Plant-wide control

The fundamental questions in plant-wide control are whether the feed rates can simply be set for a process and left unattended and whether the process is meeting the desired purity and quality specifications [1,2], What happens when common disturbances occur such as feed composition changes, production rate changes, product mix or purity specification changes, ambient temperature changes, or measurement sensors either fail or are in error This chapter covers some of the most common problem areas encountered when designing a plant-wide control scheme. 

We have discussed setting up a single feedback controller and establishing a control strategy for one unit operation a reactor, a column, etc. The next level of complexity is to look at an entire operating plant which is made up of many unit 

BUCKLEY PROCEDURE. Buckley was one of the pioneers in this aspect of control. He developed a procedure that is still widely used today. His plant-wide design methodology consists of the following steps  

Lay out a logical control scheme to handle all the liquid levels and pressure loops throughout the plant so that the flows from one unit to the next are as smooth as possible. Buckley called these the materiaUbalatice loops. If the feed rate is set into the front of the process, the material-balance loops should be set up in the direction of flow i.e., the flow out of each unit is set by a liquid level or pressure in the unit. If the product flow rate out of the plant is set, the material-balance loops should be in the direction opposite flow i.e., the flow into each unit is set by a liquid level or pressure in the unit. 

Then design the composition control loops for each unit operation. Buckley called these the product-quality loops. Determine the closedloop time constants of these product-quality loops. 

Size the holdup volumes so that the closedloop time constants of the material-balance loops are a factor of ten bigger than the closedloop time constants of the product-quality loops. This breaks the interaction between the two types of loops. 

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B. EIGENSTRUCTURE. The plant-wide control problem is still a very active area of research. In recent years there has been more emphasis on finding control structures for individual unit operations and for entire plants that are inherently simple, self-regulatory, and self-optimizing. This approach has been called a search for the eigemtructure of the plant. 

C. MAKEUP CONTROL. Another aspect of plant-wide control is the problem of "makeup control strategy. When a fresh feed stream is brought into a plant it... 

Plant-wide control is concerned with designing control systems for a large number of individual process units that may be highly interacting. A typical plant-wide control system will consist of many single-loop controllers as well as multi-variable controllers such as Model Predictive Control (MPC),1 10 and may involve thousands of measurements, hundreds to thousands of manipulated variables and hundreds of disturbance variables. Fortunately, a plant with a large number of processing units can be analysed as smaller clusters of units. 

The publication of an actual industrial plant-wide control problem, the Tennessee Eastman challenge process (Downs and Vogel 1993) generated several... 

McAvoy (1999) advanced the use of optimization calculations at the controller design stage, proposing the synthesis of plant-wide control structures that ensure minimal actuator movements. The initial work relying on steady-state models (McAvoy 1999) was recast into a controller synthesis procedure based on linear dynamic plant models (Chen and McAvoy 2003, Chen et al. 2004), whereby the performance of the generated plant-wide control structures was evaluated through dynamic simulations. 

Remark 3.4. In the context of the present chapter (and of the remainder of the book), the term hierarchical control structure reflects the use of two (or multiple) coordinated tiers of control action, and should not be confused with hierarchical plant-wide controller design strategies (see, e.g. Ponton and Laing 1993, Luyben et al. 1997, Zheng et al. 1999, Antelo et al. 2007, Scattolini 2009, and references therein), which use the term hierarchy to denote a set of guidelines, to be followed in sequence, for designing the control system for a chemical plant. 

The presence of three distinct time horizons in the process dynamics, as evinced by the analysis above, warrants the use of a hierarchical control structure that addresses the distributed (unit-level) and plant-wide control objectives separately. 

Antelo, L.T., Otero-Muras, I., Banga, J.R., and Alonso, A. A. (2007). A systematic approach to plant-wide control based on thermodynamics. Comput. Chem. Eng., 31, 677-691. 

Banerjee, A. and Arkun, Y. (1995). Control configuration design applied to the Tennessee Eastman plant-wide control problem. Comput. Chem. Eng., 19, 453-480. 

Zhu, G. Y., Henson, M.A., and Ogunnaike, B.A. (2000). A hybrid model predictive control strategy for nonlinear plant-wide control. J. Proc. Contr., 10, 449-458. 

Suppose our plantwide control system requires on-demand products. As discussed in Chap. 3, the fourth step in the plant wide control design procedure is to establish where production rate is set. If the flowrate of one of the product streams leaving the column is fixed by a downstream unit or customer, the column control structure must be set up in an appropriate way. 

Develop effective plant-wide control structures... 

Jeppsson, U., Rosen, C., Alex, J., Copp, J.B., Gemaey, K.V., Pons, M.-N., Vanrolleghem, P.A., 2006. Towards a benchmark simulation model for plant wide control strategy performance evaluation of WWTPs. Water Science and Technology, 53(1), 287-295. 

Close-up of the pneumatic conveying facilities used in the operation of the dry scrubber for plant-wide control and recycling of volatile flourides. 

An Intelligent System for the Design of Plant-Wide Control Configuration... 

Figure 8.5a shows the plant-wide control structure for both columns. The first two summer blocks add the three-stage temperatures. The multiplier block average multiplies by 0.3333. The last summer block adds the constant 273 to convert the signals back to K from the metric units (°C) used in Aspen Dynamics. The average temperature is 429 K, which is the set point of the TC2 controller shown on the faceplates in Figure 8.5b. Note that the solvent flow controller is on cascade because the solvent flow rate is ratioed to the flow rate of the fresh feed.

Sierra Monitor Corporation s Model 5I00-28-IT infrared Combustible Gas Sensor Module is network enabled, meeting user s and integrator s needs to communicate with any plant-wide control and monitoring system. The 5100-28-IT is a standalone module. Sierra Monitor, (800)727-4377, wwwsierramord-torjcom. Circle 263... 

The proposed hierarchical procedure for plant-wide control developed by Luyben and coworkers [14] is utilized, in our framework, as stepwise guidelines to perform the plant-wide process control strategy. The procedure essentially decomposes the plant-wide control problem into various levels and tries to satisfy the two fundamental chemical engineering principles, i.e. the overall conservation of energy and mass. The design procedure consists of the following main steps ... 

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