Propane Vaporization Help

[colemaj1]

I will have a propane burner that burns vapor at a max rate of 140 MMBtu/hr. The Application is a large grain dryer which only operates in summer in the south when lowest ambient temp is around 70F at night and much higher during the day (90-100F). I'm looking at two options: the first is going with a 30,000 gallon tank and a vaporizer, and the second option is getting multiple 30,000 gallon tanks and relying on ambient air to do the vaporization. To figure the cost difference in the two options, I need figure out how many 30,000 gallon tanks I would need to do the required vaporization. I found a spreadsheet online that helped me calculate the tank vaporization using ambient temp, tank dimensions, and percent fill. The sheet only went up to 20F for ambient, but I interpolated up to 70F since it looks like a linear relation. For a 30,000 gal tank with a length of 792" and diameter of 108", at a 60% fill, at 70F ambient, I find that the tank can vaporize approximately 42.8 MMBtu/hr. Assuming I keep my tanks topped off at least 60% full, I could get by with (4) 30,000 tanks.

Option 1:
30,000 tank cost = $45,000
Water bath vaporizer cost = $100,000
total = $145,000

Option 2:
(4) 30,000 tanks = $180,000

So, in option 2, for $35,000 I could do the same vaporization with no moving parts, maintenance issues, or additional energy input plus gain 90,000 gallons of additional storage. I'm thinking about going with option 2. Can anyone punch some holes in this logic for me? Also, I know code say we can't paint our tanks black, but it would seem like that would help vaporization and could decrease tank count. Perhaps paint it black and have a shade that is in place when we aren't pulling from it. I've also thought of other creative methods to decrease tank count, such as bathing the tanks with water from a nearby pond that gets as hot as bathwater in August, or solar water heaters. It has to work on cloudy days and at night though! Any other ideas?

John

 

[TD2K]

That's about 25 gpm of propane, a good sized flow and a fair bit of latent heat to account for.

What is the pressure relief set at the tank for? That would be the main driver whether you want to try coloring the tank black for example and someone doesn't move it into shade for example. Also, are you renting these tanks or buying them?

The pond is an interesting idea, are the ponds large enough to provide the heat you'll need to vaporize this much propane? Can you just back the tanks into the pond so they are partially submerged (you need to be able to get them out later of course)? You'll get a lot better heat transfer water to tank than air to tank. However, a rental pump with a distributor header to distribute water over the top of the tank wouldn't be that difficult to fabricate either and give you much the same effect.

What sort of propane pressure do you need for the burner? I'm thinking 10 to 25 psig would be ample and that corresponds to a pretty low temperature in the tank or are you constrained to a higher minimum pressure because of a minimum design temperature. I'd be interested in seeing what the basis behind the spreadsheet calculations are.

Water vaporization systems I've seen have been pretty low maintenance, I'm not sure I'd be all that worried about it.

Final thought. Can you get a single tank and burner and do a field check to see what a tank would supply? You could then try the water spray idea and see how it works?

 

[colemaj1]

I'll have to check on the pressure relief. I'm pretty sure the tanks are rated at 250psi. We will be purchasing the tanks. My burner calls for 12-15 psi.

I guess the whole idea is: why spend a whole of lot of money on vaporizers if I can do the same thing with more tanks at about the same price and gain a whole lot of storage capacity - of course this only works since I'm running during the summer. Wouldn't work if we had to run during the winter, which is ok though.

John

 

[Compositepro]

The flaw in your logic is that you if the extra tanks are needed for vaporization you cannot use their storage capacity. They have to stay full (or 60%). That just makes them expensive vaporizers with unusable propane inventory.

If your vaporizer costs $100,000 you are using the wrong equipment or vendor. Hot air balloons have vaporizers that are simply a coil of tubing around the burner flame. I would expect this to be pretty standard equipment for large burners.

 

[colemaj1]

Good point CompositePro - thanks for your input. My thoughts on the increased storage capacity was that it wouldn't be taken advantage of so much for the grain dryer during the summer, but afterwards in the wintertime we would have the option of getting into the retail propane business. We would be dealing with liquid and could empty out the tanks.

Trust me, large vaporizers are pricey. They are a few brands that can handle the truckloads/day volumes I'm talking about - algas, alternate energy, etc. if you experience in other brands that are less expensive please feel to mention them - ill take a look.

 

[Compositepro]

http://www.algas-sdi.com/pdfs/Algas-SDI_DF_Brochure 0312.pdf

 

[colemaj1]

Yes I've priced algas.

 

[colemaj1]

So is there anyone else out there who is familiar with propane systems who can chime in on this?

The main question is: can I substitute either more tanks or ambient summer heat in lieu of off-the-shelf vaporizers.

Related questions:

Is my calculation of how much a 30,000 tank will vaporize at 70F ambient correct?
Are there creative ways (off the shelf or not) to warm the tank so that it vaporizes more than at just 70F ambient, which will decrease my tank count?

 

[ash9144]

Another possibility is to feed liquid from the tank and use fin exchanger to vaporize. Similar to what is used on nitrogen systems.

 

[colemaj1]

Hey ash9144, I was thinking the same thing. I ran that trap and talked with the sales guy from Thermax who makes those fin systems, but they weren't real excited about it.

 

[Latexman]

Why not?

Good luck,
Latexman

 

[ash9144]

So what if the sales guy said no. Ask another one or ask their application eng. Or design it yourself. I don't think it would be an overly complex design.

 

[colemaj1]

I'm not sure why the guys Thermax weren't excited about it.

I've done some more research on this subject and now the central question seems to be what's the vaporization capacity of a 30,000 gal tank @ 70F and 60% filled. Most rule-of-thumb charts I've seen online are from the ASME and only go up to 20F. I interpolated and came up with 42.7MBtu for 70F which is where I'm basing on how many tanks I would need to obtain the amount of vaporization. I'm now questioning my logic, because I had a local propane guy say that the charts only go to 20F, because you hit a wall on how much the tank can vaporize, and that I must buy vaporizers with my burner capacity. My answer to him was that the charts only go up that far because most people in North America use their propane tanks all year long and 20F is a good place to stop the charts. So can anyone shed some light on this? How do we take the charts out to 70F or higher temps? I've attached an example chart so you can see what I'm talking about.

 

[SNORGY]

Have you considered putting a thermal blanket on the tanks and then heating the shell of the tank using a slipstream of propane to a space heater under the blanket? I used to buy a lot of those kinds of things for applications up north, thereby dispensing with vapourizers altogether.