Enthalpy of vaporization | EPFL Graph Search

In thermodynamics, the enthalpy of vaporization (symbol ∆Hvap), also known as the (latent) heat of vaporization or heat of evaporation, is the amount of energy (enthalpy) that must be added to a liquid substance to transform a quantity of that substance into a gas. The enthalpy of vaporization is a function of the pressure at which the transformation (vaporization or evaporation) takes place. The enthalpy of vaporization is often quoted for the normal boiling temperature of the substance. Although tabulated values are usually corrected to 298 K, that correction is often smaller than the uncertainty in the measured value. The heat of vaporization is temperature-dependent, though a constant heat of vaporization can be assumed for small temperature ranges and for Reduced temperature Tr ≪ 1. The heat of vaporization diminishes with increasing temperature and it vanishes completely at a certain point called the critical temperature (Tr = 1). Above the critical

Process monitoring of alumina nanoparticle synthesis by inductively coupled RF thermal plasma

Jong-Won Shin

Since the first inductively coupled plasma (ICP) torch was developed in the 60's, RF thermal plasmas have found a variety of applications in material processing such as crystal growth, thermal spray coatings, spheroidization and vaporization of refractory materials. In recent decades, inductively coupled thermal plasmas have been used for the synthesis of high purity nanoparticles, thank to their remarkable advantages such as high energy density, variable operating pressures and low product contamination. The formation of nano-sized particles in thermal plasmas particularly using solid refractory precursor is a complex process. Hereby the injected precursor powders are heated by the high temperature of the thermal plasma. The heated powders are vaporized and decomposed into vapor species. During the following cooling, the vapor species are condensed to nano-sized particles. In this synthesis process, characterized by evaporation-condensation, the properties of the synthesized particles such as particle size, size distribution, phases and morphology are affected by various process parameters. Gas pressure, flow rates of the different torch gases, size of the precursor powders and powder loading in the plasma are considered as the influencing process parameters. In order to optimize and control the process for tailor-made nanoparticle synthesis, it is necessary to understand the effects of the process parameters on plasma properties and on the particle-plasma interactions resulting in final properties of the synthesized powders. In the present work, firstly, the enthalpy probe technique, a well-established plasma diagnostic tool for high pressure thermal plasma, has been applied to characterize the inductively coupled Ar-H2 thermal RF plasma used for alumina nanoparticle synthesis at different operating conditions. The plasma enthalpy has been determined, and the plasma temperature under a given gas pressure could subsequently be calculated assuming local thermodynamic equilibrium (LTE), once the gas composition has been determined by mass spectrometry. The gas velocity of the plasma jet could also be obtained by using the Bernoulli equation and stagnation pressure. Secondly, the influence of flow rates of central gas and precursor feed rates on the Al2O3 precursor evaporation has been investigated by process monitoring. Optical emission spectroscopy (OES) and laser light extinction (LE) measurements have been carried out to monitor in-situ the injection of the alumina precursors and to obtain information on the ongoing precursor evaporation. The emission line intensities of the aluminum vapor were used to determine the dependence of process parameters on precursor evaporation. Furthermore, the laser light extinction was used to calculate the number density of non plasma-treated powders, from which the number fraction of evaporated powders could be deduced. The size and morphology of the synthesized particles have been characterized ex-situ. Finally, the influence of quenching conditions on the condensation and formation of the nano-sized alumina particle has been studied by process analysis of the gas phase reactions of alumina. Optical emission spectroscopic was performed at different axial positions to monitor the gas phase reactions of the vaporized particles. Axial profiles of the emission line intensities of the main vapor species of alumina, Al(g) and AlO(g), were compared with the results of the enthalpy probe measurements and the thermal decomposition reactions of alumina found in literature. The results allow explanation of the alumina reactions in the thermal plasma as well as gives indications for the optimal axial position of the quenching gas injection. In addition to the determination of the optimal quenching position, the influence of the flow rates of quenching gas on the nanoparticle formation has been investigated. The synthesis of the submicron alumina using vapor-condensation principle predominantly leads to various thermodynamically metastable crystal structures called transition alumina. Therefore, the occurrence of different transition aluminas is discussed with regards to the flow rates of the quenching gas. The results presented in this thesis show that the synthesis process of the nanoparticle can be described on the basis of experimental results and basic information on the powder materials. The explanation of the influence of process parameters on powder evaporation and nanoparticle formation shows the usefulness of the in-situ plasma process monitoring, and the large potential for optimizing and controlling the nanopowder synthesis process in an inductively coupled RF thermal plasma.

EPFL2006

Improved United-Atom Force Field for 1-Alkyl-3-methylimidazolium Chloride

Ting Chen, Berend Smit

We have developed a united atom (UA) nonpolarizable force field for 1-alkyl-3-methyl-imidazolium chloride ([Cnmim][Cl], n = 1, 2, 4, 6, 8), a potential solvent for the pretreatment of lignocellulosic biomass. The charges were assigned by fitting the electrostatic potential surface (ESP) of the ion pair dimers. The Lennard-Jones parameters of the hydrogen atoms on the imidazolium ring were adjusted to agree with the ab initio optimized geometries of isolated ion pairs. Molecular dynamics (MD) simulations were performed for a wide range of temperatures to validate the force field. Substantial improvements were found in both the dynamical properties and the fluid structures, as compared to those predicted using our previously developed UA force field (UA2006) (Phys. Chem. Chem. Phys. 2006, 8, 1096). Liquid densities were found to lie within 2% experimental data. The simulated heats of vaporization decreased about 30% relative to that predicted using the UA2006 force field. The site-site radial distribution functions between the hydrogen atoms on the imidazolium ring and the chloride anions were in good agreement with those determined by ab initio molecular dynamics. The newly developed force field gives a much better description of the self-diffusion coefficients and shear viscosities, which usually deviate by 1 order of magnitude when determined using other force fields. © 2010 American Chemical Society.

2010

Heat Transfer and Pressure Drop in the Dryout Region of Intube Evaporation with Refrigerant/Lubricant Mixtures.

Daniel Favrat, John Richard Thome, Olivier Zürcher

A summary of the objectives and conclusions reached in this project are: •A litterature search on post dryout heat transfer for pure refrigerants and for refrigerant-oil mixtures was performed and a comprehensive review written (Chapters 9, 10, and 11); •The correct thermodynamic definition of the heat transfer coefficient for evaporation of refrigerant-oil mixtures was presented and then used in the project, i.e. defining the heat transfer coefficient using the local bubble point temperature Tbub of the refrigerant-oil mixture rather than with the saturation temperature of the pure refrigerant Tsat incorrectly used in prior studies (Chapter 3); •A new generalized approach, that is simple to implement, was developed for predicting bubble point temperatures of refrigerant-oil mixtures for miscible oils that can be applied to any refrigerant-oil combination and is accurate over the local oil concentration range from 0-50 wt.% oil range confronted in direct- expansion evaporators with 0-5 wt.% oil in their refrigerant charge (Chapter 4); •A new thermodynamic method for preparation of temperature-enthalpy- vapor quality (T-h-x) curves for refrigerant-oil mixtures for general application was developed and validated against test data (Chapters 5 and 6), including a simple expression for calculating local oil concentrations as a function of vapor quality; •New, accurate thermodynamic methods for preparation of temperature-enthalpy-vapor (T-h-x) curves for the refrigerant blend R-407C and R-407C/oil mixtures were developed for thermal design of evaporators and for the present experimental test, and was validated against measured test data (Chapter 12); •Recommendations were made on how to include oil effects and refrigerant-oil T-h-x curves into evaporator design methods (Chapter 7); •The effects of oil on set-point parameters for control devices were analyzed and some recommendations made (Chapter 8); •An online oil concentration measurement system was perfected using a high accuracy density flowmeter that allowed oil concentrations to be measured accurately and continuously during experiments, such that oil holdup in the heat transfer test sections at high vapor quality could be identified and the correct inlet oil concentrations be measured, and thus utilized to reduce experimental data; a simpler industrial method for applying the measurement system was also developed and presented (Chapter 15); •Experimental results (local heat transfer coefficients, two-phase pressure drops and some two-phase flow patterns) were obtained for R-134a and R-134a/oil mixtures evaporating in plain and microfinned tubes over a wide range of test conditions (Chapters 16 and 17); •Experimental results (local heat transfer coefficients, two-phase pressure drops and some two-phase flow patterns ) were obtained for R-407C and R- 407C/oil mixtures evaporating in plain and microfinned tubes over a wide range of test conditions (Chapters 18 and 19); •The recently proposed flow boiling model and flow pattern map of Kattan-Thome-Favrat, that importantly models heat transfer coefficients based on local flow pattern and predicts the onset of dryout in horizontal tubes and local heat transfer coefficients in partial dryout regimes, was described in detail; the flow pattern map is also useful to designers wishing to design in specific flow pattern regimes and avoid inefficient heat transfer regimes, such as mist flow and stratified flow (Chapters 20 and 21); •The stratified-wavy flow regime model was modified based on the new test data at high vapor quality for pure R-134a and R-407C, which increased the accuracy of the method significantly in this flow regime, now referred to as the “modified” Kattan- Thome-Favrat flow boiling model (Chapter 22); •The modified Kattan- Thome-Favrat flow boiling model and flow pattern map were shown to accurately predict R-407C heat transfer and flow pattern data and hence is recommended for industrial use for designing direct-expansion evaporators with refrigerant blends [the original version was also verified for R-402 and R-404A blends, see Chapter 21] (Chapter 22); •The effect of local refrigerant-oil viscosity on liquid-phase convection in flow boiling heat transfer and on flow patterns was successfully incorporated into the modified Kattan-Thome-Favrat model, establishing the first flow boiling design method that predicts oil effects on heat transfer (with quite reasonable accuracy too), and included a simple method for calculating the values of µref-oil for use in the model (Chapter 23); •The oil effects on microfin heat transfer coefficients were analyzed and heat transfer augmentation ratios were determined for R-134a/oil and R-407C/oil mixtures (Chapter 25); •Plain tube, two-phase pressure drop gradients for R-134a and R-407C at three mass velocities were compared to the Friedel frictional two-phase pressure drop correlation, attaining accurate results and also microfin pressure drop augmentation ratios were determined (Chapter 25); •The ratio of [µoil/µref]mw, derived from the Arrhenius logrithmic viscosity mixing rule and local oil concentration w, was shown to accurately predict the effect of oil on two-phase pressure drop gradients at high vapor qualities, with an empirical exponent m = 0.18355 found for R-134a/oil mixtures (no foaming) and an empirical expression for m for R-407C/oil mixtures (with foaming). This ratio incorporated into the Friedel correlation accurately predicts two-phase pressure drops of evaporating refrigerant-oil flows (Chapter 25).