I'm looking for an Aspen Plus theory guide, where you can find all the basic equations which describe the modules of the program. Does it exist and is it available for an undergraduate student?
To the best of my knowledge, there is no such material. However, once you have already installed your Aspen software you have access to the help guide. Even when you will not find a very detailed explanation of the algorithms, you can get some useful information. For example, you can find the kinetic models for chemical reactions (eg. power law, LHHW, etc). For the case of the kinetic reactors, you can find the calculation pathway as well. The same for the property sets calculations. In general, the calculations are based on the same material and energy balances as well as design equations that you can find out in any chemical engineering book. I hope this could help you.