U.S. patent application number 10/432657 was filed with the patent office on 2004-05-13 for method for alkaline cooking of fiber material.
Invention is credited to Fant, Thomas, Lammi, Lari, Svedman, Mikael.
Application Number | 20040089430 10/432657 |
Document ID | / |
Family ID | 8559577 |
Filed Date | 2004-05-13 |
United States Patent
Application |
20040089430 |
Kind Code |
A1 |
Fant, Thomas ; et
al. |
May 13, 2004 |
Method for alkaline cooking of fiber material
Abstract
A continuous, alkaline process for the production of pulp from
wood chips, wherein the preheated chips are subjected to an
extended impregnation step for at least 60 min, preferably longer,
at a temperature not exceeding the impregnation liquor boiling
point at atmospheric conditions, and a rapid heating and cooking
period of less than 65 min, preferably shorter, followed by cooling
to below reaction temperature. Fresh alkali is added both during
impregnation and the heating/cooking period.
Inventors: |
Fant, Thomas; (Pori, FI)
; Svedman, Mikael; (Vaasa, FI) ; Lammi, Lari;
(Pori, FI) |
Correspondence
Address: |
LERNER, DAVID, LITTENBERG,
KRUMHOLZ & MENTLIK
600 SOUTH AVENUE WEST
WESTFIELD
NJ
07090
US
|
Family ID: |
8559577 |
Appl. No.: |
10/432657 |
Filed: |
December 18, 2003 |
PCT Filed: |
November 21, 2001 |
PCT NO: |
PCT/FI01/01009 |
Current U.S.
Class: |
162/19 ; 162/236;
162/241; 162/29; 162/38; 162/39; 162/47 |
Current CPC
Class: |
D21C 3/24 20130101; D21C
1/06 20130101; D21C 7/00 20130101; D21C 3/26 20130101 |
Class at
Publication: |
162/019 ;
162/047; 162/029; 162/038; 162/039; 162/236; 162/241 |
International
Class: |
D21C 003/26; D21C
007/00; D21C 011/00; D21C 007/14 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 24, 2000 |
FI |
20002587 |
Claims
1. A continuous, alkaline process for the production of pulp from
wood chips, said process comprising the steps of: preheating the
chips; impregnating the chips in an impregnation step with an
alkaline liquor at a temperature not exceeding the boiling point of
said liquor at atmospheric conditions, during a time period of at
least 60 min, during which impregnation step fresh alkali is added,
the average effective alkali concentration of the added liquor
being in the range from about 0.5 to about 2.2 mol/l as OH ions and
the liquor to wood ratio during a substantial part of the
impregnation step being between 3 and 10 m.sup.3/t odw; heating the
impregnated chips to a temperature of at least about 150.degree. C.
and cooking the chips, the maximum temperature of the cooking step
being less than 185.degree. C. and the total time of the heating
and cooking steps being less than 65 min, during which period fresh
alkali is added as well as possible additional liquor, so as to
result in a liquor to wood ratio of at least 2.5 m.sup.3/t odw
during a substantial part of the heating and cooking steps, and
cooling the cooked material to a temperature where cooking
reactions essentially cease.
2. A process according to claim 1, wherein at the most 90% of the
total fresh alkali calculated as total titrable alkali per unit
charged wood is added during the impregnation step.
3. A process according to claim 1 or 2, wherein the time t.sub.1 of
the impregnation step is above 120 min.
4. A process according to ay claim 1-3, wherein the total of time
t.sub.2 of the heating step and the time t.sub.3 of the cooking
step is in the range from 10 to 50 min, preferably in the range
from 10 to 40 min.
5. A process according to any claim 14, wherein the time t.sub.2 of
the heating step is below 30.
6. A process according to claim 1, wherein the liquid-to-wood ratio
is in the range from 3 to 6 m.sup.3/t odw during a substantial part
of the impregnation step.
7. A process according to claim 1, wherein the liquid-to-wood ratio
is at least 3 m.sup.3/t odw during a substantial part of the
heating and cooking steps.
8. A two-vessel digesting system for performing the method
according to claim 1, comprising an impregnation vessel and a
digester in fluid communication with the impregnation vessel,
whereby the volume V of the impregnation vessel is larger than 1/1,
preferably larger than 2/1, more preferably larger than 3/1 of the
volume of the digester.
Description
FIELD OF THE INVENTION
[0001] The invention relates to the field of alkaline pulping.
BACKGROUND OF THE INVENTION
[0002] Among the chemical pulping methods, alkaline cooking
processes and especially kraft cooking are dominant in the
production of cellulose or chemical pulp because alkine cooking
provides pulp fibers which are stronger than those from any other
commercial pulping process. The lignocellulosic material, typically
chopped into wood chips, is treated in either batch or continuous
digesters.
[0003] In chemical pulping processes as alkline cooking, the
chemical reactions with wood components are heterogeneous-phase
border reactions. To ensure uniform reactions, it is vital that all
fibers in the wood get their proper share of chemicals and energy.
Non-uniformity occurs when these criteria are not fiilfilled and
this may result in a larger amount of unfiberited wood (rejects) in
the final pulp, discolored pulp, lower final yield and impaired
bleachability and paper properties. The objective of chemical
pulping is to remove lignin so that the fibers can separate.
Ideally, each fiber should receive the same amount of chemical
treatment for the same time at the same reaction site. This means
that chemicals and energy must be transported uniformly to reaction
sites throughout each chip. There are two major stages where this
can happen; (1) the impregnation phase where chips are saturated
with chemical-containing liquid before delignification reactions
begin and (2) the continuous movement of chemicals to the reaction
sites during the cooking phase. Chip dimensions are of major
importance in this context. The longer, wider and especially the
thicker the chips are, the longer is the transportation distance to
the centers of the chips.
[0004] Pores inside fresh wood chips are also partly filled with
liquid and partly with air. The ratio is, among others, determined
by the moisture content or dry content of wood. The air should be
removed from the chips before they can be fullly impregnated by
cooking liquor. This is usually done by pre-steaming the chips.
[0005] As cooking proceeds, reactive ions must diffuse into the
chips. If the transportation distance is too long and rate of
transportation too slow, the chemicals may be completely consumed
before they can reach the chip centers, resulting in non-uniform
cooking. Thus, in cooking there is a critical balance between the
rate of ion transportation, wood porosity, chip dimensions and the
rate of chennical reaction. E.g. raising the temperature increases
the rate of transportation, but the rate of reaction increases
still faster. On an average, long and thick chips will not
delignify as fast as short and thin chips when the same cook
parameters are used. In conventional batch cooks, long and thick
chips generate more screening rejects than short and thin chips and
shorter cooks and higher cooking temperatures aggravate the effect
The higher the cooking temperature, the shorter is the required
time to a certain delignification degree and the higher is the rate
difference between delignification reaction and ion transportation.
Thus it seems that cooking uniformity would require that chips
should be small and thin enough and that the size variation in the
chip furnish should be as narrow as possible. However, in practice
many other important parameters as e.g. chipping parameters,
chipping capacity, fiber cutting, fiber length, paper properties,
the bulk density of the chip column, the permeability of the chip
column, saw dust and pin chips amounts and plugging of digester
screens has also to be taken into account. Saw dust is material
passing through a 3 mm hole screen; pin chips is material that
passes through 7 mm hole screens but is retained on a 3 mm hole
screen.
[0006] It has also been proposed that the rate of heating influence
the result. A shorter heating period requires a smaller chip size
to ensure sufficient uniformity, and it has been demonstrated that
shredding of the chips will reduce the screening when using very
rapid cooks (cycle-to-cycle) at high temperatures, and almost no
heating period. Such conditions, i.e. fast heat-up and high
temperatures prevail in certain types of continuous digesters as
saw dust and pin chips digesters. In the Kamyr type of continuous
digester as well as in most batch digesters, slow cooking, i.e. low
rate of reaction, long heating period and lower maximum
temperature, allow a more uniform cooking also of ordinary mill
size chips. As both shredding and chipping to smaller size will
affect fiber length, pulp quality and efficiency of operation, it
is desirable to provide conditions that allow the normal industrial
chip size.
[0007] For processing ordinary mill size chips, it has been
proposed that cooking non-uniformity can be reduced and perhaps
eliminated by proper chip manufacturing and screening, proper
impregnation and sufficiently low cooking temperature. Thus low
rate of chemical reactions and long cooking stage in combination
with impregnation and uniform chips.
[0008] In a conventional alkaline batch cooking process, wood chips
are fed to a digester and cooking liquor is added. The chips can be
steam treated before, during or after chip filling to pre-heat the
chips and remove air. The pressure is about atmospheric after
liquor addition. When chips and liquor have been added, the cook is
immediately started by introduction of heat either indirectly or
directly by steaml Impregnation occurs during heating. The cook
itself consists of a heating period and an "at pressure" period.
Typical heat-up times and at pressure times are 60 to 150 minutes
and 60 to 120 minutes, respectively. Typical sum of heat-up and at
pressure times is about 150 minutes. At the conclusion of the cook
when the delignification has proceeded to the desired reaction
degree, a blow valve in the digester is opened and the contents of
the digester are discharged into a blow tank as the hot liquor in
the digester flashing into steam and forces the cooked pulp out of
the digester. The cooked material is cooled and defiberized.
[0009] The difficulty with conventional batch cooking is the
non-uniform and poor quality of the pulp, high energy consumption
and environmental concerns. Therefore, one of the most important
objectives has been the attempt to improve the efficiency of the
cooking process and to improve the properties of the resulting pulp
and the uniformity, especially in relation to above mentioned
conventional process. Continuous flow processes and displacement
batch processes have therefore been developed.
[0010] Displacement batch pulping processes were developed in the
80's, originally for the sake of energy economy. Following a batch
cook, the black liquor was recovered, divided into fractions
according to temperature, stored and introduced into a digester
charged with fresh lignocellulosic material in order to transfer
the heat of the completed cook to a subsequent cook. The total
duration of the black liquor impregnation stage in batch
displacement processes is typically below 30 min at temperatures
below 100 .degree. C. Heating is carried out by displacement with a
black liquor having a higher temperature than the impregnation
liquor. Following these initial stages, white liquor is introduced,
and a main cooking stage follows. Typically, the total duration of
the hot liquor fill stage, temperature adjustment and the cooking
stage is in the range 95-120 min.
[0011] Several variations of the batch displacement cooking process
have been developed to optimize utilization of cooking chemicals
and to handle variations in raw material, where it may be
advantageous to adjust the composition and/or sequence of
pretreatment liquors. In addition to energy economy, adjustment
possibility, pulp quality and flexibility are advantages when
displacement batch cooking processes are used. On the other hand,
these processes require a significant amount of tanks and piping to
handle the various liquors involved. As the trend is towards closed
circuits and lower emissions from plants, accumulation problems may
also occur.
[0012] In contemporary continuous processes, typically referred to
as the Kamyr type, energy savings are achieved by pre-heating the
chips with steam obtained from flashing the hot black liquor. In
the pre-steaming of the chips, chips are preheated and air is
removed from the chips to facilitate later liquor impregnation. In
continuous cooking, the chip impregnation zone typically involves
30-60 min or shorter chip retention at a temperature of
115-130.degree. C. and a high pressure to enhance the
pre-impregnation of the chips and the ion transportation into the
chips. Since penetration rates increase with increased pressure,
impregnation stages operate at pressures that greatly exceed the
liquor saturation pressure at the specified temperature, i.e.
typically greater than 10 bars operating pressure for impregnation
temperatures of 115-130.degree. C. Subsequent to impregnation, the
chips are heated directly in a vapor phase and/or in several liquor
heating circuits to full cooking temperature, and then typically
cooked for at least 1.5-2.5 hours in a concurrent cooking zone at
temperatures below 165.degree. C. Practical experience suggests
that the process becomes chemical transportation rate limited at
cooking times below about 1.5-2.5 hours and temperatures above
165.degree. C. Therefore, typical cooking temperatures are between
150-165.degree. C. but even cooking temperatures of 140-150.degree.
C. can also occur, see for example international patent application
WO 98/35091. Thus, a minimum of 1.5-2.5 hours of cooking is
required. In addition, subsequent to the concurrent cooking zone, a
countercurrent zone, typically referred to as the hi-heat zone,
usually follows for 24 hours at temperatures of 130-160.degree. C.
Contemporary continuous cooking as e.g. ITC, EMCC and Lo-solids
cooking retains the cooking temperature, typically 150-160.degree.
C., all through the countercurrent zone, i.e. enlarging the cooking
zone to the counter-current zone. These modem digesters have thus a
total cooking zone of about 240-360 minutes. For the countercurrent
zone, washing filtrate is pumped into the bottom of the vessel. The
vessel bottom is also a blow dilution and cooling zone. Discharge
temperature is typically 85-90.degree. C.
[0013] The continuous processes offer, compared to conventional
batch digesters: more space efficiency, less installed power, lower
volumes of inlet streams and outlet stream, steady-state operation
vs. batch fill and discharge cycles, energy efficiency, lower
environmental impact and a first stage of brownstock washing.
[0014] However, it is found that while the typical continuous
processes have the aforesaid advantages the pulp obtained has a
number of properties, e.g. strength and uniformity of the pulp,
which are inferior to e.g. pulp produced under well-controlled
laboratory conditions. The continuous process still lacks
sufficient impregnation and this has to be compensated by lower
reaction temperatures and long retention times in the digester.
This leads to expensive, large-size and huge digesters designed for
high pressures and temperatures. The scale of contemporary
digesters, typically of the type Kamyr, at higher production levels
also causes mass-transfer problems in liquid circulations and
displacement, which further is compensated by longer retention
times and lower cooling temperatures.
[0015] Impregnation theoretically requires small chips, but modern
continuous digesters are based on the principle of maintaining
sufficient liquid circulation and a good displacement efficiency.
This calls for chip properties that are in conflict with some of
the basic requirements for ensuring uniform delignification. Thus,
a large chip size must be used, which leads to inferior
impregnation and firther longer retention times in cooking and
expensive technology. Thus, the pulp maker has been trapped by his
own technology. It is stated, that the 30-60 min retention time at
115-130.degree. C. in impregnation zones of continuous mill
digesters could never provide a completely uniform distribution of
cooking chemical for all chips (mill chips) before the start of
bulk delignification.
[0016] In Swedish patent application 9602016-9, it is suggested
that the way the chips are treated before continuous kraft cooking
is disadvantageous for the strength of the pulp. It is proposed
that the pre-impregnation at 110-130.degree. C. in e.g. Kamyr
continuous digesters is unfavorable and the chips should instead be
cold impregnated. During impregnation it would only be necessary to
have enough alkali present to neutralize possible by-products, in
order to prevent formation of acidic regions that can damage the
fiber properties. The alkali required during cooking can
consequently be added after the pretreatment, and/or during
cooking, i.e. a high alkali level is not required during
impregnation. A pulping process is disclosed which comprises
so-called "cold impregnation" as its main feature. A temperature of
about 80-110.degree. C. is specified, the time period being
unlimited. However, an optimum of 2-3 hours is suggested. Pressure
may be used in order to compress gas bubbles and cause sinking of
the chips. The theory behind the cold impregnation stage is, that
acid-generating processes within the chips shall be suppressed
until the chips are filled with alkali sufficient to neutralize any
acid released when reaction commences at higher temperatures, and
the impregnation step is defined as resulting in "an alkla
concentration sufficient to neutraze all acid produced".
[0017] The proposed process preferably uses a conventional
continuous digester like MCC, EMCC or Lo-solids digester. Thus, the
retention times in the cooking stage are in the order of several
hours, typically around 2-5 hours. In addition, the figures of the
application show a residence timne in the impregnation stage
approximately of the same order as in the cooking stage. However,
it is found that while the proposed continuous processes have pulp
strength advantages, the cooking stage still has a long retention
time and low reaction temperature . This requires huge and
expensive digesters designed for, from a technical point of view,
high pressures and temperatures.
[0018] In U.S. Pat. No. 3,215,588, and in paper entitled "Rapid
alkaline cooking", Pulp and Paper Magazine of Canada, No 7, pages
T-275-T-283, a process is disclosed wherein an extended
impregnation stage is utilized followed by a rapid cooking stage.
Chip impregnation takes place at a pressure in excess of 10 bar,
using cooking liquor. Subsequently, the chips are fed into a
continuous digester having a steam zone where the chips are rapidly
heated to 170-185.degree. C. and thus cooked before entering a
liquid zone where gradual cooling takes place prior to discharge.
The paper teaches that the total cycle including impregnation is in
the range of 30 to 45 minutes, mostly using impregnation
temperatures of 130-150 .degree. C. The process results in pulp
relatively low in lignin, having good bleachability according to
the standards of the time, and the process is rapid due to the
pressurized impregnation stage and the short heating stage.
However, the screen rejects content is high and the strength
reduced in comparison with pulp produced by conventional methods of
the period. These disadvantages are addressed in U.S. Pat. No.
3,644,918, which introduces water saturation of the chips prior to
impregnation. By using water-saturated chips, according to U.S.
Pat. No. 3,644,918, it is possible to obtain complete and uniform
impregnation at atmospheric pressure at e.g. 90.degree. C. within a
period of the order of 60 minutes. The whole amount of cooldng
liquor is added in the impregnation stage. Screen rejects are
negligible, the yield is higher and the pulp shows better
properties than according to U.S. Pat. No. 3,215,588. However, the
authors have found that this process also lacks efficiency, since
the water saturation of the chips increases the evaporation demand
in the recovery cycle, the reject levels are high and the screened
yield is low.
[0019] Thus, development of continuous cooking technology has been
characterized by improvements in various fields, e.g. energy
efficiency. However, very little attention has been paid to
important issues as how to really utilize both "the front-end" and
"back-end" of the cooking process to simplify it, retain
flexibility and also to improve the pulp quality. The failure to
consider these issues has to a great extent been responsible for
the development of larger and larger equipment as well as lowering
the flexibility of the process and causing lower pulp quality. The
development of rapid cooking has failed to understand the
conditions which are required to economically produce high-quality
pulp.
SUMMARY OF THE INVENTION
[0020] According to the present invention, an improved, continuous
alkaline cooking process according to claim 1 is provided, wherein
the raw chip material is preheated and air purged, and impregnated
with a liquor at a temperature no higher than the boiling point at
atmospheric pressure of the impregnation liquor, at retention times
of more than 60 minutes. Liquors including fresh cooking liquor are
added at an effective alkali concentration in the range from about
0.5 to about 2.2 molal as OH ions; preferably said concentration is
in the range from about 0.5 to about 1.5 mol/l as OH/l; more
preferably said concentration is in the range from about 0.75 to
about 1.5 mol/l as OH/l. A liquid-to-wood ratio in the range of 3
to 10 m.sup.3/t odw (m.sup.3 per ton oven dry wood) is to be
maintained during the impregnation step; preferably said ratio is
in the range of 3 to 6 m.sup.3/t odw. The impregnated material is
subsequently heated to a temperature T2 of at least about
150.degree. C. in a time t.sub.2 after which follows a cooking
stage with a retention time t.sub.3, a maximum temperature T3 of no
more than 185.degree. C., the liquid-to wood ratio being at least
2.5 m.sup.3/t odw during a substantial part of the heating and
cooking step. The total of t.sub.2 and t.sub.3 shall not exceed 65
min. Fresh cooking liquor is added also during the steps following
impregnation. After the cooking step, the delignified material is
cooled to a temperature where significant cooking reactions no
longer occur.
[0021] In practice, a temperature decrease to about 140.degree. C.
is sufficient to end the cooking step. Preferably, the time ti for
impregnation is above 120 min, and the temperature T1 in the range
from 70.degree. C. to the boiling point at atmospheric pressure of
the impregnation liquor.
[0022] Preferably, the total of t.sub.2 and t.sub.3 is in the range
from 10 to 50 min, more preferably in the range from 10 to 40 min.
Time t.sub.2 is preferably not more than 30 min. Further, the
liquid-to-wood ratio in a substantial part of the heating and
cooking steps is preferably at least 3 m.sup.3/t odw.
[0023] Preferably, impregnation takes place at low pressure, for
the present purposes defined as up to 5 bar. More preferably,
impregnation takes place at about atmospheric pressure. Thus, low
pressure equipment may be used, which saves investment costs. Use
of pressure may be required to ensure sinking of the chips in the
liquid phase. If high pressure equipment is installed it can
natally be utilized.
[0024] The average dry-solid of the material entering the
impregnation is preferably over 40%; More preferably said dry-solid
is over 45%.
[0025] The above process has given excellent results as shown in
the examples, and it is a significant process simplification. The
differences compared to prior art continuous processes are the
unique combination of:
[0026] low temperature during impregnation, which enables
low-pressure and temperature equipment in impregnation, and long
retention times, i.e. over 60 min
[0027] alkali concentration of 0.5-2.2 mol OH/l of added liquor in
impregnation.
[0028] a short heating and cooking stage (less than 65 minutes)
making possible a low volume of cooking equipment in relation to
the impregnation equipment.
[0029] liquor-to-wood ratio over 2.5 m.sup.3/t odw and fresh alkali
addition in heating and cooking stage or both
[0030] different pressures in the impregnation and cooking stages,
which enables simpler equipment for low-pressure impregnation and a
lower volume of high-pressure equipment.
[0031] This results in process simplification and great
flexibility. Experimental results show that the process gives high
yield, improved bleachability and at least equal quality compared
to prior art methods.
[0032] The specific temperatures, effective alkali concentration
and times used in a process according to the invention are
dependent on the type of wood and the purpose of the product.
Hardwoods generally require lower maximum temperatures and
effective alkali concentrations compared to softwood. Pulp for
unbleached products also normally require lower cooldng
temperatures than for bleached products. The impregnation time and
effective alkali concentration depends mainly on the type of chip.
Material hard to impregnate, and consequently requiring longer
times, may consist of long and thick chips, or have a large
proportion of low-porosity material. The type of equipment and the
space available are other factors.
[0033] As raw material for the process according to the invention,
industrial wood chips are used. These commonly have an average
length above 10 mm, typically 15-35 mm, and an average thickness
above 2 mm, typically 3 -7 mm.
[0034] According to one aspect of the invention, a plant for
carrying out the method comprises a digesting system including an
impregnation vessel and a digester in fluid communication with the
impregnation vessel. The volume V of the impregnation vessel is
larger than 1/1, preferably larger than 2/1, more preferably larger
than 3/1 of the volume of the digester.
[0035] The system may comprise a first transfer line between the
impregnation vessel and the digester for transporting the
impregnated material to the digester, a separator, comprising a
withdrawal space, disposed in connection with the first transfer
line for separating a transport liquid from the impregnated
material; a first return line connected to the separator to conduct
the transport liquid from the separator back to the first transfer
line; a second return line attached to the first return line and to
the impregnation vessel for supplying a portion of the transport
liquid to an inlet of the impregnation vessel; a supply line
connected to a supply space adjacent the level of the material in
the impregnation vessel.
DISCLOSURE OF THE INVENTION
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1 is a graphic representation of the time-temperature
profile of prior art conventional batch cooking,
[0037] FIG. 2 is a graphic representation of the time-temperature
profile of prior art displacement batch cooking,
[0038] FIG. 3 is a graphic representation of the time-temperature
profile of prior art continuous cooking of the Kamyr type,
[0039] FIG. 4 is a graphic representation of the time-temperature
profile of an embodiment of the invention,
[0040] FIG. 5 is a flowchart showing an embodiment of the
invention,
[0041] FIG. 6 is a schematic representation of the water balance of
a traditional cooking system,
[0042] FIG. 7 is a schematic representation of the water balance of
a cooking system in accordance with the present invention,
[0043] FIG. 8 shows the brightness achieved versus consumption of
active chlorine in pulps prepared using the conditions set forth in
Table 1,
[0044] FIG. 9 shows the brightness as a function of viscosity of
pulps prepared according to the examples in Table 1,
[0045] FIG. 10 shows the active chlorine consumption against
bleached yields in the examples according to Table 1, and
[0046] FIG. 11 shows the reject percentage as a function of
impregnation time in pulp cooked to two kappa numbers according to
the invention.
DETAILED DESCRIPTION
[0047] FIGS. 1 to 4 show the temperature profiles of prior art
pulping methods as well as that of the present invention. Referring
now to FIG. 1, which shows the temperature curve against time in a
conventional batch cook, region 1 of the curve represents the
heat-up phase, region 2 illustrates cooking at about the maximum
temperature and region 3 illustrates the discharge and cooling of
the conventional batch digester.
[0048] Typically, the duration of region 1 is 60 to 150 minutes,
and that of region 2 60 to 120 minutes. The sum of region 1 and 2
is typically about 150 minutes.
[0049] In FIG. 2 showing the corresponding curve of a prior art
displacement batch process, region 5 represents the impregnation
phase, region 6 the hot liquor fill phase, i.e. hot black liquor
treatment and hot white liquor charge, region 7 represents the
temperature adjustment phase, usually carried out by circulating
the digester content and heating, region 8 illustrates the cooking
phase at cooking temperature. Region 9 represents the displacement
with cool wash liquid and region 10 represents the cold
discharge.
[0050] Typically, the duration of region 5 is typically about 30
minutes, but it can be 10 to 40 minutes depending on digester size,
at a temperature below 100.degree. C. Region 6 is typically about
30 minutes (can be 15 to 40 minutes depending on digester size).
Regions 7 and 8 are typically 65 to 100 minutes. Thus, regions 6, 7
and 8 together typically represent 95 to 130 minutes. Region 9 is
typically 45 minutes (can be 20 minutes to 60 minutes depending on,
among other factors, digester size).
[0051] In FIG. 3, illustrative of a prior art continuous process,
region 11 represents the impregnation phase, region 12 represents
heating and region 13 represents a cooking phase, which can occur
in both concurrent and countercurrent modes. Region 14 represents
displacement and cooling of the cooked material before discharge
from the digester. Region 11 is typically 30 to 60 minutes or
shorter at a temperature of 115-130.degree. C. Regions 12 and 13
are over 90 minutes, typically 240 to 360 minutes.
[0052] FIG. 4 shows the advantageous temperature profile of the
present invention. Region 20 represents the impregnation phase,
which as can be seen is substantially extended relative to
processes presently in use. Region 21 represents the heating-up
phase. Region 22 represents the short reaction time and region 23
the cooling of the cooked material. Curve 24 illustrates
schematically an optional cooling and washing method. Between
regions 20 and 21, feeding of the pre-impregnated material to the
cooking reactor takes place.
[0053] FIG. 5 is a flowchart of a process according to the
invention. Lignocellulosic material 30 in the form of industrial
wood chips enters a pre-steaming phase 31. Steam 36 is supplied to
heat the chip flow and purge air therefrom, and gases 37 are
removed from phase 31, preferably to a closed system. In
presteaming, the chips are heated to a temperature of
80-120.degree. C., preferably 95-110.degree. C. The retention time
at the above temperature should preferably be 5-40 min. Pre-steamed
material 38 enters the impregnation stage 32. The transfer method
and equipment in point 38 between phase 31 and 32 depends on the
counter-pressure in stage 32. The residence time of the stage 32 is
at least 60 minutes. It can be significantly longer, depending on
the available size of equipment. Longer times or impregnation times
of more than about 24 h may be used for example when combining the
impregnation stage with chip storage between the chipping unit and
the cooking plant. The impregnation time rarely exceeds 120 hours
in the same equipment. The impregnation equipment may be a
down-flow vertical vessel or a horizontal conveyer type vessel with
at least one inflow and at least one outflow point for the
material, known to the person skilled in the art. Installed
continuous digester vessels can be used e.g. when upgrading an
existing plant. When using longer retention times, the impregnation
device can be considered to be of the chip silo vessel type.
Several vessels can be used in series or in parallel. According to
the invention, the impregnation vessels are preferably dimensioned
for a low pressure, i.e. pressure in the area from about 0 to 5
bar, more preferably, the vessels are dimensioned for pressure in
the range from atmospheric to about 1 bar. High-pressure equipment
(over 5 bar design pressure) can also be used when for example
upgrading a plant to a process according to the invention. Fresh
alkali 39 is added to the stage. The amount may be, for example, 30
percent or more of the total fresh alkali to be added calculated as
total titrable alkali (TTA) per charged unit of wood, but
additional fresh alkali is invariably added in the cooking stage.
Spent liquor 46 is added as needed, recycled from e.g. a subsequent
liquor-separation stage. The effective alkali concentration of the
added liquors is 0.5-2.2 mol OH/l; preferably 0.5-1.5 mol OH/l.
[0054] The fresh alkali 39 and spent liquor 46 can be added
together at one addition point, or in sequences during the
impregnation. Spent liquor can be added first, and then fresh
alkali is added and some spent liquor withdrawn. Fresh alkali can
also be added first and then spent liquor. Parts of spent liquor
and fresh alkali can also be added first, and then fresh alkali is
added together with some withdrawal of spent liquor. The fresh
alkali used can be both caustisized liquor, normally referred to as
white liquor, and uncaustisized liquor, normally referred to as
green liquor, or also derivates of the above mentioned liquors,
e.g. a mother liquor from crystallization of sodium carbonate from
green liquor.
[0055] Impregnated material 40 leaves the impregnation reactor via
a transfer system 41, which may be one of various combinations of
discharge systems in the outlet part of the impregnation vessel and
feeding technology known to the person skilled in the art. The
system is supplied with liquor 45 as required e.g. for dilution.
Transfer systems to be used are pumps, chamber feeders (e.g. of the
high-pressure (BP) feeder type), screws, scrapers and injectors
etc., and combinations thereof, known to the person skilled in the
art. The stream may enter a device 43 for liquid removal, which
device may be a screen or a liquor separator of a known type. In
device 43, the free liquor 48 not absorbed by the chips is
essentially removed, or the amount of liquor surrounding the chips
is adjusted to a predefined value, after which the chips 44 enter
the cooking stage 33. The remaining required amount of fresh
cooking liquor 47 is added to the cooking stage 33 together with
spent liquor 60 in order to e.g. adjust the alkalinity and
liquor-to-wood ratio in cooking stage 33. The liquor-to-wood ratio
is at least 2.5 m.sup.3/tons odw. In practice, the liquor-to-wood
is always below 100 m.sup.3/tons odw. The temperature of the liquor
may require adjustment to hold the preferable temperature between
70.degree. C. and atmospheric boiling point.
[0056] The digester used must be able of heating the chips to the
cooking threshold temperature T2, preferably within about 30
minutes. This can be accomplished by direct steaming and/or by
pre-heating the added liquors. In practice the heating time t.sub.2
is always above about 1 minute.
[0057] After the threshold temperature T2 is reached, exothermic
reactions and possible additional heating raise the temperature to
a maximum temperature T3. The total time of heating and cooking at
maximum temperature must not exceed 65 min. The cooking zone can be
a vertical or horizontal reactor with at least one input and one
output of material. A vertical recactor can be fed from either top
or bottom, i.e. down-flow or up-flow reactor or a combination of
down-flow and up-flow reactor. A horizontal reactor can be equipped
with a conveyor device, as a chain conveyor or a screw. The cooking
zone can also be a reactor inclined at a certain angle, e.g.
45.degree.. The cooking reactor can be equipped with liquor and
steam addition points and with withdrawal points for both gas and
liquors. These addition and withdrawal points can be several. At
the end of cooking, the effective alkali concentration of the
cooking liquor can be 0.05-0.7 mol OH/l, preferably in the range
0.1-0.5 mol The cooked material 53 exits the cooking stage and
enters a washing and cooling phase 34, is in which cooler liquor is
introduced to essentially end the cooking reactions, leach out
dissolved material inside the cooked chips, and wash the cooked
material. Any suitable combination of displacement, liquor
withdrawal, dewatering and dilution may be used. Technically, this
may be accomplished using a liquor displacement and withdrawal zone
in the cooking reactor, dilution of the cooked material, dewatering
and displacement stages occurring outside the cooking reactor in a
subsequent vessel, or pulp washing equipment known to persons
skilled in the art. The washing stage is supplied with a
substantially aqueous liquid, which can be e.g. water, condensate
or suitable diluted process liquor. Spent liquor 52 from the
washing stage may be re-circulated 60 to the digester, preferably
through a heating device (not shown) and possibly mixed with an
additional liquor which may be a liquor from the impregnation
stage, e.g. from point 55. Spent liquor 56 and 57 from the cooking
and washing stages, respectively, is removed from the process. The
heat of the combined liquor 62 is recovered either separately of at
least one the liquors representing liquor 62, i.e. liquors 56, 57
and 58, or of the combined liquor 62. The recovery of heat can take
place by flashing or heat exchange in one or several stage. The
recovered heat can be used to heat the material 30 in the
pre-steaming stage 31, and/or the recovered heat can be used to
heat the fresh alkali 47 or the spent liquors 60, 52 or 59, to heat
other process liquids, e.g. in evaporation, or to heat water.
[0058] The liquor balance of the process may firther include
circulation of liquor 48 and 49 from the separation device and from
any phase of the impregnation stage, respectively, to the beginning
of the impregnation stage. Liquor 55 from the impregnation stage
may also be conducted to the chemicals recovery through line
58.
[0059] The washed material 61 proceeds to further treatment stages
as e.g. delignification, bleaching, refinig, screening etc.
depending on the purpose of the final product.
[0060] FIG. 6 shows a typical water balance of a prior art process,
compared with the water balance of a process according to the
present invention in FIG. 7. It can be seen, that the method
according to the invention does not affect the water balance of the
plant, but the difference in relation to conventional cooking is
clear. A substantial part of the white liqiuor is added to a
pre-treatment stage to completely impregnate the chips prior to
cooking. In addition, some residual liquor after the pre-treatment
is re-circulated to the liquors introduced in the impregnation to
dilute the EA of added liquors and thereby avoid too strong liquors
being applied on the wood chips, which may lower the quality of the
pulp. Thus, because of the efficient pre-impregnation, a minor
amount of white liquor, that is, small compared to conventional
cooking, is added to the cooking stage to adjust the allity of the
cooking stage. In conventional cooking, substantially all white
liquor is added to the cooking stage. Therefore, conventional
cooking with minor pre-impregnation lacks efficiency in
pre-impregnating the chips prior to heating and cooking the
chips.
[0061] Table 1 shows the results of, on the one hand, comparative
laboratory cooking experiments (1-4) using various typical
conditions for prior art continuous and batch cooking, and on the
other hand experiments (5-11) using conditions according to the
present invention.
1 Example 1 2 3 4 5 6 7 8 9 10 11 Cooking Prior-art Prior-art
Prior-art Prior-art Invention Invention Invention Invention
Invention Invention Invention Continuous Continuous Continuous
Batch lab lab lab lab lab lab lab Mill mill lab lab Impregnation
Temperature, .degree. C. 90 80 95 95 95 95 95 95 95 EA, mol
OH.sup.-/l 3.05 0.3 1.25 1.25 1.25 1.25 1.25 1.25 1.25 Time, min 60
30 4320 60 60 180 4320 60 60 Pressure, bar 0 5 0.5 0.5 0.5 0.5 0.5
10 10 Liquor-to-wood ratios m.sup.3/t 4 5 4.6 4.6 4.6 4.6 4.6 4.6
4.6 Heating and Cooking EA of added liquor, mol OH.sup.-/l 0.54
0.69 0.62 0.64 0.41 0.62 0.64 Liquor-to-wood ratio, m.sup.3/t 1.6 5
3.5 3.5 3.5 3.5 3.5 3.5 3.5 Heat-up, min 7 50 15 15 15 15 15 15 15
Heat-up + cooking time, min 29 91 25 25 25 25 25 25 27 Max cooking
temperature, .degree. C. 175 160 169 176 169 168 160 174 168
End-of-cook residual EA, mol OH.sup.-/l 0.23 0.28 0.22 0.24 0.27
0.31 0.24 0.29 Unbleached pulp Kappa Number 15.2 15.4 19.8 20.3
16.4 17.8 26.9 30.6 32.5 19.7 27.4 Brightness, ISO % 31.1 32.1 41.7
41.8 47.2 44.8 39.7 42.6 42.0 42 40 Total Yield, % nd nd 54.2 55.4
54.3 55.3 57.2 57.6 57.5 54.5 56.5 Total Reject, % nd nd 2.66 0.97
0.12 0.88 1.2 1.16 1.23 0.96 1.4 Screened yield, % nd nd 51.5 54.4
54.2 54.8 56.3 56.4 56.3 53.5 55.1 Oxygen delignification Time, min
60 20/60 nd 60 30/120 30/120 30/120 nd nd nd nd Temp, .degree. C.
100 90/100 nd 100 90/110 90/110 90/110 nd nd nd nd NaOH charge,
kg/odt 15 10 nd 18 25 25 35 nd nd nd nd Oxygen pressure, Mpa 0.5
0.8/0.5 nd 0.6 0.8/0.5 0.8/0.5 0.8/0.5 nd nd nd nd Residual pH 12
9.7 nd 11.8 11.7 11.4 11.8 nd nd nd nd Kappa Number 9.2 9.7 nd 12
8.6 8.5 11.3 nd nd nd nd Kappa reduction, % 39 37 nd 41 48 52 58 nd
nd nd nd Viscosity, dm.sup.3/kg 895 908 nd 1065 1014 896 979 nd nd
nd nd Brightness, % ISO 54.7 50.4 nd 59.6 72.0 71.4 68.0 nd nd nd
nd ECF Bleaching Stages used D(EOP)(DnD) D(EOP)DnD nd D(EOP)DnD
D(EOP)DD D(EOP)DD D(EOP)DD nd nd nd nd Tot. conc. Act Cl, kg/odt
42.8 41.3 nd 38 13.8 17.5 18.7 nd nd nd nd Tot. conc. Act. Cl mult
0.46 0.43 nd 0.31 0.17 0.22 0.17 nd nd nd nd Brightness, % ISO 90.5
91 nd 92.0 92.0 91.7 91.5 nd nd nd nd Viscosity, dm.sup.3/kg 838
779 nd 800 922 818 871 nd nd nd nd Bleached yield, % 52.0 52.0 52.3
52.6 nd nd nd nd PFI beating results Brightness, % ISO nd 91.0 nd
90.5 92.0 91.7 91.5 nd nd nd nd SR 30 Tensile index, Nm/g nd 82.8
nd 83.2 91.9 92 92.7 nd nd nd nd Tear index, mNm.sup.2/g nd 10.8 nd
9.0 11.1 10.6 10.7 nd nd nd nd
[0062] The following abbreviations are used in the examples:
2 EA Effective alkali = NaOH + {fraction (1/2 )}Na.sub.2S,
expressed as NaOH equivalents nd not determined ECF Elemental
Chlorine Freee D Chlorine dioxide bleaching step EOP Alkali
extraction step reinforced by oxygen and peroxide n neutralization
(DnD) Chlorine dioxide bleaching step with intermediate
neutralization
EXAMPLES 1 and 2
[0063] Mill-scale production according to prior-art Kamyr
continuous cooking of industrial eucalyptus chips to kappa numbers
typical of eucalyptus cooking. Sampled pulps were oxygen
delignified and ECF bleached in the laboratory. Bleaching chemicals
demand for a given pulp brightness was determined and the pulp
strength measured by beating and testing.
EXAMPLE 3
[0064] Production of eucalyptus pulp according to prior-art process
disclosed in U.S. Pat. No. 3,664,918 (vapor phase pulping of water
saturated lignocellulosic materials) and example 1 of U.S. Pat. No.
3,664,918.
[0065] The industrial eucalyptus chips (5.5 kg oven dry weight)
were first submerged in water overnight at 2 bar overpressure and
room temperature. The excess water was separated. The water
saturation resulted in chips of 44.6% dry solids. The
water-submerged chips were metered into a chip basked positioned in
a 32-liter jacketed displacement digester with liquor circulation.
The chips were impregnated with white liquor (EA charge of 33.7%
NaOH calculated on wood, EA 122 g NaOH/l and sulfidity 30%) at a
liquor-to-wood ratio of 4 m.sup.3 per ton of dry wood at 90.degree.
C., 60 minutes and atmospheric pressure. After impregnation of the
chips and removal of excess liquor, the impregnated chips were then
subjected to steaming and the temperature of the chips was
initially raised to 100.degree. C. for 20 min and subsequently
treated at 175.degree. C. for a total of 36 min, including
heating-time of 7 minutes. After cooking the digester content was
cooled with water. After the cook the pulp was wet disintegrated
and screened Kappa number, yield, reject, brightness were
determined on the cooked pulp.
EXAMPLE 4
[0066] Production of normal eucalyptus pulp.
[0067] Example 4 show laboratory simulation data of a process
simulated according to prior-art displacement batch cooking of
industrial Eucalyptus.
[0068] 4.5 kg eucalyptus chips (oven dry basis) were metered into a
chip basket positioned in a 26-liter jacketed displacement digester
with liquor circulation. The same chip raw material as shown in
Example 3 were used. The chips were pre-steamed for 10 minutes at
100.degree. C. Then impregnation liquor fill was conducted with an
impregnation liquor containing 0.29 mol OH/l of EA. After 30
minutes impregnation, hot black liquor treatment occurred for 20
minutes with a HBL containing 0.205 mol OH/l of EA and a
temperature of 148.degree. C. Then hot white liquor (105 g NaOH/l
as EA, sulfidity 40%) at a charge of 11.6% as NaOH (EA) was added
for 10 minutes. The digester content was then heated for 20 minutes
to the cooking temperature of 160.degree. C. The time at cooking
temperature was 41 minutes. After the cook the pulp was wet
disintegrated and screened. Kappa number, yield, reject, brightness
were determined on the cooked pulp. The cooked pulp was then oxygen
delignified and ECF bleached in the laboratory. Bleaching chemicals
demand for a given pulp brightness was determined and the pulp
strength measured by beating and testing.
EXAMPLE 5
[0069] Production of eucalyptus kraft pulp in accordance with an
embodiment of the present invention.
[0070] 5.5 kg of oven dry eucalyptus chips was metered into a chip
basket in a 32-liter jacketed displacement digester with liquor
circulation. The same chip raw material was used as in Examples 3
and 4. The chips were first pre-steamed at 100.degree. C. for 30
minutes. Impregnation occurred for 3 days at a temperature of
95.degree. C. and a small overpressure of 0.5 bar. The alkalinity
of the added liquor was 1.25 mol OH/l and the liquor-to-wood ratio
was 4.6 dm.sup.3 per kg of dry wood. The added impregnation liquor
contained white liquor at a sulfidity of 40% and spent liquor
drained from previous impregnations using the same process. After
impregnation of the chips and removal of excess liquor, pre-heated
cooking liquor at various alkali concentrations (EA) was added for
5 minutes and the liquor-to-wood ratio was simultaneously adjusted
to 3.5 m.sup.3 per ton of dry wood. The digester content was heated
to the cooking temperature in about 10 minutes and the temperature
was kept at temperature for 10 minutes. After cooking, the digester
content was cooled and the liquor was drained. After the cook the
pulp was wet disintegrated and screened. Kappa number, yield,
reject, brightness were determined on the cooked pulp. The cooked
pulp was then oxygen delignifled and ECF bleached in the
laboratory. Bleaching chemicals demand for a given pulp brightness
was determined and the pulp strength measured by beating and
testing.
EXAMPLE 6
[0071] The experiment was carried out as disclosed in Example 5,
but the impregnation time was 60 min and the cooking conditions
were adjusted to give about the same kappa number as in Example
5.
EXAMPLE 7
[0072] The experiment was carried out as disclosed in Example 6,
but the cooking conditions were adjusted to give a higher cooking
kappa number.
EXAMPLE 8
[0073] The experiment was carried out as disclosed in Example 7,
but the impregnation time was adjusted to 180 minutes and the
cooking conditions were adjusted to give slightly higher kappa
number than in Example 7.
EXAMPLE 9
[0074] The experiment was carried out as disclosed in Example 8,
but the impregnation time was adjusted to 3 days and the cooking
conditions were adjusted to give slightly higher kappa number
compared to Example 8.
EXAMPLE 10
[0075] The experiment was carried out as disclosed in Example 6,
but the impregnation pressure was adjusted to 10 bar and the
cooking conditions were adjusted to give slightly higher kappa
number compared to Example 6.
EXAMPLE 11
[0076] The experiment was carried out as disclosed in Example 8,
but the impregnation pressure was adjusted to 10 bar and the
cooking conditions were adjusted to give slightly lower kappa
number compared to Example 8.
[0077] Table 1 shows the cooking characteristics, unbleached pulp
results and the subsequent oxygen delignification, ECF bleaching
and PFI beating results. All oxygen delignifications, ECF
bleachings, PFI beatings and tests were performed in the
laboratory.
[0078] Based on the results in Table 1 and FIGS. 8 to 11, the
present invention offers the following surprising benefits over a
state-of-the-art cooking process:
[0079] By using over 60 minutes pre-impregnation below 100.degree.
C. using 0.5-2.2 mol OH/l of added liquors and when cooking at a
liquor-to-wood ratio over 2.5 as shown in examples 5 through
11:
[0080] remarkably shorter residence time in heating and cooking can
be used compared to over 1.5 hours in prior-art continuous
digesters
[0081] the required cooking volume is considerable reduced
[0082] the unbleached and oxygen bleached pulp is brighter pulp at
same kappa number
[0083] lower or equal rejects amounts at same or higher kappa
number. In a process according to the invention and according to
the methods described, the reject level depends on the impregnation
time and kappa number target (see FIG. 10 showing reject levels of
pulps at kappa numbers 20 and 25 and impregnation times of 0-3 days
using a retention time of 25 minutes in heating and cooking).
[0084] the reject level is independent on impregnation pressure in
the range 0.5 bar to 10 bar for pre-steamed chips implementing that
low-pressure impregnation equipment can be used in impregnation
[0085] higher unbleached screened yield
[0086] higher kappa number reduction in oxygen delignification.
Example 7 used more NaOH in oxygen delignification, but the
additional cost for this is minor since oxidized white liquor from
the recovery cycle, i.e. low-cost NaOH, is usually used in oxygen
delignification.
[0087] considerable lower active chlorine chemical consumption in
ECF bleaching by about 50-65%
[0088] bleached pulps gives a pulp of higher viscosity, see example
5,
[0089] higher bleached yield at lower bleaching costs.
[0090] higher tensile strength although the brightness of the pulps
are higher
[0091] The effect of impregnation time is demonstrated in FIG. 11.
The reject percentage is shown as a finction of impregnation time
as puip is cooked according to the invention to kappa numbers 20
and 25 using a total heat-up and cooking time of 25 min. It is
seen, that a satisfactory level is reached when 1 hour impregnation
residence time is used; a further half percent decrease is achieved
by extending impregnation with a further hour. The improvement due
to extension even to three days is marginal. However, table 1 shows
that the bleaching chemical consumption is significantly lower and
bleached pulp viscosity is higher when using 3 days impregnation
compared to 60 minutes impregnation.
* * * * *