(715h) Long-Term Maintenance and Production Planning for the Integrated Chemical Enterprise

We address an enterprise-wide strategic maintenance turnaround and production planning problem. For networks of large, continuous plants, maintenance and production planning clearly affect each other and here we consider them together for the full network. It is clear that non-safety-related tasks performed for routine maintenance turnarounds and outages significantly disrupt production and profits, and that high production targets can prompt evaluation to defer routine maintenance during periods of high profitability. Consequently, the timing of maintenance turnarounds can affect both production and profitability significantly, especially when a facility is running at capacity. For example, production rates are commonly affected by fouling, catalyst activity, and overall unit reliability---and these depend on the cumulative production or time since the last scheduled maintenance turnaround. Separately from conversion rates and throughput, selectivities may also change with time and production rate and have an impact on production in integrated site networks. Further, deferring a scheduled outage/turnaround can increase the scope of work required for the next scheduled turnaround, and may also increase the cost and duration of the turnaround. There may also be restrictions and requirements on site imports/exports, material swaps, and mandatory inspections that prevent a deferral.

In order to create a rigorous enterprise production and profitability plan, these factors must be taken into account. They require the tracking of time and production since the previous turnaround, the timing of which is itself a decision variable. Additionally, the large time intervals between routine maintenance turnarounds necessitate very long planning horizons due to end effects, significantly adding to problem size. In this work, an MILP-based framework is presented that includes the above conditions, and, in addition, is able to consider long- and short-term financial performance by providing recommendations that not only maximize long-term NPV, but also balance quarter-to-quarter profits while respecting strict safety requirements and standards. Though profits from sales may be booked soon after delivery, costs for turnarounds are usually incurred several months prior to and several months after execution---this needs to be considered in the formulation when there are quarterly budget constraints or profitability targets.

To summarize, attention to safety, mandatory inspections, production technologies, reliability, maintenance, finance and accounting, and supply chain are all critical to production planning, and we afford these consideration in the current context. Effective reformulations, modeling strategies, and associated case studies are presented to address this real-world large-scale problem.