Hydrocarbon Processing - June 2021 - GP-33

TREATING AND NGL
Energy savings for a propane
refrigerant compressor using simulation
H. Y. NOH, Saudi Aramco, Yanbu, Saudi Arabia
Saudi Aramco's Yanbu fractionation
plant utilizes deethanizers, depropanizers,
debutanizers and an RVP column.
These distillation columns are equipped
with overhead air fan coolers, except for
the deethanizer columns, which have a
propane refrigerant closed-loop system.
The system essentially consists of a steamoperated
compressor, an air cooler, expansion
valves, a refrigerant flash drum and an
evaporator.
Propane refrigerant is highly compressed
by the compressor, cooled by the
air cooler and then flashed through the expansion
valve in the refrigerant flash drum
under medium pressure. Medium-pressure
liquid propane is let down to further
reduce pressure and is then vaporized in
the evaporator, after being used as a cooling
medium for the deethanizer overhead
ethane vapor stream. The vaporized refrigerant
is recycled back to the compressor, in
addition to the low-pressure flashed vapor
propane, through an overhead letdown
valve after the refrigerant flash drum.
In two 1980s-era deethanizer modules,
the existing compressors have a single
suction side only. This configuration
is inefficient in terms of compressor input
energy because single suction is designed
to receive only the combined low-pressure
streams: one from the evaporator
and one from the refrigerant flash drum.
The newest module of the deethanizer at
the propane refrigerant compressor has
two different suctions.
Based on this information, a simulationa
study was conducted to modify the
old configuration to correspond to the
newest module to determine whether energy
improvement could be achieved.
Existing compressor design. The Yanbu
fractionation plant receives C2
+ that
is transferred to deethanizer columns for
FIG. 2. Mass and energy balance for the existing design, using simulation.a
Gas Processing & LNG | MAY/JUNE 2021 33
separation. Normal operating pressure
in the columns is 390 psia, and the temperature
required for near-pure ethane
condensing on the overhead is approximately
41°F. FIG. 1 shows the deethanizer
overhead cooling system with typical operating
conditions.
In the existing design mode, the overvapor
stream is partially conrefrigerant
system.
head C2
densed by the liquid C3
The liquid refrigerant C3
absorbs heat from
177°F
322 psia
30.31
42°F
390 psia
61 psia
29°F
61 psia
38°F
210,500 lb/hr
C2+ feed:
155,000 bpd
Deethanizer
41°F
29°F
466,500
lb/hr
C3+
from others
157,600 bpd
Depropanizer
FIG. 1. Existing deethanizer overhead cooling system and required steam flowrate to turbine.
Comp Disch
Temperature
Pressure
Comp Suct
Comp Suct
Temperature
Pressure
Mass Flow
R
RCY-1
9
29.16
61.00
677.0
F
psia
Mlb/hr
Comp
Comp Work
Comp Work
Heat Flow 30.31 MMBtu/hr
5
MIX-100
8
Temperature
Pressure
Mass Flow
25.12
61.00
466.5
F
psia
Mlb/hr
5
Temperature
Pressure
Mass Flow
8
7
Heat Absorbed
Heat
Heat Flow 71.47 MMBtu/hr Absorbed
Evaporator
VLV-102
6
C2 Vapor
38.04
61.00
210.5
F
psia
Mlb/hr
PCV
4
Temperature
Pressure
Mass Flow
43.42
83.00
210.5
F
psia
Mlb/hr
Flash
Drum
3
VLV-101
2
Turbine
MPS
HPS
HPS
Mass Flow 295.9 Mlb/hr
4
177.7 F
322.0 psia
Comp Disch
Air Cooler
3
Temperature
Pressure
Mass Flow
43.42
83.00
677.0
F
psia
Mlb/hr
C2
Economizer
2
Temperature
Pressure
116.0 F
309.0 psia
VLV-101
Heat
Released
1
Heat Released
Heat Flow 87.54 MMBtu/hr
1
Temperature
Pressure
Vapor Fraction
141.5
314.0
0.0000
F
psia
Ethane
Temperature
Pressure
122.6
360.0
F
psia
Ethane
83 psia
43°F
677,000
lb/hr
116°F
41°F
HPS MPS
MMBtu/hr
Air
295,900 lb/hr
Existing
122°F
360 psia
C2
product
0.5 mol% C3
C2 and is converted into C3
vapor. In the
existing system, the low-pressure (61-psia)
vapor from the evaporator is designed to
combine with another low-pressure (61psia),
cloud-marked vapor stream from
the flash drum (83 psia) in FIG. 1. Then, the
combined stream is used as suction for the
compressor. The refrigerant vapor is pressurized,
fully condensed, subcooled and
flashed. The propane refrigerant circulates
in the closed loop.
Air
141°F

Hydrocarbon Processing - June 2021

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