Rate-based modeling of steam ethane cracker

Gündür, Selin
Ethylene production is the main building block of petrochemical industry and it is produced via thermal cracking process which does not require a catalyst and takes one of the refinery white products, that is straight run naphtha, as a feedstock. In some processes, ethane which is produced as a result of naphtha cracking is fed into a separate ethane cracker which also yields same products as ethylene and propylene etc. The main reason to process produced ethane in a separate cracker is that, ethane which is harder to crack when compared with naphtha requires higher residence times within the cracker tubes to achieve the desired conversion. When crackers or furnaces used in refineries and petrochemical plants are considered, process gas temperature within the tubes is very crucial to be able to monitor the cracking process. In general temperature measurements are done at the inlet and outlet of the furnaces, as well as at some points on tube surface. However, temperature profile of process gas within the tubes cannot be monitored. Since ethane cracking process has a run length of approximately three months followed by a decoking period, in order to maintain operational safety, accurate modeling of the process has to be done to find gas temperature profiles. At high temperatures coking rate increases and coke formed deposits on the tube internal walls and prevents efficient transfer of heat. Because of this inefficient heat transfer, cracker duty is increased at a level of maximum allowable tube surface temperature in order to obtain desired ethane conversion. When that maximum surface temperature is obtained, process is shut down and decoking period is started. Thus, modeling of ethane cracker is crucial to find the temperature profile of the process gas within the coil as well as coke formation profile within the tubes. In this study, three different reaction network models were constructed and separate mass, energy and momentum balance equations were solved simultaneously using an algorithm developed in MATLAB software for each case. Model validation is achieved using two studies found from literature. In the three models that are considered in this study, in Model-I basic reaction mechanism are included in which neither coke formation nor removal reactions are involved , whereas in Model-II only coke formation and in Model-III additionally coke removal reactions are included. In each model different number of components and reactions are considered. The input data from PETKİM and reactor specifications are used and outputs, such as conversion, temperature, pressure are compared. Program outputs are compared with the industrial data that is provided by PETKİM Co. Since Model-III considers more number of reactions and coke formation and removal, it yielded similar results with the real plant data and therefore selected as the appropriate model. Using this model, process gas temperature profile can easily be evaluated by the input plant data and control of operation can be done in a better way.


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Citation Formats
S. Gündür, “Rate-based modeling of steam ethane cracker,” M.S. - Master of Science, Middle East Technical University, 2015.