(133a) Taking Optimal Advantage of Feedstock Flexibility with Coilsim1D

Authors: 
Van Geem, K. M., University of Ghent
Pyl, S., Ghent University
Reyniers, M. F., Ghent University

The petrochemical industry is continually searching for higher performance and increased selectivity to increase their profit margins. In this search accurate simulation models such as COILSIM1D have become an indispensable tool. COILSIM1D is a recently developed single event microkinetic model (SEMK) for simulating the steam cracking process.1,2 The core of this model consists of the single event reaction network based on the radical chemistry of the cracking process. The reaction network consists of two parts, a monomolecular µ network and a β network, and is the most extensive reaction network ever generated for the steam cracking process. Over 500 molecules with a carbon number up to 33 allow representing a very broad range of industrially relevant feedstocks, from ethane to LPG, over naphtha to gas oil and vacuum gas oils. The fundamental approach results in a more detailed product distribution than the one obtained with other reaction networks. Special attention is given to an accurate description of the C4 fraction, the pygas fraction and the fuel-oil composition.

The model equations are based on a 1-dimensional reactor model, in which no radial gradients are assumed, except for the temperature in a thin film close to the wall in which all resistance to heat transfer is located. The flow is assumed to be of the plug flow type. The model equations contain the continuity equations for the different species, an energy balance and a momentum equation. These equations can be solved rapidly and accurately using a stiff solver DASSL3 resulting in the product yields and the pressure, concentration and temperature profiles along the reactor coil for a wide variety of industrial steam cracker configurations.

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 Fluent3 or from one of the in house developed software codes FURNACE5 or FLOWSIM6. Moreover, desired reactor outlet conditions can also be specified such as 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.

One of the major challenges for the use of this SEMK model in an industrial environment was the requirement of a detailed molecular feedstock composition while it is only seldom available in industrial practice. The development of a method for feedstock reconstruction based on the global characteristics of the hydrocarbon feeds, the so-called commercial indices allows to tackle this issue. This method was originally developed for naphtha fractions but has been extended to condensates, kerosenes and VGO's. The combination of the feedstock reconstruction method with COILSIM1D allows to obtain accurate simulation results1,2 and provides a valuable tool to take optimal advantage of feedstock flexibility.

COILSIM1D is validated using over 300 pilot plant experiments from the experimental database of pilot plant experiments that cover a wide range of feedstocks and process conditions and over 30 different industrial furnace simulations. Excellent agreement is obtained between the simulated and experimental product yields. Also for difficult feedstocks such as VGO, heavy naphthas and gas oil mixtures a good agreement between the simulated and experimentally determined product distribution is obtained.

References:

1. Kevin M. Van Geem, Damien Hudebine, Marie Françoise Reyniers, François Wahl, Jan J. Verstraete and Guy B. Marin, Molecular reconstruction of naphtha steam cracking feedstocks based on commercial indices. Comp. & Chem. Eng., 31 (9): 1020-1034, 2007

2. Kevin M. Van Geem, Marie Françoise Reyniers and Guy B. Marin, Challenges of Modeling Steam Cracking of Heavy Feedstocks, Oil & Gas Science and Technology-Revue de l'IFP, in press

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

4. 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, In Press, Corrected Proof, Available online 3 May 2007

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

6. 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.

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