During a chemical plant turnaround operation (plant startup, shutdown, or process upset), off specification (off-spec) product streams may be resulted. These off-spec streams usually have to be sent for flaring to insure the safety of chemical plant personnel and equipments, meanwhile to protect the environment from sever pollutions. Flaring is crucial to the chemical process industry. However, excessive flaring will cause negative environmental and social impacts and also results in tremendous material and energy losses. Thus, flare minimization has become a major concern for chemical process industry. By far, flare minimization almost exclusively depends on the experienced operators, engineers, and administrators; and a well planning, scheduling, and training in the plant. But that is not enough for chemical process industry today because of increasingly strict environmental regulations and economic competitions.

Facing this challenge, a new philosophy of zero discharge for off-spec products is proposed in this paper to advance current endeavors in flare minimization. Since off-spec products are inevitable during the plant turnaround operation, they must be either recycled to the upstream process for online reuse, or stored somewhere temporally for future reprocessing when the plant manufacturing becomes stable. Therefore, the off-spec products can be saved (i.e., zero discharged) instead of being flared. To tackle the design and operational problems for zero discharge, such as where, when, and how to recycle a stream, a dynamic simulation based general methodology is developed.

The methodology starts with the upper-level concept design and operational procedure development, where first-principle analysis and human expertise are employed. In the lower-level, rigorous dynamic simulations will be conducted to critically check the feasibility and operability of the design or operational procedures from upper level. When infeasible or unstable conditions are identified, the dynamic simulation model will also be iteratively used to test new design and operational candidates until the optimal one is identified. The philosophy of zero discharge for chemical plant turnaround operation via dynamic simulation is virtually demonstrated by the startup of an ethylene plant.