Positioning Control for Process Through - put

The application of a valve positioning controller (VPC) on top of a basic regulatory plantwide control structure for maximizing the process through-put to increase plant profitability is demonstrated. The input to the VPC is a measurement of the bottleneck constraint and its output is the set-point of the through-put manipulator. The HDA process is used as an example case study with the feed hydrogen compressor considered as bottleneck constraint that limits production. Results show that the automatic adjustment of the through-put manipulator allows the process to be operated close to (or at) the bottleneck constraint. De-rating of the plant through-put due to the possibility of disturbances is then avoided resulting in enhanced throughputs and plant operating profit. The increase in the through-put and plant profit over the de-rated operation is quantified with respect to variation in the magnitude of the principal disturbance that causes the bottleneck constraint to be hit. Results show that significant enhancement of more than 20% increase operating profit can be achieved. VPC is thus demonstrated to be a simple and effective means for increasing plant profitability by maximizing through-put. Key-words: Plant-wide Control, Valve Positioning Control, Through-put Maximization Introduction Research in plant-wide control has received much attention since the early nineties, aided to a large extent by the ready availability of commercial packages such as HYSYS and Aspen, for rigorous plant-wide dynamic simulations. Much of the research in plant-wide control has focused on methodologies for synthesizing effective decentralized control structures. Luyben and coworkers have done seminal work on the design of effective plant-wide regulatory control structures for complex chemical processes culminating in a nine step heuristic procedure for (Luyben and Luyben, 1997). The application of the procedure for effective plant-wide regulatory control has been illustrated on a variety of chemical processes such as the hydro-dealkylation of toluene, the Tennessee Eastman challenge process, a vinyl acetate process, an isomerization process etc (Luyben et. al., 1999). Given a basic regulatory structure, the next step is to adjust the key process set-points to maximize the plant operating profit. Surprisingly, there are very few articles in the open literature that explicitly address the issue of plant-wide control for maximum operating profit. This is even as modern chemical plants are known to routinely employ constrained NLP optimization of the key set-points to optimize plant profit within the framework of model predictive control or real time optimization. For continuous chemical processes producing bulk chemicals, maximizing plant profitability many-a-times boils down to maximizing the process through-put with the increased production rates translating directly to higher profits due to the raw material – product price differential. Constraints such as column flooding, maximum available flow rates (material, heating or cooling) etc typically act as bottlenecks limiting the achievable production rate. Maximizing the production rate then requires controlling the plant in a manner such that it operates as close as possible to the bottleneck constraint. The application of * Corresponding author. Email: [email protected]; Phone: 91-512-2597513; Fax: 91-512-2590104.