(71g) Non-Isothermal Modeling of Ethane Thermal Cracker

The production of olefins is dominated by thermal cracking of a wide range of feed stocks, ranging from natural gas to heavy ends. Thermal cracking is a simpler technique compared to catalytic cracking. Unlike catalytic cracking, thermal process is not dependant upon feed pretreatment or the compatibility between the catalyst and the feed. Thermal cracker consists of empty tubular systems embedded in a fired furnace. The furnace caters to the heat requirements of the endothermic process. Radiative heat transfer heats up the tube walls and heat is further conveyed to the reaction mass from the heated tube walls. The transport of heat in the tube dictates the variation in reaction rates both in lateral and axial directions, which result in concentration gradients being setup in both the directions. In thermal cracking, the majority of modeling studies available in literature [Behlolav et al., 2003; Niaei et al., 2004;Pant and Kunzru, 1996; Ramana Rao et al.,1988] are based on the plug flow assumption i.e. a 1-dimensional model where the lateral gradients are neglected. Sundaram and Froment (1979) have compared the predictions of a 1-dimensional model to a 2-dimensional model for a single irreversible reaction. It is shown that 2-dimensional model gives better predictions out of the two model approaches. In another study by the same authors [Sundaram and Froment, 1980], a 2-dimensional model has been used for ethane cracking under turbulent conditions and it is shown that temperature and concentration profiles exist in lateral and axial directions. The reported results do not include the product distribution within the reactor. In the present work, a 2-dimensional model for ethane cracking is established and results are simulated with an aim of obtaining the product distribution in the reactor and showing the effect of various operational conditions. The model is formulated under the assumptions of constant wall temperature, axial symmetry, laminar flow and gradients in q direction not being considered. Based upon a molecular reaction scheme proposed by Sundaram and Froment (1977) for ethane cracking, the model consists of mass balances for the eight species involved and overall energy balance. These 9 PDEs form a strongly coupled system of equations, which have to be solved simultaneously. These model equations along with the boundary and initial conditions are solved numerically by finite difference method. Backward implicit numerical scheme is utilized. Here the tridiagonal matrix of the order M x M was changed to M x 3 using Srivastava's subroutine(1983).To validate the model predictions, the results are compared to analytical results for temperature profiles in absence of chemical reaction effect. A high level of agreement between analytical and model predictions is obtained. The coupled set of equations is solved simultaneously by developing a FORTRAN program. The kinetic data for the reaction scheme is as reported by Sundaram and Froment (1977). The physical properties data is adapted from standard sources. The simulated results predict the radial and axial temperature and concentration gradients in the reactor. The effect of variation of wall temperature, tube diameter and flow rate is also studied. These parameters are varied in the range of 900-1100^oC for wall temperature, 1-2.5 cm for tube diameter and 1.25-2.5 kg/hr for inlet flow rate. A higher conversion for ethane with a corresponding increase in product yields is obtained with an increase in wall temperature and/or tube diameter and a decrease in flow rate.

Keywords:Non-isothermal cracking, Finite difference method, Coupled partial differential equations

References:

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