TCE Species Mapping
|Thermodynamic Calculation Engines (TCEs)||TCE Options|
|TCE Functionality Overview||TCE Species Mapping||TCE Configuration Options||AQSol||ChemApp||PHREEQC||OLI|
Latest SysCAD Version: 15 November 2023 - SysCAD 9.3 Build 139.34246
Mapping between SysCAD streams and TCE streams is done as shown in the diagram below. The focus here is on mapping of mass (conserving elemental mass balance). The dotted areas represent the control volumes applicable to each type of thermodynamic calculation. The Unit Operation, or the black control volume, incorporates forward mapping, calculation, and reverse mapping. Unit operations would include reactors, evaporators, reverse osmosis units, solvent extraction units, and more. The Side Calculation, or the red control volume, is used to read one or more streams and perform a specific calculation. It can be used as a virtual sensor in the model, and is often used to determine reaction extents for SysCAD reaction blocks. The Direct Calculation, or green control volume, does not use mapping at all. As mentioned previously, it is a direct interface to the thermodynamic software. The Direct Calculation is a user interface for performing single point calculations and generating trends, similar to how one would typically use thermodynamic software directly.
Each of the thermodynamic calculation engines that link to SysCAD require mapping to convert a SysCAD stream into a representation suitable for the thermodynamic engine. In general, mapping between SysCAD species and ions, and species contained within the thermodynamic model, consists of two steps:
- Forward Mapping is used to convert SysCAD species to species within the thermodynamic model
- All unit operations that take data from SysCAD streams require a forward mapping step. This includes all TCE unit types except Direct Calculations.
- Reverse Mapping is used to convert calculated speciation from the thermodynamic model solution back into SysCAD species.
- When all species predicted by a TCE equilibrium solution are directly mapped to SysCAD species, reverse mapping is quite straightforward. In this case, each species is simply passed into the mapped SysCAD stream species.
- However, when there are ion mappings involved, the situation is much more complex. In this case, the ion breakdown reactions for each SysCAD species are used to reassemble the SysCAD species.
- These are done using specialized algorithms to determine the most likely apparent species from ionic species. Details on this algorithm are provided in Reverse Mapping Algorithms.
The mapping types are:
- Direct mapping: there is a TCE species with matching phase, elemental composition, and charge to that of a SysCAD species
- Ion mapping: there is a TCE species with matching phase, elemental composition, and change to a SysCAD Ion (defined in the IonList.txt file)
- Note that in the case where a species can be directly or ion mapped, direct mapping always takes precedence.
The following applies in Forward Mapping:
- All non aqueous species will be mapped using direct mapping. The exception to this rule is AQSol, where all non-aqueous species are broken down.
- Aqueous species will be mapped using (in order of priority):
- direct mapping, if possible
- ion mapping, if possible, using ions in the IonList file
- All aqueous species which do not have a direct mapping, but have a corresponding ion breakdown pathway, will be broken down into ions using the IonList file
- Any species which do not have direct mapping or ion mapping to the thermodynamic engine will be bypassed and a warning will be issued to the user.
- The Ionlist file is stored within the project cfgfiles folder, see Ion Definitions.
- Please note that ChemApp typically only uses direct mapping, unless an aqueous database is used and the user selects the option to use ions using the Ionlist file.
A typical set of breakdown reactions is given below:
H2O(l) = OH- + H+ H2SO4(aq) = SO4-- + 2H+ H2SO4(l) = SO4-- + 2H+ HCl(aq) = H+ + Cl- Na2SO4(aq) = SO4-- + 2Na+ NaCl(aq) = Na+ + Cl- NaOH(aq) = OH- + Na+ HSO4- = SO4-- + H+ NaSO4- = SO4-- + Na+
Species that are mapped by ions undergo a reaction like this to form an ion that is recognized by the TCE.
- An example of forward mapping to an OLI reactor is shown below:
- In this case, the first access window shows the composition of the SysCAD feed stream. The second access window shows the OLI stream composition that is a result of forward mapping and will be used by OLI to calculate the equilibrium composition.
- In this case, there is a ion map between NaCl(aq) in the SysCAD stream and the Na+ and Cl- ions of the OLI model.
The following applies in Reverse Mapping:
- Used when TCE species need to be converted back into SysCAD species, such as any of the TCE flowsheet unit operations (TCE Reactor, etc.).
- Direct mappings are done in a similar way to forward mapping
- For reverse mapping requiring assembly of TCE ions into SysCAD species, the following steps are done:
- Product ions are first broken down using ionic breakdowns to their primary ions. Primary ions are ions which cannot break down further.
- The primary ions are then used to generate SysCAD species by a user-selected algorithm Reverse Mapping Algorithms
- In this case, the PHREEQC equilibrium composition predicts a number of different ions.
- The reverse mapping algorithm uses various ionic breakdowns to calculate the apparent species, which in this case, is dissolved CaSO4(aq).
- Any additional H+ and OH- species are combined to form water.
- Full elemental balance checks are provided to ensure that all elemental mass has been properly accounted for.
A typical set of reactions involved in the formation of species is given below:
Reaction Reaction Extent (kmol/s) Limiting Reagent SO4-- + 2Na+ = Na2SO4(aq) 0.0001019753503061328 Na+ SO4-- + 2H+ = H2SO4(aq) 4.216684916817301e-13 SO4--
Reverse Mapping Algorithms
The user-selected Reverse Mapping Algorithms are (selected from the TCE_ModelCfg model):
- Simple Extent of reaction
- Using initial vector of primary ions, determine which reversed breakdown reactions, i.e. formation reactions, have the greatest possible extent
- For greatest possible extent, we completely consume the limiting reagent
- Salts First (recommended method)
- Now we first process only species which are salts, i.e. do not contain H+ or OH-
- For this smaller group, do as per Simple extent of reaction
- Then do the same for acids/bases, i.e. those that contain H+, OH-
Option for reverse mapping algorithm:
- RevIonMap.BreakdownFirst: Available from Build 139.32979. When checked (original behaviour), the algorithm breaks all ions into their smallest parts before forming salts, acids, and bases. When not checked (recommended), the algorithm first attempts to form species from ions predicted by the equilibrium solution, followed by breaking down into their smallest parts.
- The recommendation is to use Salts First algorithm and NOT select RevIonMap.BreakdownFirst.
- Acids and bases will react violently to form a salt in almost all cases.
- Therefore, must prioritise salts to avoid possibility of forming bases and acids together.
- Please see example 2 below for a reverse mapping example containing brine.
- In both algorithms, the last reaction to be executed is always the formation of water from H+ and OH-.
- The reverse mapping functionality in SysCAD is a tool to help the user to predict salts and bases or acids which form, but it requires the user to add the appropriate species to the SysCAD species database.
- To assist the user, it provides warnings if acids and bases are present together in a solution, as this is not usually expected. In this case, the user must determine what additional species are required.
|Feed to Reactor||Product reverse mapping using
Simple extent of reaction
|Product reverse mapping using|
Ca++ = 1 mole SO4-- = 2 moles Na+ = 1 mole H+ = 1 mole
|Results order is:
Ca++ + SO4-- = CaSO4 H+ + 0.5 SO4-- = 0.5 H2SO4 Na+ + 0.5 SO4-- = 0.5 Na2SO4
|Resulting order is:
Ca++ + SO4-- = CaSO4 Na+ + 0.5 SO4-- = 0.5 Na2SO4 H+ + 0.5 SO4-- = 0.5 H2SO4
- Using an AQSol example, we have the following feed: 55% Water, 15% H2SO4 and 30% NaCl. The resulting SysCAD species are quite different depending on the reverse mapping algorithm selected, as shown in the following table. P_003 is the feed, P_004 is the product.
Unmapped/Bypassed Species in Forward Mapping
When a SysCAD stream contains a species for which there is no equivalent species in the TCE, it bypasses the TCE solver and is added to an Unmapped stream. The user gets a warning in this case. When a user specifically specifies a bypass, this is added to the Bypass stream. The user does not get a warning in this case, as it is based on a user specification. Both of these streams are added to the final TCE stream.
A third type of stream is the reaction bypass stream. This is used primarily when a TCE (that does not have its own VLE capabilities) is used in conjunction with the SysCAD VLE model. In this case, both a TCE solution and a vapour stream in equilibrium with the TCE solution is produced.
The solution here is to select or generate a different database for use with the TCE which includes the species you are trying to map. Alternately, the user may purposely bypass the species (or ensure they are not in the feed). Note that in the case where SysCAD VLE is used together with a TCE, species may be generated which are not represented within the TCE. These form a reaction bypass stream which is considered part of the TCE solution. This can be used where a TCE is defined for the aqueous phase only.
Unmapped Species in Reverse Mapping
When a TCE solver predicts the formation of a species for which there is no equivalent species in SysCAD, this results in the unmapped species being reported as an error. The model is switched to a side calculation.
The ideal solution here is to add species to the SysCAD database to enable mapping to a SysCAD stream. Alternate solutions include: suppress solids in ModelCfg so they do not form (e.g. trace amounts and/or slow kinetics); check inputs and settings to review if this is a valid case; bypass or use CFE on some species to change the conditions; or use the TCE calculation as a side calculation.
Unmapped Ions in Reverse Mapping
When a TCE solver predicts the formation of ions for which there is no species that can be formed from them in SysCAD, this results in the unmapped ions being reported as an error and the model switched to a side calculation.
The ideal solution here is to add liquid/aqueous phase species to the SysCAD database to enable formation of the SysCAD species from the ions (ions are aqueous). Alternate solutions include: check inputs and settings to review if this is a valid case; bypass or use CFE on some species to change the conditions; or use the TCE calculation as a side calculation.
For example, if the ions Na+ and Cl- are reported as unmapped ions, the solution is to add NaCl(aq) to the SysCAD database.
Common Error Messages
Some of the common errors users may encountered are:
After load of a new project or new TCE database
- Missing Ions: this occurs when there are ions in the TCE database which are not in the IonList.txt file. If this happens, you will be notified by error conditions on the TCEModelCfg (1st tab), as shown in the AQSol example below .
- To fix the error, go to the TCEConfig tab (2nd tab) and press . This action will add the missing ion for you automatically. You will get a notification in the Message Window.
- For example: AMC_001 1 Ions Added
- Then close and reopen the project, on the 2nd tab of TCEModelCfg model, press to make sure all the ions are mapped.
- Ions are mapped to SysCAD species and ions: this occurs when there are ions in the TCE database which can map to both ions in the IonList.txt file and ions defined as SysCAD species in the SysCAD database.
- If this happens, remove those ions from the SysCAD database so that they only occur in the IonList.txt file OR remove those ions from the IonList.txt file so that they only occur in the SysCAD database.
- Then close and reopen the project, on the 2nd tab of TCEModelCfg model, press to make sure all the ions are mapped.
After solve of the project
- Unmapped Species: this usually means the species maybe missing in the database (or not mapped in the TMC model). In the example shown below, CO2 is unmapped, to fix, add CO2(g) to the SysCAD.93.db3 and try again.
- Unmapped Ion: this usually means there are ions left over that cannot reform into an aqueous species. In the example shown below, we have 3 orphan ions, H+, F- and Br-, this would indicate that HBr(aq) and HF(aq) are missing in the database, add these two species to the SysCAD.93.db3 and try again.
The use of TCE calculations at discrete locations within an overall flowsheet introduces the use of different enthalpy models at different locations in the flowsheet. Within any TCE unit, the enthalpy values reported on the Input and Output tabs are generally calculated by the TCE. On the other hand, the enthalpy values reported by SysCAD streams entering and leaving the unit are calculated by SysCAD Species Property Models. We have two systems for enthalpy calculations. It is important to understand this interaction for correct interpretation of simulation results.
The best location for a quick check on comparative enthalpy values between TCE and SysCAD is in the Reverse Mapping Summary display on TCE results tab page where. This shows, after a successful mass reverse mapping, the enthalpy reported by TCE and SysCAD displayed side by side together with the difference. Ideally these numbers should be close for consistency across the flowsheet between the two systems. Where there are significant differences (and the user accepts the TCE Output stream and species mapping as valid), then the user should investigate updating the project SysCAD species database enthalpy (Cp) data to achieve better compatibility between the systems.
For some cases, depending on TCE used, stream composition, input stream mixing and settings in unit model used, the displayed Input TCE enthalpy value may not necessarily reflect the expected or valid enthalpy value from the TCE. Take care using the reported TCE Input stream enthalpy value - some understanding of the TCE enthalpy calculations and the specific use case is required.
The PHREEQC TCE does not include an enthalpy model. For PHREEQC all enthalpy values are calculated by SysCAD. It is therefore recommended that the user takes care entering and reviewing SysCAD species database enthalpy (Cp) data for species of interest. The PHREEQC Input and Output enthalpy values reported are simply a copy of the SysCAD stream enthalpy before/after mapping. Completely successful mapping is required to use these reported enthalpy values.
Enthalpy Values Displayed
The figure below, for a simple case, shows the calculation of enthalpy during a TCE calculation. The four enthalpy values indicated are:
- Feed SysCAD enthalpy
- Feed (Input) TCE enthalpy
- Product (Output) TCE enthalpy
- Product SysCAD enthalpy
It is important to note in this example that the calculated enthalpy values are not equal, but are quite close. This is the desired case, and expected when there is "good" enthalpy (Cp) data for species in the Species Database for the project. If there are significant differences, the SysCAD species data for the project should be reviewed!
Individual versus combined streams
In many cases, the way that streams are added will affect the calculated TCE feed enthalpy, and thus, the results. For streams which are expected to react when mixed in the reactor, users should have these as separate input streams to the unit, rather than combining them into a single stream. This is because for many situations, it is appropriate to calculate the TCE enthalpy of the incoming stream as if it were at equilibrium. Consider the following example where sulphuric acid solution and lime slurry are being added to a tank using OLI. In both cases shown below, EquilibrateFeedStreams is checked in the OLI Reactor 2 TCE Tab Page, meaning that the equilibrium composition of the feed stream(s) at the specified temperature and pressure is used to calculate the feed stream enthalpy.
Below, a tie has been used upstream to mix two streams containing acid and lime. No reactions are defined in the tie. This is incorrect and the temperature of the TreatedSolution (OLI reactor output) is predicted to be 25.1 C:
In this example, the two streams are added directly to the OLI reactor, this is correct and the TreatedSolution temperature (OLI reactor output) is predicted to be 43.6 C:
For the first case, the OLI reactor calculates the TCE equilibrium enthalpy of the single inlet stream. For the second case it separately calculates the TCE equilibrium enthalpy for each inlet stream, which gives a total TCE feed enthalpy different to the first case.