Aspen HYSYS vs Aspen Plus: Which Simulator Should You Use?

A direct, no-nonsense comparison of the two industry-standard simulators and when to use them.

This is one of the most common questions we get from engineers starting out in process simulation — and one that experienced engineers sometimes get wrong too.

Both are AspenTech products. Both do steady-state and dynamic simulation. Both are industry standard. But they are built for different problems, and using the wrong one wastes time or produces unreliable results.

This article gives you a direct answer, not a marketing comparison.

The Short Answer

Aspen HYSYS is built for oil and gas, refining, and upstream applications — hydrocarbon-dominated systems at a wide range of pressures.

Aspen Plus is built for chemical processes — specialty chemicals, pharmaceuticals, polymers, batch operations, and systems involving reactions, solids, and complex non-hydrocarbon chemistry.

That’s the rule of thumb. The rest of this article explains why, and covers the exceptions.

Why the Two Tools Exist Separately

AspenTech did not build two simulators by accident. HYSYS and Aspen Plus have different underlying architectures, different default thermodynamic models, and different user interfaces — because they were designed to solve fundamentally different engineering problems efficiently.

HYSYS uses a sequential-modular architecture optimised for hydrocarbon systems. It excels at rapid flowsheet building for gas and oil processing, with fast convergence on the equations governing vapour-liquid equilibrium of hydrocarbon mixtures.

Aspen Plus was built with a broader, more flexible modelling architecture — capable of handling solids, electrolytes, polymers, and complex reaction kinetics that HYSYS is not designed for.

Side-by-Side Comparison

When Aspen HYSYS Is the Clear Choice

• Upstream oil and gas facilities — wellhead separation, gas gathering, compression, gas processing. HYSYS handles multiphase hydrocarbon flow and separator design natively and efficiently.
• Refinery process units — crude distillation, FCC, hydrocracker, reformer. HYSYS’s Oil Manager and petroleum assay characterisation tools are purpose-built for handling crude oil as a mixture of pseudocomponents, something Aspen Plus does not do well.
• LNG and gas processing — cryogenic separation, NGL fractionation, acid gas removal. HYSYS’s hydrocarbon-focused thermodynamic packages (Peng-Robinson, SRK) are tuned for this application.
• Pipeline and flare system studies — HYSYS integrates directly with Aspen Flarenet for flare header hydraulics and offers BLOWDOWN Technology for depressurisation studies — both essential tools for oil and gas process safety work.
• Compressor and pump-heavy systems — HYSYS’s pressure changer models and dynamic simulation capability are well suited to compressor surge analysis, a common requirement in gas processing and refining.

When Aspen Plus Is the Clear Choice

• Specialty chemical manufacturing — processes with complex reaction chemistry, multiple reaction steps, or custom kinetics. Aspen Plus’s reactor models (RCSTR, RPFR, RGibbs, RYield, RStoic, Requilibrium) offer far more flexibility than HYSYS’s reactor options.
• Batch processes — pharmaceutical intermediates, specialty chemical batch reactors, batch distillation. Aspen Plus has dedicated batch modelling capability that HYSYS simply does not offer.
• Processes involving solids — crystallisation, drying, solids handling and processing. Aspen Plus’s solids modelling framework handles particle size distribution and solid-phase behaviour that HYSYS cannot.
• Polymer production — Aspen Polymers, a specialised module within Aspen Plus, handles the unique thermodynamics and kinetics of polymerisation reactions.
• Electrolyte and aqueous chemistry — processes involving strong electrolytes, pH-dependent chemistry, or complex aqueous systems benefit from Aspen Plus’s electrolyte NRTL and OLI-integrated thermodynamic models.
• Distillation of non-hydrocarbon systems — azeotropic separations, extractive distillation, and systems with strong non-ideal liquid behaviour are generally handled with more model flexibility in Aspen Plus.

Where the Line Gets Blurry

A few applications sit in the overlap zone, and the right tool depends on specifics:

• Amine gas treating — Both tools can model amine absorption for acid gas removal. HYSYS has a dedicated Acid Gas property package built specifically for this. Aspen Plus uses the Electrolyte NRTL framework, which is more rigorous but requires more setup. For straightforward gas treating in an oil and gas context, HYSYS is usually faster to set up. For complex or unusual amine chemistry, Aspen Plus’s rigor may be worth the extra setup time.
• Glycol dehydration — Similarly, HYSYS has a dedicated glycol package tuned for TEG dehydration, making it the practical first choice for this specific application even though Aspen Plus could technically model it.
• Carbon capture — Both tools are used in industry for CCUS process modelling. HYSYS is common when carbon capture is integrated with an upstream or midstream hydrocarbon process. Aspen Plus is common when the capture process is modelled independently with detailed absorber-stripper kinetics and rate-based column modelling — which Aspen Plus supports more rigorously.
• Fertiliser and syngas processes — These often sit at the intersection of hydrocarbon processing (feed gas) and chemical reaction engineering (synthesis reactions), and the choice depends on which part of the process is the primary focus of the study.

Can You Use Both on the Same Project?

Yes — and on complex projects, this is common practice. A typical example: a gas-to-chemicals project might use HYSYS to model the upstream gas processing and feed conditioning, then hand off validated stream conditions to Aspen Plus for the downstream chemical synthesis and separation train.

AspenTech supports this workflow through its integrated environment — models can exchange stream data between HYSYS and Aspen Plus, allowing each tool to be used for the part of the process it handles best.

At CHEMKLUB India, our engineers are certified in both platforms, and we routinely select — or combine — the appropriate tool based on the specific process rather than defaulting to whichever tool is more familiar.

A Practical Decision Framework

Ask these questions in order:

1. Is the primary stream hydrocarbon-based (oil, gas, condensate)?
   Yes → Start with HYSYS.
   No → Go to question 2.
2. Does the process involve significant reaction chemistry, solids, or batch operations?
   Yes → Use Aspen Plus.
   No → Go to question 3.
3. Is petroleum assay / crude characterisation required?
   Yes → HYSYS (Oil Manager is purpose-built for this).
   No → Go to question 4.
4. Does the process involve electrolytes, polymers, or complex non-ideal liquid chemistry?
   Yes → Aspen Plus.
   No → Either tool will generally work; choose based on team familiarity and available thermodynamic package fit.

Common Mistakes

• Using Aspen Plus for a straightforward oil and gas separation or compression train — technically possible, but slower to set up and lacking the hydrocarbon-specific conveniences (Oil Manager, tuned property packages) that HYSYS provides natively.
• Using HYSYS to model a batch pharmaceutical process — HYSYS has no batch modelling framework. This is not a matter of preference; the tool genuinely cannot do it.
• Assuming the same thermodynamic package works equally well in both tools — Peng-Robinson in HYSYS and Peng-Robinson in Aspen Plus are implemented similarly but not identically. Binary interaction parameters and defaults can differ. Always verify, don’t assume.
• Choosing the tool based on team familiarity alone rather than process fit — this is understandable given training investment, but it produces slower, less reliable models when the tool is genuinely mismatched to the process.

Frequently Asked Questions

What is the difference between Aspen HYSYS and Aspen Plus?

Aspen HYSYS is optimised for oil and gas, refining, and hydrocarbon-dominated processes, with built-in tools for petroleum assay characterisation and hydrocarbon thermodynamics. Aspen Plus is optimised for chemical processes involving reactions, solids, batch operations, polymers, and electrolyte chemistry. Both are steady-state and dynamic simulators from AspenTech, but they are built for different process types.

Can Aspen Plus be used for oil and gas simulation?

Technically yes, but it is not the practical choice. Aspen Plus lacks HYSYS’s Oil Manager for crude assay characterisation and its hydrocarbon-tuned property packages, making oil and gas modelling slower and less convenient in Aspen Plus compared to HYSYS.

Which simulator is better for a refinery?

Aspen HYSYS is the standard choice for refinery simulation — crude distillation, FCC, hydrocracker, and reformer modelling all benefit from HYSYS’s petroleum characterisation tools and hydrocarbon-focused thermodynamic packages.

Which simulator should I learn first as a process engineer?

This depends on your target industry. If you plan to work in oil and gas, upstream, or refining, learn Aspen HYSYS first. If you plan to work in specialty chemicals, pharmaceuticals, or industries involving batch processes and complex reactions, learn Aspen Plus first. Many experienced process engineers eventually become proficient in both.

Can HYSYS and Aspen Plus models be connected on the same project?

Yes. AspenTech’s integrated environment allows stream data to be exchanged between HYSYS and Aspen Plus models, which is useful for projects that span both hydrocarbon processing and chemical synthesis — for example, gas-to-chemicals or gas-to-liquids projects.