Surface FOAMS

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. 

Houghton et al. (HI3) have reported data on the size, number, and size-distribution of bubbles. Distinction is made between bubble beds, in which bubble diameter and gas holdup tend to become constant as the gas velocity is increased (these observations being in agreement with those of other workers previously referred to), and foam beds, in which bubble diameter increases and bubble number per unit volume decreases for increasing gas velocity. Pore characteristics of the gas distributor affect the properties of foam beds, but not of bubble beds. Whether a bubble bed or a foam bed is formed depends on the properties of the liquid, in particular on the stability of bubbles at the liquid surface, foam beds being more likely to form in solutions than in pure liquids. 

Sub-surface foam injection, Foam to Tank NFPA 11-3.2.6.3 640 1050 NFPA 11-3.2.8.2,1 hose, 50... 

Foam—stable aggression of small bubbles of lower density than oil or water, which shows tenacious qualities in covering and clinging to vertical or horizontal surfaces. Foam flows freely over a burning liquid surface, forming a tough, air-excluding continuous blanket to seal volatile combustible vapors from access to air. 

Froth flotation is an application of foams that is used to separate mineral components from each other based on their having different surface properties, typically their wettability and surface electrical charge. For example, froth flotation is the classic process used to separate copper from lead ore. The process involves having hydrophobic particles attach to gas bubbles which rise through a turbulent suspension to create a surface foam called a froth. Figure 10.2 shows an illustration of a mechanical flotation cell. This is the classic flotation device [53,91,625], First, the flotation feed particles are well dispersed into a particle suspension. Together with chemical flotation aids, such as collectors and frothers, this constitutes what is called the flotation pulp. In a mechanical flotation cell, air is fed in the form of fine bubbles and introduced near the impeller (see Figure 10.2). In addition to mechanical flotation cells, there are also pneumatic cells and cyclone flotation cells. Pneumatic... 

For absorption in a mechanically agitated gas-liquid dispersion in the absence of surface foam, they proposed... 

The main features of this technique are the absence of contact between the foam and the ambient space (i.e. no foam/gas interface) and constant capillary pressure along the whole foam height. This technique allows to study the kinetics of internal foam destruction at various capillary pressures, i.e. decrease in the specific foam surface area without destruction of the foam column. Thus, the influence of surface foam films on foam lifetime and the character of foam destruction can be estimated. 

Owing to their dense, closed surface, untreated foamed PS slabs are acoustically ineffective. Small absorption effects can be achieved by slotting, perforating or corrugating the surfaces. Foamed PS slabs are suitable for footfall-sound insulation only if the dynamic E modulus is lowered by elastifi-cation. 

Figure 22. Aging of rigid urethane foam with cut surfaces (foam blown with CFC-11) (212).

For foam separation processes, adsorption takes place in solution, the essential basis exists for solute separation by foaming. Foam consists of gas bubbles separated by thin liquid films. The liquid films are often formed by the mutual approach of two already existing liquid surfaces (e.g., two bubbles below the surface). Foam structures may vary between two extreme situations. The first is wet foam, which consists of nearly spherical bubbles separated by rather thick liquid films. The second is dry foam, which may develop from the first type as a result of drainage (i.e., foam drainage). 

Fast-mixing oxy/fuel burners can overheat the glass locally, leading to numerous glass defects such as cord and seeds. On the melt surface, foam formation or reboil is... 

Napolitano GE, Richmond JE. 1995. Enrichment of biogenic lipids, hydrocarbons and PCBs in stream-surface foams. Environ Toxicol Chem 14(2) 197-201. 

Adhesive surface Foamed adhesive layer Adhesive surface... 

An important parameter of the catalysts is the value of the specific surface which equals, for p ure foam m etals, a pproximately 0.01-0.1 m /g. F or t he m ajority o f c atalytic processes, taking place in the kinetic region, this is insufficient to produce active catalysts. Therefore, a number of methods have been developed for increasing the specific surface by direct deposition on the foam support of a catalytically active phase (up to 3-8 m /g), or using a layer of the intermediate support Y-AI2O3 (up to 20-50 m /g). Low-surface foam metals may be used in high-temperature external diffusion processes. 

Surface foam and entrained air can indeed be a serious hindrance to operations and quality in a paper miU. This can be a major economic problem recently it was estimated that the use of defoamers in the US paper industry alone in 2001 was over US 250 million and growing at over 5% annually. That is not the only cost of the problem - a much more significant impact to the papermaker would be the lost production and product quality problems that require the use of these products to control foam. 

Free air bubbles in the range of 70-100 im in diameter tend to rise to the surface of the pulp suspension, resnlting in the formation of surface foam. Bound or trapped air bubbles less than 70 pm are attached to the fibre or trapped within a fibre bundle or floe. As the entrained air bubbles rise, they may coalesce to form larger bubbles, subsequently bringing fibres and fines with them to the surface. This results in surface foam that contains fibres and fines. 

As mentioned above, problems related to surface foam and/or entrained air can manifest in many different ways. Here are just a few specific examples from paper mills. 

One option to manage entrained air and surface foam is the use of chemical additives. In many cases a chemical approach is the only viable option. These additives are commonly referred to as defoamer or antifoams. Defoamers and antifoams are complex products comprised of many components and chemistries. Considerable progress has been made over the years in the technology of foam and entrained air management in the pulp and paper industry since the introduction of improved technologies as a defoaming aid. 

Surface foam Dispersion of gas in an aqueous system at the surface Entrained air Dispersion of air in an aqueous system in the body of the systan Static foam Old surface foam from which most of the liquid has drained/dried... 

Surface foam has a non-spherical shape that may be described as polyhedral. Compared with spherical bubbles, polyhedral foam has a larger volume of air in a relative small volume of water. It contains a large number of individual air cells separated by thin water layers (shown in Fig. 4.5). 

Although there are effective mechanical means to manage entrained air and surface foam, such as the use of deculators and resolving mechanical issues, the focus here will be on chemical means to destabilise and destroy snrface foam and remove entrained air from the system. 

Fig. 4.6 Diagram depicting entrained air and the different bubbles sizes as they rise to cause surface foam...

Technology developments are moving in the direction of environmentally friendly water-based products to control surface foam and deaerate paper stock systems. Current technology allows formulation chemists to readily develop products that are capable of effectively combating both surface foam and entrained air through formulation modifications incorporating fatty alcohols in combination with other specific components. 

The first phase is to establish knowledge of the current entrained air and foam levels in the system. Key sample points should be selected and with the use of a portable EGT, the entrained air level can be established at each point in the system. Sample points identified can be before and after fan pumps, screens, cleaners, chests, and deculators. A visual audit of surface foam and deposition should also be carried out. This should include noting levels of turbulence in situations where stock and white water flows into chests which give opportunities for the introduction of air into the system. 

To identify the appropriate programme and select the best products, two laboratory test methods are useful. The first laboratory procedure measures the defoamer s effect upon surface foam from the perspectives of immediate knockdown and performance endurance. Figure 4.11 shows a photograph of the laboratory equipment used for the test which re-circulates the process sample while maintaining the actual process temperature of the sample. The sample container is transparent which allows the tester to monitor and measure foam levels as they are reduced... 

There is no substitute for hard work to survey the process systems and learn the pulp and papermaking process to understand potential incompatibilities and synergies with other wet-end additives and the process conditions, followed by laboratory studies evaluating different defoamer chemistries. Defoamers should be evaluated for their immediate effect to knock down surface foam and achieve longevity in performance, as well as impact entrained air. 

As a final note, when faced with the problans related to entrained air and surface foam the following will assist toward successful evaluation of a selected defoamer product in a pulp or papermaking systan baseline data collection, identification and quantification of avenues that lead to a positive return on investment, and a mutually agreed-to evaluation plan along with implementation phases following the evaluation plan that allow for day-to-day operation changes with contingencies. 

Uses Defoamer, drainage aid for kraft pulp mill brownstock washing operations, cold-stock systems, pulp mill bleaching and screening grind and letdown defoamer for gloss and deep tint paints Features Removes entrained air and surface foam Regulatory FDA compliance... 

Uses Defoamer, processing aid for elimination of entrained air and surface foam for pulp/paper industry, wet-end processes, size press sol ns., calender stack, waste treatment and effluent systems, paints, food-con-tact paper/paperboard Features Water-based high-efficiency Regulatory FDA 21 CFR 176.210... 

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