Simulation convergence problems

There is a great difference between various simulators (5) in terms of how easily and how well the hypothetical calculation units can be incorporated in the simulation. The trial-and-error calculations, which ate called iterative calculations, do not always converge for every flow sheet being simulated. Test problems can be devised to be tried with various simulators to see if the simulator will give a converged solution (11). Different simulators could take different numbers of iterations to converge and take different amounts of computet time on the same computet. 

One of the most frustrating problems that you may encounter using the Transient Analysis is convergence problems. When PSpice is simulating a differential equation, it calculates a data point and estimates the error associated with the calculation. If the error is larger than a specified maximum, PSpice reduces the time step and recalculates the point and the error for the new point. Reducing the time step usually reduces the error. PSpice will continue reducing the time step until the error is within acceptable limits, or until PSpice reaches the limit on the number of times it is allowed to reduce the time step. 

This dialog box shows the settings that affect convergence problems. You can change some of these settings and then resume the simulation. 

Convergence problems have been observed in some transient simulations of circuits with switches, such as the circuit on page 334. You will notice this error because the transient simulation terminates prematurely. If you look at the output file, an error message near the end of the file will indicate a convergence problem. The problem can be fixed by choosing a smaller value of Maximum Step Size, or by using one of the methods mentioned in Section 6.A.3. 

These adjustments are based on the most commonly available. OPTIONS parameters and features in all of the simulators. Each simulator has several more convergence-related parameters that can be adjusted. Please see the individual syntax manuals for your program for more details on how to handle convergence problems. 

Verify that DC convergence has been achieved. View the error statements in the text editor to verify that the convergence problem pertains exclusively to the transient simulation. 

To avoid convergence problems during feed perturbation runs of the FCC simulator, we have made adjustments to the size of each feed property change and reset all initial estimates before each feed perturbation to the values computed by the simulator for the base FCC operation. 

No convergence problems were found in linking the hydrocracker simulator to the preprocessor and running on a wide range of alternate operations and feed property changes. 

When simulating this flowsheet convergence problems appear. These may be attributed to the following reasons ... 

Feed temperature Too cold or too hot a feed may disturb one or more stages and may also cause a composition pinch. The temperature of the lean oil to an absorber will affect the removal of the heavies from the vapor along with losses of the lean oil overhead. Convergence problems in the simulation may represent actual physical problems, suggesting that a feed exchanger is needed. 

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. 

We have a natural ally in nature, writes columnist Sanjaya Maniktala. Design problems occur, he says, when we schedule a confrontation with natural forces. Nature doesn t have convergence problems, like PSpice often does. Try simulating the air flow over a heat sink. 

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