M.A. Latifi

The use of micro-electrodes in the study of the flow regimes in a packed bed reactor with single phase liquid flow

Abstract The utilization of micro-electrodes in a non-conducting wall with subsequent signal analysis allowed the determination and the study of the flow regimes in a packed bed reactor with single-phase liquid flow. In effect, the analysis of the rate of fluctuation of the liquid—solid mass transfer coefficient (velocity gradient) as a function of liquid flow rate clearly showed the transition between inertial flow regime (nonlinear steady-state laminar) and turbulent flow regime. This transition, located between Reynolds number values of 110 and 370, corresponds to an unsteady-state laminar flow regime. Examination of the power spectral densities of these fluctuations led to a better understanding of liquid flow behaviour during this transition.

Optimization and non-linear control of a batch emulsion polymerization reactor

The optimal temperature policy which minimizes the terminal time in a batch emulsion polymerization reactor of styrene and α-methylstyrene was determined by means of orthogonal collocation techniques combined with a general non-linear programming method. The constraints concern the final latex properties and the thermal limitations of the pilot plant. An experimental validation has been realized. The optimal temperature profile was tracked using a non-linear geometric control technique which is particularly adapted to polymerization reactor control. An extended Kalman filter was used to estimate the non-measured state variables. Experimental results showed excellent agreement with predictions for this complex system. A good temperature tracking was observed and the product quality was well predicted and controlled.

Hydrodynamics of liquid flow in packed beds: an experimental study using electrochemical shear rate sensors

A literature survey on the electrochemical sensor technique shows that the simple relation between the average measured diffusional current and the local shear rate of the liquid flow holds even for extremely non-homogeneous flow conditions with high-amplitude fluctuations. Even though the frequency response of electrochemical probes is unsatisfactory, their transfer function has been recently determined and low-amplitude—high-frequency fluctuations of the instantaneous local shear rate can be investigated. Small circular probes have been successfully used to study hydrodynamics of liquid flow in packed-bed reactors. The local flow is extremely non-homogeneous in space and, for high Reynolds numbers, quite fluctuating in time. The onset of chaotic, time-dependent flow occurs at superficial Reynolds numbers of about 110–150. The analysis of the characteristic times of the velocity gradient fluctuations in the chaotic flow regime indicates the existence of liquid agregates having the characteristic dimensions of the porous media.

Dynamic optimisation of small size wastewater treatment plants including nitrification and denitrification processes

In this paper, dynamic optimisation of small size wastewater treatment plants is studied. The problem is stated as a hybrid dynamic optimisation problem which is solved using a gradient-based method. The aeration policy which minimises the energy consumption and satisfies discharge requirements under specified constraints (process and physical constraints) is then determined. The comparison between usual rule-based control policies and optimised aeration strategies showed that the optimised aeration profiles lead, to reductions of energy consumption of at least 30%.

Hydrodynamics and liquid—solid mass transfer mechanisms in packed beds operating in cocurrent gas—liquid downflow: An experimental study using electrochemical shear rate sensors

Overall and local hydrodynamics and liquid—solid mass transfer mechanisms were investigated in a laboratory scale packed bed reactor operating in gas—liquid downflow. The mean liquid saturation and the liquid—solid mass transfer rate were determined using classical electrochemical techniques and the local instantaneous hydrodynamics were analyzed using electrochemical shear rate sensors. The experimental results as well as theoretical considerations enabled us to contribute to the elucidation of gas—liquid flow mechanisms, especially in high-interaction regimes. In pulse flow at low liquid flow rates (L < 10kg m−2 s−1) the wall is, on average, not entirely wetted, this might explain hot spot occurence in industrial fixed-bed reactors. In dispersed bubble flow and in the liquid rich slugs of pulse flow, the local instantaneous liquid-wall shear rate is characterized by high-amplitude—high-frequency fluctuations. The comparison of the space averaged shear rate measurements with the overall mass transfer rate indicates that the liquid—solid mass transfer mechanism is laminar in nature and may be modelized by a succession of developing laminar boundary layers. An overall mechanical force balance on the liquid shows that the average drag of the liquid by the gas is very small, compared to the total energy dissipated by the gas in the reactor. All the experimental results obtained in this work as well as several literature data can be explained by a flow mechanism in dispersed bubble flow, where the liquid flow is dominated by viscous forces whereas the gas bubbles pass through the packed bed by pressure pulses.

Analysis of two-phase flow distribution in trickle-bed reactors

Abstract In the present paper, a phenomenological one-dimensional model of a two-phase gas and liquid flow in a trickle bed reactor is developed. Based on some realistic assumptions specific to tickling flow regime, the original equations of continuity and momentum are reformulated in terms of liquid saturation and gas pressure equations. The computational method used is the finite volume technique combined with Godunovs method. It is shown that the pressure drops derived from the model solution are in good agreement with those measured in the same operating conditions and with previous literature results.

The use of microelectrodes for the determination of flow regimes in a trickle-bed reactor

The utilization of microelectrodes in a non-conducting wall with subsequent signal analysis allowed the determination of flow regime transitions: trickling/pulsing, trickling/dispersed bubble and dispersed bubble/pulsing in a trickle-bed reactor by the analysis of the rate of fluctuation of the liquid—solid mass transfer coefficient (velocity gradient) variations as a function of liquid and gas flow rates.

Multicriteria dynamic optimization of an emulsion copolymerization reactor

Abstract A multicriteria optimization approach based on an evolutionary algorithm has been developed to determine the optimal control policy for a fed-batch emulsion copolymerization reactor, particularly for styrene and butyl acrylate in the presence of n-C12 mercaptan as chain transfer agent. The process model was elaborated and validated experimentally in order to predict the global monomer conversion, the number and weight average molecular weights, the particle size distribution and the residual monomers mass fraction. The process objectives were to produce core–shell particles (hard core and smooth shell) with specific end-use properties and high productivity. This has been achieved by the maximization of the monomers overall conversion at the end of the process and the minimization of the error between the glass transition temperature and a designed profile subject to a set of operational constraints. The nondominated Pareto solutions obtained were ranked according to a decision making aid method based on a decision maker preferences and experience using multi-attribute utility theory. Finally, the best solution was implemented experimentally.

Experimental investigation of the liquid/solid mass transfer at the wall of a trickle-bed reactor: influence of Schmidt number

In this paper, an experimental study of liquid/solid mass transfer at the wall of a trickle-bed reactor is presented. Three observed flow regimes were mainly analysed: trickling, pulsing and dispersed flows. The pressure drops in the trickling flow regime were interpreted by means of relative permeabilities approach. New mass transfer correlations are proposed in each of the flow regimes. A statistical analysis showed that all the parameters involved in these correlations were determined very accurately. The influence of Schmidt number was particularly investigated. It was found, through a qualitative analysis, that the experimental results were consistent with the theory of laminar boundary layer over a plane plate.

Liquid—solid mass transfer in a packed bed with downward cocurrent gas—liquid flow: An organic liquid phase with high Schmidt number

Abstract An electrochemical technique was used to measure the overall liquid—solid mass transfer coefficients at high Schmidt number in downward cocurrent gas