delay coking foam and surface tension 1

[JesseLai]

Hi
I am trying to figure out how foam growth in hydrocarbon is related to surface tension. My understanding is that as surface tension decreases, foam height grows with injection of N2 gas in isotherm environment. Then after injection of silicon antifoam, the liquid's surface tension increases. Is that right?
My second question is what happens to surface tension at higher temperature? For example, I heated the vacuum residuum to 850F. The residuum started to crack at 750F and produced vapor;subsequently, foaming occurred. Is it still the same trend,i.e. decreasing surface tension increases foam height in high temperature isotherm like 850F ?

Regards,

Jesse Lai

 

[Compositepro]

There is no direct correlation between surface tension and foam (bubble) stability. Foams can occur with high or low surface tension. Foam stability is a function of liquid film elasticity, i.e., will the film stretch without thinining and breaking. Surfactants reduce surface tension and often create stable foams. But it is not the lower surface tension that stabilizes the foam. It is the elasticity of the surfactant film on the liquid surface.

Most defoamers work by introducing a send phase particle into the surface film which will not thin and stretch so it cause the film to rupture.

When you say cracking I guess you mean chemical cracking and not physical cracking. In your case I would guess that polymerization of the residuum is a large factor in foam formation. Molecular weigh and chain length are very important factors in being able to make films and fibers. This means being able to stretch a liquid without it breaking.

 

[JesseLai]

Thank you for your reply, Compositepro.
I don't add any chemical to crack the vacuum residuum from refinery. Heat is applied for cracking.

Foam stability is a function of liquid film elasticity. What does film elasticity depend upon? Does the film elasticity increase with higher bulk viscosity or bulk surface tension of the material?