Theoretical evaluation of lower explosion limit of hybrid mixtures

Lower explosion limits of hybrid fuel mixtures are usually determined through time consuming and expensive experiments. Although, mathematical expressions like Le-Chatelier's Law and Bartknecht curve have been used by many researchers to predict the LEL of hybrid mixtures, significant deviations remain unexplained. This research work, presents a more sophisticated and general approach for the determination of LEL of hybrid mixtures.Assuming that the combustion kinetics of pure species are independent and unchanged by the presence of other combustible species, complete conversion of the reactants and no heat losses, a simple mathematical model has been derived from the enthalpy balance of the whole system. For the experimental validation of the modelled values, modified version of 20L sphere has been employed, following the European standard (EN 14034-3: 2011) as experimental protocol. Hybrid mixtures of three dusts with two gases were selected for the scope of this publication. By analyzing the modelled as well as the experimental values, it can be concluded that the LEL values of the individual components in the hybrid mixture set the upper and lower limit for the LEL of the hybrid mixture provided the total amount of fuel in the system is considered as the concentration of the hybrid mixture. Moreover, the amount of dust or gas required to render the hybrid mixture flammable mainly depends on the energy contribution upon combustion of the individual species to raise the temperature of the whole system from ambient to the flame temperature.Le-Chatelier's Law and Bartknecht curve are empirical relations, which might hold true for a first-order approximation of LEL of hybrid mixtures, but do not represent the most conservative values of LEL reported in literature. This implies that there is a non-zero probability of occurrence of an explosible mixture in the non-explosible concentrations ranges defined by these relations. Considering these arguments, the authors suggest to employ the model presented in this paper – which presents reasonably conservative values of LEL of hybrid mixtures – for theoretical calculation of LEL of hybrid mixtures, when no precise experimental data is available.     

Turbulent deflagrations of mildly flammable refrigerant-air mixtures

With current concerns around global climate change, new hydrofluorocarbons with low Global Warming Potential (GWP) are being evaluated as alternative refrigerants. These alternative refrigerants, however, may be mildly flammable (as defined by the A2L safety group classification) and pose safety concerns for the heating, ventilation, air conditioning, and refrigeration (HVAC/R) industry. Consequently, careful assessments of different flammability characteristics and risks for these refrigerants are essential for their safe use in actual applications. In this study, deflagration propagation measurements for different mildly flammable refrigerants, including difluoromethane (R-32) and 2,3,3,3-tetrafluoropropene (R-1234yf), were undertaken in different geometries including a 9.1-m long conduit test rig and a closed cubical 12.5 m³ volume. Different tests were conducted for full volume deflagrations as well as with and without obstructions. Turbulent deflagration speeds for well-mixed, refrigerant-air mixtures have been shown to be orders of magnitude larger than their corresponding laminar flame speed values that are used in classifying flammable refrigerants in safety standards. Testing has also quantified the resulting severity as measured by the event overpressure which was shown to worsen with increased congestion or confinement as a consequence of increased induced turbulence. This work illustrates the importance for severity evaluations for actual large-scale or congested geometries of concern in practical applications. Even for mildly flammable refrigerants characterized by laminar flame speeds <2 cm/s, which is lower than the 10 cm/s limit for A2L refrigerants, relatively fast deflagrations can be generated for very congested geometries where downstream turbulence is generated as the flame front passes over obstacles in these situations.     

Combining product engineering and inherent safety to improve the powder impregnation process

The functionalization of nonwoven textiles can be realized by dry powder impregnation. In order to develop and improve this process, two complementary approaches have been combined: product engineering and inherent safety. It consists in integrating ab-initio consumers' requirements, production constraints as well as safety and environmental considerations. This case study is focused on the proposal, the characterization and the selection of powders mixtures of flame retardants and copolyesters, which will be used to create fire-proofed textiles. The influences of the chemical natures of the flame retardant (e.g. calcium carbonate, aluminium trihydroxide, ammonium polyphosphates), their respective concentrations, particle diameters and the addition of silica to flame retardant/polymer mixtures on their minimum ignition energy has been investigated. It has been determined that ammonium polyphosphates are far more efficient than other flame-retardants and that a minimum of 20%wt. concentration is needed to generate a powder mixture that will be almost insensitive to ignition by an electrostatic source. Modifying the particle size distribution and introducing glidants play also a significant role on flame retardant/polymer interactions, on powder dispersibility and has a strong impact on the minimum ignition energy. Finally, the formulations which have been selected fulfill the requirements for fire resistance, flowability, prevention of dust explosion; they are non-toxic, environmentally friendly and their cost is reduced.     

Fume emissions during gas metal arc welding

The control of exposure to welding fumes is of increasing importance in promoting a healthy, safe and productive work environment. This article describes the effects of shielding gas composition on the amount and composition of welding fumes produced during gas metal arc welding (GMAW). The amount of fumes generated during welding was measured for steady current over a range of wire-feed speeds and arc voltages using the standard procedures contained in ANSI/AWS F1.2 [American Welding Society. ANSI/AWS F1.2. Laboratory method for measuring fume generation rates and total fume emission of welding and allied processes. Miami, Florida; 1992]. Results of these measurements show that the fume formation rates (FFRs) increase with CO₂ and O₂ in the shielding gas mixture. The lowest FFRs were obtained with the mixtures of Ar?+?2%CO₂ and Ar?+?3%CO₂?+?1%O₂. The highest FFRs were obtained with the mixtures of Ar?+?18%CO₂ and Ar?+?5%CO₂?+?4%O₂. The welding fumes contains mainly iron, manganese, silicon, titanium and sodium under oxide forms. The fume cluster particles have dimensions between 0.5 and 2?µm. The FFR was found to be governed by the transfer modes of molten metal, i.e. the current intensity and arc voltage, as well as by the shielding gas mixtures composition. Thus these parameters have to be taken into consideration before designing a welding process. Whenever possible, users of GMAW should use the lowest current intensity. However, when this is not possible, due to the constraints of process productivity, welders should use higher currents, but with Ar?+?2%CO₂ and Ar?+?3%CO₂?+?1%O₂ shielding mixtures, which will lead to smaller fume emissions.     

Combined effects of 2,3,7,8-tetrachlorodibenzo- p -dioxin and polychlorinated biphenyl congeners in rats

The objective of this work was to evaluate potential interactions between 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and polychlorinated biphenyls congeners (PCBs) in rats. Groups of five adult female rats were given 0, 2.5, 25, 250, or 1000?ng TCDD/kg body weight/day or TCDD in combination with a mixture of PCB congeners at a concentration of 2 or 20?µg?kg^?1 body weight/day by gavage for 28 days. After the 28-day treatment period, the rats were killed for the analysis of biochemical, liver enzyme activities, and hematological and pathological end points. Growth suppression, increased absolute and relative liver weights, and decreased thymic weight were observed in the 1000?ng TCDD group alone, or the groups receiving a mixture of 1000?ng TCDD and 2 and 20?µg PCBs. TCDD-increased liver and thymic weights were not altered by PCBs; however, growth suppression was more pronounced in animals receiving 1000?ng TCDD and 2?µg PCBs. Increased hepatic microsomal methoxy resorufin-O-demethylase and ethoxy resorufin-O-deethylase activities occurred in 250 and 1000?ng?kg^?1 TCDD-treated animals, which were antagonized by PCBs. Effects of 250?ng TCDD on serum cholesterol and liver uridine diphosphate glucuronosyl transferase activity were reduced by 20?µg PCBs. Treatment with 1000?ng TCDD increased serum albumin, decreased liver vitamin A, increased kidney vitamin A, and liver microsomal glutathione-S-transferase activity, which were not affected by PCBs. Decreased hemoglobin, platelet, packet cell volume, and red cell indices were observed in TCDD-treated rats, but no interactive effects were seen. Histopathological evaluation revealed that liver, thyroid, and thymus were the target organs, but the effects of co-exposure to PCBs and TCDD were variable. These results indicate that the mixture effects of PCBs and TCDD may be additive, synergistic, or antagonistic depending on the dose level and end points measured.     

Dilution with air to minimise consequences of toxic/flammable gas releases

Dilution has long been considered a solution to many problems of toxic/flammable material releases. It implies diluting to a concentration that is below physiologically dangerous levels for a toxic substance (generally below TLV), or to a level below LFL for a flammable material release, ensuring that the process adopted for dilution does not itself enhance the risks.

In this paper, we discuss the dilution of a gaseous release by deliberate and cautious mixing with air to reduce its concentration to a harmless level. The idea bears its origin to the Bhopal Gas Tragedy where some families saved themselves by turning the ceiling fans on when MIC reached their bedrooms at the dead of very cold night on December 2–3, 1984. The air pushed in by the fans diluted the MIC to below the harm level.

Some of the advantages of using air dilution are: no cost of air, no air storage needed, no need to treat the air after use as in case of water curtains; required equipment, its maintenance and staff training in its use are very likely to cost less than in other ways of handling a release.

Air dilution may not be feasible in all cases, such as gaseous release within a congested equipment layout, release that forms a liquid pool, etc. The method needs to be evaluated for each case.     

A new method for the prediction of flash points for ternary miscible mixtures

The flash point is one of the most important physicochemical parameters used to characterize the fire and explosion hazard for flammable liquids. The flash points of ternary miscible mixtures with different components and compositions were measured in this study. Four model input parameters, being normal boiling point, the standard enthalpy of vaporization, the average number of carbon atoms and the stoichiometric concentration of the gas phase for mixtures, were employed and calculated based on the theory of vapor–liquid equilibrium. Both multiple linear regression (MLR) and multiple nonlinear regression (MNR) methods were applied to develop prediction models for the flash points of ternary miscible mixtures. The developed predictive models were validated using data measured experimentally as well as taking data on flash points of ternairy mixtures from the literature. Results showed that the obtained average absolute error of both the MLR and the MNR model for all the datasets were within the range of experimental error of flash point measurements. It is shown that the presented models can be effectively used to predict the flash points of ternary mixtures with only some common physicochemical parameters.     

Experimental study on the influence of flame retardants on the flammability of R1234yf

As an alternative refrigerant, R1234yf shows great environmental friendliness. However, the flammability has a negative impact on its use and promotion. To lower the combustible risk, it's a proper way to mix with nonflammable refrigerants in applications, but the research on the flammability of these blends is still limited. Therefore, the influence of three typical nonflammable refrigerants CO₂, R1336mzz(E) and R1233zd(E) as flame retardants on the flammability of R1234yf was experimentally studied in this paper. With the measurements of flammable limits of R1234yf/CO₂, R1234yf/R1336mzz(E) and R1234yf/R1233zd(E) at different concentrations, the parameters of regression equations for their flammable limits were obtained. The critical suppression concentrations of retardant were determined as 0.575, 0.35 and 0.6, respectively. The results showed that their suppression abilities could be ranked as R1336mzz(E)>CO₂>R1233zd(E). Then the inhibition mechanisms were analyzed, and the differences in flame colors at flammable limits were compared. With the establishment of estimation model for calculating critical suppression concentrations of ternary blends, the critical suppression lines of CO₂/R1234yf/R1336mzz(E) and CO₂/R1234yf/R1233zd(E) were obtained, by which the nonflammable zones of them were preliminarily determined. The results will provide practical guidance for the applications utilizing R1234yf and its blends in safety.     

Flammability hazard analysis of imidazolium-based ionic liquid binary mixtures under high temperatures

Ionic liquid (IL) mixtures are promising because they can optimize the involved properties according to industrial needs. It has already been demonstrated that IL flammability is due mainly to IL decomposition generating flammable substances. Four different ILs, 1-Butylimidazolium tetrafluoroborate ([BIM][BF₄]), 1-butylimidazolium nitrate ([BIM][NO₃]), 1-butyl-3-methylimidazolium tetrafluoroborate([BMIM][BF₄]), and 1-butyl-3-methylimidazolium nitrate ([BMIM][NO₃]), were selected as the parent salts to form the different imidazolium-based IL binary mixtures. These mixtures were tested via isothermal thermogravimetric analyzer (TGA) at different temperatures (120, 150, 180, 210, and 240 °C), then tested by the flash point analyzer after isothermal heating pretreatment at the above temperatures. Results show that the mixtures' flash point values decrease with the heating temperature increase. Vaporization of the IL mixtures’ decomposition products results in a higher concentration of flammable gases and a flash point decrease, which lead to the flammability hazard increasing. Moreover, results show that the flash points of the studied binary imidazolium IL mixtures are more similar to those of the more unstable IL in their parent ILs. Also, the flammability hazard of IL binary mixtures may obviously increase under the high temperature environment for a long time.