Note: Descriptions are shown in the official language in which they were submitted.
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B-1098
METHOD FOR SULFITE PULPING USING SURFACTANTS
8ACKGROUND OF THE INVENTION
The sulfite pulping of wood is a well known process and is
described in Pulp and Paper Chemistry and Chemical Technology,
Volume 1, Chapter 4, edited by James P. Casey, Wiley Interscience
1980, the disclosure of which is incorporated herein by reference. Sulfite
puiping is similar to Kraft (i.e., sulfate) pulping in that the pulp yield and
reject level are a function of the degree of delignification. The
delignification process depends on the penetration and diffusion of
cooking chemicals in the wood substance. Usually, the cooking liquor
moves much more rapidly in the longitudinal direction than in the
transverse direction of the fibers; with more rapid heating, cooking tends
to be uneven. Insufficient penetration and diffusion of cooking liquor
causes the centers of wood chips to be undercooked or burnt. As a
result, higher screen rejects and lower yields are obtained.
Satisfactory sulfite pulping, especially the acid sulfite process, can
be achieved most readily if the cooking liquor is brought into intimate
contact with the wood chips. Free sulfur dioxide in acid sulfite liquor
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enters wood more rapidly than does combined sulfur dioxide; therefore,
the centers of chips may sometimes contain a sulfur dioxide solution but
no base. At temperatures above about 11 0~C, such an occurrence is
expedited, and chips become brown (due to lignin thermal condensation)
at the center, and are soon rendered uncookable. In order to avoid this
occurrence, presteaming is often utilized to improve the penetration of
cooking liquor. In addition to presteaming, penetration and diffusion can
also be facilitated by hydrostatic pressure and by increasing chip
moisture content.
Many studies have been conducted to understand the cooking
mechanisms and to improve the efficiency of the cooking process. A
number of digester aids were found to be highly effective in enhancing
the efficiency of the sulfite process. These chemicals include various
quinone derivatives, dimethylamides, esters of formic acid, and
amphoteric surfactants.
Esters of formic acid (i.e., formates) function as inhibitors for the
condensation or polymerization of the lignin during pulping, which in turn
improves the cooking process. Dimethylamides and amphoteric
surfactants are surface active agents, and function as penetrating and
dispersing agents to enhance cooking liquor penetration.
Several factors are believed to be responsible for the mechanisms
of cooking liquor penetration, including interfacial tension, wettability, and
emulsification. The interfacial tension between the cooking liquor and
resin must be decreased in order to increase the penetration rate of
cooking liquor into the wood chips. There are two mechanisms
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corresponding to the lowering interfacial tension: Deformation of resin
and formation of a resin-in-water emulsion.
Low interfacial tension reduces the work of deformation necessary
for resin droplets to emerge from the narrow necks of pores. A low
water/resin interfacial tension is desirable for the movement of resin
through the narrow spaces. This mechanism demonstrates how the
cooking liquor can penetrate deeply into the chips.
o Alternately, a very low interfacial tension is needed to form an
emulsion of resin in the cooking liquor. If resin, which blocks the pores,
can be emulsified by a surface active agent, the cooking liquor can easily
pass through the pores. This leads to a good cooking liquor penetration,
while also preventing redeposition of dissolved resin particles back onto
the fibers.
The increased wettability of wood chip surfaces by a surface active
agent also creates more favorable conditions for cooking liquor
penetration. The spreading of cooking liquor on the chip surface is
governed by the surface tension of cooking liquor, the surface tension of
the chip, and the interfacial tension between the cooking liquor and the
chip. In general, the lower the surface tension of the cooking liquor, the
easier the spreading occurs.
Free sulfur dioxide in acid sulfite liquors enters wood more rapidly
than does combined sulfur dioxide. It is desirable to facilitate cooking
liquor penetration in a more uniform, thorough pattern. An increase in the
moisture content of wood chips and the wetting of wood chip surfaces will
CA 0221741S 1997-10-27
.
alleviate non-uniform penetration. The addition of surface active agent in
the cooking liquor can further enhance the penetration mechanism.
The present invention demonstrates that surfactants with high
s HLBs (where such measurements are appropriate) are effective in
facilitating the spreading of cooking acid on wood chip surfaces. HLB is
an abbreviation for hydrophile-lipophile balance as related to the oil and
water solubility of a material. A high HLB indicates that the hydrophilic
portion of the molecule is dominant, while a low HLB indicates that the
10 hydrophobic portion of the molecule is dominant. The water solubility of
materials increases with increasing HLB.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to an improved method for sulfite
pulping of wood chips in which wood chips are digested in an acid sulfite
pulping liquor to produce a wood pulpl the improvement comprising
adding to the chips, prior to mixing with the acid sulfite pulping liquor, an
effective amount of a nonionic or anionic surfactant, or mixtures thereof,
20 the method enhancing the penetration of the acid sulfite pulping liquor
into the chips. The treatment of the present invention demonstrates
efficacy as a digester aid for sulfite pulping by improving the penetration
of cooking liquor via emulsification and wetting mechanisms.
In a preferred embodiment, the present invention allows for the
use of ethoxylated surfactants as digester aids for acid sulfite pulping
processes. The present invention reduces reject levels andtor provides
higher yield production rates by reducing cooking time, cooking
chemicals and/or cooking temperature.
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Nonionic ethoxylated surfactants, e.g., linear alcohol ethoxylates,
alkylphenol ethoxylates and E0/P0 block copolymer surfactants, as well
as anionic surfactants, e.g., alcohol ether sulfates, phosphate esters,
ether phosphates, etc., are all effective for purposes of the present
s invention. Dosage ranges of from about 0.1-10 pounds of treatment per
ton of oven dry wood chips are considered effective, with about 0.25-5
pounds/ton being preferred.
An HLB range for the nonionic surfactant treatment of from about
o 10-15 is preferred. A surfactant with an HLB below about 8 is oil soluble,
and will not be effective for purposes of the present invention.
Particularly preferred nonionic surfactants include nonylphenol
ethoxylates and oleyl alcohol ethoxylates.
The following laboratory results demonstrate the effectiveness of
the present invention on the reduction of pitch-water interfacial tension
and an increase in pitch dispersability.
The contact angle of surface active agents was measured using a
dynamic contact angle analyzer. The wettability of the surface active
agents is associated with the degree of the contact angle. The smaller
the contact angle, the better the liquid spreads on the solid surface.
The dynamic interfacial tension was measured using a Drop
Volume Tensiometer. Mineral oil was employed as a model system to
simulate pitch components such as fatty acids, resin acids, etc.
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The capability of chemical agents to emulsify pitch particles was
tested by emulsifying pitch particles in a brown stock washer filtrate,
which was obtained from an acid sulfite mill. The pitch particles (greater
than 10 mm) were organic deposits originally collected from the sulfite
mill. The effectiveness of the chemical agents was determined by
measuring the amount of pitch being removed (i.e., washed out), turbidity
of the emulsion, and the degree of pitch deposition on the surface.
Several surface active agents were tested. A description of these
o materials is found in Tables I and ll, below.
Table I
Avg. Mol. Wt.
Sample Chemical Name (~/mol)
comPonent (comP~) 1 comPonent2 Comp. 1 Como. 2
Unsaturated alcohol Unsaturated alcohol
ethoxylate ethoxylate 1148 1060-1104
2 Alkylphenol ethoxylate Alkylphenol ethoxylate 484 748
3 Branched alcohol
ethoxylate 862
4 Secondary alcohol
ethoxylate 834-890
Polyoxyethylene tridecyl
ether phosphate 590-1082
6 Linear alcohol ethoxylate 666
7 Linear alcohol ethoxylate 714-742
8 Alcohol ether sulfate 400-428
9 Linear alcohol ethoxylate Linear alcohol ethoxylate 666 714-742
30 10 Unsaturated alcohol Secondary alcohol
ethoxylate ethoxylate 1148 834-890
11 Polyoxyethylene tridecyl Alcohol ether sulfate
ether phosphate 590-1082 400-428
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Table ll
Sample Chemical Name HLBValue
ComPonent 1 ComPonent2 ComP. 1 comP. 2
Unsaturated alcohol Unsaturated alcohol
ethoxylate ethoxylate 15.3 11.3
2 Alkylphenol ethoxylate Alkylphenol ethoxylate 10.9 14.1
3 sranched alcohol
o ethoxylate 15.3 ----
4 Secondary alcohol
ethoxylate 15.4 ----
Polyoxyethylene tridecyl
ether phosphate ----
6 Linearalcohol ethoxylate 13.0 ----
7 Linear alcohol ethoxylate 14.5 ----
8 Alcohol ether sulfate ---- ----
g Linear alcohol ethoxylate Linear alcohol ethoxylate 13.0 14.5
Unsaturated alcohol Secondary alcohol
ethoxylate ethoxylate 15.3 15.4
11 Polyoxyethylene tridecyl Alcohol ether sulfate
ether phosphate ---
The contact angle of cooking acid (pH 1.5) on Teflon~ was 112
degrees (see Table lll, below). This indicates that the cooking acid
cannot wet a hydrophobic surface (e.g., aged wood chip) very well. In
order for the cooking acid to spread on the wood chips quickly and
completely, the contact angle of cooking acid should be reduced. As
shown in Table lll, all tested surfactants were able to reduce the contact
angle from 112~ to 66~ or lower. Thus, the spreading of cooking acid on
wood chip surfaces will be significantly improved with the addition of
these surfactants.
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Table l l l
Contact Angle of Cooking Acid Containing Various
Surface-Active Agents on Teflon Surface
Contact Angle on Teflon
Sample Concentration (ppm) (De~rees)
Untreated ------ 112.34
1 100 66.42
2 100 18.18
3 100 61.71
4 100 64.39
100 28.29
6 100 62.98
7 100 51.39
8 100 50.00
9 100 51.59
100 66.41
11 100 33.19
A sulfuric acid solution (pH 1.5) was used as a cooking aid to
evaluate the effect of various surfactants on the reduction of interfacial
tension (IFT) at the oil-cooking acid interface. Results as shown in
Table IV below demonstrated that these chemicals were very effective in
reducing the IFT. The IFT decreased from 48 dynes/cm (untreated) to
less than 10 dynes/cm (treated). The lower the IFT, the easier the
formation of an oil-in-water emulsion. It is expected that these chemicals
30 will function effectively during the pulping process in terms of emulsifying
and dispersing fatty/resin acids. As previously discussed, the reduction
of IFT at the oil-cooking acid interface will enhance the penetration of
cooking acid into the wood chips, while preventing redeposition of
dissolved resin particles back onto the fibers. This latter aspect will
improve the efficiency of the brown stock washing process.
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Table IV
Dynamic Interfacial Tension at Oil-Cooking Acid
(pH 1.5) Interface
Dynamic Interfacial Tension
(dyne/cm)
SamPle Concentration (ppm) Infusion Rate at:
0.2 ml/hr. 0.5 ml/hr. 1 ml/hr.
Untreated ------ 47.87 47.97 48.18
100 6.27 7.44 8.82
2 100 7.63 8.55 9.60
3 100 8.22 8.74 9.79
4 100 6.61 7.56 8.66
100 6.20 7.50 9.16
6 100 6.31 7.93 9.44
7 100 4.08 5.17 5.44
8 100 7.45 8.33 9.86
9 100 6.36 8.10 9.54
100 6.21 7.61 8.99
11 100 5.47 7.31 8.96
Table V summarizes the turbidity measurements of the pitch
emulsion, the percentage of pitch being washed out via 100 mesh sieve,
and the pitch deposition tendency on the Teflon surface. A higher
turbidity indicates that more pitch particles are emulsified in the filtrate.
As shown in Table V, all tested chemicals produced pitch emulsions with
30 high turbidities compared to the untreated pitch containing filtrate. This
was predicted by the dynamic IFT measurement. Since turbidity value is
proportional to the amount of pitch emulsified in the filtrate, more pitch
particles are expected to be removed from the filtrate during the washing
process. As shown in Table V, a significant amount of pitch was removed
35 by treatment with these chemicals. In addition, no deposition on the
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Teflon surface was observed for these treatments. The results
demonstrate that the treatments of the present invention will prevent pitch
from depositing onto a hydrophobic paper machine surface.
Table V
Capability of Surfactants to Emulsify Pitch Particles
Deposition
% of Pitch Tendency on
o SamPleTurbidity (NTU) Washed Out Teflon Surface
Untreated 180 2.9 Heavy Deposition
567 34.6 No Deposition
2 1026 51.1 No Deposition
9 559 31.1 No Deposition
11 776 34.5 No Deposition
It is known that most ethoxylated surfactants are unstable when
subjected to the conditions of the acid sulfite/bisulfite pulping processes,
because the ethylene oxide groups contained in the surface active
agents are decomposed at low pHs (pH<3) and high temperatures
(>1 00~C). Consequently, the surface active agents lose their wetting
ability, as well as other functions, such as emulsification capability.
Table Vl below demonstrates that after cooking at a temperature of 95~C
for 15 hours, a significant reduction of surface activity of sample 2 in acid
solution is observed. It is expected that the ethoxylated surfactants will
decompose more rapidly at a normal sulfite pulping temperature (130-
1 40~C).
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.
Table Vl
Surface Tension of Surfactants in Acid Sulfite Cooking Liquor
Sample 2 Surface Tension (dyne/cm)
s Acid Sulfite Cooking Liquor H2SO4Solution
Concentration (ppm)(pH = 1.49) (pH = 1.45)
Before After Before After
35.34 43.36 34.71 38.36
33.12 36.97 31.00 36.04
An experiment was conducted at a sulfite mill. The mill has three
digesters with a total production of 170 tons per day using a calcium
based sulfite pulping process. Two of the digesters have 35 ton chip
capacities, while the other digester has a 65 ton capacity. The wood
species was 100% birch, which was aged for one year. The moisture
level of the wood chips was approximately 32 wt.%. The cooking acid
had a pH of 1.45.
Sample 2 was applied at the rate of three pounds per ton, diluting
prior to injection in the steam packing line with three gallons per minute of
fresh water. The diluted chemical was added during the entire
presteaming process of chip filling to treat chips as they entered the
digester. The chip filling time was about 25 minutes for the small
digesters and 45 minutes for the third digester. Following the fill, the
cover was closed and cooking acid was pumped into the digester. Acid
was then precirculated prior to raising to a cooking temperature of 140~C.
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The chemical addition was controlled mechanically by a series of
interlocked outlets energized by the opening of the steam packing valve.
Neat chemical was injected into a stream of fresh water; this mixture was
injected directly into the steam line for the steam packer. When the chip
filling process was complete the steam valve closed, deenergizing the
chemical pump.
A minimum of 20 minutes per cook reduction in cooking time and a
25% reduction in rejects (11.3% untreated vs. 8.48% treated) was
o observed after the start of the trial. The pulp mill also experienced about
a 4% reduction in bleaching chemicals, mostly chlorine and caustic.
An additional experiment was conducted at a sulfite mill. The mill
used magnesium bisulphite solution (pH 2-3) as the cooking acid.
5 Sample 1 was added to the cooking acid feed pump that supplies the
digesters. The treatment was added at 2 Ibs./ton of wood chips. Pre-
trial, trial, and post-trial results were compared to determine if increases
in production occurred when the treatment was used. Table Vll below
contains pre-trial, trial and post-trial averages of blows/day and
20 tons/blow. The data demonstrates that the pulp production did not
increase as compared to pre and post trial data; this is attributable to the
fact that the chemical was added directly to the cooking acid, as opposed
to the treatment of the present invention.
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,
TABLE Vll
Avera~e Tons/Blow Avera~e Blows/Day
Pre-trial 11.54 18.69
Trial 10.81 17.25
Post-trial 11.50 17.93
While this invention has been described with respect to particular
o embodiments thereof, it is apparent that numerous other forms and
modifications of this invention will be obvious to those skilled in the art.
The appended claims and this invention generally should be construed to
cover all such obvious forms and modifications which are within the true
spirit and scope of the present invention.
