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Method of separation of a gas from a gas mixture

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Publication number
CA1115648A
CA1115648A CA 312623 CA312623A CA1115648A CA 1115648 A CA1115648 A CA 1115648A CA 312623 CA312623 CA 312623 CA 312623 A CA312623 A CA 312623A CA 1115648 A CA1115648 A CA 1115648A
Authority
CA
Grant status
Grant
Patent type
Prior art keywords
gas
mixture
method
barrier
separation
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired
Application number
CA 312623
Other languages
French (fr)
Inventor
Robert Benoit
Jacques Machefer
Jacques Mauvisseau
Pierre Plurien
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Commissariat a l'Energie Atomique et aux Energies Alternatives
Original Assignee
Commissariat a l'Energie Atomique et aux Energies Alternatives
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Grant date

Links

Classifications

    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D53/00Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/22Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by diffusion
    • B01D53/225Multiple stage diffusion
    • B01D53/226Multiple stage diffusion in serial connexion
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D5/00Condensation of vapours; Recovering volatile solvents by condensation
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D53/00Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/002Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by condensation
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D57/00Separation, other than separation of solids, not fully covered by a single other group or subclass, e.g. B03C

Abstract

ABSTRACT OF THE DISCLOSURE

A gas which is the most readily condensable of the gases of a mixture is separated by passing the mixture in contact with at least one microporous barrier having a permeability of 100 x 10-7 to 1000 x 10-7 moles of air/cm2/min./cmHg, a pore thickness of 10 to 100 .ANG., by adjusting the partial pressure of the most readily condensable gas and/or the temperature of the gas mixture as a function of the mean pore radius of the barrier in order to ensure capillary condensation of the condensable gas at the pore inlets of the barrier and to induce a flow of condensed gas along the pores and across the barrier, the gas which has thus condensed being then collected.

Description

This invention relates to a method of separation of a gas from a gas mixture as applicable in particular to the purification of uranium hexafluoride derived from an isotopic enrichment plant.
It is known that, in the petrochemical industry, problems often have to be solved in regard to the separa-tion of gas mixtures. For example at the outlet of a gas-phase synthesis reactor, it may be required to collect the synthesis gas and to recover the reaction gases which have not reacted in order to recycle these latter in another reactor together with a make-up or supplement gas.
It may also be necessary to carry out purification or concentration of a gas in a synthetic or natural mixture.
In order to separate gas mixtures of this type, known separation processes include the following :
- condensation : the gas mixture is cooled in heat exchangers and in atmospheric coolants, then in partial condensers, - absorption : the mixture is cooled and selectively absorbed on molecular sieves, - dissolution : the mixture is cooled and selectively dissolved in suitable solvents.
These processes are subject to disadvantages from an economic standpoint since they are costly and presuppose considerable capital investments ; moreover, from a thermo-dynamic point of view, they call for the use of a large amount of mechanical or heating energy.
In regard to the purification of uranium hexa-fluoride delivered by an isotopic enrichment plant, it is-known that uranium hexafluoride contains a certain number of impurities such as F2, ClF3, HF, ClF, N2, etc...
The separation of UF6 from these light gases is usually performed by methods such as :
- crystallization : the gas mixture is cooled in heat exchangers or crystallizers, thus resulting in crystal-lization of the UF6. It is necessary to have banks of these crystallizers in series in order to prevent trapping o~ the light gases at the time of excessively rapid formation of UF6 crystals and at the same time in order to permit of continuous purification. The different steps of cooling, reheating and vaporization which are necessary consume energy and leave the gases and vapors in contact with each other over long periods of time ;
- distillation : this process is continuous but is fairly dlfficult to control and problems arise from the need to handle the overheated corrosive liquids which are present in this case ;
- the use of cascade-connected gaseous diffusion barriers :
this process makes it possible to obtain fairly high separation factors but entails considerable power consumption without thereby permitting complete separation.
In order to separate a binary gas mixture, especially a mixture of hydrogen and sulphur dioxide, it has also been endeavored to utilize the phenomenon of 1`-capillary condensation in the vicinity of a porous mass of graphite powder by modifying the conditions of tempera-ture and pressure. This method didnot prove satisfactory, however, since the graphite powder employed did not really make it possible to obtain an appreciable continuous flow of the sulphur dioxide through its mass and did not really constitute a physical barrier for the hydrogen of the mixture.
The precise object of the present invention is to provide a method of separation of a gas fro~ a gas mixture which overcomes the disadvantages of all the methods recalled in the foregoing and makes it possible to obtain a continuous and efficient separation of the gases which are present.
The method according to the invention for the separation of a gas from a gas mixture in which said gas is the most readily condensable of the gases of the mixture essentially consists in passing said gas mixture in contact with at least one microporous barrier having a permeability between lO0 x 10 7 and 1000 x 10 7 moles of air/cm2/min./cmHg, a thickness between a few microns and a few tens of microns and pore radii between lO and lO0 A; the partial pressure of said most readily condensable gas and/or the temperature of said gas mixture is adjusted as a function of the mean pore radius of said barrier in order to ensure capillary condensation of said most readily condensable gas at the pore inlets of said barrier and a flow of said condensed gas along the pores and across said barrier and ~,~
, i .

said gas which has thus condensed is then collected.
In accordance with the invention, the micro-porous barriers are of metal, of ceramic material or of fluorinated polymer. The unit of permeability of these microporous elements as expressed in moles of air/cm2/min/
cmHg means that x moles of air pass at a temperature of 20C per cm2 of microporous element surface area per minu~e and in respect of a pressure diference of 1 cmHg between the exterior and the interior of the microporous element.
The phenomenon of capillary condensation employed in the method according to the invention is carried into effect in accordance with Kelvin's law :

Pv ~ M
g P pL.R~T.m wherein :
Pv = saturated vapor pressure at the temperature T, P = partial vapor pxessure in the mixture, a = surface tension of the condensate, M = molar mass of the condensate, PL = specific volume of the condensate R = ideal gas constant, m = radius of curvature of the meniscus.
It is recalled that the radius of curvature m of the meniscus of the condensate as given by the Kelvin relation and the corresponding pore radius yp assumed to be cylindrical are related by the following relation :

YP

where ~ corresponds to the angle of contact of the condensate with the wall of the pore. This relation clearly points to the importance of wettability of the condensate with respect to the material which constitutes the barriers.
The nature of the microporous barriers employed in accordance with the method of the invention as well as their characteristics (permeability, thickness, pore radius) make it possible to obtain a continuous and selective flow of the most readily condensable gas and -- - therefore eficient separation; -In fa~t, the pores of the microporous layer are completely obstructed by the con-densate and thus constitute a barrier for the other gases which are noncondensable or less readily condensable than the first gas.
The application of the method in àccordance with the invention to the purification of uranium hexafluoride obtained from an isotopic separation plant is particularly advantageous. In fact, uranium hexafluoride is far more ~0 readily condensable than the other gases constituting impurities which accompany UF6 at the outlet of an isotopic enrichment plant. Thus, in accordance with the method o~
the invention, the pores of the microporous layer are rapidly clogged by the condensed uranium hexafluoride and form a barrier for the other gases which constitute the impurities. Moreover, by virtue of the method according to the invention, the contacting time between the condensed llquid and the mixture to be separated is reduced to a very considerable extent: the possibilities of dissolution of the other gases in liquid UF6 are in that case very limited. The rates of permeation in accordance with the method of the invention are considerably higher than those obtained when UF6 is purified by gaseous diffusion. Thus the ratio is between 5 and 10 according to the microporous element employed.
In order to ensure efficacious purification of UF6 in accordance with the method of the invention, steps are taken to ensure that the mixture is entirely and con-tinuously homogeneous and that the conditions of partial pressure of UF6 are uniform throughout the length of the microporous element employed.
By way of example and without any limitation ! being implied, the description given below relates to the practical application of the method in accordance with the invèntion to the purification of uranium hexafluoride derived from an isotopic enrichment plant.
The accompan~ing figure is a diagrammatic illustration of the device for carrying out the method contemplated in the present Application.
A mixture of UF6 and ClF3 is ~ithdrawn through the duct 1 at a rate of 2500 g/.5 at the level of the stage n of an isotopic enrichment plant at the point correspond-ing to the highest pressure. The proportions of the mixture are 7.1 moles of UF6 for 0.11 mole of ClF3. The pressure is 1500 millibars and the temperature is higher than 100C.

1~ ~5~

This mixture is passed into a heat exchanger 2 and cooled therein to a temperature of 90C, then passed through the duct 3 into the diffuser 4 which comprises 540 microporous barriers.
At the outlet of said diffuser 4, practically pure UF6 is delivered through the pipe 5 at a flow rate of 2400 g/s, namely 6.95 moles/second of UF6; the temperature is 90C and the pressure is 275 millibars. This uranium hexafluoride is sufficiently free of impurities to permit 10 withdrawal at 6 of the quantity desired for pxoduction and to reintroduce the remainder at 7 for feeding back to the stage n of the enrichment plant at a point corresponding to the lowest pressure.
At the outlet of the diffuser 4, a mixture of UF6 and r ClF3 is obtained through the duct ~ in proportions of 0.13 mole of UF6 in respect of 0.11 mole of ClF3 ; the temperature is 90C and the pressure is 1450 millibars.
This mixture is cooled to a temperature of 75C by passing through the heat exchanger 9, then introduced 20 through the duct 10 into the diffuser 11 which is constit-uted by 50 microporous barriers.
At the outlet of said diffuser 11, practically pure ~F6 is obtained through the duct 12 at a flow rate of 35 g/s, namely 0.10 mole/s of UF6 ; the temperature is 75C, the pressure is 275 millibars. It is possible either to effect a further withdrawal of said uranium hexafluoride for production or to reintroduce this latter into the stage n of the enrichment plant at a point corresponding to low 1~.15~
pressure.
At the outlet of the diffuser 11, a mixture of ClF2 and uranium hexafluoride is obtained through the duct 13 in proportions of o.ll mole of ClF3 in respect of 0.03 mole of UF6; the temperature is 75C, the pressure is 1400 milli-bars. This final mixture which therefore contains approx-imately 20 % UF6 and 80 % ClF3 can either be discharged or undergo further separation by means of conventional methods such as condensation or undergo further separation by means of the method in accordance with the invention.
The barriers of the diffuser 4 and of the difuser 11 are tubular barriers of ceramic material and have a dia-meter of 1~5 cm, a pore radius of 25 A and a sweep length of 1 m. The barriers are placed in parallel in each diffuser. Barriers having the same dimensions and either metallic or of fluorinated polymer have also been employed with comparable results.
By way of comparison, it can be noted that separation of the same mixture by means of the conventional method of gaseous diffusion would entail the need for approximately twenty stages and that each stage calls for a compression o the entire flow and approximately five times the number of barriers per diffuser.
Compared with the conventional methods of separation by crystallization steps, the method in accordance with the invention offers the following advantages :
- the products are available at pressures which can be directly utilized either for pro~uctioA or ~or recy ling - ~
-in thè cascade ;
the set of two diffusers replaces the first bank of crystallizers and has the further advantage of dispensing with the need for an evaporator ;
in the method according to the invention, approximately 35 x 103 kcal/h are required for cooling purposes whereas 100 x 10 kcal/h are necessary for cooling in accordance with the crystallizer process ;
the process is continuous and avoids the need for storages of crystallized products.
.

..

Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
1. A method of separation of a gas from a gas mixture in which said gas is the most readily condensable of the gases of said mixture, wherein said gas mixture is passed in contact with at least one microporous harrier having a permeability of 100 x 10-7 to 1000 x 10-7 moles of air/cm2/min./cmHg, a pore radii of 10 to 100 .ANG., the partial pressure of said most readily condensable gas and/or the temperature of said gas mixture is adjusted as a function of the mean pore radius of said barrier so as to ensure capillary condensation of said condensable gas at the pore inlets of said barrier and to induce a flow of said condensed gas along the pores and across said barrier and the gas which has thus condensed is then collected.
2. A method according to Claim 1, wherein the barrier or barriers are of ceramic material.
3. A method according to Claim 1, wherein the barrier or barriers are of metal.
4. A method according to Claim 1, wherein the barrier or barriers are of fluorinated polymer.
5. The application of the method according to any one of claims 1 to 3 to the purification of uranium hexafluoride derived from an isotopic enrichment plant.
CA 312623 1977-10-06 1978-10-03 Method of separation of a gas from a gas mixture Expired CA1115648A (en)

Priority Applications (2)

Application Number Priority Date Filing Date Title
FREN7730069 1977-10-06
FR7730069A FR2405081B1 (en) 1977-10-06 1977-10-06

Publications (1)

Publication Number Publication Date
CA1115648A true CA1115648A (en) 1982-01-05

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ID=9196166

Family Applications (1)

Application Number Title Priority Date Filing Date
CA 312623 Expired CA1115648A (en) 1977-10-06 1978-10-03 Method of separation of a gas from a gas mixture

Country Status (6)

Country Link
US (1) US4239507A (en)
JP (1) JPS5462182A (en)
CA (1) CA1115648A (en)
DE (1) DE2843416A1 (en)
FR (1) FR2405081B1 (en)
GB (1) GB2005153B (en)

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US4482360A (en) * 1982-05-29 1984-11-13 Nippon Steel Corporation Porous materials for concentration and separation of hydrogen or helium, and process therewith for the separation of the gas
FR2540396B1 (en) * 1983-02-04 1988-09-23 Petroles Cie Francaise Process for gas dehydration
GB8327143D0 (en) * 1983-10-11 1983-11-09 Petrocarbon Dev Ltd Purification of helium
JPS642408B2 (en) * 1983-11-04 1989-01-17 Toyoda Chuo Kenkyusho Kk
US4710205A (en) * 1987-03-16 1987-12-01 Honeywell Inc. Stabilized liquid films
US4961758A (en) * 1988-11-15 1990-10-09 Texaco Inc. Liquid membrane process for separating gases
US4906256A (en) * 1989-03-23 1990-03-06 Membrane Technology & Research, Inc. Membrane process for treatment of fluorinated hydrocarbon-laden gas streams
US5256296A (en) * 1990-12-28 1993-10-26 Membrane Technology & Research Membrane process and apparatus for removing a component from a fluid stream
US5256295A (en) * 1990-12-28 1993-10-26 Membrane Technology & Research Two-stage membrane process and apparatus
US5236474A (en) * 1991-09-13 1993-08-17 Bend Research, Inc. Membrane-based removal of condensable vapors
US6083297A (en) * 1995-12-13 2000-07-04 Whatman, Inc. Gas dehydration membrane with low oxygen and nitrogen permeability
US5730779A (en) * 1996-10-31 1998-03-24 Air Products And Chemicals, Inc. Fluorochemical recovery and recycle using membranes
DE19850557A1 (en) * 1998-11-03 2000-05-04 Univ Bremen Process for the separation of condensable substances in gases or gas mixtures
CA2428280A1 (en) * 2000-11-13 2002-05-16 Mcmaster University Gas separation device
US7758671B2 (en) * 2006-08-14 2010-07-20 Nanocap Technologies, Llc Versatile dehumidification process and apparatus
US20120125020A1 (en) 2010-05-25 2012-05-24 7Ac Technologies, Inc. Methods and systems for desiccant air conditioning using photovoltaic-thermal (pvt) modules
US9101874B2 (en) 2012-06-11 2015-08-11 7Ac Technologies, Inc. Methods and systems for turbulent, corrosion resistant heat exchangers
EP2929256A4 (en) 2012-12-04 2016-08-03 7Ac Technologies Inc Methods and systems for cooling buildings with large heat loads using desiccant chillers
WO2014134473A1 (en) 2013-03-01 2014-09-04 7Ac Technologies, Inc. Desiccant air conditioning methods and systems
WO2014152888A8 (en) 2013-03-14 2015-08-20 7Ac Technologies, Inc. Methods and systems for liquid desiccant air conditioning system retrofit
US9470426B2 (en) 2013-06-12 2016-10-18 7Ac Technologies, Inc. In-ceiling liquid desiccant air conditioning system

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CA625947A (en) * 1961-08-22 United States Atomic Energy Commission Diffusion separation method
GB860099A (en) * 1958-01-10 1961-02-01 Atomenergi Ab Method of producing permeable membranes
FR1255382A (en) * 1960-01-23 1961-03-10 Commissariat Energie Atomique Improvements to methods and devices for coating a microporous metal layer a macroporous metal support
NL298334A (en) * 1962-09-25
US3420069A (en) * 1967-05-01 1969-01-07 Nasa Condenser-separator
US3614855A (en) * 1967-12-26 1971-10-26 Gen Electric Continuous production of calibration gas flows
DE1925582C3 (en) * 1969-05-20 1974-07-04 Linde Ag, 6200 Wiesbaden

Also Published As

Publication number Publication date Type
US4239507A (en) 1980-12-16 grant
FR2405081A1 (en) 1979-05-04 application
DE2843416A1 (en) 1979-04-12 application
CA1115648A1 (en) grant
GB2005153A (en) 1979-04-19 application
GB2005153B (en) 1982-04-28 grant
FR2405081B1 (en) 1980-08-01 grant
JPS5462182A (en) 1979-05-18 application

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