Modular simulation mode

The older modular simulation mode, on the other hand, is more common in commerical applications. Here process equations are organized within their particular unit operation. Solution methods that apply to a particular unit operation solve the unit model and pass the resulting stream information to the next unit. Thus, the unit operation represents a procedure or module in the overall flowsheet calculation. These calculations continue from unit to unit, with recycle streams in the process updated and converged with new unit information. Consequently, the flow of information in the simulation systems is often analogous to the flow of material in the actual process. Unlike equation-oriented simulators, modular simulators solve smaller sets of equations, and the solution procedure can be tailored for the particular unit operation. However, because the equations are embedded within procedures, it becomes difficult to provide problem specifications where the information flow does not parallel that of the flowsheet. The earliest modular simulators (the sequential modular type) accommodated these specifications, as well as complex recycle loops, through inefficient iterative procedures. The more recent simultaneous modular simulators now have efficient convergence capabilities for handling multiple recycles and nonconventional problem specifications in a coordinated manner. 

In the simulation mode, the system of equations (8.1) may contain several thousand equations and variables. As it may be very difficult to solve a unique model that includes the entire plant, the solution procedure demands a systematic and modular approach. The strategy adopted by sequential modular process simulator is to write balance equations separately for each unit. This leads to rewrite the system of equations (8.1) in the following form ... 

The problem they chose for their prototype is part of the life support system, specifically the portion that removes COg from the cabin atmosphere. This system already has been constructed, and NASA engineers are already familiar with its operation and how it can fail. Using this information they were able to build as part of their knowledge base a simple simulation for the modes of failure of each of the components in the system. The life support system is modular, in that portions of it can be replaced, once a problem has been isolated. The graphical representation chosen for the instrument schematic and panel is shown in Figure 3. 

When there are multiple recycles present, it is sometimes more effective to solve the model in a simultaneous (equation-oriented) mode rather than in a sequential modular mode. If the simulation problem allows simultaneous solution of the equation set, this can be attempted. If the process is known to contain many recycles, then the designer should anticipate convergence problems and should select a process simulation program that can be run in a simultaneous mode. 

To initiate the global simulation, the CHEOPS server as well as the wrappers of all participating simulators are started. Next, the simulation expert specifies the input file for CHEOPS and runs the simulation. CHEOPS solves the simulation in a sequential-modular mode Each simulator is run in turn, where the simulation results are passed from one simulator to the next. In the case of feedback loops, this process is iterated until a steady state is reached (i.e., the global simulation converges). 

Flowsheeting is still dominated by the Sequential-Modular architecture, but incorporates increasingly features of the Equation-Oriented solution mode. A limited number of systems can offer both steady state and dynamic flowsheeting simulators. 

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