|Publication number||US2249624 A|
|Publication date||Jul 15, 1941|
|Filing date||Jul 26, 1939|
|Priority date||Jul 26, 1939|
|Publication number||US 2249624 A, US 2249624A, US-A-2249624, US2249624 A, US2249624A|
|Inventors||Francis R Bichowsky|
|Original Assignee||Dow Chemical Co|
|Export Citation||BiBTeX, EndNote, RefMan|
|Referenced by (29), Classifications (10)|
|External Links: USPTO, USPTO Assignment, Espacenet|
July 15, 1941. F, R. BIYCHOWSKY METHOD FOR DRYiNG Filed July 26, 1959 M w o M T/ N5 E. v@ 5 C m ATTORNEY Patented July 15, 1941 UNITED STATES PATENT OFFICE METHOD FOR DRYING Application July 26, 1939, Serial No. 286,536
This invention relates to methods of drying materials. More particularly it concerns an improved method for removing moisture from materials by bringing into contact therewith a relatively dry atmosphere of controlled temperature and humidity, this atmosphere being subsequently conditioned for re-use by contact with a hygroscopic liquid.
One of the objects of the invention is to provide a highly efficient method of drying moist materials. Another object is to define a cyclic process for conducting drying operations under conditions which are substantially adiabatic and largely independent of the surrounding atmosphere. 1
With these and other objects in view, the present invention contemplates passing a stream of air of predetermined temperature and relative humidity in a closed system into contact with materials to be dried and directly thereafter into contact with a hygroscopic solution of controlled temperature and concentration, whereby the air is brought to the proper condition for recirculation in the process without having to be rejected or replaced in part with fresh air. This continued re-use of the same air permits large heat savings in the process.
The invention may best be understood with reference to the accompanying drawing which illustrates diagrammatically a preferred embodiment of the invention.
In the apparatus illustrated, drying is conducted in a chamber I with shelves 2 for holding the, material to be dried. Air is circulated through the drier in a closed circuit comprising a motor-driven air-circulating fan 3, an air delivery duct 4 connecting the fan to the chamber l, and an outlet duct 5 connected through a damper 6 to the air-conditioning chamber 1 and through a damper 8 to the by-pass 9, the opposite end of both the chamber 1 and the by-pass 9 being connected to a duct Ill and by it to the inlet side of the fan 3. The conditioning chamber 'I is part of a circulatory system for the hygroscopic liquid, the system also comprisinga reservoir II, a liquid sump l2, and a pump l3 connecting the sump through lines l4 and I5, and. a valve Hi to the reservoir ll. circulating system comprising the chambers l and I and their associated ducts, and the liquid circulating system comprising pump l3 and connecting piping, are preferably provided with insulating lagging (not shown) to prevent transmission of heat into or out of the system.
Inasmuch as the hygroscopic solution in the liquid circulating system becomes diluted during operation, a small portion of the solution is withdrawn from the line H through a valve I1 and is conveyed through a line [8 and a heatexchanger jacket I9 into an evaporator or regenerator 20 which is fired by a gas burner 2| controlled by a gas valve 22. trated liquor from the bottom of the regenerator is then returned to the liquid circulating system through a heat-exchanger coil 23, or a by-pass 24 controlled by a valve 25, and thence through a return line 26 leading to the sump I2. This line 26 is provided with a small heat exchanger 21 to which cooling water may be admitted as needed through a valve 28.
The concentration of the hygroscopic solution in the apparatus may be controlled by any desired means, suitably by varying the rate of evaporation .in the regenerator 20, e. g. by adjusting the gas-flow to the burner 2|. This control may be efiected automatically by means of a density indicator 29 receiving hygroscopic liquid through a by-pass 30, the density-responsive plummet 3| of the device being connected to the burner valve 22 (as shown by the solid line) in such manner that the gas flow to the burner is varied in accordance with the indications of the plummet 3|. The temperature and humidity of the air circulated through the driers are regulated by a thermostat 32 and a wet-bulb thermometer 33' located within the drying chamber l, the thermostat 32 being adapted to actuate the valve l6 controlling the rate of circulation of the hygroscopic liquid, and the Wet-bulb thermostat 33 being operatively connected to valve 24 controlling the gby-pass 23 (as shown by the full line), and simultaneously or alternatively to the valve 28 controlling the cooler 21 (as shown by the broken line).
In operating the process according to the invention, the materials to be dried are placed upon the shelves 2 or otherwise conveyed into the drying chamber, and the fan 3 is set in motion causing the air in the system to circulate through the chamber I in contact with the material to be dried. In this chamber, since in 0p- Both the aireration the partial pressure of water in the air is less than the vapor pressure of the moisture in the material being dried, this moisture will evaporate into the air stream, such evaporation cooling the air and increasing its humidity. Inasmuch as the chamber l is thermally insulated, this evaporation and cooling occurs under essentially adiabatic conditions, and the air remains The reconcen- I at substantially constant total heat, 1. e. at constant wet-bulb temperature.
The cooled moist air leaving the chamber I passes through the duct 5 and damper 6 into the conditioning chamber 1, the by-pass damper 8 being closed in this mode of operating. In the chamber I the moisture-laden air is brought into contact with a hygroscopic liquid circulated by the pump I3, either in the form of a shower or spray, or in distributed form upon a suitable medium such as cotton strings suspended from the reservoir H, as shown. In this air-liquid contact zone the hygroscopic liquid absorb part of, usually most of, the moisture from the air, such absorption being accompanied by a 1ibera-. tion of the heat of condensation and solution of the water vapor, this heat raising the temperature of the hygroscopic liquid, which, in turn, transfers a part of this heat back to the air in contact therewith. The air, thus warmed and dried, is ready for recycling in the process through the duct ID to the inlet of the fan 3. As operation is continued, the hygroscopic liquid being circulated into contact with moist air in the chamber 1 continues to heat up, as explained, until it reaches and remains at some temperature above that of the incoming air, at which tem perature the amount of heat transferred from the liquid to the air is exactly equal to the heat liberated in the dehumidification of air. Now, since the chamber '1 is thermally insulated and since, as will be explained, the hygroscopic liquid circulating through the reservoir ll, sump l2, pump l3, and pipes l4 and I5 does not change in temperature appreciably and is maintained at constant composition, it is seen that the heating and dehumidification of the air occurring in the chamber 1 takes place under adiabatic conditions, i. e. the air remains at constant heat content and hence at a constant web-bulb temperature.
As hereinbefore explained, the hygroscopic liquid being circulated through the chamber 'lis maintained at constant concentration by withdrawing a small portion, e. g. 5-10 per cent of the total, and removing water therefrom, as in the evaporator 20, which is controlled by the density-responsive plummet 3|. The reconcentrated liquor returned from the evaporator through the pipe 26 should be at substantially the same temperature as the diluted portion withdrawn through line l8 in order to avoid adding or subtracting heat from the otherwise adiabatic circulatory system for the hygroscopic liquid. This necessary'temperature control may given off by the material being dried. If a condition of inequality prevails, the temperature and humidity of the air passing into the drying chamber I will not remain constant, but will vary somewhat as the drying proceeds. In certain instances this variation is of no great consequence, but for drying numerous materials, especially heat-sensitive products, such as gelatine, it is highly desirable that the drying air be supplied at constant conditions of temperature and humidity.
In order to maintain a constant temperature and humidity of the drying air, it is necessary to insure: (1) that the air-circulation system remain essentially adiabatic, that is, at constant wet-bulb temperature, and (2) that the airreconditioning step in chamber 1 be carefully controlled. These requirements will now be considered in detail.
(1) The process of the invention, since it involves closed cycles of both the air and the hygroscopic liquid, would of necessity remain adiabatic if it were not for the possibility of heat loss or gain in the liquid drawn off to the rgenerator 20, for heat transfer by radiation between the room and the walls of the chambers l and I and connecting ducts and piping, and for slight frictional effects. In the preferred operation of the process, there is little heat loss or gain in the regenerator circuit since the reconcentrated liquor may, by means of the heatexchanger 23, be returned through the pipe 25 at substantially the same temperature as the diluted liquor withdrawn through the pipe l8, as hereinbefore explained, However, although heat transfer to and from the room is minimized by covering the apparatus with heat insulation, it can rarely be entirely eliminated. To this end, a control instrument responsive to variations in the total heat content of the system, such as a wet-bulb thermometer 33, is placed in the drying chamber 1. When the apparatus is to be run at a temperature above that of its surroundings, so that the total heat content of the system tends to decrease because of radiation losses, the wetbulb thermometer 33 is connected operatively to the valve 25 controlling the heat-exchanger by-pass 24, as illustrated; the valve 28 controlling the cooler 21 is kept closed. Then, when the total heat of the system falls, the thermometer 33 drops slightly in temperature, opening the valve 25, and allowing some of the reconcentrated liquor from the evaporator 20 to by-pass be efiected by any desired means, as by operat it has been found that unless means are employed to control the air-reconditioning step in the chamber I there is no assurance that the amount of water absorbed by the hygroscopic solution will precisely equal the amount of water theheat-exchanger 23. Under this condition the. reconcentrated liquor returns to the circulatory system through the pipe 25 at a higher temperature than the diluted liquor leaving throng ,jthe line It. Heat is accordingly added to the liquid circulatory system and is, of course, transferred to the air-circulatory system, since the two are in contact in the chamber 1, so that the total heat content of the entire apparatus rises until the desired value is reached, at which time the wet-bulb thermometer operates to close the valve 25. On the other hand, when the drying apparatus is to be run at a temperature lower than that of its surroundings so that the total heat of the system tends to increase by radiation into the chambers l and 1, the wet-bulb thermometer is connected operatively to the valve 23 controlling the cooler 21, and the by-pass valve 25 is closed. Thus, when the total heat of the system rises, the thermometer 33 operates to open the valve 28, cooling the reconcentrated liquor returning to the system in the pipe 25 to a temperature below that of the .diluted liquor leaving the pipe I3. Heat is extracted from the entire system until the desired heat content is attained, when the thermometer 33 closes the valve 28.
In an alternative structure of the apparatus, not illustrated, the heat-exchanger 23 may be omitted, in which case the small heat-exchanger 21 is connected ,so as to be either a heater or cooler in response to the demands of the wetbulb thermometer 33. In fact, in the broadest sense of the invention, the wet-bulb thermometer 33 may be used to control make-up heating or cooling means located anywhere within the air or hygroscopic liquid circulating systems, since these systems are inter-communicating and are otherwise adiabatic. It is thus evident that the wet-bulb thermometer 33 is essentially a means responsive to variations in the total heat content of the air and hygroscopic liquid circulatory systems and adapted to actuate means for increasing or decreasing the total heat content of the two systems.
(2) As hereinbefore noted, if the drier is to" be operated on air of constant temperature and humidity, not only must the system remain at substantially constant total heat, but in addition the air reconditioning step in the chamber 1 must be carefully controlled so as to maintain constant conditions. That is, the air-hygroscopic liquid contacting step must be operated so that the effectiveness thereof, i. e. the quantity of moisture removed from the total quantity of air after each passage through the chamber I, is such that the amount of water absorbed by the hygroscopic solution equals the amount of water given oil by the'material being dried. This effectiveness depends upon the rate of flow of air through the chamber 1, the rate of flow of hygroscopic solution from the reservoir II into the sump I2, and the concentration of the solution. In the apparatus illustrated, the rate of air flow remains constant as long as the damper 8 is closed and the fan 3 operates at constant speed; the concentration of the hygroscopic solution is held constant by the density-responsive plummet 3I. The effectiveness of the reconditioning step is then dependent only on the rate of flow of hygroscopic liquid. Accordingly, to provide automatic control, the valve Iii governing this flow is connected operatively to a dry-bulb thermometer or thermostat 32, placed in the drying chamber I, as shown, and set at the drying temperature desired. Then, when the temperature of the air-reconditioning step in chamber 1.
Thus, if the valve I6 and density-device 29 be kept at a constant setting, the thermostat 32 may be connected so as to vary the speed of the fan 3, or to control the quantity of air passing through the chamber 1 by automatically controlling the relative positions of the dampers 6 and 8. Again, if the valve l6, fan 3, and dampers 8 and 8 are at a' constant setting, the thermostat 32 may be connected to control the concentration of the hygroscopic liquid, e. g. by varying the addition of heat to the evaporator 23 by means not illustrated, or by adding additional quantities of one of the components to the solution. In these instances, the density control 29 and auxiliary parts are omitted. In any of these modifications, the thermostat 32 may be considered as a means responsive to variations in temperature in the drying chamber I and adaptditioning operation in the chamber I.
As will be evident from the foregoing, the ultimate function of the thermostat 32 and the wetbulb thermometer 33 is to control the temperature and humidity of the air entering the drier I. The wet-bulb thermometer 33 has hereinbelore been discussed as means responsive to the total heat content of the system, i. e. wet-bulb temperature. However, as long as the dry-bulb temperature is under control, the total heat content and humidity are interdependent, so that the wet-bulb thermometer 33 may equally well be considered as means responsive to the humidity of the air. Thus, it has been found that any other humidityor moisture-responsive instrument, e. g. any hygrostat, will operate equally aswell as the wet-bulb thermometer 33. In-
deed, since in the apparatus the temperature and humidity are both under independent control in an essentially adiabatic system, it has been found possible to interchange the connections of the thermostat 32 and humidity device 33 without greatly affecting the operation of the apparatus. These interchanged controls are not, however; quite as rapidly responsive as controls connected .as illustrated, and hunting may at times occur.
The process as particularly described above has been with reference, first, to adiabatic drying in the absence of any careful control of temperature or humidity, and, then, as is preferable, with temperature and humidity carefully maintained constant by suitable controls. However, with certain materials it is desirable to change gradually the state of the air entering the chamber I during the course of the drying operation. This can be accomplished according to the present invention by constantly or intermittently changing the setting of the thermostat 32 and hygrostat 33 in any prescribed manner, the operation being otherwise as described.
The process of the invention may be carried out using any hygroscopic liquid or solution, as sulfuric acid, glycerine, aqueous solutions of calcium and lithium halides, and the like. The process is not limited to the removal of water by air drying, but is equally applicable to the removal of any volatile liquid frommaterials moist therewith, using any inert gas as the circulating medium, and any suitable absorbent liquid capable of being reconcentrated.
It will be appreciated that in the process of the invention the drying operation in the chamber I, the air-reconditioning operation in the chamber I, and; the circulating system for the hygroscopic liquid all are maintained under sub-= stantially adiabatic conditions. It is entirely unnecessary to provide external means for heating the air going into the drying chamber or to discard any of the air leaving such chamber, as is necessary in most drying methods hitherto known. The only heat requirement of the process, besides that for slight radiation losses, if any, is the heat'required to reconcentrate the hygroscopic solution. This quantity of heat my co-pending application Serial No, 245,401,-
filed December 1'7, 1938, which, in turn is a continuation-impart of my prior application Serial No. 13,968, filed March 30, 1935. I
Other modes of applying the principle of the invention may be employed instead of those explained, change being made as regards the details disclosed, provided any of the steps or means stated in any of the following claims or the equivalent of such stated steps or means be employed.
1. The method of removing a volatile liquid from a material wet therewith which comprises: circulating a body of an inert gas in a closed system through a drying zone wherein it is contacted with the wet material and removes part of the volatile liquid therefrom; as vapor, then at least in part through a reconditioning zone wherein it is contacted with an absorbent liquor which removes part of the vapor of the volatile liquid and reconditions the gas for re-use, and finally back to the drying zone; circulating the absorbent liquor employed in the reconditioning zone into re-use in said zone; and withdrawing absorbent liquor from the liquor circulatory system, reconcentrating the same to an extent suf: ficient to maintain the concentration of the entire body of absorbent liquor substantially conthe gas and liquor circulatory systems and the surrounding atmosphere and thus to maintain both the said systems at substantially constant total heat content, and returning the reconcentrated liquor to the liquor circulatory system;-
2. The method of removing a volatile liquid I from a material wet therewith which comprises:
circulating a body of an inert gas in a closed system through a drying zone wherein it is contacted with the wet material and removes part of the volatile liquid therefrom as vapor, then at least in part through a reconditioning zone wherein it is contactedwith an absorbent liquor which removes part 01' the vapor of the volatile liquid and reconditions the gas for re-use, and finally back to the drying zone; circulating the absorbent liquor employed in the reconditioning zone through a closed system into re-use in said zone; and withdrawing a small portion of the absorbent liquor from. the closed liquor circulatory system, reconcentrating the same to an extent suflicient to maintain the concentration of the entire body of absorbent liquor substantially constant, adjusting the temperature of the reconcentrated liquor to a degree sufllcient to compensate for small heat exchange effects between the gas and liquor circulatory systems and the surrounding atmosphere and thus to maintain both the said systems at substantially constant total heat content, and returning the reconcentrated liquor to the liquorcirculatory system. FRANCIS R. BICHOWSKY.
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|U.S. Classification||34/473, 34/86, 34/77|
|International Classification||F24F3/14, F26B21/08|
|Cooperative Classification||F24F2003/144, F26B21/083, F24F3/1417|
|European Classification||F24F3/14C1, F26B21/08B|