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Structured Packing Performancesexperimental Evaluation of Two Predictive Models

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Structured Packing Performancesexperimental Evaluation of Two Predictive Models 1788 Ind. Eng. Chem. Res. 2000, 39, 1788-1796 Structured Packing PerformancesExperimental Evaluation of Two Predictive Models James R. Fair,*,† A. Frank Seibert,† M. Behrens,‡ P. P. Saraber,‡ and Z. Olujic‡ Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712, and Laboratory for Process Equipment, Delft University of Technology, 2628 CA Delft, The Netherlands A comprehensive total reflux distillation study of sheet metal structured packings was carried out with the cyclohexane/n-heptane test mixture. The experiments covered a wide range of pressures, two corrugation angles, two surface areas, and two surface designs. Experimental results include pressure drop, capacity, and mass-transfer efficiency. The database generated has been used to evaluate generalized performance models developed independently at The University of Texas Separations Research Program (SRP model)) and at Delft University of Technology (Delft model). This paper reports the experimental results and compares them with predictions from the two models. Deviations of predictions from measurements are discussed in terms of likely contacting mechanisms in the packed bed. Introduction consistent data. And, of course, the data had to be made available to the public if studies could be continued by Corrugated structured packings are used widely in others. distillation columns. Early versions were fabricated Recent experiments at the SRP have provided the from metal gauze and had a specific surface area of needed data. To achieve meaningful and consistent 2 3 about 500 m /m . Later versions were fabricated from comparative results, we felt it necessary to use the the more economical sheet metal and had a specific packings of a single manufacturer. To this end, the 2 3 surface area of about 250 m /m and a corrugation angle Julius Montz Co., of Hilden, Germany, agreed to furnish (with respect to the horizontal) of 45°. This geometry a set of packings with two corrugation angles (45° and became a rather standard design of several manufactur- 60°), two surfaces (shallow embossed, nonperforated and ers/vendors, and most of the reported performance data expanded metal, perforated), and two specific surface 2 3 have been based on tests with the 250 m /m surface areas. The characteristics of these packings are shown and 45° angle. More recently, the manufacturers have in Table 1. This unique data set has been used to begun to offer sheet metal packings with various surface validate SRP and Delft performance predictive models.1-6 areas and a larger corrugation angle to satisfy growing In the present paper we present comparisons between needs for more efficiency or capacity. However, pub- model and experimental values, shortcomings of the lished data on the performance of such structured models, and the potential for their improvement. packings are scarce, and current models for predicting their performance have not been validated adequately. The purpose of this paper is to provide new performance SRP and Delft Models data for sheet metal packings that embody two corruga- A summary of working equations of the SRP and Delft tion angles and different combinations of surface area models is given in Tables 2-4. Table 2 shows expres- and surface treatment, to ascertain how well available sions common to both methods, which include equations predictive models fit the data. for predicting load point as well as the effect of loading A model often used for predicting mass transfer and on pressure drop. Characteristic expressions for the hydraulic performance of structured packings was de- individual models are given in Tables 3 and 4. One veloped several years ago at the Separations Research should note that the expression for the hydraulic Program (SRP) of The University of Texas at Austin.1-3 diameter of a gas flow channel (eq 11) requires liquid The basis for the model is a modified wetted-wall flow film thickness, which is calculated using the appropriate arrangement through the channels of the packing. An correlation from Table 3 or 4, depending on the method alternate model has been developed more recently at used. The same approach is valid for liquid holdup in the Delft University of Technology4-6 and is also founded eqs 6 and 7, to provide effective gas and liquid velocities, on liquid film flow down inclined corrugated plates and and in eqs 14 and 15 for the heights of mass-transfer in addition takes into account explicitly several macro units for the gas and liquid phases. geometrical parameters which can affect packing per- SRP Model. This model (Bravo et al.1-3) has been formance. For a thorough evaluation of predictive modified recently by Gualito et al.7 to better represent models, it has been necessary to await experimental higher pressure performance. Basically, the SRP model work at a semiworks scale using a consistently fabri- considers the bed as a series of parallel, inclined wetted- cated set of packings that could provide internally wall columns with a characteristic cross section and an equivalent diameter equal to the side dimension of the * To whom correspondence should be addressed. E-mail: corrugation. This macroscopic parameter does not ac- [email protected]. count for the effects of proprietary designs of corruga- † The University of Texas at Austin. tions and surface; such are taken into account by using ‡ Delft University of Technology. coefficients regressed from experimental data coupled 10.1021/ie990910t CCC: $19.00 © 2000 American Chemical Society Published on Web 05/06/2000 Ind. Eng. Chem. Res., Vol. 39, No. 6, 2000 1789 Table 1. Characteristics of the Packings Tested surface void fraction corrugation element height channel dimensions (m) 2 3 3 3 (m /m ) (m /m ) angle hpe base height side packing ap (deg) (m) b h s description B1-250 244 0.980 45 0.194 0.0225 0.0120 0.01645 shallow embossed, unperforated B1-250.60 245 0.978 60 0.211 0.0223 0.0120 0.01645 shallow embossed, unperforated B1-400 394 0.960 45 0.197 0.0140 0.0074 0.01033 shallow embossed, unperforated B1-400.60 390 0.960 60 0.196 0.0143 0.0074 0.01029 shallow embossed, unperforated BSH-400 378 0.970 45 0.194 0.0151 0.0074 0.01058 expanded metal, perforated BSH-400.60 382 0.970 60 0.215 0.0148 0.0074 0.01047 expanded metal, perforated Table 2. Common Equations, SRP, and Delft required input variables: R, , h, b, hpe, npe, FG, FL, µG, µL, σ, DG, DL, λ, and FG (A) Packing Geometry and Flow-Related Parameters height of the packed bed: hpb ) npehpe (1) installed specific surface area of packing: ap ) 4s/bh (2) corrugation side length: s ) x(b2/4) + h2 (3) superficial gas and liquid velocities: ) F (4) uGs FG/x G uLs ) uGs(FG/FL) (5) effective gas and liquid velocities uGe ) uGs/[(1 - hL) sin R] (6) uLe ) uLs/[hL sin R] (7) for liquid holdup, hL, different expressions are utilized by SRP and Delft methods. (B) Pressure Drop full operating range: ∆p ) ∆p Fload (8) (∆z) (∆z)preload pressure drop enhancement factor for the loading region: 2/sin R 2 0.13 FG uLs F ) 3.8 (9) load ( ) ( 2 ) F G,lp gdhG loading point F-factor, i.e., the point of departure from preloading conditions: - u F 0.25 0.5 F ) 0.0532gd (F -F ) Ls L (sin R)1.15 (10) G,lp ( hG L G ( xF ) ) uGs G hydraulic diameter of triangular gas flow channel: 2 (bh - 2δs) bh - 2δs 2 bh - 2δs 2 0.5 bh - 2δs d ) { }/{ + + } (11) hG bh [( 2h ) ( b ) ] 2h (C) Mass-Transfer Efficiency HETP ) [(ln λ)/(λ - 1)]HTGGo (12) HTUGo ) HTUG + λHTUL (13) HTUG ) uGs/(kGae) (14) HTUL ) uLs/(kLae) (15) stripping factor for total reflux (L/V ) 1) situation: 2 λ ) m )Rlk/[1 + (Rlk - 1)xlk] (16) with a friction factor for dry pressure drop. The effect Delft Model. This model4-6 utilizes zig-zag triangu- of liquid flow on pressure drop utilizes liquid holdup as lar flow channels with a corresponding hydraulic diam- a major variable, directly related to average film thick- eter at crossings of corrugations. The open side at a ness and effective surface area. Because the film thick- crossing of corrugations is an interface between two gas ness depends on the effective gravity, the calculation flows and is responsible for a considerable loss of energy. procedure is implicit. A pressure drop of 10.25 mbar/m Friction at the gas-liquid interface is a much smaller is generally recommended as a flooding point pressure contributor to pressure drop. Sharp bends between drop, which in some cases has to be increased to about elements also contribute significantly to pressure drop. 20 mbar/m to ensure convergence. The spreading ten- Importantly, the model is based on complete wetting of dency of the liquid is accounted for by the contact angle the metal surface; thus, liquid holdup is determined between the liquid and the particular surface used. from the packing area and the average film thickness. Enhancement of the surface by texturing is represented This approach gives lower values of holdup than those by a correction factor based on laboratory tests. Because given by the SRP method (see Figure 1). an experimental value of the correction factor was not available for the packings used in this study, an Experimental Method assumption/interpolation was made (Fse ) 0.35, see eq 27) on the basis of a comparison of surface character- The experimental test system at SRP has been istics of packings of this study and other packings from described in previous papers.3,8 Briefly, it comprises a the SRP database.
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