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Biomedical subjects

Massimo Morbidelli

Publications and source records attributed to Massimo Morbidelli.

27 records · Page 2Linked to original sources
PubMed2003

Hydrodynamic radius of fractal clusters.

The Kirkwood-Riseman theory has been used to derive an analytical formula for the evaluation of the hydrodynamic radii of fractal clusters. The proposed relation is based on knowledge of the particle-particle correlation function and can be applied to clusters containing any number of particles larger than 4. The calculated values of the hydrodynamic radius are compared with the results of other theoretical approaches proposed in the literature, as well as several experimental data. Finally, the developed relation has been used in connection with a population balance equation model that computes the clusters' mass distribution to estimate the average hydrodynamic radius for a population of fractal clusters of different sizes. The obtained results have been compared to suitable experimental data for a silica colloidal suspension aggregating under reaction-limited conditions.

Journal Article↗
PubMed2003

PowerFeed operation of simulated moving bed units: changing flow-rates during the switching interval.

A possible way to improve the separation performance of simulated moving bed (SMB) units is to change the internal and external liquid flow-rates during the switching period. This operation mode, referred to as PowerFeed, is examined in this work through a model analysis. Similar to the Varicol process, which allows for the asynchronous movement of the ports, the PowerFeed process exhibits more degrees of freedom than the classical SMB process and therefore allows more room for optimization. Using an optimization technique based on a genetic algorithm, all three processes have been optimized for a few case studies in order to determine their relative potentials. It is found that PowerFeed and Varicol provide substantially equivalent performances, which are however significantly superior to those of the classical SMB process.

Algorithms↗
PubMed2003

Aggregation kinetics of polymer colloids in reaction limited regime: experiments and simulations.

The kinetics of reaction-limited cluster aggregation of fluorinated polymer colloids in a broad range of particle volume fractions has been investigated experimentally by measuring independently the Fuchs stability ratio W and the time evolution of both the average radius of gyration and the average hydrodynamic radius of the aggregates mass distribution. The W value is determined from the aggregation rate at the very initial stage of the aggregation, where the presence of triplets is negligible. The time evolutions of and are then simulated using the cluster mass distribution calculated from the population balance equations with various aggregation kernels proposed in the literature. It is found that, when the measured W value is used, the only kernels that can correctly simulate the experimental results are the product kernel and the one derived by Odriozola et al. (Europhys. Lett. 53 (2001) 797), with some proper tuning of the exponent in the kernel. For the particle volume fraction phi<1%, the obtained value for the exponent is 0.4 and independent of phi, while it tends to decrease for larger phi values, most likely indicating a significant effect of multi-body interactions on the aggregation kinetics.

Journal Article↗
PubMed2003

Multiobjective optimization of simulated moving bed and Varicol processes using a genetic algorithm.

The size of the packing material, the total number of columns and the total feed concentration have significant impacts on the economics of a preparative chromatographic separation, through their effects on column pressure drop, column efficiency and thermodynamics. In this work, the role of these parameters on the performances of a simulated moving bed and a Varicol process is investigated on a chiral separation system from literature, using an equilibrium stage model. A multiple objective optimization technique based on a genetic algorithm is adopted, which allows to maximize simultaneously the purity of the extract and productivity of the unit. In this way, it is possible to optimize and compare the performances of both processes in a wide range of parameter values, so as to assess their relative potential under equally optimized conditions. The optimization results, i.e. the so-called Pareto sets, have been discussed in the frame of equilibrium theory and the roles of these three parameters have been clarified.

Algorithms↗
PubMed2002

Experimental investigation of the behavior of gas phase simulated moving beds.

The preparative, continuous gas chromatographic separation of the enantiomers of the inhalation anaesthetic enflurane has been studied on a chiral stationary phase based on octakis(3-O-butanoyl-2,6-di-O-n-pentyl)-gamma-cyclodextrin, dissolved in polysiloxane SE-54 and coated on Chromosorb particles. This has been carried out in a gas chromatographic simulated moving bed (GC-SMB) unit, equipped with eight columns, yielding a total volume of 1.128 l. With reference to this particular system, the separation performance and the behavior of GC-SMB units have been analyzed in depth. We have varied the internal flow-rates in the four sections of the unit in the range between about 1 and about 5 std l/min, the temperature between 20 and 45 degrees C, the switch time between about 2 and about 14 min, and the feed concentration in the range 0.32-0.90 mol%. Similarities and differences in the behavior of GC-SMBs as compared to conventional liquid phase SMBs have been described, and discussed. Operating conditions leading to more than 99% purity in one or both outlet stream have been identified, together with those achieving optimal throughput. Under such optimal conditions, about 20 g of each enflurane enantiomer with enantiomeric purity larger than 98% have been prepared.

Adsorption↗
PubMed2002

Solvent gradient operation of simulated moving beds. I. Linear isotherms.

The simulated moving bed (SMB) is a multi-column chromatographic separation process, which--with respect to the single-column preparative batch process--allows for a continuous separation with larger productivity and smaller solvent consumption at the same time. The benefits of this process have been shown for several different applications in fine chemistry, particularly for the separation of enantiomers. In general, SMBs are operated under isocratic conditions. However, separation performance can be further improved by applying some sort of gradient mode operation, in order to optimize the operating conditions of each individual section of the unit. This can be achieved by tuning the retention behavior of the solutes to be separated along the unit, namely by enforcing weak adsorption conditions in sections 1 and 2, and strong adsorption conditions in sections 3 and 4. This can be achieved by applying a temperature gradient (high temperature in section 1, and low temperature in section 4), a pressure gradient (e.g. in the supercritical SMB, when pressure is high in section 1, and low in section 4), or a solvent gradient, which is the aim of this work. In the solvent gradient mode the mobile phase consists of a mixture of two or more solvents. To different mobile phase compositions corresponds a different retention behavior of the solutes, i.e. different adsorption isotherms. In this work we study a closed loop SMB unit with solvent mixtures of two different compositions entering the unit at the feed and desorbent inlet ports, respectively. Thereby two different mobile phase compositions are established in sections 1 and 2, and sections 3 and 4, respectively. To optimize this process the equilibrium theory design criteria for non-linear SMBs are extended to describe this operation mode. It is shown how the region of separation is derived and how the optimal operating conditions can be found. Finally the solvent gradient mode is compared with the isocratic mode in terms of productivity and solvent consumption.

Adsorption↗
PubMed2002

Separation of binaphthol enantiomers through achiral chromatography.

Chromatography is a key technique for the analytical, preparative, and production scale separation of enantiomers, particularly in the pharmaceutical and fine chemicals industries. Although it is common belief that this separation can be accomplished only using a chiral stationary phase, it has been recently shown that under certain circumstances a non-racemic mixture of specific chiral compounds can be separated in two fractions which differ in enantiomeric excess (e.e.) also on an achiral stationary phase. In this work we show that in the case of the enantiomers of binaphthol in chloroform achiral chromatography on LiChrospher 100 NH2 furnishes two fractions constituted of the pure enantiomer present in excess and of the racemic mixture, respectively. This is demonstrated by on-line monitoring the concentration of both enantiomers at the outlet of a chromatographic column fed with a non-racemic pulse of the two enantiomers by using a UV detector and a polarimeter in series. Furthermore, we provide experimental evidence of the presence of homo- and hetero-dimers in solution through NMR experiments and develop a consistent physico-chemical model of the solution itself and of the competitive achiral adsorption equilibria. When combined with a standard rate model of the chromatographic column this not only confirms the possibility of achieving 100% e.e. through achiral chromatography, but also allows for a qualitative and quantitative description of all the experimentally observed phenomena. Among these, the effect of the enantiomeric excess and of the overall concentration of the injected pulse on the chromatographic behaviour are worth mentioning.

Adsorption↗
PubMedDate not supplied

Kinetics of cold-set diffusion-limited aggregations of denatured whey protein isolate colloids.

The CaCl2-induced cold-set aggregation kinetics of the denatured whey protein isolate (WPI) colloids has been investigated under dilute diffusion-limited cluster aggregation (DLCA) conditions, using small-angle light scattering. In particular, the structure factor, the scattered intensity at zero angle and the average radius of gyration have been measured for the aggregating system as a function of time. It is found that the fractal dimension of the clusters is df= 1.85, in the range typical of clusters aggregated under DLCA conditions. The aggregation kinetics in this transition region can be described by a power law relation in the initial stage of the aggregation, but the exponent of the power law is equal to 0.7, i.e., significantly larger than 1/df= 0.54, which is the typical value of the DLCA kinetics. Since it is found that the average gyration radius of the clusters has reached a value of 80 microm, leading to a cumulative volume fraction of clusters equal to 0.25, it is legitimate to expect that the process is in the region of transition from aggregation to gelation. This confirmed by the fact that, at the later stage of the aggregation, the growth of the average cluster size further accelerates with time and eventually becomes explosive, leading to gelation. The observed aggregation kinetics has been compared with that reported in the literature from DLCA Monte Carlo simulations, and a good agreement has been found with the data corresponding to the transition region from aggregation to gelation. Numerical simulations using the Smoluchowski kinetic model have also been carried out in order to support the experimental findings.

Biocompatible Materials↗