In OLI Flowsheet: ESP v12.5, sub flowsheets emerge as a game-changing feature that allows engineers to break down large simulations into modular, reusable process sections. The goal is to help simulation engineers, process engineers, and technical teams leverage sub flowsheets to “model smarter and scale faster” in their organizations.
Business & Technical Benefits of Sub Flowsheets
Sub flowsheets deliver value on multiple fronts:
- Manage Complexity: By nesting a section of the flowsheet inside a sub flowsheet block, large process models become easier to visualize and understand, which enhances clarity and maintainability. Instead of one monolithic flowsheet with a “spaghetti bowl” of connections, engineers can compartmentalize process sections (e.g. a pretreatment unit or recycle loop) into neat sub flowsheets. This makes it simpler to navigate and troubleshoot complex simulations.
- Reusability and Standardization: A sub flowsheet acts like a function or subroutine in a program – defined once and reused anywhere. Teams can create a library of validated sub-process models and drop them into new flowsheets without rebuilding from scratch. This ensures consistent modeling standards across global teams and projects, capturing best practices in a template that can be shared organization wide. The result is faster project setup and less duplicate work.
- Improved Solver Performance: Complex models often struggle to converge. Sub flowsheets can be solved (run) independently of the main flowsheet, allowing engineers to test and optimize sections in isolation before integration. This localized iteration can reduce convergence issues and speed up overall solution time. In essence, sub flowsheets divide-and-conquer simulation complexity, which often leads to more robust and faster simulations.
- Collaboration and IP Protection: Because sub flowsheets can be saved as separate files or templates, organizations can better manage and protect their intellectual property. For example, an R&D team might develop a proprietary reactor model as a sub flowsheet and share it with plant engineers without exposing all underlying details. Moreover, multiple engineers can work on different sub flowsheet sections of a model concurrently, enhancing collaboration efficiency especially in large-scale projects.
Technical Setup Guidance for Sub Flowsheets
Setting up a sub flowsheet in OLI Flowsheet: ESP v12.5 is straightforward, but it introduces a few key concepts to understand. Sub flowsheets act as modular sections of your main (parent) flowsheet, enabling cleaner designs and easier troubleshooting.
To connect a sub flowsheet to the main flowsheet, you must manually add inlet and outlet portals inside the sub flowsheet. These portals serve as the connection points between the two environments. Once created, they allow main flowsheet streams to flow seamlessly in and out of the sub flowsheet, linking the two models together.
In this example, the inflows include the following components:
- H₂O
- C₂H₆O
- CO₂
- NaCl
- NaOH
- HCl
- C₁₀H₂₂
Figure 1 – Sub Flowsheet Example – Inflow
Main Flowsheet Layout
For this example, the main flowsheet will include the following streams:
- 2 Inlet Streams:
-
- Inlet_H2O+NaCl
- Inlet_organics
- 4 Outlet Streams:
- Outlet_Vapor
- Outlet_Liquid1 (aqueous)
- Outlet_Liquid2 (organic)
- Outlet_Solids
At the center of the flowsheet is a single Sub flowsheet block (Sub-1), which connects all six streams.
This configuration keeps the parent flowsheet clean and organized, while all detailed operations are contained inside the sub flowsheet. The overall layout will resemble the figure below, clearly showing how the Sub flowsheet block serves as a bridge between the main process and the modular, internal model:
Figure 2 – Sub Flowsheet – Connections
The sub flowsheet will look like this:
Figure 3 – Sub Flowsheet Configuration
Step-by-step guide: set up a Sub flowsheet (OLI Flowsheet: ESP v12.5)
- Adding the Sub flowsheet Block: From the unit operations palette, add a Sub flowsheet block to the parent (main) flowsheet and name it (e.g., Sub-1).
Figure 4 – Sub Flowsheet Selection
- Create main-flowsheet streams
Add and clearly label the external connections you’ll use in this example:
- Inlets: Inlet_H2O+NaCl, Inlet_organics
- Outlets: Outlet_Vapor, Outlet_Liquid1 (aqueous), Outlet_Liquid2 (organic), Outlet_Solids
Figure 5 – Create the Main Flowsheet
- Open the sub flowsheet editor
Double-click Sub-1 (or use Open Sub flowsheet). A blank sub flowsheet canvas will open.
Figure 6 – Open the Sub Flowsheet Editor
- Add portals inside the sub flowsheet
Place 2 Inlet Portals and 4 Outlet Portals (matching the two inlets and four outlets you created on the main flowsheet). These portals are the connection points between parent and sub flowsheet.
Figure 7 – Sub Flowsheet – Adding Portals
- Build the internal model
Inside the sub flowsheet, add and connect:
- Mixer (Mix-1)
- Separator (Sep-1)
Rename internal streams for clarity and connect all blocks to the appropriate portals.
Figure 8 – Add Blocks and Connect
- Specify block settings
- Mixer: Keep default settings (adiabatic; minimum inlet pressure).
- Separator: Set vapor flow = 5 mol/hr and minimum inlet pressure.
Figure 9 – Sub Flowsheet – Blocks and Connections
- Specify main-flowsheet inlet conditions
On the parent flowsheet, define temperature, pressure, flow, and composition for both Inlet_H2O+NaCl and Inlet_organics.
(If following the example components discussed earlier: H₂O, C₂H₆O, CO₂, NaCl, NaOH, HCl, C₁₀H₂₂.)
Figure 10 – Main Flowsheet – Stream Conditions
- Map streams to portals
In Sub-1’s properties, link each main-flowsheet stream to the correct Inlet/Outlet Portal (via dropdown) or use drag-and-drop. Verify every inlet/outlet is mapped exactly once.
Figure 11 – Sub Flowsheet – Map Main Streams
It will look like this:
Figure 12 – Sub Flowsheet Connections
- Connect outputs and run
Complete any remaining connections on the parent flowsheet, then run the model. Review results (callouts/reports) as needed.
Figure 13 _ Sub Flowsheet – Run & Analyze the Results
- Troubleshooting & best practices
- You can run the sub flowsheet independently from its Properties pane to isolate and resolve convergence issues.
- Ensure the number of portals matches the number of external streams.
- Use clear, consistent names for portals and streams (e.g., SF_Inlet_Org, SF_Outlet_Aq).
- Initialize with reasonable guesses (flows/T/P) and keep recycles inside the sub flowsheet when possible to improve convergence.
This workflow keeps the parent flowsheet clean while the detailed logic lives inside a reusable, well-scoped sub model—making it faster to test scenarios, troubleshoot, and standardize best practices across projects.
Figure 14 – Run Sub Flowsheet
Use Case Examples Across Industries
Sub flowsheets are broadly applicable – here are a few examples of how different industries can leverage them in practice:
- Downstream O&G: Hydrodesulfurization units and diesel stripping columns often create convergence challenges due to complex recycle loops and tightly coupled operations.
By leveraging sub flowsheets, engineers can decouple these systems into manageable, modular blocks:
- HDS Units: Separate the reactor section, effluent cooling and gas-liquid separation train, and amine sweetening unit into dedicated sub flowsheets. This allows the reactor with its kinetics and heat release to converge independently, while the cold-end and amine system are solved sequentially. As a result, engineers can troubleshoot faster, test multiple scenarios, and scale models across refinery operations with greater confidence and efficiency.
- Upstream Oil & Gas (MEG Regeneration Loop): In offshore gas production, Monoethylene Glycol (MEG) is injected to prevent hydrates, and later regenerated. The regeneration process (glycol reclaiming and recycle) is a complex sub-process with flash tanks, distillation, and filters. Using a sub flowsheet, an engineer can model the entire MEG regeneration loop as one reusable module. Upstream production teams benefit by easily integrating or removing the MEG loop in field simulations depending on need. This promotes standardized modeling of hydrate control strategies across assets. The technical setup is easier to manage, and each piece (production well, MEG loop, gas processing) can be solved separately then combined.
- Carbon Capture (Amine Unit in Power Plant): Consider a power plant with a CO₂ capture unit (amine absorber and regenerator). The capture system can be built as a self-contained sub flowsheet (including the absorber column, stripper column, cross heat exchangers, etc.). By doing so, power engineers can attach this “carbon capture” sub flowsheet to different plants without rebuilding it, or swap in updated versions as technology improves. It eases collaboration between the technology provider (who develops the carbon capture model) and the plant engineers (who integrate it into the overall power plant model). For adoption, one tip is to treat the sub flowsheet like a vendor skid – clearly define its battery limits (inlet flue gas, outlet treated gas, captured CO₂ stream, etc.) and document those interfaces for users.
- Chemicals & Critical Materials (Batch / Multi-Step Processes): Many chemical processes involve sequential operations that can be grouped. For example, in battery recycling (critical materials recovery), there might be a leaching step followed by solvent extraction and precipitation. Each of these could be a sub flowsheet, or the entire train could be a sub flowsheet embedded in a larger recycling facility model. OLI’s internal experts found sub flowsheets particularly beneficial for semi-batch cases like autoclave corrosion tests and ion exchange cycles, where a series of steps is needed. By using sub flowsheets, one can isolate these steps, iterate on them easily (e.g. optimize the autoclave conditions independently), and then plug them into continuous plant models. This modular approach helps technical teams evaluate complex sequences (like charging and heating an autoclave) with less error.
- Mining & Metals (Hydrometallurgy): In hydrometallurgical flowsheets, certain circuits like solvent extraction or crystallization can be modeled as sub flowsheets. For instance, a solvent extraction circuit for cobalt could be a sub flowsheet that includes multiple extraction, scrub, and strip stages. Mining companies can maintain a library of these circuits for different metals. When developing a new project, they mix-and-match sub flowsheets (one for nickel SX, one for cobalt refining, etc.) to quickly prototype the whole flowsheet. This boosts agility in process design. Technically, it also means each circuit can be optimized by specialists independently (e.g. the SX chemist focuses on the SX sub flowsheet) and then integrated, which de-risks the simulation.
These examples scratch the surface. Across industries – from oil & gas to renewable energy to minerals – sub flowsheets are empowering engineers to build complex models with greater confidence and efficiency. Early adopters have noted how it “keeps the model clean and readable” and significantly cuts down time spent on repetitive modeling tasks. By modularizing simulations, organizations can drive consistency, reduce errors, and ultimately accelerate innovation in their process design and optimization work.
Conclusion
Sub flowsheets in OLI v12.5 represent more than just a new software feature; they reflect a shift toward smarter simulation practices. By simplifying complexity, promoting reuse, and easing collaboration, sub flowsheets help technical teams do more with less effort – a clear win for businesses aiming to improve agility and retain proprietary knowledge. Embracing sub flowsheets is a step toward digital transformation in engineering simulation, enabling your organization to respond to challenges with unprecedented speed and consistency. In short, it’s time to sub flowsheet your way to smarter process modeling.