US1951184A - Art of separating mixed gases - Google Patents

Description

' Mamfi 13, 19340 Filed June 16 1931 INVENTOR ATTORNEYS teale111... 13, 1934 UNITED STATES PATENT OFFICE;

8 Claims.

This invention relates to improvements in the art of separating mixed gases and is especially useful in the separation of air into relatively pure oxygen and/or nitrogen.

One of the primary objects of the invention is to effectively eliminate impurities such as lubricant carried into the system from the compressors, with a minimum loss in liquid. 1

Another object is to secure the return through the system of vaporizedliquid oxygen in a pure state.

More specifically it is an object of the invention to provide in the system a rectifying column having an internal vaporizing section for vaporizing the liquid produced in the maintray section and a plurality of external vaporizer sections for completing such vaporization, all of the vaporizing sections being connected in series, and so connected to each other and to a withdrawal pipe that the liquefier in the system will taken such vaporized liquid from the last vaporizer as it needs, the vaporized liquid in the remaining vaporizer sections flowing either in whole or in part either to such withdrawal pipe and/or to the tray section as the demands of-the system vary. p

I accomplish the foregoing, together with such other objects and advantages as may hereinafter appear, or are incident to the invention, by means of an arrangement which is illustrated in the preferred form in the accompanying drawing, the figure being a diagrammatic view partly in side elevation and partly in section illustrating the preferred embodiment of the invention.

Referring now to the drawing the air separation plant comprises two interchangers A and B, a liquefier C, an expansion engine D, exchangers K1, K2, and L, a purifier M and a rectifying column made up of a pot E, a preliminary tray section F, a vaporizer section G, a main tray section H and a top tube section J.

Air compressed to the necessary operating pressure, say 500 lb. gage, enters the system at the air inlet 1 at, say, centigrade. This compressed air is nearly free of carbon dioxide which has been removed in purification apparatus not shown.

The compressed air is saturated with water va-T por, however, and contains oil-vapors from the lubricant used in the compressor.

With valves 2, 5 and 7 open, and valves 3, 4 and 6 closed, the compressed air passes up around the tubes in interchanger A and then up around the tubes in interchanger B, leaving the latter through pipe 8. Valves 9, 10 and 11 should be closed and valves 12, 13 and 14 open. Then the oxygen and nitrogen separated in the rectifying colsection.

umn will be returning through'interchanger B and not through interchanger A. If the valves 2, 3, 4,

5, 6, 7, 9, 10, 11, 12, 13 and 14 have just been reversed, in'terchanger A will be cold and the compressed air will be more or less cooled therein 00 by warming up the interchanger and thawing out the accumulated ice. After the ice is completely thawed out and interchanger A warmed up, the compressed air will not be appreciably cooled therein, but will enter interchanger B at about 65 30 centigrade. In interchanger B the compressed air'will be cooled by the returning oxygen and nitrogen flowing through the tubes therein to, say, minus 110 centigrade.

After leaving the interchangers through pipe 8, the compressed air divides, part going through pipe 15 to the liquefier C, while the larger portion passes to expansion engine D through pipe 16. The portion entering the liquefier is liquefied therein by the returning oxygen and nitrogen from the rectifying column, the level of the ac-- cumulated air being indicated on level gage 17. The portion of the compressed air passing to expansion engine D is expanded therein to, say, '75 lb. gage and from thence passes through pipe 18 to the lower end of the preliminary tray section F.

The preliminary tray section F contains a number of trays arranged 'to cause the rising vapor from the expansion engine to bubble through liquid which is flowing down from tray to tray. With a single acting expansion engine as indicated in the figure, it was found that the fluctuations in pressure due to the intermittent discharge were sufficient to prevent the liquid flowing down from tray to tray. .Therefore, in order to reduce the fluctuations of pressure within the preliminary tray section, expansion chambers 19 and 20 were provided with orifices at 21' and 22. The restrictions to flow following chambers for accumulat tion of pressure, reduced the fluctuations in pressure sufiiciently to permit the liquid to flow down from tray to tray. These chambers may be made an integral part of the rectifying column.

Leaving the uppermost tray, the vapor passes up through a large central tube 23 inthe vapo 10c izer section to the space under the dome 24. The vapor then passes down through a large number of small tubes 25 in the vaporizer section and is partly condensed. The vapor portion leaves through pipe 26. The liquid portion passes through liquid sealed tubes 27 and flows down from tray to tray through the preliminary tray The interaction of the down. flowing stream of liquid with the rising vapor bubbling through it in each tray results in the rising vapor the rising vapor until it finally contains about percent oxygen by the time it reaches pot E. At some intermediate tray, the oxygen percentage will be about 21 percent. as in atmospheric air. As near this point as practicable, the liquefied air from liquefier C will be introduced through pipe 28 as determined by the control of valve 29.

" The oxygen-rich liquid from pot E is throttled through pipe 30.:by means of valve 31. This liquid enters the main tray section at 32 and overflows from tray to tray until it reaches the space above dome 24. From this space the liquid flows through pipe 33 to the space surrounding the tubes in the vaporizer. The liquidwill boil by heat transfer from the condensing vapor within the tubes and the resulting vapor will rise and leave the vaporizer section through pipe 34, enteringthe main tray section somewhat above the dome 24. The vapor will bubble through the downfiowing liquid on the trays until it reaches the inlet tray at 32. Here it will be joined by any vapor formed by throttling the oxygen-rich liquid and the total vapor will pass up through the trays above the inlet until it reaches the top tube section. It passing through the tubes in this section, the vapor will be partly liquefied, the vapor fraction passing out through pipe 35 while the liquid fraction returns as a reflux liquid through the trays above the inlet to the main tray section and joints the oxygen-rich liquid admitted a 32.

The vaporizer and top tube sections thus produce refluxes of vapor and liquid respectively through t -e trays of the main tray section. The interaction between the rising vapor streamand the downfiowing liquid stream results in the oxygen-nitrogen mixture throttled into the main tray section being separated into nearly pure nitrogen vapor, say 96 percent pure, and nearly pure oxygen liquid, say 99.5 percent pure.

The oxygen-rich liquid entering the vaporizer section through pipe 33 is fractionated by boiling so that's. portion of this liquid leaving through pipe 36 is of higher purity than the entering liquid. Pipe 36 is connected to exchanger K1 at a point somewhat above the bottom. In exchangers K1 and K2, further fractionation occurs "by boiling of the liquid in condensing a portion of the nitrogen vapor discharged from the prelimi- .nary tray section through pipe 26. The remaining oxygen liquid passes through pipe 37 containing valve 38 into exchanger L where it is completely vaporized. The vapor formed in ex changer L passes through pipe 39 to'liquefier C. As long as less oxygen vapor returns to the liquefier than is formed in exchanger L. only the highest purity oxygen from exchanger L. so returns, the excess oxygen formed in exchanger L passing through-pipe 40 to the main tray section H of the rectifier through pipe 41. Should, however, more oxygen vapor return to the liquefier than is formed in exchanger L, then some oxygen from exchanger K2 will return through pipe 40. The remaining oxygen formed in exchanger K2 will fiowto the main tray section thr gh p p Only in case more oxygen vapor returns to the liquefier than is formed in both exchangers L and K2, will any oxygen vapor return from exchanger K1. In this way, oxygen vapor of the highest purity only will return to the liquefier under all conditions. In passing down through the tubes in exchangfirs K1 and K2, the nitrogen entering from pipe 26 is partlyliquefied. The mingled liquid and vapor pass through pipe 42 to't'he pot 43 at the lower end of exchanger L. The vapor passes upward through the tubes in exchanger L, where it is nearly all liquefied, the liquefied portion fiowing back into plot 43. The nitrogen liquid collecting in but 43 is throttled by valve 44 through pipes 45 .and 46 into the top tube section of the rectifying column. Here the nitrogen liquid is vaporized by imparting refrigeration to produce a refiux liquid in the main tray section, and the vapor passes through pipe 47 to the liquefierC.

The gas remaining after the liquefaction of nearly all the nitrogen from the preliminary rectification, contains large portions of neon, hydrogen, helium and other highly volatile gases in the original atmospheric air compressed and cooled. This gaseous mixture is further purified in the purifier M by bringing it into heat, exchange relation with the liquid nitrogen after being throttled to a lower pressure through valve 44. It had been previously in heat exchange with 1(15 liquid oxygen which vaporizes at an appreciably higher temperature under the same pressure.

.The more volatile gases are further purified of ing these gases through the liquefier and inter- 1 5 changers.

Instead of throttling the liquid air from liquefier C by valve 29 through pipe 28 into the preliminary tray section, this liquid air may bethrottled by valve 52 through pipe 53, into the -main tray section, the *entering point being so selected that the composition of the liquid on '"the tray at that point is about the composition of the liquid admitted.

A 'by-pass valve 54is provided between the 25 expansion engine exhaust pipe 18 and the return pipe 47 from the top tube section J to the liquefler C.

The peculiar advantages of the process and arrangement shown will be brought out in de--13 scribing the cooling down to operating temperatures and the normal 'operatien.

A certain length of time'will be required to cool down the apparatus to operating temperatures. Before starting the compressor, by-pass- 5 valve 54 is opened. Valve 38 should also be open and valves 31 and 44 may be open, Valves 29 and 52 should be closed. Valves 2, 5, 7, 12, 13 and 14 may 'be open while valves 3, 4, 6, 9, 10 and 11 may be closed to have the compressed air pass through interchanger A before passing through interchanger B.

ihe air compressor is then started and the compressed air passed into inlet 1 after being freed of carbon dioxide either before or after be- .15 ing compressed. As soon as the compressed air attains sufilcient pressure, expansion engine D is started. Most, if not all of the air after expansion passes through by-pass valve 54 and returns through the liquefier C and the interchanger B.

through valve 29 into the trays of the preliminary tray section F. overflowing from tray to tray, this liquid air cools the preliminary tray section and liquid finally accumulates in pot E, as shown on level gage 55. The. vapor rising from the liquid air passes through pipe 26 to start cooling the exchangers K and L and the top tube section J of the rectifying column.

When some liquid has accumulated in pot E, valve 29 ispartly closed .and valve 52 partly opened to supply liquid air to the trays of the main tray section H. As the trays fill with liquid it overflows from tray to tray and finally accumulates in vaporizer section G and exchangers K and L.

After considerable accumulation of liquid has occurred, valve 54 is gradually closed to get the plant into normal operation.

When valve 54 is closed, the pressure in the preliminary tray section rises until liquefaction begins within'the tubes of vaporizer G and exchangers K1, K2 and L due to imparting heat to boil the liquid outside of these tubes. Equilibrium is finally reached where the heat trans fer is just sufficient to liquefy all the air entering from the expansion engine, with the exception of that discharged by valve 51.

The liquid accumulating in the pot at the bottom of exchanger L is throttled through pipe 45 and valve 44 into top tube section J of the rectifying column. Here the liquid vaporizes around the tubes in condensing vapor rising within the tubes. and the plant reaches steady operation.

During steady normal operation, the height of liquid in top tube section .1 will depend upon the temperature difference available for heat transfer between the nitrogen vapor condensing within the tubes and the nitrogen liquid vaporizing around the tubes. This temperature difference will, in turn, depend upon the rectification pressure (at the rectifier top) and the pressure under which the nitrogen liquid vaporizes. pressure will depend upon the resistance en- 'I'he latter Very soon the normal refiuxes are set up countered by the nitrogen upon returning through liquefienC and interchanger B and connecting piping. The rectification pressure will be determined by the adjustment of automatic pressure relief valve N. The adjustment of this valve may be changed until the height of liquid in the top tube section J is any desired amount. It is desirable to operate at as high a level as possible in order toutilize to the greatest advantage the surface available and reduce the rectification pressure to a minimum value.

The liquid in pot E will be throttled through valve 31 as fast as it accumulates.

its operation continues, solid particles of oil from the compressors and other impurities gather in the liquid oxygen, in vaporizer G. These impurities are carried by the flow through pipe 36 into exchanger K1 and from thence into exchanger K2 and finally from thence through pipe 37 into exchanger L where they accumulate. At intervals, valve 38 may be closed and the liquid in exchanger L permitted to partially evaporate. Then the remaining liquid may be discharged space which could be shut off at will and drained.

through a drain pipe provided for that purpose. thereby carrying out of the system these accumulated impurities with minimum lossof oxygen liquid. Plants heretofore built have included no 8o It was, therefore, impossible to remove the accumulated impurities without seriously affecting the operation of theplant by removing a very large quantity of oxygen liquid.

The provision of" a number of external exchangers reduces the volume of liquid oxygen in which the impurities finally accumulate to a much smaller quantity than if only one such exchanger were employed. Hence, when the last exchanger in the series is shut off, there is a very small effect upon the normal operation of the rectifying column. Also, but a small quantity of oxygen liquid need be discharged from the system to rid it of all accumulated lubricant. This disturbance will occur but for a very short time by quickly closing-valve 38 and draining out the liquid in exchanger L through drain 53 without waiting for much of the liquid to boil away. This can be done by reason of the small amount of liquid normally in exchanger L.

The amount of heating surface provided in exchanger L is smaller in comparison with the heating surface provided in vaporizer G and exchangers K1 and K2 than would correspond to the amount of heat required to vaporize the quantity 135 of oxygen to be withdrawn from the plant. Consequently some excess vapor is always passing through pipe 40 from exchanger K1. The oxygen vapor withdrawn through pipe 39 is, therefore, contaminated but slightly with vapor from exchanger K1 produced from the less pure oxygen liquid therein.

;As the amount of, heating surface in the exchangers K1, K2 and L approximately equals the amount of surface provided invaporizer G, the several exchangers make a more compact apparatus than a single exchanger would.

Valve 51 is opened to maintain such a flow throu h it as will prevent the accumulation of highly volatile gasesfrom reducing appreciably the heat transfer in exchanger L.

1. In a rectifying column having a tray section, the combination of a vaporizer means for vaporizing the liquid reflux from the tray section, said means comprising more than two compartments connected in series for liquid flow, means for shutting oif the last one from the preceding ones and means for withdrawal of liquid therefrom. Y 0

2. In a rectifying column having a tray section, and a vaporizer section for vaporizing part of the liquid reflux from the tray section, the combination of an external vaporizer of multiple N sections connected in series with each other and with the vaporizer in the column for liquid flow, and means for withdrawing the vaporized liquid from the last external section, and means for withdrawing liquid from said last external seci3) tion.

3. In a rectifying column having a tray section, and a vaporizer section for vaporizing part of the liquid reflux from the tray section, the

combination of an external vaporizer of multiple 1.;5

sections connected in series with each other and with the vaporizer in the column for liquid flow, and means for withdrawing the vaporized liquid from the last external section and connections from preceding sections to said withdrawal means, and means for withdrawing liquid from said last external section.

4. In a rectifying column having a tray section, and a vaporizer section for vaporizing part of the liquid reflux from the tray section, the combination of an external vaporizer of multiple sections connected in series with each other and with the vaporizer in the column so connected that the vaporized liquid may flow from each-section to a withdrawal means and to the tray section. and such withdrawal means leading from the last section of the series.

5. In a rectifying column, the combination of a preliminary tray section, a main tray section, and an intermediate vaporizer section having an internal large pipe for leading vapor'from the preliminary tr'ay section thereto to the upper part thereof for down flow therein, a connection for leading vapors from the vaporizer-section to the main tray section, and a connection for withdrawing liquid from the vaporizer section.

6. In a rectifying column, the combination of a preliminary tray section, a main tray section, and an intermediate vaporizer section having an internal large pipe for leading vapor from the preliminary tray section thereto, and a liquid seal for preventing fiow of vapor from the preliminary tray section to the vaporizer section save through said pipe.

'7. In a system for separating air by liquefaction and rectification, the combination of interchangers, a liquefier, a rectifying column having a tray section and a vaporizer section for vaporizing part of the liquid reflux from the tray' section, an external vaporizer of two or more sections arranged in series with one another and the vaporizer section 01' the column, a pipe for withdrawing the vaporized liquid of the last external section and returning it to the liquefier,

and connections whereby vaporized liquid irom the other external section or sections may flow to said withdrawal pipe or to the tray section.

8. In a system for separating air by liquefaction and rectification, the combination-of interchangers, a liquefier, a rectifying column having a tray section and a vaporizer section for vaporizing part of the liquid producedin the tray, an external vaporizer of two or more sections arranged in series with one another and the vaporizer section of the column, a pipe for withdrawing the vaporized liquid of the last external section and returning it to the liquefier, and connections whereby vaporized liquid from the other external section or sections may flow to said withdrawal pipe or to the. tray section, together with means for withdrawing uncondensed vapor from thelast external section and purifying the same by liquid from the last section, and means for leading such liquid to the column for abstracting heat therefrom.

WM. L. DE BAUFRE.

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