US2480093A

US2480093A - Method of and apparatus for pumping liquid oxygen

Info

Publication number: US2480093A
Application number: US488650A
Authority: US
Inventors: Carl R Anderson
Original assignee: Air Products Inc
Current assignee: Air Products Inc
Priority date: 1943-05-27
Filing date: 1943-05-27
Publication date: 1949-08-23
Anticipated expiration: 1966-08-23
Also published as: FR926711A; BE487786A; USRE23799E

Description

c. R. ANDERSON 2,480,093

METHOD oF AND APPARATUS FOR PUMPING LIQUID OXYGEN 6 Sheets-Sheet l Aug. 23, 1949.

Filed May 27, 194:5

Aug. 23, 1949. c. R. ANDERSON METHOD 0F AND APPARATUS FOR PUMPING LIQUID OXYGEN Filed May 27, 1943 e sheets-sheet 2 Aug. 23, 1949. c. R. ANDERSON METHOD OF AND APPARATUS FOR PUMPING LIQUID OXYGEN 6 Sheets-Sheet 3 Filed May 27, 1943 INVENTOR. AR A. ANofRso/V BY /mfmyzm/w Aug. 23, 1949. c. R. ANDERSON METHOD oF AND APPARATUS FDR PUMPING LIQUID OXYGEN Filed May 27, 194s 6 Sheets-Sheet 4 mme INVENTOR.

CARL R. ANDERSQN ATTORNEY Aug. 23, 1949. Q R, ANDERSON 2,480,093

METHOD OF AND APPARATUS FOR PUMPING LIQUID OXYGEN Filed May 2'7, 1943 6 Sheets-Sheet 5 CARL l2 ERSU/Q BY y ATTOBNE Y l- 23 1949 c. R. ANDERSON 2,480,093

METHOD OF AND APPARATUS FOR PUMPING LIQUID OXYGEN Filed May 27, 1945 6 Sheets-Sheet 6 CARL. R. ANDERSON ATTORNEY Patented Aug. 23, 1949 METHOD F AND APPARATUS FOR PUMP- lNG LIQUID OXYGEN Cari R. Anderson,

Detroit, Mich., assignor to Air Products Incorporated, Detroit, Mich., a corporation o! Michigan Application May 27, 1943, Serial No. 488,650 Claims. (Cl. 62-122) l This invention relates to a method of pumping liqueed gases and to an apparatus adapted to that use.

An object of the invention is to provide a method and means for withdrawing a liqueed gas from a vessel in which it is stored or is being collected, in such manner as to avoid the POS- sibility of gas locking the pump.

An object of the invention is to provide a method and means for withdrawing a stream of liquefied gas of any desired constant quantity from a collecting pool in a gas fractionating column.

An object of the invention is to provide a method and means for withdrawing a stream of gas from a fractionating tower and of reducing the gas to liquid form for delivery under pressure by means adapted to the pumping of liquids.

An object of the invention is to provide a method and means for pumping liqueed oxygen directly from a. pool of commercially pure oxygen in` a fractionating tower to cylinders or pipe lines in which gaseous oxygen is transported under high pressure, thereby avoiding the requirement for an oxygen storage tank and a gaseous oxygen compressing system.

An object of the invention is to provide a method and means for withdrawing oxygen in gaseous form from the pure oxygen vapor space in a fractionating tower, for liquefying the gaseous stream and for delivering the oxygen into pressure cylinders or pipe lines, thereby retaining in the tower any lubricating oil or other combustible substances which may enter the tower with the air stream.

While the invention is applicable to the handling of all liqueed gases (liquids having a boiling point so much below atmospheric temperature that heat leakage into insulated apparatus is likely to produce difficulties in pumping), it is found most useful in connection with the pumping of liquid oxygen because of the very low atmospheric-pressure boiling point of this liquid and the fact that the presence, in the apparatus containing the compressed gas at atmospheric temperature, of any trace of carbonaceous substances is the source of extreme danger.

The invention will therefore be described in connection with the manipulation of oxygen, it being understood that such description is illustrative and not limiting.

In the attached drawings the invention is illustrated schematically, to wit:

Fig. 1 illustrates a form in which oxygen in the liquid state is pumped from the pure oxygen pool in a. single fractionating column and the stream is cooled below its boiling point, at the minimum 2 pressure existing in the pump cylinder, by heat exchange against gaseous product nitrogen from the top of the column.

Fig. 2 illustrates a form in which the same steps are performed in connection with a double or two-stage fractionating col Figure 3 illustrates a modification of Figure 1 wherein product oxygen is taken off as a. vapor and condensed against the feed air going to the top of the column.

Fig. 4 illustrates a modication of Fig. 1

wherein the product nitrogen fromv a single fractionating column passes directly to the heat interchanger, and the entering liquid air feed after being cooledt in the boiling coil immersed in the lower section of the column is used to condense the gaseous oxygen stream as well as to subcool the condensed oxygen stream and cool the pump.

Figs. 5 and 6 illustrate modiications in which all the steps of cooling are applied to the oxygen product of a double fractionating column by interchange with intermediate fractionation prod= ucts of the column.

The fractionating equipment illustrated is conventional and any preferred form of either single or double column may be used.

For the purpose of illustration, the single column apparatus of Figs. 1 and 3 consists essentially of a, double pass heat interchanger I0, having two banks of tubes II-Ii and Ill-I2, together with a fractionating column I3 provided with a plurality of bubbling plates A, and a boiling coil I4 in the base.

The ilows through this system, which also are conventional, are as follows: Air under pressure, from a source not shown, enters the system through feed pipe I 5, passes through tubes II-IL thence through pipe I6 to the boiling coil Il, thence through pipe l1 and expansion valve I8 to the top of the column, which it enters in a form largely liquid. Flowing downwardly through the column itis fractionated in the wellknown manner, pure liquid oxygen collecting in a pool I9 in the base of the tower while an impure gaseous nitrogen leaves the top of the tower through pipe 20.

The gaseous nitrogen, after a portion of its refrigerating eiect has been utilized in a manner which will be described, flows through pipe 2| to the shell of interchanger I0. The oxygen is withdrawn from the column through pipe 22, and after various manipulations which will be described, passes through pipe 23 to the tube bank |2|2 of interchanger I0.

In passing through the interchanger in counterow to the entering air stream, the product gaSDuS Oxygen,

oxygen and nitrogen are brought substantially to atmospheric temperature and pressure in withdrawing heat from the air feed, which is further cooled and liqueied in supplying heat to the pure oxygen in pool I9.

The double column system illustrated in Figs. 2, and 6 may have the same interchanger l0, but the fractlonating column consists of two sections 24 and 24', each supplied with bubbling plates A and A'. The upper section is provided with a condenser 25, the condensate from which drains into the lower section of the tower.

The cooled high pressure air from interchanger l0 passes through boiling coil i4, immersed in a pool 26 of crude (e. g. 35%) oxygen in the base of the lower section and thence, through pipe l1 and expansion valve i8, into the lower section at a medial height. This section fractionates the feed in the well-known manner, substantially pure gaseous nitrogen rising into the condenser 25, which is immersed in a pool 21 of pure oxygen collecting in the base of the upper section. As this section is maintained at a materially lower pressure than the lower section, the condenser acts as a reboiler for the pure oxygen and returns the nitrogen as a liquid into pool 28, from which a portion overflows to act as reflux in the lower section of the column while the remainder is transferred through pipe 20 and expansion valve 30 to the top oi' the upper section, in which also it acts as reiiux. The crude oxygen is transferred through pipe 3l and expansion valve 32 to a medial point in the upper section, in which it is fractionated in the well-known manner to substantially pure oxygen and slightly impure nitrogen.

At this point in either system we have two products-nitrogen and oxygen-each at a temperature which slightly exceeds its atmospheric pressure boiling point. These temperatures are approximately 193 centigrade (at 5 pounds gauge) for nitrogen and -1'79 centigrade (at 6 pounds gauge) for oxygen.

It is very desirable in many cases to conduct the oxygen product directly to the cylinders or pipe lines in which it is transported as a compressed gas, at pressures ranging up to 2500 or more pounds per square inch. As it is unduly costly (though common practice) to bring the liquid oxygen to the gaseous form and thereafter compress it, it is highly desirable to pump it as a liquid and vaporize it prior to entering the pipe line or storage vessel, thus saving an important amount of power.

A further advantage in pumping the oxygen in liquid phase lies in the avoidance of use of the aqueous lubricants required in compressing these lubricants introducing water vapor which must be removed by chemical drying and adsorption to obtain dry oxygen in the transport cylinder or pipe line.

The step of pumping liquid oxygen has Proven in practive to be one of great difilculty. The liquid is, in the nature of the case, at its boiling point at the existing pressure. From this it iollows that any reduction in pressure, such as is occasioned by fluid friction in the pump suction, or any increase in temperature due to leakage of heat into the pump body or to frictional heat transmitted into the liquid, will cause the evolution of gas which locks the suction and puts the pump out of commission. A further cause of vapor lock is back leakage through the discharge valve, the high pressure leakage liquid partially flashing to the gaseous state.

A'the pressure existing in Many attempts to solve this problem have been made, but so far as I am aware, no method has heretofore been proposed which gives satisfactory results in pumping directly from the pool in the fractionating column, without the interposition of a storage vessel.

I have solved this problem by two steps which are preferably used together but may be used individually. The first is to utilize a small portion of the cooling effect available in the gaseous product nitrogen for cooling the steam of liquid oxygen, on its way to the pump, to a temperature below that corresponding to its boiling point at the pump cylinder during the suction stroke. The second is to utilize another small portion of the refrigerating value of the nitrogen in cooling the pump cylinder.

In the various figures of the instant application, the oxygen pump 33 may be any pump capable of handling liquid at high pressure but is here illustrated as a single acting plunger pump, having a suction valve 34, a discharge valve 35, a cylinder 36, a plunger 31, a rod 3l, a crosshead 39. a connecting rod 40, a crank 4I, a worm gear 42, a driving pinion 4l and an actuating motor 44.

In the form of the invention shown in Fig. l, the pure oxygen collecting in pool Il flows through pipe 22 to one side of an interchanger 45, thence through pipe 46 and suction valve 34 into the pump cylinder on the up-stroke of the plunger. On the down stroke the liquid passes through discharge valve 35 and pipe 23 to tube bank I2 of interchanger i0, in which the stream is brought to atmospheric temperature and the gaseous condition. and is discharged at any desired pressure through pipe 4l. If desired, the stream in pipe 23 may be directed to a storage vessel in liquid condition, but more usually it will be passed through the interchanger and delivered by pipe 41 to pressure cylinders or other pressure vessels or to pipe lines in which it is transported under pressure.

The stream of gaseous nitrogen in pipe 20 is passed through the opposite side of interchanger 45, preferably in counter-flow to the stream of liquid oxygen, and the liquid is thus cooled to a temperature below its boiling point at column pressure, preferably from 6 to 8 C. below.

The stream of nitrogen which, because of its relatively great mass, has been only slightly elevated in temperature, iiows through pipe 4l and a coil 49 wrapped around the pump cylinder, in

-which it acts to Withdraw any heat transmitted to the liquid in the pump and tends to maintain the low temperature imparted to the liquid in interchanger 45.

From this coil the gaseous nitrogen passes through pipe 2l to the shell of interchanger Il, from which it is 'delivered in gaseous form and at substantially atmospheric temperature through pipe 50.

In the form shown in Fig. 2 the flows are identical with those above described with the exception that the liquid oxygen is withdrawn from a pool 21 in the upper section of the tower instead of from pool I9 in the base of the single column.

By the use of this cooling cycle, a. properly designed and insulated pump may be caused to operate at full stroke capacity for extended periods and without any risk whatever of gas locking. The refrigerative value lost by the nitrogen in cooling the liquid oxygen is largely recovered in the evaporation of the oxygen in interchanger i0. The heat absorbed by the nitrogen from the pump. which is due mainly to packing friction, causes a small loss of refrigerating eiect which may be compensated by a. correspondingly slight increase in air feed pressure.

Figures 3 and 4 are modiiications of the form of invention shown in Figure 1 and Figures 5 and 6 are modifications of the form of invention in Figure 2, which include, among other features, provision for withdrawing the product oxygen in vapor form and condensing the same before passing it to heat exchanger 45.

The product oxygen condensing feature shown in Figs. 3, 4,.5 and 6 is designed to eliminate any danger of oil being carried with the oxygen into the filled cylinders or into any part of the apparatus in which detonative combustion might occur. This feature permits the use of oil lubricated primary air compressors and avoids the difficulties attendant on water or soap-water lubrication.

Referring ilrst to Figs. 3 and 4, oxygen is Withdrawn through pipe 22 which is connected into the base of the column at the point indicated at 5i, above the liquid level of pool i9.

The gaseous pure oxygen flowing through this pipe passes through a condenser 52, in which it is liquefied by heat exchange with the air feed, down-stream from expansion valve I8. The liqueded oxygen then nows through pipe 53 to interchanger 45. in which it is cooled as above deo of condensing the gaseous oxygen has the maior advantage of preventing the passage of any combustible impurities into any part oi the system containing compressed oxygen.

In addition to the feature of condensing vaporous oxygen product, Fig. 4 includes a modification which in some cases may be very desirable. In this iol-m of the invention, instead of using the product nitrogen to subcool the liquid oxygen product in condenser it and to cool the pump di, the ieed air downstream of expansion valve it is utilized for these purposes. Referring particularly to the drawing, pipe il conducting the feed air from coil il immersed in the boiling oxygen at the bottom of the column, passes the ieed air through expansion valve it to heat exchanger tk The expanded feed air subcools the liquid oxygen product in conduit d6 and then passes out of heat exchanger t5 through conduit tt to the coil It@ surrounding the pump. The expanded iced air refrigerates the pump through the medium of this coil and then passes on through conduit 2l and conduit Bt through condenser di tc the top of the column. In condenser di, the expanded feed air condenses the vaporous oxygen product in the same manner as in the modification of Fig. 3.

In the form shown in Fig. 5, oxygen withdrawal pipe it is connected into the base oi upper column section td', at a point 5d above the liquid level of pool ti. The gas thus withdrawn is reiiquened in condenser di by heat interchange with the stream oi crude oxygen owing through pipe di, the condenser being downstream from expansion valve d2. The condensed liquid oxygen same as those oi' Figs. l and 2, re-l spectiveiy, and further description of them would densation of the pure gaseous oxygen product is accomplished by heat interchange with the crude oxygen intermediate product and with the intermediate liquid nitrogen product from the lower section of the double column.

Crude oxygen from the base of the lower section flows through pipe 3|,

Nitrogen from pool 2B iiows through pipe 29, and expansion valve to interchanger 45, pipe 48 to coil I9 and returns to I claim:

l. In combination with a fractionating column for separating a mixture of low boiling point gases: a pump adapted to handle liquids and a channel connecting the suction ci said pump with a relatively warm vapor space in said column; a condenser interposed in said channel and a heat interchanger interposed in said channel between said condenser and said pump; means for conducting a relatively cool stream of the mixture through said condenser toward said column in heat exchange relation with a stream of vapor passing through said condenser; means i'or consaid condenser and said pump; ducting a relatively cool stream means for con of the mixture vessel.

3. In combination with a fractionating column for separating a mixture of low boiling point interchanger interposed in said channel between said condenser and said pump; means for conducting a relatively cool stream of the mixture through said condenser toward said column in heat exchange relation with a stream of vapor passing through said condenser; means for conducting a relatively cold uid from said column through said interchanger in heat exchange relation with a liquid stream flowing from said condenser toward said pump, and means for bringlng said relatively cold uid into a further heat exchange relation with the liquid-conveying end of said pump.

4. In combination with a fractionating column for separating a mixture of low boiling point gases: a pump adapted to handle liquids and a channel connecting the suction of said pump with a relatively warm vapor space in said column; a condenser interposed in said channel and a heat interchanger interposed in said channel between said condenser and said pump; means for conducting a relatively cool stream of the mixture through said condenser toward said column in heat exchange relation with a stream of vapor passing through said condenser, and means for conducting a relatively cool fluid from said column through said interchanger in heat exchange relation with a liquid stream flowing from said condenser into said pump,

5. In combination with a fractionating column for separating a mixture of low boiling point gases: a pump adapted to handle liquids and a channel connecting the suction of said pump with a relatively warm product-collecting space insaid column; a heat interchanger 'interposed in said channel; means for directing a stream of a relatively cold product from said column through said interchanger in heat exchange relation with the relatively warm column product flowing through said channel toward said bringing said relatively cold product into a further heat exchange relation with the liquid-conveying end of said pump.

6. In combination with a fractionating column flor separating a mixture of low boiling point gases: a pump adapted to handle liquids and a channel connecting the suction of said pump with a relatively warm product-collecting space in said column; a heat interchanger interposed in said channel, said interchanger providing separated flow paths for fluids passing therethrough; means for directing a stream of a relatively cold product from said column through said interchanger in heat exchange relation with the relatively warm column product flowing through said channel toward said pump, and means for conducting said column product from said interchanger to a point exterior to said column.

7. In combination with an air fractionating column wherein oxygen is separated in liquid form: a pump adapted to the pumping of liquids; a channel connecting the suction of said pump with the liquid oxygen collecting space in said column, and means for bringing the liquid-conveying end of said pump into heat exchange relati'on with a product of said column colder than said liquid oxygen.

8. In combination with an apparatus producing liquid oxygen and a gaseous, nitrogen-rich product; a liquid-oxygen pump; means for bringing a stream of said liquid oxygen into'heat interchange relation with said nitrogen-rich product and thereby cooling said oxygen stream; means for producing a further heat interchange between said nitrogen-rich product and said pump, and means for conveying said cooled oxygen stream to said pump.

9. In combination with an air fractionating column wherein oxygen is separated in liquid pump, and means for' form: a pump adapted to the pumping of liquids; a channel connecting the suction of said pump with the liquid oxygen collecting space in said column, and means for bringing the liquid-conveying end of said pump into heat exchange relation with a fluid from said column colder than said liquid oxygen.

10. The method which comprises withdrawing a stream of oxygen vapor from an air fractionating operation; eecting a heat interchange between said oxygen vapor and another product of said fractionating operation which is colder than said oxygen vapor to condense the same to liquid form, reducing the temperature of said liquid stream below the boiling point of oxygen at the minimum momentary pressure reached in an ensuing pumping step, and pumping said oxygen stream in substantially liquid form.

11. In a method of producing oxygen and conditioning it for delivery to receiving means, in which air after compression and cooling is rectified at a relatively low temperature and reduced pressure thereby producing a cold nitrogen product and a liquid oxygen product having a temperature corresponding to its boiling point at said reduced pressure; the set of steps comprising subiecting fluid from said oxygen product to heat exchange with a colder uid derived from said rectification and thereby forming a sub-cooled liquid oxygen product; pumping such sub-cooled liquid oxygen product to a desired higher pressure, said sub-cooling reducing the liquid oxygen temperature at least sufliciently to prevent the same from iiashing into vapor during such pumping; and converting the liquid oxygen at said higher pressure into a gas by heat exchange with the compressed air to be liquefied.

l2. The process of separating air into its constituents, oxygen and nitrogen, and conditioning the oxygen for delivery to a receiving system, said process comprising compressing and subjecting to a refrigerating eiect air at high pressure, subiecting the air to a reduction in pressure and llquefaction into two portions, one rich in oxygen and the other rich in nitrogen, subsequently rectifying said portions to produce separate fractions, one consisting essentially of oxygen and the other consisting essentially of nitrogen, withdrawing oxygen from said one fraction in the gaseous phase and subjecting it to a condensing operation by heat exchange with a cold product developed during the separation process, Withdrawing liquidoxygen from the condensing operation, sub-cooling the withdrawn liquid oxygen by heat exchange with said other fraction, pumping the liquid oxygen to a relatively high pressure, utilizing the cold of the liquid oxygen at high pressure to produce said refrigerating effect and thereby vaporizing the oxygen.

13. In a method of producing oxygen and conditioning it for delivery to'receiving means, in which air after compression and cooling is rectified at a relatively low temperature and reduced pressure thereby producing a cold nitrogen product and a liquid oxygen product having a temperature corresponding to its boiling point at said reduced pressure; the set of steps comprising' subjecting fluid from said oxygen product to heat interchange with a stream of gaseous nitrogen obtained as one of the final products of said rectiiication and thereby forming a sub-cooled liquid oxygen product; pumping such sub-cooled liquid oxygen product to a desired higher pressure, said sub-cooling reducing the liquid oxygen temperature at least sufficiently to prevent the same from iiashing into vapor during such pumping; and converting the liquid oxygen at said higher pressure into a gas by heat exchange with the' compressed air to thereby cool the air.

14. The method of transferring the product oxygen from an air fractionating operation into a pressure vessel which comprises: withdrawing a stream of oxygen vapor from said operation; condensing said vapor and reducing the temperature of said condensed stream to a point below the boiling point of oxygen at the minimum momentary pressure reached in an ensuing pumping step; stantially liquid form to a pressure materially higher than that at which said vapor was condensed; applying an additional external cooling eiect to that portion of the liquid stream in which said higher pressure is created; heating and thereby gasifying said liquid stream, and charging the resultant gas into a pressure vessel under the pressure initiated in said pumping step.

15. The method which comprises: withdrawing a stream of oxygen vapor from an air fractionating operation; condensing said vapor and cooling the condensed stream to a point below the boiling point of oxygen at the minimum m mentary pressure reached in an ensuing pumping step, and pumping said cooled oxygen stream in substantially liquid form to a desired destination.

16. In a procedure in which a stream of oxygen is Withdrawn from an air fractionating operation, the steps comprising: pumping said withdrawn stream in liquid condition, and cooling said stream in transit to said pumping step to a temperature below the boiling point of oxygen at the minimum momentary pressure reached in said step, by heat interchange with a stream of the gaseous nitrogen obtained as one of the final products of said fractionating operation.

17. In a procedure in whiclra stream of oxygen is withdrawn from an air fractionating opera-l tion, the steps comprising: pumping said withdrawn stream in liquid condition, and cooling said stream in transit to said pumping step to a temperature below the boiling point of oxygen at the minimum momentary pressure reached in said step, rst by heat interchange with the cooled and expanded air feed entering said fractionating operation and thereafter by heat interchange with a stream of the gaseous nitrogen obtained as one of the final products of said operation.

18. The methodof transferring liquefied product of a fractionating operation, in which operation a mixture of component gases having boiling points substantially below atmospheric tem-' y perature is compressed and cooled, the compressed and cooled mixture expanded and the eiuent of the expansion step subjected to the fractionating operation to produce a liquefied higher boiling point fraction and a gaseous lower boiling point fraction, comprising withdrawing a stream of the liquefied higher boiling point fraction from the fractionating operation, subcooling the stream of liquefied higher boiling point fraction by heat exchange with a relatively colder product from the fractionating operation to reduce the temperature of the higher boiling point fraction to a point below the boiling point oi' the higher boiling point fraction at the minimum momentary pressure reached in an ensuing pumping step. and pumping the subcooled higher boiling point fraction in liquid phase to a relatively high pressure.,

pumping said oxygen stream in sub-v 19.' I'he method of transferring liquefied product of a fractionating operation, in which operation a mixture of component gases having boiling points substantially below atmospheric temperature is compressed and cooled, the compressed and cooled mixture expanded and the eiiiuent of the expansion step subjected to the fractionating .operation to produce a liquefied higher boiling point fraction and a gaseous lower boiling point fraction, comprising withdrawing a stream of the liqueed higher boiling point fraction from the fractionating operation, subcooling the stream of liqueed higher boiling point fraction by heat exchange with a relatively colder product from the fractionating operation to reduce the temperature of the higher boiling point fraction to a point below the boiling point of the higher boiling point fraction at the minimum momentary pressure reached in an ensuing pumping step, pumping the subccoled higher boiling point fraction in liquid phase to a relatively high pressure and heat exchanging the higher boiling point fraction from the pump with the mixture of gases going to the fractionating operation to cool the mixture and vaporize the higher boiling point fraction.

20. The method of transferring liqueiied product of a fractionating operation, in which operation a mixture of component gases having boiling points substantially below atmospheric temperature is compressed and cooled, the compressed and cooled mixture expanded and the efiluent of the expansion step subjected to the fractionating operation to produce a liquefied higher boiling point fraction and a gaseous lower boiling point fraction, comprising withdrawing a stream of the liquefied higher boiling point fraction from the fractionating operation, subcooling the stream of liquefied higher boiling point fraction by heat exchange with a relatively colder product from the fractionating operation to reduce the temperature of the higher boiling point fraction to a point below the boiling point of the higher boiling point fraction at the minimum momentary pressure reached in an ensuing pumping step, pumping the subcooled higher boiling point fraction 4in liquid phase to a relatively high pressure, and subjecting the higher boiling point fraction during the pumping step to heat exchange with a relatively cold product of the fractionating operation to prevent vaporization of the higher boiling point fraction during the pumping step.

CARL R. ANDERSON.

REFERENCES CITED The following references are of record in the file Yof this patent:

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