This memoir documents the design and simulation of a grassroots ammonia production facility targeting a nameplate capacity of 400 metric tons per day (MTPD) of anhydrous ammonia. The process follows the conventional steam methane reforming (SMR) / Haber-Bosch route, which accounts for the majority of global ammonia production.
Natural gas (predominantly methane, CH₄) is reformed with steam to generate synthesis gas (H₂ + CO), shifted to maximise H₂, purified, compressed, and finally reacted over iron catalyst to produce NH₃.
Design Basis
Nameplate Capacity
400 MTPD
Anhydrous NH₃
NH₃ Molar Production
271 mol/s
= 400 × 10⁶ / (17.03 × 86400)
Feed
Natural Gas
~98% CH₄; desulfurised
SMR Pressure
30 bar
Process side
HB Loop Pressure
150 bar
Synthesis loop
H₂ : N₂ Feed Ratio
3 : 1
Stoichiometric
Steam : Carbon Ratio
3.0
Molar (S/C)
HB Recycle Ratio
5.0
Recycle / Makeup
Process Route Summary
#
Section
Key Equipment
Purpose
1
Feed Pretreatment
Desulfurizer (ZnO), preheater
Remove H₂S, preheat NG & steam
2
Primary Reforming
Fired tubular reformer (R-101)
CH₄ + H₂O → CO + 3H₂
3
Secondary Reforming
Adiabatic reformer (R-102)
Combust residual CH₄; add air-derived N₂
4
HTS / LTS
Two adiabatic shift beds (R-201, R-202)
CO + H₂O → CO₂ + H₂
5
CO₂ Removal
Absorber (T-301), Regenerator (T-302)
Remove CO₂ to < 500 ppm
6
Methanation
Single-bed Ni reactor (R-401)
Trace CO/CO₂ → CH₄ (catalyst protection)
7
KO / Drying
KO pots V-401/402/403, chiller
Remove H₂O from syngas
8
Compression
Multi-stage compressor (K-501)
25 → 150 bar makeup gas
9
HB Synthesis
Multi-bed converter (R-501/502)
N₂ + 3H₂ → 2NH₃ (Fe catalyst)
10
Product Recovery
Separator (V-501), refrigeration
Condense & recover liquid NH₃
2. Process Flow Diagram (PFD)
The PFD below illustrates the full ammonia production chain from natural gas feed to liquid ammonia product. Key stream conditions are annotated at each major unit boundary. Dashed lines represent utility connections (steam, cooling water, refrigerant).
Note on PFD scope: This PFD covers the main process train. Utility systems (steam headers, cooling-water loops, refrigeration circuits, flare, instrument air, boiler feed-water treatment) are shown as dashed connections and detailed in Section 10.
The primary reformer is a top-fired furnace housing catalyst-packed tubes (Ni/α-Al₂O₃). Heat is supplied by natural gas burners at a volumetric flux of 5.2 MW/m³. The reforming reaction is strongly endothermic (ΔH = +206 kJ/mol), driving high conversion at elevated temperature.
Catalyst Volume
10 m³
Inlet Temperature
500°C
773 K
Outlet Temperature
640°C
913 K (approx.)
Operating Pressure
30 bar
Heat Flux
5.2 MW/m³
Steam:Carbon Ratio
3.0
Molar (H₂O / CH₄)
Activation Energy (Eₐ)
240.1 kJ/mol
Pre-exp. Factor (A)
4.22 × 10¹⁵
Primary Reformer Inlet Stream (Scaled for 400 MTPD)
Component
Flow (mol/s)
Mole Fraction
CH₄
211.6
0.250
H₂O (steam)
634.8
0.750
CO (trace)
0.137
~0.000
CO₂ (trace)
0.137
~0.000
H₂ (trace)
1.365
~0.002
Total
848.0
1.000
3.2 Secondary Reformer (R-102) — Adiabatic
Process air is injected into the secondary reformer to combust a fraction of H₂ from the primary outlet, raising temperature to ~850–900°C and completing methane conversion. Critically, the nitrogen content of the injected air sets the final H₂:N₂ = 3:1 stoichiometric ratio required for the Haber-Bosch synthesis.
Two adiabatic fixed-bed shift reactors operate in series to convert CO to CO₂ + H₂. An interstage heat exchanger (E-201) cools gas from the HTS outlet to the LTS inlet temperature, recovering heat as medium-pressure steam.
4.1 High Temperature Shift (R-201 — Fe/Cr Catalyst)
Inlet Temperature
350°C
623 K (after WHB cooling)
Outlet Temperature
~220°C
Adiabatic rise
Pressure
30 bar
Catalyst Volume
25 m³
Kinetic Factor
1.2 × 10⁶
Activation Energy
48 kJ/mol
4.2 Low Temperature Shift (R-202 — Cu/Zn Catalyst)
Inlet Temperature
210°C
483 K (after E-201)
Outlet Temperature
~190°C
Pressure
30 bar
Catalyst Volume
25 m³
Kinetic Factor
5.0 × 10⁵
Activation Energy
40 kJ/mol
4.3 WGS Reactor Profiles
Approximate CO mole fraction reduction through the two-stage system:
Stage
T In (°C)
T Out (°C)
CO Inlet (mol%)
CO Outlet (mol%)
CO Conversion (%)
Secondary Reformer Outlet
850
—
~12–14
—
—
HTS (R-201)
350
220
12–14
~3–4
~70–75%
Interstage Cooler (E-201)
220
210
—
—
—
LTS (R-202)
210
190
3–4
0.2–0.4
~90–92%
Overall WGS
—
—
12–14
<0.5
>97%
4.4 Equilibrium Constant
Both reactors use the van't Hoff expression:
K_eq(T) = exp(4577.8 / T − 4.33) [T in K]
5. CO₂ Absorption Tower — T-301
CO₂ (approximately 13 mol% of the shifted syngas) is removed by counter-current contact with a chemical solvent (MDEA or potassium carbonate, "Benfield" configuration) in a packed absorber column. The Kremser–Brown–Souders method is used for stage calculation.
Tower Designation
T-301
Theoretical Stages
20
Kremser Factor (A)
1.45
Operating Temperature
40°C
313 K
Operating Pressure
27 bar
CO₂ Feed Concentration
~13 mol%
CO₂ Product Spec.
<500 ppm
CO₂ Removal Efficiency
>99.5%
5.1 Absorber Stage Profile (Simplified)
Stage
T (°C)
CO₂ Gas (mol%)
Note
Inlet (Stage 1 — Bottom)
40
13.0
Rich syngas enters
Stage 5
40
~4.5
Stage 10
40
~0.8
Stage 15
40
~0.05
Outlet (Stage 20 — Top)
40
<0.05
<500 ppm; meets spec
5.2 Regenerator (T-302 — Not Simulated)
Rich solvent from T-301 is pumped to a regeneration column where CO₂ is stripped using low-pressure steam. Lean solvent is cooled and recycled. The CO₂ vent stream can be compressed and sold or sequestered. See Section 10 for auxiliary equipment details.
Trace CO and CO₂ remaining after CO₂ absorption (both catalyst poisons for the HB iron catalyst) are hydrogenated to methane over a Ni catalyst bed. The reactions are exothermic, raising the gas temperature by approximately 150–200°C.
Inlet Temperature
260°C
533 K
Outlet Temperature
~430°C
Adiabatic rise
Pressure
26 bar
Catalyst Volume
5 m³
Catalyst Type
Ni / Al₂O₃
CO Inlet
3.5 mol/s
CO₂ Inlet
0.15 mol/s
CO Outlet Target
<100 ppm
Methanation Reactions
Reaction
ΔH (kJ/mol)
CO + 3H₂ → CH₄ + H₂O
−206.0
CO₂ + 4H₂ → CH₄ + 2H₂O
−165.0
6.2 Knockout (KO) Pots — V-401, V-402, V-403
Water produced in the methanator and any residual steam must be removed before compression. Three sequential KO drums operate at progressively lower temperatures to knock out liquid water. The final chilled drum brings moisture below 330 ppm, safe for the HB iron catalyst.
Vessel
T (°C)
P (bar)
Diameter (m)
L/D
H₂O Removal
V-401 (KO-1, Warm)
80
27
~1.8
3.5
~95% cumul.
V-402 (KO-2, Cold)
30
27
~1.9
3.5
~99% cumul.
V-403 (KO-3, Chilled)
5
25
~1.9
3.5
<330 ppm H₂O
Sizing method: Souders-Brown K = 0.107 used for vertical vapour-liquid disengagement. L/D = 3.5 for all drums. Water vapour pressure from Antoine equation: log₁₀(p_sat) = 5.11564 − 1687.537 / (T°C + 230.17).
6.3 Makeup Gas Composition (After Drying)
Component
Flow (mol/s)
Mole %
H₂
427.5
73.9
N₂
142.5
24.6
CH₄ (inert)
5.9
1.0
Ar (inert)
1.8
0.3
H₂O
<0.2
<330 ppm
Total Makeup
577.9
100.0
7. Syngas Compression System — K-501
Dry makeup gas at ~25 bar is compressed to the synthesis loop pressure of 150 bar. A separate synthesis gas circulator (K-502) compensates for loop pressure drop (~8 bar) to maintain loop circulation.
Inlet Pressure
25 bar
Outlet Pressure
150 bar
Pressure Ratio
6.0
Isentropic Efficiency
75%
Makeup Gas Flow
578 mol/s
Circulator Pressure Lift
142→150 bar
7.1 Compression Stages
For a pressure ratio of 6.0 with H₂-rich gas (MW ≈ 6.8 g/mol), a minimum of three compression stages with intercooling are required to control discharge temperatures and achieve acceptable stage efficiencies.
Stage
Inlet P (bar)
Outlet P (bar)
Stage Ratio
Intercooling?
Stage 1
25
55
2.2
Yes (to ~40°C)
Stage 2
55
120
2.2
Yes (to ~40°C)
Stage 3
120
150
1.25
Yes (to ~40°C)
8. Haber-Bosch Synthesis Loop — R-501 / R-502
8.1 Loop Configuration
Compressed makeup gas is mixed with recycled gas (recycle ratio = 5.0) and fed to a multi-bed adiabatic converter operating at 150 bar. Quench gas (15% of feed) is injected between beds to control temperature. The loop includes a waste-heat boiler (E-501) on the converter outlet and an ammonia separator (V-501) operating at −10°C.
Loop Pressure
150 bar
Reactor Inlet T
280°C
553 K
Reactor Outlet T
404°C
677 K
Once-Through Conv.
15–20%
of N₂
Overall Loop Conv.
~97%
With recycle
NH₃ Production
271 mol/s
400 MTPD
Recycle Ratio
5.0
Quench Fraction
15%
Cold gas bypass
8.2 Loop Feed Composition (Including Recycle)
Component
Flow (mol/s)
Note
N₂
710
142.5 × 5 (recycle)
H₂
2137.5
427.5 × 5 (recycle)
NH₃ (recycle slip)
34.1
Residual in recycle stream
CH₄ (inert)
~35
From recycle accumulation
Ar (inert)
~8
From recycle accumulation
Total Loop
~2925
8.3 Temkin-Pyzhev Kinetics
r = A·exp(−Eₐ/RT) · [ p_N₂ · p_H₂³ / p_NH₃² ]^0.5 − reverse term
K_p(T) = exp( 91,800 / (R·T) − 19.5 ) [T in K, p in Pa]
A = 1.0 × 10¹² mol/(m³·s·Pa^1.5) Eₐ = 165 kJ/mol
8.4 Waste-Heat Boiler (E-501)
Hot Inlet T
404°C
Hot Outlet T
200°C
Duty
~24.5 MW
Steam Generated
~37,500 kg/hr
Steam Pressure
~40 bar
Saturated
8.5 Inert Purge
CH₄ and Ar (introduced via air in secondary reforming) build up in the loop. A continuous purge stream (~1–3% of recycle) is taken to prevent excessive inert accumulation, which would reduce NH₃ partial pressure and conversion. The purge is sent to the reformer fuel header.
Converter effluent at 200°C (after the WHB) is further cooled by the makeup gas feed-effluent heat exchanger and then by refrigeration (propane/ammonia cascade) to −10°C. At 150 bar and −10°C, essentially all NH₃ condenses as liquid product while unreacted H₂, N₂, and inerts remain gaseous and return as recycle.
Separator T
−10°C
263 K
Separator P
150 bar
NH₃ Production
271 mol/s
NH₃ Purity
>99.8%
Production Rate
400 MTPD
9.2 Product Specification
Parameter
Value
Standard
Purity (NH₃)
>99.8 wt%
Anhydrous grade
Water content
<0.2 wt%
Residual oil
<5 ppm
Compressor lube carryover
Storage pressure (refrigerated)
Atmospheric
−33°C, 1 atm
Storage pressure (pressurised)
8–10 bar
Ambient temperature
10. Auxiliary Equipment & Utilities — Not Covered in Simulation
The Python simulation covers the main process reactions and equipment sizing. The following auxiliary systems are required for a complete plant and have not been modelled. Each subsection identifies the system, its function, and approximate sizing basis.
10.1 Feed Gas Pretreatment
Desulfurizer (D-101) — ZnO Guard Beds:
Natural gas may contain up to ~10 ppm H₂S and COS. ZnO beds (typically two in lead–lag configuration) reduce sulfur to <0.1 ppm to protect downstream Ni (reformer) and Cu/Zn (LTS) catalysts. Sizing: GHSV ≈ 1000–1500 h⁻¹; vessel diameter ~1.5 m; temperature 350–400°C. A cobalt-molybdenum (CoMo) hydrodesulfurizer upstream converts organic sulfur to H₂S first.
Feed Gas Saturator / Pre-reformer Preheat:
Feed gas is preheated from ambient to ~500°C via the reformer flue gas (convection section). Fired-side duty ~15 MW. A process steam saturator may be integrated to minimise external steam consumption.
Pre-Reformer (optional):
An adiabatic Ni pre-reformer between the preheat coils and the primary reformer can convert C₂+ hydrocarbons in the natural gas feed, reducing the risk of carbon lay-down in the primary reformer tubes. Not included in this simulation since feed is assumed to be essentially pure methane.
Radiant Box & Burners:
The primary reformer furnace (top-fired configuration) contains ~300–400 catalyst tubes in a radiant box with roof burners firing natural gas. Tube metallurgy: HP alloy (25Cr-35Ni). Flue gas exits the radiant box at ~1000°C. Approximate duty: ~160 MW (combined radiant + convection), accounting for 60–65% thermal efficiency.
Convection Section Heat Recovery Coils:
Multiple coils recover heat from flue gas (1000°C → 200°C): (1) process gas preheat, (2) steam superheating to 510°C / 105 bar, (3) boiler feed water preheat, (4) combustion air preheat. Net fuel requirement ≈ 70 MW LHV for 400 MTPD plant.
Flue Gas Stack & Induced Draft Fan:
Stack height ~60–80 m; induced draft (ID) fan ~500 kW; stack exit temperature controlled to ~150°C to avoid acid dew point condensation. Continuous NOx/SOx monitors required.
10.3 CO₂ Removal Regeneration Loop
Rich Amine Pump (P-301):
Transfers rich solvent from T-301 sump to regenerator. Flow ≈ 150–200 m³/hr; differential head ~50 m; ~50 kW motor.
Lean/Rich Heat Exchanger (E-301):
Cross-exchange between hot lean and cold rich solvent, recovering ~8–12 MW and reducing reboiler duty.
Regenerator Column (T-302):
Stripping column, typically 20–25 trays; operates at ~120°C / 1.5 bar with low-pressure steam reboiler. CO₂ product is available for compression and sale (~400 t/day CO₂ byproduct).
Regenerator Reboiler (E-302):
LP steam at ~5 bar; duty ~20–25 MW. This is the largest single utility consumer in the CO₂ removal section.
Lean Amine Cooler (E-303) & Lean Amine Pump (P-302):
Cool lean solvent to 40°C before recycle; pump returns to top of absorber.
10.4 Process Condensate Treatment
Process Condensate Stripper (T-401):
Condensate from LTS outlet, CO₂ absorber sump, and KO pots contains dissolved NH₃, CO₂, and methanol traces. A small stripper column (6–8 trays, LP steam stripping) recovers condensate as boiler-feed water quality. Stripped gases are recycled to the reformer.
10.5 Steam System
High-Pressure Steam Header (105 bar / 510°C):
Generated in the primary reformer convection section. Drives the main syngas compressor and/or steam turbine generator. Flow ≈ 250–300 t/hr.
Medium-Pressure Steam Header (40 bar):
Generated in the HB waste-heat boiler (E-501) — ~37,500 kg/hr. Used for process preheating and CO₂ regeneration reboiler.
Low-Pressure Steam Header (5 bar):
Letdown from MP header via turbines or PRVs. Used for CO₂ regenerator reboiler, process condensate stripper, and tracing.
Boiler Feed Water (BFW) Treatment Unit:
Deaerator, ion-exchange softening/demineralisation, oxygen scavenging (hydrazine or DEHA dosing). Feed water quality: silica <20 ppb, total dissolved solids <0.1 ppm. Capacity ≈ 350 t/hr.
10.6 Cooling Water System
Cooling Tower(s):
Counter-flow induced-draft cooling towers; supply temperature 30°C, return 45°C. Total cooling duty ~80–120 MW including interstage compressor intercoolers, LTS outlet cooler, absorber gas cooler, and make-up gas cooler. Two-cell tower (duty + 1 spare cell).
Cooling Water Pumps & Chemical Treatment:
Biocide (chlorine or isothiazolone), corrosion inhibitor (phosphonate), scale inhibitor. Conductivity controls blowdown at ~4000 μS/cm.
10.7 Ammonia Refrigeration System
Refrigeration Compressor (K-601):
Ammonia vapour refrigeration cycle to condense product NH₃ at −10°C and to provide chilling duty for the KO-3 pot (5°C). Two-stage compression typical: LP (−33°C / 1 bar) → HP (120°C / 15 bar) condensation. Total refrigeration duty ≈ 10–15 MW. Compressor driver: ~4 MW electric motor or steam turbine.
NH₃ Condenser (E-601) & Refrigerant Receiver (V-601):
Air-cooled or water-cooled condenser; refrigerant receiver (accumulator drum) with level control to product rundown.
10.8 Ammonia Product Storage & Export
Refrigerated Storage Tanks (TK-701A/B):
Two flat-bottom atmospheric refrigerated storage tanks, each ~5,000–10,000 MT capacity (≈7,300 m³ at 0.68 t/m³), operating at −33°C / 1 atm. Inner tank: 9% Ni steel; outer tank: carbon steel with 300 mm urethane foam insulation.
Refrigerated NH₃ Transfer Pumps (P-701):
Vertical submerged pumps; 200–300 m³/hr; 80 m head; 150 kW; explosion-proof motor.
Loading Facilities:
Rail car loading arms (pressurised), truck-loading skid (pressurised or refrigerated), and (if coastal) marine loading arm with vapour return line.
Flare & Blowdown System:
Closed-loop blowdown drum (V-801) collects process reliefs, liquid NH₃ drain, and high-pressure purge. NH₃ vapours are scrubbed (dilute H₂SO₄ or water) before incineration; H₂/N₂ purges are flared. Flare tip rated for peak relief load ≈ 100 MW.
10.9 Instrument Air & Plant Nitrogen
Instrument Air Compressors (K-901A/B):
Screw compressors (duty + standby), 7–8 bar, 100% oil-free class; dew point −40°C at line pressure. Capacity ≈ 2 × 500 Nm³/hr.
Plant Nitrogen System:
PSA nitrogen generator or liquid N₂ vaporiser for catalyst blanketing, purging during shutdowns, seal gas for rotating equipment, and positive pressure in NH₃ storage vents. Purity ≥ 99.99 mol% N₂.
10.10 Electrical & Rotating Equipment Summary
Equipment
Tag
Estimated Power / Duty
Driver
Syngas Compressor
K-501
~4.1 MW
Steam turbine (HP steam)
Synthesis Circulator
K-502
~0.5 MW
Steam turbine or electric motor
Refrigeration Compressor
K-601
~4 MW
Electric motor
Cooling Water Pumps
P-801A/B
~2 × 500 kW
Electric motor
BFW Pumps
P-901A/B
~2 × 200 kW
Electric motor
Rich Amine Pump
P-301
~50 kW
Electric motor
NH₃ Transfer Pumps
P-701
~150 kW
Electric motor (explosion proof)
ID Fan (Reformer)
FN-101
~500 kW
Electric motor
Instrument Air Compressors
K-901A/B
~2 × 100 kW
Electric motor
Total Plant Power Import
~8–12 MW
Grid (remainder steam-driven)
10.11 Safety Systems
Safety Instrumented System (SIS): IEC 61511 SIL-2 rated shutdown system for reformer burner management, loop high-pressure protection, and NH₃ storage. Separate from DCS.
NH₃ Leak Detection: Electrochemical point detectors at all potential release points; open-path IR detectors in storage area. Alarm at 25 ppm, shutdown at 100 ppm.
Pressure Relief Valves (PSVs): Sized per API 520/521 for blocked outlet, fire case, and control valve failure scenarios on all major vessels and reactors.
Emergency Depressurisation: Manual and automatic blowdown valves to flare on synthesis loop (>150 bar inventory) to prevent vessel over-temperature during fire scenarios.
Catalyst Protection Interlock: Automated shutdown of HB loop on high CO/CO₂ at methanator outlet (>50 ppm) to prevent irreversible poisoning of Fe catalyst.
10.12 Environmental Controls
DeNOx (SCR or Low-NOx Burners): Reformer flue gas NOx controlled to <100 mg/Nm³ via low-NOx burner design or selective catalytic reduction (SCR) unit with urea injection.
Process Condensate Stripper: Removes NH₃ and CO₂ from process water to meet discharge limits (<10 ppm NH₃, pH 6–9).
NH₃ Scrubbers on Vents: All NH₃-containing vent streams pass through a dilute H₂SO₄ or water scrubber before atmospheric release, ensuring <10 mg/Nm³ NH₃ at stack exit.
CO₂ Export / Sequestration: ~400 t/day of high-purity CO₂ from the CO₂ absorber regenerator is available for compression and export (urea synthesis, EOR, or CCS).