US2871250A - Method for continuous multistage countercurrent contacting of liquids with vapors - Google Patents

Description

Jan. 27, 1959 E. w. ECKEY 2,

- METHOD FOR CONTINUOUS MULTISTAGE COUNTERCURRENT CONTACTING OF LIQUIDS WITH VAPORS 7 Filed Aug. 12, 1955 2 Sheets-Sheet l INVENTOR Eddy mmjc QBWM BYQiu/nu, 114; 7L

ATTORNEYS Jan. 27, 1959 E. w. ECKEY 2,871, METHOD FOR CONTINUOUS MULTISTAGE COUNTERCURRENT CONTACTING OF LIQUIDS WITH VAPORS 2 Sheets-Sheet 2 Filed Aug. 12, .1955

INVENTOR I Eddy WEcJwy 9 \bmw ATTORNEYS METHOD FOR CONTINUOUS MULTISTAGE' COUNTERCURRENT CONTACTING OF LIQ- UIDS WITH VAPORS Eddy W. Eckey, Wyoming, Ohio Application August 12, 1955, Serial No. 528,102

12 Claims. (Cl. 260-4285) This invention relates to a method for continuous multistage contacting of liquid with vapors. More particularly, the invention relates to a process for componentinterchange between vapors and high viscosity liquids, for example, such as are encountered in refining fats by continuous countercurrent treatment with a stripping vapor to remove free fatty acids and odoriferous components.

In general, the present process finds its greatest applicability in the contacting of vapors such as stripping vapors, or vapors of the liquid material undergoing treatment, with viscous liquid materials whose vapor pressures are so low as to require low absolute pressures Which may be about atmospheric or slightly above, but generally are below atmospheric, for example, down to about 0.5 mm. of mercury. Among the uses to which the process may be put is the treatment of fats and oils of the glyceride type such as vegetable oils including soybeam oil, linseed oil, cottonseed oil, peanut oil, corn oil,-

etc.; marine oils including whale oil, menhaden oil, sardine oil and the like; animal oils such as lard; hydrogenated vegetable, marine and animal oils produced by catalytic hydrogenation of the aforementioned oils; reduction of tall oil to recover volatile components and pitch; fatty acid distillation; removal of tocopherols and sterols from natural oils; and other similar services particularly those in which the material to be distilled from the viscous oil is present in relatively low concentrations and must be reduced to a small fraction of the original concentration as will appear more fully hereinafter.

The process is likewise useful in connection with the removal of solvent from solvent extracted vegetable, animal and marine oils and fats, or from solvent extracted mineral oils such as lubricating oils, particularly wherein the removal of all except traces of the solvent is important and cannot be readily accomplished by other more conventional treatment.

The process may also be used for steam-stripping of tars and the production of lubricating oil fractions from bright stocks, etc., either before or after refining by other conventional methods.

The process is also useful for stripping absorbed hydrocarbons from absorber oils such as those used in connection with the recovering of desirable hydrocarbons from natural gases, refinery gases and the like; or for stripping small quantities of accumulated lower boiling contaminants from absorber oils.

By the same token, the process can be used under some conditions for the countercurrent continuous extraction of higher boiling constituents from natural or refinery gases (e. g. propane, butane, pentane, etc.), particularly, under conditions in which a substantial number of theoretical plates are required but in which a low pressure drop through the system is particularly desirable or necessary. In this latter service the process is of particular advantage in the relatively low pressure operations, generally less than about 100 p. s. i. g., where the ratio of liquid to vapor is relatively low, and wherein it is difii- Patented Jan. 27, 1959 2 cult to obtain efficient operation in the conventional countercurrent tray-type columns.

The process and apparatus are useful in extractive-distillation'operations. In this type of operation a liquid which is non-volatile under the conditions of the distillation is passed through the apparatus along with the material to be separated by distillation, for the purpose of altering the relative volatilities of the materials being separated.

As used throughout the specification and claims herein the term non-volatile liquid means a liquid which is non-volatile under the conditions of operation.

The process finds its greatest applicability in the treatment of liquids having a viscosity not less than about 0.6 centipoise at the average temperature of the distillation.

The term viscous liquids as used herein refer to liquids having such viscosity characteristics.

There is a rule-of-thumb that most pure liquids have nearly the same viscosity at the temperature of their normal boiling points at one atmosphere pressure, namely about 0.2 to 0.3 centipoise (Perrys Chemical Engineers Handbook, 3rd ed., pp. 612 and 698, McGraw-Hill, 1950). It follows that the non-volatile liquids encountered in the various uses to which the present process may be put are well below their boiling points and will be relatively viscous to a point which does not permit of high plate efficiencies when the operations are carried out in conventional equipment. As may be seen from the table on page 699 of the Perry Handbook, efiiciencies may be as low or lower than 10% in conventional equipment, which means that ten or more actual plates would be needed to attain the effect of one theoretical plate. The graph on page 613 of Perrys Handbook shows that in petroleum refinery distillation in conventional equipment the plate efiiciency falls below 30% when the average viscosity of the mixture being treated is greater than 06 centipoise at the average temperature of the treatment' In the case of steam refining and deodorization of fatty oils the viscosity of the oil being treated will ordinarily be in the range of 0.9 to 3.5 centipoises at the operating temperature. Distillation of fatty acids involves about the same lower limit, say about 0.8 centipoise. The viscosity of absorber oils used for component recovery from hydrocarbon gases, whether considered from the standpoint of the absorption or the strippingoperations, is high under operating conditions since these are well below the temperatures at which the oil boils and consequently high plate efficiencies are not obtained in conventional equipment. Lower boiling absorber oils have been used in order to improve the plate efiiciency but this brings in the necessity of using care in stripping operations to avoid carrying over the lower boiling ends of the absorber oil. The present invention permits of the use of higher molecular weight, i. e. higher boiling absorber oil and at the same time obtain high plate efliciency.

When fractional distillations are carried out in the apparatus of this invention at pressures substantially below one atmosphere absolute, the viscosities of the liquids are substantially above the 0.2 to 0.3 centipoise value which would prevail if the operation were carried out at atmospheric pressure.

The movement of molecules from within the body or particle of liquid to the interface between the liquid and the gas in a stripping or distillation operation is tremendously slow compared with the movement of the molecule from the liquid to the gas at the interface. The diffusion or migration of molecules to the interface becomes slower as the size of the molecules of the distillate and distilland increases. Thus, it is known that exposure of a large liquid surface to flowing vapors is desirable.

, Constant exposure of new surfaces likewise increasesthe efficiency of component interchange between liquid and gas. Conventional operations employ various means for taking advantage of these factors by the use of mechanical or vapor-induced agitation as in most conventional batch operations; or by flowing vapors through shallow pools of liquid as in perforated plate and bubble tray operations; or by flowing vapors across films of liquid, often through a tortuous path, as in packed columns, baffle plate columns, or the type of apparatus wherein the liquid flows as a film down the walls of a tower.

Under ordinary conditions of distillation some molecules of distillate reenter the distilland until a point is reached where the rate of escape of vapor and the rate of reentry of molecules to the surface produce an approximate equilibrium between liquid and vapor phases. In this type of distillation or stripping the efficiency of the process increases depending upon the rapidity with which the vapor and liquid approach to attain equilibrium. This applies to both high and low pressure operations.

In order to handle high molecular weight, very viscous materials there has been developed in more or less recent years the so-called molecular still in which molecules of the distillate are evaporated from the surface of a film of hot liquid. The molecules fiy a short distance to the surface of a condenser and because of the short distance and the tremendously high vacuum employed, encounter no obstruction either by structural parts of the still or with other molecules of vapor or residual gas. In molecular distillation, as contrasted with ordinary distillation of which the present invention may be considered a form, the distillate molecules do not reenter the distilland and no equilibrium is reached between liquid and vapor. Pressures employed in molecular distillation ordinarily are of the range of 0.1 to microns, i. e. 0.0001 to 0.005 mm. of mercury, the latter being the approximate upper limit of effective operation.

The present process extends the practical range of materials which can be fractionally distilled or stripped, without resorting to molecular distillation, to include materials having higher boiling points, lower vapor pressures and higher viscosities than those presently distilla ble in conventional distillation apparatus.

It is one of the principal features of the present process that the liquid and the vapor are maintained continuously close to equilibrium in the vapor-liquid contacting zone by means wherein the advantage of an ideal countercurrent distillation process can be approached to a degree that is unattainable in ordinary stills, particularly when the materials being handled are viscous and high boiling. By the present process the size of apparatus for a given capacity is smaller, thereby reducing cost, particularly when the apparatus must be fabricated from stainless steel or other costly materials. There also results a reduction in the weight-quantity of stripping vapor required, both because of the equilibrium effects obtained, and the negligible pressure drop through the system. Consequently there is likewise a reduction in cost of maintaining a vacuum in operations at low absolute pressures, say as low as about 0.5 mm. of mercury. There results an improved quality of product through more precise fractionation or stripping and, as previously mentioned, the practical range of materials that can be fractionally distilled or stripped without resorting to molecular distillation is considerably extended. It is particularly useful where a required degree of, refinement or fractionation is high; that is, where the ratio of concentration of volatile component in. the feed stock to its concentration in the stripped stock, or in the less volatile fraction, is large, say and upward.

An understanding of the process is best acquired by first describing the appended figures wherein a preferred apparatus is shown in Figures 1 to 3 inclusive.

Figure 1 is a longitudinal section of the apparatus;

Figure 2 is a transverse section of Figure l Figure 3 is a fragmentary view of one design of an impeller.

Referring to Figure 1, the appaartus comprises a cylin drical tube 10 jacketed by another tube 11, of larger diameter. The annular space 39 between these two tubes is sealed by means of flange 36 at the upper end of the apparatus and flange 37 at the lower end. The tempera ture in the vapor-contacting zone can be controlled by admitting steam, Dowtherm, or any other suitable heating or cooling medium, either vaporou-s or liquid, into the jacket thus formed. When a hot condensing vapor such as steam is being used to heat the column, it enters the jacket through the pipe 26 at the top and the condensed vapor drains from the pipe 27 at the bottom of the apparatus. Tube 10 is generally inclined at a slight angle to the horizontal, namely from right to left as viewed in the drawing, to assist liquid fiow therethrough. Tube 11 may be inclined from left to right to permit adequate draining of the condensate from the jacket. Alternatively the design and location of inlet 26 and outlet 27 may be altered to suit any particular circumstance.

The stripping or fractionating process takes place within the inner tube 10. This tube 18 is sealed at the lower end by means of the flanged head which is bolted to flange 37 by suitable means such as stud bolts 33. A vacuum or pressure tight seal is obtained, for example, by the use of a suitable gasket between the faces of the flanges. The flanged head 25 seals the upper end of tube 10 in a similar manner.

The material to be processed is fed to the column through the inlet tube 17. The rate or" feed can be maintained constant by means of a positive displacement pump or any other suitable controlling device, not shown. If it is necessary to heat the material to be processed before it enters the column, as is usually the case in the deodorization of lard, vegetable oil and the like, this can be done in a conventional type of heater. The inlet tube 17 enters the column through a stuffing box assembly, 21 and 34, in the flanged head 25. This stufling box is not necessarily of the design shown in Figure l but can be of any suitable design whi h will provide a leakproof seal under the temperature and pressure conditions encountered in the process. The point in the column at which the feed is discharged from the inlet tube 17 can be varied to suit various operating requirements by sliding the inlet tube 17 one way or the other through the stuffing box 21. In a stripping operation employing a stripping vapor, such as in fat deodorization, the feed point is near the upper end of the column. When the column is being used for continuous fractional distillation, the feed may be further down the column. In this case means are provided for returning part of the condensed overhead as reflux to the upper end of the distillation zone, for example, by means of a pump and a separate reflux inlet line from the reflux condenser to the distillation zone. If it is desirable to have the feed enter the column at a point below the center, the inlet tube 17 could come through the flanged head 24 on the lower end of the apparatus as will be apparent. If the taken on the line 2-2 apparatus is to be built for a particular operation in which the feed to the column. is to be at the same point at all times, the stuffing box 21 could be eliminated and the inlet tube 17 would then be welded or otherwise fixed into the flanged head 25; or the liquid feed could be introduced through a line extending down the walls ofthe cylinder it at an appropriate point.

The stripping or fractionating section of the column is divided into a number of compartments by the rings 16 which may fit into circular grooves machined in the tube 10, as illustrated, or may be welded, bolted or otherwise suitably secured at spaced intervals to the inside of the tube'10; The inside diameter of these rings 16 is smaller than the inside diameter of the tube 10 so. that they formweirs at the 'bottomi'ofthe cylinder over which the liquid flows as it passes from the upper to the lower end of the column. Thus, with the column in operation, there will be a pool of liquid in each compartment along the bottom of the tube the depth of the liquid layer in each compartment depending upon the inside diameter of the rings 16. The relationship of diameter of the rings to the inside diameter of tube 10 is generally approximately as shown although this may vary to some extent. It will be apparent that the depth of the pools of liquid between the rings will be greater as the diameter of the tube is increased. These rings are generally spaced equidistant from each other although there could be some variation. The inside diameter of therings 16 should not be so small as to obstruct to any substantial extent the flow of vapors through the column. They are in no sense of the word vapordirecting balfles. Their function is (1) to confine the liquid flowing along the bottom into pools, and (2) to confine the liquid coalesced on the walls so that it is returned to the pool from whence it emanated, as will be more fully explained.

A rotating shaft 12 extends through the column 10. The lower end of the shaft is mounted for rotation, for example, in a ball bearing 23 or other suitable hearing. The shaft extends through the stuffing box 22 in the flanged head 25 at the upper end of the column where it can be connected with a power source, such as a motor, steam turbine or the like, for rotation. Obviously the shaft need not be mounted exactly as shown. In Figure 1, the bushing 35 serves as a bearing for the shaft at the upper end but this can be replaced by any other practical arrangement, such as a ball bearing mounted in an external yoke. Any suitable design of sealing ring or stufiing box which will provide an effective seal under the operating conditions of temperature and pressure can be used.

On the shaft 12 are mounted in any suitable manner a multiplicity of discs or impellers 13. These'are separated on the shaft by washers 14 and 15, the thickness of which will depend upon the desired spacing between the rings 16 and also upon the number of impellers to be mounted on the shaft between any adjacent pair of rings. The impellers 13 are held in place atthe upper end of the shaft by the shoulder 40. They are then placed on the shaft as shown in Figure 1 and held securely by the lock nuts 28 adjacent the lower end of the shaft. If desired, the shaft may be splined.

Thus, the assembly can be easily dismantled after removing the head 25 and removing the assembly from the cylinder. The impellers can then be rearranged on the shaft as desired, or replaced. It may be desirable in some instances to mount the impellers permanently on the shaft as by spot welding, in which case the nuts 28 and shoulder 40 would not be necessary.

The shaft 12 is mounted parallel to the axis of tube 10, but eccentric thereto, as shown in Figure 2. The distance between the center line of the shaft 12 and the center line of the tube 10 depends on the diameter of the impellers 13. When mounted on the shaft, the impellers 13 are close to the bottom of the tube 10 (e. g. about ,4 inch), leaving a large unobstructed crescent-shaped area along the top of the tube through which the vapors can freely pass. The ratio of cross-sectional area of this free space to the cross-sectional area of the tube may vary, but should be great enough so that there is no substantial pressure drop on the vapor from one end to the other when the vapor is passed through the zone at operating velocities in the absence of liquid.

The rotating impellers 13 pick up liquid from the shallow pools between the dividing rings 16 and project it in the form of flat transverse sprays of fine droplets completely around the inside of the tube 10. Intimate contact with the stream of stripping vapor such as steam,

is obtained as it-passcs in succession through these maultiplef'ctutains of fine droplets, The liquid droplets strike the wall of the cylindrical tube 10, coalesce, and flow downwardly along the walls between the rings 16 and into the pool of liquid on the bottom of the column. Here it is picked up once more by the impellers 13 and again sprayed outwardly, whereby contact with the vapors or stripping steam is repeated. The portions of dividing rings 16 extending around the cylinder above the level of the pools, confine the coalesced liquid on the walls of the tube 10 so that it flows back to the pool from whence it was sprayed. Thus, most of the liquid in a given pool at a given time is sprayed into the stripping vapor repeatedly before it flows over the weirs 16 and/or through the gaps 38 to the next downstream pool.

The column can be operated in a horizontal position, as shown in Figure 1, but, as previously mentioned, it is generally preferred to tilt the apparatus at a small angle, say up to about 20 to 30 and preferably about 1 to about 10, to the horizontal, the upper end, through which the feed inlet tube 17 enters the column, being the higher. In a stripping operation, the material to be processed enters the column at the upper end through the inlet tube 17. It is picked up preferably two or more times and sprayed by the impellers 13 in the first compartment, then flows to the next compartment where this spraying action is repeated. On reaching the lower end of the column, the processed liquid flows through outlet tube 19 to a cooler or storage tank, or if it is to be further processed or refined, may be pumped directly to the next processing unit.

Steam for the stripping process enters. the column through the tube 20 in flanged head 24 and travels through the column counter to the general flow of the liquid material being processed, but transverse the flow of impelled droplets. While passing through the column, the steam comes in repeated contact with the thin flat sprays of liquid droplets produced by the action of the impellers 13, as well as with the coalesced liquid- This intimate contact covering the walls and the discs. between the steam and extensive liquid surfaces, particularly as provided by the droplets, which surfaces are constantly being renewed, together with the countercurrent.

flow of the liquid and vapor streams, results in an exceedingly efficient stripping action. It has been found that the material to be distilled is in substantial equilibriumbetween the liquid and vapor phases at all times despite the high linear velocity of vapor flow which may be employed.

The steam carrying stripped vapors after leaving the contacting section, passes through a series of bafiles 29 and 30 at the upper end of the column. These serve as mist extractors to remove entrained droplets of oil. The

Instead of the bafiies 2? and 30, any other suitable mist extractor which is physically adaptable to use in or with the apparatus may be used to remove entrained liquid droplets from the vapor stream. Such extractors are well known and need not be described in detail.

The liquid flow from compartment to compartment is principally over the weirs formed on the bottom of the tube 10 by the dividing rings 16. The spacing between the rings may vary somewhat but they are always relatively close together. Where several impellers areused in each space, they may be farther apart than when one is used. In larger apparatus the distance between rings may be greater to compensate for the larger volume of liquid and the thicker gauge of the impellers.

A small amount of liquid, 'as it drains down the wall of the column 10, will flow through the opening 38 in the dividing rings 16, as shown in Figure 2. Opening 38 lies adjacent to but above the normal liquid level of the '7 pools, for example, about 60" above the lowest point in the cylinder, 'as'illustratcd. The column can be operated efficiently without these openings.

Tube 31. in the upper flanged head 25 and tube 32 in the lower flanged head 24 provides a means to which instruments for the measurement of the absolute pressure as well as the pressure drop through the column may be attached. The pressure differential between the upper and lower ends of the column can be accurately measured by connecting tubes 31 and 32 to opposite sides of a differential manometer using oil or some other non-volatile, low density fluid in the manometer. The low pressure drop of less than 3 mm. and generally less than 2 mm. of mercury obtained in this apparatus is an important feature of the invention and is made possible by the fact that a large unobstructed passage is provided for the flow of stripping vapors between the outer periphcry of the impellers and the inner periphery of the dividing rings, above the normal level of liquid in the pools.

It is possible, with the apparatus described above, to obtain efli'cient results employing any one of a number of. impeller designs. Two of the many possible designs are shown in Figures 2 land 3. A plain metal disc has been found to operate satisfactorily. More complicated designs have also been used. One such impeller was fabricated from a perforated metal disc by bending the outer edges of .the disc circumferentially to form a rounded cup-like member. This type of impeller picks up the liquid and sprays is outwardly through the small perforations as small droplets. Paddle wheels or wire brushes may also be employed.

The number of impellers l3 and dividing rings 16 in the column will generally depend on the available space. In general, it is desirable to extend the impellers l3 and rings 16 along the entire central section of the column, leaving the ends of the tube open where the tubes 18 and. 19' enter, as illustrated in Figure 1. While Figure 1 shows three impellers in each compartment in the tube, this is not necessarily the best arrangement under all conditions for the treatment of all materials. It may be better, in some instances, to have a greater number of compartments with as few as one impeller in each compartment. In other cases, the number of impellers may be increased and the number of compartments decreased. The apparatus shown has been found to give excellent results, particularly with the materials described in the following examples, which are given for purposes of illustration and not intended to be limiting.

The vapors are recovered in a conventional manner. If the vapors are homogeneous, as is the case when ordinary distillation is carried out, they may be condensed and recovered in the usual way. If, was is often the case when stripping vapors are employed, the condensate forms a two-phase system, the material stripped from. the liquid being treated may be recovered by decantation or by dissolving or extracting them with a suitable solvent or by other conventional methods where the material is of sufijcient value to warrant recovery.

The number of dividing rings in the vapor-contacting section will vary considerably, depending upon the material undergoing treatment and the degree and precision of separation desire For the removal of components comprising a fraction of one percent of the total liquid treated, as often occurs when refining fats and oils, the minimum number of rings is about ten and is preferably forty or above.

The minimum number of successive locations from which the liquid is sprayed should be not less than twentyfive and there should be at least one spray means in each of the compartments formed by adjacent rings. Preferably two to five rotating discs are used in each compartment.

The-minimum aggregate spraying capacity of the device should be at least fifty times the rate of feed to the finite number of theoretical plates.

8 apparatus. Preferablythe-spraying capacity is one hundred or more times the feed rate.

The pressure drop through the apparatus is less than 0.2 mm. of mercury per theoretical plate. In general, the pressure drop throughout the apparatus is less than 3 mm. of mercury and, for an apparatus having from six to thirty theoretical plates, is generally less than 2 mm. of mercury across the distillation or vaporcontacting section.

The droplet sizes in general depend upon the type of spray equipment employed. For rotating discs, the droplet sizcs (diameter) will range from about 0.05 to 2 mm., the average droplet size, as well as the minimum and maximum, depending upon the particular design of the discs, as well as upon the speed of rotation of the discs. If the average drop size is less than about 20 microns particle entrainment may become excessive and it is, therefore, preferred that the average drop sizes be not smaller than about 50 microns (0.05 mum).

The desired droplet size can be obtained when using rotating discs with a peripheral speed of about eight to one hundred feet per second and preferably about eight to fifty feet per second. Efficiency of the apparatus increases considerably with the speed of rotation of the discs, particularly in the range of about eight to twenty feet per second peripheral speed. At speeds greater than 100 ft. per sec., power consumption becomes excessive so as to be impractical.

The slope of the column may be varied considerably depending upon a number of factors. Within limits the capacity of a given column increases with increased slope. Thus, in a given'apparatus under given operating conditions, when the liquid charge rate is increased to a point just short of flooding at a 3 slope, the degree of entrainment is markedly decreased by increasing the slope to 4".

The linear vapor velocities employed may be varied over a wide range from relatively low velocities below ten feet per second to relatively high velocities of 30 feet per second or more. The maximum is something short of the point where entrainment of liquid becomes excessive. The optimum vapor velocity will depend upon the job to be done, the pressure conditions, the size of droplets, the slope of the apparatus, the rate of liquid feed, the vapor density and the like. For example, linear velocities can be considerably higher at 1 mm. pressure than at atmospheric.

The apparatus employed has a very small H. E. T. P. (height equivalent to one theoretical plate). The number of theoretical plates to be employed for any given service is dependent upon a number of factors, including the nature of the material undergoing treatment, the amount of material to be distilled from the feed stock, and the degree and precision of separation required, as well as the presence or absence of a stripping vapor. In general,

when removing fatty acids and odoriferous materials from fats and oils wherein the material to be removed amounts to less than about one percent by weight of the feed, about 10 to 20 theoretical plates are sufiicient when using steam as the stripping agent. As may be seen from Examples I and ll, an apparatus having six theoretical plates, or about one plate for every three inches of length in the vapor contacting zone, had a steam utilization effectiveness better than seventy-seven percent of an ideally efiicient continuous countercurrcnt process with an in- This was true even when operated under a wide range of feed rates. The larger number of theoretical plates provides improved steam utilization and a better margin of safety as to quality of product. When operating with as many as thirty theoretical plates, the efficiency of removal of fatty acids from the oil and of steam utilization is still further increased, exceeding and approaching When using the device as a fractional distillation column, the number of theoretical plates employed will depend upon the material being fractionated and the degree of fractionation required! Examples VI and VII show various arrangements and operating conditions under which the effect of six and seven theoretical plates can be obtained. Obviously the number of theoretical plates can be increased still further by lengthening the vaporcontact section and adding more rings andimpellers. It will be observed from these examples and from the discussion herein that about the same effectiveness can be obtained in a number of ways, such as varying the number of the dividing rings, the number of successive locations for spraying in each compartment, and the speed of the rotor.

It should be noted that when the material being treated has, say, less than one percent of distillable material present the residence time and therefore the number of times at which the oil is sprayed from any given compartment is substantially the same from the first to the last compartment. Thus, if the rate of feed is one pound per minute and the-spraying capacity per compartment is two pounds per minute, the oil will be sprayed from each compartment approximately two times before it flows to the next compartment.

The advantages of the process and apparatus may be briefly summarized as follows:

(1) Low pressure drop through the column of less than 0.2 mm. of mercury per theoretical plate;

(2) Effective mixing of liquid and renewal of liquid surfaces in contact with the vapor, repeated sufiiciently to result in high effectiveness per unit'length of column (small H. E. T. P.).

(3) Lowered capital and operating costs;

(4) Ease of disassembly for cleaning and repair;

(5 Good control of temperature;

(6) Good heat transfer;

(7)"Hold up in the column may be varied down to small values allowing use of short time and high tem perature for processing materials otherwise difficult to distill or refine without overheating;

' (8) Flexibility making the apparatus adaptable to a wide variety of materials and feed rates by varying the angle of tilt and speed of rotation of the impellers;

(9) Close approach to ideal countercurrent performance in stripping operations.

For purposes of simplification and comparison with conventional processes, the invention will be described in greatest detail in connection with the refining of fats and oils to remove free fatty acids and odoriferous materials to improve the odor, fiavor'and other characteristics of fats and oils intended for human consumption. The process finds one of its greatest advantages in this field. It should be understood, however, that the process is not to be interpreted as strictly limited to this particular field of utility. It will be described in connection with the preferred apparatus shown in Figures 1-3, but it will be apparent how it can be adapted to the alternative but not necessarily equivalent modifications.

REFINING FATS AND OILS The refining of fats to remove free fatty acids and odoriferous materials therefrom is a widely practiced art.

These can be generally classified as (l) batch processes,

(2) continuous non-countercurrent processes, and (3) continuous countercurrent processes.

The. essential operation in batch fat refining processes entails merely the vpassing of steam or other stripping vapor through a pool of hot fat maintained in a batch still, norma lly under subatmospheric pressure, until the desired quantity of free fatty acids and odoriferous materials has been removed. Conventional batch refining of fats and oils has changed little in its essentials in the last several decades and in many respects leaves much to be desired, particularly because of the excessive steam requirements to achieve the desired results. To effect a simple deodorization of fat, ten pounds or moreof stripping steamplus some -100 pounds of steam tornaintain vacuum, is required for each 100 pounds of fat cle odorized. Furthermore, the demand for heat, cooling water and steam varies from time to time in batch operations with the result that capacity to supply these materials must be geared to the maximum demand, which results in increased capital outlay.

Continuous non-countercurrent stripping processes entail flowing the fat continuously through an apparatus in which it encounters successive jets of fresh steam. Such processes are-best represented by the Wecker process which was developed in Germany and which is described inter alia in Wecker Patent 1,766,863. An analogous process is described 'in Bissel Patent 1,508,769. Such processes have had little success in deodorization of alkali refined oils to edible quality. They have been utilized for removing fatty acid from high acid oils but when used for this purpose they normally leave at least about the last one percent of the free fatty acid and a substantial amount of the odoriferous materials in the oil, these being the most difiicult to remove. Hence this type of process does not offer a commercially satisfactory solution to the fat refining problem.

Because of the deficiencies of batch and continuous non-countercurrent processes, considerable attention has been devoted to continuous countercurrent fat refining processes. Under ideal conditions the quantity of stripping vapor required is greater in batch than in countercurrent processes when operated at the same efiiciency to achieve the same result. This difference increases with the degree of precision and completeness of removal of the volatile components. Thus, it may be calculated that when reducing a volatile component initially present in the fat in relatively small concentrations to one-tenth that of the original concentration, an ideal batch process requires about two and one-half times the quantity of stripping steam required in an ideal countercurrent process. When the ratio of final to initial concentration of the volatile material to be removedis l to 100, the batch process requires more than four times as much stripping vapor as the ideal countercurrent process. The foregoing factors are of particular importance in the case of the refining of fats for the reason that the volatile less than one-hundredth as great as that in the untreated oil.

Y The prior art continuous countercurrent fat refining processes are restricted essentially to the use of packed columns, or plate and bubble cap columns. Since fat refiining processes must be operated at low absolute pressures, such apparatus does not permit even aclose approach to steam consumption possible in an ideal countercurrent stripping operation. The large expansion of steam in low pressure batch stills produces such violent agitation of the oil that the efiiciency of the stripping steam may exceed of theoretical efficiency for batch operation. On the other hand, much of the theoretical advantage of the countercurrent system is lost because most of the energy of expansion of the steam is expended as it enters the bottom stage, and the energy in succeeding tom of the column where a large volume of steam is most needed, maybe three to four times the pressure at the top.

Additionally, when processing viscous materials such as fats, plate efficiency is small, several actual plates being required to produce the effect of one theoretical plate. Hence, a longer column of actual plates is required to provide the necessary number of theoretical plates to approach ideal steam efficiency. Generally there is about 1 to 3 mm. of mercury pressure drop across each actual plate. Thus, the use of a longer column further aggravates the stripping vapor pressure-drop above referred to, thereby increasing actual steam requirements.

Thus, conventional bubble cap or packed columns show little or no actual advantage over batch systems insofar as the aggregate quantity of stripping steam plus steam required to maintain the vacuum are concerned. The use of such columns can be justified only by other advantages such as steadiness in the demand for steam, heat and cooling water.

It is an object of this invention to provide a process for the continuous countercurrent refining of fats with stripping vapors which more nearly approaches ideal steam efiiciency for such countercurrent operations than is possible by methods known to the prior art.

It is a further object of the invention to provide a continuous process for refining fats wherein the fat is repeatedly and effectively contacted with the counterilowing stripping vapor to achieve highly effective distillable-component removal per unit length of column with a minimum of pressure drop, and maximum ease and precision of control of temperature and processing time, thereby producing products of improved quality through more precise, fractionation or stripping.

In accordance with this invention, there is provided a continuous countercurrent process for refining fats which comprises introducing a stream of hot liquid fat or oil adjacent one end of a distillation zone, introducing a hot stripping-vapor into the other end of said zone, dispersing said fat in droplet form into said vapor, collecting coalesced droplets in pools and again dispersing the resultant liquid as droplets into said vapor, repeating said coalescina and dispersing operations a plurality of times as the fat flows through the column, Withdrawing refined fat from the end of said zone remote from the fat-feed end, and withdrawing the stripping vapors and volatilizcd components of the feed from the opposite end.

in a preferred practice of the invention, the fat is preliminarily treated in conventional manner to effect deaeration and is thereafter heated to desired processing temperature, normally a temperature of about 150 C. to about 300 C., and preferably a temperature of about 200 C. to about 270 C. The fat temperature which is employed is conventional in the fat refining art and does not per se constitute a novel aspect of the invention. The so prepared hot fat is then treated in an apparatus such as described heretofore.

The fat refining process of this invention can be practiced at any desired pressure including atmospheric pres sure, but is preferably practiced at a pressure of below about 25 mm. of mercury. A preferred pressure range is from about 25 to about 0.5 mm. of mercury. It will be appreciated that the combined volume of strippingvapor and volatile materials stripped from the fat will be greater at lower pressures than at higher pressures, hence, the stripping effect per unit weight of stripping redia is greater at lower absolute pressures, for the type of materials being discussed.

Steam is preferred as the stripping vapor although other vapors or gases which exert no harmful effect on the fat can be used. These include lower boiling alcohols, low boiling hydrocarbons such as hexane, etc.

Example I The effectiveness of the refining process was determined with a test oil consisting of a mixture of refined corn oil with pure lauric acid. The principal part of the apparatus was substantially as shown in Figure 1, consisting of a jacketed 3' inch I. D. stainless steel tube inclined toward the outlet end at an angle of 3 to the horizontal. It was provided with an internal rotor, having its axis eccentric to the axis of the tube. The inner wall of the tube for a length of 18 inches in its central part was divided into a series of 36 shallow compartments A inch Wide by a series of dividing rings set into shallow grooves cut into the wall of the tube. These rings were inch wide and of such a thickness that when the position in the grooves they projected inwardly for a distance of inch from the inner wall of the tube throughout its circumference except for an open section cut from each ring, leaving a gap of inch at a place 60 from the lowest part of the tube.

The rotor consisted of a motor-driven shaft, on which were mounted a series of aluminum discs, two inches in diameter, of the style shown in Figure 3, separated from each other by spacers, and spaced so that three discs dipped into each of the aforementioned compartments. The position of the shaft was such that the lowest point on the circumference of each disc was inch from the lowest point on the circumference of the tube.

The tube was flanged at each end to accommodate heads with suitable bearings and stuffing box for mounting the rotor; it was fitted also with suitable inlets and outlets for admitting the oil feed near the upper end and withdrawing the refined oil at the opposite end; for admitting steam in counterflow to the oil; and for withdrawing steam and distillate vapors and conducting them to condensers. Small openings in the head were connected with either side of a differential oil manometer, which provided precise measurement of the pressure drop of the vapors during their passage through the apparatus. Suitable feed and receiving tanks, condensers and vacuum pump were provided, as well as accessories for measuring the pressure, temperature and rate of steam flow and for withdrawing samples of the refined oil. The oil feed was. drawn from the feed tank by a gear pump and passed through a tubular pre-heater to bring it to operating temperature before it entered the Working section of the apparatus. The temperature in the stripping section was controlled by vapors of a mixture of diphenyl and diphenyl oxide passing through the jacket.

The oil feed consisted of refined corn oil, having an acid value of 0.08, mixed with pure laurie acid in quantity sufficient to raise the acid value of the mixture to 2.90. The mixture therefore contained 1% by weight of free lauric acid and 0.04% of the fatty acids of corn oil. This oil was treated in the apparatus described, under the following operating conditions: temperature, 36l Fa; pressure, measured at the condenser, 10.9 mm. of mercury; oil feed rate, 132.6 grams per minute (17.5 lbs. per hour); stripping steam, 1.99 grams per minute; rotor speed, 1680 R. P. M. The pressure diiferential between the ends of the tube was 0.15 mm. of mercury.

A sample of the treated oil taken when the apparatus was operating smoothly had an acid value of 0.356. Calculations, based on the equation of Garbcr and Lerman, Transactions of the American Institute of Chemical Engineers, volume 39, pages 113-31 (1943), and on the assumption that the vapor pressures of dilute solutions of lauric acid in corn oil are in accordance with Raoults law, show that the effectiveness of the steam, in comparison with an ideally efiicient continuous countercurrent process with an infinite number of theoretical plates, was greater than 77%. In terms of number of theoretical plates, the effectiveness of the apparatus in this experiment was six theoretical plates, or one plate for every three inches of length of the working section of the tube. The quantity of steam that would have been required to produce the same result in an ideally efficient batch stripping'operation would have been nearly twice as great as the quantity used in this test, as calculated by the equation given by Garber and Lerman.

An apparatus having a stripping section eight feetlo'ng';

Example II The test oil described in Example I was treated in the same apparatus under substantially the same conditions, exceptthat a higher rate of feed and a correspondingly higher. rate of steam flow were maintained than was the case in Example I. Pressure, 10.2 mm. Hg; temperature, 361 F.; oil feed rate, 177.9 g./min. (23.5 lbs./hr.); steam rate, 2.66 g. per min.; rotor speed, 1680 R. P. M. The pressure difference between the two ends of the tube was 0.3 mm. of mercury. Acid value of the treated oil was 0.316.. The calculated efliciency and number of theoretical plates was approximately the same as in Example I. Y

When the angle of tilt was increased to 4 degrees, the apparatus'was operated at the same feed rate with no indication of flooding. At 3 tilt the feed rate was about the maximum attainable in this piece of equipment without flooding.

Example Ill The test oil described in Example I was treated in the same apparatus under difierent operating conditions. Pressure, 7.3 mm.; temperature, 394 F.; oil feed rate, 109.3 g. per min; steam rate, 0.73 g. per min; rotor speed, 1680R. P. M. The pressure diiference was 0.08 mm. Acid value of the treated oil was 0.142. This corresponds with 0.05% by Weight of lauric acid, if no correction is made for the less volatile acids in the original corn oil. If it is assumed that the free fatty acid of the corn oil remainedin the treated oil, the calculated value for residual lauric acid is reduced to 0.02%.

Example IV for'deodorization of fat was determined 'by treating a batch of shortening obtained in the undeodorized state from the regular production of a large-scale shortening manufacturer. The shortening was of the hydrogenated vegetable oil type, consisting principally of soybean oil together with small percentages of cottonseed oil and other vegetable oils. The shortening had the following characteristics: iodine value, 80.5; acid value, 0.09.

' The general procedure resembled that described in EX- ample I," except that the melted fat in the feed tank was deaerated before the beginning of the test. Operating conditions'were as follows: temperature, 448 F.; pressure, 6.0 mm.; oil feed rate, 72.6 grams per minute; steam rate, 1.09 grams per minute; rotor speed, 1680 R. P.- M angle of tilt, 3. Pressure drop on the vapors passing through the stripping section amounted to only.

Example V *"Unrefined lard was refined in the apparatus described iri'ExampleI. The lard, as obtained from a commercial packing house, was of the grade designated as pure laid and consisted of steam rendered lard which had had no refining other than clarifying by mixing with a small proportion of fullers earth and filtering. The lard had an acid value of 0.49, corresponding with 0.25% free fatty acid calculated as oleic acid.

' ture of n-decane and Aftertreatment in the apparatus described in Example I under the two sets .of conditions shown in the following tabulation, the lard was found to have been fully deodorized. Comparison with a sample of deodorized vegetable shortening to which 1% of the untreated lard had been-added, showed that this control sample had perceptibly more lard odor and flavor than the treated lard.

Temperature, C 232 235 Pressure at upper end of working section, mm. Hg 7. 2 7. 2 Pressure drop in working section, mm. Hg 0.075 0.15 Oil Feed rate, g./min 72. 6 72. 6 Stripping steam rate, g./min 1. 04 1.35 Stripping steam rate, g./ g. of fat treated 1. 43 '1. 86 Acid value of fat before treatment 0. 49 0. 49 Acid value of fat. aiter treatment 0. 043 0. 026 Free fatty acid (as oleio) after treatment 0.022 0.013

' The data show that the efiectiveness of the steam for removing the free fatty acid from the lard under these conditions was substantially greater than would be expected for the same quantity of steam used in a batch operation at the same temperature and pressure; this is true whether the comparison is made with the calculated value for steam used in an ideal batch operation at 100% efiiciency or with the experimental results published by Bailey for a highly efiicient batch stripping of high acid lardflakes (A. E. Bailey, Journal of the American Oil Chemists Society, volume 26, pages 166-170 (1949)).

Example VI The effectiveness of an apparatus similar to that described in Example I, when operated as a fractional distillation column, was determined by distilling a test mixthalene).

The apparatus differed from that described in Example I in length, materials of construction, and in some of the details of the dividers and impellers. The central part of the working tube or column consisted of a 12 inch length of glass pipe approximately 3 inches in internal diameter, mounted in nearly horizontal position, and connected at each end to an aluminum section 3.5 inches long. The aluminum sections were provided with suitable bearing, stuffing box, inlet and outlet for vapors and suitable openings for insertion of thermometers and connection to pressure gages. i

A glass flask, used as still pot for boiling the test mixture, was connected to a vapor inlet at the bottom of the aluminum section at one end of the column, and a refluxing still-head was connected to the vapor outlet at the top of the aluminum section at the opposite end. The still-head in turn was connected to suitable traps, manostat and pump for maintaining reduced pressure.

Within the 12 inch glass section, a series of cast-iron piston rings were placed, to form a series of shallow troughs; the thickness of these rings, measured inwardly from the wall of the tube, was inch, and their width was inch. A gap of A inch remained between the ends of each ring when they were in place. Toothed discs of the type shown in Figure 3, approximately two inches in diameter, were mounted on a shaft placed so that the clearance between the discs and the bottom of the column wall was about inch.

The apparatus was operated as a fractionating column,

with vapors passing from the still pot through the fractionating section to the still-head, where they were refiuxed as liquid back to the fractionating section, except for the portions taken for analysis of distillate. Conditions were as follows: pressure, 18 mm. of mercury; rotor speed, 1565 R. P. M.; angle of tilt of fractionating section, number of dividing rings in fractionating section, 11; numberof rotating discs, 98. Analysis of samples taken duringsteady operation showed that the mole fraction' of n-decane in the distillate was 0.315 as compared with 0.147 in the distilland, corresponding with a fractrans-decalin (decahydro-naph-- tionating: effect of better than 6 theoretical plates, or one theoretical plate for each two inches of length of the working section of the column.

Example VII When equipped with 31 instead of 11 dividing rings, and only 46 rotating discs, the apparatus described in Example V1 had the effect of 7 theoretical. plates when tested as described in Example V1.

1 claim as my invention:

1. A vapor-liquid contacting process which comprises passing vapors continuously through an elongated, substantially straight vapor-liquid contacting zone having an axis disposed at no more than a small angle from the horizontal wherein continuous interchange of components occurs between the vapors and a series of transverse sprays of liquid droplets, said vapors passing through said zone in an essentially straight through flow, introducing a viscous liquid continuously into a zone below and parallel to said vapor-liquid contacting zone, conducting the liquid through a series of liquid stages at least ten in number in a general direction counter to the direct ion of flow of the vapors in said contacting zone, mixing the liquid as it enters each stage with the liquid in that stage, withdrawing the liquid from each stage and discharging it as droplets across said vapor-liquid contacting zone in a direction substantially normal to said axis, thereby effecting intimate contact between said vapor and said droplets, returning the liquid by gravity to the same stage and repeating said withdrawal and discharge at least once before the liquid progresses to an adjacent downstream stage, the aggregate volume of liquid Withdrawn from the several stages and discharged across said contacting zone being at least fifty times the volume of liquid entering the process, and withdrawing liquid from one end of said contacting zone'and vapor from the other.

2. The process of claim 1 wherein the axis of said vapor-liquid contacting zone is disposed at an angle of no more than about ten degrees from the horizontal.

3. The process of claim 2 wherein the liquid is selected from the group consisting of glyceride fats and glyceride oils containing small proportions of fatty acids and odoriferous components.

4. A process for effecting component interchange between a vapor and a viscous liquid which comprises passing the vapor continuously through an elongated, substantially straight vapor-liquid contacting zone having an axis disposed at no more than a small angle from the horizontal wherein continuous interchange of components occurs between the vapor and a series of transverse sprays of liquid droplets, said vapors passing through said zone in an essentially straight through flow, introducing the viscous liquid continuously into an elongated liquid zone below and parallel to said vapor-liquid contacting zone, conducting the liquid in a general direction counter to the direction of flow of the vapors in said contacting zone through a series of at least ten individual liquid pools disposed in a line extending longitudinally in said liquid zone, mixing the liquid as it enters each pool with the liquid in that pool, projecting most of the liquid as droplets across said vapor-liquid contacting zone in a direction substantially normal to said axis at least about twice from eachpool before the liquid progresses to an adjacent downstream pool, thereby effecting intimate contact between said vapor and said droplets, returning by gravity the liquid projected from each pool and directing to the same pool the liquid to be again projected therefrom, the aggregate volume of liquid withdrawn from the several pools and discharged across said contacting zone being at least fifty times the volume of liquid entering the process and withdrawing liquid from one end of said contacting zone and vapor from the other.

5. The process of claim 4 wherein the liquid is selected from the group consisting of glyceride fats and '16 glyceride oils containing small proportions of fatty acids and odoriferous components, and wherein the treated liquid is deodorized and contains less than about one percent of the fatty acids contained in the untreated liquid. v

6. The process of claim 4 wherein the pressure within said vapor-liquid contacting zone is maintained at from about 0.5 millimeters of mercury to about atmospheric.

7. The process of claim 4 wherein the pressure drop from one end to the other of the vapor-liquid contacting zone is less than about three millimeters of mercury.

8. The process of claim 4 wherein said liquid is discharged across said vapor-liquid contacting zone as droplets having an average diameter of not less than about fifty microns.

9. The process of claiin 4 wherein the axis of said vapor-liquid contacting zone is disposed at an angle of no more than about ten degrees from the horizontal.

10. The process of claim 9 wherein the liquid is selected from the group consisting of glyceride fats and glyceride oils containing small proportions of fatty acids and odoriferous components.

11. A process for efiecting vapor-liquid component interchange which comprises introducing a hot, high molecular weight liquid having relatively volatile and relatively non-volatile components and a viscosity of at least about 0.6 centipoise at the temperature employed into an elongated zone having an axis disposed at no more than a small angle from the horizontal and conducting the liquid by gravity through a plurality of at least ten individual pools disposed in an axial line in the lower portion of said z'one, passing hot stripping vapors longitudinally through an elongated, substantially unobstructed upper portion of said zone in a direction countercurrent to the flow of said liquid, said substantially unobstructed portion comprising a vapor-liquid contacting zone, projecting most of the liquid as droplets having an average diameter of not less than fifty microns across said vapor-liquid contacting zone in a direction substantially normal to said axis at least about twice from each pool before the liquid progresses to an adjacent downstream pool, thereby effecting intimate contact between said vapor and said droplets, returning by gravity in a direction substantially normal to said axis the liquid projected from each pool and directing to the same pool the liquid to be again projected therefrom, the aggregate volume of liquid withdrawn from said pools and discharged across said contacting zone being at least fifty times the volume of liquid charged to said elongated zone, withdrawing stripping vapors mixed with vapors of said volatile component from said elongated zone and withdrawing refined liquid from the opposite end thereof.

12. The process of claim 11 wherein said liquid is selected from the group consisting of glyceride fats and glyceride oils, the axis of said elongated zone is disposed at an angle of no more than about ten degrees from the horizontal, and said vapor-liquid contacting zone is maintained at a pressure of from about 0.5 millimeters of mercury to about atmospheric.

References Cited in the file of this. patent UNITED STATES PATENTS

Claims (1)

1. A VAPOR-LIQUID CONTACTING PROCESS WHICH COMPRISES PASSING VAPORS CONTINUOUSLY THROUGH AN ELONGATED, SUBSTANTIALLY STRAIGHT VAPOR-LIQUID CONTACTING ZONE HAVING AN AXIS DISPOSED AT NO MORE THAN A SMALL ANGLE FROM THE HORIZONTAL WHEREIN CONTINUOUS INTERCHANGE OF COMPONENTS OCCURS BETWEEN THE VAPORS AND A SERIES OF TRANSVERSE SPRAYS OF LIQUID DROPLETS, SAID VAPORS PASSING THROUGH SAID ZONE IN AN ESSENTIALLY STRAIGHT THROUGH FLOW, INTRODUCING A VISCOUS LIQUID CONTINOUSLY INTO A ZONE BELOW AND PARALLEL TO SAID VAPOR-LIQUID CONTACTING ZONE, CONDUCTING THE LIQUID THROUGH A SERIES OF LIQUID STAGES AT LEAST TEN IN NUMBER IN A GENERAL DIRECTION COUNTER TO THE DIRECT ION OF FLOW OF THE VAPORS IN SAID CONTACTING ZONE, MIXING THE LIQUID AS IT ENTERS EACH STAGE WITH THE LIQUID IN THAT STAGE, WITHDRAWING THE LIQUID FROM EACH STAGE DISCHARGING IT AS DROPLETS ACROSS SAID VAPOR-LIQUID ZONE IN A DIRECTION SUBSTANTIALLY NORMAL TO SAID AXIS, THEREBY EFFECTING INTIMATE CONTACT BETWEEN SAID VAPOR AND SAID DROPLETS, RETURNING THE LIQUID BY GRAVITY TO THE SAME STAGE AND REPEATING SAID WITHDRAWAL AND DISCHARGE AT LEAST ONCE BE-

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