Naphtha feed, pyrolysis

The breakeven price defines the unit price one could afford to pay for the gas oil to realize the same ethylene production cost as for a naphtha feed. If for a given naphtha price, the gas oil can be obtained at a price below the indicated gas oil breakeven price, gas oil would be the more attractive feed. If the gas oil can only be obtained at a price above the breakeven, naphtha would be the desired feed. Thus, with naphtha at, say, 1 /lb, the heavy gas oil would have to be priced below 0.7 /lb to be more profitable than naphtha as a cracking feed, assuming premium by-products prevail. Valuing the by-products as fuel has only a slight effect on the breakeven levels. The curves cross as nonaromatics in the pyrolysis gasoline have been valued the same as naphtha feed. 

Ross and Shu [38] report a model for naphtha pyrolysis. Naphtha feed is treated as a single lumped constituent A, which decomposes by a pseudo-elementary process of the first order into products By, viz. 

The feedstock consists of a mixture of C8 aromatics typically derived from catalytically reformed naphtha, hydrotreated pyrolysis gasoline oran LPG aromatization unit. The feed may contain up to 40% ethylbenzene, which is converted either to xylenes or benzene by the Isomar reactor at a high-conversion rate per pass. Feedstocks may be pure solvent extracts or fractional heartcuts containing up to 25% nonaromatics. Hydrogen may be supplied from a catalytic reforming unit or any suitable source. Chemical hydrogen consumption is minimal. 

Semikinetic modeling is illustrated by a generalized model for naphtha pyrolysis. The empiricism associated with the semikinetic model dictates the need for an extensive data base for parameter estimates. The naphtha data base consists of about 400 tests covering pure components and their mixtures and 17 naphthas (25). The pure components studied were normal and isopentanes, cyclohexane, and n-heptane. The wide range of naphtha feed properties is summarized in Table III. 

Naphtha feed is treated as a single pseudo species. Naphthas, used as pyrolysis feedstocks, are mainly composed of paraffins and naphthenes, with lesser amounts of aromatics. Olefin content is usually very small. Consistent with observed pyrolytic behavior of paraffins and naphthenes (15,16,26,27,28), feed decomposition is assumed to follow first-order kinetics. Equation 3 of the reactor model can be simplified as follows. 

Most olefins of petrochemical interest are produced by thermal cracking of naphtha feed stock, yielding about 12-14 million metric tons/year ethylene[l]. A Hungarian olefin plant, completed in 1975, is also operated on a naphtha feedstock. Yields and relative amounts of the main products greatly depend on the qualities of the naphtha feedstock pyrolyzed and the parameters of the cracking operation[2,3]. A detailed study of the pyrolysis is, therefore, of great industrial significance. 

The combination of low residence time and low partial pressure produces high selectivity to olefins at a constant feed conversion. In the 1960s, the residence time was 0.5 to 0.8 seconds, whereas in the late 1980s, residence time was typically 0.1 to 0.15 seconds. Typical pyrolysis heater characteristics are given in Table 4. Temperature, pressure, conversion, and residence time profiles across the reactor for naphtha cracking are illustrated in Figure 2. 

The U.S. ethylene industry has been based primarily on the cracking of ethane and propane derived from natural gas. The quantities and liquid contents of U.S. natural gases have been such as to permit substantial quantities of these light hydrocarbons to be recovered for use as economically attractive ethylene feedstocks. In Europe and Japan, however, naphthas have been generally the available and preferred feeds to pyrolysis. 

Under the assumptions of naphtha price and aromatics value stated above, naphtha pyrolysis clearly would be superior to light hydrocarbon pyrolysis at their current feed prices. A similar analysis can probably be made also for gas oil. Thus, if the possible developments discussed above do materialize, the heavier feeds could probably dominate almost all new U.S. ethylene plant construction in the future. 

For the primary step in the overall pyrolysis plant, the feedstock must be vaporized, if in liquid form, then mixed with steam, and finally preheated to the reactor temperature. When the feedstock (e.g., ethane or propane) is in gaseous form, vaporization is usually achieved by simple heat exchange with other product components such as condensing propylene. To vaporize liquid stocks such as naphthas, higher temperatures must be used. These feeds may be partially preheated before entering the furnace itself and then fully vaporized as they flow through the convective zone of the furnace. Typical furnace and tube geometries are shown in Fig. 4. 

In a previous publication (12,13) the development of pyrolysis models for gas and liquid feeds was discussed. In a more recent paper, Shu and Ross (14) described a generalized model for predicting the rate of thermal decomposition of naphthas. These models become key components of Braun s computational system. 

Shu, W. R., Ross, L. L., "A Feed Decomposition Model for Naphtha Pyrolysis". Paper No 62c, the 71st Annual Meeting of AIChE, Miami Beach, November 1978. 

Feedstocks For either gaseous (ethane/propane) or liquid (C4/naphtha/ gasoil) feeds, this technology is based on Technip s proprietary Pyrolysis Furnaces and progressive separation. This method allows producing olefins at low energy consumption with particularly low environmental impact. 

The KSF severity index is defined as a logarithmic function of the conversion A/ of a reference hydrocarbon present in the feed. Zdonik selected M-pentaoe. a compound that is always present in naphthas, and which offers the advantage that it cannot be formed in the pyrolysis of the other components by a side reaction. 

It must be pointed out that the severi of steam cracking affects not only the conversion of the feed aod the overall yield of C3- products, but also the distribution of the different compounds obtained. As illustrated by Fig. Z6 for the pyrolysis of naphthas, the results are as follows ... 

Light naphtha can produce over 30% ethylene with about half this yield of propylene. Methane yield is also high at over 17% with produetion of pyrolysis gasoline lower than the heavier liquids in the region of 14%. This is considerably more than the yields of pyrolysis gasoline (Cs+ aliphatie moleeules plus BTX) from gaseous feed stocks discussed above. 

Gas oil cracking has all of the characteristics of naphtha cracking. The typical statistics are given in Table 9.4. Relative to naphtha, the gas oil cracking requires considerably more feed for the same ethylene output (500 kt/y) and at the same time produces increased volumes of pyrolysis gasoline and more particularly pyrolysis fuel oil. This requires an increase in the fixed capital to naphtha for the same scale of operation. 

These reactions are reversible, and there is a dynamic equilibrium between carbon formation and removal. Under typical steam reforming conditions, reactions (46) and (48) are carbon - removing, whilst reaction (47) leads to carbon formation in the upper part of the tube [503]. With naphtha as steam reformer feed, irreversible pyrolysis (as in a steam cracker for ethylene production) with the sequence naphtha —> olefins—> polymers—- coke will occur. The mechanism of carbon formation and the determination of the risk areas in the reformer operating conditions on the basis of relevant equilibrium data are discussed in some detail in various publications [362], [363], [418]-[420]. 

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