Papers by Héctor Hernández
Physical Review Letters, 2007
Inhomogeneous cosmological perturbation equations are derived in loop quantum gravity, taking int... more Inhomogeneous cosmological perturbation equations are derived in loop quantum gravity, taking into account corrections in particular in gravitational parts. This provides a framework for calculating the evolution of modes in structure formation scenarios related to inflationary or bouncing models. Applications here are corrections to the Newton potential and to the evolution of large scale modes which imply non-conservation of curvature perturbations possibly noticeable in a running spectral index. These effects are sensitive to quantization procedures and test the characteristic behavior of correction terms derived from quantum gravity. PACS numbers: 98.80.Cq,04.60.Pp,98.80.Bp Cosmology has provided a successful paradigm for structure formation in our universe through an inflationary phase [1] in early stages. Conceptually, however, the scenario is incomplete due to the presence of past singularities . At such a singularity, the classical theory of general relativity breaks down and has to be replaced by an extended framework which remains well-defined even at very high curvatures. Since this requires modifications to general relativity at early stages of cosmic evolution, there can then also be corrections to the usual scenario of structure formation which might eventually be observable. While dimensional arguments and low energy effective theory indicate that effects are very small, given by the tiny ratio of the Planck length ℓ P = √ G to the Hubble length H −1 , a detailed analysis is required and may reveal more sizeable effects. This is what we provide in this letter in the framework of loop quantum gravity [3], a non-perturbative background independent approach to quantize gravity.
Chemical Engineering and Processing, 1995
Aromatic nitrations by mixed acid have been selected as a specific case of a heterogeneous liquid... more Aromatic nitrations by mixed acid have been selected as a specific case of a heterogeneous liquid-liquid reaction. An extensive experimental programme has been followed using adiabatic and heat-flow calorimetry and pilot reactor experiments, supported by chemical analysis. A series of nitration experiments has been carried out to study the influences of different initial and operating conditions such as temperature, stirring speed and sulphuric acid concentration. In parallel, a mathematical model to predict the overall conversion rate has been developed. In this paper the mathematical modelling and the implementation and experimental validation for benzene, toluene and chlorobenzene mononitration in the kinetic control regime (slow liquid-liquid reaction) are presented and discussed.
Computers & Chemical Engineering, 1996
In this paper the use of neural networks for fitting complex kinetic data is discussed. To assess... more In this paper the use of neural networks for fitting complex kinetic data is discussed. To assess the validity of the approach two different neural network architectures are compared with the traditional kinetic identification methods for two cases: the homogeneous esterification reaction between propionic anhydride and 2-butanol. catalysed by sulphuric acid. and tbe heterogeneous Iiquid-liquid toluene mononitration by mixed acid. A large set of experimental data obtained by adiabatic and heat flux calorirnetry and by gas chromatography is used for the training of the neural networks. The results indicate tbat tbe neural network approach can be used to deal with tbe fitting of complex kinetic data to obtain an approximate reaction rale function in a Iimited amount of time. which can be used for design improvement or optimisation when. owing to small production levels or time constraints. it is not possible to develop a detailed kinetic anaIysis.
Chemical Engineering and Processing, 1996
Aromatic nitration by mixed acid was selected as a specific case of heterogeneous liquid-liquid r... more Aromatic nitration by mixed acid was selected as a specific case of heterogeneous liquid-liquid reaction. An extensive experimental programme was followed using adiabatic and heat flow calorimetry and pilot reactor experiments, supported by chemical analysis. A series of nitration experiments was carried out to study the influence of different initial and operating conditions, such as temperature, stirring speed, feed rate and sulphuric acid concentration. In parallel, a mathematical model to predict the overall conversion rate was developed. In this paper, the mathematical modelling, implementation and experimental validation for mononitrations of benzene, toluene and chlorobenzene in the mass transfer controlled regime of fast liquid-liquid reactions are presented and discussed.
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Papers by Héctor Hernández