(75g) Effect of Operating Conditions and Feedstock Composition on Run Length of Steam Cracking Coils

One of the main problems
of the steam cracking process is the formation of a coke layer on the inner
walls of the reactor coils. This leads on the one hand to a decrease of the
heat flux to the reacting gas mixture and on the other hand to an increase of
the pressure drop over the reactor. Hence, periodically the furnace operation
has to be stopped and the coke has to be burnt off by means of a mixture of
steam and air. To allow for simulation of the run length of industrial steam
cracking coils the fundamental simulation model COILSIM1D ^1,2,3
incorporates two coking models. The coking model of Plehiers ^4-6 is
developed for predicting coking rates for steam cracking of light hydrocarbon
feedstocks.  The model of Reyniers et al. ⁷ allows to simulate the
coking rate of heavier feedstocks ranging from light naphtha fractions until
heavy condensates. Both coking models account for the heterogeneous
noncatalytic or so-called asymptotic coking only. The contributions of the
heterogeneous catalytic coking and the homogeneous noncatalytic coking to the
total amount of coke formed during the complete run length are assumed to be
negligible. ^4,5 The coking kinetics are coupled to the 1-dimensional
reactor model equations solved in COILSIM1D.

These equations can be
solved rapidly and accurately using a stiff solver DASSL⁸ resulting
in the product yields and the pressure, concentration and temperature profiles,
coking rates and coke layer thicknesses along the reactor coil for a specific
reactor configuration. The boundary conditions can be either an external tube
wall temperature profile, a process gas temperature profile or a heat flux
profile. The latter can be obtained from Fluent⁹ or from one of the
in house developed software codes FURNACE¹⁰ or FLOWSIM¹¹.
Moreover, the desired cracking severity can also be specified by means of e.g.
the propylene/ethylene ratio, the methane/propylene ratio, a key component
conversion, or a fixed yield of ethylene or methane. The program then returns
the process conditions required to obtain the desired outlet specifications by
solving the resulting two points boundary condition problem using a shooting
method. Several examples of run length simulations will be presented for
feedstocks ranging from ethane over n-butane to light and heavy naphtha's
cracked over a broad range of operating conditions. Good agreement is obtained
between simulated and observed run lengths of several industrial-cracking
units. These simulations can be used in an industrial setting to optimize
furnace operation for various feedstocks and operating conditions. To that
purpose a graphical user interface has been developed. Its use will be
demonstrated.

1.    
K.M. Van Geem, D. Hudebine, M-F. Reyniers, F.
Wahl, J.J. Verstraete and G.B. Marin, Molecular reconstruction of naphtha steam
cracking feedstocks based on commercial indices. Comp.
& Chem. Eng., 31 (9): 1020-1034, 2007

2.    
K.M. Van Geem, M-F. Reyniers, and G.B. Marin,
Challenges of Modeling Steam Cracking of Heavy Feedstocks, Oil & Gas
Science and Technology-Revue de l'IFP, 63, 79-94, 2008.

3.    
K.M. Van Geem, M-F. Reyniers, and G.B. Marin,
Taking optimal advantage of feedstock flexibility with Coilsim1D, AICHE Spring
meeting, New Orleans, LA, 2008.

4.    
P.M. Plehiers, G.C. Reyniers and  G.F. Froment,
?Simulation of the Run Length of an Ethane Cracking Furnace?, Ind. Eng. Chem. Res., 29, 636-644, 1990.

5.    
P.M. Plehiers and  G.F. Froment, Chemicla
engineering Communications, 80, 81-99, 1989.

6.    
M.V.R. Rao, P.M. Plehiers and  G.F. Froment,
Chemical Engineering Science, 43, 6, 1223-1229, 1988.     

7.    
G.C. Reyniers, Froment G.F., F.D. Kopinke and G.
Zimmerman, Coke formation in thermal cracking of hydrocarbons. 4. Modeling coke
formation in,  naphtha cracking, Ind. Eng. Chem. Res. 33, 2854-2850, 1994

8.    
S. Li, L.R. Petzold, Design of New DASPK for
Sensitivity Analysis, UCSB Technical report, 1999.

9.    
G.D. Stefanidis, K.M. Van Geem, G.J. Heynderickx
and G.B. Marin, Evaluation of high-emissivity coatings in steam cracking
furnaces using a non-grey gas radiation model, Chemical Engineering Journal,
137 (2), 411-421, 2008.

10.  G.J. Heynderickx, M. Nozawa, Banded gas and nongray surface
radiation models for high-emissivity coatings. AICHE JOURNAL  51 (10):
2721-2736 OCT 2005

11.  G.D. Stefanidis, B. Merci,  G.J. Heynderickx and G.B. Marin, CFD
simulations of steam cracking furnaces using detailed combustion mechanisms. Comp. & Chem. Eng. 30 (4): 635-649,
2006.