Process design modelling

As indicated in the previous chapter, models are used for a variety of applications, such as study of the dynamic behavior, process design, model-based control, optimization, controllability study, operator training and prediction. These models are usually based on physical fundamentals, conservation balances and additional equations. In this chapter the conservation balances for mass, momentum, energy and components are introduced. 

There are different challenges and obstacles to achieve robust enterprise safety management as integrated with enterprise management solutions. The cornerstone of such integrated view is the development of standard and systematic model formalization method that can facilitate the construction of the underlying process design model and build around it other model elements that represent lifecycle activities. Also... 

While many methods for parameter estimation have been proposed, experience has shown some to be more effective than others. Since most phenomenological models are nonlinear in their adjustable parameters, the best estimates of these parameters can be obtained from a formalized method which properly treats the statistical behavior of the errors associated with all experimental observations. For reliable process-design calculations, we require not only estimates of the parameters but also a measure of the errors in the parameters and an indication of the accuracy of the data. 

In many process-design calculations it is not necessary to fit the data to within the experimental uncertainty. Here, economics dictates that a minimum number of adjustable parameters be fitted to scarce data with the best accuracy possible. This compromise between "goodness of fit" and number of parameters requires some method of discriminating between models. One way is to compare the uncertainties in the calculated parameters. An alternative method consists of examination of the residuals for trends and excessive errors when plotted versus other system variables (Draper and Smith, 1966). A more useful quantity for comparison is obtained from the sum of the weighted squared residuals given by Equation (1). 

Figure 1.6 The onion model of process design. A reactor design is needed before the separation ind recycle system can be designed, and so on. (From Smith and Linnhoff, Trans. IChemE, CkERD, 66 195, 1988 reproduced by permission of the Institution of Chemical Engineers.)...

When an oil or gas field has just been discovered, the quality of the information available about the well stream may be sparse, and the amount of detail put into the process design should reflect this. However, early models of the process along with broad cost estimates are needed to progress, and both design detail and cost ranges narrow as projects develop through the feasibility study and field development planning phases (see Section 12.0 for a description of project phases). 

Industrial scale polymer forming operations are usually based on the combination of various types of individual processes. Therefore in the computer-aided design of these operations a section-by-section approach can be adopted, in which each section of a larger process is modelled separately. An important requirement in this approach is the imposition of realistic boundary conditions at the limits of the sub-sections of a complicated process. The division of a complex operation into simpler sections should therefore be based on a systematic procedure that can provide the necessary boundary conditions at the limits of its sub-processes. A rational method for the identification of the subprocesses of common types of polymer forming operations is described by Tadmor and Gogos (1979). 

The use of the computer in the design of chemical processes requires a framework for depiction and computation completely different from that of traditional CAD/CAM appHcations. Eor this reason, most practitioners use computer-aided process design to designate those approaches that are used to model the performance of individual unit operations, to compute heat and material balances, and to perform thermodynamic and transport analyses. Typical process simulators have, at their core, techniques for the management of massive arrays of data, computational engines to solve sparse matrices, and unit-operation-specific computational subroutines. 

Flow-sheet models are used at all stages in the life cycle of a process plant during process development, for process design and retrofits, and for plant operations. Input to the model consists of information normally contained in the process flow sheet. Output from the model is a complete representation of the performance of the plant, including the composition, flow, and properties of all intermediate and product streams and the performance of the process units. 

For optimisation of process design and process control, the efficiency and effectiveness of the various methods depend on the process being modeled and the process modeling software that is used. 

REIST Aerosol Science and Technology, Second Edition RHINE, TUCKER Modeling of Gas-Fired Furnaces and Boilers and Other Industrial Heating Processes ROSSITER Waste Minimization Through Process Design SAMDANI Safety and Risk Management Tools and Techniques in the CPI... 

Process Design vs. Accounting Linear Program Models... 

The process design linear program model is best written with flexibility in mind, such as extra matrix rows to provide flexibility in recycling, adding outside streams intermediate in the process, and determining component incremental values at each processing stage. This subject is discussed more fully later in this chapter. 

The method of approach given here works best if a diagram of the process model is first produced, such as shown in Figure 1. To simplify matrix development and look at the problem as a process design, do not think of the matrix in terms of equations. Rather, the rows are... 

Figure 1. Process model for illustrating how process design linear programming can be achieved.

The example used here does not represent any particular process and is simpler than most real life plant cases. It does, how ever, have the elements needed to talk our way through the development of a process design linear program model. All models should have the following ... 

During process design, the greatest opportunity to benefit from dynamic simulation is after adequate design information is available to develop the model,... 

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