Vaporization Propane without LPG vaporizer 2

[Lijantropo]

Good Morning.

I'd like to know how I can estimate the rate of vaporization without a vaporizer or without applying the heat directly to the tank.

This is the case:

I have a propane tank that storage saturated propane. When the regulator let de vapour above the liquid to go out, the system lost the equilibrium and part of the liquid have to vaporize to reach the equilibrium again. The liquid takes the energy from itself and so, the temperature inside the tank drops.

my question:
how i can size the tank to let the system vaporize as many liquid as vapour is going out (without any external device)?

 

[TD2K]

Your propane consumption sets the rate liquid propane has to vaporize to replace what you are using under steady state conditions. That flow rate, along with the latent heat of vaporization sets the duty transferred from the outside air (I'm assuming the tank is outside) into the liquid. If the amount of heat from the atmosphere isn't sufficient to maintain the vaporization rate and you are drawing off more propane than can be replaced, the pressure in the tank will decrease. That will reduce the temperature of the liquid propane and increasing the heat transfer from the air since you now have a larger dT. That can only go so far depending on what minimum propane pressure you need.

You have several variables to consider:
1. As you use propane the amount of liquid decreases. Your heat transfer I suspect is primarily through the wetted area of the tank so as the tank level decreases your heat transfer may becomed limited. You may need to have external finned piping to ensure your heat transfer area.
2. The 'minimum' hot side temperature you want to design for
3. The minimum pressure and thus temperature you can accept the propane vapor at.

Natural convection is going to transfer heat from the ambient air to the heating surface.

The same concept is used for vaporizing liquid N2 for plants where no external heating medium or energy is used, ambient air is used with finned pipes to vaporize the liquid nitrogen.

 

[Lijantropo]

Thank you TD2K for your answer.

I had read that the area of the tank must be large enough to vaporize. I thought that area was the interface area (the area between the liquid and its vapor)and that is why i thought the energy was taken from the liquid itself and later compensated from the ambient (it is the same process but different order). In this case, the rate of vaporization depends only of the velocity to achieve the equilibrium (the amount of vapor leaving the tank) and the convection heat transfer will set the temperature inside the tank.

What do you think about it?

----
My consumption isn't continuous, so the vaporization is just for a few minutes (five or six). That is why I don't want use any kinf of external advice for the vaporization, but I am going to evaluate the use of fins .

 

[TD2K]

The interface area isn't an issue for what you are concerned with, it's the flow of energy from the surrounding environment.

Let's say you had an equilibrium vapor/liquid at some pressure and temperature. You suddenly draw out vapor and drop the vapor space pressure to half of its original pressure. The liquid is still at its original temperature and its vapor pressure is now greater than the pressure in the vapor space. Some of the liquid vaporizes using its own internal enthalphy causing the liquid's temperature to drop. Enough liquid vaporizes until the liquid's vapor pressure and the pressure in the vapor space are the same.

 

[Compositepro]

The important factor is what pressure you require. As you consume propane The temperature of the liquid propane will fall due to the heat of vaporization. This will drop the pressure in the tank according to the vapor pressure curve of propane. As the liquid temperature drops the rate at which heat is absorbed from the ambient air increases. The liquid temperature where propane has zero gage pressure (15 psia) is -40F. Heat transfer is from the outside of the tank to the liquid wetted area inside.

 

[Lijantropo]

Thanks TD2K and Compositepro for your answer.

 

[thalathb]

Why dont you design the tank as per the normal consumption, and let the vapour vent up thorught compressor , then gets condensation in the way back to tank as liqued again.

 

[TD2K]

And that would do what?

He wants to ensure a sufficient supply of propane vapor.

 

[Lijantropo]

Thanks thalathb for your suggestion, but there is the big question: how to design the tank for a normal consumption?. I mean, I know the normal consumption so I could design a lot of tanks with enough volume to storage propane for a week, a month, etc., but the purpose is TO GUARANTEE the rate of propane, and this depends of the geometry of the tank (because I am not going to use a vaporizer).

In short, I think there could be a lot of tanks with enough volume for my normal consumption, but only a few of them will be have the required geometry for this operation.

 

[786392]

Dear Lijantropo Hello/Good Afternoon,

Although lot of useful and guiding accurate info is given to you by our learned/experienced professional Colleagues.

I feel to alert you for a somewhat serious(potential)hazard of negative pressure inside(vacuum as indicated indirectly by Compositepro in the 27th June 14:42 Hrs post)

This may endanger the containment's mechanical integrity if proposed storage containments(vessels,Bullets or spheres) are not designed for vacuum!

Hope this helps with way forward on process design safety.

Best Regards
Qalander(Chem)

 

[Lijantropo]

Hello 786392,

Thank you for the alert. I am going to take into account the vacuum inside the tank.

Best Regards,

L.

 

[TD2K]

That's got to be some cold conditions to start to pull a partial vacuum in a propane tank since propane boils at about -40F at atmospheric pressure.

 

[786392]

T2K Although you are 100% correct but envisage continuous propane vapors Consumption(i.e. eat-up).

With little or no big fresh liquid inflow in comparison to the eat-up rate(s)(leaving aside any heat energy inputs).

In-effect this may create low pressure,low temperature conditions with a possibility of some partial vacuum;that needs to be taken care-off.

Best Regards
Qalander(Chem)

 

[Lijantropo]

Good morning,

Another question related:
In the tank I have propane liquid & vapor in equilibrium. Then, when the propane begin to flow, due to pressure difference, some liquid vaporizes to reach the equilibrium again. If the amount of heat from the atmosphere isn't sufficient, the temperature and the pressure in the tank drops until the heat transferred from atmosphere can maintain certain temperature or until the pressure reach the atmospheric pressure.

Assume that the temperature and the pressure drops only a little and the propane flow stops. In that point the propane will take heat from the atmosphere, due to temperature difference,and the temperature will increase. However, as you have a fixed volume, the pressure in the tank will also increase.

My question is: it would reach the original pressure?
(If I had a pressure indicator in the tank I would see the original pressure again?)

I think that it is possible if the amount of liquid is sufficient, but it "sounds" strange, and I can not "play" with a real propane tank in order to verify this .

Regards,

Lij

 

[Compositepro]

The vapor pressure curve of propane controls everything. If the temperature of the liquid propane is X the pressure will be Y. Vapor leaving the tank is a heat flow out because that vapor comes from boiling the liquid. The liquid propane thus drops below ambient temperature resulting in a heat flow into the tank through the walls.When the two heat flows balance you will reach an equilibrium temperature.

If you stop vapor flow, then tank will rise to ambient temperature and the pressure will rise to the vapor pressure at that temperature. This will be your "original" pressure if you are at you original temperature.

 

[rmw]

Your ambient conditions drive the rate of the process described by Compositepro above. Are you in a climate where you have to deal with a wide variety of ambients? If so, you have to take that into account as well. Can you locate the tank in the sunshine? Or is that variable too? You could paint your tank black if it is in the sun to enhance the heat gain from the sun.

I used to have to deal with a similar problem when putting Freon in a large bus. The liquid would become so cold after a while that there wasn't enough pressure to drive any more gas into the system to continue charging it, so I would put the jug right in the exhaust flow path and it would heat the tank enough to keep the pressure up. Fortunately the charging point was right ajacent to the engine exhaust. Without doing that, I would have had to wait for the cycle that Compositepro describes above to bring the pressure back up enough to continue.

rmw

 

[dcasto]

HD 5 propane has about 5% ethane and boils at anywhere e amount of propane in the tank, which changed the composition. In northern Montana, we had electric heaters on our smaller (25 gallon) tanks that would add heat in order not to create a vacuum. I've open a propane tank and had it suck in air before. In the deep winter there, we made 240 VP propane and not the HD 5 spec of 208 VP.

At a loading dock in north Texas in the winter, we could not load the trucks from a single 30,000 gallon tank during the winter without adding steam heat to the tank. With 3 bullets opened to the loading dock loading at 350 gpm, the tanks would get enough solar and atmospheric heat transfer to not cavitate the pumps. 2 30,000 gallon tanks were on the border line.

 

[Lijantropo]

Thank you all for your responses!
This has been a great help for me.

Regards,

Lij