(220e) Boosting the Energy Efficiency of Ethylene Production

Due to the large amount of different equipment and the complex set-up of heat exchanger-networks in order to increase the recovery of heat and cold, ethylene plants are among the most complex petrochemical processes. Changes with positive effects in one part of the process can lead to effects in other parts of the plant with negative impacts on the whole process that are difficult to detect and to quantify. Process simulation is a powerful tool to display these correlations. A detailed model of the crack-gas-separation part of an ethylene plant was generated with KBC-PetroSim^® based on process flow sheets, equipment data sheets, laboratory tests of products and intermediates and process data obtained from the process control system during a three day test run under full load. The complete model contains 268 devices and 995 streams with 74 recycles. About 81% of the energy to run the gas separation is delivered by the crack-furnaces, the rest has to be imported from other sources within the refinery. The cryogenic processes for the separation of the light gases are counted among the major energy consumers and are composed of a 3-stage propylene refrigeration system and a 4-stage ethylene refrigeration system. Due to economic and technical issues the plant cannot always be operated at the design point. Especially partial load conditions are crucial for the compressors of the cryogenic systems. Recycle streams, which ensure a minimum flow rate to critical equipment cause a reduced energy efficiency of the whole process. The simulation of the plant shows how to adapt the control strategy in order to move energy demand from sections that are working under full capacity to these critical sections and obtain energy savings of 2% based on the imported energy. Using the model of the plant as a tool for pinch and capacity analysis it is possible to develop different scenarios for the modification of the heat exchanger network, which allows a higher interaction between the two refrigeration systems. The so obtained savings of energy are up to 7.6% based on the imported energy and 8938 t/a of CO₂ emissions.