Experimental Investigation and Correlation of Liquid-Side Mass Transfer in Pilot-Scale Rotating Packed Beds

Abstract

Mass transfer in rotating packed beds (RPBs) is intensified by centrifugal force compared to conventional columns. The resulting significant reductions of equipment size may be advantageous, e.g., for off-shore applications or retrofits. Volumetric liquid-side mass transfer coefficients k L a e are one of the key parameters for modeling. A database of k L a e for pilot scale RPBs, i.e., with radial bed lengths significantly greater than 100 mm, was compiled from the literature and supplemented by own water deoxygenation experiments. Available correlations for k L a e , which are based on lab-scale RPBs mostly with bed lengths below 60 mm, could not adequately describe the data. Hence, a new correlation was developed, which is capable of describing the majority of the data within ±30%. The correlation incorporates effects of liquid as well as gas flow rate and captures the local maximum in separation efficiency, which was observed at intermediate centrifugal accelerations.

Zickzack Packings for Deaeration in Rotating Packed Beds─Improved Rotor Design to Counter Bypass Flows

“…Thus, at very high centrifugal accelerations, the mass transfer in the RPB equipped with the MFP approaches that of the empty-rotor measurements. This is in line with the findings of Hilpert et al for countercurrent deaeration in pilot-scale RPBs. It can be explained by a shortened liquid residence time available for the mass-transfer process in the metal foam packing at higher centrifugal accelerations.…”

“…Despite the fact that mass transfer inside an RPB does not take place exclusively inside the packing, the volume of the packing is commonly chosen as the reference volume for the calculation of k L a . , For better comparability of the mass transfer in the RPB equipped with the different ZZ packings, the same reference volume ( R i , R o , z ) in eq was chosen for all ZZ packings. This way, the difference in k L a values between the three packings under study is solely a function of the total mass transfer in the RPB.…”

“…To achieve maximum mass transfer in the RPB equipped with the ZZ packing, n rot must thus be set much less precisely compared to metal foam packings. This is a significant advantage for operating the RPB, especially at high liquid loads; under these conditions, the maximum in mass-transfer performance in metal foam packings is quite pronounced …”

“…Similarly, this observation was also made in the work of Hilpert et al on deaeration in RPBs with both a metal foam packing and an empty rotor; they found that an increase in k L a with gas-flow rate can be observed at sufficient n rot , possibly due to a combination of increased specific effective surface area and enhanced gas− liquid interaction, and thus mass transfer on a microscale. 42 Furthermore, it has been proven in the literature that the interfacial area increases with the gas flow; examples are a variety of countercurrently operated rotor internals like metal foam packings, 54 wire mesh packings, 14,20,28,55,56 blade supported packings 57 as well as the comparable rotor-stator reactor, 50 and RZB. 58 For all operating points above n pmin , the k L a value using 2SZP was highest among the packings studied.…”

“…Notwithstanding this complex mass-transfer behavior, significant differences in the mass transfer of the same RPB equipped with different packings can be attributed to different packings used. Experimental determination of volumetric liquid-side mass-transfer coefficients ( k L a ) in the desorption process of oxygen from water is a precise and well-established method for quantifying mass transfer in RPBs. , In this work, the average k L a for the total mass transfer in the RPB was evaluated as a measure for the mass transfer specific to each packing. Therefore, an expression was derived using a differential material balance of oxygen in the liquid phase from the inlet to the outlet of the RPB.…”