Browsing by Author "Franken, Hendrik Hermanus"
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- ItemEstablishment of a supercritical pilot plant and the hydrodynamics of supercritical countercurrent columns(Stellenbosch : Stellenbosch University, 2014-12) Franken, Hendrik Hermanus; Schwarz, C. E.; Knoetze, J. H.; Stellenbosch University. Faculty of Engineering. Department of Process Engineering.ENGLISH ABSTRACT: Supercritical fluids are enjoying ever increasing popularity as a solvent medium for extraction, stripping and absorption processes. Being readily tuneable and able to achieve sharp, highly efficient separations, supercritical fluids present an attractive alternative to traditional solvents, while using less intrinsically harmful compounds. Although the potential of supercritical fluids as solvents have been known for more than a century, there are still several areas of uncertainty, one being the hydrodynamics of extraction columns operating under supercritical conditions. This shortcoming can be attributed to the satisfactory performance of modified standard hydrodynamics to approximate column design, along with a predominant culture of overdesign in process engineering. Even though modified subcritical hydrodynamic models provide a good approximation they do not successfully predict the effect of changes in density, viscosity and surface tension of a supercritical fluid, leading to inaccuracies in column design. This study investigates the state of hydrodynamics under supercritical conditions in counter current packed columns discussed in literature, identifies shortcomings in existing literature and devises a way of addressing the said shortcomings. The primary objective of this study is to establish a multipurpose supercritical pilot plant capable of measuring hydrodynamics under supercritical conditions, followed by the secondary objective of measuring preliminary hydrodynamic data to prove the plant can deliver on its design requirements in measuring reliable hydrodynamic data. During a survey of available literature it was found that very little experimental work has been performed on hydrodynamics under supercritical conditions and especially on random packings. Further it is found that the systems investigated in literature were conducted under conditions of significant mass transfer. As mass transfer directly affects flow rates and fluid properties of the fluids in the column, it is vital to use systems with very little to no mass transfer. This ensures the most accurate approach possible when investigating fundamental hydrodynamic behaviour. Finally it was found that there are no well-defined correlations available for a wide range of packings, fluid properties and hydrodynamic phenomena for columns under supercritical conditions. To remedy the shortcomings in hydrodynamic data it was decided that more pilot plant work is required. It was found that no pilot plants available can measure hydrodynamic data. An investigation was performed into retrofitting available pilot plants, plants used by other research groups and commercially available plants. It was concluded that the best option was to salvage the major parts of an existing old pilot plant and use them to construct a new, customized pilot plant. This provides the opportunity of constructing a custom, multipurpose pilot plant capable of use in future research. After an initial concept design a full design of the new pilot plant was performed. The plant consists of two columns of 17 mm and 38 mm inside diameter and 3.5 m and 1.5 m packed height, respectively, and is capable of pressures and temperatures of up to 300 bar and 200°C. Furthermore the pilot plant can measure liquid hold-up, pressure drop, flooding and entrainment in accordance with the objective of measuring supercritical hydrodynamic data. Liquid hold-up was determined by stopping the process and allowing the column to drain, after which the volume drained was measured. To measure the pressure drop an Endress+Hauser Deltabar S PMD75 DP cell was used. Flooding was determined using the measured pressure drop and volumetric rate of column overheads, from where a hydrodynamically inoperable state is defined. Overall entrainment, although unlikely due to the presence of a demister in the column, was investigated by comparing the column overheads to literature phase equilibria. Preliminary hydrodynamic testing was performed using the 38mm diameter column packed with 1/4” Dixon rings. Testing is performed with at 120 bar and 40°C with a CO2 supercritical phase and polyethylene glycol liquid phase with an average molar mass of 400 (PEG 400). The hydrodynamic data gathered showed expected trends, but showed discrepancy with literature due to differences in liquids used, column packing and experimental system between the respective studies.
- ItemThe hydrodynamics of supercritical packed countercurrent columns(Stellenbosch : Stellenbosch University., 2020-03) Franken, Hendrik Hermanus; Schwarz, C. E.; Knoetze, J. H.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.ENGLISH ABSTRACT: Supercritical fluids are enjoying ever-increasing popularity as a solvent medium for extraction, stripping, absorption and fractionation processes. Although the potential of supercritical fluids as solvents have been known for more than a century, there are still several areas of uncertainty, one being the hydrodynamics of fractionation columns operating under supercritical conditions. Supercritical fractionation columns are readily tuneable and able to achieve sharp, highly efficient separations, presenting an attractive alternative to traditional solvents in specific niche applications. Robust hydrodynamic models are key to the design of supercritical fractionation processes, but no such models are available in the open literature. To create models, investigations into fundamental hydrodynamics are required. Two aspects are of particular concern when investigating hydrodynamics. Firstly, the fluid properties of the respective phases involved must be known. Secondly, mass transfer should be minimised or quantified. No study found in the literature presents hydrodynamics under supercritical conditions with measured, not estimated, saturated fluid properties taken into consideration. This study had the overarching aim to investigate hydrodynamics in countercurrent columns operating under supercritical conditions while laying groundwork for the eventual development of accurate predictive models and design methods. This aim was broken down into five objectives to remove obstacles and generate the needed data to achieve the aim. To address the lack of fluid property data, the first objective was to develop new equipment capable of concurrently measuring the required fluid properties of density and dynamic viscosity. The equipment setup (Pmax 35 MPa; Tmax 393 K) included a variable volume view cell to determine bubble/dew points and density, with a quartz-crystal resonator to measure dynamic viscosity. The equipment was validated, firstly using pure component fluid property measurements with n-dodecane and benzene at 0.1 - 30 MPa and 313 - 353 K and, secondly, using CO2 + ethyl tetradecanoate (ET) for binary phase equilibrium measurement. New data were presented on the dynamic viscosity and density of both saturated CO2 and ET phases. Using the developed equipment, it was possible to evaluate binary systems implicitly for use in supercritical hydrodynamics, addressing the second objective of the study. Two Poly [dimethylsiloxane] (PDMS) fluids graded at 100 cSt and 200 cSt were selected. Saturation pressure was determined for 1 – 70 wt% PDMS in CO2 at 313 – 353 K. The density (~900 – 800 kg.m-3) and dynamic viscosity (~0.7 – 7 mPa.s) were measured at saturation. The selected systems exhibited low mutual solubility and fluid properties in the ranges desired for further work. The third objective, to conduct hydrodynamic pilot plant studies, could be planned and executed using the measured fluid property and phase data. The equipment was operated at 14 MPa and 333 - 323 K to investigate the hydrodynamic operability for a 38 mm diameter column packed with ¼” Dixon rings. The temperature/pressure combination was selected to investigate a wide array of fluid properties while presenting the opportunity to differentiate between the influence of viscosity and density. The pressure drop, liquid hold-up, mass flow and massfractions were measured. Importantly, it was found that liquid hold-up and pressure drop are not reliable indicators of operability in supercritical systems. Three distinct types of inoperability were identified, namely liquid layer flooding, bubble column flooding and entrainment. The influence of the density and dynamic viscosity on hydrodynamics was found to be complex, yet significant. Further, no observable loading operating regime was observed, with the column only operating in the pre-loading or inoperable (flooded/entrained) regimes. The fourth objective was to evaluate three hydrodynamic models for their ability to predict experimental and literature data. The Stichlmair et al. model was able to predict the liquid hold-up after a modification was made to take supercritical fluid properties into account, and the empirical constants were recalculated. The model could, however, not predict the experimental pressure drop or operability limits regardless of any modification. Maćkowiak’s SBD model failed to predict any hydrodynamic properties. The model by Woerlee provided an order of magnitude estimation of liquid hold-up and pressure drop for specific conditions using very little empiricism. The model could not predict flooding at all. None of the models could present an accurate view of the hydrodynamics of the system regardless of attempted adjustments and modifications, with the models displaying different qualitative trends than the gathered experimental data. The final objective tested the literature hypothesis that supercritical hydrodynamics are fundamentally similar to ‘classical’ hydrodynamics. Three conclusions made during this study cast doubt on the fundamental similarity. Firstly, the lack of a detectable loading zone was in contradiction with classical hydrodynamics where the loading zone plays a significant role. Secondly, pressure drop and liquid hold-up were found to be unreliable predictors of operability in the supercritical systems investigated, in contrast to classical systems. Thirdly, the investigated hydrodynamic models cannot predict pressure drop, flooding, or hydrodynamic capacity for the supercritical systems investigated. The thesis presents the following novel contributions: a)Equipment, presenting a novel combination of measurement techniques for concurrent determination of phase equilibria, density and viscosity at fluid saturation,as published in J. Supercrit. Fluids, 133 (2018) 444-454. The publication also contains novel data on benzene viscosity, phase equilibria, and saturated phase density and dynamic viscosity for CO2 + ethyl tetradecanoate. It further presents the first measurement of saturated fluid properties of the supercritical phase found by the author in the literature. b) New data on two PDMS + CO2 systems, 100 cSt and 200 cSt, including phase equilibria,and saturated phase density and viscosity for both phases, as published in J. Supercritical Fluids, 139 (2018) 1-7. Given the lack of saturated fluid properties under supercritical conditions and the complexity of measurement, this represents a valuable contribution, especially being the only measurement of saturated supercritical fluidphase properties found in the literature. c) New data on supercritical hydrodynamics allows various observations and conclusions. Observations include the identification of three different inoperability modes, the lack of an observable loading zone, as well as describing the influence of fluid properties on the hydrodynamic behaviour. Published in part in Chem. Eng. Trans., 69 (2018), with a second publication planned. d) The full evaluation of the Stichlmair et al. model, Maćkowiak’s SBD model and Woerlee’s model against the gathered data showcases the inability of the investigated classical hydrodynamic models to predict supercritical hydrodynamics. This finding, along with other findings mentioned above, highlight the possibility of a fundamental difference between classical and supercritical systems. However, there is insufficient information to make a definite statement.