Defoamer and defoamer in papermaking industry

Defoamer: in the pulp and paper industry (short paper industry), foam treatment is a thorny problem in production. The operation of the pulping unit from the steam ball to the coating process (washing, filtering, bleaching, dewatering, papermaking, sizing, coating, etc.) has different degrees of foam. Foam is a dispersed system with a large number of bubbles dispersed in liquid continuous phase. Its dispersed phase is gas, which brings difficulties to industrial production, such as reducing production capacity, wasting equipment capacity, affecting product quality and affecting normal production. Therefore, how to control foam effectively in the production process has long been emphasized by researchers. This paper briefly discusses the causes of foam formation, the mechanism of eliminating bubbles and the commonly used defoamer.
1. Foaming reason
When a liquid containing a surfactant or a liquid with large viscosity is stirred, a lot of bubbles that are not easy to disappear are often produced. The reasons why these bubbles are relatively stable and not easy to disappear are as follows:
1.1 membrane elasticity
The resistance of liquid membranes to local thinning during general thinning process is membrane elasticity. When there is a thin point in the liquid film, this point is the possible rupture site. However, when this point is further stretched, the surfactant molecules in this area will be more reduced, and the surface tension will increase, resulting in unbalanced force, pulling the surrounding surface to move toward the thinner point to balance the surface tension. The movement of the surface layer will pull the liquid of the lower layer together, thus preventing further thinning of the initial weakness and the resulting bubble rupture. This effect can also be called "self-healing effect". Of course, when the surface tension is balanced, it may be that the molecules in the main body fluid move out without having to move the molecules by the adjacent surface. But if this happens, there will be no return to rarefied parts or further thinning, resulting in foam bursting. However, most of the foaming surfactant molecules move very slowly from the main body to the surface, so the self-healing effect is the main one.
1.2 Surface Viscosity
Surface viscosity is a two-dimensional form of total viscosity, which is caused by the interaction between adjacent molecules on the liquid surface. For example, in the typical non-ionic surfactant solution, the polyethylene glycol end of the surfactant molecules can form hydrogen bonds to prevent or slow down the loss rate of the foam wall and stabilize the foam. If the viscosity of the liquid itself is high, the loss rate of the foam wall will be slow and stable, and the foam will be stable.
The 1.3 electric double layer mutual exclusion effect on the ionic surfactant, the thinning of the foam wall will continue until the charged groups inside the inner and outer walls become sufficiently close and cause electric mutual exclusion. This mutual exclusion stopped the further thinning of the bubble wall. Of course, this effect is important only for very thin bubbles.
1.4 Entropic Double-Layer Mutual Exclusion
For non ionic surfactants, when the thinning of the foam wall reaches a certain extent, the mixing entropy of the peg end will be too large to penetrate into each other to prevent further thinning of the foam wall. Of course, this effect is important only when the bubble is very thin.
Reduction of gas diffusion between 1.5 bubbles
For the thickness of foam film exceeding 10nm, the first two items are the main ones.
2. Defoaming mechanism
One is to form a double-layer membrane on the foam wall by dispersing and diffusing in the foam through the defoaming agent. During the diffusion process, the surfactant with stable effect will be discharged, and the tension on the surface of the foam will be reduced, the self-healing effect of the foam will be destroyed, and the bubble will burst. The two is that the defoamer can enter the foam wall, but only spread to a very limited extent, forming a mixed sheet with the foaming agent. If the cohesion of these monolayers is poor, the foam will burst.
The common principle of these two mechanisms is that the defoamer must first diffuse into the foam, which can be expressed by the infiltration coefficient E. When defoamer enters the membrane, the dispersion ability depends on the expansion coefficient S. E and S, which can be expressed by the surface tension and interfacial tension of defoamer and foaming medium.
In E = rF-rFZ-rAS = rF-rFA-rA, rF and rA are the surface tension of foaming medium and defoamer respectively, and rFA is the interfacial tension between them. Of course, the infiltration coefficient and expansion coefficient of the best defoamer are both positive, i.e. with lower surface tension rA. However, it may also have larger rFA term value, so that E may be positive and S becomes zero or negative. At this point, the defoaming agent enters the foam wall but does not expand, but if the mixed film formed lacks cohesiveness, it will also play a defoaming role. On the contrary, if the cohesive film is similar to or stronger than that of the original foam film, then there will be no defoaming effect. In addition, because the defoamer droplet acts on the foam wall and destroys the foam, if the defoaming agent has low water solubility, it can stay at the liquid air interface for a longer time and maintain a long time defoaming activity. In summary, an ideal defoamer should have the following characteristics: lower surface tension than foaming medium; lower water solubility, and resistance to emulsification and chemical decomposition; higher diffusion rate; lower intermolecular polymerization force, which will not improve the surface viscosity of the system; basically non-toxic to people and the environment; and will not significantly increase BOD, COD and TOD of waste liquor.