US3397547A

US3397547A - Avoidance of explosion hazards in air fractionation by halogenated hydrocarbon additin

Info

Publication number: US3397547A
Application number: US690684A
Authority: US
Inventors: Erb Ezra
Original assignee: Union Carbide Corp
Current assignee: Union Carbide Corp
Priority date: 1965-05-17
Filing date: 1967-12-14
Publication date: 1968-08-20
Anticipated expiration: 1985-08-20
Also published as: DE1551627A1; GB1103532A; FR1526618A; BE726251A; NL6707500A

Description

Aug. 20, 1968 E. ERB 3,397,547

AVOIDANCE OF' EXPLOSION HAZARDS IN AIR FRACTIONATION BY HALOGENATED HYDROCARBON ADDITION INVENTOR 37 EZRA ERB ATTORNEY Aug. 20, 1968 E. ERB 3,397,547

AVOIDANCE OF EXPLOSION HAZARDS IN AIR FRACTIONATION BY HALOGENATED HYDROCARBON ADDITION Original Filed May 17. 1965 2 Sheets-Sheet 2 IIOO nooo

INERT SOLUBLE DILUENT CONCENTRATION TO ELIMINATE BOILING OF LIQUID OXYGEN 500 600 700 MONOCHLOROTRIFLUOROMETHANE CONCENTRATION IN LIQUID OXYGEN (PARTS PER MILLION) &

O O O O O O IOG INVENTOR EZRA ERB 831mm@ ATTORNEY United States Patent O AVOIDANCE OF EXPLOSION HAZARDS IN AIR FRACTIONATION BY HALOGENATED HYDRO- CARBON ADDITION Ezra Erb, Alden, N.Y., assignor to Union Carbide Corporation, a corporation of New York Continuation of application Ser. No. 456,140, May 17,

1965. This application Dec. 14, 1967, Ser. No. 690,684

8 Claims. (Cl. 62-20) ABSTRACT OF THE DISCLOSURE To avoid an explosion hazard resulting from ethane concentration above an obstruction in a passageway for partial vaporization of oxygen-enriched liquid, CClF3 or GF4 is added to the liquid in concentration below 1000 p.p.m. to eliminate boiling in Ithe ethane concentration zone before the 4 mol percent explosion level is reached.

The present application is a continuation of application Ser. No. 456,140 tiled on May 17, 1965.

This invention relates to the avoidance of explosion hazards in low temperature air fractionation plants, and more particularly to the hazards resulting from the accumulation of ethane `as an atmospheric air impurity in an obstructed heat exchanger passageway.

There is a long-standing problem of occasional explosions in the condenser-reboiler separating the low and higher pressure rectication columns of air separation plants. Such explosions are believed caused by various hydrocarbon contaminants in the -feed air stream, e.g. acetylene, methane and ethane, being concentrated in the boiling oxygen-enriched liquid to produce explosive mixtures which the become inadvertently ignited. The accumulation usually occurs in an obstructed passageway of the condenser-reboiler, the obstruction resulting from solid materials which inadvertently enter the plant and tend to gravitate and become lodged in the passageway. For example, the solid material may be powder insulation, adsorbent or regenerator packing dust, or perhaps slugs of carbon dioxide and water which are not completely eliminated in the contaminant removal equipment upstream of the rectification column.

The prior art has employed various methods for controlling this explosion hazard, including (1) air feed stream purication means such as hydrocarbon scrubbers or adsorption traps, (2) periodic drainage of a portion of the oxygen-enriched liquid from the bottom of the condenser-reboiler to reduce the total quantity of explosive hydrocarbon material therein, and (3) by oxygen recirculation systems which selectively remove most of the dangerous hydrocarbons from the oxygen-enriched liquid by adsorption in regenerable traps. However, such methods of contamination control have not been entirely effective for all contaminants, particularly those only weakly adsorbed such as methane and ethane.

It has been proposed to desensitize bodies of liquid oxygen by adding thereto a soluble inert halogenated hydrocarbon in quantity suicient to reduce the explosive tendency of such liquid, i.e. an amount between about 0.5 and 50 mol percent. There are important drawbacks to this proposal, for example substantial contamination of the oxygen-enr-iched product by the fluorocarbon desensitizer, which may not be tolerated for many uses, e.g. breathing oxygen. Also the loss of the expensive uorocarbon desensitizer in the product is very significant. Finally the addition of 0.5-50 mol percent tluorocarbon to the oxygen-enriched liquid raises the boiling point of this liquid by an appreciable extent and necessitates a very significant increase in the air feed inlet pressure to achieve 3,397,547 Patented Aug. 20, 1968 the necessary cooling, fractionation and rectification. This in turn adds greatly to the plant operating costs.

An object of this invention is to provide an improved method for avoiding the explosion hazards in low temperature air fractionation plants.

Another object is to provide such a method which is effective against accumulation of the weakly adsorbed ethane impurity contained in the air feed and which builds up in the oxygen-enriched liquid reboiler.

Still another object is to provide an improved method which is not characterized by high contamination of the oxygen-enriched product stream, excessive consumption of `diluent and high power costs from increased air feed inlet pressure.

Other objects and advantages of this invention will be apparent from the ensuing disclosure, appended claims, and the drawings in which:

FIG. 1 is a longitudinal cross-section of a normally operating oxygen-enriched liquid reboiler passageway (without obstruction), utilizing the primary principle of recirculation wherein the liquid body enters the lower end;

FIG. 2 is a longitudinal cross-section of an alternative, normally operating oxygen-enriched liquid reboiler passageway (without obstruction) wherein the liquid body enters the upper end;

FIG. 3 is a longitudinal cross-section of the FIG. l passageway with an obstruction therein;

FIG, 4 is a graph showing the concentration of monochlorotriiluoromethane diluent required to avoid an explosion hazard for various ethane concentrations and various temperature differences across the obstructed passageway of FIG. 3; and

FIG. 5 is a schematic drawing of a nitrogen condenseroxygen reboiler operating according to the present inven1 tion.

One aspect of this invention relates t0 an atmospheric `air fractionating operation wherein the air feed containing high boiling point hydrocarbon impurities including ethane is cooled, partially freed of these impurities other than ethane, and partially condensed. The 'mixture is fractionated in the usual manner into a nitrogen-rich vapor and an oxygen-enriched liquid body containing the impurity. The liquid body flows into one end of a multiplicity of passagewaysl having a vertical component for partial vaporization therein by heat exchange with a warmer fluid thermally associated with the passageways and establishing a temperature difference of below about 3.5 C. The oxygen-enriched iluid is discharged from the opposite end of the passageways. Oxygen-enriched liquid is provided at the upper end of the passageways, and an obstruction forms in at least one passageway intermediate the upper end lower ends thereof.

The specific novelty relates to the improvement of avoiding an explosive ethane-oxygen mixture in the obstructed passageway due to ethane accumulation immediately above the obstruction by adding an inert soluble low volatility diluent. This diluent is added in a concentration below about 1,000 parts per million relative to the oxygenenriched liquid body, and in sufficient quantity to raise the boiling point yof the ethane-enriched, oxygen-containing liquid immediately above the obstruction sufliciently to eliminate boiling of the liquid before its ethane concentration attains 4.0 mol percent. The diluent is a member selected from the group consisting of monochlorotriuoromethane and' tetrauoromethane.

Referring now to the drawings, FIG. l illustrates a normally operating oxygen-reboiler passageway 10, at least the lower part of which is lled with liquid from the oxygen-enriched pool 11 beneath the lower end of the passageway. Pool 11 is forced into the passageway lower end by the hydrostatic head, and is partially evaporated by heat exchange with a warmer Huid 12 in the space 13 thermally associated with passageway 10. This fluid may for example be cold nitrogen-rich vapor from the higher pressure rectication section of the column, which is condensed by the heat exchange. Since nitrogen is more volatile than oxygen at atmospheric pressure, (their respective boiling points are 196 C. and 183 C.), it is necessary to provide the former at substantially higher pressure than the oxygen-enriched liquid to achieve the necessary temperature difference (AT) between the two uids of less than about 3.5 C. For instance, the nitrogen-rich cold gas entering space 13 may be at 75 p.s.i.g. and 178 C., and the pool 11 may contain 99.5 percent oxygen at 181 C. and 5 p.s.i.g.

Partial vaporization of liquid in passageway produces a low density two-phase mixture 14 which rises rapidly and is ejected from the upper end into chamber 15. The vaporized portion 16 then preferably passes through a series of rectication stages for counterow against descending liquid, and further oxygen-enrichment. In normal practice the unvaporized part of the mixture emerging from the upper end of passageway 10 is preferably withdrawn through conduit 17, directed through an adsorption trap for further removal of adsorbable air impurities, and returned to oxygen-enriched liquid pool 11. However, this unvaporized part of the liquid falls downwardly onto the header sheet 18 connecting adjacent passageways. Part of this falling liquid is capable of flowing backwardly by gravity through any passageway in which boiling does not occur. Another possible source of oxygen-enriched liquid for backward flow downwardly through passageway 11 is liquid draining from the rectication stages above (not illustrated).

In the FIG. 2 arrangement, the oxygen-enriched liquid pool 11 is over the oxygen reboiler passageway header sheet 18, so that the liquid enters the upper end of passageway 10 and the two-phase mixture formed therein ows downwardly. This mixture is ejected from the passageway lower end into chamber 19. The oxygen-enriched vapor portion is withdrawn through conduit 20 and the unvaporized portion is withdrawn through bottom conduit 21. The liquid portion is preferably pumped through an adsorption trap for further removal of air impurities and returned to pool 11 in chamber 15 at the top end of passageway 10.

In either the FIG. 1 or 2 arrangement, uid flow through the passageway 10 is interrupted in the event that a solid obstruction forms intermediate the ends. In both arrangements a source of oxygen-enriched liquid is available at the upper end of the passageway for gravity flow down the walls. With an obstruction present, the vapors generated from the down-llowing liquid rise countercurrent to this liquid and rectification occurs. For example, it has been determined that fewer than two theoretical stages are required to concentrate ethane from p.p.m. to the 4 mol percent explosive limit within the passageway. It has also been determined that the height of a typical passageway equivalent to a theoretical stage may be as low as 4-5 inches.

FIG. 3 illustrates the phenomenon occurring when a solid obstruction 22 is formed in the FIG. 1 passageway and the present invention is used to avoid an explosive mixture from the resultant ethane accumulation. It should be understood that the passageway 10 must have a vertical component or projection for the ethane accumulation to occur. That is, the passageway is either vertical or inclined less than 90 degrees from the vertical alignment, so that oxygen-enriched liquid enters the upper end thereof and descends by gravity through the passageway until stopped by obstruction 22.

During normal operation, the oxygen-enriched liquid and the vapor formed from the heat introduced to this liquid through the walls of passageway 10 ow concurrently, i.e. in the same direction. This direction can be either upward (FIG. 1) or downward (FIG. 2). However, when an obstruction 22 is formed in passageway 10, the two phases ow countercurrently, i.e. in opposite directions. Also, in normal operation only a minor part of the oxygen-enriched liquid is vaporized whereas in the obstructed passageway a major fraction of the liquid is vaporized.

Referring again to FIG. 3, oxygen-enriched liquid containing perhaps 15 p.p.m. ethane in chamber 15 drains downwardly through the upper end of obstructed passageway 10 and is rectied against the vapors rising through the passageway section 23 above obstruction 22. Ethane is less volatile than oxygen, and thus tends to accumulate in the passageway section 24 immediately above the obstruction 22. That is, passageway upper section 23 tends to act as a wetted wall tower or stripping column. The ethane depleted oxygen vapor (also containing still lower boiling components such as nitrogen) rises from the passageway upper end through chamber 15 as stream 16. If sufficient theoretical stages are provided in upper section 23, the ethane enrichment at the lower end thereof will continue until either of two events occur: (l) The lower explosion limit for ethane in oxygen (40,000 ppm. or 4%) is reached or exceeded, or (2) The local temperature difference (AT) between the boiling oxygen-enriched liquid and the warmer fluid 12 drops to zero and boiling ceases in the obstructed passageway. The latter may occur because the boiling point of the liquid in the passageway upper section 23 progressively increases as the concentration of ethane and other soluble constituents increases. Assuming then that the initial temperature difference is below about 3.5 C., this temperature difference will progressively decrease and boiling will cease when the AT vanishes. It will be apparent from the foregoing discussion that it is preferable for event (2) to occur and thus prevent event (l) from occurring.

If the boiling oxygen-enriched liquid in passageway upper section 23 also contains other components which have higher boiling points than oxygen and are inert (non-explosive), they assist the ethane in reducing the AT. Xenon is such a component, and the normal xenon content in atmospheric air aids in reducing the AT before 4% ethane concentration in the oxygen-enriched boiling liquid is reached. However, the xenon `cannot raise the liquids boiling point suiciently to eliminate the AT before the ethane concentration reaches 4% because of its low solubility in the oxygen-enriched liquid.

According to this invention, a very small quantity of an inert diluent selected from the group consisting of monoohlorotriuorornethane and tetrafluoromethane is introduced to the liquid Ibody in a concentration below about 1000 p.p.m. relative to the oxygen-enriched liquid body. This inert diluent may for example be introduced to the liquid on passageway closure means 18 through conduit 25, and thus passes downwardly through the passageway upper section 23 with the oxygen-enriched liquid. Alternatively the inert diluent may be introduced with any stream entering the rectification equipment. Each of these diluents is (l) non-explosive in admixture with oxygen, (2) soluble in liquid oxygen to the extent of greater than 10 mol percent under prevailing operating conditions, and (3) less volatile than oxygen. These characteristics are summarized in Table A.

Other halogenated hydrocarbons are not suitable for use as the diluent of this invention because of poor solubility in liquid oxygen at prevailing conditions, i.e. less than 10 mol percent. Another reason why other lhalogenated hydrocarbons are not suitable is their relatively high freezing points and their tendency to freeze in the `feed conduit to the cold fractionating equipment. These low solubility compounds would also accelerate passageway obstruction if added in concentrations exceeding their solubility limits.

The inert diluent-containing-oxygen-enriched liquid descending in passageway upper section 23 is rectied by the rising oxygen-rich vapors, and the higher boiling soluble diluent is progressively concentrated in the lower end 24 of upper section 23 along with ethane. The boiling point of this liquid increases by virtue of the increasing concentrations of inert soluble diluent, ethane and xenon (if present), and the decreasing oxygen concentration. The resulting small liquid pool 26 accumulates in section 24 immediately above obstruction 22. It has been found that the boiling point of liquid pool 26 rises suiciently to eliminate boiling thereof before the ethane concentration reaches 4%. That is, the practice of this invention actually eliminates the explosion hazard by preventing the ethane concentration from attaining the hazardous level. This is in contrast to many previously employed and ineifective methods in which the hydrocarbon concentration in the liquid oxygen body is monitored, and apprporiate measures are taken when the ethane concentration increases. However, the present invention recognizes that the explosion hazard does not usually occur in the bulk liquid oxygen but rather in an undetectable obstructed passageway in which boiling ethane-containing oxygen-enriched liquid is accumulating.

It should be understood that the present invention does not by itself eliminate the problem of explosion hazards due to hydrocarbon buildup in an obstructed liquid oxygen-enriched passageway. Other means such as adsonption traps must be provided to remove the bulk of the acetylene in the air feed. Without such other means acetylene will lreadily concentrate in the oxygen-enriched liquid (even without passageway obstruction) to about 1.2 p.p.m. or higher. ln an obstructed passageway acetylene need concentrate inthe oxygen-enriched liquid to only a yfew p.p.m. to begin formin-g an easily detonated solid phase. It is necessary to add a far greater amount of inert diluent than contemplated by this invention to avoid formation of solid phase acetylene. On the other hand, when all the air feed is cleaned by adsorption and when the bulk oxygenenriched liquid is further cleaned by adsorption, the acetylene concentration in this liquid will be about 1 2 parts per billion. At such low level of concentration acetylene does not pose an explosion hazard even in an obstructed liquid oxygen reboiler passageway.

This invention does not desensitize the oxygen-enriched liquid provided at the upper end of the obstructed passageway or the oxygen-enriched liquid body entering the lower end of this passageway. That is, a concentration of less than about 1000 p.p.m. soluble diluent is far below the concentration necessary to significantly aiect the explosive propensity or characteristic of ethane, oxygen-enriched liquid. A diluent concentration of at least 5000 p.p.m. (0.5%) and preferably at least 20% is necessary to desensitize a body of oxygen-enriched liquid.

The specific concentration of inert soluble diluent needed to avoid an explosion hazard in an oxygen-enriched liquid boiler-heat exchanger with an obstructed passageway depends not only on the ethane concentration of this liquid, but also on the temperature difference between the boiling liquid and the warmer heat exchanging lluid. A minimum required ratio exists between the total inert diluent material (e.g. monochlorotriuorometh-ane and atmospheric xenon) and the ethane present, so that for increased amounts of ethane in oxygen-enriched liquid an increased amount of externally introduced inert diluent material will be needed. It also follows that a relatively large AT will require a larger concentration of diluent to raise the boiling point of the oxygen-enriched liquid suiciently to overcome this AT, eliminate boiling and further ethane concentration to the 4% level.

These relationships are graphically illustrated in FIG. 4, which is a family of curves for various AT values between 1.5 C. and 3.5 C., the monochlorotritiuoromethane concentration being plotted as the abscissa and the ethane concentration as the ordinate. The curves indicate the concentration of monochlorotriuoromethane needed in substantially pure liquid oxygen to eliminate the temperature difference and boiling of liquid oxygen at a certain ethane concentration in this liquid. For example, if the temperature difference is 2.0 C. and the boiling liquid oxygen has an ethane concentration of 200 p.p.m., a mono- Y chlorotrifluoromethane concentration of 600 p.p.m. is needed in this liquid to eliminate the boiling. Similiarly, at the same ethane concentration but a AT of 2.5 C., the monochlorotriuorometh-ane concentration should be about 800 p.p.m. A similar relationship exists when tetrauoromethane is employed as the inert diluent, except that relatively more diluent is needed because of its lower boiling point and higher relative volatility with respect to oxygen (see Table A). Also, a similar relationship exists when the boiling liquid contains significant quantities of nitrogen, e.'g. 50% O2, with the balance N2 and atmospheric inerts.

It should be understood that the specific type of air cooling, partial condensing and fractionating system does not enter into the present invention except that such system must remove the bulk of other hydrocarbon impurities such as acetylene, and provide an oxygen-enriched liquid to be boiled in multiple passageways. Suitable low temperature air fractionation systems are well known to those skilled in the art, as for example described in First et al., U.S.P. 2,918,801 and Matsch et al., U.S.P.

FIG. 5 illustrates a liquid oxygen boiling-nitrogen vapor condensing heat exchanger 30 comprising a series of oxygen passageways 31 in alternating sequence with nitrogen passageways 32 and bonded thereto for heat exchange by solid conduction between the passageway walls. Each oxygen passageway 31 is thermally associated with, and separates adjacent nitrogen passageways in a sandwich-type assembly. The nitrogen passageways are manifolded together (by means not illustrated) at both the upper inlet end in communication with inlet conduit 33, and the lower discharge end in communication with the discharge conduit 34. The oxygen passageways 31 are open at both their lower and upper ends, the former communicating with bottom chamber 35 having liquid oxygen inlet means comprising downcomer 36 from a series of rectication stages. The oxygen passageway upper ends are joined by nitrogen passageway closure means 18 forming the base of upper chamber 15. The inert soluble diluent is introduced to 4bottom chamber 3S through conduit 37 and control valve 38 therein.

Liquid oxygen at about -181 C. and 5 p.s.i.g. is forced by hydrostatic head through the lower end of passageways 31 to a level preferably at least two-thirds of the distance between the lower and upper end. Cold nitrogen vapor at about 178 C. and 75 p.s.i.g. is introduced through inlet conduit 33 at the upper end of the heat exchanger 30 and flows downwardly through passageways 32. Heat is transferred from the nitrogen vapor to the oxygen liquid through the thermally associated `passageways 31 and 32, by the temperature difference of 3 C. for boiling of the oxygen liquid and condensation of the nitrogen vapor, the latter being withdrawn through discharge conduit 34. The resulting oxygen liquidvapor mixture emerges from-the upper ends of passageways 31 and the vapor may be withdrawn as product through conduit 39 or passed upwardly through a series of rectification stages (not shown) in communication with upper chamber 15 for contact with a downwardly iiowing liquid.

During this normal operation of heat exchanger 30, the ethane concentration of the liquid oxygen pool 11 in bottom chamber 35 is monitored. This for example may be accomplished by liquid phase sampling conduit 40 communicating with analyzer 41, as for example comprising a gas chromatograph and ilame ionization detector. The concentration of the inert soluble diluent, introduced through conduit 37 and control valve 38, is monitored in the same manner.

Assume now that an obstruction 22 forms in at least one oxygen passageway intermediate the lower and upper ends thereof and the liquid oxygen descending into this passageway is rectied by the rising vapors. As previously described in conjunction with FIG. 3, the ethane concentration of the liquid immediately above the obstruction increases.

The soluble inert diluent concentration in the upper section of obstructed passageway 10 is sufficient to eliminate the AT across this particular passageway and eliminate liquid oxygen boiling therein. The remaining unobstructed passageways 31 continue to operate in the normal manner and are substantially unaffected by the inactivity of passageway 10. The ethane and soluble inert diluent concentrations in the remaining passageways 31 will remain constant lbecause they are not dead-ended.

It has been previously stated that the advantages of the present invention over the prior art scheme of introducing sufficient soluble inert diluent to desensitize the entire body include (1) extremely low losses of diluent in the product oxygen, (2) extremely low contamination of the product oxygen by the diluent, (3) negligible increase in the air feed pressure and plant operating cost. These advantages are illustrated in the following table.

TABLE 3 Diluent* in Diluent Increase in Lower L02 Diluent in Consumption Column's- O2 Product, Mol Ppm. p.p m. Lb./100 .l/ton Pressure, Power Percent tons Oz O2 p.s.i. Cost,

Percent *Diluent is n1onochlorotrifluoromethane.

Although preferred embodiments of this invention have been described in detail, it is contemplated that modications of the described method may be made and that some features may be employed without others, all within the scope of the invention.

For example, although the invention has been specically described in terms of boiling an oxygen liquid containing essentially no nitrogen, it is equally suitable for avoiding explosion hazards in any oxygen-enriched liquid from an air fractionation plant. As used herein the term oxygen-enriched liquid refers to a liquid containing more than the normal -concentration of oxygen in air, i.e. 21%. As one exemplitication, the liquid accumulating in the kettle or lower end of a rectication column contains about 50% O2, and thus is an oxygen-enriched liquid. This liquid is often boiled in a heat exchanger to extract heat from nitrogen vapor, and the liquid is further enriched in oxygen as it boils. Rectification of the further enriched liquid in an obstructed passageway of the heat exchanger still further concentrates the oxygen along with ethane impurity from the atmosphere. This concentration is sufficient to create an explosive iluid, and the present invention may be etfectively employed to avoid this hazard.

As a further alternative, higher pressure air may be at least partially condensed as the warmer fluid in heat exchange with the boiling oxygen-enriched liquid.

I claim:

1. In an atmospheric air fra-ctionating operation wherein the air feed containing high boiling point hydrocarbon impurities including ethane is cooled, partially freed of said hydrocarbon impurities other than ethane, partially condensed, fractionated into a nitrogen-rich vapor and an oxygen-enriched liquid body containing said ethane impurity, which body flows into one end of a multiplicity of passageways having a vertical component for partial vaporization therein lby heat exchange with a warmer uid thermally associated with said passageways establishing a temperature difference of below about 3.5 C. and discharges from the opposite end thereof as a liquidvapor mixture, wherein oxygen-enriched liquid is provided at the upper end of said passageways, and an obstruction forms in at least one passageway intermediate the ends thereof: the improvement of avoiding an explosive ethaneoxygen mixture in the obstructed passageway due to ethane accumulation immediately above them obstruction, by adding an inert soluble diluent to said oxygen-enriched liquid body at concentration below about 1000 parts per million relative to said oxygen-enriched liquid body and based on the temperature difference between the oxygenenriched liquid body and the said warm fluid associated therewith and the observed ethane concentration in the oxygen-enriched liquid body, the diluent addition being in sufficient quantity to increase the diluent to second higher concentration in ethane-enriched, oxygen-containing liquid immediately above the obstruction and raise the boiling point of such liquid sufficiently to eliminate boiling of the liquid before its ethane concentration attains 4.0 mol. percent, the said diluent being tetraiiuoromethane.

2. A method according to claim 1 in which said nitrogen vapor is said warmer fluid.

3. A method according to claim 1 in which said nitrogen vapor is said warmer liuid, and the nitrogen vapor is condensed by said heat exchange.

4. In an atmospheric air fractionating operation wherein the air feed containing high boiling point hydrocarbon impurities including ethane is cooled, partially freed of said hydrocarbon impurities other than ethane, partially condensed, fractionated into a nitrogen-rich vapor and an oxygen-enriched liquid body containing said ethane impurity, which body flows into one end of a multiplicity of passageways having a vertical component for partial vaporization therein by heat exchange with a warmer fluid thermally associated with said passageways establishing a temperature difference of below about 3.5 C. and discharges from the opposite end thereof as a liquidvapor mixture, wherein oxygen-enriched liquid is provided at the upper end of said passageways, and an obstruction forms in at least one passageway intermediate the ends thereof: the improvement of avoiding an explosive ethaneoxygen mixture in the obstructed passageway due to ethane accumulation immediately above the obstruction, by adding an inert soluble diluent of monochlorotrii'luoromethane to said oxygen-enriched liquid body at rst concentration below about 1000 parts per million relative to said oxygen-enriched liquid body and based on the temperature difference between the oxygen-enriched liquid body and the said warmer iluid associated therewith and the observed ethane concentration in the oxygenenriched liquid body and consistent with the chart illustrated in FIGURE 4, the diluent addition being in suicient quantity to increase the diluent to second higher concentration in the ethane-enriched, oxygen-containing liquid immediately above the obstruction and raise the boiling point of such liquid suciently to eliminate boiling of the liquid Ibefore its ethane concentration attains 4.0 mol. percent, the said diluent being monochlorotriuoromethane.

5. A method according to claim 4 in which said nitrogen vapor is said warmer fluid.

6. A method according to claim 4 in which said nitrogen vapor is said warmer fluid, and the nitrogen vapor is -condensed by said heat exchange.

7. In an atmospheric air fractionating operation wherein the air feed containing high boiling point hydrocarbon impurities including ethane is cooled, partially freed of said hydrocarbon impurities other than ethane, partially condensed, fractionated into a nitrogen-rich vapor and an oxygen-enriched liquid body containing said ethane impurity, which body flows into one end of a multiplicity of passageways having a vertical component for partial vaporization therein by heat exchange with a warmer iluid thermally associated with said passageways establishing a temperature difference of below about 3.5" C. and discharges from the opposite end thereof as a liquid-vapor mixture, wherein oxygen-enriched liquid is provided at the upper end of said passageways, and ethane accumulates in at least one passageway: the improvement of avoiding an explosive ethane-oxygen mixture in the one passageway due to ethane accumulation by adding an inert soluble diluent of monochlorotriuoromethane to said oxygen-enriched liquid body at first concentration below about 1000 parts per million relative to said oxygen-enriched liquid body and based on the temperature difference between the oxygen-enriched liquid body and the said warmer uid associated therewith and the observed ethane concentration in the oxygen-enriched liquid body and consistent with the chart illustrated in FIG- URE 4, the diluent addition being in sufficient quantity to increase the diluent to second higher concentration in the ethane-enriched, oxygen-containing liquid in said one passageway and raise the boiling point of such liquid sufficiently to eliminate boiling of the liquid before its ethane concentration attains 4.0 mol. percent, the said diluent being monochlorotriuoromethane.

8. In an atmospheric air fractionating operation wherein the air feed containing high boiling point hydrocarbon impurities including ethane is cooled, partially freed of said hydrocarbon impurities other than ethane, partially condensed, fractionated into a nitrogen-rich vapor and an oxygen-enriched liquid body containing said ethane impurity, which body ows into one end of a multiplicity of passageways having a vertical component for partial vaporzation therein by heat exchange with a warmer uid thermally associated with said passageways establishing a temperature difference of below about 3.5 C.

and discharges from the opposite end thereof as a liquidvapor mixture, wherein oxygen-enriched liquid is provided at the upper end of said passageways, and ethane accumulates in at least one passageway: the improvement of avoiding an explosive ethane-oxygen mixture in the one passageway due to ethane accumulation, by adding an inert soluble diluent to said oxygenenriched liquid body at yconcentration below about 1000 parts per million relative to said oxygen-enriched liquid body and based on the temperature difference between the oxygen-enriched liquid body and the said warm fluid associated therewith and the observed ethane concentration in the oxygen-enriched liquid body, the diluent addition being in su'icient quantity to increase the diluent to second higher concentration in ethane-enriched, oxygen-containing liquid in said one passageway and raise the boiling point of such liquid sufficiently to eliminate yboiling of the liquid before its ethane concentration attains 4.0 mol. percent, the said diluent being tetrauoromethane.

References Cited UNITED STATES PATENTS 2,541,409 2/ 1951 Cornelius 62--13 X 2,918,801 12/1959 First et al. 62-14 3,080,724 3/ 1963 Gordon et al. 62-20 3,081,157 3/1963 `Gordon et al 23--221 3,124,443 3/1964 Hellin'gman et al. `62--14 X OTHER REFERENCES Preliminary Investigations of Carbon Tetratluoride as an Inert Diluent Gas to Prevent Explosion of Mixtures of Cyclopropane and Oxygen, by C. S. Jones et al., November 1950, Anesthesiology, pp. 562-566.

NORMAN YUDKOFF, Primary Examiner.

V. W. PRETKA, Assistant Examiner.