(399c) Flare Minimization for Cracking Gas Replacing Process of Ethylene Plant Start-up Via Dynamic Simulation

The start-up operations of ethylene plants, which are usually conducted every 3 to 6 years,  generate large quantities of off-spec products that have to be flared for safety. It has been estimated that an ethylene plant with a capacity of 1.1 billion pounds of ethylene production per year will easily flare about 5.0 million pounds of ethylene during one single start-up. Flare minimization has become one of the focuses of many enterprises and researchers.

Currently, one of the flare minimization strategies is to start compressors and precool the refrigeration system before feeding cracking furnaces with start-up working mediums (SWM) such as mixed gas. The start-up operation with mixed gas is called Mixed Gas Run (MGR). Different from traditional start-up operation with cracking gas as the SWM, it is quite difficult for even experienced engineers to make a detailed start-up plan for MGR with the objective of minimum emission and energy based on their experiences. Unsuitable schemes and operations will cause considerable emission and energy loss or even severe accidents. In order to avoid above situations, a methodology of flare minimization by dynamic simulation was proposed by Xu et al to decide and validate start-up plans. However, the coupling relationships among the compression system, the refrigeration system, and the separation system have not been considered comprehensively in previous studies.

Generally MGR includes three steps: nitrogen starting-up and preliminary precooling process, mixed gas replacing and further precooling process, and cracking gas replacing and starting separation process (CGRP). In this paper, the cracking gas replacing process was quantitatively calculated and analyzed via dynamic simulation. Compression system models with performance curves and refrigeration system models with heat capacity data were developed for observing dynamic changes during the CGRP. Effects of energy saving and emission reduction of total reflux method for different distillation columns were compared and the start-up time of each key column was given. The operation sequences of key devices, the settings and interactions of critical operating parameters during the CGRP were discussed. This paper, taking a real ethylene plant as an example, demonstrates the method for determining each device operation scheme, the gantt chart of start-up operating procedures, and the quantified replacing time and emission reduction effect of the whole process.

Reference:

1.      Xu, Q.; Yang, X.; Liu, C.; Li, K.; Lou, H. H.; Gossage, J. L., Chemical plant flare minimization via plantwide dynamic simulation. Industrial & Engineering Chemistry Research 2009,48, (7), 3505-3512.

2.      Song, G.; Qiu, T.; Zhao, J., Flare Minimization Model for Ethylene Splitter System’s Shutdown. Industrial & Engineering Chemistry Research 2013,52, (26), 9180-9188.

3.      Yang, X.; Xu, Q.; Li, K.; Sagar, C. D., Dynamic Simulation and Optimization for the Start-up Operation of An Ethylene Oxide Plant. Industrial & Engineering Chemistry Research 2010,49, (9), 4360-4371.

4.   Zhao, Y.; Zhang, J.; Qiu, T.; Zhao, J.; Xu, Q., Flare Minimization during Start-Ups of an Integrated Cryogenic Separation System via Dynamic Simulation. Industrial & Engineering Chemistry Research 2014, 53, (4), 1553-1562.