US 2535902 A
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Description (OCR text may contain errors)
Dec. 26, 1950 w. H. BAILEY, JR
GAS DRIER Filed March '7, 194'? mgm..
Y customary to dry these gases by Patented Dec. 26 1950 GAS DRIER William H. Dailey, Jr., Library,
Carnegie-Illinois Steel Corporation,
ration of New Jersey Pa., assigner to a corpo- Application March 7, 1947, Serial No. 733,019
This invention relates to gas dryers and more particularly to such gas dryers as are used to dry the gases used for protective atmospheres in the annealing of steel, although not limited thereto.
In the annealing of steel the gases which are used forlprotective atmospheres are usually of such a nature that they require more or less com plete drying before being used. When the protective atmosphere comprises nitrogen gas, or nitrogen containing small percentages of deoxidizing gases such as carbon monoxide or hydrogen, it is particularly important `that there be a very low water vapor content. lt has been means of rcfrigeration, thus condensing -out water vapor; or by using desiccants such as activated alumina, silica jell, etc. Frequently desiccants are used following refrigeration.
Desiccants lof the type described possess the property of adsorbing water vapor onto their surfaces in quantities up to to 15% of their weight. The residual watervapor content of the gas-to be dried may be reduced to 0.01% or less, depending upon the particular desiecant, the quantity of incoming moisture, and other factors.
It is customary to employ two containers or adsorber towers containing the desiccant, and to operate one tower While the other `is disconnected and subjected to heat in order to drive ori the water vapor. After ythe water vapor is driven on?, the desiccant is cooled to a temperature sunlciently low to permit satisfatcory operation and is then ready to dry additional gases.
Drying equipment of the type `described is not entirely satisfactory due to the diiliculty in heating and 'cooling the desiccant. It is usually the practice to embedcheating and cooling coils in the charge of desiccant. The heating means may be steam, electricity, etc. In other cases, the desiccant is heated by Ablowing hot air or hot combustion gases through it. Cooling is accomplished by coils embedded in the desiccant, or, by external cooling coils through `which air or other gas `is circulated. The principal objection to these systems ,is the length of time that Ais required for reactivating contamination due to air r or combustion gases, and the dii'n'culty in maintaining the embedded heating and cooling surfaces.
'lt is among 'the objects of `the present invention to provide an improved gas dryer which `is not possessed of the shortcomings of the prior art, as recited hereinbefore.
,Another object is to provide a completely closed system of the class described, together with means for preventing the entry of air or other contaminating gas.
The foregoing and other objects will become' more apparent after referring to the accom,-
panying specialisation and drawing, in the latterof which:
llhe drawing is a flow diagram illustrating an arrangement of equipment suitable for carrying a unit comprising a frame, not shown, on which there is mounted a pair `of `vertically extending.
y desiccant containers or drying towers :3A and 3B which are disposed 'ln adjacent relationship and constructed and arranged :for easy access to the interiorly disposed desiccant. A gas inlet manie: fold extends horizontally between :and connects. the upper ends of the drying towers 3A and 3B,- tlie said manifold being provided with a gas inietpipe 6 and valves iA and 7B located at either side of the pipe t. A dr-y gas outlet manifold S extends horizontally between, and connects the lower ends of the drying towers 3A and SB, valves.'
HA and H2B being provided for disconnecting said towers and a gas outlet pipe it `being con--i nected to the gas outlet manifold at a point which lies between the said valves IIA and i I'B.
The above arrangement is conventional. Itpermits one tower to 'be placed iin drying service and when the desiccant in this tower becomes saturated shifting to a second tower. thus unin-V terrupted .drying service is obtained. The satu' rated deslccant in the first tower is yreactivated during the time the second tower is in drying service.
The unit `of the present invention includes an improved reactivating system including an exs.
ternal convection heater i2 and an external convection cooler i4 through both oi .which gas.
may be circulated as by means of the pump or blower it which is shown as being intermedia ately disposed. vThe heater l2 and cooler it mayL be combined as a single heat transfer surface,-y
or they may take `the form of separate units, as shown, uwhich I prefer.
The towers 3A and 3B are connected to the outlet side of 4the heater i2 `through a pipe 2t and a common lheader 2,2. Located inthe header- 22 and on either side of the pipe 20 are disconnect valves 24A and 24B. The `towers are con@ nected to the inlet ol the cooler i4 vthrough a pipe 25 and a common return header Dief connect valves 28A and 28B are 4provided in the pressure relief valve 29 is provided in the return header 20 between the valves 28A and 28B. The relief valve 29 exhausts to the atmosphere and is set to open at a pressure above atmospheric but below the pressure of the dry gas supplied from the heater 9 through the purge line 2T. In this manner the water removed from the desiccant is gradually expelled from the reactivating p system. An orice plate can be substituted fo the relief valve 29 if desired. .i Y
The normal cycle of operation is as follows:
The deoxidizing gas enters the drying appara-f.
valve IIA being open and IIB closed. Normal operating procedure is to use one tower for drying while the other tower is being reactivated, thus insuring a constant now oi dry gas to the annealing furnaces.
When the desiccant in tower 3A becomes saturated, a suitable procedure for the operation of the hereinbefore described apparatus is to close the gas inlet and outlet valves, iA and IIA, and open valves iB and IIB, thus bringing tower 3B into drying service. At the same time valves 29B and 28B are closed and valves MA and 28A are opened which will cause the gas remaining in tower 3A to be circulated through desiccant container of tower 3A and the heat exchangers (I2-I4) by means of the blower I9. The heat exchanger l2 is controlled to heat the gas to some 4 through the cooling heat exchanger, and additional steam to be removed by condensation in the cooler. This will allow a somewhat greater rate oi reactivation, since the fan or blower I6 would be operating at a low temperature and is able to circulate a greater mass of drier gas. The heat required for reactivation is not greatly increased by this modification since during the major portion of the reactivating time the majority of the heat imparted to the gas by the heater I2 is used in vaporizing water from the desiccant, i. e., the gas which enters the tower at about 600 F. leaves at about 200 to 250 F.
maximum temperature, usually 600 F., when aci tivated alumina is used as the desiccant. The hot gas heats the desiccant. As the temperature of the desiccant approaches 200 F., water vapor is4 given off in very large quantities, almost proportional to the amount of heat applied, so that aj'mixture of steam and gas leaves the tower 3A at .the bottom thereofthrough header 20.
1 A portion of this mixture is bled oir through the relief valve 29 and is replacedby fresh dry gas from the header 9 through the purge line 2l.l
The'steam is, therefore, gradually replaced with dry gas from `the purginginlet connection. As the drying o the desiccant proceeds the temperature of the gas leaving the tower rises. When the gas leaving the alumina reaches a predetermined ternperature of, for example, 550 F. (indicating that all of the desiccant has approached or exceeded this temperature), the drying is substantially complete and the heat is turned o and cooling is started, as by turning water into the cooler I4. Coolingis continued until the gas leaving the desiccant reaches a temperature low enough to insure efficient adsorption of the water vapor. For activated alumina, this temperature is 80 to 100 F. When this temperature is attained, tower 3A is ready for drying service, and tower' 3B can be removed from drying service and its desiccant reactivated. Means for measuring the temperature of the gas in the pipes 20 and 95 areprovided; such means are well known to the art and have not, therefore, been shown.
v The above mode of operation is necessary when the heater I2 and the cooler I4 are combined in a single unit, however, as previously stated I prefer to use separate heat transfer units for cooling and `heating as this permits water or other cooling medium to continuously flow vThe system thus far described, including the modification thereof, is a single stage system insofar as the desiccant is concerned and requires-that the temperature of the entering wet gas be low enough so that the entering temperature plus the rise in temperature due to condensation of moisture on the desiccant does not exceed the effective operating temperature of the desiccant as a moisture absorbent.L
rIhe entering temperature of the wet gas is commonly reduced to an acceptable value by the use of a conventional refrigerating machine which, as such, forms no part of the present invention and accordingly is neither shown nor described.
When using the apparatus of the prior art, it is extremely diicult to determine when the desiccant is approaching saturation, since usually the moisture content of the exit gas remains at an extremely low value until saturation of the desiccant occurs, and then rises very rapidly. The teachings of the present invention contemplate the control of the time in drying service by means of conventional temperaturemeasuring devices which are located at various levels in the desiccant to detect the level at which maximum adsorption occurs by measuring the temperature dierence between the said levels. For example, thermocouples 35A and 31A, and 39B and 37B or other temperature-measuring devices may be embedded in the desiccant at two different levels near the exit end of the towers 3A land 3B, respectively. The thermocouples A and 91A `(or 36B and 31B when tower 3B is in drying service) will read substantially the same until the zone of maximum adsorption passes the first of the two or thermocouple 36A. At this time, its temperature will drop, while that of the thermocouple 3A which is nearer the exit of tower 3A remain at a higher temperature, thus Vindicating that the zone of adsorption is between the two'and near the exit end of thel tower A.
In addition, it is also proposed to automatically turn ol the heater I2 during reactivation (when the `exit gas reaches the desired temperature), turn on the vcooling water, and automatically switch the inlet, outlet land recirculating valves when the adsorption zone reaches a predetermined level in the drying tower by means of these couples and conventional control equipment not shown.
While I have shown and described certain specic embodiments, of my invention, it will be understood that these embodiments are merely for the purpose of illustration and description and that various other forms Ymay be devised within the scope of my invention, as deiined in the appended claims.
. I claimz.
l. The method of providing a continuous supply of dry gas wherein at least two containers of desiccant are used, which method comprises passing the wet gas through 'a container of active desiccant while reactivating a second container of moisture saturated desiccant, said reactivating step including heating a portion of the gas to a temperature suicient to vaporize moisture in the saturated desiccant, forcibly circulating said heated gas through the moisture saturated desiccant to Vaporize said moisture, gradually purging said circulating gas of vaporized moisture by continually bleeding-off a portion of the gas after it leaves the container of desiccant being reactivated and replacing said bled-oii portion with dry gas issuing from the container of active desiccant; said heating, forced circulation and purging being continued until substantially all moisture is removed from the desiccant being reactivated, and then cooling the reactivated desiccant to its operating temperature by continuing the forced circulation and purging While cooling the circulating gas.
2. The method of providing a continuous supply of dry gas wherein at least two containers of alumina are used, which method comprises passing the wet gas through a container of active alumina while reactivating a second container of moisture saturated alumina, said reaotivating step including heating a portion of the gas to a temperature of about 600 F., forcibly circulating said heated gas through the moisture saturated alumina to Vaporiae said moisture, gradually purging said circulating gas of vaporized moisture by continually bleeding-off a portion of the gas after it leaves the container of alumina being reactivated and replacing said bled-off portion with dry gas issuing from the container of active alumina; said heating, forced circulation and purging being continued until the temperature of the gas leaving the container of alumina being reactivated approaches the temperature of the gas entering said container, and then cooling the reactivated alumina to a temperature of about 100 F. by continuing the forced circulation and purging while cooling the circulating gas.
3. The method of providing a continuous supply of dry gas wherein at least two containers of desiccant are used, which method comprises passing the `wet gas through a container of active desiccant while reactivating a second container of moisture saturated desiccant, said reactivating step including heating a portion of the gas to a temperature sufficient to vaporize moisture in the saturated desiccant, forcibly circulating said heated gas through said moisture saturated desiccant to vaporize said moisture, bleeding-off a portion of the moisture containing gas after it leaves the container of desiccant being reactivated and replacing said bled-oir portion with dry gas issuing from the container of active desiccant whereby the vaporized moisture is gradually purged from the circulating gas, cooling the circulating gas to further lower its moisture content; said heating, forced circulation, purging and cooling being continued until substantially all moisture is removed from the desiccant being reactivated and then discontinuing the heating but continuing the forced circulation, purging and cooling until the temperature of the reactivated desiccant is lowered to its operating temperature.
4. The method of providing a continuous supply of dry gas wherein at least two containers of alumina are used, which method comprises passing the wet gas through a container of active alumina while reactivating a second container of moisture saturated alumina, said reactivating step including heating a portion of the gas to a temperature of about 600 F., forcibly circulating said heated gas through said moisture saturated alumina to vaporize said moisture, bleeding-o a portion of the moisture containing gas after it leaves the container of alumina being reactivated and replacing said bled-01T portion with dry gas issuing from the container of active alumina whereby the vaporized moisture is gradually purged from the circulating gas, cooling the ciroulating gas to further lower its moisture content; said heating, forced circulation, purging and cooling being continued until the temperature of the gas leaving the container of alumina being reactivated approaches the temperature of the gas entering said container, and then discontinuing the heating but continuing the forced circulation, purging and cooling until the temperature of the reactivated alumina is lowered to about 100 F.
5. Apparatus for providing a continuous supply of dry gas comprising at least two containers of desiccant, a wet gas main, a dry gas main, a heat exchanger, a blower connected in series flow relationship with said heat exchanger, means for selectively connecting said containers to said gas mains, means for selectively connecting said containers to said heat exchanger and blower, means connected to said last named connecting means for bleeding-off moisture containing gas, and means connected to the last named connecting 'means for replacing said bled-off gas with ldry gas.
6. Apparatus for providing a continuous supply of dry gas comprising at least two containers of desiccant, a wet gas main, a dry gas main, a
i heat exchanger unit adapted to heat gas, a blower,
WILLLIAM I-I. DAILEY, JR.
REFERENCES CITED The following references are of record in the file of this patent:
UNITED STATES PATENTS Number Name Date 1,863,656 Hartman June 21, 1932 2,083,732 Moore June l5, 1937 2,160,831 Colby et al June 6, 1939 2,359,660 Martin et a1. Oct. 3, 1944 FOREIGN PATENTS Number Country Date 520,709 Germany Mar. 13, 1931