Cristina M. S. G. Baptista

Effective interfacial area in agitated liquid–liquid continuous reactors

A chemical method has been used to quantify the effective interfacial area in a baffled continuous stirred liquid-liquid reactor. Two and four straight paddle impellers were used in the experimental runs, at 34°C, with hold-up fractions of dispersed organic phase between 0.061 and 0.166 and stirring speed ranging from 360 to 1500 rpm. Influence of the residence time on the formation of the interfacial area generated in this system was not registered; however, differences were reported between continuous and batch mode operations. The interfacial area was correlated to hold-up fraction and Weber number by a new empirical model proposed in this work. This model allows to use only one equation to calculate the interfacial area in this continuous stirred reactor in the wide range of operating conditions tested (490 < We < 9600), which include different flow regimes. This is a relevant contribution as previous studies in this field only contemplate turbulent flow. In the transitional regime the mean drop size diameter decreases abruptly with Weber number, but this pattern changes in the higher range of Weber where the dispersed drops become smaller very smoothly. This pattern does not depend on the agitator used or hold-up fraction. The mean drop size diameter is smaller for the four paddle impeller and increases with hold-up fraction. The model developed may be applicable to dispersions in aromatic nitration reactors, improving its operation and design.

Photocurable bioadhesive based on lactic acid.

Novel photocurable and low molecular weight oligomers based on l-lactic acid with proven interest to be used as bioadhesive were successfully manufactured. Preparation of lactic acid oligomers with methacrylic end functionalizations was carried out in the absence of catalyst or solvents by self-esterification in two reaction steps: telechelic lactic acid oligomerization with OH end groups and further functionalization with methacrylic anhydride. The final adhesive composition was achieved by the addition of a reported biocompatible photoinitiator (Irgacure® 2959). Preliminary in vitro biodegradability was investigated by hydrolytic degradation in PBS (pH=7.4) at 37 °C. The adhesion performance was evaluated using glued aminated substrates (gelatine pieces) subjected to pull-to-break test. Surface energy measured by contact angles is lower than the reported values of the skin and blood. The absence of cytoxicity was evaluated using human fibroblasts. A notable antimicrobial behaviour was observed using two bacterial models (Staphylococcus aureus and Escherichia coli). The cured material exhibited a strong thrombogenic character when placed in contact with blood, which can be predicted as a haemostatic effect for bleeding control. This novel material was subjected to an extensive characterization showing great potential for bioadhesive or other biomedical applications where biodegradable and biocompatible photocurable materials are required.

Prehydrolysis of Eucalyptus globulus Labill. hemicelluloses prior to pulping and fermentation of the hydrolysates with the yeast Pichia stipitis 10th EWLP, Stockholm, Sweden, August 25–28, 2008

Abstract The aim of this work was to evaluate the potential of Eucalyptus globulus hemicelluloses, mainly xylan, for bioethanol production. Hemicelluloses have been removed prior to pulping by auto-hydrolysis and an acid-catalysed hydrolysis. As the hydrolysates obtained were rich in xylose, the yeast Pichia stipitis was selected for fermentation. It was confirmed that the yeast performance is strongly dependent on pH and the presence of inhibitors, such as lignin. The addition of Ca(OH)2 was successful for lowering the concentration of inhibitors and adjusting the pH. The strain was grown in culture media with increasing volumetric percentages of treated hydrolysates up to 100% (v/v), supplemented with nutrients other than the carbon and energy source. This methodology shortened the lag phase of fermentation and improved the performance of yeast. Maximum ethanol concentration (12 geth l-1), productivity (0.22 geth l-1 h-1), and yield (0.48 geth gxyl eqs -1) were achieved with treated acid-hydrolysates. These results are similar to those obtained by a synthetic medium with an equivalent xylose concentration.

Modelling multitubular catalytic reactors : the influence of shell side flow

Abstract The main purpose of the paper is to highlight some important features of modelling multitubular catalytic reactors. In this study a two dimensional model is used to describe the flow of the coolant on the shell side. The unit is divided in cells over the cross section of the assembly as well as in the axial direction. With this approach the coolant flow is made up of two different streams: one in pure crossflow perpendicular to the tubes and the other in parallel to the bundle. The performance of the tubes in each cell is computed with a two dimensional heterogeneous model and has shown to be sensitive to the number of cells used, particularly in the baffle window zones that are critical areas and therefore require a large number of cells. The influence of the operating conditions upon production and yield for the co-current and counter-current coolant flow regimes is examined and it is shown that carrying out the analysis on a single tube basis is not adequate to predict the optimum operating conditions.

A contribution to the study of runaway and parametric sensitivity in fixed bed catalytic reactors

Abstract The aim of the present work is to contribute to the analysis of parametric sensitivity in fixed bed reactors using an heterogeneous two dimensional model. The sensitivity functions are calculated along with the state variables and the bound of the region of parametric sensitivity is drawn by the maximum of the sensitivity of the particle temperature at the hot spot. The results show that with a two dimensional heterogeneous model runaway risk can be detected for operating conditions the one dimensional model qualifies as stable. This method can be applied to any reaction scheme, simple or complex. The runaway limit proved to be coincident whichever the parameter in question allowing to centre the analysis on simple operating parameters such as inlet gas concentration or temperature which are easily manipulated. In addition it highlighted the influence of the coolant temperature upon reactor stability and showed to be in good agreement with other criteria to foresee runaway.

Functionalization and photocuring of an L-lactic acid macromer for biomedical applications

ABSTRACT L-lactic acid was the starting material for obtaining bioahesives. Reaction with 1,4-butanediol provided a telechelic lactic acid prepolymer with hydroxyl end groups further functionalized with 2-isocyanatoethyl methacrylate. Films were produced upon UV irradiation, 2 min, after addition of Irgacure 2959. This was a solvent and catalyst free process. Thermal characterization of films confirmed stability at physiological temperature, enabling photocuring. Adhesion properties were assessed with good results. In vitro degradation tests showed moderate hydrolytic instability dependent on thickness. SEM images revealed a uniform and compact structure. Thrombosis tests confirmed the materials’ thrombogenicity while biocompatibility experiments showed fibroblast viability and antimicrobial behavior. GRAPHICAL ABSTRACT

Using moving finite elements method to solve population balance equations comprising breakage terms

Abstract This paper introduces a Moving Finite Elements Method (MFEM) based on cubic Hermite polynomial local approximations to handle Population Balance Equations (PBE) modeling breakage phenomena. The adequacy and performance of the MFEM framework are tested in two case studies involving breakage: a benchmark problem with known analytical solution and the hydrolysis of starch. Both examples emphasize the stability and accuracy of MFEM in handling PBE models, thus proving its suitability for this kind of problem.

Safe design of industrial multitubular fixed bed reactors based on parametric sensitivity studies

Abstract The parametric sensitivity of a multitubular fixed bed catalytic reactor processing a highly exothermic complex reaction is examined here. The study combines two distinct models in order to foresee any risk in operation. For the multitubular reactor a cell model is used which takes into account the interaction between coolant flow on the shell side and the reacting gas in the tubes. The generalized sensitivity region is delimited by the maximum of the sensitivity of the particle temperature hot spot to different inlet parameters. The sensitivity functions are calculated along with the state variables adopting a two dimensional heterogeneous model. This strategy highlighted non uniformities in the sensitivity region boundaries for tubes in different rows of the bundle and proved to be in good agreement with other methods to foresee runaway. Moreover, it confirmed that coolant flow rate and its temperature have a strong influence on operation safety in either co- or counter-current coolant flow regimes.