24 Kinetic-Mathematical Model for Naphtha Pyrolyzer Tubular Reactors V. ILLES, O. SZALAI, and Z. CSERMELY
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Hungarian Oil and Gas Research Institute, Budapest-Veszprem, Hungary
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 i s , therefore, of great industrial significance. Calculation of the ethylene yield of naphtha cracking using the severity function introduced by Linden et al.[4] is wide spread. Recently, Zdonik et al.[5] introduced the so called kinetic severity function (KSF) characterizing the cracking severity with the n-pentane conversion achieved within the experimental conditions, or, alternatively, with the term ∫ K5dT calculated for the n-pentane from the first order kinetic equation. Several scientists including [6] and [7] have accepted this method. There are a few publications dealing with the statistical mathematical models, based on the results of experimental naphtha feedstock cracking operations[8,9]. 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 During the cracking experiments, samples were withdrawn from several sample points located along the reactor.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]. Several investigators discuss the determination of the overall decomposition kinetics of hydrocarbons on the basis of nonisothermal experimental data. Kershenbaum and Martiη[10], Kunzru et al.[11], and Leftin and Cortes[12], studied the pyrolysis pro-
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Albright and Crynes; Industrial and Laboratory Pyrolyses ACS Symposium Series; American Chemical Society: Washington, DC, 1976.
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INDUSTRIAL A N D L A B O R A T O R Y PYROLYSES
ω
11
0
Figure 1.
1
0,4
1
ι
1
0,6 0,8 Relative length of reactor, ζ
1,0 the
Naphtha cracking temperature and pressure as a function of relative length of the reactor
Expt.No. 44 45 46
1
0,2
Symbol
A
V +
Expt.No.
Symbol
47 48 49
X
0
*
Albright and Crynes; Industrial and Laboratory Pyrolyses ACS Symposium Series; American Chemical Society: Washington, DC, 1976.
24.
ILLES ET AL.
425
Kinetic-Mathematical Model
cess of propane, n-nonane, and isobutene, respectively in bench scale flow reactors. Experiments were conducted with identical temperature profiles and feed-rates but different hydrocarbon partial pressures. Samples were taken only at the reactor outlet. According to our best knowledge, a descriptive method has never been published that i s based on the true decomposition processes of naphtha feedstocks. A kinetic-mathematical model i s presented here that was developed based on experimental pyrolysis data obtained on straight run Romashkino (Soviet Union) crude naphtha cuts. Characteristics of this naphtha feedstock are summarized in Table 1. Table 1 Characteristics and group analysis data of the naphtha feedstock used for the pyrolysis experiments Group analysis
Characteristics Density, 20°C: 0.7224 g/cm
3
Average molecular weight: 100 g/mole
weight %
n-paraffins
30.9
iso-Daraffins
29.5
naphthenes
28.1
aromatics
11.5
Engler d i s t i l l a t i o n i n i t i a l temp., °C
50.5
10% d i s t . temp.
80.2
50% d i s t . temp.
110.5
90% d i s t . temp.
137.0
end point temp.
174.7
hydrogen content
14.68
sulphur content
0.035
Pyrolysis experiments were conducted in the pressure, temperature, and residence time range of commercial i n t e r e s t , under 50 weight percent steam in the feed stream. Figure 1 presents the temperature and pressure profiles kept throughout the experiments. The six experiment series were made using two different temperature-profile shapes ( I . and II.) and 820 C and 850 C reactor e x i t temperatures, respectively.
Albright and Crynes; Industrial and Laboratory Pyrolyses ACS Symposium Series; American Chemical Society: Washington, DC, 1976.
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INDUSTRIAL AND LABORATORY PYROLYSES
Description of Overall Decomposition Rate of Naphtha Feedstocks Determination of the rate equation and rate constants» Numer i cal characterization of the decomposition of naphtha feedstocks was made with the so called "degree of decomposition" term, introduced formerly by the authors to characterize the decomposi tion of hydrocarbon mixtures[13], defined for naphtha feedstocks as follows: J
