Acetylene limit mixtures

Acetylene-chlorine mixtures explode violently under even weak irradiation. The explosion limit lies at a 10% chlorine content. To minimise the danger of explosion chlorine must be added continuously and with cooling. 

Flashback is more likely with an acetylene-oxygen mixture than with any of the other fuel-oxidant combinations in common use. Flame propagation in this system is at 1130cm/sec. This imposes severe limitations on the use of acetylene and oxygen with premixed burners. The relatively low flame propagation rate of nitrous oxide and acetylene (180cm/sec) is a definite advantage for the system. 

Figures 2 and 3 demonstrate calculations of the chemical-equilibration process in the detonation products of a stoichiometric acetylene-oxygen mixture at T = 3160 K and density equal to 0.11 kg/m (this point lies on the isentropic line passing through the C3 point of detonation in a mixture at the initial pressure 0.1 MPa and temperature 298 K). As seen from the figures at the first state CO2 forms by reaction CO-I-OH— C02+H since CO and OH are in superequilibrium concentrations. This increases rapidly the hydrogen atom concentration (Fig. 2) so that it reaches a superequilibrium value and then drops to the equilibrium level. Concentrations of the components involved in all the bimolecular reactions satisfy the detailed balance conditions after 1 fis (Fig. 3). At later states, recombination reactions with he rate limiting step CO-j-O-l-M—> C02+M start to be prevailing.

Acetylene burns in air with an intensely hot, luminous, and smoky flame. The ignition temperatures of acetylene and of acetylene-air and acetylene-oxygen mixtures vary according to composition, initial pressure, initial temperature, and water vapor content. As a typical example, an air mixture containing 30 percent acetylene by volume at atmospheric pressure can be ignited at about 581 °F (305°C). The flammable limits of acetylene-air and acetylene-oxygen mixtures similarly depend on initial pressure, temperature, and water vapor content. In air at atmospheric pressure, the upper flammable limit is about 80 percent acetylene by volume and the lower limit is 2.5 percent acetylene. Some references list the upper flammable limit as 100 percent, which is due to the de-... 

Let us focus on flame acceleration investigations in tubes with obstacles [16]. Some variations of this process (flame acceleration after ignition having passed several obstacles, the flame either dies out or reaches a steady constant velocity) have been recorded. The self-quenching mode is observed in lean near-limit mixtures (hydrogen + acetylene mixtures are an exception), which is explained by the fast turbulent mixing of hot combustion products with cold reagents [20]. 

Separation of acetylene from mixtures with other gases is a difficult problem. Cull coordination polymers of the type [Cu2(pzdc)2(L)] (L=pillar ligand) called CPL (coordination pillared layers) were demonstrated to adsorb selectively acetylene from the mixtures with CO (CPL-1 with 4,4 -bpy as a ligand) [214]. Acetylene adsorption increased drastically at low pressures, whereas CO adsorption followed the slowly rising curve, so that at P= 1.1 kPa the ratio of the adsorbed amount of to that of CO was about 26 at 270 K. The density of adsorbed acetylene was 0.434g/cm which is 200 times larger than the compression limit for the safe use of acetylene at 300 K and 0.2 MPa. 

Alternatives to oxychlorination have also been proposed as part of a balanced VCM plant. In the past, many vinyl chloride manufacturers used a balanced ethylene—acetylene process for a brief period prior to the commercialization of oxychlorination technology. Addition of HCl to acetylene was used instead of ethylene oxychlorination to consume the HCl made in EDC pyrolysis. Since the 1950s, the relative costs of ethylene and acetylene have made this route economically unattractive. Another alternative is HCl oxidation to chlorine, which can subsequently be used in dkect chlorination (131). The SheU-Deacon (132), Kel-Chlor (133), and MT-Chlor (134) processes, as well as a process recently developed at the University of Southern California (135) are among the available commercial HCl oxidation technologies. Each has had very limited industrial appHcation, perhaps because the equiHbrium reaction is incomplete and the mixture of HCl, O2, CI2, and water presents very challenging separation, purification, and handling requkements. HCl oxidation does not compare favorably with oxychlorination because it also requkes twice the dkect chlorination capacity for a balanced vinyl chloride plant. Consequently, it is doubtful that it will ever displace oxychlorination in the production of vinyl chloride by the balanced ethylene process. 

Ma.nufa.cture. The principal manufacturers of A/-vinyl-2-pyrrohdinone are ISP and BASF. Both consume most of their production captively as a monomer for the manufacture of PVP and copolymers. The vinylation of 2-pyrrohdinone is carried out under alkaline catalysis analogous to the vinylation of alcohols. 2-Pyrrohdinone is treated with ca 5% potassium hydroxide, then water and some pyrroHdinone are distilled at reduced pressure. A ca 1 1 mixture (by vol) of acetylene and nitrogen is heated at 150—160°C and ca 2 MPa (22 atm). Fresh 2-pyrrohdinone and catalyst are added continuously while product is withdrawn. Conversion is limited to ca 60% to avoid excessive formation of by-products. The A/-vinyl-2-pyrrohdinone is distilled at 70-85°C at 670 Pa (5 mm Hg) and the yield is 70-80% (8). 

Although acetylene is considered to be a material having a very low toxicity, a threshold limit value (TLV) of 2500 ppm has been estabUshed by NIOSH. In the presence of a small amount of water carbide may become incandescent and ignition of the evolved air—acetylene mixture may occur. Nonsparking tools should be used when working in the area of acetylene-generating equipment. 

Both aliphatic and aromatic terminal alkynes reacted with aliphatic aldehydes giving exclusively a mixture of ( ,Z)-1,5-dihalo-1,4-dienes and disubstituted ( )-a,p-unsaturated ketones, the former being the major products in all cases. When nonterminal aromatic acetylenes were used, the trisubstituted ( )-a,p-unsat-urated ketones were the exclusive compounds obtained. The procedure was not valid for ahphatic and unsaturated alkymes. However, the catalytic system was found to be compatible with alcohols and their corresponding acetates although limited yields were obtained. 

The procedure described below is quite general and uses finely, freshly machine--powdered KOH, which is added to a solution of the primary or secondary (acetylenic) alcohol and a 10-15% molar excess of tosyl chloride in Et20, kept around 0 C The excess of tosyl chloride is destroyed during the reaction of the excess of KOH. Side- and subsequent reactions ("saponification of the ester by KOH and 1,2-elimination of p-toluenesulfonic acid from thtfester) can be suppressed by keeping the temperature of the reaction mixture below 5 C. This procedure can be carried out within 2 h and generally gives excellent (often almost quantitative) yields of the tosylates. Purification of acetylenic tosylates by distillation, which is risky because of the limited thermal stability of the esters, is not necessary because the... 

The impregnation of porous nickel discs with CoPc was difficult because of the limited solubility of the chelate in the usual solvents. CoPc cathodes with carbon as substrate were therefore prepared for use in H2/O2 fuel cells. A mixture of 72 mg CoPc and 48 mg acetylene black, with PTFE as binder, was pressed into a nickel mesh of area 5 cm2. Electrodes of this type were tested in an H2/O2 fuel cell with 35% KOH electrolyte in an asbestos matrix at 80° C. Figure 5 compares the current/voltage characteristics of CoPc cathodes (14 mg/cm2) with those of other catalysts, including platinum (9 mg/cm2), silver (40 mg/cm2), and pure acetylene black (20 mg/cm2). An hydrogen electrode (9 mg Pt/cm2) was used as the anode in all tests. To facilitate comparison of the activity of different cathodes, the pure ohmic internal resistance of the cells (of the order of 0.02 ohm) was eliminated. 

Levedahl (106) noted that aliphatics from acetylene to octane all gave hot flames at 600° C. He suggested that some reaction, such as thermal decomposition, which was common to all aliphatics, became important. Benzene showed a very high, inconsistent hot flame limit, while cyclohexane was low and variable. Levedahl believed the cool flame and subsequent reaction, along with compression, served to raise the mixture temperature to the critical value. Acetylene was believed to play a major role in the ignition reaction. 

A warning was given that the 5 molar solution in ether used as a solvent for Diels-Alder reactions would lead to explosions [1], Such a reaction of dimethyl acetylene-dicarboxylate and cyclooctatetraene in this solvent exploded very violently on heating. The cyclooctatetraene was blamed, with no supporting evidence [2]. It would appear desirable to find the detonability limits of any reaction mixture before any attempt is made to scale up. Initial studies have not shown detonability in any lithium perchlorate solution in an organic solvent, while adiabatic calorimetry showed exothermicity only above 150°C. Further testing is recommended [4]. A safe alternative to lithium perchlorate/ether as a solvent for Diels-Alder reactions is proposed [3]. 

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