US3210948A

US3210948A - Method for fractionating gaseous mixtures

Info

Publication number: US3210948A
Application number: US260342A
Authority: US
Inventors: Clarence J Schilling
Original assignee: Air Products and Chemicals Inc
Current assignee: Air Products and Chemicals Inc
Priority date: 1958-05-19
Filing date: 1963-01-29
Publication date: 1965-10-12
Anticipated expiration: 1982-10-12
Also published as: GB864855A

Description

Oct. 12, 1965 c. J. SCHILLING METHOD FOR FRACTIONATING GASEOUS MIXTURES Original Filed May 19, 1958 m 5 Y bntam u M 1/ mZ M. R m m r O T N M M 2 a mm m R A L c Y B g 3 o umnmmmmm I2:

United States Patent 3,210,948 METHOD FOR FRACTIONATING GASEOUS MIXTURES Clarence J. Schilling, Allentown, Pa., assignor, by mesne assignments, to Air Products and Chemicals, Inc.,

Trexlertown, 1821., a corporation of Delaware Continuation of application Ser. No. 736,327, May 19, 1958. This application Jan, 29, 1963, Ser. No. 260,342

3 Claims. (Cl. 6213) This application is a continuation of applicants copending application Serial No. 736,327, filed May 19, 1958, for Method and Apparatus for Fractionating Gaseous Mixutres, now abandoned.

The present invention relates to methods and apparatus for separating the componets of gaseous mixtures, and more particiularly to such methods and apparatus wherein the enthalpy of a fluid stream is reduced with the performance of work. The invention has particular utility in connction with the separation of components of gaseous mixtures by low temperature liquefaction and fractionation and will be illustrated by way of example in connection with the liquefaction and fractionation of air for the separation of oxygen and nitrogen components thereof.

In the liquefaction and fractionation of air and other gaseous mixtures, it is common practice to subject the gaseous mixtures to a relatively high pressure, and then to pass it through and orifice to a region of relatively low pressure. Upon throttling in this manner, a drop in temperature of the gaseous mixture is observed; and this phenomenon is the well known Joule-Thomson effect. At least one such temperature drop, after heat exchange with relatively cold fluids, is utilized to bring the gaseous mixture to a desired temperature and pressure within its liquefaction range.

Thereafter, the gaseous mixture, in liquid phase or in two-phase liquid-vapor relationship, is fed to a fractionating column in which it is fractionated in known manner to segregate the components of the gaseous mixture each in the phase appropriate to its boiling point. Inasmuch as at least aone of the components as separated in a single column is relatively impure, it is common practice to feed them in liquid phase to a second fractionating column at a temperature and pressure lower than the temperature and pressure of the first column where the liquid phase and the vapor phase collecting in the final stage will contain the components of the gaseous mixture with at least one in a highly segregated, commercially pure condition. Liquefaction of the first-stage vapor phase is ordinarily accomplished by heat exchange with the second-stage liquid phase. Thus both interstage streams are in liquid phase and are commonly throttled through an expansion valve to drop the temperature and pressure thereof, as described above. Not only in the case of the mixture fed to the first stage but also in the case of the interstage streams, the expansion through the expansion valves is adiabatic, that is, the enthalpy remains constant. Therefore, in a conventional fractionating operation there are a plurality of streams of material passing from regions of high pressure to regions of relatively low pressure and concurrently undergoing adiabatic expansion so that the enthalpy of each stream remains constant.

In the present invention the energy requirements of a fractionating operation are substantially reduced. This is accomplished by withdrawing energy from liquid phase material and vapor phase material as these materials are passing from zones of relatively high pressure to zones of relatively low pressure thereby to utilize the available energy of any or all of the material. By available energy is meant any portion up to and including all of the energy of material at any stage in excess of that which the material must possess at the next succeeding stage of the process. By this operation the reduction in energy requirements of the overall operation resulting from this energy withdrawal will be substantially greater than the energy absorbed by the expansion devices. The present invention also contemplates applying the energy withdrawn from the liquid phase and from the vapor to at least one other stream of fluid in the operation to raise the pressure of that other stream to a required level thereby further to reduce the energy requirements of the overall operation.

The present invention provides methods and apparatus for passing streams of liquids and vapors at high pressure through work machines to lower levels of pressure to reduce the enthalpy of the streams thereby to effect the greatest possible practical reduction in the energy requirements of the overall operation. The invention also provides methods and apparatus for utilizing the withdrawn energy for raising the pressure of a fluid stream of the operation.

The specific features and objects of the present invention will appear more fully below from the following description, taken in conjunction with the accompanying drawing, of which the single figure is a diagrammatic view of a two-stage fractionating cycle incorporating the principles of the present invention.

Referring to the drawing in greater detail, the cycle there illustrated comprises a fractionatingzone including a high-pressure column or section 10 and a low-pressure column or section 12, each having a plurality of bubble plates 14- orother conventional liquid vapor contact means therein. The sections 10 and 12 comprise fractionating zones, and in a sense they may be considered as sub-zones of the total fractionating zone comprising the overall column.

A stream of gaseous mixture, such as air which for the purposes of the present invention has been substantially freed from water and carbon dioxide in any desired manpool 28 of liquid phase collects in the lower portion of separator 26.

Liquid phase at its boiling point and at substantially the pressure of the air leaving exchanger 22 is withdrawn from the bottom of separator 26 and passed through conduit 30 past control valve 32 therein and introduced into the casing of a Pelton wheel 34 where it impinges on a displaceable member comprising a vaned rotor 36, thereby turning the rotor to perform work, in a manner described below. A reduction in the enthalpy of the liquid phase takes place, which is equal in magnitude to the work performed by the Pelton wheel, including friction losses. By this means, energy is withdrawn from the stream of liquid phase, with a corresponding decrease in both the temperature and the pressure of the stream. Thus, the stream of liquid phase passes from a zone of relatively high pressure to a zone of relatively low pressure, the Pelton wheel dividing the zones from each other and providing the means whereby the pressure decrease between the zones is effected. In this way, the liquid phase is expanded with work, the expansion of a liquid with work as referred to in this application signifying a decrease in pressure accompanied by a reduction of enthalpy corresponding to the work done by the expanding liquid through the medium of the displaceable member and including friction losses.

From Pelton wheel 34, the stream of liquid phase of reduced enthalpy is conducted through a conduit 38 past a point of junction 40, through a conduit 42 and into high-pressure section wherein the liquid phase collects in a boiling pool 44 of crude oxygen.

From the phase separator 26, the vapor phase is withdrawn as overhead through conduit 46 and is passed through the cold end only of heat exchanger 22 where it is warmed against input air. The overhead then passes through conduit 48 to point of division 50 from which at least a portion passes through conduit 52 as determined by control valve 54 therein to a gas phase expander 56 having a displaceable member therein on which the vapor phase impinges and does Work as pointed out below, thereby to reduce the enthalpy of this branch of the vapor phase phase with a corresponding decrease in temperature and pressure. The expanded vapor phase then passes through conduit 58 to point of junction 40 where it joins and merges with the expanded liquid phase passing through conduit 42 into high-pressure section 10. The expanded fluid leaving expander 56 through conduit 58 is in vapor phase and provides at least a part of the ascending vapors in the liquid-gas contact section of high-pressure section 10.

From point of division 50 the other branch of the vapor phase passes through conduit 60 having a control valve 62 therein and into a second gas phase expander 64 in which it is expanded with work as in expander 56. The fluid expanded in expander 64 then passes through conduit 66 into low-pressure section 12 at the appropriate composition level.

The flow of vapor phase through conduit 52 may be regulated relative to that through conduit 60 by manipulation of valve 54 or valve 62 or both. If desired, one of these two valves may be closed entirely so that the entire vapor phase passes through the conduit controlled by the other valve.

In high-pressure section 10, the upwardly flowing vapors are liquefied in a reboiler condenser 68 in heat exchange relationship with the pool 70 of liquid product in lowpressure section 12. A portion of the liquid condensed by condenser 68 falls back onto the top plate of section 10 as wash liquid or reflux therefor and another portion collects in a pool 72 established on an annular shelf 74 about the side walls of section 10. In the case of the liquefaction and fractionation of air, the pool 44 will be crude oxygen, pool 72 will be substantially nitrogen, pool 70 will be oxygen of desired purity, and the overhead vapors in the vapor dome of section 12 will be commercially pure nitrogen.

The principal feed stream for section 12 in the fractionation of air is provided in the form of crude oxygen which has been withdrawn from pool 44 through a conduit 76 at substantially the pressure of section 10 and at its boiling point at that pressure and has been directed in liquid phase into the casing of a Pelton wheel 78 where it impinges on a displaceable member or rotor 80 to thereby reduce the enthalpy of the crude oxygen stream in liquid phase in accordance with the same principles as in the case of wheel 34. The crude oxygen, at a reduced temperature and pressure, then passes through conduit 82 and is introduced into section 12 at an appropriate composition level.

In like manner, nitrogen in liquid phase is withdrawn from pool 72 and conducted through conduit 84 to Pelton wheel 86 having a displaceable member or rotor 88 where it is expanded with work with a corresponding reduction in enthalpy, and therefore in temperature and pressure, as also in the case of

wheels

34 and 78. The expanded fluid leaving wheel 86 is preferably substantially entirely in liquid phase and passes through conduit 90 and into the top of low-pressure section 12 as wash liquid.

One product stream of the operation may comprise nitrogen gas, or equivalent lower boiling component, withdrawn from the vapor dome of section 12 through conduit 92 and passed through heat exchanger 22, in which it is warmed against the air supply and finally removed from the operation as product nitrogen gas at about atmospheric pressure and temperature.

A portion of the product oxygen or equivalent higher boiling component, is removed from section 12 in vapor phase from just above pool 70 through a conduit 94, and then passed through another passageway of heat exchanger 22 with the same result as in the case of the product nitrogen, also leaving exchanger 22 at about atmospheric temperature and pressure as a stream of low-pressure product oxygen.

Product oxygen is also removed in liquid phase from pool 70 through conduit 96 by which it is conducted to the input end of a pump 98 which may be of a conventional reciprocating type. By means of pump 98 the pressure of the liquid product oxygen may be raised to any pressure required in the commercial utilization of oxygen, that is, from 150 p.s.i. up to around 3000 p.s.i. The pressurized liquid oxygen passes through conduit 100 and still another passageway of heat exchanger 22 in which it is interchanged in the same way as the other two product streams and leaves as the high-pressure oxygen product.

In known manner a stream of high-pressure low boiling point product, in this case nitrogen, may be withdrawn from the vapor dome of condenser 68 through conduit 101, which conduit is controlled by valve 103, and passed through the cold end of exchanger 22 to a gas expansion turbine 105. The exhaust side of the turbine is at a pressure near that of the low boiling point product in line 92 and is connected therewith through line 107. By adjusting valve 103 any desired amount of nitrogen may be sent to expander 105 or where desired none.

There are thus provided by the present invention means for expanding with work each of the input and interstage fluid streams of the cycle to effect a corresponding reduction in the overall input energy requirements of the cycle. In addition, means are provided for utilizing the work done by all of the expanding fluids thereby to utilize the total available energy of the cycle and even further to reduce the overall input energy requirements of the cycle. Referring again to the drawing, wheel 34 is drivingly interconnected with a generator 102 by means of a mechanical coupling 104, generator 102 being connected in electrical circuit with trunk lines 106 and 108 by means of connectors 110 and 112. Similarly, expanders 56, 64 and 105 are drivingly interconnected with

generators

114, 116 and 109, respectively, and are also similarly connected in electrical circuit with lines 106 and 108 to feed power thereto. Finally, generators 110 and 120 are driven by wheels 78 and 06, respectively and also are connected to feed into trunk lines 106 and 103.

For the purpose of utilizing the electrical energy thus produced, motor 122 is drivingly interconnected with pump 98 by means of mechanical coupling 124 and draws power from lines 106 and 108 through

connectors

126 and 128. Similarly, motor 130 is drivingly connected with compressor 10 and draws power from lines 106 and 108 to assist in the initial compression of the air supply in conjunction with the usual motor 132 driven by an external power source (not shown).

It will thus be seen that by means of

generators

102, 114 and 116, means are provided by which substantially the total available energy of the initial column feed may be utilized to supply at least a portion of the cold requirements of the cycle. In addition, the withdrawn energy may be utilized in devices driven by the electrical energy generated in this way to supply power requirements for energy is utilized to reduce the cold requirements of any part or all of the cycle. In addition, the abstracted kinetic energy is utilized to perform useful work in the system by means of the generators and motors described above, thereby to reduce the power requirements of the cycle.

When the abstracted kinetic energy is utilized to perform useful work in the system it will be appreciated that the total available energy of the cycle may be utilized in this manner so as to reduce to a minimum the power which need be supplied to the operation from external sources such as by compressor motor 132. However, it is also to be understood that any fractional portion of this total available energy may also be utilized, thereby providing a reserve of available power up to the limit of the total available energy so as to provide for peak power requirements arising from normal fluctuations in the operation of the cycle without providing for such peak requirements by means of standby equipment operated from external power sources.

There is thus provided by the present invention novel methods of and apparatus for separating different boiling point components of gaseous mixtures in a fractionating operation, such as the fractionation of air into oxygen and nitrogen, in which the overall energy requirements of the operation are greatly reduced by extracting kinetic energy from an initial or interstage feed stream or both and utilizing the extracted kinetic energy to perform work. Desirably, the extracted kinetic energy is also utilized to perform useful work on at least one other stream of the cycle.

Although the present invention has been described and illustrated in connection with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit of the invention, as those skilled in this art will readily understand. For example, expansion engines of the positive displacement type in which work is done on a displaceable member in the form of a reciprocating element such as a piston, may be substituted in appropriate circumstances for the illustrated Pelton wheels and expansion engines. Such modifications and variations are considered to be within the purview and scope of the present invention as defined by the appended claims.

What is claimed is:

1. Method of separating gaseous mixtures in a fractionating operation, in which operation cooled gaseous mixture is subjected to fractionation in a relatively high pressure fractionating zone and a relatively low pressure fractionating zone thereby to produce cold components of the gaseous mixture, comprising the steps of cooling compressed gaseous mixture by heat interchange with at least one cold component of the gaseous mixture to effect partial liquefaction of the gaseous mixture, separating the partially liquefied gaseous mixture into a vapor portion and a liquid portion, said portions being in contact with each other, expanding the liquid portion to a lower pressure with production of external work, superheating the vapor portion by heat interchange with compressed gaseous mixture during the cooling and partial liquefaction of the gaseous mixture, expanding the superheated vapor portion to a lower pressure with production of external work, the expansion of the liquid portion and the expansion of the vapor portion being conducted in separate zones, and passing the expanded liquid portion and at least a portion of the expanded vapor portion, at equal pressures, to said relatively high pressure fractionating zone.

2. Method of separating gaseous mixtures in a fractionating operation, in which operation cooled gaseous mixture is subjected to fractionation in a relatively high pressure fractionating zone and a relatively low pressure fractionating zone thereby to produce cold components of the gaseous mixture, comprising the steps of cooling compressed gaseous mixture by heat interchange with at least one cold component of the gaseous mixture to effect partial liquefaction of the gaseous mixture, separating the partially liquefied gaseous mixture into a vapor portion and a liquid portion, said portions being in contact with each other, expanding the liquid portion to a lower pressure with production of external work, superheating the vapor portion by heat interchange with compressed gaseous mixture during the cooling and partial liquefaction of the gaseous mixture, passing the expanded liquid portion to said relatively high pressure fractionating zone, expanding the superheated vapor portion to a lower pressure with production of external work, the expansion of the liquid portion and the expansion of the vapor portion being conducted in separate zones, and subjecting at least a portion of the expanded superheated vapor portion to fractionation in said relatively low pressure fractionating zone.

3. Method of separating gaseous mixtures in a fractionating operation, in which operation cooled gaseous mixture is subjected to fractionation under low temperature to produce cold components of the gaseous mixture, the fractionation taking place in a column including a high pressure fractionating zone and a low pressure fractionating zone and a refluxing condenser connected between the top of the high pressure fractionating zone and the bottom of the low pressure fractionating zone, comprising the steps of cooling compressed gaseous mixture by heat interchange with at least one cold component of the gaseous mixture to effect partial liquefaction of the gaseous mixture, separating the partially liquefied gaseous mixture into a vapor portion and a liquid portion, said portions being in contact with each other, expanding the liquid portion to a lower pressure with production of external work, expanding the vapor portion to a lower pressure with production of external work, the expansion of the liquid portion and the expansion of the vapor portion being conducted in separate zones, introducing the expanded liquid portion and at least a portion of the expanded vapor portion, at equal pressures, into the high pressure fractionating zone wherein the gaseous mixture is separated into a liquid high boiling point fraction collecting in the bottom of the high pressure zone and a gaseous low boiling point fraction collecting in the top of the high pressure zone, withdrawing liquid high boiling point fraction from the high pressure zone, expanding withdrawn liquid high boiling point fraction to a lower pressure with production of external work, introducing expanded liquid high boiling point fraction into the low pressure zone wherein the separation of the gaseous mixture is continued producing liquid high boiling point component collecting in the base of the low pressure zone and gaseous low boiling point component collecting in the top of the low pressure zone, the refluxing condenser establishing heat interchange be tween the gaseous low boiling point fraction and the liquid high boiling point component to effect liquefaction of the gaseous low boiling point fraction, expanding liquefied low boiling point fraction to a lower pressure with production of external work, introducing expanded liquefied low boiling point fraction into the low pressure zone as reflux, and warming the vapor portion by heat interchange with a relatively warm fluid of the operation.

(References on following page) References Cited by the Examiner UNITED STATES PATENTS Van Nuys 6230 Patterson 6239 X Houvener 6239 X Rice 6214 Rice 6214 X Deanesly 6238 X Yendall 6238 X Yendall 6214 Matsch 6214 Potts 6214 Tsunoda 6214 NORMAN YUDKOFF, Primary Examiner.

Claims

1. METHOD OF SEPARATING GASEOUS MIXTURE IN A FRACTIONATING OPERATION, IN WHICH OPERATION COOLED GASEOUS MIXTURE IS SUBJECTED TO FRACTIONATION IN A RELATIVELY HIGH PRESSURE FRACTIONATING ZONE AND A RELATIVELY LOW PRESSURE FRACTIONATING ZONE THEREBY TO PRODUCE COLD COMPONENTS OF THE GASEOUS MIXTURE, COMPRISING THE STEPS OF COOLING COMPRESSED GASEOUS MIXTURE BY HEAT INTERCHANGE WITH AT LEAST ONE COLD COMPONENT OF THE GASEOUS MIXTURE TO EFFECT PARTIAL LIQUEFACTION OF THE GASEOUS MIXTURE, SEPARATING THE PARTIALLY LIQUEFIED GASEOUS MIXTURE INTO A VAPOR PORTION AND A LIQUID PORTION, SAID PORTIONS BEING IN CONTACT WITH EACH OTHER, EXPANDING THE LIQUID PORTION TO A LOWER PRESSURE WITH PRODUCTION OF EXTERNAL WORK, SUPERHEATING THE VAPOR PORTION BY HEAT INTERCHANGE WITH COMPRESSED GASESOUS MIXTURE DURING THE COOLING AND PARTIAL LIQUEFACTION OF THE GASEOUS MIXTURE, EXPANDING THE SUPERHEATED VAPOR PORTION TO A LOWER PRESSURE WITH PRODUCTION OF EXTERNAL WORK, THE EXPANSION OF THE LIQUID PORTION AND THE EXPANSION OF THE VAPOR PORTION BEING CONDUCTED IN SEPARATE ZONES, AND PASSING THE EXPANDED LIQUID PORTION AND AT LEAST A PORTION OF THE EXPANDED VAPOR PORTION, AT EQUAL PRESSURES, TO SID RELATIVELY HIGH PRESSURE FRACTIONATING ZONE.