Petrochemicals are chemicals made from petroleum and natural gas. These compounds are made up of hydro carbons. From the Oil field, crude oil and natural gas are brought to the refinery. In the refinery, through distillation and cracking, the products like naptha, kerosene, lubricating oils, LPG etc., are made.
In the petrochemical plant, further processing of refinery products into aromatics, ethylene, propylene, alcohols, butadiene, paraffins etc are done. In short, the primary petro-chemicals of the refinery process are converted into intermediates.
Out of the petrochemicals, ethylene, propylene and buta-diene are lefins. Benzene, Toluene and Xylene monomer are aromatics . Petrochemical intermediates are generally produced by chemical conversion of primary petrochemicals to form derivative products. Some of the petrochemical intermediates are Vinyl Acetate which is used for paint, paper and textile coatings. Vinyl Chloride is used for poly vinyl chloride resin manufacturing. Ethylene glycol is used for polyester textile fibres. Styrene is used in rubber and plastic manufacturing.
Ethylene is the largest volume hydrocarbon used in the petrochemical industry. It is produced by steam cracking from a wide range of hydrocarbons including ethane, propane, butane, naphtha, LPG andgas oils. The hydrocarbon stream is heated and then diluted by mixing with steam and pumped thorough a tube which is heated in a heater. Depending on the feedstock used, cracking takes place at a temperature of 750 C to 870 C.
The presence of steam helps to reduce the amount of coking in the reactor tubes. The reaction is endothermic requiring heat input. The resorting gas stream is compressed for product distillation/separation wherein the compressed gas liquifies and is separated according to its boiling temperature. Uncracked gases are recycled back to the cracking sections. The choice of feedstock and cracking conditions used determines the ratio of products obtained.
This is manufactured by polymerisation of ethylene. Polymerisation is a chemical process that combines several monomers to form a polymer or polymeric compound. Ethylene is an unsaturated hydrocarbon. The process of polymerization involves breaking of the double bonds of a monomer and the formation of a long chain compound known as the polymer. Polyethylene has two variations : High Density Polyethylene and Low Density Polyethylene. LDPE is made of branched molecules and HDPE is made of unbranched ones which permits the molecule to pack more tightly resulting in slightly higher density. LDPE is made at high temperatures and pressures whereas HDPE uses a catalyst and is made at moderate temperature and pressure.
High Density Polyethylene
Suspension, solution and gas phase polymerization can be used for production of HDPE. Now-a-days, gas phase processes are more common since gas phase processes can be used for all applications whereas slurry based materials mainly find outlets in blow moulding, pipe and film. The process of HDPE mfg involves dissolving ethylene in a diluent such as cyclo hexane, catalyst and hydro and is fed into a reactor at a pressure of 80 bar. The catalyst is fed into the reactor to maintain polymerization rate required. Special care is required in respect of catalyst since there is high susceptibility to contamination from water, oxygen etc. Polymerisation takes place at 200 C to 300 C. The polyethylene produced dissolves in cyclohexane. This then flows into a flash tower where the unreacted ethylene is recovered. In a precipitator, plastic is separated from the cyclohexane. The polyethylene is dried, extruded, pelletised and stored for shipment.
The raw material for the production of methanol is the synthesis gas. It is a mixture of carbon monoxide and hydrogen resulting from the thermal decomposition of hydrocarbons in the presence of steam. This process is called steam reforming and the reformer is nothing but a large furnace.The synthesis gas is fed into a reactor. In the presence of a catalyst and under pressure, methanol and water vapour are formed. The crude methanol is further purified into chemical grade by distillation.
Loss Exposures in Petrochemical Plants
Loss exposures in petrochemical plants are by and large similar to that of petroleum refineries. However, loss exposure of petrochemical plants are more compared to refinery operations due to the following reasons :
The processes in a petrochemical plant are more complex compared to refining and are more hazardous having "run away" characteristic. This is because, the substances handled are more reactive substances. Chances of VCE (Vapour Cloud Explosion) and BLEVE (Boiling Liquid Expanding Vapour Explosion) are quite high.
More high value equipments used
Temperatures in many processes upto 1200 C which puts enormous metallurgical challenges in process equipment fabrication. High value alloys are used for reactor fabrication.
Pressure upto hundreds of bar. This creates high creep and fatigue damage possibililty
Catalyst in the process is critical. Values of the catalyst used in the process is quite high
High concentration of values.
Risk Management Recommendations
The location and construction of control room is one of the most important factors. Control room should be minimum 25 m away from all the plants,15 m with pressurised by 6 mm. water guage and of blast proof RCC construction. In modern refineries, the control room is quite critical since most of equipments are remotely operated type. If the control room is snuffed out in the first place, it becomes that more difficult to isolate unaffeacted areas from affected areas. Pressuring of control room and setting it an elevation relative to the plant areas prevents entry of flammable vapours and fluids entering the control room. Type process control being employed is also very important. Modern plants use the latest DCS system eliminating the need for human interface with equipments to a large extent.
Equipment design according to renowned and world-wide accepted codes and standards eg., NFPA.
Typically the petrochemical plants are of outdoor construction with various equipments housed in a frame-work of steel. In the event of a fire due to the intense heat generated by the combustion of hydro-carbons, the steel columns buckle and collapse. In view of the same, it is recommended that the load bearing structural steel work are encased in RCC to protect against heat damage.
Explosion walls should be constructed around dangerous reactors. Explosion venting arrangements should be done for buildings where confined vapour cloud explosion is existent. For this a portion of the building (either one wall or the roof is constructed weak to give way and relieve the pressure.
Fire or gas detection systems in process areas is recommended. Flame detection facility should be provided at critical locations. Special fire fighting featues : deluge systems for critical equipment such as pumps compressors and vessels should be provided.
Flare Stack should be located such that it is with 10 m more in height than largest column in the plant. Flare stack should be given special attention so that flaring rate is maintained properly and adequate draft is available for flaring.
All superfluous depression, gulley, trench or any level differences which may collect flammable liquids should be levelled. Effluent channels of flammable liquid and gas handling plant should be designed with flame trap facility. Adequate emergency power supply should be provided for safe shut down of plant and cooling water pump. Infra-red thermography of electrical equipment is strongly recommended. There should be regular testing of trips and interlocks provided in equipments and the testing should be done off-line as well as on-line.
In respect of tank farms the construction of dykes should be properly done and should be cleared of vegetation at regular intervals. The level of tank farms should not be a higher level than the process areas to avoid spread of liquid to process areas. Tanks to be equipped with deluge systems and rim fire extinguishing system. Also tanks to be equipped with foam chambers directly fed by extinguishing system.
Management of process changes is critical particularly in cases where the complexity of process is enhanced. Lat but not the least, there should be investigation of near misses in addition to accident investigations. There should be efforts for simulation of process upsets along with formal safety training so that operators know what to do in the event of such upsets. Similar to this is the simulation of emergency shutdowns.
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