Volume Flows - Who's Who
The specification and calculation of Volume Flows in Material Streams is a common practice in engineering calculations. Volume depends on reference pressure and temperature conditions and, the fluid phase.
There are many ways of defining a Volume Flow (standard liquid, standard gas, actual stream conditions); but only one is correct depending on the engineering application. The objective of this article is to guide you in choosing the right option by understanding the different Volume Flow definitions and how to make Volume specifications for bulk or phase flows in the Symmetry process software platform.
Bulk Volume Flows
The Material Stream in Symmetry allows the direct specification of bulk Volume Flows without the need of using control tools like in other process engineering applications. If a Mass or Mole flow is specified, then the Volume Flows are calculated based on actual or reference conditions.
There are three types of bulk Volume Flows that can be specified or calculated:
This is the actual volumetric flow rate at the temperature and pressure defined in the stream. It is calculated using the selected Volume method from the Settings tab of the Property Package configuration window.
For instance, if the selected Property Package is Advance Peng-Robinson (APR), the Volume method for liquid and vapor phases will be based on the volume calculated from the APR equation of state.
Depending on the selected volume method, the calculated value can include volume translation and mixing effects. This volume calculation considers the phases formed by the fluid (Liquid, Vapor, Solid) and uses a mixing rule on the volume phases to calculate the bulk value.
Std Liq Volume Flow
This flow rate assumes that the stream contains a liquid fluid. This computation is based on a molar average of the standard liquid molar volumes of the individual components that comprise the mixture. No volume change on mixing is calculated.
The standard liquid molar volumes of the individual components are calculated using the Rackett equation at the standard liquid volume reference temperature (60 F as default, the value can be changed in the Main Flowsheet settings) and 1 atm.
Also note that some components are supercritical and cannot exist in the liquid phase at standard conditions (for example Hydrogen, Nitrogen, Methane, Ethane, Propane, etc.); for these components, estimated values from GPA Standards are used for their standard liquid molar volumes.
Std Gas Volume Flow
This volumetric flow rate is based on the assumption that the stream is an ideal gas. It represents the volume that an ideal gas would have at the standard gas reference temperature and pressure (60 F and 1 atm as default respectively, the values can be changed in the Main Flowsheet settings).
For instance, the volume that 1 mol of Water would have as an ideal gas at standard gas reference temperature and pressure (60 F and 1 atm) would be:
V = nRT/P = 1 mol * 8.31439 kPa m3 / (mol K) * 288.71 K / 101.325 = 23.69 m3
Let's Take a Quiz
Why is the Volume Flow of a liquid fluid not equal to the Std Liq Volume Flow if the stream conditions are equal to the standard reference conditions?
The Std Liq Volume Flow is calculated using the average molar volume of the components, these molar volumes are calculated with the Rackett equation at reference conditions (except for those who are supercritical at reference conditions). The Volume Flow is calculated based on the active Volume method of the property package and considers volume translation and mixing effects. Since we are using different methodologies for the volume calculation, different results will be obtained.
Even if we calculate the Volume Flow using the Rackett equation for a pure component, there will be a difference with respect to the Std Liq Volume Flow due to the volume translation effect.
Phase Volume Flows
Sometimes engineers need to specify the Volume Flow of a specific phase (Gas, Oil, Water) in multiphase systems. The Symmetry process software platform has this capability and can be found in the Phase Flows tab of the Material Stream, the tab can be activated in the More Properties tab.
If the bulk flow is not specified in the Material Stream, then the Phase Flows tab can be used to specify the flow of a specific phase. Once a phase flow is specified, the other phase and bulk flows are back-calculated based on the stream or reference conditions. The phase flows can be specified in Mass, Mole and Volume (at Stream or Liquid Reference conditions) basis.
Since this article deal with Volume Flows, we will focus our attention on the two options for Volume specification: Volume @ Stream Conds. and Volume @ Ref. Conds.
To understand how these options work let’s use an example that consists on a Material Stream with an equimolar mixture of Methane, n-Octane and Water (each one representing the Gas, Oil and Water phases). The pressure and temperature specified in the stream are 25 C and 100 kPa respectively. For this example, we are using the Advanced Peng-Robinson property package.
Let’s enable the Phase Flows tab and use the Volume @ Ref. Conds. basis option to specify 100 m3/h of the oil phase.
In this case, the mole flows of the gas, water and bulk phases are back-calculated based on the mole flow of oil corresponding to 100 m3/h after flashing the stream at 25 C and 100 kPa. The mole flow of water is converted to volume flow based on the stream conditions, the gas mole flow is translated to standard gas volume flow and the bulk mole flow is specified in the Material port.
The Equilibrium Results tab contains the calculated properties of the fluid flashed at the stream conditions with the bulk mole flow specified from the Phase Flow tab calculations. If we observe the flow rates from the Equilibrium Results tab, the reported Volume Flow of Oil (Liq0 phase) and Water (Liq1 phase), and the Std Gas Volume Flow (Vap phase) validate the calculated values from the Phase Flows tab.
Usually, Volume Flows are measured at specific reference conditions, if the volume needs to be specified at these conditions, then the flow basis needs to be changed to Volume @ Ref. Conds.
When this option is selected, the first thing that is noted is that two new frames are enabled, one to specify the liquid reference conditions and one to calculate ratios based on phase flow results (Gas to Oil ratio, Gas to Liquid ratio, Water Cut).
The liquid reference temperature and pressure can be modified by using the Source option in the Reference Conditions frame, the conditions can apply to the whole flowsheet (Global option) or just to the Material Stream of interest (Local option).
If we specify 100 m3/h of oil at reference conditions (15.6 C and 101.325 kPa), the flowing results are obtained:
The calculation procedure is similar to the previous case but now the mole flows of the gas, water and bulk phases are calculated based on the mole flow of oil corresponding to 100 m3/h after flashing the stream at the liquid reference conditions: 15.6 C and 101.325 kPa.
The volume flows of the Equilibrium Results tab are different than those from the Phase Flow tab because the Equilibrium Results tab has the flashed fluid properties at the stream conditions (25 C and 100 kPa). If we specify 15.6 C and 101.325 in the Summary tab, then we can validate the results from the Phase Flows tab.
The previous example specified the Oil Phase Flow, but any of the other two phases (Std. Gas or Water) can be specified instead. The Phase Flow variables work like any other variable in Symmetry and they can be connected to other tools and applications of the simulator (Case Study, Model Regression, Controller, Process calculator, etc.).
To learn more please contact your local office.
Herbert Loria, Ph.D., P.Eng.