Abstract:
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.

Description:
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 A dried gas is withdrawn from the head of the scrubbing tower in this process.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° 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° 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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a liquid-solid phase diagram of a monoethylene glycol-water mixture. 
     FIG. 2 is a schematic drawing of a defogging system. 
     FIG. 3 shows crystallization temperatures for aqueous mixtures of diethylene glycol and triethylene glycol. 
    
    
     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° C., is subjected to the process of this invention, it is sufficient to cool the solution to -12° to -15° 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° 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&#39;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 &#34;nutsch operation&#34; 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° C. The saving in glycol is 90%. Table 1(b) represents the values for diethylene glycol (outside temperature -4° C.; saving: 90%), and Table 1(c) shows the values for triethylene glycol (outside temperature -4° C.; saving: 88%). 
     
                       TABLE 1(a)______________________________________      Without     With      Regeneration                  Regeneration             Glycol            Glycol      Quantity             Content  Quantity Content      1./h.  vol.-%   1./h.    vol.-%______________________________________Runoff fromcooler 2 andmist precipitator 3        3455     21.0     3455   21.0Discharge from 6         576     21.0      466   2.5Reflux to the        2879     21.0     2989   23.8spray nozzle 4Glycol feed 9         121     100.0    11     100.0Sprayed 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 fromcooler 2 and 3315     27.5     3315   27.5mist precipitator 3Discharge from 6         625     27.5      472   4Reflux to the        2690     27.5     2840   31.6spray nozzle 4Glycol feed 9         175     100      17     100Sprayed 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 fromcooler 2 andmist precipitator 3        3280     32.5     3280   32.5Discharge from 6         665     32.5      480   5.5Reflux to the        2615     32.5     2798   37.2spray nozzle 4Glycol feed 9         218     100      26     100Sprayed 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° C. ± 0.5° 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______________________________________     Glycol     Content    Glycol     Separated     of Cen-    Content of Amount of IceCentrifuging     trifuged   Separated  Based on Ini-Period    Matter     Ice        tial Solutionmin.      vol.-%     vol.-%     vol.-%______________________________________0.3       22.5       4.0        101         24.0       3.7        165         23.9       2.6        15.510        22.5       1.0        9.520        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°-8° 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                                   ofGlycol Final     Ice      of     of     IceContent  Cooling   Point    Centri-                            Sepa-  Basedof Initial  Tempera-  of Initial                     fuged  rated  on InitialSolution  ture      Solution Product                            Ice    Solutionvol.-% ° C.            ° C.                     vol.-% vol.-% vol.-%______________________________________ 5     - 4.5     - 2.0    7.5    1.0    3510     - 6.0     - 4.0    13.8   1.8    2915     - 9.0     - 7.0    21.0   2.4    24.520     -11.5     - 9.5    24.5   2.5    19.025     -15       -12.5    28.2   3.0    14.530     -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  ContentCentri-  Water    of Centri- of Centri-                               of Sep-fuging Sprayed  fuged Prod-                      fuged Prod-                               aratedTime   on       uct I      uct II   Icemin.   ml.      vol.-%     vol.-%   vol.-%______________________________________1      --       27.8       --       5.56.5    --       28.2       --       3.01+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° 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°-10° 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.