ethane recovery levels for GSP, SCORE
and SRC designs with the residue compression power limited to 16,000 HP (the
maximum required for 95 percent ethane
recovery using GSP) and refrigeration
compression limited to 7,000 HP. At 30
percent ethane recovery, the propane recovery for a GSP design falls to approximately 95 percent for the Lancaster
design basis. In contrast, the propane recovery for SCORE and SRC is more
than 99 percent. This equates to 35,000
gallons of propane a day for each train.
The triangular-shaped area between
the GSP line and the SRC/SCORE line
illustrates what is given up in propane
recovery when a GSP design is used for
partial ethane rejection. As expected, the
largest difference is at full ethane rejection
at the left end of the graph, but there is a
notable difference in propane recovery
at the graph's right end.
Also important is the difference in
compression power for these three process
designs. The total compression horsepower

SCORE

SRC

At Full Rejection:
SCORE 18,220 HP 99.5% C3
GSP
20,250 HP 90.2% C3
delta
-2,030 HP +9.3% pts

GSP

96
95
94
At 30% C2 Recovery:
SCORE 15,300 HP 99.3% C3
GSP
18,800 HP 95.2% C3
delta
-3,500 HP +4.1% pts

93
92
91
90
0

40
50
60
Ethane Recovery

100

MARCH 2017 61

American Oil and Gas Reporter - March 2017

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