FLOWSHEET OPTIMIZATION FOR MULTI-PRODUCT
AIR SEPARATION UNITS
Bruce K. Dawson, Scott C. Siegmund, Zhang Yonggui,
Air Products and Chemicals, Inc.
The First Baosteel Annual Academic Conference, Shanghai, China, May 27-28 2004
Abstract: An Air Separation Unit (ASU) produces oxygen and nitrogen by the cryogenic
distillation of air. The majority of modern ASUs produce oxygen by the pumped LOX
process (internal compression process), where the product oxygen is produced at an elevated
pressure by boiling high-pressure liquid oxygen against high-pressure air. This flowsheet
substitutes a Booster Air Compressor (BAC) for a product oxygen compressor. Some
applications require a large quantity of nitrogen at an elevated pressure, as well as highpressure oxygen. It is possible to produce nitrogen as a low pressure gas from the upper
column of the ASU, as a medium pressure gas from the lower column, or as a high-pressure
gas by pumping liquid nitrogen and warming it against high-pressure. A turbo expander is
typically used to expand air or nitrogen from a higher pressure to a lower pressure to produce
refrigeration for the process. Various expander configurations are possible. The best choice
depends on the quantity and pressure of the high-pressure gaseous products, as well as
whether or not liquid co-production is required.
The machinery selection has a significant effect on the total cost of the ASU, and
therefore must be carefully considered when evaluating competing ASU flowsheets. When
nitrogen is produced at elevated pressure, the optimum ASU flowsheet may require
additional feed air compared to when the nitrogen is produced at low pressure and
compressed externally. Minimizing the total number of compression stages often is the
minimum capital cost solution. The applicability of vendor standard compression equipment
may also be an important factor.
This paper discusses the advantages and disadvantages of co-producing nitrogen in
varying quantities via each of the methods listed. Possible turbo expander configurations are
considered. Operability advantages and disadvantages of various alternatives are discussed.
A recent case study for a large fertilizer project is presented to illustrate the method of
evaluating several competing ASU flowsheets.
Keywords: Oxygen, Nitrogen, Pumped LOX, Pumped LIN, Optimization
1.
Background
For many years the most common air separation unit (ASU) flowsheet produced oxygen
via cryogenic distillation of air at a pressure only slightly greater than atmospheric pressure.
The Double Column Cycle has been used for over one hundred years to produce oxygen from
air. The feed air is compressed to approximately 600 KPa , cooled to near its dew point, and
introduced at the bottom of the high-pressure column (HPC). Nitrogen, being more volatile
than oxygen, concentrates in the vapor as it rises through the column. The vapor from the top
of the column is condensed against boiling liquid oxygen in the reboiler/condenser. The
condensed overhead is divided into a reflux stream that is returned to the HPC, and a reflux
stream that is sent to the top of the low-pressure column (LPC). The oxygen-enriched HPC
bottoms is sent to an intermediate stage of the LPC. Oxygen concentrates in the bottom of
the LPC.
In the Low Pressure GOX Cycle (Figure 1), the product oxygen is taken as a gas from the
bottom of the LPC. In this flowsheet, the oxygen product is compressed to the required
pressure via a product compressor. This is an energy efficient method of producing oxygen,
but oxygen compressors have inherent safety issues, and are therefore more costly, less
efficient, and less reliable than air or nitrogen compressors of an equivalent capacity.
HP GOX
LP GO X
LP G AN
HP GAN
LP G AN
W ASTE
LPC
W ASTE
E xpander
A ir
GOX
L IN
HP GAN
HPC
CLO X
Figure 1 – Low Pressure GOX Cycle
The refrigeration required to compensate for heat leak from the environment, as well as
for the temperature difference between the feed air and the product and waste streams leaving
the main heat exchanger, is provided by expanding a portion of the feed air into the LPC.
The work done by the expander can be recovered as electricity in a generator linked to the
expander, or the portion of the feed that is to be expanded can first be compressed to a higher
pressure in a compressor mounted on the same shaft as the expander. This compressor and
expander combination is commonly referred to as a compander. Using a compander results
in a higher pressure ratio across the expander, and lower expander flow to produce the same
amount of refrigeration compared to a generator-loaded expander. The optimum choice of
expander configuration depends on the total amount of refrigeration required.
If co-product nitrogen is required, it can be taken from either the top of the HPC, or the
top of the LPC. Removing nitrogen gas from the HPC reduces the duty of the
reboiler/condenser, and reduces the boilup in the LPC. For a given oxygen product flow and
purity, there is a minimum boilup required in the bottom of the LPC. The amount of gaseous
nitrogen (GAN) withdrawn from the HPC, as well as expander flow bypassing the HPC,
affects the oxygen recovery. Lower oxygen recovery implies that more feed air must be
compressed to obtain the required oxygen product flow.
Producing nitrogen at low pressure from the LPC generally doesn’t affect the oxygen
recovery, but more energy is consumed to compress the nitrogen in a product compressor.
Choosing the optimum flowsheet entails balancing the operating cost and capital cost. The
number of compressor stages directly influences the capital cost as well as the energy
consumption. Compressor vendors have standard frame sizes. Often the lowest compressor
cost per KW of compressor power is achieved by maximizing the capacity of a standard
frame.
2. Pumped LOX Cycle
Most of the recent ASUs built within the last 5 years have utilized the Pumped LOX
Cycle (Figure 2) to produce oxygen at elevated pressure directly from the coldbox, instead
of via an oxygen compressor. In this flowsheet oxygen is taken from the bottom of the LPC
as a liquid. It is pumped to an elevated pressure and then warmed to ambient temperature
against high-pressure air feed. A portion of the feed air from the Main Air Compressor
(MAC) is further compressed in a Booster Air Compressor (BAC) to supply this highpressure air.
For product pressures below the critical pressure, the air pressure must be significantly
higher than the oxygen pressure in order to efficiently exchange heat between the air and the
oxygen. Figure 3 shows the minimum air pressure as function of the oxygen pressure.
Waste
7000
MAC
BAC
HP GOX
6000
MP GAN
WASTE
LAIR
LPC
LOX
Air Pressure (KPa)
LP GAN
5000
4000
3000
2000
1000
LAIR
HPC
MP AIR
0
0
LAIR
Figure 2 – Pumped LOX Cycle
1000
2000
3000
4000
5000
6000
GOX Pressure (KPa)
Figure 3 – Minimum HP Air Pressure
vs. GOX Pressure
For oxygen pressures near or above the critical pressure there is more flexibility to vary
air pressure. In general higher air pressures give more thermodynamically efficient heat
exchange, but practical considerations such as minimizing total number of compression
stages may limit the air pressure.
The cold HP Air leaving the main heat exchanger becomes mostly liquid when its
pressure is reduced before entering the distillation columns. The reduction in vapor feed air
to the bottom of the HPC results in a reduction in LPC reboiler duty. Because the oxygen
product leaves the LPC as a liquid instead of a gas, the net boilup to LPC is the same as for
the LP GOX cycle. Because less nitrogen is condensed, however, there is less pure nitrogen
reflux available for producing pure nitrogen co-product.
3. Pumped LOX/Pumped LIN Cycle
When high-pressure nitrogen product is required, it may be economic to pump liquid
nitrogen to an elevated pressure and warm it against high-pressure air instead of compressing
it in a product nitrogen compressor. Figure 5 shows the flowsheet for this Pumped
LOX/Pumped LIN process. This flowsheet condenses a larger fraction of the total air feed.
Therefore it is often more economical to take the compander flow from an intermediate stage
of the BAC, and direct the expander discharge to the HPC instead of the LPC. Some
additional power is required for the BAC, but the expander flow does not bypass the HPC. A
higher oxygen recovery is possible because the expander flow produces boilup for the LPC.
MAC
BAC
W a s te
HP GOX
HP GAN
W a s te
L A IR
LPC
LOX
L IN
HPC
L A IR
Figure 5 – Pumped LOX/Pumped LIN Cycle
Nitrogen product compressors don’t have the safety or reliability drawbacks associated
with oxygen compressors, but if the required nitrogen pressure is compatible with the air
pressure needed to vaporize the oxygen product, a significant cost savings may be realized by
eliminating the nitrogen compressor. For GAN pressures greater than 2000 KPa, an air
pressure equal to 90% of the nitrogen absolute pressure may be used.
4. Expander Configuration and Additional Liquid Production
For any of the flowsheets presented so far, the optimum choice of expander configuration
may depend on the total amount of high-pressure nitrogen product, as well as the total
refrigeration requirement. If additional liquid products are required for backing up the
gaseous product supply, or for the merchant liquid market, the expander flow may become
too high to expand into the LPC. It may be desirable to compress the expander flow to a
higher pressure in the BAC in order to increase the pressure ratio, and therefore the work
available per unit of expander flow. Designing for a large refrigeration requirement increases
the minimum power consumption for the ASU, since machinery turndown is limited.
Understanding how the co-product nitrogen demand varies in relation to the oxygen demand,
as well as how variable the total refrigeration requirement will be is helpful in evaluating
competing process flowsheets.
5. High Nitrogen/Oxygen Requirements – A Case Study
A typical large fertilizer project requires a large amount of high-pressure oxygen, as well
as an even larger amount of nitrogen at four different product pressures. Table 1 gives the
approximate product flow requirements and pressures for a typical project.
Table 1 - Product Requirements
Product
GOX
HP GAN
MP GAN
LP GAN1
LP GAN2
Flow
NM3/hr
50,000
25,000
28,500
18,000
20,000
Max GAN Pressu re =
Pressure
KPa
4600
8100
3300
710
550
HP Air Pressu re
0.9
Consideration should be given to
producing at least a portion of the nitrogen
from the HP column to reduce the total
number of compressor stages. In order to
produce oxygen at 4600 KPa, the minimum
air pressure from Figure 3 is 6600 KPa. The
nitrogen product pressure that can be
produced as pumped LIN with this air
pressure is:
(1)
From Equation (1) a GAN pressure of 7333 KPa is consistent with the minimum air
pressure needed to efficiently vaporize the pumped LOX. A slightly higher air pressure
would result in slightly more efficient heat transfer, and would allow the HP GAN to be
produced as pumped LIN. Based on the required HP GAN pressure of 8100 KPa, an air
pressure of 7290 KPa is needed. The number of BAC stages needed to achieve this discharge
pressure can be estimated from the overall pressure ratio for the BAC. The BAC inlet
pressure is MAC discharge pressure, less the pressure drop in the front-end air purification
system.
Optimizing the overall ASU capital cost vs. energy consumption requires an
understanding of the present value for 1 KW of power consumption. Typical values range
from $1000 to $3000 per KW. Where power cost is high, a value near the upper end of this
range should be used. For projects where power is generated internally, values nearer the
lower end of this range are more common.
The optimum MAC discharge pressure ranges from 550 to 700 KPa, depending on the
present value for power. Typical values range from 600 to 650 KPa. For the case study we
will assume power is relatively inexpensive, and that the BAC suction pressure is 625 KPa.
The average compression ratio per stage for the BAC can be estimated for n stages using
equation 2.
1
P2 ⎞ n
⎟
⎛
(Avg Ratio per Stage) = ⎜⎜
⎟
⎝ P1 ⎠
(2)
For P2 of 7290 KPa and P1 of 625 KPa, the Average Ratio per Stage is 1.85 for a four
stage BAC, and 1.63 for a five stage BAC. A value of 1.85 is near the maximum average
ratio per stage (the actual pressure ratio will be slightly higher due to intercooler pressure
drop). The cost savings for a four stage BAC will usually justify the higher power
consumption.
The LP GAN2 product pressure is low enough that it could be supplied directly from the
HPC without further compression. One feasible flowsheet is to produce HP GAN as pumped
LIN at 8100 KPa , and the LP GAN2 as gas from the HPC. The MP GAN and LP GAN1
could be produced as gas from the LPC and compressed to the required pressures in a product
GAN compressor (Case 1). Alternatively, HP GAN and MP GAN could both be produced
from the HPC as pumped LIN and both the LP GAN1 and LP GAN2 are produced as gas
from the HPC (Case 2). For Case 2 only a single stage GAN compressor for the LP GAN1 is
needed. Producing so much nitrogen from the HPC results in a reduction in oxygen recovery.
A larger MAC and BAC are required, but much of the capital cost of the product GAN
compressor is eliminated. Incremental capacity in the MAC and BAC may be less expensive
than the multi-stage product GAN compressor. We can estimate the overall power for both
flowsheets to determine if the compressor cost savings result in a lower overall product cost.
5.1 Estimating Oxygen Recovery vs. HPC GAN production
When a large amount of GAN is produced either as pumped LIN or gas directly from the
HPC, there is a reduction in boilup in the LPC. As stated earlier, there is a minimum boilup
required to produce a given quantity of oxygen product at a given purity. The required
boilup can be estimated based on the L/V in the bottom of the LPC.
Once we know the boilup, the condensing flow from the top of the HPC required to
produce this boilup can be calculated based on the latent heat ratio of oxygen at LPC pressure
to that of nitrogen at HPC pressure.
The vapor feed to the HPC is slightly less than the vapor flow from the top stage, since
the latent heat of nitrogen is slightly lower than for oxygen. For our case study, no liquid
products are being produced. For screening the two flowsheets, we will use a value of 1.093
for the HPC Vapor Factor in .
To optimize the design of an ASU, a process engineer must specify the heat exchanger
area and pressure drop to balance capital cost against power consumption. More area will
result in tighter temperature differences and lower power consumption. For screening
competing flowsheets, the BAC flow may be estimated from typical ratios of HP Air to HP
GOX or HP GAN. The total air feed flow (the MAC flow) is the sum of the HPC Vapor
Feed and the HP Air Flow.
In order to estimate the total power consumption for each flowsheet, we must estimate the
expander flow. The expander flow is similar for both alternates, although producing MP
GAN at 3300 KPa directly from the coldbox (Case 2) does require slightly more refrigeration
than when the MP GAN is compressed in a product GAN compressor. The expander flow
for both alternatives is withdrawn after the second stage of the BAC. The estimated
compressor flows for each alternate are summarized in Table 2.
Table 2 – Case Study Calculated Flows
O2 Product
Flow
Pressure from coldbox
HP GAN
Flow
Pressure from coldbox
MP GAN
Flow
Pressure from coldbox
LP GAN1
Flow
Pressure from coldbox
LP GAN2
Flow
Pressure from coldbox
Total Pumped LIN
HPC GAN Vapor Product
V (LPC Boilup)
N2 Condensing Flow
HPC Vapor Feed
HP Air Flow
MAC Flow
Expander Flow
BAC 1st Stage Flow
BAC 3rd Stage Flow
Case 1
Case 2
50,000
4600
50,000
4600
25,000
8100
25,000
8100
28,500
105
28,500
3300
NM3/hr 18,000
KPa
105
18,000
550
NM3/hr
KPa
NM3/hr
NM3/hr
NM3/hr
NM3/hr
NM3/hr
NM3/hr
NM3/hr
NM3/hr
NM3/hr
NM3/hr
20,000
550
53,500
38,000
125,000
173,600
193,600
119,400
313,000
40,700
160,100
119,400
NM3/hr
KPa
NM3/hr
NM3/hr
KPa
NM3/hr
NM3/hr
KPa
For the MAC and BAC the
power for each stage can be
calculated based on the inlet
temperature, pressure ratio, and an
average heat capacity using
equations 3 and 4. In equation 3,
∆hs is the enthalpy change for an
adiabatic compression stage. For
either air or nitrogen the heat
capacity ratio k = 1.4. The value of
C for the MAC is 29.1 J/gmole/°Κ.
For the BAC the value of C is 28.5
J/gmole/°Κ. T1 is the inlet
temperature in °Κ. We will use a
value of 300 °Κ. The units for ∆hs
are J/gmole. Flow in equation 4 has
the units NM3/hr. The typical
value of the stage efficiency η is
0.85.
∆hs = C x T1 x
k −1
⎡
⎤
⎛
⎞
⎢ P2 k
⎥
− 1⎥
⎢⎜⎜ P ⎟⎟
⎢⎝ 1 ⎠
⎥
⎣
⎦
KW/Stage =
20,000
550
25,000
20,000
125,000
173,600
177,100
91,800
268,900
34,000
125,800
91,800
5.1 Estimating Power
Consumption
(3)
Flow x ∆h s
3600 x 22.4 x η
(4)
Compressor powers for each of the alternatives in the case study are summarized in Table 3.
Table 3 – Case Study Calculated Power
Consumption
Case 1
Case 2
MAC
Flow
NM3/hr 268,900
313,000
P2/P1
KPa
655/100
655/100
No. of Stages
3
3
Average Ratio per Stage
1.87
1.87
KW / Stage
KW
6706
7806
Power
KW
20,119
23,419
BAC Stages 1 and 2
Flow
NM3/hr 125,800
160,100
P2/P1
KPa
2134/625 2134/625
No. of Stages
2
2
Average Ratio per Stage
1.85
1.85
KW / Stage
KW
3079
3918
Power
KW
6,157
7,836
BAC Stages 3 and 4
Flow
NM3/hr 91,800
119,400
P2/P1
KPa
7290/213 7290/2134
4
No. of Stages
2
2
Average Ratio per Stage
1.85
1.85
KW / Stage
KW
2247
2922
Power
KW
4,493
5,844
GAN Compressor 1
Flow
NM3/hr 48,500
18,000
P2/P1
KPa
710/105
710/550
No. of Stages
3
1
Average Ratio per Stage
1.89
1.29
KW / Stage
KW
1232
173
Power
KW
3,696
173
GAN Compressor 2
Flow
NM3/hr 28,500
0
P2/P1
KPa
3300/710
No. of Stages
3
Average Ratio per Stage
1.67
KW / Stage
KW
573
Power
KW
1,718
0
Total Compressor
36,184
37,272
Power
6.1 Conclusions
•
•
•
Case 2 requires
approximately 1090 KW
or 3% more power than
Case 1, but it replaces a
large 6 stage product
GAN compressor with a
small single stage
machine. The cost
savings may well be
more than the value of
the additional power
consumption.
Other combinations of
pumped LIN and HPC
GAN could also be
evaluated. The
methodology given here
is useful for screening
competing flowsheets.
The machinery cost is an
important consideration
in optimizing the total
ASU. Adding
incremental capacity to
the MAC and BAC may
reduce the total cost of
the machinery.
The lowest MAC flow is
not necessarily the
lowest overall cost or the
best flowsheet when
large quantities of
nitrogen product are
required.