18 Pyrolysis Bench Scale Unit Design and Data Correlation J. J. L E O N A R D , J. E . G W Y N , and G . R. M c C U L L O U G H
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Shell Development Co., Westhollow Research Center, P.O. Box 1380, Houston, Tex. 77001
Ethylene cracking plants represent large capital investments, and the cost of feedstock to such plants is a major operating expense. Hence, there is considerable justification for evaluating potential feedstocks and for determining optimum operating conditions for each cracker feedstock. One approach to fulfilling such goals is to develop a reactor model that predicts yields for virtually any cracking coil and any operating condition using a bench scale unit to obtain feedstock-dependent parameters required for the model. A further extension is to have the feedstock-dependent parameters as functions of some measurable feedstock properties. In this paper, the design and operation of a bench scale unit are described. The yield results from the unit are compared to commercial yields with emphasis on the correlation of cracking severity. One approach to reactor modeling is described. Feedstock parameters are obtained from bench unit data and residence time plus profiles of pressure, fluid and metal temperatures, heat flux and methane yields for commercial conditions are calculated using these parameters. This model can then be used in conjunction with a yield selectivity model, not described here, to predict a full spectrum of yields. Experimental The bench scale unit was designed to have temperature and pressure profiles and residence times similar to conventional commercial crackers. Thus, comparative data for different feeds can be used directly. However, some sort of empirical approach would be necessary to be able to stipulate what commercial furnace operating conditions the data represent. A schematic of the bench scale unit is shown in Figure 1. The feed system is fairly conventional. Light feeds are pressured into the unit and liquid feeds are pumped from weighed feed tanks. The radiant coil is divided into four sections of small diameter stainless tubing. The outlet bulk gas temperature of each zone 311
Albright and Crynes; Industrial and Laboratory Pyrolyses ACS Symposium Series; American Chemical Society: Washington, DC, 1976.
Albright and Crynes; Industrial and Laboratory Pyrolyses ACS Symposium Series; American Chemical Society: Washington, DC, 1976.
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 22, 2018 | https://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0032.ch018
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18.
LEONARD E T A L .
Pyrolysis Bench Scale Unit
313
i s measured with thermocouple and the temperature i s controlled by four radiant e l e c t r i c a l heaters that parallel each straight length of tubing. Pressure i s controlled only at the furnace outlet. However, pressure p r o f i l e across the reactor can be cont r o l l e d . The hot gases from the radiant zone are passed through a short insulated tube and blown into condensed l i q u i d product for quenching. In this fashion, l i t t l e d i f f i c u l t y with coking i s experienced. Three product streams are recovered; a heavy oil/water quench l i q u i d , a l i g h t oil/water l i q u i d condensed at about 10°C, and a gas stream. Routine product analysis i s shown in Table I. The c a p i l l a r y gas/liquid chromotography (GLC) on the product gas is solely for the purpose of obtaining isomer d i s t r i b u t i o n s . The c a p i l l a r y GLC on the l i g h t o i l is primarily for C isomer breakdown and for benzene, toluene, and xylenes (BTX) analysis. 5
Table I.
Product Analysis
Gas Duplicate Mass Spectroscopy Capillary Gas Chromotography Light Liquid True Boiling Point" - Liquid Chromotography Capillary Liquid Chromotography Carbon/Hydrogen/Sulfur Heavy Liquid "True Boiling Point" - Liquid Chromotography Carbon/Hydrogen/Sulfur The TBP-GLC's on the liquids are for calculation of yields of various boiling range l i q u i d products. The C/H/S analyses are used for elemental material balance, calculation of heats of reactions and calculation of hydrogen d i s t r i b u t i o n , i . e . , hydrogen content of l i q u i d product. From the material balance and the product analyses, a f i n a l y i e l d report is made as shown in Table II. With each y i e l d report, the operating conditions are reported which include tube i d e n t i f i c a t i o n , hydrocarbon and water feed rates and pressure and bulk gas temperature p r o f i l e s . Severity of Cracking There are numerous expressions that are useful for representing severity of cracking.{]) The most commonly used terms in our work, i . e . , for liquids cracking, are shown in Table I I I , and are discussed below. Methane y i e l d increases monotonically with increased furnace f i r i n g , pressure and residence time. It i s feedstock dependent, but, because i t i s a dependent variable, i t i s useful for comparing y i e l d s for similar feeds at equal severities without
Albright and Crynes; Industrial and Laboratory Pyrolyses ACS Symposium Series; American Chemical Society: Washington, DC, 1976.
314
INDUSTRIAL
Table II.
%wt
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4.3
Methane
C -450°F-
10.0
450-615°F • - • C 5
Isomer Breakdown
^-•C6
Isomer Breakdown
5
3.4 615+
Ethane
5.4 Benzene
