Severity in the pyrolysis of petroleum fractions. Fundamentals and industrial application

Abstract

In thermal cracking, use is made of severity factors to characterize the process conditions and to relate these in a concise way to the product slate. A fundamental, generalized severity factor, kV,IF,, based upon the equivalent reactor volume concept is derived from the model equations representing the cracking coil operation.It relates a modified exit conversion in a unique way to operating conditions (temperature and pressure profile, steam dilution ratio), no matter what the feedstock may be. In commercial units temperature and pressure profiles are in general insufficiently defined, so that only easily accessible exk yields can be used as a severity factor. From an inspection of a large data set on naphtha, kerosene, and gas oil cracking, it follows that the C3to propylene yield ratio is the best measure of the severity of operation.Evaporation and Wling heat-transfer coefficients are presented for thin water HLms flowing over the outsii of horizontal, electrically heated brass tubes. Tests were conducted with a 5.08-cm-dlameter smooth tube, a 5.08-cmdlameter clrcumferentially grooved tube, and a 5.08-cm-diameter axially grooved tube. Both local and average heat-transfer data were obtelned for nonboillng and bolilng conditions correspondi to feed-water temfrom 1.16 to 3.79 cm3/s per centimeter length of tube. Correlations of the average heat-transfer coefficients for nonboiling and boiling conditions were developed and compared. The results indicate that both enhanced tubes provlded hlgher heat-transfer coefficients than the smooth tube.peratures ranglng from 49 to 127 O C end heat-flux values ranging from 30 000 to 80 000 W/m 9 . Flow rates ranged

Two Severity Indices for Scale-Up of Steam Cracking Coils

“…27 Indeed, ratios of yields are more reliably determined than absolute values such as the methane yield. 13 In industry, the P/E ratio is usually fixed at about 0.55. At lower severities, before the propylene yield has reached its maximum, not much cracking has occurred and large amounts of liquid byproducts are formed.…”

“…In addition to the P/E ratio as an alternative for the methane yield, other severity indices can also be used, for example, the C 3 -/C 3 ) yield ratio. 13 The C 3 -fraction contains propylene, propane, propadienes, C 2 components, methane, and hydrogen. C 3 ) is the yield of propylene.…”

Multivariable optimization of thermal cracking severity

“…Then, a tradeoff exists between the plant throughput and the frequency of the down times and, thus, an optimum figure should exist for ω. To this complexity, one can add the nonlinear behavior of the yields of propylene, butene, butadiene and gasoline along the reactor coil as demonstrated in Lichtenstein (1964), Chambers and Potter (1974a), Froment et al (1976), Szepesy (1980), Van Damme et al (1981), Van Camp et al (1984), Van Camp et al (1985), and Van Geem et al (2005) as a consequence of the temperature profile and the coil geometry. Then, oversimplification must be avoided in order to assure an optimal set-point collection of the three process variables employed in this research.…”

“…Theoretically, the concept of cracking/pyrolysis severity has been coined to represent the reaction conversion for a hydrocarbon mixture. For example, Van Camp et al (1985) proposed a generalized fundamental severity factor based upon the equivalent reactor volume concept (vide infra). This factor related a modified exit conversion to operating conditions (temperature profile, pressure profile, and steam dilution ratio), regardless of the feed type.…”

Severity Parameters for Steam Cracking

“…Ideally, one would like to have some independent controllable measure of severity which would function as an independent variable to be preset before a test run. Froment et al identified a number of measures with an emphasis on the C3 fraction normalized to propylene yieldchosen for its pressure independence and industrial severity measure. , There are three such possible measures which allow one to assess the effects of widely differing mixed feeds while at the same time allowing differing amounts of control of the reactor conditions. These are (1) the methane make, (2) the propylene/ethylene ratio (PER), and (3) the hydrogen/carbon ratio (HCR) of the C5+ cracked effluent.…”