Process Flow Diagram of a HALDOR TOPSOE process Ammonia plant

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Process Description

The SMR process of M/s. HTAS involves replacement of front-end with conventional SMR together with revamping of synthesis loop by replacing present S-200 converter with S-300 to maintain the production of Ammonia at current level. The process steps followed in this scheme are:

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Desulphurization Reforming Conversion CO Carbon dioxide removal Methanation and compression Ammonia Synthesis and Refrigeration

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Hydrogenation-

Desulphurization

The natural gas feedstock contains upto 10 wt. ppm of

sulphur compounds which have to be removed in order

to avoid poisoning of the reforming catalyst in the

primary reformer, and the low temperature shift

catalyst in the CO converter. After desulphurization,

the the content of sulphur is less than 0.1 wt. ppm. The

desulphurization takes place in two stages. The organic

sulphur compounds are converted to H2S in

Hydrogenator and the H2S absorption will take place

in Sulphur Absorber.

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The reaction involve in hydrogenator:

RSH + H

2

→ RH + H

2

S

R

1

SSR

2

+ 3H

2

→ R

1

H + R

2

H + 2H

2

S

R

1

SR

2

+ 2H

2

→ R

1

H + R

2

H+ 2H

2

S

(CH)

4

S + 4H

2

→ C

4

H

10

+ H

2

S

COS + H

2

→ CO + H

2

S

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The reaction took place in ZnO

absorber:

ZnO + H2S  ZnS + H2O

ZnO + COS  ZnS + CO2

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Primary reformer

Radiant Zone

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REFORMING

The reforming of the hydrocarbon feed takes place in two stages, a direct fired primary reformer and an autothermal catalytic secondary reformer. The hydrocarbon feed coming from the desulphurization unit is mixed with steam. The steam/carbon ratio is 3.0. The reaction mixture is preheated and taken to the primary reformer, where it is decomposed into hydrogen, carbon monoxide, and carbon dioxide over a nickel catalyst by heat supply. In the secondary reformer preheated air is added, and the heat thus generated by burning of the gas decomposes the methane. The methane concentration in the outlet gas from the secondary reformer is 0.6 mol% (dry basis).

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E -205 -1 E -202 -2 E -204 -2 E -205 -2 E-201 ID fan E-201 Inlet Pigtail Hot collector Cold collector Outlet Pigtail 8/17/2009 10 1st floor 2nd floor 3rd floor Burner Block E-202-1 E-203 E-204-1 E-206

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The steam reforming process can be

described by the following :

C

n

H

2n+2

+ 2H

2

O  C

n-1

H

2n

+ CO

2

+ 3H

2

- heat

CH

4

+ 2H

2

O  CO

2

+ 4H

2

O - heat

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Burner details :

It is a side fired furnace. There are 6 rows of

burners along with furnace walls.

Total number of burners

216

No. of burners in each radiant zone

108

No. of burners along each wall

54

No. of radiant burners wall

2x2

No. of burner row in each wall

6

No. of burners in each row

9

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CO Shift Conversion

The CO conversion takes place in two adiabatic stages. The high temperature CO converter contains a Cu-promoted high temperature shift catalyst, which features high activity and high mechanical strength. The low temperature CO converter is loaded with low temperature shift catalyst, which is characterised by high activity, high strength, and highly sensitive towards sulphur poisoning. After reforming, about 12.7% CO is present in the gas (dry basis). In the high temperature CO converter, the CO content is reduced to approximately 3.1 vol%, and the temperature increases from 360oC to 438oC. It is then cooled to 205oC and passed on to

the low temperature CO converter, in which the CO content is reduced to approximately 0.3 vol%, while the temperature increases to 227oC.

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The carbon monoxide in the process gas leaving

the reformer section is converted into carbon

dioxide and hydrogen according to shift reaction:

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Carbon dioxide removal

For removal of the carbon dioxide, the activated MDEA process is used. Main equipment in the MDEA process is the CO2 absorber and the CO2 stripping column. CO2 is removed from the process gas by counter-current absorption in two stages. In the lower part of the absorber, flash-regenerated solution is used for bulk CO2 removal. In the upper part of the absorber, strip- regenerated solution is used for scrubbing. Thus, nearly a complete removal of CO2 with only 0.05 vol% CO2 (on dry basis) left in the treated gas.

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R

3

N + H

2

O + CO

2

 R

3

NH+ HCO

3

-

2R

2

NH + CO

2

 R

2

NH

2+

+ R

2

N-COO

-

The CO2 is removed from the gas by absorption in the aMDEA solution Containing 40% aMDEA. The aMDEA solution contains an activator, which increases the mass transfer rate of CO2 from gas to the liquid Phase. The rest of the solution is water. The over all reactions occurring during the CO2 absorption process are :

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Methanation and compression

After the CO2 removal, the gas contains 0.05% CO2

and 0.3% CO (dry basis). These compounds are

poisons to the ammonia catalyst and must be removed

before the gas is taken to the synthesis section. This is

done in the methanator, where CO and CO2 react with

H2 to form CH4, which is harmless to the ammonia

catalyst. The reaction takes place over a nickel-based

catalyst. The content of CO + CO2 is reduced to less

than 5 ppm. The inlet temperature to the reactor is

300

o

C, and the outlet temperature is 322

o

C.

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Methanator has one catalyst bed loaded with PK-7R nickel based catalyst. The methanation reaction starts at 280 Deg C & causes temperature increase in the catalyst bed. The inlet temp must be controlled to ensure a sufficiently low content of CO & CO2 in the effluent gas. the catalyst should not be exposed to more than 420Deg over an extended period of time.

The synthesis gas is compressed to 177 kg/cm2g in the existing centrifugal type two-casing synthesis gas compressor. In order to match the required suction pressure of the existing synthesis gas compressor, GB601, a synthesis gas booster, is foreseen to be installed upstream GB601, increasing the pressure of the make-up gas from 25 to 39.6 kg/cm2g. The discharge stream from synthesis gas booster is chilled to 8oC with evaporating ammonia in the

process gas chiller before further compression in GB601, thereby reducing the load on the existing compressor. Any traces of impurities in the make-up gas, such as H2O and CO2, are removed in a molecular sieve installed between synthesis gas booster and GB601. In this way the ammonia synthesis catalyst is protected against poisoning by H O and CO and in addition any risk of plugging the equipment in the loop with ammonium carbamate is eliminated.

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The methanation process take place in the

Methanator (R-301), and the reaction involved

Are the reverse of the reforming reaction:

CO + 3H

2

 CH

4

+ H

2

O + heat

CO

2

+ 4H

2

 CH

4

+ 2H

2

O + heat

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AMMONIA SYNTHESIS

The Ammonia synthesis process takes place in the ammonia converter according to the following reaction scheme. 2H2 + N2 = 2NH3 + Heat. The reaction is reversible and only part of hydrogen and nitrogen is converted to ammonia when the gas passes through the catalyst bed. In ammonia converter, about 20% of the nitrogen and hydrogen is converted into ammonia. The unconverted remainder is recycled to the converter after separation of liquid ammonia product. The ammonia synthesis loop has been designed for a maximum pressure of 260kg/cm2g. The normal operating pressure would be about 187 kg/cm2g at inlet to the ammonia converter depending on the load and the catalyst activity. At reduced load, the loop pressure would decrease. Normal operating temperatures would be in the range of 370 to 510 deg C for the 1st bed, 425 to 480 deg C

for the second bed and 420 to 460 oC for the third bed in R-501. After synthesis gas has

passed through R-501, the effluent gas is to be cooled down to a temperature at which most of the ammonia condenses. A considerable amount of heat released by this reaction is utilized to produce medium pressure steam in the MP steam Boiler (E-501) & in the synthesis economizer (A-EA601 A/B) to pre-heat boiler feed water for auxiliary boilers. The mixture of synthesis gas & liquid ammonia passes from Cooled condenser to ammonia separator, in which the liquid ammonia is separated at a temp of 10 deg C. The liquid ammonia is depressurized and taken to product let down tank in which the main part of the gases dissolved in ammonia is liberated and ammonia product is drawn from the bottom

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Purge gas recovery section

A small amount of purge is taken from synthesis

section in order to maintain inert concentration

in loop. Otherwise :

a) loop pressure will increase

b) N2/H2 ratio will disturbed

c) conversion will disturbed

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Process condensate stripper

Process

condensate

from different section of

process is treated here for the impurities.

NH

3

+ H

2

O  NH

4+

+ OH

-

CO

2

+ H

2

O  H

+

+ HCO

3-

HCO

3-

 CO

32-

+ H

+

NH

3

+ HCO

3-

 NH

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Catalyst Type & Volume

Hydrogenator

• TK-250

• Co Mo based

• Vol- 4.9 m

3

Sulphur

Absorber

• HTZ-5

• ZnO based

• 10.7 m

3

Primary

Reformer

• RK-211

(2.9 m3)

• R-201

(6.7 m3)

• R-67-7h

(9.6 m3)

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Catalyst Type & Volume

Secondary

Reformer

• RKS-2P

(2.1 m3)

• RKS-207h

(15 m3)

• RKS-2 (2.1 m

3)

HT CO conv.

• SK-201-2

• 33.5 m

3

Fe/Cu/Cr

LT CO Conv.

• LSK

(3.4 m3)

• LK-821-2

(46.1 m3)

Cu/Zn/Cr

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Catalyst Type & Volume

Ammonia

Syn. Conv.

• KMIR

(8.9 M3)

• KMI

(39.4 M3)

• Iron based

Methanator

• PK-7R

• Ni> 23 wt. %

• 14.3 m

3

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Figure

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References