Non-azeotropic Mixtures Research Articles

Analysis and experimental study of the heterogeneous nucleation process in the boiling of mixed refrigerants

The heterogeneous nucleation process is important for the study of the nucleate boiling of the mixed refrigerants. In this paper, the onset of heterogeneous bubble nucleation of non-azeotropic mixtures was analysed based on the change in the Gibbs free energy. From the calculated results, the critical radius, change in availability, onset of boiling (ONB) superheat and heat flux were determined. The results showed that the critical radius and the maximum change in the availability of the mixtures was higher than that of the corresponding pure fluids. The ONB superheat first increased and then decreased when the mole fraction of the high boiling point component was increased. In addition, experimental tests were conducted to measure the ONB temperature and heat flux of R124, R22 and R124/R22 mixtures on a copper surface at the pressure of 0.7–0.85MPa. The ONB superheat and heat flux first increased and then decreased when the concentration of R124 was increased. Additionally, the experimental ONB superheat and heat flux were used to validate the correlations of other models as well as the model presented here. Our model shows much better agreement with the experimental data. Most of the experimental results had an error of +20% to −40%.

Numerical simulation of tubes-in-tube heat exchanger in a mixed refrigerant Joule–Thomson cryocooler

Mixed refrigerant Joule-Thomson (MRJT) cryocoolers can produce cryogenic temperatures with high efficiency and low operating pressures. As compared to the high system pressures of around 150–200 bar with nitrogen, the operational pressures with non-azeotropic mixtures (e.g., nitrogen-hydrocarbons) come down to 10–25 bar. With mixtures, the heat transfer in the recuperative heat exchanger takes place in the two-phase region. The simultaneous boiling and condensation of the cold and hot gas streams lead to higher heat transfer coefficients as compared to single phase heat exchange. The two-phase heat transfer in the recuperative heat exchanger drastically affects the performance of a MRJT cryocooler. In this work, a previously reported numerical model for a simple tube-in-tube heat exchanger is extended to a multi tubes-in-tube heat exchanger with a transient formulation. Additionally, the J-T expansion process is also considered to simulate the cooling process of the heat exchanger from ambient temperature conditions. A tubes-in-tube heat exchanger offers more heat transfer area per unit volume resulting in a compact design. Also, the division of flow in multiple tubes reduces the pressure drop in the heat exchanger. Simulations with different mixtures of nitrogen-hydrocarbons are carried out and the numerical results are compared with the experimental data.

Theoretical Investigation on the Properties of R744/R290 Mixtures

At present, an interesting research is to find promising environmentally-friendly refrigerants in auto-cascade refrigeration cycle. This paper proposed a refrigerant mixture of R744 and R290 due to its zero ozone depletion potential and negligible global warming potential. The properties of R744, R290 and the mixtures refrigerant were analyzed. And performances comparisons between R744, R290, R22 and R134a were investigated based on the simulation data. Then, the vapor-liquid phase equilibrium properties of mixture were calculated. And the conversion relations of total mole fraction, vapor phase mole fraction, liquid phase mole fraction, quality, equilibrium temperature and pressure were systematically analyzed. The results show that R744 and R290 are potential refrigerant. But each pure refrigerant has obvious disadvantage, it is a good choice that R744 and R290 are mixed together into non-azeotropic mixture, which is suitable for auto-cascade refrigeration cycle due to a large temperature glide during the phase changing.

Performance Research on Heat Pump Using Blends of R744 with Eco-friendly Working Fluid

In order to protect the environment and save energy, new refrigerants with zero ozone depleting potential, low global warming potential have been investigated by more and more researches to substitute HCFCs/ HFCs for eco-friendly working fluid. Among alternatives, non-azeotropic mixtures are becoming the more potential and important candidates. In this research, the transcritical system performances of water heater heat pump using R744/DME (dimethyl ether) binary mixture as working fluid were theoretically analyzed. On mix R744 with DME in a smaller mass fraction, both the optimum heat rejection pressure and the heating coefficient of performance (COP) are decreased. The mean relative reduction rate of optimum heat rejection pressure, however, is greater than that of heating COP. Also, the influence of superheat degree at the inlet of compressor upon the optimum heat rejection pressure and the heating COP is also discussed. The results show that the R744/DME can work as a promising alternative to replace with current widely-used Freons.

Experimental and Numerical Study of the Radiant Induction-Unit and the Induction Radiant Air-Conditioning System

In this paper we proposed the novel air-conditioning system which combined induction ventilation and radiant air-conditioning. The indoor terminal device is the radiant induction-unit (RIDU). The RIDU is the induction unit combined with the pore radiant panel on which the copper pipes with rigid aluminum diffusion fins are installed. The two-stage evaporator chiller with the non-azeotropic mixture refrigerant is utilized in the system to reduce the initial investment in equipment. With the performance test and the steady state heat transfer model based on the theory of radiative heat transfer, the relationship between the induction ratio of the RIDU and the characteristic of the air supply was studied. Based on this, it is verified that the RIDU has a lower dew-point temperature and better anti-condensation performance than a traditional plate-type radiant panel. The characteristics of the radiation and convection heat transfer of the RIDU were studied. The total heat exchange of the RIDU can be 16.5% greater than that of the traditional plate-type radiant terminal.

Error estimation of single phase effectiveness and LMTD methodologies when applied to heat exchangers with phase change

Single phase formulas based on logarithmic mean temperature difference (LMTD) or Effectiveness (ɛ-NTU) are widely and wrongly used for the thermal analysis of evaporators and condensers. Those formulas do not take into account that temperature variation during phase change is due to pressure variation and/or concentration changes when using nonazeotropic refrigerant mixtures. This paper first presents the correct evaluation of the mean temperature difference and effectiveness, for parallel and counter flow arrangements, under the hypothesis of linear temperature variation, for both evaporator and condenser cases. Then, the analytical solution is employed to evaluate the error of applying the single phase formulas of LMTD and Effectiveness to the phase change part of evaporators and condensers.

Experimental evaluation of the in-tube condensation heat transfer of pure n-pentane/R245fa and their non-azeotropic mixture as an ORC working fluid

The existence of temperature gliding in non-azeotropic mixture working fluids provides a promising opportunity for greatly improved resource utilization in organic Rankine cycle (ORC) systems for geothermal applications. The condensation heat transfer degradation associated with working fluid mixtures is an unavoidable penalty that may reduce the utilization and system benefits by necessitating larger heat exchangers. The objective of the current study is an experimental investigation of condensation heat transfer for R245fa, n-pentane, and a composition of their mixture. Quasi-local condensation heat transfer coefficients are measured in a straight horizontal smooth tube with flow conditions at mass flux G=100 and 150kg(m2s)−1, local equilibrium quality x=0.05–0.80, and reduced pressures Pr=0.05–0.15. The reduced condensation heat transfer coefficients from the test are compared with the predictions from already established mixture correction methods in the literature. Pure components R245fa and n-pentane condensation heat transfer data are well predicted by Shah (1979) correlations for all test conditions. However, their non-azeotropic mixture condensation heat transfer data require additional correction parameter to account for the heat transfer degradation. This correction parameter is correlated by the temperature gliding gradient of mixture fluids. Test data with mixture working fluids from current study are reasonably well predicted by the corrected in-tube correlation.

Adsorption isotherm of non-azeotropic solution onto porous adsorbents

Adsorption isotherm is essential component in the understanding of the adsorption process. Several methods of the measurements, analysis and interpretation of adsorption from solution have been reported in the literature. Most of the measurements of adsorption isotherm from solution were involved the measurement of excess isotherm conducted at low region of sorbates concentration. Direct interpretation of excess adsorption isotherm as adsorption isotherm is always been practice. Therefore, in this work a study on the measurement of the adsorption isotherm from solution of non-azeotropic organic solvent mixture onto porous adsorbents for whole range of liquid concentration was conducted. The study included the measurement of excess adsorption isotherm using conventional technique. Theoretical analysis and interpretation of adsorption isotherm from the excess isotherm were conducted using Pseudo Ideal Adsorption, Gibbs Dividing Plane Model and Langmuir-Fruendlich binary isotherm model. For organic solvents, acetone and propanol were chosen as the adsorbates due to the non-azeotropic properties in the mixture. Activated carbon and silicalite were chosen as adsorbents due to the different in their porosity such as macro porous and micro porous structure. The result of the study has revealed that the adsorption isotherm of non-azeotropic mixture onto activated carbon and silicalite can be interpreted as monolayer type of adsorption.

The exploitation of the physical exergy of liquid natural gas by closed power thermodynamic cycles. An overview

The world trade in LNG (liquefied natural gas) has tripled in the last 15 years and the forecasts are for its further rapid expansion. Although the cryogenic exergy of the LNG could be used in many industrial processes, it is recognized also as a source for power cycles.When using the low temperature capacity of LNG for power production, several thermodynamic cycles can be considered. This paper reports the state-of-the art of the most relevant solutions based on conventional and non-conventional thermodynamic closed cycles. Moreover, a novel metrics framework, suitable for a fairer comparison among the energy recovery performances of the different technologies is proposed. According to the defined indicators the compounds plants with gas turbine and closed Brayton cycles perform really better, with an almost full use of LNG available cold temperature and a fuel consumption with an efficiency better than that of the current combined cycles. The Rankine cycles with organic working fluids (pure fluids or non-azeotropic mixtures) using seawater or heat available at low temperature (for instance at 150 °C) also perform in a very satisfactory way. Real gas Brayton cycles and carbon dioxide condensation cycles work with very good thermal efficiency also at relatively low maximum temperatures (300 ÷ 600 °C) and could have peculiar applications.

Predicting Methods for Flow Boiling Heat Transfer of a Non-Azeotropic Mixture Inside a Single Microchannel

At this time, a widely accepted model that can predict flow boiling heat transfer in microchannels with different fluids, geometries, and operative conditions is still missing. Depending on the working fluid, a predicting correlation can lead to accurate estimation or give rise to errors up to 50% and higher. The situation is further complicated when the working fluid is a zeotropic mixture of two components, due to the additional mass transfer resistance that must be estimated. In the recent years much attention has been paid to the possible use of fluorinated propene isomers in substitution for high-global-warming-potential refrigerants. The available hydrofluoroolefins cannot cover all the air-conditioning, heat pump, and refrigeration applications when used as pure fluids because their thermodynamic properties are not suitable for all the operating conditions, and therefore some solutions may be found using blends of refrigerants, to satisfy the demand for a wide range of working conditions. The adoption of new mixtures poses the problem of how to extend the correlations developed for pure fluids to the case of flow boiling of mixtures in microchannels. In this work, a mixture of R1234ze(E) and R32 (0.5/0.5 by mass) has been considered: The local heat transfer coefficient during flow boiling of this mixture in a single microchannel with 0.96 mm diameter has been measured at a pressure of 14 bar, which corresponds to a bubble temperature of around 26°C. This flow boiling database, encompassing more than 300 experimental points at different values of mass velocity, heat flux, and vapor quality, is compared with available correlations in the literature. The introduction of a correction to account for the additional mass transfer resistance is discussed, and such correction is found to be necessary and proper to provide a correct sizing of the evaporator.

Numerical analysis of the two-phase heat transfer in the heat exchanger of a mixed refrigerant Joule–Thomson cryocooler

The recuperative heat exchanger is the most critical component of a mixed refrigerant Joule–Thomson cryocooler. The heat transfer process in such a heat exchanger takes place under two-phase conditions due to simultaneous boiling of the cold stream and condensation of the hot stream. This results in higher heat transfer coefficients as compared to single phase heat exchange. Moreover, depending on the composition of non-azeotropic mixtures, the boiling and condensation take place over a range of temperatures. In this work, the two-phase heat transfer in the recuperative heat exchanger of a mixed refrigerant Joule–Thomson cryocooler is studied. A numerical model is developed to simulate the heat transfer in a helically coiled tube-in-tube heat exchanger with nitrogen–hydrocarbons mixtures. The heat transfer coefficients for the two-phase flow under boiling and condensation are evaluated with the correlations available in the literature. The physical properties of the mixtures are evaluated at local conditions of temperature and pressure. The numerical results obtained with the developed model are compared with the experimental data reported in the literature. Additionally, the model predictions are also compared with new experimental data reported in the present work.

Two-phase pressure drop and condensation heat transfer of R32/R1234ze(E) non-azeotropic mixtures inside a single microchannel

Much attention has been paid in the recent years to the possible use of fluorinated propene isomers for the substitution of high global warming potential refrigerants. Among the fluorinated propene isomers, R1234ze(E) may be a substitute of R134a for refrigeration applications. R1234ze(E) has a global warming potential lower than 1 (considering a period time of 100 years), and it is receiving some attention also as a component of low global warming potential mixtures. In this article, a mixture of R1234ze(E) and R32 has been investigated at two different mass compositions (23/77% and 46/54% by mass) with regard to the performance at the condenser. The local heat transfer coefficients during condensation in a single microchannel with 0.96 mm diameter are measured and analyzed. The frictional two-phase pressure drop in the same channel is also investigated. The present tests are carried out on the experimental apparatus available at the Two-Phase Heat Transfer Lab of the University of Padova. The new experimental data are compared to those of pure R1234ze(E) and R32. This allows the heat transfer penalization due to the mass transfer resistance occurring during condensation of these zeotropic mixtures to be analyzed and suitable predicting models to be assessed. The knowledge of heat transfer coefficient and pressure drop allows evaluation of the overall performance of these mixtures when used in condensers.

Novel Procedure for Assessment of Feasible Design Parameters of Dividing-Wall Columns: Application to Non-azeotropic Mixtures

Dividing wall columns (DWCs), as a subset of fully thermally coupled distillation systems (FTCDS), is considered as one of most appealing distillation technologies to the chemical industry, because it can bring about substantial reduction in the capital investment, as well as savings in the operating costs. This study targets on how to improve the energy efficiency of DWCs by achieving their well-designed feasible parameters. Two methods are applied to study the effect of liquid and vapor split ratios including a shortcut method and a method of systematic calculations by using differential equation profiles. In the latter approach, differential composition profiles in each column section are obtained by considering feasible key design parameters. The finding of pinch points for each section profiles allowed determining the limiting values of the operating parameters. The intersections of these profiles are used to get well-designed feasible parameters of the liquid and vapor split ratios in an attempt to obtain the desired purities of the top, bottom, and side-stream products. The obtained parameters are validated by rigorous simulations. Three types of case studies involve the separation of hydrocarbons (n-pentane, n-hexane, n-heptane), aromatics (benzene, toluene, p-xylene), and alcohols (ethanol, propanol, butanol).

Nucleate boiling at the forced flow of binary non-azeotropic mixtures in horizontal tubes

Analysis of experimental values of heat transfer coefficients obtained through investigation of nucleate boiling of the two-component non-azeotropic mixtures inside the horizontal smooth tubes by various authors is presented. In the zone of nucleate boiling, the experimental data are in good agreement with the calculation dependence.

A numerical investigation of flow boiling of non-azeotropic and near-azeotropic binary mixtures

The flow and heat transfer characteristics of binary refrigerant mixtures in a heated horizontal tube were investigated numerically. The pressure drop, temperature profile, and heat transfer coefficient for non-azeotropic and near-azeotropic mixtures of different bulk compositions were obtained. It is found that the non-linear physical properties of the mixtures strongly affect the pressure drop characteristics. Both the fluid properties and mass transfer resistance are responsible for the heat transfer characteristics. The mass transfer resistance has a more significant influence on the nucleate boiling than the convective evaporation for non-azeotropic mixtures, while the resistance can be neglected for near-azeotropic mixtures.

열전달 촉진관에서 HFC32/HFC152a 혼합냉매의 외부 응축열전달계수

In this study, external condensation heat transfer coefficients (HTCs) of two non-azeotropic refrigerant mixtures of HFC32/HFC152a at various compositions were measured on both 26 fpi low-fin and Turbo-C enhanced tubes, of 19.0 mm outside diameter. All data were taken at the vapor temperature of 39℃, with a wall subcooling of 3∼8 K. Test results showed that the HTCs of the tested mixtures on the enhanced tubes were much lower than the ideal values calculated by mass fraction weighting of the pure component HTCs. Also, the reduction of HTCs due to the diffusion vapor film was much larger than that of a plain tube. Unlike HTCs of pure fluids, HTCs of the mixtures measured on enhanced tubes increased, as the wall subcooling increased, which was due to the sudden break-up of the vapor diffusion film with an increase in wall subcooling. Finally, the heat transfer enhancement ratios for mixtures were found to be much lower, than those of pure fluids.

Effect of Ultrasound on Liquid Phase Adsorption of Azeotropic and Non-azeotropic Mixtures: Generation of Adsorption Isotherms According to Gibbs Dividing Plane Theory

Effect of Ultrasound on Liquid Phase Adsorption of Azeotropic and Non-azeotropic Mixtures: Generation of Adsorption Isotherms According to Gibbs Dividing Plane Theory

Modified basic distillation configurations with intermediate sections for energy savings

Basic distillation configurations have been well studied, comprising of energy efficient (n − 1)‐column configurations that can be used to separate an n‐component nonazeotropic feed mixture into its components. In this article, we present new (n − 1)‐column configurations which differ from the conventional basic configurations due to the introduction of extra intermediate sections and the additional transfer‐streams associated with submixtures of these sections. We demonstrate using four‐component examples that these small differences lead to some interesting nonintuitive physical effects in the new configurations, resulting in large energy savings compared to the basic configurations. The proposed configurations offer more operable and energy efficient options for onsite implementation than the corresponding optimally operated basic configurations. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1091–1097, 2014

Theory and Experimental Study of Two-stage Auto-cascade Low Temperature Refrigerator

It is increased gradually in scientific research and daily life of two-stage auto-cascade low temperature refrigerator. Study and improve the refrigeration system performance plays an important role; characteristics of energy saving and environmental protection research also has important significance. Single-stage compression two-stage auto-cascade refrigeration system using non-azeotropic mixture refrigerant, the low temperature environment could be realized according to the pressure and temperature changes of refrigerant in the low temperature refrigerator and the relationship of vapor-liquid equilibrium. Three dimensional pressure-enthalpy diagram and enthalpy-composition diagram of two-stage auto-cascade cycle are analyzed using refrigerant properties analysis software NIST Refprop 8.0. The mixed refrigerant R600a and R23 is discussed and selected suitable for –60 ℃low temperature refrigerator, and the operation temperature and pressure of system are analyzed. The temperature and pressure and the refrigerant substances at the conjunction changes with time of two-stage auto-cascade low temperature refrigerator are studied through experimental methods. A relatively complete concept of two-stage auto-cascade refrigeration system for low temperature refrigerator is established; a rational realization of the temperature and pressure changes and vapor-liquid equilibrium is recognized. It is important to improve the performance of this kind of refrigerator and the products popularization.

Flow boiling heat transfer coefficients at cryogenic temperatures for multi-component refrigerant mixtures of nitrogen–hydrocarbons

The recuperative heat exchanger governs the overall performance of the mixed refrigerant Joule–Thomson cryocooler. In these heat exchangers, the non-azeotropic refrigerant mixture of nitrogen–hydrocarbons undergoes boiling and condensation simultaneously at cryogenic temperature. Hence, the design of such heat exchanger is crucial. However, due to lack of empirical correlations to predict two-phase heat transfer coefficients of multi-component mixtures at low temperature, the design of such heat exchanger is difficult.The present study aims to assess the existing methods for prediction of flow boiling heat transfer coefficients. Many correlations are evaluated against available experimental data of flow boiling of refrigerant mixtures. Silver-Bell-Ghaly correlation and Granryd correlation are found to be more suitable to estimate local heat transfer coefficients. A modified Granryd correlation is recommended for further use.