2 replies to this topic

[Mariouma]

Hello Everyone, I found this old exam, and I am not able to resolve it, Not sure if information is missing... anyway here's the text, maybe someone can help on how to proceed with such a problem:


"In a Petrochemical factory hydrocarbons for cracking are transported by pipes.The pressure and the temperature in the pipes are kept at 5atm and 56 C respectively.

Calculate the maximum quantity of pentane that can be added to an equimolar mixture of propane and n-butane without causing condensation".




Thanks folks

[pavanayi]

Mariouma,

Since I have no idea what tools you have/had to solve this problem, I will provide a general guideline.

1. Assume an initial flow rate in the pipeline of propane and n-butane. Ex: 50 moles/hour of each, totalling an initial flow rate of 100 moles/hour. Find the dew point of this mixture.

2. Now assume you are adding pentane to this mixture. I would start with an initial guess of 20 moles/hour. Find the dew-point of the mixture at 5atm. If it is less than 56°C, the mixture is still a vapour, which means it can take more pentane. If the calculated dew-point is more than 56°C, it means the mixture is already condensing and there is too much pentane already in the pipeline.

3. Modify the initial pentane flow rate and repeat step 2 untill you have an answer of acceptable tolerance.

4. When you have the maximum additional pentane flow rate in moles/hour, you can have the answer in any format (ex: mole fractions)

Edited by pavanayi, 27 September 2011 - 03:21 PM.

[S.AHMAD]

Dear Mariouma

Pavanayi has explained the method. I have run PRO-II and I got folllowing answer:

We can add 6.8 kmol of pentane to every 100 kmol of the equi-molar mixture of propane and butane. This gives a molar composition of C3=C4=0.4682 and C5 = 0.0636

The 5 atm is absolute pressure. If the 5 tam is gauge pressure then the answer is that we can add 4.8 kmol of pentane to 100 kmol of the propane-butane equimolar mixture.

The answer may be different if you are using different method of VLE equation. I am using BWR EOS.

For approximate answer, you can use Dalton's law by taking vapor pressure of each component at 56 C.