Acetylene Gas- its Characteristics and Safety Requirements


Acetylene Gas- its Characteristics and Safety Requirements

Acetylene is  a gas used in steel plants for oxy-acetylene gas cutting and welding and also in flame cutting machines of continuous casting machines. It is sometimes used for carburization of steel, flame heating, flame gouging, flame hardening, flame cleaning, flame straightening, thermal spraying, spot-heating, brazing, texturing and profile-cutting, and carbon coating.

Acetylene is the chemical compound with the formula C2H2. It is an unsaturated hydrocarbon and the simplest alkyne. An acetylene molecule is composed of two carbon atoms and two hydrogen atoms. The two carbon atoms are held together by what is known as a triple carbon bond having CH bond angle of 180 deg. This bond is useful in that it stores substantial energy that can be released as heat during combustion. However, the triple carbon bond is unstable, making acetylene gas very sensitive to conditions such as excess pressure, excess temperature, static electricity, or mechanical shock.

Acetylene is a flammable and colourless gas. Its molar mass is 26.04 g/mol. It is unstable in pure form and thus is usually handled as a solution. Pure acetylene is odourless, but commercial grades usually have a marked garlic like odour due to impurities. It has a density of 1.11 kg/cum at 1 atmosphere pressure and 15 deg C. The specific gravity is 0.91 (air=1).  It is lighter than air so does not accumulate at low levels, where it could cause a potential hazard. The boiling point of gas is – 84.7 deg C and melting point is -80.75 deg C. The adiabatic flame temperature (AFT) in air at atmospheric pressure is 2534 deg C. The auto ignition temperature of acetylene gas is 305 deg C.  Its lower and upper explosive limits in air under STP conditions (0 deg C and 1.02 kg/sq cm) are 2.4 volume % to 83 volume %. The upper limit can reach 100 %. Its solubility in water is 1.2 g/litre.

The  gross and net heating values of acetylene gas is 11932 kcal/kg (13980 kcal/N cum) and 11514 kcal/kg (13490 kcal/N cum) respectively.

Acetylene gas is considered to be a hazardous gas. It may react explosively even in the absence of air at elevated pressure and/or temperature. It forms explosive mixture with the air. Acetylene cylinders are to be used and stored only outdoors or in a well ventilated place. A back flow preventive device is used in case of piping the gas. Accidental releases pose a serious fire or explosion hazard.

Acetylene leaking gas fires are not to be extinguished unless leak is stopped safely.  The area is to be promptly isolated by removing all persons from the vicinity of the incident. The containers are moved from the fire area if this can be done without risk. Water spray is used to keep fire exposed containers cool. If involved in fire, the gas flow is shut off immediately if it can be done without risk. If this is not possible then withdraw from area and the fire is allowed to burn. Fire-fighting of the  fire  is to be done from protected location or maximum possible distance. All ignition sources are to be eliminated if safe to do so. Fire-fighters are to wear appropriate protective equipment and self contained breathing apparatus (SCBA) with a full face piece operated in positive pressure mode.

Acetylene gas displaces oxygen and cause rapid suffocation. In case of inhalation the affected person is removed to fresh air and is kept at rest in a position comfortable for breathing. If not breathing, if breathing is irregular or if respiratory arrest occurs, artificial respiration or oxygen by trained personnel is provided.

Acetylene is not especially toxic but when generated from calcium carbide it can contain toxic impurities such as traces of phosphine (PH3) and arsine. Concentrated or pure acetylene can easily react in an addition type reaction to form number of products like benzene, vinyl acetylene etc. These reactions are exothermic and unlike other common flammables do not require oxygen to proceed. Consequently, acetylene can explode with extreme violence if the absolute pressure of the gas exceeds above 2 kg/sq cm. The safe limit for acetylene is 1.03 kg/sq cm. Hence it is shipped and stored by dissolving in acetone or dimethylformamide (DMF), contained in a metal cylinder with a porous filling.

Acetylene must be stored under special conditions because of its unstable nature. This is accomplished by dissolving the acetylene in liquid acetone. The liquid acetone is then stored in the acetylene cylinder, which in turn, is filled with a porous (sponge like) cementitious material.

Copper catalyses the decomposition of acetylene and as a result acetylene is not to be transported in copper pipes. Brass pipe fittings are also to be avoided.

Production of acetylene

There are three processes for the production of acetylene. These processes are (i) by reaction of calcium carbide with water, (ii) From paraffin hydrocarbons by pyrolysis (Wulff process), and (iii) From natural gas by partial oxidation (Sachasse process).

Production from calcium carbide

Calcium carbide (CaC2) is manufactured from lime and coke in 60:40 ratio in electric furnace at 2000 deg C to 2100 deg C temperature. The size of the calcium carbide is first reduced to fine powder in pulverizer. The pulverized calcium carbide is then added though a gas tight hopper valve arrangement to the acetylene gas generator in which the quantity of water used is sufficient to discharge the calcium hydroxide as lime slurry containing 85 % to 90 % water. In the gas generator the temperature is kept below 90 deg C while the pressure is maintained at 2 atm.

Acetylene is produced in the gas generator by the hydrolysis reaction of calcium carbide with water. During hydrolysis the following chemical reaction take place.

CaC2 + 2H2O = Ca(OH)2 + C2H2     Delta H = – 32.5 kcals

The crude acetylene gas from generator contains traces of H2S, NH3 and PH3. It is scrubbed with water in a scrubber then sent to purifier where the gas is purified and dried with iron oxide and silica gel. The dry gas is filled into cylinders or sent through pipe line to continuous casting machines.

The flow diagram of the process is given in Fig 1.

Fig 1 Flow diagram of production of acetylene from calcium carbide

Flow diagram for acetylene production from CaC2

The production process is environmental friendly. The byproduct of the process is slaked lime which has several uses such as in construction activities as well as for Ph adjustment.

Production from paraffin hydrocarbons by pyrolysis

In the Wulff process acetylene is produced by thermal decomposition of hydrocarbons such as methane, ethane, propane, butane, ethylene etc. as per the following reactions.

C4H10 = C2H2 + C2H4 + CO + H2

C2H4 = C2H2 + H2

2 CH4 = C2H2 + 3 H2    Delta H = + 96.7 kcals

Pyrolysis is carried out in the Wulff regenerative furnace which is a rectangular steel box filled with refractory bricks checker work.

Yield of acetylene (98.5 % to 99.3 % purity) varies with the hydrocarbon feed stock used. The off-gas is principally ethylene, carbon monoxide, hydrogen and methane.

Production from natural gas by partial oxidation (Sachasse process).

These days acetylene is mainly manufactured by the partial combustion of methane or appears as a side product in the ethylene stream from high temperature cracking of hydrocarbons. The feed stock can be a variety of hydrocarbon such as natural gas, LPG, naphtha, fuel oil, even crude oil etc. Heat for the cracking operation is developed by partial oxidation of the feed stock with oxygen. The heat evolved cracks the excess hydrocarbon to acetylene. After rapid quenching with water, the acetylene is separated from the gas stream by absorption-desorption in a suitable solvent. The process is known as Sachasse process using natural gas as raw material. the reactions are as follows.

CH4 +2O2 = CO2 + H2O

2CH4 = C2H2 + 3 H2       Delta H= + 79.8 kcals