## temperature drop in one hour in a LNG tank

## temperature drop in one hour in a LNG tank

(OP)

Hi,

I am working on calculating the gas rate formation in a tank into which LNG is being filled. I want to calculate the heat going into the LNG at -163 from the natural gas vapor existing at a high temperature initially, say +20 degrees, in one hour. For this purpose, I am focusing on calculating the temperature of the cooled down vapor from +20 degrees to some temperature in 1 hour and will then calculate the heat it has given out to the LNG for evaporation. How do I arrive at what will this final temperature of the vapor be at the end of one hour, assuming the LNG is at saturated state and whatever heat it absorbs is used in evaporating into vapor. We can neglect the effect of the fresh evaporated vapor for the time-being.

Any insights are welcome. Thank you.

I am working on calculating the gas rate formation in a tank into which LNG is being filled. I want to calculate the heat going into the LNG at -163 from the natural gas vapor existing at a high temperature initially, say +20 degrees, in one hour. For this purpose, I am focusing on calculating the temperature of the cooled down vapor from +20 degrees to some temperature in 1 hour and will then calculate the heat it has given out to the LNG for evaporation. How do I arrive at what will this final temperature of the vapor be at the end of one hour, assuming the LNG is at saturated state and whatever heat it absorbs is used in evaporating into vapor. We can neglect the effect of the fresh evaporated vapor for the time-being.

Any insights are welcome. Thank you.

## RE: temperature drop in one hour in a LNG tank

Dejan IVANOVIC

Process Engineer, MSChE

## RE: temperature drop in one hour in a LNG tank

The papers you suggested look good. Will update here.

Thanks.

## RE: temperature drop in one hour in a LNG tank

Dejan IVANOVIC

Process Engineer, MSChE

## RE: temperature drop in one hour in a LNG tank

If so you cannot just pour LNG into a tank without spending hours or days slowly cooling it down. This question is therefore irrelevant. Just search first fill lng storage tank.

Evan if by some miracle your tank can stand going from 20c to –163 in a short period of time, the evolved vapour will not be trivial and should not be neglected.

Remember - More details = better answers

Also: If you get a response it's polite to respond to it.

## RE: temperature drop in one hour in a LNG tank

Dejan IVANOVIC

Process Engineer, MSChE

## RE: temperature drop in one hour in a LNG tank

Remember - More details = better answers

Also: If you get a response it's polite to respond to it.

## RE: temperature drop in one hour in a LNG tank

Also, prior to commissioning the facilities are at ambient temperature, so this is how cooldown is performed for all lines and equipment - circulating (and evaporating) LNG till normal operating temperatures are achieved.

Dejan IVANOVIC

Process Engineer, MSChE

## RE: temperature drop in one hour in a LNG tank

## RE: temperature drop in one hour in a LNG tank

As of now, yes LNG tanks are spray cooled before gradually increasing the LNG inflow. I did some rough iteration in excel and find that the rate of temperature drop for a 12000 m3 tnk is almost 10 degrees per hour. So, it makes the calculation a bit easier. The total heat input to the LNG is from external environment, the heat to cool the tank and the heat from the LNG vapor already in the tank at a higher temperature. All these will then lead to vaporization and temperature rise and expansion of the total vapor present in the tank. From this subtracting the tank empty volume remaining after cargo inflow will give the gas which has to be discharged ashore through the vapor return line.

## RE: temperature drop in one hour in a LNG tank

## RE: temperature drop in one hour in a LNG tank

Dejan IVANOVIC

Process Engineer, MSChE

## RE: temperature drop in one hour in a LNG tank

Could you suggest me how to calculate temp. drop per hour to calculate total cooling down rate for LNG tank? I am also in same boat and I need to calculate required amount of LNG for cooling down and how much time it may require to cool down the tank up to -130.C. I was told that it might take 10-12 hours for cooling down operation alone.

Thanks

Rishi

## RE: temperature drop in one hour in a LNG tank

I did some kind of calculations for LNG tank cooling down before to start loading process for LNG carrier.

Normally, cooling down time is recommended 12-15 hr based on site conditions. However my calculation sheet is showing bit high time and I am bit confuse whether I am on right track or not.

Attached is calculation sheet for your reference. Your comments are highly appreciable.

Regards

Rish

## RE: temperature drop in one hour in a LNG tank

Here is the attached file.

Regards

Rish

## RE: temperature drop in one hour in a LNG tank

There will be a lot of factors coming into play. First of all, as per regulations there should not be a drop of 10 C per hour, so your first calculation should be based on a dT= 1 hour. Then you should find how much liquid you should pump in to extract the above heat. Another interesting part you have to consider and which is why your calculations show such a long time is that you are injecting liquid fluid(LNG) which will first absorb the heat and evaporate to gas, without any change in temperature (latent heat of vaporization) and then this evaporated vapor will further absorb heat and reach certain temperature. Assuming that all the process is so slow that at end of each hour your system is in equilibrium and the temperature drop is no more than one hour. In short, the heat to be extracted from the tank to achieve a temperature drop of 10 degrees per hour is being taken by the liquid (latent heat of vaporization) and then by the evaporated liquid till it reaches a 10 degrees increase, so that the vapor inside and the tank are at the same temperature.

Also, in your sheet you are not considering the fact that the temperature gradient is changing as the tank is cooling. The temperature gradient is not always between 32 and -130. It starts from 32 when the sea and tank are at same temperature. At the end of one hour (since we need only 10 degrees drop), the gradient will be between 32(sea) and 22(the new tank temperature after 10 degrees drop) and so on.

Hope this helps.

## RE: temperature drop in one hour in a LNG tank

## RE: temperature drop in one hour in a LNG tank

Thank you for your help.

I am still wondering how to restrict the calculation based on dT= 1 hr. Would you mind to share any file or reference calculation on rishi.rga at gmail.com? It will be highly appreciable.

Thanks

Rish

## RE: temperature drop in one hour in a LNG tank

Hope this helps.

## RE: temperature drop in one hour in a LNG tank

Thank you once again. I have few clarifications, would appreciate if you could assist me;

I will calculate Q= M_tank*cp*dT where M-Tank is Mass volume of tank, cp - specific heat of LNG and dT is 10.C ( 32-10= 22.c)

when I want to calculate , ( m*L+ m*cg*dT'). You confirmed m is mass of LNG which need to pump and I want to get this quantity. Here are 2 “m” and I am bit confuse. I applied the formula as bellows

Q= m*L, where I have already calculated Q and I can find the value of m, since latent heat of vaporization is given 511KJ/kg.

For example; Q = 3700000 KJ (based on 10 .C delta T), Mass of tank 1000 kg, cp= 2.02 KJ/kg

M_tank*Cp*dT = (m*L + m*Cg*dT')

3700000 = 3700000/511(m =Q/L) + m*2.02*178

I am using cp & cg value same. Please suggest if my understanding is correct or not.

## RE: temperature drop in one hour in a LNG tank

Your equation should essentially be:

M_tank*Cp*dT = (m*L + m*Cg*dT')= m*(L + Cg*dT')

## RE: temperature drop in one hour in a LNG tank

Am I supposed to use Cp and Cg values same? Sine you said Cp is specific hear of LNG so Cg suppose to same or different?

Thanks again for your guidance.

## RE: temperature drop in one hour in a LNG tank

In---M_tank*Cp*dT ---Cp is the specific heat of the material you are extracting heat from, here the tank, say steel.

In---(m*L + m*Cg*dT'), L is latent heat of LNG and Cg is the specific heat of the vaporized LNG (gas form).

## RE: temperature drop in one hour in a LNG tank

Noted. Many Thanks for your guidance.

I guess, I can find LNG consumption from (+45.c to -20.C) and each time consider Delta T 10.c but how I will establish the time for total cooling down.

Also heat ingress due to pump and heat ingress due to loading line length can be calculated but these values will co-relate with cooling down process?