Foam
is always a thorny issue in the pulp&paper industry.
Foaming
reasons
Once liquid which contain a surfactant or this
kind of liquid having a large viscosity is agitated, a large amount of foam
which is not easily disappeared is often produced. The reasons why these
bubbles are more stable and not easy to disappear are as follows:
n Film
elasticity
The
ability of the liquid film to resist local thinning during the general thinning
process is film elasticity. When the liquid film has a thin point, this point
is a possible breakage.
However,
when this is further stretched, the surfactant molecules in this part will be
more reduced, and the increase in surface tension will result in a force
imbalance, and the surrounding surface will be moved toward a thin point to
balance the surface. tension. The movement of the surface layer pulls the
liquid of the lower layer together, thus preventing further thinning of the
initial weakness and, in turn, foam rupture.
This
effect can also be called the “self-healing effect”. Of course, when the
surface tension is balanced, it is also possible that the molecules in the body
fluid move out without having to move the molecules from adjacent surfaces.
However,
if this happens, there will be no activity to return to the thin part, nor will
it prevent further thinning, which will cause the foam to rupture. However,
most foaming surfactant molecules move relatively slowly from the bulk to the
surface, so the self-healing effect is dominant.
n Surface
viscosity
Surface
viscosity is a two-dimensional form of overall viscosity due to interactions
between adjacent molecules on the liquid surface. As in a typical nonionic
surfactant solution, the polyethylene glycol end adjacent to the surfactant
molecule can form a hydrogen bond that prevents or retards the rate of loss of
the foam wall and stabilizes the foam. If the viscosity of the liquid itself is
high, the rate of loss of the foam wall is slow and stable, and the foam is
stabilized.
n Electrical
double layer
Mutual repulsion For ionic surfactants, the thinning of the
foam wall continues until the charged groups on the inner and outer walls
become sufficiently close to cause electrical mutual repulsion. This mutual
repulsion prevents further thinning of the foam wall. Of course this effect is
only important for very thin foams.
n Entropy
double layer mutual exclusion
For
nonionic surfactants, when the thinning of the foam wall is carried out to a
certain extent, the mixing entropy of the polyethylene glycol end of the
surfactant is too large to penetrate each other, preventing further thinning of
the foam wall. Of course this effect is only important when the bubble is very
thin.
n Reduction
of gas diffusion between bubbles
For
foam film thicknesses exceeding 10 nm, the first two items are dominant.
Defoaming
mechanism
One
is to diffuse in the foam through the defoaming agent, and form a two-layer
film on the foam wall during diffusion. During the diffusion process, the
stabilizing surfactant is discharged, thereby reducing the tension of the local
surface of the foam and destroying the self of the foam. The effect is to
rupture the foam; the second is that the defoaming agent may enter the foam
wall, but only to a very limited extent, together with the blowing agent to
form a mixed single layer, if the cohesiveness of such a single layer is not
good, The bubble will burst.
The
commonality of these two mechanisms is that the defoaming agent must first
diffuse into the foam. This ability can be expressed by the infiltration
coefficient E. When the antifoaming agent enters the film, the ability to
spread is determined by the expansion factor S. E and S can be expressed by the
surface tension and the interfacial tension of both the defoaming agent and the
foaming medium:
E=rF-rFZ-rA
S=rF-rFA-rA
Wherein
rF and rA are each a surface tension of a foaming medium and an antifoaming
agent; rFA is an interfacial tension between the two. It is of course preferred
that the infiltration coefficient and the expansion coefficient of the
antifoaming agent are both positive, i.e. have a lower surface tension rA.
However, it is also possible that the value of the rFA term is large, so that E
may be a positive value and S becomes a zero or a negative value.
At
this time, the defoaming agent enters the foam wall but does not spread, but if
the formed mixed film lacks cohesiveness, it also acts as a defoaming effect.
Conversely, if the cohesiveness of the mixed film is similar to or stronger
than that of the original foam film, then there is no defoaming effect. When
both E and S are negative, the defoamer is definitely ineffective.
In
addition, since the defoamer droplets act on the foam wall to destroy the foam,
if the antifoaming agent has a lower water solubility, it can stay at the
liquid-air interface for a longer period of time and maintain a longer period
of time. Foam activity.
In
summary, an ideal antifoaming agent should have the following characteristics:
low surface tension than foaming medium; low water solubility, resistance to
emulsification and chemical decomposition; high diffusion rate; low
intermolecular polymerization force It does not increase the surface viscosity
of the system; it is basically non-toxic to humans and the environment; it does
not significantly increase the BOD, COD and TOD of the waste liquid.
Defoamer
Defoamers,
also known as antifoams, are used to eliminate and inhibit harmful foams
produced in industrial production. The defoaming methods are mainly physical,
mechanical and chemical. It is usually referred to as a chemical method in
which certain chemical agents are added to the foaming liquid to eliminate or inhibit
the formation of foam.
Defoamers
commonly used in the industry are generally classified into three types:
organic defoamers, silicone defoamers, and polyether defoamers. Among them,
silicone defoamers are more and more popular because of their strong defoaming
power, low concentration and non-toxicity to humans and the environment.
n Organic
defoamer
The
organic defoaming agent refers to a kind of defoaming agent mainly composed of
organic compounds such as fatty acid amide, phosphate ester, alcohol and ether.
Such as fatty acids and their glycerides, ethyl hexanol with higher carbon
number and medium carbon alcohol with diisobutyl group, as well as surfactant
sorbitan fatty acid ester. The organic antifoaming agent is suitable for use
under conditions in which the liquid shearing force is small and the foaming
ability of the surfactant is mild.
However,
the ability to eliminate dense foam is poor, and the market share has been
shrinking, so there are limitations in application. Some special industries,
such as strong acids and alkalis, require acid-base organic defoamers like
polytetrafluoroethylene.
n Silicone
defoamer
The
silicone defoamer can be compounded by a certain ratio of dimethyl silicone oil
and SiO2. The antifoam thus prepared has the characteristics of being insoluble
in water, difficult to emulsify, low in surface viscosity, low in surface
tension than some surfactants, and capable of interfering with the surface
elasticity of the foam film.
Especially
for the oil-soluble solution, the defoaming effect is better; the modified
composite silicone defoaming emulsion has better diffusibility, defoaming
ability and performance. Most of the defoamers currently used at home and
abroad are of this type.
Silicone
type defoamer is currently the most widely used, its dosage is small (10 ~
100μg / g), strong defoaming ability, low volatility, low oxidation resistance,
non-toxic, odorless, no side effects.
According
to the state, it can be divided into oil type, solution type, emulsion type,
modified oil type and powder type. The emulsion type is the most widely used
and the largest amount, and its active ingredients are mainly methyl silicone
oil, dimethyl silicone oil and phenyl group. Silicone oil, hydrophobic silica
(silica), polysiloxane polyether (-Si-O-C-), and the like.
The
silicone oil type defoaming agent has high defoaming efficiency, and the
emulsification problem is complicated. If the emulsification is incomplete, the
demulsification during use will seriously affect the application effect. It is
satisfactory for the defoaming of oil-soluble solutions, and the modified
silicone oil defoaming agent is well defoamed in the water system.
In
practical applications, it has been found that when the diameter of the
silicone particles in the emulsifier is less than 2 μm, the defoaming ability
is weak. This is because the antifoam droplets are too small to be easily
emulsified or diffused into the body fluid and cannot be defoamed at the
liquid-air interface. On the contrary, when the particle diameter is larger
than 50 μm, the storage stability is poor unless the emulsion is concentrated
to be delaminated.
In
addition, the particles are too large, and the silicone may also "oil
out" when diluted into the foaming system to reduce the defoaming ability
and produce oil stains. The particle size can be controlled by the selection of
suitable surfactants and emulsifying equipment.
In
addition, some concentration agents may be used in formulating the antifoaming
liquid. The concentration agent acts to increase the viscosity of the emulsion
and prevent the particles from coagulating with each other. Concentration
agents suitable for silicone antifoaming agents are hydroxyethyl cellulose,
alginate derivatives, synthetic hydroxyvinyl polymers, and the like.
n Polyether
defoamer
Polyether
defoamer is a kind of defoamer developed in recent years with the rapid
development of polyether industry. In the preparation, the water solubility and
oil solubility can be improved by adjusting the ratio and molecular weight of
oxyethylene and oxypropylene. It greatly reduces the surface ability of the
foaming liquid, rapidly disperses the foam, and has good defoaming and foam
suppressing ability.
The
polyether type defoaming agent mainly has the following ones: 1 linear
polyether, such as polyoxyethylene, polyoxypropylene, etc.; 2 end group is
composed of alcohol, ammonia (amine), or esterified polyether derivative, such
The antifoaming agent has strong surface activity and high defoaming ability.
Molecular weight is one of the important properties of polyethers, while the
cloud point of polyethers is related to the molecular weight and the pH of the
aqueous solution.
In
general, the polyether type defoamer has a large expansion coefficient, so the
foaming effect is strong, but the foam suppression effect is very poor; the
expansion coefficient of the silicone type is small, and the simple silicone
such as dimethylpolysiloxane The alkane has no defoaming effect, and after
emulsification, the surface tension is rapidly lowered, and a very small amount
of foaming and foam suppression can be achieved.
Dosage
and usage of defoamer
The
volume fraction of the organic antifoaming agent is generally from 0.1 x 10-3
to 4 x 10-3 (based on the substance having antifoaming activity). The silicone
defoaming agent with a defoaming active material fraction of 100% is less
directly used in the production process, which is not only costly, but also
difficult to work in a small amount, and the amount of use may cause pollution
problems. Therefore, most of the commonly used defoaming emulsions have been
formulated into silicones with a mass fraction of 1% to 2%. The amount thereof
is appropriately changed depending on the process conditions.
For
example, it is generally believed that when the dyeing plant starts to dye, the
silicone with a mass fraction of 5×10-5～3×10-4 can control the foam. In the waste liquid or waste
treatment system, it is sufficient as long as 1 × 10 -6 to 10 × 10 -6 .
Therefore,
the safest and most effective way to use defoamer is to add a low concentration
of dilute emulsion in a continuous or semi-continuous manner during the
production process. This will prevent foaming and prevent oil contamination
problems caused by silicone. When the operation is started, it is necessary to
avoid excessive addition of the silicone antifoaming agent.
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