Condensing methane by nitrogen vaporization

[dMemphys]

Hello everyone!

I'm currently working in cryotechnics and would need a new liquid methane line. We're investing the possibility to use our stock of Liquid Nitrogen to liquefy Methane coming in bottles (180bars). [Note: this project is temporary, so we'd like to go as low-cost as possible]

Thus I'm trying to design a heat exchanger that would vaporize nitrogen on one side and liquefy methane on the other. I'm currently going for a Shell&Tubes 'Kettle-type' with Nitrogen being Shell side. But I've got very little experience with two-phase flows in HX's...

Would anyone have a few tips for me, or book references?

Thank you in advance for your help!

Denis

 

[Montemayor]

Denis:

I believe what you are proposing to do is impractical and the wrong way to approach the problem.

You don’t reveal all of your scope of work, but you tell us enough for me to arrive at that opinion. Although you don’t tell us the flow rates involved, I have to assume they are relatively small – as compared to industrial production rates. But you do tell us that us that this is “temporary, so we'd like to go as low-cost as possible”. That tells me the flow rates are low, and not meant for fully-rated industrial production especially since you are using compressed methane cylinders.

You should steer away from the concept of applying a TEMA-type heat exchanger – especially a BKU (or any other type of kettle). In cryogenics applications the name of the game is to always supply abundant expansion capability – due to the dramatic and huge temperature changes occurring during warm up and shutdowns. Cryogenic applications call for another type of construction – one that allows free expansion and is hermetically sealed.

I would recommend you apply a simple coil-within-sealed tank design. It is the simplest, the lowest cost, and fastest to fabricate. I would introduce the methane gas at the top and allow it to condense inside the coil(s) and drop by gravity to the bottom outlet. The sealed shell would contain a positive level of liquid nitrogen that vaporizes. This design complies with total flexibility and hermetic construction to give you a safe operation. Unless there are other factors you haven’t mentioned, that should suffice. You can pack a lot of surface area in coils. Copper coils should do just fine. I've used them before.

 

[dMemphys]

Hello!

And thank you for your answer!

The flow rates are, indeed, relatively low (max: 50gr/s). And I agree I didn't really say much about the work we're used to do. In fact, we usually have aerospace contracts and are required to test joints or bearings for oxygen pumps and other critical pieces. For those tests, we are supplied by refrigerated LN2 and LOx tanks. Future tests might require the use of Liquid Methane, hence our little investigation on CH4 liquefaction and its comparison with the cost of a liquid methane tank.

But anyway, thanks for the advice, I actually makes a lot of sense! I'll look into it.

Denis