Monday 28 March 2011

DIFFERENT ROUTES TO OLEFINS PRODUCTION



Although olefins are produced by various methods, only a few are commercially proven  thermal cracking of hydrocarbons, catalytic pyrolysis, membrane dehydrogenation of ethane, ox dehydrogenation of ethanol, oxidative coupling of methane, methanol to ethylene, dehydration of ethanol, ethylene from coal, disproportioning of propylene, and olefins as a bye product.         
In addition to conventional thermal cracking in tabular furnaces, other thermal methods and catalytic methods to produce olefins have not been commercialized.

Advanced Cracking Reactor

The selectivity of olefin is increased by reducing the residence time. This requires high temperature or reduction of the hydrocarbon partial pressure. The key to the process is high temperature, short residence time, and low partial pressure. Superheated steam is used as a heat carrier to provide the heat of reaction. The burning of fuel (H2, CH4) with pure oxygen generates temperatures of 2000°C, and the cracking reaction is carried out at 950 to 1050°C, with the residence time of less than 10 milliseconds. Since the residence time is low, a specially designed Ozaki quench cooler for rapid quenching is required. Selectivity of olefins is increased by reducing residence time, High temperature, Low H/C PP.



 Disadvantage
Unfortunately, all very high temperature processes produces amounts of acetylene (>2 wt %). Acetylene hydrogenation is significantly cost factor if acetylene has no market.

Adiabatic Cracking Reactor

This principle is based on the injection of hydrocarbon feed-stock into the flue gases at elevated temperatures. Because of high temperatures (1200°C), the feed can be instantaneously vaporized, and a very high rate of decomposition can be achieved. The temperature of flue gas can be controlled by varying the oxygen/fuel ratio at the combustion chamber, and by the injection of steam in the combustion chamber. The endothermic nature of the cracking processes causes the temperature to drop rapidly after injection of feed. A substantial increase (over 10 wt %) in olefin yield can be expected, but the quenching reaction in desired conditions is still the problem. In addition, the economics of the process is not profitable.
Disadvantage
Economics still not profitable.

Catalytic Pyrolysis

Catalytic pyrolysis is aimed as primarily producing ethylene. Almost all catalyst produces higher amounts of CO and CO2 then normally obtained by conventional pyrolysis. This indicates that water gas reaction is also very active with the catalysts, and usually this yields to some deterioration of the olefin yield. Significant amount of coke have been found in these catalysts, and thus there is further reduction in olefin yield with on-stream tome. Most of these catalysts are based on low surface area alumina catalysts. Cracking temperatures are somewhat less than those observed with thermal pyrolysis. Most of these catalysts affect initiation of pyrolysis reaction and increases the overall rate of feed decomposition. Appreciably processes to olefin cracking are questionable since equilibrium of ethane to ethylene and hydrogen is not altered by catalyst, and hence selectivity to olefins at lower catalyst temperature may be inferior to that of conventional thermal cracking. Suitability of this process for heavy feeds like condensates and heavy oil has yet to be demonstrated.
Disadvantage
·        Water gas reaction is active which deteriorates olefin yield.
·        Applicability to Ethane cracking is questionable since equilibrium of ethane to ethylene and hydrogen not altered by catalyst; hence selectivity of olefins at lower catalyst temperature is inferior;
·        Also yet not demonstrated for heavy feed.

Fluidized Bed Cracking

Since many fractions of crude oil are used industrially as feed stocks in the conventional cracking furnaces, logically many researchers aimed at cracking crude oil directly. This cannot be done in conventional coils because of severe fouling in convection section and the radiant coils, and in the transfer line exchanger. Therefore various fluid-bed processes have been devolved (Coke particles as a fluidizing medium, inorganic oxides as a heat carrier, and fluidized bed with coke as a heat carrier.)  Thermal regenerative cracking jointly developed by Gulf Chemicals (now- Chevron) and Stone and Webster uses solid heat carrier in bed.
Disadvantage
For crude oil, feed stock only.

Membrane reactor

Membranes used in ethane dehydrogenation to shift the ethane equilibrium.
Disadvantage
Membrane not works at high temperature.

Dehydrogenation

The dehydrogenation of paraffin is equilibrium-limited and hence requires high temperatures. Using this approach and conventional separation methods, both Houdry and UOP having commercialized dehydrogenation of propane and propylene. A similar concept is possible for ethane dehydrogenation.
Disadvantage
But an economically attractive and commercial reactor had not been built.

 Oxy dehydrogenation

Because of limitations of ethane dehydrogenation equilibrium, research has focused on ways to remove one of the products, namely Hydrogen, by chemical methods. In this way hydrogen is oxidized to water and there are no equilibrium limitations.
Disadvantage

However, the same oxygen also oxidizes ethane and ethene to CO2 and other oxygenated products. Therefore, selectivity to olefins is a serious consideration for methane catalytic pyrolysis.

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