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Unformatted text preview: Fall 2004 ICE Topics: Process Control by Design 10.492 Lecture Notes 1: Analyzing the Shower Process revised 2004 Dec 16 Dr. Barry S. Johnston, Copyright 2004. 1 what we will be doing in this module The broad context for this module is the commonsense notion that, when designing something, one should plan for the off-normal conditions that may occur. The particular context is the design of continuous chemical processes . Design starts with some product in mind (a simple molecule, a complex substance, or a structured product), and possibly several distinctive paths to reach it (the variations in chemistry, sequence, and processing that comprise a chemical process). The alternative processes are examined in a cursory fashion and unpromising ones discarded. As the design proceeds, increasing effort is spent on fewer alternatives in the approach to detailed design. A continuous process is conceived and designed as a steady-state operation. However, the process must start up, shut down, and operate in the event of disturbances, and so the time- varying behavior of the process should not be neglected. A proper dynamic simulation of the process requires that a number of design details be in place, and thus must take place in later stages of design. Even so, it is helpful to consider the operability of a process earlier in the design, when alternatives are still being compared. In this module, we will examine some tools that will help to evaluate the operability of the candidate process at the preliminary design stage, before substantial effort has been invested. Thus, these are screening tools. The ideas presented in these notes are derived from the texts by Marlin (1), McAvoy (2), and Seider et al (3). a few ways in which processes can go wrong bad operation inadequate procedures mis-tuned controllers malfunctioning instruments bad implementation of the design backwards flowmeters and other installation mistakes valve in wrong place wrong type of valve mistakes in construction/check-off field decisions that should have been specified in the design bad design mis-sized equipment poor controller scheme prone to instability poorly located or selected instrument insufficient number of instruments we will begin by analyzing a simple process Its only an ordinary shower, but well dress it up as a chemical process: two feed streams enter and mix to form an outlet stream of flow F and temperature T. Instruments are provided on the exit line to measure these quantities, and valves regulate the feed streams. Fall 2004 ICE Topics: Process Control by Design 10.492 Lecture Notes 1: Analyzing the Shower Process revised 2004 Dec 16 2 F T F h T h F c T c F T F T F T F h T h F c T c F T Our shower should operate as a steady process. To design it, we must specify steady conditions, valve sizes, pipe size, supply pressure, instruments, and so forth. Mindful of the discussion above, well also want to examine its operability. above, well also want to examine its operability....
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- Spring '03