US3397548A - Method for supplying a gaseous product to meet a variable demand - Google Patents

Description

g- 20, 1963 s. ERGENC 3,397,548

METHOD FOR SUPPLYING A GASEOUS PRODUCT TO MEET A VARIABLE DEMAND 2 Sheets-Sheet 1 Filed April 20, 1966 RECTIFYING F 9- COLUMN GASEOUS 101 NITROGEN 700 HEAT 5 OXYGEN Inventor: Suhubetfln Ergenc BY W, I Wham, M45 MM ATTORNEYS Aug. 20, 1968 s. ERGENC METHOD FOR SUPPLYING A GASEOUS PRODUCT TO MEET A VARIABLE DEMAND Filed April 20, 1966 2 Sheets-Sheet 2 HEAT EXCHANGERS PER-COOLER ITROGEN 130 124 3 LOW DEMAND OXYGEN ON WAY TO LIQUEFACTION Fig-2 lnvenfor: Schubefi' E2 BY 6 In I enc m Ja lwlM laflv ATTORNEYS United States Patent 3,397,548 METHOD FOR SUPPLYING A GASEOUS PRODUCT TO MEET A VARIABLE DEMAND Sahabettin Ergenc, Zurich, Switzerland, assignor to Sulzer Brothers Limited, Winterthur, Switzerland, a Swiss company Filed Apr. 20, 1966, Ser. No. 543,823 Claims priority, application Switzerland, Apr. 30, 1%5, 6,012/ 65 1 Claim. (Cl. 62-52) ABSTRACT OF THE DISCLOSURE A portion of a first gas obtained from rectification, when not required by a variable user, is liquefied by a liquefied second gas obtained from said rectification step, said second gas being compressed, cooled, work expanded and liquefied, said liquefaction of the second gas being accomplished by heat exchange with liquefied first gas. The liquefied second gas is thereafter throttled and vaporized by passing in heat exchange with condensing first gas after which vaporized second gas is passed in heat exchange with air feed which air feed then passes to said rectification step.

The present invention pertains to a method and apparatus for the storage, to meet a variable demand, of an atmospheric or air-constituent gas, in particular oxygen, obtained at a constant rate by a low temperature rectification from a plant for the separation of atmospheric air into its constituent gases.

The usual installations for separation of atmospheric air into its constituent gases can be economically operated only in continuous fashion by reason of the long start-up times thereof. It is therefore customary to keep such plants in continuous operation whether the product gases can be applied to the intended use therefor or not. For example, steel plants are frequently shut down over weekends or holidays so that during these times there exists no need for oxygen. Moreover, in such plants the demand for oxygen varies widely during the working day.

In air separation plants in which oxygen is obtained in gaseous form, storage thereof in gas holders when the gas is produced beyond instantaneous requirements is expensive and uneconomical, especially when no demand for oxygen may exist for a long period of time. Consequently, in such cases the gaseous product is frequently vented to the atmosphere instead of being stored. With such practice however the plant must be designed and dimensioned to meet the peak rather than the average demand.

To avoid these disadvantages, it has heretofore been proposed to operate an air separation plant in which an air-constituent gas such as oxygen is obtained at a constant rate by low temperature rectification, and to match the operation to a varying demand for oxygen by liquefying and storing in tanks excess oxygen available in times of low demand. In this prior art process the heat of vaporization of the oxygen to be so stored is abstracted therefrom by passing it in heat exchange relation with previously liquefied raw gas, i.e. air, or with a previously liquefied residual or by-product gas of the operation of the plant such as nitrogen, part of the liquefied gas (e.g. air or nitrogen) which thus serves as a coolant being thereby vaporized. Further according to this prior art process, in times when the demand for oxygen exceeds the current rate of production thereof, the excess demand is met by withdrawing liquid oxygen from storage. The liquid oxygen so withdrawn is vaporized and employed as a cooling agent to cool, by countercurrent flow heat exchange, new raw or residual gas after 3,397,548 Patented Aug. 20, 1968 ICC compression thereof. The raw gas (e.g. air) or residual gas (e.g. nitrogen) is then condensed and finally after expansion to its storage pressure is stored in liquid form in a storage vessel therefor.

In this prior art process however it is necessary, in order to meet the refrigeration load which the stored liquefied gas represents, i.e. to compensate for the ingress of heat thereinto, either continually to withdraw liquid oxygen from the separating plant for use as a refrigerant, or else to compress the raw gas or the residual gas in the compressor to a very high pressure and then to expand it through a throttling valve. In the known process, in the event of a low or zero rate of utilization of oxygen, the oxygen obtained in the separation plant beyond current requirements is condensed by heat exchange in a condenser with liquefied residual gas withdrawn from a storage reservoir therefor. The excess oxygen is then stored in a liquid storage vessel. The residual gas vaporized in the condensation of the oxygen is lost and constitutes part of the cost of operation of the system.

In accordance with the present invention the product gas of the separation plant, usually oxygen, is withdrawn exclusively in gaseous form, and it is an object of the invention to provide a process or method for storing the product gas of substantially improved thermodynamic efliciency over the storage methods of the prior art.

This object is achieved in accordance with the invention in that, at times when the product gas is being produced faster than it is required, the excess thereof is withdrawn in gaseous form directly from the rectifying column and is passed through a condenser disposed in a storage vessel for liquefied raw gas or residual gas of the kind employed in or obtained from that rectification column. The product gas so withdrawn is thus liquefied by heat exchange with previously liquefied raw or residual gas and is then stored in a storage vessel, the fraction of the raw or residual gas vaporized upon this heat exchange being fed through at least one heat exchanger upstream of the rectifying column for precooling of the air input to the column. On the contrary, in times when the product gas is to be delivered directly from the rectifying column for use as fast as it is produced, the product gas itself is employed as refrigerant or coolant in this exchanger to cool the raw air being supplied to the rectifying column.

Since when the product gas is to be stored in liquid form, this gas is drawn directly from the rectifying column at the corresponding low temperature (approximately that of liquefaction) and introduced for condensation into a condenser disposed in the storage vessel for the liquefied raw or residual gas, the fraction of the raw or residual gas (as the case may be) which is vaporized as a result of this condensation can be employed in one or more heat exchangers upstream of the column to abstract heat from the air passing into the column for separation. At times when product gas is being withdrawn from the storage system where it is stored in liquid form, it is vaporized by heat exchange with raw or residual gas and thereby effects condensation of the latter. Under these circumstances the refrigeration load represented by the storage system may advantageously be met by compression, cooling, and workperforming expansion of part of the raw or remainder gas. Optionally, for extraction of heat from the portions of the system below room temperature by means of raw or residual gas, that gas may be cooled by one or more external refrigerants at an appropriate point in the storage system.

The invention will now be further described in terms of a non-limitative example, in which oxygen is the product gas and nitrogen is the residual or by-product gas, taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic diagram of an air separation plant useful in practice of the invention; and

FIG. 2 is a diagrammatic representation of a storage system according to the invention, suitably connected to the separating plant of FIG. 1. FIG. 2 is to be positioned to the left of FIG. 1, the lines 5, 121 and 129 being continuous across the two figures.

In FIG. 1 reference character 100 identifies a rectifying column which may have one or plural stages. Three heat exchangers 101, 102 and 103 are disposed upstream thereof for the air to be separated by the column into its constituent gases. These heat exchangers are traversed by the air to be rectified in the column. The air to be so rectified passes through the exchangers via branch lines 104, 105 and 106 of a line 99, and after flowing through the exchangers the lines 104-106 join at a line 107 which passes into the column 100 at an appropriate level. A line 108 emerges from the lower part of the column for withdrawal of cold gaseous oxygen from the column. The line 108 divides into three branch lines 109, 110 and 111 which constitute the inputs to the exchangers 103, 102 and 101 for the refrigerant (cold gaseous oxygen) flowing therein countercurrent to the air to be cooled as the latter passes through those exchangers via the lines 104, 105 and 106. The lines 109, 110 and 111 include each a valve indicated at 112, 113 and 114 respectively. Lines 115, 116 and 117 connect into the lines 109, 110 and 111 respectively upstream of these valves (in the sense of the flow of oxygen through the lines 109-111), and the lines 115 to 117 include respectively valves 120, 119 and 118. Through these valves 120, 119 and 118, the lines 115 to 117 connect into a common line 121 which leads to the storage plant shown in FIG. 2.

Downstream of the valves 112, 113 and 114 in the sense of oxygen flow from the column 100 to the individual exchangers 101, 102 and 103, separate lines 123, 124 and 125 branch off from lines 109, 110 and 111 respectively and connect through individual valves 126, 127 and 128 to a common line 129. The common line 129, having a valve 130 therein, is for the supply to FIG. 1 of nitrogen, in the case assumed, and connects into the storage system of FIG. 2.

Although in the embodiment illustrated three heat exchangers are provided, the number of such exchangers for cooling of the air to be rectified may be either larger or smaller. The common line 121 for the product gas (oxygen) and the line 129 for the residual gas (nitrogen) connect through valves 122 and 130 into the storage system of FIG. 2. While the air separation plant of FIG. 1 may include other elements of structure, those significant for an understanding of the present invention have been described.

The storage system of FIG. 2 comprises lines 2, 4 and 121 for conduction of the oxygen and lines 5, 6, 8 and 129 for the nitrogen. Line connects into the column 100 at an appropriate level therein for the withdrawal therefrom of nitrogen obtained as an air-constituent gas by rectifying action in the column. Beyond these, the system of FIG. 2 comprises essentially the following elements: a storage vessel 9 for liquid nitrogen, a storage vessel 10 for liquid oxygen, a nitrogen compressor 11, a cooler 12 for dissipation of the heat of compression of that compressor, two counterflow heat exchangers 13 and 14, an expansion turbine 15, a supercooler 16 for liquid nitrogen, a throttling valve 18, and condensers 19 and 20 disposed in the storage vessels 9 and 10 respectively.

At times when liquid oxygen is to be stored, the process of the invention is carried out as follows: If no oxygen at all is being currently consumed, the oxygen available in gaseous form and at or near its liquefaction temperature is withdrawn from the column 100' and is passed through the line 108 and thence through lines 115, 116 and 117 and thence through the line 121 directly into the condenser 19 of the liquid nitrogen storage vessel 9 in FIG. 2, valves 112 to 114 being closed, valves 118 to 120 being open,

1 and valve 122 being likewise open. The valve 131 in line 2 of FIG. 2 is also closed. Valves 126 to 128, and valve 130 are open, as will be presently explained in further detail.

Let it be assumed that the oxygen is withdrawn from the column at a pressure somewhat above one atmosphere. The nitrogen in the storage vessel 9 will be available there at a pressure of one atmosphere and hence will be at a temperature of 77 K. The oxygen withdrawn from the column condenses in the condenser 19 and fiows into the storage vessel 10. Optionally the liquefied oxygen can be further supercooled in the lower portion of the condenser 19 if the condenser is made large enough.

The quantity of nitrogen vaporized in the storage vessel 9 as a result of the heat exchange involved in this condensation passes out of the storage system through the lines 6 and 129, valve 130 being open and valve 133 being closed. It thence passes in equal proportions through valves 126 to 128 and through the branch lines 123, 124 and 125 of the separating plant of FIG. 1 and thence 'as a refrigerant to the left, in FIG. 1, via lines 109411, through the heat exchangers 101, 102 and 103. The nitrogen is heated in the exchangers 101 to 103 by heat exchange with raw air, the nitrogen emerging therefrom at nearly ambient temperature. The nitrogen s emerging at the left side of the exchangers 101-103, at lines 109- 111, can be either retained for further use or vented to the atmosphere. The nitrogen thus serves to cool the raw air flowing through those exchangers via lines 104 to 106 toward the column 100. When instead the oxygen separated out of air in column 100 is consumed as fast as it is produced, exchangers 101-103 are traversed at lines 111, 110 and 109 by oxygen alone.

For additional precooling of the raw air passing toward the column, the exchangers 101, 102 and 103 are traversed by nitrogen withdrawn from the upper end of the column. Except for the line 5 which connects to the storage plant of FIG. 2, the conduits for this purpose have been omitted from the drawing for the sake of simplicity.

The storage vessels 9 and 10 may be spherical in shape and are heat insulated, as for example by means of a layer of granular, solid insulating material. All portions of the system, including conduits, operating below ambient temperature will also be heat insulated.

Referring again to FIG. 2, a throttling valve 21 is disposed in the discharge line 132 downstream of the condenser 19. This valve is operated by a pressure sensitive device 22 in response to the pressure in the vessel 10*. If for example there arrives from the separation plant of FIG. 1 via line 121 a smaller quantity of oxygen than that which is appropriate to the exchange surface of the condenser 19, the valve 21 permits the conduit system 121, 132 to be emptied so as to produce a negative pressure therein and in the vessel 10, via pressure equalization line 4. As soon as the pressure sensitive device 22 detects a pressure below one atmosphere, the valve 21 will thus be operated to reduce the cross-section of the opening therein, and a specified quantity of liquid oxygen will accumulate in the lower end of the condenser 19, so that the surface area of the condenser effectively available for condensation will be reduced. The consequence of this is that the temperature difference between the condensing oxygen and liquid nitrogen will be larger and hence the pressure in the conduit system for oxygen will increase.

If the oxygen is being partly consumed and partly stored, e.g. if for example only two-thirds of the currently produced oxygen is being consumed, then in the example shown the valves 113 and 114 in lines 110 and 111 will be opened and correspondingly the valves 127 and 128 in the nitrogen lines 124 and 125 will be closed. Valves 118 and 119 will also be closed. On the other hand valve 112 will be closed, valve 120 will be open, and valve 126 in nitrogen line 123 will be open. Hence the heat exchangers 101 and 102 will be traversed on the refrigerant side thereof, i.e. at lines 111 and 110, by gaseous oxygen from the column 100, which oxygen is thereby raised to ambient temperature before being delivered to the consumption device. Suitable valving is provided, 'although not shown in the drawing, so that the ends 109', 110' and 111' of lines 109-111 at the left of exchangers 101- 103 can be selectively connected either to a line for delivery of oxygen to a consumption device or, optionally, to a nitrogen collection line or to the atmosphere. In the case of two-thirds consumption assumed, lines 110 and 111 will be employed for the delivery of oxygen.

The one-third of the oxygen passing into line 109 will be transmitted via lines 115 'and 121 into the storage system of FIG. 2 and there liquefied in the manner already described. To this end valve 112 in the oxygen line of the separation plant will be closed and valve 120 and the line 115 will be open, so that this one-third of the oxygen will flow through line 121 to condenser 19. Correspondingly, the refrigerant in exchanger 103 will be nitrogen, flowing through line 129, open valve 126 and into line 109 to the left of closed valve 112. The nitrogen emerging from exchanger 103 at 109 may be preserved or vented to the atmosphere. Advantageously, however, the valves are not simply cut-off valves but are adjustable in order to make possible a suitable continuously variable distribution among the various flows.

Specifically, the quantity of nitrogen vaporized in the vessel 9 as a consequence of condensation there of the one-third of the oxygen being stored will be fed into the heat exchanger 103 via lines 129, 123 and 109, valve 133 being closed and valve 130 being open. In the heat exchanger 103 this nitrogen will flow counterflow to the quantity of air passing through that exchanger. The nitrogen will hence cool this air and be vented out of the system at approximately ambient temperature.

As appears from the foregoing, it is possible by appropriate setting of the valves to deliver the oxygen currently produced in the column either in its entirety to the consumption device (not shown) via all three of the heat exchangers 101-103 or partly to the consumption device via one or more of the heat exchangers 101-103 and partly directly to the storage system, according to the current demand for oxygen.

In the event of a demand for oxygen exceeding the rate of production of the separation plant, with the oxygen so produced being passed via lines '108 to 111 and thence via the exchangers 101 to 103 directly to the consuming device, the valves 1-22 in oxygen line 121 and 130 in the nitrogen line 129 will be closed, and the valves 131 in the oxygen line 2 and 133 in the nitrogen line 6 of FIG. 2 will be open. A quantity of liquid oxygen corresponding to the excess of the demand over current production rate will then be vaporized in the vessel 10 and will be passed around the condenser '19 through line 4 and thence to the consuming device through the line 2, passing through exchangers 14 and 13 in the process and being warmed thereby. With the valve 24 open, gaseous nitrogen will be drawn from the separating plant of FIG. 1 through the line 5 and delivered'to the compressor 11, this nitrogen having been warmed to the vicinity of ambient temperature in heat exchangers of the separating plant. This nitrogen will be aspirated by the compressor and compresed thereby, and after dissipation of the heat of compression thereof in cooler 12 the nitrogen will flow in counter-flow heat exchange relation with oxygen in the heat exchanger '13, the cold oxygen flowing upwardly through exchanger 13 at line 2. The nitrogen will then be expanded to the vicinity of condensation pressure in the turbine 15, being cooled in the process to approximately 140 K. The nitrogen will then be further cooled in the exchanger 14 and will thereafter be brought to about 90 K. by heat exchange with liquid oxygen by passage through the condenser 20 in the storage vessels 10. The nitrogen thus condensed will be supercooled in the supercooler 16 by heat exchange with nitrogen vapor passing out of the vessel 9 upwardly through line 6, and the nitrogen so supercooled will in passing through the throttling valve 18 be reduced to its storage pressure of one atmosphere and delivered into the vessel 9. That is to say, the nitrogen will be expanded to a lower pressure than it would be if it were introduced into the fluid space of the storage vessel where there prevails a higher pressure due to the column of liquid above that suppositious point of introduction into the liquid. With introduction of the expanded nitrogen into the vapor space, there will consequently be produced a larger quantity of liquid nitrogen, which represents a further extraction of heat from the nitrogen vessel which may be regarded as a storage of refrigerant capacity.

The quantity of oxygen vaporized and delivered out through line 2 to the consuming device is adjusted by control of the quantity of nitrogen compressed. In the embodiment illustrated this is effected by operation on the control valve 27 in the bypass line 8. Alternatively the compressor 11 may be driven at a variable speed corresponding to the instantaneous demand for oxygen, or the quantity of nitrogen delivered to the compressor may be adjusted by operation on the valve 24. Line 6 connects into line 5 immediately upstream of compressor 14.

:In embodiments in which both storage vessels are disposed at the same height it is advantageous to provide a pump in the liquid oxygen line 2, downstream of the oxygen condenser 19 in the nitrogen vessel 9, in order to overcome the static pressure due to the height of the liquid column in the oxygen liquid storage vessel.

The embodiment of the invention described may operate with raw gas, i.e. air, in place of nitrogen.

The invention thus provides a method of supplying and storing, in the face of variable demand therefor, an air-constituent, i.e. atmospheric, gas produced by low temperature rectification of air. In accordance with the invention, when the demand is below the rate of production, the gas in excess of demand is condensed into liquid form by passing it in heat exchange relation with a stored liquefied atmospheric gas obtained from the rectification process. This liquefied stored atmospheric gas may be liquid air or a fraction of air other than the gas to whose supply and storage the invention is directed. In either event, it will be termed an air fraction gas in the appended clairns. If the invention is applied to the supply and storage of oxygen, the air fraction gas may be nitrogen or air. The vapor resulting from vaporization of the liquefied air fraction gas by absorption of heat from the condensing atmospheric gas, e.g. oxygen, is according to the invention passed as a coolant in heat exchange relation with additional air preliminary to rectification thereof. When the demand for the atmospheric gas, e.g. oxygen, is equal to or greater than the rate at which that gas is produced by the retcification process, that gas, instead of being partially liquefied and stored, is passed as a coolant in heat exchange relation with that additional air. When the demand for the atmospheric gas is greater than the rate at which it is produced, the excess of the demand over the rate of production is met by withdrawal of that gas from storage, and the liquefied gases in storage are held in liquid condition, i.e. refrigerated, with the help of the air fraction gas as it is obtained from the rectification process, this fraction being compressed, cooled, and expanded with performance of external work, being optionally so cooled by passage in heat exchange relation with the atmospheric gas withdrawn from storage and being moreover condensed into liquid form at a condenser immersed in the storage vessel for the liquefied, stored atmospheric gas.

The invention further provides apparatus for supplying and storing an atmospheric or air-constituent gas, such as oxygen, in the face of a variable demand therefor. This apparatus comprises, in the embodiment illustrated, a rectifying column which supplies the atmospheric gas and also an air fraction gas such as nitrogen. It further comprises at least one heat exchanger such as one or more of the exchangers 101 to 103 for preliminary cooling of air prior to rectification thereof in the column. It further comprises a first vessel 10 for storage of the atmospheric gas in liquefied form and a second vessel 9 for storage of the air fraction gas in liquefied form. It further comprises means such as the condenser 19 to condense the atmospheric gas by heat exchange with the air fraction gas in liquefied form. The apparatus further comprises a compressor 11 to compress the air fraction gas, heat exchangers 13 and 14 to cool it, a turbine 15 to expand it, and a condenser to liquefy it. The apparatus further comprises means including lines 129 and 123 to 125 to apply to those heat exchangers as a refrigerant, upon storage of the atmospheric gas, the air fraction gas as vaporized upon condensation of the atmospheric gas, and it comprises means including lines 108 to 111 to apply to those heat exchangers as a refrigerant, upon withdrawal of the atmospheric gas from storage, the atmospheric gas as withdrawn from the rectifying column.

While the invention has been described hereinabove in terms of the presently preferred practice of the method thereof and in terms of a presently preferred embodiment of the apparatus thereof, the invention itself is not limited thereto but comprises all modifications on and departures from that practice and embodiment properly falling within the spirit and scope of the appended claim.

I claim:

1. A method of storing, in the face of variable demand therefor, an atmospheric gas obtained at substantially constant rate by low temperature rectification, said method comprising, when said demand is below said constant rate, condensing into liquid form the excess of said atmospheric gas, over said demand, by heat exchange with a liquefied air fraction gas obtained from said rectification, and passing the vapor of said liquefied air fraction gas vaporized on such condensation as a coolant in heat exchange relation with additional air preliminary to rectification of said additional air, said method further comprising, when said demand is equal to or greater than said rate, passing said atmospheric gas as a coolant in heat exchange relation with said additional air, and said method further comprising, when said demand is greater than said rate, vaporizing a portion of the previously condensed and liquefied atmospheric gas by compressing, cooling and expanding wit-h performance of external work said air fraction gas as obtained from said rectification, by passing said expanded air fraction gas in heat exchange relation with said liquefied atmospheric gas to liquefy said expanded air fraction gas and to vaporize said liquefied atmospheric gas, and then throttling to a lower pressure the air fraction gas so passed in heat exchange relation with said liquefied atmospheric gas, thereby to obtain a liquid body of air fraction gas which is heat exchanged with the atmospheric gas for liquefaction of the latter in said condensing step at times when said demand is below said constant rate.

References Cited UNITED STATES PATENTS 2,685,181 8/1954 Schlitt 62--30 X 3,058,315 10/1962 Schuftan 62-53 X 3,079,759 3/1963 Schilling. 3,144,316 8/1964 Koehn 62-40 X 3,273,349 9/1966 Litvin et al. 6252 X NORMAN YUDKOFP, Primary Examiner.

V. W. PRETKA, Assistant Examiner.

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