Article pubs.acs.org/IECR
Experimental Determination of Bubble Size Distributions in Laboratory Scale Sieve Tray with Mesh Weiwen Wang, Suya Li, and Jianlong Li* College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, People’s Republic of China S Supporting Information *
ABSTRACT: Most applications on sieve tray internals were internals additions for increasing bubble area and changing gas liquid contact form to intensify the mass transfer in a tower. An easy improvement to set up a mesh on the sieve tray was proposed in this paper. It was confirmed that foam layer on the sieve tray acts as the main role of mass transfer. The mesh here could turn large bubbles into such small ones that gas would contact with the liquid sufficiently and also could avoid many small bubbles coalescing into large ones. Meanwhile, interfacial area was enlarged greatly, which was of benefit to mass transfer. In addition, the mesh could make small bubbles have a long stay time in trays, an advantage for mass transfer. In this paper, the gas hold-up and tray pressure drop were determined in an air−water system under isothermal conditions. Mean bubble diameter was measured by CCD in a 0.12 m diameter tower, and bubble radial distributions and probability density functions were also determined. The experimental results proved that mesh could obtain more total gas hold-up and reduce the bubble mean diameter greatly but increased the tray pressure drop severely also. The existence of mesh did not change the trend of bubble flow on the tray. The greatest bubbles still concentrated in the center and gradually reduced to the wall along the radial direction; the small bubbles turned into large ones along the height, and the distribution became wider at higher positions.
1. INTRODUCTION Sieve trays have become widely used since 1950s. Because of low-cost and steady operation, many researchers pay more attention to the plate technology. Generally, distillation and mass transfer could be intensified by equipment modification, quality separating agent addition, and energy separating agent addition.1 The most critical plate technology is internals design that determines the performance of the tower. For example, the emerging F1 float valve caused the revolution of innovation and efficiency of trays. In the 1960s, Linder Sieve Tray was launched by a subsidiary company of Union Carbide Corporation. It was more remarkable that it added a bubbling device and lead holes.2 At the same time, the Multiple Downcomers Sieve Tray,3 which was developed by the same company, was applied instantly due to its high liquid capacity. After that, new and improved plates such as VGMD, ECMD, and EEMD Series Trays were gradually developed. VGMD4 combines a fixed valve and rectangular downcomer. ECMD and EEMD5,6 were added into guide holes and anti-short-circuit plates to smooth the gradient of the liquid surface and to improve tray efficiency. Nye et al. introduced Nye trays, which added sieve pores or floating valves beneath downcomers at the AIChE Spring National Meeting in 1992.7 DJ Series Tray8 was initiated and patented by China. Though it inherited Multiple Downcomers Sieve Tray’s rectangular downcomers, it changed the liquid flow pattern. Nowadays there are three kinds of models, DJ-1, DJ-2, and DJ-3. According to the analysis of liquid velocity distribution on the tray, Nanjing University initiated the 95-type tray. It changed downcomers into the crescent model and added fair water fins on the tray.9 In conclusion, the optimization of the sieve tray structure has been concentrated on three areas: liquid inlet or outlet © 2012 American Chemical Society
modification, downcomers alteration, and internals for changing gas−liquid contact state. All the above improvements would expand fluxes and intensify efficiency but result in trays more sophisticated and expensive. The study of the fluid dynamic behavior in a bubble column was published widely. For example, Lin et al. measured bubble size distribution and local gas holdup.10 Wang et al. measured the liquid velocities and turbulent intensities of air/water two-phase current and countercurrent flows in a bubble column.11 Wang et al. inspected the radial profiles of gas holdup at high gas superficial velocity values.12 Lv et al. regressed the expression among local void fraction, space position, and gas flow based on experimental data.13 Lage and Esposito estimated kinds of models of mean bubble diameter in a homogeneous regime in the bubble column.14 However, the articles about sieve trays had not been sufficiently studied, especially with respect to bubbles. Ding et al. determined bubble properties on the sieve tray with a diameter of 1.2 m.15 Song et al. proposed an accurate, simple mathematical model for predicting gas−liquid interfacial area.16 An advanced online measuring technique which has played an indispensable role at present made the works above come true. The predecessors used the various measuring techniques that include conductivity probe, manometric method, dynamic gas disengagement technique (DGD), photographic technique method, and γ-ray computed tomography (CT). Taking advantage of different conductivity between gas and liquid, the conductivity probe Received: Revised: Accepted: Published: 7067
September 23, 2011 April 25, 2012 May 2, 2012 May 2, 2012 dx.doi.org/10.1021/ie202179d | Ind. Eng. Chem. Res. 2012, 51, 7067−7072
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Article
of a centrifugal pump. The experiments were carried out over a range of superficial gas velocities via holes from 2.31 to 3.15 m·s−1. Superficial gas velocity via holes UG is defined by: UG = QG/AO. Where, QG is gas flow rate and AO is the hole area which equals the single-hole area multiplied by the number of holes. Wet tray pressure drop was determined by U-tube manometer. Measurements were taken respectively above and below a sieve tray 0.03 m, when the indicator was kept level before the experiment. The gas distributor had 55 holes of 5 mm diameter, arranged in a triangular pitch. The sieve tray had 42 holes of 5 mm diameter (fractional holes area on tray is 9.5%). The weir length was 74 mm, and the weir height was 50 mm. The so-called mesh was pictured in Figure 1. The initial bubble diameter
could directly measure bubbles, especially when using a large sample, but data processing could be complicated.17 The manometric method was the usual method used to measure gas hold-up18 which was deduced from pressure difference. DGD could be used in opaque systems.19 Photographic technique with deliberate equipments was employed widely in bubble size determination owing to its simplicity.20 γ-Ray CT was an advanced and noninvasive measurement technique, used to measure the time-average cross-sectional gas hold-up distribution.21 CCD that is adopted in this paper belongs to the area of photographic techniques and has the advantage of leaving the flow field undisturbed. It can transfer optical signal into an electrical signal. Connected with a computer, it can realize the function of image acquisition, storage, processing, and analysis. In our opinion, no study in the open literature has investigated the effect of bubble characteristics on interfacial mass transfer of the precise distillation where heavy constituents concentrations were very low but still not meeting the separation requirements. Therefore, the goal of this present work was to investigate a way of strengthening the gas−liquid contact, especially for the precise separation of the chemical substances with a low concentration or a low relative volatility. In the experimental device, a piece of mesh, which penetrates the foam layer with much smaller size than that of sieve orifice, was arranged on the tray. Strong resistance compelled bubbles to break, updating the interfacial surface. According to the solution penetration− surface renewal model,22 the coefficient of mass transfer Ky would increase and then increase mass transfer efficiency. The mesh exerting the influence on bubble size distribution had been studied, such as radial and axial regularity of bubble mean diameter, and these results were compared with those obtained without mesh.
Figure 1. Photo of mesh.
2. EXPERIMENTAL PROCEDURE The experimental setup was carried out in an organic glass column of 0.13 m in diameter and 0.6 m in height. Scheme 1 is
depends on the inertial force rather than the hole size at high velocity.23 So an ordinary piece of stainless steel mesh was picked up with 0.47 mm side length and 0.14 mm wire diameter. The mesh was placed 0.05 m above the sieve tray and covered the holes area. Figure 2 shows a detailed sketch of teh
Scheme 1. Experimental Flow Charta
1, tank; 2, pump; 3, 12, ball valve; 4, 11, flow meter; 5, mesh; 6, sieve tray; 7, U pressure meter; 8, distributor; 9, liquid outlet; 10, air inlet; 13, fan; 14, CCD camera and computer. a
a schematic diagram used in this work. Air as the gas phase was introduced from the bottom of the column, while tap water was used as the liquid phase was introduced from the top. The liquid from the tank is fed to the downcomer by means
Figure 2. Front view of sieve tray with mesh. 7068
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tray with mesh. The mesh was fixed with five pairs of bolts and nuts for adjusting the height easily. But the stainless mesh was too soft to maintain a level in impulsive force. On the basis of a single laser sheet source and one camera lying in the vertical direction against the laser, the particle image could be an on-line test in the flow field. The highdefinition photos were taken by CCD, and the bubble number and the size were analyzed with the help of INSIGHT 3G software and image-pro Plus 6.0. The mean bubble diameter do was the average length of diameters measured at 2° intervals and passing through the object’s centroid.
Figure 3. Effect of superficial gas velocity via holes on wet tray pressure drop, ■ without mesh, △ with mesh.
3. RESULTS AND DISCUSSION 3.1. Wet Tray Pressure Drop. Figure 3 shows the effect of superficial gas velocity via holes UG on the wet tray pressure drop at a liquid flow rate of 0.0573 m3·h−1. The pressure drops of the mesh sieve tray are much greater than that of the sieve tray, increasing 56% on average, but both of their tray pressure drops followed a similar regularity. The data measured on the sieve tray can be presented in an analytical form Δp = 0.69134 − 0.32123UG + 0.06543UG2, while the mesh sieve tray was presented as Δp = 2.79462 − 1.77638UG + 0.33491UG2. The tray pressure drop increasing instantly indicated gas−liquid contact state transition. It was obviously observed on the mesh sieve tray. So this kind of modified structure is not adequate for the normal distillation due to its high energy cost. It can be only used for certain precise distillation cases. 3.2. Total Gas Hold-Up. The total gas hold-up was measured by change in overall foam layer height compared to the clear liquid height εg = (H − HL)/H, where, H is the overall foam layer height and HL is the clear liquid height. To evaluate the effect on total gas hold-up of the mesh, the clear liquid height HL was controlled at a constant value that is 11 mm. The heights were reported from the visual scales of the tower. For the sieve tray, the gas hold-up was found to be more sensitive to superficial gas velocity via holes. Its polynomial fit was: εg = 0.50194 − 0.09456UG + 0.03332UG2. An average increase of 2.54% was observed in the total gas hold-up using the mesh whereas the mesh
Figure 4. Effect of superficial gas velocity via holes on total gas hold-up at clear liquid height 11 mm, ■ without mesh, △ with mesh.
Figure 6. Effect of superficial gas velocity via holes on probability density functions at clear liquid height HL = 0.011 m, with mesh.
Figure 5. Radial profiles of the mean bubble diameter at UG = 3.03 m·s−1, HL = 0.011 m. (a) ■ z1 = 0.02 m, ○ z2 = 0.05 m, △ z3 = 0.08 m, without mesh. (b) ■ z1 = 0.02 m, ○ z2 = 0.05 m, △ z3 = 0.08 m, with mesh. 7069
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Figure 7. Different axial heights distributions of probability density functions without mesh at UG = 3.03 m·s−1, HL = 0.011 m. (a) ■ z1 = 0.02 m, ○ z2 = 0.05 m, △ z3 = 0.08 m. (b) ■ z1 = 0.02 m, ○ z2 = 0.05 m, △ z3 = 0.08 m.
broke big bubbles into smaller ones which were suffering less buoyancy and remaining more time in liquids. It is noteworthy that total gas hold-up of the mesh sieve tray increased moderately, εg = 0.00519 − 0.3224UG + 0.05063UG2, as shown in Figure 4. 3.3. Mean Bubble Diameter. Figure 5 shows the effect of axial position on the mean bubble diameter radial profile for the studied air−water system at a superficial gas velocity via holes of 3.03 m·s−1. Plane of the tray was defined by z = 0 m. This work chose three different heights to determine the bubble distribution: z1 = 0.02 m, z2 = 0.05 m, and z3 = 0.08 m. Obvious bubble coalescence occurred in the center of the tray when axial height increased. However, a significant decrease was found at z2 = 0.05 m of the mesh sieve tray. This illustrated that the mesh managed to break big bubbles. For example, it is clear by how the mean bubble diameter was affected by radial position at z1 = 0.02 m. With increasing r/R, the mean bubble diameter decreased, indicating that the minimum value of the bubble presented in the wall region. Similar trends were found at two other heights, but the radial distribution of the bubble mean diameter had a tendency to be more uneven with increasing height. The reason is that the resultant force of shear force, surface tension, buoyancy, and static pressure was different everywhere. Three orders polynomial fits of mean bubble diameter were as follows:
For mesh sieve tray: z1 = 0.02 m,
⎛r⎞ ⎛ r ⎞2 do = 2.3595 − 0.8029⎜ ⎟ − 1.40125⎜ ⎟ ⎝R⎠ ⎝R⎠ 3 ⎛r⎞ + 1.56139⎜ ⎟ ⎝R⎠
z 2 = 0.05 m,
⎛ r ⎞2 ⎛r⎞ do = 2.20587 − 3.44378⎜ ⎟ + 5.08863⎜ ⎟ ⎝R⎠ ⎝R⎠ ⎛ r ⎞3 − 3.09961⎜ ⎟ ⎝R⎠
z 3 = 0.08 m,
⎛r⎞ ⎛ r ⎞2 do = 3.41317 − 5.27297⎜ ⎟ + 5.23744⎜ ⎟ ⎝R⎠ ⎝R⎠ ⎛ r ⎞3 − 1.87896⎜ ⎟ ⎝R⎠
The range of mean bubble diameter was of 20 mm to 57 mm on the sieve tray while it dropped down to 1 mm to 3.5 mm on the mesh sieve tray. And the smaller bubble size was good for interfacial mass transfer. In summary, it has been experimentally proven that the mesh substantially affects the bubble characteristics. 3.4. Mean Bubble Diameter Probability Density Function. The sizes of the bubbles were divided into m intervals by diameter magnitude. The total number of bubbles is N. The mean diameter of the ith (i < m) interval is di and the number is Ni so that the f n(di), probability density functions (PDF), is defined by f n(di) = Ni/N. The bubble size PDF of the mesh sieve tray follows a log-normal distribution as shown in Figure 6. The results clearly show an increase in the mean bubble diameter as the superficial gas velocity via holes increases. The major mean bubble diameter with UG = 2.53 m·s−1 exhibits a value of 2.1 mm, compared to 2.6 mm and 2.8 mm for the case of UG = 2.86 m·s−1 and UG = 3.03 m·s−1, respectively. It is noteworthy that the major range of mean bubble diameter is narrower for UG = 3.03 m·s−1 than for the rest, revealing that higher superficial gas velocity via holes played a major role in bubble breakage. The cross-sectional distributions of bubble probability density functions of the three heights z1, z2, and z3 at UG = 3.03 m·s−1
For sieve tray: z1 = 0.02 m,
⎛r⎞ ⎛ r ⎞2 do = 37.48268 + 18.10068⎜ ⎟ − 90.83248⎜ ⎟ ⎝R⎠ ⎝R⎠ 3 ⎛r⎞ + 54.49515⎜ ⎟ ⎝R⎠
z 2 = 0.05 m,
⎛r⎞ ⎛ r ⎞2 do = 51.30229 − 57.35121⎜ ⎟ + 12.39703⎜ ⎟ ⎝R⎠ ⎝R⎠ ⎛ r ⎞3 + 15.76217⎜ ⎟ ⎝R⎠
z 3 = 0.08 m,
⎛r⎞ ⎛ r ⎞2 do = 57.69667 + 4.0436⎜ ⎟ − 116.6025⎜ ⎟ ⎝R⎠ ⎝R⎠ ⎛ r ⎞3 + 82.40982⎜ ⎟ ⎝R⎠ 7070
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are shown in Figure 7. Figure 7a illustrates that as the axial height increased, the mean bubble diameter increased but the share of main bubble decreased in the sieve tray. Compared the results of Figure 7a,b, it is clear that the mesh made the share of small bubbles increase. It is noteworthy that the variance was lower for the mesh sieve tray case than for the sieve tray, revealing that mesh could make more uniform bubbles.
ASSOCIATED CONTENT
S Supporting Information *
Additional details about the PLIF Test System. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
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4. CONCLUSIONS This paper proposed a novel internal structure that a mesh was set up in the foam layer of the sieve tray to intensify gas liquid mass transfer in a distillation tower. Experiment results of total gas hold-up and tray pressure drop showed that those of the mesh sieve tray were greater than those of the sieve tray, increasing 56% on average for tray pressure drop and 2.54% on average increase for total gas hold up. The CCD method was used to investigate bubble characteristics. Both kinds of trays followed a similar trend for radial distributions. Clearly, the mean bubble diameter for the mesh sieve tray was much smaller than that of the sieve tray. As a simple method of industrial application, further development and discussion of mass transfer are strongly recommended. Once its thermodynamic function is proven, this would spread swiftly in the future distillation applications.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: 0086-532-84022752. E-mail address:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The financial support during this work by Natural Science Foundation of Shandong Province (ZR2009BM011) and Institute of Chemical Engineering of Qingdao University of Science and Technology is gratefully acknowledged.
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NOMENCLATURE AO = hole area, m2 do = mean bubble diameter, mm di = the mean diameter of ith interval, mm f n(di) = probability density function H = total foam layer height, m HL = clear liquid height, m N = the total number of bubbles Ni = the number of ith interval bubbles Δp = tray pressure drop, Pa QG = gas flow rate, m3·s−1 r = radius, m R = radius of the tower, m UG = superficial gas velocity via holes, m·s−1 z = axial height, m εg = gas hold up 7071
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(22) Ma, Y. G.; Bai, P.; Yu, G. C. The Development of Gasliquid Mass Transfer Theory. Chem. Eng. (China) 1996, 24 (6), 7−10. (23) Sherwood, T. K.; Pigford, R. L.; Wilke, C. R. Rates of Mass Transfer at Surfaces with Simple Geometry. Mass Transfer; McGrawHill: New York, 1975; p 232.
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