US3686828A - Fractionation method and apparatus - Google Patents

Abstract

Fractionation method and apparatus in which an angular velocity is imparted to a vapor to form a rotating vapor stream and liquid droplets are introduced into the rotating stream to provide a centrifugal force to impel the droplets in a path through the stream, the path being of a length to attain vapor-liquid equilibrium. The liquid droplets are collected by impingement. High mass transfer and high flow rates are achieved.

Description

United States Patent Drake et al.

[ 11 Aug. 29, 1972 FRACTIONATION METHOD AND APPARATUS Inventors: Elisabeth M. Drake, 31 Gardner St., Allston, Mass. 02134; William E. Gifford, 829 Ostrom Ave., Syracuse, N.Y. 13210 Assignee: Arthur D. Little, Inc., Cambridge,

Mass.

Filed: June 8, 1962 Appl. No.: 201,051

US. Cl ..55/90, 55/241 Int. Cl. ..B01d 53/24 Field of Search ..55/36, 37, 46, 52, 90, 92,

[56] References Cited UNITED STATES PATENTS 2,817,415 12/1957 Sykes ..55/92 Primary Examiner-Reuben Epstein Attorney-Bessie A. Lepper [57] ABSTRACT Fractionation method and apparatus in which an angular velocity is imparted to a vapor to form a rotating vapor stream and liquid droplets are introduced into the rotating stream to provide a centrifugal force to impel the droplets in a path through the stream, the path being of a length to attain vapor-liquid equilibrium. The liquid droplets are collected by impingement. High mass transfer and high flow :rates are achieved.

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SHEEIBUFS VA POR iii LIQUID VAPOR OUT Elisabeth M. Drake William E. Gifford INVENTOR /im Arrg ney PATENTED 3.686.828

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Elisabeth M. Drake William E. 'Gifford INVENTOR /iw flgzpney PATENTEDAUKZQIQIZ 3.686.828 SHEET 5 BF 5 VAPOR OUT AIR 1% ACETONE Vg 200 FPS e=2oeo MOLS HR FT 2 P =0.oe LB FT3 WATER SPRAYED THROUGH 50)! HOLES AT 95 FPS 3% OPEN AIR SCREEN 4540 LB MOL/HR FT COLLECTOR Elisabeth M. Drake William E. Gifford INVENTOR BY /l AfTf r ey FRACTIONATION METHOD AND APPARATUS This invention is concerned with fractionation method and apparatus. More particularly it is concerned with a device wherein a high velocity gas stream is contacted with a liquid spray and the two phases separated; the device being capable of achieving high mass transfer rates between phases while operating at high flow rates. Usually the streams are of different composition and thus the device operates in such a manner that the outflow streams tend to be in thermal and in thermodynamic equilibrium.

A number of methods and apparatus are known for separating mixtures of fluids. Typical among these are the distillation columns, where two-phase countercurrent conditions are established by forcing the vapor phase upwardly against downwardly flowing liquid. Separation of components by distillation is possible whenever the relative volatilities of the components differ from unity. However, several factors limit the flow rate throughput of a distillation column. Since the phases must move countercurrently, the relative velocity between the vapor and liquid is limited by fluid dynamics. If the liquid flow is induced by gravity, the maximum allowable vapor counter flow velocity ranges from about 1 to 3 feet per second. As velocity increases in this range, the smaller fragments of liquid are entrained by the vapor and a loss in separation efficiency occurs. As soon as a maximum allowable velocity is surpassed, flooding takes place. In a flooded column no phase counter flow is possible and all liquid is carried out of the column by the vapor.

In the prior art, other devices are, of course, available for vapor-liquid fractionation. These devices are suitable for distillation, absorption and other interphase mass transfer processes. Among these, the most common are packed or tray columns wherein liquid flows down through the column and in contacted by upward flowing vapor. The vapor phase becomes enriched in the more volatile components as it flows against the saturated liquid stream. These columns have found extensive application although flooding in a phenomenon which limits their volumetric throughput.

In another category of fractionation apparatus, rotary devices are used to improve column performance by promoting even liquid distribution in each stage or by removing entrained liquid from the vapor stream between stages.

A third device is one which may be termed a falling film device which incorporates the use of centrifugal force to increase the rate of liquid film flow counter to vapor flow. Wetted-wall multiple tube columns are also included in this class. However, they have not been found generally suitable for high capacity operation.

For many operations, it would be highly desirable to have a method and apparatus for successively equilibrating and then separating vapor and liquid phases, the apparatus being one which occupies a minimum volume and requires a minimum amount of weight. It should also be highly efficient, that it is should attain high mass transfer and high flow rates. This in turn requires mixing the phases so that a high in terfacial area is obtained and/or maintaining a high relative velocity between the phases so that mass will be transferred by convection rather than by diffusion alone.

As examples of the application of such a method and apparatus, we may cite the washing of gases, the preparation of drugs where it is desirable for the material to be processed over a minimum amount of time, the distillation of radioactive species (where minimum volume reduces shielding requirements), the fractionation of mixtures in an airborne or space environment, and the purification of solvents in field equipment, and other similar special two-phase mass transfer processes. The apparatus of this invention is believed to met these requirements and to be capable of serving the uses indicated above. It is, moreover, extremely compact, light and efficient.

It is, therefore, a primary object of this invention to provide a method of equilibrating and separating vapor and liquid phases, the method being one which achieves extremely high mass transfer and high flow rates. It is another object of this invention to provide a method of the character described which can be performed in a very small volume and by apparatus of light weight. It is another object of this: invention to provide a method of the character described'which requires only an extremely short time of contact between the vapors and liquids thus making it particularly suitable in the manufacture of materials such as pharmaceuticals. It is another object of this invention to provide a method of separating liquids and vapors which is reliable, relative inexpensive, and yet efficient.

It is another primary object of this invention to provide distillation apparatus which achieves a high mass transfer and a high flow rate in a minimum amount of volume. It is yet another object to provide apparatus of the character described which may be easily cleaned and which is formed of parts which may be readily interchanged. It is still another object to provide such an apparatus on which repairs may be easily made while the apparatus is still in operation. Other objects of the invention will in part be obvious and will in part be apparent hereinafter.

The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others, and the apparatus embodying features of construction, combinations of elements and arrangement of parts which are adapted to effect such steps, all as exemplified in the following detailed disclosure. The scope of the invention will be indicated in the claims.

Briefly the method of this invention may be described as comprising the steps of imparting an angular velocity to a vapor thereby to form a rotating vapor stream, introducing liquid droplets into the rotating stream which provides a centrifugal force to impel the droplets in a path through the stream, the path being of a length sufficient to attain essentially complete vaporliquid equilibrium, collecting the liquid droplets by impingement, and transporting the liquid to a prior stage while vapor flows on to a subsequent stage.

The apparatus of this invention comprises means for imparting an angular velocity to a vapor to form a rotating vapor stream, means: for impelling liquid droplets through the vapor stream, means for collecting theliquid droplets subsequent to their passage through the vapor stream, means for transporting vapor and liquid between stages in a multistage unit, and means for directing the vapor and liquid streams from the ap paratus. The apparatus may be so constructed as to comprise a plurality of units to achieve extremely high separation, each unit receiving vapor from the preceding unit, while liquidis transported in a stepwise, countercurrent manner.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings in which:

FIG. 1 is a diagrammatic representation of a spray separator stage of this invention showing the mechanism by which mass transfer is accomplished;

FIG. 2 is a perspective view of a single unit of the spray separator;

FIG. 3 is a cross-section of the unit of FIG. 2 along line 33 illustrating the incorporation of a filtering screen and a preferred method by which the droplets may be formed;

FIG. 4 is a perspective view partially in cross-section of a multi-unit separator according to this invention;

FIG. 5 is an end view of the separator of FIG. 4;

FIG. 6 is a cross-sectional view of the apparatus of FIG. 4 along lines 6-6 of that figure;

FIG. 7 is a view along lines 77 of FIG. 6;

FIG. 8 is a detail of a portion of the interior of the separator of FIG. 4;

FIG. 9 is a cross-sectional view of the apparatus of FIG. 8 along lines 9-9 of that figure;

FIG. 10 is a cross-sectional view of a multi-unit separator having a variable radius of curvature; and

FIG. 11 is a cross-section of a portion of a separator used as an example to illustrate the invention.

A single mass transfer stage constructed in accordance with this invention is illustrated in cross-section in FIG. 1 and a perspective view of a single stage unit in FIG. 2. These figures also illustrate the method by which vapor-liquid separation is achieved in accordance with the teaching of this invention. In FIGS. 1

and 2 it will be seen that there is provided a tube 10 which contains a curved section 12 defined by a bend in tube 10. Communicating with the circular portion 12 of the apparatus is a liquid feed channel 13 which, in turn, is connected to a liquid conduit 14. Where liquid feed channel 13 joins with the circular bend portion 12 there is positioned a means for forming a liquid spray, such as a nozzle or a perforated member 15 as will be described later in conjunction with FIG. 3. Somewhat downstream from the point at which the liquid contacts the vapor there is a liquid collector 17 which has, as will be seen in FIG. 2, a draw-off line 18. By impelling the stream of vapor around the curved portion 12 of the apparatus, it will be seen that the vapor is given an angular velocity and there is formed a rotating gas stream. Into this stream the liquid droplets, formed by passing through the member 15, are forced with a pressure sufficient to cause them to pass in contact with the vapor stream and through it to be collected against the wall and to flow into the collecting tube 17. It will be appreciated that the-liquid droplets have a forward force due to the centrifugal action of the vapor and their initial velocity represented by the vector F in FIG. 1, and that at the same time they experience a downward force due to drag forces exerted by the vapor represented by the vector D in FIG. 1. The resultant force R serves to impel the liquid droplets outwardly to the wall of the circular section 12 and there to collect and fiow into the collector 17. The vapor stream of course, passes through the second leg of the tube 10 and is removed.

The distance traveled by the liquid droplets is critical, for it must be sufficient to complete the mass transfer and to bring about a stage of equilibrium between the liquid and gas, and at the same time be of sufficient length to effect easy mechanical separation of the liquid droplets and the vapor. It will be appreciated that the distance d shown in FIG. 1 is in fact a function of the droplet size and the gas velocity as well as the initial droplet velocity. Thus it is conceivable that for a fixed distance, d, and fixed velocities, the drops could be so small that the downstream distance required for mechanical separation would be far greater than that desired to bring about equilibrium. Likewise the drops could be so large that the mechanical separation could be effected in a distance which was much less than that required to bring about the equilibrium conditions. This therefore indicates an optimum liquid droplet size which is determined in turn by the liquid and vapors being handled. The liquids which have low surface tensions will of course more readily form smaller drops. Typical velocities, drop sizes and geometries will be illustrated in the example given below.

FIG. 2 illustrates the single unit of this apparatus in somewhat more detail showing in addition to the components discussed in connection with FIG. 1 a liquid pump 20 and a gas compressor 22.

FIG. 3 is a much enlarged cross-sectional view of a fragment of that portion of the tubular section which is circular in shape, that is the bend in the tube. In addition to the elements shown and described above in connection with FIGS. 1 and 2, there is also shown a filtering screen 25 which in this case, is placed ahead of the point in the liquid stream where the droplets are formed. This FIG. 3 also illustrates one preferred way of forming droplets, namely the use of a perforated or foraminous disc or screen with the conically shaped perforations as illustrated. It has been found for example, that a material sold as Electro-plate mesh by Perforated Products, Inc., is particularly well suited to the formation of droplets ranging in the size from 10 to microns.

FIG. 4 illustrates an embodiment which a multi-unit spray separator of this invention may take. For a multistage unit, a single gas compressor and a single liquid filter will service the entire unit. The unit is generally designated by numeral 30 and in the form of FIG. 4 it is made up of an outer cylindrical housing 31, and an inner cylindrical housing 32 defining between them an annular space 33 in which equilibration and separation of vapor and liquid is performed as will be apparent in the following description. The unit comprises end plates 34 and a gas inlet portion 35 which feeds the vapor through a shaped section 46 at the point where it joins with the annular internal space 33 of the unit. A vapor outlet 37 is provided at the opposite end of the cylindrical housing from the vapor inlet as illustrated in FIGS. 4 and 5. It is preferably a constricted configuration to collect and direct the discharged vapors. A liquid inlet channel 40 is positioned to discharge liquid into the first sprayer and the channel is conveniently fed by a liquid conduit 41 (see FIG. 7). A liquid outlet channel 42 is positioned to communicate with the last collector and to discharge the liquid through a suitable conduit 43 as shown in FIGS. 4 and 5.

Returning now to FIG. 4 it may be seen how the units, consisting of sprayers and collectors, may be positioned in the apparatus of FIG. 4. Within the annular space 33 defined in the housing 30 are an inner support ring 47 and an outer support ring 48. Between these is an annular vapor passage 46 through which vapor is passed and given the required angular velocity. Inner ring 47 supports the sprayers 50 and the atomizers 51 while outer ring 48 supports the collectors 54. A typical sprayer-collector arrangement is illustrated in detail in FIG. 8. There it will be seen that the collector 54 has operable within it a pump system represented in this drawing by paddles 55 mounted on a suitable shaft 56 which is driven by a motor or other device not shown. The collector 54 has a collector channel 57 and periodically along the collector here are supplied small transfer or liquid return lines 58 which communicate with the succeeding sprayer 50. The liquid is pumped from the collector to the successive sprayer and introduced into thesprayer through an opening 59. The direction of liquid flow is shown in FIG. 9 which is a cross-section along a series of the return lines 58 of FIG. 8. Because the liquid strikes the outer wall 48 with a certain velocity the liquid entering collector 54 has a certain amount of kinetic energy. It will be appreciated that it is forced around the periphery of the collector and that actually very little power is required to raise the liquid and pump it to the next successive sprayer.

The introduction of the liquid and its subsequent removal after treatment is shown in FIGS. 6 and 7. There it will be seen how the liquid inlet channel 40 communicates with the first sprayer 50 and how the liquid outlet channel 42 is in communication with the last collector 54. Various modifications to achieve this communication between the inlet and outlet channels and the respective sprayers and collectors are possible.

For example in FIG. 7 it is shown how the liquid inlet channel may be tapered from the inlet to the terminal end and may introduce liquid into the sprayer 50 through series of small ports 62. In a like fashion liquid may be discharged into the liquid outlet channel through a series of ports or through lengthwise communication between the liquid outlet channel 42 and the collector 54.

An alternate configuration, depicted in FIG. 10, is similar in basic operation to the system already described. However, the vapor path is not truly circular. The vapor flows in a path 65 having a constantly varying radius of curvature 66. Either the collectors 54 can be spaced to compensate for variation in drop trajectory due to varying centrifugal force as shown in FIG. 10, or the vapor velocity can be varied by tapering the vapor flow area to maintain a constant centrifugal acceleration. Drop size may also be altered from stage to stage to achieve correct performance. This might be achieved by the use of different size spray meshes 15 (FIG. 1) in each stage. 1

Application of the spray separator concept to an actual mass transfer process, permits a comparison between the predicted performance of spray separator configuration and the performance of a conventional packed tower. This is illustrated in the following example which is meant to be illustrative and not limiting, either with respect to the system used, to the operational parameters, or to the apparatus dimensions.

As an example, we may take a process in which acetone is stripped from an air stream by water F, 1 atm). The following spray separator design'is obtained for.this process example. An air velocity of 200 ft/sec and a l00,u./droplet cross-spray of water are assumed. The inner radius of the spray separator is selected as 2 feet.

The configuration shown in FIG. 11 represents this theoretical design. The droplets travel across the air stream at a terminal velocity of about ft/sec. At a liquid to a vapor mol ratio of 2.2 and for a crossflow distance of 1 inch, nearly perfect equilibration would be attained. If the water spray were formed by passing through a 3 percent open area screen, the screen width, along the direction of gas flow, would be I-% inches. This system provides a vapor mol flow rate, G, equal to 2,060 mols/hr ft and a liquid mol flow rate, L equal to 4,540 mols/hr ft (L/G 2.2) and has an equivalent stage volume of about 2 X 10' ft /lb air/sec.

The performance of the separator of this invention may be compared with that of a typical distillation column which is representative of the prior art. The column chosen for comparison is a tower packed with i-inch Raschig rings. According to the published literature (Zabban and Dodge, Chemical Engineering Progress Symposium Series 10, Collected Research Papers, Vol. 50, pp. 61-72) for such a tower acetone absorption at 70 F and 1 atmosphere pressure (identical conditions as above), G equals 10.5 mols/hr ft' and L equals 23 mols/hr ft (L/G 2.2) and the stage volume is about 20 ft llb air/sec. This means, in effect, that the spray separator of this invention is capable of handling the same amount of gas in an area only about l/200th that required in a typical. distillation column. Because of the material decrease in equilibration distance achieved by the separator illustrated in FIG. 1 the reduction in volume achieved by our separator is even more striking. For the example given above, it will be seen that the equivalent stage volumes for our spray separator and the packed tower are 2 X 10' ft /lb air/sec and 20 ft /lb air/sec. Even if multi-staging equip ment in the separator of this invention should increase spray separator volume by a factor of 10, the spray separator volume is still orders of magnitude less than the volume of conventional contactors as represented by the packed tower.

From the above description and example it will be seen that this invention provides a highly efficient, compact, and relatively simple fractionation method and apparatus. The apparatus is moreover flexible in design and adaptable to many varied applications.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in carrying out the above method and in the constructions set forth without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

We claim:

1. Fractionation method, comprising the steps of vapor stream flow;

a. imparting an angular velocity to avapor thereby to e. means for removing liquid from said last of said form a rotating vapor stream; collection means; and

b. introducing a plurality of streams of liquid droplets fmeans for removing said vapor stream.

under pressure into said rotatin v r stream t 5 5. Fractionation apparatus in accordance with claim successive i t along id vapor stream id- 4 wherein said means for transferring said liquid from i id d l t i id li id Streams i 3 said collection means to the preceding droplet forming trifugal force sufficient to impel them in paths means comprises Ponduit means Communicating through Said stream, Said paths being f a length between said collection and said droplet forming means sufficient to attain essentially complete vaporl and Pump f assoclated wlth 531d f P f 'f ]iquid equilibrium; I f 6. Fractionation apparatus, comprising in combinaa. a housing formed of an outer and inner wall defining therebetween an elongated enclosed space I having a curved configuration;

b. means for introducing a vapor under pressure into said housing whereby said vapor is converted into a rotating vapor stream; c. means for removing said vapor stream subsequent to its passage through said housing;

(1. a plurality of droplet forming means located within said housing and adapted to introduce successive streams of liquid droplets into said vapor stream c. collecting by impingement said streams of liquid droplets subsequent to their passage through said paths; and

d. transferring the collected liquid from each of said streams to the preceding point of introduction whereby the transferred liquid is used as one of said streams of liquid droplets thereby providing stepwise countercurrent flow of vapor and liquid.

2. Fractionation method in accordance with claim 1 wherein said streams of liquid droplets are formed by passing said liquid through foraminous members prior to their introduction into said rotating vapor stream.

3. Fractionation method in accordance with claim 1 Streams of dro lets through Said vapor strea including the step of filtering at least the first of said a l ralrt 0 li id lle ti n an locat d plurality of streams of liquid thereby to remove impuriwithin said housing and adapted to collect said tiessuccessive streams of liquid droplets;

4. Fractionation apparatus, comprising in combinaf. means for transferring the liquid received by each tion of said collection means, except said last collection a. means for imparting an angular velocity to a vapor means, to the preceding droplet forming means to formarotating vapor stream; upstream from it relative to said vapor stream b. a plurality of droplet forming means adapted to inflow;

troduce successive streams of liquid droplets into gmeans located eternal 0f Said g. commusaid vapor stream with sufficient forward velocity hicafihg with the first of said P F formihg to cause said streams of droplets to be impelled means and adapted to introduce liquld into 531d centrifugally through said vapor stream; first droplet forming means; and c. a plurality of liquid collection means adapted to 'h h f external f Said h g collect said successive streams of liquid droplets; mcanhg wlth the last of salfi cpllectlfm means and d. means for transferring the liquid received by each 40 to fefhove the hquld dehvered thereto of said collection means to the preceding droplet from 531d houslhgforming means upstream from it relative to said with sufficient forward velocity to impel said

Claims (5)

1. 2. Fractionation method in accordance with claim 1 wherein said streams of liquid droplets are formed by passing said liquid through foraminous members prior to their introduction into said rotating vapor stream.
2. 3. Fractionation method in accordance with claim 1 including the step of filtering at least the first of said plurality of streams of liquid thereby to remove impurities.
3. 4. Fractionation apparatus, comprising in combination a. means for imparting an angular velocity to a vapor to form a rotating vapor stream; b. a plurality of droplet forming means adapted to introduce successive streams of liquid droplets into said vapor stream with sufficient forward velocity to cause said streams of droplets to be impelled centrifugally through said vapor stream; c. a plurality of liquid collection means adapted to collect said successive streams of liquid droplets; d. means for transferring the liquid received by each of said collection means to the preceding droplet forming means upstream from it relative to said vapor stream flow; e. means for removing liquid from said last of said collection means; and f. means for removing said vapor stream.
4. 5. Fractionation apparatus in accordance with claim 4 wherein said means for transferring said liquid from said collection means to the preceding droplet forming means comprises conduit means communicating between said collection and said droplet forming means and pump means associated with said collection means.
5. 6. Fractionation apparatus, comprising in combination A. a housing formed of an outer and inner wall defining therebetween an elongated enclosed space having a curved configuration; b. means for introducing a vapor under pressure into said housing whereby said vapor is converted into a rotating vapor stream; c. means for removing said vapor stream subsequent to its passage through said housing; d. a plurality of droplet forming means located within said housing and adapted to introduce successive streams of liquid droplets into said vapor stream with sufficient forward velocity to impel said streams of droplets through said vapor stream; e. a plurality of liquid collection means located within said housing and adapted to collect said successive streams of liquid droplets; f. means for transferring the liquid received by each of said collection means, except said last collection means, to the preceding droplet forming means upstream from it relative to said vapor stream flow; g. means located external of said housing, communicating with the first of said droplet forming means and adapted to introduce liquid into said first droplet forming means; and h. means located external of said housing, communicating with the last of said collection means, and adapted to remove the liquid delivered thereto from said housing.

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