foam is a small bubbles of gas surrounded with a thin film of oil, this phenomena reduces the separation efficiency, foam exists in the gas oil interface when separator pressure decreases, or because of the surface tension, viscosity, and some impurities, because of that, small droplets are generated, these droplets are covered with oil, this happens when gas is liberated. in general; the quick motion inside the separator will cause foaming, this foam will cover the fluid surface and prevent gas from escaping, foam also reduces gas space inside the separator, and can pass the demister and escape to the gas outlet in the form of mist and liquid droplets.
Foam at the interface may occur when gas bubbles are liberated from the liquid. Foam can severely degrade the performance of a separator.
the foam may cause many problems such as:
– foamy crude reduces the separator volume because it occupies a lot of space, this will affect the oil gas separator efficiency by reducing the retention time, unless the it is designed to treat foamy crude.
– the foam density is between oil and gas density, and that will confuse the level controller.
– when there is a lot of foam in the crude oil, it may go out with oil phase or gas phase, this will reduce the separation efficiency, or cause a “Carry Over” when some droplets escape to gas phase when the oil level goes up, sometimes it causes gas blowby with liquid phase when the level is down.
the major cause of foam in crude oil is the presence of impurities other than water, which are impractical to remove before the stream reaches the separator. One impurity that almost always causes foam is CO2. Sometimes completion and workover fluids, that are incompatible with the wellbore fluids, may also cause foam. Foam presents
no problem within a separator if the internal design ensures adequate time or sufficient coalescing surface for the foam to break.
Foaming in a separating vessel is a three-fold problem:
1. Mechanical control of liquid level is aggravated because any control device must deal with essentially three liquidphases instead of two.
2. Foam has a large volume-to-weight ratio. Therefore, it can occupy much of the vessel space that would otherwise be available in the liquid collecting or gravity settling sections.
3. In an uncontrolled foam bank, it becomes impossible to remove separated gas or degassed oil from the vessel without entertaining some of the foamy material in either the liquid or gas outlets.
Alternatively, the oil may be saturated with its associated gas and then expanded in a gas container.
This alternative test more closely models the actual separation process. these tests are qualitative. There is no standard method of measuring the amount of foam produced or the difficulty in breaking the foam. Foaming is not possible to predict ahead of time without laboratory tests. However, foaming can be expected where CO2 is present
in small quantities (1–2%). It should be noted that the amount of foam is dependent on the pressure drop to which the inlet liquid is subjected, as well as the characteristics of the liquid at separator conditions.
Comparison of foaming tendencies of a known oil to a new one, about which no operational information is known, provides an understanding of the relative foam problem that may be expected with the new oil as weighed against the known oil. A related amount of adjustment can then be made in the design parameters, as compared to those found satisfactory for the known case.
The effects of temperature on a foamy oil are interesting. Changing the temperature at which a foamy oil is separated has two effects on the foam. The first effect is to change the oil viscosity. That is, an increase in temperature will decrease the oil viscosity, making it easier for the gas to escape from the oil. The second effect is to change the gas–oil equilibrium. A temperature increase will increase the amount of gas, which evolves from the oil.
It is very difficult to predict the effects of temperature on the foaming tendencies of an oil. However, some general observations have been made. For low API gravity crude (heavy oils) with low GORs, increasing the operating temperature decreases the oils’ foaming tendencies. Similarly, for high API crude (light oils) with high GORs, increasing the operating temperature decreases the oils’ foaming tendencies. However, increasing the operating temperature for a high-API gravity crude (light oil) with low GORs may increase the foaming tendencies. Oils in the last category are typically rich in intermediates, which have a tendency to evolve to the gas phase as the temperature increases. Accordingly, increasing the operating temperature significantly increases gas evolution, which in turn increases the foaming tendencies.
Foam depressant chemicals often will do a good job in increasing the capacity of a given separator. However, in sizing a separator to handle a specific crude, the use of an effective depressant should not be assumed because characteristics of the crude and of the foam may change during the life of the field. Also, the cost of foam depressants for high-rate production may be prohibitive. Sufficient capacity should be provided in the separator to handle the anticipated production without use of a foam depressant or inhibitor. Once placed in operation, a foam depressant may allow more throughput than the design capacity.
Foaming Tendency
For foaming to occur it is necessary for gas bubbles to be formed, and for the drainage of the liquid films surrounding the bubbles to be retarded. Drainage of the films is slower in highly viscous liquids, but the chief causes of foaming are surface properties which are usually unpredictable. For this reason the foaming tendency is best judged on the basis of experience of similar cases. Laboratory tests may also give an indication of the foaminess of the system.
Examples of foaming systems are some crude oils, heavy residues, absorption and extraction solvents.
Foaming in the separator may lead to carry-over of liquid (when foam reaches the gas/liquid separation internal and/or the gas outlet) or to carry-under of gas. It will also upset the level control system.
Note that foaming is more likely to be a problem at high liquid loads, when flow in the inlet pipe is in the froth or intermittent flow regimes.
How to overcome this foamy crude phenomenon?
there are many ways to overcome this phnomenon in oil and gas separators, such as installing Defoaming Plates which are inclines parallel plates which are to close to each other, when foamy crude flows through these plates, oil droplets are collected when hitting these plates and the gas is liberated. provide additional surface area, which breaks up the foam and allows the foam to collapse into the liquid layer.
some resources say that Installation of defoaming plates to combat foam is normally not effective and may even be counterproductive.
Foaming in the vessel is minimized by decreasing the downward liquid velocity, for instance by increasing the diameter of the separator vessel.
we can also use foam depressants or defoamers, these chemicals are rather expensive, we can also reduce or break foam by other means such as heat or centrifugal force.
References:
1. Gas – Liquid and Liquid – Liquid separators / Maurice Stewart.
2. Two Phase separator Design / Manish Shah.
3. Design o Oil Gas Separators from Hydrocarbon Stream/ University of Lima – Peru.