There are three important olefins used for the production of petrochemicals & polymers i.e. ethylene, propylene and butadiene. All three are produced in various ratios by the cracking of feed stocks.
Ethylene
Ethylene (ethene) H2C=CH2 is the largest building block for many petrochemicals. This olefin is used to produce many end products such as plastics, resins, fibers etc. Ethylene is produced mainly from petroleum based feed stocks by thermal cracking,
In Europe about 90% is obtained by cracking naphtha, in both the United States and Europe heavier feed stocks are playing a great role in ethylene production as supplies of traditional feed stocks (ethane, LPG and naphtha) failed to keep pace with olefin demand.
Physical Properties
Ethylene is the largest olefin. It is a colorless, flammable gas with a slightly sweet odor.
Chemical Structure
Ethylene is a planer molecule with a carbon-carbon bond distance of .134nm, which is shorter than C-C bond length of .153nm found in ethane. The C-H bond distance is .110nm, and the bond angles are <HCH = 117.2° and <HCC = 121.4° consistent with the sp2 hybridized state.
Propylene
Propylene is the second most important olefin. Its growth rate is the fastest of any petrochemical raw material, appreciably higher than the growth rate of any other olefin. The demand for petrochemical propylene is now almost equal to its fuel uses and it will soon be the larger of the two.
Physical Properties
It is a colorless, flammable gas having melting point -185oCand boiling point -48oC.
Butadiene
It runs a poor third in the production and demand race with ethylene and propylene. As with propylene, butadiene is a co product with ethylene. It almost produced at a rather constant 4-5 % when the feed stock is butane or higher .The overall ratio of ethylene to butadiene is 1-0.08.
Physical Properties
It is colorless, flammable gas having boiling point -5oC.
Different Feed Stocks
Gaseous feed stocks
· Ethane
· Propane
· Ethane-propane mixture
· N-butane
· LPG
Liquid Feed Stocks
· Light virgin naphtha (LVN)
· Full range naphtha (FRN)
· Reformer raffinate
· Atmospheric gas oil (AGO)
· Vacuum Gas Oil (VGO)
As feed stock progress from ethane through heavier fractions with lower hydrogen contents the yield of ethylene decreases and feed per pound of ethylene increases markedly, the total amount of propylene, butadiene and BTX also increases with increase in molecular weight of the feed stocks, the yield of propylene as well as that of ethylene can be important consideration in feed stock selection.
Full Range Naphtha
One of the advantages of naphtha over gaseous feed stocks is the wider spectrum of the co products provided, of course, we wish to obtain a variety of co products. An increase in severity increase the production of Ethylene, at he expense of propylene and butenes, both methane and BTX also increase. The naphtha feedstock required for high severity cracking is 15-20% higher than that for moderate severity cracking. Pyrolysis gasoline accounts for about 2/3 of this additional naphtha feed. The additional pyrolysis gasoline accounts for the additional BTX formed. The overall product Distribution is determined by the naphtha characteristics.
Storage
Pure ethylene has a normal boiling point of -103°C. At ambient temperature it is a superheated vapor at about the density of atmospheric air. Its critical temperature is 9.2°C. To store substantial volumes of ethylene, it must be liquefied and held under refrigeration or, for in-ground reservoirs; it must be compressed well into the supercritical range and held there during pressure changes.
Above-ground liquid storage near its boiling point is normally provided only as shipping storage at seaside terminals and in very limited volume at plant sites because of the high cost of such storage. Operational liquid storage within the plant is in the order of 4 to 6 hours of the plant's production capacity in horizontal or vertical cylindrical pressure tanks or, if larger, in heavily insulated spherical tanks. In areas where the subterranean strata contains impervious salt layers or salt domes, the cheapest method of storing ethylene in bulk is at supercritical pressure in leached cavities.
Transportation by Pipelines
Most ethylene is consumed by the producers themselves to turn into intermediate and finished products. The cheapest conveyance of bulk movement is the pipeline. The critical temperature of ethylene is a mere 9.2°C. The temperature of the soil in which the line is buried will, in temperature climates, straddle this temperature by at least ±15°C, which implies that a liquid phase may form in the line during the cold season, a situation that is definitely to be avoided, it follows that ethylene can be stably moved by pipeline in only one or the other of two operating modes; namely, either at supercritical pressures irrespective of temperature, which is the preferred mode, or at sub critical pressure by a safe margin lower than the vapor pressure corresponding to the temperature at any point of the line to avoid the formation of a liquid phase.
A supercritical line has the following advantages
· Due to the five-to eightfold higher density, the transport capacity for the supercritical line is several times that of the sub critical line for the same pressure gradient.
· Over longer distances the supercritical lines capacity can be increased by providing booster stations at 0 fraction of the energy needs of the sub critical line and, finally,
· A large portion of the compression energy can be recovered by expanders at the gates of the plant that consumes the ethylene.
