US3554501A - Mass transfer media and method of operation - Google Patents

Abstract

The invention is concerned with mass transfer media or devices such as are used to obtain effective contact between two fluids, for example in fractionating columns, washers, scrubbers, countercurrent liquid-liquid extractors and reactors. The invention is particularly concerned with a mass transfer device of the dished helical fin type which is rotated during operation with the axis inclined to the vertical, and by providing a line or band of perforations extending around the fin or fins of the device in the outer half thereof ensures improved wetting of the underside of the fin or fins and consequently improves the effectiveness of the device.

Description

United States Patent [72] Inventor Ralph W. King 1,273,030 7/1918 Campbell 19Grosvenor Place. London, England 2,216,722 10/1940 Denson [21] App]. No. 719,357

FOREIGN PATENTS 698,246 10/1953 Great Britain................

Primary ExaminerTim R. Miles [22] Filed Apr. 8, 1968 [45] Patented Jan. 12, 1971 [32] Priority Apr 14, 1967 [33] Great Britain Art0rneyDelio and Montgomery 2 [3i No. 17,237/67 [54] MASS TRANSFER MEDIA AND METHOD OF gfigg gw Figs. ABSTRACT: The invention is concerned with mass transfer media or devices such as are used to obtain effective contact between two fluids, for example in fractionating columns, washers, scrubbers, countercurrent liquid-liquid extractors and reactors. The invention is particularly concerned with a mass transfer device of the dished helical fin type which is rotated during operation with the axis inclined to the vertical, and by providing a line or band of perforations extending around the fin or fins of the device in the outer half thereof ensures improved wetting of the underside of the fin or fins and consequently improves the effectiveness of the device.

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ATTORNEK miss TRANSFER MEDIA ND METHOD or OPERATION This invention is concerned with mass transfer media, i.e. elements or devices of the kind used to obtain effective contact between two fluids, usually between a liquid and a gas or vapor, for example in fractionating columns, washers, scrubbers and also in countercurrent liquid-liquid extractors in which a substance initially in solution in one liquid is transferred into solution in a second liquid which is not appreciably miscible with the first liquid. The invention is more'particularly concerned with must such mass transfer media or elements of the dished helical type comprised of one or more dished helical fins bounded on the inside and outside by concentric cylindrical walls so as to form one or more continuous passages which, when seen as a cross section through the axis, have the appearance of being substantially parallelogrammatic or trapezoidal. The expression dished is intended to mean that, when the axis of the helical fin is vertical, a point on the outer edge of the fin is higher than a point on the inner edge of the fin. In operation the dished helical element is mounted with its longitudinal axis inclined to the vertical and rotates about this axis. Such a dished helical element and fractionating apparatus incorporating it are described in British Pat. Specification No. 698,246 and in pending British Pat. application No. 3315 8/65.

The invention is also concerned with fractionation and other apparatus incorporating a mass transfer medium of the dished helical fin type.

In operation as a mass transfer medium, the dished helical element is rotated slowly about its axis which is inclined at an angle to the vertical and liquid is supplied continuously to the top of the element whilst gas, vapor. or a second immiscible liquid of lower density passes upwardly through the helical passage or passages formed by the fin or fins and the cylindrical retaining walls. The liquid supplied to the top of the helical elements tends to form pools on the top surface of the element on the side to which its axis is tilted, but rotation of the element causes the pools to move over the surface and down the element, so that the upper surfaces of the fin or fins of the eleof the element.

Though in the known constructions the entire upper surfacemost of the liquid passes through the pores or perforations rather close to the inner cylinder, so that the outer part of the fin is not effectively wetted.

Careful observation of the flow of coloured liquids over the surface of a rotating fractionating element the axis of which was tilted in the above manner has. revealed a radial flow pattern. This is additional to and superimposed on the flow of liquid pools down the helix.

A continuous outward flow of liquid occurs from the inner boundary of the fin to the outer boundary near the line of maximum depth for stationary pools.

Whilst moving outward, the liquid is dragged by rotation of the fin to a higher point where the radial slope of the fin is inward. The liquid then flows back over'the fin to the inner boundary, and along the inner boundary to its starting point (disregarding movement in a helical direction with respect to the moving fin).

At low speeds of rotation the inward and outward flows merge and are discernible mainly bythe movement of liquid at the edge of the pools. As the speed of rotation increases, the streams of liquid flowing outward and inward separate; as the speed is further increased, the liquid flowing outward may stop short before reaching the outer boundary of the fin and ment are completely wetted by the liquid once per revolution begin to return towards the center. This is noticeable when the volume of liquid is small or the surface is rough.

It has now been found that both upper and lower surfaces of the fin or fins of a dished helical element, rotating as described above, can be wetted to obtain increased efficiency in operation by providing a line or band of perforations extending around each fin in the outer half of the fin, the area of the fin between the perforations and the inner edge of the fin being substantially free of perforations.

The line or bank of perforations should occupy a strip of narrow width and preferably be close to the outer edge of the fin and spaced therefrom. The perforations may, however, be cut out of the outer edge of the fin sothat they have open ends along said outer edge. By appropriate choice of the size of the perforations it can be ensured that, liquid flowing outwardly along the upper surface of the fin will pass through the perforations and along the underside of the fin to the inner boundary of the fin. Within a certain range of rotational speeds this return is regular and positive, and depending on the size 'of the perforations little, if any, drip will take place through the perforations to the upper fin surface lying below.

By locating the perforations at or close to the outer edge of the fin, the wetted area will constitute most of the fin area. Thus, the line or band of perforations should desirably be located in the outer quarter of the width of the fin and preferably in the outer one-tenth of the width. Thus with a fin 75 cms. in width, the.perforationspreferably lie in a band or strip of width 0.75 cms. extending around the fin inwardly from the outer edge thereof. The perforations are preferably so arranged that, during the rotation of the fin, liquid flowing over the outer surface of the fin towards the outer edge thereof will not bypass the perforations.

The perforations should be large enough to allow the liquid reaching them to pass through, but not so large as to allow the about 6 mms. long and they are desirably set at an angle to the radius.

Hitherto fractionating or otherapparatus incorporating a dished helical element of the type referred to has been operated with an angle of tilt of the element equal to the angle of the fin, i.'e. the angle between a straight generating line on the surface of the fin and a plane at right angles to the axis of It has now been found that by increasing the angle of tilt of the dished helical element by up to 15, and preferably by from 2 to 4, an improved performance is obtained, especially when helical fins having the perforations described above are used.

It is believed that the additional tilt compensates for the centripetal force on the i liquid caused by the frictional drag on the liquid by the surface of the rotating fin. This drag increases with the linear velocity of the fin'on proceeding from the inner to the outer radius of the fin. Though the magnitude of the additional tilt required is not critical, it has been found that at least 2 to 3 are necessary to secure the desired improvement.

The dished helical element or fin mentioned herein may also be referred to as a coned helical element or fin.

One embodiment of a dished helical mass transfer element according to the invention incorporated in a fractionator is shown in the accompanying drawings in which:

FIG. 1 is a longitudinal section through the fractionator;

FIG. 2 is is a fragmentary perspective view illustrating the dished helical fin element of the fractionator of FIG. 1, showing the bank of perforations close to the outer edges of the fins; and

of a reflux Condenser 10, a reflux splitterll, a helical fin assembly l2 and a reboil section 13 mounted one below the other in a housing 14 formed of separate shell members 15, 16

and 17 bolted together and a closure plate 18 at the top thereof.

The condenser comprises cooling coils 10 19 provided with a liquid inlet 20 and a liquid outlet (not shown) extending through the plate 18, the coils 19 being disposed in an annular space between the shell 17 and a cylindrical baffle 21 attached toa rotatable hollow shaft 22 extending along the axis of housing 14 by a boss 23, so that the baffle 21 will rotate with the shaft 22. q

Below the cooling coils 19 is a stationary annular collecting trough 24 attached to the outer shell 17 and comprising a cone-shaped outer wall 25 with an inner cylindrical wall 26 surrounding and spaced from the shaft 22. A nozzle 27 extends from and communicates with the collecting trough 24, through which nozzle liquid collected by the trough 24 will flow.

The reflux splitter 11 is formed of two flat discs 28 and 29 in contact with a thin spacing sheet 30 of Teflon or other material having a low coefficient of friction. The spacing sheet 30 may be replaced by a coating on one of the discs. Each of the discs 28 and 29 has four openings (not shown) therein, disposed symmetrically in the outer half of the disc. Means (not shown) are provided for moving the discs 28 and 29. relatively to one another and setting them in a position in which the openings in the one disc overlap those in the other disc to a desired extent. The reflux splitter 11 is mounted to rotate with shaft 22. 1

Mounted below the refluxsplitter 11 in line with the nozzle 27 is a collecting dish or tundish.3ladaptedto receive all the liquid from nozzle 27 which passes through the openings in the discs in the reflux splitter 11 as they rotate beneath the nozzle 27. An outlet pipe 32 from the tn tundish 31 passes out through the shell member 17;

The openings in the discs28and 29 of the reflux splitter 11 are such that by relative movement of the discs the reflux ratio may be varied from 1:1 to infinity, i.e., from half to none of the condensate from nozzle 27 may be permitted to pass to the tundish 31, the remainder flowing downwardly over the free edge of the reflux splitter. Thus the fractionator may be operated under total reflux downto a reflux ratio of 1:1.

Also mounted on the shaft 22 below the reflux splitter 11 is the helical fin s assembly 12 which constitutes the mass transfer or liquid vapor contacting device, i.e. the fractionating element, of the fractionator.

The helical fin assembly 12 is composed of a two-start helix mounted between innerandouter cylinders 33 and 34, respectively 3 inches and 10 inches in diameter, the inner cylinder 33 being attached to the shaft 22 bymeans of bosses 35, 36, and the outer cylinder 34 being attached to the inner cylinder by arms 37 so that the whole assembly rotates with the shaft 22.

The outer cylinder 34 extends downwardly beyond the fin assembly to form a skirt or shroud 40 surrounding a cylinder 41, and to provide a liquid seal between the skirt; 40 and the cylinder 41 and also between theskirt 40 and the outer shell member 16. A deflecting ring 42 projecting from the shell member 17 inwardly beyond the cylinder 34 prevents liquid falling down between cylinder 34 and the shell member 16.

The shaft 22 extends don down to the bottom of the fractionator through a support bearing 43 carried by arms a 44 and carries a stirrer 45 at its lower end.

The reboil section l3is heated electrically by electric wiring (not so shown) surrounding the section and encased in heat insulating material (notshown).

Electric heating means encased in heat insulating material may also be provided for that part of the fractionator between the reboiler and the condenser to compensate for heat losses and to assist rapid startup.

The shaft 22 passes through a gland box 46 fitted in the plate 18 and through a bearing 47 and is fitted with a pulley 48 adapted to receive a driving belt for driving the shaft.

The housing 14 is provided with a gas outlet 49 through which vacuum may be applied to the interior and/or noncondensables vented, a feed inlet 50, a residue drain 51 and a liquid seal drain 52. The liquid or product takeoff line isthe pipe 32.

In operation, the fractionator is inclined at an angle to the vertical. Vapor from the reboil section 13 passes up through the rotating fin assembly 12 andthe reflux splitter 11 and condenses in condenser 10. The liquid condensate runs down into the collecting trough 24 and through the nozzle 27 towards the rotating reflux splitter 11;? part passing through the openings in the discs 28, 29 of reflux splitter 11 and part contacting the solid part of the discs. That part which passes through the openings is collected in the tundish 31 and passes out through pipe 32. That part which falls on the solid part of the discs runs off the outer edge of the discs to drop down on to the upper turns of the helical fin assembly 12 and then runs down around the finsto the reboil section 13. In operation, a liquid seal of condensed vapor forms between the skirt 40 and cylinder 41.

The fractionator described has a low pressure drop and may advantageously be used for distilling heat sensitive materials.

The dished helical fin element 12 is formed by attaching two continuous curved helical fins 53'and 54 to the inner cylinder or cone 33. The outer cylinder 34 fits tightly around the fins. Each fin comprises 28 complete turns and has a pitch of one inch so that the distance between fins (less thickness of the metal) is one-half inch, measured in a line parallel to the axis. The angle of fin or cone, i.e. the angle between a straight generating line on the surface of the fins and a plane are at right angles to the axis is 20. The fins may be made by welding together split annular discs of 24 gauge sheet metal. The none nonperforated area of the fins is embossed to provide a dimpled effect.

The I helical fins a helical fins 53 and 54 are provided with three rows of holes 55 of diameter 0.144 inches in circles of pitch diameters 9.4, 8.4 and 7.4 inches as shown in FlG. 2. Each complete turn of each helix contains 43 holes per row or 129 holes in total. An alternative form or perforation is shown in FIG. 3 in which the perforations are in the form of slots 56 arranged at an angle of 45 to the radius.

The benefit of providing a narrow band of perforations close to the outer edge of the fins of a dished helical fin fractionating element of a fractionator is illustrated by the following example. The improved result obtained by increasing the angle of tilt is also illustrated.

EXAMPLE The apparatus used was the fractionator shown in FIG. 1 and described above. That part of the fractionator between the reboil section 13 and the condenser 10 was lagged and compensated by electric heating on the outside to prevent heat losses. The space the condenser 10 was connected via outlet 49 and an automatic control valve, which allowed the absolute pressure to be automatically controlled, to a vacuum pump. Suitable instruments were fitted to measure the temperature and pressure of vapor above and below the element 12. The seal ring at the lower end of shroud 40 was filled with liquid during operation. The fractionator was mounted in a frame enabling it to be tilted with its axis at various angles to the vertical.

Three sets of experiments were carried out, in the first of which the fins of the helical fin element 12 were devoid of perforations but were otherwise identical with those described above in connection with theaccompanying drawings. In the second and third sets of experiments the fins of the helical fin element were provided with the three rows of holes of the size and disposition specified above in connection with FIG. 2.

In each experiment a mixture of 4 litres of pure diethyl phthalate and 1 litre of pure dimethyl phthalate was fed to the kettle or reboiler 13 through inlet 50 and heat was applied to the reboiler so vapor passed up through the helical passages of element 12 to the condenser 10 where it was totally condensed, the liquid returning through the passages of the element 12 to the rebont All experiments were carried out at a condenser pressure between 5.5 and 6.5 torr at a constant vapor top temperature of 138 C.

The latent heat of vaporization of the mixture under these conditions was calculated as 130 B.t.u. per pound and the relative volatility of the mixture as 1.45, and these figures were used in all subsequent calculations.

The efficiency of the fractionator was determined under each set of conditions by withdrawing and analyzing samples of the overhead condensate and kettle liquid under d conditions approximating to total reflux." The efficiencies were calculated as number of theoretical plates at total reflux" by the well-known Fenske Underwood equation.

The vapor throughput in all experiments was maintained at 28 lbs/hour l percent) by adjustment of the heat input to the kettle. This was checked periodically by measuring the rate of flow and temperature rise of the water entering and leaving the condenser.

The first set of experiments (with the nonperforated fins) comprised a series of experiments carried out at rotational speeds from to 60 r.p.m., with the axis inclined at angles of 20, 23 and 26 to the vertical. The direction of rotation was creasing the angle of tilt from 20 to 23 raised the maximum efficiency from 4.9 to 5.5 theoretical plates (approximately 12 percent). Further increase in the angle of tilt from 23 to 26 produced no further increase in fractionating efficiency, although the pressure drop was somewhat increased. The pressure drop under conditions of maximum efficiency at an angle of 23 was 0.51 torr per theoretical plate.

The second set of experiments in which the perforated fins were used comprised a similar series of experiments. The fractionating efficiency was measured at a series of rotational speeds as before. The effect of the perforations on the fractionating efficiency at an angle of tilt of 23 was to raise the maximum efficiency to 8 theoretical plates, this number appearing as a sharper maximum at a rotational speed of r.p.m. Changing the angle of tilt to 20 and 26 with the perforated fins had the same relative effect on the fractionating efficiency as before. The pressure drop per theoretical plate at a speed of l0 r.p.m. and an angle of 23 amounted to 0.41 torr.

The increase in fractionating efficiency resulting from the rows of perforations near the periphery was undoubtedly due to the wetting of the underside of the fins which now occurred. This wetting effect was shown in a series of subsidiary experiments in which the fractionating element was removed from the casing and its close fitting cylindrical wrapper, and rotated in an inclined position whilst a stream of water containing a small amount of detergent was directed on to the top of the element. V

In the third set of experiments, also using the perforated fins, the direction of rotation was reversed so as to cause the pools of liquid on the fins to move upwards instead of downwards. The effect of this was to increase very considerably the amount of liquid passing through. the holes and flowing across the underside of the fins. The element-now showed a maximum efficiency of 7.6 theoretical plates at an angle of tilt of 23 and at the same speed of rotation as before,

' tions, although the degree of wetting of all surfaces of the heli-' cal passages was best with the reversed direction of rotation.

Reverse rotation would not normally be used for fractionators. but may be advantageous in cases where it is very important that all surfaces of the helical element be fully and positively wetted and flushed. e.g. in cases where both mass transfer and chemical reaction takes place. A typical case of this kind is the sulfonation of an organic liquid in which the liquid flows downwardly through the helical element in contact with an upwardly flowing airstream containing sulfur trioxide.

in operating apparatus incorporating the dished helical fin element, the speed of rotation of the dished helical fin element should be below that at which a substantial part of the liquid is removed from the fins to the inner surface of the oute cylinder by centrifugal force.

Though in the apparatus shown in HO. 1 an outer cylinder 34 is fitted to the helical fin element, this cylinder may be omitted an and the fins may instead extend close the the wall of the housing or outer casing 16 without actually making contact with the wall, the outer casing 16 then constituting the outer cylinder of the helical fin assembly.

The invention has been described above in detail as applied to fractionating apparatus, but it has been found to be of advantage generally for any apparatus in which effective contact between two fluids is required. Thus, the dished helical fin ele-' ment of the invention may advantageously be used in scrub: bers and washers to obtain effective contact between a liquid and a vapor, in countercurrent liquid-liquid extractors in which a substance in solution in one liquid is transferred into solution in a second liquid which is not appreciably miscible with the first liquid, in reactors in which mass transfer of a substance between two fluid phases is followed by chemical reaction with the transferred substance, and in other apparatus for performing similar operations. I

Examples of apparatus in which both mass transfer and chemical reaction take place are oxygenators for the oxygenation of blood and sulfonation reactors for carrying out the sulfonation reaction mentioned above.

Furthermore, improved results are obtained not only when compared with nonperforated fins as shown by the above experiments but also when compared with fins uniformly perforated over the whole of their areas.

I claim:

1. For apparatus of the kind referred to in which mass transfer between two fluids is obtained by bringing the fluids into effective contact with one another, a mass transfer device for effecting such contact consisting of a dished helical fin element comprising at least one helical fin, characterized in that each helical fin has at least one row of perforations extending around the fin in the outer half thereof, the area of the fin between the perforations and the inner edge thereof being substantially free of perforations.

2. A mass transfer device according to claim 1, in which the perforations are located in the outer quarter of the width of the fin.

3. A mass transfer device according to claim 2, in which the perforations are located in the outer one-tenth of the width of the fin.

4. A mass transfer device according to claim 1, in which the fin is provided with one or more rows of circular perforations.

5. A mass transfer device according to claim 1, in which the fin is provided with a row of perforations in the form of slots.

6. A mass transfer device according to claim 5, in which the slots are inclined at an angle to the radius of the fin.

7. Fractionating or other apparatus. including the mass transfer device of claim I mounted for rotation about its axis within the apparatus.

8. A method of operating apparatus according to claim 7 with the mass transfer device rotating continuously, in which the apparatus is tilted so that the mass transfer device rotates with its axis at an angle to the vertical equal to the angle of the fin or greater than the angle of the fin by up to 15.

9. A method according to claim 8, in which the angle offtilt is greater than the angle of the fin by from 2 to 4.

10. A method according to claim 8,'in which the direction of rotation is such as to cause liquid on the upper surface of the fin to move downwards. i

11. A method according to claim 8, in which the direction of rotation is such asto cause liquid on the upper surface of the fin to move upwards.

Claims (11)

1. For apparatus of the kind referred to in which mass transfer between two fluids is obtained by bringing the fluids into effective contact with one another, a mass transfer device for effecting such contact consisting of a dished helical fin element comprising at least one helical fin, characterized in that each helical fin has at least one row of perforations extending around the fin in the outer half thereof, the area of the fin between the perforations and the inner edge thereof being substantially free of perforations.

2. A mass transfer device according to claim 1, in which the perforations are located in the outer quarter of the width of the fin.

3. A mass transfer device according to claim 2, in which the perforations are located in the outer one-tenth of the width of the fin.

4. A mass transfer device according to claim 1, in which the fin is provided with one or more rows of circular perforations.

5. A mass transfer device according to claim 1, in which the fin is provided with a row of perforations in the form of slots.

6. A mass transfer device according to claim 5, in which the slots are inclined at an angle to the radius of the fin.

7. Fractionating or other apparatus including the mass transfer device of claim 1 mounted for rotation about its axis within the apparatus.

8. A method of operating apparatus according to claim 7 with the mass transfer device rotating continuously, in which the apparatus is tilted so that the mass transfer device rotates with its axis at an angle to the vertical equal to the angle of the fin or greater than the angle of the fin by up to 15*.

9. A method according to claim 8, in which the angle of tilt is greater than the angle of the fin by from 2* to 4*.

10. A method according to claim 8, in which the direction of rotation is such as to cause liquid on the upper surface of the fin to move downwards.

11. A method according to claim 8, in which the direction of rotation is such as to cause liquid on the upper surface of the fin to move upwards.

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