Method for evaporation of liquids

Process for the evaporative concentration of liquids. Vaporization is carried out by indirect heat exchange with a heating fluid in an evaporator. The vapor produced in the evaporator is thereafter condensed in a condenser by indirect heat exchange with a cooling liquid which is mainly composed of the liquid to be concentrated or the liquid which has already been concentrated in the evaporator. The vapor pressure over the cooling liquid in the condenser may be lowered by causing gas to flow in contact with the cooling liquid.

TECHNICAL FIELD 
The present invention relates to a method and apparatus for evaporative 
concentration of liquids and particularly to a method and apparatus 
utilizing the heat energy of the vapor leaving the evaporator to evaporate 
the liquid. 
BACKGROUND AND SUMMARY OF THE INVENTION 
One aspect of the present invention is directed to the treatment of an 
absorption liquid, such as a concentrated salt solution and the like which 
may be used in the dehumidification of gas such as air. During such 
dehumidification process the concentrated salt solution is being diluted 
by absorbing moisture from the air. The diluted salt solution is 
reconcentrated prior to being reintroduced into the absorption process. 
Another aspect of the present invention is directed to concentrating spent 
liquor from pulping processes in evaporating plants in which the waste 
liquor is concentrated to a degree of dryness which permits the combustion 
thereof to recover the pulping chemicals. In conventional evaporating 
plants the liquor is generally concentrated by evaporation in one or more 
separate evaporation stages. The vapor from the last stage is usually 
condensed by bringing the vapor into indirect contact with cooling water 
in a condenser as, for example, the one disclosed in U.K. Patent 
Application No. GB 2 000 584. 
The present invention is directed to a method and apparatus for 
concentrating a liquid by evaporation resulting in high thermal efficiency 
by utilizing the heat energy of vapor and preferably by utilizing the heat 
energy of the vapor leaving the evaporator instead of removing heat energy 
from the evaporating system to an external cooling water system. It will, 
however, be understood that the heat required for the evaporation of 
liquid may be derived from sources other than the vapor exiting from the 
evaporator. The present invention can thus be applied to the evaporative 
concentration of liquids in general and the specific examples given herein 
should thus not be construed to limit the scope of the present invention 
in any manner. 
According to one embodiment of the invention, air is dehumidified by direct 
contact with a water-absorbing liquid. An aqueous solution of an easily 
soluble salt such as potassium acetate, sodium acetate, potassium 
carbonate, calcium chloride, lithium chloride and lithium bromide or the 
like or mixtures thereof is used as the absorption liquid. These 
concentrated salt solutions exhibit great affinity to water. Consequently, 
the water vapor pressure above the solution is correspondingly low. 
If air at a certain temperature and of a certain relative humidity is 
brought into contact with such a concentrated salt solution, water vapor 
from the air is absorbed by the solution as long as the water vapor 
pressure above the salt solution is lower than that reached during the 
state of equilibrium. 
When air is dehumidified by absorption of water vapor, the absorption 
liquid will become increasingly diluted by the absorbed water. As the only 
volatile component of the absorption liquid is water, the absorption 
liquid can be regenerated by evaporation. This is usually accomplished by 
heating the absorption liquid to a temperature at which the water vapor 
pressure thereof exceeds the atmospheric pressure thus causing the water 
to evaporate. The boiling point elevation of the concentrated salt-water 
solution suitable for absorption purposes is high. Generally, the dilution 
of the absorption liquid by absorption of water vapor is relatively small 
and, consequently, evaporation in more than one stage is usually not 
feasible so that the diluted absorption liquid is usually regenerated by 
evaporation in a single stage evaporator. 
To regenerate the absorption liquid in an evaporator an amount of energy 
corresponding to the heat of vaporization is required. Additional energy 
is needed to heat up the liquid to the boiling temperature thereof and to 
compensate for heat losses and the like. 
According to a preferred embodiment of the present invention, the heat 
energy of the vapor leaving the evaporator is used to evaporate water from 
the absorption liquid. This is accomplished by bringing the vapor into 
indirect contact with the absorption liquid either before or after the 
absorption liquid is concentrated in the evaporator or both. The vapor 
exiting from the evaporator is brought into contact with one surface of a 
heat exchange element while the absorption liquid is brought into contact 
with the other surface of the heat exchange element. The absorption liquid 
is preferably caused to flow down in form of a thin film on the surface of 
the heat exchange element. In addition, air is caused to flow in contact 
with the absorption liquid to lower the water vapor pressure above the 
absorption liquid thus enhancing the evaporation of water from the 
absorption liquid. The air will be saturated by water vapor and the 
vaporization heat is removed from the surface of the heat exchange 
element. 
By condensing the vapor from the evaporator by means of the absorption 
liquid which thus will be concentrated before the evaporator and/or after 
the evaporator, a higher coefficient of performance as well as significant 
energy savings are obtained. An additional advantage of the invention is 
that the need of an external cooling water system is eliminated. The 
method and apparatus of the present invention thus provide a process 
similar to an evaporation in two stages or two effects with its 
considerably lower specific energy consumption. Although only one 
evaporator is shown in the accompanying drawings, it will be understood 
that more than one evaporator can be used. The preferred process of the 
present invention, i.e. when the vapor from the last evaporator stage is 
utilized for concentrating the solids containing liquid such a the 
absorption liquid or the spent liquor, results in an evaporation of n+1 
effects, whereby n can be one or an integer greater than one, and is 
preferably below about ten. For example in a system with three evaporators 
in series (i.e. n equals three) the process of the present invention 
effectively results in evaporation corresponding to four stages or 
effects. 
This and other objects of the present invention will become clear from an 
inspection of the drawing, the detailed description of the invention and 
from the appended claims.

DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
As shown in FIG. 1, the evaporation system comprises an evaporative 
condenser 1 and an evaporator 2. The evaporative condenser preferably 
includes one or more preferably vertically disposed spaced apart 
conventional heat exchange elements 3 which are preferably composed of 
pairs of substantially parallel plates which are connected at their edges 
to form a plurality of closed spaces within housing or casing 4. Other 
suitable configurations of heat exchange elements such as a radially 
extending arrangement of elements or tube-type heat exchangers may also be 
used. Open channels are formed between the heat exchange elements 3. The 
interiors of the heat exchange elements are connected at their upper end 
to an inlet 5 for the admission of water vapor and at their lower end to 
an outlet 6 for the removal of condensate. Although the water vapor used 
for the evaporative concentration of liquids is preferably taken from 
evaporator 2, vapor or steam from suitable sources outside the system may 
also be used. As pointed out above, however, the use of the vapor from 
evaporator 2, or if more than one evaporator, the use of the vapor exiting 
from the last evaporator, will advantageously result in n+1 evaporation 
effects. The letter n is either one or an integer greater than one but is 
preferably below about ten. A distributor means 7 provided with a 
plurality of openings or spray nozzles 8 extends lengthwise across the 
casing above each heat exchange element so as to form means for 
distributing absorption liquid preferably substantially uniformly over the 
outer surfaces of the heat exchange elements. The housing or casing is 
provided with one or more air inlets 9 preferably located at a level below 
or about the vicinity of the lower ends of the heat exchange elements 3 
and an outlet 10 for air located in the upper end of the casing. An 
impeller 11 or fan is disposed preferably adjacent the outlet 10 to effect 
an upward flow of air through the casing. 
The bottom of the housing forms a reservoir 12 below the air inlets 9 
collecting the absorption liquid dropping from the lower ends of the heat 
exchange elements. The liquid is subsequently withdrawn from the reservoir 
through a discharge conduit 13 at the bottom of the reservoir. A branch 
conduit 14 connected to discharge conduit 13 and a pump 15 are provided 
for recirculating at least a portion of the liquid. Feed liquid, e.g. 
spent liquor from the pulping process, is introduced into distributor 
means 7 through conduit 16. Conduit 16 is connected to conduit 14 for 
permitting the admixture of recirculating liquid flowing through conduit 
14 into conduit 16 due to the action of pump 15. 
The level 17 of the liquid in reservoir 12 is preferably controlled and 
maintained substantially constant in a manner known per se. 
The evaporator 2 includes disposed within housing or casing 18 preferably a 
plurality of spaced apart heat exchange elements 19 having interior and 
exterior surfaces and which may be of similar design as the heat exchange 
elements 3 of the condenser 1 also having an inlet 20 and outlet 21 for a 
heating fluid such as flue gases or steam. 
A distributor 22 forms means for distributing liquid preferably uniformly 
over the outer surfaces of the heat exchange elements. Concentrated liquid 
collected at the bottom of the housing is withdrawn from the evaporator 
through a discharge conduit 23. At least a portion of the concentrated 
liquid may be recirculated to distributor or distribution conduit 22 
through conduit 24 while the remainder may be removed through conduit 25 
and transferred to heat exchanger 26. The vapor produced in the housing 18 
by evaporation of absorption liquid is transferred to the interior of the 
heat exchange elements 3 of the condenser 1 through conduit 27. 
The liquid to be concentrated by evaporation, such as for instance black 
liquor from a pulping process, is fed to the condenser 1 through conduit 
16 and introduced into the distributor 7. From there the black liquor is 
flowing down preferably uniformly over the outer surfaces of the heat 
exchange elements 3 and will thereby be heated by indirect contact with 
the hot water vapor introduced into heat exchange elements 3 from the 
evaporator 2 or other suitable outside sources. Gas, preferably preheated, 
which is supplied through inlet 9 and flowing through the housing along 
the outside of the heat exchange elements will, by direct contact with the 
black liquor, lower the boiling point of the black liquor and cause 
evaporation of water therefrom. The water vapor together with the air is 
removed from the condenser through the outlet 10. The heat energy required 
for the evaporation of water from the black liquor is removed from the 
water vapor inside the heat exchange elements causing the vapor to 
condense. The preconcentrated black liquor is discharged from the 
reservoir 12 through conduit 13 and transferred to the evaporator 2 via 
heat exchanger 26 to raise the temperature thereof prior to entry into the 
evaporator by indirect heat contact with concentrated liquor exiting from 
evaporator 2. 
In the evaporator the preconcentrated black liquor is heated to its boiling 
point on the outer surfaces of heat exchange elements 19 by means of, for 
example, hot flue gases or steam introduced into the heat exchange 
elements through inlet 20 and removed together with the respective 
condensate through outlet 21. The black liquor will be concentrated by 
evaporation and the water vapor formed thereby is preferably led to 
condenser 1 through conduit 27 to be condensed and to serve as a heating 
medium for preconcentrating the black liquor. 
EXAMPLE 1 
Black liquor from a pulping process containing about 20-25% solids by 
weight resulting in a boiling point elevation of about 4.degree.-5.degree. 
C. enters evaporative condenser 1 through conduit 16 and is substantially 
evenly distributed as a thin-film over the outside of heat exchange 
elements 3 by means of distributor 7. While flowing over the heated 
exchange surfaces water is evaporated from the liquor due to the heat 
transfer from the vapor condensing on the inside of the heat exchanger. 
Moreover, the water vapor pressure of the liquor is reduced by an air flow 
contacting the black liquor film. 
The incoming liquor has a temperature of about 55.degree. C. and a 
discharge temperature of about 55.degree. C., while the air temperature 
has increased from about 20.degree. C. to about 30.degree. C. 30,000 kg/h 
of vapor from evaporator 2 have been introduced into condenser 1 via 
conduit 27. About 23,000 kg/h of water has been evaporated from the black 
liquor by contact with air in condenser 1. Thus, compared to a 
conventional six stage (effect) evaporator the capacity of the process of 
the present invention has been increased by about 12-15% without 
additional thermal energy. 
Turning now to FIG. 2, the regenerating system for an absorption liquid 
comprises a cooler 101, an evaporative condenser 102 and an evaporator 
103. Concentrated absorption liquid is fed through conduit 104 to absorber 
105 where it is brought into direct contact with moist air flowing through 
conduit 106 so as to remove moisture therefrom. At least a portion of the 
absorption liquid which has absorbed moisture from the contacting air is 
directed through conduit 107 to cooler 101 while another portion is 
directed through branchconduit 108 to condenser 102. 
The diluted absorption liquid exiting the absorber and to be concentrated 
by evaporation is thus first fed to condenser 102 through branchconduit 
108 where it is introduced into distributor 122 disposed above one or more 
heat exchange elements 112 of the condenser so as to cause the absorption 
liquid to flow down in form of a thin film preferably uniformly over the 
outer surface of the heat exchange element. Distributors 7, and 22 of FIG. 
1 and 110, 122 and 133 of FIG. 2 may be of any suitable construction such 
as a perforated pipe or a container having a perforated bottom plate and 
form means for preferably evenly distributing the respective liquid over 
the outer surfaces of the heat exchangers. The heat exchange elements of 
FIG. 2 may be of similar design as that described above in connection with 
condenser 1 of FIG. 1. Water vapor preferably supplied through conduit 123 
connected to the top of the housing or casing 124 enclosing one or more 
the heat exchange elements 125 of evaporator 103 is introduced into inlet 
126 to one or more of the heat exchange elements 112. As mentioned above, 
water vapor or any other suitable heating fluid from sources other than 
evaporator 103 may be utilized as heating medium in condenser 102. It is 
also important to note that the cooling liquid utilized in condensers 1 
and 102 is not limited to the absorption liquid (concentrated salt 
solution), or the spent liquor from the pulping process as exemplified but 
that any solids containing liquid or solution is suitable as a cooling 
liquid in the process of the present invention provided the liquid or 
solution can be concentrated to increase the solids content thereof. In 
this context solids containing liquids includes liquids containing 
dissolved solids. 
Thus, while pure water could also be a suitable cooling liquid, solids 
containing liquids such as a salt solution, black liquor or white liquor 
are preferred, since these liquids, if utilized as cooling liquids are 
concentrated by evaporation so that the solids content of the liquid is 
increased thereby. The water vapor contacting the inner surfaces of the 
heat exchange elements 112 condenses by indirect contact with the 
absorption liquid flowing over the outer surfaces of the heat exchange 
elements and may then be removed through outlet 127 as condensate which is 
fed to liquid pool 119 located in the bottom of the housing 124 through 
conduit 128. 
Absorption liquid preconcentrated by evaporation of water therefrom in 
condenser 102 is dropping down into vat 139 preferably disposed below the 
lower ends of the heat exchange elements 112. The preconcentrated 
absorption liquid is then transferred to evaporator 103 through conduit 
129 preferably via heat exchanger 130 to raise the temperature thereof 
prior to entry of the liquid into the evaporator by indirect contact with 
the concentrated absorption liquid previously withdrawn from the 
evaporator. 
The evaporator comprises preferably a plurality of heat exchange elements 
125 which may be of similar construction as those of condenser 102 having 
an inlet 131 and an outlet 132 for a heating fluid such as flue gases or 
steam. A distributor 133 preferably disposed above the upper ends of the 
heat exchange elements supplies absorption liquid to the outside surfaces 
of the heat exchange elements 125 in the same manner as distributors 7, 
110 or 122. The absorption liquid will be heated to its boiling point 
whereupon water will evaporate from the liquid which is flowing down over 
the outer surfaces of the heat exchange elements. The concentrated 
absorption liquid collected at the bottom of the casing 136 is withdrawn 
from the evaporator through discharge conduit 134. At least a portion of 
the concentrated liquid may be recirculated to the distribution conduit or 
distributor means 133 through conduit 135. The remainder of the 
concentrated liquid is transferred through conduit 137 via the heat 
exchanger 130 to the cooler 101. Alternatively, and depending on the 
degree of required cooling of the moist air or gas flowing through conduit 
106 all of the concentrated absorption liquid or a portion thereof may be 
directly fed from heat exchanger 103 to absorber 105 (not shown). 
The water vapor generated in housing 136 of the evaporator by evaporation 
of the absorption liquid on the outer surfaces of the heat exchange 
elements 125 is withdrawn from the evaporator and transferred through 
conduit 123 to the evaporative condenser 102 to be condensed therein and 
to serve as a heating medium for preconcentrating the absorption liquid in 
the manner described above. 
The portion of the concentrated absorption liquid fed to the cooler 101 is 
introduced into inlet 121 of the heat exchange elements 109 which may be 
of similar design as those of the condenser 1 of FIG. 1. The concentrated 
absorption liquid is brought into indirect heat exchanging contact with a 
cooling liquid supplied by distributor means 110 disposed above the heat 
exchange elements 109 and flowing down preferably in form of a uniform 
thin film over the outer surfaces of the heat exchange elements. The 
cooling liquid preferably comprising the condensate from the condenser 102 
is collected at the bottom of casing 111 enclosing the heat exchange 
elements 109 of cooler 101 and the heat exchange elements 112 of condenser 
102 which are disposed preferably above elements 109. Additional cooling 
water may be added at valve 140 or to liquid pool 138. In contrast to 
known cooling devices at least a significant part of the cooling water 
used for evaporative cooling at condenser 102 and cooler 101 thus 
originates from the moist air stream flowing through conduit 106. Heat 
exchange elements 109, 112 and casing 111 form cooling tower 113 through 
which air is drawn by means of impeller 114. The cooling liquid 119 
collected in a pool 138 at the bottom of the casing is preferably 
recirculated by a pump 117 through conduit 118 to distributor 110. The 
level of the liquid 119 is preferably controlled and maintained 
substantially at a constant level. 
The air, sometimes also called scavenger air, flowing through casing 111 
over the outside surfaces of heat exchange elements 109 of cooler 101 is 
in direct contact with the outside surfaces wetted by the cooling liquid 
and will cause evaporation of water from the cooling liquid. The 
evaporated water is removed by the air flow. Evaporation of water causes 
the removal of heat from the absorption liquid. The cooled absorption 
liquid is withdrawn from the heat exchange elements 109 through outlet 120 
and returned to the absorber 105 through conduit 104. The amount of 
scavenger air required for cooling the absorption liquid and flowing over 
heat exchange surfaces 109 of cooler 101 without evaporative cooling is 
about ten times greater than with evaporative cooling as described above. 
The amount of scavenger air flowing through casing 111 through inlet 115 is 
thus carefully balanced. Generally, 90% of the air introduced into inlet 
115 ma be withdrawn prior to the air coming into contact with condenser 
102 The withdrawn air stream is not indicated in FIG. 2. 
EXAMPLE 2 
Air is introduced into absorber 105 at a rate of 8,100 kg/h dry air and 
under the following conditions: t=30.degree. C. dry bulb, 27.degree. C. 
wet bulb; x=0.021 kg H.sub.2 0/kg dry air. 
After absorption, air is exiting from absorber 105 at a rate of 8,100 kg/h 
dry air under the following conditions: t=37.degree. C. dry bulb, 
20.degree. C. wet bulb; x=0.0065 kg H.sub.2 O/kg dry air. The amount of 
absorbed water is calculated as 8,100 (0.021-0.0065)=117 kg. The amount of 
heat transferred to cooler 101 is approximately 200,000 kJ/h in 38,000 
kg/h absorption liquid. is approximately 200,000 kJ/h in 38,000 kg/h 
absorption liquid. 
During the absorption step the liquid stream in conduit 10B has taken up 
approximately 117 kg/h of water from the air. If the absorption liquid is 
evaporated in a single step evaporator after increasing the temperature of 
the absorption liquid to the temperature of the evaporator by heat 
exchange the energy consumption would be approximately 1 kg of steam per 
kg of evaporated water Using the evaporative condenser in accordance with 
the present invention as pre- and/or post-evaporator for the absorption 
liquid reduces the amount of energy required for the evaporation by about 
1.5-1.9 times as compared to the use of only the evaporator. 
Instead o the described heat exchange elements other tube-type heat 
transfer elements may also be used. Equally, if the concentration of the 
diluted salt solution is low and, the boiling point elevation thereof is 
therefor moderate thus enabling the evaporation in two conventional 
evaporation stages, the invention can also be used to condense the water 
vapor from a second or subsequent stage. In addition the heating medium in 
condenser 102 such as water vapor or steam may be generated and obtained 
from a source other than evaporator 103. Of course, the water vapor from 
evaporator 103 may be mixed with one or more heating fluids coming from 
other suitable sources. Preferably, however, the water vapor utilized in 
condensers 1 and 102 originates from evaporator 2 and 103, respectively. 
Thus, it should be understood that the preferred embodiments and examples 
described above are for illustrative purposes only and are not to be 
construed as limiting the scope of the invention which is properly 
delineated in the appended claims. While the invention has been herein 
shown and described on what is presently conceived to be the most 
practical and preferred embodiment thereof it will be apparent to those of 
ordinary skill in the art that many modifications may be made thereof 
within the scope of the invention.