How heat will be distributed inside of the tank?

[Jency]

I am working on a crude oil tank heating system which will be used to keeping the crude oil temperature not drop down because of the heat loss to Ambient. The heat loss through the tank wall at the coldest date had been calculated to be about 1 MMBtu/hr. The shell and tube type Glycol/Crude heat exchanger with a crude circulation pump had been sized to achieve this application.

My question is: The tank is a 500,000 BBL tank which is about 75m diameter x 18 m height. With this heating system, a small amount of crude will be drawn off from one side of the tank. After heat exchange, the heated crude will flow back to tank with additional 1MMBtu/hr heat. But how does the heat be distributed inside of the tank? How can I make sure the cooler crude which is the partial close to tank wall be heated more than the warm crude in middle?

Had any one has this type of the experience to know if this system will work OK to keep the tank oil in certain temperature and without to causing partial of the tank are heated up, but the other partial is is still cooling down? Note there will be mixer installed in tank. But the tank is big.... Is there any thoughts for how to arrange the piping to make better heat circulation inside of the tank? Or any other way to achieve this application?

Thanks in advance

Jency

 

[Compositepro]

All you need is to run a loop of glycol piping close to the bottom tank and near the circumference. Natural convection will take care of the rest. Insulating the tank would help.

 

[Jency]

Compositepro,

Thanks for your input. The tank will be insulated.

The reason for not using the glycol heating coil inside of this tank is considered the system maintainance issue. The tank will be expect to operating at mainly full condition most of the time. And also, the required heat is not that much.

Or maybe I should consider to run a loop of heated oil piping?

Jency

 

[StoneCold]

Jency
You are going to need a way to evenly disperse the heat to the tank. Compositepro idea is a good one, it takes advantage of natural convection and applies the heat at the areas of the tank with the largest heat loss. If you pump out of the tank it would seem that you need to pump back into the tank at about 10 or 12 locations to try to distribute the heat to the tank. Otherwise you may have a short circuiting problem where your hot liquid just recirculates, due to it's reduced viscosity and you don't get good dispersion throughout the tank.
You could pump the oil around the ring and heat it outside of the tank if you don't want the actual exchange to be in the tank. But it would seem that you need some type of ring dispersion of the heat.

Regards
Stonecold

 

[Compositepro]

It all depends on how uniform you want the temperature. You can pump out of one side and return to the other side, at the bottom of he tank. Hot fluid will rise and mix naturally. An electric immersion heater in one location at the bottom of the tank is often adequate. No pumps or piping to purchase or maintain.
Natural convection works great as long as the fluid viscosity too high, in which case pumping may also be a problem.

 

[Jency]

Thanks StoneCold and Compositepro,

Yes, the evenly heat dispersion is excetly what I was concerning with this normal tank heating system since the tank is 75m diameter. The purpose of heating is keep the crude in certain temperature (summer at about 18 C, winter at about 7.5 C) which is the temperature the crude has viscosity less than 400 cSt at those temperature and can be pumped into pipeline.

So basically, the crude which required to be heated is about 400cSt. And hopefully the temperature can be kept as uniform as possible.

With this tank size, if the crude along the tank wall can be kept warmer, I would expect the crude in the middle has a very little temperature fluctuation.