Engineering design physical models

The traditional approach used in engineering design and modeling is based on the assumption that the entire system/process can be described by mathematical equations. This type of model is called a white box model. Such a model is based on the first engineering principles, i.e., physical, chemical, biological, or economic laws. No ejqierimental data is needed. 

Physical models have long been used by engineers to gain a better understanding of complex phenomena. Such models probably constitute the oldest method of structural design. Physical models have also been used for many years in the fields of hydraulics, hydrodynamics, and aerodynamics. 

Intended Use The intended use of the model sets the sophistication required. Relational models are adequate for control within narrow bands of setpoints. Physical models are reqiiired for fault detection and design. Even when relational models are used, they are frequently developed bv repeated simulations using physical models. Further, artificial neural-network models used in analysis of plant performance including gross error detection are in their infancy. Readers are referred to the work of Himmelblau for these developments. [For example, see Terry and Himmelblau (1993) cited in the reference list.] Process simulators are in wide use and readily available to engineers. Consequently, the emphasis of this section is to develop a pre-liminaiy physical model representing the unit. 

One must understand the physical mechanisms by which mass transfer takes place in catalyst pores to comprehend the development of mathematical models that can be used in engineering design calculations to estimate what fraction of the catalyst surface is effective in promoting reaction. There are several factors that complicate efforts to analyze mass transfer within such systems. They include the facts that (1) the pore geometry is extremely complex, and not subject to realistic modeling in terms of a small number of parameters, and that (2) different molecular phenomena are responsible for the mass transfer. Consequently, it is often useful to characterize the mass transfer process in terms of an effective diffusivity, i.e., a transport coefficient that pertains to a porous material in which the calculations are based on total area (void plus solid) normal to the direction of transport. For example, in a spherical catalyst pellet, the appropriate area to use in characterizing diffusion in the radial direction is 47ir2. 

Based on factors such as cast, mechanical properties, physical properties, ease of fabrication (design) and the corrosion resistance data available in the literature the choice of materials can lead to a short list of two or three materials. At this stage it is prudent that the engineer design prototype laboratory scale model equipment from the short-list of selected metals or alloys, and determine the corrosion rates in the environment of interest. These accelerated tests will enable the engineer to select the best candidate material, making proper allowance for the corrosion of the metal or alloy over the lifetime of the equipment. 

The statistical techniques which have been discussed to this point were primarily concerned with the testing of hypotheses. A more important and useful area of statistical analysis in engineering design is the development of mathematical models to represent physical situations. This type of analysis, called regression analysis, is concerned with the development of a specific mathematical relationship including the mathematical model and its statistical significance and reliability. It can be shown to be closely related to the Analysis of Variance model. 

The engineering design process begins with a concept and continues to completion using various tools of engineering, such as mathematics, physics, engineering sciences, computers, and models. 

One must understand the physical mechanisms by which mass transfer takes place in catalyst pores to comprehend the development of mathematical models that can be used in engineering design calculations to estimate what fraction of the catalyst surface is effective in promoting... 

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