H2Tech - Q1 2022 - 27

ADVANCES IN HYDROGEN TECHNOLOGY
genator. This section is usually seen as two vessels in a lead-lag
arrangement or as a single vessel when sulfur levels are very
low. In addition to pressure drop, KPIs include the following:
* Outlet H2
S should be less than 0.1 ppmv vs. inlet H2
* The catalyst life depends on pickup/capacity.
The process engineer should have the information
of expected pickup from the catalyst supplier.
Note: The process engineer should ask the catalyst supplier
for actual pickup and not theoretical pickup. The theoretical
pickup might be much higher than the actual pickup.
For example, Case 1-1,000 kmol/hr of (NG + recycle
H2) with 25 ppmv of H2
600 kgS/m3
and an actual pickup of 500 kgS/m3
the pickup figures are an assumption and do not correspond
to any specific product.
The volume of each bed is 10 m3
. Eq. 8 is used to calculate
the amount of sulfur to be removed:
(25/1,000,000) × 1,000 kmol/hr × 24 = 0.6 kmol H2
Therefore, the expected life is calculated using Eq. 9:
500 kgS/m3
× 10 m3/19.2 = 260 days
S/day
= 20.4 kg H2S/day = 20.4 × 32/34 = 19.2 kgS/day (8)
(9)
The most economical way of operating a lead-lag arrangement
is to allow the lead bed to operate until near saturation-i.e.,
until the outlet H2
S from the lead bed is 90% of the inlet H2
S.
Feed purification, Section 3: Ultra-purification. This section
is not common in all flowsheets. The primary function
of this section is to purify H2
S to parts per billion (ppb) levels.
It is usually seen as a small layer beneath the zinc oxide
absorber(s). KPIs include the following considerations:
* It may be difficult to measure very low ppb levels
of H2
S by using detector tubes or the laboratory.
However, the economic benefit of having this layer
can be realized in terms of extended life of the
pre-reformer catalyst. Ultra-purification also
helps extend the life of primary reformer catalysts
in highly stressed reformers.
* H2S pickup/capacity is another KPI.
Pre-reformers. Pre-reformers are not found in all flowsheets.
The most important benefit of a pre-reformer is the
flexibility to process different feeds. The primary function of
the pre-reformer is to convert C2
+ in the feed to C1
and H2
.
The key parameters include the steam-to-carbon (S/C) ratio,
inlet temperature, pressure, Z90
drop. Z90
progression and pressure
feeds are shown in FIGS. 1 and 2.
KPIs include C2
is an important trend that needs to be monitored
plots for NG and naphtha
progression.
+ slip, pressure drop and Z90
ATE is not a KPI for the pre-reformer, as the SMR's ATE would
be close to zero throughout the expected life of the catalyst if
the C2
+ is within the guaranteed limit. However, the SMR and
WGS equilibrium temperatures can be calculated and compared
with the observed temperatures.
FIG. 2. Z90 plot for naphtha feed.
H2Tech | Q1 2022 27
closely, since it determines the timing of the pre-reformer
catalyst changeout. The typical Z90
FIG. 1. Z90 plot for NG feed.
S.
SPECIAL FOCUS
Mass balance across the pre-reformer. Case 1-1,000
kmol/hr of treated (NG + recycled H2
composition:
* H2
* N2
* CH4
* C2
* C3
H6
H8
= 2%
= 5%
= 90%
= 2%
= 1%.
The inlet S/C ratio (mol/mol) is 3, the outlet pressure is
S-would have a theoretical pickup of
. To reiterate,
24.2 bara (23.9 atma), the inlet temperature is 450°C, and the
outlet temperature is 400.2°C. The outlet composition (dry
basis) reported by the laboratory is:
* H2
* N2
= 18%
= 4%
* CH4
=73%
* CO = 0.03%
* CO2
= 5%.
The unknowns include the outlet dry flow rate and the outlet
steam, and it is not known how much steam has been consumed
in the reactions to make products.
The inlet and outlet moles for Case 1 are shown in TABLE 1.
In this example, d is the outlet dry flow rate, and s represents
the moles of steam at the outlet. To calculate d using carbon
balance, Eq. 10 is used:
900 + 20 × 2 + 10 × 3 = 0.73d + 0.05d + 0.0003d
d = 1,243.11 kmol/hr
To calculate s using oxygen balance, Eq. 11 is used:
2,910 = 2 × 0.05 × 1,243.11 + 0.0003 × 1,243.11 + s (11)
s = 2,785.3 kmol/hr
The total wet outlet flow (TABLE 2) is calculated in Eq. 12:
(10)
)-has the following
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