Belkacem Benadda

Batch extractive distillation: The process and the operational policies

The batch realisation of the extractive distillation and the role played by the different steps in the process are analysed and on the basis of this analysis several operational policies are presented. The different BED policies are studied by simulation and compared (with the example of the mixture acetone–methanol+water). A method is suggested for the comparison of the different BED operational policies. In a BED process the mole fraction of the more volatile component (A) in the distillate (xd,A) can be influenced not only by the variation of reflux ratio (R) but also by changing the flow rate of the solvent (F). The simulation results are experimentally investigated and verified for each operational policy. The original R=const.policy is modified by shortening the second preparatory step of the BED (R=∞,F>0). The possibilities of performing a constant distillate composition (xd,A=const.) policy are discussed. In order to increase the efficiency of the modified R=const.policy further, it is combined with the xd,A=const. policy, maintaining a high value of xd,A for a longer period during the production of A by gradually increasing R or F or R and F simultaneously. The feasibility of the cycling policy is also studied for the BED.

Interfacial area and volumetric mass transfer coefficient in a bubble reactor at elevated pressures

Abstract The present study deals with the pressure effects on mass transfer parameters within a bubble reactor operating at pressures up to 5 MPa . The gas–liquid systems are N2/CO2-aqueous solution of Na2CO3–NaHCO3 and N2/CO2-aqueous solution of NaOH. A sintered powder plate is used as a gas distributor. Three parameters characterizing the mass transfer are identified and investigated with respect to pressure: the gas–liquid interfacial area a, the volumetric liquid side mass transfer coefficient kLa and the volumetric gas side mass transfer coefficient kGa. The gas–liquid absorption with chemical reaction is used and the mass transfer parameters are determined by using the model reaction between CO2 and the aqueous solutions of Na2CO3–NaHCO3 and NaOH. For a given gas mass flow rate, the interfacial area as well as the volumetric liquid mass transfer coefficient decrease with increasing operating pressure. However, for a given pressure, a and kLa increase with increasing gas mass flow rates. The mass transfer coefficient kL is independent of pressure. Furthermore, the pressure increase results in a decrease of kG and kGa for a given gas mass flow rate. The values of the interfacial area, which are obtained from both chemical systems are found to be different. These discrepancies are attributed to the choice of the liquid system in the absorption reaction model.

Homoazeotropic distillation of maximum azeotropes in a batch rectifier with continuous entrainer feeding I. Feasibility studies

Abstract Our former method for the assessment of the feasibility of extractive distillation in a batch rectifier was extended for the investigation of the separation of maximum azeotropes. The method is based on the calculation of the still path and possible composition profiles of the column sections. The sequence of the separation steps was determined, then limiting values and the influence of the most important parameters were investigated. The feasibility calculations performed to the mixture acetone—chloroform using benzene and toluene as solvent verified the benefits of the continuous feeding of the separating agent.

Feasibility studies for batch extractive distillation with a light entrainer

Abstract Our former method for the assessment of the feasibility of batch extractive distillation (usually performed in a rectifier with a heavy entrainer) was extended to the case where a light entrainer is fed continuously into a rectifier or a stripper. The method is based on the calculation of the vessel path and possible composition profiles of the column sections. The method is demonstrated for both configurations on the examples of separation of minimum boiling point azeotropic mixtures. For the rectifier the most important conclusions were also experimentally verified.

Influence of pressure on the gas/liquid interfacial area a and the coefficient kLa in a counter-current packed column

Abstract The effects of pressure on the volumetric coefficient kLa and the interfacial area a in a counter-current packed column were studied in the pressure range 105 to 12 × 105 Pa. The method of gas/liquid absorption with chemical reaction was applied. The influence of the gas/liquid system on the interfacial area was also studied using three different chemical systems. It is shown that a and kLa decrease when the total pressure is increased. The authors have attempted to explain the differences between the values of a obtained under atmospheric pressure when the chemical system is changed.

Influence of Pressure on the Hydrodynamics and Mass Transfer Parameters of an Agitated Bubble Reactor

The present study deals with the pressure effects on the hydrodynamic flow and mass transfer within an agitated bubble reactor operated at pressures between 10 5 and 100x10 5 Pa. In order to clarify the flow behavior within the reactor, liquid phase residence time distributions (RTD) for different operating pressures and gas velocities ranging between 0.005 and 0.03 m/s are determined experimentally by the tracer method for which a KCl solution is used as a tracer. The result of the analysis of the liquid-phase RTD curves justifies the tank-in-series model flow for the operating pressure range. Good agreement is obtained between theoretical and experimental results assuming the reactor is operating as perfectly mixed. Two parameters characterizing the mass transfer are identified and investigated in respect to pressure: the gas-liquid interfacial area and volumetric liquid-side mass transfer coefficient. The chemical absorption method is used. For a given gas mass flow rate, the interfacial area as well as the volumetric liquid mass transfer coefficient decrease with increasing operating pressure. However, for a given pressure, a and k L a increase with increasing gas mass flow rates. The mass transfer coefficient k L is independent of pressure.

Validation of the gas-lift capillary bubble column as a simulation device for a reactor by the study of CO2 absorption in Na2CO3/NaHCO3 solutions

Abstract Validation of the gas-lift bubble column, a new laboratory device, has been achieved by studying the chemical absorption of carbon dioxide into carbonate/bicarbonate solutions. This reaction has been conducted over a range of temperatures extending from 293.2 K to 343.2 K, and using the hypochlorite ion as a catalyst, with corresponding NaClO molarities ranging from zero to 0.2 M. The Danckwerts model was used.The surface renewal rate s , the liquid-side mass-transfer coefficient k L and the rate constant K 2 wer determined. During the hydrodynamic study, the gas-liquid exchange area was evaluated by a photographic method.

Experimental Study on a Co-current Gas–Liquid Down Flow Contactor with Gas Entrainment by a Liquid Jet

Two-phase flow co-current vertical downflow reactor with gas entrainment by a liquid jet is investigated in an air-water system. Experiments are carried out in order to clarify the flow behavior of the reactor under various conditions. Gas entrainment flow rates and gas holdup are quantified experimentally and their dependency on the liquid jet flow rates are shown. The experimental program also included determination of liquid phase residence time distribution (RTD) characteristics for different liquid jet flow rates. The result of the analysis of the liquid phase RTD curves justified the tank-in-series model flow for the liquid phase. On the basis of these analyses, the reactor hydrodynamics are modeled by the tank-in-series model including dead zones. Good agreement is obtained between theoretical and experimental results assuming the reactor is operating as perfectly mixed. The volumetric mass transfer coefficient k L a L is determined experimentally by a gasing out method. The interfacial area is deduced from the bubble diameter measurements which are determined by visualization experiments.

Modelling mass transfer during venting/soil vapour extraction: Non-aqueous phase liquid/gas mass transfer coefficient estimation

We investigate how the simulation of the venting/soil vapour extraction (SVE) process is affected by the mass transfer coefficient, using a model comprising five partial differential equations describing gas flow and mass conservation of phases and including an expression accounting for soil saturation conditions. In doing so, we test five previously reported quations for estimating the non-aqueous phase liquid (NAPL)/gas initial mass transfer coefficient and evaluate an expression that uses a reference NAPL saturation. Four venting/SVE experiments utilizing a sand column are performed with dry and non-saturated sand at low and high flow rates, and the obtained experimental results are subsequently simulated, revealing that hydrodynamic dispersion cannot be neglected in the estimation of the mass transfer coefficient, particularly in the case of low velocities. Among the tested models, only the analytical solution of a convection-dispersion equation and the equation proposed herein are suitable for correctly modelling the experimental results, with the developed model representing the best choice for correctly simulating the experimental results and the tailing part of the extracted gas concentration curve.

Validity of the Use of the Mass Transfer Parameters Obtained with 1D Column in 3D Systems during Soil Vapor Extraction

AbstractThe efficiency of soil vapor extraction (SVE) depends primarily on mass transfer parameters. The objective of this work was to determine whether mass transfer parameters obtained from calibrating a model against unidirectional column experiments could be used for three-dimensional (3D) applications. A 3D experimental setup was designed and constructed to simulate a decontamination system at the bench scale. A mathematical model simulating multiphase flow and multicomponent transport of contaminants in the subsurface under nonequilibrium transient mass transfer conditions was developed and used. Results showed that the mass transfer parameters obtained from unidirectional column assays were several orders of magnitude greater than those obtained with the 3D trials. Quantitative analyses of the experimental data showed that mass transfer limitations occurred in 3D. Mass transfer coefficients are related to the distribution of air velocity in the 3D reactor. Consequently, unlike the 1D column, mass t...