Note: Descriptions are shown in the official language in which they were submitted.
4~8
PRESSURIZED P~LP CHLORINATION, AND
_L RINE INTRODUCTION IN MINUTE BUBBLES
BACKGROUND AND SUMMARY OF THE INVENTION
In typical medium consistency chlorination facilities, such
as in the bleaching or delignification of the chemical pulps,
chlorine is injected through nozzles into a mixer, and then is
passed to a retention vesse] which is at approximately
atmospheric pressure. During the residence of the pulp in the
mixer and the retention vessel the chlorine reacts with
impurities and encrustants (such as lignin and bark) in the pulp
slurry, to effect desired bleaching or delignification.
According to the present invention, a method and apparatus
are provided for significantly enhancing the efficiency and
efficacy of the treatment of comminuted cellulosic fibrous
material slurries with chlorine gas, particularly for the
bleaching or delignification of chemical pulps. According to the
present invention it has been found that if the pressure within
the mixer and/or the retention vessel is increased, such as to
the range of about 15-150 psig, the solubility of the chlorine
gas increases. Also at increased pressure the chlorine gas has a
tendency to take the form of minute bubbles thus increasing the
overall reactivity area, and the network of interlocked fibers at
this pressure inhibits the coalescence of gas bubbles. Also,
according to the present invention, the chlorine gas may be
introduced into the mixer through porous material discs, such as
porous ceramic discs or porous sintered metal discs having
G~V~3
a pore size of about 5-175 microns, so that from the time of
introduction the chlorine is in the f`orm of minute bubbles.
One aspect of the invention consists of a method of treating
a slurry of comminuted fibrous cellulosic material with chlorine
gas, comprising the steps of intimately mixing chlorine gas with
the slurry; feeding the slurry to a retention vessel; and
maintaining the pressure in the retention vessel between about 15
psig - 150 psig to enhance the solubility of chlorine gas in the
slurry and thereby enhance the efficiency and/or efficacy of the
chlorine treatment.
Another aspect of the invention consists of an apparatus for
treating a slurry with chlorine gas comprising a fluidizing mixer
for intimately mixing chlorine gas witn a slurry, said mixer
including at least one chlorine gas introduction nozzle, and a
discharge line extending therefrom; a first throttling valve
disposed in said discharge line for throttling the slurry`
discharged from the mixer to thereby control the pressure within
the mixer; a retention vessel operatively connected to said
discharge line on the opposite side of said first throttling
valve from said mixer, said retention vessel including an inlet
line connected to said mixer discharge line, and a retention
vessel discharge line; and a second throttling valve disposed in
the retention vessel discharge line for controlling the pressure
within the retention vessel.
7~
2A
The improvement,s in solubility of the chlorine gas in the
pulp slurry that are achieved according to the present invention
result in a number of beneficial effects, including decreased
overall chemical consumption, use of more chlorine relative to
chlorine dioxide (which is also added during treatment, and which
is more costly than chlorine gas), decreased volume of slurry in
the retention vessel, treatment to lower Kappa numbers, less
chemical wastage, and decreased consumption of bleaching agents
in subsequent stages.
According to the present invention it is preferred to
intimately mix the chlorine gas with the pulp slurry in a
fluidizing mixer, the pulp having a consistency of about 2.5-16%,
and preferably about 7-12%, and then pass the slurry through a
discharge line into a retention vessel. At the discharge line
from the mixer a throttling valve is utilized to throttle the
discharge and thereby increase the pressure within the mixer and
maintain it at the range of about 15-150 psig. A similar
throttling valve is placed at the discharge of the retention
vessel to also maintain the pressure therewithin within the range
of 15-150 psig. The chlorine gas when introduced into the mixer
may be introduced through porous material discs so as to be in
the form of minute bubbles.
It is the primary object of the present invention to provide
for the more efficient treatment of comminuted cellulosic fibrous
material slurries with chlorine gas. This and other objects of
the invention will become clear from an
~%7~
inspection of the detailed description of the
invention, and from the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a schematic view of exemplary
apparatus according to the present invention, for
the bleaching of chemical pulp according to the
present invention;
FIGURE 2 is a detail end view of the front
of the mixer of the apparatus of FIGURE 1, showing
the interconnection thereof to chlorine gas and
chlorine dioxide liquid sources;
FIGURE 3 is a side cross-sectional vi.ew of
the structure 44 of FIGURE 2;
FIGURE 4 is a side cross-sectional view of
an introduction nozzle 47 of FIGURE 2; and
FIGURE 5 is an end view of the nozzle of
FIGURE 4.
DETAILED DESCRIPTION
The invention is applicable to a wide
variety of procedures for the treatment of
comminuted cellulosic fibrous material slurries with
chlorine gas. In particular the invention is useful
as the first stage of a multistate bleach feed in
the bleaching of deciduous wood chemical pulps,
coniferous wood chemical pulps, mixtures of chemical
pulps, and non-wood fibrous chemical pulps; as the
firs~ stage of a delignification procedure as a
~7641~)~
stage subse~uent to an initial delignification using
oxygen; and in like bleaching, delignification, and
related procedures.
For the purposes of description, the method
according to the invention will be described with
respect to the chlorine bleaching, along with
chlorine dioxide, of chemical pulp, but it is to be
understood that the invention has wider
applicability.
Chemical pulp is provided in a conventional
bleach feed storage tank 10, the pulp being
introduced therein through an inlet 11, and being
pumped from the bottom of the tank 10 utilizing a
conventional fluidizing centrifugal pump 12, such as
the type shown in U.S. Patent 4,435,193. The pulp
has a consistency of about 2.5-16~, preferably 3.5-
13~, and most preferably about 7-12%.
The temperature of the pulp slurry
typically will be from 50F-160F, more preferably
80-150F. The pulp is pumped in line 13 to
conventional fluidizing centrifugal mixer 14. The
mixer 14 preferably is capable of fluidizing pulp
even if it has a consistency up to about 16~, to
effectively intimately mix the pulp with chemicals
that are added to it within the mixer. A form such
a mixer 16 might take is shown in Canadian Patent
1102604, and such a mixer also is commercially
available from Kamyr, Inc. of Glens Falls, New York
and sold under the trademark "MC".
Chlorine gas from source 15 passes through
barometric loop 16' in line 16 and is added to the
mixer 14, as is chlorine dioxide from source 17
through line 18. Typically the chlorine gas will
have a temperature of about 75F-140F, and more
35 commonly in the range of about 90F-125F. The
~7~S4~
pressure at source 15 and in line 16 typically will
be in the range of about 60-125 psig, and
conventional safety devices, valves, and interlocks
will be used with the chlorine gas system.
Chlorine dioxide from source 17 will
typically have a temperature of 35F-100F, more
commonly 40F-55F. The chlorine dioxide will be
added in liquid form, with concentrations ranging
from 4-14 gpl, preferably 6-10 gpl.
Total retention time in the pressurized
zone ~mixer 14 and/or vessel 20) is about .01-60
minutes, preferably about 0.1-20 minutes.
The invention is particularly appli~able to
the bleaching of deciduous wood chemical pulps with
Kappa numbers in the range of 10-25 (more commonly
15-20), bleaching of coniferous wood chemical pulps
with Kappa numbers in the range of 25-150 (more
commonly 28-38), bleaching of mixtures of chemical
pulps, and bleaching of non-wood fibrous chemical
pulps with Kappa numbers in the range of 14-150.
From mixer 14 the pulp is passed to
retention vessel 20 and flows upwardly therein with
dilution liquid added through introduction structure
21. The pulp is discharged by a scraper into the
discharge line 23 at the top of the retention
vessel, and then passes into a stock line for
further treatment (e.g. subsequent bleaching
stages).
According to the present invention it has
been found that by pressurizing the mixer 14 and/or
the vessel 20 the solubility of chlorine in the pulp
is significantly increased thus causing more
efficient reaction with the impurities in
encrustants (such as lignin and bark) on, with, and
in the pulp fibers. Also the applica~ion of
~ ~7$~
pressure favors the production of minute bubbles of chlorine gas,
and the network of interlocked fibers coupled with the increased
pressure inhibits the coalescence of gas bubbles. The end result
is decreased overall chemical consumption, decreased degree of
substitution of chlorine dioxide for chlorine (chlorine dioxide
being more expensive), a decreased volume of slurry in the vessel
20, lower extracted Kappa numbers for the pulp, less chemical
wastage, and decreased consumption of bleaching agents in
subsequent stages.
Pressurization of the mixer 14 and vessel 20 is preferably
effected utilizing a valve 30 at the dischargQ 23 from the tank
20. The valves 30, 31 may be selected from a wide variety of
available throttling valves, such as valves having all wetted
parts of materials resistant to the corrosive-aggressive
conditions that are present. For instance the valve 30 may
comprise a four inch diameter ball valve, and the valve 31 may
comprise an eight inch diameter ball valve. Typical throttling
ball valves that can be utilized in this regard are available
from WKM of Houston, Texas, and are sold under the trademark
DynaSeal 350. Of course a wide variety of other throttling
valves that are commercially available may be u~ilized. The
valves 30, 31 are operated, either manually or automatically, to
control the pressure within the mixer 14 and/or the tank 20 so
that it is within the range of about 15-150 psig (preferably
about 3-120 psig).
In order to ensure that the chlorine gas, from the time of
.i. its introduction, is in the form of minute bubbles, the
-5~` structures illustrated in
~ ~7~a~
FIGURES 2 through 5 preferably are utilized. FIGURE
2 shows the chlorine source 15 connected through
line 16 to the mixer 14. The particular intercon-
nection structures illustrated in FIGURE 2 include
the branched connection 40 which has branch conduits
41, 42 extending therefrom. In each branch conduit
is disposed a filter 44 and ~ne or more valves 45,
and each branch conduit terminates in a chlorine gas
injecting nozzle 47. Two such structures are
utilized in case one malfunctions (in which case it
can be closed off and the other utilized until
repair is effected), and also to evenly introduce
the chlorine gas into the mixer. The injection
nozzles 47 (any reasonable number of nozzles may be
provided) in the embodiment illustrated in FIGURE 2
are connected to the mixer 14 at opposite sides of
the circular cross-section pulp inlet portion 50 of
the mixer.
A typical filter 44 is illustrated in
FIGURE 3, and includes the solid annular metal body
52 which mounts a circular disc 53 of porous
material. Typical porous material for the disc 53
include porous ceramics and sintered metals.
A typical chlorine gas injection nozzle 47
is illustrated in FIGURES 4 and 5 and includes a
metal housing including annular flange 55 and
annular body 56, with a conical surface 57 formed at
the innermost end o~ the body 56 ~the end closest
to, or extending into, the mixer 14). Disposed
within the body 56 and having a conical surface
abutting the conical surface 57 is disc 58. The
disc 58 comprises means for introducing the chlorine
gas in the form of minute bubbles, and comprises a
porous material having pore sizes ranging from about
5-175 microns. Typical porous materials utilizable
~.~7~4~3
for the formation of the disc 50 are conventionally
available porous ceramics and porous sintered
metals.
The disc 58 is held in place against the
conical surface 57 by the gasket 59, locking ring
60, and locking clip 61. The locking ring 60 can be
screw-threaded into engagement with the body portion
56 (which has internal threads therein), with the
clip 61 engaging the body 56 and the ring 60 to hold
them in the position illustrated in FIGURE 4 wherein
the ring 60 presses against the gasket 59 and the
disc 58.
It will thus be seen that according to the
present invention a method and apparatus are
provided for increasing the efficiency and efficacy
of the treatment of comminuted cellulosic fibrous
material slurries with chlorine gas. While the
invention has been herein shown and described in
what is presently conceived to be the most practical
and preferred embodiment thereof, it will be
apparent to those of ordinary skill in the art that
many modifications may be made thereof within the
scope of the invention, which scope is to be
accorded the broadest interpretation of the appended
claims so as to encompass all equivalent methods and
apparatus.
