Hydrocarbon Processing - May 2022 - 60

Process Optimization
For simplicity, the reaction is assumed
to be a first-order, irreversible reaction
(Eqs. 5 and 6):
N2
+ 3H2 → 2NH3
rN2 = KCN2
where K = 0.005
N2
(5)
(6)
is the base component for the reaction.
Since the reactor is an adiabatic reactor,
no heat goes into or exits the reactor;
therefore, the value of energy term is zero.
Simulation results and discussions.
Plug flow reactor simulations were conParameter
ducted
with DWSIM software and values
were calculated using steady-state assumptions.
These results were obtained
by using the Peng-Robinson thermodynamic
package and entering the values
given in the problem statement.
TABLE 2 shows the results from the
simulation of the plug flow reactor in
DWSIM.6
FIG. 1 depicts the pressure, temperature
and concentration profile of the
reactor. Over the length of the reactor,
N2
and H2
as they combine to form NH3
Temperature
Pressure
Mass fl ow
TABLE 2. Simulated results for the plug fl ow reactor
Product
821.051
199.852
3000
Molar fraction (mixture)/H2
Mass fl ow (vapor)/H2
Molar fraction (mixture)/N
Mass fl ow (vapor)/N
Molar fraction (vapor)/NH3
Mass fl ow (vapor)/NH3
0.193745
59.7411
0.5
2142.47
0.306255
797.792
concentration will increase. Due to the
Feed
400
200
3000
0.5
201.391
0.5
2798.61
Units
°C
bar
kg/hr
kg/hr
kg/hr
kg/hr
concentrations will decrease
, whose
adiabatic nature of the plug flow reactor,
which has no energy exchange with
the surroundings, the temperature of the
constituents inside the reactor rapidly
increases over its length, reaching 820°C
(1,508°F) before it exits the reactor. As
the plug flow reactor only contains gases
that are reacting over a small distance,
there is only a small pressure drop along
the reactor's length and the products
leave the reactor at approximately the
same pressure that they entered.
Sensitivity analysis approach. The
sensitivity analysis was performed to investigate
the effects of changing various
parameters inside the plug flow reactor
on the conversion of H2
. This simulation
incorporated changes in feed temperature,
feed pressure, feed mass flowrate
and reactor volume over specified intervals.
FIG. 2 shows the variation in H2
conversion
by changing the input parameters
individually.
Case 1: Variation of conversion of
H2 with the inlet temperature of the
feed. In FIG. 2, the H2
conversion is shown
as a function of the feed temperature,
which varies between 100°C (212°F)
and 600°C (1,112°F) without affecting
any other parameters. It can be seen from
the graph that H2
does not react until approximately
240°C (464°F). This is due
to the fact that the activation energy required
by the constituents to trigger the
reaction has not yet been reached. The
reaction takes place as soon as the threshold
is crossed. Because the gases expand
in the same amount of space in the plug
flow reactor when the temperature of the
inlet feed is increased, the conversion
percentage slowly begins to decrease.
Case 2: Variation of the conversion
FIG. 1. Pressure, temperature and concentration profile across the length of the reactor.
of H2
of the feed. FIG. 2 shows H2
with the change in the pressure
conversion
as a function of feed pressure, all other
parameters remaining constant. The
feed pressure varies between 50 bar and
400 bar. From the curve, it is evident that
the H2
conversion percentage inside the
FIG. 2. Variation in the H2
conversion by changing input parameters individually.
60 MAY 2022 | HydrocarbonProcessing.com
reactor becomes increasingly higher as
the feed pressure increases due to the
gases being compressed as the feed pressure
increases. In this reaction mixture,
however, there is a point around 325
bar at which maximum conversion is
achieved. In subsequent steps, the conversion
of H2
increasing the pressure.
is only slightly altered by
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