Propane, thermal cracking

Hydrocarbon and steam mixture flows from the convection section into the radiant section at approximately 525-700°C, depending on the feedstocks and the heater design. This is called the crossover temperature. For ethane and propane thermal cracking, the crossover temperature is approximately 700° C. For vacuum gas oil (VGO) and AGO cracking. 

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). 

An isopropyl carbocation cannot experience a beta fission (no C-C bond beta to the carbon with the positive charge).It may either abstract a hydride ion from another hydrocarbon, yielding propane, or revert back to propene by eliminating a proton. This could explain the relatively higher yield of propene from catalytic cracking units than from thermal cracking units. 

This chapter contains a discussion of two intermediate level problems in chemical reactor design that indicate how the principles developed in previous chapters are applied in making preliminary design calculations for industrial scale units. The problems considered are the thermal cracking of propane in a tubular reactor and the production of phthalic anhydride in a fixed bed catalytic reactor. Space limitations preclude detailed case studies of these problems. In such studies one would systematically vary all relevant process parameters to arrive at an optimum reactor design. However, sufficient detail is provided within the illustrative problems to indicate the basic principles involved and to make it easy to extend the analysis to studies of other process variables. The conditions employed in these problems are not necessarily those used in current industrial practice, since the data are based on literature values that date back some years. 

The thermal cracking of propane is practiced industrially for the primary purpose of making ethylene and propylene, but other reactions also occur. A scheme worked out by Sundaram Froment (Chem Eng Sci 32 601, 1977) consists of the nine reactions of the table. Equilibrium constants were deduced from thermodynamic data and the other constants by nonlinear regression from the extensive data on this topic in the literature and laboratory. 

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. 

Feed stock for the first sulfuric acid alkylation units consisted mainly of butylenes and isobutane obtained originally from thermal cracking and later from catalytic cracking processes. Isobutane was derived from refinery sources and from natural gasoline processing. Isomerization of normal butane to make isobutane was also quite prevalent. Later the olefinic part of the feed stock was expanded to include propylene and amylenes in some cases. When ethylene was required in large quantities for the production of ethylbenzene, propane and butanes were cracked, and later naphtha and gas oils were cracked. This was especially practiced in European countries where the cracking of propane has not been economic. 

Table I shows the products from a well-designed gas-recovery unit in a typical refinery having a catalytic-cracking unit and a thermal-cracking unit. Where only the propane propylene is charged to the polymerization unit a depropanizer is added to separate the Cs and lighter from the C and heavier, shown in the last column of the table.

Propane is concerted to ethylene and methane in a thermal cracking operation by the reaction... 

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... 

To illustrate the complexity of the process of thermal cracking. Table 2.1 lists some of the more important reactions in the cracking of propane. 

Rates of thermal cracking are first-order in good approximation for propane, butane and still higher hydrocarbons [21], This is remarkable because chain mechanisms with initiation by break-up of a reactant normally result in reaction orders of one half or one-and-a-half, depending on which radical is consumed by termination. First-order behavior can result from "mixed" termination, which, however, can in most cases be ruled out as dominant mechanism (see Section 9.3). A more probable explanation is a combination of effects that key hydrocarbon radicals participate in several steps of different molecularities, that some steps are reversible, and that some unimolecular ones require collision partners. 

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