Ethane, thermal cracking

Example 9.2. Thermal cracking of ethane. Thermal cracking of ethane at temperatures in the range of 800 to 1200 K and ambient or lower pressures yields mainly ethene and hydrogen ... 

For example, a large number of theoretical kinetic models have been suggested for describing ethane thermal cracking. Let us consider one of simplified mechanisms... 

Olefins are produced primarily by thermal cracking of a hydrocarbon feedstock which takes place at low residence time in the presence of steam in the tubes of a furnace. In the United States, natural gas Hquids derived from natural gas processing, primarily ethane [74-84-0] and propane [74-98-6] have been the dominant feedstock for olefins plants, accounting for about 50 to 70% of ethylene production. Most of the remainder has been based on cracking naphtha or gas oil hydrocarbon streams which are derived from cmde oil. Naphtha is a hydrocarbon fraction boiling between 40 and 170°C, whereas the gas oil fraction bods between about 310 and 490°C. These feedstocks, which have been used primarily by producers with refinery affiliations, account for most of the remainder of olefins production. In addition a substantial amount of propylene and a small amount of ethylene ate recovered from waste gases produced in petroleum refineries. 

As indicated in Table 4, large-scale recovery of natural gas Hquid (NGL) occurs in relatively few countries. This recovery is almost always associated with the production of ethylene (qv) by thermal cracking. Some propane also is used for cracking, but most of it is used as LPG, which usually contains butanes as well. Propane and ethane also are produced in significant amounts as by-products, along with methane, in various refinery processes, eg, catalytic cracking, cmde distillation, etc (see Petroleum). They either are burned as refinery fuel or are processed to produce LPG and/or cracking feedstock for ethylene production. 

The most important commercial use of ethane and propane is in the production of ethylene (qv) by way of high temperature (ca 1000 K) thermal cracking. In the United States, ca 60% of the ethylene is produced by thermal cracking of ethane or ethane/propane mixtures. Large ethylene plants have been built in Saudi Arabia, Iran, and England based on ethane recovery from natural gas in these locations. Ethane cracking units have been installed in AustraHa, Qatar, Romania, and Erance, among others. 

Thermal Cracking. / -Butane is used in steam crackers as a part of the mainly ethane—propane feedstream. Roughly 0.333—0.4 kg ethylene is produced per kilogram / -butane. Primary bv-pioducts include propylene (50 57 kg/100 kg ethylene), butadiene (7-8.5 kg/100 kg), butylenes (5-20 kg/WO kg) and aromatics (6 kg/ToO kg). 

About 35% of total U.S. LPG consumption is as chemical feedstock for petrochemicals and polymer iatermediates. The manufacture of polyethylene, polypropylene, and poly(vinyl chloride) requires huge volumes of ethylene (qv) and propylene which, ia the United States, are produced by thermal cracking/dehydrogenation of propane, butane, and ethane (see Olefin polymers Vinyl polymers). 

Although ethylene is produced by various methods as follows, only a few are commercially proven thermal cracking of hydrocarbons, catalytic pyrolysis, membrane dehydrogenation of ethane, oxydehydrogenation of ethane, oxidative coupling of methane, methanol to ethylene, dehydration of ethanol, ethylene from coal, disproportionation of propylene, and ethylene as a by-product. 

Cracking temperatures are somewhat less than those observed with thermal pyrolysis. Most of these catalysts affect the initiation of pyrolysis reactions and increase the overall reaction rate of feed decomposition (85). AppHcabiUty of this process to ethane cracking is questionable since equiUbrium of ethane to ethylene and hydrogen is not altered by a catalyst, and hence selectivity to olefins at lower catalyst temperatures may be inferior to that of conventional thermal cracking. SuitabiUty of this process for heavy feeds like condensates and gas oils has yet to be demonstrated. 

Cracked Gas Drying. Ethylene and propylene are two of the most important petrochemical raw materials today. They are manufactured by a thermal cracking of ethane, propane, or naphtha. One of the important separation-purification steps in the production of ethylene and propylene is removal of water before low temperature separation. Although alumina has been the most commonly used desiccant in drying cracked gas in the past, 3A molecular sieve adsorbents have an overall economic advantage 32), and many cracked gas plants are using the 3A molecular sieves today. 

Figure 1.5 shows ways of designing tubular reactors to include heat transfer. If the amount of heat to be transferred is large, then the ratio of heat transfer surface to reactor volume will be large, and the reactor will look very much like a heat exchanger as in Fig. 1.5b. If the reaction has to be carried out at a high temperature and is strongly endothermic (for example, the production of ethylene by the thermal cracking of naphtha or ethane—see also Section 1.7.1, Example 1.4), the reactor will be directly fired by the combustion of oil or gas and will look like a pipe furnace (Fig. 1.5c).

Ethylene is manufactured on a very large scale 1 by the thermal cracking of ethane in the gas phase ... 

The reactor impregnated with nickel showed inferior performance again. Deactivation was observed, which was assumed to originate from coking, sintering, oxidation of the nickel or even losses of volatile nickel species. With increasing temperature, enhanced formation of by-products, namely methane and ethane, was observed in the reformate both under partial oxidation conditions and in the autothermal mode, which was attributed to thermal cracking. 

Thermal cracking investigations date back more than 100 years, and pyrolysis has been practiced commercially with coal (for coke production) even longer. Ethylene and propylene are obtained primarily by pyrolysis of ethane and heavier hydrocarbons. Significant amounts of butadiene and BTXs (benzene, toluene, and xylenes) are also produced in this manner. In addition, the following are produced and can be recovered if economic conditions permit acetylene, isoprene, styrene, and hydrogen. 

Ethylene is a key building block for the petrochemical industry. It is usually made by thermally cracking gases—ethane, propane, butane, or a mixture of these—as they exist in refinery off-gases. When gas feedstocks are scarce or expensive, naphthas and even whole... 

Methane-based commercial production of ethylene via oxidative coupling has been investigated, but to date the lower per pass conversions required for acceptable ethylene selectivities combined with purified oxygen costs make this process noncompetitive with thermal cracking of ethane from natural gas liquids. 

Application To produce polymer-grade ethylene and propylene by thermal cracking of hydrocarbon fractions—from ethane through naphtha up to hydrocracker residue. Byproducts are a butadiene-rich C4 stream, a Cg— Cg gasoline stream rich in aromatics and fuel oil. 

Technip Ethylene/cracking furnaces Ethane to HVGO Thermal cracking of hydrocarbons in the presence of steam by highly selective GK and SMK cracking furnaces 500 2001... 

As an example, thermal cracking of ethane will be examined here in detail. 

Thermal cracking of ethane is an excellent example of an intricate mechanism that leads to a kinetic behavior obeying a simple, first-order rate law in good approximation over a fairly wide range of conditions. It also serves to show how easily such a deceptively simple rate law is misinterpreted. Moreover, the example illustrates an important general point ... 

Examples include the hydrogen-bromide reaction, thermal cracking of ethane and n-butane, oxidation of cyclohexane, and the hydrogen-oxygen reaction. 

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