US 2475255 A
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Patented July s, 1949 METHOD OF DRYING GASES Walter F. Rollman, Cranfotd, N. J., assignor to Standard Oil Development Company, a corporation of Delaware ApplicationMarch 17, 1944, Serial No. 526,907
3 Claims. (Cl. 62-470) The present invention relates to improvements in the drying of gases and, more particularly, relates to an improved process in which gases, such as air, carbon dioxide. nitrogen, petroleum and natural gases, and others, are dried by contact with a cooled, granular, inert, solid material which is suspended in the water-saturated or partially water-saturated gas during the drying operation. This application is a continuation-inpart of my abandoned application, Serial No.
472,297, filed January 13, 1943, entitled, Chemical process.
Prior to my invention, a commonly used method of drying air to a very low moisture content, as
for example in blast furnace service, consisted merely of freezing out the water by passing the air over refrigerated tubes. This method is inefficient because the temperature differential between the refrigerant and the air must be high, due to the low thermal conductivity of the frost on the tubes; and two units must be used alternately to permit continuity of operation since the ice deposited on the tubes must be periodically removed, and in order to do this the refrigeration plant being defrosted must be out of service.
In the dehydration of natural gas, absorption, adsorption or refrigeration are the methods commonly employed. One of the earliest methods was that in which brine served as the means for chilling the gas to remove water vapor. Under high pressures of gas, the formation of gas hydrates,
crystalline compounds of water and gas, is likely to occur.
Th main object of this invention is to provide a method for drying gases, particularly air and natural gas, continuously, emciently and relatively at a much lower cost than heretofore possible.
A specific object of my invention relates to an improvement in the drying of air to make it suitable, for example, to support the combustion of a blast furnace, by contacting the moisture-laden air with a cooled circulating granular solid material at temperatures sufficiently low to cause particles of ice to form whereby the moisture is removed from the air, while at the same time the movement of the solid material prevents the ac cumulation of ice films anywhere in the dryer.
A further specific object is the provision of improved methods for drying natural gas, to increase pipe line capacity by preventing the formation of clogging ice deposits and to reduce corrosion due to the presence of excessive amounts of water.
Other and further objects of my invention will appear from the following more detailed description and claims.
In the accompanying drawing, I have shown a diagrammatic layout in which in Fig. 1, I have indicated a complete process for drying air, utilizing this as typical of other uses ,of my invenm exit pipe 45.
2 tion; and in Fig. 2, I have shown an alternate form of the dryer element proper.
In order to increase the understanding of my invention, I shall now describe a preferred modification thereof in detail, and in so doing I shall refer to the drawings in detail. In the views similar reference characters refer to similar parts.
In Fig. 1, wet air, say 80% saturated at a temperature of F., is discharged into the system through line I and thence passes in series through two heat exchangers 3 and 5 where it is cooled by heat interchange with ice and cold air from a subsequent portion of the system, as will subsequently appear. The chilled air at a temperature of about 40 F. is withdrawn through an eductor M which must be at the extreme bottom of the dryer to prevent plugging of the inlet line with frost. The moist air contacts in a centrally disposed tube I5 an inert, chilled granular solid material such as sand, the said material having a particle size of, say, from 20-50 mesh. The incoming air is very quickly cooled by contact with the granular solid material in tube l5 and by indirect heat transfer with a coolant which is discharged into cooling space 20 through inlet line 25 and withdrawn through line 30. The coolant material may be brine and may have an inlet temperature of 40 F. The air and granular solid material in tube l5 move concurrently upward at a superficial gas velocity of 2-20 feet per second where the particle size is from, say, 20-50 mesh. Due to the combined chilling effect of the cold solid and the coolant in space 20 the water in the air is frozen, but the motion of the particles prevents any accumulation of frost on them or on the surface of the tubes.
There may be some slippage of air past the solid inert particles. but in order that the particles have suflicient scouring action to prevent deposition of ice on the tube surface, superficial air velocity should not be much above that required to suspend the inert solid in the air stream. The ice is carried through tube It by the air as a fine dust mixed with the inert granular solid. From the top of tube IS the mixture of cold dry air, ice dust. and inert granular solid passes into a disengaging space 40. Here, due to the lowering of the superficial gas velocity to the order of say 0.2 to 3 feet per second, the inert granular solid separates from the air and gravitates through tubes 22, where it is rechilled, into the lower chamber 24 of the dryer l2 whenceit is again sucked into the lower end of tube i5 by the incoming air. The ice dust, which is finely divided and has a much lower specific gravity than the inert solid, is retained in suspension in the disengaging space and is withdrawn with the cold dry air from the top of the dryer I2 through From the dryer thecold dry air amazes with suspended ice dust passes into the centrifugal separator mounted on the top of the tower 50 superposed on and in open communication with heat exchanger 3. The ice dust removed from the air by the separator gravitates through the tower 50 and contacts the tubes 4 of heat exchanger 3, whereupon the ice is melted and withdrawn through a condensate outlet pipe 8.
The cold substantially dry air freed of ice is then withdrawn from tower 50 through pipe 55 and passed into heat exchanger 5 in the space surrounding the tubes I therein, where it abstracts heat from the incoming wet air passing through the tubes 1 and finally the dried air is recovered from the system through line 60. Water condensed out of the incoming air in exchanger 5 is withdrawn beyond the header 6| through pipe 62.
In Fig. 2, I have shown a modification of my invention in which a bank of tubes 26, shown in cross section, containing the coolant are disposed horizontally within the dryer and circulation of the solid inert material is indicated by the arrows in this figure. In this alternate method of operation, the circulating inert solid passes around tubes containing coolant, rather than through tubes surrounded by coolant. In the drawings, I have shown merely one riser tube I5 in the dryer, but it is to be understood that two or more such tubes may be used. Also, both the riser tube i5 and the coolant tubes 20 may be provided with spiral bafiies or fins in order to cause a turbulent fiow of the fluids through the same, and any other known expedient may be used which will increase the efilciency of the process.
In summary, the method of operation of.the dryer is as follows: The incoming air is very rapidly chilled by contact with the granular solid as the air passes through the heat exchanger, as indicated previously, and the moisture is removed from the air as ice. However, the ebullient and turbulent state of the solid granules prevents the deposition of frost or ice on the tube wall, and, to a large extent, on the surface of the particles of inert solid, and also prevents agglomeration of the ice particles themselves. The inert particles are carried through the central tube and into the disengaging space, from which they gravitateback down through the heat exchanger where they are rechilled and thence discharged to the bottom of thedryer and the cycle is repeated. The ice dust, because of its small particle size and comparatively low specific gravity, is not separated in the disengaging space but is entrained in the cold dry air passing out of the dryer, from which it is subsequently separated, as explained. It is an important feature of my invention that the velocity of the air and the particle size of the inert material are so adjusted that the ice dust is carried overhead with the air, but the inert solid is not withdrawn overhead through outlet pipe 45.
It should be pointed out that any inert material may be used provided it is dense and hard and does not dust readily. Such solids include, for example, sand, quartz, sintered clays, hard and rust-resistant metal shot, and the like.
Assume air at 70 F. and containing 1.1 lb. of water per 1000 cubic feet is to be dried. We may assume that the solid material in pipe ll is sand, since this material is perhaps as good as any from the standpoint of non-dusting, etc. In a representative run, 0.5 lb. of ice are melted in the'first heat exchanger 3 per 1000 cubic feet of air, during the prechilling of the entering air. Conditions are also controlled so that the incoming air in pipe I is at a temperature of about 40 F. The sand or other contact material which is at a temperature of about 20 F. to 30 F. in pipe l5, preferably is of a size of from about 20 to 50 mesh. The coolant in space 20 which is preferably brine solution (although other coolants such as liquefied sulfur dioxide, ammonia, etc.,'may be used) is preferably at a temperature of about 40 F. The air velocity in tube l5 may vary from 2 to 20 feet per second (superficial velocity) and from 0.2 to 3 feet per second in the disengaging space. The air leaving dryer i2 is at a temperature of about 10 F. It is possible to reduce the wet air mentioned above (1.1 lbs. water per 1000 cubic feet air) to a water content of about 0.2 lbs. per 1000 cubic feet of air.
Numerous modifications of my invention may be made by those who are skilled in the art without departing from the spirit thereof.
What I claim is:
1. A method of drying gas which comprises passing a moisture-containing gas upwardly through a cooling zone containing a body of granular material at a velocity sufiicient to keep said granular material in suspension within said .gas, maintaining said cooling zone below the temperature of said incoming gas and sufiicient to freez the moisture contained in said gas, removing gas containing ice particles from the upper end of said cooling zone and separating the ice particles from said gas.
2. A method of drying gas which comprises passing a moisture-containing gas upwardly through a cooling zone containing a body of granular material at a velocity sufficient to keep the granular material in suspension, maintaining said cooling zone below the temperature of the incoming gas stream and sufilcient to freeze moisture contained in said gas, removing a suspension of granular material, cooled gas and ice particles from said cooling zone, separating the granular material from said gas and ice particles, cooling said granular material below the temperature maintained in said cooling zone, returning the cooled granular material to said cooling zone and removing ice particles from said cooled gas.
3. In the process defined by claim 2, the further improvement which comprises reducing the velocity of the gas after removal from said cooling zone to cause said granular material to separate therefrom.
WALTER F. ROLLMAN.
REFERENCES CITED The following referenioes are of record in tho file of this patent:
UNITED STATES PATENTS Number Name Date 83,255 Chicester Oct, 20, 1868 1,148,331 Olsson July 27, 1915 1,974,145 Atwell Sept. 18, 1934 economy, availability.