Hydrothermal damaging of molecular sieve and how to prevent it
Peter B. Chr. Meyer
Natural Gas Marketing Manager
CECA / ATOFINA
Paris La Défense
France
Paper presented at the Gas Processors Association Europe
February 2003, Paris
Hydrothermal damaging of molecular sieve and how to prevent it
Peter B. Chr. Meyer
Natural Gas Marketing Manager
CECA / ATOFINA
Paris La Défense
France
ABSTRACT
Natural gas treating units utilising molecular sieve technologies are usually optimised relative to the
available adsorption time and the required regeneration time. The total cycle time is usually such that
at the end of the adsorption a limited time is available for adequate regeneration of the adsorbent.
This leads in many cases to the section of bed at the inlet of the adsorber being subject immediately to
high temperatures from the start of the regeneration without any heating ramp. Water condensation
can occur in certain layers of the molecular sieve which are not heated up sufficiently causing boiling
of molecular sieves in liquid water at high temperature. The consequence might be attack of the binder
(dust formation) and hydrothermal damaging (loss of crystalline structure and “pore closure” effect of
the molecular sieves) which leads to a poor adsorption behaviour and a short life time. To prevent this
kind of damaging a good molecular sieve formulation (binder and zeolite) is necessary. Further
improvements can be done by optimising the regeneration conditions.
This article discusses with an example of an existing unit how to prevent the hydrothermal damaging
by changing the regeneration conditions based on simulations of the unit with the proprietary
simulation program SIMATEP.
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1
Hydrothermal damaging of molecular sieve and how to prevent it
Introduction
This article discusses how to make molecular sieve last longer by optimising the design package:
product and operating conditions.
In a previous paper presented on the GPA Europe conference in February 2002 [1] different causes
for damaging of molecular sieves leading to a shorter life time of the adsorbent are discussed.
Except the problem of different kind of liquid carry-over (Amines, glycols, caustic soda …) and in
consequence the chemical attack and/or the fouling of the molecular sieves, or problems due to non
perfect molecular sieve formulation (zeolite + binder) the operating conditions, especially the
regeneration conditions, may cause severe damages. Especially the hydrothermal damaging of
zeolites is discussed in this paper and how to prevent it.
1) Natural gas treating units : regeneration conditions in general
Natural gas treating units utilising molecular sieve technologies are usually optimised relative to the
available adsorption time and the required regeneration time. Compared to other MS units Natural Gas
treating units (drying or purification like H2S, CO2 or mercaptan removal) have a relatively high flow
rate of gas to be treated per adsorber. The adsorbent bed has usually a very huge diameter and a bed
height of usually not more than 2-3 times the diameter. Thus, in order to limit the pressure drop to a
reasonable value helping to economize compressor power and energy consumption. The adsorption is
usually done from top to bottom, the regeneration in the inverse direction.
The adsorption time is minimized to reduce the vessel size and the amount of adsorbent used. One of
the design limits is the time available for an adequate regeneration of the adsorbent. The time is
determined by the energy needed for the regeneration which is brought in the system by the flow rate
and the temperature of the regeneration gas. Energy is needed to heat up the vessel, the adsorbent,
the support material and to remove the adsorbed molecules. Depending on the type of molecular sieve
and the regeneration pressure, the inlet regeneration temperature can vary between 200°C and 300°C
(392°F to 572°F). Very often the regeneration gas flow rate should be minimized as the regeneration
gas is recycled to the feed of the adsorber(s) and in consequence can play an important role for the
design of the bed (quantity of adsorbent needed). As the regeneration gas is usually cooled down only
with an air cooler the water content can be significantly higher than the water content of the feed gas.
The optimisation of the energy to be put in is an optimisation between flow rate and temperature
depending on the physical and chemical resistance of the molecular sieve. While a temperature of
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300°C (572°F) for a regeneration at high pressure (far above 30 bar, 435 psi) could be possible for a
4A molecular sieve, the life time of a 3A molecular sieve would be shortened extremely using such a
temperature. In this case, the choice would be rather to use a higher flow rate and a lower
temperature.
If the available regeneration time is very short, there is no time to heat up the adsorbent by heating up
the regeneration gas with a ramp. The regeneration gas will flow in the adsorber almost immediately
with the maximum regeneration temperature.
2) Hydrothermal damaging of molecular sieve [2]
Heating up the adsorber without using a heating ramp leads to a strong temperature difference in the
vessel. At the bottom, the molecular sieve will be very hot and will desorb rapidly the adsorbed water
while the layers at the top of the adsorber will be still at adsorption temperature. The water desorbed in
the bottom layer will condense in the top layer. This phenomena is called refluxing or retro
condensation. Retro condensation in this paper does not mean condensation of hydrocarbons what
may happen for associated gas at hydrocarbon dew point. The heating going on will heat up the liquid
water and boil the molecular sieves in liquid water. Hydrothermal damaging will appear in
consequence. Liquid water means in this article the appearance of condensed water due to
oversaturation of the gas.
The binder of the zeolite might be attacked or weakened leading to dust formation or formation of
agglomerates. Usually the binder is more sensitive to chemical attacks like Amine carry-over for
example. How to make molecular sieves stand this type of attack is described in previous articles [3].
Hydrothermal damaging is different depending on the type of molecular sieve:
-
X type molecular sieves (10 A or 13X): a loss of crystalline structure occurs with a roughly
proportional loss in adsorption capacity
-
A type molecular sieve (3A, 4A, 5A): the external crystal surface is attacked resulting in a
“pore closure” effect that affects the kinetics of adsorption. Nevertheless, the substantial
adsorption capacity persists. In general, 3 A molecular sieves (potassium A type) are more
sensitive to hydrothermal damaging than 4A (sodium type) and 4A type more than 5A (calcium
A type).
The different behavior of A type zeolites, compared to X zeolites, can be demonstrated in a laboratory
with a simple test: an A type zeolite is hydrothermally degraded by steaming. By abrasion the outer
layer of crystals is removed (the layer that is supposed to be affected by "pore closure"). Tests then
show the zeolites have recovered their original adsorption kinetics.
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The higher the temperature of regeneration, the heavier the damaging of the molecular sieves. In an
industrial unit it is important, too, to limit the quantity of water appearing in liquid phase (condensing
water due to oversaturation of the gas phase) as this decreases the teperature where hydrothermal
destruction may occur with the water acting as stabilizer for intermediates formed by the dissolution of
the zeolites. [4]
The SIMATEP simulation program determines for a given unit the quantity of liquid water, the section
of the molecular sieves and the temperature when it appears for a given regeneration procedure.
3) SIMATEP Simulation program
Based on an approach of heat and mass transfer using different types of adsorption isotherms this
program has been developed in the R&D center of the TotalFinaElf-group in collaboration with French
engineering schools and universities.
The program simulates the conditions at different steps of the adsorption and regeneration process.
Information like temperature and water profile in the bed, appearance of free water can be achieved.
The following case study gives an example how to optimise a unit (prevent damaging of molecular
sieves) and which results may be obtained by SIMATEP in order to lengthen the lifetime of molecular
sieves.
4) CASE Study: Natural Gas Drying unit (Middle East)
Description of the unit:
The unit consists of three adsorbers, two adsorbers in adsorption and one in regeneration. Each
vessel contains 35 t (approx. 77 000 lbs) of 4A molecular sieve.
Flow rate per vessel 234 000 Nm3/h (approx. 210 MMSCFD)
Temperature of 19°C (67°F) and 32 bar (464 psi), saturated with water
Adsorption time 32 hours
Regeneration conditions (initial): flow rate 32 400 Nm3/h (approx. 29 MMSCFD), pressure 32 bar,
maximum heating temperature 265°C (510°F).
Heating time 450 minutes, cooling time approx 100 minutes. No intermediate heating step, almost no
heating ramp (determined by the heater).
There is almost no stand-by time during the regeneration (remaining time used for valve operation).
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The plant people noted molecular sieve damaging and a short lifetime of the installed molecular
sieves. Analysis of the installed molecular sieve showed hydrothermal destruction of the upper layers
of the adsorbers.
For replacement of the molecular sieves by CECA SILIPORITE molecular sieves the plant was
simulated with SIMATEP and different regeneration procedure where proposed and tested.
The simulation of the existing regeneration procedure gives the following results for the appearance of
water. The diagrams show the temperature in the adsorber and the quantity of water per bed volume
appearing at different bed length in function of the elapsed regeneration time.
Diagram 1: Original regeneration procedure, temperature profiles
300
Bed Temperature profiles
250
200
T°C 150
112.5
100
90
67.5
50
45
0
112.5
67.5
22.5
5.5
6.0
4.6
5.0
bed length
4.1
3.1
3.6
2.2
2.6
1.2
1.7
0.2
0.7
22.5
elapsed
time min
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Diagram 2: Original regeneration procedure, retro condensed water
1
Retrocondensed Water in the bed during heating
0.9
0.7
0.6
0.5
0.4
90
45
67.5
5.5
0
6.0
22.5
4.6
5.0
3.6
4.1
0.1
3.1
2.2
2.6
1.2
bed length
1.7
0.2
0.7
0.2
0.3
moles of water/m3 ms bed
0.8
s
elap
ed t
ime
The liquid water phenomena lasts from approximately the 20th till to the 90th minute.
In order to compare the different regeneration procedures and the improvement to prevent
hydrothermal destruction the time where the maximum peak of water appears was chosen.
The diagram below shows the temperature profile in the adsorber and the maximum quantity of liquid
water in the adsorber in function of the bed height.
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Diagram 3: Original regeneration procedure, maximum water with temperature profile
Instantaneous condensed water in the bed - minute 68
300
1.0
250
0.8
0.7
200
0.6
Liquid water
0.5
150
Temperature
0.4
100
0.3
0.2
Temperature °C
moles of liquid water/m3 of MS
0.9
50
0.1
0.0
0
0.2
0.7
1.2
1.7
2.2
2.6
3.1
3.6
4.1
4.6
5.0
5.5
6.0
bed length (m)
The instantaneous maximum water appears approximately at the 68th min of the regeneration time and
the zone where the liquid water appears is at a temperature between 170 and 200°C. The level of this
temperature is very important.
An improvement of the regeneration procedure would be to prevent the contact of a large amount of
water at a high temperature with the molecular sieve.
A regeneration procedure with an intermediate heating step and the same maximum temperature and
same regeneration flow rate was proposed. Heating plateau at 130°C (266°F) for 90 minutes before
heating up to 265°C. Total heating time 510 minutes. The temperature profiles are the following:
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Diagram 4: Intermediate regeneration procedure, temperature profile
300
Bed Temperature ramp up
250
200
T °C
150
Série7
Série6
100
50
Série4
0
b ed leng t h
elapsed
t ime min
Diagram 5: Intermediate regeneration procedure, retro condensed water
R e tr o c o n d e n s e d w a te r in th e b e d d u r in g h e a tin g
1 .0
0 .9
0 .7
0 .6
0 .5
175
150
125
75
100
50
6.0
25
5.5
5.0
0 .0
4.6
4.1
2.2
0 .1
3.6
3.1
b e d le n g th
0 .2
2.6
1.7
0.7
1.2
0.2
0 .4
0 .3
moles of water/m3 ms bed
0 .8
se
elap
e
d tim
The liquid water phenomena lasts from approximately the 25th to 150th minutes of the heating time but
the maximum instantaneous water peak is lower (0.76 moles/m3 versus 0.9 moles/m3) and the
temperature where it appears is lower, too (125 – 170 °C). This is shown on the diagram 6 below.
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Diagram 6: Intermediate regeneration procedure, maximum water with temperature profile
I n s ta n ta n e o u s c o n d e n s e d w a te r in th e b e d - m in u t e 1 2 5
0 .9
300
0 .8
250
0 .7
0 .6
200
0 .5
L iq u id w a t e r
T e m p e ra tu re
0 .4
150
0 .3
100
0 .2
0 .1
50
0 .0
0 .2
0 .7
1 .2
1 .7
2 .2
2 .6
3 .1
3 .6
4 .1
4 .6
5 .0
5 .5
- 0 .1
6 .0
0
The final regeneration procedure proposed was the following one: flow rate of 38000 Nm3/h (increase
of 18% versus original one), maximum heating temperature 250°C (decrease of 6% versus original
one) and a heating ramp at 130°C during 90 minutes. Total heating time 510 minutes.
The corresponding diagrams are attached below.
Diagram 7: Final regeneration procedure, retro condensed water
Retrocondensed water in the bed during heating
1.0
0.8
0.7
0.6
0.5
175
125
75
6.0
25
0.0
5.3
4.6
0.1
3.8
3.1
bed length
0.2
2.4
1.0
0.3
1.7
0.2
0.4
moles of water/m3 ms bed
0.9
s ed
elap
time
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Diagram 8: Final regeneration procedure, maximum water with temperature profile
Istantaneous condensed water in the bed - minute 102
200
0.6
160
140
0.4
120
Liquid water
0.3
100
Temperature
80
0.2
Temperature °C
moles of liquid water/m3 of MS
180
0.5
60
40
0.1
20
0.0
0
0.2
0.7
1.2
1.7
2.2
2.6
3.1
3.6
4.1
4.6
5.0
5.5
6.0
bed length (m)
The liquid water phenomenon lasts from approximately the 25th to 125th minute of the regeneration
time. The instantaneous water peak is lower again, 0.53 versus 0.76/0.9. The temperature where the
liquid water appears is approximately 100 - 115°C versus 125 – 170°C/170 – 200°C.
Results after the change of the regeneration conditions:
–
Peak water retro-condensation has been reduced by 42%
–
Temperature range when water retro-condensation peak occurs has been reduced by 42%
–
Water retro-condensation lasts longer but quantity and temperature range (when the
phenomena occur) have been significantly reduced
–
Due to higher gas velocity most of the droplets will be carried out of the bed – through higher
flow rate more energy is given to the system in the same time and therefore either soak
temperature or soaking time can be reduced (less thermal stress to the MS for the first option
and less overall regeneration time for the second option)
The customer recently replaced the installed molecular sieve again by CECA SILIPORITE molecular
sieve after a life time of 4 years instead of 2-3 years.
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5) Conclusion
In order to prevent hydrothermal damaging of molecular sieves it is not only important to choose the
right formulation of the molecular sieve (binder and zeolite) but the operating conditions especially the
regeneration conditions should be carefully determined. The proprietary product SIMATEP of CECA
allows these detailed studies.
6) Acknowledgments
I would like to thank you all my colleagues from CECA / ATOFINA for the critical review of this paper
and their help putting it together.
7) References cited
1.
Archiaston Musamma / Sutopo, “Eliminate the molecular sieve problem and extent the life
time to nine years”, GPA Europe conference, London Feb 21st, 2002
2.
Guido
Dona,
customer
information,
“Hydrothermal
Stability
of
Molecular
Sieves”,
http://www.atofinachemicals.com/adsorbents/sub4.cfm
3.
R. Le Bec, R. Voirin, D. Plee, S. Brunello, "New type of Molecular Sieve With longer life for
Natural Gas Drying", B 3 presentation at LNG 12, 4-7 May 1998, Perth (Australia).
4.
M. Suckow, W. Lutz, J. Kornatowski, M. Rozwadowksi and M. Wark, “Calculation of the
hydrothermal long-term stability of zeolites in gas-desulphurization and gas-drying
processes”, Gas Separation & Purification, Vol 6 No.2, 101-108, 1992.
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11