Ethylene largely consumed in the production of polyethene (HDPE, LDPE and LLDPE) and used as a major feedstock for the manufacture of Ethylene Di-chloride, Polyvinyl Chloride, Vinyl Chloride and Ethylene Oxide (EO). Modern petroleum refinery generates ethylene from its process units along with gasoline, naphtha, diesel etc. Naphtha from the refineries widely transported to petrochemical plants to produce ethylene and convert into to above valuable chemical products. One of the technologies used in present industries for ethylene production by naphtha cracking technology. On controversy to the commercial plant, the process flowsheet is modified to make it simple.
Process Description of Naphtha Cracking technology
Straight run or hydrocracked naphtha used as feedstock in commercially established ethylene production industries. To simplify the process an attempt on naphtha cracking which is steam pyrolysis technology is discussed aiming to produce concentrated ethylene with fewer by-products like acetylene, BTX etc. A cracking furnace having a separate convection section for preheating and a radiant section is used. Its interior contains burners placed along the sidewalls or at the bottom of the furnace. The temperature in the furnace continuously maintained between 950 to 1000oC by the series of burners controlling. 5 bar pressure is maintained in the tubes by the naphtha feed pumps.
By a heat exchanger, fresh feedstock (Naphtha) is preheated with cracked products stream that comes out from the furnace. The preheat naphtha is mixed with steam and passes to the convective section. Its temperature is raised to 300oC temperature and pass to the radiation section of the furnace for further increasing to 800oC. This is the condition where naphtha is cracked into simple compounds.
Steam is added to dilute the feedstock at a ratio of 0.6 kg/kg HC to prevent the coke formation at the cracking zone. High temperature product gas is cooled by removing the latent heat of water in steam generators that produce HP steam with 10 to 14Mpa. Transfer line heat exchangers operate with high thermal efficiency during cooling the product gas. Products from C2 to C4 group are formed during cracking along with some quantity of BTX and ethylbenzene, Hydrogen and fuel oil. Optimum values of residence time, the steam ratio as well as temperature and pressure affect the by-products formation.
Simplified process flow sheet for the manufacture of ethylene by naphtha cracking
Ethylene purification and by-products separation and recycle
Oil quenching mechanism is used to cool the furnace effluent gas and the recovered heat is used to produce low-pressure steam in the plant utility. Gas oil and fuel oil are obtained when the gas is passed to the primary fractionators.
The volatile components from the primary fractionators are cooled and compressed to 5 to 6Mpa. Heavy hydrocarbons (above C3) are liquefied and separated by separators.
The cracked products are passed through coolers and compressors and compressed to 30 atm and 30 oC and separation of C3, C4, C5 and C6 components takes place by partial fractionation and liquefaction. This can be done in four stages:
Cooling the whole mass of gas to 30°C under 35 atm pressure to liquefy C4 and heavier constituents.
The uncondensed gas is subjected to severe conditions, i.e.; up to +30°c and 20-atmosphere pressure, whereby propane condenses leaving ethane and ethylene in gaseous form.
Dry gas constituting CH4 and H2 is separated from ethane /ethylene mixture this mixture is used as a refrigerant. Acidic constituents like CO, CO2, H2S, and SO2etc are removed in acid gas-removing unit, by caustic wash not shown in process flow sheet. After the treatment the left out in the gas are methane, hydrogen, ethylene, ethane and a slight amount of acetylene (1%). Hydrogen is separated initially than the gas is liquefied. Hydrogen gas is purified and part of it is sent to hydrogenation units, to convert acetylene and propadiene to ethylene and propane respectively. The tail gas obtained from demethaniser is abundant in methane and used in the fuel system. The gases are sent to acid gas removal unit, Demethaniser for methane removal and hydrogen purification unit. Tail gas is removed and then traces of acetylene are converted into ethylene by hydrogenation. Then it is sent to ethylene splitter for separation of ethylene.
Ethane and ethylene are liquefied and fractionated
The heavy bottoms of first stage unit are processed for C3 and heavy ends. This stage is not shown in the process flowsheet.
Other chemical processes technologies available for ethylene production
- Thermal cracking
- Fluidized bed cracking
- Catalytic pyrolysis and catalytic partial oxidation
- Membrane dehydrogenation of ethane
- Oxidative dehydrogenation of ethane by using Nickel Oxide based catalyst
- Methane oxidative coupling technology
- Dehydration of ethanol
- Methanol conversion to ethylene
- Disproportionation of propylene
- Ethylene from coal by FT process
- Ethylene reclamation from the by-product and off-gases generated during the cracking of petroleum in gasoline refining