Mixtures Of Isobutane Research Articles

Fine tuning of gas system and performance studies of RPC detectors in the INO's mini-ICAL detector

The India-based Neutrino Observatory's (INO) Iron CALorimeter (ICAL) experiment will be instrumented with about 28,800 large area glass Resistive Plate Chambers (RPCs) as active detector elements. The mini-ICAL, which is a 600-times scaled-down prototype version of the ICAL, is currently in operation at the Inter Institutional Centre for High Energy Physics (IICHEP), a transit campus of the INO project in Madurai, India. The 4 m×4 m detector comprises an ∼ 85 t magnet, built using 11 layers of iron plates to accommodate 10 layers of RPCs — two RPCs per layer. A closed-loop gas system (CLS) is designed to supply gas mixture of R134a (95.2%), isobutane (4.5%), and SF 6 (0.3%) to the mini-ICAL, collect the return gas from the RPCs, purify it using filters, and recirculate the mixture into the detector. Contamination of any kind into the gas mixture will severely affect the characteristics of the RPC detectors, such as the dark current and noise rates, and consequently deteriorate performance of the RPC detector. The contamination is mainly caused due to leaks within the gas system, in the gas lines and in the RPC gas gap volume. This paper deals with systematic identification and mitigation of leaks within the closed-loop gas system and performance improvement of the RPCs installed in the mini-ICAL detector.

Isothermal (vapor ＋ liquid) equilibrium measurements and correlation of the binary mixture 3,3,3-trifluoropropene (R1243zf) ＋ isobutane (R600a) at temperatures from [formula omitted] to [formula omitted] K

The restrictions set by the F-gas Regulation and the Kigali Amendment to the Montreal Protocol have pushed an extensive research into alternatives for fluorinated greenhouse gases in air conditioning and refrigeration. Hydrofluoroolefins (HFOs) are emerging as promising substitutes for hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) in HVAC and refrigeration. Amongst these, R1243zf raised interests as low-GWP substitutes for R134a. The pursuit of low GWP refrigerants has also led to the examination of hydrocarbons (HCs), like propane (R290) and isobutane (R600a), due to their efficiency, low charge, and affordability, despite their flammability. The mixtures of R1243zf and isobutane could be of interest for medium temperature heat pump applications. Moreover, blending HFOs with HCs usually leads to azeotropic mixtures. While HCs benefit of a wide range of data and reliable Equations of State (EoS), HFOs, and in particular their mixtures, lack comprehensive information. This study reports vapor–liquid equilibrium (VLE) measurements for the 3,3,3-trifluoropropene (R1243zf) + isobutane (R600a) binary system in the temperature range between 283.15 K and 323.15 K. The measurements have been carried out by means of a vapor-recirculation apparatus combined with gas-chromatographic analysis. This work reports 53 experimental data with an expanded uncertainty (k=2) of 0.003 mol mol−1. The reported data, along with the data available in the literature, have been used to developed two new mixture models based on the Helmholtz free energy EoS, both leading to a significant improvement in the VLE prediction, with RMSE values lower than 0.005 mol mol−1 for both the liquid and the vapor phase composition.

Phase Behavior of Isobutane + CO2 and Isobutane + H2 at Temperatures Between 190 and 400 K and at Pressures Up to 20 MPa

Mixtures containing isobutane, carbon dioxide, and/or hydrogen are found in various industrial processes, green refrigerant systems, and the growing hydrogen industry. Understanding the thermophysical properties of these mixtures is essential for these processes, and depends on reliable experimental data. Making use of an automated static-analytical apparatus, measurements were made of the phase behavior of binary mixtures of isobutane with CO2 and with H2, extending the range of available data for both mixtures. Measurements of the system isobutane + CO2 were carried out along three isotherms at temperatures of (240, 280, and 310) K with pressures from the lower limit of the sampling system (~ 0.5 MPa) to the mixture critical pressure. The results exhibit good agreement with literature data. Measurements on isobutane + H2 were carried out along nine isotherms at temperatures of (190, 240, 280, 311, 339, 363, 375, 390, and 400) K with pressures up to 20 MPa, covering a much broader range of conditions than the one prior investigation. The results have been used to optimize temperature-dependent binary parameters in the Peng–Robinson equation of state with two different mixing rules. This approach was found to perform well in comparison to alternative models.

Effect of two halogenated olefins on combustion characteristics of Isobutane

To reduce the greenhouse effect, environmentally friendly working fluids such as unsaturated halogenated olefins and hydrocarbons have attracted much attention. This study focuses on the combustion characteristics of two binary mixtures of isobutane (R600a) and R1234ze(E) and R1233zd(E). The results indicate that the addition of R1234ze(E) and R1233zd(E) has different effects on the combustibility of R600a under rich and lean fuel conditions. From the perspective of flammability limit range, as the ratio of R1234ze(E) and R1233zd(E) increases, the flammability limit range of both mixtures first increases and then decreases. For R1233zd(E)/R600a containing chlorine (Cl), the maximum flammable limit range corresponds to (nF + nCl)/nH equal to 1/3, which is significantly smaller than the nF/nH ratio of 1 in R1234ze(E)/R600a. Secondly, through calculation and simulation, it was found that the flame retardancy of R1233zd(E) is stronger than that of R1234ze(E), as the Cl in the former has a stronger ability to capture active radicals. Finally, based on Chemical kinetics analysis, it is found that adding R1234ze(E) to R600a during oxygen enrichment will increase the concentration of active radicals H, O and OH during combustion. This leads to higher combustion temperature and smaller flame thickness, thereby promoting the combustion process. The results will have significant implications for the safe application of new mixed working fluids and the analysis of internal reaction mechanisms of the mixture flammability.

Propane-isobutane mixtures in heat pumps with higher temperature lift: An experimental investigation

The shift towards renewable energies leads to an increased interest in heat pumps, not only for floor heating at temperatures around 40 °C, but also for providing hot water at higher temperatures (>60 °C). To reach the goal of a climate neutral process, it is important to use working fluids with a low global warming potential (GWP). Zeotropic mixtures are discussed to potentially increase the coefficient of performance (COP) of a heat pump compared to pure fluids. The present work experimentally analyses a compression heat pump process of such fluids; five mixtures of propane (R290) and isobutane (R600a) are investigated here. The influence of composition, compressor frequency, inlet temperature of the working fluid into the evaporator and the inlet temperature of the secondary fluid water into the condenser are researched. Besides the COP, the destroyed exergy in the components are evaluated, to find the most relevant losses. It is shown that the COP for a temperature lift of 33 K are above 3 for the mixtures, while at a higher temperature lift of 53 K the COP is around 2. Furthermore, a strong impact of the overall compressor efficiency on the process was found. The results show that the advantage of the zeotropic mixture within the heat exchangers is often limited by their low contribution to the total specific exergy destruction compared to the compressor. The compressor efficiency changes with the composition, due to composition dependent pressure and density variations. The often-neglected changes in electro-mechanical losses get important, when the pressure ratios are changing with composition, as shown experimentally.

Pyrolysis mechanism of HFO-1234yf/iso-butane mixture: ReaxFF reactive molecular dynamic simulation study

HFO-1234yf/iso-butane mixture is a promising working fluid of organic Rankine cycle, but the working fluid will be pyrolyzed under the high temperature heat source. In this study, the pyrolysis mechanism of HFO-1234yf/iso-butane mixture was studied by using density functional theory method and ReaxFF reactive force field. The initial pyrolysis reactions of HFO-1234yf and iso-butane, the pyrolysis process, the effects of HFO-1234yf to iso-butane ratio and temperature on the pyrolysis of HFO-1234yf/iso-butane mixture were investigated. The results indicated that the iso-butane of the HFO-1234yf/iso-butane mixture inhibited the pyrolysis of HFO-1234yf, the thermal stability of HFO-1234yf/iso-butane mixture was better than that of pure HFO-1234yf. Therefore, HFO-1234yf/iso-butane mixture has better thermal stability than HFO-1234yf and can match higher temperature heat source.

Glazing as a bonding system for zirconia dental ceramics

Studies have reported challenges of debonding of dental zirconia crowns to from luting cement and prepared teeth. The aim of the study was to explore the application of dental glazing systems for enhancing the bonding of zirconia dental ceramics to luting resin cement. Commercial glaze powder and liquid (Vita Akzent) and experimental mica-based glaze powders were used for the study. X-ray diffraction analysis of the glaze powders (XRD) and Fourier Transform InfraRed Spectroscopy (FTIR) was done on the glaze liquid. Sandblasted sintered dental zirconia (Katana, Noritake) were the control samples. Glazed zirconia samples were coated with commercial glaze and experimental glaze powders which were further etched with 5% hydrofluoric acid. Shear bond strengths of sandblasted and glazed zirconia samples to resin composites were evaluated. XRD of commercial and experimental glaze powders revealed a broad peak confirming the amorphous nature of glass and FTIR analysis of the glaze liquid revealed symmetrical stretching (CH2-CH3) of the alcohol group indicating a mixture of iso-butane and ethanol. Glazed and etched zirconia showed significantly higher shear bond strength to resin cement compared to sandblasted zirconia. The study confirms the glassy nature of dental glaze powders and the presence of ethanol-based mixtures in the commercial glaze liquid. Glazing systems have the potential to be explored for enhancing the bonding of non-etchable zirconia ceramics to resin cement and tooth substrates.

Reasonable performance comparison for a refrigeration system using different refrigerants: Case with propane and isobutane mixtures with several compositions

Traditional refrigerants such as HCFC and HFC have a great influence on global warming. On the other hand, natural refrigerants have little impact on the environment, and low global warming potential (GWP) and zero ozone depletion potential (ODP) are key-characteristics of natural refrigerants. Hydrocarbon refrigerant is one of the natural refrigerants with higher COP than other natural refrigerants. Furthermore, performance improvement is promoted by mixing hydrocarbon refrigerants. Due to their characteristics, refrigerant mixtures have an advantageous heat exchange performance over a single refrigerant. Generally, refrigerant mixtures have temperature glide during phase change. Because of this, temperature difference can be maintained uniform during the heat exchange with secondary fluid. In this study, cooling cycle using isobutane/propane mixtures is researched to maximize the performance when changing mixing ratio by simulation. However, refrigerants with different isobutane ratios can be considered completely different refrigerants. Therefore, a good methodology is needed to compare performances of cycle. In this study, unlike previous studies, various methodologies are used to compare the performance of refrigerant mixtures. Also experiments for isobutane/propane mixtures were conducted. Various temperature conditions are used in the simulation study, and outdoor temperature of 35 °C and indoor temperature of 27 °C were used in the experiment to validate the simulation results.

Experimental Investigation of the Effect of a Recuperative Heat Exchanger and Throttles Opening on a CO2/Isobutane Autocascade Refrigeration System

An experimental evaluation of an autocascade refrigeration (ACR) system was carried out. A zeotropic mixture of isobutane and CO2 was employed as a working fluid in an autocascade refrigeration (ACR) system. An experimental system was designed and built to study the influence of the recuperative heat exchanger (RHX) and openings of the throttle valves on the system performance. The use of RHX facilitated the condensation process and improved the cycle characteristics. The working mass concentration of CO2 was higher, as it was closer to the nominal concentration and the discharge pressure was lower by 19% to even 39% when the RHX was employed in the system. An increase of up to 20% in the coefficient of performance (COP) was observed. Furthermore, the effects of the openings of the throttle valves on the system characteristics were studied. The change in the openings of the expansion valves affected the mass flows and the working mixture composition. The working CO2 mass fraction increased with higher openings of the evaporator throttle. The subcooling degree of liquid CO2-rich refrigerant increased with higher openings of the expansion valve under the phase separator. The results of the present work should be helpful for design and optimization of autocascade systems working with natural and synthetic refrigerants.

Outgassing and leak test of INO-ICAL RPC detectors

Glass Resistive Plate Chambers (RPCs) of size 2 m × 2 m, operated in avalanche mode will be used as an active detector element at the INO-ICAL experiment. The RPC system of the ICAL experiment will operate with a gas mixture of C2H2F4 (94.5%), isobutane (5%) and SF6 (0.5%). To fulfil the physics goals, about 29,000 RPCs will be used which are expected to last for 20 years or more. The quality and purity of the gas plays a vital role in the stable operation of RPC detectors. The presence of impurities in a gas mixture contribute towards the degradation of detector performance. The various assembly materials like glues, button spacers, frames, etc. used in the construction of the chamber may outgas and contaminate the gas mixture. We have performed the very first study to estimate the outgassing due to various materials used in the construction of INO RPCs. The present study includes the results obtained from gas chromatography showing the generation of impurities and dangerous radicals produced due to outgassing when RPC are operated in the cosmic stand. The study also includes a test of purity and effectiveness of the gas mixing system. Since a large amount of gas mixture is to be circulated inside the RPC during the operation of the ICAL detector, a proper leak test will help to minimize gas leaks reducing operational costs and atmospheric air pollution. A proper quantitative leak test is also performed on RPC after assembly by monitoring the absolute pressure inside the chamber, along with the atmospheric pressure and temperature to estimate the leakage rate.

The ultra light Drift Chamber of the MEG II experiment

The MEG experiment at the Paul Scherrer Institute searches for the charged Lepton Flavor Violating μ+→e+γ decay. MEG has already determined in a first data taking phase the world best upper limit on the branching ratio: BR(μ+→e+γ)<4.2×10−13 An upgrade of the whole detector has been approved to obtain a substantial increase in sensitivity. Currently MEG is in upgrade phases, this phase involves all the detectors. The new positron tracker is a high transparency single volume, fully stereo cylindrical Drift Chamber (CDCH), immersed in a non uniform longitudinal B-field, with length of 1.93 m, internal radius of 17 cm and external radius of 30 cm. It is composed of 9 concentric layers, divided into 12 identical sector of 16 drift cells. The single drift cell is approximately square, with a 20 μm gold plated W sense wire surrounded by 40 μm silver plated Al field wires in a ratio of 5:1. The total number of wires amounts to 11904 for an equivalent radiation length per track turn of about 1.5x10−3 X0 when the chamber is filled with a gas mixture of helium and iso-butane. Due to the high wire density (12wires∕cm2), the use of the classical feed-through technique as wire anchoring system could hardly be implemented and therefore it was necessary to develop new wiring strategies. The number of wires and the stringent requirements impose the use of an automatic system (wiring robot) to operate the wiring procedures. Several tests have been performed in different prototypes of the drift chamber, exposed to cosmic rays, test beams and radioactive sources, to fulfill the requirement on the spatial resolution to be less than 110 μm. In this paper we describe the CDCH construction and the first tests during the 2018 Engineering Run.

Evaluated Site-Specific Rate Constants for Reaction of Isobutane with H and CH3: Shock Tube Experiments Combined with Bayesian Model Optimization.

Evaluated site-specific rate constants for the reactions of isobutane with CH3 and H were determined in a combined analysis of new shock tube experiments and existing literature data. In our shock tube experiments, CH3 radicals, produced from the pyrolysis of di- tert-butylperoxide, and H atoms, produced from the pyrolysis of C2H5I, were reacted with dilute mixtures of isobutane in argon at 870-1130 K and 140-360 kPa, usually with a radical chain inhibitor. Propene and isobutene, measured with GC/FID and MS, were quantified as characteristic of H-abstraction from the primary and tertiary carbons, respectively. Using the method of uncertainty minimization using polynomial chaos expansions (MUM-PCE), a comprehensive Cantera kinetics model based on JetSurF 2.0 was optimized to our experiments and available literature data spanning ambient temperatures to 1327 K. Based on Bayes' theorem, MUM-PCE constrains the kinetics model to the experimental data. The isobutane literature data used for optimization included both raw experimental data and reported branching and total rate measurements. Data for ethane were also included to better define the absolute rate constant for abstraction of H from primary carbons. For both H and CH3, the optimization increased the relative rate of tertiary to primary H-abstraction compared with existing estimates, especially at higher temperatures. We combine the present data for primary and tertiary sites with previous results from our group on 1-butane to derive site-specific rate constants for the reaction of H and CH3 with generic primary, secondary, and tertiary carbons suitable for a wide range of temperatures.

Thermodynamic analysis of a novel Ejector Enhanced Vapor Compression Refrigeration (EEVCR) cycle

This paper presents a theoretical thermodynamic analysis of a novel Ejector Enhanced Vapor Compression Refrigeration (EEVCR) cycle using zeotropic mixture of propane and isobutane (R290/R600a) as a refrigerant to replace R134a in domestic refrigerators/freezers. 1 D thermodynamic model for a constant area mixing ejector is used to estimate the cycle performance under the condition of optimal operating regime. The Coefficient of Performance (COP), the Volumetric Cooling Capacity (VCC), Qv and the compressor pressure ratio are studied for the novel EEVCR cycle and the results are compared to the conventional cycle using pure fluid R134a and zeotropic mixture R290/R600a. A comparative study was carried out to determine the propane mass fraction, z in the zeotropic mixture that would be a suitable replacement for R134a. The results indicated that the cycle COP and Qv could be improved by 23% and 62.71%, respectively. In addition, the cycle COP and Qv for the proposed EEVCR cycle were higher by 70% than those for a modified ejector expansion cycle. Finally, the propane mass fraction of 60% in the zeotropic R290/R600a mixture was found to have similar saturation pressure as R134a and the cycle COP and Qv were found similar to those of R134a.

Calorimetric investigation of hydrates of pure isobutane and iso- and normal butane mixtures

This paper shows the accurate calorimetric measurements of the phase equilibrium of pure isobutane, normal butane and their mixture in water-saturated quartz powder and in bulk. The heat capacity, temperature derivative of pressure at constant volume, enthalpy and pressure, as a function of temperature, for binary and ternary systems (water-isobutane, water-normal butane, water-isobutane-normal butane) have been measured in the hydrate's forming range (the temperature range of 234–280 K and pressure up to 10 MPa). Based on experimental data of heat capacity and temperature derivative of pressure at constant volume, the phase diagrams for hydrates of isobutane and mixture of isobutane and normal butane have been constructed. The isobutane-normal butane ratio of gaseous phase has been defined. The lower and upper quadruple points of isobutane hydrate and the quintuple point of isobutane hydrate formed by mixture of butanes have been detected. Isobutane is a hydrate former. Normal butane has an effect on the phase equilibrium of hydrates. An upper quadruple point of the pure isobutane hydrate is transformed into a line of upper quadruple points of the isobutane hydrate, which is in equilibrium with the binary mixture isobutane - normal butane. A line of upper quadruple points starts at the upper quadruple point of pure isobutane hydrate and stretches up to the quintuple point of hydrate formed by the mixture of butanes. The content of normal butane at the quintuple point is 29.9 ± 0.5% mol and does not depend on its concentration in the original mixture (before hydrate formation). The stability of isobutane and normal butane concentration in mixture is provided due to partial dissociation or formation of hydrate. A three-component system (water - normal butane - isobutane) of five phases (water – ice-hydrate - liquid butanes - gaseous butanes) has zero degree of freedom at the quintuple point. There is only one value of pressure, temperature and content of butanes in which five phases can exist together in equilibrium.

Ru/CeO2 catalysts for combustion of mixture of light hydrocarbons: Effect of preparation method and metal salt precursors

Two preparation methods and two metal salt precursors (chlorine containing and chlorine-free) were used to synthesize Ru/CeO2 catalysts and their catalytic behaviour has been evaluated in combustion of propane, n-butane and isobutane mixture. In the first method, Ru nanoparticles (Ru NPs) deposited on CeO2 were synthesized by a microwave-assisted polyol method using RuCl3 as a metal precursor and ethylene glycol as a reducing agent. In the second, Ru/CeO2 catalysts were prepared by traditional incipient wetness impregnation method using RuCl3 or Ru(NO)(NO3)3. ICP-AES, N2 adsorption-desorption, H2-TPR, XRD, HRTEM and SEM-EDS techniques were used to characterize the catalysts. In all the Ru/CeO2 catalysts only highly dispersed Ru nanoparticles (1–2nm) were present. The catalytic data show that the preparation method and the nature of the metal precursor have a significant influence on catalytic performance of the Ru/CeO2 catalysts. The VOC reactivity order (iso-butane >n-butane > propane) show that propane is the most difficult to oxidize. The Ru(NPs)/CeO2 and impregnated Ru(N)/CeO2 catalyst exhibited much higher activity and stability than that of impregnated Ru(Cl)/CeO2. It was found that the Ru(NPs)/CeO2 nano-catalyst was free of chlorine species although it had been prepared from RuCl3. The differences in the activity have been ascribed to the presence of Cl− ions on the Ru(Cl)/CeO2 catalyst and to much better reducibility of the chlorine-free Ru(NPs) and Ru(N) catalysts. The Cl species have a negative impact on the redox properties of CeO2, they have some influence on Ru agglomeration during long-term activity tests (50h), and consequently, the chlorinated catalyst exhibited a poorer activity and stability compared with those of the Cl-free catalysts.

On the Hydrogen Embrittlement of Commercially Pure Alpha Titanium: An Example from the Petrochemical Industry

Grade 2 of commercially pure Ti consisting of α-phase has many applications in the petrochemical industry such as floaters of gauges used to indicate liquid levels in tanks and reaction vessels. A floater fabricated by welding of 3.5-mm-thick sheet of grade 2 Ti into a thick-walled cylinder to indicate the level of a liquid mixture of isobutane, neobutane and neopentane in a petrochemical plant has lost its structural integrity by puncturing, cracking and blistering particularly at the section in contact with the liquid. The damage has been most severe in the base metal adjacent to the weld. Detailed microstructural characterization of the damaged floater and unwelded section of the same material has been carried out using scanning electron microscopy combined with energy dispersive spectroscopy, x-ray diffraction and transmission electron microscopy, and the results have been complemented by stress analysis and microhardness measurements. It is shown that the mechanical strength of the floater has been degraded by a combination of excessive absorption of hydrogen during welding and rapid cooling from the β-phase field aided by the stresses generated by the liquid pressure. Absorption of hydrogen and rapid cooling are found to alter the desirable morphology of equiaxed grains of α-phase into a multi-phase structure with fine platelet-type morphology. The base metal adjacent to the weld is found to contain the brittle δ-phase of titanium hydride in a low-ductility matrix of α-Ti with some β-Ti. However, β-Ti is found to be the predominant constituent of the weld.

Performance enhancement of open loop gas recovery process by centrifugal separation of gases

The proposed INO-ICAL detector [1] is going to be instrumented with 28800 RPCs (Resistive Plate Chamber). These RPCs (2 × 2 m2 size) will consist of two glass electrodes separated by 2 mm and will use a gas mixture of Freon R134a, isobutane and sulphur hexafluoride (in the ratio of 95.3:4.5:0.2). An Open Ended System (OES), in which the gas mixture is vented to the atmosphere after a single passage through the detector, is most commonly used for small detector setups. However, OES cannot be used with the INO-ICAL detector due to reasons of cost and pollution. It is necessary, therefore, to recirculate the gas mixture in a closed loop. In a Closed Loop gas System (CLS) [2] the gas mixture is purified and recirculated after flowing through the RPC. The impurities which get accumulated in the gas mixture due to leaks or formation of radicals are removed by suitable filters. The Open Loop System (OLS) [3] is based on the separation and recovery of major gas components after passage of the gas mixture through the RPCs. and has the advantage that it does not need filters for removal of impurities. However a CLS is found to be more efficient than OLS in the recovery of gases in the mixture. In this paper we discuss centrifugal separation [4] as a technique to extract major gas constituents and use this technique to improve the efficiency of OLS. Results from preliminary trial runs are reported.

Design validation and performance of closed loop gas recirculation system

A pilot experimental set up of the India Based Neutrino Observatory's ICAL detector has been operational for the last 4 years at TIFR, Mumbai. Twelve glass RPC detectors of size 2 × 2 m2, with a gas gap of 2 mm are under test in a closed loop gas recirculation system. These RPCs are continuously purged individually, with a gas mixture of R134a (C2H2F4), isobutane (iC4H10) and sulphur hexafluoride (SF6) at a steady rate of 360 ml/h to maintain about one volume change a day. To economize gas mixture consumption and to reduce the effluents from being released into the atmosphere, a closed loop system has been designed, fabricated and installed at TIFR. The pressure and flow rate in the loop is controlled by mass flow controllers and pressure transmitters. The performance and integrity of RPCs in the pilot experimental set up is being monitored to assess the effect of periodic fluctuation and transients in atmospheric pressure and temperature, room pressure variation, flow pulsations, uniformity of gas distribution and power failures. The capability of closed loop gas recirculation system to respond to these changes is also studied. The conclusions from the above experiment are presented. The validations of the first design considerations and subsequent modifications have provided improved guidelines for the future design of the engineering module gas system.

Thermo-Economic Analysis of Zeotropic Mixtures and Pure Working Fluids in Organic Rankine Cycles for Waste Heat Recovery

We present a thermo-economic analysis of an Organic Rankine Cycle (ORC) for waste heat recovery. A case study for a heat source temperature of 150 °C and a subcritical, saturated cycle is performed. As working fluids R245fa, isobutane, isopentane, and the mixture of isobutane and isopentane are considered. The minimal temperature difference in the evaporator and the condenser, as well as the mixture composition are chosen as variables in order to identify the most suitable working fluid in combination with optimal process parameters under thermo-economic criteria. In general, the results show that cost-effective systems have a high minimal temperature difference ΔTPP,C at the pinch-point of the condenser and a low minimal temperature difference ΔTPP,E at the pinch-point of the evaporator. Choosing isobutane as the working fluid leads to the lowest costs per unit exergy with 52.0 €/GJ (ΔTPP,E = 1.2 K; ΔTPP,C = 14 K). Considering the major components of the ORC, specific costs range between 1150 €/kW and 2250 €/kW. For the zeotropic mixture, a mole fraction of 90% isobutane leads to the lowest specific costs per unit exergy. A further analysis of the ORC system using isobutane shows high sensitivity of the costs per unit exergy for the selected cost estimation methods and for the isentropic efficiency of the turbine.

Phase diagrams for hydrates beyond incipient condition — Complex behavior in methane/propane and carbon dioxide/iso-butane hydrates

We have recently developed an algorithm for multiphase equilibrium calculations and stability analysis involving hydrates. This algorithm allows phase equilibrium calculations in scenarios presenting different number of phases that may also be in distinct physical states.Here, we present P×T phase diagrams calculated using our algorithm that involve mixed hydrates of natural gas beyond incipient condition and show complex behavior. Our phase diagrams show a global overview of the behavior of phase equilibrium involving hydrates in a mixture of water, methane and propane and in a mixture of water, carbon dioxide and iso-butane. We accomplish that using a global phase diagram in a wide range of pressure and temperature to show diverse phase equilibrium regions.We present an approach to tune the Peng–Robinson equation of state to improve the calculations of the chemical potential of water in liquid state at high pressure and therefore, improve the prediction of the phase equilibrium between this phase and ice or hydrates, up to pressure conditions as high as 100 MPa.Our calculations predict 3-phase equilibrium regions and 4-phase equilibrium univariant lines in binary hydrates of methane and propane and in binary hydrates of carbon dioxide and iso-butane. We use local phase diagrams highlighting details in regions with complex behavior of phase equilibrium to show how the phase boundary isopleths and univariant lines relate in a phase diagram for water, methane and propane.Our calculations for a mixture of water, carbon dioxide and iso-butane predicts structural transitions occurring below the dew point line of the guest components, in a mixture with excess of guest components, and 5- phase equilibrium invariant point. We show equilibrium lines that present retrograde behavior in the phase diagram for this mixture.