Steam cracking Temperature

The principal sources of feedstocks in the United States are the decant oils from petroleum refining operations. These are clarified heavy distillates from the catalytic cracking of gas oils. About 95% of U.S. feedstock use is decant oil. Another source of feedstock is ethylene process tars obtained as the heavy byproducts from the production of ethylene by steam cracking of alkanes, naphthas, and gas oils. There is a wide use of these feedstocks in European production. European and Asian operations also use significant quantities of coal tars, creosote oils, and anthracene oils, the distillates from the high temperature coking of coal. European feedstock sources are 50% decant oils and 50% ethylene tars and creosote oils. 

Butadiene is obtained mainly as a coproduct with other light olefins from steam cracking units for ethylene production. Other sources of butadiene are the catalytic dehydrogenation of butanes and butenes, and dehydration of 1,4-butanediol. Butadiene is a colorless gas with a mild aromatic odor. Its specific gravity is 0.6211 at 20°C and its boiling temperature is -4.4°C. The U.S. production of butadiene reached 4.1 billion pounds in 1997 and it was the 36th highest-volume chemical. ... 

Selecting the naphtha type can be an important processing procedure. For example, a paraffinic-base naphtha is a better feedstock for steam cracking units because paraffins are cracked at relatively lower temperatures than cycloparaffins. Alternately, a naphtha rich in cycloparaffins would be a better feedstock to catalytic reforming units because cyclo-paraffins are easily dehydrogenated to aromatic compounds. Table 2-5 is a typical analysis of naphtha from two crude oil types. 

Steam cracking reactions are highly endothermic. Increasing temperature favors the formation of olefins, high molecular weight olefins, and aromatics. Optimum temperatures are usually selected to maximize olefin production and minimize formation of carhon deposits. 

Unalloyed steel can be directly oxidized by steam at temperatures over 750 °F (399 °C). Extensive cracks may occur, the steel surface may resemble tree bark, and magnetic oxide deposits are generated at the failure site. Intergranular oxidation may take place. In the thermal oxidation reaction, hydrogen is directly released as shown ... 

Conversion of Ethylene on H-ZSM-5 zeolite Ethylene was reacted in three distinct ways. First, it was oligomerized at room temperature on an activated (at 673 K) zeolite. Secondly, water was then adsorbed at 295 K and the sample steam-cracked at 573 K. 

Olefins are now made primarily by a process called steam cracking in which alkanes are reacted homogeneously (cracked) at high temperature. With ethane, the overall reactions are primarily... 

Figure 5.3 shows light olefin yields of DCC process in four refineries with different feedstocks at reaction temperatures of 545-565°C. The propylene yield can reach 23 wt% with paraffinic feed, and about 18-19 wt% with intermediate-based feed. The propylene/ethylene ratio is about 3.5-6.2, much higher than that of steam cracking. The DCC operation can be modified to further increase the yield of propylene. For example, recycling a part of DCC cracked naphtha to the reactor resulted in a propylene yield increment of 3.5 wt % in Jinan Refinery [16]. 

Initially, ethylene was obtained by the dehydration reaction of ethanol. Nowadays, ethylene is obtained by steam cracking from naphtha as a basic chemical. Steam cracking degrades longer aliphatic chains and introduces the double bond. Steam cracking is done at temperatures up to 900°C and leaves a wide variety of products behind. Ethylene is recovered by distillation processes. 

The major industrial source of ethylene and propylene is the pyrolysis (thermal cracking) of hydrocarbons.137-139 Since there is an increase in the number of moles during cracking, low partial pressure favors alkene formation. Pyrolysis, therefore, is carried out in the presence of steam (steam cracking), which also reduces coke formation. Cracking temperature and residence time are used to control product distribution. 

Cracking temperature and vapor residence time were the most important parameters controlling the cracking reactions. Within the range of conditions tested, other variables such as type and area of cracking surface, the vapor concentration of the feedstock and presence of steam made little difference to the yields of BTX and ethylene. Steam is used as a diluent and... 

FIG. 19-13 Noncatalytic gas-phase reactions, (a) Steam cracking of light hydrocarbons in a tubular fired heater, (b) Pebble heater for the fixation of nitrogen from air. (c) Flame reactor for the production of acetylene from hydrocarbon gases or naphthas. [Patton, Grubb, and Stephenson, Pet. Ref. 37(11) 180 (1958).] d Flame reactor for acetylene from light hydrocarbons (BASF), (e) Temperature profiles in a flame reactor for acetylene (Ullmann Encyclopadie der Technischen Chemie, vol. 3, Verlag Chemie, 1973, p. 335). 

Description Feeds are sent to USC cracking furnaces (1). Contaminants removal may be installed upstream. A portion of the cracking heat may be supplied by gas turbine exhaust. Pyrolysis occurs within the temperature-time requirements specific to the feedstock and product requirements. Rapid quenching preserves high-olefin yield and the waste heat generates high-pressure steam. Lower temperature waste heat is recovered in the downstream quench oil and quench water towers (2) and used in the recovery process. Pyrolysis fuel oil and gaso-... 

Technip Isobutylene/butacracking Butanes (field) Co-production of propylene and iso-butylene by thermal steam cracking of butane at elevated temperatures NA NA... 

In industrial practice, however, the most widespread technique consists in passmg a mixture of hydrocarbons and steam through tubes placed in a furnace. The hydrocarbons, which are raised to high temperature, are pyrolysed and the resulting products are separated after a rapid quench. Coke deposits are periodically removed by controlled combustion. This is the technology of steam cracking, which is the main focus of this chapter. 

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