Ammonia Synthesis Flowsheet

Operator Training

By Gerard B. Hawkins

Managing Director, CEO

Introduction Most modern ammonia processes are

based on steam-reforming of natural gas or naphtha.

The 3 main technology suppliers are Uhde (Uhde/JM Partnership), Topsoe & KBR.

The process steps are very similar in all cases.

Other suppliers are Linde (LAC) & Ammonia Casale.

Simplified - NH3 Plant H2O

H/C feed

H/C purification

Removes impurities (S,

Cl, metals)

Primary reforming

Converts to H2, CO, CO2 +

H2O + CH4

CO Shift

WGS reaction

Secondary reforming

Combustion + Adiabatic Reforming

+ Adds Nitrogen

Air

Ammonia synthesis NH3

Converts N2 + H2 => NH3

Syngas compression

Purification

CO2 Removal & Methanation

Ammonia Synthesis Loop

Synthesis reaction is equilibrium limited, typically 15 – 20% NH3 at converter exit.

Therefore recycle in a ‘loop’ is required. Multi-stage complex converters are

required to control bed temperatures. Various designs are used depending on

contractor. Liquid Ammonia is recovered by

refrigeration.

Simplified Flowsheet for a Typical Ammonia Plant

Natural Gas

Steamsuperheater

AirSteam

30 bar

Steam

Steamraising

350 C 200 C

Heat Recovery

Steamraising

Cooling

Cooling

Reboiler

CO

Cooling

Preheater

HeatRecovery

Steam

Boiler

Process Condensate

Quench

Quench

Liquid Ammonia

H

Hydrodesulphuriser Primary Reformer

SecondaryReformer

High Temperature

Shift

Low Temperature

Shift

Ammonia SynthesisMethanator

Carbon DioxidePurge Gas

Cooling

400 Co

390 Co

2

790 Co

550 Co

1000 Co

o

420 Co

150 Co

400 Co

470 Co

o

220 Co

290 Co

330 Co

2

CO Removal2

220 bar

Refrigeration

CondensateCooling

AmmoniaCatchpot

Ammonia Plant Steam & Power System

Waste Heat recovery is used to raise HP steam, 100 – 120 bar

Steam is used to drive the main compressors • Process air • Syn gas compression + circulator • Refrigeration

Pass-out steam is used for process.

Ammonia Flowsheet Variations 1. Uhde

Top fired reformer • Cold outlet manifold design

Secondary reformer with internal riser H P loop (200 bar) with radial flow

converter • 1 or 2 converters

Once-through synthesis section upstream of main synthesis loop for very large capacities (dual pressure Uhde process)

Ammonia Flowsheet Variations 2. KBR

Top-fired reformer • With internal risers

Several synthesis loop options: • Conventional 140 bar loop with 4bed

quench converter • Higher pressure for large-scale plants • Horizontal converter on modern plants. • KAAP design – 100 bar loop with Ru/C

catalyst Braun Purifier flowsheet

• Excess air with cryogenic ‘purifier’ to remove excess N2 and inerts from MUG

Ammonia Flowsheet Variations 3. Topsøe

Side-fired reformer Radial flow converter

• S-100 2 bed quench • S-200 2 bed intercooled • S-250 = S-200 + boiler + 2nd converter

(1 bed) • S-300 3 bed intercooled

Ammonia Flowsheet Variations 4. Linde LAC (Linde Ammonia

Concept)

Hydrogen plant + N2 addition from air separation unit

Ammonia Casale synthesis loop

Ammonia Flowsheet Variations 5. ICI (JM)

AMV • Large-scale process with excess air,

low pressure loop (80 – 110 bar) LCA

• Small-scale plant based on GHR technology

AMV / LCA technology is now part of JM’s ‘background in ammonia’

Ammonia Synthesis Mechanism

Dissociative adsorption of H2

Dissociative adsorption of N2 - Believed to be the Rate Determining Step (RDS)

Multi-step hydrogenation of adsorbed N2

Desorption of NH3

Typical Uhde Synthesis Loop

Uhde Dual-Pressure Process

C.W.Make up gas from frontend

C.W.

Steam

Once through

converter

SynthesisLoop

Purge

NH3NH3

NH3

1 2 3 R

C.W.Make up gas from frontend

C.W.

Steam

Once through

converter

SynthesisLoop

Purge

NH3NH3

NH3

1 2 3 R

Effect of Pressure on Ammonia Equilibrium Concentration

0

10

20

30

40

50

60

50 75 100 125 150 175 200 225 250 275 300

NH

3co

ncen

tratio

n %

Pressure bara

380 C

400 C

420 C

Ammonia Equilibrium Diagram

300 (572)

350 (662)

400 (752)

450 (842)

500 (932)

550 (1022)

600 (1112)

650 (1202)

0

10

20

30

40

Equilibrium

Max Rate

Temperature °C (°F)

Am

mon

ia c

onte

nt %

Effect of Catchpot Temperature on Ammonia VLE

0.0

2.0

4.0

6.0

8.0

10.0

12.0

50 75 100 125 150 175 200 225 250 275 300

NH

3co

ncen

tratio

n %

Pressure bara

0 Cminus 20 C

Synthesis Loop Principles: Mass Balance

Overall Loop Mass Balance • On a mass basis:

NH3 = MUG – Purge • On a molar basis:

NH3 = (MUG – Purge) / 2 because 4 mol -> 2 mol in the NH3

reaction. Converter balance, on a molar basis: NH3 = Inlet gas – Outlet gas

Synthesis Loop Principles: Mass Balance

Converter Molar balance: NH3 = Circ Flow x (NH3out- NH3in) 1 + NH3out NH3in is set by P & T of final

separator + position of MUG addition (before or

after separator).

Synthesis Loop Principles: Effect of Purge

Circulating composition is the same as the purge composition (like a stirred-tank reactor).

Inerts (CH4 + Ar) build-up in loop. Circulating gas H / N ratio is very

sensitive to MUG H / N ratio because the reaction consumes gas in a 3 : 1 ratio.

Synthesis Loop Principles: H2 : N2 ratio example

H / N = 3 : 1 MUG NH3 Purge H2 3000 2700 300 N2 1000 900 100 H / N 3.0 3.0 3.0 H / N = 2.95 : 1 H2 2950 2700 250 N2 1000 900 100 H / N 2.95 3.0 2.50

Synthesis Loop Principles :

Inerts Balance

Inerts (CH4 + Ar) concentrate in the loop, typically by a factor of about 10.

Note that some of the inerts (10 – 20% of the total) dissolve in the product NH3.

A few loops with purified make-up gas have a ‘self-purging loop’ where all the inerts are removed in solution in the product.

The NH3 content of the purge at the flowmeter position is required to check the loop mass balance.

Synthesis Loop Principles : Effect of H2 Recovery

Most modern loops have H2 recovery. 2 systems are used, cryogenic or

membrane. The overall effect is similar, typically 90%

H2 recovery at 90% purity. Overall loop H2 conversion to NH3

increases from about 92% to 98%. MUG H / N ratio changes from 3.0 to

approx. 2.85, and returns to 3.0 after H2 addition.

Synthesis Loop Principles : Control of Catalyst Bed Temperatures

Multi-bed design : 2, 3, or 4 catalyst beds with

intermediate cooling.

Synthesis Loop Principles : Converter Heat Balance

Older converter designs usually had an interchanger after the final bed to contain high temperatures within the converter.

Modern designs typically have no ‘overall’ interchanger because this gives better heat recovery (heat available at a higher temperature)

‘Split converter designs’ further increase the heat recovery temperature.

3 Bed Converter Example

450 C

1. Optimum Catalyst Temperatures

410 C

520 C

415 C

480 C

410 C

3 i/c design

‘Cold’ Converter

410 C

520 C

415 C

480 C

410 C

450 C

120 C

335 C

2 i/c design 410 C

520 C

415 C

480 C

410 C

450 C

‘Hot’ Converter

235 C

1 i/c design 410 C

520 C

415 C

480 C

410 C

450 C

‘Split’ Converter 305 C

Converter Heat Recovery Example

In all cases the amount of heat recovered is the same, only the available temperatures are different.

In all cases, the catalyst bed temperatures are the same:

Bed 1 410 – 520 dT = 110 Bed 2 415 – 480 dT = 65 Bed 3 410 – 450 dT = 40 Total Bed dT = Converter dT = 215

Comparison of 74 & 35 Series

30

40

50

60

70

80

90

100

110

120

0 2 4 6 8 10 12 14Time on line (years)

Rel

ativ

e A

ctiv

ity Severnside LCAStandard Catalyst

Effect of Size on Activity

Particle Diameter (mm) 14 12 10 8 6 4 2 0

Rel

ativ

e A

ctiv

ity

120 100 80

60

40

0 20

Effect of Size on Activity

Smaller pellets = high activity Therefore high production rate or

smaller catalyst volume But pressure drop will rise Either axial-radial or radial flow

beds are used to minimise pressure drop

Radial flow is the basis of many converter internal retrofits

Deactivation

Clean Gas • Thermal sintering

Contaminated Gas • Both Temporary and Permanent

Poisoning • Oxygen induced sintering • By water, CO and CO2 • Site blocking/Sintering

Typical Operating Conditions

Temperature (oC) 360-520

Pressure (bar) 80-600

Space velocity (hr-1) 1000-5000

Poisons oxygen and oxygen compounds normally < 3ppm

Catalyst Size

Grade Size A 1.5-3.0 mm B 3.0-4.5 mm C 3.0-6.0 mm D / E 6.0-10.0 mm G 14.0-20.0 mm

Catalyst Reduction

Max water in outlet gas during reduction (ppm)

Formation of water during reduction of 1te of Catalyst (kg)

Pre-reduced Oxidized

1000 3000

25 280

End

Ammonia Converter Designs

Converter Designs Objectives for modern designs are; - low pressure drop with small catalyst

particles. - high conversion per pass with high grade heat recovery. Principal types are designed by: Uhde Kellogg (KBR) - conventional, Braun,

KAAP Topsoe Ammonia Casale JM (I C I)

Uhde

Uhde design a range of converters: Modern designs use radial flow

with inter-cooling & 'split converters' with heat recovery between,

- Converter 1 : 2-bed, 1 interchanger

- Heat recovery (boiler) - Converter 2 : 3rd bed.

Uhde 3 bed NH3 Converter

M W Kellogg Converter Types

'Conventional' make-up gas and loop layout, refrigeration to low temperature (-25 C),

loop pressure typically 140 - 180 bar. Converters: 4 bed quench ; conventional Kellogg

design. Horizontal converter ;

• lower cost, low pressure drop, easier installation

• 2 bed inter-cooled layout with small catalyst

Kellogg Ammonia Quench Converter Outlet

Inlet

Kellogg Horizontal Converter

Bed 1 Bed 2A Bed 2B

Inlet

Outlet

KBR KAAP

Converter is made up of 4 beds First bed uses magnetite catalyst Ru can not be used since

temperature rise is too large Lower beds use Ru catalyst Ru catalyst has a carbon support Catalyst developed by BP

• Very high activity even at low pressure

Braun Converter Types

Purifier Process gives pure make-up gas

- low levels of poisons; H2O, CO, CO2 - Low inerts; no purge from loop Converters : Basically 2-bed intercooled with each

catalyst bed in a separate vessel Modern designs may use 3 converters

&/or radial flow

Haldor Topsøe S- Series

S-100 :Radial flow 2-bed quench S-200 :Radial flow 2-bed inter cooled S-250 : S-200, heat recovery, 2nd

converter with 1 radial flow bed S-300 :Radial flow 3-bed inter cooled

Topsøe S-200 Converter Inlet

Outlet

Cold Bypass

Ammonia Casale

Ammonia Casale - 'axial-radial' concept

- radial flow without a top cover on the beds

- simpler mechanical design

No. of beds & type of inter-bed cooling varies;

typically 3 bed, 2 interchanger.

ICI Types Lozenge quench converter :

• single bed divided into 3 parts by quench addition

• simple concept but suffered high pressure drop

ICI AMV Process : • Low pressure loop with H2 recovery at loop

pressure • range of converters in use • Terra: ICI 3-bed, 1 quench + 1 intercooler

axial flow ICI LCA Process :

• Tube-cooled + adiabatic design.

ICI Lozenge Quench Converter

ICI Tube Cooled Converter

ICI TCC Equilibrium Plot

300 (572)

350 (662)

400 (752)

450 (842)

500 (932)

550 (1022)

600 (1112)

650 (1202)

0

10

20

30

40

Equilibrium

Max Rate

Converter Profile

Temperature °C (°F)

Am

mon

ia c

onte

nt %