Process Simulation, not PLC Emulation
Hold on guys. Before someone mentions how Concept has a PLC simulator, I'm pretty sure that motomarty is looking for something like Simulink to model his process, not something to run his PLC program on.
Motomarty:
Although I have heard of some process simulation programs (but can't come with any names - I've heard of Simulink, but never used it), I usually just write my own in the PLC (or, if the PLC is full, a spare PLC of the same family that I beg/borrow/steal from the customer/vendor/future project.)
I feel that this gives me much greater control over the how the simulation responds, and allows me to build in scenarios that a canned package might not. You still wind up having to debug both your regular ladder and the simulation ("Oh, yeah - the pressure should drop when this valve is open"), and it's easier to do that if you have complete control over everything.
As far as developing an animated simulation, my projects usually include some sort of HMI or SCADA. Often there is one or more "graphic" screens that are a sort-of reproduction of the P&ID. While I've sometimes gotten fancy and given some of the valves and tanks a 3-D sort of look, it usually doesn't pay in terms of effort vs. benefit ("coolness factor").
But after creating the HMI graphics, I would never then try to create another set of simulation graphics. That's why I simulate in the PLC. The HMI/SCADA then treats the simulated data as if it were real, and if I've done a good enough job, you can't tell the difference.
Just how complicated is your process, anyway?