The Production Of Oxygen

Abstract

A plant for the production of low purity oxygen, in which a low pressure stream of incoming air is cooled against outgoing gas streams and fed into a high pressure fractionating column, and a high pressure stream of incoming air is cooled against outgoing gas streams, partially condensed against boiling liquid oxygen product in a product vaporizer, and separated into gas and liquid streams, the liquid stream being sub-cooled and expanded into a low pressure fractionating column while a major part of the gas stream is re-heated and expanded to provide plant refrigeration. Crude liquid oxygen from the bottom of the high pressure column is cooled against waste outgoing nitrogen from the low pressure column and then admitted to the low pressure column after first being used to liquefy some of the nitrogen from the high pressure column in an external reboiler/condenser. Liquid oxygen product from the low pressure column is pumped to a higher pressure before being passed through the sub-cooler and the product vaporizer. The remainder of the high pressure column nitrogen is liquefied in a second external reboiler/condenser by the separated high pressure liquid feed on its way to the low pressure column. The liquefied high pressure column nitrogen is used as reflux for the two columns, that for the low pressure column being cooled against outgoing waste nitrogen. The expander exhaust is likewise cooled against outgoing waste nitrogen before admission to the low pressure column.

Description

This invention relates to the production of oxygen, especially low purity oxygen.

There are a number of well established industrial processes using large quantities of air which may become more efficient when the air is oxygen enriched. For example, in the steel industry there is an interest in the use of oxygen enrichment of the air supply to blast furnaces. Oxygen enrichment may also be economically benficial in glass melting furnaces, the fire refining of copper and numerous processes associated with the petrochemical and petroleum cracking industries.

The oxygen from the air separation unit is often mixed with varying quantities of air to produce an enriched air stream which may contain from 25 % to 35 % oxygen. The purity of the oxygen produced from the air separation plant determines the amount of air which must be added to produce a final enriched air stream. The overall cost of producing the enriched air stream is thus a function of the cost of oxygen from the air separation unit, the cost of compressing the oxygen to the enriched air delivery pressure and the cost of compressing the air which is mixed with the oxygen to the enriched air delivery pressure.

The pressure at which the enriched air is used is usually less than 50 psia, for example, when oxygen enriched air is blown into a blast furnace producing iron, or in the fire refining of copper. It is an object of this invention to provide a means of producing an oxygen stream containing up to 70 % of oxygen at a pressure which is high enough for use directly without the need to compress the oxygen gas in an oxygen compressor.

According to the present invention, low purity oxygen is obtained from the fractionation of air in a plant operating according to a split pressure cycle, with a low pressure air feed passing, after cooling, to a high pressure fractionating column, and a high pressure feed passing, after cooling, to a product vaporizer in which it is partially condensed against boiling oxygen product, the liquid fraction then being sub-cooled and flashed down to low pressure column pressure for admission to a low pressure fractionating column.

A major portion of the vapour fraction of the high pressure feed may be reheated and expanded to provide plant refrigeration, and a minor portion condensed against waste nitrogen and combined with the sub-cooled liquid fraction.

An important preferred feature of the cycle is that a first external reboiler/condenser supplies the vapour boil-up for the low pressure column when crude liquid from the base of the high pressure column vaporizes against condensing nitrogen, and a second external reboiler/condenser condenses the remaining nitrogen from the high pressure column against a boiling liquid air stream.

A liquid oxygen stream can be withdrawn from the base of the low pressure distillation column and passed through a pump where its pressure is raised to the required oxygen product delivery pressure, and the oxygen product can be vaporized in a reboiler-condenser with heat transferred from the condensing high pressure air stream.

Such a plant can produce oxygen product at up to 70 % oxygen content. The pressure of oxygen is fixed by the pressure of the high pressure air stream, for example, 50 psia oxygen can be produced with a high pressure air pressure of 109 psia entering the cold box. The limit of 70 % oxygen purity is fixed by the equilibrium between liquid and vapour at the bottom tray of the low pressure distillation column and the mass balance around the base of the low pressure column. The use of the vaporized crude oxygen stream as the vapour feed to the base of the low pressure column sets an upper limit of about 70 % oxygen on the liquid collecting in the sump of the low pressure column.

A low purity oxygen plant in accordance with the invention will now be described by way of example and with reference to the accompanying drawings, in which:

FIG. 1 is a flow diagram of the complete plant; and,

FIG. 2 illustrates a modification of the plant of FIG. 1.

Referring to FIG. 1, air enters the plant at two pressure levels from a low pressure compressor K101 and a high pressure compressor K102. Just over half of the total plant air, the low pressure air, is delivered at 54.3 psia. It is cooled in low pressure reversing heat exchanger cores E101 and E102 and passed directly to a high pressure fractionating column C101.

The reversing heat exchangers are a well known means of cooling down the feed stream to a low temperature plant against warming product streams and at the same time causing high boiling point impurities to be removed from the cooling stream and sublimed into one of the low pressure warming streams by switching the cooling stream and the particular warming stream passages at regular intervals. This switching of the two streams between identical sets of passages in the heat exchanger is accomplished by means of switch valves at the warm end of the heat exchanger and self-acting check valves at the cold end of the heat exchanger. In the present plant the air and waste nitrogen streams are switched at regular intervals to remove water and carbon dioxide from the air streams.

The rest of the plant air, the high pressure air, is delivered at 109.0 psia. It is cooled in high pressure reversing heat exchanger cores E103 and E104 and enters a product vapourizer E109 where it is partially condensed against boiling product liquid oxygen. The resulting two-phase air mixture passes into a phase separator C110. The liquid air fraction, about 75 % of the total high pressure air stream, is passed through a sub-cooler E108 where it is cooled against the product liquid oxygen stream then flashed down to low pressure column pressure and fed into a separator C107. The liquid portion of the stream from the separator C107 passes through an external reboiler/condenser E106 in which about 88 % of the stream is vaporized against condensing nitrogen from the top of the high pressure column C101. The liquid and vapour air fractions from the separator C107 then enter a low pressure fractionating column C102. The operating pressure of the low pressure column is 21.5 psia at the base.

Crude liquid oxygen containing 44 % oxygen, withdrawn from the bottom of the high pressure column C101, is sub-cooled against waste nitrogen from the top of the low pressure column in a main sub-cooler E.105. Hydrocarbons are then removed in adsorbers C105 and the crude liquid is flashed down to low pressure column pressure and fed into a separator C108. The liquid stream from the separator C108 is mostly vaporized in an external reboiler/condenser E107 against condensing nitrogen from the top of the high pressure column, and the whole of the flow from the separator C108 enters the base of the low pressure column C102. A small liquid purge stream is taken from the separator C108 to the sump of the column C102, by-passing the reboiler/condenser E107, to prevent possible hydrocarbon buildup.

The operating pressure of the high pressure column C101 is 48.3 psia at the top. Nitrogen gas leaving the top of the high pressure column C101 is totally condensed in the external reboiler/condensers E106 and E.107 and part of it provides the reflux for the high pressure column C101. The remaining part is sub-cooled against waste nitrogen from the top of the low pressure column C102 in the main sub-cooler E105 and flashed down to low pressure column pressure to provide the reflux for the low pressure column C102.

Returning to the partial condensation of the high pressure air stream in the product vaporizer E109, about 25 % of the total high pressure air stream leaves the separator C110 as vapour. 90 % of this vapour fraction is used as a reheat stream for the high and low pressure heat exchanger cold cores E102 and E104. This method of controlling the temperature difference at the cold ends of the reversing heat exchangers E102 and E104 ensures that the solid carbon dioxide deposited from the air stream on to the surface of the heat exchangers is sublimed into the waste nitrogen stream when the exchangers are switched. The remaining 10 % of vapour is condensed in a nitrogen superheater E110 against that fraction of waste nitrogen from the sub-cooler E105 which leaves the plant through the high pressure reversing heat exchangers E103 and E104. The liquid air stream leaving the superheater E110 rejoins the main liquid air stream from the sub-cooler E108. The function of the nitrogen superheater E110 is to warm the outgoing nitrogen sufficiently to ensure that the high pressure air feed is not liquefied in the cold cores E104 of the high pressure reversing heat exchanger. This is necessary for proper removal of CO.sub.2 in the reversing heat exchangers E103 and E104.

The fraction of the high pressure air which has been reheated in the reversing heat exchangers is expanded through a turbo-expander K103 and provides the plant refrigeration requirement. A by-pass is available to pass more high pressure air through the turbine should additional refrigeration be required. The expander exhaust is cooled in the main sub-cooler E105 and enters the low pressure column C102 slightly superheated.

Outgoing waste nitrogen from the low pressure column C102 is warmed against liquid nitrogen from the high pressure column, the crude O.sub.2 liquid from the high pressure column, and the turbo-expander exhaust stream, in the main sub-cooler E105. It leaves the plant via the high and low pressure reversing heat exchangers E101 to E104.

Liquid containing 68 % oxygen is withdrawn from the low pressure column sump and raised in pressure by a pump G101 to 55 psia. The total stream is passed through hydrocarbon adsorbers C106, warmed to its bubble point against liquid air in the sub-cooler E108 and fed to a separator C109. A small liquid bleed from this separator is flashed down to low pressure column pressure and returned to the low pressure column to prevent possible hydrocarbon buildup. The remaining liquid oxygen is totally vaporized against condensing high pressure air in the product vaporizer E109. This, the product stream, passes through the reversing heat exchangers E101-E104, and leaves the plant at 50 psia.

The concentration of CO.sub.2 in the high pressure air leaving the high pressure reversing heat exchanger E104 is about 0.3 ppm, and in addition, there may be some solid CO.sub.2 carryover from the heat exchanger E104. This CO.sub.2 would tend to deposit in the air channels at the top end of the product vaporizer E109 where no liquefaction takes place unless special methods were used to prevent this. Several different methods are available to prevent this deposition.

1. The high pressure air may be fed into the vaporizer E109 at a point part way down its length as indicated in FIG. 1. This arrangement leaves some free surface area above the feed point where condensation will take place providing a falling liquid film to wash away any CO.sub.2 deposited on the surface area below or near the feed point.

2. Another method is to pump a small liquid bleed from the bottom of the separator C110 to the top of the vaporizer E109 by a pump as shown in FIG. 2. This arrangement will also ensure that the incoming air meets a falling liquid film to dissolve and wash away any CO.sub.2 deposition.

3. A further method is to pass the high pressure air stream leaving the high pressure reversing heat exchangers through carbon dioxide absorbers. These will also remove any hydrocarbons in the stream.

The CO.sub.2 concentration in the vapour fraction leaving the separator C110 is so low that there is no risk of CO.sub.2 deposition in the expander K103 or in the main sub-cooler E105. Also, the vapour leaving the separator is at its dewpoint and will immediately condense in the nitrogen superheater E110 giving no deposition problem in that unit.

It will be seen that two pairs of hydrocarbon absorbers are provided, one C105 to remove hydrocarbons from the crude liquid oxygen stream from the high pressure column, and the other C106 to remove hydrocarbons from the product liquid before it enters the vaporizer. The external reboiler/condenser E107 in which the crude oxygen is totally vaporized, is protected from the build-up of residual hydrocarbons by providing the small liquid bleed to the low pressure column from the separator C108. The other external reboiler/condenser E106 only partly vaporizes the liquid air stream and the constant liquid off-take prevents hydrocarbon build-up in that unit. The small liquid bleed from the separator C109 back to the low pressure column prevents build-up of residual hydrocarbons in the product vaporizer E109.

The plant described has been designed to eliminate problems of CO.sub.2 deposition and hydrocarbon build-up which are often critical in low purity oxygen plants.

Variations on the design described above may be considered which use other means than reversing heat exchangers such as plate-fin heat exchangers, to cool the air and remove water and carbon dioxide impurities. These would include regenerators filled with stones or other types of packing and having air sidestreams instead of reheat streams for temperature difference control.

Claims

What I claim is:

1. Plant for the production of low purity oxygen by the fractionation of air, comprising a low pressure compressor compressing a first air feed stream in a first feed line, a high pressure compressor compressing a second air feed stream in a second feed line, a first heat exchanger in which said first feed stream from said low pressure compressor is cooled against outgoing gas streams, a high pressure fractionating column receiving the cooled first feed stream from said first heat exchanger as feed, a second heat exchanger in which said second feed stream from said high pressure compressor is cooled against outgoing gas streams, a product vaporizer in which said cooled second feed stream from said second heat exchanger is partially condensed against boiling oxygen product, a first separator receiving said partially condensed second feed stream from said product vaporizer and separating it into a liquid fraction stream, a first sub-cooler receiving said liquid fraction stream from said first separator and cooling it against outgoing liquid oxygen, a low pressure fractionating column receiving said cooled liquid fraction stream from said sub-cooler as feed after flashing of said cooled liquid fraction stream down to low pressure column pressure, an outgoing liquid product line delivering liquid oxygen from the bottom of said low pressure column to said product vaporizer via said sub-cooler, and an outgoing waste nitrogen line delivering nitrogen gas from the top of said low pressure column to said first and second heat exchangers.

2. Plant according to claim 1, further comprising passages in said second heat exchanger receiving a major proportion of said vapour fraction stream from said first separator and reheating said major proportion of said vapour fraction stream, an expander receiving said reheated major proportion of said vapour fraction stream from said passages and expanding it to provide plant refrigeration, and an expander exhaust line delivering the exhaust from said expander to said low pressure column as feed.

3. Plant according to claim 2, further comprising a nitrogen superheater in which a minor proportion of said vapour fraction stream from said first separator is condensed against outgoing waste nitrogen in said waste nitrogen line, the condensed minor proportion of said vapour fraction stream from said nitrogen superheater being combined with said cooled liquid fraction stream from said sub-cooler for delivery to said low pressure column.

4. Plant according to claim 1, further comprising a first external reboiler/condenser receiving crude liquid oxygen from the bottom of said high pressure column after it has been flashed down to low pressure column pressure, and partially vaporizing it against a condensing nitrogen stream from the top of said high pressure column, the partially vaporized crude liquid oxygen being then fed to the bottom of the low pressure column.

5. Plant according to claim 4, further comprising a second external reboiler/condenser in which another portion of tne nitrogen from the top of the high pressure column is condensed against the boiling cooled liquid fraction stream from said sub-cooler before said fraction stream enters the low pressure column, the condensed nitrogen from said reboiler/condensers providing reflux streams for both columns.

6. Plant according to claim 5, further comprising a second sub-cooler in which the nitrogen reflux stream for the low pressure column is sub-cooled against the waste nitrogen stream from the top of that column.

7. Plant according to claim 2, wherein said expander exhaust line includes a cooler in which the expander exhaust is cooled against the waste nitrogen from the top of the low pressure column.

8. Plant according to claim 4, further comprising a cooler in which the crude oxygen from the bottom of said high pressure column is cooled against the waste nitrogen from the top of the low pressure column before entering said first reboiler/condenser.

9. Plant according to claim 1, further comprising a pump in said outgoing liquid product line which pumps the liquid oxygen to a higher pressure before it enters said product vaporizer.

10. Plant according to claim 1, wherein passages in said product vaporizer conduct said cooled second feed stream through said product vaporizer in a downward flow direction, said feed stream entering at a point part way down from the top of said vaporizer.

11. Plant according to claim 10, further comprising a bleed line withdrawing a small liquid bleed from said liquid fraction stream leaving said separator and delivering it to said product vaporizer passages at the top of said vaporizer.

12. Plant according to claim 1, further comprising a second separator receiving the product oxygen from said product vaporizer and separating it into a vaporized product fraction and a liquid product fraction, a line returning said liquid product fraction to said product vaporizer, a gaseous product line delivering said vaporized product fraction to said first and second heat exchangers, and a bleed line delivering a small amount of said liquid product fraction to said low pressure column.

13. Plant according to claim 5, comprising further separators respectively receiving the crude liquid oxygen from said first reboiler/condenser and said feed liquid fraction stream from said second reboiler/condenser, each of said further separators having a liquid delivery line returning separated liquid to the respective reboiler/condenser, a gas delivery line feeding separated vapour to said low pressure volumn, and a bleed line delivering a small portion of separated liquid to said low pressure column.