US2176498A

US2176498A - Vacuum rectifying column

Info

Publication number: US2176498A
Application number: US151715A
Authority: US
Inventors: Kenneth C D Hickman
Original assignee: Distillation Products Inc
Current assignee: Distillation Products Inc
Priority date: 1936-07-06
Filing date: 1937-07-02
Publication date: 1939-10-17
Anticipated expiration: 1956-10-17
Also published as: FR825680A; DE715418C; GB477955A

Description

Oct. 17, 1939. I 3, HICKMAN 2,176,498

' VACUUM RECTIFYING 'COLUMN Filed July 2, 19 37 WM 45 H 57 56 47 W1 46 ll KenneihCDHicKman INVENTOR ATTORNEYS Patented Oct. 17, 1939 UNITED STATES PATENT OFFlCE VACUUM RECTIFYING COLUMN of Delaware Application July 2, 1937, Serial No. 151,715 In Great Britain July 6, 1936 L 9 Claims.

This invention relates to improvements in the fractionation or rectification of vapors under vacuum and more particularly to improvements in vacuum rectifying columns.

The conventional rectifying columnfor use at ordinary pressures is filled with a series of bubble cap plates which ensures thorough washing of the reflux by the ascending vapors. This construction is known to be' unsatisfactory for vaclo uum and particularly high vacuum fractionation, due to the high resistance to the flow of vapors through the column. Relatively high pressures therefore prevail in the still and column, even though high capacity evacuating pumps are em- 18 ployed. To avoid these disadvantages, it has ,be-

oome common practice to employ plates which are so designed and located that the ascending vapors are substantially unconstricted in passing through the column. This construction avoids w pressure drop to a considerable extent, but does not avoid other dimculties. The vapor velocity under vacuum conditions is so high that in a column of this type condensate is swept along with the vapor stream and is, therefore, substantially prevented from freely flowing countercurrent thereto, as is necesary to obtain efiective interchange between the two. Poor fractionation and flooding of the column, therefore, takes place. This is especially troublesome at low presan sures, such as at .001 to 50 mm. where vapor velocities are extremely high.

This invention has for its object to overcome the above deficiencies of vacuum, and particularly high vacuum, rectification. Another object is to provide an improved method of vacuum rectification. A further object is to provide methed and means whereby the adverse effect of the high velocity vapor streams is substantially avoided. A still further object is to provide an proved vacuum rectifying column. Other objects will become apparent from the following detailed description.

These and other objects are accomplished by the herein described invention, which in general Q comprises collecting the condensate from an upper plate and passing it to the next lower plate in the column while protecting it from the action of the vapor stream.

To facilitate the description of my invention,

no reference is made to the accompanying drawing in which:

Fig. 1 is a vertical section in perspective'of one type of fractionating column embodying the principles of my invention:

If Fig. 2 is a vertical section of a modification of the column shown in Fig. 1, being provided with semi-circular plates;

Fig. 3 is a horizontal section taken on line 3-3 of Fig. 2; I

Fig. 4 is a vertical section in perspective of 241- 6 paratus substantially the same as that illustrated in Fig. 2, but being provided with external heating elements and means for aiding in the even distribution of condensate over the surface of the plates. 10

Referring to Fig. 1, reference numeral 2 deslgr nates a cylindrical column which is connected at the base to a source of vapor and. which is provided at the top with a jacket 3 through which cooling fluid is circulated in order to cool the upper walls of the main casing 2. Numerals t, 5, t and l designatecircular plates supported by and fixed to the walls of the cylindrical casing 2 in such a manner that they are approximately horizontal. These plates are provided with a cenat tral opening, the inside edge of which is turned under to form

gutters

3, 9, it and M, respectively. Numerals i2, 53, it and i5 designate circular plates which approximately correspond in diameter to the diameter of the central opening in plates li. The outside edges of plates i2--iii are turned under to form gutters 2d, at, 22 and 23. Plate I2 is rigidly supported in the central position, illustrated by a series of pipes 25, which communicate with gutter 8 and are rigidly fixed 5p thereto and which are also rigidly fixed to the upper surface of plate it. These pipes are provided with openings 25 at a position just above the surface of plate It, located upon the side of the pipe facing towards the center of the col- @5 umn. Plate is is similarly supported by a series of pipes 26 located about its periphery, which communicate with the gutter 9 and are provided with openings 2? at the base thereof. Another series ,of conduits it, having openings ii in the g base thereof, connect to gutter 2i and deliver liquid therein onto the upper surface of plate 3. Plate it is supported by a similar series of conduits it, which communicate with gutter ill and deliver liquid through openings it onto the up- (it per surface of plate it, and another set of lower conduits 28, which deliver liquid from gutter 22 onto the upper surface of plate 5 through openings 29. Plate i5 is supported in substantially the same way by conduits 30 communicating with gutter H and having openings 3! in the base thereof, and by a similar series of lower conduits 32 provided with openings 33 which concated openings 34-31, over each of which is suspended a cap 38. The .outside peripheries of plates 4'l are notched at 46 to provide openings for the flow of condensate from the upper side, down onto the lower side of the plate. Numeral 45 designates an annular gutter which is connected to a withdrawal conduit 41.

Referring to Figs. 2 and 3, numeral 55 designates the casing of a fractionating or. rectifying column which is provided with a dome 56 and which is connected to conduit 57 at its upper end.

Numerals

5B, 59, 66 and Bi designate approximately horizontal semi-circular. plates rigidly fixed to the wals of the main casing 55 and which are provided with a series of openings 62 about the periphery which are in close proximity tothe wall of casing 55.

Numerals

53, 64, 65 and 66 designate gutters formed at the free edge of the semi-circular plates 58-6l by turning the g edge thereof in the manner illustrated. A plurality of conduits 6'! communicate with gutter 66 and terminate a short distance above the surface of plates 66. A similar series of conduits 66 communicate with'gutter 65 and terminate a short 3; distance above the surface of plate 69. Another similar set of conduits 69 communicate with gut-. ter 64 and terminate a short distance above the surface of plate 58.

Referring to Fig. 4,

numerals

15, 16, Hand 18 a designate a serrated blade which acts as a weir in evenly distributing liquid over the surface of

plates

58, 59, 66 and 6f. Numerals l9 and 86' designate external heating units, preferably electrical heating pads, which are located on diametrically opposite sides of the column 2 and in positions corresponding to the location of the ends of the

gutters

63, 64, 65 and 66.

In operating the apparatus of Fig. 1, cooling fluid is circulated in the space between the upper walls of column 2 and jacket 3. The upper end 5 course, must be gas tight.

of the column 2 is connected to vacuum pumps having suficient capacity to evacuate the system 'to the desired low pressure. The base of the column is connected to the vapor source which, of The vapors ascend through the column passing into the space between plate l2 and plate 4. In passing through this space, the vapors necessarily must flow toward the axis of the column. The vapors then are reversed in their path of flow and pass between the plate 4 and central plate l3, the vapors passing toward the outside walls of the casing 2. This repeated reversal of the direction of fiow of the vapors takesplace until they eventually pass 5 through the central opening in the top plate I and are condensed upon the cold upper walls of column 2. Condensate flows into gutter and .is withdrawn through conduit 41.

Less volatile constituents condense upon the co various plates as the vapors pass in the zigzag path through the column. Condensate from plate 1 'flows to the openings 46 about the periphery thereof, down and onto the under surface of the plate. The vapors ascending the-col- 45 mu at this point are flushing or brushing the under surface of plate I in a direction towards the .axis of the column. Therefore, liquid passingv through the openings 46 is caused to fiow into the gutter H by this action of the vapors.

.70 At the same time, interchange between the film of condensate and the vapors take place and volatile components thereof are again vaporized and carried along with the vapor stream;*"The washed liquid collecting in gutter H flows down through conduits 30 onto the upper surface ,of plate l5.

The vapors in contact with the upper surface of this plate are also passing in a direction toward .the axis of the column and the liquid flowing from these conduits is, therefore, blown towards the opening 31 and passes therethrough onto the under surface of plate IS. The ascending vapors in contact with the under surface of plate l5 are passing at high velocity towards the ofiter walls of the column and the liquid passing through the onto the upper surface of plate 6, is blownover to'the wall of casing 2, passes through the openings 46 onto the under surface of plate 6 and is then blown into gutter I6. Substantially the same action takes place on each of the sets of plates down through the column. 4

It is apparent that, by the construction illustrated, the high velocity vapors are caused to pass into intimate contact with a large area of condensate and that effective interchange takes place in its passage through the column. It is also apparent that washing of the condensate in an upward direction is avoided by passing it in protected paths in a downward direction at intervals while intermittently exposing it to the washing action of the vapor. The high velocity vapors are also caused to help force the liquids in the downward direction instead of forcing them in an upward direction as isusual with conventional vacuum column construction.

In operating the apparatus of Fig. 3, conduit end of the column is connected to a source of vapor such as the pot of a still. The ascending vapors pass into contact with the bottom surface of plate 58 and are forced to pass to the diametrically opposite side of the column, as illustrated by the long arrows. The vapors then pass along the bottom surface of plate 59 and are forced to pass to the opposite side of the column. In this way, the vapors take a zigzag course up through the column and eventually pass into the conduit 51 and thence into the condenser. collecting upon the top surface of plate 6| flows through the opening 62, along the bottom of the plate 6|, where the force of the vapors blows it as a thin film into gutter 66. The direction of flow of condensate is shown by the short arrows.

The liquid then fiows through conduits 6'! onto plate 66 where the action of the -vapors causes it to flow as a film into gutter 65. Substantially the same action takes place on

plates

59 and 58. The condensate is, therefore, caused to flow both by gravity arid by the dynamic action of the vapors in a downward direction through the column while effective interchange between large areas of the condensate and large volumes of the vapors takes place. Due to the wide spacing between the plates of all of the figures. slight resistanceto the flow of vapors is offered.

The operation of the apparatus of Fig. 4 is substantially the same as that of Fig. 2 except that heating elements 19 and cause vaporization of condensate collecting upon the walls of the column in their immediate vicinity. Such vapor- Condensate nation is advantageous, since the high velocity 1| ing from the spirit or scope of my invention. For

instance, the entire outs'ide surface of the column a vance 'and passed with the main vapor stream into interchange with the condensate. The elements 15, l6, l1 and I8 cause even distribution of condensate over the entire surface of the plates 58 to 6|.

Many changes and variations canv be made in the above described apparatus, without departmay be jacketed to provide for the circulation of a cooling or heating fluid. Also, certain sections of the-column may be jacketed to enable the control of the temperature of any particular zone or area in the column. It is also desirable in many cases to provide part or all of the column with an insulating jacket or lagging to prevent heat losses and thusavold the necessity of adding or subtracting heat. The column will be heated by vapors to an extent depending upon the rate of distillation whch is, in turn, controlled by the temperature of the still. Heating of the column will not usually be necessary except with very low vapor pressure compounds or with very low rates of distillation. The number of plates employed depends upon the degree of separation required and the rate of distillation and can be.

varied within wide limits to suit these requirements. The plates may slant slightly upwards or' downwards, but approximately horizontal plates are preferred, since they increase the rate of downward flow caused by the action of the vapors. If it is desired to fractionate under molecular distillation conditions, the plates can be spaced in the tower at distances of less than the mean free path under the particular pressures employed.

it will be apparent that the base of the column can be connected to any well known type of vaporizing apparatus, such as an ordinary pot still. Also, the base of the column may be used as the vapor source in well known manner, by disposing a heating element therein and introducing the material to be vaporized in ordinary or preheated condition. Since materials which are usually distilled under vacuum conditions have low vapor pressures and are difficultly volatile, it is best to employ a flash vaporizer as a vapor source. These permit almost instantaneous vaporization of small amounts of the material and avoid considerable decomposition due to the fact that all of the material is not heated during the whole distillation period. A flash vaporizer in which small amounts of the material are caused to flow over a heated revolving surface by centrifugal force is particularly advantageous.

Since the velocities of vapors at pressures of below mm. such as at .001 to 50 mm. are exceptionally high, little interchange between vapor and condensate takes place in towers of the prior art. My invention is, therefore, of particular value for fractionations carried out within this pressurerange. It is especially intended for use with vapors entering the bottom of the column at pressures of .01 to 50 mm. and particularly .1 to 10 mm., and leaving at the top at pressures of .001 to 20 mm.

Due to the fact that the edges of the plates are turned under to collect any condensate on the plate, all-splashing and dropping is substantially avoided and this fact materially contributes to the avoidance of intrainment. Since the features of my invention permit the use of widely spaced plates which have been found to be necessary in vacuum, and particularly in high vacuum dlstillations, substantially all back pressure problems due to constriction of vapors are avoided. The outstanding advantage of the construction described is that obstruction to reverse passage is avoided, since the vapors are prevented from coming in contact with condensate at the time that it is flowing towards the base of the column. However, the vapors are contacted at intervals with the condensate while it is in the form of films of large surface area. During such contact the condensate travelsiin the same direction as the vapors and yet it is always being carried counter-current to the vapors by the gutters and conduits. However, even though the vapors and condensate travel in the same direction during contact, undesirable entrainment does not take place and the condensate is in fact helped to flow in a counter-current direction. At the same time very effective interchange takes place between the two.

What I claim is: I

1. A vacuum fractionating column of the character described and comprising in combination, a plurality of approximately superimposed plates, which are'widely spaced with respect to each other, which have a considerably smaller area than the cross sectional area of the column and which are adapted to retain condensate in the form of a film and means for conveying condensate from each of the plates to the next lowest to the surface of the next lowest plate while protecting it from the action of the high velocity vapor stream.

3. A vacuum fractionating column comprising in combination, a main column casing, a plurality of approximately horizontal superimposed plates which permit substantially unconstricted flow of vapor through the column which plates are so constructed that condensate is caused to flow thereover in the form of a thin film by contact with the moving vapor, and means located upon the plates for collecting condensate and passing it to the surface of the next lowest plate while protecting it from the high velocity vapor stream.

4. A vacuum fractionating column comprising in combination, a tower or main column casing, a;

5. A vacuum fractionating column comprising in combination, an approximately vertical main column, a plurality of approximately horizontal plates spaced from each other in the column and adapted to retain a film of condensate on the top and bottom surfaces thereof, the plates covering only a portion of the cross sectional area of the column and being so arranged in the column that the particular cross sectional area not covered by one plate is covered by the plates below and above it, means for collecting condensate from each plate and means for conveying the collected condensate in a protected path to the next lowest plate.

. 6. A vacuum fractionating column of the character described and comprising a plurality of approximately super-imposed plates, alternate plates being staggered in relation to one another so as to oifer slight resistance to the flow of vapors, means for allowing condensate to pass in the form of a film over the top and bottom surface of each plate substantially in the same direction of flow as the vapors, and means for conveying condensate from each of the plates to the next lowest plate, while protecting it from the vapor stream.

'7. A vacuum fractionating column of the character described, which comprises in combination, a main column casing, a plurality of approximately superimposed plates which offer slight resistance to the flow of vapors which are adapted to retain a thin film of condensate on the top and bottom surface thereof and which are so disposed in the column that the vapors and condensate travel in the same direction when in contact with the top and bottom surfaces thereof and means for conveying condensate from each of the plates to the next lowest plate, while protecting it from the vapor stream.

8. A vacuum fractionating column comprising in combination an approximately vertical cylindrical casing, a plurality of semi-circular plates having perforations about the circular edge thereof disposed at intervals upon the internal walls of the casing and in an approximately horizontal position, each alternate plate being disposed upon the opposite wall of the casing, collecting gutters located under the straight edge of each plate and conduits connecting each gutter to the top straight edge of the next lowest plate.

9. A vacuum fractionating column comprising in combination, an approximately vertical column casing, a series of plates, having small perforations about their periphery and a large central opening, disposed in a horizontal position and at spaced intervals within the main casing, collecting gutters at the edges of the central openings of each plate and so located as to collect liquid flowing along the bottom surfaces of the plates, a second series of plates arranged .in alignment with and corresponding in diameter to the central opening in the first series of plates, and which are disposed between each pair of the first series of plates, collecting gutters at the edges of the second series of plates and so located as to collect liquid flowing along the bottom surfaces thereof, a plurality of conduits connecting KENNETH C. D. HICKIWAN.