High-Melting-Point Monomer Storage Outside 1

[handee]

Hi. I've been asked to look into converting a 10,000 gal underground storage tank for use with a flammable acrylic monomer with a freezing point of 16C. The tank was designed for much lower freezing liquids - this monomer will freeze in place here in New England.

Does anyone have any experience with circulating heaters or other methods for storing such materials outdoors?

Thanks!

Fran McConville
author of the Pilot Plant Real Book

http://www.pprbook.com

 

[Montemayor]

Fran:

First, I am not an advocate of employing underground storage tanks for chemicals. I believe that you will find local regulations and environmental agencies frowning on this application – if not outright prohibiting it. There are other, more practical reasons for discarding the use of underground storage tanks – especially those made of steel. Some outstanding reasons are:

1. The inspection and control of potential corrosion is an ongoing headache; one can never be 100% sure of ensuring 100% mechanical integrity, simply because of the inability to visually inspect both the inside and the outside surfaces of the tank (I’m assuming the tank is buried, not in a concrete pit; you didn’t specify which).

2. The possibility of a leak through the walls is a major environmental event; you have literally no means to contain it. All you can offer environmentalists is that you will make all efforts to monitor potential external leaks and react with immediate pump-out of the chemical to an available and ready, above-ground tank. Contamination of the earth would take place regardless of the speedy action and you require an investment in an additional ready, available, above-ground tank. And, don't forget the obvious: how do you repair a wall leak? The answer is with a very costly shutdown and vessel entry.

3. The utilization of a reliable heating system is of prime importance for an underground tank – more than for an above-ground tank. For reliability, you pretty well have to discard the use of conventional, low-pressure steam and the customary steam trap. I am confident that you would have to employ a liquid heating fluid with recirculating pump(s) and the necessary instrumentation to ensure 100% operation and reliability in order to avoid a freeze-up within the underground equipment.

4. A required tank inspection is a dreaded job simply because of the need for a confined space vessel entry. For chemical service, this is a hard reality and a task that must be done from time-to-time in order to ensure the tank’s integrity. This is bad enough for an above-ground tank; for an underground tank, this is considered a virtual NIGHTMARE. The personnel risks are compounded due to the need to ensure emergency evacuation through vertical manways that are oriented only in one direction: up and against gravity. Safety techniques to evacuate personnel through vertical openings are harder to carry out than for those in multiple, horizontal orientation.

5. The required pump(s) needed to transport the chemical almost have to vertical, submersible type. This is a special type of pump and requires that two of them be installed – in separate locations. With a conventional, above-ground tank, you can use one common suction header and conventional, centrifugal pumps.

6. The need to clean out the underground tank – to effect inspection, for example – can be a major undertaking and a headache – operationally and economically. How do you effectively drain and flush the damn thing? Obviously, you have to dedicate a lot of positive, engineered circulation of flush fluids and purging – more contaminated fluid to safely get rid of!

There are other reasons, but the above are my major concerns.

I don’t have any problems with a 16 oC freezing point fluid. I’ve succeeded in designing and modifying Phenol above-ground, API 620 storage tanks. Phenol has a freezing point of 41 oC, is an extremely hazardous material that can cause systemic poisoning, behaves like a Class IIIA combustible liquid and has a flash point of 79 °C. In spite of its multiple bad characteristics, Phenol is still handled above-ground – where it can be monitored and controlled for operability and safety reasons. I would recommend that you model your storage system in a similar manner. For a reference on Phenol, go to:

http://www.sunocochem.com/HES/tech_manuals_phenolP.htm

I realize your concerns may not relate to toxicity, but the operability and other safety concerns are mutual and common with the handling of Phenol.

I hope the above experience is of some guidance to your application. I do not apologize for this lengthy response because I strongly feel that the latent safety issues concerning the operation and maintenance of underground tanks are worth the effort.

 

[TrevorP]

I am not as familiar with underground tanks as Montemayor obviously is, but he makes some sound points. I do not wish to add to the merits or otherwise of underground tanks.
Some things I would like to add, however, from my experience of dealing with similar fluids. Firstly, if it is a monomer, it will likely polymerise. This is probably one of the biggest issues with handling this product. For this reason it is probably unlikely that you should use steam. Hot water or thermal heating fluid would be the preferred option on an external exchanger. Secondly, you clearly need to put in at least as much heat as you will lose from your system, in the worst conditions. Use the bulk monomer temperature to control the flow of your heating fluid, and the temperature of your heating fluid to control the outlet temperature of your monomer. Finally, and perhaps most importantly keep it on constant recirculation at all times (Heating or not) to prevent blockages in your pipework.

 

[handee]

Thanks for your quick responses. Let me clarify a few things. You've raised many valid points, but this site already has 6 well-engineered underground storage tanks. One of them is used very little and an option on the table is to clean it out and use it for another raw mat (the monomer in question). The other option is to use an indoor above-ground tank and design an entirely new delivery system for it.

Indeed the monomer can polymerize, and although it's stabilized, it must not be exposed to temperatures above 40C. And not only must it be prevented from freezing in the tank, but also in the exposed pipework from the tank farm to the point of use (about 100 ft, currently insulated, but not heat traced).

I'm also going to get in touch with the supplier (a small company called BASF - perhaps you've heard of them?) and find out how they handle this material. They should have some good advice. Thanks for your input an if you have any other thoughts I'm eager to hear them.

-Fran

Fran McConville
author of the Pilot Plant Real Book

http://www.pprbook.com