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M3 Technology's Polyethylene and Polypropylene Scheduling
Polymer production scheduling is a vital issue in a petrochemical complex. The main objective in polyolefin (polyethylene, polypropylene) production is to optimize the product wheel to minimize the cost of transitions. In addition, the system needs to have high degree of flexibility to allow the scheduler to quickly adjust the production schedule due to process problems or changing customer orders. Some large gas-phase lines may only produce 2 grades during a year and only make 1 or 2 transitions a month. Whereas, a SclairTech process can make 300 grades a year and can make up to 3 grade changes a day. The drawing below illustrates the typical equipment that may be modeled in SIMTO. Of course, some lines may share equipment like extruders. Also, some polymer grades can be made on multiple lines. All of these possibilities are accounted for in SIMTO.
In SIMTO, every piece of equipment that is schedulable is capable of carrying detailed composition, production rate, inventory information and a wide variety of constraints. For a particular grade, the recipe is used in computing consumption and yields. If a production rate model is available, then it is integrated into SIMTO for computing yields. The better the production model, the better will be the simultaneous determination of feasibility during the development of a schedule. It is useless to develop an optimum product sequencing that the plant cannot execute. The current conditions of the plant are updated via baseline update function to keep the SIMTO model and constraints in tune with the plant.
The production of a grade has a start-time, end-time and production rate. The transition from grade to grade can be modeled based on operating experience. Like in a Mitsui slurry process that employs 2 reactors that can be run in parallel or series, there are easy transition and more costly transitions that require changes in equipment configuration. A sub-objective is to minimize the number of difficult transition. When is the best time to take the difficult transition? is a problem the scheduler may want to explore running what-if cases. The scheduler has more knowledge of how the plant is actually operating and what is needed by production.
Every reactor product can be segregated into multiple grades as required. Easy to designate transition material or on-spec material and assign to silos and easy to schedule hold-ups.
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