How to calculate the vapor fraction at te outlet of a pipeline at the start up conditions?

[Chemico822]

HI,
I am workin on propane loading terminal. The propane from the storage tank is flowing through pipe line of 5200 feet before it feeds the ship. at the start up conditions, the pipe will be at the atmospheric temperature ( 105 F). The propane from the storage tank (down stream of the pump) is at -141 F and 65 Psig, this will eneter a pipe which is at 105 F. Since the pipe is at the higher temperature, the propane exit temp will be higher and may also form some amount of vapor.But this vapor rate will change from minute to minute as the pipe gets cooldown. I want to know how long does it take to cooldown the peipe completly to what ever the fluid outlet temp is? and also how much vapor will be generated during this period? we are planing to send the vapor back to the storage tank, and the vapors from the storage tank will be compressed and send back to the storage tank itself. It is just a closed loop vapor recompression. I want to kknow the vapor generation rate to check whether my existing compressor can handle that rate. if not the pressure in the tank will build up. generally these tanks operates at atmpsphric pressure and can handle upto 1 Psig. This pipe lline is having 6" of polyisosyrene insulation, the wind velocity is 105 miles/hr, ambient air temp is 105 F, 24" stainless steel pipe ( O.D: 24", I.D: 23.25"), flowrate is 500000 lbs/hr. I guess for my calculation here i can ignore the insulation and the ambient temperature, because when the fluid enters 105 F pipe there won't be any heat transfer between the ambient and the propane as it has the insulation. A method for hand calcualtion is apprciated, also let me know if there is a way to do this is Hysys? let me know if you need any other info.

Thanks,
Chemico 822.

 

[Chemico822]

Sorry its not propane, its ethane and the temp is -132 F

 

[LittleInch]

My issue with this is what is your start-up flow rate and how to avoid thermal shock and thermal bending in your pipe. This is the real issue when introducing extremeny cold fluids. Is this just a commissioning issue? Normally these lines are constantly re-circualted back to the main tank and the vapour emitted as a result dealt with at the tank due to the issue over thermal shock if you empty and fill these lines on a regualr basis.

Given the realtive size of the pipeline, wall thickness and the insulation, I would have thought that the steel would cool down within a minute or less. For vapour, at least one option is to calcualte the heat energy in the steel at 105F and -132F and assume that all that energy is transferred into the latent heat of evaporation of the liquid. This should give you a total vapour amount.

By the way did you really mean 105 miles an hour wind speed?? I can't imagine much loading or unloading going ahead at that wind speed...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Chemico822]

Yes its commissioning. My point is the length of the pipe is 5200 ft and the flowrate is 500000 lbs/hr. So it will take 63 mins to exit the pipe, but i am not sure whether the outlet temp of ethane when it exits the pipe will be close to whatever it is at the begining? Based on what you said, if the pipe will cooldown in one min the problem is lot easier tahn i thought. But to prove this i need some calculations handy. i was using Dittus -boeltier equation to calculate the overall heat transfer coefficient of the pipe, and did some calculations. I diveded the entire pipe segment to 63 small segments, so that i can know what is exactly happening for every minute. The heat hold by the (63*82 = 5200) 82 ft stainless stell pipe is Q= Mass of the pipe * Cp* ( 100-(-132)). this heat will be taken by the ethane in one min, so Q = mass of the tahne *Cp* (T2-(-132)). this will give me what is the temp of the ethane inside the pipe after one min. at the same time, the same heat is lost by the pipe, so Q= hi*A*ln DT and i solved for the mettal temp for the first 82 ft. Now i know the metal temp for the first 82 ft and ethane temp before it eneters the second segement. i continued this calculation for the entire 63 segements and i calculated the ethane temp that comes out of the pipe is -128 F for the first one min and the metal temp for the last segment to be 99 F. if i do this calculation untill i see the metal temp of last segment to be equal to the etahne temp which is -132F i am approximatley calculating 40 hours, which doesn't make any sense to me. I made a mistake in my calculations and now i have to find out what that is, hence started this topic. Thanks for your reply though @LittleInch.

Thanks,

Chemico822.

 

[MortenA]

IMO you should get this simulated in a transient simulation if you dont wanna see whats happen. Its not that much work