As mentioned before, Acetylene gas is commercially generated by the chemical reaction between calcium carbide and water. The detailed description of what process occurs to produce pure commercially usable acetylene gas at each of the stages numbered in the flow diagram has been given below for a clear and more detailed understanding of the user.
Acetylene is produced by reaction of calcium carbide with water in the BDM Stationary Acetylene Generator. The raw material i.e. Calcium carbide is fed from top of the Generator through the Hoppers into sufficient quantity of water which is held in the main generator. This reaction is highly exothermic in nature and results in the following reaction:-
CaC2+2H2O = C2H2 + Ca(OH)2 + 27,000 Calories
The various essential parameters including the dome Pressure, Plant operation temperature and the water level in the Acetylene Generator are monitored continuously by the Control System.
The acetylene gas produced as a result of the reaction with water starts increasing the dome pressure and hence occupies the free volume inside the main generator till the pressure reaches the set pressure level in the control system. The pressure sensor then signals the control system to automatically stop the carbide Screw-feed motor to stop further acetylene from being produced thereby controlling the pressure build-up. Once the acetylene passes from the generator for further processing, the dome pressure comes down and the sensor signal allows the carbide screw-feed motor to start again in order to supply more carbide through the hopper for further acetylene generation. Normally 2 hoppers are used alternately for carbide supply into the generator to ensure that the acetylene production is continuous.
Acetylene is a flammable gas and hence the temperature of the Generator needs to be set to a safe limit. Since acetylene is created by an exothermic process, the temperature inside the acetylene generator needs to be controlled by circulating fresh water into the system and keeping the production process around a temperature of 60°C. The field instruments measuring the temperature in the generator signals the Control panel to shut down the Carbide feeding mechanism and allow fresh water to flow into the system through the water inlet valve once the temperature reaches the safe limit.The level of water inside the generator needs to be at the optimum level to allow maximum efficiency so BDM has introduced an automatic level controller to maintain the HIGH and the LOW level.
The waste discharged as a result of the acetylene production process is called slurry being a viscous mixture. Large volume of slurry discharge should be avoided as acetylene is mixed with the water and at this point it has to be discharged efficiently from time to time through the slurry discharge valve to avoid losing acetylene as a result of the waste discharge.To optimize the acetylene production process, we integrate an Agitator system comprising of fins on a motor driven shaft into the Main generator which ensures that the carbide-water mixing is continuously agitated to release any remaining acetylene gas which remains in solid state in the carbide. The agitator is driven in synchronization with the Screw-feed system and also timed to run in order to agitate the slurry which has a tendency to settle down and may also cause a jam in the agitator.
The acetylene gas produced can easily escape from the same route that the carbide came into the system and hence needs to be contained within the generator. This is done by the Hydraulic jack and operates automatically to seal the escape route once the carbide inside the Hopper is completely fed into the plant and the screw feed mechanism is stopped. The Acetylene gas then passes through Flash – Back Arrestors which is essentially a one way path for the gas.
The hot acetylene gas exits the main generator and enters the Condenser which consists of long stainless steel tubes and operates on a low pressure process wherein the temperature of the Acetylene gas is lowered by the heat exchange with surrounding cooling water that is circulated in the condenser. A part of the moisture in the Acetylene gas is removed once it condenses and is then drained.
The Low pressure Drier is charged with the chemical known as calcium chloride which absorbs and removes further quantities of moisture from the acetylene gas which is at a low pressure at this stage in the process. A bypass is provided in the drier to allow a certain amount of moisture to be present in the Acetylene gas for reaction in the purifier.
The purifier is essentially a Two Layer purifying chamber which is filled with chemical mixtures that are designed to eliminate the phosphorous and sulphur compounds which are present in the Acetylene gas sometimes along with acidic fumes as well. The purifying chemicals may be activated for re-use upon contact with atmospheric air. For continuous operation one extra charge of chemicals can be kept in open wooden trolleys. The gas passing through the purifier is sprayed evenly to mix with the chemicals so it is further purified and ready for the Scrubber.
This vessel is place in between the Purifier and Acetylene compressor. Here the acetylene gas is washed thoroughly with water to remove the particles of the purifying chemicals including ammonia which may be carried over with the gas from the Purifier.
The moist Acetylene gas enters the suction of the Flame-Proof acetylene compressor system which is a multi-stage water submerged compressor with intercoolers and safety relief valves at every stage. The acetylene gas is compressed to the required pressure up to 300 PSI for filling into the cylinders and then passed into the high pressure dryer.
The high pressure dryer consists of a battery of three high pressure vessels. The first vessel is filled with a metal filler of mechanical devices and the other two are filled with anhydrous calcium chloride. The acetylene gas which passes through this three-tier battery is cleansed of lube-oil vapor and moisture contained in the gas. A check-valve is introduced at a set pressure so that the valve opens to the cylinder filling system after the desired pressure is reached.
The cylinders are charged through flexible Static Free Uni-flow Valves with acetylene here at the desired filling pressure up to 300 PSI. Several non-return-valves are introduced along the pipeline for forward flow of the gas. These non-return-valves are important to the safety of the process and are to be periodically examined during operation.
The cylinders should be weighed on a scale to ascertain for the loss of acetone (if any) and then fill the cylinder with acetone by the acetone pump to ready it for acetylene filling. Acetylene cannot be transported in a pure state as it is unstable and introducing acetone mixture allows for safe and economical transportation and storage of acetylene.
During filling, a continuous spray of cooling water is kept on the cylinders to avoid temperature rise of cylinder during filling and to allow better absorption of acetylene gas in acetone with more density. After filling is complete, the cylinders are taken out by disconnecting the connections to the cylinder and the cylinders are weighed on weighing machine to determine the quantity of acetylene filled in the cylinders.