Note: Descriptions are shown in the official language in which they were submitted.
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Rauma-Repola Oy
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Method for carbonation of green liquor by means of concentrated
carbon dioxide gas in a mixing reactor and for the separation
of concentrated hydrogen sulphide only in process steps
after carbonation for the preparation of alkali carbonate
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The subject of the present invention is a method for the
carbonation of green liquor and for the separation of sulphur
from the solution mentioned above as hydrogen sulphide.
In the method in accordance with the present invention, the
sulphur is 'separated as hydrogen sulphide out of the clarified
;~ 10 green liquor obtained from burning of the waste liquor produced
in the production of pulp so that the green liquor is first
precarbonated by means of gas containing carbon dioxide and
thereupon carbonated by means of concentrated carbon dioxide
gas, whereby alkali bicarbonate is produced in the solution
. to be carbonated. Hereupon the carbonated green liquor is
passed into the apparatus for separation of hydrogen sulphide,
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wherein alkali bicarbonate and alkali bisulphide react and
liberate hydrogen sulphide.
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Several methods are known for the separation of sulphur
from an alkali sulphide solution or from an alkali sulphide
solution that contains alkali carbonate, as hydrogen sulphide.
Among these methods should be mentioned the Sivola process,
the Mead method, the Stora pro¢ess, and the Tampella process.
The method in accordance with the present invention is mainly
characterized by what is stated in claim 1.
The method in accordance with the present invention is
the following:
Green liquor clarified and precarbonated in a known way is
carbonated by means of concentrated carbon dioxide gas of
more than 50 per cent by volume in a mixing reactor under a
pressure that is higher than the total counter-pressure of
the solution. The obtained bicarbonate-containing solution
that c~ntains bisulphide and carbonate is passed into a known
apparatus for the separation of hydrogen sulphide, in which
part of the bicarbonate reacts with bisulphide and a desired
quantity of the total sulphur in the solution leaves the
solution as hydrogen sulphide.
The method in accordance with the present invention is more
specifically the following:
The following reactions take place in the carbonation of
green liquor:
(Me = alkali metal)
Me2C03 + C2 ~ H20 ~ 2 MeHC03 (1)
MeHS ~ MeH;C03 ~ Me2C03 + H2S (2)
According to the above reaction equations, when the green
liquor is carbonated, when C02 is absorbed, at the same time
desorption of H2S tends to take place. When carbonation of
the green liquor is performed in known absorption columns by
means of concentrated carbon dioxide, the conversion of
carbon dioxide to the carbonation reactions is poor and the
gas leaving the apparatuses is a mixture of carbon dioxide and
hydrogen sulphide, and this is detrimental in view of the
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known further treatment of the gas, burning of the hydrogen
sulphide and recovery of the sulphur dioxide.
The principle of the present invention is to perform carbon-
ation of green liquor with no liberation of hydrogen sulphide
taking place at the same time.
When green liquor is Garbonated, the component pressure of
hydrogen sulphide as well as the component pressure of carbon
dioxide are increased together with the degree of carbonation.
In order to aecelerate absorption of carbon dioxide and to
prevent desorption of hydrogen sulphide, a pressure must be
used in the carbonator that is higher than the summed-up total
counter-pressure of the component pressures of the carbon
dioxide and hydrogen sulphide in the green liquor solution and
the component pressure of water vapour. The further the
carbonation is extended, the higher shall the pressure in
the carbonator be made. In the procedure in accordance with
the present invention, the carbonation can be performed either
in one carbonator or in several subsequent carbonators. When
several carbonators are used in series, their operating
pressures can be staggered, whereby savings are obtained in
expenses of compressing C02 as well as in purchase expenses
affected by the structural pressures of the carbonators.
In the method in accordance with the present invention, the
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carbonation is performed by means of concentrated carbon
dioxide gas of more than 50 per cent by volume in carbonators
of the mixing-reactor type.
The carbonation reactor is a continuous stirred tank reactor,
or a back-mix reactor with a steady-state continuous flow of
feed, and product streams. The feed of gaseous carbon dioxide
and the incoming green liquor are very carefully mixed to form
a dispersion of small bubbles in green liquor, which makes
the reaction rate signifieantly high because of large area of
contact surface. The composition of mixture inside the
reactor vessel is practically uniform with high gas hold-up
and large amount of very small crystals of product bicarbonate.
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The carbon dioxide gas is dispersed in the carbonators into
small bubbles in the green liquor so that a dispersion is
formed in which the proportion of gas by volume is 10 to 50
per cent. By means of this method, the mass~transfer
area can be made as large as possible and the total mass trans-
fer coefficient of the carbonator can be made even 20 to 40
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fold as compared, e.g., with the carbonating towers of "passette"
construction used in the Sivola recovery process.l)
The very high total mass tran;sfer coefficient of the carbonator
makes the carbonators belonging to the method in accordance
with the invention so small that, despite their nature of
pressure vessel, they are economically competitive as compared
with conventional known carbonators.
- The operating temperature of the carbonator,is 0 to 100C.
When a carbonation temperature of 50 to 100C is used,
according to the solubility data, a great part of the
reaction product MeHC03 of reaction (1) remains uncrystallised
in the solution. Owing to the high speed of the carbonation
in accordance with the invention, the proportion of MeHC03
in excess of the solubility is not crystallised well, but a
very high number of Grystal nuclei are produced and the
crystal size remains very small.
Small crystal size is advantageous in the process step
following the carbonation, in which the hydrogen sulphide is
separated from the solution in a known way. When hydrogen
sulphide is removed from the solution, the reaction (2) can
proceed from the le~t to the right. The rate of dissolution
of MeHC03 (s), i.e. of crystalline MeHC03, i8 proportional
t~ the pha~e limit area between the crystals and the solution.
The carbonation in accordance with the method of the invention
can be performed without cooling, in which case,the reaction
heat liberated owing to the exothermic nature of the carbonation
reaction (1) is bound t~ the process flow and is utilized in
the next process step, of endothermic reaction type, separation
of hydrogen sulphide.
1) The construction of said previously known towers is described,
e.g., in the book by Hou, T-P, Manufacture of Soda,
Reinhold, New York 1942, pp. 134, 170, 246.
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By means of the method in accordance with the invention, it
is possible, from green liquor, to prepare an alkali carbonate
solution whose alkali sulphide content is of ~he desired
magnitude, by connecting one or se~eral carbonation and
hydrogen-sulphide separation steps one after the other.
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