nh3

4
Introduction to Ammonia Manufacture Ammonia is generally manufactured from natural gas using the steam reforming process. Other feedstocks and processes are used but these are not described here. There are several reaction stages and catalysts are key to the economic operation of modern ammonia production plants. Diagram 1 illustrates the chemistry of the ammonia process and the basic materials of the catalysts used. The first stage is purification where impurities, mainly sulphur compounds, are removed from the gas stream. Steam reforming is performed in two stages. In the primary stage, the endothermic reactions take place at pressures around 30 bar and temperatures of 800°C or higher. This is followed by an exothermic secondary reformer where air is added to the partially reformed gas stream. The carbon monoxide in the gas leaving the secondary reformer is converted to carbon dioxide in the shift reactors and then removed by scrubbing from the gas stream. Any residual carbon oxides are then converted back to methane by methanation before compression of the hydrogen and nitrogen to ammonia synthesis pressure. The final reaction stage is ammonia synthesis where the hydrogen and nitrogen combine to form ammonia. This reaction stage takes place at high pressure (100-350 bar) and is highly exothermic. Diagram 2 shows a simplified block diagram of a typical 1000 tonne/day ammonia plant including details of operating temperatures, catalyst volumes, KATALCO catalyst and gas compositions. The economics of ammonia production require careful energy management as illustrated by the flowsheet in Diagram 3 which shows many heat exchangers are necessary to optimised heat recovery as well as to generate the steam required for process purposes.

description

nh3

Transcript of nh3

Page 1: nh3

Introduction to Ammonia Manufacture

Ammonia is generally manufactured from natural gas using the steam reforming process. Other feedstocks and processes are used but these are not described here. There are several reaction stages and catalysts are key to the economic operation of modern ammonia production plants. Diagram 1 illustrates the chemistry of the ammonia process and the basic materials of the catalysts used.

The first stage is purification where impurities, mainly sulphur compounds, are removed from the gas stream.

Steam reforming is performed in two stages. In the primary stage, the endothermic reactions take place at pressures around 30 bar and temperatures of 800°C or higher. This is followed by an exothermic secondary reformer where air is added to the partially reformed gas stream.

The carbon monoxide in the gas leaving the secondary reformer is converted to carbon dioxide in the shift reactors and then removed by scrubbing from the gas stream. Any residual carbon oxides are then converted back to methane by methanation before compression of the hydrogen and nitrogen to ammonia synthesis pressure.

The final reaction stage is ammonia synthesis where the hydrogen and nitrogen combine to form ammonia. This reaction stage takes place at high pressure (100-350 bar) and is highly exothermic.

Diagram 2 shows a simplified block diagram of a typical 1000 tonne/day ammonia plant including details of operating temperatures, catalyst volumes, KATALCO catalyst and gas compositions.

The economics of ammonia production require careful energy management as illustrated by the flowsheet in Diagram 3 which shows many heat exchangers are necessary to optimised heat recovery as well as to generate the steam required for process purposes.

Page 2: nh3

CHEMISTRY OF AMMONIA PROCESS

HYDRODESULPHURISER(Sulphur Removal)

PRIMARY REFORMING(Steam Reforming)

SECONDARY REFORMING(Air Addition)

HIGH TEMP SHIFT(CO Conversion)

AMMONIA SYNTHESIS(Ammonia Formation)

METHANATORCO/CO Polishing

LOW TEMP SHIFT(CO Conversion)

RSH + H RH + H SHCI + NaAlO Al0OH + NaCLH S + ZnO ZnS + H OCatalyst: CoMo/NiMo

Modified Alumina Zinc Oxide

2 2

2

22

CO + H O CO + H

Catalyst: Iron/Chromium/Copper

2 2 2

CO + H O CO + H

Catalyst: Copper/Zinc/Aluminium

2 2 2 N + 3H 2NH

Catalyst: Fused Promoted Magnetite

2 32

2

CO Removal

K CO + H O + CO 2KHCO32 2 2 3

2KHCO K CO + H O + CO3 2 3 2 2

CO + 3H CH + H OCO +4H CH + 2H O

Catalyst: Nickel Oxide

2 2

2

4 2

4 2

Ammonia

Natural

Gas

CH + H O 3H + COCO + H O H + CO

Catalyst: Nickel Oxide

2

4 2

2 2

2 CH + H O 3H + CO2H + [O + N ] 2H O + N

Catalyst: Nickel Oxide

2

4 2

2 2

2

2 2

2

Diagram 1

Page 3: nh3

Simplified block diagram of typical 1000 tonne/day ammonia plant

Purification Primary Reforming SecondaryReforming

High TemperatureCO Shift

Low TemperatureCO Shift

Methanation AmmoniaSynthesis

CORemoval

Exit Gas Composition

NCOCOCHN + ACO + CO2

2

4

2

2

CO

Rem

ova

l

57-4

18 M

54-8

25 M

71-5

45 M

83-3

60 M

11-4

25 M

35-4/8

65 M

41-6

10 M

59-3 10M

390 Co 790 Co 1000 Co

420 Co 220 Co 330 Co 470 Co

400 Co

550 Co

350 Co 200 Co 290 Co 400 Co

Steam AirNatural Gas Feed

Hydrogen

3

3

3

3 3 3 3 3 3

69.89.3

10.510.4

--

Volume %56.512.97.50.3

22.8-

Volume %60.3

3.015.6

0.320.8

-

Volume %61.40.3

17.80.2

20.3-

Volume %74.6

0.40.10.3

24.6-

Volume %74.2

--

0.825.0

5ppm

MUGVolume %

63.4--

9.524.9

<5ppmNH3 - 2.5

LoopVolume %

Ammonia

32-5 24 M

2

2

Diagram 2

indicating KATALCO catalyst types and volumes required

KATALCO KATALCO KATALCO KATALCO KATALCO KATALCO

KATALCO

KATALCO

KATALCO

Page 4: nh3

Simplified flowsheet for typical ammonia plant

Natural Gas

Steamsuperheater

Air

Steam

30 bar

Steam

Steamraising

350 C200 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 SynthesisCO Removal Methanator

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

2

220 bar

Refrigeration

CondensateCooling

Diagram 3