Note: Descriptions are shown in the official language in which they were submitted.
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A. AHLSTR~M OSAKEYHTI~, Noormarkku
753523
Method and apparatus for the recovery of easily evaporable
components from hot gases
The present invention relates to a method and apparatus for
the recovery of heat and easily evaporable components, such as
methanol and turpentine, from hot gases, especially from the
expansion vapors of waste liquor. This invention relates
especially to a method and apparatus for the fractionation of~
the condensates in connection with the evaporation of liquids,
and it is intended mainly for the pre-evaporation of a waste
liquor, such as sulfate black liquor, emergin~ from a continuous-
workin~ di~ester, whereby the black liquor is concentrated from
approx. 15-18% to approx. 23-25% in film-evaporation devices
working according to the falling film principle, by using for
the evaporation the so-called digestion buffer vapor.
In the continuous-working cellulose digester currently in use,
the black liquor passing into the evaporator is taken out at a
minimum absolute pressure of approx. 8 atm. and at a temperature
of approx. 170C. Thus it contains a considerable amount of
thermal energy, which can be utilized in the process.
So far the black liquor has usually been pre-evaporated first by
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lowering the pressure durih~ two successive expansion stages
so that a liquor vapor at approx. 120C, suitable for the
expansion of digestion chips, is obtained from the first stage,
and the vapor at approx. 100C generated during the second
stage is used for heating water. At this time the temperature
of the liquor is approx. 100C, which is regarded as a
suitable inlet temperature for the final evaporation. The
final evaporation is performed in a multi-stage evaporator
based on the indirect transfer of heat; in this case, fresh
vapor which yields the thermal energy required by the process
is fed to the first stage. Such a use of heat is not as
economic as it could be.
If the digester expansion vapor is used as one source of heat
for the black-liquor evaporator, the need for fresh vapor
decreases. Advantages are also gained in terms of environmental
protection.
The use of black-liquor expansion vapor for the pre-evaporation
in the buffer evaporator is known p~r se from, for example, U.S.
Patent 3 286 763, and an evaporator suitable for this purpose
has been introduced in Canadian Patent 1,032,465 of the present
applicant; thls contains laminae inside which heating vapor
is fed and which serve as heat exchangers. The liquid to be
evaporated is caused to flow onto the outer surfaces of the
laminae, where it flows downwards. The direction of the vapor
flow is also downwards and the produced condensate is removed
at the lower part of the apparatus. One object of this invention
is to make the fractionation of the condensates of such a
buffer evaporator more effective.
In the evaporation of black liquor the easily evaporable
components are removed along with the vapor flow during the first
evaporatin~ stages. If a pre-evaporator, for example, a buffer
evaporator, is available, a large proportion of these easily
evaporable coMponents is separated from the black liquor in the
pre-evaporator and is condensed together with the outlet vapor.
The evaporatin~ components in the evaporator thus pass from the
liquor into tho condensate. Because the quantities of these
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components, especially of methanol and turnentine, are large
and for environmental reasons must be removed from the condensate
before it is released into the watercourse, they must be
separated from the condensate water. This separation requires
a stripping column into which fresh vapor is fed.
The object of the present invention is to provide a more
economic method and apparatus than previous ones for the
recovery of heat and easily evaporable components from hot
gases and vapors
In general terms, the present invention provides, in one aspect
thereof a method for the recovery of heat and easily evaporable
components from hot gases by means of which a liquid flowing
downwards along heat exchanger surfaces is simultaneously
heated indirectly, whereby the hot gases are fed into the
lower section of a gas space, blast gas containing evaporable
components is removed at the upper section of the gas space,
and a condensate derived from the hot gases is removed at the
bottom of the gas space, comprising further ~ringing the blast
gases into one or several indirect heat exchange contacts with
the liquid, in successive additional gas spaces, recovering
condensate accumulated at the bottoms of these additional
spaces, and withdrawing the hlast gases from the last additional
gas space.
In another aspect, the present invention provides an
apparatus for the recovery of heat and easily evaporable
components from hot gases, which comprises an evaporation unit
having means for feeding the liquid to be evaporated into the
evaporation unit, for removing the concentrated liquid from it,
and for recycling part of the li~uid which is at the bottom of
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the evaporation unit into its ul)per section onto a substantially
vertical heat exchanger in the upper section, the lower section
of the heat exchanger having an inlet for feeding the hot gases
into the heat exchanger, upwards and countercurrently to the
liquid flowing along the outer surfaces of the heat exchanger,
and an outlet pipe for condensate, the upper section of the
heat exchanger being connected to one or more successive
substantially vertical additional heat exchangers operating by
indirect heat exchange, at least one of the additional heat
exchangers having at its bottom an outlet pipe for the removal
of the produced condensate and the last additional heat
exchanger having, at the end opposite to its gas inlet, an
outlet for blast gaseS.
Thus, the buffer vapors emerging from the
heat exchanger are still exposed to an indirect heat exchange
in one or several successive evaporators operating according to ¦ -
the cocurrent or the countercurrent principle, the liquid to
-be evaporated flowing downwards along the heat exchanger surfaces
of the evaporator. Thereby the heat contained in the hot gases
can be used effectively for the evaporation of the liquid and
the easily evaporable components present in these gases can
still be separated from the other condensates.
!
The invention is described below in more detail with reference
to the enclosed drawings, in which Figs. 1 and 2 depict two
known evaporation units as diagrammatic cross sections, Figs.
3-5 depict diagrammatic cross sections of three different embodi-
ments of the invention, and Fi~. 6 depicts a schematic coupling
dia~ram of a sulfate black~liquor evaporation plant provided
with evaporation units accordin~ to the invention.
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Fig. 1 shows an evaporation unit 1 known from the aforesaid
Canadian Patent 1,032,465. Liquor to be evaporated is fed
along the pipe 6 into its lower section, liquor concentrated
by evaporation is removed along the pipe 7, and part of the
liquor to be evaporated present in the evaporation unit 1 is
removed along the pipe 13 and refed into the same evaporation
unit 1, onto the heat exchanger 2 inside the unit,possibly
together with a new liquor batch to be evaporated, so that this
new and/or recycled liquor flows along the surfaces of the heat
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exchanger 2, whereby heat is transferred to the liquor.
Through the heat exchanger 2, heating vapor 5 is fed in through
the inlet 14 in the upper section of the heat exchanger 2 in
such a manner that the vapor flows in the heat exchanger down-
wards cocurrently with the liquor to be evaporated,
whereby a part 8 of the vapor is condensed, and this part 8
together with the through-blast vapor 10, i.e., the uncondensed
part, is directed out at the lower section of the heat exchanger
2.
The vapors generated in the evaporation of the liquor are
removed from the evaporation unit through the outlet 11 in
its upper section.
A distillation effect is produced on the condensing vapor side
by making arrangements for a condensate and vapor flow, and
for a through-blast,also in a known manner, as shown in Fig. 2.
The inlet vapor 5 and the outlet condensate 8 flow countercurrent!
in the laminae of the heat exchanger 2. The cutlet condensate
8 is purified because it is in contact with the inlet vapor
5, in which the partial pressures of the easily evaporable
components are at their lowest. The evaporating components
accumulate in the upper section of the laminae of the heat
exchanger 2, from where they are removed by a through-blast. A
heat-exchanger lamina thus serves as a kind of distillation
device.
It has been observed that in the apparatus shown in Fig. 2 the
methanol present in the inlet vapor 5 is distributed in the
following manner, depending on the amount of the through-blast
10:
Through-blast 10 Methanol distribution
% of inlet vapor 5 In condensate 8 In through-blast 10
10 % 33 % 67 %
20 % 23 % 77 %
30 % 19 % 81 %
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If a recovery of 80% of the methanol is desired, 70% of the
condensates of the inlet vapor do not require an additional
treatment. In these evaporators, in wich the vapor is fed into
the upper section of the laminae and the condensate is removed
by means of a through-blast at the lower ends of the laminae,
100~ of the condensates of the inlet vapor require further
treatment.
The known solution shown in Fig. 2 has, however, certain dis-
advantages, since the thermal energy of the through-blast vapor
10 is not used for the evaporation of the liquor.
The object of the present invention is therefore to provide a
method and apparatus in which the good points of the solutions
illustrated in Figs. 1 and 2 are combined, i.e., an effective
utilization of the condensation energy of the inlet vapor 5
and its distilling effect when the vapor flows counter-
currently in relation to the produced condensate 8.
According to the invention (Fig. 3), the heating vapor 5 is fed
at the lower section of the first lamina group 2 through the
inlet 14. From there it flows upwards countercurrently to the
condensate 8, and at the upper section of the lamina group 2,
that part of the vapor which has not condensed is removed and
directed to the upper section of the second lamina group 3
in the same evaporation unit 1. From there it flows downwards
cocurrently with the liquid to be evaporated. Here the condensate
of the inlet vapor 5 is divided into two fractions 8 and 9,
of which one 9 contains the bulk of the evaporable components.
According to a more advantageous emdobiment (Fig. 4), a third
lamina group 4 is added to the same evaporation unit 1. The
vapors to be blown through are fed from the second lamina
group 3 to the lower section of the third lamina group 4; from
there they flow upwards and are removed at the upper section of
the lamina group 4 through the outlet 15. In this case the
condensate of the inlet vapor 5 can be divided into three
fractions, whereby the most important evaporable components
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of the black liquor, i.e., methanol and turpentine, are separated
from each other.
It is evident that instead the second lamina group 3, a
single pipe or parallel coupled pipes can be used for directing
the gases emerging from the upper section of the lamina group 2
to the lower section of the lamina group 4. In this case the
evaporation unit has two heat exchangers 2 and 4 operating
according to the countercurrent principle, coupled in series,
although the connecting pipe 3 also serves as a heat exchanger
to some extent.
The connecting pipe can also be led outside the evaporation unit
from the lamina group 2 to the lamina group 4, as shown in Fig.
6.
In the pre-evaporation of black liquor, a suitable through-blast
10 in the first lamina group 2 is approx. 30% and in the second
one 3 approx. 1% of the vapor entering the evaporation unit.
In this case the methanol and turpentine are distributed as
follows during the black liquor evaporation:
Pure condensateMethanol condensate Through-
8 9 blast 10
Methanol 20 ~ 38 % 42 %
Turpentine1.3 %2.4 % 96.3 %
Water 70 % 29 % 1
The pure condensate 8 does not require any further treatment.
The through-blast 10 is condensed in a turpentine condenser
(not shown in the figure) and directed to turpentine separation,
in which methanol and turpentine are separated from each other.
The methanol from the turpentine separation is combined with the
methanol condensate and fed into the stripper, where the
methanol is separated from the water. Normally it would have
been necessary to separate first the turpentine and then the
methanol from the total condensate quantity.
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The distilling effect described above can be made more effective
in the manner indicated in Fig. 5. In Fig. 5 there are,below
the laminae 2, material-transfer trays i2 where the inlet
vapor 5 strips the outlet condensates 8. This further promotes
the collection of the easily evaporable gases at the top of
the laminae. It has been observed concerning methanol that, if
three trays 12 with an efficiency ratio of some 50~ are
added, 84% of the methanol can be caused to accumulate in a
10~ through-blast 10.
Fig. 6 depicts a sulfate black liquor evaporation plant provided
with buffer evaporators and with evaporation units according
to the invention.
The black liquor 101 emerges from the digester at 170C. It
is fed into the expansion tank 102. From the expansion tank lG2
the vapor 103 and the black liquor 104 are directed into the
evaporator 105, into which fresh vapor 106 is also fed. From the
evaporator 105 the turpentine through-blast 107 passes into the
turpentine condenser 108. The pure condensate is fed through
the expansion tank 109 into the pure-condensate tank 110.
The methanol condensate is fed into the methanol condensate
tank 111, where it expands, and the expansion vapors 112 are
directed into the turpentine condenser 108. The outlet vapor
113 from the evaporator 105 and the outlet black liquor 115 are
fed into the next unit 114. As above, the black liquor then
passes further through the units 119 and 120. The products
obtained are a product black liquor 116, a pure condensate 117,
and a methanol condensate 118. The turpentine vapors are
collected in the turpentine condenser 108. It is unnecessary to
feed the through-blasts from the units 119 and 120 into the
turpentine condenser since their turpentine contents are
already quite low. The bulk of the turpentine has been separated
from the black liquor during the earlier stages.
Fig. 6 shows the water (t/h), methanol (Mkg/h), and turpentine
(Tkg/h) balances of sulfate black liquor evaporation.
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The total condensate rate is 300 t/h when the flow 101 in the
buffer and final evaporator is evaporated to a dry-matter
content of 65%. In Fig. 6 the flow into the turpentine condenser
108 is approx. 1 t/h. The turpentine is thus obtained in a
flow which is approx. 0.3% of the total quantity of condensate.
The turpentine recovery rate is nearly 98%. The methanol is
collected from the condenser 111 into a condensate quantity
which is 23 t/h, which is only 7.5% of the total condensate
quantity. The methanol recovery rate is nearly 60%.
Since the recovery rates of methanol and turpentine are thus
already high enough at the buffer evaporator, the condensates
emerging from the final evaporator can be left untreated.
