Ind. Eng. Chem. Res. 1993,32, 2373-2378
2373
Distillation Trays Using Noncircular Perforations Michael W. Biddulph’ Department of Chemical Engineering, University of Nottingham, NG7 2RD, U.K.
Alan C. Burton BOC Group, Technical Center, Murray Hill, New Jersey 07974-1002
J. Terry Lavin BOC Process Plants, Guildford, Surrey GU2 5YH, U.K.
This paper describes an investigation of the possibility of using angled slots in a distillation tray rather than the circular holes used in conventional sieve trays. The incentive to study this was to assess the possibility of using vapor momentum to drive the liquid across chordal weir trays, thus removing the problems of stagnant zones around the edges of trays. The results show that this can be achieved, and that the trays resulting can have low pressure drop performance without weeping. Fundamental bubbling studies are described as well as results from an industrial test column.
Introduction
Principle of Operation
The design of distillation tray columns has changedvery little over many years. The designer still tends to use sieve trays on which the liquid flows across by hydraulic gradient, traversing a “channel” of varying width. However, this unchanging design does not indicate completely satisfactory operation, where no improvement is possible. In fact, it is only recently that designers have realized that conventional trays could operate with higher efficiencies. Over the past 20 years it has become very clear that nonuniform flow across conventional trays has serious consequencesfor performance (Lockett, 1986). It has been demonstrated experimentally (Biddulph, 1986)that when uniform flow is present on a rectangular tray with a 1-mlong flow path, considerably higher tray efficiencies are achieved. It has also been shown that these high tray efficiencies are consistent with expectedand normal values of the point efficiency, and are due to the cross-flow effect (Biddulph, 1986). The incentive for the investigation described in this paper came from a desire to establish a tray on which the liquid is driven positively across at all points on the tray, removing the tendency, present on conventional trays, to flow preferentially across the center of the tray, leaving the sides stagnant or even in retrograde flow. The idea was to use material having angled slots rather than circular holes for the vapor to pass through. Other trays have been developed along these lines, notably the Union Carbide Slotted Sieve Tray (Lockett, 1986). This tray has angled slots punched at various points on a sieve tray to deflect some of the vapor and use the horizontal momentum to drive the liquid across. This tray has been found to improve tray efficiency, but it still has the basic performance characteristics of the original sieve tray. The expanded metal material used in the study described here deflects all the vapor and operates uniformly over the entire tray. Burton (1992) has reported studies on an air/water simulator, diameter 1.8 m, which have demonstrated the removal of stagnant zones. The tray which has resulted from this development has been tested extensively, and the noncircular perforations appear to give it some beneficial operating characteristics, in addition to the provision of uniform liquid flow.
The tray is made of two grids of expanded metal attached together (Biddulph et al., 1992). The lower grid is fine expanded metal, with slots of the order of 1mm wide and 3 mm long, and it is this grid which deflects the vapor to drive the liquid along. However, on its own, this deflection effect imparts too much sideways momentum, and would shoot the liquid straight off the tray. In order to control this, an upper grid of coarse expanded metal is attached to the lower grid, with the strands angled against the direction of flow, and this allows the right amount of driving force to remain. The relationship between the characteristics of the grids is important. A schematic diagram of the tray is shown in Figure 1,and a general top view of the tray is shown in Figure 2. One of the important features of the lower grid material is ita open area, since this determines the pressure drop and some operating characteristics. The open area is a function of the angle to the horizontal at which it is measured. This function has been measured by using a light transmission technique (Burton, 1992). A schematic diagram of the light transmission apparatus is shown in Figure 3. Light is produced by a6-V, 18-Wbulb and passed through a long tube to provide an approximately parallel beam. Below the sample is a 9-mm-diameter lightsensitive resistor to measure the light flux. The light source, tray sample, and light-sensitive resistor are housed within a light-tight box. Because the area of coverage by the light beam is quite small, multiple readings were taken at various places on the sample and average values were taken. The angle of orientation of the sample could be varied to estimate open area as a function of angle of viewing, and this is shown for various “607” (Burton 1992) grids in Figure 4. A maximum value was found to occur at an angle of about 25-30° to the horizontal. (The *607” designation is the catalogue number of Expamet Co., Hartlepool,UK.) The maximum open area of the examples shown in Figure 4 was about 185% ,and other samples tested have had even higher open areas. As will be seen, the expected result of this is a tray with a lower pressure drop than conventional sieve trays with open areas around 8-10 5%. The slot shape and angle of maximum open area are also important characteristics of the lower grid. The important dimensions of the upper grid are the distance between adjacent strands, typically around 20 mm, and the width of the strands, typically around 3 mm.
* To whom correspondence should be addressed.
OSSS-5885/93/2632-2373~04.~0I0 0 1993 American Chemical Society
2374 Ind. Eng. Chem.
Res., Vol. 32, No. 10,1993
Liquid flow
Upper arid
2o
t
t
I
Lower wid
V a w r flow
Figure 1. Principle of operation of expanded metal tray.
I
0 0
20
I 40
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I
SO
SO
I
Angle (degl
I 607M 607R 607C 607T
+ -0....Q.. +
Figure 4. Open area characteristic4 of ‘607” expanded metal. Liquid flow
Figure 2. General view of expanded metal tray.
F4
Light source
rH-1 / /
Lens
Tray sample
/
Light sensitive re6istor To D.V.M.
Figure 3.
ight transmission technique for open arf estimation.
These features determine the amount of control exerted by the grid over the moving biphase. The open area of the upper grid is so high, around 80%, that it does not affect the tray pressure drop. The two grids, when attached together, form a rigid combination which makes up into a satisfactorytray, as strong mechanically as conventional trays. Tray Hydraulics Tray Capacity. One consequence of increasing the hole area (open area) of a conventional sieve tray is that the tray can operate at higher vapor loadings. The penalty for this change is that turndown is compromised, with weeping/dumpingbecomingaseriousprohlem. It hasbeen found that the angled slob of the new tray provide a
resistance to weeping, due to the narrowness of the slots and a surface tension effect, while the higher open area affords higher vapor capacity operation. The result is highcapacity, high-turndown behavior. A similar type of characteristic can probably be obtained by using conventional sieve trays with very small holes, perhaps 1mm in diameter. Some studies have been carried out with such trays (Kalbassietal., 1987)and havedemonstratedslightly improved efficienciesin the methanollwater system compared withlargerholes. Thiswas attheexpenseofslightly higher pressure drops, but these tests were at a constant open area of 8% Such a tray with higher open area would reduce the pressure drop. The equipment used for those tests did not allow observation of the weep point, but it is likely to be lower due to the smaller holes. However, the benefit of the expanded metal tray is in the improvement of liquid flow patterns combined with lower weep point characteristics at very high open area. Industrialscale testa with the expanded metal material on the liquid air system (Urua et al., 1992) have shown an ability to operate over an F-factor range of from 0.2 to 1.6 (based on the superficialvapor velocity)without loss of efficiency. Laboratory testa on an air/water simulator have suggested that F-factors well in excess of 2 are possible with the correct selection of grid characteristics. The reason for the wide range of operation appears to be because, as the vapor loading is reduced, par& of the tray become inactive. However this does not affect the efficiency adversely, unless it becomes too severe, and it does not create weeping of liquid. It is not known at the present time whether or not this might become a problem a t large diameters. This aspect of the behavior of the tray is considered further later in the section concerned with pressure drop. A t the top end of the capacity range, eventually liquid is blown off the tray at high vapor throughput. A typical capacity chart is shown in Figure 5. It can he seen that there are two lower lines instead of the usual single lower limit line. One is the “active limit” line, which is the vapor loading at which the tray just ceases to be completely active over the whole area. The
.
Ind. Eng. Chem. Res., Vol. 32, No. 10,1993 2376 50
2 40
-
E30
-
0
rn
'E
9
1.5
-
607M/2093,
E
I
5
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______----_______------____---Lower Limit (607) 20
40
Us = 1 . 6 2 =/e
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60
80
100
120
140
-
t
160
Weir Loading cc/cm s
U,
0 1
Active Limit Upper Limit .,.#,#
Figure 5. Chart showing range of operation.
I
1.62 i / a
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I
I
I
Weir loading (cc/cm.s) Figure 6. Effect of liquid loading on liquid holdup. 50
bottom line shows the extent to which the tray can, in fact, be turned down while still retaining a good masstransfer efficiency. Liquid Holdup. Measurements of clear liquid head have been made on an aidwater simulator (Burton, 1992). Manometer connections set into the tray floor were used, with the other leg of each manometer connected to the vapor space above the tray. Seven different readings being taken along the 1.2-m flow path length used. The readings of most interest are those for the case of no outlet weir. It has been found that, due to the sideways deflection imparted by the vapor, an outlet weir is detrimental. A weir causes retrograde flow at the outlet end of the tray due to reflection off the weir, so the normal operating mode for these trays is without an outlet weir. The principle of operation of the expanded metal tray is to control the liquid head locally, at every point on the tray, by means of the upper control grid. The origin of the use of an outlet weir on conventional trays is to control the amount of liquid on the tray, and this is now not necessary. A simple correlation of liquid head as a function of liquid and vapor loadings was derived. The usual technique is to use a flow parameter, where
However, the effects of phase loadings were established by using the following forms of correlation: for a sieve tray,
while for the new tray (Burton, 1992),
A comparison of these two equations illustrates how the tray exerts much greater control over the total head of liquid on the tray, and this is taken into account at the design stage. These relationships are plotted in Figures
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1.6
1.8
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2.2
2.4
Superficial air velocity Imls) Figure 7. Effect of air velocity on liquid holdup.
6 and 7. These show that the effect of liquid loading is similar to that on a sieve tray and somewhat lower at low loadings, but that the effect of vapor loading is quite different. The effect of the momentum transfer fromvapor to liquid is clear. One result of this difference is the response of the clear liquid head to column turndown at constant reflux ratio. A conventional sieve tray shows a slight reduction in clear liquid head at turndown, but the expanded metal tray shows an increase in clear liquid head. This is not detrimental since the narrow slots resist weeping, and it may be beneficial in its effect on point efficiency. The comparison is shown in Figure 8. Froth heights and densities have been measured and shown to be similar to those on a sieve tray. Tray Pressure Drop. Traditionally tray pressure drop has been modeled as the sum of three component parts, the dry tray pressure drop, the clear liquid head, and the residual pressure drop, the last comprising surface tension and other effects. The validity of this method of predicting a wet tray pressure drop is questionable, but it is used here as a means of comparing the tray with traditional sieve trays.
2376 Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993
\
\ FLOWCON7ROL
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Figure 8. Effecta of turndown on liquid head. Tabla I
tray type
eff open area
sieve expanded metal tray
0.079 0.089
orifice coeff 1.86 1.68
0
0.2
0.4
0.8 0.8
1
1.2
1.4
1.8
1.8
2
2.2
Vapour F-Factor
Figure 9. Pressure drops of trays.
The dry tray pressure drop may be modeled by a mechanical energy balance on the gas passing through the tray, resulting in the equation (4) which serves as a basis of comparison. In this equation c is the orifice coefficient and j3 is the effective open area. In the case of a conventional sieve tray the effective open area is equal to the actual hole area. For the case of the expanded metal tray, from a mechanical energy balance, the value of B is the maximum value of CP sin 8. (Here @ is the actual open area and B is the angle to the tray floor.) Experimental data from a hydraulic simulator have provided the comparative values of orifice coefficient and effective open area listed in Table I. The data in Table I represent an average 29% drop in dry tray pressure drop compared with a conventional sieve tray designed for the same service. The residual pressure drop is the third component of the total tray pressure drop, and its calculation is surrounded by a degree of uncertainty, particulary for trays with small holes. It is generally agreed that it is a strong function of the surface tension of the liquid on the tray, and the usual expression is
braid = 4uldh~fi (5) For the case of noncircular holes this approach is wholly inappropriate, and it is better to isolate the effect of surface tension by correlating directly by hrmid = kzulpfi (6) Measurements in a small test rig using pure liquids of different surface tension have provided the following expressions for residual pressure drop: for a 1.8-mm hole sieve tray, braid = 188 u/p&; for expanded metal tray,
hrmid = 409aIprg. These results show that the surface tension contribution to totaltray pressure drop is greater for the new tray, but still a small fraction of the tray pressure drop. The total tray pressure drop has been measured (Urua et al., 1992) in a rectangular distillation column using the system methanoVwater under totalreflux conditions. Some of these results are shown in Figure 9, where it can be seen that the tray pressure drop is considerably less than that for a 1.8-mm hole size sieve tray in the same rig. This is to be expected due to the much higher open area. Some other features can also be noted. Firstly, the rate of increase of the pressure drop with F-factor is less for the total pressure drop than for the dry tray pressure drop. This is because of the effect of vapor rate on the liquid head on the tray. Secondly, the rate of increase of pressure drop of the new tray is much less than for the sieve tray. There is some evidence that this is because only a fraction of the slots are allowing vapor to pass under operating conditions. However, the nonbubbling slots do not allow liquid to weep through, as nonbubbling holes on a sieve tray would. The tray appears to operate quite satisfactorily under these conditions. As the vapor loading is increased, more and more slots come into operation, which provides the tray with an in-built flexibility of operation. This behavior has been confirmed by raising the vapor loading to extremely high levels, F-factors of 3.5-4, at which point the slope of the pressure drop dependence suddenly increases, indicating that here all the slots are passing vapor. This characteristic of behavior is the explanation for the extremely wide range of satisfactory operation exhibited by the new tray. Bubbling characteristics were investigated in a singleslot test rig, using a high-speed video camera capable of operating at 10oOframes per second. Bubbling frequencies of about 401s were measured for the slotted tray, compared
Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993 2377 Stage 4 Stage 3 Stage 2 Stage 1 .nose.
Stage 1:
A hemispherical bubble expands and spreads across the surface.
Stage 2:
The bubble ceases to spread across the surface and a "nose' begins to grow where the high velocity jet of gas from the slot impinges on the inner surface of the bubble.
Stage 3:
The expansion stage; the "nose" fills out and the bubble expands radially, though much faster in the direction of the gas flow.
Stage 4:
The detachment stage; a neck forms a1 the base of the bubble as the bubble pulls away from the orifice.
Figure 10. Schematic illustration of bubble growth from an angled slot. 60
0
0.5
V
1
w
F-
1.5 (Fs)
Figure 12. Industrial test column efficiency results.
Mass-Transfer Efficiency 0
1
2
3
4
5
6
7
8
Position across tray
Figure 11. Hydraulic gradients at a weir loading of 75 X lo-' m3/(m 8).
with around 25/s for small-hole sieve trays. This increased frequency must be good for the creation of interfacial area for mass transfer. Bubble growth profiles were observed, and are illustrated in Figure 10, with the various stages noted. These were for single bubbles, and the behavior into a two-phase mixture is unknown, but they show the deflecting effect on the vapor. Hydraulic Gradient. A good illustration of the transfer of vapor momentum to the liquid can be seen in the measurements of the hydraulic gradient. The hydraulic gradient under various loading conditions was estimated by means of the series of clear liquid head tappings along the center line of the 1.2-m-long aidwater simulator mentioned above. The hydraulic gradients at a fixed liquid loading and with increasing vapor loading are shown in Figure 11. The superficial vapor velocity was varied from 1.42 to 2.1 m/s and it can be seen that the hydraulic gradient goes from a normal type, with the liquid deeper near the inlet, to a reverse one at the highest vapor rate. This reflects the momentum transfer to the liquid, and is felt to be a good indication of correct matching of upper and lower grid characteristics. An upper grid with wider openings allows too much momentum to be transferred, resulting in a reverse hydraulic gradient. An upper grid with smaller openings can be too restrictive and allow too great a normal hydraulic gradient. If the grid characteristics are correctly matched, the hydraulic gradient should be zero in the design region of loadings.
Plant tests have been carried out in an air separation plant, and overall column efficiencies measured (Urua et al., 1992) in a column of 1-m diameter. The column containing the new trays was observed to operate stably at all loadings studied. The results illustrate the wide range of operation possible, from about 16% to about 146% of nominal design throughput. Below about 37 % of design throughput, the foam heights were about 2-3 cm. In all these tests a t low temperature, no weeping of liquid was observed. Vapor F-factors in the range 0.2-1.6 were studied, with liquid weir loadings in the range 7-70 ems/ (cm s). Uniform continuous foam was observed at all loadings above 83% of design throughput. At loadings below about 37% of design, areas of clear liquid were observed around the edges of the tray. Figure 12 shows the measured overall column efficiencies, which were well in excess of 100% except for the lowest loading. At this diameter of column, namely 1m, the enhancement factor of tray efficiency over point efficiency is not great, and so these results reflect largely the high point efficiency which comes from the small perforation size. This is also true of small-holesieve trays, but the advantage of the expanded metal tray is expected to increase as column diameter increases, where the benefits of uniform liquid flow should increase.
Conclusion The principle of using noncircular perforations in a distillation tray has been investigated. The high open area, expanded metal material used provides low pressure drop operation, without the penalty of weeping, except a t very low vapor loadings. The operating range is exceptionally wide, and the efficiencies are high. The mode of operation, based on driving the liquid across the tray a t
2378 Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993
all points, removes the problems of hydraulic gradient. This may mean that single cross-flow trays can be used up to large column diameters, avoiding the complication and cost of building multipass trays. The version described here has small dimension slots, and so is likely to be useful for clean ~ystems.Another version with much bigger slots is being developed for potential use for fouling systems. In this case the wider slots do not provide the nonweeping feature of the small-slot version, but the improved flow characteristics and low pressure drop should still be present.
Acknowledgment The authors would like to express their thanks for the support for this project provided by BOC Process Plants, Guildford, UK.
Nomenclature dh
= hole diameter (m)
F, = vapor-phase F-factor ((m/s).(kg/rnV) g = acceleration due to gravity (m/s2) It,l = static liquid head (m) h b = head loss across dry tray (m) hdd = residual head loss (m) U,= superficial gas velocity (m/s) W L= liquid weir loading (mV(m 8)) ?! , = effective open area
c
= orifice coefficient
0 = angle of deflection (deg)
= liquid density (kg/m3) = gas density (kg/m3) u = surface tension of liquid (N/m) = open area (%) 'Ik = flow parameter
p~ p~
Literature Cited Biddulph, M. W. Efficiencies of T r a p in Cryogenic Distillation Columna. Cryogenics 1986,26,24-28. Biddulph, M. W.; Kler, 5. C.; Lavin, J. T. Assignors to the BOC Group plc. US Patent 6 091 119,1992. Burton,A. C. Distillation Tray Hydraulics. Ph.D. Thesis,University of Nottingham, UK, 1992. Kalbaeei, M. A.; Dribika, M. M.; Biddulph, M. W.; Mer, S.; Lavin, J. T. Sieve Tray Efficiencies in the Absence of Stagnant Zones. Znst. Chem. Eng. Symp. Ser. 1987, No.104, A611-A627. Lockett, M. J. Dktillation Troy Fundamentals; Cambridge University Prese, New York, 1986. Urua, I. J.; Lavin, J. T.; Biddulph, M. W. A New High Performance Flow Control Tray. Znst. Chem. Eng. Symp. Ser. 1992, No.128, A345A360.
Received for review January 14, 1993 Revised manwcript received May 3, 1993 Accepted June 28, 1993. a Abstract published
1, 1993.
in Advance ACS Abstracts, September
