Entrainment foam-caused

Some materials may foam during vaporization. Stable foams may cause excessive entrainment. Foaming may be caused by dissolved gases in the liquor, by an air leak below the liquid level, and by the presence of surface-active agents or finely divided particles in the liquor. Many antifoaming agents can be used effectively. Foams may be suppressed by operating at low liquid levels, by mechanical methods, or by hydraulic methods. 

Defoamers. Foam is a common problem in papermaking systems (27). It is caused by surface-active agents which are present in the pulp slurry or in the chemical additives. In addition, partially hydrophobic soHd materials can function as foam stabilizers. Foam can exist as surface foam or as a combination of surface foam and entrained air bubbles. Surface foam usually can be removed by water or steam showers and causes few problems. Entrained air bubbles, however, can slow drainage of the stock and hence reduce machine speed. Another serious effect is the formation of translucent circular spots in the finished sheet caused by permanently entrained air. 

As the water evaporates into steam and passes on to the superheater, soHd matter can concentrate in a boHer s steam dmm, particularly on the water s surface, and cause foaming and unwanted moisture carryover from the steam dmm. It is therefore necessary either continuously or intermittently to blow down the steam dmm. Blowdown refers to the controHed removal of surface water and entrained contaminants through an internal skimmer line in the steam dmm. FHtration and coagulation of raw makeup feedwater may also be used to remove coarse suspended soHds, particularly organic matter. 

BW containing some level of contaminant that is entrained and passes with the steam into the main steam-header. Carryover is always detrimental to the steam cycle process and is primarily caused by BW priming (surging), gulping, misting, or foaming. 

Air entrainment and foaming in hydrocarbon liquids can cause operational problems with high-speed machinery in physical-chemical processes such as petroleum production, distillation, cracking, coking, and asphalt processing. 

When an open tank with a free surface is stirred with an impeller, a vortex will form around the shaft. It is important to prevent this vortex from reaching the impeller, because entrainment of air in the liquid tends to cause foaming. The shape of the free surface depends upon (among other things) the fluid properties, the speed and size of the impeller, the size of the tank, and the depth of the impeller below the free surface. 

Mechanical carry-over is the entrainment of small droplets of boiler water in the separated steam. Since entrained boiler-water droplets contain solids in the same concentration and proportions as the boiler water, the amount of impurities in steam contributed by mechanical carry-over is the sum of all impurities ill the boilei water multiplied by the moisture content of the steam. Foaming of the boiler water results in gross mechanical carryover. The common causes of foaming are excessive boiler-water solids, excessive alkalinity or the presence of certain forms of organic mailer, such as oil. 

The experiments have proved that membrane distillation can be applied for radioactive wastewater treatment. In one-stage installation the membrane retained all radionuclides and decontamination factors were higher than those obtained by other membrane methods. The distillate obtained in the process was pure water, which could be recycled or safely discharged into the environment. It seems the process can overcome various problems of evaporation such as corrosion, scaling, or foaming. There is no entrainment of droplets, which cause the contamination of condensate from thin-film evaporator. Operation at low evaporation temperature can decrease the volatility of some volatile nuclides present in the waste, such as tritium or some forms of iodine and ruthenium. The process is especially economic for the plants, which can utilize waste heat, e.g., plants operating in power and nuclear industry. 

Dissolved air is not readily drawn out of solution. It becomes a problem when temperatures rise rapidly or pressures drop. Petroleum oils contain as much as 12% dissolved air. When a system starts up or when it overheats, this air changes from a dissolved phase into small bubbles. If the bubbles are very small in diameter, they remain suspended in the liquid phase of the oil, particularly in high viscosity oils. This can cause air entrainment, which is characterized as a small amount of air in the form of extremely small bubbles dispersed throughout the bulk of the oil. Air entrainment is treated differently than foam and is typically a separate problem. Some of the potential effects of air entrainment include pump cavitation, spongy and erratic operation of hydraulics, loss of precision control, vibrations, oil oxidation, component wear due to reduced lubricant viscosity, equipment shutdown when low oil pressure switches trip, microdieseling... 

Foam, on the other hand, is a collection of closely packed bubbles surrounded by thin films of oil that float on the surface of the oil. Generally, foam is cosmetic, but it must be treated if it makes oil level control impossible, if it spills onto the floor to create a safety or a housekeeping hazard, causes air locks at high points, or is so extreme that the equipment is lubricated with foam. Small amounts of foam do not necessarily need to be treated unless the system suffers from the air entrainment conditions listed here, although the presence of the foam may be symptomatic of a more serious problem. ° ... 

Carry-over can be caused by (1) entrainment of small liquid droplets by the moving vapor (2) splashing of the violently boiling liquid directly into the vapor vent line and (3) foaming of the solution to the extent that foam is carried over into the vapor vent line. 

Foaming. Some materials, especially organic substances, foam during vaporization. A stable foam accompanies the vapor out of the evaporator, causing heavy entrainment. In extreme cases the entire mass of liquid may boil over into the vapor outlet and be lost. 

When the reaction is carried out in a low-boiling-point solvent or when one of the reactants is quite volatile, heat can be removed by allowing the solvent or reactant to vaporize. The vapors are condensed in an overhead condenser, and the liquid is returned to the reactor. The limiting factor in this design may be the allowable vapor velocity in the reactor. Too high a velocity will cause foaming or excessive entrainment of liquid, and the reaction rate per unit volume of reactor will decrease because of greater gas holdup. 

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