3 replies to this topic

[sukanta87]

Hi All,

 

Am used to work with Aspen-hysys. But, according to literature point of view am facing some difficulties to reply someone's asking in a formal way. Would you please help me to clarify the following queries one by one.

 

1. How hysys work?

2. What are the Simulation techniques of hysys?

3. What are the basic assumptions for a simulation?

4. What are the boundary conditions for simulation?

 

Here am supposing to a natural gas simulation program.

 

 

Thanks in advance.

 

[processengbd]

Hysys is just a process simulator.

1&2. Means first it does property calculation for you based on fluid package you define. Then hysys does heat and material balance for you. As we all chemical engineers did in our graduate study. It has some algorithm to do solve equations regarding conservation of mass and energy around an unit operation. Then it has some specific algorithm for specific unit operation. Take distillation column for example if you did a heat a mass balance around a tray in your undergrad you know hysys does it same way. I would say more rigorously. If you have aspentech support access you can have a look at there reference guide which have majority of equation (other then proprietary) they use in the simulator.
3 & 4. Please be specific about assumption and boundary condition. Which unit operation are you referring ? I think hysys has thousands of algorithm regarding these unit operation. each has its own consideration. a question for you. what do you mean by boundary condition. as far as I know this term related to transport phenomena. where we put some restrain on differential equation to solve a problem.


I would say hysys is not a black magic box. it just do heat and material balance and some extra calculation for you.

[sukanta87]

Hi processengbd,

 

Thanks for your comments.

In any simulation some variables we must fix to have a solution, what is DOF. 

 

For 3. Some variables/ process conditions we assume based on recommended practice. For example, to do a distillation column simulation usually we declare column T and P. That's am indicating the assumption here for the distillation column. Actually it depends on the objectives and specific work. My asking was to define some general/common assumption for any simulation in hysys.

 

For 4. you are right as boundary conditions are usually used in differential and integral task with specific limitation in transport phenomena. In most cases, the boundary conditions are very important for the simulation region's physical processes. Different boundary conditions may cause quite different simulation results. Improper sets of boundary conditions may introduce nonphysical influences on the simulation system, while a proper set of boundary conditions can avoid that. So arranging the boundary conditions for different problems becomes very important. For any separator simulation we define some output streams ( assume product) from a input (assume feed). The range of product or the specification can be the boundary condition here. My asking was to define some general/common boundary conditions for any simulation in hysys. 

 

Different thermodynamics model used in hysys are the set of those algorithms as you said. Selecting a model for a purpose we are implying those things to work.

 

Actually was searching some common/general working principle, assumptions, boundry conditions and techniques for simulation in hysys if it has. May be the common isn't common or huge in terms of literature!

[processengbd]

As we know DOF means degrees of freedom. In hysys that means number of variable you need to specify if you want to solve any unit operation. or in the context of heat and mass balance we know that we need exactly equal number of equation to number of unknowns in a system to solve it. like to solve a overall mass balance around a distillation column with only inlet and two product. you have three variable and one equation. now you need to know two of the three variable to solve the equation. I think DOF is the one of themain basic knowledge needed before anyone try to simulate a process. like for a stream to be fully defined you need to specify its component, temp., pressure, and flow rate. now think if the stream is fully vefined & connected to a valve with an outlet stream. now what degree of freedom the system has? 1. you can either specify pressure drop or temp. hysys is going to solve it.

3. as you have correctly said " Actually it depends on the objectives and specific work." there is no general rule for the assumption. if you are talking about brownfield project then you have column temp &,pressure from project then your main goal is to see how the column will perform if we change some parameter in the column. In case of greenfield project you have to calculate each parameter based on previous project experience. like column pressure and temp. depends on column top product quality.

4. I have never seen anyone using the term boundary condition in the context of process simulation. in separator hysys does the flash calculation. now we usually fully define the inlet stream & the pressure drop of the vessel, that's it. the separator would be solved. so which variable are you referring as boundary condition here?

thank you

Edited by processengbd, 04 July 2014 - 12:25 PM.