Equation-Oriented Strategy
The Equation-Oriented (EO) strategy is an alternative method to solving flowsheets in Aspen Plus. Normally,
Aspen Plus uses the Sequential Modular (SM) strategy to solve flowsheets. In this strategy, each block
computes its outlet streams given values for the inlet streams, model parameters and operating conditions.
When recycle streams are present, SM must iterate on all the blocks within the recycle loop to converge the
values of the tear stream. Similarly, design specs require iterative loops.
EO, on the other hand, collects the model equations for all the blocks in the flowsheet and solves them
simultaneously using specialized solvers. This has the advantage of avoiding iterations involving recycle streams
and design specifications. It therefore allows:
 Simulation of complex recycle processes
 Simulation of highly heat-integrated processes
 Efficient economic optimization
 Efficient model tuning via parameter estimation or data reconciliation
The disadvantages of EO are:
 Difficulty in diagnosing non-convergence problems
 Complexity of model configuration
Nonetheless, there are many classes of problems that can be readily solved in EO that are not possible in SM.
EO works well when all variables are near the solution. However, EO is not suited to solving a simulation
without good estimates for all variables. Thus, before you solve your flowsheet in EO, you must initialize it in
SM.
SM initialization does not require a completely converged SM solution. The minimum requirement is that each
block be solved once. How tightly the SM flowsheet must be solved to ensure a robust EO formulation is
problem-dependent.
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