Chemical plant start-ups are very critical dynamic operations, which normally consume huge amount of energy and also generate large quantities of off-spec products for flaring, causing significant and intensive air emissions. Many studies have been individually conducted on energy savings or emission reductions under plant normal conditions. However, quantitative studies on simultaneous energy consumption and emission generation for chemical plant start-ups are still lacking. In this study, plant-wide dynamic simulations are employed to investigate energy consumption and emission generation for an ethylene plant under different start-up strategies. Dynamic modeling and simulations for two start-up designs associated with three start-up operating procedures are performed. Based on plant-wide dynamic simulations, dynamic profiles of energy consumption and emission generation during the plant start-up are obtained and analyzed. Details of energy accounting on cooling and heating duties of key distillation towers, auxiliary heat exchanger duties, and power consumption for compressor system are provided for each start-up case. Through comprehensive analysis, the most desirable start-up solution is identified. This virtual study not only characterizes the emission generation during the plant start-up, supporting flare minimization activities that benefits environmental sustainability, but also enhances critical research on energy and raw material savings during the plant start-up that will benefit the industrial sustainability.
As one of the most important petrochemicals, ethylene is produced in large quantities in 55 countries and 117 companies worldwide (Ceresana, 2010). The global ethylene production capacity on January 1, 2012, has reached about 141 million tpy, which is 2.5 million tpy over the capacity a year earlier of more than 138 million tons per year (tpy) (True, 2012). The majority of produced ethylene is used for making ethylene oxide, ethylene dichloride, and linear low and high density polyethylene. Some amounts are also used to produce ethyl benzene, alcohols, olefins, acetaldehyde and vinyl acetate. Another small part of ethylene is used as anesthetic gas, for fruit ripening and for welding and cutting metals (Inchem.org, 2008). Due to global population and market increment, it is predicted an increment of nearly 6.5 million tpy of the world ethylene production capacity in 2012, and over 2 million tpy increment in 2013 (True, 2012).
Ethylene is mainly produced through steam thermal cracking in ethylene plants. Generally, a scheduled turnaround cycle for an ethylene plant is 3–6 years, including a plant start-up and shutdown. Other scheduled causes such as emergency incidents, process upsets, equipment maintenances, market changes and severe weather situations will also result in plant shutdowns and start-ups (Liu & Xu, 2010). During an ethylene plant start-up, large amounts of off-spec products will be generated and flared. As a result, the flaring emissions generate huge amounts of CO2, CO, NOx, VOCs (volatile organic compounds), highly reactive VOCs (HRVOCs) (defined in Texas air quality regulation as ethylene, propylene, isomers of butene, and 1,3-butadiene), and partially oxygenated hydrocarbons (e.g. formaldehyde) (Xu et al., 2009a and Xu et al., 2009b). One study shows that an ethylene plant with a capacity of 1.2 billion pounds of ethylene production per year will easily flare about 5.0 million pounds of ethylene during one single start-up (Xu & Li, 2008). Based on the 98% flaring efficiency (destruction efficiency) (EPA, 2009), the resultant air emission will include at least 40.0 klb CO, 7.5 klb NOx, 15.1 klb hydrocarbon, and 100.0 klb HRVOCs (Xu et al., 2009a and Xu et al., 2009b), which can cause adverse environmental impacts, e.g. locally transient air pollution problems and local community concerns. In fact, flaring emissions cause not only potential environmental and societal problems, but also tremendous raw material and energy losses, which could have been used for making desired products. Therefore, flare minimization has become one of the major concerns for ethylene plants, especially under current stress of increasingly strict environmental regulations and economic competitions.
In this study, plant-wide dynamic simulation is utilized to investigate emission generation and energy saving performances of six different start-up plans for an ethylene plant start-up. On the basis of virtual dynamic simulations, the dynamic profiles of energy and emission during the plant start-up can be obtained and summarized. Through comprehensive analysis and comparison, the most desirable start-up solution is identified. This study not only help characterize the emission generation during the plant start-up, supporting flare minimization activities that benefits environmental sustainability, but also help enhance critical studies on energy and raw material consumptions during plant-start-up that eventually will benefit the industrial sustainability.
