IAPG Natural Gas Congress

Molecular Sieves:is your regeneration procedure optimized?

CECA – VETEKAuthor Peter Meyer

Presenter Bob Davenport

IAPG 2008

A non optimized regeneration procedure can harm the molecular sieves and reduce significantly their life time

New unit: if the regeneration gas is recycled the recycled water content has to be taken in account, if the regeneration gas flow rate is too short the unit will not work

Knowing how to optimize the procedure can help debottlenecking a unit

Why optimize the regeneration procedure?

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cap TSA

cap PSA

Industrial units

Regeneration: how does it work?

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TSA Regeneration: how does it work?

This presentation will focus on Natural Gas Drying regenerated by Thermal Swing Adsorption (TSA).

Regeneration procedure:

1) Switch including possibly pressure change (depress.)

2) Heating (Purge? Two step heating? Heating ramp?)

3) Cooling (dry/wet gas?)

4) Switch including possibly pressure change (repress.)

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Regeneration: parameters

Heating step: how much heat?

- Heat up the molecular sieves

- Heat up the vessel (internal/external insulation)

- Remove water

- Push out desorbed water

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Regeneration: temperature influence

The quantity of regeneration gas depends on the inlet temperature during the heating. Below a minimum temperature the water dew point spec could not be reached (too high residual water content), the maximum temperature depends on the type of molecular sieve.

Regeneration temperature

Quantity of regeneration gas

Minimum!!

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Regeneration: uncomplete regeneration

A sudden decrease of adsorption time is the sign for a bad regeneration (accumulation of water) happens very often shortly after start up, possibility to recover sieves

Adsorption cycles

Adsorption time

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Regeneration: uncomplete regeneration

Make sure to have a plateau at the outlet during heating.

Make sure to have a small temperature difference between inlet and outlet during heating.

Regeneration time

Tem

per

atu

re

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Regeneration: Maximum temperature

3A: 230°C (446°F) for saturated gases, up to 260°C (500°F) for unsaturated gases

4A: normally 250°C (482°F), up to 290°C (554°F) with precautions

For information

5A/13X: 300°C (572°F) in case of sweetening, but if there is NO water on the sieves

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Regeneration: pressure influence

More regeneration gas (quantity) is needed if the regeneration pressure is at a high pressure.

Two cases for pressure range:

Low pressure – heating limited

The regeneration gas has to bring in the energy for heating and desorption (regeneration temperature above boiling temperature of water at regeneration pressure)

High pressure – stripping limited

The regeneration gas has additionally to strip off (push out) the desorbed water.

The limit between both is around 30-35 bars. Example: a regeneration at 60 bar (870 psia) may require perhaps 25% more regeneration gas quantity.

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CASE STUDY

3 adsorber system, 2 in adsorption, 1 in regeneration

500 MMSCFD, 60 bar (870 psia), 30°C (86°F),

Saturated gas, 4A molecular sieve,

Adsorption time 16 hrs,

Regeneration recycled (air cooler sat. @ 55°C (131°F), 30 bar (435 psia))

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CASE STUDY

Case 1: Regeneration at 250°C, 482°F and 30 bar, 435 psia

Case 2: Regeneration at 250°C, 482°F and 60 bar, 870 psia

Case 3: Regeneration at 290°C, 555°F and 60 bar, 870 psia

Case 4: Regeneration optimization to minimize hydrothermal damage and improve molecular sieves performance.

(Case 5: Correction of case 2 supposing only case 1 flow rate available, internal heat insulation)

Fixed: pressure drop during adsorption, no stand-by time.

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CASE 1

Case 1: Regeneration at 250°C, 482°F and 30 bar, 435 psia

Procedure

Depress 15 min

Heating

Cooling

Repress 15 min

7 hrs 30 min available

100% FP

Case 1

Vessel diameter : 100%

Adsorbent weight : 100%

100% FR1

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CASE 2

Case 2: Regeneration at 250°C, 482°F and 60 bar, 870 psia

Procedure

Depress 0 min

Heating

Cooling

Repress 0 min

8 hrs 00 min available

30 min more

Case 2

Vessel diameter : 100%

Adsorbent weight : 104%

Regengasquantity : 123%

Heater /

Compressor -

Capacity ??100% FP

115% FR1

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CASE 3

Case 3

Vessel diameter : 100%

Adsorbent weight : 102%

Regengasquantity : 115%

Case 3: Regeneration at 290°C, 482°F and 60 bar, 870 psia

Procedure

Depress 0 min

Heating

Cooling

Repress 0 min

8 hrs 00 min available

Steel: max. design

temperature ??100% FP

108% FR1

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CASE STUDY

Case 1: Regeneration at 250°C, 482°F and 30 bar, 435 psia

Case 2: Regeneration at 250°C, 482°F and 60 bar, 870 psia

Case 3: Regeneration at 290°C, 555°F and 60 bar, 870 psia (possibility to reach lower dew point)

Case 4: ???

MS Quantity Regengas

quantity

Regengas

Flow rate

CASE 1 100% 100% 100% FR1

CASE 2 104% 123% 115% FR1

CASE 3 102% 115% 108% FR1

HYDROTHERMAL DAMAGING

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End of Adsorption

0

2

4

6

8

10

12

14

16

18

quan

tité

adso

rbée

(g/1

00g)

ETAT FINAL DE LA COLONNE(ADSORPTION), Q_H2O

Hydrothermal damaging

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Start of Heating

S1

Série1

0

20

40

60

80

100

120

140

T°C

bed length

elaps

ed

time

min

Bed Temperature ramp up

Cold saturated section

Heated section

Hydrothermal damaging

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Heating proceeds ...

S 1

Série2

0

20

40

60

80

100

120

140

T ° C

bed len gt h

elapsed

time min

B ed T emper atur e r amp up

Steam Fog Formation

Cold Saturated Section

Hot Section

Hydrothermal damaging

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Water Retro-condensation

during Heating

0.2

4

0.4

8

0.7

2

0.9

6

1.2

1.4

4

1.6

8

1.9

2

2.1

6

2.4

2.6

4

2.8

8

3.1

2

3.3

6

3.6

3.8

4

4.0

8

4.3

2

4.5

6

4.8

5.0

4

5.2

8

5.5

2

5.7

6

6

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

bed length

Retrocondesed Water in the Bed during heating

S 1

Série2

0

20

40

60

80

100

120

140

T ° C

bed len gt h

elapsed

t ime

min

B ed T emper atur e r amp up

Vaporization Zone

Condensation Zone

REFLUX

Water Droplets

Crust and Lumps formation

Hydrothermal damaging

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CASE 4

Case 4: Regeneration at 290°C, 482°F and 60 bar, 870 psia

Procedure

Depress 0 min

30 min intermediate heating

Heating

Cooling

Repress 0 min

7 hrs 30 min available

Case 4

Vessel diameter : 100%

Adsorbent weight : 102%

Regengasquantity : 123%

100% FP

115% FR1

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CASE STUDY

Case 1: Regeneration at 250°C, 482°F and 30 bar, 435 psia

Case 2: Regeneration at 250°C, 482°F and 60 bar, 870 psia

Case 3: Regeneration at 290°C, 555°F and 60 bar, 870 psia

Case 4: Regeneration at 290°C, 555°F and 60 bar, 870 psia, interm. heating

Advantage of case 4: lower water dew point at outlet.

MS Quantity Regengas

Quantity

Regen flow rate

CASE 1 100% 100% 100% FR1

CASE 2 104% 123% 115% FR1

CASE 3 102% 115% 108% FR1

CASE 4 102% 123% 115% FR1

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CASE STUDY

Case 1: Regeneration at 250°C, 482°F and 30 bar, 435 psia

Case 2: Regeneration at 250°C, 482°F and 60 bar, 870 psia

Case 3: Regeneration at 290°C, 555°F and 60 bar, 870 psia

Case 4: Regeneration at 290°C, 555°F and 60 bar, 870 psia, interm. heating

Case 5: keep regeneration gas flow rate and MS height of case 2, internal heat insulation , diameter 3.5 m, increase feed flow rate, decrease adsorption time

MS Quantity Regengas

quantity

Regen flow rate

CASE 1 100% 100% 100% FR1

CASE 2 104% 123% 115% FR1

CASE 3 102% 115% 108% FR1

CASE 4 102% 123% 115% FR1

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CASE 5 (versus case 2)

Case 2: Regeneration at 250°C, 482°F and 60 bar, 870 psia

Procedure

Depress 0 min

Heating

Cooling

Repress 0 min

4 hrs 30 min available

Adsorption 9 hrs

Case 2

Vessel diameter : 94.6%

Adsorbent weight : 88%

Regengasquantity : 61%

Pressure drop 0.7 bar

(233%)

140% FP

100% FR2

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CASE STUDY

Internal insulation, 3.7m 3.5 m internal diameter

Higher feed flow rate : 140%

Higher pressure drop during adsorption : 233%

Adsorption time down from 16hrs to 9hrs (shorter life time)

MS Quantity Regengas

quantity

Regen flow rate

CASE 2 100% 100% 100% FR2

CASE 5 88% 61% 100% FR2

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How to prevent hydrothermal damaging?

Hydrothermal damaging happens when liquid water is present on the molecular sieves at high temperature:

One should try to change the regeneration procedure in order to prevent desorption of water when the molsieve bed is not yet heated up almost homogenously thus limiting water condensation at top layers

intermediate heating step + higher regeneration gas flow rate

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How to prevent hydrothermal damaging?

Inlet heating temperature

0

100

200

300

400

500

600

0 50 100 150 200 250

Heating time (min)

Reg

ener

atio

n

tem

per

atu

re (

°F)

Original

Instantanious water flow

0

1000

2000

3000

4000

5000

6000

0 50 100 150 200 250

R egener ati on ti me (mi n)

Original procedure:

- int. temperature too high, too much water desorbed

- plateau of outlet temperature showing water re-vaporization

Temperature at adsorber outlet

0

50

100

150

200

250

300

0 50 100 150 200 250

Time (min)

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How to prevent hydrothermal damaging?

New procedure:

- int. temperature lower

- smoother increase of outlet temperature

Inlet heating temperature

0

100

200

300

400

500

600

0 50 100 150 200 250

Heating time (min)

Reg

ener

atio

n

tem

per

atu

re (

°F)

Original

CECA

Instantaneous water flow

0

1000

2000

3000

4000

5000

6000

7000

0 50 100 150 200 250

Time (min)

Temperature at adsorber outlet

0

50

100

150

200

250

300

0 50 100 150 200 250

Time (min)

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How to prevent hydrothermal damaging?

Comparison for water maximum

0.00 1.00 2.00 3.00 4.00 5.00

Vessel from bottom to top

0

50

100

150

200

Original w ater

CECA w ater

Original Temperature

CECA Temperature

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Conclusion

When looking at your molecular sieve unit and its regeneration procedure:

- Don’t underestimate the pressure influence on the regeneration gas quantity

- Think about hydrothermal damaging

- Optimization does not cost a lot but lengthens the life time of the molecular sieves

Don’t hesitate to ask the nice and knowledgeable guys from CECA to help you.

THANK YOU!