3 replies to this topic

[shahabn]

hi all 

how wax molecules ( heavy paraffins) are defined in  process simulation software such as pro ii and aspen?

Is ther any simulation model for lube oil dewaxing ?

Best Regards

Shahabn

[Pilesar]

Typically choose a heavy petroleum component for wax when used in distillaiton. Boiling point will be high. In real life, high temperatures can lead to degradation that is not easily reflected in simulations. I've never built lube oil dewaxing models and do not have any examples.

[latexman]

I’d choose about a C30 paraffin, unless an analysis is available.

[astro]

To be honest, I wouldn't know without in-depth research.

 

So, deferring to ChatGPT, here are responses to your first question - posited as "How are wax molecules (heavy paraffins) defined in process simulation software such as ProII, Hysys and Aspen+?":

1. The general problem

In lube oil and dewaxing systems, the heavy paraffins (C20–C60+ typically) are not single, well-defined molecules. They’re mixtures of long n- and iso-paraffins, often with ring structures, dispersed in aromatics and naphthenes.
Thus, process simulators can’t treat them as discrete pure compounds — they use pseudocomponent or lumped-component representations based on boiling range, molecular weight, or carbon number.

2. PRO/II (AVEVA / formerly SimSci)

Representation method:

• Uses pseudocomponents derived from assay or blend characterisation (TBP, D86, etc.).

• Each pseudocomponent has properties defined by average molecular weight, normal boiling point, density (or Watson K-factor), and optionally viscosity correlation.

• When characterising a lube or waxy fraction, you extend the assay to high end points (often to 1000 °F+ or 540 °C+) and define extra pseudocomponents for wax ranges (e.g. C35–C40, C40–C50, C50+).

• Thermodynamic properties extrapolated using correlations such as Lee–Kesler, Twu equations, or API methods.

Key point:
PRO/II itself doesn’t know “this is C44 n-paraffin”; it only knows that the pseudocomponent behaves like a paraffinic hydrocarbon of given boiling point, MW, and Watson K. The “wax” identity is therefore statistical, not molecular.

3. Aspen HYSYS / Aspen Plus

Representation methods differ slightly:

(a) Aspen HYSYS Refining / Petroleum mode

• Uses Assay Manager or Characterisation Tool to generate pseudocomponents from crude/lube distillation data.

• You can specify a paraffinic nature (high K-factor ~12.5–13.0) to indicate waxy character.

• The simulator assigns pseudo “hydrocarbon types” (paraffinic, naphthenic, aromatic) and calculates properties via Twu, Riazi–Daubert, or Lee–Kesler correlations.

• For very heavy ends, HYSYS introduces a “Plus Fraction” or “Resid/Wax” component with extrapolated critical properties.

( Aspen Plus (Chemical mode)

• Handles waxes through user-defined pseudocomponents or solid components.

• You can explicitly add normal paraffins up to C100+ using the built-in DIPPR database (e.g. n-C36, n-C40).

• In solvent dewaxing studies, people often create “solid wax” species for long n-paraffins (C24–C50) and define a solid phase for the crystallised wax.

• Solid properties (fusion temperature, heat of fusion, density) may come from empirical fits, e.g. Ruzicka–Domalski correlations.

Key point:
Aspen Plus can explicitly handle solid/liquid equilibrium (SLE) and thus model actual n-paraffin precipitation, while HYSYS treats it implicitly via solubility curves or pseudo-component behaviour (no true solid phase).

4. Definition methods

Approach How “wax” is defined Used in Assay-based pseudocomponents Derived from TBP curve; each cut has average MW, Tb, density, K-factor. PRO/II, HYSYS Petroleum, Aspen Plus crude models Carbon-number pseudocomponents Assigns n-C20, n-C24, n-C30 etc. using correlations for MW, Tb, Cp, etc. Aspen Plus, HYSYS (chemical mode) User-defined pure solids Defines explicit solid n-paraffins with fusion data for SLE Aspen Plus Lumped paraffin fraction One pseudo “wax” or “C40+” component representing all crystallisable paraffins PRO/II, HYSYS

5. Thermodynamic property handling

Property How it’s obtained Critical properties (Tc, Pc, Vc) Extrapolated using Twu correlations from mid-range hydrocarbons Acentric factor Derived from API correlation vs boiling point and K-factor Liquid density, Cp, enthalpy Riazi–Daubert correlations (paraffinic basis) Solid–liquid equilibrium (SLE) In Aspen Plus only, via UNIFAC-SLE or user-defined sublattice parameters Viscosity & pour point Empirical correlations (e.g. Walther or modified API) using measured data if available

6. Illustrative example

Suppose you have a lube base oil feed with 10 wt % wax above C35.
In Aspen HYSYS:

1. Characterise the feed via Assay Manager (to 700 °C+ TBP).

2. Define final pseudocomponents (say P1 = C20–C30, P2 = C30–C40, P3 = C40+).

3. Assign high K-factor (~12.8) to P3 to indicate a highly paraffinic cut.

4. In simulation, that fraction can represent “solid wax” formation onset.

In Aspen Plus:

1. Add explicit solid components n-C28, n-C32, n-C36, n-C40.

2. Define SLE with solvent (e.g. MEK/MIBK).

3. Run flash or crystallisation block to predict wax yield vs temperature.

7. Typical data sources for wax property definition

• DIPPR 801 database (pure paraffin data up to ~C100).

• API Technical Data Book, Chapter 10 (high-MW paraffin correlations).

• Riazi–Daubert correlations for critical properties.

• Ruzicka–Domalski for enthalpy of fusion and solid density.

• Literature (e.g. Assael et al., Int. J. Thermophys., 1997) for high-carbon paraffin melting points and Cp.

8. In summary

Simulator “Wax” Representation Solid phase support Typical use PRO/II Assay pseudocomponents (C30+, K ≈ 12.5–13) None Refinery lube unit M&E balance HYSYS Plus fraction / pseudocomponents from assay Limited (no explicit solids) Steady-state & dynamic flowsheet Aspen Plus Explicit n-paraffin solids (C20–C60+) or pseudocomponents Yes (SLE capable) Dewaxing thermodynamics & crystallisation

 

.......................

 

Moving on to your next point, "What simulation models are commonly recognised as reliable for lube oil dewaxing?", I'm getting:

Key Processes to Model in Lube Oil Dewaxing

To model dewaxing you typically need to represent:

1. Solid-Liquid Equilibrium / Solubility of waxes in oil + solvent (e.g. which fraction crystallises under what conditions).

2. Wax crystallization kinetics: nucleation, growth, crystal size distribution (CSD), possibly aggregation and breakage.

3. Heat transfer / cooling / chilling dynamics (including thermal gradients, heat flux limitations, cooling rate).

4. Mass transfer / solvent dilution / solvent recovery where relevant.

5. Plant-scale process features: filters, scraped‐surface exchangers (SSHEs), wash stages, steady vs transient operation.

 

From cursory observation, the above seems reasonable to me. Give it some thought for the specifics of your use case and in the absence of advice from an authoritative source, at least you've got something to get started. Hopefully someone else can chip with detail that improves on my meagre offering.

Edited by astro, 04 October 2025 - 07:58 AM.