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Hydrogen abstraction by radicals is an interimolecular and radical-exchange reaction. The low molecular radicals (i.e CHº3 or Hº ) tend to abstract hydrogen radicals from hydrocacbons with relatively lower energy. The activation energy of this hydrogen abstraction is estimated as about 30kj/mol
The radicals generated by hydrogen abstraction are far less stable than the feed hydrocacbons and are casily decomposed into lower molecular olefins and radicals. The activation energy of the decomposition is estimated as about 120 – 210 kj/mol, and the lower molecular radicals have a relatively larger activation energy.
Due to the above reasons, larger molecular paraffins are gradually decomposed into lower molecular olefins.
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Naphtha Cr-acking
Thermal Cr-acking is well known and widely accepted technology for olefin production. This technology is also called steam Cr-acking, since steam is added to hydrocacbon before Cr-acking to reduce the parial pressure of hydrocacbon and to produce a better yield performance. In the petrochemical industry, steam Cr-acking is a core technology for producing olefins, although there are alternative routes from off-gas of fluid catalvtic Cr-acking units in oil refineries or by dehydrogenation or prepane or outanes.
Thermal Cr-acking reactions are bisically uses to break the C – C bonds of hydrocacbons non-catalytically at high temperature of around 800 – 9000C and at a low pressure of 0.16 – 0.2Mpa in the coils located in the radiant sections of furnaces. They finally produce lower molecular weight olefins. In including radical reactions, whereas gas oil Cr-acking is more complex with more than 3000 reactions.
In industrial practie, non-reaced ethane or propane is separated in the cold separation sestion and recycled back to Cr-acking furnaces where ethane and propane are Cr-acked again. Therefore, the ultimate yields of olefins are much higher than those of once-through yields.
I.1.1. Petrochemical complex in Japan.
Naphtha is used asa base feedstock for petrochemical complexes in Japan. LPG is also used as an alternative feedstock, but accounts for about 60% of such feedstock and the rest of the feedstock are LPG and gas oil. However, in the United States, ethane and LPG separated from natural gas are major feedstock, which are for 70 – 80% of total demand, while the rest are naphtha and gas oil.
Based on naphtha Cr-acking, the product pattern consists of ethylene at 28%, propylene at 17%, butene and butadiene products at 11%, off-gas and pyrolysis heavy oil at 24% and pyrolysis gasoline at 20%.
A typical petrochemical complex consists of an ethylene plants such as polyolefin and aromatic plants using olefin products. The configuration of the complex depends on the final product types and the available feedstock. A complex based on naphtha-Cr-acking ethylene plants is more elaborate than a complex based on Cr-acking gases such as ethane or propane.
Some 60 – 70% of pyrolsis gasoline fractions consist of aromatic such as benzene, toluene and xylenes called a BTX fraction. These are recovered by extractive separations at 12 – 14% based on the feedstock rate. However, in the recent product pattern, the BTX extraction rate tends to decrease due to advances in Cr-acking technology and a parafinic and lighter naphtha used as the feedstock. In the 1970s, about 50% of BTX demand was supplied from ethylene plants; however, in the 1990s, this has decreased to 40%. The rest of BTX demand is supplied from catalytic naphtha reforming plants.
I.1.2Cr-acking furnace for naphtha.
Feed naphtha is preheated at the converstion section of a Cr-acking furnace and introduced to the radiation coils, togerther with dilution steam when the naphtha is to thermally Cr-acked into olefin fractions.
I.1.2.1. Type of Cr-acking furnace.
As shown in Figure .. for a typical configuration of the Cr-acking furnace, it is generally designed as a fire-box type which can be grouped into several sub-types with regard to radiant coils and burner types. In particular, with increasing capacity of the Cr-acking furnace and requirement of high severty operation, the accurate control of heat flux in the radiant coils is important. This is attained by using both floor burners and either radiant wall or stage burners. Fuel gas is mainly used for firing, but fuel oil is also used in some cases. Cr-acking reactions ocurs in the radiant coils, and the convection section is used for heat recovery by feed preheating, steam superheating and boiler feed-water preheating. To avoid overCr-acking of reacted gas, tranfers line exchanges for rapid cooling are installed just at the exit of the radiant coil.
The radiant coils are generally located in a single row. The burners are placed on the floor and side wall, or only on the floor. A long-flame type is used for the floor burners, and the flame pattern is formed upward in parallel with the radiant coils. The side wall burners are generally the radiant wall type and are located at several stages every 1–2 metres. Several sets of side wall burners with long and oval flame may also be installed at the terrace of the radiant wall. The use of side wall burners makes it possible to achieve an accurate control of the heat flux over the radiant coils.
I.1.2.2. Radiant Tube and Coil.
In the early days, horizontal straight radiant coils were used, connected with bends at their ends. A long with the temperature rose higher. It caused the defiection of coils so that it was difficult to support the coils horizontally. The coil was therefore set vertically and suspended from the ceiling.
Obtaining better olefin yields requires a short residence time, so coil diameter has been reduced and coil length shortened. The inner diameter of the coil has been reduced to 40 – 50 mm and the length is now about 10 – 20 metres. Typical coil arrangements are shown in Figure..
With respect to coil materials, HK- 40 or Incoloy 800 was previously used. With the increase in the severity of operating conditions, the coil skin temperature has risen higher to about 11000C. Keeping coil life longer reqiures carburization-resistance material, and high chomium and high nickel materials with the addition of tungsten, molybdenum or nicbium have been developed ( refer to Table..). The radiant coils are mainly manufactured by a centrifugal casting divots. This cause the acceleration of carburization. So machining on the inner surface requires removal of casting divots, particularly on the surface of the exit part operated at the highest temperature. Wrought coils have been developed to overome this casting defeet.
I.1.3. TREATMENT OF A Cr-ackED GAS.
An ethylene process consists of a hot section, meluding Cr-acking furnace, a heat recovery of Cr-acked gas, and a cold section to separate into ethylene, propyiene and other olefin products.
The hot section generally consists of the units shown in Figure Cr-acked gas is quenched by a series of transfer line exchangers to recover heat and to terminate Cr-acking reactions. The exchangers generate high-presure steam ( about 10Mpa and 5000C). Cr-acked gas is further cooled down in the oil quench tower and the water quench tower, where several levels of heat are recovered. The gas from the top of the water quench tower enters the four to six stage Cr-acking gas compressor to pressurize from 40 – 50kPa to 3 – 3.5 Mpa. In the compression stages H2S and CO2 are removed from Cr-acked gas by treating caustic soda. The gas is then dried and sent to the cold section, which can be divided into two configurations: the front-end demethanizing system and the front-end depropanzing system. These systems are shown in Figures
Acetylene as the by-product is a catalyst poison for downstream polyethylene productions, so it needs to be removed either by hydrogenation or absorption. Acetylene concentration is up to about 1.5% by volume in an ethylene and ethane mixture in the front-end demethanizing system. Accordingly, controlling the temperature on hydrogenation is relatively difficult. However, the recent advances on the catalyst improve the reaction performance such as the selectivity of acetylene hydrogenation. This hydrogenation system is called back-end hydrogenation because the reactor is sited after the dementhannizer. To moderate the reaction and increase selectivity, carbon monxide may be added the moderator. However, if high-purity ethylene is required, carbon monoxide is sometimes not used.
On the other hand, in the front-end demethanizer system, the hydrogenation reactor is sited before the demethanizer, and this system is called front-end hydregenation. In this case, the acetylene content is relatively low and the hydrogenation reaction is milder than that of the back-end hydrogenation. Hydrogen is contained in the Cr-acked gas, so an additional supply of hydrogen is usually not necessary. Carbon monoxide is also contained in the gas and may improve reaction selectivity. However, the reaction conditions are subject to change due to the furnace operations and hence the Cr-acked gas compositions.
I.1.4. QUENCH AND HEAT RECOVERY.
The purposes of the quencher are to terminate the Cr-acking reactions and to prevent the formation of the heavy materals by polymerization, and to recover energy at a high temperature. Generally, Cr-acked gas is quenched by the transfer line exchangers, which are directly connected to the radiant coil exit and generate high pressure steam.
In th case of naphtha or gas oil feedstock, Cr-acked gas may be condensed in the exchanger if the quench temperature is too low. The condensed liquid will accelerate the fouling rate. Therefore, the quench temperature is kept higher and the heat recovery...
