6 replies to this topic

[Root]

Dear Forum members,

It is common subject and reaptedly asked by many forum members but every one ask about his on problem. The problem we are facing in our LPG/NGL plant is we are using 3A Axens adsorbent for gas dehydration, after three month running plant moisture start break through. Plant is designed for 30 mmscfd but we are operating at 40% load. Dehydration inlet pressure is 62 barg and regeneration time 36 barg.

following actions were taken but still problem persist,

1. Regen gas flow increased to 3 mmscfd.
2. Regen gas tempe increased to 260^0C.
3. Heating time increaed to 6 hrs.
4. Cooling time increased to 5 hrs.

The wells are waxy in nature and during this winter temperature  remained 15^0C.

Hope some one explain and suggest some operational changes.

Thanks

Root

[Art Montemayor]

Root:

 

We need specific data if we are to help you.

1. What is the composition of the gas you are drying?  Is it essentially natural gas (methane)?  Or is it a heavier hydrocarbon - propane?  butane?
2. Are you using a Molecular Sieve adsorbent?
3. What do you call " moisture start break through"?  In other words, what are your product specifications on water content and what water content are you now having?  Basically, you need to tell us the Design Criteria for your adsorption unit.
4. We need to have a P&ID of your adsorption unit,  If you don't have one, draw a detailed sketch to show the piping, valves, and instrumentation.  This is needed to identify if you might have a valve sealing failure.
5. Molecular Sieve units are normally regenerated using dry product gas at approximately 500 ^oF (260 ^oC) or recirculated wet gas at 600 ^oF (315 ^oC).  Which regen system do you use?
6. Are you using an electric heater or a fired heater?
7. What is the quantity of Mol Sieve used in each vessel?  (How many vessels do you use?)
8. Has the installed Mol Sieve adsorbent only been used in drying service for 3 months and then started to fail drying?
9. When was the adsorbent last inspected by taking out samples?
10. Who designed and fabricated the drying unit and how long has the unit been in operation in this plant?  Has it ever failed to dry the gas to specifications before?
11. What is your temperature and pressure base for defining your "standard" gas condition?

Await your reply.

[ankur2061]

Root,

 

Refer the link below and the attachment with the post #2

 

http://www.cheresour...-rating-method/

 

Regards,

Ankur.

[Root]

Hi Art,

Document attached as you asked.

Thanks

Root

Attached Files

•  MOL Sieve.pdf   291.48KB   41 downloads

Edited by Root, 02 March 2016 - 06:21 PM.

[Root]

Root:

 

We need specific data if we are to help you.

1. What is the composition of the gas you are drying?  Is it essentially natural gas (methane)?  Or is it a heavier hydrocarbon - propane?  butane?  Natural gas consist C1 to C6
2. Are you using a Molecular Sieve adsorbent? Yes
3. What do you call " moisture start break through"?  In other words, what are your product specifications on water content and what water content are you now having?  Basically, you need to tell us the Design Criteria for your adsorption unit. Cryogenic unit requirement is less than 0.1 ppm and end of adsortion cycle it is going to more than 8 ppm
4. We need to have a P&ID of your adsorption unit,  If you don't have one, draw a detailed sketch to show the piping, valves, and instrumentation.  This is needed to identify if you might have a valve sealing failure. Design basis and P&ID attached.
5. Molecular Sieve units are normally regenerated using dry product gas at approximately 500 ^oF (260 ^oC) (Axen give us 240⁰C )or recirculated wet gas at 600 ^oF (315 ^oC).  Which regen system do you use?
6. Are you using an electric heater or a fired heater? Combination of hot oil heater and electric heater.
7. What is the quantity of Mol Sieve used in each vessel?  (How many vessels do you use?)
8. Has the installed Mol Sieve adsorbent only been used in drying service for 3 months and then started to fail drying? yes only for drying.
9. When was the adsorbent last inspected by taking out samples? not yet, problem started since one week before.
10. Who designed and fabricated the drying unit and how long has the unit been in operation in this plant?  Has it ever failed to dry the gas to specifications before? Exterren
11. What is your temperature and pressure base for defining your "standard" gas condition? please elaborate more this point.

Await your reply.

[Art Montemayor]

Root:

 

Thank you for the attached data sheets and the P&ID on the Exterran Adsorption dryer package.  This is the type of information that our members really need.  Unfortunately, the .pdf copies are not very good.  One data sheet is scanned in reverse and the P&ID is almost unreadable because it has been reduced and scanned at a very small size.  All I can make out is that there seem to be 2 adsorber towers.  The instrumentation and valving cannot be distinguished for type and labels.   We need copies that are totally readable and legible.  Check out your own attachments and you will see what I mean.

 

If you have unsaturated hydrocarbons (like benzene, ethylene, etc.) in your moist feed, they might crack and form tars or gums when heated to the necessary regeneration temperature.  I seriously doubt you can dry your gas down to 0.1 ppm(V) of water content.  I doubt if you can measure that low of a water content.  I also doubt it because your regeneration temperature of 240 ^oC is considered too low for a mol sieve, in my opinion.  Nevertheless, without clear and legible information, what I can offer you from experience is the following:

• Your mol sieve adsorbent may be plugged with impurities from the feed gas - such as waxes, gums, solids, or water droplets;
• One of more of your block valves isolating the two beds may be leaking at the valve seat and allowing wet feed gas to leak into the lower pressure regen gas side;
• Your beds may be "powdered", where the adsorbent has generated "dust" due to attrition caused by water droplets or bed movement;
• You may not be circulating sufficient regen gas to regenerate the entire spent bed.
• There are other possibilities, but with information hard to read or interpret, I can't mention more.

Your data sheets seem to state that you regenerate with dry product gas.  That means that you are recirculating dry product gas through the heater(s) and sending it to the spent bed to be regenerated.  After heating and cooling the bed, this recirculated regen gas is recycled back into the raw, moist feed stream.  However, in order to get back into the feed stream, the recycled regen gas must be boosted in pressure exceeding the feed stream's.  Are you using a compressor?  What is the pressure inside the bed being regenerated?  The lower this pressure is, the easier and faster the regeneration.

 

I would use as high a regen temperature as my equipment will safely tolerate.  That is why I have in the past always designed my adsorbers to heat the regen gas to 600 oF where possible. Exterran is a reputable firm and I have done business with them in the past.  They should be able to tell you the maximum allowable temperature you can apply to the regeneration gas stream and what you can do to remedy the situation you now have.

 

When any engineer uses the term "MMScfd", he/she should always state what is the base temperature and pressure referred to with the use of term "Standard".  There are many definitions for the term "standard cubic feet".  The GPSA uses 60 ^oF and 14.696 psia as their definition of the base conditions.  That is why I ask for what is used in your design.  If Exterran haven't noted the base condition in their drawings and calculations it is because, like a lot of engineers, they are being lazy and sloppy.

[Root]

Root:

 

Thank you for the attached data sheets and the P&ID on the Exterran Adsorption dryer package.  This is the type of information that our members really need.  Unfortunately, the .pdf copies are not very good.  One data sheet is scanned in reverse and the P&ID is almost unreadable because it has been reduced and scanned at a very small size.  All I can make out is that there seem to be 2 adsorber towers.  The instrumentation and valving cannot be distinguished for type and labels.   We need copies that are totally readable and legible.  Check out your own attachments and you will see what I mean.

 

If you have unsaturated hydrocarbons (like benzene, ethylene, etc.) in your moist feed, they might crack and form tars or gums when heated to the necessary regeneration temperature.  I seriously doubt you can dry your gas down to 0.1 ppm(V) of water content.  I doubt if you can measure that low of a water content.  I also doubt it because your regeneration temperature of 240 ^oC is considered too low for a mol sieve, in my opinion.  Nevertheless, without clear and legible information, what I can offer you from experience is the following: Exterran give 240⁰C value based on design conditions.

• Your mol sieve adsorbent may be plugged with impurities from the feed gas - such as waxes, gums, solids, or water droplets; Please elaborate how and why this happened.
• One of more of your block valves isolating the two beds may be leaking at the valve seat and allowing wet feed gas to leak into the lower pressure regen gas side;No valve is passing because moisture is going up after 6 hrs of change over (6 hrs in adsorption).
• Your beds may be "powdered", where the adsorbent has generated "dust" due to attrition caused by water droplets or bed movement; Bed are still good as no temperature shoot up recorded and bed out let dust filters delta pressure is not gone up.
• You may not be circulating sufficient regen gas to regenerate the entire spent bed.We are maintaining design flow 3.6 MMSCFD even plant is running on 40% load while vendor suggested only 1.2 MMSCFD.
• There are other possibilities, but with information hard to read or interpret, I can't mention more.​What are these possibilities.

Your data sheets seem to state that you regenerate with dry product gas.(yes regeneration gas is residual gas, from low temperature separator out. After regeneration gas is being injected in sale gas line. )   That means that you are recirculating dry product gas through the heater(s) and sending it to the spent bed to be regenerated.  After heating and cooling the bed, this recirculated regen gas is recycled back into the raw, moist feed stream.  However, in order to get back into the feed stream, the recycled regen gas must be boosted in pressure exceeding the feed stream's.  Are you using a compressor?  What is the pressure inside the bed being regenerated?  The lower this pressure is, the easier and faster the regeneration.

 

I would used as high a regen temperature as my equipment will safely tolerate.  That is why I have in the past always designed my adsorbers to heat the regen gas to 600 oF where possible. Exterran is a reputable firm and I have done business with them in the past.  They should be able to tell you the maximum allowable temperature you can apply to the regeneration gas stream and what you can do to remedy the situation you now have.

 

When any engineer uses the term "MMScfd", he/she should always state what is the base temperature and pressure referred to with the use of term "Standard"( STP is 60⁰C and 14.7 psi for this project).  There are many definitions for the term "standard cubic feet".  The GPSA uses 60 ^oF and 14.696 psia as their definition of the base conditions.  That is why I ask for what is used in your design.  If Exterran haven't noted the base condition in their drawings and calculations it is because, like a lot of engineers, they are being lazy and sloppy.

Hope you can further explain to trouble shoot moisture break problem further.