Computer modular approach

ECVAM is the leading international center for alternative test method validation. Hartung et al. (29) summarized the modular steps necessary to accomplish stage 3 (test validation). The seven modular steps are (I) test definition, (2) within-laboratory variability, (3) transferability, (4) between-laboratory variability, (5) predictive capacity, (6) applicability domain, and (7) performance standards (29). Steps 2-4 evaluate the test s reliability steps 5 and 6 evaluate the relevance of the test. Successful completion of all seven steps is necessary to proceed to stage 4 (independent assessment or peer review). This modular approach allows flexibility for the validation process where information on the test method can be gathered either prospectively or retrospectively. The approach is applicable not only to in vitro test methods but also to in silico approaches (e.g., computer-based approaches such as quantitative structure-activity relationships or QSAR) and pattern-based systems (e.g., genomics and proteomics). 

Alles, 1977] Alles, H. G. (1977). A Modular Approach to Building Large Digital Synthesis Systems. Computer Music Journal, 1(4) 10-13. 

Simultaneous. In order to circumvent the in-efficiencies associated with loop within loop struct-tures forced by the module design and sequential modular approach, there has been considerable academic effort to investigate how to perform all computations simultaneously. The potential advantages of this global (or "equation oriented") approach are generally recognized but acceptance of the approach has been slow due to a number of reasons ... 

The computational architecture is a sequential modular approach with advanced features. To model a process, each equipment module is simulated by a program module. The overall process is simulated by connecting the models together in the same way as the equipment in the flow sheet. When the input streams are known then the outputs can be calculated. The entire flowsheet can be calculated "sequentially" in this manner. Advanced features are discussed below in connection with an example. 

Johnson, A. I. (1972) Brit. Chem. Eng. and Proc. Tech. 17, 28. Computer-aided process analysis and design — a modular approach. 

In the equation-based approach, the equations for all units are collected and solved simultaneously. The natural decomposition of the system into its constituent unit operations is therefore lost. Moreover, the simultaneous solution of large numbers of equations, some of which may be nonlinear, can be a cumbersome and time-consuming problem, even for a powerful computer. For all these reasons, most commercial simulation programs were still based on the sequential modular approach when this text was written. 

I he expansion bus allows the computer to be expanded using a modular approach. Whenever you need to add something to the computer, you plug specially made circuit boards into the connectors (also known as expansion slots) on the expansion bus. The devices on these circuit boards are then able to communicate with the CPU and are semipermanently part of the computer. 

In the past, most simulation programs available to designers were of the sequential-modular type. They were simpler to develop than the equation-oriented programs and required only moderate computing power. The modules are processed sequentially, so essentially only the equations for a particular unit are in the computer memory at one time. Also, the process conditions, temperature, pressure, flow rate, etc., are fixed in time. With the sequential modular approach, computational difficulties can arise due to the iterative methods used to solve recycle problems and obtain convergence. A major limitation of sequential modular simulators is the inability to simulate the dynamic, time-dependent behavior of a process. 

Hillestad, M., and T. Hertzberg, Dynamic Simulation of Chemical Engineering Systems by the Sequential Modular Approach, Comput. Chem. Eng., v. 10, p. 377 (1986). 

For a design problem, converge the entire flowsheet (close all the recycles) for every intermediate value of the adjust variable. This is very expensive computationally and is a major drawback to the sequential modular approach. Alternative and sometimes faster approaches include... 

More recant efforts to develop computer-aided process synthesis methodologies can be cbeincterized as either sequential modular, simultaneous modular, or equation oriented, Sequeatial-modular approaches are ben for steady-state simulation where process inputs are defined and process para met ara ate available. The best feature of sequential-modular approaches is that they are flowsheet oriented, bot they are not as flexible as the other methodologies in parfonuing design and optimization tasks.39... 

Chapter 13 surveys methods of system identification in physiology, the process of extracting models or model components from experimental data. Identification typically refers to model specification or model estimation, where unknown parameters are estimated within the specified model using experimental data and advanced computational techniques. Estimation may be either parametric, where algebraic or difference equations represent static or dynamic systems, or nonparametric, where analytical (convolution), computational (look-up tables), or graphical (phase-space) techniques characterize the system. This chapter closes with a recent hybrid modular approach. 

Sequential modular approach has some clear advantages for process flowsheeting that explain why it still dominates the technology of steady-state simulation over the simultaneous or equation-oriented approach. Table 8.2 shows a list of pros and cons about sequential modular process simulators. In order to cope with the disadvantages, a few process simulators have improved the flow of information and avoid redundant computations. As an example, Aspen HYSYS has implemented the bidirectional transmission of information technology. 

In an acyclic system, the sequential modular approach is ideal from a computational viewpoint. However, most of the process plants involve recycles, particularly those highly integrated, in that case the problem cannot be solved directly and an iterative process must be used. Consider, for example, the simple flowsheet in Figure 8.8a. All the degrees of freedom are satisfied however, to calculate the mixer, we need to completely know both the feed stream and the recycle stream, but we only know the feed. To calculate the recycle stream, we should solve the compressor ... 

The simultaneous modular approach along with equation-based methods have been used to optimise three case studies. The modular approach has advantages but when optimising there is a price to pay for this. More bounding steps are required resulting in higher computation times although in the case of the water network problem, the most nonlinear of the problems, the price was small. 

---