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
Mark Pilling Sulzer Chemtech USA
s long as there are fractionating columns, there
will be issues with foaming in some of the different
chemical applications. Foaming
in columns is problematic
because it hinders the hydraulic
processes (the vapour and
liquid flows within the tower).
This is especially true with
trayed internals where liquid
and vapour are meant to
contact intimately and then
physically separate within a
stages. Figure 1 Bubbles draining in foam
Conversely, packed columns
tend to be more forgiving in foaming When the wall of a bubble ruptures, the bubble
collapses, destabilising the foam. The main cause
Generally, there are two competing issues with of bubble rupture is thinning of the liquid wall.
foaming: first, the tendency for the process to Figure 1 shows how the liquid walls thin in the
generate foam (foaminess); and secondly, the upper section of the foam as the liquid drains
tendency for the process to destroy foam (foam downward.
Foams are stabilised when liquid viscosity and
stability). As with any dynamic balance, when
production (foaming) exceeds consumption surface tension oppose the natural drainage
(foam breakage), an excess occurs. When a frac- tendency of the bubble liquid. Liquid properties
tionating column generates stable foam, the play a central role in foaming. Liquid drainage
within foam is a natural phenomenon. Liquid
column capacity will decrease.
This article will briefly discuss the fundamen- always has a tendency to drain downward (or in
tals of foams and the different types of foam the direction of any centrifugal forces). As the
formations. The effect of foaming on various liquid drains from the bubbles, the liquid walls
internals will be discussed along with design and thin and weaken, eventually rupturing the
operating methods used to mitigate foaming bubbles and breaking the foam. Any condition
that stabilises the bubble wall thickness will
and/or the effects of foaming.
stabilise the foam.
Fundamentals of foams
Surface tension gradients within localised
Foaming is essentially the encapsulation of liquid create what is known as the Marangoni
vapour within a liquid cell. Foams can be formed effect, where liquids flow from lower to higher
with a variety of methods generally associated surface tension regions. Generally, foaming
with mechanical agitation or vapour formation. tendency is proportional to this gradient. This
PTQ Q4 2015 1
Ideally. column.understanding that a smaller amount (weight) of ing the incipient formation of a second liquid smaller size particulates creates more foam phase. Ross foams Somewhat surprisingly. where the higher volatility component has a In less serious cases. To further support this conclusion. and Types of foaming shape on both viscosity and foaming in water.key factor of foaming is Without this effect. equilibrium effect. the ence of particulates tends to higher surface tension in the stabilise foam. It is widely known that the presWhen this happens. the increase in foam As discussed by Ross2. When a bubble forms in these the particles need to be removed from systems. particulate size. eliminat. the system.the particulates did not transform a non-typical rally be susceptible to foaming. Simply.3 In his troubleshooting endeavour. and instance. the lighter component evaporates from the system with filtration or totally prevented the liquid and the surface tension of the remain. Since this is an foaming system (water) into a foaming system. stable foaming is then surface tension increases and possible. the equipment can be lower surface tension. As the a stable foam. by Bolles. a hydrocarbon fluid with a high equi. Foams stabilised by surface foaming system. when a surfactant was added to the overcome by changing the system temperature. it is clear that particuFoaming can occur with or without the presence lates are generally detrimental additions to a of mass transfer. a liquid solution with an stability was not always directly proportional to incipient formation of a second liquid phase (for viscosity.1 film to thin and break. Increased tension region at the base of viscosity inhibits the drainage of the bubble. the causes the liquid to form rivulets surfactant concentration at and droplets. creating dramatic foaming within the stabilisation effects than a larger amount of column. he Also important was the reinforcement of the found that sections of the tower were approach. Figure 2 It is important to note that shows wine ‘tears’ produced as a pure liquids will not produce result of mass transfer. water and foam was produced.truth for fractionation column applications ing the incipient second liquid phase and the where the liquid solution is filtered to remove foaming subsided. An thinning bubble wall and interesting study done by Kadoi4 looks at the stabilises the foam. This ‘heals’ the Figure 2 Marangoni effect in wine foams and stabilises them.digitalrefining. the liquid surface decreases Foaming from solids/particulates as the bubble size increases. Ross foams can sometimes be However. they increase from the lower surface the solution viscosity. The potential for the presence tension gradients due to mass transfer are of particulates should always be accounted referred to as Marangoni foams. This is an unfortunate raised the temperature of the column.from forming in the process or entering the ing liquid increases and stabilises the bubble. Instead. When solids are expansion area draws liquid present in liquids. In applications for during the column engineering design stage.composition seemed to play important parts in librium amount of water or an aqueous fluid foam stability. he larger sized particles. when alcohol evaporates from the wine a surfactant is added to the on the wall of the glass. size. particulates and the worst offenders (small particles) are the most difficult to remove. the evaporaexplained well by Zuiderweg tion would have caused the bubble and Harmans. Marangoni foams All things considered.com/article/1001169 . However. be a problem. It is also important to note that with a small amount of hydrocarbon) will natu. shape. 2 PTQ Q4 2015 www. the solids stabiA good example of this in practice is discussed lised the foam. influence of particulate composition. Marangoni foaming can sized to account for the foaming.
column is high enough to bridge gaps within the and structured packings packed bed. In most systems. with about half of the reported industrial column foaming cases coming from acid gas treating units. With this near the feed. In either case. and MDEA in their pure Figure 3 Vapour and liquid flows on trays state. with the vapour remaining also affect foaming in these systems in the continuous phases and liquid rivulets and • Refinery alkylation isostrippers also can foam droplets being in the dispersed phase. DEA. foaming can substanble to foaming.digitalrefining.com/article/1001169 PTQ Q4 2015 3 . foam acts as a volumetric bubbles are stabilised. are essentially non-foaming. such as MEA. the packings are much less likely to where aqueous hydrofluoric acid is present in generate bubbles and foaming. When trays do foam. 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 Figure 4 Vapour and liquid flows in structured packing foaming • Amine degradation products: increases the froth height and causes entrainsurfactants. the vapour will flow upward through A wide variety of research has been done on the the liquid and form bubbles.relative to random packing in high vapour rate tions. ment. Reviewing random versus structured packing. the vapour flows upward through a applications. This First.6 Studies show that different tially limit tray capacity. it is accepted that packings perform we see an interesting set of counter principles. With break quickly. structured is essentially due to the nature of the devices packing has a very high capacity and efficiency (see Figures 3 and 4). However. liquid flow multiplier. www. the effective into the downcomer. Packings operate with vapour and liquid flowOther contaminants and suspended particulates ing past each other. Also.floods it. a foam layer develops on In cases where the liquid rates are low and the top of the liquid on the tray deck and then flows foaming tendency is moderate. ings are preferred in foaming applications. in systems where the foaming systems. topic of foaming with different column internals. However. when the liquid rate in a packed Difference between trays. This is why packthe hydrocarbon. In most trayed applica. The foam on the tray deck ‘liquid’ volumetric flow rate is low to moderate. the more laterally open continuous liquid layer on the tray deck and structure of random packings allows them to creates bubbles. However. This is a Ross foam condition operation. random packings. due to its streamline structure. Generally. foaming is an issue. crude types have different foaming potential. the bubbles process high liquid rates more effectively. better in foaming processes than trays do.Processes and applications that are susceptible to foam Amine contacting and regenerating systems are notorious for foaming tendencies. However.5 Acidic amines. the foam fills up the downcomer and Some other known foaming processes are: • Refinery preflash towers and long residue eventually backs onto the tray and hydraulically stripping sections are also known to be suscepti.
In amine systems. However. a ‘foam factor’ is used that derates the capacity of the device. Degradation products. the next best option is to remove the contaminant in the process itself. it should be noted that a lower surface area packing (for example. and corrosion products must be minimised. in higher liquid rate and/or highly foaming systems where the effective ‘liquid’ volumetric flow rate is higher. the foam factor for a random packed column is typically less conservative than for those used with trays. there are a variety of contaminants that can cause excessive foaming in a fractionation process. liquid hydrocarbons and greases need to be avoided. trays are used quite often in amine contactors. Similar to trays.com/article/1001169 . How to avoid foaming in the process As mentioned previously. As a first pass. Preventing these contaminants from entering the system is nearly always the most effective method to prevent foaming. For very high liquid rates (>50 m3/m2-hr) and moderate to high foaming. For very low liquid flux rates (<10 m3/ m2-hr) and low to moderate foaming. the liquid flow rate. Since they can handle foaming more effectively than trays. Although this seldom removes all the contaminants. sometimes by as much as 50%. above a liquid rate of 20 m3/ m2-hr. a known foaming application. If the contaminant cannot be removed from the feed. the standard 250 m2/m3 size) with a more vertical 60° corrugation angle would be much more 4 PTQ Q4 2015 resistant to foaming. but it must be kept in mind that the degree of foaminess and the packing geometry will have a major effect in the proper packing choice. random packings will be the preferred choice. structured packings will be the natural choice. From these studies. This effect is shown in a study by Thiele. random packings typically are also derated using a foam factor.In these cases. In the testing from Thiele. and the expected severity of foaming. Upstream contaminants (particularly from oil fields) need to be carefully monitored and removed. with some structured packing units working properly at flux rates over 100 m3/ m2-hr. In these cases. When trays are not mandatory in a foaming service. which are considered to be foaming systems. They are used mainly because some amine reactions are slow and additional residence time is desired.digitalrefining. It is important to note that structured packings with appropriate corrugation angle and hydraulic diameter have been successfully used in hundreds of gas sweetening units. For example. making random packing the better choice. However. This is because these services are often dirty and trays provide a more robust solution. www. it would seem that a reasonable ‘rule of thumb’ would be that structured packings in moderate foaming systems are typically safe at liquid flux rates below 25 m3/ m2-hr. One thing to note is that if carbon beds are used in conjunction with antifoams that the carbon beds may actually remove the anti-foam and mitigate its benefit. Structured packings may or may not use a foam factor. they are typically only used in applications where they are needed for specific purposes. The selection between random versus structured packing should be based on previous experience. the inherent hydraulic advantages of structured packing ensure that it performs well. solids. packings will be the first choice. the random packing’s ability to handle more liquid tends to overcome the vapour handling capabilities of the structured packing. but often not the most cost effective method. structured packing with a surface area of 350 m2/m3 with a 45° corrugation angle showed a significant increase in pressure drop due to foaming. this is probably a useful value for an initial review. Trays are also used in sour water strippers. Oils. With this derating factor. This is commonly done with particulates or other chemical contaminants by using a recycle stream and a mechanical filter or an activated carbon bed. another known foaming process. the amine quality must be checked and continually cleaned. it does serve to maintain them at an acceptable level. Liquid loads in these applications are typically high. the column diameters are larger so that the vapour flows are lower through the column and the tray downcomers are larger to have more residence time and lower liquid velocities. The difficulty comes where the effective liquid flux rates are between these extremes.7 What internals to use in foaming applications? Since trays are the most susceptible column internal to foaming.
it is accepted that trays operating in the froth regime are clearly more PTQ Q4 2015 5 . you can mitigate foamincreases. This is essentially just designliquid viscosity decreases. Different types of good solution is to use a feed anti-foams work well in some device with centrifugal separasystems and work poorly. While the cation is for a US West Coast correct dosage of the proper anti-foam can work refiner in the preflash tower.digitalrefining. temperature variations installation of a GIRZ in the tower feed to mitican control incipient second liquid phases. or tors (such as a Sulzer GIRZ and even promote foaming. They generally phase feed.bridging and creating a foaming opportunity.having issues with foaming when running differcal can even produce a detrimental effect. the internals need performance packing with an S shape (such as to be designed beforehand to allow for this. This removes the minimised and that the liquid surface tension gradient that feed is submerged in the normal stabilises the foams. systems. using a larger crimp size (lower surface If there is a surface tension decrease of more area packing) creates larger channels for fluid than 1 dyne/cm per theoretical stage moving up flow. All of these occur. angle can be used. The refiner was very well. vapour velocity ing by using a larger size packing that is further increases. the process components and conditions refiner can run multiple crude slates with no must be understood and correct temperature issues in the column. If it is suspected that the actual foam causing contamiincoming feed is foaming then a nants. if the system is Marangoni negative packing layer interface will also delay the onset (increasing surface tension as you move up the of foaming in the bed. First. this process can be expected to have relative liquid flux rates. care needs to be costly and is often empirical. the liquid rate decreases. Directionally. there are only a few design www. this can generally be more easily down the corrugations without predicted with a review of the physical proper. taken that the vapour and liquid with the proper formulation separate without generating being dependent upon the foam. The smooth vertical trancolumn). MellapakPlus) to minimise liquid hold-up at the Conversely. In this gate foaming. more liquid vaporises. the improper rate of the same chemi. Sulzer recommended the As mentioned earlier. this should not be serve to assure a uniform surface an issue other than to ensure tension on the liquid portion of that the feed momentum is the foam. Finally. a more vertical crimp column pressure should have a similar effect. ties of the components in the process simulation. A decrease in With structured packing. in other HiPer inlet cyclone). sition between the packing layers can be seen in Figure 6. foaming is not expected. the feed Anti-foams are commonly used distribution should not create to reduce the foaming tendency foaming. Also. using a high foaming issues.com/article/1001169 Trays For tray designs. How to minimise the effects of foaming with equipment design Packing With packings.ing with a foam factor. With a two The use of anti-foam can be phase feed.How to deal with foaming in the process changes possible. rences tend to inhibit foaming. The same can be said Figure 5 Sulzer GIRZ feed device One example of a GIRZ applifor dosage rates. as the system temperature With random packing. thus increasing their flexiadjustments need to be made as possible. The unit is now on-line and the case. With a total liquid of the process. In this case. and the away from flood. This will also delay bridging due to higher the column. distributor liquid. such as a 60° (X Style) packFor mass transfer Marangoni positive systems ing corrugation. This will allow the liquid to flow that promote foaming. ent crude slates. bility and profitability.
2-4. In the University of Texas with a weir spray regime. For trays operating in the froth when high liquid rates are present. Another helpful modification can be to increase the tray’s spacing. In destroy the foam. and using For dry drops: larger deck orifices. dry drop is a good indicator of foaming shift a tray’s operation from froth to spray susceptibility. A successful case liquid velocity does not exceed 0. that may be formed.susceptible to foaming than In froth regime applications.2 m/s).6 This would indicate that large downcomers with a slope from top to bottom would be preferred. process and equipment modifications can be www.9 At these rates.8 m/s). It is recommended that tray to more easily accommodate the foam the downcomer be large enough so that the clear build-up prior to flooding. However.8 increased downcomer residence time is less beneficial than decreased downcomer velocity. column designs can droplets above the deck. on the tray deck is blown into However. the following guidelines are depth on the tray deck. study using these techniques is discussed by Another more conservative recommendation is for a limit of 0. likely still be altered to control destroying any small bubbles Figure 6 Sulzer MellapakPlus foaming. lower the liquid depth on a tray is to increase foaming tendency is low the number of liquid passes. Conclusions Figure 7 Shell HiFi Plus Trays 6 PTQ Q4 2015 Foaming in susceptible fractionation columns is a phenomenon that is essentially unavoidable. it was found that higher continuous phase and the liquid vapour rates created more foam. It is quite calculation that represents vapour momentum difficult to force operation in the spray regime entering a tray. those operating in the spray there is too much liquid on the regime.digitalrefining. As an examthe froth regime. In amine columns where liquid regime include lowering the effective liquid rates are high. The most effective way to • <40mm H2O (vapour hole velocity of 4. Methods to regime. increasing the vapour proposed by Shiveler:10 velocity through the deck orifices. The vapour side dry pressure Unfortunately in foaming packing drop of a tray is a common applications. When foaming cannot be avoided.06 m/s. steps can be taken to eliminate and/or minimise the formation of foaming. by understanding the foaming process and the root cause of the foaming.11 Generally. it is seen that Resetarits. most trays are designed to operate in a froth regime. (vapour hole velocity of pass trays (shown in Figure 7) are often used for 4. froth and foam height through the liquid pool create studies were conducted at the foaming opportunities. allowing the foaming applications.com/article/1001169 .• 40-50mm H2O. This is due to the tray deck to allow a practical balance between foam generatransition to a spray regime and tion and foam destruction.10 m/s. low to moderate tendency Tray downcomer design is also important for this purpose. Shell HiFi multi. vapour bubbles ple. vapour is in the load of 55 m3/m-hr.
2004 AIChE Annual Meeting. Repke J U. 79. 6 Barber A D. Hydrocarbon Processing. Mar 1992. Chem. Foaming in crude distillation units. 1978. He holds a BS degree in chemical engineering from the University of Oklahoma. 3. HiPer. 2015 AIChE Spring Meeting. 89-108. 15. Oct 1967. van den Akker R M A. 1958. No. 1975. 2011. 10 Shiveler G H. Vol 9. 8 Resetarits M L. Shell HiFi and Shell HiFi Plus are trademarks of Shell. Relationship between foam stabilization and physical properties of particles on aluminum foam production. References 1 Zuiderweg F J.156. 81 Part A.com/article/1001169 A. I & EC Process Design and Development. 9 Redwine D A. 2 Ross S. Froth and foam height studies. IChemE. 1912-1919. Steps for trouble-shooting amine sweetening plants. Trays inhibit foaming. 4 Kadoi K. Foaminess of binary and ternary solutions. and MellapakPlus are trademarks of Sulzer Chemtech AG. No. The solution of a foam problem. LINKS More articles from: Sulzer Chemtech USA More articles from the following categories: Corrosion/Fouling Control Mass Transfer & Separation PTQ Q4 2015 7 . 3 Bolles W L. 1561-1565. The influence of surface phenomena on the performance of distillation columns. Hangx G W www. I ChemE. Wandke H. 5..digitalrefining. Harmens A. Navarre J L. Trans. 525-532. Chem. 52. Hydrodynamics of foaming systems in packed towers. 48-52. Process Des. Nishioka G. Chem Eng Prog. 17 (4).1/15-35. Eng. The Journal of Physical Chemistry. Mark Pilling is Manager of Technology with Sulzer Chemtech USA where he oversees the development of mass transfer equipment and specialises in applications for various process technologies. Jan 2003. GIRZ. Vol. Ogboja O. What caused tower malfunctions in the last 50 years. Wiehler H. Wozny G. 7 Thiele R. Kister Distillation Symposium. Sept 1967. 5 Kister H Z. Hydraulic studies in sieve tray columns. Vol. Materials Transactions. Eng. Monkelbaan D R. Nakae H. 429-443. Van Winkle M.made to successfully deal with the foaming to maintain a properly operating column with efficient operation. 10. 11 Thomas W J. Ind. Sci. Wijn E F.. Symposium Series No. Dev. Flint E M.