Liquid Holdup Profiles in Structured Packing Determined via Neutron

Nov 21, 2013 - packings (Sulzer Mellapak 250.Y, Mellapak 500.Y, and MellpakPlus 752.Y). Experiments were conducted with liquid loads ranging from 5.4 ...
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Liquid Holdup Profiles in Structured Packing Determined via Neutron Radiography Michael Basden,§ R. Bruce Eldridge,*,§ John Farone,§ Esther Feng,§ Daniel S. Hussey,† and David L. Jacobson† §

Separations Research Program, University of Texas, Austin, Texas 78712, United States National Institute of Standards and Technology, 100 Bureau Drive, Mail Stop 8461, Gaithersburg, Maryland 21701, United States

†

ABSTRACT: Neutron scans of an operating air−water contactor were performed at the NIST Center for Neutron Research (NCNR). Averaged radiographs were used to determine local and average liquid holdup profiles in three stainless steel structured packings (Sulzer Mellapak 250.Y, Mellapak 500.Y, and MellpakPlus 752.Y). Experiments were conducted with liquid loads ranging from 5.4 to 48.9 m3/m2·h (2.2 to 20 GPM/ft2) and F-factors ranging from 0.61 to 1.65 Pa0.5 [0.5 to 1.35 (ft/s)(lbm/ ft3)0.5] in a column with an inner diameter (ID) of 14.6 cm (5.75 in.). The average holdup in Mellapak 250.Y is validated against previous X-ray computed tomography (CT) and pilot plant experiments. Local holdup profiles exhibited a strong dependency on vertical position in the column, generating a periodic profile in the bulk of all packings. The largest values for holdup were observed at the interface (joint region) between two layers of packing. The joint region in MellapakPlus 752.Y exhibited lower holdup when compared to Mellapak 500.Y. in MellapakPlus 752.Y via X-ray CT.8 They found enhanced liquid accumulation at contact points between corrugated sheets and in sections surrounded by a wiper band. Janzen et al. utilized ultrafast electron beam X-ray tomography to examine dynamic holdup in Montz B1-350NM and Montz B1500MN.11 Liquid distribution and holdup were found to be time independent below the flooding point. Neutrons were selected for investigation due to the relatively high attenuation of neutrons by lighter elements when compared to stainless steel.13 Figure 1 shows a comparison of the relative neutron and X-ray cross sections of relevant elements in these experiments. X-ray attenuation increases with increasing atomic number, with water attenuating significantly

1. INTRODUCTION Metal structured packings are widely employed in vapor−liquid contacting devices for distillation, absorption, and stripping. The high specific surface areas (ap), regular geometries, and high void fraction (ε) of structured packing allow for low pressure drop and high efficiency in contactor devices. Distillation design equations are semiempirical at best, relying on macroscale mass transfer and pressure drop data to generate the design correlations. A more detailed understanding of the local hydraulic and mass transfer behavior inside a column will help develop new packing designs and more accurate mass transfer and hydraulic predictive models. Various imaging techniques have been applied to gain an understanding of the hydraulic behavior within an operating vapor−liquid contactor packed with structured packing. Early efforts utilized gamma ray absorption to determine holdup in Mellapak [Certain trade names and company products are mentioned in the text or identified in an illustration in order to adequately specify the experimental procedure and equipment used. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the products are necessarily the best available for the purpose.] 250.Y, 250.X, and 500.Y.1 High energy X-ray tomography has been shown to be a noninvasive and efficient method to image the interior of a packed column.2−5 Quantitative measurements of holdup and wetted area have also been found using X-ray computed tomography (CT) scans in a variety of structured packings.6−11 High resolution gamma ray systems have also been used to study the liquid spreading in structured packings.12 Green et al. used X-ray CT to determine local liquid holdup and effective wetted area in an air−water contactor containing Mellapak 250.Y, observing the bulk of the water flowed through the packed bed as a film on the packing surface.6 Aferka et al. examined the spatial distribution of interfacial area and holdup © 2013 American Chemical Society

Figure 1. Relative thermal neutron (25 meV) and X-ray cross sections (100 keV) of relevant elements.13 Received: Revised: Accepted: Published: 17263

August 7, 2013 October 4, 2013 October 16, 2013 November 21, 2013 dx.doi.org/10.1021/ie402574x | Ind. Eng. Chem. Res. 2013, 52, 17263−17269

Industrial & Engineering Chemistry Research

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fewer X-rays than iron. This can lead to poor contrast between lighter elements and the background.14 Neutrons do not exhibit such behavior, allowing for a higher contrast between the liquid films present in an operating air−water contactor than achievable with X-rays. For this reason, neutron tomography was recently used to determine the distribution of liquid film thickness in adiabatic annular flow through a model fuel rod geometry.15,16 Zboray et al. also evaluated the impact of spacer choice on the liquid holdup within the channel and gas core.16

2. EXPERIMENTAL EQUIPMENT 2.1. Neutron Facility. Experiments were conducted at the NCNR on thermal beam tube 2.17,18 A schematic of the imaging system is provided in Figure 2. The neutron beam was

Figure 2. Schematic for packed column and neutron radiography setup. Figure 3. Schematic showing the dimensions of the air−water contactor used for the neutron imaging experiments.

attenuated by the heavy water (i.e., D2O) flowing inside the column, the packing, and the column itself. Neutrons not attenuated were detected by an amorphous silicon flat panel detector with an area of 25 cm × 20 cm. In this study, Radiographs were acquired under a neutron flux of 3 × 107 cm−2 s−1, and an aperture size was chosen to give an L/d ratio of 450. Images were acquired at a frame rate of 30 Hz with a pixel pitch of 0.254 mm, yielding a spatial resolution of roughly 0.5 mm. 2.2. Air−Water Contactor. Experimental conditions required special consideration regarding the materials of construction and process fluid utilized. Due to the high neutron scattering cross-section of hydrogen, plastic columns and packings could not be examined. Furthermore, water with mass fractions above 0.10 H2O could not be used as the process liquid. For this reason, 0.99 by mass and 0.70 by mass D2O were combined for use as the process fluid. The mass fraction of D2O used in the system was tracked, maintaining the mass fraction of D2O above 0.95 in all cases. The schematic for the air−water column used in these experiments is shown in Figure 3. The aluminum column had an outer diameter of 15.24 cm (6 in.) and an inner diameter of 14.61 cm (5.75 in.). The aluminum column was a single 121.92 cm (48 in.) piece of aluminum tubing with a flange welded to the bottom to connect to the aluminum base. The bottom flange had a groove designed to hold an O-ring to provide a water-tight seal. Four pegs were used for packing support. They were 0.3175 cm in diameter and 1.9 cm in length and were welded inside the column at a height of 20.32 cm (8 in.) above

the flange. The aluminum base is the same as that used in previous X-ray computed tomography experiments.6 It provides for two process connections: the liquid outlet and the gas inlet. Air was fed into the column with a 1.27 cm (0.5 in.) tubing connection, where it connected to a 8.89 cm (3.5 in.) diameter sparger type gas distributor. Three types of structured packing were used for the imaging experiments: Sulzer Mellapak 250.Y (ap = 250 m2/m3, ε = 0.987), Sulzer Mellapak 500.Y (ap = 500 m2/m3, ε = 0.975), and Sulzer MellapakPlus 752.Y (ap = 500 m2/m3, ε = 0.975). For Mellapak 250.Y, four half elements with a height of 10.2 cm (4 in.) were packed into the column. For Mellapak 500.Y and MellapakPlus 752.Y, two full elements with a height of 20.32 cm (8 in.) were packed into the column. Additionally, a 5.1 cm (2 in.) piece of Mellapak 250.Y was placed at the top of the bed to help with initial liquid distribution in the Mellapak 500.Y and MellapakPlus 752.Y experiments. All packing elements were rotated 90° with respect to the previous element. Liquid flow rates utilized in this work were 5.4, 25.7, and 48.9 m3/m2·h (2.2, 10.5, 20.0 GPM/ft2). Vapor rates (measured as an Ffactor) were 0.61, 1.22, and 1.65 Pa0.5 [0.5, 1, 1.35 (ft/s)(lbm/ ft3)0.5]. The key difference between Mellapak 500.Y and MellapakPlus 752.Y is the joint geometry. In Mellapak 500.Y, the triangular channels formed from the crimped metal sheets are maintained at the lower and upper ends of the packing. In MellapakPlus 752.Y, the corrugation angle at the top and 17264

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bottom of a packing element are gradually reduced to 0° to form vertical channels. This modification provides for a smooth change of flow direction at the interface between packing elements (i.e., the joint). A diagram showing the difference in geometry between the two is shown in Figure 4.

Figure 4. Difference in joint geometry between Mellapak (left) and MellapakPlus (right). In both images, the transition between packing layers is shown in the center. Images courtesy of Sulzer Chemtech Ltd. Copyright 2000.

2.3. Flow System. The process flow sheet is shown in Figure 5. In-house instrument air at the NCNR was used in all

Figure 6. Sample averaged radiograph for dry Mellapak 250.Y. The black rectangle is the open beam section used for normalization of grayscales when averaging the set of images.

normalization, an average radiograph was generated by computing the mean grayscale of each pixel in a series of images. Averaged radiographs were computed for both the 0° and the 190° cases. A sample averaged dry radiograph is shown in Figure 6. After the dry images were acquired, the column was operated at the various liquid and vapor flow rates detailed above. Experiments were conducted in random order with fifteen minutes between experimental changes to allow the system to come to equilibrium. Once again, the column was rotated from 0 to 190° and backward with additional frames captured at the 0° and 190° angles. A sample averaged radiograph can be observed in Figure 7. Once again, each image was normalized by an open beam section before being averaged to the final radiograph shown. 3.2. CT Results. Projection data were reconstructed using the Octopus reconstruction package from inCT. Sample reconstructed images acquired from the continuous scans are shown in Figure 8. Dry images exhibit feathering artifacts away from the center of the column. The feathering is more severe near the top and bottom of the detector’s field of view. It is believed these artifacts are due to approximating the cone beam as a parallel beam during reconstruction. This could be remedied by sampling a wider range of angles and incorporating the cone angle in the reconstruction process. Dry scans also exhibit a high degree of random noise with magnitudes similar to the features of interest, making it difficult to isolate the packing from the noise via the use of a threshold grayscale. While no quantitative information was derived from the CT scans, reconstructed images from the dry scans were used in locating the elevations of packing features (e.g., perforation location) to aid in the interpretation of radiography results. The irrigated images in Figure 8 show a high degree of aliasing and noise. This is believed to be caused by the dynamic

Figure 5. Process flow sheet for the experimental system.

experiments. Due to the small piping, the maximum F-factor achieved was 1.65 Pa0.5 [1.35 (ft/s)(lbm/ft3)0.5]. Micro Motion F050 mass flow meters were used to track liquid and gas flow rates via a LabVIEW program. Liquid flow was controlled using a centrifugal pump equipped with a variable speed drive. At the top of the column, a bale of Teflon and stainless steel coknit was used as a mist eliminator to prevent liquid entrainment and minimize the loss of heavy water. The distributor feed tube was routed through the center of the mist eliminator to a liquid distributor designed by Koch-Glitsch. In the distributor, drip tubes were used to maintain a liquid head and steady flow rate. The drip tubes had a series of holes drilled at various heights to achieve the desired range of liquid flow rates. Drip point density was 240 holes per squared meter (22.3 holes per squared foot).

3. EXPERIMENTAL RESULTS AND DISCUSSION 3.1. Image Acquisition. Originally, the experiments were designed to acquire neutron CT scans of the operating air− water contactor. Two sequences of dry images were captured as the column rotated 190° forward, followed by image acquisition as the column rotated 190° backward. Additional frames at 0° and 190° were also captured for use in radiography calculations. To account for random variations in reactor output, it is necessary to normalize each radiograph by an open beam section in each image, as indicated in Figure 6. After 17265

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each angle. For a parallel beam, the minimum number of projections to ensure adequate reconstruction is π P≥S (1) 2 where P is the number of projections and S is the number of points per projection line. In practice, S is the number of pixels in a single detector row.19 For current experiments, there were 768 pixels in each row. This would require 1207 projection angles. To reduce noise to acceptable levels, a minimum of 20 integrations (i.e., number of images captured and averaged at a given angle) is recommended based on radiography results. The effect of using an increased number of integrations is shown in Figure 9. In the case of continuous scanning, more random noise is observed. Both reconstructions, however, exhibit feathering near the edges of the column.

Figure 7. Sample averaged radiograph for Mellapak 250.Y operating under air/water flow. Figure 9. Sample reconstructed images for Mellapak 250.Y. (left) Images collected via continuous scanning (1 integration per projection). (right) Images collected via incremental scanning (100 integrations per projection).

3.3. Data Analysis. To account for beam hardening due to the small amount of light water present, a modified version of the Beer−Lambert Law was used for calculations: ⎞ ⎛I −ln⎜ = μtw + βtw 2⎟ ⎠ ⎝ Io

(2)

where I is the intensity measured through the film (e.g., a wet radiograph), Io is the initial intensity (e.g., a dry radiograph), μ is the linear attenuation coefficient (mm−1), β is a quadratic correction factor (mm−2), and tw is the water thickness. Both the attenuation coefficient and the correction factor are functions of the volume fraction of D2O in the system: μ = −0.3564VD2O + 0.4086

(3)

β = −0.0058VD2O − 0.0058

(4)

Using the above equations, the amount of water in each pixel can be computed by multiplying tw by the pixel size. Holdup at each elevation was calculated by summing the amount of water in a row of pixels and dividing by the empty column volume. To validate this method, comparisons between previous data acquired via high energy X-ray tomography and traditional methods (i.e., measuring the volume of liquid drained from the packing after flow to the column was stopped) were made.6 In Figure 10, holdup data from current experiments and two other data sets are shown. The X-ray results are from a 15.22 cm (6 in.) plastic column.6 The other set of data is from the 45.7 (18 in.) column belonging to the Separations Research Program (SRP). It should be noted all data gathered for these

Figure 8. Sample reconstructed images for Mellapak 500.Y. (top left) Dry packing near the top of the detector’s field of view (FOV). (top right) Dry packing in the center of the detector’s FOV. (bottom left) Irrigated packing near the top of the detector’s FOV. (bottom right) Irrigated packing near the center of the detector’s FOV. Irrigated images were taken at a liquid load of 48.9 m3/m2·h (20 GPM/ft2) and an F-factor of 0.61 Pa0.5 [0.5 (ft/s)(lbm/ft3)0.5].

nature of the system and insufficient integrations at each angle during the continuous scans. This would be remedied by using an incremental scanning procedure and several integrations at 17266

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peak in liquid holdup was found at roughly 2 mm above the actual joint. At this point, liquid holdup can be over double of that observed in the bulk of the packing. Liquid holdup profiles in the bulk of the packing also have a periodic profile. The actual period corresponds to the distance between perforations of the packing. When examining reconstructed neutron images of the dry column, the peaks in liquid holdup were found to be slightly above (