SOME SPECIAL FEATURES OF OUR ACETYLENE PLANTS

A C E T Y L E N E  P LA N T  L A Y O U T

Special precautions to be taken into account when making the Acetylene Plant Building:

• A safety zone of 15 m. all around the Acetylene Plant, from any other building should be provided, and a minimum of 9 m. from any slurry pit.
• A separate Acetone storage should be constructed and must be provided with a 12 m. safety zone all around.
• A distance of 90 m. must be maintained from any Oxygen Plant in the vicinity.
• All non-flame and explosion proof switches must be provided separately in an enclosure away from the Acetylene Plant Building.
• No flammable material should be used for construction purposes of the Acetylene Plant Building.
• Both the Acetone go down and Calcium Carbide Store as well as the Acetylene Plant Building should be well-ventilated with ventilators on the roof as well as ground level. All ventilators must be protected with non-corrosive, galvanized non corrosive mesh, with a minimum gauge of 28 mesh to the linear inch. Covers should be provided on the ventilators so that rain water may not enter through these openings.
• All electrical wiring should be of low voltage in the Acetylene Plant Premises. Electrical lines must not cross any pipe conveying water or steam or acetylene.

PROCESS DESCRIPTION

 In a Stationary Carbide to Water type Automatic Acetylene Generator acetylene is produced by reaction of calcium carbide with water. Adequate quantity of water is held in the generator shell to which calcium carbide is fed from top. The following reaction takes place:-

CaC₂+2H₂O = C₂H₂ + Ca(OH)₂ + 27,000 Calories

The generated acetylene gas occupies the free volume inside the generator shell over the water level and pressure of gas goes up till it reaches the set pressure level when by action of the pressure controller carbide feed motor is cut-off. If the acetylene gas is taken out from the generator, the pressure inside the generator shell will go down and by action of the pressure controller carbide feed motor will start and feed further carbide through the screw feed mechanism from the hopper into the shell and further acetylene shall be generated. Thus the process shall continue till the carbide filled in the hopper under operation is exhausted. At that stage change over takes place and second hopper which was filled with carbide when first hopper was under operation now starts feeding carbide into the generator shell and first hopper is filled with carbide.

As the process of acetylene generation is exothermic, there is a temperature rise of the generator. The generator temperature cannot be allowed to go high because acetylene can catch fire. Moreover, generation of acetylene is optimum at a particular temperature around 60ºC. Hence it is necessary to keep the generator temperature around 60ºC. This is done automatically by the temperature controller which is preset at a temperature of 60ºC. Since the generator has a tendency of increased temperature, the temperature controller acts on the water inlet valve and opens the valve to bring in fresh process water which reduces temperature of the generator and thus the generator temperature is kept at about a particular predetermined set point.

With inlet of fresh water to bring down the generator temperature the water level inside the generator goes up and this water level is maintained between a high and low set point by pneumatically operated level controller. Slurry discharge valve is made to open automatically by the level controller to discharge some quantity of slurry to lower the waster level inside the generator when the water level tends to rise above the maximum level. As soon as the level of water goes down the slurry discharge valve automatically closes. Thus only minimum quantity of slurry is discharged at a time. Water inside the generator absorbs certain quantity of acetylene and hence this water already saturated with acetylene should not be discharged continuously or in large volume but should be retained inside the generator and thus prevent loss of acetylene with the slurry.

The Agitator is continuously driven when the carbide feed motor is running. However, when the generator is in dormant state and there is no carbide feed, there is a tendency of the slurry to settle down and jam the agitator. A timer controlled electric motor drive keeps the agitator under operation when the generator is in dormant state. Thus slurry is always kept mixed with water without sedimentation. Agitator is a very important part of the generator as it keeps the carbide and water intimately mixed for total reaction of the carbide and thus prevent loss of carbide in the form of solids with the slurry.

Passage of acetylene gas from the generator to the carbide chamber (hopper) under filling is kept shut off by hydraulically operated shut off device which operates automatically when the carbide in the hopper is exhausted and screw feed motor is stopped.

The generated acetylene passes from the generator chamber through Wet Type Flash Back Arrestors and then reaches Low Pressure Condenser where the temperature of gas is lowered by cooling water and part of the moisture in the acetylene gas is taken out in the form of waster by condensation. Gas then passes through low pressure dryer filled with calcium chloride to remove further quantities of moisture from acetylene gas before the gas reaches purifying chamber. Gas then passed into purifying chamber with optimum quantity of moisture in the gas (gas is not completely dry). Purifying chamber is filled with purifying chemicals to remove Phosphorus and Sulphur compounds from the gas and if necessary to remove acidic fumes also. Thereafter gas passes to water filled scrubber where acetylene gas is washed with water to remove particles of purifying chemicals which may be carried over with the gas from purifying chamber. The wet gas then enters suction of acetylene compressor, compressed and passed to high pressure dryer filled with mechanical devices and chemicals to remove lube oil vapor and moisture from the gas. Gas after passing through the High Pressure Dryer is arrested by Back Pressure Valve on the pipeline leading to the manifold. Back Pressure Valve is set at a pressure between 12 to 14 kg/cm2g so that no gas till the pressure rises above the set pressure is allowed to pass on to the manifold. This ensures efficient drying of acetylene gas in the high pressure dryer. Thereafter gas passes through Dry type Flash Back Arrestor to manifold and then passes to the cylinders hooked on the manifold through Static Free Uni-flow Valves meant for each cylinder.

Before hooking the cylinders on the manifold for filling they are weighed on a balance, loss of acetone if any ascertained, loss is made up by filling acetone preferably by hand pump directly from acetone drum and then only cylinders are made ready for connecting to the manifold for filling.

Water Meter is provided with each generator to check the water consumed by the generator in a given time and to compare the ratio of carbide and water to determine the proper operation of the generator.

Hoist with skip and chain pulley block is provided to lift carbide from the ground to the generator hopper for filling of hopper.

A small bank of 4 to 6 Nos. 6 M3 Nitrogen cylinders are kept in manifold with piping to various points for flushing the various parts of the plant and inter-connecting pipelines.

Electrical lines are to be provided by customer from MCC located at a safe distance to feed carbide screw feed motors, compressor motor controls and instrumentation.