ABSTRACT

Praxair currently has a development program in place to automate the control of its pipeline systems using proprietary control technology. Closed loop control offers a number of benefits over current manual pipeline operation, including continuous analysis of the pipeline and improved decisions regarding which plants to load or unload as pipeline demand changes. These benefits lead to more efficient pipeline operation and maximized profits. As closed loop control of pipeline networks is a relatively new problem for Praxair, it is important to ensure that the techniques being applied are well understood and tested prior to any field implementation. Towards this end, a dynamic pipeline simulation was used in place of the real pipeline during control system development. This allows for rigorous testing of the pipeline control methods without the risk associated with testing on the actual pipeline. Furthermore, the pipeline control simulation integrates dynamic pipeline models with Praxair's standard SCADA software, resulting in a realistic representation of the actual on-line control system, which is used for further system development, operator training and offline analysis. Introduction As one of the world's largest industrial gas manufacturers, the bulk of Praxair's revenue comes from the sale of oxygen, nitrogen, and other industrial gases. These products are then used as feedstock for a variety of industries including steel, petrochemical and semiconductor manufacturing. The majority of Praxair's sales result from the products produced in its Air Separation Plant's (ASP). The pipelines fed by ASP's are the focus of this paper. Figure 1 shows a simplified diagram of an Air Separation Plant. Compressed air feeds a series of heat exchange and distillation units, operating at cryogenic temperatures, to separate the air into its individual components of oxygen, nitrogen and argon. While the argon is typically produced in relatively small quantities as a liquid, the other products can be produced as liquid or gas depending on the prevailing market and/or customer needs. Figure 1 shows the simplest Praxair/customer relationship, where it is assumed a single ASP is feeding customers directly connected to that plant. In this scenario, the goal of ASP operation is to produce just enough oxygen and nitrogen gas to meet the local customer demand. As shown in Figure 1, liquid products are produced simultaneously from the same plant and are stored in tanks at the site. The liquid products are distributed by trailer or railcar to customers with on-site liquid storage tanks. While the available liquid storage is used to buffer fluctuations in liquid demand, little additional gas storage capability exists. Therefore, the ASP production must be continuously adjusted to account for gas pipeline demand changes. To ensure that operating costs are continually minimized, the ASP usually has a closed loop control system that monitors pipeline pressures and automatically adjusts production as customer demands change. In addition to meeting customer demand, the individual plant control system will ensure that safety, purity and other constraints are not violated as production levels change.

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