US2443072A - Packed fractional distillation column - Google Patents

Description

June 8, 1948. R. R. KAISER PACKED FRACTIONAL DISTILLATION COLUMN Filed Oct. 20. 1944 n Superficial Vapor Velocity F/Sec.

INVENTOR. Ralph R. Kaiser BY Patented June 8, 1948 PACKED FRACTIONAL DISTILLATION COLUMN Ralph R. Kaiser, St. Matthews, Ky., assignor to Joseph E. Seagram & Sons, Inc., Shlvely, Ky., a corporation of Indiana Application October 20, 1944, Serial No. 559,619

4 Claims. 1

This invention relates to improvements in packed fractional distillation columns, that is to say columns having a .phase contacting medium or packing, such as fiber glass, Raschig rings and other like materials, providing a maze of spaces, the size of which determine its free space area. These columns are universally packed to provide a free space area which is of aconstant and predetermined value from one end to the other. It is known that packed columns operate most efficiently when the vapor velocity, or throughput, is maintained at a point just below the flooding point. The flooding point is reached when the vapor velocity becomes high enough to pile up the descending liquid at some point in the column causing the liquid to bubble, froth and be entrained. It is accompanied by asharp rise in pressure drop and a sharp decrease in efllciency.

It is well appreciated that various factors may cause, .or contribute to, flooding. Thus the Wible patent, No. 2,271,671, suggests that flooding may be occasioned: atthe bottom of the column, due to the abrupt increase in the velocity of the vapor entering the packing; at various levels throughout the column, due to improper packing or fouled packing or packing supports or liquid distributing devices between the sections of a sectional packing, all of which interfere with the free flow of vapor; and at the top of the column, due to the abrupt decrease in the velocity of vapor leaving the packing, an improper distribution of reflux entering the packing and a slight excessof liquid which occurs at the top of the packing when the reflux is returned at a temperature below the top boiling temperature. The Wible patent asserts that in a clean and well packed column, the levels at which initial flooding begins are at the top or the bottom of the packing. It notes that the flooding point at the bottom and top levels can be substantially raised, and the column operated at a substantially higher vapor velocity, hence higher capacity and efliciency, by'causing the vapor to engage the liquid at the bottom, and disengage it at the top, of the packing gradually instead of abruptly. This is accomplished by progressively packing the bottom and top end portions of the packing for a distance of two to three inches so that the free space area in the bottom end portion progressively decreases from a high value downwardly to the mean value in the direction ofvapor flow while such area in the top end portion progressively increases from the mean value to a higher value in the direction of vapor flow.

It is believed accurate to say, however, that, heretofore, the prevailing impression has been 2 n that when the vapor velocity at the top of the column approached the point at which the top of the column would flood, the vapor velocities at all other sections of the colunm, including the bottom, approached the points at which such other sections would flood; hence the column was just as apt to flood at one section as it was at another. It is my view that this impression is incorrect. I have found that the most pronounced flooding tendency in a column occurs at the top and that such tendency decreases downwardly through the column in a progressive manner. In other words,

I I have determined that the vapor velocity and flooding points at correspondingpoints along the column form separate curves which diverge downwardly through the column so that, when the vapor velocity at the top of the column is close to the flooding point at the top of the column, the vapor velocity at the bottom of the column is sub stantially below the flooding point at the bottom of the column. The significance of this is that the ordinary packed column normally operates with decreasing efficiency from top to bottom.

I have discovered that a packed column can be made to operate at substantially the same efliciency through its length, or at predetermined efilciencies ranging up tomaximum at selected points along its length, and such forms the principal object of the present invention.

I have discovered that as the efficiency downwardly through the column is raised, the height of the column can be correspondingly decreased without decreasing its capacity and such result forms another important object of this invention.

Another object is to provide a packing having a vapor velocity curve which closely parallels the flooding point curve so that maximum efiiciency is attained at substantially all points along the column.

The invention is illustrated in the accompanying drawing wherein:

Figure 1 is a vertical sectional view of a column embodying the present invention;

Figure '2 is a similar view of another form of column embodying the invention;

Figure 3 is a similar view of another form of column embodying the invention; and

Figure 4 is a graph showing the correlation between the H. T. U. in feet and superficial vapor velocity in feet per second for different concentration ranges of an ethanol water mixture.

The carrying out of my invention involves the use of 'a packing which is progressively packed in a manner such thatthe free space area of the packing increases upwardly through the column.

This may be accomplished in various ways. For

example, in the embodiment shown in Figure 1 the other in a manner determined wholly by the progressive increase in diameter.

In the embodiment shown in Figure 2, a progressive increasein the free space area is obtained upwardly through the fractionating zone by packing a tapered shell I with fiber glass packing 60f the samedensity, while in Figure 3 the desired increase in area is obtained upwardly through the fractionating zone of a shell 8 of constant diameter by progressively packing the zone so that the density of the resulting packing 9 decreases in the upward direction. While the packing of Figure 3 is illustrated as a unitary body of continuously decreasing density, or increasing free space area, in the upward direction, it will be appreciated that it may be composed of a series of fiber glass sections, superimposed one over the other with each higher section being less dense but of uniform density. In place of fiber glass, other well known packing materials may of course be employed.

A distinction between the actual velocity of the vapors flowing through a column and the superificial vapor velocity should be observed. The actual velocity is of course affected by various well known factors. The superficial vapor velocity (per second) is obtained by dividing the cubic foot (per second) volume of vapor actually flowing through the column by the square foot area of the column, omitting all other factors such 'as the packing, etc.

In prior packed columns, the superficial vapor velocity is constant since the free space or cross sectional area of the column is constant. In the present case, however, the progressive increase in free space area necessarily results in a corresponding decrease in superficial vapor velocity. Accordingly, the rate at which the free space area increases must be carefully predetermined if the best results are to be secured. The change in area should be such as to produce in each section, under a given operating condition,. a vapor velocity which is but slightly below the flooding point of that particular section. each section will then operate at, or in the neighorhood of, the maximum efliciency at which it is possible to operate such section. Heretofore, only the top section could be so operated, all progressively lower sections necessarily operating at progressively lower efficiencies.

The fact that diflerent sections of a column have different flooding points can be demonstrated by the correlation that exists between the height of a transfer unit, or H. T. U. as it is commonly called, and the superficial vapor velocity through the column. This relationship between H. T. U. and superficial vapor velocity is indicated in the curves of Figure 4. From these curves it will be apparent that, as thesuperficial vapor velocity increases in any one section, the height of a transfer unit decreases in that particular section. However, since some investigators have concluded that the rate of throughput has no When this is achieved,

. 4 appreciable effect on the performance character istics of a packed column (Trans. Am. Inst. Chem. Engrs. 39, 813-846, Dec. 1943), it may be helpful to note that Figure 4 is taken from a published article on this invention (Trans. Am. Inst. Chem. Engrs. 40, 487-495, Aug. 1944) and to indicate how such curves were obtained.

ethanol water mixture was distilled in a column 1 foot in diameter and 8 feet tall, having a fiber glass packing of predetermined height (5' 8") and of predetermined density, the apparent density being 5 pounds per cubic foot. This distillation was conducted with a constant heat input at one value, for example, and continued with a constant predetermined reflux until the distillation system reached equilibrium. The concentration of the vapor at the bottom and top of the still was then measured and found to be 4 mol percent (Y1) at the bottom and 28 mol percent (Y2) at the top. These values were used to calculate the number of transfer units in the section by the method of Chilton and Colburn (Industrial Engineering Chemistry 2'7, 255-260, 1935). The H. T. U. was then obtained by dividing the height of the packing by the number of transfer units thus found. The superficial vapor velocity corresponding to this height of a transfer unit was then obtained in the normal manner. With the superficial vapor velocity and a height of a transfer unit thus obtained, one point on curve A for a concentration of 4 to 28 mol percent ethanol was thus established; The foregoing was then repeated for each of a number of different values of heat input to establish curve A.

An ethanol-water mixture of higher concentration was next employed which produced base and top concentrations of 38 and 56 mol per cent ethanol, the procedure being repeated to establish curve B for this range of concentration. The foregoing procedure was again repeated for the concentration range of 55 to 67 mol per cent ethanol to establish curve C and of '14 to 87% to establish curve D.

It may be noted: that curve A was extended to a velocity approximating 6.4 without reaching a flooding point; that curve B was extended to a velocity approximating 5.8 feet per second without reaching flooding point; that curve C was extended to a velocity of less than 5 but could have been extended to more than 5.0 without reaching a flooding point; and that curve D was extended to a velocity approximating 4.7 but immediately beyond this velocity, flooding occurred. It will thus be seen that as the concentration range increases, the velocity at which flooding occurs decreases. This may be explained in part on the ground that the molecular weight of the liquid increases upwardly through the column, such weight being at a minimum in the lowest concentration range A at the bottom of the column and a maximiu'n in the highest concentration range D at the top of the column. While the molecular weight of the liquid thus increases upwardly through the column, the number of mols of liquid is constant throughout the column; hence, the volume of liquid necessarily increases upwardly through the column.

The foregoing should make clear that the ratio of liquid volume to vapor volume in the column increases upwardly through the column due to gresslvely higher sections of the column with progressively lower superficial vapor velocities. This condition necessarily obtains as long-as the ratio of the volume of one mol of the lower boilingiiquid. to the volume of one mol of the higher boiling liquid is greater than one (1) and it becomes progressively more pronounced as such ratio becomes progressively greater than (1).

The foregoing also makes apparent that the most economically designed still requires that advantage be taken of the highest vapor velocity possible for each range of concentration. To demonstrate this more clearly, four calculations were made to determine the comparative heights of one column having a constant free space area and another column having an upwardly increasing free space area, first with total reflux into both columns and next with a reflux ratio of 3 to 1. The calculations were based one system having feed of 7 mol per cent ethanol, a product of 84 mol per cent and a waste of .005 mol per cent. The method used in all of these calculations was that of Chilton and Colburn above noted. The results of these calculations are tabulated in Tables. I-IV wherein V/L indicates the reflux ratio, Y1 and Y2 indicate the base and top vapors of the particular section of the column involved, and NT the number of transfer units. These tables follow:

Table I VIL=1 s rfllal r a 't P ked l1 0 ill 611 80 apor I222? Ethanol igfig N Height Velocity Feet Ft T Required Ft./sec. Feet Table II V/L=l s 0131 P r t P k d u l 0 er en 80 8 81311111; a Ethanol AF NT RHelghtd eoo y 6 H0 FtJsec. Feat eet Table III V/L=l.33

s 1131 r 1 P k 1 11 I 01 8! 8H ac e 6 2 13; Ethanol gg NT es n 900 y squire FtJsec. Feet Feet Tables I and II show that, with total reflux, the total packed height for a column, having a constant diameter of 1.0 feet, is 33.2 feet while the packed height for a column of the same capacity having a packing of the same density but a. diameter varying upwardly from 0.87 to 1.0 feet,

is only 24.2 feet, a savings in height of 27.1%. From Tables III and IV, in which these same data are applied to columns operating at a reflux ratio of 3 to 1 (V/L equals 1.33) the reduction in heightextends from 63.7 feet to 47.0 feet equaling a savings of some 26.2%. As applied to Figure 1, Tables II and IV show the Height of each of the shell parts I through 5 for a given operation, while Tables I and III show how much longer each of the shell parts must be made for the same operation when the only change from Figure 1 is to increase the diameter of shell parts i, 2 and 3 up to one foot.

Having described my invention, I claim:

1. Apparatus for fractionating a fluid mixture comprising; a' fractionating column of the type wherein a liquid is fed into the top thereof and a vapor into the bottom thereof and brought into countercurrent relation to effect distillation in which a. low boiling component of the mixture is progressively concentrated upwardly along the countercurrent path; and packing in said column, occupying substantially the entire zone in which fractionation takes place and providing a free space area which increases progressively upward from substantially the bottom of the packing to substantially the top thereof.

2. The apparatus of claim 1 wherein the crosssectional area of the column containing the packing increases upwardly in a progressive manner.

3. Apparatus for fractionating a fluid mixture, comprising; a fractionating column of the type wherein a liquid is fed into the top thereof and a vapor into the bottom thereof and brought into countercurrent relation to effect distillation in which a low boiling component of the mixture is progressively concentrated upwardly along the countercurrent path; and means, forming over substantially the entire zone of said column in which fractionation takes place, a packing providing a free space area which increases upwardly in a progressive manner substantially from the lower end of the packing to the upper end thereof.

4. Apparatus for fractionating a fluid mixture, comprising; a fractionating column of the type wherein a liquid is fed into the top thereof and a vapor into the'bottom thereof and brought into countercurrent relation to eflect distillation in which a low boiling component of the mixture is progressively concentrated upwardly along the countercurrent path; and a series of superposed packing sections extending through substantially the entire zone in which fractionation takes place,

each section having a predetermined space area,

the free space area of the packing section adjacent the bottom of the series being less than the free space area of the packing section which lies adjacent the top of the series. the interposed packing sections being formed with predetermined space areas arranged to provide a progressive increase in free space area upwardly from the bottom of the series to the top thereof.

RALPH R. KAISER.

REFERENCES crrEn The following references are of record in the,

file of this patent:

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