Chemical Engineering February 2021 - 29

B-002
Superheated Steam
To stack
Recycled H2
S2
COM-001
DSU-001
CS-003
NG Feed
Air
B-001
99.9 wt% H2
Mains
water
Legend
AC
B
COM
CS
Demineralization
Package
Air Cooled HEX
Blower
Compressor
Convective Section
DEA-001
P-003
DEA-001 Deaerator
DSU-001
PSA
HEX
P-003
Pump
Desulfurizer
Pressure Swing Adsorption
Heat Exchanger
R-001
R-003
SMR-001
WHB-001
Pre-reformer
R-002
High-temperature WGS
Low-temperature WGS
Steam Methane Reformer
Waste Heat Boiler
FIGURE 2. The process-integration strategies outlined here relate to a steam-methane reforming (SMR)
plant. Note that streams S1, S2, S3 and S4 are placed where the process-integration strategies
are employed
(a)
(b)
1,200
1,100
1,000
900
800
700
600
500
400
300
200
100
15
00
QH, min=4.83 MW
Pinch
5
10 15 20
Net heat fl ow, MW
1,200
1,100
1,000
900
800
700
600
500
400
300
200
100
Pinch
CS-002
R-001
CS-001
Desulfurized Natural Gas
(Fuel)
SMR-001
S3
PSA offgas
CS-006
PSA
Phase
Separator
Wastewater
Boiler Feedwater
AC-001
HEX-002
S1
WHB-001
R-002
HEX-001
R-003
S4
Steam Drum
CS-005
CS-004
QC, min=11.17 MW
5
10 15 20
Net heat fl ow, MW
FIGURE 3. The GCCs displaying the minimum utility targets are shown for (a) the base case; and (b) the
flowsheet with subsequent process-modification strategies, which reveals excess energy availability
Example - H2 production
The analysis outlined in this article
relates to a hydrogen-production
plant situated in a petroleum-refinery
complex in Northwest England.
However, the general principles will
be applicable to many types of heatexchanger
networks.
At this site, hydrogen (H2) is produced
for use in the refinery's hydrotreating
and hydrocracking units,
and also for export to a nearby fertilizer
manufacturing facility. The steam
methane reforming (SMR) process is
employed for the production of hydrogen.
Figure 2 shows the process
flow diagram (PFD) of the SMR plant.
A fresh feedstream of natural gas
(NG) is mixed with a small portion
of recycled hydrogen product before
the mixture is pre-treated within
the desulfurizer (DSU-001) and prereformer
(R001)
units.
In the latter
unit, hydrogen
sulfide,
mercaptans
and heavier
hydrocarbons
are removed.
The desulfurized
NG
then enters
the tubular
furnace reactor
(SMR001),
which
is operated at
1,056°C. The
NG serves
two purposes
in the SMR001
unit: as
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supplementary fuel; and as a reactant
for the steam-reforming process,
along with steam. As a result,
synthesis gas (syngas) is produced
from the reaction between water and
methane. The heat from the fluegas
is recovered in the convective section
of SMR-001 (down to 550ºC) before
stack discharge.
Syngas effluent from SMR-001 is
next cooled in a waste-heat boiler
(WHB-001) before passing to a hightemperature
water-gas shift (WGS)
reactor (R-002) and then a low-temperature
WGS reactor (R-003). In
both reactors, carbon monoxide and
water react to form carbon dioxide
and hydrogen. The hydrogen-rich
stream then undergoes air-cooling
(in AC-001) in order to condense any
water vapor, while its gas mixture
passes through a pressure-swing
adsorption (PSA) unit for the separation
of hydrogen. The offgas from the
PSA is sent to SMR-001 as fuel.
Figure 3(a) shows the GCC of the
base case, which indicates that the
minimum hot utility target (QH, min) of
the process is identified as 4.83 MW,
for a minimum approach temperature
(∆T) of 10°C, without any cold
utility requirement (data for the heat
integration study is given in Table
1). Note that this situation is termed
as a " threshold problem " in process
integration literature [6]. Figure 3(b)
shows the GCC for the process,
Stream Description Stream
Cold
1
2
3
4
5
6
8
9
10
11
12
Demin. water
SMR-001 air Cold
Natural gas
feed
Produced
hydrogen
R-001 feed
Cold
Cold
TABLE 2. HEAT INTEGRATION DATA FOR THE REVISED CASE
∆H
Tsupply
(ºC)
Ttarget
(ºC)
30.00 90.00
(MW)
0.79
15.00 850.00 13.12
40.00 360.00 1.96
40.00 360.00 0.02
Cold 360.00 550.00 1.37
R-001 steam Cold 350.00 550.00 0.13
7 SMR-001 feed Cold 550.00 800.00 2.34
SMR-001
steam
R-002 feed
R-003 feed
PSA feed
SMR-001
fluegas
FEBRUARY 2021
Cold 350.00 800.00 2.01
Hot
Hot
Hot
800.00 350.00 -7.32
350.00 215.00 -2.13
215.00 40.00 -2.86
CP
(MW/°C)
0.013
0.016
0.006
0.000
0.007
0.001
0.009
0.004
0.016
0.016
0.016
Hot 1,056.00 200.00 -21.15 0.025
29
Temperature, ºC
Temperature, ºC
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Chemical Engineering February 2021

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