Influence of Operating Variables on Functional Parameters of Ammonia Synthesis Reactor

Influence of Operating Variables on Functional Parameters of Ammonia Synthesis Reactor
Influence of Operating Variables on Functional Parameters
of Ammonia Synthesis Reactor
1Chie-Amadi, Grace Orlunma and 2*Amagbo, Lloyd Godpower
1,2Department of Chemical and Petrochemical Engineering, Rivers State University, P.M.B. 5080, Port Harcourt, Rivers
State, Nigeria
The synthesis of ammonia involves several parameters that require appropriate control
measures. Hence, this study investigated the effects of operating temperature and pressure on
size, space velocity, mean residence time and heat generated per unit reactor volume at capacity
of 430,000 tons of ammonia production per year. The analysis was performed at temperature and
pressure ranges of 673 – 773 K and 150 – 300 atm. At constant conversion, the volume and mean
residence time decreased with increase in operating temperature and pressure, while space
velocity and heat generated per unit reactor volume increased with increasing temperature and
pressure. For instance, from 698 – 773 K and at constant pressure of 250 atm and 0.32 conversion,
volume and mean residence time decreased from 3.717 to 0.501 m3
and 7.01 x 10-5
to 9.46 x 10-6
hr, while space velocity and heat generated per unit reactor volume increased from 1.698 x 104
to
1.259 x 105
hr-1
and 1.40 x 107
to 1.57 x 108
kJ/hr.m3
respectively. The temperature and pressure
ranges may be realistic, but 698 K and 250 atm is considered for the designed capcity based on
the process economics, size and heat requirement.
Keywords: Ammonia synthesis, Plug Flow Reactor, Operating Variables, Reactor Functional Parameters
List of Symbols
D Reactor Diameter (m)
Fc Fco Inlet and Outlet Coolant flowrate (m3/hr)
f Catalyst activity
FN Flowrate of nitrogen (kmol/hr)
H2 Hydrogen
k1, k2 Forward and reverse rate constant
Mr Mean residence time (hr)
N, N2 Nitrogen
NH3 Ammonia
Pi, P Component and total pressure (atm)
q
Heat generated per reactor reactor volume
(kJ/hr.m3)
rN rate of nitrogen (kmol/m3.hr)
R Universal gas constant (kJ/kmol.K)
Sv Space velocity (hr-1)
Tco, Tc inlet and outlet cooling temperature (K)
To, T inlet and outlet reactor temperature (K)
vo volumetric flow rate (m3/hr)
V Reactor volume (m3)
XN Nitrogen conversion
yi Component feed composition
ƐN Fractional change in volume
ŋ Catalyst effectiveness factor
ΔHr Heat of reaction (kJ/kmol.)
INTRODUCTION
The manufacture of ammonia is essential, especially as it
is widely use in the agricultural and allied industries. The
utilization of ammonia for the manufacture of urea,
ammonium phosphates, ammonium nitrate and calcium
ammonium nitrate as fertilizers has made it most useful to
the agricultural sector that accounted for over 88% usage
in the production of ammonium fertilizers globally, with
China alone producing about 32.6% (Pattabathula and
Richardson, 2016).
In Nigeria, over dependence on oil and gas for income and
foreign exchange has caused low participation in the
agriculture which was the main stay of her economy before
the discovery of oil/gas in the country. According to
Nigeria’s Federal Ministry of Agriculture and Rural
Development (FMARD, Nigeria is faced with two key gaps
in agriculture – the inability to meet domestic food
requirements, and inability to export at levels required for
internal market. The first gap was due to productivity
*Corresponding Author: Amagbo Lloyd Godpower;
Department of Chemical and Petrochemical Engineering,
Rivers State University, P.M.B. 5080, Port Harcourt,
Rivers State, Nigeria. E-mail: amagbolloyd@gmail.com
Co-Author 1
Email: gchieamadi@yahoo.com
Research Article
Vol. 2(1), pp. 008-017, May, 2020. © www.premierpublishers.org, ISSN: 2257-1869
Research Journal of Chemical Engineering and Processing

Chie-Amadi and Amagbo 9
challenges which include farmers’ inability to access
fertilizer (FMARD, 2016). Hence, suggested the building of
more fertilizer plants in the country to revitalize the
agricultural sector. This declaration was followed by key
industry players to increase their output capacity. One of
such industry is Notore Chemical Industry, Nigeria, which
proposed a production capacity of 430,000 metric ton of
ammonia to be produced per year. However, the
production of fertilizer required some chemical ingenuity
and expertise, therefore the need to explore an effective
design of ammonia synthesis reactor.
The processes and routes for ammonia production varied
depending on raw materials used and capacity. However,
on commercial scale, the Haber-Bosch process is mostly
used, which made use of hydrogen produced from natural
gas and nitrogen extracted via the distillation of air as raw
materials (Pattabathula and Richardson, 2016; Chen et al.,
2019). Ammonia synthesis is exothermic, reversible and
occurs at very high temperatures and pressures typically
ranging from 673-723 K and 200-300 atm (Clark, 2013;
Yancy-Caballero et al., 2015; Albers et al., 2017). The
reaction takes place over an iron catalyst, which is
promoted by fusing with oxide of potassium, aluminium or
calcium (Albers et al., 2017). According to Nguyen et al.
(2017), temperature does not only affect conversion and
yield of ammonia, it also affects the catalyst zone, which
can reduce the performance of the overall process. Also,
in ammonia synthesis, increase in pressure favours the
reaction, though pressure varies from plant to plant
depending on the design capacity (Pattabathula and
Richardson, 2016). However, operating a reactor at high
pressure will not only increase cost, it will also require
construction materials that will withstand the pressure,
which are expensive to acquire and maintain (Clark, 2013).
Notable studies on modelling, optimisation and design of
ammonia reactor have been reported. Babu and Reddy
(2012) studied the modelling of ammonia converter using
one and two internal collocation points to predict the
temperature and mole fraction at the outlet of the reactor
beds. They reported that the errors between actual and
predicted data decreased substantially when the two
internal collocation points was used. Also, Umair et al.
(2013) in their modelling reported an increase in ammonia
yield when the inlet temperature and pressure were
increased. Similar trends were also reported by Akpa and
Raphael (2014) while investigating the performance of a
model developed to study an existing ammonia converter.
Thus, 6.7% deviation between the predicted and the
existing ammonia converter was reported.
Because of the heat requirement in ammonia synthesis,
several authors have performed heat analysis on ammonia
synthesis reactor. Thus, Florez-Orrego and de Oliveira
Junior (2017) showed that appropriate control of reactor
inlet temperature resulted to higher conversion of
synthesis gas, while in another study it was reported that
a reliable result on heat analysis was obtained when
ammonia reactor was incorporated with compressor and
refrigeration system (Florez-Orrego and de Oliveira Junior,
2016).
It has also been shown that the performance of ammonia
synthesis reactor is influenced by its configurations.
Khademi and Sabbaghi (2017) investigated three reactor
scenarios: internal direct cooling reactor, adiabatic quench
cooling reactor and adiabatic indirect cooling, and found
that maximum conversion was achieved at different
temperatures for the respective reactor types. However,
the internal direct cooling reactor outperformed the other
reactor configurations with maximum conversion of
nitrogen achieved at feed gas temperature of 495 K. On
the contrary, Penkuhn and Tsatsaronis (2017) reported
that the synthesis of ammonia in an indirect-cooled reactor
performed better than the direct-cooled reactor, with lower
power demand as well as higher steam generation.
However, Jorqueira et al. (2018) compared the
performance of adiabatic reactor with three beds in series
and autothermal reactor using the compositional approach
based on cubic equations. A maximum error in terms of
temperature between plant data and the reactors were
reported as 1.6% and 2.7% for adiabatic reactor model and
autothermal reactor respectively.
In Demirhan et al. (2018) alternative source of ammonia
production from biomass was investigated and compared
with natural gas. They argued that ammonia production
from biomass reduces green house gas emission and also
compete favourably with natural gas route. This
technology may not be easily adopted in developing
counties like Nigeria. Also, in another study,
thermodynamic analysis of an integrated energy system
for power, steam and ammonia production was developed
and simulated via Aspen Plus by Chehade and Dincer
(2019). This study was performed as a potential
replacement for the conventional Haber-Bosch process.
They reported a total work rate of 12631 kW and ammonia
conversion rate of 38%, with overall and turbine efficiency
of the integrated system recorded as 71 % and 92 %
respectively. Similarly, in a recent study, the simultaneous
production of gasoline and ammonia in thermally coupled
reactor from catalytic naphtha reforming plant was
reported by Shakeri et al. (2019), where the endothermic
reaction for naphtha reforming process carried out in the
shell side of the reactor while the exothermic reaction for
ammonia synthesis process took place in the tube side.
This configuration according to the authors increased the
thermal efficiency and reduction in operational costs due
to the elimination of interstage heaters thereby, reducing
thermal load of condensers for ammonia synthesis unit.
Although the simulated yield of reformate was higher than
value from conventional naphtha reforming reactors, but
conversion of nitrogen was slightly lower than value
obtainable from conventional ammonia synthesis process.
It is therefore, necessary to note that the synthesis of
ammonia involves several process conditions that if

Res. J. Chem. Eng. Process. 10
appropriately controlled may lead to improve yield of
ammonia. Although, previous studies had dealt adequately
with modelling and simulation of ammonia synthesis
reactor with different specific objectives, but studies on
functional parameters like space velocity and mean
residence time, which are performance measures for
reactor design, have not been thoroughly looked into
(Levenspiel, 2007). Therefore, this study investigated the
effects of inlet temperature and pressure on the functional
parameters of ammonia synthesis reactor capable of
producing 430,000 tons of ammonia per year.
DEVELOPMENT OF DESIGN EQUATIONS
Design equations were developed for the computation of
functional parameters of a plug ammonia synthesis flow
reactor. Figure 1 represents a schematic diagram of a plug
flow reactor configuration designed with coolant to
maintain the reactor temperature.
Figure 1: Differential element of plug flow reactor
where: dV Differential volume (m3)
NF Flowrate of nitrogen (kmol/hr)
CoF Coolant inlet flowrate (m3/hr)
CF Coolant outlet flowrate (m3/hr)
NX Nitrogen conversion (%)
oT Inlet temperature (K)
T Outlet temperature (K)
CoT Coolant inlet temperature (K)
CT Coolant outlet temperature (K)
Volume of Reactor
Taking material balance on the differential reactor with
respect to nitrogen, we obtained as follows:
Where:
NFInput 
NN dFFOutput 
dVrncedisappearaofRate N )(
dt
dN
onaccumulatiofRate N

Upon substitution into equation (1) yields:
dt
dN
dVrdFFF N
NNNN  )(
But for plug flow reactor at steady state, 0
dt
dNN
Hence,
dVrdFFF NNNN )( 
Or
NN dFdVr  )(
And )1( NNoN XFF 
After substitution and simplification, the volume, V of the
reactor can be expressed as:
 

N
No
X
X N
N
No
r
dX
FV
)(
(2)
In Yusup et al. (2006), the rate in terms of nitrogen was
expressed as:








 5.12
5.1
1
2
3
3
22
H
NH
NH
HN
N
P
P
k
P
PP
kfr (3)
So, substituting Nr in equation (3) into equation (2)
gives:
N
X
X
H
NH
NH
HN
N
dX
P
P
k
P
PP
k
f
F
V
N
N

























0
2
3
3
22
0
5.12
5.1
1
1

(4)
Space Velocity
The space velocity is the number of reactor volume of feed
at specified condition that can be treated per unit time. It is
mathematically expressed as:
V
v
S o
v  (5)
From equation (2), it implies that
 N
N
o
N
vNo
r
F
fv
dX
dS
dV
dXv

0

(6)
Hence, combining equations (4) and (6) gives
N
X
X H
NH
NH
HN
N
o
v dX
P
P
k
P
PP
k
F
fv
S
N
No
 







 5.12
5.1
1
2
3
3
22
0

(7)

Mean Residence Time
The mean residence time (Mr) is the final time required to
process one reactor volume of feed measured at specified
condition (Levenspiel, 2007),and it is expressed as:
)1(0 NN
r
Xv
V
M

 (8)
Thus, equation (8) becomes
 NNo
r
Xv
dV
dM


1
(9)
Hence, combining equations (4) and (9) gives
 
dX
Xrfv
F
M
N
N
X
X NNNo
N
r  

0
0
)1(
1

dX
X
P
P
k
P
PP
k
fv
F
M
N
N
X
X
NN
H
NH
NH
HNo
N
r 

























0
2
3
3
22
0
)1(
1
5.12
5.1
1 

(10)
Heat Generation per unit Volume of Reactor
The generation per unit volume of plug flow reactor is
giving by:
V
XFH
q
NNr 0
)( 
 (11)
From equation (11),
dV
XFH
dq
NNr 0
)( 
 (12)
Again, combining equations (4) and (12) gives
  NNr
X
X H
NH
NH
HN
dXfXH
P
P
k
P
PP
kq
N
No









  5.12
5.1
1
2
3
3
22
(13)
The heat of reaction was calculated using the equation
developed by Mahfouz et al. (1987) as presented in
equation (14).
















 



09.9157101069197102525.0
34685.5
10734.459609.846
54526.0
3623
3
6
TT
TP
TTHr
(14)
Where T and P are the reaction temperature and pressure
Evaluation of Temperature and Pressure Effects
Ammonia synthesis reaction is dependent on temperature.
So, Yusup et al. (2006) developed an expression for the
forward and backward specific rate constants according to
equations (15) and (16). Therefore, effect of temperature
on the reactor functional parameters was evaluated by
varying the temperature term in equations (4), (7), (10) and
(13) after substitution of equations (15) and (16). Thus,
temperature range of 673 to 773 K and pressure range of
150 to 300 atm were used for the evaluation.





 

RT
k
20800
exp1078954.1 4
1 (15)





 

RT
k
47400
exp105714.2 16
2 (16)
where:
R Universal Gas constant
However, the partial pressure exerted by the reaction
components is expressed as a fraction of the total system
pressure as given follows.
PyP NN 22
 (17)
PyP HH 22
 (18)
PyP NHNH 33
 (19)
The feed composition used to evaluate the partial
pressures of nitrogen, hydrogen and ammonia were
obtained from the work of Akpa and Raphael (2014) as
shown in Table 1.
Table 1: Feed Composition (%).
2H 2N 4CH 3NH Argon 22 / NH
63.32 21.01 10.48 2.08 3.11 2.78
Design Basis
In 2013, Notore Chemical Industry in Rivers State of
Nigeria proposed a design capacity of 430,000 metric
tonnes per year of ammonia production, which is
equivalent to 430,000,000 kg per year of ammonia. So, this
capacity was used as basis for the calculations. Taking
cognizance of anticipated reactor shut down for
maintenance and other factors, 330 working days have
been chosen for this design. Thus, on hourly production,
we have:
yearhr
yearkg
yearkg
/7920
/000,000,430
/000,000,430 
hrkg/104293.5 4

The chemistry of ammonia synthesis reaction is shown in
equation (20)
322 23 NHHN  (20)
From the stoichiometry, 1 mole of nitrogen combined with
3 moles of hydrogen to produce 2 moles of ammonia.

Hence, the molar flowrate of nitrogen is obtained as
follows:
2
2
ofweighmolecular
offlowratemass
N
N
FNo  (21)
The mass flow rate of nitrogen can be obtained from the
expression:
hrkghrkg
kgmolkg
kgmolkg
mole
mole
/104712.4/104293.5
/17
/28
2
1 44

Hence, molar flowrate of nitrogen is:
hrkgmol
kgmolkg
hrkg
/85.1596
/28
/104712.4 4


The volumetric flow rate can be expressed with the
assumption that the flowrate into the reactor is the same
as that exiting the reactor. Thus,
)/( 3
hrm
densityfluid
flowratemass
vo 
Hence,
)/103131.6
/860.0
/104293.5 34
3
4
hrm
mkg
hrkg
vo 


The input parameters used for the analysis are
summarised in Table 2.
Table 2: Summary of input data
Data Symbol Value Unit
Reaction density ρ 0.86 kg/m3
Flow rate of nitrogen FN0 1596.595 kmol/hr
Volumetric flow rate vo 6.3131 x 104 m3/hr
Fractional change in
volume
N -0.5 -
Universal gas
constant
R 8.314 kJ/kmol.K
Operating
temperature
T 673-773 K
Operating pressure P 150-300 atm
RESULTS AND DISCUSSION
The results of temperature and pressure effects on
volume, mean residence time, space velocity and heat
generated per reactor volume in the synthesis of ammonia
are presented in this section.
Effect of temperature on reactor parameters
Effect of temperature was evaluated at constant pressure
of 250atm and catalyst effectiveness factor of 0.3. The
result analysis revealed that variation in reactor operating
temperature has effect on the system volume, mean
residence time, space velocity and heat generated per unit
reactor volume.
Figure 1: Profile of reactor volume versus nitrogen
conversion at varying temperature
From the analysis, as shown in Figure 1, operating
temperature has effect on reactor volume. Thus, increase
in reactor operating temperature decreases the volume of
reactor at a specified nitrogen conversion. This implied
that at higher operating temperature, the volume of reactor
required to convert a specific degree of nitrogen to
ammonia will be smaller compared to when it is operated
at lower temperature. This observation has earlier been
reported by Moodley et al. (2005), which further stated that
increased reactor volume at low temperature led to high
catalyst requirement. However, from the analysis at
operating temperature of 673 to 773 K, the reactor volume
decreased from 0.929 to 0.125 m3, 1.858 to 0.251 m3,
2.788 to 0.376 m3, 3.717 to 0.501 m3 and 4.646 to 0.627
m3 at nitrogen conversion of 0.08, 0.16, 0.24, 0.32 and
0.40 respectively. Though, the reduction in reactor size will
definitely decrease cost, but may not be realistic
operationally if it becomes too small. Previous studies
have also acknowledged the effect of temperature on the
size of ammonia synthesis reactor (Khademi and
Sabbaghi, 2017; Nguyen et al., 2017). In Khademi and
Sabbaghi (2017), an optimum reactor size was reported at
635 K for quench reactor and 696 K for indirect cooling
adiabatic reactor, while 7.8 to 4.75 m3 was obtained at inlet
temperature of 658.15 to 706.15 K by Jorqueira et al.
(2018).
Figure 2: Profile of space velocity versus nitrogen

Space velocity as reactor functional parameters, is rarely
considered in the study of ammonia synthesis. However, it
is regarded as one of the performance measures for
continuous flow reactor (Levenspiel, 2007). In Figure 2, the
profile of space velocity versus nitrogen conversion was
shown at various reactor operating temperatures. Unlike
volume, increase in reactor operating temperature
increases the space velocity at a specified nitrogen
conversion, while it decreases with increase in nitrogen
conversion at constant temperature. This implied that the
volume of feed that will be charged into ammonia synthesis
reactor per hour at higher temperature will increase.
Hence, at operating temperature of 673 to 773 K, the
space velocity increased from 6.794 x 104 to 5.036 x 105
hr-1, 3.397 x 104 to 2.518 x 105 hr-1, 2.265 x 104 to 1.679 x
105 hr-1, 1.698 x 104 to 1.259 x 105 hr-1 and 1.359 x 104 to
1.007 x 105 hr-1 at nitrogen conversion of 0.08, 0.16, 0.24,
0.32 and 0.40 respectively. Although, increase in
temperature is favourable to reactor space velocity, overall
process analysis must be examined to balance the entire
process performance due to the impact of excessive heat.
Figure 3: Profile of mean residence time versus nitrogen
Figure 3 shows the profiles of mean residence time versus
nitrogen conversion at various operating temperatures.
Like volume, increase in reactor temperature reduces the
mean residence time of the reactants (nitrogen and
hydrogen) in the reactor. Thus, at operating temperature
of 673 to 773 K, the mean residence time decreased from
1.53 x 10-5 to 2.07 x 10-6 hr, 3.20 x 10-5 to 4.32 x 10-6 hr,
5.02 x 10-5 to 6.77 x 10-6 hr, 7.01 x 10-5 to 9.46 x 10-6 hr
and 9.20 x 10-5 to 1.24 x 10-5 hr at nitrogen conversion of
0.08, 0.16, 0.24, 0.32 and 0.40 respectively. The decrease
in residence time can be attributed to increase in reaction
rate as a consequence of temperature increase. The
advantage of decreased residence time in ammonia
synthesis was reported by Moodley et al. (2005). The
authors reported that a decrease in residence time
increases productivity of a reactor. Other studies have also
reported the influence of residence time on ammonia
synthesis. Thus, Chengyue et al. (1985) observed that the
yield of ammonia depends on the rate of nitrogen adsorbed
onto the catalyst surface, which in turn was influenced by
the residence time, while Azarhoosh et al. (2014) observed
that the total feed flow rate and molar fraction of ammonia
were affected at reduced residence time. Meanwhile,
reduced mean residence time significantly affected the
overall energy efficiency of ammonia process positively
(Gómez-Ramírez et al., 2015).
Figure 4: Profile of heat generated per unit volume of
reactor versus nitrogen conversion at varying temperature
The profile of heat generated per unit volume of reactor
versus nitrogen conversion at varying temperatures is
shown in Figure 4. Like space velocity, heat generated per
unit volume of reactor increases with increase in reactor
operating temperature at a given nitrogen conversion.
However, heat generation per reactor volume decreases
as nitrogen conversion was increase at a given operating
temperature. As obtained from the analysis, the heat
generated per unit reactor volume at operating
temperature range of 673 to 773 K increased from 5.60 x
107 to 6.29 x 108 kJ/hr.m3, 2.80 x 107 to 3.14 x 108 kJ/hr.m3,
1.87 x 107 to 2.10 x 108 kJ/hr.m3, 1.40 x 107 to 1.57 x 108
kJ/hr.m3 and 1.12 x 107 to 1.26 x 108 kJ/hr.m3 at nitrogen
conversion of 0.08, 0.16, 0.24, 0.32 and 0.40 respectively.
While the objective and methodology of this study may be
different from other studies, Gómez-Ramírez et al. (2015)
showed that appropriate operating temperature resulted to
an improve capacity of ammonia produced with efficient
heat generation. Similarly, Cheema and Krewer (2019)
reported that heat generation in ammonia reactor system
was a function of inlet temperature, which determines the
outlet reactor temperature. In the study, a high production
rate of ammonia was recorded at higher temperature,
while at low temperature there was overlap of reactants
conversion, leading to low ammonia production. They
attributed the maximum ammonia produced to heat
generated in the reactor system. Although, high heat
generation may lead to higher yield, it can also increase
the overall residence time distribution in ammonia
synthesis reactor system (Gómez-Ramírez et al., 2015;
Cheema and Krewer, 2019).
Effect of operating pressure on reactor parameters
The effect operating pressure on reactor volume, mean
residence time, space velocity and heat generated per unit
reactor volume was studied at constant temperature of 698

K and catalyst effectiveness of 0.3. Like operating
temperature, change in reactor operating pressure has
effect on the selected parameters.
Figure 5: Profile of reactor volume versus nitrogen
conversion at varying pressure
Figure 5 shows the profiles of reactor volume versus
nitrogen conversion obtained by varying the operating
pressure. The analysis revealed that increase in pressure
reduces the volume of reactor at every given nitrogen
conversion. This implied that the size required to process
a product of interest like ammonia depends on the process
pressure. Thus, a smaller reactor size is needed at high
pressure to convert nitrogen to ammonia. Hence, at
operating pressure range of 150 to 300 atm, the reactor
volume decreased from 1.484 to 0.370 m3, 2.968 to 0.740
m3, 4.451 to 1.109 m3, 5.935 to 1.479 m3 and 7.419 to
1.849 m3 at the corresponding nitrogen conversion of 0.08,
0.16, 0.32 and 0.4 respectively. These numerical values
show that, at 150 atm, the reactor size was almost twice
the size at 200 atm, and it increased further to about 4
times at 300 atm. Like temperature, pressure increase
favours conversion. This is in agreement with previous
studies on pressure effect on ammonia synthesis (Umair
et al., 2013; Akpa and Rahael, 2014). In most reported
works, optimal performance of ammonia synthesis reactor
was often reported at pressure between 200 atm and 250
atm (Yancy-Caballero et al., 2015; Khademi and
Sabbaghi, 2017; Jorqueira et al., 2018; Shakeri et al.,
2019). However, some authors have equally reported
improved ammonia production at pressure less than 200
atm. For instance, Dashti et al. (2006) reported at 136.5
atm, Babu and Reddy (2012) between 151 and 170 atm,
while Azarhoosh et al. (2014) and Rabchuk et al. (2014)
reported at 125.82 and 178 atm respectively. In Florez-
Orrego and de Oliveira (2017), 29.4-39.3m3 was obtained
in a packed bed reactor between 150 and 200 atm.
Figure 6: Profile of space velocity versus nitrogen
Figure 6 shows the profile of space velocity versus
nitrogen conversion at various reactor operating
pressures. Again, increase in reactor operating pressure
also increases the space velocity at every given nitrogen
conversion. However, there was decrease in space
velocity as nitrogen conversion was increased at constant
pressure. This implied that at constant conversion, the
more volume of nitrogen feed equivalent to the reactor
volume would be fed into the reactor per hour as pressure
increases. Hence, at 150 to 300 atm, the space velocity
obtained increased from 4.255 x 104 to 1.707 x 105 hr-1,
2.127 x 104 to 8.536 x 104 hr-1, 1.418 x 104 to 5.691 x 104
hr-1, 1.064 x 104 to 4.268 x 104 hr-1and 8.509 x 104 to 3.414
x 104 hr-1 at nitrogen conversion of 0.08, 0.16, 0.24, 0.32
and 0.40 respectively. Although, increase in temperature
is favourable to reactor space velocity, overall process
analysis must be examined to balance the entire process
performance due to the impact of excessive heat. Space
velocity as reactor functional parameters, is rarely
considered in the study of ammonia synthesis.
Figure 7: Profile of mean residence time versus nitrogen

Figure 7 shows the profiles of mean residence time versus
nitrogen conversion at varying operating pressures. Again,
increase in reactor operating pressure also decreases the
mean residence time of the reactor, but at constant
pressure, the mean residence time increases with
increase in nitrogen conversion. Thus, at operating
pressure of 150 to 300 atm, the mean residence time
decreased from 2.45 x 10-5 to 6.10 x 10-6 hr, 5.11 x 10-5 to
1.27 x 10-5 hr, 8.01 x 10-5 to 2.00 x 10-5 hr, 1.12 x 10-4 to
2.79 x 10-5 hr and 1.47 x 10-4 to 3.66 x 10-5 hr at nitrogen
conversion of 0.08, 0.16, 0.24, 0.32 and 0.40 respectively.
Like temperature, pressure influences the amount of
average time the reacting materials spent in the reactor.
Hence, the decrease in residence time at increased
pressure is an indication that equilibrium will be
established at a faster rate (Moodley et al. (2005), which
will favour the rate of ammonia production. In Azarhoosh
et al. (2014), it was observed that at every increase in
pressure there was an optimal temperature, which
decreases the residence time.
Figure 8: Profile of heat generated per unit volume of
reactor versus nitrogen conversion at varying pressure
The profile of heat generated per unit volume of reactor
versus nitrogen conversion at varying pressures is shown
in Figure 8. Thus, like temperature, increase in reactor
operating pressure also increases the heat generated per
unit reactor volume, but decreases with increase in
nitrogen conversion at constant pressure. Again, the heat
generated per unit reactor volume at operating pressure of
150 to 300 atm increased from 3.91 x 107 to 1.57 x 108
kJ/hr.m3, 1.96 x 107 to 7.85 x 107 kJ/hr.m3, 1.30 x 107 to
5.23 x 107 kJ/hr.m3, 9.78 x 106 to 3.92 x 107 kJ/hr.m3 and
7.82 x 106 to 3.14 x 107 kJ/hr.m3 at nitrogen conversion of
0.08, 0.16, 0.24, 0.32 and 0.40 respectively. Earlier studies
had reported the synergy between operating pressure and
heat generation in ammonia system. According to Akpa
and Rahael (2014), if pressure of ammonia system is
increased, the system is adjusted such that equilibrium will
shift to the right, resulting to higher conversion and hence,
more ammonia yield due to increased heat generation in
the system. However, Florez-Orrego and de Oliveira
(2017) in an optimisation study noted that high pressure
increased power consumption in the refrigeration system
of ammonia synthesis. This is as results of the rate of heat
generated per the reactor volume. In addition, Shakeri et
al. (2019) observed that a decrease in total pressure of the
exothermic side of a combine process for naphtha and
ammonia production, increased reaction for nitrogen
production, but with reduced conversion towards ammonia
production. These, in summary implied that, while heat is
a necessity for ammonia synthesis, excessive pressure
could also impair on the overall performance of ammonia
system if not properly controlled. The summary of
parameters obtained at temperature of 698K and pressure
of 250 atm are presented in Table 3.
Table 3: Summary of parameter at 698 K and 250 atm
Parameter Nitrogen Conversion
0.08 0.16 0.24 0.32 0.40
Volume (m3) 0.53 1.07 1.60 2.13 2.66
Length (m) 2.71 5.43 8.14 10.85 13.57
Mean residence time (hr) 8.79 x 10-6 1.83 x 10-5 2.88 x 10-5 4.02 x 10-5 5.27 x 10-5
Space velocity (hr-1) 1.19 x 105 5.93 x 104 3.95 x 104 2.96 x 104 2.37 x 104
Heat generated per unit reactor volume (kJ/hr.m3) 1.09 x 108 5.45 x 107 3.63 x 107 2.72 x 107 2.18 x 107
CONCLUSION
The synthesis of ammonia is critical in agricultural industry
and the process involves high temperature and pressure.
Therefore, it is imperative to thoroughly analyse the entire
process based on projected capacity and designed
configurations to avoid overshooting the operating
variables that may lead to excessive cost and low
performance. Hence, the effects of reactor operating
temperature and pressure on key reactor functional
parameter were evaluated in the synthesis of ammonia.
The analysis shows that temperature and pressure has
effects on reactor size, space velocity, mean residence
time and heat generated per unit reactor volume. Thus,
increase in operating temperature and pressure reduced
the reactor size and the mean residence time of reactants
in the reactor, while the space velocity and heat generated
per unit reactor volume increased as operating
temperature and pressure are increased. Although, the
result analysis obtained for the reactor functional
parameters at 673 to 773K and 150 to 300 atm may be
ideal, the study considered the operating temperature of
698 K and pressure of 250 atm as appropriate for the
designed capcity based on factors such as process
economics, sizing and heat requirements.

REFERENCES
Akpa, J.G., & Raphael, R.N., (2014). Simulation Study of
Smmonia Synthesis Converter, SENRA Academic
Publishers, British Columbia, 8 (2), 2913-2923.
Albers, P.W., Lennon, D., & Parker, S.F. (2017).
Experimental Methods in the Physical Sciences:
Catalysis: Vol. 49. Retrieved from:
http://dx.doi.org/10.1016/B978-0-12-805324-9.00005-
4 [Accessed 1st April, 2020]
Azarhoosh, M.J., Farivar, F., & Ebrahim, H. (2014).
Simulation and Optimization of a Horizontal Ammonia
Synthesis Reactor using Genetic Algorithm, Royal
Society of Chemistry, 4, 13419–13429.
Babu, S. A., & Reddy, V. G. (2012). Mathematical
Modeling of Ammonia Converter, International
Conference on Chemical, Civil and Environmental
Engineering (ICCEE2012), Dubai, 301-304.
Cheema, I.I., & Krewer. U. (2020). Optimisation of the
Autothermal NH3 Production Process for Power-to-
Ammonia, Processes, 8, 38-58.
Chehade, G., & Dincer, I. (2019). Exergy Analysis and
Assessment of a New Integrated Industrial Based
Energy System for Power, Steam and Ammonia
Production. Retrieved from: https://dx.doi.org/10.1016/
j.energy.2019.116277 [Accessed 1st April, 2020]
Chen, S., Perathoner, S., Ampelli, C., & Centi, G. (2019).
Electrochemical Dinitrogen Activation: To Find a
Sustainable Way to Produce Ammonia, Studies in
Surface Science and Catalysis, Volume 178, 31 – 46.
Retrieved from: https://dx.doi.org/10.1016/B978-0-444-
64127-4.00002-1 [Accessed 1st April, 2020]
Chengyue, L., Hudgins, R.R., & Silveston, P.L. (1985).
Modelling of Non-Stationary Ammonia Synthesis -
Kinetics Over an Iron Catalyst, Canadian Journal of
Chemical Engineering, 63, 795 – 802.
Clark, J. (2013). Modified Haber Process. Retrieved from:
http://www.chemguide.co.uk/physical/equilibria/manuf
actureofammonia.html [Accessed 21th December,
2019]
Dashti, A., Khorsand, K., Marvast, M. A., & Kakavand M.
(2006). Modeling and Simulation of Ammonia Synthesis
Reactor, Journal of Petroleum & Coal, 48, Issue 2, 15-
23.
Demirhan, C.D., Tso, W.W., Powell, J.B., & Pistikopoulos,
E.N. (2018). Sustainable Ammonia Production through
Process Synthesis and Global Optimization, American
Institute of Chemical Engineers (AIChE). Retrieved
from: https://doi10.1002/aic.16498. [Accessed 5th
April, 2020]
Florez-Orrego, D., & de Oliveira Junior, S. (2017a)
Modeling and Optimization of an Industrial Ammonia
Synthesis Unit: An Exergy Approach. Retrieved from:
https://doi:10.1016/j.energy.2017.06.157. [Accessed
6th April, 2020]
Flórez-Orrego, D., & Oliveira Junior, S. (2016). On the
Efficiency, Exergy Costs and CO2 Emission Cost
Allocation for an Integrated Syngas and Ammonia
Production Plant, Energy, 117, Part 2, 341-360.
FMARD (2016). The Agriculture Promotion Policy:
Building on the Successes of the Agricultural
Transformation Agenda, Closing Key Gaps, Federal
Ministry of Agriculture and Rural Development, Nigeria.
Gómez-Ramírez, A., Cotrino, J., Lambert, R.M., &
González-Elipe, A.R. (2015). Efficient Synthesis of
Ammonia from N2 and H2 alone in a Ferroelectric
Packed-Bed DBD Reactor, Plasma Sources Science
and Technology, 24, http://dx.doi:10.1088/0963-
0252/24/6/065011. [Accessed 5th May, 2020]
Jorqueira, D.S.S., Neto, A.M.B., & Rodrigues, M.T.M.
(2018). Modeling and Numerical Simulation of
Ammonia Synthesis Reactors Using Compositional
Approach, Advances in Chemical Engineering and
Science, 8, 124-143.
Khademi, M.H., & Sabbaghi, R.S. (2017). Comparison
between three Types of Ammonia Synthesis Reactor
Configurations in Terms of Cooling methods, Chemical
Engineering Research and Design, 1 2 8, 306–317.
Levenspiel O. (2007). Chemical Reaction Engineering, 3rd
Edition, Oxford, London: Hienneman Publishers.
Mahfouz, A.T., Elshishini, S.S., & Elnashaie, S.S.E.H.
(1987). Steady State Modeling and Simulation of an
Industrial Ammonia Synthesis Reactor, ASME press,
10(3), 1.
Moodley, A., Kauchali, S., Hildebrandt, D., & Glasser, D.
(2005). Debottlenecking the Ammonia Synthesis
Reactor System with the aid of Attainable Region
Theory, Proceedings of the First International
Conference on Modelling, Simulation and Applied
Optimisation, Sharjah, UAE, 1st – 3rd February, 2005.
Nguyen, T. A., Nguyen, T.A. Vu, T.D., Nguyen, K., Dao, T.K., & Huynh,
K.P. (2017). Optimization of an Auto-Thermal Ammonia
Synthesis Reactor using Cyclic Coordinate Method,
IOP Conference Series: Materials Science and
Engineering, 206, 012059. Retrieved from:
doi:10.1088/1757-899X/206/1/012059 [Accessed 4th
April, 2020]
Pattabathula, V., & Richardson, J. (2016). Introduction to
Ammonia Production, American Institute of Chemical
Engineers (AIChE), 69 – 75.
Penkuhn, M., & Tsatsaronis, G. (2017). Comparison of
Different Ammonia Synthesis Loop Configurations with
the Aid of Advanced Exergy Analysis. Retrieved from:
http://dx.doi.org/10.1016/j.energy.2017.02.175
[Accessed 2nd April, 2020]
Rabchuk, K., Lie, B., Mjaavatten, A., & Siepmann, V.
(2014). Stability Map for Ammonia Synthesis Reactors,
Proceedings from the 55th Conference on Simulation
and Modelling (SIMS 55), 21-22nd October, 2014,
Aalborg, Denmark, 159 - 166.
Shakeri, M., Iranshahi, D., & Naderifar, A. (2019). A
conceptual Investigation for the Simultaneous
Production of Gasoline and Ammonia in Thermally
Coupled Reactors, Chemical Engineering &
Processing: Process Intensification, 138, 15–26.
Umair, Z., Balasubramanian, P., & Shuhaimi, M. (2013).
Kinetic Model for Ammonia and Urea Production
Processes, Proceedings of the 6th International

Conference on Process Systems Engineering (PSE
ASIA), Kuala Lumpur, 325-330.
Yancy-Caballero, D. Y., Biegler, L. T., & Guirardello, R.
(2015). Optimization of an Ammonia Synthesis Reactor
using Simultaneous Approach, Chemical Engineering
Transactions, 43. Retrieved from: http://www.aidic.it/cet
[Accessed 1st May, 2019]
Yusup, S., Zabiri, H., Nyusoff, N., & Chinyew, Y. (2006).
Modeling and Optimization of Ammonia Reactor using
Shooting Methods, Proceedings of the 5th WSEAS
International Conference on Data Networks,
Communications & Computers, Bucharest, Romania,
258-268.
Accepted 20 May 2020
Citation: Chie-Amadi GO, Amagbo LG (2020). Influence
of Operating Variables on Functional Parameters of
Ammonia Synthesis Reactor. Research Journal of
Chemical Engineering and Processing, 2(1): 008-017.
Copyright: © 2020: Chie-Amadi and Amagbo. This is an
open-access article distributed under the terms of the
Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any
medium, provided the original author and source are cited.

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