17 replies to this topic

[Ra v]

Can any one tell me...

What might be the reasons for the Liquid CO2 storage tank filled with liquid CO2 turns in Solid (ICE inside).

 

 

rgs

A Ravi

[thorium90]

The liquid CO2 in the storage tank turned into a solid and has ice inside? I hope I read that wrong

Edited by thorium90, 06 February 2013 - 09:15 AM.

[markymaark]

The air used to pressurize the vessel is not dry enough causing water vapor to freeze?

[thorium90]

one doesnt use air to maintain pressure in a liquid CO2 tank. The liquid is at equilibrium with the vapor and the pressure is dependent on the temperature only. Use more liquid and the remaining vaporizes to maintain the pressure. I dont get how ice ( i assume you mean water ice) gets into the tank?

Edited by thorium90, 06 February 2013 - 09:50 AM.

[Art Montemayor]

 

Ravi is writing about solid CO₂ (otherwise known as “Dry Ice”).

 

I have written countless posts on this subject through the years and even given several seminars on liquefied gas storage – mostly to mechanical and civil engineers.  I would have thought all chemical engineers would be up to speed on what is happening in a compressed liquefied gas storage tank.  At least I would like to think that all you guys were smart enough not to sleep through all your thermodynamic courses in university.  This is very, very basic stuff and you should know it backwards and forwards because it has latent hazards and many problems for process, project, and plant engineers.

 

First, get a hold of a CO₂ Mollier Diagram or a Temperature-Entropy Diagram.  Look at it carefully and remember all you were hopefully taught about it in university:

 

thorium90 is precisely correct.  Liquid CO₂ exists in equilibrium with its vapor inside the tank because it is in the saturated state.  Although Ravi fails to identify it, the conditions inside his tank should be approximately 250 psig and -8 ^oF.  If you look at the Mollier diagram, you will see that CO₂ has a peculiar characteristic – it has a “Triple Point” where the liquid can coexist with liquid and vapor.  This happens at 60 psia and -70 ^oF.  Below this condition you get into an equilibrium between vapor and solid dry ice.  Liquid does not exist below the triple point.  In fact, it is impossible to have liquid CO₂ at atmospheric pressure – not matter how cold you get it.  This is very important for you guys to grasp and remember because if you allow a liquid CO₂ storage tank (are you listening Ravi?) to vent its vapor pressure (either because a relief valve popped and stayed open or because some incompetent opened a vent valve and left it open, the liquid contents will convert into solid CO₂ snow inside the tank.  When the tank is vented to atmospheric pressure, the temperature inside the tank will be that of the dry ice snow (-109 ^oF).  That means the tank’s steel (usually ASTM 516 with a minimum allowable temperature of -20 ^oF) will be subjected to extreme stresses and may suffer a fracture failure during or after this exposure to such a low temperature.  This puts you, your personnel and all surroundings in a very hazardous situation.

 

The same phase change in CO₂ is seen every day when you activate a common, portable CO₂ fire extinguisher or invert one of Ravi’s filled CO₂ cylinders and open its valve:  the HP liquid CO₂ (yes, all CO₂ cylinders have liquid inside and not just gas) expands adiabatically and converts itself to dry ice and cold (-109 ^oF) vapor.  The white “smoke” you see is really water vapor that has been instantaneously condensed by the cold CO₂ vapor produced.  The reason CO₂ fire extinguishers don't have to be inverted to expand the Liquid CO₂ contents is that they have a syphon tube inside of the cylinder as part of the cylinder valve.

 

I hope this responds to Ravi’s query and also alerts him to what can happen or what has already (I hope not!) happened to his storage tank.

 

 

 

[Ra v]

Dear Art,

 

Thanks for your reply.

 

let us assume the Conditions in tank was around 320 pisg and temperature -4 Def F. then as per mollier Diagram i can see that there is no solid CO2 phase near to this. But at the same condition of tank Is the moisture carry over to the CO2 storage tank from the Drying unit is also might be one reason for Dry icing in Tank.. Am i wrong...?

 

And you are right this was happened to CO2 Condensor which was filled with Dry ice.  So i was worried that if moisture carries to storage tank will be also a problem to Storage tank.Is it necessary to check mositure content and how to check thatand how much it should be in ppm...????

 

As you know in our plant we have installation in which first CO2 gas passes through  >>KmNo4 Tower >>Heater..>> Carbon filter >> Co2 compressor >>Drying Unit>> Condensor CO2.  Is the sequence is right.????

because i have seen in some plants the sequence is like first  CO2 Compressor >> Drying unit >>Carbon filter. Why is it so... ???

 

 

And please explain the procedure for thawing of CO2 Condensor and Storgae tank if any.

what should be checked after thawing...???

 

Rgs

A Ravi

[Art Montemayor]

 

A Ravi:

 

Your tank cannot have Liquid CO₂ at the conditions you cite.  If your saturated liquid is truly at -4 ^oF, then your storage tank pressure is really 270 psig and NOT 320 psig as you state.  Something is wrong in your plant data or either your thermometer/pressure gauge is wrong.  Additionally, what is the MAWP of your storage tank?  Normally these tanks are designed for 300 psig.  I am concerned that you are exceeding the MAWP of your storage tank.  Please  answer this question as well as ALL the other previous questions that I have written in this thread – well as in all the other threads you have originated.

 

Now, you have changed your claim of having solid CO₂.  You now state: “Is the moisture carry over to the CO₂ storage tank from the Drying unit is also might be one reason for Dry icing in Tank”.  What is it that you are claiming?????  Are you reporting dry ice (solid CO₂) or WATER ICE present in your system??  These are two different "ices".  Please make up your mind and report the actual facts.

 

Water ice in liquid CO₂ is a totally different story.  That can only mean you are not operating the CO₂ adsorbent dryers properly and have a high dew point in the product CO₂ going to the CO₂ Condenser.  If that happens, you will start to clog your condenser and storage tank with super cooled water ice.  Therefore, the pressure of the system will start to go up at the CO₂ compressor’s discharge.  That is common sense.  What is also common sense is that you should have a detailed Operating Manual for this plant that clearly tells you how to operate it as well as the Dryers.  It should clearly tell you about testing the exit CO₂ from the dryers for dew point every 8 hours (at the very least).  In fact, you should have been supplied with a dew point tester together with the CO₂ plant.  Don’t you have one?  Again, I have repeatedly asked you if you were supplied all these instructions with your plant and if you are working to obtain them.  You have not responded.

 

I don’t know what CO₂ plants you have visited.  Who designed and built these plants?  Where are they located at?  Why haven’t you obtained their operating manuals and instructions?  I see no reason to have a “heater” after your permanganate scrubbers.  As I have stated in another of your threads, your plant is designed and fabricated by inexperienced and perhaps incompetent engineers who have little or no experience with CO₂ plants.  Carbon “filters” are usually used directly after your CO2 compressor if it is lubricated with oil.  This is to remove any oil before the CO₂ enters the dryers.  You should already know this, but you have not even told us if your compressor is lubricated or if it is a non-lube model.  I don’t know because you won’t answer.

 

The basic procedure for thawing out the CO2 Condenser and Storage tank is to carefully drain out ALL the contained liquid CO₂ first.  Note carefully that I said LIQUID CO₂, and not vapor.  After all the liquid is depleted, you can vent out all the vapor.  Then you slowly start to blow in hot air.  You continue this until the CO₂ Condenser and Storage tank reach ambient temperature.

 

Kindly respond and answer ALL of my questions.  I cannot continue to respond and answer your questions when you fail to reciprocate.   It is up to you.

[thorium90]

Art is right, most such tanks I've seen, people operate below 16barg. 320psig is very high. Attached a CO2 thermodynamic table for you to see. - 4F correspond to 271psig. I assume you read it from a pressure gauge and not a transmitter hence the error. Nevertheless the error is quite significant. Gauge with large range, very small dial, too many lines. Yes, people do count the lines wrongly and read the wrong pressure. Typical standards is less than 3ppm moisture but we try to go down to near 1ppm since the moisture that goes in has no way of ever coming out and therefore the moisture is cumulative. You have any portable dew point meters you can use at your plant?

Attached Files

•  co2.pdf   36.61KB   78 downloads

Edited by thorium90, 07 February 2013 - 12:55 PM.

[Ra v]

Thanks to Art & thorium90 for the response i got in this thread.

 

Art:

 

I have checked  the temperature gauge it was not working properly. but the presuure gauge was showing 320 Psig/ 24 bar .  

I have checked the Data plate on storage tank and found 
Working Pr is  19.5 Bar

Design Pr 21 Bar

Test Pr: 27 Bar

 

 

"Now, you have changed your claim of having solid CO₂.  You now state: “Is the moisture carry over to the CO₂ storage tank from the Drying unit is also might be one reason for Dry icing in Tank”.  What is it that you are claiming?????  Are you reporting dry ice (solid CO₂) or WATER ICE present in your system??  These are two different "ices".  Please make up your mind and report the actual facts."

 

Reg this i was confused between the dry ice and water ice...??? i really dont know that they are different, i used to think that ice is about only Solid CO2.

 But now i understood the difference between the water ice and solid Ice.

 

As i want to say you one most important thing regarding this plant, The supplier of this plant has not given any document  relating to plant like  the detailed operating manual, any SOPs, Work instructions and Equipment details and etc etc... i was collecting the information for the other sites and i was just trying to work hard to understand the system as well. and solve the issues in plant. Thats the reason i was not able to reply some of your threads accurately and confidently. So Please give me some time so that i can preapre myself. i will  attach the Equipment sheet that  i have prepared from the field. 

 

In congo i have visited one CO2 plant i.e supplied/designed by whittmann.i have looked the PID of the plant also first and then i visited and i saw the " Deodiser Heater"  after KMNO4 tower then CO2 Compressor, which is same as our plant  .

 

As you said i too agrees with you that "  Carbon “filters” are usually used directly after your CO2 compressor if it is lubricated with oil.  This is to remove any oil before the CO₂ enters the dryers. " But our compressor is Non Lubricated" So that means it was right.\

 

I hope you understand the problem with this plant i am facing, So please help me as you did till now.

 

 

Regards

A Ravi

 

[thorium90]

So which is it? Solid CO2 or Solid H2O? Anyway, why do you need such a high pressure in the tank? Or is it due to lousy insulation? Working pressure is 19.5bar but most people wont go too close as CO2 can boil quite alot if warmed up, say loss of vacuum suddenly, making the pressure jump. Although usage amounts will also affect the pressure.

Edited by thorium90, 07 February 2013 - 12:51 PM.

[Art Montemayor]

Ravi:

 

Now I am more concerned about your situation.  Note that your storage tank pressure gauge is probably correct and your liquid CO₂ pressure is truly:

320 psig = 22 barg

 

And your Data plate on storage tank shows:

Working Pressure = 19.5 Bar = 282.8 psig

Design Pressure = 21 Bar = 304.5 psig

Test Pressure = 27 Bar = 391.6 psig

 

As you can see, your tank is operating well above its safe limits and you must do something to stop this type of excessive pressure being imposed on it.  You are almost approaching the tank’s hydrostatic test pressure.  That is simply not permissible!

 

From your data my experience tells me that some of the following is probably has gone wrong:

1. You have accumulated air inside your liquid CO₂ storage tank; or
2. Your refrigeration unit is not working properly.

Usually – and more often than not – it is the refrigeration unit that is not performing as specified.  If you have no expertise on plant to fix that problem, then you should bring in a refrigeration expert to analyze the situation.  You may have qualified experts in Kinshasa that can address and fix any problem you may be having in not being able to liquefy the dry gaseous CO₂ at the specified pressure of approximately 250 psig (17.2 barg).  Note that I say “dry CO₂ gas”.  You must confirm that your gas is dry at the refrigeration unit.  To do that you should be taking dew point tests as I have previously stated.  Again, you failed to answer my basic question: DO YOU HAVE A DEW POINT TESTER?  ARE YOU USING IT?

 

If you don’t have a tester, then get one – immediately.  You can purchase a simple dew point cup that is operated manually, using liquid CO₂ as the refrigerant.  But just GET ONE AND USE IT.

 

You are doing the right thing by looking at a Wittemann CO₂ plant.  Wittemann knows about CO₂ and what they are doing.  However, you must know also what you are looking at and how it works.

 

[Art Montemayor]

thorium:

 

Now you are getting mixed up.  It isn't the pressure that Ravi needs.  Don't forget, he has no control over it except with his ability to liquefy the CO₂.  The conditions for the liquefaction - as you know and have previously stated - are that the liquid CO₂ is SATURATED.  Therefore, all Ravi has to do is ensure that the refrigeration unit does its job while any non-condensables are kept out of the system.  Then the operation will go very well and normal with the tank pressure at 17 barg - instead of the 22 barg.  It could be lousy insulation - which I doubt because it has operated at around 17 barg in the past (according to Ravi).  Since the CO₂ is in the saturated state, there is no "boiling".  It's either all gas or all liquid (but saturated).

[thorium90]

Ok. But I meant the boiling as a change in the vapor fraction, or in this case the amount of liquid. So the addition of heat causes more vapor and less liquid in the tank but the temperature and pressure is still the same. I was thinking the poor insulation is causing the tank contents to warm up, based on the table, 6F is about 323psig. His original question actually never mentioned about problems with the pressure, which leads me to assume he finds such pressures normal at first. Although like you said its really more likely to be a problem with refrigeration or air ingress. Actually, arent there any pressure relief devices? Im surprised you can have the pressure so high with nothing happening

 

What about the usage? Although it is not mentioned what happens to the liquid CO2 stored inside, I assume it will be used somehow, probably tanker truck filling? The removal of liquid causes some liquid to vaporize and creates some cooling in the tank and lowers the pressure. Venting some gas would also alleviate the pressure problem temporarily and provide some cooling. Not sure how much that can help though. But it can also help remove air from the tank as the air would still be in the gas. Although where could the air enter the system?

 

Ravi mentioned dry ice in the tank, but based on the table and your supplied pressures, it is likely to be water ice. Ravi, you mentioned you found ice in the tank? How? You also found dry ice at the condenser outlet? You actually opened it? Is the question simply hypothetical to find out what causes the pressure to increase in the tank? What kind of dryer do you use? Temperature swing adsorption type? How is its regeneration controlled? How old is this dryer? Is it well maintained, new adsorbent bed etc?

 

Perhaps Ravi should measure the dew point of the gas exiting the dryer and before the condenser

 

So to summarise, you opened the condenser, found something that looked like ice at the outlet, and you are worried it went into the tank, hence your original post. And you werent sure if it was water or CO2 ice at first too am I right? But along the course of the thread, it has come to the realization that the tank pressure is also much higher than normal, hence your further worries.

 

And Art suggested it could be a problem with the refrigeration unit and hence the pressure in the tank was increased. He also suggested that air containing non condensables could have entered the tank.

 

So a few more questions, how much ice did you see? Did it melt? Or did it just vaporize itself and disappear? Your condenser and all is it some kind of packaged unit? Does it come with a panel for control, including dials buttons etc? Uses some type of refrigerant?

Edited by thorium90, 08 February 2013 - 02:47 AM.

[Ra v]

Art

As i said i am new in kinshasa and learning french , because it is must to learn the language here to deal with all personels.So right now i didnt find any expert on refrigeration to solve the issue with refrigeration unit.
In our refrigeration unit , It is having a PLC but it wont work properly and no indication of temperatures pressure and flow etc etc to analyse the situation  what is happening in the process...???
 
So i spoke to my manager to get some refrigeration expert long back 2 month before.. but yet no one here. Anyhow here the operators also works based on their experience, but they dont know the process ... they look some parameter and decides based on that.
 
As we dont have a dew Point tester here .I have ordered it  too long back.And regarding  the operation of Drying unit ,It is also not proper here.The cycles are also not proper. and we dont have any tester to analyse that drying unit operation.

Thorium99

 

Actually, there are pressure relief devices, Im also  surprised why nothing happened even at 26 bar because their Set point are at 24-25 bar as said by operators here..
 
Let me tell you what happend  in  the CO2 storage tank ,It was working normally  i.e Chilling unit starts at 17 bar and stops at 14 bar of CO2 Tank Pressure ,But one day  the pressure went to 23 bar  even though chilling unit started but pressure not coming down ,so we decided to  transfer the liquid CO2 in Bottles and then the pressure went down to 21 bar , But it was not going down, We started the chilling unit but even though it was not going below 21 bar and increased upto 26 bar slowly

Then we decided to vent the vapor inside the tank by opening the top vent. and we released the vapor .

 

We didnt found any ice in the tank...But in CO2 Condensor it was the problem with watre ice, we found the ice inside CO2 condensor, as we opened the liquid CO2 outlet flange and found that water ice inside, and we have purged the system with air.

Activated Alumina with molecular seives with  Temperature swing adsorption type

How is its regeneration controlled? what does it mean:::????

1 and half yaer old is this dryer.
 

Regarding the refrigerant we use R 404 A and oil grade B 170,

I have some doubts on refrigerant too

1.Can you tell me what happens if more refrigerant flow throw the loop...

2. Oil temperature effects what happen if it low/high and how much to maintain

3. How TXV opens or controls, because we have Danfoss TES 12 fro R 404 A/ R 507

 

 

Regards

A Ravi

 

 

[thorium90]

As in the absorber is a packaged unit with its own PLC on an automatic cycle? You purged the system with air? Hmm, I'm not too familiar with refrigeration systems, but if its anything like air conditioning (since its R404a), then I suppose more refrigerant means more cooling. Im more familiar with turbines and expanders for cyrogenic purposes though

Looking through your other thread, your CO2 tank is just 5000L? Thats not very big, is it a VIE tank?

Edited by thorium90, 08 February 2013 - 09:11 AM.

[Ra v]

Thorium99

 

Yes it was automated

 

The sytem was purged with air.

 

Rgs

A Ravi

[thorium90]

normal compressed air will still contain moisture. So the dryer unit runs automatically on a timer? Not on dew point control I suppose? So it doesnt tell you the dew point? What about the regeneration temperature and cycle time? Care to define how not proper the cycles are?

Edited by thorium90, 08 February 2013 - 09:27 AM.

[Art Montemayor]

A Ravi:

 

As I have mentioned in my response on your other thread:  I believe it is time to stop scattering all these operational topics on Combustion CO₂ plant operations and consolidate the many questions you have been posting one-by one, into ONE thread.  It is getting inefficient, confusing, tiresome, and very repetitive to have so many threads on one topic: CO₂ Plant Operations.

 

You basically have a very teenie, small CO₂ plant (total capacity = 100 kg/hr) that has been designed by what I can clearly see is a company that knows little or nothing about CO₂ plants and who has tried to maximize their profit margin on the sale of the plant at your expense and troublesome operation.

 

No one who knows anything about CO₂ plants would install a PLC on such a small and simple refrigeration system.  For this size of plant you are much better off with a simple, pure manual operation using a low-cost, available refrigerant like ammonia in a simple, cast iron reciprocating compressor.  All the successful CO₂ plants I have operated and also seen out in the field have this type of design.  It is far wiser and more practical to design an ammonia system for use in the Congo where imports are hard and expensive.  Ammonia cylinders are far easier to find (& cheaper) in the Congo than R-404A.

 

You are going to have a lot of problems with what refrigeration package you have and you are going to have to make some practical modifications to it to make it perform dependably.

 

Your dryer system is probably designed by the same people and probably all screwed up.  You should have a manual operation that allows operators to switch the beds every 8 or 16 hours.  That makes it very simple, low-cost, and dependable.  Your dryer towers should be approximately 6 inches in diameter by 5 feet tall.  They should be filled with Activated Alumina H-151 (or equal) and they should be heated up to 450 ^oF during regeneration.  You should use CO₂ as the regeneration gas and you should easily obtain a dew point of -75 to -90 ^oF (which should be equal to about 2-5 ppmV of water moisture – practically nothing).  You should not have any problems with such a design.  Your regeneration heater is an electrical element.

 

If I were to operate this little plant in the Congo, I can guarantee you that I could easily obtain:

• 125 kg/hr of liquid CO₂ in the tank;
• MEA consumption would average 1.0 kg/tonne CO₂

All automation would be stripped out and I would use only manual operation of the plant.  I have done this type of design in some Latin American Plants during the 1960’s and never had one bit of trouble.  And we produced bulk liquid CO₂ delivery, cylinder filling (including fire extinguishers), and dry ice production.  We operated our plants  “24-7”, 24 hours a day, 7 days a week for 330 days a year.  Our plants ranged from 100 kg/hr to 1,000 kg/hr in capacity.

 

You can do it too, if you heed the experienced advice.