Froth Heights on Dualflow Trays with a Ternary Azeotropic System of

Apr 7, 2000 - The minimum capacity on dualflow trays was observed at an F number of 0.75 for the water−steam system. Turndown ratios, covering a giv...
1 downloads 0 Views 139KB Size
Download PDF
1430

Ind. Eng. Chem. Res. 2000, 39, 1430-1436

Froth Heights on Dualflow Trays with a Ternary Azeotropic System of Ethyl Acetate-Ethanol-Water Ian A. Furzer* Department of Chemical Engineering, University of Sydney, New South Wales 2006, Australia

Experimental froth height measurements have been obtained on nine dualflow trays in a 150 mm diameter glass column. The froth heights can be predicted from Azbel theory, given the height of clear liquid. A comparison of predicted and experimental froth heights at the boiling point temperature in the water-steam system showed constant clear liquid height over a restricted vapor velocity range. Froth heights observed when distilling a ternary azeotropic mixture of ethyl acetate-ethanol-water showed considerable scatter for nine dualflow trays over an F number range from 0.5 to 2.0. The minimum capacity on dualflow trays was observed at an F number of 0.75 for the water-steam system. Turndown ratios, covering a given range of froth heights, were high for the ternary system at 3.60. New economic conditions in the competitive global environment require a reexamination of dualflow trays for low cost, low tray spacing, and good turndown ratios. Introduction Dualflow trays are sieve trays without weirs and downcomers. They offer a potential increase of 20% in vapor handling capacity over sieve trays with downcomers. This is due to the vapor path occupying the full cross-sectional area of the column. This potential increase in capacity is important in our competitive global economy, when considering increased column throughputs. The normal tray arrangement with weirs and downcomers is well-established for sieve, valve, bubble cap, and other company trays. In this case, the capacity limitations are based on entrainment flooding, when the liquid entrainment has an adverse effect on tray and column efficiency or by flooding in the downcomers. While experimental measurements of the liquid entrainment flow rate lead to numeric evaluation of the reduction in tray and column efficiency, froth heights provide a visual observation of when entrainment effects are likely to become important. Froth heights are therefore not exactly defined, and different observers will produce different results. The major problems are the dynamic nature of the froth-spay mixture, the uncertainty of the boundary between the two-phase and single-phase regions, and whether a small amount of spray should be considered. Fair in Perry and Green1 shows the original entrainment flooding plot with the flow parameters, as well as the capacity factors, for tray spacings from 150 to 900 mm. It is useful to consider the tray spacing as being equivalent to the froth height. This plot applies to a wide variety of trays with weirs and downcomers. Certain derating factors could be introduced for the Fair plot, for different surface tensions (if they are available) and dirty and foaming liquids. It is stated the flooding plot can also be used for dualflow trays if the open area is 20% or greater. Hutchinson and Baddour2 have measured froth heights (tray spacing) on dualflow trays in a column with a diameter of 685 mm, on the air-water system. The data have been replotted as a Fair plot showing the flow parameter and capacity factor with * E-mail: [email protected].

Figure 1. Dualflow tray capacity on a Fair plot in an air-water system.

froth height as a parameter; see Figure 1. The dualflow flooding plot is noted for the high values of the flow parameter. A line is shown in Figure 1 for a typical sieve tray with weirs and downcomers and a tray spacing of 400 mm. The dualflow flooding characteristic is about 30% higher than the Fair plot line at a tray spacing of 400 mm. It is useful to use the Fair plot to estimate froth heights for dualflow trays. An estimate of the froth heights in a dualflow tray has been obtained from the Fair flooding plot for a single component in the column under boiling point temperature conditions at a pressure of 101.0 kPa at total reflux. Figure 2 shows the froth heights for water (steam) at various F numbers in the dualflow column, where the F number is given by

F ) uxFV

(1)

Figure 2 shows the single component froth heights for methanol, ethanol, and ethyl acetate. The dualflow estimated froth height characteristic is a rising froth

10.1021/ie990246g CCC: $19.00 © 2000 American Chemical Society Published on Web 04/07/2000

Ind. Eng. Chem. Res., Vol. 39, No. 5, 2000 1431

Figure 2. Estimated froth heights from the Fair plot at total reflux for single components: steam, ethyl acetate, ethanol, and methanol.

Figure 3. Dualflow tray capacity diagram for liquid and vapor loadings and air-water system.

height with F number, and the estimated froth height of 600 mm occurs when the F number is near 4. The Hutchinson and Baddour2 froth height data on dualflow trays for the air-water system can also be plotted on a liquid mass loading (L′) and vapor mass loading (V′) diagram, with froth height (tray spacing) as a parameter; see Figure 3. The L′/V′ diagram shows the very high liquid loadings that can be placed on a dualflow tray. This identifies the important industrial use of dualflow trays in wastewater treatment and in mass transfer operations involving large values of the L′/V′ ratio. It is useful to plot in Figure 3 the line where the L′/V′ ratio equals 0.620, for the air-water system. This corresponds to an L/V ratio, on a mole ratio basis, of 1 for the air-water system. This would indicate that further froth height data is needed in the region where the L/V ratio is close to 1, to ensure the capacity limitations are not exceeded in the design of distillation columns with dualflow trays, operating in the region where the L/V ratio is close to 1. Figure 4 also shows the froth height line for an L/V ratio of 1, obtained by

Figure 4. Froth height data for dualflow trays in an air-water system.

extrapolating the data in Figure 3 to the L′/V′ equals 0.620 line. This froth height line in Figure 4 indicates that the froth heights could be lower under distillation conditions when operating near an L/V ratio of 1. Lockett3 presents a major collection of research results on froth, spray and froth-spray transitions, and froth heights (dispersion heights) for trays with weirs and downcomers. Most of the results for froth height measurements have been made on the air-water system at room temperature. There is considerable variation in froth height estimation from five sets of experimental results. The estimated froth height of 500 mm occurs when the F number is near 3. Kister4 discusses the dualflow tray by a comparison with sieve, valve, and bubble cap trays. Dualflow trays have the highest capacity, but a low turndown ratio and a reduced tray efficiency. Other experimental results5 have been obtained for a comparison between turbogrid trays and a wide range of tray types. Dualflow trays have a capacity limitation when the froth height reaches the tray above, resulting in liquid entrainment and a reduced tray efficiency. The liquid must flow downward through the holes on the sieve tray as there are no downcomers. This results in a choking effect, leading to a liquid holdup, where vapor liquid contacting can take place. The dualflow trays operate with a positive weeping rate. The unusual feature of the capacity limitation of the dualflow tray is the froth height interaction with the tray above, plus the simultaneous positive weeping liquid flow rate. The high free areas of the dualflow tray have evolved to ensure the restriction on a positive weeping liquid flow rate does not limit the vapor handling capacity of the tray. One major difference between dualflow trays and sieve trays with weirs and downcomers, is the presence in the dualflow case, of a positive weeping liquid flow rate from the tray above, interacting with the froth and spray on the tray below. This type of interaction is absent in a sieve tray with weirs and downcomers operating normally, without weeping. This entry of a downflow of weeping liquid in the dualflow case into the froth-spray mixture may result in a decreased froth height. The competition for both vapor and liquid flow rates through the same holes, results in a dynamic two-phase mixture on the dualflow tray. An important part of this motion

1432

Ind. Eng. Chem. Res., Vol. 39, No. 5, 2000

Figure 5. Liquid volume fraction and Froude number (Azbel theory).

Figure 6. Vertical distribution of the liquid volume fraction (Azbel theory).

is an oscillation of the mixture, permitting a hole to flow vapor or liquid for separate short periods. The frequency of the oscillation increases at smaller column diameters. Azbel Theory and Froth Height. The vapor flow in the holes in the dualflow tray will have an essentially vertical velocity component. There is no vertical vapor flow between the holes immediately above the dualflow tray. In a tray floor zone, which may be from 10 to 20 mm above the tray floor, there will be an expansion of the vapor flow and an interaction with the vapor flows from adjacent holes. The tray floor zone region will be a complex region to analyze, particularly due to additional interactions with the liquid holdup and the competing reverse flow downward through the same holes. Azbel6 theory is concerned with the two-phase, vaporliquid mixture above the tray floor zone region. Azbel theory has the potential to predict the distribution of the liquid holdup in the vertical direction above the tray, but excluding the tray floor zone region. The liquid holdup distribution is often expressed as a distribution of the liquid volume fraction ΦL in the two-phase mixture. Of particular importance is the Azbel theory prediction of the height above the tray floor when the liquid volume fraction is zero. This predicted vertical height is the predicted froth height on the dualflow tray, subject to the assumptions in the Azbel theory. The basis of the theory is the vertical distribution of the potential and kinetic energy of the liquid and gas in the two-phase mixture. The total energy is given by an energy integral covering the height of the froth. A steady-state distribution of the liquid volume fraction is obtained by finding the minimum of the integral, subject to the constraint of a clear liquid (total) holdup, hCL. The solution is given by eq 2, where Fr is the Froude number.

6. An important characteristic is the value of z/hCL when ΦL equals zero.

ΦL ) 1 -

[

Fr/4

]

z {1 + xFr/4} hCL 2

0.5

(2)

Figure 5 shows the distribution of ΦL with Fr, with z/hCL as a parameter. A more familiar vertical distribution of ΦL with Fr number as a parameter is given in Figure

hF ) lim

ΦLf0

{ }

z h hCL CL

(3)

The froth height hF is dependent on hCL and the Fr number. A large number of experimental measurements based on the transmission of ionizing radiation have been used to generate ΦL and z characteristic curves for sieve trays with weirs and downcomers. Azbel theory has the potential to predict ΦL and z curves which match the experimental characteristics above the tray floor zone region. The solution of eqs 2 and 3, when ΦL equals zero, leads to the important result,

hF ) hCL +

[ ] hCL g

0.5

u

(4)

This is a surprisingly simple result, whereby the froth height is only a function of the clear liquid height and the mean or superficial vapor velocity, u. Figure 7 shows this linear relationship of hF with u, for values of hCL ranging from 10 to 100 mm. Xu et al.7 have measured the froth heights on dualflow trays under distillation conditions. They used a column with a diameter of 300 mm and operated both the methanol-water and methanol-2-propanol systems at total reflux. Froth heights for a 20% free area dualflow tray were 150 mm at an F number of 1.65 for the methanol-water system. Figure 7 shows the experimental froth heights under distillation conditions for the methanol-water system, plotted on the Azbel predicted froth height figure. The results indicate a predicted clear liquid height on the dualflow tray from 20 to 50 mm. The experimental results indicate that the clear liquid height on a dualflow tray will change with the mean vapor velocity. Experimental Section An experimental glass column of 150 mm diameter containing nine dualflow trays of 20% free area on 320 mm tray spacing with an 8 mm hole diameter was used to obtain visual observations of the froth heights under

Ind. Eng. Chem. Res., Vol. 39, No. 5, 2000 1433

Figure 7. Azbel plot: Froth height and mean vapor velocity. Data points: methanol-water system.

Figure 9. Experimental dualflow froth heights at total reflux for a water-steam system.

Figure 8. Experimental dualflow column and liquid flow rate measurement equipment.

total reflux conditions (L/V ) 1) at a total pressure of 101.0 kPa. A special facility was incorporated into the pipework to accurately measure the liquid flow rate from tray 9, the bottom tray in the column. Figure 8 shows the liquid, L leaving tray 9 and entering a calibrated volume, with a valve below. Closing of the valve, permitted the measurement of the time to fill the calibrated volume, giving the liquid flow rate. Opening the valve returned the liquid to a thermosiphon reboiler, which, due to the external liquid recycle loop, generated vapor without interruption. Liquid flow rate measurements were only made after the froth heights had been stabilized. It was possible to collect nine data points on froth heights, one from each tray in the column for each experimental run. The F numbers were calculated from the experimental liquid flow rate from tray 9 and the total reflux condition, when L/V equals 1. The first series of runs was based on a single component, water. The boiling two-phase mixture on the nine dualflow trays can be described as a boiling water-steam system. Figure 9 shows the experimental froth heights for 22 runs for tray 9 for the water-steam system. The froth height rises with F number, with a 200 mm froth being observed at an F number of 2.15. An important lower capacity limit has been measured for the 20% free-area dualflow trays when the froth height is 0. This condition

Figure 10. Azbel plot of experimental data points for a watersteam system.

occurs in the water-steam system at total reflux when,

F ) 0.75

(5)

If the turndown ratio, TR, at total reflux is defined as the ratio of the vapor velocities when the froth heights are 200 and 0 mm, then

TR )

u200 u0

(6)

TR )

F200 F0

(7)

For a froth height range in the water-steam system from 0 to 200 mm, the turndown ratio is 2.87 at total reflux. Figure 10 shows the froth height in this system plotted against the mean vapor velocity on an Azbel plot. The experimental froth height measurements for the water-steam system would fit a predicted clear liquid height of 10 mm over a considerable vapor velocity

1434

Ind. Eng. Chem. Res., Vol. 39, No. 5, 2000

Figure 11. Experimental dualflow froth heights for dualflow trays (numbered 1-9) in a water-steam system.

range, before rising to 40 mm at a vapor velocity of 2.80 m s-1. The comprehensive collection of froth height data for all nine dualflow trays in the column for the watersteam system is shown in Figure 11. The scatter in the measurements is relatively small, considering that the froth heights have been determined by visual observations, which are observer-dependent. The turndown ratio for the 20% dualflow trays to cover a froth height range from 50 to 200 mm is 2.25 at total reflux. The second series of dualflow runs was with a ternary mixture of ethyl acetate-ethanol-water. This mixture provides large composition changes throughout the column, and the top tray composition may approach the minimum boiling ternary homogeneous azeotropic composition. The dualflow column was operated at total reflux, where L/V equals 1, and at a total pressure of 101.0 kPa with visual observations being made of the froth height on all trays. The liquid flow rate leaving tray 9 was measured using the calibrated-volume technique. The composition of the liquid on all trays was measured by a calibrated gas chromatograph and reported by Furzer.8 Figure 12 shows the froth heights in the ternary mixture as a function of F number for trays 2 and 9 for 12 runs. Tray 2 shows considerably higher froth heights than does tray 9, the bottom dualflow tray in the column. Tray 2 compositions are closer to the ternary azeotropic composition, and a composition effect may be contributing to the increased froth heights. The lower capacity limit has not been obtained exactly, but for this ternary system, it is near an F number of 0.50. While there is a tendency for the froth height to rise with F number, the scatter in the data makes the trend more difficult to identify. Figure 12 shows the tray 2 froth height approaching 300 mm, whereas the tray 9 froth height is only 90 mm at an F number of 1.80. The maximum capacity of the dualflow column will be obtained on the upper trays when this ternary azeotropic mixture is being distilled at total reflux. The turndown ratio for the dualflow tray with this ternary mixture, to cover a froth height range from 60 to 300 mm, is about 3.60 at total reflux. Figure 13 shows the complete collection of froth height data for all nine dualflow trays for 12 runs for this ternary azeotropic mixture. The principal characteristic

Figure 12. Experimental dualflow froth heights for trays 2 and 9 in a ternary system of ethyl acetate-ethanol-water.

Figure 13. Experimental dualflow froth heights for trays 2-9 in a ternary system of ethyl acetate-ethanol-water.

is the fill of the space with data points, indicating little correlation between the variables. There must be additional variables involved which are excluded from the F number. A similar scatter would be obtained if the data were plotted on an Azbel plot. Discussion and Conclusion The maximum capacity of dualflow trays is a key concept in the effective economic design of a distillation column using these trays. The flooding condition is based on the interaction of the froth on a tray with the tray above. The froth height needs to be less than the tray spacing by including a safety factor of 80%. The froth height is the key variable in setting tray spacing. The other aspect of dualflow trays is the turndown ratio. There is a need for a reliable capacity range to permit capacity adjustments during startup, control, and shutdown. Experimental measurements on dualflow capacity fall into three classes: the air-water simulation, the distillation of a single component at total reflux, and the distillation of mixtures at total reflux.

Ind. Eng. Chem. Res., Vol. 39, No. 5, 2000 1435 Table 1. Turndown Ratios for Dualflow Trays

system water-steam water-steam ethyl acetate-ethanol-water

Figure 14. Froth heights on dualflow trays for data from the following: air-water data from Figure 3, MeOH-water data from Figure 7, water-steam data from Figure 9, and ethyl acetateethanol-water data from Figure 13.

Figure 14 is a composite plot, collecting the experimental dualflow froth height data as a function of F number from the previous figures. In Figure 14, the experimental dualflow data for the methanol-water system at total reflux is shown. Results from froth height measurements on an air-water simulator at L/V equals 1 are shown in Figure 14. Figure 14 also shows the froth heights for dualflow trays in the water-steam system under distillation conditions at total reflux. This set of data generally shows froth heights lower than those from the methanol-water and air-water data. The water-steam data is consistent on several trays in the column, and the minimum capacity could be located when the froth height was 0. Figure 14 groups the scatter of froth height data in Figure 13 into a rectangular region. This rectangular region shows the wide range of froth heights that can be expected when distilling a ternary mixture of ethyl acetate-ethanol-water at total reflux on dualflow trays. The scatter is due to additional unknown variables. There appears to be a composition effect, as the azeotropic composition is approached. It is tempting to consider surface tension to be an important variable. Values of the surface tension of this ternary mixture at the bubble point temperature are unknown and may be affected by impurities. Medina et al.9 have distilled a ternary mixture of cyclohexane-heptane-toluene in a laboratory column of 38 mm diameter, fitted with downcomers and weirs. Surface tensions at the bubble point temperature needed to be calculated in this system, and these calculated results indicated that the froth height was dependent on the composition region due to positive or negative surface tension gradients. They also show the tray efficiency may depend on froth height. Furzer10 has reported on the need for a good thermodynamic model to be fitted to the system, before a tray efficiency could be estimated. The important conclusion that can be reached with this ternary azeotropic system, ethyl acetate-ethanolwater, is that the column capacity is limited at total reflux by froth heights on the top dualflow trays. It remains extremely difficult to estimate froth heights (tray spacing) for dualflow trays for this ternary system.

froth height range, mm 0 50 60

200 200 300

F number, Pa0.5

turndown ratio

0.75 0.95 0.50

2.87 2.25 3.60

2.15 2.15 1.80

Experimental froth height data is essential for a safe design of column capacity. The minimum capacity of the dualflow tray has been observed for the water-steam system at total reflux, when the F number is 0.75. The vapor turndown ratios at total reflux have been estimated for a specified range of froth heights in a manner similar to that in eqs 6 and 7 and are shown in Table 1. Dualflow trays in a 150 mm diameter column at total reflux have good turndown ratios, better than those expected from statements in the literature. Scale-up effects for industrial columns may need further investigation. Dualflow trays need to be considered as alternatives to other trays due to their low cost, reasonable froth heights (low tray spacings), good capacity, and good turndown ratios. Acknowledgment The experimental data were collected by S. Davis, C. Ferns, and H. Kansara. Nomenclature CSB ) capacity factor ) u[FV/(FL - FV)]0.5 ms-1 F ) F number, Pa0.5 FLV ) flow parameter ) (L′/V′)(FV/FL)0.5 Fr ) Froude number ) u2/ghCL g ) gravitational acceleration, m s-2 hCL ) height of clear liquid, m hF ) height of froth, m L ) liquid flow rate, kg mol s-1 L′ ) liquid flow rate, kg m-2 s-1 TR ) turndown ratio ) u200/u0 u ) vapor mean velocity, m s-1 V ) vapor flow rate, kg mol s-1 V′ ) vapor flow rate, kg m-2 s-1 z ) vertical distance, m ΦL ) liquid volume fraction FL ) liquid density, kg m-3 FV ) vapor density, kg m-3 Subscripts 0 ) froth height 0 mm 200 ) froth height 200 mm

Literature Cited (1) Perry, H. R.; Green, D. W. Perry’s Chemical Engineering Handbook, 7th ed.; McGraw-Hill: 1997; Figure 14.25. (2) Hutchinson, M. H.; Baddour, R. F. Ripple Trays - A New Tool for Vapor-Liquid Contacting. Chem. Eng. Prog. 1956, 52 (12), 503. (3) Lockett. M. J. Distillation Tray Fundamentals; Cambridge University Press: 1986. (4) Kister, H. Z. Distillation Design; McGraw-Hill: 1992. (5) Kastanek, F.; Huml, M.; Braun, V. Measuring the Efficiency in a Column of One Metre Diameter. Inst. Chem. Eng. Symp. Ser. 1969, 32 (5), 100. (6) Azbel, D. S. The Hydrodynamics of Bubbler Processes. Int. Chem. Eng. 1963, 3 (3), 319.

1436

Ind. Eng. Chem. Res., Vol. 39, No. 5, 2000

(7) Xu, Z. P.; Afacan, A.; Chuang, K. T. Efficiency of Dualflow Trays in Distillation. Can. J. Chem. Eng. 1994, 72, 607. (8) Furzer, I. A. Distillation in a Ternary Homogeneous Azeotropic System; CHEMECA 98, 28th Australasian Chemical Engineering Conference, Port Douglas, North Queensland, Australia, Sept. 28-30, 1998; 26, 192.4. (9) Medina, A. G.; McDermott, C.; Ashton, N. Surface Tension Effects in Binary and Multicomponent Distillation; Chem. Eng. Sci. 1978, 33, 1489.

(10) Furzer, I. A. Critical Process Modelling in Separation Systems; CHEMECA 98, 28th Australasian Chemical Engineering Conference, Port Douglas, North Queensland, Australia, Sept. 2830, 1998; 26, 192.5.

Received for review April 5, 1999 Revised manuscript received January 3, 2000 Accepted February 7, 2000 IE990246G