or some distillation columns operating at high
Fthroughput, the tray itself may not be the factor
Hig Capacity Distillation EA
A
'
'
'
which limits capacity. The column vapor velocity and resulting liquid entrainment ma>-set the maximum vapor-liquid handling capacity of a column. When gravity is the only force effecting phase separation, an ultimate column capacity can be estimated. On the other hand, it is possible to achieve much higher column loadings if centrifugal force is used to aid droplet separation. Several different trays ( 3 ) were studied in which centrifugal force was utilized to separate more effectively the vapor and liquid phases. The capacity of such trays can be made greater than that of any other trays for which data ardavailable. For instance, the tray at the right at 12-inch tray spacing has a capacity equal to 15Syoof that of a perforated tray of very large open area at 24-inch tray spacing. The separating efficiency of the tral-s tested thus far is in the general range of 5070. Gravitational limit of Vapor Velocity
In the course of a study of perforated trays without downcomers, it was found that the capacity of the tray was increased as open area in the tray was increased. However, this relationship held only up to an open area of about 25 to 30 per cent, depending on the type of tray. For trays of greater open area, the tray itself was no longer the factor limiting capacity-there appeared to be a condition where the liquid could no longer fall against the rising tapor and a substantial part of the liquid was entrained. This concept led to the development of an expression for predicting the absolute maximum vapor velocity that can exist in a conventional column with phase separation after each contacting step. When a particle of liquid is suspended in a vertically rising vapor stream, there are two opposing forces acting on the particle, First, the frictional drag of the vapor on the liquid surface tends to carry the liquid particle upward; and second, the force of gravity tends to pull the particle downward. When these forces are exactly equal, the liquid particle will have zero velocity with respect to the vessel. The equation which results from equating these two forces and solving for particle diameter is : D, = -3 KV2 po 4E
(P1
-
Po)
(1)
D, = diameter, feet, of a particle which has zero velocity
with respect to vessel
K = constant in the turbulent region V = vapor \,elocity, ft./sec.
vapor density, lb./cu. ft. liquid density, lb./cu. f t . g = gravitational constant, ft./sq. sec. The constant, K , for solid particles and for high Reynolds Numbers would have a value of about 0.44. However, Hinze (2) found that, for liquid particles falling through vapors, a value of 0.7 is more nearly correct. The solution of this equation for a given system and set of operating conditions will define the particle size that will remain suspended in the vapor stream, neither rising 'nor falling. pE =
pi =
14
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
Figure l. A tray using centrifugdl farce far droplet separation. Note the separote mixing and settling zonas. Heh velocity vapor flow through the small perforated tray area ensures effective contacting. The vones direct the mixture from the contocting zone toward the column wall, which acts as the outer WOII of a cyclone. The mixture is dischorged tangentially into a settling zone, while centrifugal farce causes liquid to flow to the column WOII.The tray i s arranged for a central dawncomer to the troy below
nuV O L 5 6 NO. 4 A P R I L 1 9 6 4
IS
Now, if the particle sue that actually exists in a system could be calculated from physical properties of the system and from operating conditions, it could be determined whether the particle would fall or be entrained. Although no firm method is known for estimating the average particle size, the maximum particle size that can exist in a given system can be calculated from the Weber Number relationship. The Weber Number is a dimensionless group and can be expressed as follows: We = Weber Number D = particle diameter, feet c = surface tension, Ib./sq. sec. The Weber Number relates the forces of surface tension, which tend to hold a drop together, to the forces of the frictional drag of the vapor flowing past the particle, which tend to split the drop. As conditions are changed to increase the numerical value of the Weber Number, a critical point is reached at which time the particle will rupture. This so-called critical Weber Number varies somewhat with liquid viscosity, but for most hydrocarbons in the gasoline boiling range, the critical value is about 7 to 9 (2). Then, by rearranging Equation 2 and using the critical value of the Weber Number, the maximum diameter of a stable particle can be defined.
velocity upward, or the particles that were of maximum diameter will now rupture with the higher vapor velocity and the fragments will be entrained. In Figure 2 these two equations are plotted. The intersection of the two curves represents the absolute maximum velocity that can exist in the column under the given conditions with liquid still falling downward. Equation 4 is a solution for this intersection.
Vme== absolute maximum vapor velocity, ft./sec.
(3)
In order to attain the vapor velocity predicted by Equation 4 without entraining liquid, all liquid particles must be of uniform size-the maximum size that can exist. Since there is a range of particle sizes in the fluidized bed of a fractionating tray, the vapor velocity that can actually be reached in a column is considerably below that which would be calculated from Equation 4. Yet the extent to which the actual maximum vapor velocity approaches the calculated velocity is practically the same for several high-capacity trays, perhaps because the degree of dispersion is similar in most types of fractionating trays. Trays that are limiting because of high pressure drops, high liquid loads, etc., should not be included in this comparison. Figure 3 shows a comparison of the theoretical maximum vapor velocity (Equation 4) and the actual maximum vapor
Dmaa = maximum diameter for stable partide, feet (We), = critical Weber Number Equations 1 and 3 then define, for a given system, the diameter of a particle that will remain just suspended in the vapor stream and the maximum particle diameter that is stable. Now if both of these conditions are met (D, equals Dmar),the vapor velocity is absolutely the highest that can be tolerated. For instance, if the vapor velocity is increased, the particles will have a positive
Earl Manning, Jr., is a chrmicul engineer wha has sjecialiud in the dislillulion field fm ouer ten years. A1 the lime that this urlule was written, he was a Senior Research Engineer at Shell Oil Co.'s Houston Research Laboratory, Deer Park, Texos. He is now 6he Group Lead@, Computing, in the Technological Department of Shell's Nmco, La., rehnny. He wishes 60 acknowledge the assis6ance of C . B. Kincannon, MomeII Nager, and E. L. Ciaridge.
I
AUTHOR
Figure 3. Compa"son
of themttical maximum wpor wIm@m'!h odml&umfor conwntional Irqs
KEY: 1, Bubble cop Iray m'th bubble capx rmowd, butane system; 2. dah of Scofild (5), C,C, syrfm; 3, perforated L r q , iswctonc-lolume system at 45 p.s.i.g.; 4, pegorated tray, i m c t a ~ o l u e n cy t e m a6 5 p.f.ig.; 5, &fa of Scojtld, bcnznu-tolumc sys6m, ahspheric pressure; 6, &la of &oJield, e f h y l b e ~ e ~ ~ o p r o p y l b e " ~ ~ at 3.4 p.s.i.0.; 7, data of &ojeld, Oir-watn, afmosbheric; 8,prforatcd fray, a'r-water, afmosphric
3"
6''
velocity for several high capacity trays. The systems represented vary in vapor density from 0.057 to 1.91 Ib./cu. ft. Even with these extreme differences in operating conditions and types of contacting devices, the actual maximum vapor velocity in all cases is about 38% of the calculated value. This seems to he the upper limit common to all of these cases. These column vapor velocities are based on the column cross-sectional area available for vapor flow. For more exact considerations, allowances should be made for the fraction of the cross-sectional area that is occupied by liquid. This is to say that in the fluidized bed on a fractionating tray the dispersed droplets occupy space between two trays; therefore, the actual vapor velocity passing a droplet is greater than the calculated superficial velocity. However, at present, there is no simple way of estimating the proper correction. Phase Separation via the Inefiial Principle
%"
The most obvious means of circumventing this capacity harrier of conventional trays is the use of more than one "g" to separate the vapor and liquid phases after a contacting step. Several types of trays (3) utiliing the inertial principle for phase separation have been studied and are discussed in the following pages. Tray Performance
F i p e 4. Cms-flow hays inskalled in a rectangular column. The hoys *bow were wed in air-wafer Shrdiu
Figure 5. Ct.orsflow hays installed in (I CylindncaI column. The hays below were tested lvith a h.q&ocmbon
srsran
A physical separation process such as fractionation involves countercurrent flow of two phases with successive stages of mixing and then separating these two phases. Previous data have indicated that better mixing and therefore higher efficiencies result from greater vapor velocities until the point is reached where further increases in vapor velocity cause severe entrainment, channeling of vapor, or flooding. I t is desirable, then, to develop a means of efficient phase separation such that extremely high vapor velocities and a resulting
Figure 6. Comparison of the c@ady of fhe hays shown in Figure 4 with cqbm'ty of parfarated hays with 25% @en m a . Bofh hays are at 6 inch sp.Cing,in an air-watm systm VOL 5 6
NO. 4
APRIL 1964
17
fine liquid dispersion can be tolerated in the mixing zone, nonetheless without excessive entrainment. Only a relatively small percentage of the cross-sectional area of the column need be utilized for vapor flow through the tray since a high vapor velocity is desired at the point of mixing. CrosrFlow Tfay
Air-Water System. The first tray to be studied in this investigation was one in which the vapor-liquid mixture was caused to flow in a horizontal direction after the contacting step. An abrupt change in direction of flow of the vapor effected phase separation. The details of the trays are shown in Figure 4. Tray spacing is six inches and weir height is three inches. This permits a nineinch backup in the downcomer. Only 22y0 of the tray area is used for the vapor-flow zone, of which 31.4% is open area. This design achieves a better phase separation after mixing than do conventional trays because: -there is no upward force exerted on the liquid droplets by the horizontally flowing vapor -the sharp change in direction of flow of the vapor in the disentrainment chamber will tend to separate the phases by centrifugal force -the large cross-sectional area at the top of the downcomer (several times larger than that used with conventional bubble cap trays) will permit more nearly complete separation of the vapor which is entrained in the liquid downflow. The actual capacity of these trays with an air-water system is compared with that of a highxapacity perforated tray (25% open area) in Figure 6. Both trays were at 6-inch spacing. At a constant vapor-liquid ratio, as designated by the dashed line, the tray using centrifugal force for phase separation had a 30% capacity advantage. No attempt was made to evaluate the efficiency of these trays. HydrocarbonSystem. Trays similar tothosediscussed above were installed in a 5-foot diameter experimental distillation column. Structural dimensions are shown in Figure 5. The total open area was about 14% of the column cross section. The area at the top of the downcomer was 55% of the column area. The capacity and efficiency of these trays, distilling a mixture of 2,2,4-trimethylpentane and toluene, are 18
I N D U S T R ~ A LA N D E N G I N E E R I N G C H E M I S T R Y
compared to those of high-capacity perforated trays (ZOO/, open area) in Figure 7. The over-all performances of the two tray types are approximately the same, even though there was a very obvious hindrance to the performance of the experimental tray-the original development was carried out on a rectangular column. When trays of this particular design were installed in the round column, vapor streams flowing horizontally followed the curvature of the vessel wall. The streams from the two sides seemed to collide and oppose the streams moving across the center of the tray. Visual observation, through six-inch windows in the side of the column, revealed extreme turbulence in vapor flow just below the point at which the vapor entered the tray. The net result was severe entrainment, which decreased both capacity and efficiency. Simple baffles, to make a round column square inside, should improve the performance of such a tray. Even with this obvious malfunctioning, the performance of the tray was still creditable. Techniques of experimentation have been described (4).
Fiprc 8. Column equipped with the circufmpow hays sfiown in Fipm 1. Note thc path of thc iipuid offer it Icaves the uana
Figure 9. Comparimn of an ulfimt6 capacity pnjwafed tray u ' t h fhe urcular pow fray
Circulor Flow Troy
A more logical means of utilizing centrifugal force for phase separation would seem to be one in which the column wall is used as the outer cylinder of a cyclone. Such a tray, one foot in diameter, was constructed and studied with the air-water system. Contacting takes place on and above a small perforated tray section which receives liquid from the tray above and vapor from the tray below. The two-phase mixture, after the contacting stage, is discharged tangentially into the settling zone. The liquid is forced outward against the column wall by centrifugal force and flows into the downcomer leading to the next lower tray. The vapor enters the
tray above through a conduit located inwardly from the column shell. Details of the tray are shown in Figure 1, and column operation is shown in Figure 8. Figure 9 compares the capacity of a perforated tray of 25% open area and 24-inch tray spacing (a so-called ultimate capacity tray) with that of the 12-inch diameter tray. Even though the tray spacing of the tray using centrifugal force for phase separation was one-half that of the perforated tray, it had up to 58% greater capacity. This is rather impressive since the capacity for this particular perforated tray had not been increased appreciably by increasing open area (that is, column vapor velocity is the limiting factor). A small laboratory tray was fabricated to study contacting efficiency of high-velocity trays. A photograph of the tray and its component parts is shown in Figure 10. The trays were spaced by glas cylinders (which also served as the column wall) which were 3 inches in inside diameter and 3.5 inches long. For atmospheric pressure operation with isooctanetoluene test mixture, the tray capacity (at total reflux) was 26 gallons per hour. This is equivalent to a linear vapor velocity of 4.3 feet per second. The tray efficiency was about 50%. By comparison, a 3-inch diameter Oldershaw column (7) would have a slightly higher efficiency, but only 490/, of the capacity. Conclusions
As shown here, in most systems the column vapor velocity becomes limiting at approximately 38% of the calculated maximum velocity. The capacity of distillation columns can be extended considerably beyond that of any presently known fractionating device by installing trays that utilize centrifugal force for vapor-liquid phase separation after each contacting stage. Although the efficiencies of the trays studied were by no means spectacular, an efficiency of near 100% should be possible by approaching liquid atomization in the contacting step, followed by adequate phase separation. REFERENCES
Figm 70. Laboratq tray which uhXm ce&ifugal force
(1) Braun-Knecht-Heirnann Company, 601 O'Ncil Avenue, Bclmont, Calif., quipmcot Bmhulr Form BO1056 5 M. (2) Hinre, J. 0.. A M . Si. Ru. A-I, 273-88 (1949). (3) Manning, Earl, Jr., U. S. Patent 1,771,746 (Dec. 4, 1956). (4) Manning, Earl, Jr., M q l + Stanley, Jr., Hind., G. P., JI., INO. E m . Cmru. 19,2051-2054 (1957). (5) Scofidd, R. 0.. C k m . Enb. Pmg. 46, 405-14 (1950).
VOL 56
NO. 4
APRIL 1964
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