MeOh tank cooling 2

[imatasb]

Dear colleagues,

I've just started in a new factory and I've found a strange methodology to cool down the MeOH tanks (2000 m3). In the summer as the weather becomes very hot and during the hours of maximum sun radiation the open the water against fire system to cool them down.

I've found this a very strange thing.

The final objective is to avoid pressure increment due to MeOH vaporization. For me the good practice would be to install a condenser that will condense all the vapors.

Do any of you use the water system (with huge quantities of spent water) or do you use the condenser or any alternative methods

Thanks.

 

[Montemayor]

imatasb:

These are rather large (528,000 gal) MeOH storage tanks and the problem must be an important item for their proper operation and for fugitive emissions.

However, MeOH vapor pressure at "ambient" (70 oF) is a benign 100 mmHg. The normal boiling point (vapor pressure = 760 mmHg) is 149 oF (65 oC). I can't imagine that your storage site reaches 65 oC in the Summer - is this correct? Therefore, your storage tanks (I presume they are minimum quality equal to API 650) should easily be able to store and withstand the MeOH vapor pressure. In fact, I would have designed a Nitrogen blanket system on them, since they obviously represent a nice chunk of investment and deserve a better and environment-friendly design. Unless you have information that you haven't shared with us that shows your tanks can't take approximately 3-5 psig storage pressure, I don't see a problem in simply containing the contents.

Of course, I'm assuming a lot of things here, since you haven't given any basic data outside of the fluid's identity. I assume, for example, that you are using API 650 tanks; that you have a formal engineering design that incorporates tank relief devices for both pressure and vacuum and that your fluid temperatures are controlled and monitored. In short, I assume you've engineered the installation to be safe and dependable.

I suspect there is other information that we don't know, but I would never allow the tanks to be drenched using the fire water system. There is no need for this unless other things outside the scope of operations are happening - and then, I can see a potential hazardous situation. Fire water usage jeopardizes the Fire Water system: that is not its design scope nor intent. It also corrodes the tanks and weakens their soil foundations, causing structural problems. It is a very inefficient way to cool MeOh. I could have much better cooling with a much smaller recirculation external heat exchanger. But, as I said before, I doubt that you require the cooling; the vapor pressure of MeOH reveals that you do not need it. Besides the vapor pressure data, I can speak with authoritative knowledge since I've stored and used MeOh in that same method and I never required any storage cooling in the hot, 110 oF Texas sun. And I always employ Nitrogen blanketing on MeOH. I certainly would not resort to using a MeOH "condenser" - there simply is not enough vapor to condense, and a liquid-liquid cooler is much more efficient and responsive - besides being much less costly to install and operate.

So you must have another problem. Perhaps you have non-condensables collecting in your tank and raising your pressure. This would give a false impression of MeOH "boiling" - which is not the case with the temperature less than 65 oF!

I expect your reply and possible some bits of basic data to reinforce your observations. In the meantime, I hope this information helps.

Art Montemayor
Spring, TX

 

[sshep]

When I first saw your post I also thought that pump-around through an external cooler would be your most cost effective mitigation for bulk tank cooling, as per Montemayor's suggestion. If the spray is being done mainly to reduce outbreathing of methanol vapor for environmental reasons, you should consider an internal floating roof. If being done to mitigate pressure (as you indicate) then your breather vent is undersized and you will someday get first hand experience in popping the roof seam, or (more likely) sucking the tank in.

The external spray probably started as a temporary or desperation solution for something (not sure what), and has became permanent because your management has failed to give the long term solution the proper attention and resources. No reasonable engineer would envision this to be a long term solution for all the reasons Montemayor cited.

 

[aspearin1]

If I can piece the bits of information together, are you saying that a fire hydrant is opened up and outdoor MeOH tanks are bathed in water? If radiant heat is the big issue, give the tanks some shade. It sounds to me more like a seasonal dilution ritual, rather than cooling of the tanks. Is there history of a spill there?

aspearin1

 

[Montemayor]

aspearin1:

With liquid Methanol, whose vapor pressure doesn't get to atmospheric until it is heated to 149 oF (65 oC), how do you justify that radiant heat could be an issue here? I don't believe the noonday sun is going to heat the tank to 65 oC - even in Saudi Arabia. And even if it did, the normal, atmospheric API 650 tank can take 3-5 psig easily (160 oF saturation). There is no ritual or heat up involved with this tank that can be justified by radiant sunlight. Something else, I'm afraid, is affecting the operation and forcing some management person to react in an ignorant manner. Whoever has the firewater turned on the tank is obviously unaware of what is happening -- or possibly someone is forcing 150+ oF MeOH into the tank and trying to condense & subcool it there with the firewater spray. This is just a bad operation and leaves me with a feeling that it is an accident waiting to happen. This tank doesn't need shade; it needs a knowledgeable process engineer to take command of the situation and resolve it. Perhaps this is what imatasb is trying to do. Let's wait and find out...

I'm very concerned about this thread because of the scarcity of basic data on such a potentially hazardous material. If we indeed are dealing with free, vaporizing Methanol within in a storage agea, then this immediately raises the spectre of a Methanol Fire - and this is a serious and dangerous possibility. Engineers in Australia may recall a huge Methanol fire in a storage tank that occurred late last year, I believe. No one was killed, but it was a miracle when you see the video tape of the incident. Methanol burns with ferocity and is difficult to extinguish.

Art Montemayor
Spring, TX

 

[aspearin1]

Do you believe that an closed vehicle in the desert heat can reach a temperature of 140degF? If it's 110-120 degF outside, the combination of radiant heat most certainly can drive it over the top... This is how babys and dogs fry in the summer, when left with the windows closed. This said... my intention was not in insist on the possibility, but rather to question the plausibility of the current practice. Why bathe a tank in cool water? Why? Why? Why? The very idea seems absurd... You're absolutely right. We need more information. It just doesn't add up.

imatasb,

Is this fresh MeOH from a supplier, or recycled MeOH from an in-house process? If there's an organic acid (formic, acetic) present in the mixture, even in very small quantities, then you have an equilibrium reaction occurring (esterification) and you can expect quite a high vapor pressure in that tank, especially at elevated temperatures. Please clarify your process, if you can.

aspearin1

 

[imatasb]

Dear friends,

Hi again, I've read all your answers and I want to add a few details about the question.

First of all thanks to Montemayor for his well reasoned answer. The fact is that I'm nort working phisically at the plant I talked about (it was just to avoid a large explanation). I'm working at the headquarters of a group. The factory I've talked about belongs to this group. Yesterday I was talking with a operator from this factory to get some info and in a moment of the conversation he explained that.

At the moment I compilating all the info about these tanks but at the moment I can state that:

1. They have relief valves (for pressure and vacuum)
2. The tanks are constructed according API 650.

I want to point these things just to avoid the direction that the thread is taking.

Anyway I know that is not a good practice. What I wanted to know were the systems that are used normally in case that there is need of cooling.

Apart from that I supposse that you use a scrubber not to throw the air saturated with MeOH to the atmosphere (i'm the moment I'm getting this info also).

At the moment what I can not assure is whether they have a blanketing system or not.

I will reply again when I have all the info.

 

[25362]

I think inerting is a "sine qua non" condition, since at the normal operating temperatures the vapour pressure would bring methanol vapours well within the flammability limits of 6 to 36% in air. In the chemical industry it is common practice to provide inert gas blanketing whenever the flammable or combustible liquid temperatures may be above or within 20[sup]o[/sup]F of their flash point.

I may venture a reason for the use of water deluge on external heating. It may not be the vapour pressure of the methanol, on its heating, as Art Montemayor aptly addressed, that prompted the use of the firefighting water hydrants, but the "quick" pressure development due to gas expansion on heating in case the breathers or vents are suspected to be ineffective.

In any case, it would be of much interest to read imatasb clarifications.

 

[sshep]

I would like to add that even with all the concerns raised, the economics for reducing the working and breathing losses for this system could be very significant. Reducing these losses could easily either pay for or help justify such mitigations as a cooling loop or floating roof. It is well worth the effort to calculate how many dollars are being evaporated away.

 

[tblaser]

Montemayor:

I agree with the nitrogen blanket. It is a good idea used frequently, employing a relief device vented to a safe location.

Off the immediate subject: Forgive me if I am looking at old codes, but I read in the forward of API 650 that the maximum design pressure of API 650 tanks is 2.5 psig, and to achieve that design level requires Appx. F requirements. Or is the 3 to 5 psig you mention, derived from the voice of experience with what can typically be done with these style tanks?

Thanks,

 

[Montemayor]

tblaser:

Thanks for bringing up a very important point. I don't think you're looking at old codes; I stand corrected for a mistake in not proofing what I write. I am glad you caught it and not a certified expert like M.J. Cronin, one of the real sharp, tank guys on this fora. He probably would have justifiably nailed me for the error. To paraphrase something he has stated in the past:
The major differences between the API 650 and API 620 rules are as follows: 1)For tanks with flat bottoms and operating pressures less than 2.5 psig, API 650 is commonly used as the design code. 2)Tanks that have operating pressures between 2.5 and 15 psig or tanks in refrigerated service use API 620 as the design code.

What I was trying to communicate was that you can employ the inherent, above atmospheric MAWP available in the API 650 design (or better, API 620) for blanketting service. I've had, in past, the experience of finding my storage tanks "rated" as atmospheric, but upon examining them I found the floor, shell and roof all capable of falling under API 650 or better. Some came up to API 620 standards. This is so because in the process industry, it isn't rational or practical to use the minimum API standards like 1/4" or 3/16" wall. At these thicknesses, a good welder will produce an undesirable and weakened "puckered" or warped shell or floor. The roof will come out similarly. In order to have good penetration and weld strength while maintaining robust and mechanical integrity in the final product, one has to employ 3/8"+ steel plates. As I worked as boilermaker through college, I have always employ 3/8" as my minimum, practical limit on butt welding plate - and I always butt weld instead of lap. This, plus corrosion allowance is what gives a thicker shell, floor, and roof.

As I have come across these existing, strong storage tanks, I have had them re-rated and classified as API 650 or API 620 by an experienced mechanical engineer. I've even resorted to standardizing on using anchor lugs to assist in wind loading and avoiding uplift when N2 blanketting is employed. Prior to this, many tanks were devoid of anchor bolts. They simply relied on their weight to keep them "anchored". Re-rating to API 650 or 620 yields a very flexible, reliable design and a great solution in the process industry where we have had to confront stricter and stricter emission regulations - forcing us into installing blanketting and/or control devices on chemical storage tanks - especially Methanol.

I didn't want to turn this into a long post, but I feel that although basic and simple, the subject is a serious and important one that shouldn't be treated lightly.
Thanks again.

Art Montemayor
Spring, TX