Freeze regeneration of glycol solutions loaded with water

Aqueous glycol solutions, used for example, in the de-icing of heat exchange surfaces used in defogging systems, are regenerated by freeze crystallization, the ice being separated with relatively little loss of glycol.

BACKGROUND OF THE INVENTION 
This invention relates to a process for the regeneration of glycol 
solutions loaded with water. 
Lower glycols are normally strongly hygroscopic substances, and for this 
reason mono-, di-, and triethylene glycols have been known for years as 
desiccants. Thus, these compounds are used, for example, for the drying of 
gases (U.S. Pat. No. 2,787,451) by introducing the gases at the bottom 
into a scrubbing tower where they are met by a spray of glycol solution 
from above. A dried gas is withdrawn from the head of the scrubbing tower 
in this process. However, glycols are also employed as desiccants for 
liquids, insofar as these liquids are not miscible with glycol. 
Special significance is attributed to the glycols in low-temperature 
processes where there is the danger of the crystallization of water ice 
when processing moist liquids or gases, leading to clogging of apparatus 
parts. In these instances, minor amounts of glycol are added to the gas or 
to the liquid, before it comes into contact with apparatus parts which are 
at below 0.degree. C., thereby to prevent a freezing out of water ice. 
A typical example for the last-mentioned application is the use in 
defogging devices (DOS [German Unexamined Laid-Open Application] No. 
2,224,671). In these defoggers, utilized for the defogging of airports, 
freeways, etc., the air is blown by means of a blower over the evaporator 
and liquefier of a refrigerating machine and, due to the cooling of the 
air at the evaporator, a condensation or freezing out of the fog is 
accomplished. Since most of the moisture, in case of outside temperatures 
of below about +5.degree. C., is precipitated in the form of ice on the 
evaporator of the refrigerating machine, the heat transfer from the air 
and/or fog to the evaporator drops rapidly as soon as the ice layer has 
reached a certain thickness. Therefore, it has been desirable to suppress 
the deposition of ice on the evaporator at the outset, and for this reason 
the evaporator is sprinkled, in the conventional process, with an aqueous 
glycol solution. Since a device as utilized, for example, for airport 
defogging requires per operating hour about 100-150 kg. of glycol as the 
antifreeze agent, a regeneration of the glycol must be provided, unless 
this quantity is to be discarded. 
The heretofore customary regenerating method (U.S. Pat. No. 2,787,451) 
resides in heating the glycol solution and distilling off part of the 
absorbed water. However, this process has the disadvantage that it 
requires a large amount of heat energy and cooling water, because large 
temperature differences must be overcome for the heating and recooling of 
the solutions. A further disadvantage of the warm regeneration method is 
that the glycol can be oxidized during heating partially to glycolic acid, 
which has an extremely corrosive effect on aluminum. 
SUMMARY OF THE INVENTION 
It is one object of the present invention to provide a process making it 
possible to again separate the water absorbed by the glycol solution in a 
manner which is significantly more advantageous from an energy viewpoint. 
This object is attained in accordance with the invention by cooling the 
solution to below its ice point and separating the thus-crystallized ice 
from the remaining glycol solution. 
The regenerating method according to this invention has nowise been 
obvious, since hydrophilic compounds normally tend toward the formation of 
mixed crystals with water, and something similar was initially also to be 
expected from the glycols. However, pure water ice is deposited during the 
cooling of aqueous glycol solutions, whereby a separation of the 
components glycol and water becomes feasible. 
Furthermore, it had to be expected that the separation of the mother liquor 
from the ice slurry would become increasingly difficult in proportion to 
the quantity of frozen-out ice. However, this has likewise proven 
groundless. Consequently, to conduct the process in accordance with this 
invention, a one-stage crystallization of the ice is sufficient.

DETAILED DESCRIPTION 
To conduct the process of this invention, the water-loaded glycol solution 
must be cooled to below the temperature at which the water ice begins to 
crystallize (ice point). The dependency of the onset of crystallization on 
the concentration of a solution of monoethylene glycol and on the 
temperature can be seen from FIG. 1. In the area underneath the curve, ice 
is crystallized on the left-hand side of the eutectic point and 
monoethylene glycol is crystallized on the right-hand side thereof. For 
example, if a 20% glycol solution, the crystallizing point of which is 
about -10.degree. C., is subjected to the process of this invention, it is 
sufficient to cool the solution to -12.degree. to -15.degree. C. to attain 
shortly thereafter a rapid crystallization of water ice. 
FIG. 3 shows the onset of crystallization for diethylene glycol (curve A) 
and triethylene glycol (curve B) in dependence on the temperature and the 
glycol content of the solution. 
In this connection, it proved to be advantageous to allow the glycol 
solution to flow through a heat exchanger, the other cross section of this 
heat exchanger being charged with refrigerant from a refrigerating 
machine. 
The amount of separated ice depends on the temperature to which the glycol 
solution is cooled. This temperature can, in turn, be controlled by the 
temperature of the refrigerant and the residence time in the heat 
exchanger. When cooling a 20% strength monoethylene glycol solution to 
about -17.degree. C., an increase in the concentration of the solution to 
about 30% is attained with a sufficiently long residence time of the 
solution in the heat exchanger. Such a rise in concentration is normally 
sufficient for technical processes, since the glycol solution is then 
capable of acting as an antifreeze and of reabsorbing, for example, the 
amount of water precipitated by crystallization on the cooling surfaces of 
defoggers. 
It has been found that the size of the thus-separated ice crystals 
increases with the time spent in cooling the solution. In contrast 
thereto, a subcooling of the solution proved to be very damaging, since in 
case of subcooled solutions very fine ice crystals are suddenly produced 
which can be separated from the mother liquor only with difficulties. In 
accordance with a special embodiment of the present invention, the 
possibility has thus been provided, in order to avoid subcooling, to 
recycle a small partial stream of ice-containing mother liquor to the 
solution to be regenerated, whereby the latter is continuously inoculated. 
The crystallized ice can be separated from the mother liquor by 
centrifuging or filtering. In this connection, continuously operating 
filter centrifuges and filter presses proved to be especially 
advantageous. 
A decisive factor for the purity of the thus-separated ice and thus for the 
separating effect attained is the separation of the mother liquor from the 
crystals. The regenerating effect becomes greater with an increasing 
completeness of the separation of the mother liquor. Accordingly, other 
known solid-liquid separation techniques can be used aside from filtration 
and centrifugation, attention being directed, for example, to class 210 of 
the U.S. Patent Office Manual of Classification as well as to chemical 
engineering handbooks, e.g., Perry's, and also the ice recovery techniques 
in desalination freezing processes. 
According to a special embodiment of the idea of this invention, the 
provision has thus been made to continue the centrifuging or "nutsch 
operation" with a supply of heat until about 10-20% of the separated ice 
has been superficially melted off. The heat can be supplied by blowing 
warm air onto the precipitated ice. With the aid of the resultant water, 
the mother liquor is flushed out of the crystal slurry to an almost 
quantitative extent. 
A like effect with respect to the precipitated ice is also achieved if the 
ice is sprayed with a small amount of water (about 20%, based on the ice 
to be purified) during the centrifuging step; this process has the 
advantage of a significantly shortened centrifuging period. 
The same measures of blowing warm air into the system or spraying water 
thereon are also advantageous when utilizing filter presses. 
The invention will furthermore be explained with reference to the 
schematically illustrated embodiment. 
FIG. 2 illustrates the essential parts of a defogging device as it can be 
used, for example, to defog airports. With the aid of a blower 1, foggy 
air is taken in from the left-hand side via an evaporator 2 of a 
refrigerating machine and greatly cooled at that point. During this step, 
the atmospheric humidity is condensed on the evaporator 2. The air then 
passes furthermore through a mist precipitator 3 and is then forced 
through the blower 1 toward the right-hand side. Upstream of the 
evaporator 2, a spray nozzle 4 is disposed making it possible to spray the 
evaporator 2 with glycol solution to prevent an ice precipitation on the 
evaporator. The liquid running off from the evaporator 2 and the mist 
precipitator 3 is collected in a collecting trough 5 and conducted to a 
regenerating unit 6. In the regenerating unit 6, the glycol solution is 
subjected to the freezing-out process of this invention, and part of the 
water contained in the solution is separated in the form of ice. The ice 
is ejected via conduit 7. A conduit 8 connects the regenerating unit 6 
with the spray nozzle 4, which is fed in this way with regenerated glycol. 
Via a conduit 9, glycol losses can be replenished. 
The advantages of the process of this invention will furthermore be 
demonstrated with the aid of several tables. 
Tables 1(a) to 1(c) show comparisons between the regenerating method of 
this invention and those methods wherein, without the regeneration of the 
invention, a portion of consumed solution is continuously discharged to 
prevent the water concentration from becoming too high. 
The numerical data stem from a defogger according to FIG. 2. The values in 
Table 1(a) relate to monoethylene glycol as the solvent, and to an air 
temperature of -4.degree. C. The saving in glycol is 90%. Table 1(b) 
represents the values for diethylene glycol (outside temperature 
-4.degree. C.; saving: 90%), and Table 1(c) shows the values for 
triethylene glycol (outside temperature -4.degree. C.; saving: 88%). 
TABLE 1(a) 
______________________________________ 
Without With 
Regeneration 
Regeneration 
Glycol Glycol 
Quantity 
Content Quantity Content 
1./h. vol.-% 1./h. vol.-% 
______________________________________ 
Runoff from 
cooler 2 and 
mist precipitator 3 
3455 21.0 3455 21.0 
Discharge from 6 
576 21.0 466 2.5 
Reflux to the 
2879 21.0 2989 23.8 
spray nozzle 4 
Glycol feed 9 
121 100.0 11 100.0 
Sprayed solution 
3000 24.2 3000 24.2 
______________________________________ 
TABLE 1(b) 
______________________________________ 
Without With 
Regeneration 
Regeneration 
Glycol Glycol 
Quantity 
Content Quantity Content 
1./h. vol.-% 1./h. vol.-% 
______________________________________ 
Runoff from 
cooler 2 and 3315 27.5 3315 27.5 
mist precipitator 3 
Discharge from 6 
625 27.5 472 4 
Reflux to the 
2690 27.5 2840 31.6 
spray nozzle 4 
Glycol feed 9 
175 100 17 100 
Sprayed solution 
2855 32.0 2855 32.0 
______________________________________ 
TABLE 1(c) 
______________________________________ 
Without With 
Regeneration 
Regeneration 
Glycol Glycol 
Quantity 
Content Quantity Content 
1./h. vol.-% 1./h. vol.-% 
______________________________________ 
Runoff from 
cooler 2 and 
mist precipitator 3 
3280 32.5 3280 32.5 
Discharge from 6 
665 32.5 480 5.5 
Reflux to the 
2615 32.5 2798 37.2 
spray nozzle 4 
Glycol feed 9 
218 100 26 100 
Sprayed solution 
2825 37.8 2825 37.8 
______________________________________ 
Table 2 shows the dependence of the separating effect on the centrifuging 
time. The measurements were conducted with a 20% by volume monoethylene 
glycol solution which was cooled to -11.5.degree. C. .+-. 0.5.degree. C. 
As can be seen from Table 2, the optimum centrifuging time ranges between 
1 and 5 minutes. With a shorter centrifuging period, the amount of 
centrifuged mother liquor is still to small, whereas with a longer 
duration of the centrifuging step, the effect of the melting of the ice 
becomes noticeable. The centrifuge had a diameter of 17 cm and was 
operated at 900 rpm. 
TABLE 2 
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Glycol 
Content Glycol Separated 
of Cen- Content of Amount of Ice 
Centrifuging 
trifuged Separated Based on Ini- 
Period Matter Ice tial Solution 
min. vol.-% vol.-% vol.-% 
______________________________________ 
0.3 22.5 4.0 10 
1 24.0 3.7 16 
5 23.9 2.6 15.5 
10 22.5 1.0 9.5 
20 20.0 0.2 2.0 
______________________________________ 
Table 3 shows the dependency of the separating effect on the concentration 
of the glycol solutions utilized when the water is being frozen out. In 
the measurements on which Table 3 is based, the monoethylene glycol 
solution was cooled with a brine, the temperature of which was 
6.degree.-8.degree. C. below the ice point of the respective solution. The 
centrifuging time was about 6.5 minutes. The reduction of the separated 
amount of ice with growing glycol content of the initial solution is 
related to the stronger inclination of the curve in FIG. 1 at higher 
glycol contents. 
Table 3 
______________________________________ 
Tempera- 
ture 
of Sepa- 
Centri- rated 
fuged Glycol Glycol Amount 
Product = Content 
Content 
of 
Glycol Final Ice of of Ice 
Content 
Cooling Point Centri- 
Sepa- Based 
of Initial 
Tempera- of Initial 
fuged rated on Initial 
Solution 
ture Solution Product 
Ice Solution 
vol.-% .degree. C. 
.degree. C. 
vol.-% vol.-% vol.-% 
______________________________________ 
5 - 4.5 - 2.0 7.5 1.0 35 
10 - 6.0 - 4.0 13.8 1.8 29 
15 - 9.0 - 7.0 21.0 2.4 24.5 
20 -11.5 - 9.5 24.5 2.5 19.0 
25 -15 -12.5 28.2 3.0 14.5 
30 -19 -16.5 34.0 3.0 11.0 
______________________________________ 
Table 4 shows the scrubbing effect of ice water on the purity of the 
separated ice. These values are based on laboratory experiments, wherein 
the steps were carried out respectively with 500 ml. of solution with 25 
vol.-% of monoethylene glycol. In Experiment 1, the centrifuging time was 
1 minute, and in Experiment 2, this time was 6.5 minutes. No water was 
sprayed on. In Experiments 3 and 4, the centrifuging was conducted for 
respectively 1 minute, then water was sprayed on, and then another minute 
of centrifuging was carried out. It can be seen that the glycol content of 
the separated ice decreases at a comparable centrifuging time and, on the 
other hand, a great saving in centrifuging time can be obtained by the 
spraying on of water. 
TABLE 4 
______________________________________ 
Amount Glycol Glycol Glycol 
of Content Content Content 
Centri- 
Water of Centri- of Centri- 
of Sep- 
fuging Sprayed fuged Prod- 
fuged Prod- 
arated 
Time on uct I uct II Ice 
min. ml. vol.-% vol.-% vol.-% 
______________________________________ 
1 -- 27.8 -- 5.5 
6.5 -- 28.2 -- 3.0 
1+1 10 30 23.6 4.5 
(10 ml.) 
1+1 20 31 22.4 3.0 
(21 ml.) 
______________________________________ 
The regeneration of glycol solutions by freezing in accordance with this 
invention is, however, also markedly superior to the conventional glycol 
warm regeneration. During the warm regeneration, the glycol solution must 
be heated by about 100.degree. C. Besides, the heat of evaporation of the 
water must be provided which amounts to about 536 kcal/kg. of water. 
However, in the process of this invention, a cooling of the loaded glycol 
solution by 5.degree.-10.degree. C. is sufficient, and one must only 
provide the melting heat of the ice, which amounts to 80 kcal/kg. 
The process can be performed with mono-, di- or triethylene glycol. 
Monoethylene glycol is preferred because of its higher freezing point 
depression and lower viscosity and because the quantity by weight that is 
necessary for the process is smallest in the case of monoethylene glycol. 
The preceding examples can be repeated with similar success by substituting 
the generically or specifically described reactants and/or operating 
conditions of this invention for those used in the preceding examples. 
From the foregoing description, one skilled in the art can easily ascertain 
the essential characteristics of this invention, and without departing 
from the spirit and scope thereof, can make various changes and 
modifications of the invention to adapt it to various usages and 
conditions.