Enhance ethylene production from mixed c4 hydrocarbons and light naphtha

Abstract

In an ethylene plant, mixed C4 is the only end-product that is not in high demand. In addition, it is costly and risky to store at the plant. The coke accumulation on the furnace coils tends to increase the pressure and decrease the yield hence increase fuel consumption. Consequently, ethylene production plants that use naphtha as a feedstock constantly suffer from these drawbacks. Therefore, this study aims to reduce the cost and risk of the storage of the C4 and reduce the coke deposition. The objectives were achieved by the recycling of mixed C4 to improve the ethylene and propylene yields and the addition of CO2 in the feedstock to minimize the coke deposition on the furnace coils. The recycling study consisted of two stages/processes. In the first process, the feed contained 100% naphtha, while the second process contained 85% naphtha and 15% mixed C4 as a recycled feed. During the study, a data from several units of the ethylene plant was recorded on a daily basis for 60 days (maximum operating days for each operating furnace) in order to select the highest ethylene yield. The results showed that there were higher ethylene and propylene yields after the recycling of mixed C4, with a remarkable/significant increase in coke accumulation on the furnace coils with respect to the coke accumulation during 100% naphtha cracking. In the third process, the addition of CO2 to feedstock during naphtha cracking in an attempt to decrease coke accumulation was carried out by modeling and optimization of the key operating conditions by training a real data collected during 60 days using mathematical model ordinary differential equations (ODEs) falling under the ANN approach. It was found that the addition of CO2 improves the yield of ethylene and propylene up to 15 %, decreased coke deposition on the furnace coils (form 5.5mm to 2.4mm coke thickness), hence less energy was required to operate the furnace. The addition also minimised the operation and the maintenance costs. The results from the proposed model also showed that the run time of the furnace with the addition of CO2 was almost twice (from 45 days to 120 days) the run time with adding steam. Based on these results, this study has proven that the recycling of C4 mix accompanied with CO2 has noticeable positive results on the production of ethylene from the naphtha thermal cracking process.