Scheduling of Cracking Furnaces with Rigorous Process Models

ERICA P. SCHULZ; SOLEDAD DIAZ; ALBERTO BANDONI

Austin, EEUU

Congreso; AIChE Annual Meeting; 2004

American Institute of Chemical Engineers

  This work addresses the determination of a maintenance policy for cracking furnaces in an ethylene plant with decaying performance. Thermal cracking of ethane produces deposition of coke on the tubes internal surface with the consequent decaying performance of the furnaces due to the higher heat-transfer resistance and the reduced residence time (reduced effective area of cross section inside the tubes). Therefore, furnaces have to be periodically shut down and cleaned to restore the plant performance. The trade-off is between high production rates obtained with a frequent maintenance and the loss of production due to shutdowns and the maintenance costs. The model takes into account the interactions between the entire process plant operation and the furnace performance by the inclusion in the furnace feeds of an important ethane recycle stream from downstream. The entire ethylene production process has been modelled with ten parallel furnaces, quenching, hydrogenation reactors, separation train, ethane recycle, ethylene recycle, cracked gas compressor, ethylene and propylene compressors and utility plant. These plant sectors are represented by nonlinear correlations. The main decisions are the number of shutdowns and their location on the time horizon, as well as the operational profiles of the key process variables. A Big-M formulation is applied for the disjunctions. The resulting model is a multiperiod mixed integer nonlinear programming problem with time varying product demand and was solved with GAMS using DICOPT++ with CONOPT and OSL solvers.