Note: Descriptions are shown in the official language in which they were submitted.
CA 02368808 2002-O1-22
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F0128
METHOD OF EVAPORATION OF SOLUTIONS ACCOMPANIED BY
CRYSTALLIZATION OF SALTS
The invention relates to the field of production of alumina, soda, potash and
other salts and specifically it relates to the method of evaporation of
solutions in
evaporating units having substantially tubular processing elements.
Background of the Invention
A method of evaporation of solutions in evaporation units having
processing elements of tubular configuration and accompanied by
crystallization
of salts is known in the art (see L. P. Pertsev "Tubular evaporating units for
the
crystallization of solutions" M. Mashinostroenie 192, p. 29 and 66). This
method consists of heating of the solution by steam, accompanied by ' removal
of
condensate and evacuation of evaporated slurry containing crystals of salts
and
vapors from an apparatus.
This method is affected by the following major drawbacks. About 20 to 30
percent of the inner space of the heating tubes are clogged by solids of salts
which
are broken off from the walls of the processing tubes and other elements
during
operation of the apparatus. Such condition requires stoppages of the apparatus
CA 02368808 2002-O1-22
every 3-4 days for washing of each individual tube with water. The
encrustation
of the most efficient drip pans and clogging of the heating tubes result in
the
reduction of the evaporation unit capacity and steam utilization ratio.
Installation
of sophisticated and expensive drip pans leads to a substantial increase in
the cost
of the equipment. Evaporation of water and other liquids used for washing of
the
individual heating tubes causes further increase in the consumption of the
heating
medium such as steam.
A method and apparatus adapted for prevention of deposits formation on
the inner walls of crystallizing evaporation units are also known in the art
(see, for
example, German patent application N 1619806 filed 1972). According to this
prior art method, a non-saturated solution is supplied on the internal walls
of the
unit. Another solution capable of dissolving the solid deposits is poured down
in
the form of a film along the internal walls of the apparatus.
The following important drawbacks are common for this method. This
method is applicable for dissolving of the solid deposits situated only on
vertical
or substantially inclined walls capable of supporting the poured film-type
solution.
It is quite dii~icult to use the dissolving film according to the German
patent
application for the removal of deposits accumulated on a ceiling or other
upper
horizontally positioned sections of the unit. There are also major problems
with
applying this method for the removal of solids accumulated in the commonly
used
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drip pans of grid-type or shutter-type. Another drawback of this method is in
utilization of the complex and sensitive spraying devices used for the
formation of
the film-type solutions which become clogged very easily.
One of the main objects of the invention is to minimize formation of solids
in the separator and other parts of the processing units and to minimize
clogging of
heating tubes by the solid pieces separated from the walls and other
structural
elements of the apparatus.
Brief Description of the Drawings
FIG. 1 shows a processing unit utilizing the method of the invention.
Description of the Preferred Embodiment
As illustrated in FIG. l, a device 10 consists of a heating section 1 which
is positioned between the separation chamber 2 and a receiving chamber 7. A
plurality of substantially hollow heating elements 5 and at least one
substantially
hollow circulatlon element 6 are situated within the heating section 1, so as
to
provide fluid communication between the receiving chamber 7 and an interior
space of the separation chamber 2. Although, any conventional configuration of
the heating and circulation elements is contemplated, in the preferred
embodiment
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of the invention these elements are formed having substantially tubular
configuration. A condensate of a heating medium or heating steam is removed
from the heating chamber 1 by means of a drain line 9. As will be discussed
below, a portion of the condensate is diverted from the drain 9 by means of
the
line 3 and introduced into the interior space of the separation chamber 2
through
a nozzle 4.
The heating medium in the form of the heating steam is directed into the
inner space of the heating section 1, so as to surround the heating elements
5. A
solution is drawn from the receiving section 7 into the heating elements 5.
Upon reaching the boiling temperature, the solution in the form of a vapor-
liquid
mixture enters the interior space of the separator 2. In the separation
chamber 2
cooling occurs and an equilibrium between the liquid and vapor phases is
achieved. This results in deposits of the suspended crystals of salts on the
inner
walls of separation chamber 2. The liquid phase containing the crystals of
salts is
recirculated to the receiving chamber 7 by means of a return or circulation
element 6, and the vapor phase is evacuated from the separator through the
line
8. After being returned into the receiving chamber 7 and mixed with the
solution, the liquid phase containing crystals of salts re-enters the heating
elements
6, thus providing continuous circulation of fluids in the apparatus of the
invention. The vapor phase, after being separated from the liquid phase, is
evacuated from the separation chamber 2 through the line 8.
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In the invention, as a result of evaporation of sodium carbonate, sulfates,
etc., the concentration of soluble salts is increased accompanied by
crystallization
thereof in the separation chamber 2. The resulted mixture is constantly
removed
from the separation chamber 2 through the circulation element 6 at the level
above the upper region of heating elements 5.
In the heating section 1, the heating medium or heating steam is isolated
from the processed fluids passing through the heating elements 5 and
circulation
element 6. A condensate developed as a result of the heat exchange between the
heating medium and the processing elements 5, 6 is accumulated within the
interior space of the heating section 1 and removed through the drain line 9.
An important feature of the invention is that the condensed portion of the
heating
medium including steam is diverted from the drain line 9 into the separator 2
by
means of the line 3. The fme particles of spray of the condensed heating
medium including steam are introduced into the interior area of the separator
2
through the nozzle 4. In the preferred embodiment of the invention about 0.3
to
2 percent of the condensed portion of the heating medium or condensate in the
form of a fme spray is introduced into the separation chamber 2. In the
separator
the fine particles of condensate are combined with the particles of the liquid
phase
washing and diluting the same. Mixing of the fme spray of condensate with the
particles of liquid phase causes enlargement of the latter particles within
the
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evaporator. This significantly reduces the formation of solids on the interior
wall
of the separation chamber 2 in general, and specifically provides protection
from
excessive formation of undesirable solid deposits on the interior walls.
Because of evaporation of the fme particles of condensate, the method of
the invention practically excludes undesirable overheating of the vapor phase
in
the separation chamber. This prevents forming precipitants on the walls of the
separator and minimizes formation of solids.
The above discussed introduction of the fme particles of condensate also
excludes over-saturation of the particles of the solution which are present in
the
steam space and precipitated on the separator walls with salts. This approach
also
prevents over-saturation of the drops of the solution situated on the
structural
elements of the apparatus and drip pans as a result of mixing with the drops
of
condensate. All this prevents clogging and encrustation.
In the separation chamber, when the fme particles of condensate are merged
with the particles of the liquid phase, the vapor phase is cleaned and
purified
forming drops of larger size. The cleaning of the vapor phase also accelerates
precipitation and the increases in the efficiency of the drip pan operation.
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As indicated above, the recommended amount of the fine particles of
condensate introduced into the separation chamber should be between about 0.3
and 2.0 percent of the amount of condensate developed within the heating
compartment. Increase of the amount of the fine particles above the 2 percent
level will not result in increasing the effciency of the method. On the other'
hand, the reducing the amount of the fine particles below the 0.3 percent
level of
the produced condensate is technologically diffcult to achieve.
The largest amount of non-reacted fme particles of condensate is supplied
along with evacuated vapors into the inter-tubular space of the heating
chamber of
the next unit and removed together with condensate. A part of the fine
particles of
condensate introduced into the separator is evaporated by the overheated
vapors.
The above practically excludes any dilution of the liquid phase in the
separation
chamber.
Example
During the industrial trial of the method of the invention, an evaporating
apparatus consisting of four units was utilized. About 0.4 - 0.6 percent of
the
condensed heating medium from the first unit was introduced through the
ejectors
into the hollow separators (without drip pans). The results of the trial were
compared to the operation of the available most powerful evaporating units
having
the working space of about 800 m2 and utilized in the sodium carbonate -potash
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production accompanied by crystallization of anhydrous sodium carbonate. This
prior art evaporating unit operates without introduction of the condensed
heating
medium into the working space of the apparatus. After utilization of the
method
of the invention, time between the maintenance shut downs for washing/cleaning
of the processing tubular elements was increased up to 40 days.
Simultaneously,
the clogs in the tubular elements of the processing equipment was
substantially
reduced. In view to the increase in the efficiency of the heat transfer
process and
reduced resistance of the drip pans due to the encrustation, the ratio of
steam
use/utilization was substantially increased. The specific consumption of steam
per ton of evaporated water was reduced from 0.62 to 0.33 t/t.
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