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ABSTRACT: Separations in capillary electrophoresis and especially in micellar electrokinetic capillary chromatography are characterised by a large number of parameters and therefore difficult to optimise. This paper reviews recent approaches suitable for optimisation of selectivity in capillary electrophoresis.Typical features of optimisation strategies applicable to capillary electrophoresis and micellar electrokinetic capillary chromatography in particular are discussed. A distinction is made between statistical approaches, using fitting procedures of polynomial equations, and practical optimisation schemes, based on physicochemical models describing the migration behaviour.Besides speeding up the search in finding satisfactory separation conditions, additional knowledge may be obtained about the migration and separation mechanism(s) when a systematic approach is applied. However, due to the complexity and the number of available optimisation schemes, these approaches should not be used as black-box systems. The analyst has a crucial role in optimising a separation.
Journal of Chromatography A.
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ABSTRACT: The interactions between a turbulent flow field and discrete particles have numerous applications in biochemical engineering. On the one hand, flows have a strong influence on the particle motion, from which consequences for heat and mass transfer, mixing or even damage to particles are derived. On the other hand, the presence of the discontinuous (solid) phase is regarded as altering the turbulent field (two-way coupling). At present, no fully explained mechanism of this turbulence alteration is offered in the literature. However, the two-way coupling can no longer be considered when the particle concentration becomes sufficiently high. The dominant mechanism affecting the flow is then the particle—particle interaction. Until now, no clear definition of a demarcation between hydrodynamic (fluid—particle interaction) and viscous (particle—particle interaction) influences in liquid—solid or liquid—solid—gas systems has been given in the literature.In this paper we present first a description of the forces acting on a particle in a flow and the most relevant parameters linked to the response of a particle to turbulent stimulations. Some illustrations are given for common biochemical applications. The second part is concerned with the action of the particles on the turbulence, the main trends observed and their significance in such applications being focused on. It is also demonstrated here that the transition between the hydrodynamic and the viscous regimes is located between 20% and 30% in solid volume concentration.
The Chemical Engineering Journal and the Biochemical Engineering Journal.
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ABSTRACT: In-situ product recovery can improve the performance of a butanol fermentation. Five technologies for in-situ product recovery have been compared on the basis of design parameters and energy efficiency. These five technologies are: stripping, adsorption, liquid—liquid extraction, pervaporation and membrane solvent extraction. From these, pervaporation and liquid—liquid extraction are considered to have the greatest potentials.
Process Biochemistry.
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ABSTRACT: In waste streams containing hexose and pentose sugars, xylose is only converted when the glucose concentration is sufficiently low not to be inhibitory. In an attempt to ferment glucose and xylose simultaneously in a single reactor without the addition of oxygen, Pichia stipitis was immobilized in alginate beads. In the core of the beads, a low hexose concentration is expected, thus allowing xylose conversion. The conversion of hexose and pentose appears to be simultaneous. When P. stipitis was immobilized in alginate, a glucose conversion rate of 0.13 g g-1h-1was found. Due to this low rate, the glucose concentration in the core of the beads was too high to allow xylose conversion. Independently of the initial cell concentration, the biomass concentration only doubled once during these experiments. Another possibility to convert both glucose and xylose is the coimmobilization of Saccharmoyces cerevisiae with P. stipitis to attain higher glucose conversion rates. With coimmobilized S. cerevisiae and P. stipitis, simultaneous conversion of glucose and xylose was possible. The rate of xylose conversion depended on the dilution rate in the reactor and the initial P. stipitis concentration in the beads. Cell counts in the beads showed that S. cerevisiae formed a dense layer at the periphery of the beads, but that P. stipitis hardly grew under these conditions. The poor growth of P. stipitis in alginate beads leads to the need for high initial cell concentrations to reach a more complete conversion of mixed substrates.
Enzyme and Microbial Technology 12(11):860-864. · 2.37 Impact Factor
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ABSTRACT: A fast analysis technique for the determination of oil concentrations in soil and sediment samples is described and validated. The so-called thermal desorption mass spectrometry combination (TD-MS) proved to be a very rapid analytical method for mineral oil in soil. When compared to a soxhlet extraction and to the standard method of analysis, the latter yielded significantly different results compared to the other two methods, while the results from TD-MS and soxhlet extraction did not differ significantly from each other. Of the three methods TD-MS yielded the lowest variance for multiple analyses. Total time for the analysis of a wet soil sample was 20 min, including sample preparation.
Analytica Chimica Acta. 315:159-166.
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ABSTRACT: The migration behaviour of charged solutes in micellar electrokinetic capillary chromatography is affected by both the electrophoretic properties of the solute and its interaction with the micelles. Both must therefore be considered to obtain an adequate description of the migration behaviour and thus resolution. A novel equation is presented, in which the resolution is expressed as a function of the micellar interaction (capacity factor for charged solutes) and the electrophoretic mobility (free zone migration time) of the charged solute. Simulations reveal the dependence of the resolution of two closely migrating solutes on both distinct interactions, which is clearly different as compared to the separation of neutral solutes and general guidelines with respect to better separation are deduced. Experimental data illustrate that this equation can be used to gain more insight in the corresponding migration mechanisms. Furthermore, using this knowledge, it is possible to adjust the experimental parameters to achieve better resolution.
Journal of Chromatography A. 753(1):121-131.
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ABSTRACT: Substrates that contain hexose as well as pentose sugars can form an interesting substrate for the production of ethanol. If Pichia stipitis is used for the conversion of such substrates, the pentose sugars are only converted when the hexose concentration is very low. This is a consequence of the sequential use of substrate. A system with reactors in series might be a good process for the sequential use of hexose and pentose sugars. The concentration of the preferred substrate will be low in the last reactor, thus allowing a more optimal pentose conversion. Two different systems with reactors in series were tested. The first system had three reactors with P. stipitis as the sole yeast. The second system consisted of two reactors: Saccharomyces cerevisiae was inoculated in the first reactor to convert hexose rapidly, and the second reactor contained P. stipitis to convert the pentose sugars. In the three-reactor system with P. stipitis, an almost complete conversion of a glucose/xylose mixture was possible. Aeration of the second reactor accelerated the sugar conversion. The volumetric ethanol production rate is, however, relatively low: 0.51 g l−1 h−1. With two reactors and two yeasts, only 20% of the xylose is converted. This is caused by the competition for oxygen between the two yeasts in the second reactor. The conversion of a mixture of glucose and xylose in a system containing reactors in series is possible. Low dilution rates have to be applied to obtain sufficiently low glucose concentration for the conversion of xylose; consequently, the volumetric production rates are low. The system with a combination of S. cerevisiae and P. stipitis can only be competitive when the yeasts are separated.
Enzyme and Microbial Technology 13(10):828-833. · 2.37 Impact Factor
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ABSTRACT: The effects of medium composition and aeration on the fermentation of xylose and glucose by Pichia stipitis CBS 5773 were investigated. The addition of yeast extract (5 kg m3) has a positive effect on the growth of P. stipitis. There is a linear relation between the oxygen uptake and the biomass production. The bruto yield of biomass on oxygen (Y″ox) is 39.5 g mol−1 for growth on xylose and is dependent on the oxygen supply rate for growth on glucose. Under restricted aeration the process is determined by the oxygen supply. A negative correlation has been found between the yield of ethanol on substrate and the oxygen uptake. The biomass yield increases with increasing oxygen uptake. A low aeration rate is necessary for an optimal conversion of glucose and xylose to ethanol. The maximum specific ethanol production rates on glucose and xylose were respectively 0.35 and 0.13 g g−1 h−1. This maximum was reached at oxygen uptake rates below 0.005 mol l−1 h−1. Due to a limited substrate uptake rate, a high cell concentration will be needed to obtain high volumetric productivities.
Enzyme and Microbial Technology.
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ABSTRACT: Yeast flocculation is considered to be ruled by colloid-chemical and biological interactions. The change in hydrophobicity, surface potential and surface structures and the appearance of lectins on the cell wall point in this direction. According to the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory we calculated the net attraction energy between non-flocculent and flocculent yeast cells. There seems to be no difference between the secondary minima of the potential energy curves of both states of the cells, although most parameters used within the DLVO model change in a manner suggesting that flocculation is favoured when the cells become flocculent. Parallel with these calculations we measured the net attraction force between flocculent cells. This force was approximately 2000 times higher than the force calculated with the DLVO theory. From these findings it is concluded that the interaction between flocculent yeast cells has only biological character. The changes in the physicochemical state of the cell wall must be ascribed to changes in its biological nature.
Colloids and Surfaces B: Biointerfaces.
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ABSTRACT: Prediction of the power uptake of impellers is crucial for a successful scale-up, operation, and design of stirred tank reactors (STR). However, for gas-liquid systems equipped with multiple impellers, commonly used in industry, information on the local power uptake is scarce in literature. For single-impeller stirred vessels, inconsistency between the experimental data on power uptake under gassed conditions and available relations in literature is often reported. In this paper empirical correlations on the local gassed power uptake of impellers are derived both for single-impeller and multi-impeller systems, based on a large body of experimental data from literature. Under gassed conditions the bottom impeller of a multiple impeller configuration or a single impeller draws relatively less power than an impeller in the middle or at the top. The power uptake by impellers in a gassed STR shows two regimes. It is found that middle and top impellers reach their second regime at a much higher gas flow rate than single or bottom impellers under the same conditions.
Chemical Engineering Science.
