Marco Mazzotti

Optimal operation of simulated moving bed units for nonlinear chromatographic separations

The simulated moving bed (SMB) technology is receiving more and more attention as a convenient technique for the production scale continuous chromatographic separation of fine chemicals. Characteristic features of SMBs are improved performances with respect to preparative chromatography and nonlinear competitive adsorption behavior. The selection of the operating conditions to achieve high separation performances under nonlinear conditions is acknowledged to be the major problem in running a SMB unit for a new application. This problem is solved in this paper, where a general theory is developed which provides explicit criteria for the choice of the operating conditions of SMB units to achieve the prescribed separation of a mixture characterized by both constant selectivity Langmuir isotherms and variable selectivity modified Langmuir isotherms. The space of the operating parameters, i.e. the fluid to solid flow-rate ratios, is divided in regions with different separation regimes. The effect of increasing nonlinearity of the system on the operating conditions and the separation performances, namely desorbent requirement, enrichment, productivity and robustness of the separation, is thoroughly analyzed. The obtained results are shown to provide a very convenient tool to find both optimal and robust operating conditions of SMB units. Finally, a comparison between model predictions and experimental data dealing with the resolution of different racemic mixtures assesses the reliability and accuracy of the obtained theoretical findings.

Modeling fixed-bed adsorption columns through orthogonal collocations on moving finite elements

An orthogonal collocation method based on moving finite elements has been developed for simulating fixed-bed adsorbers. The movement of the grid is calculated explicitly before starting the integration of the discretized equations, using the results of the Equilibrium Theory for wave transitions and the shock layer theory for shock transitions. The performance of the developed moving grid technique has been extensively compared with that of the corresponding techniques based on a fixed discretization grid. Substantial improvements both in computational time as well as in accuracy of the obtained solutions have been found in all cases.

A continuous chromatographic reactor: SMBR

The results about the esterification of acetic acid with ethanol on a highly cross-linked sulphonic ion-exchange resin (Amberlyst 15) in a continuous Simulated Moving Bed Reactor (SMBR) laboratory unit are reported. The resin acts simultaneously and effectively both as a catalyst as well as a selective sorbent. This allows to obtain reaction products of high purity and, since the reaction is equilibrium limited, to achieve greater conversion with respect to that attainable without separation. Multicomponent sorption equilibria and swelling of the resin, as well as esterification kinetics, have been studied experimentally and described through appropriate models. Some experiments in a fixed-bed chromatographic reactor packed with the resin demonstrate the capabilities of the resin. The thermodynamic and kinetic descriptions of the system have been combined to develop a fully predictive mathematical model of the chromatographic reactor, able to predict its experimental behavior with reasonable accuracy.

Modeling multistage flash desalination plants

Multistage flash (MSF) desalination plants are among the major sources of potable water in the world, in particular in the Arab Gulf area [1]. These are very large, complex and expensive plants, in which large amounts of seawater and energy are consumed. Accordingly, big quantities of concentrated brine are disposed of after the desalination treatment. These characteristics of MSF plants make issues such as optimization of the operation and minimization of the corresponding environmental impact of the greatest importance. To the aim of addressing all these aspects, mathematical models provide a very useful tool. This work describes a steady-state mathematical model developed to analyze the MSF desalination process. It is based on a detailed physicochemical representation of the process, including all the fundamental elementary phenomena. In particular, the model accounts for the geometry of the stages, the variation of the physical properties of water with temperature and salinity, the different non-idealities involved in the process, the mechanism of heat transfer and the role of fouling. The developed model allows to analyze the role of operating and design variables in determining the process performances in terms of steady-state behavior. These information are used not only for design purposes, but also to support the development of a dynamical model through which the time-dependent behavior of the plant can be studied.

Performance of a six-port simulated moving-bed pilot plant for vapor-phase adsorption separations

Abstract A simulated moving-bed (SMB) pilot plant, characterized by two unique features, the number of ports (6 instead of 24, as in most industrial applications) and the fluid phase (vapor iather than liquid), has been built and operated. Such a plant has proven capable to achieve complete separation for a mixture of m- and p-xylene using isopropylbenzene as desorbent and KY zeolites as adsorbent. The dynamic behavior of the unit has been investigated and compared with model predictions, both in terms of approach to cyclic steady state as well as of responses to typical inputs. The role of the key design parameters on the steady-state separation performance of the unit has been analyzed cxperimentally. demonstrating the consistency with the predictions of the Equilibrium Theory.

Long chain branching in emulsion polymerization

A kinetic model for evaluating the degree of polymerization of a branched polymer produced in emulsion is developed. Only chain branching occurring through chain transfer to polymer has been considered. The model accounts for chain compartmentalization and, when coupled to a model able to describe the evolution of the polymerization system, allows to evaluate the cumulative properties of the produced polymer both in the pre- and post-gel phases. The difficulties related to the description of a heterogeneous polymer chain population and of gel formation are overcome by using the “numerical fractionation” technique. A parametric analysis of both instantaneous and cumulative properties is reported and discussed, with special attention to the role of radical compartmentalization in determining the molecular weight properties of an emulsion produced polymer.

Shock layer analysis in multicomponent chromatography and countercurrent adsorption

Abstract Shock layer(constant patterns), which propagate with constant velocity without changing their shape, can develop in adsorption units under suitable operating conditions. They have been observed both in experiments as well as in numerical computations. By extending the approach and the results of two solute chromatography (Rhee and Amundson, 1974, Chem. Engng Sci.29, 2049-2060), in this paper we develop the shock layer theory for multicomponent adsorption processes in fixed-bed and in countercurrent columns, where the adsorption equilibria are described by Langmuir-type adsorption isotherms. Two different models to describe the adsorption-diffusion process in the solid particles have been considered: the solid diffusion model and the pore diffusion model. Explicit expressions for the concentration profiles across the shock layer as well as for its propagation velocity are reported for the general class of multicomponent systems and column configurations mentioned above. The reliability of the theoretical results has been assessed by comparison with both numerical results and experimental data.

CHROMATOGRAPHIC RESOLUTION OF ENANTIOMERS

Abstract In the pharmaceutical industry, the increasing demand of new single enantiomer products has raised a great interest towards chromatographic separation methods. While analytical applications of high performance liquid chromatography and gas chromatography are well established, the scaling up to preparative applications still needs a better comprehension and rationalization of the elementary phenomena involved. In this work the analytical and preparative separation of an isoxazoline racemic mixture on triacetylcellulose has been studied. In order to determine the thermodynamic properties and the equilibrium isotherms of adsorption several experiments at very low concentration (in a HPLC column) and at higher concentrauon (using the frontal analysis technique) have been performed. Moreover, pulse experiments on a preparative column have been run at different temperature and at different racemic concentration. The comparison between experimental data and model results is discussed.

C5 Separation in a Vapor Phase Simulated Moving Bed Unit

The separation of n-pentane and iso-pentane is carried out in a six-port vapor phase SMB pilot plant, using n-heptane as desorbent and 5A-Zeolite as adsorbent solid. The results of a number of experimental runs illustrate the role and the effect of the different operating parameters and demonstrate the possibility of achieving high purity separation. They are discussed in the light of the known criteria for selecting robust and optimal operating conditions for SMB units.

Analysis of a Six Port Simulated Moving Bed Separation Unit

Abstract The optimal design and operation of a Simulated Moving Bed separation unit is discussed with reference to a six port pilot plant operating in the vapor phase. The separation performance is illustrated by considering various mixtures of the fraction C-8 isomers, and using Isopropyl-benzene and KY Zeolites as desorbent and adsorbent, respectively. A model based on the Equilibrium Theory is used to design the optimal operating conditions. The consistency of model predictions and experimental results is shown and discussed.