I’ve added some more material to the web site – solving mass balances for processes with recycle streams, using Comsol. In process simulators, one usually solves the equations by taking the input to a unit, solving that unit, taking the output and putting it into the next unit. If there are no recycle streams, you are done. If there are recycle streams, though, you do this over and over until the problem converges (it usually does). When using Comsol, however, you are writing algebraic equations like you would in Excel (i.e. like the formulas you’d put in the cells there). Then you must solve those algebraic equations, which are usually nonlinear. Sometimes that type of calculation does not converge, and you need to supply a reasonable guess to get a solution. You can see in the example when that was necessary for the problems solved. Go to the book web site, www. ChemEComp.com and look at Download or system mass balances.

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