SAFETY ENGINEERING OF ANTHROPOGENIC OBJECTS ASSESSMENT OF THREAT ARISING BY EXPLOITATION OF ACETYLENE

In the paper base reactivity parameters of acetylene are considered. The terms of safe usage of acetylene are interpreted and related to their molecular structure. Unusual susceptibility of acetylene to exothermic self-decomposition is analysed. Chemical composition and parameters of acetylene decomposition at various initial states are estimated in thermodynamic calculations. Combustion and explosion of acetylene/air and acetylene/oxygen mixtures are estimated in full range of acetylene concentration.


INTRODUCTION
Acetylene is a technical gas preserving their usability in many areas of technology and everyday life. Acetylene mixtures with oxygen offer exceptional energy outcome (high heat of combustion) as well as a stream of high-temperature combustion products. Due to mobility acetylene containing devices become to be an easy-to-delivery energy source. Reliably to ignite acetylene effectively burns at wide concentration ranges. But in reverse, exceptional reactivity and flammability constitute a considerable combustion and explosion hazard.
A unique feature, in comparison to other hydrocarbons, is the susceptibility of acetylene to exothermic self-decomposition. That imposes special precautions for storage and transport of acetylene cylinders. Safety of exploitation of acetylene is regulated by technical documents [1 ÷ 3] and is a subject of numerous investigations, e.g. [4 ÷ 7] and others.
However, the hazardous incidents that occasionally are also accompanied with life losses are still noted [7,8]. The proper recognition of a hazard and right forecast of possible upshots is a primary task in ensure of safe technical specification of anthropogenic objects [9].
In the paper exceptional reactivity, low ignition energy and wide flammability range of acetylene are recalled and discussed. A special attention is paid the phenomenon of selfdecomposition of acetylene. The process is self-excited and occurs violently with massive emission of energy. The molecular structure of acetylene that leads to that special behavior is discussed. Exact parameters of acetylene decomposition at constant pressure and volume are estimated. As in practical applications acetylene is reacting at various concentrations in relation to air or oxygen evaluation of combustion and explosion parameters of acetylene mixtures with air and oxygen is performed.

Extreme ignitability and flammability of acetylene
The specific characteristic of acetylene, in comparison to other hydrocarbons, is their low minimal ignition energy. The facility to ignite is closely related to extreme combustibility as well as explosion and detonation hazard of acetylene/oxidizer mixtures. Several parameters characterizing combustion and flammability properties of acetylene are set up in Table 1 [10  12] Characteristic feature of acetylene is their low minimal ignition energy that is at least ten times lower than that of typical hydrocarbons. That is a serviceable factor that eases technical application. Acetylene-oxygen or acetylene-air bunches that are formed during welding, cutting etc. are easy to ignite and effectively burn-out enabling full fuel consumption. But by the same exceptional ignitability imply increased hazard degree.
The outstanding propriety of acetylene in comparison to other hydrocarbons is their high value of laminar combustion velocity ( The special feature of acetylene is also their high upper flammability limit. Two aspects of limiting acetylene flammability are to be discerned. In standard definition combustion occurs by fuel reactions with oxidizer. The maximal values of acetylene concentration at which the combustion may be supported by acetylene reactions with oxygen are of 80% to 82% ( [2], [10,13]). On the other hand, combustion is the process in which energy is produced the fuel being a source. In the case of acetylene, the energy-producing reactions may occur independently to oxidizer (oxygen) presence. Acetylene may emit energy solely, as result of their decomposition, without oxygen presence. Therefore, in many literature sources the value of 100% is stated as the upper flammability limit of acetylene ( [12,14]).

Self-decomposition susceptibility as a special threat arising by exploitation of acetylene
The susceptibility to auto-decomposition creates serious threats in exploitation of acetylene. At increased temperature, by compression as well in contact with contaminates the break of intermolecular bonding and decomposition to elemental carbon and hydrogen is observed. The limiting temperature of auto-decomposition of acetylene is of 180-190C however the loss of stability may to occur at even lower temperatures [2]. The reaction may to start at even ambient temperature after compression to pressure of about of 15 bar [7].
The The specificity of acetylene physicochemical properties is closely related to their molecular structure.

Characteristic features of molecular structure of acetylene
Carbon atom has six electrons, four of them are taking part in building of intermolecular bonds. Valence electrons are placed 2s (two) and 2p (other two) orbitals. Orbital 2s is of spherical shape with zeroth momentum while 2p orbitals are perpendicular and adherent electrons (each pair) differ in angular momentum (l x , l y , l z , Fig. 2.1a).
In most common cases the binding orbitals are formed by sharing of elections of adjoining atoms. In e.g. methane four bonding orbitals and are formed each sharing of one election originating from carbon and the other from hydrogen. Carbon atom gains of four pair of electrons and the octet rule i.e. formation of noble gas valence shell is fulfilled (Fig. 2.1b).

Figure 2.1. Schemes of carbon (a) and methane (b) electron orbital structures
The process of averaging of properties of electrons originating from 2s and 2p orbitals is named as hybridization [15]. Hybridized orbitals in methane are denoted sp3 what mean that they result from mixing of one orbital 2s and three 2p orbitals. The electron cloud of methane takes a tetrahedral form.
The hybridization, in various forms, occurs by formation of electron bonds in other chemical species containing of carbon atoms. Among many of chemical species in which carbon is present the important is the case when between carbon atoms two bonds are to be formed as e.g. in ethylene (Fig. 2.2).

Figure 2.2. Structural chemical formulae of ethylene
Denoted by parallel lines, double in ethylene or triple in acetylene various bindings in multiple bonds differ from each other in a considerable manner. The scheme of acetylene molecule that contains triple bond between carbon atoms is depicted in Fig. 2.3.

Figure 2.3. Space structure of bonding orbitals of acetylene
The H-C and one C-C bonds in acetylene are formed in hybridized way. In H-C bond hydrogen atom brings one electron and one is shared with carbon. Two elections from different carbon atoms create the C-C orbital. In both cases common binding electron clouds are formed ( [15]). The resulting hybridization is of sp type (Fig. 2.3).
Accounting of hybridization, the number of electrons adjacent to each carbon atom increases to 6 but it is still to less to attain of noble shell configuration. The completion of octet rule occurs as result of interaction between electrons remaining in 2p orbitals (two at each carbon atom). But the arising bonds are significantly different from hybridized H-C and C-C orbitals. In general, the electrons remain in their initial displacement and only long-range interactions are emerging. That kind of chemical bond is named as  (Fig. 2.3).   The relatively large negative formation enthalpy of methane is to be noted. That means that However, uncontrolled polymerization of stored materials may lead to incidents of fire or explosive nature [4]. Especially, the threat may arise when outflow of energy liberated in breaking of doubled bonds will be hindered (by casing, obstacles, etc.). The thermal explosion [10] may then to succeed.
Auto-polymerization of acetylene is also to happen. Some portion of energy is required to initiate the reaction [2]. However once process starts it accelerates and takes explosive form.
The issue is that by acetylene polymerization only one  bond (Fig. 3.3) is to break. The remaining  orbital is still unstable and a potential source of energy. Then, origination of polymerization leads to further and full decomposition of acetylene molecules that is accompanied will excessive liberation of energy and is to occur violently.

Parameters of acetylene decomposition
In simple way the decomposition reaction of acetylene may be written as where the evolved energy is equal to enthalpy of formation at constant pressure (Table 2.   In last line of the Table 2

ACETYLENE-OXYGEN MIXTURES
In practical applications acetylene is used in various concentrations in relation to air or oxygen. Results of evaluation of combustion and explosion of acetylene/air mixtures are presented in Fig. 3.1.   Therefore, a potential threat arising by acetylene exploitation is manly demarcated by parameters of possible uncontrolled decomposition of acetylene.
The course of combustion and explosion of acetylene/oxygen mixtures in dependence of acetylene concentration is presented in Fig. 3.3. The symbols and notation are the same as in  4,5MJ/kg however, it is still lower than by auto-decomposition. The pressure maximum is attained at about of 50% concentration of acetylene. 50% is a threshold over which the elemental carbon (graphite) is to be formed. As at higher acetylene concentrations the number of moles in gaseous phase becomes to diminish, the magnitude of explosion pressure (p v ) is to decrease (Fig. 3.3).

CONCLUSIONS
In the paper unusual reactivity, i.e. ignitability, wide flammability limits, proneness to