U.S. patent application number 10/432656 was filed with the patent office on 2004-05-13 for method for alkaline batch cooking of fiber material.
Invention is credited to Fant, Thomas, Lammi, Lari, Svedman, Mikael.
Application Number | 20040089431 10/432656 |
Document ID | / |
Family ID | 8559576 |
Filed Date | 2004-05-13 |
United States Patent
Application |
20040089431 |
Kind Code |
A1 |
Fant, Thomas ; et
al. |
May 13, 2004 |
Method for alkaline batch cooking of fiber material
Abstract
An alkaline batch process for the production of pulp from wood
chips, wherein the preheated chips are subjected to an extended
impregnation step outside the digester for at least 60 min,
preferably longer, at a temperature not exceeding the impregnation
liquor boiling point at atmos-pheric conditions, and a rapid
heating and cooking period in the digester of less than about 90
min, preferably shorter, followed by cooling to below reaction
tempera-ture. Fresh alkali is added both during impregnation and
the heating/cooking period.
Inventors: |
Fant, Thomas; (Bole, 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: |
8559576 |
Appl. No.: |
10/432656 |
Filed: |
December 29, 2003 |
PCT Filed: |
November 21, 2001 |
PCT NO: |
PCT/FI01/01010 |
Current U.S.
Class: |
162/29 ; 162/249;
162/45; 162/47 |
Current CPC
Class: |
D21C 1/06 20130101; D21C
3/26 20130101 |
Class at
Publication: |
162/029 ;
162/047; 162/045; 162/249 |
International
Class: |
D21C 007/14; D21C
011/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 24, 2000 |
FI |
20002586 |
Claims
1. An alkaline batch process for the production of pulp from wood
chips, said process comprising the steps of: preheating the chips;
impregnating the chips outside the batch digester with an alkaline
liquor at a temperature not exceeding the boiling point of said
liquor at atmospheric conditions during a time period t.sub.1 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 0.5 to 2.2 mol/l as OH.sup.- ions and the
liquor to wood ratio during a substantial part of the impregnation
step being between 3 and 10; transferring the chips to a digester,
heating the chips in the digester in a heating step to a
temperature of at least 150.degree. C. during a time period t.sub.2
beginning after the charging of the chips when heat supply to the
digester is started, during which period fresh alkali as well as
possible additional liquor is added; cooking the chips in a cooking
step during a time t.sub.3 ending when cooling of the digester
starts, the total of t.sub.2 and t.sub.3 not exceeding about 90
min, the maximum temperature not exceeding 185.degree. C., and the
liquid-to wood ratio wood ratio during a substantial part of the
heating and cooking steps being at least 2.5 m.sup.3/t odw; and
cooling the cooked material to a temperature where cooking
reactions essentially cease.
2. A process according to claim 1, wherein the total of t.sub.2 and
t.sub.3 does not exceed 80 min, preferably 70 min.
3. A process according to claim 1, wherein the total of t.sub.2 and
t.sub.3 is between 10 and 60 min.
4. A process according to any claim 1-3, wherein t.sub.2 is between
1 and 40 min, preferably between 1 and 30 min.
5. A process according to any claim 14, wherein the liquid-to-wood
ratio during a substantial part of the heating and cooking steps is
at least 3 m.sup.3/t odw, preferably at least 3.5 m.sup.3/t
odw.
6. A digesting system for performing the method according to claim
1, comprising at least one impregnation vessel; batch digesters in
fluid communication with the impregnation vessel; transfer lines
between the impregnation vessel and the bottom of each digester for
transporting the impregnated material to the digester, a separator
comprising a withdrawal space disposed in connection with each
digester for separating a transport liquid from the impregnated
material; first return lines attached to each separator to conduct
the transport liquid from the separator back to the transfer lines;
second return lines connected to the first return lines and to the
impregnation vessel for transferring a portion of the transport
liquid to an inlet of the impregnation vessel; a supply line for
fresh alkali connected to an inlet of the impregnation vessel;
whereby the volume V of the impregnation vessel is larger than 1.5
times the volume of each digester, preferably larger than 3 times
the volume of each digester; more preferably larger than 5 times
the volume of each 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 alkaline 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 alkaline cooking, the
chemical reactions with wood components are heterogenous-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 fulfilled and
this may result in a larger amount of unfiberized 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 fully 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 chemical 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, produced 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 steam. 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
lo 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 and chemical content, 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. Practical experience shows that the process becomes
chemical transportation rate limited at total heating and cooking
times below about 95 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
cirquits 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 2-4 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 modern 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 cooking 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 further 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 unfavourable 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 alkali
concentration sufficient to neutralize 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 time 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 a paper titled "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 cooking
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 both batch and 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
recognize the conditions, which are required to economically
produce high-quality pulp.
SUMMARY OF THE INVENTION
[0020] According to the present invention, an improved, alkaline
batch cooking process 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
to result in a concentration in the range from about 0.5 to 2.2
mol/l as OH.sup.- ions; preferably said concentration is about 0.5
to 1.5 mol/l as OH.sup.-/l ions; more preferably said concentration
is about 0.75 to 1.5 mol as OH.sup.-/l ions. 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.
[0021] The impregnated material is subsequently transferred to a
batch digester and heated to a temperature T2 of at least about
150.degree. C., preferably in not more than 40 min (t.sub.2), after
which follows a cooking stage at a time t.sub.3 with a maximum
temperature T3 of no more than 185.degree. C., and a liquid-to wood
ratio of at least 2.5 m.sup.3/t odw during a substantial part of
the heating and cooking steps. The total of t.sub.2 and t.sub.3
shall not exceed 90 min. Fresh cooking liquor is added during the
heating step, the cooking step or both. After the cooking step, the
delignified material is cooled to a temperature where significant
cooking reactions no longer occur.
[0022] In practice, a temperature decrease to about 140.degree. C.
is sufficient to end the cooking step. Preferably, the time t.sub.1
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. Preferably, the total of
t.sub.2 and t.sub.3 is less than 80 min. More preferably, the total
of t.sub.2 and t.sub.3 is less than 70 min. Even more preferably,
the total of t.sub.2 and t.sub.3 is in the range 10-60 min.
Further, the liquid-to wood ratio during a substantial part of the
heating and cooking steps is preferably at least 3 m.sup.3/t odw;
more preferably, it is at least 3.5 m.sup.3/t odw.
[0023] According to the invention, batch digesters over 10 m.sup.3
are used. The heating and cooking time total in minutes may be
expressed in relation to the digester volume V in m.sup.3 as
follows:
t.sub.2+t.sub.3.ltoreq.(0.09 V+63) min when V.gtoreq.100
m.sup.3,
t.sub.2+t.sub.3.ltoreq.70 min when V is between 10 and 100
m.sup.3.
Preferably, t.sub.2+t.sub.3.ltoreq.(0.09 V+53) min when
V.gtoreq.100 m.sup.3, and
t.sub.2+t.sub.3.ltoreq.60 min when V is between 10 and 100
m.sup.3.
More preferably, t.sub.2+t.sub.3.ltoreq.(0.09 V+43) min when
V.gtoreq.100 m.sup.3, and
t.sub.2+t.sub.3.ltoreq.50 min when V is between 10 and 100
m.sup.3.
Even more preferably, t.sub.2+t.sub.3.ltoreq.(0.09 V+33) min when
V.gtoreq.100 m.sup.3, and
10 min.ltoreq.t.sub.2+t.sub.3.ltoreq.40 min when V is between 10
and 100 m.sup.3.
[0024] The heating time of a batch reactor is heavily dependent on
size. A batch reactor of industrially significant size cannot be
considered as a whole with regard to temperature, but each region
within the reactor should ideally have the same temperature
history. In a liquid displacement reactor equipped for
bottom-to-top displacements, the material at the bottom reaches hot
liquor displacement temperature long before the material at the
top, but is correspondingly cooled earlier. Thus, the various parts
of the digesters experience the same temperatures during
approximately the same periods, but at a given point of time, the
temperature may be different in various regions of the batch
reactor.
[0025] According to the present invention, the heating step during
period t.sub.2 is preferably carried out by means of liquid
displacement, whereby the amount of heat delivered to each region
of chips is essentially uniform throughout the digester. The
cooking step during period t.sub.3 is preferably carried out by
liquor exchange as e.g. liquid circulation or displacement.
[0026] According to the present invention, the cooling step is
preferably carried out by means of liquid displacement.
[0027] The average dry-solid of the material entering the
impregnation stage is preferably over 40%; more preferably said
dry-solid is over 45%.
[0028] Preferably, impregnation takes place at low pressure, for
the present purposes defined as up to 5 bar. 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 may be
utilized as expedient.
[0029] 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.
[0030] According to one aspect of the present invention, a
digesting system is provided for carrying out the process of the
invention. The digesting system comprises at least one impregnation
vessel, batch digesters in fluid communication with the
impregnation vessel; transfer lines between the impregnation vessel
and the bottom of each digester for transporting the impregnated
material to the digester; a separator, comprising a withdrawal
space, disposed in connection with each digester for separating a
transport liquid from the impregnated material; first return lines
attached to each separator to conduct the transport liquid from the
separator back to the transfer lines; second return lines connected
to the first return lines and to the impregnation vessel for
transferring a portion of the transport liquid to an inlet of the
impregnation vessel; and a supply line connected to an inlet of the
impregnation vessel. Due to the relation between the time periods
for impregnation and cooking, the volume V of the impregnation
vessel is larger than 1.5 times the volume of each digester,
preferably larger than 3 times the volume of each digester, more
preferably larger than 5 times the volume of each digester.
[0031] 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 batch processes are the unique
combination of:
[0032] low temperature during impregnation, which enables
low-pressure and -temperature equipment in impregnation, and long
retention times, i.e. over 60 min, in a separate stage outside the
digester
[0033] alkali concentration of 0.5-2.2 mol OH.sup.-/l of added
liquor in impregnation
[0034] a short heating and cooking stage making possible a lower
volume of cooking equipment
[0035] liquor-to-wood ratio over 2.5 m.sup.3/t odw and fresh alkali
addition in heating or cooking stage or both
[0036] different pressures in the impregnation and cooking stages,
which enables simpler equipment for low-pressure impregnation and a
lower volume of high-pressure equipment, i.e. batch digesters.
[0037] This results in process simplification and great
flexibility. The batch cycle is significantly shortened compared to
prior art processes. Experimental results show that the process
gives high yield, improved bleachability and at least equal quality
compared to prior art methods.
[0038] The specific effective alkali concentrations, temperatures
and times used in a process according to the invention are
dependent on the type of wood and the purpose of the product.
Hardwood cooking generally requires lower maximum cooking
temperatures than softwood cooking. Pulp for unbleached products
also normally require lower cooking temperatures than for bleached
products. The impregnation time depends mainly on the type of chips
and raw material. 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.
DISCLOSURE OF THE INVENTION
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] FIG. 1 shows schematically the time-temperature profile of
prior art conventional batch cooking,
[0040] FIG. 2 shows schematically the time-temperature profile of
prior art displacement batch cooking,
[0041] FIG. 3 illustrates schematically the time-temperature
profile of prior art continuous cooking of the Kamyr type,
[0042] FIG. 4 illustrates schematically the time-temperature
profile of an embodiment of the invention,
[0043] FIG. 5 is a schematic representation of the tank farm
required combined with a flowchart showing an embodiment of the
invention and the various liquor streams occurring during the
production cycle,
[0044] FIG. 6 is a representation corresponding to FIG. 5 showing
an alternative embodiment of the invention,
[0045] FIG. 7 is a representation corresponding to FIG. 5 showing a
further alternative embodiment of the invention,
[0046] FIG. 8 shows the brightness achieved versus consumption of
active chlorine in pulps prepared using the conditions set forth in
Table 1,
[0047] FIG. 9 shows the brightness as a function of viscosity of
pulps prepared according to the examples in Table 1,
[0048] FIG. 10 shows the active chlorine consumption against
bleached yields in the examples according to Table 1, and
[0049] 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
[0050] FIGS. 1 to 4 show the temperature profiles of prior art
pulping methods compared to 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.
[0051] 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.
[0052] 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.
[0053] 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). The stages of regions 5-9
occur in the same batch equipment.
[0054] 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.
[0055] 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 before discharge from the
digester. Between regions 20 and 21, feeding of the pre-impregnated
material to the batch digester takes place.
[0056] FIG. 5 is a schematic representation showing the various
tanks used in an embodiment of the invention, and a flowchart of
the process together with the liquor streams occurring and their
relation to the tanks. Wood chips are pre-steamed (point 1) and
charged into the impregnation vessel. The chips can preferably be
pre-heated to a temperature of 95-110.degree. C., the retention
time at that temperature being preferably 5-40 min. The transfer
method and equipment between the presteaming phase and the
impregnation vessel depends on the counter-pressure in the
impregnation stage. The residence time of the impregnation stage
(point 2) 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
atmospheric to 5 bar. Atmospheric conditions can be used.
High-pressure equipment (over 5 bar design pressure) can be used
when for example upgrading a plant to a method according to the
invention. Liquor A is added to the stage. The liquor contains
fresh alkali (point WLimp) and spent liquor from tank 4. The amount
may be, for example, 30 per cent 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 (from tank 4) 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.sup.-/l; preferably, in the range 0.5-1.5 mol OH.sup.-/l; more
preferably, in the range 0.75-1.5 mol OH.sup.-/l.
[0057] The impregnation liquor A is a mixture of fresh alkali and
spent liquor. The fresh alkali and spent liquor 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. The temperature of liquor A may require adjustment to
hold the preferable temperature between 70.degree. C. and its
atmospheric boiling point.
[0058] Impregnated material is transferred from the impregnation
reactor to the batch digester via a transfer system (points 3 and
4), 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 A1 as required e.g. for dilution. Transfer
systems to be used are for example pumps, chamber feeders (e.g. of
the high pressure (HP) feeder type), screws, scrapers and injectors
etc., and combinations therof, known to the person skilled in the
art. Preferably, the digester is charged hydraulically by e.g.
pumping from the bottom. Other methods, e.g. charging from the
digester top after liquid separation, may also be used. Surplus
liquor is removed at A1 from for example the digester screen
girdle, and is conducted to tank 4.
[0059] Following chip charge, hot black liquor B from tank 1 and
hot white liquor C from tank 3 are charged in a hot liquor fill
stage (point 5), initially displacing liquor A2 to tank 4 and then,
as the temperature rises above boiling point, D to tank 2. The
temperature is adjusted by means of circulation-based direct or
indirect steam heating or direct steam heating of the digester
(point 6.1). In accordance with the invention, at the end of the
cooking stage (point 6.2), the effective alkali concentration of
the cooking liquor can be 0.05-0.7 mol OH.sup.-/l, preferably in
the range 0.1-0.5 mol OH.sup.-/l. Cooking is completed, and the
batch is cooled by displacing the cooking liquor with cooler liquor
(point 7), e.g. wash filtrate E from tank 5, possibly containing
also liquor from tank 4. Displaced liquor is divided according to
temperature and chemical content into fractions B1 and D1, to tanks
1 and 2 respectively. When the temperature has decreased below
about 100.degree. C., the digester is discharged (point 8),
preferably by pumping using additional filtrate F from tank 5 as
required.
[0060] In accordance with the flow balance, flow G, filtrate from
the wash plant, may be used to dilute the white liquor, which is
conducted to tank 3 while being heated by black liquor from tank
2.
[0061] FIG. 6 is analogous to FIG. 5, but no circulation heating is
used in the digester, which consequently requires no heating
circuit piping. Instead, the digester heating takes place using hot
liquor displacement (point 5), whereby the temperatures of black
liquor B and white liquor C are preferably adjusted by heat
exchange before introduction into the digester. Liquors D and B
displaced during the hot displacement stages are conducted to tanks
2 and 1, respectively, depending on temperature and/or chemical
content. According to this embodiment of the present invention,
also cooking is carried out by displacement (point 6).
[0062] FIG. 7 shows an embodiment where heating occurs by direct or
indirect steam heating to the digester circulation or direct steam
heating of the digester (point 6.1). Other differences are
pressurized blow of the digester content at the end of cooking
(point 8).
[0063] Table 1 shows the results of, on the one hand, comparative
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 Co king Prior-art Prior-art Prior-art
Prior-art Invention Invention Invention Continuous Continuous
Continuous Batch lab lab lab mill mill lab lab Impregnation
Temperature, .degree. C. 90 80 95 95 95 EA mol OH.sup.-/l 3.05 0.3
1.25 1.25 1.25 Time, min 60 30 4320 60 60 Pressure, bar 0 5 0.5 0.5
0.5 Liquor-to-wood ratio m.sup.3/ft 4 5 4.6 Heating and C oking EA
of added liquor, mol OH.sup.-/l 0.54 0.69 0.62 Liquor-to-wood
ratio, m.sup.3/t 1.6 5 3.5 3.5 3.5 Heat-up, min 7 50 15 15 15
Heat-up + cooking time, min 29 91 25 25 25 Max cooking temperature,
.degree. C. 175 160 169 176 169 End-of-cook residual EA, mol
OH.sup.-/l 0.23 0.28 0.22 0.24 Unbleached pulp Kappa Number 15.2
15.4 19.8 20.3 16.4 17.8 26.9 Brightness, ISO % 31.1 32.1 41.7 41.8
47.2 44.8 39.7 Total Yieid, % nd nd 54.2 55.4 54.3 55.3 57.2 Total
Reject, % nd nd 2.66 0.97 0.12 0.88 1.2 Screened yield, % nd nd
51.5 54.4 54.2 54.8 56.3 Oxygen delignification Time, min 60 20/60
nd 60 30/120 30/120 30/120 Temp, .degree. C. 100 90/100 nd 100
90/110 90/110 90/110 NaOH charge, kg/odt 15 10 nd 18 25 25 35
Oxygen pressure, Mpa 0.5 0.8/0.5 nd 0.6 0.8/0.5 0.8/0.5 0.8/0.5
Residual Ph 12 9.7 nd 11.8 11.7 11.4 11.8 Kappa Number 9.2 9.7 nd
12 8.6 8.5 11.3 Kappa reduction, % 39 37 nd 41 48 52 58 Viscosity,
dm.sup.3/kg 895 908 nd 1065 1014 896 979 Brightness, % ISO 54.7
50.4 nd 59.6 72.0 71.4 68.0 ECF Bleaching Stages used D(EOP)(DnD)
D(EOP)DnD nd D(EOP)DnD D(EOP)DD D(EOP)DD D(EOP)DD Tot. conc. Act
Cl, kg/odt 42.8 41.3 nd 38 13.8 17.5 18.7 Tot. conc. Act. Cl mult
0.46 0.43 nd 0.31 0.17 0.22 0.17 Brightness, % ISO 90.5 91 nd 92.0
92.0 91.7 91.5 Viscosity, dm.sup.3/kg 838 779 nd 800 922 818 871
Bleached yield, % 52.0 52.0 52.3 52.6 PFI beating results
Brightness, % ISO nd 91.0 nd 90.5 92.0 91.7 91.5 SR 30 Tensile
index, Nm/g nd 82.8 nd 83.2 91.9 92 92.7 Tear index, mNm.sup.2/g nd
10.8 nd 9.0 11.1 10.6 10.7 Examples 8 9 10 11 Co king Invention
Invention Invention Invention lab lab lab lab Impregnation
Temperature, .degree. C. 95 95 95 95 EA mol OH.sup.-/l 1.25 1.25
1.25 1.25 Time, min 180 4320 60 60 Pressure, bar 0.5 0.5 10 10
Liquor-to-wood ratio m.sup.34.6 4.6 4.6 46 /ft Heating and C oking
EA of added liquor, mol OH.sup.-/l 0.64 0.41 0.62 0.64
Liquor-to-wood ratio, m.sup.3/t 3.5 3.5 3.5 3.5 Heat-up, min 15 15
15 15 Heat-up + cooking time, min 25 25 25 27 Max cooking
temperature, .degree. C. 168 160 174 168 End-of-cook residual EA,
mol OH.sup.-/l 0.27 0.31 0.24 0.29 Unbleached pulp Kappa Number
30.6 32.5 19.7 27.4 Brightness, ISO % 42.6 42.0 42 40 Total Yieid,
% 57.6 57.5 54.5 56.5 Total Reject, % 1.16 1.23 0.96 1.4 Screened
yield, % 56.4 56.3 53.5 55.1 Oxygen delignification Time, min nd nd
nd nd Temp, .degree. C. nd nd nd nd NaOH charge, kg/odt nd nd nd nd
Oxygen pressure, Mpa nd nd nd nd Residual Ph nd nd nd nd Kappa
Number nd nd nd nd Kappa reduction, % nd nd nd nd Viscosity,
dm.sup.3/kg nd nd nd nd Brightness, % ISO nd nd nd nd ECF Bleaching
Stages used nd nd nd nd Tot. conc. Act Cl, kg/odt nd nd nd nd Tot.
conc. Act. Cl mult nd nd nd nd Brightness, % ISO nd nd nd nd
Viscosity, dm.sup.3/kg nd nd nd nd Bleached yield, % nd nd nd nd
PFI beating results Brightness, % ISO nd nd nd nd SR 30 Tensile
index, Nm/g nd nd nd nd Tear index, mNm.sup.2/g nd nd nd nd The
following abbreviations are used in the examples: EA Effective
alkali = NaOH + 1/2Na.sub.2S, expressed as NaOH equivalents nd not
determined ECF Elemental Chlorine Free 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
[0064] Mill-scale production according to prior-art "Kamyr"
continuous cooking of industrial eucalyptus chips to typical kappa
numbers 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
[0065] 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.
[0066] 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 jacketed displacement digester with liquor circulation. The chips
were impregnated with white liquor (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
[0067] Production of normal eucalyptus pulp.
[0068] Example 4 show laboratory simulation data of a process
simulated according to prior-art displacement batch cooking of
industrial Eucalyptus.
[0069] 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 at 80.degree. C. was
conducted with an impregnation liquor containing 0.29 mol
OH.sup.-/l of EA. After 30 minutes impregnation, hot black liquor
treatment occurred for 20 minutes with a HBL containing 0.205 mol
OH.sup.-/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
[0070] Production of eucalyptus kraft pulp in accordance with an
embodiment of the present invention.
[0071] 5.5 kg of oven dry eucalyptus chips was metered into a chip
basket in a jacketed displacement digester with liquor circulation.
The same chip raw material was used as in Example 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.sup.-/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 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 6
[0072] 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
[0073] 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
[0074] 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
[0075] 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
[0076] 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
[0077] 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.
[0078] Table 1 shows the cooking characteristics of Eucalyptus
hardwood chips, 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.
[0079] The effect of impregnation time is shown in FIG. 11. The
reject percentage is shown as a function of impregnation time as
pulp 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 per cent decrease is
achieved by extending impregnation with a further hour. The
improvement due to extension to even 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.
[0080] 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:
[0081] remarkably shorter residence time in heating and cooking can
be used compared to over 1.5 hours in prior-art Kamyr-type
continuous digesters and in prior-art displacement batch
digesters
[0082] the required cooking volume is considerable reduced
[0083] the unbleached and oxygen bleached pulp is brighter pulp at
same kappa number
[0084] 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)
[0085] 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
[0086] higher unbleached screened yield
[0087] 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.
[0088] considerable lower active chlorine chemical consumption in
ECF bleaching by about 50-65%
[0089] bleached pulps gives a pulp of higher viscosity, see example
5,
[0090] higher bleached yield at lower bleaching costs.
[0091] higher tensile strength although the brightness of the pulps
are higher
[0092] The following examples make clear some advantages of the
present invention over prior art kraft batch cooking when cooking
industrial softwood chips.
EXAMPLES 12 AND 13
[0093] Production of a normal "reference" softwood kraft pulp by
using prior-art displacement kraft batch technology.
[0094] 4.2 kg Scandinavian softwood chips (oven dry basis) were
metered into a chip basket positioned in a 26-liter jacketed
displacement digester with liquor circulation. Industrial chips
were used consisting of 10% over-thick chips (fraction retained on
a 8 mm wide bar) and 90% of so-called accept chips (chip fractions
retained between 8 mm wide bars and 13 mm holes). The lid of the
digester was closed. The impregnation liquor (IL) was pumped into
the digester. The amount of the IL was 4.5 l/kg o.d. wood and EA
0.3 mol/l. The conditions in the impregnation step were total time
20 min, temperature 90.degree. C. and pressure 3 bar. After the
impregnation stage followed immediately the hot black liquor stage
and hot white liquor stage. The hot black liquor and hot white
liquor displaced the IL. The amount of hot black liquor was 4.0 l
per kg o.d. wood and EA 0.45 mol/l. The conditions in the hot black
liquor and hot white liquor stage were: Total time 30 min,
temperature 5.degree. C. below cooking temperature and pressure 7.0
bar. Then temperature adjustment and cooking by circulation
followed. The hot white liquor was also split charged, so that 70%
was charged at the hot black liquor fill and 30% after 15 min at
cooking temperature. The cooking time was varied by having
different cooking temperatures. At the target H-factor,
displacement liquor was pumped into the digester cooling the pulp.
The conditions in the final displacement were: Temperature
80.degree. C., time 50 min and total amount of liquor 7.0 l/kg o.d
wood. After the cook, the pulp was wet disintegrated and screened.
Kappa number, yield, reject, brightness and viscosity were analyzed
on the pulp.
[0095] Cooking characteristics are shown in table 2.
2TABLE 2 Results of Example 12 and 13 EXAMPLE 12 EXAMPLE 13
Impregnation liquor, min 20 20 Impregnation liquor, EA mol
OH.sup.-/l 0.3 0.3 Hot black liquor + hot white liquor 30 30
addition time, min Hot black Liquor, EA mol OH.sup.-/l 0.45 0.45
Hot White Liquor Charge, EA % 16.9 16.6 NaOH Temperature adjustment
+ cooking 80 60 time, min Cooking temperature 169 172 H-factor 1050
1000 Total heating and cooking time, min 110 90 End of cook
residual, EA mol OH.sup.-/l 0.45 0.5 Kappa Number 31.5 31.1 Total
Yield, % on wood 47.7 47.6 Coarse reject, % on wood 1.03 1.98 Fine
reject, % on wood 0.42 0.66 Screened yield, % on wood 46.3 45.0
Viscosity, dm.sup.3/kg 1154 1094 Brightness, % ISO 32 30
EXAMPLE 14
[0096] Production of softwood kraft pulp by using displacement
kraft batch technology according to an embodiment of the
invention.
[0097] 4.2 kg Scandinavian softwood chips (oven dry basis) were
metered into a chip basket positioned in a 26-liter jacketed
displacement digester with liquor circulation. The same chips as in
examples 12 and 13 were used. The lid of the digester was closed.
The chips were steamed for 20 min at 100.degree. C. Impregnation
liquor (IL) was pumped into the digester and circulation put on for
20 minutes. After 20 minutes of circulation was the circulation
stopped and the chips were impregnated for a total time of 180
minutes. The temperature of the impregnation stage was 95.degree.
C. and the effective alkali concentration of added impregnation
liquors was 1.3 mol OH.sup.-/l. The overpressure was 0.3 bar in the
top of the digester. After the impregnation stage, a hot black
liquor and hot white liquor fill stage followed immediately. The
hot black liquor and hot white liquor displaced the spent
impregnation liquor. The amount of hot black liquor was 4.2 l/kg
o.d. wood and EA 0.45 mol OH.sup.-/l. The conditions in the hot
black liquor and hot white liquor fill stage were: Total time 30
min, temperature 5.degree. C. below maximum cooking temperature and
pressure 7.0 bar. Then lo temperature adjustment and cooking by
circulation followed. The total heating and cooking time was 70
min. At the target H-factor displacement liquor was pumped into the
digester, cooling the pulp. The conditions in the final
displacement were: Temperature 80.degree. C., time 50 min and total
amount of liquor 6.7 l/kg o.d. wood. After the cook, the pulp was
wet disintegrated and screened. Kappa number, yield, reject,
brightness and viscosity were determined on the pulp.
3TABLE 3 Results of Example 14 EXAMPLE 14 Impregnation liquor, min
180 Impregnation liquor, BA mol OH.sup.-/l 1.3 Hot black liquor +
hot white 30 liquor addition time, min Hot black Liquor, EA mol
OH.sup.-/l 0.45 Hot White Liquor Charge, EA % NaOH 6.4 Temperature
adjustment + 40 cooking time, min Cooking temperature 174 H-factor
808 Total heating and cooking time, min 70 End of cook residual, EA
mol/l 0.45 Kappa Number 30.6 Total Yield, % on wood 48.1 Coarse
reject, % on wood 0.76 Fine reject, % on wood 0.22 Screened yield,
% on wood 47.1 Viscosity, dm.sup.3/kg 1138 Brightness, % ISO 33
[0098] Based on the results shown in tables 2 and 3, the present
invention offers the following surprising benefits over a
state-of-the-art cooking process:
[0099] By using over 60 minutes pre-impregnation in a vessel
outside of the displacement kraft batch digester according to
method presented in example 14,
[0100] the heat-up and cooking time can be reduced by over 50%
compared to a prior art process at the same kappa number and reject
level (sum of coarse and fine reject). Decrease of total cycle time
by at least 40 min, which for a reference installation with total
cycle time of 220 min means a production increase of at least 18%.
A lower number of batch digesters or lower total batch digester
volume can be used to reach a given production level.
[0101] lower reject level although 40 min shorter heating and
cooking time
[0102] the screened yield is significantly higher.
[0103] the unbleached brightness is slightly higher.
[0104] The following examples make clear some advantage of the
present invention over prior art kraft batch cooking in terms of
the equipment required.
EXAMPLE 15
[0105] Production of 1800 air dry tons softwood kraft pulp per day
by using prior-art displacement kraft batch technology
[0106] The total batch digester volume required is about 4000
m.sup.3 using 10 times 400 m.sup.3 digesters.
EXAMPLES 16-18
[0107] Production of 1800 air dry tons softwood kraft pulp per day
by using a displacement kraft batch cooking process according to
the invention.
[0108] Examples 16 to 18 show that the same production can be made
with a total batch digester volume of 2400 to 2800 m.sup.3 using a
lower number of digesters. The examples show, that the volume
ratios between the individual batch digesters and the impregnation
vessel are about 2 to 6.8.
4TABLE 4 Results of Example 15-18 EXAMPLE EXAMPLE EXAMPLE EXAMPLE
15 16 17 18 Production, 1800 1800 1800 1800 adt/d Digester size,
400 350 350 300 m.sup.3 Number of 10 8 8 8 batch digesters Total
volume 4000 2800 2800 2400 of batch digesters, m.sup.3 Impregnation
60 120 180 retention time, min Impregnation 700 1370 2050 vessel,
m.sup.3 Ratio between 2 3, 9 6, 8 impregnation vessel and batch
digester volume
[0109] Based on the results shown in table 4, the present invention
offers the following surprising benefits over a state-of-the-art
cooking process:
[0110] A lower number of batch digesters can be used which also
means less pressure vessels, piping, instruments etc.
[0111] A lower total batch digester volume can be used to reach a
given production level.
[0112] In order to lower the batch digester volume and the number
of batch digesters, an impregnation vessel is used which according
to the invention can be designed for much lower pressure and
temperature conditions. In addition, the building requirements are
much lower with a method according to the invention as the digester
is preferably filled with chips hydraulically from the bottom. The
chip silo and capping valve above the batch digesters can also be
eliminated. The ratio between the impregnation vessel digester
volume is important in order to obtain sufficient
pre-impregnation.
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