Hydrocarbon Processing - January 2022 - 23

Sustainability
to isobutane. In commercial operation,
however, the opposite occurred-isobutane
was supplied in excess relative to the
C4
olefin content.
TABLE 2 compares the commercial and
design mass balances and alkylate yield.
The design basis required a supplementary
isobutane stream (3 tph) to fully
react with C4
olefins that were in excess
in the design feed. The commercial application
did not require utilization of the
supplementary isobutane stream. Due to
the commercial feed containing relatively
lower quantities of C4
olefins, excess isobutane
and relatively higher quantities of
non-reactive n-butane, the alkylate production
and yield underperformed compared
to the design case. As a result, the
effective alkylate production rate for the
unit was 5,600 bpd, as compared to the
designed 7,400 bpd.
In commercial operation, 0.86 tph of
flare gas was produced, representing 2.2
wt% of the feed. The flare gas was attributed
to unrecovered isobutane, which,
under the design scenario, was intended
to be captured and recycled using the
compressor system flash tank. As isobutane
in the commercial feed was supplied
in excess, the original design load
of the recovery system was insufficient
to handle the additional throughput;
therefore, flaring was required. At present,
a corresponding retrofit scheme has
been proposed to add a pumping system
to increase the recovery capacity of
flashed isobutane.
It should be noted that the deviation
in feed composition of commercial feed
from that of the design feed experienced
by the operator is not unusual, since the
typical feedstocks for the alkylation process
are off-gases from upstream FCC or
MTBE units. The off-gas compositions
are dependent on FCC/MTBE feed and
their operating conditions.
TABLE 3 compares the commercial
and design alkylate product specifications.
Key metrics-such as the research
octane number (RON), motor octane
number (MON) and endpoint distillation
temperature-met the design
basis specifications during commercial
operations, indicating that the process
produces superior-quality alkylate. The
observance of a higher commercial product
Reid vapor pressure (RVP) was due
to the operator's inclusion of up to 10
wt% n-butane alongside alkylate prodTABLE
3. Alkylate product specifications
Specification
RON
MON
Initial boiling point, °C
10% distillation temperature, °C
50% distillation temperature, °C
90% distillation temperature, °C
Endpoint, °C
Vapor pressure (37.8°C), kPa
Density (20°C), g/cm3
Chloride content, ppm
Sulfur content, ppm
TABLE 4. Alkylate composition
Constituent, wt%
C3
C4
C5
C6
C7
C8
C9
C10
C11
C12
4.23
3.48
2.16
3.59
82.15
1.22
0.81
2.36
uct to balance the low RVP in the plant's
existing gasoline pool. Modifying the nbutane
content in the alkylate product
was achieved by adjusting the process
operating conditions of the n-butane extraction
column. Complete removal of
the n-butane fraction from the alkylate
product would result in an alkylate RVP
of less than 30 kPa.
TABLE 4 shows the composition of the
commercial alkylate product. The process
achieved 100% olefin conversion. No
C12
were detected in the product stream. The
C8
+ or higher boiling point compounds
content in the alkylate was 82%, which
is comparable to, or exceeding the performance
of, best-in-class competitive acidbased
alkylation processes. The narrow
distribution of the alkylate composition
indicates that the process is highly selective
for alkylate yield.
TABLE 5 shows the commercial utilities
consumption for the unit. The absolute
consumption rates for steam,
electricity, instrument air and recycled
Design basis
96.5 ± 0.5
94 ± 0.5
Unspecified
Unspecified
Unspecified
Unspecified
≤ 200
35
0.69
< 1
Unspecified
Commercial
97.5
95
37
80
105
113
168
46.6
0.7
0.3
< 0.2
TABLE 5. Utilities consumption
Utilities
1 MPa steam, tph
3.5 MPa steam, tph
Electricity, kWh/hr
Instrument air, Nm3
Recycled water, tph
Condensed water
discharge, tph
Total, kgEO/t alkylate
Commercial quantity
36.88
3.21
/hr
5,480
199.67
1,801.54
-40.09
154.51
TABLE 6. Recommended contaminant
limits
Contaminant
Water
Butadiene
Methanol
Dimethyl ether
MTBE
Total sulfur
Limitation, ppm
≤ 10
≤ 50
≤ 50
≤ 100
≤ 50
≤ 20
water were in alignment with other operators
using the CIL-catalyzed alkylation
technologya
. However, when energy
use was normalized to kilograms of oil
equivalent (kgEO) per ton of alkylate
produced, the energy consumption for
this installation (154.51 kgEO/t alkylate)
was 20%-30% higher than the peer
group, as expected. The discrepancy was
attributed to the lower-than-anticipated
alkylate production due to feed quality
issues (low olefins and high non-reactive
n-butane contents). The operator has
identified feed control as an area of optimization
and is considering measures to
Hydrocarbon Processing | JANUARY 2022 23
Hydrocarbon Processing - January 2022

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