Hydrocarbon Processing - October 2021 - 53

Hydrogen
K. R. RAMAKUMAR, Johnson Matthey,
United Arab Emirates
Monitoring hydrogen plant performance-Part 1
Process monitoring is an indispensable practice to keep track
of key performance indicators (KPIs) of the hydrogen (H2
) plant.
A good system of process monitoring not only ensures safe and
reliable plant operations, but also helps in making strategic decisions
such as 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 that of the entire refinery complex, since the downstream
hydrotreated/hydrocracked product yields are affected, as well.
This will have a direct implication on the overall refinery margin.
For better monitoring and control, it is imperative to know
the fundamentals of performance indicators. Performance monitoring
requires operating data inputs from the plant, which are
normally accessible from the intranet servers or from the distributed
control system. It also requires stream analyses from
the laboratory (or from online analyzers) to understand how
each reactor is performing with respect to conversion/yield.
The laboratory testing frequency of such streams is normally
decided by the process engineering department in conjunction
with the laboratory section.
The main objective of this article is to guide H2
plant process
engineers in:
* Performing a detailed mass balance across the H2
flowsheet 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
is being made 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.
* Monitoring other critical parameters and KPIs across
each reactor in the H2
flow sheet.
Feed definition and characterization. Two types of feed
are considered for studying plant performance: Case 1 is a natural
gas (NG) feed, and Case 2 is a naphtha feed. Note: The flow,
composition and operating conditions are purely assumptions
based on the author's experience and do not correspond to any
specific plant.
) = 2%
H6
) = 5%
) = 2%
H8
) = 90%
) = 1%.
For Case 1, feed comprising 1,000 kmol/hr of (NG + recycled
H2) is assumed to have the following composition:
* Hydrogen (H2
* Nitrogen (N2
* Methane (CH4
* Ethane (C2
* Propane (C3
This combined stream of NG and recycled H2
of hydrogen sulfide (H2
cle H2
contains 10 ppmv
S) and 5 ppmv each of methyl mercaptan
and dimethyl disulfide (DMDS).
For Case 2, 80 kmol/hr of naphtha, with 20 kmol/hr of recy,
is assumed. The most common laboratory information
for naphtha is the ASTM D86 standard covering distillation
and specific gravity. The ASTM D86 cut points of the assumed
naphtha feed are:
* T10%
* T30%
* T50%
* T70%
* T90%
= 110°F
= 128°F
= 135°F
= 145°F
= 155°F.
The reported specific gravity is 0.716. Naphtha must be characterized
further based on the above information. Degrees Fahrenheit
and Kelvin will be used only for the following estimations.
Naphtha characterization. Eq. 1 is used to find the volume
average boiling point (VABP):
(T10%
+ T30%
+ T50%
+ T70%
The correction factor is:
* Slope = (T90%
* ∆ = 3.825°F.
The mean average boiling point (MeABP) is shown in Eq. 2:
MeABP = VABP - Δ = 134.6 - 3.825 =
130.8°F = 328.03°K
The molecular weight is calculated using Eq. 3:
1.6607 x 10-4
(specific gravity)-1.0164
(328.03)2.1962
× (MeABP)2.1962
× (0.716)-1.0164
×
= 1.6607 × 10-4
×
= 78.2 kg/kmol
-2.725 × (specific gravity)-6.798
]
(3)
The carbon/hydrogen (C/H) weight ratio is calculated using Eq. 4:
3.4707 [exp {(0.01485 Tb + 16.94x (specific gravity) - 0.012492 Tb × (specific gravity)}
× Tb
(4)
Substituting Tb = MeABP; therefore, the C/H weight is 6.022.
Hydrocarbon Processing | OCTOBER 2021 53
(2)
- T10%
+ T90%
)/ 5 = 134.6°F
* Ln ∆ = [-0.94402 - 0.00865 (VABP - 32)0.6667
(2.99791 x slope0.333
)/ 80 = 0.563
)]
+
(1)
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