Note: Descriptions are shown in the official language in which they were submitted.
10610S8
FIELD OF THE INVENTION
.. . . ..
The present invention relates to oxygen pulping.
More specifically the present invention relates to an im-
proved process of oxygen pulping by relieving and building
up the pressure during pulping.
DESCRIPTION OF THE PRIOR ART
It has been known for many years that wood can be
digested to liberate cellulose by using oxygen gas in the
presence of an aqueou~ alkaline medium such as sodium hy-
droxide or sodium carbonate. In the early 1950's a process
was proposed wherein wood was treated in an aqueous alkali
solution of pH 7 - 9 at a temperature between 120 - 175C.
under an atmosphere of oxygen carrying gas with a partial
oxygen pressure of at least 800 p.s.i.; however, the results
obtained were not commercially satisfactory (see U.S. patent
2,673,148 dated March ~3, 1954 - Harris).
U.S. patent 2,926,114 dated February 23, 1960
issued to Grangaard & Saunders describes a process of oxygen
pulping using sodium bicarbonate (50% on dry wood) with
oxygen at 200 p.s.i. and at a temperature of 120 - 160C.
This process suffered from relatively high chemical consump-
tion.
More recently improvements have been proposed to
the oxygen pulping process as exemplified by Canadian
patents 935,957 and 935,958 issued October 30, 1973 to
Samuelson et al and Samuelson respectively. Samuelson et al
patent describes a process using progressive addition of
alkali to maintain the pH in the range of 9.5 to 13 during
the oxygen pulping stage. The Samuelson patent on the other
hand teaches maintaining the partial pressure of oxygen sub-
stantially constant by continuously bleeding the gaseous
phase containing oxygen and carbon dioxide and recovering
the oxygen. These processes represent improvements over the
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prior art but are still not commercially satisfactory.
SUMMARY OF THE PRESENT INVENTION
.
The present invention provides a further improve-
ment in the art of oxygen pulping and is predicated on the
finding that pressure cycling the digester at least every
30 minutes during the cook by venting gases from the di-
gester thereby lowering the pressure by at least 50 psi.
followed by returning the digester to pressure by adding
an oxygen containing gas so that the operating partial
pressure of oxygen is at least 95 psi, substantially re-
duced the rejects content of the resultant pulp.
Broadly, the present invention relates to a methodof cooking cellulosic material with oxygen in one or more
stages in an alkaline medium in a digester at a temperature
in the range 125-170C at an oxygen partial pressure of
at least 95 psi, wherein the improvement comprises pressure
cycling at least once every thirty minutes by venting gas
to reduce the total operating pressure in the digester by
at least 50 psi. followed by raising the pressure to at
least 95 psi.oxygen partial pressure; the digester being
maintained at said oxygen partial pressure of at least
95 psi. for at least 70% of the cooking time.
Generally the operating pressure (pressure between
pressure cycles) during the oxygen digestion will be a
minimum of about 95 psi oxygen partial pressure. High
oxygen partial pressure during the cook results in a more
uniform pulp but may require higher capital cost and thus
the selection of the maximum operating pressure will be
based on economics.
The cooking temperature for the oxygen cook will
be in the range of 125 to 170C.
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Each pressure cycle (purging and replenishing
gases in the system) requires that the pressure be reduced
significantly below the operating pressure. A reduction
of at least 50 psi is required to provide significant
results. Obviously the pressure should be reduced by the
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minimum amount necessary commensurate with obtaining the
desired results. The partial pressure of oxygen will
normally not be reduced to zero.
There will be at least one pressure cycle (re-
duction and increase in pressure) every 30 minutes during
the cook and preferably one every 15 minutes. The time of
each cycle m~st be controlled to insure at least 70% of the
time the digester is provided with an oxygen partial pres-
~ure of at least 95 p.s.i. Preferably at least 85~ of the
time the digester will be operating with an oxygen partial
pressure of at least 95 p.s.i. Clearly it is best to complete
a pressure cycle in as short a period of time as possible to
ensure the best operation of the system; however, it will be
apparent that after each purge the pressure need not be neces-
sarily returned to its original operating oxygen partial
pressure provided the oxygen partial pressure is always
maintained above the 95 p.s.i required. It is contemplated
that in each pre~sure cycle there will be a significant re-
duction in pressure followed by a significant increase in
pressure although the increase in pressure may not return
the digester to its original operating pressure but may
return the digester to a pressure above or below original
operating pressure. In any event the operating pressure
between cycles will be regulated to ensure that there is at
least 95 p.s.i. oxygen partial pressure in the vessel.
Cooking may be done in stages using the same or
different pulping conditions in each stage and including,
if desired, mechanical treatment of the wood materials be-
tween the stages. The pressure cycling technique of the
present invention may be used in all or only some of the
stages.
BRIEF DESCRIPTION OF' DRAWINGS
Further features, objects and advantages will be
1061058
evident from the following detailed description of a pre-
ferred embodiment of the present invention taken in con-
junction with the accompanying drawings in which:
Figure 1 schematically illu~trate~ lab apparatus
S used for carrying out experiments on oxygen pulping in ac-
cordance with the present invention.
Figure 2 ~chematically illustrates one form of
apparatus that may be u~ed commercially with the pre~ent
invention.
DESCRIPTION OF PREFERRED EMBODIMENT
In the arrangement shown in Figure 1 a dige~ter
vessel 10 having an agitator or cavitator 12 i~ provided
with a screened area 14 for holding the chips to be digested.
The cavitator 12 has a hollow ~haft 9 communicating with the
space at the top of the digester 4 and with the hollow vanes
13. Alkali may be introduce~ to the digester via line 16,
oxygen via line 18 and gases may be periodically vented via
line 20.
In operation, chip~ to be digested are placed
into the basket 14 and the vessel 10 is filled with liquor
to level L. The liquor may carry 3ufficient alkali for the
dige~ting process although if desired alkali may be pro-
gressively added during the ~cok to aid in maintaining the
pH within the de~ired range. It is preferred to maintain
the p~ in the range of 7 to 10. Oxygen gas is introduced
to the vessel 10 Yia line 18 to maintain a high oxygen
partial pressure within the digester. During the cook oxy-
gen partial pressures of at least 95 p.s.i. and up to 400
p.~.i. and higher depending on the equipment is used. During
~he cooking operation the agitator 12 i~ rotated thereby
vigorou~ly agitating the liquor and gas within the vessel 10
to ensure proper dissolving of the oxygen in the liquid and
proper contact between the liquid and the chip~. Gas is
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drawn through the hollow shaft 9 which is open to the gas
above level L and this gas is drawn through the shaft 9 and
is dispersed through holes in vanes 13.
Periodically gas in the digester i8 pressure cycled
by venting via line 20 followed by increasing the pressure.
The vented ga~ will normally contain water vapour and CO2
evolved from the digestion process together with oxygen.
In a 2 hour cook it haæ been found that, for example, pres-
sure cycling 7 times in the course of the cook at uniformly
spaced intervals was satisfactory, i.e. every fif~een
minutes. When the valve in the line 20 is opened the pres-
sure in the vessel 10 is reduced substantially (by at least
50 psi.) so that some of the dissolved gases in the liquid
phase will also be purged. It has been found that the
pressure must be lowered at least 50 psi. when cooking at
an operating oxygen partial pressure of about 100 psi.
In the arrangement shown in Figure 2 the cooking
process may be operated in several ways. The apparatus as
shown comprises a vessel 100 having: a sparger 102 adjacent
the bottom thereof for introduction of oxygen gas; a re-
circulation line 104 with a pump 106; a gas discharge line
108; a digested pulp outlet 110; and suitable inlets for
liquor and chips as schematically illustrated at 112 and
113 respectively.
The system illustrated in Figure 2 may be operated
in various waysO One way is to maintain a liquid level as
indicated at 114 thereby to completely submerge any chips in
the digester and to circulate the liquor via line 104 while
introducing oxygen via the sparger 102. The oxygen bubbles
up through the chips and liquor to ensure good contact and
dissolving of the oxygen in the liquor.
An alternative mode of operating the device of
Figure 2 is to maintain the liquid level as indicated at 116
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well below the chip level as indicated at 118 so that liquor
is recirculated via line 104 and trickles down through the
chips that extend above the level 116. Again oxygen is
introduced via the sparger 102 and passes upwardly through
the liquor and chips at the bottom of the vessel 100 and
through the chips extending above the level 116.
In the Figure 2 arrangement the pressure in the
vessel 100 is periodically reduced by opening the valve in
the line 108 to purge some of the gases from the system and
reduce the pressure in the vessel 100 substantially, fol-
lowed by raising the pressure to above the minimum operating
pressure of 95 psi. oxygen partial pressure and preferrably
to the original operating pressure. The pressure cycle in
this arrangement is essentially the same as that described
hereinabove with respect to Figure 1. After the cook has
been completed the gas pressure may be relieved through line
108 and the line 110 opened to blow the pulp to a suitable
receiver (not shown).
The vented gases from the process contain sub-
stantial quantities of oxygen and these gases after proper
treatment may be used in various ways, for example, the gases
may be recycled or used in wet oxidation of the spent liquor
or for oxygen bleaching or in any other suitable manner.
One effect of the pressure cycle technique des-
cribed above is to purge carbon dioxide from the system,
thus preventing the pH of the liquor from dropping to a
lower value at which the rate of reaction becomes less. A
further effect, which is not completely understood, involves
an increase in the uniformity of the cook, and a lower re-
ject level. It is thought that the drop in pressure which
occurs during the venting results in expansion of the
structure of the cellulose material allowing easier pene-
tration of liquor and oxygen during the subsequent part of
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the cooking. Also, liquor containing a high concentration
of reaction products is thought to be voided from the ~truc-
ture of the cellulose material durin~ the venting allowing
the penetration of fresh liquor and resulting in an in-
creased rate of cooking.
Following are ~pecific example~ of the present in-
vention. In all ca~e~, a total operating pre~sure of 175
p8i (including ~team pres~ure) was u~ed, with a total alkali
charge of 31% sodium carbonate on wood (18.1~ as Na20) and
a liquor to wood ratio of 25:1. A mixture of maple and elm
wafer3 of 0.035~ thickness was used in all examples. Ap-
proxi~ately 110 psi oxygen pres~ure was added to a 1 gallon
reaction vessel similar to that shown in figure 1 containing
the wood and alkaline cooking liquor. The ve~sel and con-
tents were heated ~o the required cooking temperature in
60 minutes. The total pre~sure was then adjusted to the 175
p~s.i. operating pressure by addition of oxygen to the vessel.
Example l(a)
In a reference control, the chips were treated for
2 hours at 151C, and the oxygen in the vessel was replaced
7 times during the cook by bleed~ng off gas for 2 minute~
and simultaneously introducing fre h oxygen without signi-
ficantly changing the total pressureO This venting proce-
dure wa~ carried out every 15 minutes and the amount of
oxygen introduced each time was approxLmately equivalent to
the amount of gas already present in the vessel. Carbon
dioxide produced in the reaction was removed with the vented
gas, and the pH of the cooking liquor therefore remained at
a value of 80 1 + 0. 3 throughout the major portion of the
cooking period. The results of ~his digestion procedure are
as follows:
Total Accepted Rejects Kappa NQO Tappi Elrepho
Yield Yield _ of screened pulp Viscosity Brightness
54.4% 39.7% 14.7% 24.6 14.9 cps 53.8
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Example 1 (b)
A digestion was carried out under the same reaction
conditions as those of part (a), except that in accordance
with the present invention the oxygen was replaced by purg-
ing to 75 psi and then raisinq the pressure to 175 psi
using fresh oxygen. The total time required for each vent-
ing cycle was 2 minutes. The frequency of venting was the
same as in part (a), as was the pH of the liquor. The re-
sults of this digestion procedure are summarized below:
Total Accepted Rejects Rappa No. Tappi Elrepho
Yield Yield Viscosity Brightness
51.9% 50.6% 1.3% 16.5 11.8 cps 62.9
It can be seen that by using the pressure cycling
technique of the present invention, significantly lower re-
jects are obtained together with a lower Kappa number. The
pulp produ~ed had the following properties after 1000 re-
volutions of PFI mill treatment:
Freeness Bulk Burst Tear Breaking
~Canadian Standard) Factor Factor Length
346 1.49 51 58 8.5 km
Example 2
A digestion was performed at 146C for a period
of 2 1/2 hours using the same pressure cycling technique
described in Example 1 (b). The results of this digestion
are summarized below:
Total Accepted Rejects Kappa No. Tappi Elrepho
Yield Yield _ Viscosity Brightness
59.0% 53.9% 5.1% 25.2 14.6 cps 55.5
It will be noted that the Kappa number for the
pulp of example 2 is substantially the same as for example
1 (a), but the rejects of example 2 are significantly lower.
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Example 3
A digestion was performed in two ~tages at 151C,
u~ing half of the total alkali charge in each stage. The
fir~t stage of cooking was 1/2 hr., and the second stage
1 1/2 hr. The same pressure cycling technique described in
Example 1 Ib) was used in both stages, except that gas re-
lief was more frequent in the first stage (every 8 minutes).
Before each cooking stage, the wood material was impregnated
at 85C for 20 minutes with a solution containing the re-
quired amount of sodium carbonate at a concentration of 30
gmJ litre. The alkaline solution wa~ then diluted with
water for use as cooking liquor in ~he digestion. After the
first cooking stage, the partially digested wood material
was disintegrated into smaller particles using a Waring
blender. The results obtained after the second dige~tion
stage are a~ follows:
Total Accepted Rejects Kappa No. Tappi Elrepho
Yield Yield ~iscosity Brightness
56.7~ 56.6% 0.1% 17.8 11.8 cps 64.3
It will be apparent from example 3 that a good
pulp is produced by staged cooking with mechanical working
between the stages. Mechanical working is not essential.
Modifications may be made without departing from
the spirit of the invention as defined in the appended
claims~
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