Numerical Modeling, Simulation Of Tubular Furnace For Naphtha Cracking

Ethylene is the one of the most important building blocks used in the petrochemical industry. Thermal cracking of hydrocarbons (e.g. naphtha) is the main route for the manufacturing of ethylene and propylene. Thermal cracking furnace is the'heart'of the whole ethylene manufacturing process. Building of cracking furnace mathematical model to investigate the flow, heat transfer, mass transfer and cracking reactions is becoming an effective way to decide furnace design and optimal production operation.The first part is the development of cracking furnace steady-state mathematical model considering cracking reactions, heat transfer and pressure drop when process gas flow through tube reactor. The mathematical model consists of a continuity equation for each component, an energy equation and a pressure drop equation. Kinetics data for reverse reactions were not given in Kumar paper. Reverse reaction kinetic parameters were calculated. The reaction equilibrium constants for these reverse reactions were extracted from original reference. Experiment data and industrial production data were used to verify steady-state mathematical model. Simulation results were consistent with experiment data and industrial production data very well.The second part is the development of cracking furnace dynamic-state mathematical model on the base of strict chemical process conservation mechanism. Coke buildup on the internal tube wall was also included using Kumar coke deposit model. Dynamic production process of an industrial furnace was simulated. Run length (i.e. the time between two consecutive decoking operations) and other production parameters changing with manufacture time were predicted, including product yields, pressure drop, process gas residence time, energy consumption and coke rate. The simulation results matched the industrial production data very well.Based on the cracking furnace dynamic-state mathematical model, various case studies were then carried out to investigate the impact of the process gas temperature profile, inlet steam to naphtha ratio, pressure and diameter ratio of second pass to the first pass so that the ethylene/propylene product yields, run length, residence time, energy consumption and other operation parameters can be evaluated. The results show that(1) With the same operation parameters, increasing process gas temperature along tube can raise ethylene yield and energy consumption, shorten process gas residence time and manufacture time.(2) Fixing naphtha feed and with the same operation parameters, increasing inlet steam to naphtha ratio can decrease ethylene yield and process gas residence time, raise energy consumption, prolong manufacture time.(3) Fixing total feed and with the same operation parameters, increasing inlet steam to naphtha ratio can raise ethylene yield and process gas residence time, decrease energy consumption, prolong manufacture time.(4) With the same operation parameters, increasing outlet pressure can raise ethylene yield and energy consumption, prolong process gas residence time, shorten manufacture time.(5) With the same operation parameters, increasing diameter ratio of second pass to the first pass can raise ethylene yield, decrease pressure drop of the whole tube, prolong process gas residence time, shorten manufacture time.Process optimization was applied to the operation of this industrial tubular reactor. Operation profit was used as objective function. The process gas temperature profile and steam to naphtha ratio in the feed were used as optimisation variables. The effects of coking on reduction of manufacturing time and the decoking cost have been considered. After optimization, the furnace profit can be improved obviously.Numerical simulation model for cracking tube reactor was developed. Naphtha free radical reaction model and computation fluid mechanics model were considered at the same time in the cracking tube reactor simulation model. FLUENT physical property data library was set up for naphtha free radical reaction model. Cracking tube reactor can be simulated considering fluid flow, heat transfer, mass transfer, and free radical reactions contemporaneously.