Feedstock cracking

This chapter focuses on the economics of cracking naphtha and gas oil. The cracking of liquid feedstock produces most of the world s ethylene. This is dominated by naphtha cracking, the character of which has been discussed previously. Where available, there is some cracking of gas oil. 

The cracking of naphtha is carried out in all regions. The nominal capacity of the operations ranges from about 250 kt/y ethylene to operations producing over 1 million tonnes ethylene. There is good economy of scale and today s world scale crackers have a typical scale of 500 to 1000 kt/y ethylene, typically 850 kt/y. 

Feedstock (after pre-treatment if necessary) is passed along with steam to the pyrolysis furnace. This cracks the compounds in the naphtha, producing a full range of products which are extremely complex. As with gas feedstock, heavier products are produced, but in increased volumes. After quenching a primary fractionator (not present in gas crackers) separates the heavy pyrolysis fuel oil from the cracked gases. 

One of the issues that concern liquid feedstock cracking operations is a higher rate of fouling. This is not only a consequence of heavier coke forming precursors, but also as a consequence of long lived free radicals which act as agents for the formation of a polymer (often referred to as pop-corn polymer) in the primary fractionator and downstream units. For instance, free radicals based on styrene or indene have sufficiently long half-lives to pass from the pyrolysis section into the primary fractionator. These can concentrate in this unit and produce polymer (free radical polymerisation) when sufficient amounts of suitable olefins are present, in particular styrene itself and di-olefins such as cyclo-pentadiene or butadiene. 

One of the features of liquid feedstock cracking is the production of large volumes of ethane. These are collected by the de-ethanizer tower 

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Table 6 compares the total production of butylenes in the United States, Western Europe, andjapan. Included in this table are relative amounts of productions from different processes. In the United States, about 92% of the butylene production comes from refinery sources, whereas only about 45% in Western Europe andjapan are from this source. This difference arises because the latter cracks mostiy petroleum distillates in the steam crackers that produce larger amounts of butylenes than the light feedstocks cracked in the United States. 

Feedstock Cracked gas Naphtha Light gas oil Fleavy gas oil Residue... 

Unlike gas feedstock cracking the economics cracking of liquid feedstock such as naphtha is complicated by the ability to change the product slate by changing the cracking conditions (severity) and to select... 

Liquid Feedstock Cracking Table 9.1 Statistics for Naphtha Cracking... 

K is a function of feedstock, cracking severity, dilution steam ratio and other system variables it is an empirical factor. 

As with residence time, the correlating definition of hydrocarbon partial pressure must consider the partial pressure history in the coil. For a given feedstock cracked at a fixed conversion and gas outlet pressure in a pyrolysis coil of a given configuration, the average hydrocarbon partial pressure, PhCA determined by ... 

This present paper presents the kinetic-mathematical model developed to describe the overall decomposition rate and yields of the naphtha feedstock cracking process. The novelty and practical advantage of the method developed lies in the fact that the kinetic constants and yield curves were determined from experiments carried out in pilot-plant scale tubular reactors operated under non-isothermal, non-isobaric conditions and the reactor results could readily be applied to simulate commercial scale cracking processes as well. During the cracking experiments, samples were withdrawn from several sample points located along the reactor. Temperature, as well as pressure were also monitored at these points[2,3]. 

Product Distribution of Plant-scale Naphtha Feedstock Cracking Operation, Description of Yield Curves... 

Kinetic-Mathematical Model of Naphtha Feedstock Cracking... 

Previous sections oresented the so-called kinetic model of naphtha feedstock cracking, which allows the calculation of the degree of decomposition, X, the overall kinetic severity function, BKSF, and the true residence time, T, as functions of the relative length of the reactor, z. 

Figure 11 presents the measured yield values and the calculated yield values of the more important reaction oroducts (drawn in full line) obtained from the kinetic-mathematical model applied for the naphtha feedstock cracking experiment No. 44. 

Furthermore, it can be established, that to obtain maximum ethylene yield in the naphtha feedstock cracking process, (and the process is advantageously interrupted at that point) the reaction mixture should have a true residence time between the 0.46 - 0.52 seconds limits, depending on the severity of cracking (exit teinjerature, temperature-profile). 

Similar compositional changes of the two feedstocks cracked on GX-30 are reported by Kraemer (1991) plotting each oil lump versus reaction time for the three temperatures. The same general trends for product yields with temperature and reaction time as discussed for Octacat catalyst were also observed with GX-30. 

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