This is a continuation-in-part-in-part of application Serial No. 07 769,602 filed October 1, 1991.

FIELD OF THE INVENTION

This invention relates to the control of foam generated during papermaking operations. Of specific interest is the foam found in black liquor residual containing process streams.

BACKGROUND OF THE INVENTION

Foam can cause a serious problem in pulp mills, paper mills and in effluent treatment. Lack of adequate foam control may result in curtail¬ ment of production or diminished product quality. Foam is a colloidal system in which a gas is dispersed in a liquid. Foam can exist either as bubbles of entrained air in bulk or as a combination of entrained gases and surface foam. Foams are thermodynamicaliy unstable and are stabi¬ lized by two basic mechanisms in pulp and paper process stream:

1 The adsoφtion at the air/water interface of surface active materials such as salts of rosin acids, fatty acids and lignin;

2. The concentration of finely divided solid particles around each bubble such as starch, cellulose, fines and fillers, etc.

Foam caused by the presence of residual black pulping liquor is believed to be stabilized primarily by the first mechanism whereas foam in paper machine white water is stabilized primarily by the second mechanism

Defoamβrs, which, when added to a foaming liquid, prevent the formation of bubbles or cause small bubbles to coalesce are well known to those skilled in the art In general, defoamers/antifoams are usually comprised of either high melting point hydrophobic materials such as stearic acid, ethylβne-bistearamide, saturated fatty alcohols or concen¬ trates of various low HLB surfactants such as propoxylated and/or ethoxylated esters of various fatty acids and/or polyhydric alcohol fatty acid esters.

Surfactant-based and hydrophobic particle antifoams behave quite differently Two different mechanisms have been proposed to explain la¬ mella rupture for these two types of antifoams In the mechanism of hydro- phobic antifoams the particle tends to adsorb at the air/liquid interface in the lamella When the lamella drains sufficiently, due to capillary and gravitational forces, the particle begins to bridge the other side of the la¬ mella, which causes a hole in the film and in turn, initiates rupture. The contact angle between the film and the particle, the melting point and the particle size and shape are all very important in this mechanism

The surfactant-based mechanism is quite different. Initially, foam stabilizers adsorb at the air/liquid interface and defoamers disperse in the medium. Before the film starts to burst, its surface usually will be indent¬ ed, which will cause weak spots to form. Indentation causes a local in- crease of surface area, which disturbs the equilibrium distribution of sur¬ factant (i.e., concentration of surfactant in the stretched area is lower than that in the undisturbed areas) and produces a surface tension gradient. Thermodynamicaliy, a new equilibrium must be established. This can be done in two ways. First, the foam stabilizers from the areas of lower surface tension are pushed toward the weak spot until the sur¬ face tension is uniform. The movement of the surface toward the weak spot carries an appreciable volume of underlying liquid along with it. Thus, the weak spot in the film, which was made thin by indentation, is restored to its original thickness and the film becomes stable. Second, when the antifoam molecules diffuse much faster than foam stabilizers, the surface tension gradients will be erased so that the indented spot cannot be restored to its original thickness and the region will remain mechanically weak. Having diffused to the weak spot, an effective anti¬ foam must form a loosely packed noncoherent film. Loose packing of the surfactant molecule in the film increases the rate of diffusion of the gas between bubbles, which in turn increases the drainage rate, causing the film thickness to thin out faster. In other words, the antifoam must have a high surface area per molecule. When the thickness of the lamella be¬ comes small enough (due to capillary and gravitational forces), surfac- tant-based antifoams begin to spread onto the surface and eventually rupture the film. It should be kept in mind that the surface viscosity also affects the defoaming performance. A high surface viscosity tends to retard the drainage rate and spreading rate which, in turn, decreases the antifoam effectiveness.

The above mechanism is applicable for mediums which have a low surface density (i.e., fine paper white water systems). When the surface charge density is highly positive or negative (e.g., black liquor), the anti¬ foam, which was found to be quite effective in low surface charge density, loses its effectiveness. The rationale can be explained as follows. In mediums which have a high negative surface charge density, such as black liquor, foam is caused by lignin, resin and fatty acids from the wood. Sulfate soaps derived from pitch may also act as foaming agents. Kraft lignins have been shown to be surface active and good foam stabi- lizing agents. Lignin residues stabilize the foam via steric stabilization. Thus, in black liquor, the zeta potential is much more negative than is found in fine paper white water due to sulfate soap and lignin residue. In other words the electrostatic repulsion, due to sulfate soap, and steric stabilization, due to lignin residue, contribute to foam stabilizers in black liquor and thus, retard the foam drainage rate. Therefore, a conventional antifoam (e.g., polyether surfactant and/or polyhydric alcohol fatty acid ester), which works well in fine paper white water, loses its effectiveness in black liquor systems.

GENERAL DESCRIPTION OF THE INVENTION

The above and other problems in the field of paper processing antifoams are addressed by the present invention. It has been discov¬ ered that the ability of conventional antifoamers to reduce foaming, particularly in black liquor, was found to be considerably enhanced by combining them with a small amount of a polymer. The most effective polymers are those that are cationic and are either homopolymers or condensate polymers.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures I - VIII are plots which show the defoaming capabilities of numerous defoaming compositions under simulated paper processing conditions.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, it has been discovered that the antifoam characteristics of conventional antifoams are signifi¬ cantly augmented by the addition of cationic polymers to the foaming medium. Conventional antifoaming compounds include, among others, polyether surfactants, polyhydric alcohol fatty acid esters and polybutene. The polymers useful in the practice of the present invention are certain homopolymers and condensate polymers. The desired homopolymer is polyethyleneimine, and the desired condensation polymers are the products of dimethylamine plus epichlorohydrin, and a terpolymer of epichlorohydrin plus dimethylamine plus ethylene diamine (EPI/ DMA/EDA). The number average molecular weights of the above identified polymers should be between 5,000 and 2,000,000.

The defoamer treatment composition according to the present invention comprises at least one conventional defoamer plus at least one polymer as defined above. The amount of polymer present in the de- foamer composition is from 0.1 % to 20%, by weight, of the total compo¬ sition with the remainder consisting of conventional defoamer compounds as defined above. If necessary or desired, the chemical components may be diluted or dissolved in water.

Once the treatment composition is prepared it is then added to the pulp process stream at or prior to locations where foaming, especially from black liquor, is a problem. Such a location might be the digester or in the feed lines or reservoirs containing black liquor. The defoamer composition of the present invention should be added in a sufficient amount to maintain a concentration in the black liquor of from 0.1 to 100 ppm, by weight. The most preferably concentration range would be, however, from 1.0 to 50 ppm, by weight.

The following mechanism is proposed to explain why this novel de¬ foamer composition works so effectively in the foaming medium, especial¬ ly black liquor. Due to opposite charges, cationic polymers will tend to adsorb on the bubble surface to "bridge" another surface or to form "patches" at the bubble surface. When "patches" or "bridges" are formed, the drainage rate will improve, causing the conventional defoamer to enter the lamella, spread and rupture the bubble. However, replacing the polymers with various cationic surfactants does not have a significant effect on foam destabilization. This is because most of the cationic surfactants have low charge densities and their surface head group area is small, thus, they cannot adsorb effectively onto the bubble surface to form a "patch" or to "bridge" another bubble surface like polymers.

Examples

To illustrate the efficacy of the invention, diluted pulp mill black liquor is used as the foaming medium. The medium is circulated from a calibrated reservoir (in centimeters) via a pump and is returned back to the reservoir. This action agitates the medium which, in turn, causes foam. A known amount of the defoamer is introduced into the test cell

before the pump is turned on. The calibration of the test cell ranges from 0 to 295 cm; the medium usually occupies the first 135 cm. A longer time required for the foam to reach a certain level indicates a better defoamer. The time is recorded when the foam reaches the 290 cm level. The sources of the compounds tested are disclosed in the legend following Example 8.

Example 1 :

An experiment comprised of six defoamers was conducted using an antifoam recirculation test cell as described above. Various defoam¬ ers were added to 1.5% diluted black liquor, pH-adjusted with sulfuric acid (pH=4.8, T=140°F) at a dosage of 75 ppm and the foam height was recorded over time. The defoaming composition of the six samples were as follows:

1. 33% solids Polyethyleneimine solution polymer (M. W. approximately 20,000)

2. A 100% active blend of a polyglycol ester and a polyether surfactant.

3. 50% Polybutene L14, 50% Peg 600 DO

4. 44.4% Polybutene L14, 44.4% Peg 600 DO, 8.9% of 0.50%

Poly-N-vinyl pyrrolidone (M.W. approximately 360,000), 2.2% of 33% solids Polyethyleneimine (M.W. approximately 20,000)

8

5. 48.5% Polybutene L14, 48.5% Peg 600 DO, 2.9% Polyethyleneimine (33% solids) (M.W. approximately 20,000)

6. 47.6% Polybutene L14, 47.6% Peg 600 DO, 4.8% Polyethyleneimine (33% solids) (M.W. approximately 20,000)

The results of the recirculation test cell are plotted in Figure 2. It can be seen that there was considerable reduction in foam height with the defoamers of the present invention (defoamer compositions 5 and 6). (See Example 2 concerning defoamer composition 4.)

Example 2:

Defoamers of the present invention (defoamers 4, 5, 6; i.e., a blend of a Polybutene, Peg 600 DO and polyethyleneimine and/or poly¬ vinylpyrrolidone) were tested against 100% active Polybutene/Peg 600 DO (defoamer 3) using a recirculation test cell. These defoamers were added to 1.5% diluted black liquor, pH-adjusted with sulfuric acid (pH= 4.8, T=130°F) at a dosage of 75 ppm and the foam height was recorded. The formulations for these defoamers are listed in Example 1. Figure 2 clearly shows that the defoamer compositions of the present invention (defoamers 4 and 6) control the foam much better than defoamer 3 and thus, have a much greater product performance.

A comparison of the results, defoamers 4 and 5 in Figures 1 and 2 revealed that replacing a small amount of polyethyleneimine (PEI) with poly-N-vinyl pyrrolidone (PVP) increases the defoaming performance sig¬ nificant!/ at low temperature (T=130°F, Figure 2) the effect of PVP on

efficacy is much less pronounced. The result indicates that low tempera¬ ture enhances nonionic polymer-anionic surfactant complex.

Example 3:

An antifoam recirculation test cell was used to test additional de¬ foamer compositions of the present invention (defoamers 8 and 9) against a 100% active conventional surfactant-based antifoam (defoamer 7). These defoamers were added to 1.5% diluted black liquor, pH-adjust- ed with sulfuric acid (pH=4.6, T=135°F) at a dosage of 45 ppm and the foam height was recorded. The defoaming compositions for these four samples are listed below:

7. A blend of polyglycol ester and a polyether surfactant

8. A blend of polyglycol ester and polyether surfactants and 2.5% (active) dimethylamine epichlorohydrin condensate polymer (M.W. approximately 20,000)

9. A blend of polyglycol ester and polyether surfactant and

3.7% (active) EPI/DMA EDA polymer (M.W. approximately 500,000)

10. Dimethylamine epichlorohydrin condensate polymer (40% solids) (M.W. approximately 20,000)

Results are plotted in Figure 3. It can be seen that the defoamers of the present invention (samples 8, 9) are far superior to a conventional surfactant-based antifoam and to the polymer by itself.

Example 4:

An experiment comprised of four defoamer compositions was con¬ ducted using an antifoam recirculation test cell. The defoamers were added to 1.5% diluted black liquor pH-adjusted with sulfuric acid (pH=4.6, T=135°F) at a dosage of 45 ppm and the foam height was recorded. The four samples were as follows:

11. Ethoxylated fatty triglyceride

12. Ethoxylated fatty triglyceride and 1.6% (active) polyethyleneimine polymer (M.W. approximately 20,000)

13. Ethoxylated fatty triglyceride and 4.9% (active) EPI/DMA/EDA polymer (M.W. approximately 400,000)

14. Dimethylamine epichlorohydrin condensation polymer (40% active solids) (M.W. approximately 10,000)

Results are plotted in Figure 4. Addition of a small amount of cationic polymer to ethoxylated fatty triglyceride (samples 12, 13) increases the efficacy considerably.

Example 5:

The following defoamers were tested in 1.5% diluted black liquor, pH-adjusted with sulfuric acid (pH=4.8, T=140°F) at a dosage of 75 ppm:

15. A blend of high molecular weight unsaturated fatty ester, PEG 400 DO, and water

16. A blend of high molecular weight unsaturated fatty ester, PEG 400 DO, water, POE (40) sorbitol hexaoleate, and 1.5%

(active) EPI/DMA/EDA polymer (M.W. approximately 500,000)

17. A blend of high molecular weight of unsaturated fatty ester, PEG 400 DO, POE (40) sorbitol hexaoleate, and 2.8% (active) EPI/DMA/EDA polymer (M.W. approximately 400,000)

18. EPI/DMA/EDA polymer (50% active solids) (M.W. approximately 500,000)

Results are plotted in Figure 5. It can be seen that a small amount of cationic polymer with a conventional defoamer increases the perform¬ ance considerably.

Example 6:

A defoamer composition of the present invention (defoamer 20) was tested against conventional defoamers (defoamers 19, 21 , 22 and 23) using a recirculation test cell. These defoamers were added to an actual paper machine linerboard white water media (pH=4.8, T=125°F) at a dosage of 7.5 ppm and the foam height was recorded. The defoaming compositions for these five samples are listed below.

9. A blend of high molecular weight unsaturated fatty ester, PEG 400 DO and water

20. A blend of high molecular weight unsaturated fatty ester, PEG 400 DO, water and 1.5% (active) EPI/DMA/EDA polymer (M.W. approximately 500,000)

21. 1 % ethylenebissterylamide, 30% hydrocarbon oil in water emulsion antifoam

22. 10% active of a blend of polyglycol ester and polyether surfactant

23. Water-based emulsion defoamer containing 13% high melting point fatty alcohols

Figure 6 schematically illustrates the results. It can be seen that the defoamer of the present invention (defoamer 20) is superior to all of the conventional, prior-art defoamers.

Example 7:

An experiment comprised of eleven defoamer compositions was conducted using an antifoam recirculation test cell. The defoamers were added to 1.5% diluted black liquor pH-adjusted with sulfuric acid (pH=4.6, T=135°F) at a dosage of 60 ppm and the foam heights were recorded. The eleven samples were as follows:

24. Ethoxylated fatty triglyceride

25. Ethoxylated fatty triglyceride and 5.3 (active) aminomethyl- ated polyacrylamide (M.W. approximately 1x10 ⁶ )

26. Ethoxylated fatty triglyceride and 5.3% (active) medium charge density copolymer (AM/AETAC) (M.W. approximately 4x10 ^β )

27. Ethoxylated fatty triglyceride and 5.3% (active) high charge density copolymer (AM/AETAC) (M.W. approximately 3x10 ^β )

28. Ethoxylated fatty triglyceride and 5.3% (active) polyamine (condensation polymer of HMDA still bottoms and EDC) (M.W. approximately 10, 000)

29. Ethoxylated fatty triglyceride and 5.3% (active) high charge density polyamine (condensation polymer of DMAPA/EPI) (M.W. approximately 200,000)

30. Ethoxylated fatty triglyceride and 5.3% (active) polyamine- amide (condensation polymer of DETA/EPI) (M.W. approximately 250,000)

31. Ethoxylated fatty triglyceride and 5.3% (active) copolymer

(AM/Sipomer 25) (M.W. approximately 4x10 ^β )

32. Ethoxylated fatty triglyceride and 5.0% (active) dimethyl¬ amine epichlorohydrin condensation polymer (M.W. approximately 20,000)

33. Ethoxylated fatty triglyceride and 4.9% (active) polydimethyl diallyl ammonium chloride (M.W. approximately 20,000)

34. Ethoxylated fatty triglyceride and 4.9% (active) polytrimethyl allyl ammonium methyl sulfate (M.W. approximately 30,000)

Results are plotted in Figure 7. It can be seen that addition of a small amount of condensate polymers (28, 29, 30, 32) and homopolymers (33, 34) to a conventional defoamer (24) increases the efficacy consid¬ erably. It is noteworthy that presence of copolymers (26, 27, 31 ) did not improve the defoaming performances.

Example 8:

The following defoamers were tested in 1.5 diluted black liquor, pH adjusted with sulfuric acid (pH=4.8, T=135°F), at a dosage of 60 ppm:

35. Ethoxylated fatty triglyceride

36. Ethoxylated triglyceride and 1.3% (active) EPI/DMA/EDA polymer (M.W. approximately 600,000)

37. Ethoxylated fatty triglyceride and 2.5% (active)

EPI/DMA/EDA polymer (M.W. approximately 600,000)

38. Ethoxylated fatty triglyceride and 1.3% (active) dimethylamine epichlorohydrin condensation polymer (M.W. approximately 20,000)

39. Ethoxylated fatty triglyceride and 5.0% (active) dimethylamine epichlorohydrin condensation polymer (M.W. approximately 20,000)

40. Ethoxylated fatty triglyceride and 1.3% (active) high charge density copolymer (AM/AETAC) (M.W. approximately 3x10 ^β )

41. Ethoxylated fatty triglyceride and 5.0% (active) EPI/DMA/EDA polymer (M.W. approximately 600,000)

Figure 8 shows the results. This example compared the effect of various molecular weights on defoaming performance. It can be seen that increasing the active amount of high molecular weight (approximately 600,000) dimethylamine epichlorohydrin condensation polymer decreas¬ es the defoaming performance (36, 37, 41), presumably the high molecu¬ lar weight polymer is so bulky and thus has a difficult time to orient or adsorb at the air/liquid interface. At the same amount of active (1.3%), the condensation polymers (36, 38) are much more efficacious than the copolymer (40).

LEGEND: The sources of the compounds used in the above examples are disclosed below:

polyethyleneimine: Cordova Chemical polybutene L14: Amoco poly-N-vinyl pyrrolidone: GAF dimethylamine epichlorohydrin condensate polymer: Betz Laboratories epichlorohydrin plus dimethylamine plus ethylene diamine terpolymer (EPI/DMA/EDA): Betz Laboratories ethoxylated fatty triglyceride: Buckman 454c

PEG 400 DO: Mazer

POE (40) sorbitol hexaoleate: Henkel

AM/AETAC: Betz Laboratories

HMDA/EDC: Betz Laboratories DMAPA/EPI: Betz Laboratories DETA/EPI: Betz Laboratories AM/Sipomer: Betz Laboratories polydimethyl diallyl ammonium chloride: Betz Laboratories