Ammonia converter

1 Evaluationperformanceof a radialflowammonia converter, Dr.M.T.ElTohfa
ABU QIR FERTILIZERS CO.
Alexandria - Egypt
Dr. Mohamed tarek El-Tohfa
AmmoniaProcessDept.Manager

Introduction
Ammonia converter configuration
Performance of a radial flow ammonia converter
Ammonia converter modelling, simulation and optimization
Model development and verification
Ammonia converter catalyst Evaluation
Addition of fourth bed as a booster converter
Summary

Egypt
Located at the cost of
Abu-Qir bay
20 Km east
Alexandria – Egypt
1975
Abu Qir fertilizers company is one of the
largest nitrogen fertilizers production
complexes in the Middle East. It comprises
three plants producing 6000 tons daily.

1979 1990 1991 1998 2005 2006
Abu Qir I Abu Qir II
Abu Qir IIIMarine Line UAN
NPK

ABU QIR plant III was commissioned in October
1998, it consists of two main plants, ammonia plant
with capacity 1200 ton/day and Urea plant with
capacity 1750 ton/day.

Exothermic

Features of UHDE 3-bed radial flow ammonia
converters:-
 low pressure drop.
 small catalyst size
 maximum conversion rate
 optimum temperature control
 no reduction of ammonia concentration
 Three radial flow type catalyst beds
 Indirect cooling by heat exchangers
 The catalyst type BASF S6-10/S6-10 red, is a
magnetite (Fe3O4) catalyst, Size (1.5-3) mm and
the operating temperature range is 350 –520°C.

Two differential equations describe mathematically the steady state
behavior of the ammonia converter:-
I. Concentration-Position relationship for transformation of
the reactants to products (Conversion profile).
II. Temperature-Position behavior of the reacting synthesis gas,
the catalyst, and vessel internals (Temperature profile).
The derivation of the model equations on the basis of bed
radius (dr) enables us to follow-up the temperature
distribution inside the catalyst bed more accurate ,
especially the temperature transmitters passes inside the
catalyst bed.

10 Evaluationperformance of a radialflowammonia converter, Dr.M.T.ElTohfa
Material balance:
Based on nitrogen conversion:









A
F
R
dr
dX
No
NHN
2
32
2

1
η = 1 : Effectiveness factor (diffusion resistance)
: The initial molar flow rate of nitrogen, kmol/hr.
A : Surface area of catalyst bed, m2
r : Radius of catalyst bed, m
RNH3 : Reaction rate, kmol/m3.hr
XN2 : Fractional conversion of nitrogen
2NoF
Where:

Energy balance:
 
p
NHr
ρuc
ηRΔH
dr
dT 3

 2
  ipp Ycc imix
(Redlich-Kwong thermodynamic relation). : Gas Density, kg/m
ΔHr : Heat of reaction, kJ/kmol.
cp : Specific heat of reacting gas mixture, kJ/kg. K
u : Fluid velocity, m/hr.
RNH3 : Reaction rate, kmol/m3.hr
 : Effectiveness factor
T : Temperature, K
r : Radius of catalyst bed, m
Where:

Reaction Rate:


























α1
3
H2
2
NH
α
2
NH
3
H
2
2
a2NH
a
a
a
a
aNKkβR 3
3
2
3
Where:
α (Constant) = 0.5-0.75
K2 Rate constant for reverse reaction
Ka Equilibrium constant
ai Activity of component i
β Catalyst activity coefficient
(Modified Temkin equation)

Reaction Rate Constant K2
K2 = K0 exp (-E/RT)
K0 Arrhenius coefficient
E Activation energy
R Gas constant
Equilibrium Constant Ka
6899.2
6.2001
10848863.110519265.5log691122.2log 275
1010  
T
TTTKa
Heat of reaction:
   
  0.14215P51.21P101.928108.3395T
TP104.455214.35953.1619P6.329.3T100.11625
T100.03356T100.349967.2949T91840ΔH
632
349
3522
r







r = ro
XN2 = 0
T = Ti
The sets of model differential equations were solved in
MATLAB employing 4th order Runge Kutta approach with
initial conditions:
The temperature and conversion profiles for the three beds obtained
from the present developed model were compared to operating data
of a radial flow ammonia converter in Abu-Qir plant III in order to
validate these results and estimate the percent deviation.

rori
507.4 0c

463.30c

414.4 0c

15.09%

Plant data
Model
calculation
results
%Deviation
Exit temperature of 1st
bed, (°C)
510 507.4 0.50
Exit temperature of 2nd
bed, (°C)
464 463.3 0.15
Exit temperature of 3rd
bed, (°C)
418 414.4 0.86
Converter exit
temperature, (°C)
425 421.1 0.91
Exit % ammonia 15.2 15.09 0.72
Ammonia Make, TPD 1208 1206.8 0.099

Effect of
inlet flow
rates
Effect of
inert content
Effect of
operating
Pressure
Effect of
inlet
ammonia
conc.

1206.8
1220.3
1233.6
1246.7
1259.7
1272.5
1285.2
1297.6
1309.9
1322
1334
1200
1220
1240
1260
1280
1300
1320
1340
1360
620,000 630,000 640,000 650,000 660,000 670,000 680,000 690,000 700,000 710,000 720,000 730,000
AMMONIAPRODUCTION,TPD
INLET FLOW RATE, NM3/HR

1206.8
1193.1
1179.3
1165.3
1151.1
1136.8
1122.3
1107.7
1092.9
1077.9
1062.8
950
1000
1050
1100
1150
1200
1250
630,000 620,000 610,000 600,000 590,000 580,000 570,000 560,000 550,000 540,000 530,000
INLET FLOW RATE, NM3/HR

1000
1020
1040
1060
1080
1100
1120
1140
1160
1180
1200
1220
15.73 16.73 17.73 18.73 19.73 20.73
1206.8
1182.9
1158.2
1132.8
1106.8
1080.3
% Inerts

1140
1160
1180
1200
1220
1240
1260
1280
1300
1320
15.73 14.73 13.73 12.73 11.73 10.73
1206.8
1230.1
1252.7
1274.6
1296
1317.5
% Inerts

1206.8
1233.3
1259
1284.1
1308.6
1200
1220
1240
1260
1280
1300
1320
175 180 185 190 195 200 205
PRESSURE,BAR

1206.8
1179.5
1151.4
1122.5
1092.7
1062
1030.5
900
950
1000
1050
1100
1150
1200
1250
180 175 170 165 160 155 150
PRESSURE, BAR

1060
1080
1100
1120
1140
1160
1180
1200
1220
3 3.2 3.4 3.6 3.8 4
1206.8
1189.6
1171.1
1152.6
1133.8
1115.1
%NH3

1160
1180
1200
1220
1240
1260
1280
1300
3 2.8 2.6 2.4 2.2 2
1206.8
1225.5
1243
1260.3
1277.4
1294.3
%NH3

0
0.2
0.4
0.6
0.8
1
1.2
0 2 4 6 8 10 12 14 16 18 20
Catalystactivitycoefficient,β
time, years

Fourth bed length Vs Ammonia conversion

Fourth bed length Vs Ammonia production

A successful model was developed to represent the performance of a
radial flow ammonia converter in ammonia plant III at Abu-Qir
Fertilizers Company.
The model was verified and the results show that the calculated
values are in very good agreement with the plant data.
The model can predict the first bed outlet temperature which is
considered the highest temperature in the reactor (can't be practically
measured).
The model is utilized to calculate the catalyst activity coefficient (β)
which is used for the catalyst performance evaluation with time, and
compare between different types of catalysts.

Steady state simulator is used as a guide to adjust the synthesis loop
operating conditions at different operation modes, in both normal and
emergency operation to improve the ammonia converter performance.
The model is utilized to determine the optimum inlet bed temperature
to obtain higher production rate and maximize ammonia converter
efficiency.
Addition of a new catalytic fourth bed (Booster Converter) was
studied to increase the conversion per pass of the ammonia synthesis
reaction, increase ammonia production rate and save energy. The
present developed model was utilized to figure out the length of the
fourth bed with the ammonia conversion and ammonia production.

Dr. Mohamed tarek El-Tohfa
Dr.Eng.mtarek@gmail.com
AmmoniaPlantIII
AmmoniaProcessDepartmentManager

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