Investigation of CL‐20 and RDX Nanocomposites

Nanocrystalline explosives offer a number of advantages in comparison to conventional energetics including reduced sensitivity and improved mechanical properties. In this study, formulations consisting of 90 % hexanitro‐hexaazaisowurtzitane (CL‐20) or cyclotrimethylene trinitramine (RDX) and 10 % polyvinyl alcohol (PVOH) were prepared with mean crystal sizes ranging from 200 nm to 2 μm. The process to create these materials used a combination of aqueous mechanical crystal size reduction and spray drying. The basic physical characteristics of these formulations were determined using a variety of techniques, including scanning electron microscopy, X‐ray diffraction, and Raman spectroscopy. Compressive stress‐strain tests on pressed pellets revealed that the mechanical properties of the compositions improved with decreasing crystal size, consistent with Hall‐Petch mechanics. In the most extreme case (involving CL‐20/PVOH formulations), crystal size reduction from 2 μm to 300 nm improved compressive strength and Young’s modulus by 126 % and 61 %, respectively. These results serve to highlight the relevance of structure‐property relationships in explosive compositions, and particularly elucidate the substantial benefits of reducing the high explosive crystal size to nanoscale dimensions.     

The Formulation Design and Performance Test of Gas Generators Based on Guanidinium Azotetrazolate

Six types of gas generators based on guanidinium azotetrazolate (GZT) were designed into six formulations having different oxidants: GZT‐LiNO₃ (1), GZT‐NaNO₃ (2), GZT‐KNO₃ (3), GZT‐Mg(NO₃)₂ (4), GZT‐Sr(NO₃)₂ (5) and GZT‐KMnO₄ (6), respectively. The properties of these formulations were investigated in terms from gas production, appropriate combustion temperature and nontoxic gaseous emission. REAL software calculation program [1] was used to calculate the combustion heat at constant pressure, combustion heat at constant volume and specific volume in standard state. It showed that gas generators based on GZT with nitrate salts as oxidant exhibited better performance. Thus its thermal behavior and combustion temperature were studied further and the experimental results were consistent with the theoretical calculation results. Therefore, it can be concluded that formulation 3 has comprehensive optimal performance: low moisture content, insensitivity to friction, heightened vacuum stability, high combustion heat and specific volume. Namely, formulation 3 exhibited the most promising indications of commercial application, such as using in air bags of motor vehicles.     

The Development of a New Synthesis Process for 3,3′‐Diamino‐4,4′‐azoxyfurazan (DAAF)

Process optimization studies were performed for the preparation of the high explosive 3,3′‐diamino‐4,4′‐azoxyfurazan (DAAF). These process studies were pursued to address issues such as problematic waste generation products, particle size, impurities, and manufacturability. This paper describes the original synthesis method and inherent issues. An optimization process was designed to investigate the issues with purity and manufacturability. Particle size effects were addressed by adding a recrystallization step to the synthesis. Ultimately, a complete solution to all observed issues was found with a new synthesis process, which now allows DAAF to be prepared without any impurities, with good particle size and without the need for recrystallization. Importantly, the new synthesis process can be performed in an environmentally friendly manner, with the production of non‐hazardous waste.     

Investigation of HMX‐Based Nanocomposites

Advanced munition systems require explosives which are more insensitive, powerful, and reactive. For this reason, nano‐crystalline explosives present an attractive alternative to conventional energetics. In this study, formulations consisting of 95 % octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine (HMX) and 5 % polyvinyl alcohol (PVOH) were prepared with mean crystal sizes ranging from 300 nm to 2 μm. The process to create these materials used a combination of mechanical particle size reduction and spray drying, which has the advantages of direct control of crystal size and morphology as well as the elimination of ripening of crystals (which occurs during slurry coating of nanomaterials). The basic physical characteristics of these formulations were determined using a variety of techniques, including scanning electron microscopy and X‐ray diffraction. Compressive stress‐strain tests on pressed pellets revealed that the mechanical properties of the compositions improved with decreasing crystal size, consistent with Hall‐Petch mechanics. The 300 nm HMX/PVOH composition demonstrated a 99 % and 129 % greater strength and stiffness, respectively, than the composition with 2 μm HMX. The formulations were subjected to the Small Scale Gap Test, revealing a significant reduction in shock sensitivity with decreasing crystal size. The formulation containing 300 nm HMX registered a shock initiation pressure 1.6 GPa above that of the formulation with 2 μm HMX, a 44 % improvement in sensitivity. These results serve to highlight the relevance of structure‐property relationships in explosive compositions, and particularly elucidate the substantial benefits of reducing the high explosive crystal size to nano‐scale dimensions.     

Properties of Gas‐Generating Mixtures Related to Different Fuel and Oxidizer Compositions

This work focuses on solid energetic materials designed to produce high‐pressure gas for pressurizing or inflating devices. In small gas generators sodium azide is often used. Unfortunately, this chemical exhibits drawbacks concerning toxicity and yield of gas. Other classical gas‐generating agents are double base propellants. However, they deliver toxic and reactive gases and their combustion temperatures are high. In previous work a series of alternative gas‐generating compositions have been proposed, fuelled with double base propellants, azodicarbonamide, nitroguanidine or guanidine nitrate and oxidized with potassium nitrate or potassium perchlorate. They were theoretically and experimentally evaluated on a series of combustion properties, such as ignition delay, burning rate, vivacity, specific energy, etc. The purpose of this paper is to experimentally examine the gas production of the previously proposed compositions. The yield of gas is determined through static pressure measurements after a closed vessel test, while the composition of the combustion gases is investigated through gas analysis. The addition of an oxidizer causes a significant drop in the yield of gas, but avoids the formation of hazardous gases, such as H₂ and CO, in most of the studied cases. The only exception is the mixture of a double base propellant with potassium nitrate: potassium nitrate does not fully react with the double base propellant and therefore the formation of CO and H₂ is not prevented.     

Combustion Measurements of Fuel‐Rich Aluminum and Molybdenum Oxide Nano‐Composite Mixtures

Fuel rich nano‐composite powders of aluminum and molybdenum oxide were tested for ignition and combustion behind the incident and reflected shock waves in a shock tube. The powders consisted of approximately 10 μm particles, each of which contained Al and MoO₃ mixed by mechanical alloying on the nano‐scale. These powders were aluminum rich with composition ratios of 4 : 1, 8 : 1, and 16 : 1 Al : MoO₃ by mass. Ignition tests were performed behind incident shocks for temperatures in the range of 900 to 1500 K. From these tests, ignition delay times were obtained, and some information on combustion duration was also derived. Samples were tested in air at 0.2 MPa, and compared against nano‐Al, 2.7 μm Al, and 10 μm Al baselines. Ignition results for the baseline Al cases were as expected: 10 μm Al not igniting until 2000 K, 2 μm Al igniting down to ∼1400 K, and n‐Al igniting as low as 1150 K. The thermite samples showed considerable improvement in ignition characteristics. At the lowest temperature tested (900 K), both the 8 : 1 and 4 : 1 samples ignited within 250 μs. The 16 : 1 sample (94% Al) ignited down to 1050 K – which represents an improvement of roughly 1000 K over baseline Al with only a small energetic penalty. In all cases, the ignition delay increased as the amount of MoO₃ in the composite was reduced. The 4 : 1 nano‐composite material ignited as fast or faster than the n‐Al samples. Ignition delay increased with decreasing temperature, as expected. Emission spectra and temperature data were also taken for all samples using high‐speed pyrometry and time‐integrated spectroscopy. In these cases, measurements were made behind the reflected shock using end‐wall loading, though the conditions (temperature, pressure, and gas composition) were identical to the incident shock tests. Spectroscopy showed strong AlO features in all the samples, and the spectra fit well to an equilibrium temperature. Broadband, low resolution spectra were also fit to continuum, gray body temperatures. In general, the observed temperatures were reasonably close to 3500 K, which is similar to the combustion temperatures of pure aluminum under these conditions.     

Ways to Create Fuels for Stoichiometric Gas‐Generating CHNO‐Compositions with Low Ammonium Nitrate Fraction

The investigation is aimed to study the possibility of the creation of new CHNO‐fuels for smokeless stoichiometric gas‐generating compositions for airbag inflators. New fuels must have a rather high content of oxygen (to decrease the ammonium nitrate content) as well as a moderate enthalpy of formation in order to prevent an increase of the combustion temperature and of the amount of toxic gases (CO and nitrogen oxides) in the combustion products. Ways to create such new fuels are examined, mainly by introducing low‐enthalpy oxygen containing groups together with oxidizing groups (such as NO₂, ONO₂, NNO₂) into molecules. For several hypothetic substances, the enthalpy of formation has been calculated, thermal stability has been qualitatively estimated, combustion temperatures of stoichiometric compositions have been calculated, and possible ways of their synthesis have been considered.     

Syngas Production Process Development and Economic Evaluation for Gas‐to‐Liquid Applications

The range of syngas composition produced by autothermal reforming of natural gas makes it suitable and in compliance with the gas‐to‐liquid technology for subsequent conversion to synthetic crude oil. A modified and improvised design for this reforming technique is proposed in which oxygen consumption is reduced as compared to the conventional process. Simulative optimization helps in optimal designing of the new process subject to a set of constraints applied to both the processes for uniform and homogeneous comparison. Aspen HYSYS is used for simulation and optimization of the process and the economics are evaluated by an Aspen Process Economic Analyzer.     

Mid‐Infrared Monitoring of Gas‐Liquid Reactions in Vitamin D Analogue Synthesis with a Novel Fiber Optical Diamond ATR Sensor

A novel mid‐infrared optical sensor enabling in situ ATR measurements was applied to investigate several steps of a vitamin D analogue synthesis. The probe based on silver halide fibers coupled to a diamond prism was connected to a conventional FTIR spectrometer with internal MCT detector. All steps of the reaction were monitored by real‐time in situ FTIR measurements. The steps carried out were the dissolution of SO₂ in a CH₂Cl₂/CH₃OH solvent mixture as well as the addition of SO₂ to a vitamin D analogue, the subsequent ozonation of the double bond in the SO₂ addition product, and the following reduction of the formed hydroperoxide with triphenylphosphine. The dissolving process of SO₂ and the addition of SO₂ to the vitamin D analogue were monitored by changing the characteristic ν_as(SO₂) and ν_s(SO₂) modes of dissolved and incorporated SO₂. It was found that during ozonation of the SO₂ addition product the formation of hydroperoxide is accompanied by the simultaneous formation of the corresponding aldehyde identified by the typical ν(C=O) band at 1720 cm^–1. Extended ozone exposure favors the formation of the corresponding acid detected by an additional carbonyl band at lower wavenumbers. During the reaction with triphenylphosphine the increasing intensity of the aldehyde band and the appearance of the ν(P=O) mode of the formed triphenylphosphine oxide indicate the progressive reduction of hydroperoxide. The hydroperoxide band disappears completely during the reaction whereas the ν_as(SO₂) band remains unaffected.     

Microwave‐Assisted Heterogeneous Gas‐Phase Catalysis

This article reviews microwave‐assisted heterogeneous gas‐phase catalysis. To date, this special means of non‐classical energy input by microwave radiation is still a fringe area of catalysis research, and alternative reaction engineering in chemistry and chemical engineering. However, microwave‐assisted heterogeneous gas‐phase catalysis is expected to gain significant popularity in academia and industry in the near future. Experimental set‐ups that have been described in literature are critically reviewed, and concepts for the design of improved experimental set‐ups are provided in this article. Historical developments, current tendencies, and a short introduction to the theory of dielectric heating are discussed.     

Gas‐Liquid Direct‐Contact Evaporation: A Review

Gas‐liquid direct‐contact evaporators are characterized by the bubbling of a superheated gas through the solution to be concentrated. In other words, they are nonisothermal bubble columns. Despite their simplicity of construction, these units exhibit rather complex hydrodynamics and, similar to what occurs to isothermal bubble columns, the design of such units still poses a problem. The present paper reviews the literature regarding this kind of equipment, addressing both experimental studies and modeling efforts. The covered issues include classic and potential applications, bubbling regimes, gas holdup and bubble size distributions, as well as mathematical models proposed for simulating the unit. Additionally, pertinent literature on isothermal bubble columns is also discussed. Recommendations are made for future research.     

Heat Transfer from Immersed Vertical Cylinders in Gas‐Liquid and Gas‐Liquid‐Solid Fluidized Beds

The heat transfer coefficient, h, was measured using a cylindrical heater vertically immersed in liquid‐solid and gas‐liquid‐solid fluidized beds. The gas used was air and the liquids used were water and 0.7 and 1.5 wt‐% carboxymethylcellulose (CMC) aqueous solutions. The fluidized particles were sieved glass beads with 0.25, 0.5, 1.1, 2.6, and 5.2 mm average diameters. We tried to obtain unified dimensionless correlations for the cylinder surface‐to‐liquid heat transfer coefficients in the liquid‐solid and gas‐liquid‐solid fluidized beds. In the first approach, the heat transfer coefficients were successfully correlated in a unified formula in terms of a modified j_H‐factor and the modified liquid Reynolds number considering the effect of spatial expansion for the fluidized bed within an error of 36.1 %. In the second approach, the heat transfer coefficients were also correlated in a unified formula in terms of the dimensionless quantities, Nu/Pr^1/3, and the specific power group including energy dissipation rate per unit mass of liquid, E^1/3D^4/3/ν_l, within a smaller error of 24.7 %. It is also confirmed that a good analogy exists between the surface‐to‐liquid heat transfer and mass transfer on the immersed cylinder in the liquid‐solid and gas‐liquid‐solid fluidization systems.     

Combustion Characteristics of Silicon‐Based Nanoenergetic Formulations with Reduced Electrostatic Discharge Sensitivity

This paper details the synthesis and combustion characteristics of silicon‐based nanoenergetic formulations. Silicon nanostructured powder (with a wide variety of morphologies such as nanoparticles, nanowires, and nanotubes) were produced by DC plasma arc discharge route. These nanostructures were passivated with oxygen and hydrogen post‐synthesis. Their structural, morphological, and vibrational properties were investigated using X‐ray diffractometry, transmission electron microscopy (TEM), nitrogen adsorption‐desorption analysis, Fourier transform infrared (FTIR) spectrometry and Raman spectroscopy. The silicon nanostructured powder (fuel) was mixed with varying amounts of sodium perchlorate (NaClO₄) nanoparticles (oxidizer) to form nanoenergetic mixtures. The NaClO₄ nanoparticles with a size distribution in the range of 5–40 nm were prepared using surfactant in a mixed solvent system. The combustion characteristics, namely (i) the combustion wave speed and (ii) the pressure‐time characteristics, were measured. The observed correlation between the basic material properties and the measured combustion characteristics is presented. These silicon‐based nanoenergetic formulations exhibit reduced sensitivity to electrostatic discharge (ESD).     

Design and Numerical Simulation Analysis of an Integrative Gas‐Liquid‐Solid Separation Hydrocyclone

Normally, a gas‐liquid‐solid separation includes both degassing and desanding processes, which means a relatively higher facility investment and larger energy consumption. Based on an inner‐cone hydrocyclone developed before, an integrative degassing and desanding hydrocyclone was designed. Its design idea and process are described in detail. By means of a hollow inner cone (IC), the separated liquid enters into the cone through holes on it and then flows to the liquid‐phase outlet. Due to integrative separation and tangential solid outlet, the separator has a more compact size. Simulation analysis of the effect of IC diameter and IC height on separation performance was carried out. Results indicate that with a larger IC diameter the gas content in the solid outlet decreases, while as the IC height rises, the gas content in the liquid outlet increases.     

Ways to Create New CHNO‐Oxidizers for Stoichiometric Gas‐Generating Compositions with Low Combustion Temperature

The investigation is aimed to study the possibility of creation of new CHNO‐oxidizers for smokeless gas‐generating compositions for airbag inflators. For ensuring low amount of CO and nitrogen oxides in combustion products it is necessary to create stoichiometric compositions with a relatively low combustion temperature. Ways to create new oxidizers acceptable to this requirement are examined, mainly by introducing low‐enthalpy oxygen‐containing groups into the oxidizer molecule. Standard enthalpy of formation (Δ_fH°) has been calculated for substances with unknown Δ_fH°, thermal stability has been qualitatively estimated, and combustion temperatures of stoichiometric compositions have been calculated.     

Gas‐Lift Reactors for Rapid Reactions with Appreciable Gas Consumption

Gas‐lift reactors offer important advantages for a number of gas/liquid and gas/liquid/solid reactions. However, the design and operation of these reactors can be complex when there is a substantial change in the molar gas flow rate along the length of the reactor, e.g., when a gaseous reactant is converted into a liquid product. In this situation, there is a strong coupling between reactor hydrodynamics and reaction kinetics, which arises from the fact that the rate of liquid circulation through the reactor and the longitudinal profile of gas holdup in the riser are mutually dependent. Several one‐dimensional models have been developed to describe kinetic/hydrodynamic coupling in gas‐lift reactors. These models offer useful insights into the parameters that affect reactor performance. The models can also be used to explore different approaches to scale‐up.     

Nanoenergetic Composites of CuO Nanorods,Nanowires, and Al‐Nanoparticles

This paper reports on the synthesis of the nanoenergetic composites containing CuO nanorods and nanowires, and Al‐nanoparticles. Nanorods and nanowires were synthesized using poly(ethylene glycol) templating method and combined with Al‐nanoparticles using ultrasonic mixing and self‐assembly methods. Poly(4‐vinylpyridine) was used for the self‐assembly of Al‐nanoparticles around the nanorods. At the optimized values of equivalence ratio, sonication time, and Al‐particle size, the combustion wave speed of 1650 m s⁻¹ was obtained for the nanorods‐based energetics. For the composite of nanowires and Al‐nanoparticles the speed was increased to 1900 m s⁻¹. The maximum combustion wave speed of 2400 m s⁻¹ was achieved for the self‐assembled composite, which is the highest known so far among the nanoenergetic materials. It is possible that in the self‐assembled composites, the interfacial contact between the oxidizer and fuel is higher and resistance to overall diffusional process is lower, thus enhancing the performance.     

A Pressurized Vessel Test to Measure the Minimum Burning Pressure of Water‐Based Explosives

It is well known that water‐based commercial explosives locally ignited in closed vessels do not undergo self‐sustained combustion when the pressure is lower than some threshold value. The latter is usually referred to as the Minimum Burning Pressure (MBP) of the explosive and is now being used by some manufacturers as a basis of safety for many associated manufacture, transport, and handling processes. In the present work, both an apparatus based on hot‐wire ignition and an associated methodology were developed to measure the MBP of water‐based explosives. Typical results for various emulsion and water‐gel explosives are also reported and discussed. It is also shown that the technique could be used to characterize very insensitive explosive substances normally used as explosive precursors.     

Optimal Design of a Gas‐to‐Liquids Process with a Staged Fischer‐Tropsch Reactor

The optimal design of a natural gas‐to‐liquid hydrocarbons (GTL) process with a multistage cobalt‐based Fischer‐Tropsch reactor and interstage product separation is considered. The objective function is to maximize the wax (C₂₁₊) production rate at the end of the reactor path. Sectioning of the Fischer‐Tropsch reactor increases the chain growth probability inside the reactor which results in a higher production of wax. The carbon efficiency of the two‐stage reactor is distinctly higher than that of the single‐stage reactor.