Hydrocarbon Processing - October 2021 - 55

Hydrogen
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
. Eq. 8 is used to calculate
S/day
× 10 m3
/19.2 = 260 days
(9)
S.
the amount of sulfur to be removed:
(25/1,000,000) × 1,000 kmol/hr × 24 = 0.6 kmol H2
= 20.4 kg H2S/day = 20.4 × 32/34 = 19.2 kgS/day (8)
Therefore, the expected life is calculated using Eq. 9:
500 kgS/m3
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
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
progression and pressure drop. Z90
. The key paramis
an important
eters include the steam-to-carbon (S/C) ratio, inlet temperature,
pressure, Z90
trend that needs to be monitored closely, since it determines the
timing of the pre-reformer catalyst changeout. The typical Z90
plots for NG and naphtha feeds are shown in FIGS. 1 and 2.
KPIs include C2
+ slip, pressure drop and Z90
progression.
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
TABLE 1. The inlet/outlet moles for Case 1
Inlet moles, kmol/hr
H2
N2
CH4
C2
C3
H6
H8
CO2
CO
H2
O
20
50
900
20
10
3 x [900 + 20 x 2 + 10 x 3] = 2,910
Outlet moles, kmol/hr
0.18d
0.04d
0.73d
0.05d
0.0003d
s
FIG. 2. Z90 plot for naphtha feed.
Hydrocarbon Processing | OCTOBER 2021 55
FIG. 1. Z90 plot for NG feed.
the C2+ is within the guaranteed limit. However, the SMR and
S-would have a theoretical pickup of
. To reiterate,
the pickup figures are an assumption and do not correspond to
any specific product.
The volume of each bed is 10 m3
WGS equilibrium temperatures can be calculated and compared
with the observed temperatures.
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
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
(10)
)-has the following
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