Table of Contents
What is a Material Balance Group (MBG)?
Comparison: MBG vs. True Species vs. Apparent Species
Example: NaCl-CH4-NaCN-H2O Stream
Objective
This article explains the concept of Material Balance Groups (MBG) in OLI software.
MBG variables are used to quantify and track the total amount of an element at a specific oxidation state, or at times, a collection of elements, across all phases.
This concept applies consistently across all OLI products, including OLI Studio: Stream Analyzer, OLI Flowsheet: ESP, OLI Cloud APIs, and the OLI Engine.
What is a Material Balance Group (MBG)?
A Material Balance Group (MBG) represents the total of all species that share a common chemical identity, such as:
- The same oxidation state of an element, or
- A specific functional or organic group that behaves as a conserved unit in mass balance calculations.
While MBGs are most often associated with individual element oxidation states (e.g., Fe(+2) vs. Fe(+3)), they can also apply to:
- Chemical groups (e.g., CH4, C6H13S-1, or CN-1) that represent a conserved mass balance unit (typically organic chemical groups)
OLI uses MBGs internally to maintain accurate mass and charge balance across all system species and phases, enabling consistent accounting of redox transformations, complexation, and molecular group conservation. This representation is helpful for applications in wastewater treatment or rare earth extraction to track ions and contaminants across phases, or for the CCUS industry to monitor changes in oxidation state through a process.
Comparison: MBG vs. True Species vs. Apparent Species
| Concept | Definition | Example | Primary Use in OLI |
| Material Balance Group (MBG) |
A sum of either a) all species of an element in a particular oxidation state, or b) all species that share a group of elements.
|
Na(+1), Cl(-1), CH4, CN(-1) | Tracks redox or chemical group totals across all phases. |
| True Species |
The chemically distinct entities (ions, complexes, molecules) actually present in the phase according to the OLI thermodynamic model.
|
Na(+1), OH(-1), H3O(+1), NaOH | Used for equilibrium calculations and detailed chemical speciation. |
| Apparent Species | A molecular, ion-paired representation of the True Species in a system. This is typically a non-physical representation of the system, important for maintaining electroneutrality. | NaCl, H2O, HCl | Used for reporting and converting True Species to neutral molecules for subsequent stream Inflows. This is particularly helpful for the OLI Engine in Alliance software, where molecular species representation is required. |
For more information on True Species and Apparent Species, please refer to our Support Center article.
Example: NaCl-H2O Stream
For a stream of 55.5082 moles of H2O and 1 mole NaCl, OLI predicts the following True Species, Apparent Species, and Material Balance Groups (referred to as “Element Balance” in the OLI Studio Report)
The total moles reported for True Species is 57.5082, whereas the Apparent Species table reports 56.5082 total moles. This discrepancy exists because the True Species table accounts for the dissociation of NaCl in H2O, which generates 1 mole of Na(+1) and 1 mole of Cl(-1).
In contrast, the Apparent Species representation maintains the ion-paired, neutral form of the system; in this case, the Apparent Species list includes 1 mole of NaCl.
The MBG (Element Balance) table outlines the unique oxidation states in the system.
Please note: to view the list of Apparent Species in the OLI Studio Report, please select the Customize button > check the Molecular Output checkbox.
Example: NaCl-CH4-NaCN-H2O Stream
For a stream of 55.5082 moles of H2O, 1 mole NaCl, 1 mole CH4, and 1 mole NaCN, OLI predicts the following True Species, Apparent Species, and Material Balance Groups:
In this example, the MBG table includes individual element oxidation states, as well as CH4 and CN(-1). Both CH4 and CN(-1) exist as consolidated units for mass balance in OLI.
Conclusion
The Material Balance Group (MBG) is a key OLI variable that represents a conserved chemical grouping across all phases. It typically corresponds to the total of an element in a particular oxidation state but may also represent a functional group or organic unit. Understanding MBGs—alongside True and Apparent Species—helps users interpret chemical transformations and assess redox or mass or ion balance.