Acetylene Plant And Generator
Acetylene Plants are used in the production of ethylene or various chemical processes and are designed for maximum carbide yield and high efficiency.
The acetylene plant is constructed and inspected with a best practice policy that meets the intent of design principles and manufacturing practices from Section VIII of the ASME codes. It is also designed for NFPA 51A: “Standards for Acetylene Charging Plants ”.
Acetylene plants are manufactured with complete in-house know-how. Every part has been carefully designed by experienced engineers. Parts have been accurately made and carefully inspected before assembly. Each unit, after assembly, is charged and worked continuously to check its performance.
The acetylene plant is of an advanced design, incorporating several features not ordinarily found in other generators enabling them to attain the highest efficiency in the world — that is 98%.
The plants are fully automatic with twin hopers. All operations are performed by pneumatically controlled valves, assisted by highly sensitive sensors, integrated with the temperature control instruments. These open the water inlet valve or residue drain valve.
Two pneumatic operated cylinders help to open and close the feed of calcium carbide which is fed into the Acetylene generator by screw feed conveyors. The screw feed mechanism and the agitator are operated by flame-proof motors and fittings. The entire operation is perfectly synchronized to give maximum efficiency and simplicity in the operation of the Acetylene plant.
Any failure, whether it is of the power supply, water, instruments, or the air is signaled visually as well as audibly with the onboard alarm to alert the acetylene plant operator.
The design of the entire acetylene plant has been made keeping the utmost safety in mind. Auto controls, flashback arrestors, high-pressure reverse flow check valves, safety valves, and complete visual gauging helps to protect both equipment and personnel.
Acetylene Plant Process Description.
In a "stationary carbide to water type automatic acetylene generator", acetylene gas is produced by the reaction of calcium carbide with water. An adequate quantity of water is held in the generator shell in which calcium carbide also known as calcium acetylide is fed from the screw feed. Where upon the following reaction takes place:
|CaC2 + 2H2O = C2H2 + CA(OH)2 + 27000 CALORIES.|
The generated acetylene gas occupies the free volume inside the acetylene generator shell over the water level and the pressure of this gas is allowed to rise till it reaches the preset pressure level.
At this point, the carbide feed motor is cut off by the pressure controller.
When the acetylene gas is removed from the acetylene generator shell the pressure inside the acetylene generator shell is reduced whereby the pressure controller reactivates the carbide feed motor and further carbide is fed by the screw feed mechanism from the hopper into the acetylene plant shell and more acetylene, also known as Ethyne is generated.
This process continues till the carbide in the hopper under operation is exhausted. This operation can be made continuous by using a second hopper which comes into operation as soon as the carbide in the first is exhausted. While the second hopper is being utilized to supply carbide to the acetylene generator shell, the first can be replenished and kept ready for replacement.
Passage of acetylene gas from the acetylene generator to the carbide chamber (hopper) while being filled is kept shut by a pneumatically operated shut-off device which operates automatically when the carbide in the hopper is exhausted and the screw feed motor is stopped.
Since the process of acetylene generation is exothermic, there is a temperature rise in the acetylene generator. This temperature rise cannot be allowed to go unchecked as acetylene is inflammable.
Moreover, the optimum generation of acetylene takes place around 60ºC. Hence it is necessary to maintain the acetylene generator temperature around 60ºC. This is achieved automatically by the temperature controller which is preset to 60ºC.
As the temperature inside the acetylene generator rises beyond 60ºC, the temperature controller activates the water inlet valve which lets in fresh process water and thereby reduces the temperature of the acetylene generator to the desired level.
As freshwater is introduced to lower the acetylene generator temperature, the water level inside the generator rises and a slurry discharge valve opens automatically to discharge some slurry and thus lower the level of water to an acceptable level when the slurry discharge valve closes automatically. This slurry discharge valve is operated by a pneumatically operated water level controller which is preset for minimum and maximum water levels inside the generator.
Its operation, in conjunction with that of the discharge valve, ensures that only a small quantity of slurry is discharged at a time. The advantage of such action is that the water inside the generator, which dissolves a certain quantity of the acetylene generated, is not discharged continuously, or in large quantities, so that the dissolved acetylene is not discharged with the slurry.
The acetylene generator is also incorporated with an agitator which is continuously driven during the operation of the carbide feed motor to keep the calcium carbide and water intimately mixed for complete reaction and thus prevents loss of carbide in the form of solids with the slurry.
When the acetylene generator is dormant there is a tendency of the slurry to settle down and jam the agitator so a timer-controlled electric motor keeps the agitator running even when there is no calcium carbide being induced to the generator.
The acetylene generated is passed from the acetylene generator chamber through a wet type flashback arresters into a low-pressure condenser where the temperature of the gas is lowered by water cooling.
This also removes part of the moisture in the acetylene in the form of condensed water. The gas is then passed through a low-pressure dryer filled with anhydrous calcium chloride to remove further quantities of moisture before the gas enters the purifying chamber with an optimum quantity of moisture.
The purifying chamber is filled with chemicals that remove phosphorus and sulfur compounds and any acidic fumes. Thereafter the gas passes through a water scrubber so that the particles of the purifying chemicals which may have been carried over with the gas from the purifying chamber are removed.
The wet gas then enters the acetylene compressor where it is compressed and passed to the high-pressure dryer where mechanical devices and chemicals remove moisture and any oil vapor from the gas.
After passing through the high-pressured dryer, the gas is arrested by a backpressure valve on the pipeline leading to the manifold. This also ensures efficient drying of the gas in the high-pressure dryer. Thereafter, the gas is passed through a dry type flashback arrester to the cylinders hooked onto the manifold through static-free uniflow valves
Before hooking the cylinders to the manifold, they have to be made ready by ascertaining that the quantities of acetone contained in them are as per requirement.
This is done by simply weighing them. During the filling of cylinders, they are cooled by water spray so that any temperature rise due to the filling operations and the absorption of gas into the acetone is maintained at an optimum level.
There may be some drop in pressure after filling the cylinders for the first time which may necessitate a second or third filling. After disconnection, the cylinder is weighed and the quantity of acetylene in them is ascertained. Cylinders are also tested for leaks before storage.
Water meters are provided with the generators to check the quantity of water consumed to determine that the ratio of carbide and water is correct for proper utilization 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 the hopper.
An instrument air compressor is provided for operating the pneumatic instruments and valves. Apart from the instruments mounted on the generator, some of the control instruments are located on a panel that is installed at a safe distance along with MCC.
It is recommended that the customer keeps a small bank of 4 to 6 numbers of 6M³ Nitrogen cylinders and pipe nitrogen gas to various points of the plant to be able to purge the plant with nitrogen before startup or maintenance.
Electrical lines are to be provided by customers from MCC located at a safe distance to feed carbide screw feed motors, compressor motor controls, and instrumentation. Water requirements are also to be noted and catered for.
Special precautions when making the Acetylene Plant building (As per PESO guidelines):
- 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 godown should be constructed and must be provided with a 9 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 the construction of the Acetylene Plant building.
- Both the acetone godown 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 noncorrosive, galvanized mesh, with a minimum gauge of 11 mesh to the linear cm. Covers should be provided on the ventilators so that rainwater may not enter through these openings.
- All electrical wiring should be of low voltage in the Acetylene Plant premises. Electrical lines must not cross or run in the same trench used for conveying water or steam or acetylene.