الفهرس يوجد فقط 14 صفحة متاحة للعرض العام
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المستخلص
The world-wide demand for lower olefins (ethylene, propylene, butenes and butadienes) is higher than any other chemical, but due to their high reactivity they are only found in very low concentrations in crude oil. It is therefore necessary to ‘crack’ saturated hydrocarbons into unsaturated hydrocarbons using the large-scale catalytic cracking or steam cracking processes.
Ethylene, which is a cornerstone in the petrochemical industry, is produced commercially by thermal cracking of ethane, ethane-propane mixture, or naphtha in the presence of steam. Cracking is also known as pyrolysis. The pyrolysis of hydrocarbon feedstock involves two parallel reaction pathways. The primary reactions lead to desirable products and the secondary reactions to undesirable products such as methane and coke. This coke deposits on the walls of the reactor, reducing the overall heat transfer coefficient and increasing the pressure drop across the reactor.
Steam cracking is the most energy-consuming process in the chemical industry and globally uses approximately 8% of the sector’s total primary energy use, excluding energy content of final products excluded.
The transfer line exchangers (TLEs) are used to quench and rapidly cool high temperature effluent from the cracking furnace to prevent degradation by secondary reactions, and to recover sensible heat from the cracked gas in the form of high pressure steam.
The present work deals with the modeling of primary transfer line exchangers used in the ethylene plant of Sidi Kereer Petrochemicals Company (SIDPEC) located in Alexandria, Egypt. These exchangers are vertical exchangers of special annular tube-in-tube design. The hot cracked gas enters the inner tubes, and super high pressure boiler feed-water enters the outer tubes. The steam/water mixture flows upward cocurrently with the process gas. The generated steam in the form of super high pressure steam (SHP steam) is then superheated and used to drive the major compressors.
A mathematical model of the TLEs was developed and used to predict the cracked gas outlet temperature, the mass flow rate of the generated SHP steam, and the pressure drop in the tube side of the exchanger. The TLEs inlet conditions are determined by the furnace outlet operation. The results obtained from the model were compared with different operating cases corresponding to different furnace loads. The agreement between the calculated results and the operating data showed reasonable accuracy of the mathematical model.
In addition, the model was used to simulate the TLEs performance at different furnace loads, different cracked gas inlet temperatures, different cracked gas compositions, and different boiler feed water temperatures. Finally, a sensitivity analysis was used to determine the most effective parameter in the performance of the transfer line exchanger.