Chemical Engineering February 2021 - 30

Demin Water
MPS
22.9
3.6
Process Streams
Deaerator
HEX-001
HEX-n001
Demin Water
MPS
11.3
1.3
From R-002
HEX-001
*
Deaerator
To R-003
From R-003
HEX-002
FIGURE 4. The Strategy 1 process modification involves utility distribution for the process; (a) displays the
original utility distribution; and (b) displays the revised utility distribution (flowrate in ton/h). Note that the
unit marked with the * symbol represents the four HEX units shown in Figure 8
Saturated Steam
Steam Drum
From
DSU-001
BFW
From
DSU-001
R-001 *
SMR-001*
BFW
R-001 *
SMR-001*
WHB-001
BFW
* Steam-consuming reactor
BFW
R-002 *
BFW
R-002 *
R-003 *
R-003 *
To PSA
(b)
To PSA
FIGURE 5. The configuration in (a) shows steam distribution in the base case, while (b) shows the revised
setup described in Strategy 2, which resulted in a reduction in reactor heating requirements
following the modifications that are
outlined in the following sections.
These modifications reveal excess
energy availability.
Energy reduction strategies
In general, the energy efficiency of a
process can be improved by the following
techniques [7]:
1. Reducing waste and losses
2. Optimizing process operation
3. Heat-recovery improvement
4. Identifying process changes
5. Optimizing energy-supply systems
These techniques are the underlying
concepts used in
developing
the process-improvement strategies
outlined in this article.
According to Ref. 6, heat recovery
can be further increased by exploiting
Strategy Parameter
1
2
3
4
30
BFW flowrate
(feed to DEA)
Reactor feeds
heating utility
Heat flow
supplied by NG
Fluegas
heat flow
After
12.6
ton/h
16.16
MW
3.81
MW
78.9
MW
the process conditions that possess
flexibility and freedom to be changed
(within process limits). The SMR design
evaluated in this article consists
of several units that can be explored
for improving energy efficiency, such
as furnaces, boilers, reactors, stacks
and so on. The following section
describes four such strategies for
enhancing heat recovery, leading to
overall lower energy consumption.
Stream data for these revised heatintegration
steps are given in Table 2.
Strategy 1: Utilities. In the base
case, medium-pressure steam
(MPS) is fed to the deaerator (DEA) in
order to remove the oxygen content
of the demineralized (demin.) water
before it is heated in heat exchangers
HEX-001 and HEX-002, as well
TABLE 3. RESULTS FROM THE FOUR STRATEGIES
Before
26.5
ton/h
17.86
MW
22.63
MW
70.6
MW
Remarks
Additional 4.98 MW of excess
energy is available for heat
integration
Reduction of 1.7 MW in the
heating requirements
Reduction of 1,440 kg/h of NG
An extra 8.3 MW is available for
heat integration
Air 15
150
(a)
12.6
HEX-002 Convective
Section (CS)
Steam
Drum
12.6
13.9
To R-001, SMR-001,
R-002, R-003
CS
(b)
To R-001, SMR-001,
R-002, R-003
To Phase Separator
(a)
Superheated
Steam
as the furnace convective section, in
order to produce steam. However,
an excess amount of steam is fed
to the DEA, so it can be sold as superheated
steam after further heating.
Upon inspection, only 48% (12.6
ton/h) of the 26.5 ton/h of generated
steam is actually required as a reactant
in the plant.
Next, the boiler feedwater (BFW)
that was originally used as the cooling
medium in HEX-001 and HEX002
is removed. Doing this leads to
a lower demin. water flowrate, as it
is no longer constrained by the heattransfer
requirement of the heat exchangers.
The heated demin. water
stream from the DEA is treated as individual
cold streams for steam generation
in the reactors (see Strategy
2 below for details). An extra heat
exchanger (HEX-n001) is used for
heating the demin. water from 30 to
90°C before it enters the DEA. Following
the data-extraction heuristics
in Ref. 6, HEX-001 and HEX-002 are
extracted as hot streams, while HEXn001
is extracted as a cold stream
in pinch analysis. As a result, an
extra 4.98 MW of excess energy is
available for heat integration. These
process modifications are shown in
Figure 4.
Strategy 2: Reactor steam supply.
In the base-case process, the saturated
steam from the steam drum
(shown in red in Figure 5) is mixed
with the feed of the pre-reformer.
Four reactions consume steam (R001,
SMR-001, R-002 and R-003).
The saturated steam is supplied to
the pre-reformer in excess such that
no additional makeup is required in
subsequent reactors. Consequently,
a large proportion of steam is present
in each of the reactor streams.
The excess steam is heated and
(a)
To
SMR-001
Air 15
850
To
SMR-001
FIGURE 6. The process modifications in Strategy 3 show that higher air temperature
of 850°C (b) enables better heat integration over the base case (a)
SMR-001 1,056
Fluegas
550
To
(a)
stack
SMR-001 1,056
Fluegas
200
To
FIGURE 7. With Strategy 4, shown in (b), fluegas exhaust temperature (°C)
can be reduced, leading to energy savings over the base case (a)
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(b)
stack
(b)

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Chemical Engineering February 2021

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