Table of Contents
Example Calculation in OLI Software
- Example Case Set-Up
- Creating a Callout for the Vapor Product Stream
- Duplicating Callouts for Liquid Product Streams
- Adding Entrainment to a Vessel
- Calculating an entrainment value
- Entering an entrainment value
Objective
This article describes how to use the entrainment feature in OLI Flowsheet: ESP to represent carryover between process phases in vessels such as separators and reactors.
By specifying entrainment among vapor, liquid 1 (aqueous), liquid 2 (organic), and solid phases, users can model cases where physical carryover influences downstream conditions. Examples include chloride transport in vapors or solid fines entrained in liquid streams.
This article also outlines best practices for defining entrainment ratios and interpreting their impact on simulation results.
Disclaimer
The user interface, calculations, and results displayed in this article are from OLI Flowsheet: ESP Version 12.5. Other software versions may appear different due to continual developments to the software.
Background
What is entrainment?
Entrainment refers to the unintended physical carryover of one phase into another during separation—such as liquid droplets in vapor or fine solids in a liquid stream. In OLI Flowsheet, the entrainment feature allows users to specify the extent of this carryover as a mass-based ratio, improving the realism of process simulations.
When to include entrainment in a model
In real process systems, entrainment can affect product purity, corrosion behavior, and equipment performance. Incorporating entrainment parameters enables the user to simulate these effects accurately. For example, modeling vapor-phase chlorides or solids carryover can help predict impacts on downstream heat exchangers and dead legs.
Where to find the entrainment feature
The entrainment option is available for two unit operations: the Separator and the Reactor.
To locate it:
- Select the desired unit operation on the flowsheet.
- Navigate to the Properties window.
- Under the Equilibrium Calculation section, find the Entrainment field.
- Click the arrow to the right of the Entrainment field to open the entrainment settings
Figure 1. Location of the Entrainment option under the Equilibrium Calculation section in the Properties window.
Entrainment is phase-specific, meaning one phase can be entrained into another. However, even though OLI distinguishes between multiple liquid phases, the entrainment setting applies to the combined total liquid phase when entraining into solids or vapor. Within the liquid region, liquid 2 can be entrained into liquid 1, or vice versa.
All entrainment inputs are mass-based ratios (g/g). Each field defines the number of grams of one phase entrained per gram of another—for example, grams of solids entrained per gram of liquid. This ensures consistent, mass-normalized definitions across all entrainment specifications.
Figure 2. Entrainment input options available for the Separator unit in OLI Flowsheet: ESP (Example: Sep-1).
The highlighted input fields show where users can specify mass-based entrainment ratios for solids, liquids, and vapors.
Example Calculation in OLI Software
The following example demonstrates a typical case where entrainment parameters are included to model phase carryover within a separator.
Detailed setup instructions and guidance for entering entrainment ratios are provided in the next section.
Example Case Set-Up
This example models a separator in a gas plant receiving unit. The separator’s primary function is to separate vapor, condensate, and liquid phases from an incoming mixed stream.
In this example, it is assumed that a portion of the liquid is carried over into the vapor phase. The goal is to model this entrainment and evaluate its effect on downstream equipment in the vapor line.
To follow along, enter the inflow species listed below in the Chemistry tab.
Figure 3. Example inflows defined in the Chemistry tab for the separator case
Next, create a new stream and enter the composition as shown below. This stream will serve as the feed to the inlet separator.
In this example, the stream has been labeled “Feed Gas.”
Figure 4. Feed Gas stream definition showing composition, temperature, pressure, and total flow.
Next, select a Separator from the unit operation library and place it on the flowsheet.
Connect the Feed Gas stream to the separator inlet and attach new streams to each of the four outlets: Liquid 1 Product, Liquid 2 (Organic) Product, Vapor Product, and Solid Product.
Assuming the separator’s operating pressure and temperature are known, set the Calculation Type to Isothermal and specify the operating temperature.
Similarly, set the Pressure Specification to Absolute Pressure and enter the operating pressure in the separator properties, as shown below.
Figure 5. Separator configuration showing feed and outlet connections, with operating pressure and temperature defined in the Properties window.
At this point, the simulation can be run. The separator will produce vapor, liquid 1, and liquid 2 phases under the specified conditions. No solids are formed in this case.
Creating a Callout for the Vapor Product Stream
To easily monitor stream data and later visualize the effect of entrainment, add a callout to the vapor product stream.
- Right-click on the Vapor Product stream in the flowsheet.
- Select Add Callout to create a callout box.
- Right-click on the new callout box and choose Edit.
- In the Edit Callout window, expand the Phase Flows section.
- Select the following variables to display:
- Mass
- MassLiquid-1
- MassVapor
- MassLiquid-2
- Click OK to save and close the selection window.
The callout now displays key mass flow information for the vapor product stream, which will make it easier to compare results before and after adding entrainment.
Figure 6. Variable selection window for customizing the Vapor Product callout display.
Duplicating Callouts for Liquid Product Streams
Once the callout box is created for the Vapor Product stream, right-click on the vapor product callout and select Copy.
Next, right-click on the Liquid 1 Product stream and select Paste Callout.
A new callout box will appear on the flowsheet for the Liquid 1 Product stream, displaying the same selected variables.
Repeat this process for the Liquid 2 (Organic) Product stream.
This setup allows you to display and compare key mass flow information for each outlet stream directly within the flowsheet.
Figure 7. Callouts created for the vapor, liquid 1, and liquid 2 product streams showing temperature, pressure, and phase mass flow values
The resulting Vapor Product stream, shown above, consists entirely of vapor from the ideal separation. It is important to note that the separator in this configuration represents an ideal separation, meaning no entrainment or carryover between phases occurs.
In real systems, however, vessel efficiency and internal design can result in partial phase carryover, such as liquid entrained in the vapor stream.
To represent this phenomenon more accurately, the next step is to define entrainment parameters for the separator.
Figure 8. Vapor Product stream results showing 100% vapor composition under ideal separation conditions.
Adding Entrainment to a Vessel
Calculating an entrainment value
It is common to define entrainment based on a target percentage of liquid carryover. For example, if a vessel operates at 99% efficiency, an estimated 1% of the liquid may be entrained into the vapor phase.
Because entrainment parameters in OLI Flowsheet are entered as mass ratios, a short calculation is required to determine the correct value to input. The callout boxes created earlier can help by displaying the total liquid and vapor mass flows from the separator.
In the ideal separation case (without entrainment), the outputs show that each outlet contains only its designated phase (see Figure 7). The total liquid leaving the separator can be calculated as follows:
Total Liquid (lb/hr) = Mass Liquid 1 (lb/hr) + Mass Liquid 2 (lb/hr)
Total Liquid = 0.38 (lb/hr) + 81.01 (lb/hr) = 81.39 (lb/hr)
If we assume that 1% of this total liquid is entrained into the vapor phase, the target entrained mass is:
Target Entrained Liquid = 81.39 (lb/hr) * 0.01 = 0.8139 (lb/hr)
Next, use the vapor mass flow rate from the ideal separation case (1,000 lb/hr) to calculate the entrainment ratio:
Entrained Liquid in Vapor (mass/mass) = 0.8139 (lb/hr) ÷ 1000 (lb/hr) = 8.139e-4
This calculated ratio (8.139e-4) can then be entered into the Liquid in Vapor (g/g) field in the separator’s Entrainment section.
Entering an entrainment value
In this example, liquid is entrained into the vapor phase to represent more realistic operating conditions.
To define the entrainment:
- Select the Sep-1 unit operation in the flowsheet.
- Open the Properties window.
- Under the Equilibrium Calculation section, locate the Entrainment field and click on the arrow to expand the settings. (Refer to Figure 1 above.)
- In the Entrained Liquid section, enter 8.139e-4 in the box next to Liquid in Vapor (g/g), as shown below.
- Run the simulation.
This setting defines the amount of liquid that appears in the vapor outlet based on a mass ratio.
For example, an entrainment value of 8.139e-4 indicates that 0.0008139 grams of liquid are carried over for every gram of vapor in the outlet stream.
Figure 9. Entrainment parameters defined for the Separator, showing the Liquid in Vapor (g/g) ratio.
After running the simulation with the defined entrainment, the updated results are shown below.
In this case, approximately one percent of the total liquid is now present in the vapor product stream. This reflects the modeled entrainment effect, where a small portion of the liquid phase is carried over into the vapor outlet.
The callouts display the updated mass flow distribution for each outlet stream, confirming that liquid has been successfully included in the vapor phase as specified by the entrainment ratio.
Figure 10. Simulation results showing liquid carryover into the vapor product stream after entrainment is applied.
With the entrainment parameter applied, the results now reflect non-ideal separation behavior, where a small portion of the liquid phase is carried into the vapor stream. This demonstrates how entrainment can be used to simulate equipment efficiency in real processes.
Conclusion
The entrainment feature in OLI Flowsheet: ESP allows users to represent non-ideal phase behavior and model carryover effects between vapor, liquid, and solid phases.
By defining entrainment ratios, users can simulate real-world vessel performance where efficiency losses or design limitations lead to partial phase mixing.
In this example, adding an entrainment ratio to the separator successfully demonstrated liquid carryover into the vapor phase, showing how entrainment impacts downstream mass distribution.
Using calculated mass ratios based on expected equipment efficiency (for instance, 1% liquid entrainment) provides a practical approach for building realistic process simulations.
Incorporating entrainment into vessel models improves prediction accuracy for downstream units, product quality, and potential corrosion or fouling behavior.
This workflow can be extended to other unit operations, such as reactors, where entrainment between multiple liquid or solid phases influences process performance.