7 replies to this topic

[Miss secret]

Good night..

in my project .. i have to add hydrogen to the hydrocarbon stream , that will decrease the vapor pressure of the hydrocarbon.. by this relation:

total pressure = [x .p*]HC+ [x .p*]H2

where x is the mole fraction of hydrocarbon, hydrogen.

p* the vapor pressure..

 

My problem is how to find the vapor pressure of hydrogen, to find the boiling point of the mixture ,

the total pressure is 190 kg/cm2

[Steve Hall]

You aren't thinking clearly on this problem. Vapor pressure is, basically, a fixed property just as boiling point and density are fixed. When I say "fixed" I mean they are determined at a particular temperature or pressure.

 

The relationship you give for total pressure is true for ideal compounds that don't interact with each other in the liquid phase. No bonding, reactions, etc. And it presumes that the vapor space above the liquid is fully enclosed and contains only the same compounds that are in the liquid (i.e., no air).

 

The mixture won't have a "boiling point" in the same sense as the pure HC. As you raise the temperature, the vapor pressures increase and the equilibrium concentrations between liquid and vapor change. If the total pressure is higher than the combined vapor pressures then there will be phase transfer but it will be slow and not result in an observed "boiling" appearance. But if the total pressure is lower than the vapor pressures then there will be rapid phase transfer which is observed as "boiling".

 

What happens in a closed vessel as the temperature is raised? The total pressure increases.

 

What happens in a closed vessel with pressure control (relief valve) as the temperature is raised? You can do the calculations according to your formula - and because the VP of H2 is so much higher than the HC I think you'll find that the H2 is stripped out of the mixture first until the temperature reaches the boiling point (at the controlled pressure) of the HC, at which point you'll observe a "boiling" effect.

 

Having said all of that, when dealing with slightly dissolved gases in liquid, the usual relationship to use is Henry's Law. 

 

Hope this helps!

Steve

[Miss secret]

thank you for helping me,

but there is some points are not clear,

I want to evaporate the heavy hydrocabon, to introduce it to a gas phase reactor.

and I want to reduce the boiling point by adding hydrogen.

[Bobby Strain]

Actually, you probably want to use hot hydrogen to vaporize the hydrocarbon mixture. Or add hydrogen to a hydrocarbon furnace feed. It's a tedious calculation without a simulator.

 

Bobby

Edited by Bobby Strain, 30 December 2013 - 08:11 PM.

[PingPong]

I want to evaporate the heavy hydrocabon, to introduce it to a gas phase reactor.

and I want to reduce the boiling point by adding hydrogen.

Which heavy hydrocarbon are we talking about here?

 

What is the inlet temperature of the reactor?

[Miss secret]

Actually, you probably want to use hot hydrogen to vaporize the hydrocarbon mixture. Or add hydrogen to a hydrocarbon furnace feed. It's a tedious calculation without a simulator.

 

Bobby

yes, I want to add hydrogen to a hydrocarbon furnace, but i have to do it without simulators :'(

[Miss secret]

 

I want to evaporate the heavy hydrocabon, to introduce it to a gas phase reactor.

and I want to reduce the boiling point by adding hydrogen.

Which heavy hydrocarbon are we talking about here?

 

What is the inlet temperature of the reactor?

 

the heavy hydrocarbon is heavy vacuum gas oil

the reactor inlet temperature is 375 degree C

[PingPong]

Heavy vacuum gasoil (HVGO) is not one component but a mixture of many hydrocarbons with atmospheric boiling points ranging from roughly 400 to 650 ^oC.

 

To vaporize that mixture completely at 375 ^oC and 190 kg/cm² would require an enormous amount of hydrogen relative to the HVGO. I do not know of any process that requires completely vaporized HVGO at that high pressure. It is simply not feasible.

 

Maybe you are referring to a Hydrocracker, but a Hydrocracker does not vaporize the HVGO reactor feed completely, but only partly. The amount of vaporization is simply resulting from the 375 ^oC at 190 kg/cm² in combination with the amount of hydrogen required to obtain the right hydrogen partial pressure. The hydrocracker reactor catalyst therefor operates on a mixture of vapor and liquid.

Edited by PingPong, 01 January 2014 - 06:13 AM.