I
DONALD F. OTHMER and ROBERT D. BEATTlEl Polytechnic Institute of Brooklyn, Brooklyn,
N. Y.
Vacuum-Compression Distillation Column Design and Pressure Losses Reboiler pressure can be lowered by using an impeller in each distillation stage V A c r n m D I s n L L A T I o x s in industry use molecular stills below 1 mm. of Hg absolute and plate or packed columns above 50 mm. of Hg absolute, but efficient rquipment of adequate capacity for the intermediate range is lacking for heat-sensitive materialu. Pressure drop in the column must be minimal to give a low pot pressure (and temperature) ; hence, a contacting unit was built with a slight compressing effrct in each stage to overconie friction loses, giving a lower pressure in the reboiler than at the head so that heat-sensitive liquids are not decomposed. This efficient vacuum fractionator was found to be suitable for distillation and gas absorption at 1 to 20 mm. of Hg absolute pressures. A multi-bladed impeller compresses the vapor in each stage, so that pressure loss is less than 10% of that in usual vacuum distillation towers, and may be reduced to zero or even negative values-Le., lower pressure at the base. The impellers disperse the liquor as a spray, so that mass transfer efficiencies during vacuum operation of 1 Present address, 320 Lexington St., Watertown 7 2 , Mass.
the new design may be as high as those during atmospheric operation of bubble cap, sieve, or packed towers. Investment and operating costs may be slightly higher, but only a small improvement in yields of heat-sensitive products is economic justification for its use. Experimental work covered several aspects of performance; the present article discusses design factors, fluid mechanics, and pressure drops at various operating conditions. Equipment
Figure 1 shows the equilibrium stage with a vapor compressing imprller and integrally mounted liquor distributor, The 7.5-inch diameter impeller, E, was built up starting with an available compressor impeller, obtained commercially as a machined bronze casting. Four vertical troughs, A, semicircular in cross section and with multiple notched sides for weirs were spaced 90" apart on top of the impeller, to spray the liquor out radially against the counterflowing vapor stream. These weirs were fed from collector rings, C, with liquor coming from the plate above by a cylindrical sleeve, D. Studs, F, supported the top gutter plate above the bottom gutter plate, which was bolted to the top of the impeller. The impeller assembly was keyed, B, to a 1.75-inch diameter shaft. Vapor flow is indicated by long sweeping arrows and liquor flow by short arrows,
0
1
2 3 INCHES
4
The right side of Figure 1 shows three different section? of a stage. Section X - X shows one-third of a cross section through the Iiquor distribution section with the relative positions of the four posts and four vertical weirs all extending between upper and lower gutter plates. Section Y-1' shows one-third of a cross section through the impeller blades; and Section 2-2 shows one-third of the upper plate looking down just below an impeller. The central 4-inch diameter hole to the stage above has the drip ring D. Milled into this plate are eight radial troughs 45' apart and 0.25 inch wide X 0.125 inch deep, which allow liquor flow toward the center beneath the rotating impeller. Four holes drilled into alternate troughs allowed represeatative liquid samples to be obtained via connection to the outside (not shown). This milled top plate was welded to the wall section (a piece of 10-inch n.p.s. steel pipe); and there were flanges on both ends so that successive stage shells were bolted together with gaskets between flange faces. To avoid confusion, diffuser stator vanes are not shown in Figure 1. However, Figure 2 shows helical stator vanes surrounding an impeller assembly. Experimental Variables Stages: 10.5 inches I.D. X 8.5 inches high with a 7.5-inch diameter compressor impeller. Rotation rate: 220 to 1737 r.p.m.
5
Figure 1. Vertical cross section of vacuum-compression distilling column shows impeller assembly in typical stage; ihree horizontal cross sections show deiail of impeller and plate A. Vertical weirs.
B. Shaft & key.
C. Collector ring gutter.
D. Drip ring.
E. Impeller.
F. Support studs.
G. Radial troughs,
VOL. 53, NO. 10
OCTOBER 1961
779
COOLiNG WATER TO VACUUM SYSTEM
A Figure 2. Vertical cross section of stage with elevation of impeller shows vanes and liquor distributing weirs, also stator vanes, surrounding impeller
Liquor rate (water): u p to 50 lb. per hr.-sq. ft. Gas rate (air): 86.5 to 409.0 lb. per hr.-sq. ft. Liquor distributor: 4 equally spaced vertical weir troughs, 2.25 inches long with multiple notching on both edges. Number of stator vanes: 6 and 12 with pitch of: straight vanes, 2 " to 90" angle from the impeller blade tip tangent; circular vanes, 9 " to 77" angle between cylindrical axis and impeller blade tip tangent; and helical vanes, 10 " to 45 " angle of helix with the floor of s8tage. Design Factors Atmospheric pressure drop tern on a single contacting stage with air-water system showed the allowable vapor rate at zero apparent pressure drop to increase linearly with rotation rate as expected (8-70) u p to about 600 r.p.m., but interference effects, as windage from the impeller, caused a departure from expected linearity near 1000 r.p.m. Static, diffuser vanes improved per-
--STEAM
Figure 3. Set-up for vacuum-compression distillation studies under vacuum operation shows three contacting stages under test between other sections
I
1. Eqilibrium stage. 2. Speed reducer. 3. Motor, 1 hp. 4.-Drive structure. 5. Inlet section. 6. Exit section. 7. Thermometers.""& Sampling arrangement. 9. G a g e trap. 10. Zimmerli gage. 1 1 . 30 degrees inclined manometers. 12. Condenser. 13. Reflux drum. 14. Throttle valve. 15. Rotameters, 16. Canned pump. 17. Control valve. 18. Pressure gage. 19. Reboiler. 20. Liquid returns
formance and helical ones were better than either flat or cylindrical vanes. A 10" angle of helical generation was best, increasing allowable vapor rates by as much as 40%. At a 54.5 lb./hr./ ft.2 water rate, and 1000 r.p.m. impeller speed, capacities at no apparent pressure loss were 53 c.f.m. air for 10" helical vanes, 34 c.f.m. air without vanes. Pressure drop results, obtained while distilling, began with negative values a t low superficial vapor velocities, increased parabolically, passed through zero loss, and leveled off at positive losses, once the liquid seal was established. Pressure drop performance is characterized by u,, which increased linearly with rotation rate until interference effects reduced impeller efficiency a t higher rotation rates. During
A simplified design of impeller i s shown here; no operating data for it are reported. Vapor rises and flows out between the impeller blades, while liquid drops through spouts, attached to the troughs of the upper plate, into a built up simple gutter ring which passes it through small holes drilled in the top of the impeller, thence down and in between the impeller vanes and out b y centrifugal action. Or small holes may b e drilled through the curb to discharge the liquid centrifugally above the top of the impeller
780
INDUSTRIAL A N D ENGfNEERlNG CHEMISTRY
I
fractionation, ub decreased with pressure, and values, excluding liquid seal considerations, were correlated within 7.27' average deviation by the following empirical equation. Vapor rate at zero pressure loss = uo = 8
X
2'0.6
T W
(w+
0.3)
Liquor rate had little observable effect on pressure drop, as would be expected. During distillation of the two-ether and chloroform-benzene systems, power consumed increased negligibly with vapor rate, while liquor rate caused sharp increases in power drawn to maxima during liquid seal formation of about 0.1, 0.17, and 0.30 hp. per stage (including drive friction losses) at respective rotation rates of 700, 1000, and 1520 r.p.m. Also power increased rapidly with rotation rate, and the incremental power for liquid pumping correlated linearly with rotation rate on a logarithmic plot. Pressure drops observed for a vacuumcompression stage were less than 10yc of the minimum pressure drop reported for conventional fractionators ( 4 , 6, 8, 9, 7 7. 74, 76) ; and proper design of the impeller should allow the reduction of these pressure losses to zero or negative values. A special bubble cap tower at 56 mm of Hg on an ethyl benzenestyrene separation gave a pressure drop of 2.0 mm. of Hg per plate, whereas the vacuum-compression unit at 30 mm. of Hg and about 1500 r.p.m. gave an observable pressure drop of 0.045 mm. of Hg per stage. Preliminary estimates show the vacuum-compression unit might cost about lOyo more than a bubble cap
'VACUUM COMPRESSION
4: 13
Figure 4. Set up for air-water pressure drop studies has flowmeters and test section between entrance section and exit section 1 . Equilibrium stage. 2. Speed reducer. 3. Motor, 1 hp. 4. lifting bar. 5. Entrance section. 6. Exit section. 7. Drive structure. 8. Thermometers. 9. Static pressure manometers. 10. Orifice meters. 11. Blowers, 2 hp. 12. Slide valve. 13. Air discharge openings. 14. Ducts, 4-inch diameter. 15. W a t e r line in. 16. Drain
tower of equivalent performance. Operating costs will be slightly higher because of the power cost of the drive motor. Elimination of as lo~w as 1% loss in products due to decomposition of heat-sensitive materials would usually justify use of the fractionator.
Operation T h e hydromechanics of gas and liquid flows as affecting gas pressure drops were tested during actual fractionation tests in the set-up of Figure 3. also in an assembly of several sections for airwater operation as in Figure 4. The operation is described below to include sampling of liquid streams, although these samples were not used in calculating pressure losses determined here. Pressure drops, also separation efficiencies, during the vacuum fractionation were tested in a column using three contacting stages, whose internals are illustrated in Figures 1 and 2. Figure 3 illustrates the over-all flow plan for the vacuum fractionation apparatus. As shown in Figure 3, three contacting stages, 1, were assembled between a
vapor inlet, m t i o n 5, and an overhead vapor exit, section 6. T h e impeller assemblies for the three stages were rotated om a common 1.75-inch diameter steel shaft which was driven from a speed reduction unit, 2 , and a Ihp. electric motor, 3. The speed reduction unit included a variety of sheave sizes whose combinations gave fonr desired speeds-300, 700, 1000, and 2550 r.p.m. The drive components were mounted vertically on a channel iron structure, 4. Steam flow at 25 p.s.i.g. was conbscolld with a V-port hand valve and passed inra any or all of three heating coiB (3.2 square feet per coil) within a 21-gallon copper reboiler, 19 (16-inch diameter X 24 inches between dished heads): T h e reboiler had a 3-inch diameter vapor pipe connected from the top oF6he reboiler to the vapor inlet, section. 5, and liquid returns from the bottom of rhe column to the reboiler were made t h o u g h a 0.75-inch nominal copper tube. Distillate was returned also to the reboiler. Vapor entering the column was contacted with liquid in three successi\e stages, 1, previously described. Each stage had a n inclined socket well for 0 to 220' F. by 1' F. thermometers, 7 . Differential pressures were measured by 30" incliried manometers, 1 2 , whose fluid was dioctyl phthallate. Dynamic sampling was achieved by maintaining a small finite rate of flow through the sampling holes (Figure 1, Section 2-2)to follow the equilibration of the column until samples were withdrawn into preevacuated chambers. Figure 3 shows that vapor leaving the top contacting stage passed through a 3inch overhead vapor line into a watercooled shell and tube condenser, 12, with a 14 square foot surface. 'The entire system was evacuated through the condenser using a conventional vacuum source-an ice trap, a dry ice-acetone trap, and a mechanical vacuum pump. Flow rates of distillate and reflux were measured in two rotameters (capacity: 60 to 2650 ml. per min.), and a pump, 16, in the distillate line transferred liquid into the reboiler, 19. Absolute Dressure
A I R
RATE
was meamred at the middle contacting, stage using a Zimmerli gage, IO. Abospheric fractionation w a ~tested > inmentially the same apparatwmrangement as for the vacuum tests except that the ice trap, dry ice trap, and vacuum pump were not used. Three contacting stages (Figures 1 and 2) were used with the same steel shell (Figure 3). T h e equipment was started UP in> sequence: Vacuum pump was started, steam was admitted to the reboiler, water was admitted to the condenser, and the impeller motor was started. Aftetsteady state conditions were attained and before sampling the column, the following data were observed and recorded: the impeller speed in r.p.m.; temperatures in ' F. for the overhead vapor line, the reflux and distillate lines, the three contacting stages, and the reboiler line; the absolute pressure in mm. of Hg at the middle stage; and the reflux and distillate rotameter readings i n centimeters. Then samples were collected in pre-evacuated chambers and drained into prechilled 50-ni1. flasks. Immediately after sampling, readings were taken of the differential pressures across the contacting stages, the power drawn by the impeller motor, and refractive indices on the samples. Tests on pressure drop or losses such as dhose encountered in a gas absorption unit were carried out with the system airwater in the assembly shown in Figure 4 using a single equilibrium stage, assembled between a gas entrance section and a gas exit section, each having 4-inch nozzles. The impeller assembly, mounted on a 1.75-inch diameter shaft, was belt-driven from a speed reducer and a 1 hp. electric motor. Air flow was measured by an orifice meter, 10, with a 30' inclined water manometer and was adjusted by a bypass slide valve, excess air being wasted. T h e metered air passed into the bottom, through the contacting stage, and out through the top of the tower through an exhaust duct, 14. Static pressure manometers, 9, in the entrance and exit ducts and adjacent thermometers were installed.
C.F.N.
Figure 5. Pressure drops for air-water contacting at constant water rate of 53.1 pounds water per hour per square foot a t indicated r.p.m. of impeller. Both curves (left) for 38" helical stator vanes and lower curves (right) for 45" helical stator vanes show lower pressure drops for higher rotational speeds, with top speeds causing inversion of this trend due to windage or other losses
A steady, measured flow of water was distributed evenly on the plate above the contacting stage by a sparger ring. The liquid ran through the contacting stage and was removed through a drain, 16, in the next lower section. Three types of stator vane assemblies were constructed: flat or straight, cylindrical, and helical. Tests \vere also made without any stator vanes. Straight vanes were used in positions both with and against the direction of rotation a t angles defined between the tip tangents to the impeller blades and the vane surface. I'Vith the direction of rotation, angles Tvere 15" and 25"; while against the direction of rotation, angles were 2", 32"; GO", 80', and 90". Twelve equally spaced vanes were used. Cylindrical vanes were used a t angles of 9" and 22' with the direction of rotation, also 37" and 77": against the direction of rotation. This angle was that subtended between the axis of the cylinder and the tip tangents to the impeller blades. All angles were tried using 6 vanes and those a t 22' were also tested with 12 vanes. Six helical vanes were always used; tests were made a t angles of generation subtended with the horizontal of lo", 15', 23", 30": 38", and 45". Figure 2 shows that the helical vanes terminated in a cylindrical transition surface to curl the gases inward in counterflow to the sprayed liquor. Air-water pressure drop values on various stator vane assemblies were determined using instruments previously calibrated. Flow rates were expressed in cubic feet of gas per minute and pound liquid per hour per square foot. Pressure drop data were recorded directly in centimeters of water. T h e experimental results considered in this paper are of pressure drop and power consumption. These results, as are those of mass transfer studies, are affected by total pressure, rates of gas or vapor flow, liquid flow, and rotational speed. Pressure Drop Studies Using Air and Water Flows
The air-water system was used at atmospheric pressure to determine the optimum stator vane assembly in the contacting stage for maximum gas flow at minimum pressure drop, with flat, ,cylindrical, and helical vanes. Studies were made a t a constant liquor (water) rate of about 50 lb. per hr.-sq. ft. Effect of Gas Rate, Figure 5 is typical of the many made in showing the .effect of gas rate on pressure drop. Many times as much data as are plotted were accumulated in optimizing the tower design. T h e liquor rate chosen was purposely low in order to obtain friction loss results for wet surfaces within the contacting stage, but not to include friction losses for liquid pumping by the
782
rotating impeller. Liquid pumping occurred a t higher rates, when liquor collects in the stage. Pressure drop increases with gas rate a t constant rotation rate and, for helical vane stator assemblies, also with increase of angle of generation with the horizontal plane from 38"to 45". The relation of pressure drop to gas rate was also determined in scores of runs for helical vanes a t lower angles of generation, as well as for stator assemblies with cylindrical vanes, also for flat vane surfaces. A control series of pressure drop tests was also run without any stator vanes. These data show that pressure drop increases with increase of gas rate, while liquor rate was held constant a t 50 Ib. per hr.-sq. ft. Effect of Rotation Rate. The spinning impeller a t higher speeds applied greater mechanical work to gases flowing through it. Hence, at constant gas rate, the pressure drop observed across the contacting stage decreased with increasing speed of rotation. Figure 5 shows the pressure drop curves to be approximately equidistant throughout their lengths. However, a t rates of 1000 r.p.m. and above, interference effectse.g., windage--cause an inversion in the trend of displacement of the curve with changing rotation rate. Effect of Vane Angle. Helical vanes in the stator were specified by the angle of vane surface with the horizontal. Figure 5 shows that a reduction in the vane angle from 45" to 38" caused a considerable displacement of the curves. When a pressure drop line for any fixed rotation rate passes through zero pressure drop, the corresponding gas ryte is indicated. At about 700 r.p.m., the gas rate at zero pressure drop is about 18 c.f.m. at a vane angle of 45" and about 19 c.f.m. ar 38". Similar increase in the gas rate a t which there is zero pressure drop is noted at even lower generating angles for helical vanes in many other runs. For vanes with cylindrical surfaces, as the vane angle increases in the direction of rotation, the gas rate a t zero pressure drop was found to increase. As the vane angle is increased against the direction of rotation, the gas rate at zero pressure drop decreased. Similarly, for straight vanes, the gas rate at zero pressure drop increased with an increase in vane angle in the direction of rotation and decreased with an increase in vane angle against the direction of rotation. Number of Vanes. A reduction in stator vanes from 12 to 6 increased the gas rate a t zero pressure drop from about 27.5 to almost 30 c.f.m. Comparison with Control Runs. Pressure drop data, under different flow rates, ~ v e r etaken with no stator vanes surrounding the impeller, and gave a maximum gas rate at zero pressure drop
INDUSTRIAL AND ENGINEERING CHEMISTRY
of about 36.0 c.f.m. None of the vane assemblies tested with either flat or cylindrical surfaces has gas rates this large at zero pressure drop, and only helical vane assemblies with angles of generation less than 15 " have maximum gas rates larger than 36.0 c.f.m. A helical vane stator assembly with a 10" angle of generation gave a maximum gas rate of 50.0 c.f.m. a t zero pressure drop. Presumably, a stator assembly with helical vanes a t a lower angle would exhibit an even larger allowable gas rate. but this was not determined. Pressure Drops during Fractionation
Fluid friction losses were determined across the contacting stages during distillation operations using a mixture of n-butyl ether and dichloroethyl ether as the test mixture. Pressure drop readings were made across each of the three contacting stages while operating at a steady state. Under several different rotation speeds, pressure drops were observed a t total reflux under vacua of 20, 30, and 50 mm. of Hg absolute and also a t finite reflux ratios a t 30 mm. of H g absolute. Similar readings were made while distilling chloroform-benzene mixtures at atmospheric pressure with total reflux. Also, studies were made a t constant liquor rate using the two-ether system a t 1000 r.p.m. and 30 mm. of Hg absolute. Compressive performance of the column was evaluated in terms of the pressure drop in millimeters of water. The variables studied were vapor rate: liquor rate. pressure, and rotation rate. Effect of Vapor Rate. Figure 6 shows the typical effect of vapor rate (superficial vapor velocity) on pressure drop using average of 1006 r.p.m. a t total reflux using the two-ether system. Data were also taken at other rotation speeds and operating pressures and gave similar plots. Pressure drop increases with increasing vapor rate. Usually, the pressure drop curve begins at a negative value for low vapor rates and increases parabolically. At higher rates, the pressure drop curves tend to level off; and this is always true at total reflux, Although not plotted because data deviated widely, a leveling off trend was indicated for the top stage at much higher pressure drop levels than either the middle or bottom stages. The pressure drop for the top stage was always greater than that for the middle stage, and the middle stage pressure drop was greater than that for the bottom stage. The leveling off trend probably results from the impeller design. When sufficient liquor collects within a stage to give a liquid seal around the rotor blades, the impeller begins pumping gas more efficiently as it is better able to compress with the aid of liquid.
VACUUM COMPRESSION The three contacting stages were, as nearly as possible, identical, even though the pressure drop curves vary. The pressure drop was lower across the bottom stage, probably because the impeller had more favorable suction conditions. The bottom impeller sucked from an undisturbed reservoir of vapor (from the reboiler) while the other two drew vapors from the contacting zones of lower stages. Turbulence was created in the contacting zones not only by counterflow of vapor and liquid spray, but also by movement of the vertical weirs and support studs. Consequently, the middle and top impellers operated less efficiently, and a higher pressure drop was observed. The top stage had an even higher pressure drop, possibly because its impeller was discharging against the overhead vapor pipe. The middle and bottom impellers discharged into the suction of the impeller in the stage above. The pressure drop curve for the middle stage might be assumed as representative of most of the stages that would be found in a multistage vacuum-compression fractionating column, and the top and bottom (of this or larger column) might average at about the same values. Effect of Rotation Rate. Consistent with air-water pressure drop studies, those during distillation also decreased with increasing rotation rate a t constant pressure. The vapor rate a t the point of zero pressure drop, u,, measures the effect of rotation rate. Values of u, shouId increase as observed pressure drop decreases; hence, u, should increase with rotation rate a t constant pressure. This was confirmed by plots of much data (not shown). For example, at 20 mm. of Hg constant pressure, values of u, for the bottom stage are: 2.2, 4.9, and 5.5 feet per second at respective rotations of 301, 1012, and 1544 r.p.m. Effect of System Pressure. For the same volumetric flow of vapor, pressure drop was found in many tests to increase with total pressure a t constant rotation speed, as would be expected. For example, values of u, at 1000 r.p.m. for the bottom stage a t 20, 30, and 50 mm. of Hg are, respectively, 4.9, 3.9, and 3.2 feet per second. Effect of Liquor Rate. Figure 7 shows the effect of liquor rate using the two-ether system a t 30.0 mm. of Hg total pressure, an average rotation rate of 1010 r.p.m. and liquor rates of 49.7, 104.5, and 176.4 lb. per hr.-sq. ft. Curves for the lower two liquor rates are similar to those of Figure 5. No leveling off in the pressure drop curves is found as for those at total reflux in Figure 6. However, in Figure 7, the curves for the higher liquor rates show only the relatively level portion of the pressure
drop curves for the middle and bottom stages; and the curves are similar to the high vapor rate portions of curves at total reflux in Figure 6. This indication of the relation of liquor rate to the leveling off of pressure drop curves a t total reflux was confirmed by many other tests. Here again, a liquid seal around the impeller blades permitted the impeller to operate more efficiently as a compressor, at liquor rates between 104.5 and 176.4 lb. per hr.-sq. ft. The necessary liquor rate depends on the design of the radial liquor troughs in the plate, the physical properties of the system being handled, the rotation rate, and the clearance of the impeller above the plate. Aside from liquid seal considerations, at liquor rates below 104.5 Ib. per hr.-sq. ft. where there is no liquid seal, pressure drop curves of Figure 7 remain unchanged with increasing liquor rate.
6 w
s 5 4 P
bz w 3 VI
:0
a 2
0
4
SUPERFICIAL
VAPOR
6 VELOCITY
8 FT/SEC
Figure 6. Pressure drops during distillation of the two-ether system a t total reflux, 30 mm. of Hg, and 1006 r.p.m. show the curve for middle section, probably average conditions, to vary from those of top and bottom sections which were probably not normal
Power Consumption during Distillation
While the coIumn was a t steady state conditions, wattmeter readings were taken for different shaft speeds: using the two-ether system at total reflux and under vacua of 20, 30, and 50 mm. of Hg absolute; also a t fixed reflux ratios and 30 mm. of Hg absolute; and using the chloroform-benzene system at atmospheric pressure and total reflux. Data were taken also during fractionation a t constant liquor rate using the two-ether system a t 1000 r.p.m. and 30 mm. of Hg absolute. Effect of Vapor Rate. The typical effect of vapor rate on power consumption during distillation a t total reflux was shown in many studies using the twoether system and the chloroform-benzene system to be practically unaffected by increasing vapor rate, except at a critical vapor rate, where an abrupt step to a higher value was noted. Power consumption begins a t the minimum power level caused by mechanical friction and remains about constant until the liquid seal is established, where it increases and then levels off again. No other increases in power consumption occurred with increasing vapor rate, and friction masked the effect of varying total pressure. Effect of Rotation Rate. The power required increased rapidly with rotation rate. Effect of Liquor Rate. At high liquor rates, a sharp increase in power consumption occurred, showing that liquor collects in the stages to establish a seal around the impeller. Incremental Power. By plotting the power consumption a t very low vapor rates and also a t high vapor rates under total reflux, two different curves were obtained for a first condition of no liquid
8 W IL
9 6
2 L
24
% 3 2 W 0.
0
I
,
2
,
4
,
,
6
,
,
8
SUPERFICIAL VAPOR VELOCITY FThEC
Figure 7. Pressure drops for each of three stages during distillation of the two-ether system always a t 30 mm. of Hg VOL. 53, NO. 1 0 ,
OCTOBER 1961
783
"sloshin?" or pumping and a second where there was such liquid pumping. The mechanical losses and gas compression power requirements might b e assumed to be the same, and the differences between these two curves thus represented the "incremental power'' due to liquid pumping and may be shown in Figure 8 as a logarithmic plot. T h e upper straight line represents the twoether system a t pressures below 50 mm. of Hg. while the lower line represents the chloroform benzene system at atmospheric pressure.
0.5
3.2
3 L
w 1
50.1
a
2
c
z 0.05
ul
a * " 002
530
200
1000
PCTATION > A T E
2000
RP4
Figure 8. Incremental power (over mechanical losses and gas compression) increases greatly with rotation rate and varies with properties of iiquids being distilled; upper line, two-ether system below 50 mm. of Hg and lower line chloroform-benzene system a t atmospheric pressure
L 40 LL
0
w c
2 20 C
Correlation of Data Gas rate, rotation rate, vane angle, and number of stator vanes were seen to have marked effects on the pressure losses through the stages of the vacuumcompression fractionator. Pressure drop may be related to rotation rate by an orifice form of the equation developed by Rogers and Thiele ( 7 7 ) ;simplified to : H = C'u* (1) As shown previously by White and Othmer (79) and others for packed columns, Equation 1 shows the pressure results to correlate linearly with vapor rate on a logarithmic plot. Next, V V was related to vapor rate. Equation 2 for calculating pressure drop is well known (76).
8 3
A
10
200
600
400
2000
IO00
Also, the relationship of work pressure drop has been shown (2).
Combining Equations 2 and 3, k E = C' - Au3p2 200
600
400
g
2000
IO00
to
(4)
From Stepanoff (18), the Tvork production by a turbo machine on a gas kuApa2r2 E=-
(5)
g
Equating the expressions for work in Equations 4 and 5, F k r2 C' .4u3p2 = - Aupa2r2 or uz = - a2 ( 6 )
C 200
400
600
i3C0
, I
200
I
l
l
l
l
,
l
400 600 I000 ROTATION RATE RP.M
2000
,
I
D
2000
Figure 9. Vapor rate which results a t zero pressure drop usually passes through a maximum a t increasing rotational rates of impeller a t any given vane design and angles indicated: A. Helical vanes, angle of generation of helix with the horizontal. B. Cylindrical segment vanes, angle indicated i s that of cylindrical axis with impeller tip tangent. C & D. Straight vanes, angle indicated i s that subtended b y vanes a n d impeller blade tip tangent.
784
g
g
C'P
Thus the vapor rate a t zero pressure drop seems to be related linearly to the rotation rate, Figure 9 sh0.rz.s a logarithmic plot for flat cylindrical and helical vane surfaces. 'The results fall smoothly on parabolic cun'es for the several vane shapes, Below 400 to 600 r.p.m., the curves are substantiall? straight for all vane shapes, as indicated by Equation 6 ; but at higher rotation rates, the parameters pass through maxima and then decrease. I n general, values for flat vane surfaces decrease gradually Lvith increasing rotation rate beyond the maxima, T h e parabolic parameters for cylindrical vane surfaces are more sharply defined, while the skewed parabolic parameters for helical vane surfaces decrease rapidly with increasing rotation rate for rates above 1000 to 1500 r.p.m.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Other effects related to rotation rate must cause departure from the expected linearity indicated by Equation 6 . As the impeller spun faster? loss in efficiency was caused by increasing turbulence in every part of the vapor path which had not been designed for minimum losses. T h e narrow vapor entrance port and lack of streamline sections-e,g., no conical impeller h u b for guidance for suction vapor flowcaused a friction loss in impeller efficiency a t the entrance to the impeller blades. Unpolished blade surfaces caused another decrease in compressor efficiency. Minor dimensional differences in the diffuser assemblies caused turbulence a t the blade tips, contributing another loss in efficiency. Windage from the spray struclure on top of the impeller probably contributed major interference a t higher rotation rates, and the interfering turbulence increased exponentially Lvith rotation rate. I t is well known that turbomachinery becomes efficient as the result of many empirical modifications. Presumably inefficiencies could have been minimized by improved design; however, the turbulence from liquor contacting the vapor was probably controlling and is needed for the desired mass transfer efficiency. Vane Angle. Selection of the optimum discharge diffuser was made by cross-plotting the maximum points from the curves exemplified by the helical vanes in Figure 9 at constant vane angle. The vane angle for helical surfaces refers to the angle of generation of the helix \vith the floor of the contacting stage. Pressure Drop during Distillation. Pressure drops during fractionation were obtained using the two-ether system at 20, 30, and 50 mm. of Hg absolute and the chloroform-benzene system at 760 mm. As explained before, the pressure drop behavior of the unit during distillation may be characterized from the vapor rate a t zero apparent pressure loss: u,. Pressure drops increase with vapor rate and total pressure and decrease with increasing rotation rate. A decrease in apparent pressure drop with rotation rate means an increase in u,. As shown above, the vapor rate at zero pressure drop follows linearly the rotation rate (Equation a), partially confirmed by Figure 9. However, windage from the impeller and other interference effects probably caused departure from linearity above 600 r.p.m., with a sharp maximum. Figure 10, a plot of values of u,: the vapor rate a t zero pressure loss against rotation rate, shows a similar linear relation for each constant pressure for the two-ether system. Above 1000 r.p.m.: turbulence or other interference effects again caused departure from straight lines. Figure 9: for values of u, using 10" helical vanes in the air-water system, showed a better correlation with the
VACUUM COMPRESSION I
1
" l " 1
Pressure Drop Comparison F-factor = 1.0
Itotation
Type of Tray 18 inch dia. sieve, l / g inch holes (6.7% free area) 18 inch dia. bubble cap, 41/4 inch caps 8 ft dia. bubble cap, 6 inch caps 21/2 ft. dia. bubble cap, 3 inch caps 2 ft. dia. bubble cap, 11/~inch caps 27-inch ripple 10 ft. dia. bubble cap, 3 inch caps 8 X 2 inch Kaskade trays 1-inch Raschig rings in 10 inch n.p.s. tower atL = G 66 inch Uniflux Tower
System Acetic acid-water
Pressure 1 atm.
Pressfire Drop/Stage 2 . 5 mm. Hg
Acetic acid-water
1 atm.
5 . 2 mm. Hg
Gas oil-lube dist.
50 mm. Hg
2.4 mm. Hga
Light ends-gasoline Acetone-benzene
200
6.4 mm. Hg
... ...
Air-water Ethyl benzenestyrene Methylcyclohexane-toluene Acetone-air-water
1 atm. 3.2 mm. Hg 56 mm. Kg 2.0 mm. Hg
... ...
Dry plate Absorption oil inch liquid over weir n-butyl etherdichloroethyl ether
mm. f t . for the two-ether system data against rotation rate. There was little deviation of points from the straight lines plotted, again confirming Equation 6. T h e following equations represent these lines for the two-ether system:
T
I
2
-
in
-
l
:: m
5
2
2
-
4
t
5
W
3
3.
(7a)
WO.6a
(30 mm. of Hg pressure)
m
3
W0.m'
(20 mm. of Hg pressure)
P ~ 0 , - = 0.00522
...
0. I
p.s.i.a.
300
1 atm.
2.4 mm. Hg
1 atm.
1.0 mm. Hg/
(7b)
l
1 atm.
4 . 7 mm. Hg
1 atm.
6.8 mm. Hg
...
...
300 1000 1520
load, static and running seal, vapor and liquid
P T
= 0.0124
~0.-
Wo.451
(7c)
Also, the exponents in Equations 7a, 7b, and 7c correlate on'a straight line when plotted against l/PO.G7 on arithmetic scales. 0 0268 p0.67
or by combining
8X
uo =
PO.6
TW
+ 0.300
(8)
T h e help of Hoffmann-LaRoche, Inc., and Godfrey L. Cobot, Inc., is gratefully appreciated, as is also the early interest of E. G. Scheibel, a t that time Chief Chemical Engineer at Hoffmann-LaRoche, Inc. Nomenclature
A = cross sectional area, sq. ft. C' = coefficient, a constant
('e
-!- 0.30)
D (9)
Values of u, calculated by Equation 9 correlated with experimental results within 7.2% average deviation. Discussion. I t was shown that u, increases linearly with rotation rate until interference effects apparently cause a decrease in capacity at higher rotation rates; an empirical equation was derived relating u, to P, T , and VV.
E F H
L P T W
f g
k I
r
'
-
20
T h e pressure drop results for distilling equipment are made best a t the same velocity head or F-factor. Accordingly, the pressure drop results measured during fractionations of the two-ether system at 30 mm. of H g absolute, total reflux, and rotation rates of about 300, 1000, and 1520 r.p.m. have been compared in the table with results from the literature for sieve trays (Q), bubble cap trays (6, 9, ? I , 76), ripple trays ( 8 ) , Uniflux (72) and Kaskade trays ( I 7), as well as packed towers (74), all at 1.0 F-factor and usually a t total reflux. T h e systems reported petroleum fractions, water, ketones, and aromatics; the distilling pressures varied from 50 mm. of H g absolute to 200 p.s.i.a. Acknowledgment
(50 mm. of Hg pressure)
Exponents =
1000 2000 RATE R P M .
Figure 11. UP/T is seen to b e given linear plot against rotation rate on logarithm paper
... ...
30 mm. Hg 0.117 mm. Hg 30 mm. Hg 0.081 mm. Hg 30 mm. Hg 0.045 mm. Hg
600
ROTATION
...
5 . 0 mm. Hga
93 p.s.i.a.
u
I . .
ft.
experimental data than the u, data from fractionation tests with the same vanes in Figure 10. Because composition (air), pressure, and temperature were always constant during the air-water pressure drop studies, while composition and temperature varied with each distillation, a density correction term (pressure/ temperature) was used. Figure 11 is a logarithmic p l o ~ of the quantity
T
&I3
Ref.
0.06
10 inch dia. vacuum Compression Contacting stage (bottom) a Calculated from operating data on vapor densities, and cap specifications.
P ~ 0 , - = 0.00252
Rate, R.P.M.
Figure 10. Vapor rate which results a t zero - pressure drop i s seen to - b e lower a t increasing pressures (mm. of Hg) - during vacuum distillation - of two-ether system
u
uo p w
diameter in feet energy or work of rotation, ft. Ib. sec. = superficial F-factor = upO.5 = pressure drop or loss, inches of water = length in feet = pressure in mm. of H g = temperature in Rankin = rotation rate in r.p.m. = fanning friction factor = gravitational constant, 32.16 ft./ sec2 = a constant = radius of impeller, ft. = superficial gas or vapor rate, ft./ sec. = gas or vapor rate a t no pressure loss, ft./sec. = vapor density, lb./cu. ft. = angular velocity, radians/sec.
= =
literature Ciied See companion article which follows.
RECEIVED for review August 10, 1961 ACCEPTED August 11, 1961 Presented at A.1.Ch.E. International Meeting, Mexico City, June 1960.
