206
INDUSTRIAL A S D ENGINEERING CHEMISTRY
T’ol. 22, s o . 3
Design of Fractionating Columns‘ With Particular Reference to Petroleum Distillation R. B. Chillas and H. M . Weir THE ATLANTICREPININGCOMPANY, PHILADELPHIA. PA
HE problems involved in the design of fractionating Confronted with the great complexity of the raw material, equipment for complex natural mixtures, such as pe- the practical designer must be satisfied with an analysis of troleum crude oil, differ in magnitude and kind from his problem in terms of average or maximum and minimum those relating to apparatus for industrial separation of binary boiling-point characteristics of fractions, as a substitute for mixtures. Whereas the engineer concerned with one of the exact chemical and physical data easily obtainable from most latter problems thinks of production in terms of pounds, the binary mixtures. The practical efficiency of the fractionating petroleum technologist speaks of thousands of pounds per equipment is rated very largely upon its performance in rehour. The difference in kind between the two problems is spect to production of a series of controlled boiling-range related to the fact that the fundamental concepts by which fractions. Since the equipment makes a physical separation the distillation of binary mixtures can be ordered are well only, it is obvious that control of the boiling range of fractions known. Some of the imoortant factors can be deduced from from various crudes implies wide flexibility in the amount of simple graphical computaeach cut taken from the t i o n s based upon equally c o l u m n ; a n d that, even simple and direct laboratory u n d e r t h i s circumstance. Despite many articles on the design of fractionating determinations. On the certain restrictions in the columns, little information is available to which the other hand, generalizations choice of boiling ranges are designer of such equipment can turn for help in selectof debatable applicability imposed by the character of ing the proper combination of plate area, spacing beare the nearest approach to the crude itself. tween plates, and individual plate design. fundamental facts relating Figure 2 presents a flow This paper presents a method (applicable also to to the separation of petroof a type of fractionchart other distillation problems), which was used in the deleum fractions. F u r t h e r ating equipment which will sign of a 30-plate column to separate 8000 gallons per more, the character of the make separations such as hour of crude petroleum into six commercial fractions. r a w m a t e r i a l (crude oil) are indicated in Figure 1, Observations on bubble-plate action using vapor veusually changes a number and the discussion of column locities of 2 to 6 feet per second through the free space design will be based on this of times during the life of of the column, heat-balance calculations, and a secany commercial separating so-called single-flash proctional type of plate which requires neither machine equipment, and the specifiess. work nor the use of any packing in its assembly are cations for the s e p a r a t e d Crude petroleum heated presented, together with brief notes on some accessory in a pipe still to 750-800” products may be altered acapparatus. F. is introduced a t F into cording to market demands column A . which urovides which bear no relationship for continuous f r a c t i o n a l to the changes in the raw separation of the first five distillate cuts and for removal material. These, in part, are the differences in the very nature of the of undistilled residue from the bottom of the column. The problems, but many features of the fractionating equipment low-boiling naphtha is taken as vapor from the top of the are similar. It is fairly well recognized that the plate and column, condensed in tubular condenser-exchanger, C, further bubble cap type of column has the widest useful working cooled by water in tubular unit C’, and then passed through range of capacity in both branches of the art of fractionation a continuous water separator, W . A portion of the waterand certain principles of mechanical design for this type of free naphtha is pumped back to the top of the column, the column are more or less independent of the kind of mixture rest being taken from the system as product. The higher boiling distillates are removed as liquid “side being distilled. This paper is a discussion of the mechanics of design of streams” from flow boxes FB. These flow boxes are proporbubble cap and plate fractionating columns for petroleum tioning devices through which all reflux must flow before passcrude oil. As a framework for the observations on desirable ing further down the column. The operator adjusts each and undesirable features in such columns, actual calculations flow box to remove from the system any desired and fixed fractional part of the reflux as a side stream. CJf the principal requirements for a column will be given. Proper adjustment of the volume ratio of forward flow t o While the argument and illustration specifically refer to petroleum, it will be evident, as noted above, that many of return flow a t the top of the column and at each flow box will the fundamental considerations apply to the separation of result in a series of product streams fractionated to meet the required end points, as determined by laboratory assay discomponents from any mixture. The illustration refers to the continuous separation of six tillations. The initial point shown by the assay of any stream is not commercial fractions from a typical mixed base petroleum crude. Figure 1 is a graph of the data obtained from labora- independently variable, but is conditioned by the end point tory assay distillations (4) of the crude itself and of each of of the next lower boiling material and by the amount and the six fractions. Arranged in the order of increasing boiling- character of constituents boiling in the range of the two fracpoint range, these fractions are: naphtha, refined oil distil- tions. It is also always lowered by a small proportion of lowlate, gas oil, paraffin distillate, high-viscosity or “hivis” dis- boiling constituents corresponding to the requirements of equilibrium with vapors which rise through the section of the tillate, and residue or bottoms. column from which the side stream is taken. The effect Presented a t the meeting of the American 1 Received January 22, 1930. of such low-boiling material on the distillation curve is Institute of Chemical Engineers, Philadelphia, P a , June 19 to 21, 1029.
T
March, 1930
ISDUSTRIAL AND EiYGIiVEERING CHE;VISTRY
207
plainly shown by the gas oil and paraffin distillate curves in Figure 1. Whether or not this small amount of low-boiling material is objectionable in a given stream depends upon a number of considerations. However, each forward flow or side stream may be provided with a stripper, S , into which a small amount of steam can be injected, so that the contamination can be removed by steam distillation when desired. One method of
He must. therefore, design to secure the best conditioiis with respect to the major factors. The height of the column is closely related to the number of plates required to effect separation. Fixed and simple relationships between the quality of vapor arising from binary mixtures at their boiling point serve as the basis for a satisfactory mathematical ans~verto the question of the number of plates required for separation of these mixtures. Unfortunately, it is not possible to crystallize experience in the separation of petroleum fraction. from one another or from crude oil into the form of a useful set of rules or formulas. Actual experiment with the crudes in a semi-works column served the writers in fixing the number of plates in the columns of the type being discussed. The arrangement is as qhown by the plate nunibering within the column in Figure 2 . While methods have been proposed ( 2 ) for determination of the number of plates t o 5eparatc complex mixture\, these are not particularly helpful and a critical experimental and mathematical itudy of this important question would be of very great practical value to the petroleum industry. It i b a problem which is entirely suitable for investigation in a university laboratory. Before discussing the other major factors in design of a column, it will be advisable to deDistillation Temperature #E ocribe the mechanism of bubble plate operation. Figure I-A. s. T.M. D i s t i l l a t i o n s o f Fractions, w i t h Y i e l d s f r o m S e m i n o l e Crude Consi&r a plate as in Figure 3 with a llumber of vapor uptakec, C , each covered by a bubble &posing of the steam and hydrocarbon vapors rising from cap, C, the Ion-er rim of which is notched for distribution of the stripper is to introduce them into the column one or two vapor. Liquid flows from the inlet downpipe, I D , across the plates above the point of origin of the side stream. To avoid plate to the overflow downpipe, OD. The space back of thc complication of Figure 2, no method for disposing of vapors inlet weir, I W , forms a seal pan to prevent flow of vapor up the downpipe. From the downstream side of the weir, O W , the i. indicated. Steam strippers should bc operated to remove only the low- reflux is delivered to the plate below through the do~vnpipc. boiling contamination from any stream. Lower boiling conA stituents from a portion of the body of the cut can be distilled, but this requires uneconomically large quantities of steam. PI 30 Such control should be effected by flow-box adjusiment rather than by the stripping operation. The bottoms or residue is c' always similarly con1aminated to a considerable extent with I lower boiling constituents. To remove these, either a n external stripper may be used or steam can be introduced below conrentional exhausting or stripping plates in the column itself as shown in Figure 2 . It need hardly be inentioned that the practical design of a column and accessoricls, such as are indicated in part by Figure 2 , involves a study of the probable maximum duty of each item of the equipment. This in turn necessitates collection of data such as is shown in Figure 1 for each crude which may later be processed. The method of handling such data will 1Ahphth6 be evident from the discussion of design for the crude of Figure 1 , and no further specific mention of the requirements of the column for wide applicability will be made. I I As a mechanism for converting heat into work the fractionResidue ating column is grossly inefficient; but since this fact is inFigure 2-Flow D i a g r a m of F r a c t i o n a t i n g C o l u m n a n d Accessories herent in the principle by which separation is effected, it A , column, FB, flow boxes, S , strippers, C , tubular condenserneed not concern the present discussion. From the present exchanger, C', tubular water cooler, W, continuous water separator standpoint, the ideal column would be of small diameter and height, and each plate section in the column would provide Despite the most careful leveling of caps to secure an apfor true equilibrium of vapor and liquid. The pressure drop parent equal submergence of notcheb. vapor bubbling alwayi through the column would be small, so that the difference in atarts locally. That is, vapor will emerge from notches temperature a t the top and bottom of the column would be a around the entire periphery of one cap, or in the space between function only of the difference in composition between the two caps which present an extended row of opposed notches. mixed vapors and liquid. The designer cannot secure a set of There is a definite reduction in resistance to flow through chanconditions which represent even the practical ultimate in all nels in which vapor starts to rise. Additional caps or pair5 of these factors a t once, since they are mutually incompatible. caps start to bubble as the supply of vapor increases, because
1
-
INDUSTRIAL ARiD ENGINEERIiVG CHEMI8TRY
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flow through original channels demands higher pressure differential than is offered by the solid head of liquid a t other points on the plate.
ID
C
b
Q
I
,A I
T
i
lDI
bJ F i g u r e 3-Diagrammatic S k e t c h of Bubble P l a t e I , inlet side, 0, overflow side; WW, distributing weirs; DD, downpipes
Whenever sufficient vapor is passing through the plate to induce bubbling through a number of caps, three fairly distinct zones can be observed. Zone 1 consists of a coarse foam, which extends some 6 inches above the normal surface of the liquid. Zone 2 is a region of large drops thrown about in all directions to a height of 15 to 18 inches. Zone 3 contains very small droplets, many of which do not fall back to the plate, but are carried upward with the vapor. Ipcrease in the volume of vapor passing through the plate does not greatly alter these conditions until all the caps are bubbling. When the vapor supply is increased beyond this point, the combined height of the three zones increases and the relative proportions vary until finally, a t velocities of 5 to 6 feet per second in the free space of the column, the liquid appears t o be sprayed entirely off the plate, and the zone of large drops extends to a height of 4 to 8feet. Fractionation can properly be said to take place mainly nithin zones 1 and 2, where a very large surface of liquid is exposed for interchange with the vapor. So far as the diameter of the column is concerned, it appears. therefore, that a step can be made in the direction of the ideal column by allowing for free space velocities of the order of perhaps 4 to 6 feet per second, provided extrained liquid can be removed before the vapor stream enters the next plate. Excellent contact of vapor and liquid is insured by this velocity, but this advantage is purchased at the expense of increase in pressure drop. The latter is occasioned almost entirely by frictional resistance as vapor passes through the comparatively small vapor uptake area in each plate. Heat Balance
I n order to reduce these generalities to concrete figures, the designer must strike a heat balance for his particular set of
Vol. 22, No. 3
conditions. The method of doing this will be illustrated by carrying through the calculations for the column of Figure 2, operating on 8000 gallons of crude charged per hour, and producing the percentage volume and quality of products shown in Figure 1. An 8000 gallon per hour unit is of moderate size, as petroleum columns go, and can be handled without special provision along railway rights of way. The heat data used for the calculation are taken from Figure 4. These curves represent the weighted averages of laboratory and test data from practical unjts. The curve for heat of the liquid in the lower temperature range represents averages of careful laboratory determinations, but the justification for the extrapolationin the range of 800" F. must rest almost entirely on successful use of these data in column design. The heat at any point on the total-heat curve is the sum of the heat of the liquid plus the average latent heats of paraffin hydrocarbons boiling below this point. Material streams enter and leave the column a t temperatures shown in Table 11. These figures are actual results taken from the log sheet of a large operating column. In designing this columa, a similar set of figures was a t hand as a result of experimental \Tor!< n-ith a column 20 inches in diameter and capable of handling some 200 gallons per hour of crude. Whenever the designer must rely entirely on experimental data obtained in glassnare in the laboratory, the temperature of the material streams can be approximated as follows: (1) Temperature of crude entering column (5). (2) Temperature of side streams. Corresponds closely with 5 per cent point on the assay-distillation curve of the stream, provided steam is not present in excess of 1 pound per gallon of charge. (3) Temperature of residue leaving the column. This is always less than the temperature of the "hivis" side stream, but the actual temperature is materially affected by the use of steam to strip this residue. If the percentage residue is small as in the present case, considerable error in estimating this temperature will not seriously affect the heat balance. (4) Temperature of naphtha vapors leaving the top of the column.
James ( I ) has shown that the temperature at which naphtha is completely vaporized a t atmospheric pressure corresponds closely with the 75 per cent point of the assay distillation. However, steam, from stripping the residue or from sidestream strippers, is usually present to reduce the temperature a t the top of the column below this 75 per cent point. When the relative proportions of steam and naphtha vapors are known, the true temperature of complete vaporization can be estimated from partial-pressure relationships. Since this is not the case until the first approximate heat balance is struck, a method of trial and error must be used to approximate the true top-of-column temperature. As a matter of fact, in nearly all cases where steam is present in practical amounts, assumption of a top vapor temperature equal t o the BO per cent point of the assay curve meets all the requirements of a heat balance without recourse to the trial-and-error method. If no steam is present, the 75 per cent point should he used. Table I-Material
MATERIAL
Crude Naphtha Refined oil distillate Gas oil Paraffin distillate Hivis Residue
I1
Balance i n Fractionating C o l u m n a
EXPERIMENTAL
I
CALCD. PER ON HOUR BASISCHARGE 8000 GALLONS
volume Gravity Weight Volgallonper ume L?, I "
100 28.6 20.7 10.3 25.1 7.1 8.2
.1.p. I.(;undr 38.6 6.925 64.5 6.01 42.0 6.79 36.6 7.01 28.2 7.38 21.0 7.73 11.5 8.24
Total weight
Gallons Lbs. 9er h o w 8000 55,400 2284 13,720 1656 11,230 820 5,750 2010 14,860 570 4,410 660 5,430
a Gas yield is ignored as of second order, being 0.2 to 0.3 cubic foot per gallon charge.
I N D UXTRIAL AiVD E-\’GINEERING CHEIVISTRY
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Table 11--Heat Balance for Fractionating Column
1
1-
TEMPERATURE
MATERIAL MATERIA&
Crude
F. 800
5,4380 4,410 14,860 5,750 11,230 2,000 13.7:!0
2,000
240
Outlet
F.
~~
1
57.400
HEATABOVE 60’ F.
I
1
600 720 600 470 355 228 228 228 100
Totals a b
1-
55,400
Residue Hivis Paraffin distillate Gas oil Refined oil distillate Steam Naphtha Naphtha Naphtha
Inlet
OUtDllt
h D U t
Input
Heat content
B. 1. u. p e r Ib. Vapor 608 599Q Liquid 5181 Liquid 335 Liquid 440 Liquid 335 Liquid 235 Liquid 155 4 v . 1135 Vapor 215 Liquid SO Liquid 15
1.82 1.94 4.97 1.35 1.74 2.27 2.95
2.27
0 . 8 3 Radiation b 17.60 By difference
i
5i,4(1OI
output
Million B . 1. n. p n b o a 33.2
35.47
35.47
Assumption: Vaporization is as shown by yields; hence (0.916 X 608) f (0.084 X 518) = 599 mean. 2.25 t o 2.5 per cent.
Tables I and I1 give the material and heat balances for the column. The heat out of the column “by difference” is the heat which must be carried to the condenser by naphtha vapors which are condensed and cooled and returned to the column a4 reflux. (Figure 2) The volume of this “external” reflux, assuming it is cooled to 100’ F., is necessarily 17,600,000 B. t. u. per hour - 88,000 lbs, per hour (215 - 15) B. t. u. per hour
When this cool material is introduced into the top of the column, it condenses an additional volume of vapor. making the total “internal” reflux 215 215
- 15 - 80 X 88,000 lbs. per hour
=
130,500 lb::. per hour
The liquid downpipes for the top plate of the column must handle this weight of mat’erialper hour. It is interesting to note a t this point that the heat carried to the top of the column by vapors is equivalent t o almost two-thirds of the total heat input, thus: .Ifillioits S t e a m . . . .., , , , , , , , . . , , . . . . . , , , . . , , , . , . , , , . . Naphtha . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vapors e q u i v a h t t o reflux.. . . , . . . . . , . , , . . . . , . .
(1) Specific volume of hydrocarbon vapor mixture4 throughout a column in which crude oil is being distilled is 4 cubic feet per pound at the 50 per cent point shown by assay distillation of the condensed vapors. (2) Vapors obey ideal gas laws with respect to expansion.
In the majority of instances sectional heat balances for the column will show that the volume of vapor a t the top of the column will be larger than a t any other point. This is the case in the present example (Table 111). Design of Column
Having thus obtained the maximuin volumes of vapor arid liquid passing any section of the column, the determination of the details of the design may be undertaken. Msny of these details are the subject of patents or patent applications.
B. t . u . p e r hour 2.27 2.95 17.60
__
22.82
Application of the means indicated by Peterkin (3) to reduce the heat to the top of column does not alter the principles of the method used herein, and therefore this point need not be further expanded. The heat balancl: serves two purposes. It indicates the duty on naphtha condensers and on side-stream and residue coolers, and it serves as a basis for design of the column itself. KO discussion of heat exchange and cooling equipment will be given here, since the design of t’hese it’ems follows wellknown practice. I n applying the heat balance to the design of the column, the first conPideration is to determine the volume of vapors a t any point. Table 111--Vapor Volume a t Top of Column WEIGHT OF
SPECIFIC
VAPOR
VOLUME
TOTAL
VOLUME
3
2
‘00 -03
4110
500.
Teem??
Naphtha forward flow Internal reflux equivalent Steam (superheated) Total
Lbs. p e r h o w 13,720 130,500
2,000 146,220
Average density of vapor at top
Cu. ft. per lb. Cu. ft. per h r . 4.0 55,000 4.0 522,000 29.0 58,000
--
635,000
= 0.23 lb. per cu. f t
These vapor volumes are obtained with sufficient accuracy for calculation of pressure drop through orifices, lines, plates in a column, etc., by making use of-the following simple assumptions:
7
c53 re ‘ F
‘’C
YOO
q30
KXV
Content of Oil above 6 0 ° P. (760 mm. Hg) Fixed oints mean specific heat 0 5 a t 300° F 0 7 a t 800° latent heat (B t u per p6und) 130 a t 300°F , 90 a t 800’ F.
Figure 4-Heat
2;
The diameter of the column can be fixed on the basis of the general observations and suggestions given under the discussion of Figure 3 and in more detail herewith. Though the vapor volume per hour may be considerably smaller than the maximum at certain points in the column for one ‘given set of conditions, it is nearly always found that the location of
INDUSTRIAL AXD ENGINEERING CHEMISTRY
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zones of lower vapor volume changes with differences in crudes and in operating procedure. For this reason and in the interest of simplicity in general construction, the column diameter should be the same over its entire height even though this entails considerable variation in vapor velocity from point to point. The writers’ observations of bubble-plate operation show that, unless unusual attention is paid to leveling the plate of a large-diameter column, uniform bubbling of vapor from all caps will not occur until a vapor velocity approaching 12 feet per second through the notches in the bubble caps is obtained. For a well-designed plate this vapor notch (and vapor uptake) velocity will correspond to something like 1.4 feet per second velocity through the free space of the column. I n the literature one frequently sees the maximum free space velocity in a fractionating column set a t from 1.0 to 1.3 feet per second. I n the opinion of the writers this is a minimum rather than a maximum velocity for a good design of largediameter fractionating column operating on petroleum vapors a t atmospheric pressure. The corsiderations which really limit the column velocity will be discussed in order below. For the illustration we will arbitrarily choose a maximum column free-spare velocity of 2.25 feet per second and show how other factors are designed to meet this condition, which is not a high velocity in the writers’ experience. Referring to the heat balance and the calculation of vapor volume, Table 111. the cross-sectioaal area of the column must be:
Vol. 22,
xo. 3
635,000 cu. ft. per hour = 78.5 sq. ft. 2.25 ft. per sec. X 3600 sec. per hour
Hence a 10-foot diameter column is required. Type of Plate
The designer should criticize the plate which he finally evolves from several standpoints. It should combine as high percentage of vapor-uptake area (compared with free space in the column) as is consistent with a low liquid gradient across the plate when the maximum volume of reflux is traversing the path between inlet downpipe ID and overflow downpipe OD of Figure 3. Careful attention should be paid to means for obtaining good distribution of reflux across each plate, since any irregularities will impair the fractionation to a surprising extent. I n certain instances it is advantageous to provide small downpipes more or less uniformly distributed over the plate; but when side streams are to be taken from a column, simplicity in design dictates grouping the inlet and overflow downpipes on opposite sides of the plate. This arrangement would be quite unsatisfactory from the standpoint of liquid gradient, particularly in large columns, if caps were designed so that they protruded above the normal liquid level. By using caps of the order of 2l/4 to 2l/2 inches high and providing an overflow weir ‘/4 to ‘/z inch higher, the conditions tending toward low liquid gradient are decidedly better than can be obtained with the higher caps.
.
’Vote-With a reasonably level plate, the liquid height is that of the vapor uptake under the caps when no vapor I S flowing On the other hand if vapor is passing through any cap, the liquid level is held by the weir
\
\
U Figure 5-Detail
J
of 10-Foot D i a m e t e r Bubble Plate
1
The shape of the rather tortuous passage for the vapor under the bubble cap should be smooth, well rounded, and of as nearly uniform cross section as can be consistently obtained. For columns over 4 feet in diameter designs can be worked out so that these passages, designated in a group as the “vapor uptake,” have an area of a t least 11 to 12 per cent of the cross-sectional area of the column. At the same time ample provision can be made for inlet and overflow weirs, joints between plate sections, etc., and adequate thickness of metal allowed for corrosion of bubble caps and vapor uptakes. It has been found that cylindrical vapor uptakes larger than about 4 inches in diameter, or long slots wider than 2 inches, do not lend themselves to the low cap design described above. With 1 to l l / z inches slot width, a well-balanced and low design can be readily obtained with the desirable 11 to 12 per cent vapor uptake. By incorporating a peripheral cap in the design, especially in columns under 36 inches in diameter, a good design with 17 to 22 per cent vapor uptake is practicable. Even in columns over 4 feet in diameter the peripheral cap is highly advantageous, not only for the vapor-uptake area thus obtained, but also because it provides a simple method for taking up minor inaccuracies in diameter, such as are almost inevitable in the shells of the larger columns. The limits mentioned may be helpful as a basis for criticizing a newly designed plate. Whenever much larger vapor-uptake areas in
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211
design for large plates with grouped downpipes are obtained, the designer should examine the liquid gradient characteristic of the plate with special care, as it may be quite high. Before passing to the illustrative calculation for the plate, it may be remarked that no inconsistency is involved in speaking of column velocities which “lift the liquid from the plate” and a t the same time stressing the importance of low liquid gradient. The effect of a high liquid gradient persists, even under conditions of most violent agitation due to vapor passage through the plate, and tends to distribute the vapor unevenly. Calculations for Design of Plate Let us assume that an acceptable design of plate can be developed in which the total vaporuptake area is 11 per cent of the plate area as is the case in the plate shown in Figures 5 and 6. The velocity through the uptakes for a flow of 635,000 cubic feet per hour of vapor (of 0.23 pound per cubic feet) will be 635,000 cu. ft. per hour v = 0.11 X 78.54 sq. f t . X 3600 sec. per hour =
20.5 f t . per sec.
The velocity head, h , is then given by the formula
vz
h =2gc2 F i g u r e 6-Vertical Cross S e c t i o n of 10-Foot D i a m e t e r F r a c t i o n a t i n g C o l u m n The discharge coefficient, e, for a dry plate of several different designs has been found to be Weirs 0.38 to 0.54. For the plate of Figures 5 and 6, c = 0.45; hence, (20.5)2 Assume that the 10-foot diameter plate can be designed with = 32.3 f t . = T X 32.16 X (0.45)2 inlet and overflow weirs (ITV and OW, Figure 3) which are Wilson and Bahlke (6) have shown that the specific,gravity 7 5 per cent of the diameter, or 90 inches long. The weir of normal paraffin hydrocarbons a t their boiling points is formulas do not apply strictly for low liquid head, but are approximately 0.6, equivalent to 5 pounds per gallon. This useful in obtaining some idea of the flow. The Francis density will be used in all calculations involving flow of liquid formula, expressed in the following units: in the column. Q = liquid volume, in gallons per minute The hot oil equivalent of 32.3 feet head of vapor is h = head of liquid, in inches
’’
32.3 ft. X 0.23 11). per cu. ft. X 12 in. per ft. (62.4 X 0.6) lbs. per cu. ft.
:=
2,38 in,
which is the height t o which hot oil will be backed up in downpipe OD by the velocity head of vapor through the plate. The area of the downpipes must be sufficient to pass the maximum volume of reflux through the entering orifice on the floor of the plate behind OW, Figure 3. The orifice formula Q = cads becomes Q = 4.4adh7 when Q is expressed in gallons per minute, t,he area, a, and head, h, in inches, and the sharp-edged orifice coefficient c = 0.61 is used. To pass the reflux a t the top of the column, or 130,500 lbs. per hour = 435 gal. per min. 5 lbs. per gal. X 60 min. per hour
the minimum downpipe area with a liquid head equal to the height of the overflow weir is 4.4Vh
435 4.4V2.75
59.6 sq. in.
This is definitely the minimum size of the downpipe. An apparent available suction head within the downpipe should be disregarded, since the liquid flowing is a t its boiling point. Actually the downpipe should be much larger, so that a t maximum flow the orifice should never be filled, even in the event of considerable irregularity in operation. An area not less than 2.5 times, and preferably 4 times, the calculated area affords a safe design.
1
= length of weir, in inches
c
= discharge coefficient = 0.62
becomes Q = 2.981ha/2. At the top of the column the flow of the internal reflux requires a head h, calculated from 435 = 2.98 X 90 x h ‘I2,of 1.38 inches. At the bottom of the column the flow is only that of the hivis stream, or 4410 lbs. per hour = 14.7 gal. per min 5 lbs. per gal. X 60 gal. per min.
The head over a 90-inch weir would be so very small that the slightest departure from level would completely spoil the distribution. Several small and shallow V-notch weirs equally spaced in the main overflow weir will assist in maintaining distribution. Using the V-notch weir formula Q = 1.191h in which the units are as above, and assuming that four notches each 4 inches long are used, the depth of each V becomes 0.85 inch. Pressure Drop through the Plate
If the final design actually incorporates the various parts of a plate assembly according to the dimensions just calculated, the maximum pressure drop through the plate will be made up of the following: Velocity head (flowing vapor) Bubble cap submergence (no flow) Head on overflow weir Total head
HOTOIL Inches 2.38 2.75 1.38 6.51
I N D U S T R I A L B S D ENGINEERISG CHEMISTRY
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The type of calculation illustrated above for the 10-foot column orients the designer as to the requirements of the plate. The final design selected by the writers for a case similar to that chosen here for illustration is shown in Figure 5. Spacing between Plates
The remaining important factor influencing the size of the column is the spacing between plates. With the 2.25 feet per second velocity a t the top of the column, the spacing demanicd for flow of reflux is of the order of 7 inches as just calculated. Since a t even much lower velocity, fine spray, possibly sufficient to impair fractionation and certainly sufficient to impair the color. will be carried as high as 36 inches, ,
i 20,
,
,
,
,
,
,
,
,
,
I
I
,
,
D,stance between Plates l6inches Raho Air = 50 Cu Ft Water
Equivalent ‘i=:“Raho
I Gal
bosed o n 4 c u ~ t / l h o f o i I
$$
Vapor (30cu R /@,Ir=
Figure 7-Effect of “Venetian Blind” Baffles on Removal of Entrained W a t e r Spray from Air
Vol. 22. No. 3
inches with one set of baffles. but 24-inch spacing with two sets of baffles should be allowed if possible. The spacing between plates required for the installation of B flow box with downpipes equal in length to those for 18-inch plate spacing is about 32 inches. Side-Stream Flow Boxes
The essential features of the side-stream flow boxes are a flat-bottomed trough, which forms a spillway for the liquid from the downpipes, and an adjustable vertical dividing vane, which splits the flow into side stream and reflux for lower plates in the column. The relation between the volume of these two does not vary with changes in the rate of flow; hence the quality of the side stream remains substantially constant despite changes in rate of input of feed to the column. An impression of the general features of design can be obtained from Figure 6. Miscellaneous Details of Design
The water separator, W , and the side-stream strippers, S, of Figure 2 need little more than mention. The former is a conventional continuous separator which receives a mixture of oil and condensed steam and discharges the liquids after they are allowed to separate b y difference in specific gravity. The side-stream strippers are essentially small columns in which liquid is fed to the top plate. Three plates give the required good contact of steam with the liquid. The principles of design are the same as for the large fractionating column. One important feature of the plate design in the large column has not been mentioned. Such plates should be made in sections which will go through a manhole for ease in initial installation and also for subsequent cleaning to remove coke. Owing to the limited space in which such work must be done, plate sections should be made small enough so that they can be handled by one man. Time will be saved if the joints betreen sections can be so constructed that no machine work or packing is needed in their assembly. A joint which meets these requirements consists of an inverted channel section which covers the two upturned edges or rims of adjacent plate sections as indicated in Figures 5 and 6. During operation this joint forms an effective mutual seal against the flow of either liquid or vapor. Conclusions
adequate means for spray removal should be placed beneath each plate in the column. A baffle for this purpose has been developed which combines high efficiency and reasonable cost with small actual height and low resistance to vapor flow. It is somewhat similar in appearance to a Venetian blind placed horizontally and consists of a series of blades 3 inches wide and inch thick placed a t an angle of 30 degrees from the vertical and spaced 11/* inches apart. Figure 6 is a vertical cross section of a portion of a column fitted with these baffles. Some idea of the effectiveness of this type of baffle can be obtained from Figure 7 , which depicts results of an experiment in a model in which water was substituted for petroleum liquid and air used in place of vapor. Reflux (water) was passed over the lower of two plates assembled in an open shell through which air was flowing. The upper plate was used to collect the liquid which was carried into it by entrainment. The volume of liquid so collected is expressed on the ordinate of Figure 7 as the per cent of the reflux. The results are suggestive with respect to development of a really high-capacity column. In the opinion of the writers the minimum spacing for column velocities of more than 2 feet per second is about 18
The writers believe that the trend of column design in the future will be in the direction of smaller diameter and higher vapor velocity for a given throughput. While they have had but little experience with columns operating continuously a t more than 3 feet per second vapor velocity a t atmospheric pressure, there seems to be no reason why this is a limiting velocity. I n the design of columns to be operated a t high pressure or a t very greatly reduced pressure the same principles of design apply. In the latter columns the use of baffles of the type described and proper attention to the design of plates will enable the designer to obtain surprisingly high vapor velocity with a pressure drop not to exceed 1 mm. of mercury per plate. Whenever possible a new plate should be observed in operation under conditions which approximate those for which it is designed. This is relatively simple in the case of atmospheric pressure columns where air and water can conveniently be substituted for hydrocarbon liquid and vapor. Very minor changes in the arrangement of the elements in a plate often indicated as desirable by such a test will sometimes make considerable improvement in its operation.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
March, 1930
Literature Cited
213
(4) Peterkin and Ferris, IND.ENG.CHEM.,17, 1248 (1925). (5) Piroomov and Beiswenger, Am. Petroleum Inst., Rept. Meeting, December, 1928, p. 52. (6) Wilson and Bahlke, IXD. Eac. CHEM.,16, 115 (1924)
(1) James, J . SOC.Aulontoliwe En!., 18, 501 (1926). (2) Murphree. IND.ENG.CHEM.,17, 747 (1925). (3) Peterkin, Proc. 4 m . Petroleum I n s t . , 9, 142 (1927).
9th Annual
Our Last Year’s Foreign Trade in Chemicals and Related Products’ Otto Wilson NATIONAL PRESSCLUB,W A S E I N G ~ OD. N , C.
A
YEAR of steady, prosperous activity in American chemical industries is indicated by the Government’s figures of our 1929 foreign trade in cheriicals just released. A marked increase in the value of exports of American chemicals and related products accompmied a small gain in imports. -4s a rewlt an adverse balance of trade of nearly $A,000,000 in 1928 Tyas changed to a favorable balance of 18,000,000 in 1929. The significant feature of the returns, however, lies in the fact that the gain of $15.000,000 in the export trade was due, not to the swelling of a few outstanding items, but to increases in nearly all the large groups of chemicals. In the import trade, on the other hand, a gain of $6,000,000 in industrial chemicals was balanced by a like decrease in fertilizers, with the other groups showing no very wide fluctuations. U n i t e d S t a t e s F o r e i g n T r a d e i n C h e m i c a l s and R e l a t e d P r o d u c t s YEAR IMPORTS EXPORTS BALANCE OF TRADE 1928 1929
$143,234,000 144,062,000
$137,331,000 152,162,000
-$5,903,000 +$8,100,000
CHEMICALS AND RELATED P R O D U C T S
I n compiling tht: foreign-trade statistics the Government includes most imports and exports of chemicalis in one major statistical group, Group 8, headed “Chemicals and Related Product’s.” This is divided into eight subgroups. The total value of the imports and exports in each of these subgroups in the last two years was as follows: I m p o r t s a n d Exports by Subgroups IMPORTS
SUBGROUP 1928 1929 Coal-tar products SS3,907,000 522,824,000 Medicinals and pharniaceuticals 5,179,000 6,422,000 Industrial chemical speciala ties Industrial chemicals 24,165,000 30,644,000 Pigments, paints, and varnishes 3 766 000 3 821 000 Fertilizers and materials 78:118:000 72.’340:000 Explosives, fuses, etc. 979,000 960,000 Soap and toilet preparations 7,121,000 6,941,000 a Not separately stated.
EXPORTS
19% 1929 514,113,000 $18,059,000 20,522,000
21,282,000
14,551,000 25,501,000
14,457,000 28,194,000
25 614 000 16:095:000 5,165,000
29 119 000 20:444:000 4,549,000
15,721,000
16,059,000
While the group totals in 1929 were thus, for the most part. fairly close t o tho,se of the previous year, the trade in particular commodities often showed wide variations. The more important of these are noted below. 1 Received February 5 . 1930. All 1929 figures are ?reliminary but in general may be taken as correct, as the final figures seldom show changes of importance. Values of imports (except coal-tar products) are those declared a t foreign port of shipment and d o not include o w a n freight and insurance. Export valcations are those declared by American exporters. Further details relating to given imports or exports may b,e obtained through this publication or by addressing the writer of the article. Government statistics on which these annual reviews are based are not available for the calendar year until the end of the following January. As heretofore. we have delayed publication of this review until it could be based on the full twelve-months figures.
Coal-Tar Products
Imports of coal-tar products fell off somewhat in 1929, while exports registered a large gain. But the lower total of the incoming trade was due almost wholly to the drop in a single item, creosote oil. Purchases of this coal-tar crude dropped to 79,301,000 gallons from 88,385,000 gallons in 1928. Lower prices cut the total value in still greater proportion, from $13,928,000 to $i10,119,000. The average valuation of all imports was 12.8 cents per gallon as against 15.8 cents in 1928. (This valuation differs from that of all other chemical imports in that in the case of intermediates and finished products it is based on the American selling price. In the case of crudes, which come in free, it is based on the wholesale price abroad.) Last gear’s decline both in quantity and average value was a continuation of a tendency of the last three years. Other crudes and intermediates in general showed good increases, while imports of finished coal-tar derivatives were more than 20 per cent higher than in 1928. Finished colors, dyes, stains, etc., from Europe continued to grow in volume. The gain in the trade with Germany and Switzerland noted in 1928 carried on through 1929. Purchases from these and other sources in the last two years were as follows: I m p o r t s of Colors, Dyes, Stains, Color Acids, and Color Bases COUNTRY 1928 1929 Pounds Value Pounds Value Belgium 115,000 I 137,000 76,000 $ 96,000 France 74,000 90,000 108,000 160,000 Germany 4,056,000 4,250,000 4,685,000 5,019,000 Italy 60,000 12,000 14,000 65,000 Switzerland 1,639,000 2,563,000 92,000 3,000.000 2,005,000 United Kingdom 98,000 101,000 96,000 Other countries 205,000 55,000 239,000 60,000 Total
6,252,000
$6,877,000
-_
___
7:593,000
88,448,000
The average valuation of the imports from Switzerland, $1.17 per pound, was 5 cents lower than in 1928, but that of the German products slightly increased. Coal-tar medicinals registered a gain of 36 per cent in volume last year, imports of 153,000 pounds, valued a t $332,000, comparing with 113,000 pounds valued a t $209,000 in 1928. Exports of coal-tar products showed a healthy gain of nearly 30 per cent in value, the total rising from 114,113,000 in 1928 t o $18,059,000 last year. But this was very largely due to increased sales of a single article, benzene, exports of which went to 33,346,000 gallons valued at $8,537,000, as compared with 21,338,000 gallons valued a t $4,963,000 in 1928. This gain much more than off-set a decline which the 1928 trade had experienced. Germany, the United Kingdom, and Belgium are the largest buyers of American benzene. Exports of other crude distillates showed both gains and losses, the chief decreases being in sales of coal-tar, pitch, and coke. Coal-tar dropped from an export total of 138,000 barrels in 1928 to 109,000 barrels in 1929, and pitch and coke
