CA1073613A - Method of processing wood chips - Google Patents

Abstract

Inventors: JENS OLOV RYHMAN
NILS ANTON VOGT

Title: METHOD OF PROCESSING WOOD CHIPS.

Abstract: Air is circulated through a body of wood chips at a rate sufficient for maintaining a uniform temperature and oxygen content throughout the body of wood chips. Fresh air is supplied at a rate sufficient for keeping the oxygen content of the circulating air at 16 % by volume, at least. The temperature is main-tained at 35 - 80°C. This process results in a rapid seasoning of the natural resin, of the wood chips, making it possible to use the wood chips for pulping after 1 - 4 days only.

Description

~0~36~

The present invention relate~ to a method Or processing wood chips. It ;s the main object of the invention to provide a method for producing a rapid seasonin~ of the natural resin in the wood chips. It is another object o~ the invention to provide a resin seasoning process which maintains a uniform moisture content in the entire body of wood chips being pro-cessed. ~t is a particular object of the invention to provide a method which makes the wood chips use~ul ~or a ~ubsequen~
wood pulping process, particularly a sulphite pulping process.
Resin seasoning tWhich may also be referred to as resin ageing) means that the natural resin in the wood undergoes such alteration that it does not disturb the following manufact-uring processes, and that an increased portion of the resin.
can be removed during digestion and bleaching. The alterations occur by biochemical oxidation and are accompanied by com-bustion o~ easily reactin~ substances in the wood, primarily hemicellulose and sugar. If the resin is allowed to season completelyj which is generally considered desirableg almost 30 % of the resin can be removed before the digestion. ~he 2Q subs~ance loss of the wood is in that case 3 ~ 5 %. The pulp loss is often negligible, however, since similar substances can also easily be dissolved durin~ di~estion and bleaching~.
If cellulose pulp is to have the desired uniform quali-ties, the first requisite is a uniform chip quality, par-ticularly for sulphite digestion. Previously the wood wasstored irl lo~ piles for a year or so~ after which it could be chopped to chips, which were immediately digested. Modern machinery for felling, transport and handling, as well as in-creased demands for rapid capital returns, have resulted in timber nowadays being chopped as quickly as possible into chips

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which are then blown into a stack where, due to the larger contact area of the chips with the air, the resin is seasoned considerably more quickly than previously. The seasoning as well as the moisture equalization will be especially rapid if the stack is made so large that the heat developed can be subs~antially retained in the stack. According to information available from various sources of literatuTe, when the resin is fully seasoned about 20 m3 normal volume (900 mol) carbon dioxide is developed per ton of dry wood which, at an optimal temperature of about 35C, requires 40 days. Any shor~er period of storage has been considered out of the question. AEter storing at 50C for 72 hours, for ins~ance, only about 0.8 m (36 mol~ carbon dioxide is produced, and the alterations in wood and resin have not been measurable. Added to this, in practice there are always a number of problems; extreme cold or the formation of ice may delay or prevent heating to the optimum storing tempera-tura; the surface layer of the stack may be chilled and acquire different qualities; rot may set in. The storage must therefore be carefully watched, and since even tearing down the stack is troublesome, handling will require relatively hard work and high costs. The storage time is often 2 - 3 months, and interest costs are thus also still high.
According to the present invention, it has been found that the resin Z0 can be seasoned within a few days, thus eliminating the drawbacks mentioned above.
The method of the invention con~rises providing a body of wood chips in a container, maintaining a temperature of 35 - 80C in said container, main-taining a circulation of air in said container to produce a uniform tempera~
ture and oxygen content in said container, withdrawing part of said circulating air, and substituting fresh air in a quantity to maintain an oxygen content of at least 16~ by volume of the air in the container.

~ 1 _3_

3~

The invention will now be described with reference to ~igs. 1 and 2 and Tables 19 2 and 3.
Fig. 1 shows a wood c~ps container for operating the method of the invention.
Fig. 2 shows another wood c~ps container.
Table 1 shows the consumption and production of heat a~ter 72 hours processing, Table 2 relates to digestion and bleaching experiments on a laboratory scale and records of the practical result of the invention. Table 3 shows that the invention0~rated on full production scale gives resin seasoning results at least as good as conventional storage of wood chips over long periods. The consumption of oxygen, air and heat is usually given per metric ~on of dry wood.
1 mol gas = 22 liters at 0C and 100 kPa tkilopascal).
1 m3 air - approximately 1.3 kg.
The container 1 of Fig~ 1 is provided with a conveyor 9 and a rotary feeder 2 for supplying wood chips~ and with means 3~ 10 for removing chips. The wood chips move through the container in a substantially continuous flow. A fan 4 circulates air from the top to the bottom of the container.
Heat is supplied by blowing steam through a pipe 5 into the circulating air. Alternatively, the circulating air may be heated indirectly via a heat-exchanger. Part of the air is blown out at an outlet 6. The same quantity of fresh air is supplied through an inlet 7. The quan~ities of air and heat at 5, 6 and 7 are regulated in known manner by valves, either manually or automatically. The wood chips are withdrawn from the container through a bottom outlet by means of a rotary feeder 3~ and are transported to the pulping process on a conveyor 10.

~36~3 Fig. 2 shows an example of how the equipment can be modified. In this case the recirculating air is drawn off some distance from the top of the container through outlets 8 placed around the container at the lowest level normally reached by the top surface li of the column of chips. The space above the outlets 8 serves as a buffer, whereas the space below the outlets 8 is normally filled with wood chips. The out-let 6 for consumed air- is still at the top of the container, however, so tha~ the air heat is recov~red during the passage lQ of the air through the cold chips above the outlets 8.
The equipment can of course be varied in many other ways.
For exampleg the circulating air may be passed through the body of wood chips in a direction transverse to the flow of chips.
The chips roay be heated when or before they are fed into the container. Such heating~ howeverg must be performed sufficient-ly slowly and without local overheating. The conditions necessary for heating will be described below.
Table 1 below specifies the consumption of heat, air, and oxygen for heating two metric tons of wood chips, having a water content of 50 % by weight a from 0C to 50C,,and for processing said wood chips according to the invention for 72 hours. It i9 required that air shall be supplied to maintain an oxygen conten~ of at least 16 % by volume, and that the air shall be circulated at such a rate as to avoid any local over--heatin~ in the body of wood chips. ~he heating of 1 metic ton of dry wood chips requires 1.25 MJK 1 (megajoule per Kelvin).
The heating of 1 metric ton of water requires 4.19 MJK 1. The heating of 1 kg air requires 1.0 kJK 1 (kilojoule per Kelvin).
The evaporation of 1 kg water requires 2400 kJ. The figures given in Table 1 do not include any loss of heat due to, for exampleS a poor heat insulation.

~0~3~3 Table 1 Heating 1 metric ton dry wood chips62.5 MJ (megajoule) Heatin~ 1 metric ton ~ater in the chips 210.0 MJ
Hea~ing 20x120 mol air (70 k~) 3.5 MJ
Evaporatin~ water (6 kg)1500 MJ Gross 291 MJ

Heat produced by oxidation:
60 mol 2 to form C02 24.0 MJ
60 mol 2 to form oxidation products other than C02 12.0 MJeduct 36 MJ
Net -255 MJ

According to laboratory experiments, 15 - 30 mol oxygen were consumed per metric ton of dry wood during the first 24 hours. If the oxygen content in the air falls to or below 16 %
there will be serious diskurbance in the resin seasoning pro-cess, which cannot later be rectified. Allowing for a goodmargin of safety, therefore 3 it has been assumed that 120 mol oxygen per metric ton dry wood chips should be supplied during the specified 72~hour period, and only half of thîs quantity of oxygen, it is assumed, will be able to be completely con-verted to ~orm C02g thereby producing 0.4 MJ h~at per moloxygens. The oxy$en content may not be permitted to drop more than 5 units, i.e. from 21 % to 16 % by volume. The mînimum air requirement for the supply of oxygen will thus be 120~0.05 = 2400 mol. or about 54 m3 air per metric ton dry wood. The minimum air circulation for the heat supply will be 255~(15+3.5) = 14 times 2400 mol, i.e. 760 m3 (0C) per metric ton dry wood.
The loosely packed chips take up a volume of about 7.5 m3 per metric ton of dry chips. The air volume between the chips lS

approximately 5 m3 per metric ton of dry chips. Said air, therefore, has to be chan~ed approximately 150 times during a 72-hour period~ or about twice an hour. It has been found that ,, , , ~

~0~3~3 the temperature increase from 0C to 50C takes place in a relatively narrow zone of the chips column. However, it is important to keep the position of said temperature increase zone under control. An easy way o~ doing this is to move the position of said zone up to the outlet for the circulating air by increasing the air velocity. In this case a regulator should be arranged ~o control the air circulation so that the temperature at outlet 8 in Fi~. 2 is kept at 25C, for instance~
and another regulator should be arranged to control the steam supply so that the temperature of the air enterin~ the contain-er is kept at 50C. The air circulation will then be approxi-mately 1000 m3 (0C~ per metric ton of dry wood. If the temperature in the container is increased the circulation can be substantially reduced, since the air will now transport a larger quantity of water vapour. At a temperature of 80C the required air quantity is only 250-300 m3 per metric ton of dry wood. In the embodiment according to ~ig. 2 the air circulat-ion rate must also be sufficient to ensure a uniform temperat-ure. The risk of a local over-heating or under-heating increases with the temperature, and wiI1 primarily affect the re~ion above the temperature-increasing zone. Said risk can be limited by lowering the temperature-increasing zone. Said risk c~n be limited by lowering the temperature and increasing or revers-ing the air circulation, as well as by making the container tall in relation to its bottom area.
Table 1 also shows that the heating of the chips, which o~ course should be performed without a heat-consuming drying of the chips, is only dependent to a slight degree on the air consumption and on the heat produced by oxidation. It is also 0 evident that the heat produced by oxidation is sufficient to `~

~07~ a 3 cover both the air heating and the normal heat losses to the surroundings. Admittedly in comparison with storage in stacks, there is an immediate increase in heat consump~ion of 255 NJ, corresponding to 0.1 metric toD steam per metr~c ton dry chips, S or more at higher temperatures. However, the heat added by this additional s~eam is completely utilized in the steaming of the chips prior to the digestion process. The total thermal economy is therafore considerably bet~er than with outdoor storage of chips in stacks3 where the loss of heat is con-siderableO
The technical results are clear from Table 2, showing a selection of results from laboratory experiments perfarmed over processing periods of from 0 to 16 days at temperatures ranging from 50 to 80C and with varying supplies of oxygen.
The experiments showed that most of the resin seasoning is accomplished within 24 hours. Processing periods exceeding 4 days have not, therefore, been included in the table. Further-more, the table only records results obtained at 60C. Satis factory resin seasoning was obtained at all temperatures, probably somewhat better at higher temperatures. However, 80C was considered to be of less interest, partly bec~use it was found that i~ might cause a risk of lignin condensation.
Excessively high temperatures should also be avolded from the point of view of thermal economy, and about 60C may therefore be preferable. ThP following applied in the experiments re-corded in Table 2~
Seasoning: 0, 1, 2, and 4 d~ys at a temperature of 60C, with a full supply of air, and with a limited air supply. In the lat~er case the oxygen content had decreased ~o 15 % after 2 days. Different wood samples were used for the two series.

. - ~ .' ' ~36~3 Digesting The processed wood was digested with a magnesium--based digestion liquor. The liquor had a content of combined S2 of 1.0 %~ and a total content of S02 of 6 %. The ratio of wood to liquor was 1 to 4. The maximum temperature was 130C.
The time to reach the maximum temperature was 300 minutes. The maximum temperature was maintained for 210 - 270 minutes.

_eaching: The pulp was bleached with chlo~ne~ referred to as l'C1' in the table below, was subsequently subjected to an extraction with alkali~ referred to as "E~, and was finally bleached with hypochlorite, referred to as ~IHl'. The"charge"
relates to the quantity of bleaching agent~ in kilogramsg per metric ton of dry pulp. The letter "K" represents the kappa number, because the quantity of added chlorine depended on the kappa number of the pulp. The quantity of alkali is given as the equivalent quantity of sodium hydroxide. The quantity of hypochlorite is given as the equi~alent quantity of chlorine.
The~'exce~s alkali~' means that alkali~ defined as sodium hydro-hydroxide~ was added in a quantity amounjtin~ to 50 % by wei~ht of the bleachin~ a~ent~ defined as chlorine.

_C E H
Pulp concentration, % ~y wei~ht 3 10 10 Temperature~ C 20 20 45 Time 3 minutes 60 120 240 Charge 2.4 K 10 8 Excess alkali~ % - - 50 ~ 3~3 Table 2 __ , Wood processing with rapid resin Wood seasoning accordin~ to the in- stored ~vention ~ for 1 Limited ~ Full 3year ; oxy~en supply oxygen supply ~piles ~ime at 60C3 days 0 2 4 0 1 2 4 ~olids content of ^ . ~ . . .
~ood, % .54.1. 55.3 53.9 45.8 44.8 44.4 43.0 ~nbleached pulp:
Time at max.
. temp., minutes 270 210 210 210 240 240 240 Kappa number 13.9 28.1 35.7 11.8 15.4 15.9 22.8 DKM-extract, % 1.18 0.75 0.86 0.89 0.68 0.67 0.75 1.10 ~leached pulp: ¦
Wood yield, % ~, 49.7 51.2 50.5 48.1 48.5 49.8 49.7 Bri~htness Scan,~ 88.2 84.9 83.7 88.8 87.5 88.1 88.8 ~KM-extract, % 1.09 0.75 0.75 1.09 0.72 0.67 0.60~ o.8 Extrack-bound chlorine mg/kg 2180 '1480 1435 2240 1230 1190 1160 1640 . . . , _ The "DKM-extract~' in Table 2 above relates to the quantity of resin dissolved when the pulp is extracted with dichloro-methane~. The term "extract-bound chlorine" is used to define the quantity of resin which has been chlorinated during the bleach-in~ process.
The limited oxygen supply was achieved by processing the wood in a closed vessel containing air. The oxygen content of the air dropped in two days from 21 % to values between 16 and 11 %. The oxygen consumption was 25 - 50 mol oxygen per metric ton dry chips. I'he air was subsequently replaced by fresh air.
This method of substituting ~resh air for the consumed air was not strictly in accordance with the invention3because there was no circulation of air through the closed vessel. This in-consistency is not importantj however, because the experiment ', ', ' ' . ' : . . - :: : ' . -1~3~3 was made with a small scale closed vessel, ensuring a uniform temperature, moisture content a~d oxygen content throughout the whole vessel. Therefore, it is clear ~rom the results given in Table 2 that the damage caused to the resin season-ing during the first days is difficult to remedy later. pn the other handJ it can be noted that in spite of this damage the processing result with a limited oxygen supply was acceptable.
Thus already after about one day the resin content of the bleached pulp, which may be said to represent the result of the wood processing,had decreased to a level well below that obtainable with the use of conventionally stored wood, under comparable conditions during the pulping process. It is clear from Table 2 that it is preferred to supply air at such a rate that its oxygen content does not drop below 16 % by volume during the processing of the wood chips.
An experiment was also performed on full production scale. The wood chips were processed at a temperature of 55C
in a container holding about 240 metrîc ton wood-chips and with a fan capacity of 20,000 m3/h Some of the heating was performed by adding steam to the circulating aîr~ the rest by heating the circulatin~ air in a heat exchan~er. The processing time was planned to be 60 hours i.e. the chips flow was 4 metric tons per hour and the air circulation was 5000 m3 per metric ton chips. This high rate of air circulation gave uniform heating. It was found, how-ever, that the container used was not entirely s~table for this proces~ng of wood chips~ because the flow of chips was not entirely uniform. Therefore, the effective processing time was shorter than the desired 60 hours period. In spite ~ 3613 of this unsa~isfactory processing container the results obtain-ed were very goodg which is evident f'rom Table 3 below.

Table 3 Date Type of wood Unbleached Bleached Extract-bound of used in pulp pulp chlorine pulp-' pulping DKM extract % DKM extract % mg/kg ing ., process .B~tch Average Butch'Avera~e B~tch Average '22 Wood stored , 1.28 1.04 1750 23 fnrlOgepyelears ' 1.30, 0.92' , 1500 '24 ' 1.24, 1.08 1900' 1.40 1.20 2200 26 ' 1.3ll 1.06, 1600, , 27 , 1.36 , o. g6 . 1400 ,28 , 1.32 1.32 1.06', 1.05 1800 1750 , i29 ,Wood processed 1.34 , 1.00,' 1700 ''30 according ,to , 1.38 ' 1.12' 2000 , ,the inventlon 31 ', , 1.26 , 1.06 , 1600' ', 1 , 1.30 1.02 1600', , 2 1.36, 1.331.06 1.05' 2000' 1800 , 3 Wood stored 1.26 1.06' 1900

4 ''fnrlogepYielears 1.26 1.261.12 1.09 2100 2000 Table 3 discloses that7 to serve as a comparison, pulping was made for ~even consecutive days, to wit 22nd - 28th of the relevant month~ usin~ wood which had been stored ~or one year in log piles. Subsequently7 the same pulping process was con-tinued for five consecutive days 5 to wit from the 29th to the 2nd of khe next month, usin~, wood which had been processed for the 60 hours period referred to above. As a co~parison~ pulp-ing was now continued during the 3rd and 4th using wood storedfor one year n log piles. The various batches of pulp thus produced were analyzed. The results are given in Table 3. It ~7~6~3 ~

is evident from Table 3 tha~ the results obtained wi~h the wood processed according to the inventlon are as good as those obtained with the wood which had been stored for one year. Consequently, the rapid resin seasoning process of the invention can replace the conventional long time stor ing of the wood before the pulping process.

Claims (7)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of processing wood chips for producing a rapid resin season-ing, comprising providing a body of wood chips in a container, maintaining a temperature of 35 - 80°C in said container, maintaining a circulation of air in said container to produce a uniform temperature and oxygen content in said container, withdrawing part of said circulating air, and substituting fresh air in a quantity to maintain an oxygen content of at least 16% by volume in the air in said container.

2. A method as claimed in claim 1, comprising adding wood chips to the top of the container, withdrawing wood chips from the bottom of the container, and maintaining the circulation of air by withdrawing air from the top of the container, replacing part of said air by fresh air, and re-entering the air to the bottom of the container.

3. A method as claimed in claim 1, comprising adding wood chips to the top of the container to produce a body of chips in the container, withdrawing wood chips from the bottom of the container, withdrawing part of the circu-lating air from the top of the container, withdrawing the rest of the circulating air from the container at a level intermediate the top and bottom of the body of wood chips, adding fresh air to said rest of circulating air, and re-entering the air to the bottom of the container.

4. A method as claimed in claim 1, comprising maintaining a temperature of 60°C in said container.

5. A method as claimed in claim 1, comprising maintaining the temperature in the container by heating the air entering the container.

6. A method as claimed in claim 5, comprising heating the air by adding steam to it.

7. A method as claimed in claim 1, comprising retaining the wood chips in said container for at least 15 hours.