Why do hydrocarbons cause foaming? 5

[JohnCalvin]

All,

I wish to take issue with someone about the popular theory that hydrocarbons cause foaming and to find out why. I have read quite a bit on the subject esp. in Ullman's Encyclopaedia of Industrial Chemistry. I have come to the conclusion that there are two general kinds of foam causes, viz. mechanical (or plant operation related) and chemical.

For chemical induced foam to occur, according to Ullman's, the chemical has to have surfactant properties. The reason for this is that a foam is basically a dispersion of a gas within a liquid. Surfactants will stabilise/promote this since their hydrophilic portion will hold onto the aqueous phase while their hydrophobic portion holds onto the gas phase. But hydrocarbons, which are completely hydrophobic, DO NOT FIT THIS SOUND THEORY.

Furthermore, if one wants to break chemically induced foam one needs to add an appropriate antifoam. Since the foam consists of bubbles, i.e. gas with entrained liquid, one needs to locally de-stabilise the surface tension of that bubble in order for it collapse/rupture so that it doesn't pose a problem. So in amine units it is usually accepatable practice to add silicone antifoams. And why not? Silicone antifoams are very hydrophobic. Their ability to disperse and to destabilise foam formation in a highly hydrophilic amine unit is precisely what makes them attractive antifoams. This fits the theory.

BUT WHY SHOULD HYDROCARBONS BE ANY DIFFERENT? ARE THEY NOT ALSO COMPLETELY HYDROPHOBIC? HOW DO THEY STABILISE A FOAM? HOW DO THEY STABILISE THE SURFACE TENSION OF A BUBBLE WHICH CONSTITUTES THE ENTIRE FOAM? IT APPEARS TO ME TO GO COMPLETELY AGAINST THE ENTIRE THEORY OF FOAM FORMATION.

If I stand corrected, please feel free to do so. For me this is a matter of the laws of chemistry/physics which I cannot see applying in a case of hydrocarbons causing foam in an amine unit. I know that experience marks it as a fact, point taken, but I want to see the theory for that as well.

Yours in theory and in practise,

JohnCalvin

 

[25362]

To John Calvin, this is a complex phenomenon, theories to explain it appear from to time in the literature. Anyway, FWIW, I"ll try to summarize some points that may satisfy your query, or not at all.

First of all we are not speaking of pure hydrocarbons, but of mixtures of hydrocarbons. Ullmann is right in that surface acitivity effects must be there to form foams. Solid impurities such as FeS in amine solutions can stabilize foams. Mixtures of hydrocarbons may show foaming activity and foam stabilizing effects as we shall see herebelow.

Foams (sometimes called froths) form when bubbles rise to the surface of a liquid and persist w/o coalescing, or w/o rupture. They consist of closely packed arrays of gaseous polyhedral cells separated by thin liquid films. Foams' lives (stabilitites) can be short or long. There are important forces affecting foam stability: gravity and interfacial tension that induce foam disappearance, and capillary forces, and viscosity that oppose them and tend to stabilize a foam.

Pressure differences across bubble boundaries are similar.
However, at the junctions (called Plateau borders) regions of lower pressure exist. These form the causeway for liquid drainage.

The prime mechanism of "film rupture" is gravitational drainage, and it is directly related to "bulk" viscosity. The "bulk" liquid hydrocarbon appears to be sandwiched between the surfactant molecules at the liquid-gas boundary. Bulk viscosities are sometimes much higher than those of the pure hydrocarbons, either because of lower temperatures due to evaporation, or due to the presence of tiny solid trace impurities that increase the liquid viscosity making the fluid non-newtonian, or any other reason.
These viscosities retard drainage and provide a measure of stability to the foam. These effects are more strongly felt in lubricant hydrocarbon oils.

There are other factors such as the Marangoni effects resulting in a flow from a nonthinned (lower-surface-tension) surface to the thinned (high-surface-tension) surfaces, which counteract film drainage restoring the films. These effects disappear when the film thickness drops below 20-30 nanometers such as on the upper level of a foam.

There is sometimes gelatinization of the films by intermolecular or chemical interactions that immobilize the liquid inside the film layers, so that capillary or gravitational stresses aren't sufficient to cause flow.

Then there is the Marangoni mass-transfer-induced effect. A film is stabilized when the ST increases due to transfer of the less-volatile hydrocarbon component which has a higher ST than the more-volatiles. The film has a higher ST than the bulk of the liquid, thus "sucking" liquid into the film between bubbles, counteracting drainage.

The Ross-type of foaming activity appear when a weak solution tends to separate into two liquid phases. It has been attributed to concentration gradients.

When the foam drainage is almost complete, a point is reached at which the monomolecular surfactant layers come close enough to create repulsive forces that delay film rupture. As you say, either mechanical, thermal or chemical methods are used to displace the stabilising monomolecular surfactants so as to create a hole in the layer.
Silicone-based fluids of the correct MW have limited solubility in oils and sufficiently low ST, lower than that of the hydrocarbons or the surfactants, and can displace them.

BTW, foaming is not only undesirable in distillation or absorption columns, it is as bad for oil lubrication. However, its use is exploited, for example, in industrial extractions, in the refining of metal ores, and in fire-fighting.

If you have the time, I'd advise you read the chapter 14.2 on foaming in the book titled Distillation-Operation by Henry Z. Kister (Mc Graw-Hill).

I hope the above is useful.

 

[22082002]

This was a quite informative question and answer...

Thanks,
SAA

 

[JohnCalvin]

Thank you for the informative reply. If I have understood you correctly then hydrocarbons are not the cause of foaming per se, but they do have the propensity to stabilise it. Is this correct? The primary cause in chemically induced foam would be the surfactant like chemicals. Am I right?

I enjoy the correspondence

Kind regards,

John Calvin

 

[25362]

To make bubbles, i.e., to create surface area takes work against the surface tension. Thus "aereated" liquids will tend to seek the more stable, lower-energy-level-state associated with the smallest (equilibrium) surface area.

As I see it, all liquids have the capacity to foam. The leitmotif is the foam duration. Pure liquids, including hydrocarbons, will sustain the foam as long as the gas is introduced; when this stops the short-lived foam may collapse. The tendency of systems to seek a lower energy level is the underlying principle.

Physical chemistry books tell us that surface tension is not affected by bubble curvature as long as the thickness of the liquid layer is small compared with the radius of curvature. The thickness of the lamellae of common foams is of only a few molecules, and in the range of 10^-7 cm under most conditions.

The bubbles are more spherical when poorly drained and polyhedral in shape when well-drained and their films are thin.

Viscosities such as found in lubes, chemicals that lower surface tension, Marangoni effects, a mixture of "pure" hydrocarbons having different surface tensions, etc., tend to "stabilize" foams.

 

[JohnCalvin]

To 25362

I see an area which we may be approaching from different sides. I agree fully that due to the very nature of a gas passing through a liquid that foaming will always be present. However, analytically, i.e. in laboratory tests, a foam has been defined as having a certain height (> 10 cm I think) and a certain collapse time (> 7 sec) after aeration. Anything less than this should not be a problem foam for stability reasons and is not constituted as a foam according to those analytical tests.

Hydrocarbons would tend to generate / stabilise those sorts of foams which have low foam heights and short collapse times. But there other foams which are relatively stable with respect to gravitational forces having long collapse times and large foam heights. It is more these that I was referring to in my inquiry. I do not think that hydrocarbons could be a culprit for forming these foams. Would you agree with that statement?

Regards,

John Calvin

 

[25362]

To John Calvin, here again we are dealing with hydrocarbons of differing MWs and MW ranges. You may be right regarding HCs of lower MWs in regard to distillations and absorption.

Kister gives examples of a crude oil: its foams made by gas velocities of 0.5 fps may have decay times of 4 minutes and froth heights of more than 40 cm. There are other fields in which HC foaming exists. In extractive distillations the addition of a heavy homologue can suppress foam caused by the stripping off of light components that lower the surface tension.

Sometimes foaming is used on purpose (foam fractionation and flotation), and at times foaming of hydrocarbons is undesirable. Kister on his book Distillation -Operation- (McGraw-Hill) says a quick-breaking foam may reach a life of 5 s; a moderately stable foam persists 2-3 min.

Concentrating on your last para. an example concerning HC lubes used as hydraulic fluids could be appropriate. It is a fact that mineral HC oils are relatively incompressible, but all dissolve a certain amount of air, the amount increasing with pressure. When this pressure is reduced in a hydraulic system millions of small air bubbles are released. As the hydraulic "virgin" oil doesn't have good "defoaming" properties, the trapped air will circulate in the system together with the oil. Then, although the oil is relatively incompressible a small number of air bubbles will increase its compressibility and lead to spongy hydraulics.

That's the reason of using anti-foam (inhibiting) agents such as silicones in ppm dosages; over-use would tend to stabilize foams. There are lab-bench standardized foaming tests such as IP 146 or ASTM D 892-03 used to measure the foaming characteristics of lubes.

Does this example answer your query ? I hope so.

As you see, it all depends on what HC, or mix of HCs, one speaks about, the foaming gas or vapour, the operating conditions, etc., etc. Often lab tests showing foam decays of less than 5 s have failed to detect actual severe foaming in the plant system. That's the reason to look for tests and techniques that better reproduce plant conditions.

 

[JohnCalvin]

To 25362, thank you once again for your correspondence and experience. Foaming is definitely a complex field with many variables.I just want to ask one more question concerning this last response of yours and that is are the examples which you cite all in the context of amine units? I just want to quite convinced that this is the case in our plant setup. What sort of HC MW would one look at as a foaming culprit?

Thank you for the correspondence,

John Calvin

 

[25362]

No, I didn't refer in my last message to amine units. And when I spoke of higher hydrocarbon MWs (lubes) which are accompanied by higher viscosities, I should have added a higher molecular complexity as obtained by isomerism and chain ramification.

 

[JohnCalvin]

To 25362,

Thank you for the lengthy debate - I think I have a better understanding now.

Regards,

John Calvin