Foams - American Chemical Society

12 Antifoaming and Defoaming in Refineries

Downloaded by UNIV LAVAL on July 8, 2014 | http://pubs.acs.org Publication Date: October 15, 1994 | doi: 10.1021/ba-1994-0242.ch012

V. E. Lewis and W. F. Minyard Refinery Process Chemicals, Nalco Chemical Company, P.O. Box 87, 7701 Highway 90-A, Sugarland, TX 77487 This chapter is intended as an introduction to defoaming and antifoaming in refinery and petrochemical applications. A brief look at defoaming theory is presented along with chemistry of antifoams. This is followed by examination of antifoam applications in refineries and petrochemical plants. The causes of foam formation,fromdemulsifying surfactants to dispersants, are dealt with starting with the desalter and proceeding to the distillation towers and to the coker. Unit diagrams are provided, and a description of each process and how antifoams are used in the particular process is presented. XuAMING IN REFINERY PROCESSES leads to a number of operating problems. Foaming can occur in both aqueous and nonaqueous systems and in a variety of process types such as distillation, extraction, gas and liquid scrubbing, and other separations. The economic consequences of uncontrolled foaming can be significant. Foaming can cause serious losses in throughput and require expensive and impractical operating changes. When crude oil enters a refinery, thefirstvessel in which it is treated is the desalter. Here it is mixed with water to remove any salts that may be present. In this process, an emulsion is formed, and it must be broken prior to further treatment. The majority of the emulsion is broken by an electric grid; however, chemicals called emulsion breakers (organic salts) are also used to aid this process. The desalted crude is then fractionated by using a series of distillation towers at pressures from atmospheric to vacuum. In each of these towers, potential impediments to processing exist. These take the form of corrosion and fouling (deposit formation). Corrosion inhibitors are used 0065-2393/94/0242-0461$08.54/0 © 1994 American Chemical Society

In Foams: Fundamentals and Applications in the Petroleum Industry; Schramm, L.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.


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FOAMS: FUNDAMENTALS & APPLICATIONS IN THE PETROLEUM INDUSTRY

to combat corrosion on the metal surfaces in the refinery or petrochemical plant. There are two classes: filmers and neutralizers. Filmers prevent corrosion by spreading a thin film on the metal surface, and neutralizers actually react with any acidic species that may cause corrosion. The bottoms (highest boiling material) from the vacuum distillation tower are sent to the coker, where the heavy, viscous residual oil is cracked to produce coke, light naptha, and distillate. Corrosion can still occur in this vessel and is treated with corrosion inhibitors. In addition, each of the chemicals described can end up in the coker because they are very high in molecular weight and boiling point. Corrosion inhibitors, emulsion breakers, and dispersants, which are substances that prevent the deposit of polymers or other insoluble particles on vessel surfaces, all serve a valuable purpose in that the refinery could not be operated effectively, or in many cases at all, without them. However, they do have one drawback. They all have surface activity and can stabilize foam formation in any of the vessels in which they are present. In addition, the crude oil itself may contain elements that would stabilize foam formation. Although sometimes petrochemical processes differ vastly from refinery processes, they also have conditions that can favor foam stabilization. Foam formation can be as costly as any of the other types of problems such as corrosion, fouling, or emulsions. A distillation tower, whether a refinery atmospheric tower or a petrochemical debutanizer (a distillation tower that removes hydrocarbons that boil in the range of butanes, butènes, or butynes) that is filled with foam can no longer provide the desired separation of compounds. Quite simply, in refinery or petrochemical operations, foam costs money; either in reduced capacity or contamination of products. To combat problems in these and other vessels, a group of compounds known as antifoams or defoamers have been developed. Although these terms are used interchangeably, they actually have specific definitions that apply to the method by which they are used. A defoamer is a compound added to a system that is already foaming. Its effectiveness is often expressed in terms of its "initial knockdown" capability, which is its ability to minimize foam production. An antifoam, on the other hand, is added to a system prior to foam formation and thus inhibits this process. Figure 1 shows a graphic representation of defoamers and antifoams. The particular stream that was treated, styrene-butadiene latex, is outside the scope of this text, but the example is useful to help define defoamer and antifoamer. In this evaluation, a glass column with a steam inlet attached to the bottom is used. The column is graduated in centimeters; 90 cm is the top of the column. Latex was added, and steam was introduced into the bottom of the column, producing foam. Foam height was allowed to build up to 70 cm; then the defoamer was added. The initial knockdown time period for each defoamer was about 2 s (given by the second


12.

LEWIS & MTNYARD Antifoaming and Defoaming in Refineries


80 1

463

I

0

7 2

20 14

60 120 180 240 300 30 90 150 210 270 TIME IN SECONDS

Figure 1. Comparison of hydrophobic silica defoamer action using a styrène—butadiene latex.

point on the graph for each compound). After this initial knockdown, the products then continued to inhibit foam production for the entire test period. At this point, they were acting as antifoams. Before discussing specific classes of chemicals used as antifoams, a brief discussion of general antifoam characteristics is in order. Because foam formation is a surface phenomenon; that is, it occurs at a gas-liquid interface, an antifoam must concentrate at the surface. It must also be located at the gas—liquid interface. If an antifoam is to be effective it must be able to enter the film that makes up the foam bubbles and spread across the film surface. Equations 1 and 2 define the entering coefficient, E, and the spreading coefficient, S, of an antifoam with respect to a particular foaming medium.

In these equations, a

m

Ε = a

m

+ a

S = a

m

- o

ma

m&

- σ

Ά

~ σ

Ά

(1) (2)

represents the surface tension of the foaming


464


medium, σ is the surface tension of the antifoam, and a is the interfacial tension between the two (1). For the antifoam to enter the film, Ε > 0, and if the antifoam is to spread across the film bubble surface, then S > 0. As the antifoam spreads, it takes large amounts of the film and surfac tant with it. This spreading leads to a weakened film and eventually bub ble rupture (2, 3). Although accurate, these two equations represent experiments performed on static systems. In a dynamic system, such as a delayed coker or other refinery vessel, the equations may not be com pletely representative. Regardless of how well an antifoam penetrates a foam film or how well it spreads, one other criterion is necessary for good foam inhibition. The antifoam must be insoluble in the foaming medium. If a compound is soluble in a foaming system, it cannot act as an antifoam. In fact, in some cases, it may even increase foaming. Shearer and Akers (4) have shown this effect by using silicone oils as antifoams for different hydrocarbon oils. The silicone oil used was a poly(dimethylsiloxane) with a viscosity of 1000 centistokes (1 cSt = 1 mm /s). At low concentrations, this antifoam is soluble in No. 555 oil and actually increases foaming. After the hydro carbon is saturated at about 300 ppm, it is insoluble and acts as an anti foam. The same silicone oil is insoluble in No. 702 oil at all concentra tions and inhibits foam formation. As concentration increases, foam inhi bition gets better.


Ά

m a

2

Chemistry of Antifoams

Hydrophobic Silicas. Because foaming is a surface phenomenon, any antifoam used must concentrate at the surface (or gas—liquid inter face). Hydrophobic silicas, which are silicas that have been treated with a compound that causes them to float on the top of water, have been used to fulfill this function for almost 30 years. U.S. Patent 3 408 306 (5) dis closes the use of a hydrophobic silica dispersed in a hydrocarbon oil. Hy drophobic silica for this composition, which is still in use today, is made either by continuous ("dry roast") or batch process. In either process, precipitated silicas rather than silica gels or fumed silicas are typically used to make antifoams. During a continuous process, silicone oils, usual ly poly(dimethylsiloxane), are sprayed onto a bed of hydrophilic silica. The bed is heated to temperatures ranging up to 300 °C, and reaction times are up to 20 h. At these temperatures and reaction times, bond for mation between the silica particle and silicone oil may occur in addition to simple coating of the particle. After treatment, the silica particles may be blended with a hydrocar bon oil and any other additive as desired. Blending with hydrocarbon oil



12.

LEWIS & MINYARD

Antifoaming and Defoaming in Refineries

465

is critical for maximizing effectiveness of hydrophobic silica. Hydrophobic silica by itself or blended as an emulsion in water has no defoaming ability (6, 7). This phenomenon is depicted in Figure 2, which shows a compari son between two hydrophobic silicas when evaluated in a latex-foaming application (the blank is a sample of untreated latex). Product A is a silica—silicone blend that has been emulsified in water. Product Β is a hy drophobic silica in a naphthenic oil. For this evaluation latex samples were dosed with Product A and B, separately, at levels of 250 ppm. As the graph indicates, A had almost no activity in this evaluation, and Β kept the foam height at about 40 cm throughout the full 5-min test. Hydrophobic silica production via batch processes is somewhat dif ferent. A slurry of hydrophilic silica in oil is treated with the hydrophobic agent and a catalyst. The hydrophobic agent may be either silicone poly mer containing a reactive end group or a small reactive silicon-containing molecule such as trimethylchlorosilane. Typical silicone polymers used in this process are hydroxy-terminated poly(dimethylsiloxane)s. The catalyst

100

Time in Seconds

Figure Ζ Comparison of hydrophobic silica antifoam properties. The blank is an untreated styrene—butadiene latex. Compound A is an aqueous blend. Compound Β is a hydrocarbon blend



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is alkaline in nature. Organic polyethyleneamines are often used for this purpose. Figure 3 shows an example of the reaction as carried out using a hydroxy-terminated poly(dimethylsiloxane). Extent of reaction is measured for this, and the continuous process, by hydrophobic index (HI). HI is measured by placing hydrophobic silica in water that contains varying percentages of an alcohol. The percent alcohol at which the silica starts to become wet (sink in the solution) is defined as its hydrophobic index. Methanol and 2-propanol have both been used for this measurement. 2-propanol, which has two more carbons than methanol, is the less polar of the two alcohols. Owing to this fact, treated silicas become "hydrophilic" in 2-propanol-water solutions sooner than they do in methanol-water solutions. Thus, the same silica will have a higher HI in a methanolwater solution than in a 2-propanol-water solution. In Figure 3, the hydrophobic index was measured using a 2-propanol-water solution, and this hydrophobic silica just started to sink in a 20% solution of 2-propanol in water. Small reactive silicon-containing molecules, such as chlorosilanes or silazanes, are also used to make hydrophobic silicas. Used in place of large silicone polymers, these silanes or silazanes react rapidly with the reactive sites on the silica particle. During this reaction, there is less steric crowding at the reactive sites on the silica particle, which allows for a greater portion of the silica to react with the hydrophobic agent. U.S. Patent 3 338 073 (8) discloses the use of dimethyldichlorosilane to form a unique bridging silicon ether. In a batch process, hydrophilic silica (typically containing ~0.5% moisture) is added to the carrier oil and dehydrated to a water content of

Foams - American Chemical Society

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