US2477737A - Method for dehydrating liquid carbon dioxide - Google Patents

Description

wr 2 m949- c. A. GETZ METHOD FOR DEHYDRATING LIQUID CARBON DIOXIDE Filed Jan. 28, 1944 N 1V mm.

denied ug. 2, 1949 METHOD FOR DEHYDRATING LIQUID CARBON DIOXIDE Charles A. Getz, Glen Ellyn, Ill., assignor, by mesne assignments, to Cardox Corporation, Chicago, Ill., a corporation of Illinois Application January 28, 1944, Serial No. 520,095

7 Claims.

This invention relates to new and useful improvements in methods and apparatus for dehydrating low temperature and low pressure liquid car-bon dioxide.

During the last few years the use of liquid carbon dioxide i'or certain purposes and operations has been increasing at a rapid rate because of the commercial development of methods and apparatus for handling, storing, and using liquid carbon dioxide at pre-selected low temperatures, and their corresponding low vapor pressures. The temperature most frequently selected at this time for commercial operations is F., which provides a vapor pressure of 305.5 pounds per square inch (absolute). However, temperatures ranging 'as low as 40 F. are at times employed for certain operations. l

Probably the greatest expansion in the use of this low temperature, low pressure liquid carbon dioxide, has occurred in the field of re extinguishment where the improved method of storing in large quantities, at controlled temperatures and pressures, and 'discharging at high rates has greatly enhanced the extinguishing characteristics of and has materially increased the kinds of hazards which can lbe protected by this medium.

New, or greatly expanded, uses of any material or substance invariably creates new handling and/or treatment problems and this increase in the use of liquid carbon dioxide is no exception. The present invention is directed to the problem of dehydrating low temperature, low pressure, liquid carbon dioxide to such a state of dryness that water will not be released in pipe lines, flow controlling valves, or the like, as a result of the flashing to vapor of a sufcient percentage of the low` temperature liquid carbon dioxide to provide the pressure diierential that is essential to eiect its flow through pipe lines, etc., under its own vapor pressure. It will be appreciated that even if only a very small quantity of water were to .be released into pipe lines and ilow control valves of a. re extinguishing system, employing this low temperature, low pressure liquid carbon dioxide as the extinguishing medium, each time the system were used, the water that would accumulate from several operations would be such that proper functioning of valves having small dimensional clearances between their moving parts would be prevented by the freezing of the water.

It is believed that a discussion at this point oi some of the physical properties of carbon dioxide will be helpful in arriving at a better under- 2 standing of the importance of the problem involved and the manner in which it is solved by the present invention.

Liquid carbon dioxide is a volatile liquid. This means that when liquid carbon dioxide is in equilibrium with vaporous carbon dioxide the pressure and temperature are inter-dependent. If the temperature of the liquid varies, the pressure exerted also varies. Carbon dioxide exists in liquid form only in the temperature range of from 69.9 F. to +87.8 F. This temperature range corresponds to a pressure range of approximately 60.4 pounds per square inch, gage pressure, to 1051.5 pounds per square inch, gage pressure. At the lower limit of this range, we arrive at what is called the triple point. Below this temperature and pressure range, liquid carbon dioxide does not exist; only carbon dioxide vapor and/or snow can be present. Above +8'7.8 F. liquid carbon dioxide does not exist. This upper temperature limit is known as the critical temperature.

An understanding of what takes place when low pressure, low temperature liquid carbon dioxide is discharged through a pipe line is irnportant. Obviously the only reason liquid carbon dioxide flows through a pipe line is because of a pressure diierential between any two given points along the length of said line. In order for a pressure of say 80 pounds per square inch to exist at a point of discharge, it is necessary, that the liquid which is present at the discharge be refrigerated to the corresponding temperature of F. If the temperature of the stored liquid is 0 F., it should be apparent that considerable heat must -be removed from the liquid before it reaches the point of discharge. The liquid carbon dioxide is lowered in temperature by evaporation. Part of the liquid carbon dioxide iiashes to vapor and the latent heat of vaporization cools the remaining liquid down to -60 F.

As the liquid carbon dioxide stored at 0 F., and 305.5 pounds pressure (absolute) is discharged irom a pipe line, part of the liquid evaporates to cool the remaining liquid to a lower temperature. When the pressure near the discharge point approaches the value of pounds per square inch (absolute) over 20 percent of the liquid carbon dioxide must have changed to vapor.

Each pound of liquid carbon dioxide in storage fills a volume of about .015 cubic foot. As soon as the liquid carbon dioxide begins to iiow through a pipe line, part of the liquid flashes to vapor and the total volume per pound of carbon dioxide increases materially. The volume of one pound of carbon dioxide as it approaches the point of discharge is about twelve times the volume of the pound of carbon dioxide as it leaves its storage. This, of course, means that for any given rate of tiow the velocity of the medium is much greater near the point of discharge than near the point of storage. That is to say, the pressure drop for any given length of pipe is greater near the discharge than near the storage.

The solubility of water in liquid carbon dioxide is given as .05 percent by weight in Quinn and Jones book entitled Carbon Dioxide," published as the American Chemical Societys Monogram Series No. '72. The range covered for this solubility is given in this book as from F. to 70 F. The solubility of water in liquid carbon dioxide has been studied in connection with this invention by means of a special fifty pound capacity liquid carbon dioxide storage tank having transparent plastic ends making it possible to observe the liquid carbon dioxide inside of the tank over a temperature range of from 88 F. on down to the triple point. Wet carbon dioxide was condensed in the tank and itwas observed that a layer of water formed below the layer of liquid carbon dioxide at 2 F. The density of liquid carbon dioxide at this temperature is 63.3 pounds per cubic foot. The maximum density of water is known to be 62.5 pounds per cubic foot. Consequently, one would expect that the liquid water present in the tank would float on top of the liquid carbon dioxide. As pointed out above, this was not the case under the conditions that the experimental tank was operated. Furthermore, it is rather surprising that liquid water exists at this low temperature of 2 F. It is apparent that the water is in a strange form.

The solubility of water in liquid carbon dioxide was also studied using this experimental tank. Water in excess of the solubility was added to a quantity of liquid carbon dioxide in the tank at near the critical temperature. The tank was agitated vigorously and the water allowed to settle to the bottom of the liquid carbon dioxide. A sample of liquid carbon dioxide was then decanted into a sample cylinder. This quantity of liquid carbon dioxide was evaporated and the vapor passed over a very e'icient drying agent. The increase in weight of the drying. agent gives the quantity of water present.

The volume of the vapor was measured, which through the density tables -gives aus the weight of the sample taken. This procedure of sampling for the determination of water was continued as the temperature of the liquid carbon dioxide was lowered. Eventually, a pressure of 60.4 pounds per square inch (gage) was reached in the tank. This corresponds to the triple point. The solubility of the water in the liquid carbon dioxide was found to vary from around 1.00

percent by weight at 70 F. to around .01 at With the above information on the solubility of water in liquid carbon dioxide in mind, let

us assume that 100 pounds of liquid carbon dioxide is discharged through a pipe line and that the storage pressure of the liquid is 300 pounds per square inch (gage) while the pressure at the discharge orifices is 100 pounds per square inch (gage). If this liquid carbon dioxide is saturated with water at the storage pressure it will contain at least .033 of a. pound of water.

As the pounds of carbon dioxide ow through the pipe line, approximately 18 pounds of the liquid carbon dioxide will flash to vapor. This means that 82 pounds of the original 100 pounds remains as liquid carbon dioxide. The 18 pounds of vapor cannot hold more than 0.00005 of a pound of water while the 82 pounds of liquid carbon dioxide cannot hold in solution more than 0.00656 of a pound of water. Consequently, the amount of water that is precipitated is equal to the original quantity (at least 0.033 of a pound). minus the water remaining in the vapor (0.00005 of a pound), minus the water held in solution in the liquid carbon dioxide (0.00656 of a pound), or 0.026 of a pound. As the temperature is below 0 F. throughout the length of the pipe line, the precipitated water has a great tendency to freeze to the pipe walls, and particularly to the intricate moving parts of fiow controlling valves.

From the above it should be apparent that if low pressure liquid carbon dioxide is saturated with water, there is grave danger that the moving parts of control valves will fail to function. In actual practice, this has been found to be the case.

It isthe primary object of this invention to provide methods and apparatus for removing water from low temperature, low pressure liquid carbon dioxide to an extent suflicient to avoid deposition of the .water when the liquid is caused to flow under its own vapor pressure through pipe lines, and the like, i

A more specic object of the invention is to provide methods and apparatus for dehydrating low temperature and pressure liquid carbon dioxide in connection with its storage in insulated and refrigerated tanks, or in connection with its transfer from one such tank to another.

Other objects and advantages of the invention will be apparent in the following description.

In the accompanying drawings which form a part of this specification and in which like numerals designate like parts throughout the same,

Figure 1 is a diagrammatic view illustrating the invention in a form wherein the dehydrated liquid carbon dioxide is kept separate from the' untreated carbon dioxide. and

Figure 2 is a partial diagrammatic view showing the invention in a form wherein the dehydrated liquid carbon dioxide is mixed with the untreated carbon dioxide.

In the drawings, wherein for the purpose of illustration are shown preferred forms of the invention, and first particularly referring to Figure 1, the reference character II designates an insulated storage tank containing wet liquid carbon dioxide from which water is to be removed. The liquid carbon dioxide is maintained at a desired low temperature, say 0 F. and its corresponding vapor pressure by a suitable cooling coil, not shown, in accordance with the disclosures of the -Geertz and Taylor patent, No. 2,180,231, issued November 14, 1939. A pipe line I2 connects tank II to evaporating chambers I3 and I4, which are arranged in parallel with respect to the pipe line, and flow from tank II to the two chambers is controlled by valves I5 and I6 in the pipe line. At the bottoms of the chambers I3 and Il for draining them are pipes provided with control valves I1 and I8 respectively. A pipe line I9 leads from chambers I3 and Il to a drying chamber 20 which may contain a drying agent, such as magnesium perchlorate or silica gel. Flow to chamber 20 from provided.

chambers I3 and I4 is controlled by valves 2I and 22, respectively, in pipe line I9. From chamber 20 a pipe 23 provides communication to a condensing chamber 24| from which a pipe 25 leads to an insulated tank 26 in which the dehydrated liquid carbon dioxide is stored at the same controlled temperature as in tank II by means of a cooling coil, not shown.

In order to supply heat to evaporating chambers I3 and I4 and to abstract heat from condensing chamber 24, refrigerating apparatus is This apparatus includes a heat transfer coil 21 in chamber I3 and another heat transfer coil 2B in chamber I4. Flow under high pressure of warm refrigerant vapor to coils 21 and 28 yis controlled by valves 29 and 30, respectively, along with control of refrigerant lflow from these coils, by corresponding valves 3l and 32, through a conduit 33 which connects coils 21 and 29 to a receiving chamber 34. A conduit 35 extends upwardly from chamber 34 to carry refrigerant to an expansion valve 36, from which the refrigerant proceeds under reduced pressure through expansion coil 31 located in chamber 24. `.A tube 38 leads from expansion coil 31 to a compressor 39. From the high pressure side of the compressor refrigerant vapor flows through a conduit 40 to coils 21 and 28 under the control of valves 29 and 30. Preferably a precooling coil 4I is placed in conduit 40 and tube 38 is in close heat exchange relation with a portion of conduit 40 between coil 4I and valves 29 and 30. The coil 4I, however, is not necessary and, if it is included as part of the apparatus, the close heat exchange rela- I#tion of tube 38 with conduit 40 may or may not also be included.

In the operation of this preferred form of the invention, wet liquid carbon dioxide is admitted from tank II, where it may be kept at F. and 305.5 pounds per square inch absolute pressure, to one of the evaporating chambers I3 and III by way of pipe line I2 and the corresponding control valve I5 or I6. The purpose of the two parallel chambers I3 and I4 is to permit use of one for evaporating -carbon dioxide, while the Vother is being thawed out so as to melt and purge out, through valve I1 or I8 as the case may be, the ice accumulated during its previous use. In chamber I3 or I4, the liquid carbon dioxide from tank II evaporates and absorbs heat from corresponding coil 21 or 28. In evaporating, the carbon dioxide loses most of its water because saturated carbon dioxide vapor can contain only a very small part of the water that saturated liquid carbon dioxide holds. Thus at 0'F. and 305.5 pounds per square inch absolute pressure, carbon dioxide vapor can contain at most 0.00191 percent water vapor while liquid carbon dioxide can dissolve about 0.05 percent water. Consequently all water over the 0.00197 percent separates out during the evaporation of the liquid carbon dioxide taken from tank I I. Incidentally, substantially all oil which in practice may be present in the liquid carbon dioxide is also removed. The water which separates out in the evaporating chamber I3 or I4 deposits in the form of ice and the carbon dioxide vapor passes off through pipe line I9 to drying chamber 20 Where more water may be removed from the vapor by means of a drying agent if it is desired to further dehydrate the carbon dioxide. From chamber 20 the carbon dioxide vapor proceeds through pipe 23 to condensing chamber 24, where it gives up Heat to the expansion coil or evaporator 31 of the refrigerating apparatus with the result that it coning chamber 34.

6 denses. From the chamber 24 the condensed dehydrated carbon dioxide flows through pipe 23 to storage tank 26 where it is maintained at approximately 0 F;

After a period of use for evaporating wet car- 4bcn dioxide from tank II, each evaporating chamber I3 or I4 requires purging because of the ice which has accumulated therein as a result of the continued evaporation of carbon dioxide and separation therefrom of water. which at the prevailing low temperature deposits in the form of ice. If Vfor example chamber I3 is considered as being in need of purging, valves I5 and 2I are closed and valves I6 and 22 are opened. whereby flow or carbon dioxide through chamber I3 is stopped and flow through chamber I4 is established. After the ice in chamber I3 thaws, valve I1 is opened and purging therethrough is eected of the water and whatever oil separated from the carbon dioxide during evaporation in chamber I3.

The cycle in the refrigerating apparatus may conveniently be considered starting with receiv- Liquid refrigerant ows up.i wardly from chamber 34 through conduit 38 to expansion valve 36, which is set to reduce pressure so as to create a temperature of about -10 F. in the refrigerant evaporator 31, in order to condense the carbon dioxide vapor in chamber 28 at a pressure of approximately 300 pounds per square inch. Vaporous refrigerant is drawn from expansion coil 31 through tube 38 to compressor 39. From the compressor the vaporous refrigerant passes through pre-cooling coil 4I and conduit 40 to valves 29 and 30. If chamber I3 is in use, valve 29 is open and the vaporous refrigerant, in passing through condenser or heat transfer coil 21, gives up heat to the carbon dioxide evaporating in chamber I3 and condenses. From coil 21 the condensed refrigerant flows through open valve 3| and conduit 33 to receiving chamber It 'for recycling. When chamber I3 is to be purged, ivalves 29 and 3l are closed and valves 30 and 32 are opened, whereby refrigerant passes through condenser or heat transfer coil 28 with a similar result.

Pre-cooling means 4I forms a part of the refrigerating system although it is not necessary that it take the form of a coil as shown. The pre-cooling means 4I may be used in coil form to increase operating eflciency somewhat. The refrigerating apparatus has more heat than is necessary to vaporize the liquid carbon dioxide passing through chamber I3 or I4, since heat is derived, not only from the compressor, but also from the atmosphere because of the low temperature at which the apparatus isoperating. The cooling coil is intended to dissipate the extra heat. Inasmuch as the entire appartus operates at temperatures near 0 F., pre-cooling coil 4I may be at too low a temperature to dissipate heat to the room air. If this condition should obtain, the temperatureof the pre-cooling coil may be raised by transferring heat from conduit 40 to tube 33, whereby the refrigerant vapor compressed by compressor 39 is superheated and the temperature of pre-cooling coil4 4I will be substantially above room temperature, even though the temperature of the refrigerant entering condensing coil 21 or 28 will be cooled to 10 F.

Since the .pressure differential between the condensing coil 31 and the vaporizing coil 21 or 28 can be relatively small, the efficiency of the compressor 39 will be extremely high. That is,

I also at F. and, therefore, only a small temperature differential is required between the said condensing and vaporizing coils.

The form of the invention shown in Figure 2 differs from that shown in Figure 1 in that from the condensing chamber 24 the condensed dehydrated carbon dioxide is carried back to storage tank il, by way of pipe line I2, tovbe commingled and recycled with the liquid carbon dioxide already therein. Hence the liquid carbon dioxide stored for use does not have the high degree of dehydration of that obtained by practicing the form of the invention illustrated in Figure l, since the dehydrated product on being returned to storage tank Il is contaminated with the wet carbon dioxide therein. Yet, after the liquid has been cycled s'everal times, a fairly dry liquid carbon dioxide is obtained which is suiiiciently dry for most purposes in connection with which the use of wet or bulk liquid carbon dioxide has not been satisfactory. Thereafter, because part of the product is consumed and replenished -with wet liquid carbon dioxide, the liquid stored in tank Il is maintained in a fairly dry condition without reaching the highly dehydrated condition of the product stored in tank 26 of Figure 1, although eventually a similar high degree of dehydration would be attained if there should be no occasion to add wet carbon dioxide to tank Il. This form of the invention is advantageous, when a very dry form of liquid carbon dioxide is not required, because of the lower initial cost resulting from the need of only one storage tank.

From the teachings of the aforesaid patent to Geertz and Taylor, it will be apparent that tank Il of either Fig. 1 or Fig. 2 may be a fixed storage tank or the tank of a transport vehicle while tank 26 of Fig. 1 may be either a fixed storage tank or the tank of a transport vehicle. That is to say, the dehydrating of the low temperature and pressure liquid carbon dioxide may be accomplished while transferring the liquid from one xed storage tank to another xed storage tank; from a fixed storage tank to a transport tank; from a transport tank to a fixed storage tank; or from one transport tank to another transport tank with the apparatus of Fig. 1. The apparatus of Fig. 2 may be considered as functioning to dehydrate the carbon dioxide while it is being stored in either a fixed storage tank or in a transport tank. A

It is to be understood that I do not desire to be limited to the exact order of steps as they have been disclosed, for variations and modifications 0f the same, which fall within the scope of the accompanying claims, are contemplated. It further is to be understood that the particular types of apparatus herein shown and described are to be taken only as preferred examples of the invention, and that various changes in the shape, size, and arrangement of parts may be resorted to without departing from the spirit of the invention or the scope of the subjoined claims.

Having thus described the invention, I claim:

1. A method of removing water from carbon dioxide, comprising passing wet liquid carbon dioxide in such relation to a higher temperatured liqueflable gas refrigerant that is in vapor phase as to eil'ect therebetween suiiicient heat exchange to liquefy the vaporous refrigerant and to vaporize the liquid carbon dioxide, collecting the water that is released by the vaporizing of the carbon dioxide, lowering the temperature of the refrigerant, passing the vaporous carbon dioxide in such relation to the lower temperatured refrigerant as to effect therebetween sumcient heat exchange to liquefy the vaporous carbon dioxide and to vaporize the liquid refrigerant, withdrawing the dried liquid carbon dioxide, and reconditioning the vaporous refrigerant so that it again can be passed in heat exchange relation to wet liquid carbon dioxide.

2. A method of removing water from carbon dioxide, comprising passing wet liquid carbon dioxide in such relation to a higher temperatured liqueiiable gas refrigerant that is in vapor phase as to eifect therebetween suiiicient heat exchange to liquefy the vaporous refrigerant and to vaporlze the liquid carbon dioxide, collecting the water that is released by the vaporizing of the carbon dioxide, passing the vaporous carbon dioxide through a drying agent to remove water vapor therefrom, lowering the temperature of the refrigerant, passing the vaporous carbon dioxide that has passed through the drying agent in such relation to the lower temperatured refrigerant as to effect therebetween suicient heat exchange to liquefy the vaporous carbon dioxide and to vaporize the liquid refrigerant, withdrawing the dried liquid carbon dioxide, and reconditioning the vaporous refrigerant so that it again can be passed in heat exchange relation to wet liquid carbon dioxide.

3. A method of removing water from carbon dioxide, comprising passing wet liquid carbon dioxide in su-ch relation to a higher temperatured liqueiiable v.gas refrigerant that is invapor phase as to effect therebetween suilicient heat exchange to liquefy the vaporous refrigerant and to vaporize the liquid carbon dioxide without raising its temperature, collecting the water that is released by the vaporizing of the carbon dioxide, lowering the temperature of the refrigerant, passing the vaporous carbon dioxide in such relation to the lower temperatured refrigerant as to effect therebetween sufficient heat exchange to liquefy the vaporous carbon dioxide without raising its temperature and to vaporize the liquid refrigerant, withdrawing the dried liquid carbon dioxide, and reconditioning the vaporous refrigerant so that it again can be passed in heat exchange relation to wet liquid carbon dioxide.

4. A method of removing water from carbon dioxide, comprising withdrawing from storage liquid carbon dioxide containing water in solution, evaporating the withdrawn liquid carbon dioxide by applying thereto only the latent heat of evaporation, said heat being obtained from a separate fluid medium of higher temperature than the liquid carbon dioxide, collecting the water that is released from the carbon dioxide as the latter is evaporated, liquefying the evaporated carbon dioxide by extracting therefrom only the latent heat through absorption thereof by the aforesaid separate fluid medium that has passed in heat exchange relation with the liquid carbon dioxide and which is of lower temperature than the carbon dioxide vapor as a result of having given up heat to the liquid carbon dioxide, and passing the dried liquid carbon dioxide to storage.

5. A method of removing water from carbon dioxide, consisting of flowing through a path of therefrom only the latent heat.

6. A method of removing water from carbon dioxide, comprising withdrawing from storage liquid carbon dioxide containing Water in solution, evaporating the withdrawn liquid carbon dioxide by applying thereto only the latent heat of evaporation, said heat being obtained from a separate fluid medium of higher temperature than the liquid carbon dioxide, collecting the water that is released from the carbon dioxide as the latter is evaporated, passing the owing carbon dioxide vapor through a drying agent to remove water vapor therefrom, liquefying the evaporated carbon dioxide by extracting therefrom only the latent heat, through absorption thereof by the aforesaid separate fluid medium that has passed in heat exchange relation with the liquid carbon dioxide land which is of lower temperature than the carbon dioxide vapor as a result of having given up heat to the liquid lcarbon dioxide, and passing the dried liquid carbon dioxide to storage.

'7. A method of removing water from carbon dioxide, consisting of flowing through a path of confinement liquid carbon dioxide containing water in solution, evaporating the owing liquid 10 carbon dioxide by applying thereto only the latent heat of evaporation, withdrawing from the flowing carbon dioxide the water that is released during said evaporation, passing the flowing carbon dioxide vapor through a drying agent to remove water vapor therefrom, and liquefying the flowing dried carbon dioxide vapor by extracting therefrom only the latent heat.

` CHARLES A. GETZ.

REFERENCES CITED The follofwing reierenlces are of record in the file of this patent:

UNITED STATES PATENTS Number Name r Date 1,119,011 Grosvenor Dec. 1, 1914 1,986,863 Terry Jan. 8, 1935 2,093,805 DeBaufre Sept. 21, 1937 2,341,697 Dennis Feb. 15, 1944 2,341,698 Dennis Feb. 15, 1944 FOREIGN PATENTS Number Country l Date 8269/32 Australia Aug. 3, 1933

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