Gas drier

Dec. 26, 1950

W. H. DAILEY, JR

GAS DRIER

Filed March 7, 1947

2,535,902

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William H Dailey Jr.

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Patented Dec. 26, 1950

2,535,902

UNITED STATES PATENT OFFICE

2,535,902

GAS DRIER

William H. Dailey, Jr., Library, Pa., assignor io Carnegie-Illinois Steel Corporation, a corporation of New Jersey

Application March 7, 1947, Serial No. 733,019

Claims. (Cl. 183—4.5)

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. δ

In the annealing of steel the gases which are used for. protective atmospheres are usually of such a nature that they require more or less complete drying before being used. When the protective atmosphere comprises nitrogen gas, or io 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. It has been customary to dry these gases by means of re- 15 frigeration, thus condensing out water vapor; or by using desiccants such as activated alumina, silica jell, etc. Frequently desiccants are used following refrigeration.

Desiccants of the type described possess the 20 property of adsorbing water vapor onto their surfaces in quantities up to 5 to 15% of their weight. The residual water, vapor content of the gas to be dried may be reduced to 0.01% or less, depending upon the particular desiccant, the 25 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 off the 30 water vapor. After the water vapor is driven off, the desiccant is cooled to a temperature sufficiently low to permit satisfatcory operation and is then ready to dry additional gases.

Drying equipment of the type described is not ₃₅entirely satisfactory due to the difficulty in heating and cooling the desiccant. It is usually the practice to embed heating and cooling coils in the charge of desiccant. The heating means may be steam, electricity, etc. In other eases, the desic- 40 cant is heated by blowing 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 45 these systems is the length of time that is required for reactivating, contamination due to air or combustion gases, and the difficulty in maintaining the embedded heating and cooling surfaces.

. It is among the objects of the present inven- 50 tian 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 55 •with means for .preventing the entry of air pr other contaminating gas.

The foregoing and other -objects will beeome ’ more apparent after referring to the accompanying specification and drawing, in the latter of which:

δ The drawing is a flow diagram illustrating an arrangement of equipment suitable for carrying out the methods of the present invention.

Referring more particularly to the drawing, the apparatus of the invention takes the form of 10 a unit comprising a frame, not shown, on which there is mounted a pair of vertically extending: . desiccant containers or drying towers 3A and 3B which are disposed in adjacent relationship and^;constructed and arranged for easy access to the 15 interiorly disposed desiccant. A gas inlet mani-fold S extends horizontally between and connectsthe upper ends of the drying towers 3A and 3B, the said manifold being provided with a gas inlet pipe 3 and valves 7A and 7B located at either 20 side of the pipe 8. A dry gas outlet manifold 9 extends horizontally between, and connects the lower ends of the drying towers 3A and SB, valves· ilA and (IB being provided for disconnecting said towers and a gas -outlet pipe ί 3 being con25 nected to the gas outlet manifold at a point which lies between the said valves 11A and 1 IB.

The above arrangement is conventional. It permits one tower to be placed in drying serviceand when the desiccant in this tower becomes 30 saturated shifting to a second tower, thus uninterrupted drying service is obtained. The saturated desiccant in the first tower is reactivated during the time the second tower is in drying service.

The unit of the present invention includes an. improved reactivating system including an external convection heater 12 -and an external convection cooler 1-4 through both of which gas. may be circulated as by means of the pump or 40 blower 18 which is shown as being intermediately disposed. The heater 12 and cooler 44 may be combined as a single heat transfer surface,’ or they may take the form of separate units, -as shown, which I prefer.

The towers 3A and 3B are connected to the outlet side of the heater 12 through a pipe 29 and a common header 22. Located in the header22 and on either side of the pipe 20 are disconnect valves 24A and 24B. The towers are con50 nected to the inlet of the cooler i 4 through a pipe 25 and a common return header 26. . Disconnect valves 28A and 28B are provided in theheader 2.6 on either side of the pipe ,26. .Pipe 27 of relatively small bore connects the dry gas 55 manifold 9 to the inlet side of the cooler 14. The pipe 27 can be termed a purge line which supplies fresh dry gas to the reactivating system. A

2,536,902 pressure relief valve 29 is provided in the return header 26 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 21. In this manner the water removed from the desiccant is gradually expelled from the reactivating system. An orifice plate can be substituted for the relief valve 29 if desired.

The normal cycle of operation is as follows:

The deoxidizing gas enters the drying apparatus through the common inlet connection 6 and is directed into one or the other of the drying towers. In Figure 1 valve 7B is shown closed and valve 7A is shown open; the gas to be dried is, therefore, directed into tower 3A. The gas then passes downward through the desiccant bed where the water vapor content is absorbed, and leaves the tower at the bottom from where it is directed into the common outlet connection 10, valve HA 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 flow of 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, 7 A and 11 A, and open valves IB and 1 IB, thus bringing tower 3B into drying service. . At the same time valves 24B and 28B are closed and valves 24A 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 (12—14) by means of the blower iS. The heat exchanger 12 is controlled to heat the gas to some maximum temperature, usually 600° F., when activated alumina is used as the desiccant. The hot gas heats the desiccant. As the temperature of the desiccant approaches 200° F., water vapor is. given off in very, large quantities, almost proportional to the amount of heat applied, so that a '.mixture of steam, and gas leaves the tower 3A at the bottom thereof through header 26.

; A portion of this mixture is bled off through the relief valve 29 and is replaced by fresh dry gas from the header 9 through the purge line 27. The steam is, therefore, gradually replaced with dry gas from the purging inlet connection. As the drying of the desiccant proceeds the temperature of the gas leaving the tower rises. When the gas leaving the alumina reaches a predetermined temperature 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 off and cooling is started; as by turning water into the cooler 14. Cooling is 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 25 are provided; such means are well known to the art arid have not, therefore, been shown.

The above mode of operation is necessary when the heater 12 and the cooler 14 are combined in a single uhit, 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 through the cooling heat exchanger, and additional steam to be removed by condensation in the cooler. This will allow a somewhat greater rate of reactivation, since the fan or blower 16 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 12 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.

The 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.

The 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 difficult 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 difference between the said levels. For example, thermocouples 36A and 37A, and 36B 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 and 3B, respectively. The thermocouples 36A and 37A (or 36B and 37B 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 37A which is nearer the exit of tower 3A remain at a higher temperature, thus indicating that the zone of adsorption is between the two and near the exit end of the tower A.

In addition, it is also proposed to automatically turn off the heater 12 during reactivation (when the exit gas reaches the desired temperature), turn on the cooling water, and automatically switch the inlet, outlet and 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 specific 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 may be devised within the scope of my invention, as defined in the appended claims.