Likewise, all the systems can be modeled and the beauty is you get to understand the material and heat flow in the process and based on that components can be simulated. In some process simulators such as EMSO (Environment for modeling, simulation, and optimization) there is an option for the user to develop codes for modeling and then simulate the system, however, in other process simulators the system is either customized or we have to develop models by scribbling pen in our notebooks. Another suggestion, keep the models as simple as possible, for example, don't write so many codes and make it look like exaggerated and end up seeing it as a difficult task to simulate the system. Oh yeah, if I have bored you enough thinking that you know all these things so let me share some interesting tips on modeling. First of all keep a gentle look at the dimension of the system you are modeling, because it is easy dealing with cylindrical and rectangular systems, however, when conical systems or systems similar to that comes into the picture, you have to take a minute back and think. Yeah another point "THE SIGNS" which is very important which comes into picture while considering the rate of reaction and heat of reaction as well, so decide wisely depending on the situation or pay for it at the end. Same goes for in energy balance equations where the sign depends on whether heat is realized to a system (+ve) or heat is absorbed (-ve). If the system is assumed to be a steady state then the rate of accumulation is considered as Zero. Oh yes, if you try to model continuous systems such as tanks in series or something like that, keep a sharp look that a constituent coming from the first tank, goes exactly into the second tank and the process goes on.