Modelling of heat transfer and pyrolysis reactions in an industrial ethylene cracking furnace

Abstract

The development of a relatively simple mechanistic model for an industrial ethylene cracking furnace is described, including the estimation of selected model parameters to improve model predictions. Energy balance equations are developed to account for radiative, conductive, and convective heat transfer in the radiant section, and for convection and conduction in the ultra-selective heat exchanger (USX) and in the transfer line exchanger (TLE). Kinetic schemes by Ranjan et al. and Sundaram and Froment are used to model the cracking reactions. [1,2] The heat transfer model is combined with mass and momentum balances to model gas composition, pressure, and temperature changes as a function of position along the reactor tubes. Initial values and uncertainty ranges are assigned to 44 model parameters based on information in the literature and our industrial sponsor. A sensitivity-based technique and a mean-squared-error (MSE) criterion are used to select the appropriate subset of 22 parameters for tuning. Parameters are estimated and model predictions are validated using industrial data. Model predictions provide a good match to data that were not used for estimation.

Modelling coke formation in an industrial ethane‐cracking furnace for ethylene production

“…Algebraic Equation (23) is used to calculate the inner coil wall temperature. The heat flux profile required for solving Equation (23) is determined using equations from the pyrolysis model . The model equations were solved numerically using PREDICI …”

“…The momentum balance in Equation (29) predicts the pressure drop of the process gas inside the tube . An energy balance ODE for calculating the temperature profile of the process gas inside the tube is provided by Equation (31) . The reaction schemes for the small amount of cracking that occurs inside the USX and TLE coils are the same as in the coils of the radiant section of the furnace and are shown in Table .…”

“…In this equation δ is the total coke thickness, r c , Cat is the catalytic coke formation rate from Equation (27), r c , Pyro is the pyrolytic coke formation rate from Equation (28) and L c is the characteristic thickness that determines whether further catalytic coke formation is significant. In all, the proposed coke formation model has three parameters ( L c , α f , Cat , and α f , Pyro that require estimation (on top of the 17 parameters that were estimated previously in the pyrolysis model) …”

“…The total number of radiant coils is 16. It is assumed that all 16 coils have the same temperature, pressure, and concentration profiles . As such, the model development and simulations in the current article focuses on one coil that contains one sixteenth of the furnace throughput.…”

“…Operating conditions at x s = 0 for the concentrations of all species and for the process gas temperature, process gas pressure, and furnace temperature are known from industrial data. Boundary conditions (BCs) used in the model are explained in detail in our previous work . PREDICI was chosen by NOVA Chemicals for model implementation because of its capability in handling complicated reaction schemes, its parameter estimation tools, and its user interface .…”