Adjusting the sulphur balance of a sulphate cellulose plant by heat treating black liquor in a last evaporation stage

The invention relates to a method of adjusting the sulphur balance of a sulphate cellulose plant, in which method black liquor is concentrated and heat treated by keeping it at a temperature higher than the cooking temperature for a certain time period to separate the sulphur compounds contained in the black liquor as gaseous sulphur compounds therefrom in the last evaporation stage of a series of evaporation stages.

FIELD OF THE INVENTION 
The invention relates to a method for adjusting the sulphur balance of a 
sulphate cellulose plant. Sulphur values are removed as a gas from black 
liquor by heat treating the liquor at a temperature higher than cooking 
temperature and by adjusting the temperature and/or the delay time of the 
heat treatment. 
BACKGROUND OF THE INVENTION 
When manufacturing cellulose by sulphate cooking method, wood is cooked in 
an alkaline liquor containing sodium hydroxide and sodium sulphide. The 
lignin of the wood is separated from actual cellulose fibres. The cooking 
liquor and cellulose pulp are separated, and the pulp is then washed. The 
fibres from the pulp are then bleached and/or finished otherwise. The 
residual liquor, referred to as "black liquor", is further processed for 
recovery and reuse. 
Black liquor contains many chemicals which can be recycled back to the 
cooking process following retreatment to reduce the amount of new 
chemicals required for the process. Black liquor also contains organic 
materials which can be burned for energy recovery provided that the liquor 
is sufficiently concentrated. 
Black liquor is usually concentrated by utilizing the vapor produced at 
various stages of the process and the heat energy contained therein to 
evaporate water from the black liquor. The concentrated black liquor is 
then fed into a soda recovery boiler where it is burned and forms a smelt. 
Green liquor is prepared from the smelt from the soda recovery boiler and 
changed into white liquor, i.e. alkaline cooking liquor, by adding lime. 
Sulphur enters a sulphate cellulose plant in the form of sulphuric acid 
from the manufacture of bleaching chemicals and for splitting tall oil in 
a tall oil cooking plant. This sulphur in a variety of resulting forms 
enters the pulp cooking process and must be addressed therein. 
Particularly troubling are the sulphurous gases generated during the 
process. Examples of sulphur gas discharge points include the cellulose 
cooking stage, the evaporation plant where black liquor is concentrated, 
and in the soda recovery boiler when black liquor is burned. 
Excess sulphur poses a number of problems in a sulphate cooking process. 
Sulphur deposits foul equipment and reaction contact surfaces. Sulphur is 
also an odor nuisance and is considered to be a harmful emission to the 
environment. A reduction in sulphur gas emissions (e.g., by increasing the 
dry content of the liquor or by collecting all odor gases of the plant 
together to be destroyed by burning) causes an increase in sulphur in the 
liquor cycle. It would be desirable to have a process that would remove a 
desired amount of sulphur from the liquor cycle in order that the sulphur 
content of the cycle could be maintained as desired. 
From the Finnish Published Specification 85515 (U.S. Pat. No. 5,277,759), 
for instance, sulphur is removed as a gas from black liquor by heating the 
black liquor before the last concentration stage to a temperature higher 
than the cooking temperature and maintaining that temperature for at least 
20 minutes. Thereafter, the black liquor is permitted to expand and vapors 
containing gaseous sulphur compounds are released from the black liquor. 
The sulphurous gases are destroyed by combustion. The black liquor is 
further concentrated by further evaporation. Though obviously functional, 
the equipment required for the process is very large, is expensive, and 
has a very poor energy economy. 
On the other hand, U.S. Pat. No. 2,711,430 teaches that gaseous sulphur 
compounds can be separated with water vapor from the black liquor by 
heating the black liquor to a temperature higher than the cooking 
temperature, maintaining that temperature for a sufficient time, and 
allowing the black liquor to expand. The amount of the sulphur separated 
is varied based on the temperature and especially on the delay time. 
It would be desirable to have a process that would allow excess sulphur to 
be removed from the liquor cycle while preserving energy values where 
possible. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a method wherein the 
sulphur levels in a cellulose plant can be adjusted as desired and as 
efficiently as possible. 
The method according to the invention is characterized in that the heat 
treatment is performed in connection with or just after the last 
concentration stage of the black liquor. By this process, the black liquor 
can be concentrated to at least 80% solids while using the last 
concentration stage as the holding vessel for the heat treating process. 
By this process, the liquor concentration and heat treating steps can be 
performed in existing equipment or with very small changes to the process. 
Specifically, a process according to the invention is directed to sulphate 
cellulose processes in which wood pulp is cooked in a liquor circulating 
within a liquor cycle of the process. The cooking process produces a pulp 
slurry and a residual black liquor which is concentrated to produce a 
concentrated black liquor having a solids content of about 70%. In the 
present invention, this concentrate is heated to a temperature of at least 
140.degree. C. (the specific temperature is selected to be higher than the 
cooking temperature) and further concentrated to a solids content of at 
least about 80% while aging the concentrate in a holding tank to allow 
sulphur values to form sulphurous compounds that can be separated as a gas 
with a drop in pressure. Thereafter, the pressure over the liquor is 
reduced to vaporize the formed sulphur compounds. These sulphur compounds 
are then removed from the process. 
An advantage of the present invention is that the benefits can also be 
realized with the existing equipment and without significant additional 
investments while maintaining a high energy efficiency. At the same time, 
the sulphur level in the whole cellulose cooking process can be easily and 
simply adjusted. With suitable recovery and treatment, the sulphur values 
can be recycled as sulphuric acid for the bleaching process or to the tall 
oil splitting process thereby reducing the need for the purchase of 
sulphuric acid and increasing the economics of the process.

DETAILED DESCRIPTION 
In the solution of FIG. 1, concentrated black liquor of about 70% solids is 
fed through line 1 into mixing tank 2, from which it is fed further 
through line 3 into concentration reactor 4 which may be the last in a 
series of concentration reactors, preferably evaporators. Into the mixing 
tank 2 are also fed ash and possibly make-up chemicals through line 5. In 
the concentration reactor 4, liquor is concentrated either by direct or 
indirect heating from steam in line 6 to about 80% solids. Condensate is 
removed through line 7. Secondary steam separated from the liquor can be 
removed, if desired, from the upper part of the concentration reactor 
through line 8 by valve 32. Black liquor concentrate is removed from 
concentration reactor 4 with pump 10 and line 9. Part of the concentrated 
liquor is brought forward by means of a valve 11 into a holding tank 12, 
and a part is returned into the concentration reactor 4. 
The black liquor in the concentration reactor 4 is heated to about 
140.degree. C., typically about 150.degree.-170.degree. C. with a 
corresponding pressure. In concentration reactor 4, the viscosity of the 
liquor decreases so that the liquor can be pumped forward despite a dry 
solids content of 80%. A delay time, e.g. 20 to 60 min. in the holding 
tank 12 is adjusted such that the sulphur begins to be separated from the 
liquor as gaseous compounds including, e.g., dimethyl sulphide. 
From holding tank 12, the concentrated and aged black liquor is brought via 
line 14 through valve 15 into expansion tank 16. In expansion tank 16, the 
concentrated aged liquor is permitted to expand. Sulphurous gases and 
steam are separated and are led out of the upper part of the tank over a 
valve 17 into a 18. The discharge of gases and steam is adjusted in a 
simple manner by means of a pressure regulator 19, which controls the 
valve 17. From the expansion tank 16 the liquor is brought by means of a 
pump 21 through a line 20 over a valve 22 to be injected into a soda 
recovery boiler 23, in which it is burned in a known manner. A part of the 
liquor can be returned into the expansion tank 16 over a valve 24, if 
necessary. 
As the pressure is reduced when heated concentrated liquor moves into 
expansion tank 16, the temperature of liquor in the holding tank falls a 
little with respect to the temperature of the concentration reactor. To 
counteract such a reduction in temperature, holding tank 12 can be heated 
by hot steam in line 13 through valve 33. Such steam can be taken from a 
suitable place of the cellulose process, e.g. from soot blowing steam of a 
soda recovery boiler. The concentrated black liquor temperature should be 
maintained at a temperature higher than the original cooking temperature 
of the pulp cooking stage (not shown). This will usually mean that is a 
corresponding pressure in the delay tank. 
Steam removed from concentration reactor 4 through line 8 can be used at 
previous concentration and evaporation stages for heating the liquor. 
Steam containing sulphurous gases removed through line 18 can be used in a 
suitable manner by bringing it to a combustion plant or a catalyst plant 
to be retreated in order that the sulphur can be changed into a state as 
utilizable as possible, such as sulphuric acid. 
The method according to FIG. 1 can also be realized without holding tank 12 
in such a way that the holding time of the liquor in the concentration 
reactor 4 is lengthened to 20-60 minutes, in which time the sulphur 
compounds are separated from the liquor. A separate holding tank 12 is 
then not needed, but the liquor is brought directly into the expansion 
tank 16 in which the sulphur compounds are released and can be removed as 
gaseous compounds. This embodiment is shown in FIG. 2, which corresponds 
to FIG. 1 as far as the reference numerals are concerned, except for that 
holding tank 12 with associated parts is missing, because the liquor is 
fed after the delay in the concentration reactor 4 directly into the 
expansion tank 16 through line 15. The consequence is then that the size 
of the concentration reactor 4 increases to some degree which is not 
significant. 
FIG. 3 shows a system suitable for realizing a second embodiment of the 
method of the invention. (The reference numerals used correspond to those 
of respective parts of the equipment according to FIG. 1.) In FIG. 3, the 
concentrated liquor is brought directly through line 3 into concentration 
reactor 4. 
In the system shown in FIG. 3, the method of the invention can be effected 
by using a two-stage expansion in such a way that the liquor is heated in 
the concentration reactor 4 to a sufficiently high temperature, e.g., 
about 190.degree. C., in which case the pressure in the reactor naturally 
corresponds to the temperature. When the dry solids content of the liquor 
is suitable, at least about 80%, the liquor is led into an expansion tank 
16 where the liquor temperature falls with simultaneously falling 
pressure. 
The concentrated liquor stays in the expansion tank 16 for a suitable time, 
i.e. 20 to 60 minutes, to age and allow the sulphur to separate from the 
liquor as gaseous compounds. Part of the gaseous sulphur compounds can be 
removed together with steam as gaseous compounds 18. The remainder passes 
together with the aged and concentrated liquor into an ash mixing tank 25 
wherein the aged concentrated liquor is allowed to expand further to a 
lower pressure. In connection with the expansion, the main part of the 
sulphurous gases is separated from the liquor with the steam and removed 
over a valve 26 into a line 27. The expansion can naturally be carried out 
in more than two stages, in which case expansion tanks are added. 
The pressure control in the ash mixing tank 25 occurs by means of a 
regulator 28, which controls the valve 26. In the ash mixing tank, the 
liquor is mixed with ash and/or make-up chemicals coming through a line 5. 
Mixing is performed as efficiently as possible by means of a mixer 29 in 
the lower part of the tank. From the ash mixing tank 25, the aged and 
concentrated liquor that has been freed of gaseous sulphur compounds is 
passed through a line 30 over a valve 31 to soda recovery boiler 23 for 
conventional burning. 
FIG. 4 is a schematic view of a conventional evaporating plant at a 
sulphate cellulose plant following integration with the system shown in 
FIG. 1. Such a plant would be upstream of the systems depicted in FIGS. 
1-3. The evaporating plant is made of a series of evaporators 34-38 where 
the black liquor is heated indirectly with steam to increase the dry 
solids content of the black liquor and remove water. Five evaporators are 
depicted in FIG. 4, but more or less than five evaporators can be used. 
Dilute black liquor 39 with a dry solids content within the range from 
about 15-18% is fed to evaporator 36. A relatively more concentrated black 
liquor 40 is removed and passed to evaporator 37 for additional 
concentration. Concentrated black liquor 41 is further concentrated in 
evaporator 38. Concentrated black liquor 42 has a solids content within 
the range from about 25-30% and is passed to intermediate liquor tank 43. 
Thereafter, stored liquor 44 is passed to evaporator 35 and evaporator 34. 
Concentrated black liquor 46 having a solids content of about 70% is 
passed to tank 47 and forms concentrated black liquor feed 1 described in 
FIGS. 1-3. 
Final evaporator 4 is operated to further concentrate the black liquor from 
70% solids to 80% solids. See, U.S. Pat. No. 5,112,441 the disclosure of 
which is herein incorporated by reference. 
Steam is used to concentrate the black liquor in a countercurrent exchange 
pattern by feeding fresh, hot steam 48 into evaporator 34. The contact 
will form steam 49 that is reduced both in temperature and pressure 
relative to steam 48. Steam 49 is then passed to evaporator 35 to make 
steam 50. This pattern continues to make streams 51-53 containing steam 
from each subsequent evaporator 36-38. Through each evaporator, the 
pressure and temperature of the steam stream is reduced while water is 
evaporated from the black liquor. 
As described with reference to FIGS. 1-4, the temperature of the 
concentrated black liquor in evaporator 4 is raised to a temperature 
higher than that of the pulp cooking stage (not shown) which also permits 
evaporation to be performed to a solids content of at least about 80-%, 
preferably higher than 80%. Once at temperature, the 80% concentrated 
liquor is held and aged for a period of about 20-60 minutes (either within 
evaporator 4 or in holding tank 12) before being allowed to expand in 
expansion tank 16 and separate gaseous compounds 18 including sulphur 
compounds. 
By the present process, the desired amount of sulphur can be removed from 
the liquor by controlling the treatment temperature and time as well as 
the amount of incoming sulphur that is allowed to accumulate in the 
process liquor. If less sulphur is desired, a greater amount of sulphur is 
removed after the last concentration stage (e.g., by increasing the 
holding time at temperature in tank 12) relative to the amount of new 
sulphur that may be introduced into the process. Sulphur levels can be 
increased by reducing the holding time and removing less sulphur. By a 
combination of these mechanisms, the desired sulphur balance can be 
achieved in the process liquor circulating through a sulphate cellulose 
plant. 
The invention can be applied especially well to the process described in 
Finnish Patent 73474 (U.S. Pat. No. 5,112,441) in which the final liquor 
concentration step occurs under a pressure higher than atmospheric 
pressure and at a temperature higher than the boiling point of the 
atmospheric pressure of the liquor. As modified by the present invention, 
sulphur would be separated from the concentrated liquor merely by adding a 
holding tank between the last evaporator and the expansion tank. The 
liquor in the holding tank would be heated, e.g., by hot steam, if needed. 
The realization of the method as an investment is thus rather cheap and 
easily applicable. 
The invention can also be applied to the solution of the Finnish 
Application Ser. No. 894049 (U.S. Pat. No. 5,230,773), in which is 
described a process for obtaining a high dry solids content. In that 
process, ash is not fed to the black liquor until a separate ash mixing 
tank positioned between the system of a last evaporator and an expansion 
tank, and the boiler. With the present process, sulphur removal can be 
performed by adding a holding tank before the expansion tank. 
Alternatively, the expansion tank after the last concentration stage can 
be used for the heat treatment as well as the holding tank with separation 
of the sulphurous gases in the last ash mixing tank wherein the black 
liquor is also expanded. 
In the specification and drawings above, the invention has been described 
by way of example only and it is by no means restricted to it. The 
suitable heat treatment temperature and delay time depend on the equipment 
to be used and on the sulphur separating capacity needed, but the most 
substantial thing is that an already existing concentration reactor 
equipment with its expansion tanks can be used almost as such for the heat 
treatment of liquor by using the method of the invention and no extra 
heating and/or delay tanks and expansion tanks have to be built 
separately.