Overview
OLI software calculations are based on closed systems, meaning mass is conserved within the system unless explicitly defined otherwise. This implies that air or any external environment is not automatically considered part of the system unless you create it within the model. Understanding how to simulate air headspace is essential when modeling systems in contact with air, such as evaporation or gas absorption processes.
This guide outlines how to simulate an open system within OLI software by creating air headspace using an air stream. We'll explain how to approximate an "open" system since true open systems are not feasible in OLI due to the need for a defined total mass. Images will help visualize the setup and results.
Disclaimer: The example provided below utilizes version 12 of the software. The interface, features, and options may differ in older or newer versions.
Closed vs. Open Systems in OLI
In OLI, a closed system is one where mass cannot leave or enter the system. This ensures that the mass balance is conserved. Energy transfer, such as heat transfer, can occur, but the total mass stays constant. For example, in a system of 10% methanol in water, all components remain within the system, and no evaporation into the air takes place.
On the other hand, an open system allows for mass transfer between the system and its surroundings, such as methanol evaporating into the air. However, OLI requires a defined mass for calculations, so a fully open system cannot be modeled directly. Instead, you can approximate an open system by creating an air stream and combining it with the liquid stream.
Simulating an Open System with Air Headspace
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Creating an Air Stream: To simulate an open system in OLI Studio: Stream Analyzer, you need to create an air stream. This can be done by selecting one of the pre-defined air streams from the Object Library (Menu: View > Toolbars > Object Library).
There are two options:-
Dry Air: A stream consisting of a standard dry air composition:
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Moist Air: A stream with 50% humidity:
Both streams have a default amount of 1 mole (approximately 24 liters at 25°C). You can adjust this amount as needed to simulate a larger air space.
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Moist Air: A stream with 50% humidity:
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Dry Air: A stream consisting of a standard dry air composition:
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Combining Air with Liquid: Once you’ve created the air stream, combine it with your liquid stream (e.g., a water-methanol mixture) using a Mixer object. This simulates the interaction between the liquid and the air. The image below shows 1 kg of water in contact with 1 mole (~24 liters) of air, approximating an open system where the liquid can exchange gases with the surrounding air.
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Checking the Gas Composition: After running the simulation, review the final gas composition. In the example above, the concentration of CO₂ is 0.033 mole%, which is close to the standard atmospheric value of 0.038 mole%. This ensures that the air stream used is sufficient to approximate an open system.
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Adjusting the Air Amount: If the CO₂ concentration is too low (e.g., 0.025 mole%), it indicates that the air stream is too small to properly simulate an open system. To resolve this, increase the amount of air by adjusting the Multiplier value of the air stream in the Definition tab of the Mixer. For example, increasing the air amount to 5 moles (approximately 120 liters) may provide a better approximation, as shown in the following image.
Conclusion
Although true open systems cannot be created in OLI due to mass conservation requirements, it is possible to approximate an open system by introducing an air stream and combining it with your liquid stream. By adjusting the air amount, you can simulate conditions where mass transfer occurs between the liquid and the surrounding air. This method allows for accurate modeling of processes like evaporation or gas absorption, ensuring your simulations align with real-world behavior. If you need further precision, adjusting the air amount can help you match the composition more closely to the expected atmospheric values.
By following this approach, you can effectively simulate air headspace within OLI, making your models more robust and reflective of actual conditions.