Foaming in fractionation columns
By understanding the foaming process and its root causes, steps can be taken to eliminate or minimise the formation of foaming.
Sulzer Chemtech USA
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Fundamentals of foams
Foaming is essentially the encapsulation of vapour within a liquid cell. Foams can be formed with a variety of methods generally associated with mechanical agitation or vapour formation. When the wall of a bubble ruptures, the bubble collapses, destabilising the foam. The main cause of bubble rupture is thinning of the liquid wall. Figure 1 shows how the liquid walls thin in the upper section of the foam as the liquid drains downward.
Foams are stabilised when liquid viscosity and surface tension oppose the natural drainage tendency of the bubble liquid. Liquid properties play a central role in foaming. Liquid drainage within foam is a natural phenomenon. Liquid always has a tendency to drain downward (or in the direction of any centrifugal forces). As the liquid drains from the bubbles, the liquid walls thin and weaken, eventually rupturing the bubbles and breaking the foam. Any condition that stabilises the bubble wall thickness will stabilise the foam.
Surface tension gradients within localised liquid create what is known as the Marangoni effect, where liquids flow from lower to higher surface tension regions. Generally, foaming tendency is proportional to this gradient. This key factor of foaming is explained well by Zuiderweg and Harmans.1
It is important to note that pure liquids will not produce a stable foam. However, when a surfactant is added to the system, stable foaming is then possible. Simply, the surfactant concentration at the liquid surface decreases as the bubble size increases. When this happens, the higher surface tension in the expansion area draws liquid from the lower surface tension region at the base of the bubble. This ‘heals’ the thinning bubble wall and stabilises the foam.
Types of foaming - Ross foams
As discussed by Ross2, a liquid solution with an incipient formation of a second liquid phase (for instance, a hydrocarbon fluid with a high equilibrium amount of water or an aqueous fluid with a small amount of hydrocarbon) will naturally be susceptible to foaming. Since this is an equilibrium effect, Ross foams can sometimes be overcome by changing the system temperature. A good example of this in practice is discussed by Bolles.3 In his troubleshooting endeavour, he found that sections of the tower were approaching the incipient formation of a second liquid phase, creating dramatic foaming within the column. To further support this conclusion, he raised the temperature of the column, eliminating the incipient second liquid phase and the foaming subsided.
Foaming can occur with or without the presence of mass transfer. Foams stabilised by surface tension gradients due to mass transfer are referred to as Marangoni foams. In applications where the higher volatility component has a lower surface tension, Marangoni foaming can be a problem. When a bubble forms in these systems, the lighter component evaporates from the liquid and the surface tension of the remaining liquid increases and stabilises the bubble. Without this effect, the evaporation would have caused the bubble film to thin and break. Figure 2 shows wine ‘tears’ produced as a result of mass transfer. As the alcohol evaporates from the wine on the wall of the glass, the surface tension increases and causes the liquid to form rivulets and droplets.
Foaming from solids/particulates
It is widely known that the presence of particulates tends to stabilise foam. When solids are present in liquids, they increase the solution viscosity. Increased viscosity inhibits the drainage of foams and stabilises them. An interesting study done by Kadoi4 looks at the influence of particulate composition, size, and shape on both viscosity and foaming in water. Somewhat surprisingly, the increase in foam stability was not always directly proportional to viscosity. Instead, particulate size, shape, and composition seemed to play important parts in foam stability. It is also important to note that the particulates did not transform a non-typical foaming system (water) into a foaming system. However, when a surfactant was added to the water and foam was produced, the solids stabilised the foam.
Also important was the reinforcement of the understanding that a smaller amount (weight) of smaller size particulates creates more foam stabilisation effects than a larger amount of larger sized particles. This is an unfortunate truth for fractionation column applications where the liquid solution is filtered to remove particulates and the worst offenders (small particles) are the most difficult to remove.
All things considered, it is clear that particulates are generally detrimental additions to a foaming system. The potential for the presence of particulates should always be accounted for during the column engineering design stage. In less serious cases, the equipment can be sized to account for the foaming. Ideally, the particles need to be removed from the system with filtration or totally prevented from forming in the process or entering the column.
Processes and applications that are susceptible to foam
Amine contacting and regenerating systems are notorious for foaming tendencies, with about half of the reported industrial column foaming cases coming from acid gas treating units.5 Acidic amines, such as MEA, DEA, and MDEA in their pure state, are essentially non-foaming. However, amine systems tend to have a variety of potential contaminants such as:
• Liquid hydrocarbons: Ross foams
• Oil field chemical contaminants: Ross foams and surfactants
• Corrosion products (such as iron sulphide): particulate foaming
• Amine degradation products: surfactants.
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