DE2710700A1 - PROCESS FOR CHEMICAL WOOD DIGESTION - Google Patents

Description

The invention relates to a method for increasing the material yield from wood chips or wood pulp while maintaining good fabric quality by pretreating the wood chips with oxygen before chemical digestion. In particular the invention relates to a method for pretreating wood chips with oxygen under controlled conditions Temperature and pressure conditions for a period of time sufficient to yield the material in the subsequent chemical Unlocking to increase.

The wood digestion to remove lignin and other such Materials and for obtaining the pulp can be used in a known manner as chemical digestion, for example in Sulphate or force pulping process and in the soda pulping process carry out. It is true that some mechanical digestion processes lead to higher material yields than chemical digestion; however, pulps are obtained which lead to a paper with poor strength properties. The low strength restricts the use of such papers for many applications

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purposes are decisive. As a result, it will be on a larger scale worked with chemical digestion processes.

The Kraft process is one of the chemical pulping processes generally preferred because it results in acceptable pulp at a competitive cost and because the cellulose pulp obtained in the Kraft process is at the same time gives a high quality paper of excellent strength. The most noticeable disadvantage of the power procedure is the related problem of air and water pollution, which is due to the sulfur-containing chemicals used. The baking soda process has one result in less pollution than the force method; However, if the soda process is carried out in such a way that a material yield comparable to the Kraft process is achieved is, the result is a pulp from which only paper of lower quality with lower strength can be made can. As a result, the soda process is currently little used compared to the Kraft process. But because a reduction the pollution is urgently desired, great efforts have been made, the soda process is not only in terms of paper quality, but also in terms of * to improve the yield.

In the case of chemical digestion, the yield is far below the theoretically possible value. Typically, in a commercial digestion process only about 60 54 the

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carbohydrates present in the wood in the pulp. As a result, an increase in the material yield in chemical digestion processes in general and in particular in the Kraft process and the soda process is theoretically possible and urgently desired. The size of the corresponding need can be seen from the fact there & would correspond to an increase in the yield of material from 48% to 50% savings of more than 1.8 million tons of wood per year, if it all carried out in the United States chemical pulping processes would be affected . It goes without saying that only processes are suitable for increasing the material yield which also produce a material slurry of good quality at competitive costs.

Numerous modifications have been made to increase the material yield the chemical digestion process proposed. For example, there was a pretreatment with hydrogen sulfide recommended to modify the carbohydrate end units and to achieve an increased material yield. The application of However, hydrogen sulfide leads to pollution and is expensive. As a result, the pretreatment of Do not establish wood chips with hydrogen sulfide commercially can.

Also the use of oxygen during the soda digestion process is known. While this method avoids the sulfur pollution problem, it is the Kraft method

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in terms of stock yield and / or paper quality inferior. In contrast, the invention makes a non-polluting, relatively inexpensive one Process for increasing the material yield in wood digestion created, according to which the wood chips are pretreated with oxygen in the absence of sodium hydroxide, This is followed by a chemical digestion process, which is either the soda process or the Kraft process can act. The pulp produced in this way does not contain an excessive amount of lignin and can be processed into paper the quality of which is comparable to that obtained from untreated wood using the same pulping process is obtained.

The oxygen pretreatment according to the invention is used in known Wood chips or wood shavings applied as they are currently used in commercial digestion processes. The pretreatment takes place under controlled temperature and pressure conditions and with a specified treatment duration. However, the limits within which these parameters are specified are not particularly narrow. The optimal treatment conditions can with changes in the natural properties of the treated wood chips as well as with variations the systems and processes used in the individual case vary somewhat.

As for the temperature, a range of temperatures was found

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from about 70 ° C. to about 120 ° C. is particularly favorable. Significantly lower temperatures do not lead to the desired increase in the material yield, while significantly higher temperatures are undesirable because they increase the probability of an overall oxidation of the wood without significant advantages can be achieved. Let it be noted that the present process does not depend on extensive oxidation of the wood; the one through the wood chips Oxygen consumption caused is actually so small that it is difficult to measure. Although the chemical mechanism on the basis of which the oxygen pretreatment of the wood chips leads to the subsequent digestion the material yield is increased is not known and a corresponding theoretical clarification is also not essential for the practical utilization, the fact of the low oxygen consumption is compatible with the hypothesis, that the effect is associated with the oxidation of end groups of the cellulose molecules.

As far as the pressure is concerned, it turned out to be the most favorable to carry out oxygen pretreatment in a pressure reactor to increase the pressure of oxygen and, equally important, to hold back the water that evaporates from the wood chips. Any practice that causes excessive dehydration of the wood chips is undesirable. It goes without saying that for economic reasons it is advisable to use the lowest pressure

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apply that delivers the desired effect. A total reactor pressure from about 4 at to about 30 at was found to be useful. Since in the temperature range from about 7O C to about 120 ° C the vapor pressure of water between about 0.3 and approximately 2.0 at, the calculated oxygen is sufficient partial pressure from about 2.0 at to about 29.7 at. Commercially pure oxygen was found to be beneficial. Even though the lower limit of oxygen purity is not experimental is based on previous experiments with other low temperature wet oxidation reactions It is to be expected that oxygen of low purity, which is typically at least about 40% by volume of oxygen contains, is useful. In addition to oxygen, such a gas would normally be inert gases such as nitrogen and / or argon contain.

As for the duration of treatment, i.e. H. the length of time during which the wood chips are at the desired temperature and maintained in oxygen at the selected pressure, the preferred length of time is clearly that to achieve the minimum time required to achieve the desired effect. Treatment times of about 10 to about 60 minutes have been shown proven to be effective. It can be assumed that somewhat shorter or longer periods of time can also be used are.

It also turned out to be cheap to get the wood chips quickly

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to heat to the desired reaction temperature. Heat-up times in excess of about 20 minutes are as found became less effective in increasing the fabric yield. Heat-up times are less than about 10 minutes not easy to achieve using the systems and measures previously used. As a result, it is preferred Heating time between approximately 10 minutes and approximately 20 minutes. With other equipment and procedures, if necessary shorter periods of time can be used effectively. The achievement of very short heating times, i.e. H. Time spans of significantly less than 10 minutes, however, should not be important for the economy of the process. It has also been found that the heating phase is carried out in oxygen or in an inert gas, for example nitrogen can be.

The process of the invention can be carried out on a commercial scale using various types of equipment and Carry out process flows. For example, the wood chips be moved continuously through a reactor in which the heating takes place in a first zone, while the wood chips are kept at the above-mentioned temperatures in a second zone.

The chips can be moved through the reactor with the aid of a screw, for example. The front part the screw can be equipped with indirect steam heating.

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be equipped, which is designed so that the wood chips be quickly brought to the desired temperature, while in the middle and in the outlet section of the reactor Heating devices can be arranged, which keep the wood chips at the target temperature. In order for the desired To provide pressure, oxygen is supplied to the reactor. Different types of pressure locks can be used to feed the wood chips into and out of the pressure reactor without excessive gas leakage. The reaction time can be determined via the speed of the screw control, while temperature and pressure can be measured and controlled in a known manner. Only a small amount of oxygen flow is required because only small amounts of oxygen are consumed; the needs of the oxygen flow are primarily directed Line following (1) the leakage losses and (2) the requirement to remove non-condensable gases.

After the pre-treatment with oxygen, the wood chips become one chemical digestion process, for example the Kraft process, the soda process or the like, subjected.

The oxygen pretreatment according to the invention can be used the fabric yield for both hardwood and Boost softwood.

The invention is described below on the basis of experimental results

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and examples explained in more detail.

Yellow pine wood chips approximately 20 mm long were cut into tumbled a rolling drum for about 10 hours, allowing for a substantially uniform water content was taken care of. The wood chips were then weighed out in portions of approximately 300 g, bagged and packed in stored in a refrigerator at a temperature of approximately 5 C until use. By storing the wood chips biodegradation is delayed in a cool state.

Fifteen samples of wood chips weighing approximately 50 g each were taken out weighed the batch tumbled in a roller drum for 10 hours, in an oven at a temperature of 105 ° C Dried for 18 hours, cooled to room temperature and then weighed again. In each case the weight was of the 50 g sample after drying in the oven, approximately half the initial weight. The mean of these results was the exact solids content of the entire batch of wood; it was used to approximate the amount of solids in each Calculate a 3OO g sample of wood chips, which was then treated with oxygen and digested. This procedure was necessary to enable an exact measurement of the material yield.

The oxygen pretreatment of the wood chips was carried out in one

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- if-

cylindrical swivel reactor made of stainless steel of 3 1 capacity carried out. The reactor could with the help of external Heating coils, an electronic controller and a thermocouple seated in a recess in the reactor wall can be regulated to a preselected temperature. The reactor was also equipped with an electric auxiliary heating element, which protruded into the reactor along the axis of the reactor and the acceleration of the heating of the wood chips served. The energy supply to this heating element could be precisely selected with the help of a variable transformer constant value can be set. The reactor was with a Gas line connected through which the reactor is flushed and with Gas of the desired composition could be kept at the desired pressure. The temperature of the gas in the Reactor was measured by means of a thermocouple that was inserted into the gas space in the reactor above the wood chips was. The cylindrical reactor was mounted so that his Axis was horizontal; it was moved back and forth in order to maintain the thermal balance of the wood chips with each other, with to promote the gas phase in the reactor, with the reactor walls and with the auxiliary heating rod. The bringing about of the thermal Equilibrium was further promoted by the fact that stainless steel balls were placed in the reactor together with the wood chips Steel were introduced, which supported the stirring effect.

Each of the four test variables below could be accurate to

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a preselected value.

T _D «reaction temperature - equilibrium temperature

Γλ

of gas and chips during oxygen treatment

Reaction pressure * Total pressure in the reactor during the oxygen treatment

t _u «initial heating time = time span that the H

Schnitzel and the gas phase after the introduction of the wood chips into the reactor need to To reach T_

t_ - time during which the chips were exposed to _{oxygen at T R} and P _R.

Each attempt consisted of the following stages:

Step 1; The reactor was preheated to a gas temperature equal to the desired value T _D.

Step 2: A sample of wood chips approximately 3CXD g gross weight, weighed to the nearest 0.1 g, was placed in the reactor. Air was then purged from the reactor using nitrogen; the reactor was

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fabric pressurized. The reciprocating or rocking motion was then initiated; the power supply to the auxiliary immersion heater was turned off set to the value that was required in order to achieve the desired initial heating time t ,,. The appropriate setting was made on the basis of a calibration curve, which was described in previous Calibration tests was recorded. Without additional heat input, a value of t would be approximately

Achieved 30 minutes. The auxiliary heating was therefore always required when heating times of less than approximately 30 minutes were required.

Stage 3: After the initial heating-up was complete, the heating rod was switched off. Nitrogen was purged from the reactor with oxygen. The reactor was pressurized to pressure P _D with oxygen. The gas temperature and the total pressure were then kept at the values T _R and P_ _{for the desired duration t R.}

Stage 4: After the desired reaction time t _{n had} elapsed, the pressure in the reactor was reduced; the schnitzels were taken out.

Stage 5: The chips were cooled and used for determination

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reweighed the moisture loss. Loss of moisture was due to the addition of water balanced. The re-moistened schnitzel was then stored under refrigeration until the Unlocking procedures could be carried out. This usually happened on the same day.

Because the chips typically contained 50% water and only small amounts of purge gas were used in stages 2 and 3, the water loss was low; it was typically below 20% by weight of the original water content. Because there was always excess water, the oxygen treatment was always carried out in an atmosphere saturated with water vapor. By subtracting the vapor pressure of water at T _R from the pressure P_, the partial pressure of oxygen can easily be calculated.

To determine the fabric yield, each sample was the pretreated Schnitzel in an externally heated reactor with liquid circulation using the soda or force digestion process subjected. Control samples of the schnitzel that had not undergone any oxygen pretreatment, but otherwise were identical were unlocked in exactly the same way. The digestion conditions were set as follows.

Soda Process - The temperature of the reactor was in a

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Hour increased from 2OC to 17OC and lasted for two hours held at 170 ° C while the wood chips digested became. Digestion was done using 0.32 g NcOH per g wood (on a solids basis). Expressed as aqueous NaOH, this corresponds to 5.62 g of 1.51 molar aqueous NaOH per g of wood (on a solids basis).

Kraft Process - The temperature of the reactor was increased from 20 ° C to 170 ° C in one hour and held at 170 ° C for 1.5 hours while the wood chips were digested. The digestion was carried out using 0.196 g of NaOH per g of wood (on a solids basis), 0.082 g of Na ₂ S per g of wood (on a solids basis) and a total of 0.222 g of water per g of wood (on a solids basis). This corresponds to a suIfidity of 30% and an amount of active alkali of 0.28 g NaOH per g wood.

After the end of the digestion phase, the pulp became out removed from the reactor, washed with distilled water, partially dried by squeezing and weighed. Afterward the water content and the permanganate number were determined. For some samples the acid content was also determined insoluble lignin determined. The water content of the pulp was measured by weighing duplicate 40 g samples, the samples were dried in an oven at 105 ° C for 18 hours and the samples then reweighed. According to knowledge the dry weight of the pulp obtained from a

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Al

known wood dry weight is produced, the material yield is simply obtained by division.

The permanganate number was determined according to the TAPPI standard method T21 4 m-50 determined while determining the content of acid insoluble lignin according to the TAPPI standard method T222 M-54 took place. The graph of Fig. 1 shows the expected linear relationship between the permanganate number and the lignin content and justifies the use of the permanganate number, which is easier to measure, as a characteristic value for the Lignin content.

The data obtained in the swivel reactor tests are summarized in Tables I and II. Table I shows a statistical comparison of 7 samples of wood chips pretreated with oxygen and 7 samples of untreated wood chips, all 14 samples being digested with soda under identical conditions. Every effort was made to ensure that the data for the two groups of samples were exactly comparable. The average yield for the chips pretreated with oxygen was 51.6 %, while the chips that were not pretreated led to a yield of 49.6%. This increase is large enough to be economically significant. The quality of the substances obtained, characterized by the permanganate numbers, was not significantly different. As a result, the present

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AS

Oxygen pretreatment explained below, the material yield the subsequent digestion in the soda process without deterioration to increase the quality of the pulp.

Table II shows individual experiments which reveal the influences of certain process parameters. The attempt 15 shows that 65 C is too low a temperature, while experiments 17 and 18 show that higher temperatures lead to the desired effect. According to experiments 19 to 21, an increase in the material yield can be achieved within a range of treatment times can be achieved. Tests 22 to 24 show that slow heating is undesirable is. From the experiments 25 and 26 it can be seen that the oxygen pretreatment the subsequent material yield in the power process increases and that the increase in the yield about the same as in the case of the soda digestion process is.

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TABLE I. Initial duration of Average no oxygen pretreatment Gesarz- with procedure without pre-treatment material heating gas phase oxygen 8th no oxygen pretreatment pressure.P yield Swing reactor tests duration, t _u temp., T _n treat., t 9 no oxygen pretreatment (at) ^R Type of Permanent (Wt.%) Statistical comparison of oxygen pretreatment (min) ^H (° C) ^R (min) ^R 1O no oxygen pretreatment 4.5 Unlocking ganat- 52.7 11 89 45 11 no oxygen pretreatment 4.4 ßens number 52.1 11 9O 45 12th no oxygen pretreatment 4.4 Baking soda 37.9 52.5 attempt 11 88 45 13th no oxygen pretreatment 4, ε Baking soda 35.8 51, 9 O
OO number 11 88 45 14th Average 4.4 Baking soda 39.0 51.6 OD 1 11 89 45 4.3 Baking soda 38.7 51, 1 CO 2 11 89 45 4.4 Baking soda 38.8 49.2 -J 3 11 89 48 Baking soda 38.6 51.6 ** » 4th Baking soda 37.6 51, 3 O 5 38.1 50.1 CD 6th Baking soda 40.6 49.8 CC 7th Baking soda 37.5 50.1 Baking soda 38.7 49.7 Baking soda 41, O 48.7 Baking soda 39.4 47.3 Baking soda 4O.O 49.6 Baking soda 35.5 39.O

attempt TABLE II Treatment conditions Duration of 90 45 f- total Type of recognize permit number Oxygen no oxygen pretreatment t pressure, P Unlocking 15th Swivel reactor tests Gas phase treat., (at) ßens 16 Individual experiments, the influence of special process parameters temp., T (min) 4.4 Baking soda Permanent material 17th (° c) ^R 62 4.4 Baking soda ganat- yield 18th At first 65 60 4.4 Baking soda number (Wt.%) 19th heat up 87 45 4.4 Baking soda 34.9 46.5 20th take,, 90 60 4.4 Baking soda 37.0 49.8 O
/O 21 /. \ H
(mm j 97 15th 4.4 Baking soda 35.8 52.1 OO 22nd 10 87 45 4.4 Baking soda 35.3 51, 3 CjJ 23 2O 89 70 4.4 Baking soda 36.0 51, 7 -J 24 11 89 6O 4.4 Baking soda 38.5 53.0 * - » 9 15th 92 60 4.4 Baking soda 37.3 50.3 O 25th 12th 91 6O Baking soda 39.9 46.8 IO 26th 12th 94 no oxygen pretreatment 4.4 force 35.8 48.5 to
«£> 15th 11 force 38.1 48.1 25th 37.5 50.1 45 25.3 49.6 90 26.9 47.8

a *

The yellow pine wood chips were in quantities of approximately
6CXD g packaged and in a refrigerator at approximately 5 ° C
saved. Immediately before use, two 25 g samples were weighed out of the bag to determine the water content; the sample weight was measured.

The oxygen pretreatment was carried out in a cylindrical stainless steel vessel with a capacity of 3 liters. One
Copper heat sink encompassed the lower half of the reactor and was in good thermal contact with it; a water-cooled copper pipe was wrapped around the top. The heat sink was surrounded by a band-shaped heating element to which energy was supplied via a variable transformer. The reactor was upright. The heating was carried out by supplying a predetermined amount of energy for a set period of time. This energy quickly enters the reactor, heating the wood chips and causing the temperature to peak and then drop. A typical representation of the temperature profile as a function of time is shown in FIG.

Large temperature gradients occur during the operation of this reactor, while the reactor heats up and cools down. Water evaporates from the schnitzel, condenses on the cool upper walls, and falls back into the lower ones
Part.

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The reactor was pressurized with oxygen via a gas pipe will; the pressure was measured throughout the tests. Via thermocouples in the middle of the schnitzel bed, in the heat sink and at the upper end of the reactor, the temperatures were determined during the tests.

In this reactor, the experiments with the four below can be carried out describe defined parameters.

T _M = maximum temperature reached in the middle of the pulp bed;

P_ = pressure to which the reactor was initially brought will;

t. = required after the initial energy supply

before time until the maximum bed temperature is reached; this parameter is similar but not equal to the initial heating time (t _u ) of the swivel reactor;

t. = Period of time during which the middle part of the Schnitzel bed was above 70 ° C; this parameter is similar but not identical to the reaction time (t) for the swivel reactor.

Each experiment included the following stages:

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Step 1: A sample of approximately 550 grams of wood chips was placed in the reactor. The reactor was then tightly closed and, after the air had been flushed out with oxygen, it was pressed _{to the desired pressure P 1.}

Level 2: One to achieve the desired temperature T ..

M.

sufficient voltage was applied to the heating element for approximately 8 to 10 minutes, after which the
Voltage has been switched off. The reactor temperature was then allowed to pass through the maximum value, whereupon the reactor was allowed to cool to almost outside temperature. The pressure and temperature were monitored during stage 2 and recorded as a function of time.

Stage 3: After the temperature had almost reached the outside temperature again, the chips were removed from the reactor. This generally happened about 90 minutes after placement. The chips were stored refrigerated until the digestion process could be carried out.

To determine the fabric yield, each sample was the pretreated Subjected pulp in the digestion reactor described above to a force digestion process. The temperature of the reactor was increased from 20 ° C. to 165 ° C. within one hour and for the duration indicated in Table III

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Maintained 165 ° C. Unlocking was done using the amounts of liquid given in Table III. All further treatment of the pulp was in exactly the same way carried out in the same way as in the case of the swivel reactor experiments.

The data obtained from the tests with the static reactor are summarized in Table III. Let this data realize that the yield of the Kraft process by the Oxygen pretreatment to about the same extent as was increased in the swivel reactor tests. The results of tests carried out with paper that made from some of the substances listed in Table III are summarized in Table IV. These data show none Considerable differences in strength between papers made from pretreated and non-pretreated chips. Consequently is the explained oxygen pretreatment able to which was achieved in the subsequent force digestion process To increase the material yield without there being a significant deterioration in the strength properties of the this pulp made paper comes.

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TABLE III

Static reactor experiments

Attempt number

Time to highest bed temperature, t (min)

Duration over 70 ° C, t

(min)

highest bed temperature, T °

At first Permanent Fabric total ganat- prey pressure, P _T number (Wt.%) (at) ^X

The following samples were digested in the kraft process for two hours at 165 C with a liquid having a sulphidity of 36% and a content of active alkali of about O ₁ 32 g / g wood.

28 29 3O

18 22

18 27

no oxygen pretreatment

85 28 , 2 22nd 4th 51, 9 1O1 28 18 49, 7th 20th 48, 1

The following samples were Kraft for two hours at 165 C with a Liquid with a sulfidity of 30% and an active alkali content of approximately 0.29 g / g wood digested.

31 18 3O 99 28 , 2 23.3 50, O CD
_ η 32 17 32 1O4 28 , 2 21, 8 51, 5 O 33 no oxygen pretreatment 22.7 47, 2 O 34 no oxygen pretreatment 23.9 48, 3

TABLE IV

Comparison of the strength properties of paper made of pulp with and without oxygen pretreatment

O
CD attempt
number
(see table
le III for full
constant
writing) Type of
Attempt Fabric
prey
(Wt.%) Tear length
(km) Tear
number Burst
number Tensile energy
absorption
g cm / cm 837 / 28 oxygen-
pretreated 51 .9 9.6 144 74 142 O
CO
tf ^ 29 oxygen-
pretreated 49.7. 10.3 140 78 143 CG
<£ 30th no pissed off
fabric 48.1 10.4 148 77 142 33 no pissed off
fabric 47.2 1O.5 142 74 134

Swatches were ground in a PFI mill to different degrees of freeness. After this strength tests were carried out according to the TAPPI method T2O5 hand sheets were produced carried out according to the TAPPI procedure T22O. These trials were for comparison purposes extrapolated to a freeness of 330 CSF.

Claims (7)

Expectations ^. Process for the chemical digestion of hydrous Wood chips, characterized in that the wood chips are treated prior to chemical digestion, by bringing the wood chips into contact with heat and oxygen in a pressure reactor, that water remains in the wood chips. 2. The method according to claim 1, characterized in that the heat is maintained in the range of about 70 ° C to about 12OC will. 3. The method according to claim 2, characterized in that it is carried out with a reactor pressure of about 4 at. 4. The method according to claim 3, characterized in that the Oxygen is contained in a gas mixture and makes up at least about 40% by volume of this mixture. 5. The method according to claim 4, characterized in that the wood chips before being brought into contact with the oxygen rapidly heated to a temperature within the desired temperature range. 6. The method according to claim 5, characterized in that the 709837/0999 ORIGINAL INSPECTED -tf- 27107 JL rapid heating is carried out in an inert gas. 7. The method according to claim 5, characterized in that the rapid heating within about 20 minutes he follows. 709837/0999