Note: Descriptions are shown in the official language in which they were submitted.
12~27~ Z
PUS
HEAT RECOVERY IN BLACK LIQUOR OXIDATION
TECHNICAL FIELD
The present invention relate to the treatment of
black liquor from pulp mills.
BACKGROUND OF THE INVENTION
In the standard process for treatment of black
liquor from the Raft Pulping Process, the weak black
liquor containing about 15% solids is concentrated,
typically in a multiple effect evaporation facility,
to a concentration of about 45-50% solids. The thus
obtained "ztron~" or "heavy" black liquor is further
evaporated to a solid concentration of about 65% or
more and in such highly concentrated state is
subjected to combustion in a recovery boiler. In
such systems wherein a direct contact evaporator it
employed for concentrating the liquid from the
multiple effect facility, flue gas from the recovery
boiler is employed to supply heat to the direct
contact evaporator (DOE). The smelt recovered from
the boiler is sent to further processing to reclaim
available chemical values therein, for use in the
pulping process.
In other known Raft pulp mill recovery processes
the strong black liquor from the multiple effect
evaporation is further concentrated to about 65%
solids level by indirect contact (forced circulation)
evaporator, before being fed to the recovery
boiler.
327~2
In recent years it has been the prevailing
practice to subject the black liquor to oxidation at
some stage in the evaporation sequence, thereby
converting sulfur values therein to more stable form
such as thiosulfates and/or sulfates, which among
other benefits obtained, significantly reduced the
release of malodorous sulfur compounds to the
atmosphere.
In US. Patents Nos. 4,239,589 and 4,313,788,
black liquor treating systems are disclosed in which
the oxidation of the black liquor is integrated into
the multiple effect evaporation stage of the pulp mill
recovery sequence. The heat of the oxidation reaction
is recovered as flash steam and is employed in
providing part of the heat requirement of the multiple
effect evaporation system, thus reducing demand for
outside steam.
In these prior art processes for treatment of
black liquor, whether or not oxidation of the liquor
is practiced, the concentrated liquor which is fed to
the recovery furnace will normally contain about 35%
water, which reduces heat recovery efficiency by
carrying away heat unproductively with the flue gases
as latent heat of vaporization. In addition to the
thermal effect, the presence of water poses an ever
present danger of recovery boiler explosion because of
reactions occurring in the smelt bed of the recovery
boiler, which are difficult to control.
The pulping industry has been aware of the
explosion hazard as well as of the relatively low heat
recovery efficiency of the above described black
liquor treating systems. A number of attempts have
been described in prior art patents and scientific
literature, directed to development of technology for
solving the aforesaid problems. The most notable of
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these attempt have become known as: (1) St. Aegis
Hydropyroly6i~ Process to Weyerhaeuser Dry Solids
Pyrolyzes Process; and (3) Zimpro Wet Oxidation
Process. None of Tao advocated prowesses has as yet
received a general acceptance by the industry.
Details of the first two of these named processes are
set out in the report by T. M. Grace, FORUM ON RAFT
RECOVERY ALTERNATIVES, Inst. of Paper Chemistry 1976,
at pages 228 to 268. Halpern: (1975) Pulp Mill
Processing, Notes Data Corporation, Park Ridge, NJ
1975, pp. 3Z2-325.
The Zimpro process was initially developed for
the treatment and disposal of municipal sewage sludge
and later extended for application to other areas of
industrial waste disposal. In use of the Zimpro
process for wet air oxidation of black liquor, the
oxidation unit is said to achieve over 98% oxidation
of COD in the liquor, while replacing conventional
evaporators, furnaces, smelt dissever and other
usual appurtenances of conventional black liquor
treating systems. Steam recovered in the Zimpro
process, in certain of toe designs, is recycled for
ufie in the paper manufacturing process. The Zimpro
process is described in Paper Trade Journal,
May 15, 1979 at pages 34-36 and in Ellis, C. E., TAIPEI
PROCEEDINGS - 1982 Pulping Conference at pages
glue.
Whereat in the described St. Aegis and
Weyerhaeuser processes non-oxidative pyrolyzes it
employed, in the Zimpro process the weak black liquor
is completely oxidized by use of air as oxidant. All
of these suggested alternative processes are
comparatively complex and high in capital costs and
represent significant departures from current
technology of the industry.
1~32 I
SUMMARY OF THE INVENTION
In accordance with the present invention
molecular oxygen of high purity it employed for
partial oxidation of strong black liquor and the heat
5 of oxidation is utilized to flash the oxidized black
liquor to dry or semi-dry old, which solids can be
fed to a conventional recovery boiler for complete
combustion of organic and recovery of contained
inorganic chemicals. Part of the steam recovered in
flashing the oxidized liquor is used upstream in
concentration of the black liquor and the remainder
made available for power generation or other desired
use in the pulping plant or elsewhere. By operation
in accordance with the invention improved efficiency
of heat recovery is achieved, and, since most or all
of the water normally contained in black liquor
charged to the recovery boiler is eliminated, the
explosion hazard is avoided or largely reduced.
The oxygen based recovery process of the present
invention (OBRP) can be applied to the unoxidized
strong black liquor effluent from the direct or
indirect contact evaporator of a standard Raft
recovery system, as well as in modified systems in
which partial oxidation of the black liquor is carried
out after the final stage in the multiple effect
concentration of the liquor or at an intermediate
stage of the multiple effect evaporation. In other
hereinafter disclosed embodiments the process of the
invention can be operated for partial oxidation of the
strong black liquor effluent discharged from the
multiple effect evaporation, thus replacing or
eliminating the need for a direct or indirect contact
evaporator in the recovery system.
~232'7~
GRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings illustrate by process
flow diagrams systems for practice of the invention as
compared with known prior art systems. In these
drawings:
Figure 1 is a schematic flow diagram of a
standard prior art black liquor treating system
employing a direct contact evaporator:
Figure 2 ill a schematic flow diagram of a prior
art system similar to that of Figure 1 but further
incorporating means for partial oxidation of the black
liquor prior to its introduction into the direct
contact evaporator;
Figure 3 is a schematic flow diagram similar to
that of Figure 2 wherein further oxidation of the
black liquor is applied, in accordance with one
embodiment of the invention, to the concentrated black
liquor effluent discharged from the direct contact
evaporator;
Figure 4 is a schematic flow diagram of a
standard black liquor recovery system employing
indirect contact evaporation (ICE);
Figure 5 is a schematic flow diagram similar to
that of Figure 4 in which the concentrated liquor
discharged from the indirect contact evaporation is
subjected to partial oxidation in accordance with
another embodiment of the invention:
Figure 6 is a schematic flow diagram in
accordance with another embodiment of the invention
wherein the conventional direct or indirect contact
evaporator is eliminated and partial oxidation applied
directly to the strong black liquor leaving the
multiple effect evaporation system.
~23Z7~Z
DETAILED DESCRIPTION
In each of the 6y~tems illustrated in the
accompanying drawings the numerical values shown are
bayed on the treatment applied per ton of pulp in the
black liquor discharged from the pulp worry. The
heat content of the several streams it given in
million of But (MM Btu3.
A shown in Figure 1, in a known standard system
for treatment of black liquor from a Raft pulping
plant the weak black liquor from the pulp wishers,
containing a pulp concentration of about 15% by weight
is brought to 50~ concentration by evaporation of
liquid in a multiple effect evaporation system. The
obtained strong black liquor it then further
evaporated to a higher concentration and charged to
the recovery boiler. -In the system illustrated in
Figure 1, the strong black liquor from the
multiple-effect evaporation system it brought to about
65% concentration in a direct contact evaporator and
charged to the recovery boiler for combustion
therein. As products of combustion in the recovery
boiler there are obtained, (1) an inorganic smelt from
which the contained chemical are recovered by caustic
treatment, (2) flue gay at a temperature of about
600F which it utilized a the heat Ursa for the
direct contact evaporator and recoverable exhaust
team calculated to have a heat content of
11.7 million But. Of the team available from the
recovery boiler 2.8 million But are used to supply
required heat to the multiple effect evaporation
system, leaving an available net steam product of I
million But for use in heat or power production in the
pulping plant or elsewhere a desired. The spent flue
gas discharged from the direct contact evaporator it
~X327~2
at a temperature of 325F (=163C) and will contain in
the order of about 4175 pounds water.
The system of Figure 2 of the accompanying
drawings employs an oxidizing reactor for oxidation of
the strong black liquor enroot from the multiple
effect evaporation system to the direct contact
evaporator. In that respect the illustrated system it
similar in operational sequence to systems such as are
disclosed in US. Patents Nos. 3,928,531; 4,23g,589
and 4,313,788. I disclosed in Figure 2, oxidation of
the partly concentrated black liquor with molecular
oxygen upstream of the direct contact evaporator, will
provide, from the heat evolved by the oxidizing
reaction, a flash vapor (steam) which is utilized to
provide part of the heat requirement in the multiple
effect evaporation system.
The exhaust steam from the recovery boiler,
however, will have a heat content of 11.1 million But,
which it less than that of the corresponding Figure 1
operation. Since only 2.1 million But of the total
steam product from the recovery boiler are needed for
the multiple effect evaporator system to supplement
that supplied by the oxidation reaction, there is
available a net steam product of 9.0 million But for
other desired use.
In the Figure 3 embodiment, in accordance with
the present invention, the operational sequence and
components of the system are similar to those employed
in the process diagram of Figure 2, except that
further oxidation with molecular oxygen is carried out
on the concentrated black liquor (having 65% solids)
discharged from the direct contact evaporation step.
As a result of such oxidation, designated by block
OBRP in tyke flow diagram, the liquor is further
concentrated to a dry solid content in the range of
~X327~2
80-100% and charged to combustion in the recovery
furnace. The heat balance values of the several
streams in Figure 3 are bayed on carrying the OBRP
oxidation to provide 100~ solids. While the same
total steam production is had (11.1 MM But as in the
Figure 2 example, the net steam available from the
recovery boiler (10.9 MM But) is about 21 to over 22%
greater than that available from systems according to
Figures 1 or 2.
Figure 4 illustrates a standard prior art system
employing an indirect contact evaporation step applied
to the strong black liquor discharged from the
multiple effect evaporation system. In such systems
there is a comparatively high total steam effluent
~lZ.7 MM But) discharged from the recovery boiler;
part of that discharged steam is utilized for heating
the indirect contact evaporator (3.2 MM But), leaving
a net steam make of only 9.5 MM But. A significant
part of otherwise available heat is wasted in the wet
flue gas discharged from the recovery boiler.
By partially oxidizing the concentrated black
liquor discharged from the indirect contact
evaporation step (65~ solids) in accordance with the
invention, as illustrated at block OBRP of Figure 5,
further evaporation of the liquid is had from the heat
of the oxidation reaction, bringing the liquor to a
solids concentration of 80~-100%, thus substantially
reducing or eliminating the hazard of explosion in the
recovery furnace. In the preferred example
illustrated in Figure 5, the black liquor discharged
from the indirect contact evaporation step is brought
to 96% solids concentration by partial oxidation with
high purity oxygen at OBRP before introduction into
the recovery boiler.
~2327JL2
At this high solids concentration the total team
production in the recovery furnace will provide a heat
content ox 12.7 million But. An additional 1.8
million But becomes available from the heat of the
oxidation reaction at OBRP which is utilized in the
multiple effect evaporation system. Since 1.8 million
But are withdrawn from the steam discharged from the
recovery boiler, the net steam available from the
boiler will provide 11.3 million But for other desired
use. Thus, by incorporating the oxidation step in the
line of liquid flow from the indirect contact
evaporation and the recovery boiler, there is obtained
about a 20% improvement in net available heat over the
prior art system of Figure 4.
In the embodiment of the invention illustrated in
Figure 6, the intermediate further concentration of
the strong black liquor leaving the multiple effect
evaporation system by resort to direct or indirect
evaporators is eliminated and replaced by partly
oxidizing the liquor effluent from the multiple effect
evaporation system (50% iodize concentration) before
its introduction into the recovery boiler. In the
illustrated example of Figure I, the liquor leaving
the multiple effect evaporation system is subjected to
partial oxidation with high purity oxygen at OBRP.
The heat evolved by the oxidation reaction provides a
net of 2.8 million But for use in providing the heat
input required in the multiple effect evaporation
system while evaporating the liquid during such
oxidation to a solids concentration, in the
illustrated example, to 83% solids concentration for
introduction into the recovery boiler. Thus, in
addition to savings in capital costs as a result of
eliminating the direct or indirect contact
evaporators, there is an additional plus in the
123Z~.2
available heat value of the net steam from the
recovery boiler. The net steam product of 11.51
million But made available represents a 21-29~
improvement over the prior art systems of Figures 1,
2, and 5.
The material and energy results from the
illustrated systems are summarized in Table 1 on the
basis MM But per ton of pulp.
TABLE 1
Available
Oxygen Steam
System Figure % of Pulp MM BTU
Prior Art with DOE 1 0 8.93
Prior Art with DOE
and THY 2 6.0 8.96
Using DOE. THY and
OBRP 3* 22.8 10.87
Prior Art using ICE 4 0 9.51
Using ICE with OBRP 5* 16.8 11.30
Using OBRP Alone 6* 24.6 11.51
*The processes according to the invention are
indicated by asterisk.
The process energy balances are tabulated in
Table 2. The indicated numerical values are on the
basis of MM BTU/ton of pulp.
TABLE 2 12327~
OBRP PROCESS ENERGY BALANCE
(Bests 1 Ton Pulp all Values M~BTU/Ton)
Figure # 1 2 3 4 5 6
Configuration DOE DOE DOE, THY ICE
Ho Box THY OBRP ICE OBRP OBRP
INPUTS
Gross Heat Value 19.80 19.12 19.12 19.8019.80 19.~0
Heat on I
Heat in liquor 0.63 .63 .63 .46 .46 .63
Air from SUE. .81 .81 Sal .83 .83 .72
BY Heater .19 19 --_ 08 -- --
TOTAL INPUT 21.43 20.75 20.56 21.1721.09 21.15
LOSSES
Losses Common to Both
Furnace Types 2.72 2.72 2.72 2.72 2.72 2.72
Dry Flue Gas* .97 .94 .88 1.29 1.21 1.14
~olsture from Ho in Solids 1.12 1.12 1.12 1.16 1.16 1.03
Evaporation of water in
Black liquor 3.47 3 47 1.62 1.93 14 75
TOTAL LOSSES 8.2S 8.25 6.34 7.10 5.23 5.64
Steam at SO 13.15 12.50 14.22 14.0715.8S 15.51
LESS
Soot Blower .19 .19 .19 .25 .25 .25
BY Heaters .19 .19 -- JOB .08 --
St. Air Heaters 1.04 1.04 1 04 1.07 1.07 95
TOTAL STEAM DRAW 1.42 1.42 1.23 1.40 1.40 1.20
Steam to Evans 2.80 2.12 2.12 3.16 3.16 2.80
Steam Available for Process
or Power Generatlon8.93 8.96 10.87 9.5111.30 11.51
*Temperatures of Flue Gas 325F 325F 325F ~00F 400F 400F(163C) (163C) (16~C) (204C)(204C) (204
1~327~2
12
As it teen from Table 2, by the addition of the
oxidation step of Figure 3 following the usual direct
contact evaporator, there is obtained an improvement
in net steam made available of over 21%. In the case
of systems employing indirect contact evaporation. the
oxidation step (Figure 5) obtain an improvement in
net steam production of about 19%. The system of
Figure 6 also effects an improvement in net steam
production by about 21-28~ as compared with
conventional systems.
