How to Write Models in ASCEND
Read the HOWTO's - this is a barebones guide. This is a step by step description of how you would develop models in ASCEND. We have created models for solving a few of the equations of state including the ideal gas, van der Waals, truncated virial, Redlich Kwong, Soave Redlich Kwong and the Peng Robinson equations of state. We have also written models to solve Bubble and Dew Point temperature and pressures and flash for both ideal gas, ideal liquid and for non ideal liquid mixtures.

The equations as you may see them in a textbook are presented and the way they will look in an ASCEND models are also here. In part A, all of the equations and constants and parameters and methods are within one file. In part B, we show how you can parametrize these models so they can be easily reused. We have used several example problems from Smith and Van Ness's 7th Edition. The complete, working models and scripts are also given at the end.

The procedure can be described as follows:
  1. Assemble the Equations you need
  2. Decide the kinds of variables and constants you will need
  3. Translate the equations into an ASCEND format
  4. Add methods that will properly pose your model
  5. Test the model and Methods
Ideal Gas
  1. Assemble the Equations you need
      P V = R T     (1)
  2. Decide the kinds of variables and constants you will need

    You have P which is pressure, T which is temperature, V is molar_volume and of course R which is the universal gas constant.
  3. Translate the equations into an ASCEND format
  4. Add methods that will properly pose your model
  5. Test the model and Methods
The Truncated Virial equation of State
  Z =
PV
RT
= 1 +
BP
RT
= 1 +
BPcPr
RTcTr
= 1 +
BPr
RTr
    (2)
where B is BPc/RTc B can be calculated using the correlation proposed by Pitzer and coworkers B = B0 + ω B1 and so we can rewrite Z as Z = 1 + B0Pr/Tr + ω B1 Pr/Tr and expressing Z = Z0 + ω Z1 we can write Z0 = 1 + B0Pr/Tr and Z1 = B1Pr/Tr Second virial coefficients are functions of temperature only and so B0 and B1 can be represented by B0 0.083 − 0.422/Tr1.6 and B1 = 0.139 − 0.172/Tr4.2
Redlich Kwong
Soave Redlick Kwong
Peng Robinson
This document was translated from LATEX by HEVEA.