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WHAT WE CLAIM IS:-
1 A method of fractionally distilling a liquid mixture which is carried out discontinuously.
2 A method as claimed in Claim 1, in which the fractional distillation is carried out in a rectification column having a series of substantially horizontal perforate liquidvapour contact plates spaced vertically in the column, each said plate extending across the entire plan section of the interior of the column, and in which, during a first period of the distillation, a gaseous phase of the mixture ascends through the openings in the liquid vapour contact plates and through a liquid phase of the mixture which accumulates on these plates, and, during a second period of the distillation, at least a part of the liquid accumulated on the plates passes downwardly through said openings.
3 A method as claimed in Claim 2, in which the downward passage of the liquid during the second period is caused by a pressure thrust produced in the upper part of the rectifying column.
4 A method as claimed in Claim 3, in which the pressure thrust is caused by means of a brief throttling of the gas removal from the head of the column.
5 A method as claimed in Claim 3 or Claim 4, in which the rectification is carried out as a multi-stage rectification and in which the pressure thrust is accomplished at least in part by briefly introducing the gaseous product of a rectification at a higher pressure into the head of the column.
6 A method as claimed in Claim 5, in which the gaseous product is the head product of a continuous rectifying column.
-40 7 A method as claimed in Claim 5 or Claim 6, in which the gaseous product flows by way of a throttle or choke valve into a supply container, from which it is introduced into the head of the rectifying column.
8 A method as claimed in Claim 7, in which the quantity of the gaseous product stored in the container corresponds to the quantity of liquid to be forced downwards through the column during the second period.
9 A method as claimed in Claim 2, or Claim 2 and any one of Claims 3 to 8, in which the first and second periods occur at regular time intervals.
10 A method as claimed in Claim 2, or Claim 2 and any one of Claims 3 to 9, in which the second period is shorter than the first period.
11 A method as claimed in any one of the preceding claims, in which the liquid mixture is obtained by the liquefaction of a gas mixture which is gaseous at normal atmospheric temperature and pressure, and in which the rectification is carried out at a low temperature.
12 A method as claimed in Claim 11, in which the gas mixture is cooled to the low temperature in reversible regenerators, the reversal of which regenerators is synchronised with the change-over from the 70 first period to the second period.
13 A method as claimed in Claim 11 or Claim 12 in which at least part of the gas mixture is cooled by expansion.
14 A method as claimed in Claim 2, 75 or any one of Claims 3 to 13 as dependent on Claim 2, wherein the downward passage of the liquid during the second period is caused by reducing the pressure in the foot of the rectification column 80 A method of separating a mixture of gases, substantially as described with reference to the accompanying drawing.
16 Apparatus for carrying out the process claimed in Claim 1 comprising a recti 85 fication column including a series of substantially horizontal perforate liquid-vapour contact plates vertically spaced in the column, each plate extending across the entire plan cross-section of the interior of the 90 column and an outlet conduit from the head of the column having a valve which is at least partly closable, the perforations in the liquid-vapour contact plates being of such an area that, for any particular liquid 95 mixture to be rectified in the column, the liquid cannot pass downwardly through the perforations while gas is ascending through the perforations.
17 Apparatus as claimed in Claim 16 100 in which the perforations have a cross-sectional area not exceeding a few square millimetres.
18 Apparatus as claimed in Claim 17 in which the perforations are cylindrical 105 19 Apparatus as claimed in Claim 18 in which the diameter of the cylinder does not exceed two millimetres.
Apparatus as claimed in Claim 19 in which the diameter of the cylinder does 110 not exceed 1 7 millimetres.
21 Apparatus as claimed in any one of Claims 14 to 20 in which the inlet of the rectification column is connected to the sump of a continuous rectification column 115 22 Apparatus as claimed in Claim 21 in which the inlet is connected to the sump through a throttle.
23 Apparatus as claimed in Claim 21 or Claim 22 in which the head of the con 120 tinuous rectification column is connected to the head of the first rectification column through a conduit having a closable valve.
24 Apparatus as claimed in Claim 23 in which a gas container is situated in that 125 part of the conduit which leads from the closable valve to the head of the continuous rectification column.
Apparatus as claimed in Claim 24 in which a throttle valve is disposed in that 130 857,871 part of the conduit between the gas container and the continuous rectification column.
26 Apparatus as claimed in any one of Claims 14 to 25 in which the inlet of the first rectification column is connected to a series of reversible cold regenerators.
27 Apparatus as claimed in Claim 26 in which the inlet is connected to the regenerators through a subsidiary rectification column and an expansion turbine between the subsidiary rectification column and the regenerators.
28 Apparatus for carrying out the process claimed in Claim 1 substantially as described with reference to the accompanying drawing.
G F REDFERN & CO, St Martin's House, 177 Preston Road, Brighton, Sussex.
Abingdon: Printed for Her Majesty's Stationery Office, by Burgess & Son (Abingdon), Ltd -1964.
Published at The Patent Office, 25 Southampton Buildings, London, W C 2, from which copies may be obtained.
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PATENT SPECIFICATION
DRAWINGS ATTACHED.
Date of Application and filing Complete Specification:
Jan 23, 1962 No 2356 /62.
Application made in Germany (No G 31471 Ia/17 g) on Jan 26, 1961.
Complete Specification Published: May 13, 1964; ) Crown Copyright 1964.
CO t U Index at Acceptance:-F 4 Pll; Bl BL.
International Classification:-F 25 i (B 01 di.
COMPLETE SPECIFICATION.
Fractional Distillation of Liquid Mixtures.
We,, GESELLSCHAFT FUR LINDE'S E Is M AsCHINEN AKTIENGESELLSCHAFT, a German Company, of Hildastrasse 2-10, Wiesbaden, Germany, (trading as Geselilschaft fur Linde's Eismaschinen Aktiengesellschaft Zweigniederlassung Hollriegelskreuth in Hollriegelskreuth bei Munchen), do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: -
This invention relates to methods of and apparatus for the separation of liquid mixtures by fractional distillation or rectification, and particularly, but not exclusively, to the separation of mixtures produced by the liquefaction of gas mixtures, i e mixtures of substances which are gaseous at normal atmospheric temperatures and pressures.
Continuous methods of rectifying liquid mixtures are known In order, to carry out these methods it is necessary to employ liquid-vapour contact plates which allow a simultaneous passage of vapour and liquid up and down the rectification column respectively In order to ensure that the composition of both the liquid and vapour phases at each plate corresponds as nearly as possible to the equilibrium value, it is necessary to bring the liquid and vapour into intimate contact Several types of plate are known for this purpose For instance, in one well-known form of liquid-vapour contact plate, the vapour passes upwards through small holes in the plate while the liquid flows downwards from plate to plate through a radially arranged downcomer on each plate A weir is placed on one side of each downcomer which determines the liquid level maintained on the plate during the distillation The other side of the downlPrice 4 s 6 d l comer is connected to the downcomer of the next higher plate A displacement body is positioned in the centre of each plate round which the liquid flows in a circular movement The individual liquid downcomers of the plates are staggered one above the other and as a result the liquid flows from the top of the column to its foot by means of a helical flow-path.
Conventional liquid-vapour contact plates of the type described above have the disadvantage that they are expensive to manufacture and install Moreover the quantity of liquid which can flow through a column having these plates is limited by the quantity of liquid that can flow through the downcomers The edge length of the weir at each downcomer increases linearly with the dimensions of each plate while the liquid through-put on each plate increases quadratically with the size of the plate This discrepancy may be partially compensated for by increasing the height of the weir.
It is an object of the present invention to minimise the disadvantages set forth in the preceding paragraph.
This invention consists in a method of fractionally distilling a liquid mixture, particularly a mixture of liquified gases, which is carried out discontinuously.
By the term "discontinuously" as used herein is meant that whilst the fractionation process is in operation, the vapour phase of the mixture does not continuously ascend in the fractionation zone, nor does the liquid phase of the mixture continuously descend through the rectification zone, but ascent of the vapour phase and descent of the liquid phase take place alternately, the alternations taking place at relatively short intervals of time.
This invention also consists in apparatus for carrying out the method of the inven% 57,871 tion including a rectification column including a series of substantially horizontal perforate liquid-vapour contact plates vertically spaced in the column, each plate extending across the entire plan cross-section of the interior of the column and an outlet conduit from the head of the column having a valve which is at least partly closable, the perforations in the liquid-vapour contact plates being of such an area that, for any particular liquid mxiture to be rectified in the column, the liquid cannot pass downwardly through the perforations while vapour is ascending through the perforations.
The method of the present invention may be carried out with simple apparatus and the size of the liquid-vapour contact plates does not limit the maximum through-flow of the column Moreover, the present invention ensures that the liquid and vapour phases come-into particularly intimate contact with one another.
The process may be carried out with a rectifying column which has flat horizontal plates extending across the entire plan crosssection of its interior and -which are provided with apertures of small cross-section.
: The apertures may be of any shape; round cylindrical apertures are easily produced.
The cross-sectional area of each aperture depends on the viscosity and surface tension of the liquid and also on the degree of wetting of the materials from which the plates are formed It is generally of the order of some mm 2 or less It has been found that, for the rectification of air, cylindrical apertures which have a diameter of less than 2 mm, and preferably less than 1 7 mm, are suitable.
When the present invention is carried out in a rectification column having a series of perforate liquid-vapour contact plates of the type described above, the distillation is so conducted that the liquids and gases pass through the same apertures in the liquidvapour contact plates Thus, the distillation is conducted in two periods During the first period, vapour which is produced through heating in the base of the column passes upwards through the apertures and bubbles through a layer of liquid on the plates During a second phase in the distillation at least part of the liquid is caused to flow downwardly through the apertures from one plate to another It is preferred that the change-over from one period to another should occur at regular time intervals.
Generally, the second period, in which the liquid passes down through the apertures, will be considerably shorter than the first period in which the gas flows upwards through the apertures.
Mhe downward passage of the liquid during the second period may suitably be 65 accomplished by causing a pressure thrust in the upper part of the rectifying column.
This thrust may suitably be caused by throttling or interrupting the discharge of gas from the head of the column at periodic 70 intervals, which throttling may suitably be accomplished through the action of an automatic valve inserted in the outlet from the head of the column Since the portion of the reflux liquid at the head of the column 75 continuously vaporises mainly as a result of choke relaxation, a small excess pressure is formed in the head of the column and a liquid volume corresponding to the additional gas quantity in the head of the column 80 is caused to pass through the apertures in the plates In a corresponding manner a low pressure may be caused in the lower part of the column by an increase in the removal of the gaseous sump product in the 85 lower part of the column, which low-pressure again causes the liquid to pass downwardly through the apertures in the plates.
According to one embodiment of the present invention the rectification may be car 90 ried out as a multi-stage rectification When this is the case, a gaseous product extracted from one rectification stage, for instance the head product extracted from a continuous rectification stage, which is run at a 95 higher pressure than the column, may be blown into the head of the column at regular time intervals so as to produce the requisite pressure thrust When this is done the quantity of liquid flowing downwards through 100 the column during each second period may be adjusted by adjusting the quantity of gas introduced into the head of the column.
In one preferred form of this embodiment, the gas from the high pressure column is fed, 105 preferably by way of a throttle valve, into a storage chamber, which storage chamber may be periodically connected to the head of the column, preferably by means of an automatic valve The throttle valve may be 110 adjusted in such a manner that the storage container is filled slowly and so that the pressure within the storage container only reaches the requisite value just before the valve is opened to allow the gas to flow 115 into the head of the column.
The present invention is particularly suittable for application to low temperature rectification processes such as air rectification When this is the case the air may 120 be cooled by means of reversible regenerators before introduction into the column.
When the air is cooled and purified in such regenerators it is possible to synchronise the change over from the first to the second 125 period so that it coincides with the change over of the regenerators In this manner it is possible to cause the pressure thrust in the top of the column merely by means 857,871 857,871 of the pressure thrust associated with changing over of the regentrators.
When the present invention is applied to a multi-stage process, it is not necessary that all the columns should be run discontinuously It is possible to apply the process of the present invention to only one or some of several columns.
The process of the present invention is not limited to low temperature gas rectiPfication It can also be advantageously applied to all rectifying procedures When the process of the present invention is applied to high temperatures rectification procedures, the cross-sections of the apertures in the plates will depend on the surface tension and the viscosity of the liquid and the materials of the plates, on the formation of the plates and the distance between the individual holes of the plates.
These dimensions may be found by testing and should be such that vapour and liquid cannot flow past one another through the same aperture The number of apertures of the individual plates may be adapted to the through-put of the rectification column.
The present invention may be carried out in various ways One embodiment of the invention is described by way of example -30 with reference to the accompanying drawing, which is a schematic representation of a distallation apparatus according to the present invention In the drawing only those parts of a two-stage air rectification column which are essential to the present invention are shown.
A high pressure rectification column 1 is provided with conventional liquid-vapour contact plates 2 The low pressure rectification column 3 has liquid-vapour contact plates 4 which are horizontal and which extend across the whole plan cross-section of the column and which are provided with cylindrical apertures, whose diameter is 1 7 mm or less The distances between the individual apertures are a few millimetres.
Air which is to be fractionated is led into the high pressure rectification column 1 through a conduit 5, after cooling in a -50 main heat exchanger arrangement, which is not shown The air is rectified in the column at an elevated pressure to form an oxygen enriched liquid sump product which is drawn off through the pipe 6, expanded through the valve 7 and is fed in to the low pressure rectification column 3 through the conduit 8 Liquified nitrogen is removed from the head of the column through the conduit 9 and, after expansion through the valve 10, one portion of this nitrogen is fed into the head of the column 3 to act as reflux liquid The remainder of the liquid nitrogen is expanded through the valve 11 and is delivered to the head of a secondary rectifying column 12 which is provided with conventional liquid-vapour contact plates Air is rectified in the secondary column 12, preferably after cooling in regenerators, being introduced into the column through the conduit 13 after expansion in the expansion turbine 14 The sump product of the auxiliary column 12 is led into the conduit 8 and thereby introduced into the column 3 Gaseous nitrogen is removed from the head of the column 12 through the conduit 15 The purpose of the column 12 is to separate the expansion turbine 14 from the column 3 so that it is not subjected to the pressure changes associated with discontinuous operation of the column Gaseous oxygen is removed through the conduit 16 at the foot of the column 3.
Gaseous nitrogen is removed from the head of the column 3 through the conduit 17, passes through an automatically timed valve 18 and is then mixed with the nitrogen coming from the column 12 through the conduit 15 and removed through the piping 19, from whence it is introduced into the main heat exchanger arrangement (not shown).
A portion of gaseous nitrogen is removed from the head of the column 1 by means of the conduit 20 and is conveyed through pressure reducing valves 21 and 22 into the contqiners 23 and 24 respectively These containers are connected respectively to the line 17 upstream of the valve 18 and to an appropriate point in the column 3 by means of automatically timed valves 25 and 26 respectively.
The column 3 is operated in two phases.
During the first period the valve 18 is opened and the valves 25 and 26 are closed Gaseous nitrogen can therefore flow freely from the 105 head of the column and vapour bubbles upwards in the column 3 through the apertures in the plates 4 During the second period, which is considerably shorter than the first period, and which normally lasts 110 only a few tenths of a second, the valve 18 is closed and the valves 25 and 26 are opened As a result, the gas which has accumulated in the containers 23 and 24 flows into the column and forces a quantity 115 of the liquid downwards through the plates towards the bottom of the column Where regenerators or reversing exchangers are used as the main heat exchanger arrangement it is particularly advantageous to open 120 the valves 25 and 26 in such a manner that the second phase, in which the liquid passes downwardly through the apertures in the plates, coincides with the reversal of the main heat exchanger arrangement 125 The unit consisting of the valve 22, the Container 24 and the valve 26 may be omitted, particularly if only small amounts of liquid are introduced into the column.
857,871
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