Process Improvements In Steam Cracking Of Naphtha

Abstract

Steam cracking of naphtha and light hydrocarbons such as ethane, propane and their mixtures is a major process for production of ethylene, propylene, butadiene and the aromatics. Coke formation and deposition is a major problem associated with this process due to which the furnace has to be shut down for decoking once in 30-60 days depending on the feed and process severity. Efforts and research have been done to reduce coke formation so as to increase run length between two decoking steps. This report presents the results of establishing of base run, reproducibility checking of base runs and performance of test runs which included the addition of additive mixture in a new coil made of Incoloy 800HT. The addition of these additives resulted in significant reduction of coke during cracking. Several runs were carried out in the bench scale naphtha cracker unit with different operating conditions to optimize operating conditions that would simulate commercial plant performance with respect to yields and coke sufficient to enable testing an additive in a run length of 84 h. The optimized operating conditions of coil outlet temperature (COT) are 830ºC, steam to naphtha dilution ratio is 0.32, and residence time is 0.5 sec. The corresponding naphtha and water flow rate are 63.49 g/h, and 20.32 g/h respectively. An additive recipe has been developed and tested under the optimum base run conditions and found an average coke reduction of 69%. Two kinetics based models have been developed to simulate Naphtha cracker and ethane gas cracker performance. The models were developed in MATLAB based on molecular reactions scheme and kinetics available in literature. The model integrates differential mass balance and temperature profile equations using ODE45 and model predicted performance matched well with that of plant. The model has been used to simulate the bench scale cracker performance in terms of yields successfully. Kinetic model has also been developed for ethane cracker. The model considers eight molecular reactions. The model integrates numerically the differential material, energy and momentum balance equations to generate temperature, conversion, pressure and yield profiles. The model has been tested for four design cases and found good matching between model design and actual plant design data. The case studies include prediction of reactor length for a given conversion, prediction of conversion for a given reactor length.