NH3 Syn Flowsheet

8/13/2019 NH3 Syn Flowsheet

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Ammonia SynthesisFlowsheet

Operator TrainingBy

Gerard B. HawkinsManaging Director, CEO


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Introduction

Most modern ammonia processes arebased on steam-reforming of naturalgas or naphtha.

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

The process steps are very similar in allcases.

Other suppliers are Linde (LAC) &Ammonia Casale.


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Simplified - NH3 PlantH2O

H/Cfeed

H/Cpurification

Removes

impur ities (S,Cl, metals)

Primaryreforming

Converts to

H2, CO, CO2 +

H2O + CH4

COShift

WGS

reaction

Secondaryreforming

Combustion +

Adiabatic Reforming+ Adds Nitrogen

Air

Ammonia

synthesis NH3

Converts N2 +

H2 => NH3

Syngas

compression

Purification

CO2 Removal

& Methanation


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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 arerequired to control bed temperatures.

Various designs are used depending on

contractor. Liquid Ammonia is recovered by

refrigeration.


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Simplif ied Flowsheet for a Typical Ammonia

Plant

NaturalGas

Steam

superheater

Air

Steam

30bar

Steam

Steam

raising

350 C200 C

HeatRecovery

Steam

raising

Cooling

Cooling

Reboiler

CO

Cooling

Preheater

Heat

Recovery

Steam

Boiler

ProcessCondensate

Quench

Quench

Liquid Ammonia

H

HydrodesulphuriserPrimaryReformer SecondaryReformer HighTemperature

Shift

LowTemperature

Shift

Ammonia Syn thesisMethanator

Carbon Dioxide

Purge 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

Ammonia

Catchpot


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Ammonia Plant Steam & Power

System Waste Heat recovery is used to raise

HP steam, 100 120 bar

Steam is used to drive the maincompressors Process air

Syn gas compression + circulator

Refrigeration

Pass-out steam is used for process.


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Ammonia Flowsheet Variations

1. Uhde

Top fired reformer Cold outlet manifold design

Secondary reformer with internal riser

H P loop (200 bar) with radial flowconverter 1 or 2 converters

Once-through synthesis section upstreamof main synthesis loop for very largecapacities (dual pressure Uhde process)


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2. KBR Top-fired reformer

With internal risers

Several synthesis loop options:

Conventional 140 bar loop with 4bedquench converter

Higher pressure for large-scale plants

Horizontal converter on modern plants.

KAAP design 100 bar loop with Ru/Ccatalyst

Braun Purifier flowsheet Excess air with cryogenic purifier to

remove excess N2 and inerts from MUG


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3. Topse 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


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4. Linde LAC (Linde Ammonia

Concept)

Hydrogen plant + N2 addition fromair separation unit

Ammonia Casale synthesis loop


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5. ICI (JM)

AMV

Large-scale process with excess air,

low pressure loop (80 110 bar) LCA

Small-scale plant based on GHRtechnology

AMV / LCA technology is now partof JMs background in ammonia


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Ammonia Synthesis Mechanism

Dissociative adsorption of H2

Dissociative adsorption of N2 -

Believed to be the Rate DeterminingStep (RDS)

Multi-step hydrogenation of

adsorbed N2

Desorption of NH3


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Typical Uhde Synthesis Loop


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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


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Effect of Pressure on Ammonia

Equilibrium Concentration

0

10

20

30

40

50

60

50 75 100 125 150 175 200 225 250 275 300NH3concentration%

Pressure bara

380 C

400 C

420 C


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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)

Ammoniac

ontent%


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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

3concentratio

n%

Pressure bara

0 C

minus 20 C


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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


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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 orafter separator).


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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 verysensitive to MUG H / N ratio becausethe reaction consumes gas in a 3 : 1ratio.


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H2 : N2 ratio example

H / N = 3 : 1

MUG NH3 Purge

H2 3000 2700 300N2 1000 900 100

H / N 3.0 3.0 3.0

H / N = 2.95 : 1

H2 2950 2700 250N2 1000 900 100

H / N 2.95 3.0 2.50


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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% ofthe total) dissolve in the product NH3.

A few loops with purified make-up gashave a self-purging loop where all theinerts are removed in solution in theproduct.

The NH3 content of the purge at theflowmeter position is required to check the

loop mass balance.


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Effect of H2 Recovery

Most modern loops have H2 recovery.

2 systems are used, cryogenic ormembrane.

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 H2addition.


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Control of Catalyst Bed Temperatures

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

intermediate cooling.


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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 betterheat recovery (heat available at a higher

temperature) Split converter designs further increase

the heat recovery temperature.


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3 Bed Converter Example

450 C

1. Optimum Catalyst

Temperatures

410 C

520 C

415 C

480 C

410 C


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3 i/c design

Cold Converter

410 C

520 C

415 C

480 C

410 C

450 C

120 C

335 C


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2 i/c design410 C

520 C

415 C

480 C

410 C

450 C

Hot Converter

235 C


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1 i/c design

410 C

520 C

415 C

480 C

410 C

450 C

Split Converter 305 C


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Converter Heat Recovery Example

In all cases the amount of heat recoveredis the same, only the availabletemperatures are different.

In all cases, the catalyst bed temperaturesare the same:

Bed 1 410 520 dT = 110

Bed 2 415 480 dT = 65Bed 3 410 450 dT = 40

Total Bed dT = Converter dT = 215


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Comparison of 74 & 35 Series

30

40

50

60

70

80

90

100

110

120

0 2 4 6 8 10 12 14

Time on line (years)

RelativeA

ctivit Severnside LCA

Standard Catalyst


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Effect of Size on Activity

Particle Diameter (mm)

14121086420

R

elativeAc

tivity

120

100

80

60

40

0

20


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Effect of Size on Activity

Smaller pellets = high activity

Therefore high production rate orsmaller catalyst volume

But pressure drop will rise

Either axial-radial or radial flowbeds are used to minimise

pressure drop

Radial flow is the basis of manyconverter internal retrofits


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Deactivation

Clean Gas

Thermal sintering

Contaminated Gas

Both Temporary and PermanentPoisoning

Oxygen induced sintering

By water, CO and CO2 Site blocking/Sintering


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Typical Operating Conditions

Temperature (oC) 360-520

Pressure (bar) 80-600

Space velocity (hr-1)1000-5000

Poisons oxygen and oxygen

compoundsnormally < 3ppm


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Catalyst Size

Grade Size

A 1.5-3.0 mm

B 3.0-4.5 mmC 3.0-6.0 mm

D / E 6.0-10.0 mm

G 14.0-20.0 mm


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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


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End


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Ammonia ConverterDesigns


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Converter Designs

Objectives for modern designs are;

- low pressure drop with small catalystparticles.

- high conversion per pass with high grade

heat recovery.Principal types are designed by:

Uhde

Kellogg (KBR) - conventional, Braun,

KAAPTopsoe

Ammonia Casale

JM (I C I)


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Uhde

Uhde design a range of converters:

Modern designs use radial flowwith inter-cooling & 'split

converters' with heat recoverybetween,

- Converter 1 : 2-bed, 1

interchanger

- Heat recovery (boiler)

- Converter 2 : 3rd bed.


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Uhde 3 bed

NH3 Converter


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M W Kellogg Converter Types

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

loop pressure typically 140 - 180 bar. Converters:

4 bed quench ; conventional Kelloggdesign.

Horizontal converter ;

lower cost, low pressure drop, easierinstallation

2 bed inter-cooled layout with small catalyst


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Kellogg Ammonia Quench ConverterOutlet

Inlet


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Kellogg Horizontal Converter

Bed 1Bed 2ABed 2B

Inlet

Outlet


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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


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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


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Haldor Topse S- Series

S-100 :Radial flow 2-bed quench

S-200 :Radial flow 2-bed inter cooled

S-250 : S-200, heat recovery, 2ndconverter with 1 radial flow bed

S-300 :Radial flow 3-bed inter cooled


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Topse S-200 ConverterInlet

Outlet

Cold

Bypass


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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-bedcooling varies;

typically 3 bed, 2 interchanger.


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ICI Types

Lozenge quench converter :

single bed divided into 3 parts by quenchaddition

simple concept but suffered high pressuredrop

ICI AMV Process :

Low pressure loop with H2 recovery at looppressure

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

axial flow

ICI LCA Process :

Tube-cooled + adiabatic design.


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ICI Lozenge Quench Converter


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ICI Tube Cooled Converter


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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)

Ammonia

content%

NH3 Syn Flowsheet

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