H2Tech - Q1 2022 - 29

ADVANCES IN HYDROGEN TECHNOLOGY
Process monitoring is an indispensable practice to keep
track of KPIs of an H2
plant. A good system of process monitoring
not only ensures safe and reliable plant operations, but
also helps operators to make strategic decisions, such as for
catalyst changeout schedules.
If KPIs are not monitored
closely, there can be
situations where the expected
yields are not achieved. This
affects the economics of the
H2
plant and the entire refinery
complex.
The main objectives of
this article are to guide H2
plant process engineers in
monitoring critical parameters
and KPIs across each reactor
in the H2
flowsheet, and
flowis
bein
performing a detailed mass
balance across the H2
sheet by using available information, such as dry analysis of
outlet streams. Doing so will help identify bottlenecks across
each reactor. The focus would be to see how much H2
ing produced before the final stream enters the pressure swing
adsorption (PSA) unit.
This mass balance will also help estimate the outlet stream's
composition on a wet basis, thereby facilitating the estimation
of equilibrium constants (Keq
values) for steam methane reforming
(SMR) and water-gas shift (WGS) reactions, which
will help calculate the approach-to-equilibrium (ATE) values.
These values, which are important in understanding the catalyst
activity, can then be compared with the kinetic model values
provided by the catalyst supplier.
Primary steam reformer. Every H2
process engineer knows
that the primary steam reformer is the heart of the entire H2
flowsheet. The primary function of the unit is the conversion
of methane and higher hydrocarbons (in the absence of a prereformer)
in the feed to H2
(along with CO and CO2
parameters include the steam-to-carbon (S/C) ratio, outlet
temperature, pressure, pressure drop, tube wall temperature
(TWT), reformer firing and flame characteristics, and tube appearance.
Methane conversion (as indicated by methane slip)
and pressure drop are the two major performance indicators.
Approach to equilibrium is a theoretically important parameter
in this section, as it indicates the performance of catalyst,
while other parameters (such as outlet temperature, S/C ratio
and pressure) are held constant. This value will be useful for the
process engineers during the technical evaluation stage concerning
catalyst offers. The reactions occurring in the reformer
are equilibrium limited; therefore, the methane slip observed at
zero (or close to zero) ATE indicates the minimum thermodynamically
possible methane slip at the given conditions.
Another advantage of calculating the equilibrium temperature
is that it provides an indication of the exact outlet process
gas temperature. As the WGS reaction is quick, it can be assumed
to always be in equilibrium at the reformer outlet conditions.
Normally, in most reformers, the reformer outlet temperature
indication (TI) is located a few meters away from the tube
). Key
SPECIAL FOCUS
outlet (where the catalyst ends). There is always some heat loss
that needs to be assumed from the tube end to the TI point.
Mass balance across the primary reformer. The prereformer
effluent is the feed to the reformer. Depending on
the flowsheet, there could be
some additional steam added
at
the
Mass balance is one of the crucial
checks to ensure maximum operational
efficiency of the H2
plant. This enables
the process engineer to calculate the
KPI across the whole flowsheet.
reformer
inlet. For
reformers in hydrogen and
carbon monoxide (HyCO)
plants, recycled CO2
is normally
added at the reformer
inlet, with the intention to
maximize CO yield. However,
for the examples here, let
us assume that no additional
steam is added for Case 1 and
that some steam is added for
Case 2. Note: These are purely
assumptions and bear no
similarity to any existing unit.
Case 1. The total wet flow at the outlet of the pre-reformer-as
per the calculation detailed in Part 1 of this article-
was 4,028.4 kmol/hr. The reformer inlet temperature was
500°C, and the reformer's outlet temperature was 840°C. Assuming
15°C heat less from the tube end to the TI, the tube
outlet temperature would be 855°C and the outlet pressure
would be 22 bara (21.7 atma). The outlet composition (dry
basis) reported by the laboratory was the following:
* H2
= 72.4%
* CH4
* N2
= 4.57%
* CO = 13.61%
* CO2
= 8.1%
= 1.35%.
In the following equation for this example, d is the outlet dry
flowrate and s are the moles of steam at the outlet. The inlet and
outlet moles in Case 1 are shown in TABLE 5. To calculate d using
carbon balance, Eq. 20 is used:
907.47 + 62.16 + 0.373 = 0.0457d + 0.081d + 0.1361d (20)
d (the outlet dry flowrate) = 3,691 kmol/hr
To calculate s using O balance, Eq. 21 is used:
2 × 62.16 + 0.373 + 2785.32 = s + 2 × 0.081 ×
3691 + 0.1361 × 3691
s (the outlet steam) = 1,809.73 kmol/hr
The outlet wet mol fraction is shown in TABLE 6. To calculate
the SMR equilibrium constant, Eq. 22 is used:
TABLE 5. Inlet/outlet of Case 1
Inlet, kmol/hr
H2
N2
CH4
CO2
CO
H2
O
223.76
49.72
907.47
62.16
0.373
2,785.32
Outlet, kmol/hr
0.724d
0.0135d
0.0457d
0.081d
0.1361d
s
H2Tech | Q1 2022 29
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H2Tech - Q1 2022

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