Dynamics Simulation 3

[Lblopez]

Hello,

What is a dynamics simulation?
Wich is the difference with a PRO/II conventional simulation?
Can I make it in ASPEN PLUS 11.1?

Thanks,
Lblopez

 

[madiazr]

Hi

A dynamic simulation is a simulation doing after a steady state simulation has been solved. In that type of simulation you assess the variables behaviour when a disturbance is applied on a input variable. Additionally, you can simulate control loops. ASPEN PLUS is only capable for steady state simulations... if you want to resolve in dynamic mode, you should use ASPEN DYNAMICS (included in the Aspen Engineering Suite 11.1) or, better, in HYSYS in the dynamic mode. But you must be careful in input pressure-flow relations for every stream a resistance constant to flow for every equipment. Also, valves must be added between equipments for a pressure driven simuation (and valve coefficients must be specified). In the dynamic mode, the big issue is to track the controllers of the control loops in suitable values for a process controllable and resiliable. I have some experience with hysys dynamics and aspen dynamics.

 

[UmeshMathur]

A dynamic simulation describes time-dependent behavior of a system. Therefore, the dynamic problem includes:

(1) non-linear algebraic equations (describing steady-state mass and energy balances, thermodynamic and kinetic equilibrium, or other phenomena), and
(2) ordinary differential equations (ODEs) that describe time dependent accumulation and disappearance of mass and energy, including the reaction kinetic terms.

All this should be standard material in your transport phenomena, unit operations, and reaction engineering courses.

It should be obvious that commercial simulators for dynamic simulation are far more complex than those solving pure steady-state problems. However, the generalized solution of large systems of differential equations is a highly non-trivial task, especially if some of the differential equations become "stiff" (i.e., when their time constants are small). I don't believe that the major commercially available chemical engineering dynamic simulators handle this aspect too well, as they do not incorporate the special numerical methods required for stiff ODEs.

Therefore, it is extremely important to validate the dynamic solution you get from ANY software package, a task that is neither easy nor quick.

 

[Lblopez]

Thanks,

Can I make a relief system evaluation and refinery simulation without dynamics simulation, using PRO/II or ASPEN PLUS? and dont make a big mistake?

Lblopez.

 

[UmeshMathur]

There is another recent thread called “PSVs...what's missing?” (thread135-131246) that discusses the issue of relief systems and valves. You should find this interesting, as design (or rating) of relief valves and relief systems requires consideration of PEAK loads that cannot be addressed using a simple steady-state analysis alone.

You must invoke the proper options to require determination of the proper phase conditions at all points in the piping network, using the right thermodynamic options for phase equilibria. Further, you must be sure that the proper equations are being solved for friction losses for compressible vapor or compressible vapor/liquid two phase flow. This includes determination of the two phase flow regime for horizontal, vertical or inclined pipes. These phenomena present the more complex sets of issues in fluid mechanics.

This is a non-trivial task in my opinion as the overall computation involves lots of trial and error, even at steady state.

 

[MortenA]

Lblopez

PSV and blow down system evaluation has been made for decades without the use of dynamic simulation. This is because the methodes used is based on conservative engineering - suited for safety systems.

I think that we in the years to come will see an increased use of dynamic simulation especially in conection with safety systems.

The dynamic evaluation is especially usefull in debottlenecking existing systems. You should hawever be carefull since if youd ecide to reduce your safety margins you must know and understand your design case better.

Best regards

Morten