Theory and Operation of VSG-A101 Ammonia Synthesis

by: Gerard B. Hawkins

Managing Director, CEO

Ammonia Synthesis

Discuss Ammonia Synthesis Include

• Purpose • Reactions and chemistry • Bed and system design • Operating conditions • Catalyst parameters • Performance and monitoring • Problems

Natural Gas

Steam superheater

Air Steam

30 bar

Steam

Steam raising

350 C 200 C

Heat Recovery

Steam raising

Cooling

Cooling

Reboiler

CO

Cooling

Preheater

Heat Recovery

Steam

Boiler

Process Condensate

Quench

Quench

Liquid Ammonia

H

Hydrodesulphuriser Primary Reformer

Secondary Reformer

High Temperature

Shift Low

Temperature Shift

Ammonia Synthesis Methanator Carbon Dioxide Purge Gas

Cooling

400 C o

390 C o

2

790 C o

550 C o

1000 C o

o

420 C o

150 C o

400 C o

470 C o

o

220 C o

290 C o

330 C o

2

CO Removal 2

220 bar

Refrigeration

Condensate Cooling

Ammonia Catchpot

Simplified Flowsheet for a Typical Ammonia Plant

Simplified Flowsheet for a Modern Uhde Ammonia Plant

Ammonia Chemistry

Reaction : (Exothermic) N2 + 3H2 <=> 2NH3 H(@ 700K) = - 52kJ/mol Reaction is favored by high pressure and low

temperature Pressure governed by capital and operating

cost Temperature balance of kinetics/equilibrium

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

Effect of Temperature Pressure on Ammonia Equilibrium Concentration

0

5

10

15

20

25

30

35

40

50 75 100 125 150Pressure bara

NH

3 co

ncen

tratio

n %

380 C

400 C

420 C

Ammonia Equilibrium Diagram

300 350 400 450 500 550 600 650 0

10

20

30

40 Equilibrium

Max Rate

Temperature °C

Am

mon

ia c

onte

nt %

Effect of Catchpot Temperature on Ammonia VLE

0

2

4

6

8

10

12

50 75 100 125 150

Pressure bara

NH

3 co

ncen

trat

ion

%

0 Cminus 20 C

Catalyst Requirements

High catalyst activity Low sensitivity to

catalyst poisons High thermal

resistance Reasonable

reduction time High mechanical

strength and abrasion resistance

Catalyst Formulation

The source of iron is magnetite, Fe3O4, chosen for its crystal structure

During reduction, oxygen is removed from the crystal lattice without shrinkage

This produces metallic iron which is extremely porous

A significant factor in achieving a high activity catalyst

Incorporation of Promoters

Small amounts of certain metal oxides promote activity and improves stability

Alumina and potash are the most important • They produce ‘doubly-promoted’ catalyst • Alumina is a ‘structural’ promoter • Restricts growth of iron crystallites during

reduction and operation • Increases thermal stability of the catalyst

Incorporation of Promoters

Alkali metals are ‘electronic promoters’ and greatly increase the activity of the iron particles; potassium is the most cost effective

Other promoters include calcium oxide, silica & magnesia

Contaminants in the raw magnetite must also be taken into account during manufacture to ensure the optimum concentration of promoters in the finished catalyst

Effect of Promoters and Stabilizers Conventional Catalysts

AI2O3 - stabilizes the internal surface SiO2 - stabilizes the activity in presence of oxygen

compounds during normal operation and reduction.

K2O - increases the activity - decreases the thermal stability and the

resistance against poisoning by oxygen compounds

- minimizes the neutralization of K promoter CaO - increases the stability against poisoning by

sulfur

Ammonia Synthesis - Catalyst Parameters

Parameters as follows Form Irregular particles Production Method Melt, cool and grind Size 1-3 mm Magnetite % Balance % Potash % 0.6-0.8 % Calcium Oxide % 1.4-1.8 % Alumina % 2.2-2.6 %

Ammonia Synthesis Catalyst Production

Catalyst is unusual in that it is not made via pelleting or extrusion

Unique manufacturing process A mix is made of ingredients including

promoters Feed is passed to an electric Arc furnace Then milled to give correct shape distribution

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 or small catalyst

volume But pressure drop will rise So must use either axial-radial or radial flow

beds to minimise pressure drop Basis of many converter internal retrofits

Deactivation

Clean Gas Thermal sintering Contaminated Gas Both Temporary and Permanent Poisons

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

Uhde Converter Design

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 Converter Design

Gas inlet Start up gas

Gas outlet

Second bed

First bed

Uhde Converter Design

Features of Krupp-Uhde 2-bed radial Ammonia Converters

• Easy withdrawal of the internal heat exchanger without catalyst removal

• Comfortable access for catalyst removal without removal of the cartridge

• Access to all catalyst beds without removal of intermediate heat exchanger

• Reasonable transport dimensions and weights even at high plant capacities

Uhde Converter Design

Features of Krupp-Uhde 1-bed radial Ammonia

Converters • One radial type catalyst bed

resulting in maximum conversion rate, lower recycle gas rate and low pressure drop

• Suitable large volumes of catalyst with small grain size

• Simple and reliable design • Comfortable access for catalyst

removal without removal of the cartridge

• Reasonable transport dimensions and weights even at high plant capacities

Gas outlet Gas inlet

Third bed

Ammonia Synthesis - Temperature Profile

Equilibrium curve % NH3

Heat exchanger type Quench type

Temperature °C

500 450 400 0

5

10

15

20

Typical Operating Conditions

Temperature (oC) 360-530 Pressure (bar) 100-600 Space velocity (hr-1) 1000-5000 Poisons oxygen and

oxygen compounds normally 3ppm

Catalyst Reduction

Pre-reduced Oxidised

Max water in outlet gas during reduction (ppm)

1000 3000

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

25 280

Ammonia Synthesis - Performance Monitoring

Monitor temperature profile • Adjust accordingly to optimise production

Monitor pressure drop across converter Monitor loop pressure Monitor inert levels

• Helps identify upstream problems

Ammonia Synthesis - Problems

Ammonia Synthesis is a robust catalyst • Delivers extremely long lives • Performance is a function of converter and

catalyst Must be aware of

• Effect of water • Effect of CO and CO2

• Will poison the catalyst and therefore reduce production