Units Configuration

In Units Configuration, you may select the units of measurement for the reaction system.

When changing units, numerical values entered in the project are converted accordingly in order to keep consistency with the original model. This automatic adjustment of values is done for most of the data. However, in complex models, such as those involving pseudocompounds, you may need to do some manual modifications afterwards. More details about this are available in the Notes below.

Units

The Units view displays the list of variables with their units. The available unit choices for the basic variables are:

• Time: Hours, Minutes (Default) and Seconds.
  
• Temperature: Celsius (Default), Kelvin and Fahrenheit.
  
• Pressure: Atm (Default) and Bar.
  
• Volume: Liters (Default), Milliliters and m³.
  
• Mass: Grams (Default), Kilograms and Milligrams.
  
• Moles: gmol (Default), kmol, mmol.
  
• Specific Energy: kJ/mol (Default), kCal/mol.
  
• Surface Area: cm² and m²(Default).
  
• Rate Basis: Volume (Default) and Catalyst Mass.
  
  
  Select Volume if the rate is determined per unit volume of the reaction phase, which is always true for homogeneous reactions. The reaction rate units will be (moles unit)/ [(time unit)*(volume unit)] .
  
  Select Catalyst Mass if the rate is determined per unit mass of catalyst, which is often used for solid-catalysed reactions. The rate units here will be (moles unit)/ [(time unit)*(mass unit)].
  
  
• Concentrations: Molarity (Default), Partial Pressure and Mass Density. The unit of the overall rate is independent of whether partial pressure or molarity is chosen for the concentration basis. The units of the rate constant will change depending on this choice.
  
  

Options

The option Convert Data is selected by default through the icon in the toolbar.
When enabled, any change in the Units will automatically convert numerical values in the project to preserve model consistency. If disabled, any changes in the Units view will not affect numerical values in the project.

Notes:

• On changing the units, the values loaded in several nodes will change accordingly. For example, for a batch reactor, if the Time unit is converted in this view, the PreExponential values are adjusted, along with values of experimental variables such as the batch time, Inflows and Outflows.
  
• During conversion of pre-exponential factors, when pseudocompounds are present in the rate expression or LHHW term, the converter assumes that the pseudocompounds have exactly the same units as a normal compound. The preexponential is then converted to the new units under this assumption, which is made irrespective of whether the pseudocompound is conserved or not. Therefore, the conversion is accurate only when the pseudocompound is a simple linear combination of compounds without a constant factor. When this condition does not apply, the pre-exponential conversion may be inaccurate and serves only as a mechanism for initialization.
• No changes are done in the nodes below. Therefore, after converting the units, you must review your project to see if further manual changes must be done in the following nodes:
  
  • Chemistry → PseudoCompounds
    
  • Estimation/Optimization → Reactor → Derived Quantities
    
  • Estimation → Parameters → Relationships
    
  • Estimation → Weights → Sets
    
  • Estimation → Run → Solution Options → Profile Relationships
    
  • Optimization → Objective
    
  • Optimization → Constraints
    
  In these nodes, the data values or the formulas are not automatically adjusted to the new units. You may need to modify the values here, in order to keep the model equivalency to the original project. For example, the weighting factors may need to be increased by a factor of 1000 (or regenerated), when we convert molar values from millimoles to moles, if you wish to keep the same order of magnitude for the Least Squares objective function.
  
• Experimental values of PseudoCompounds and Derived Quantities are not changed, so you may need to revise them if used in your project.
• The ability to change the Concentration basis of the rate equation from Partial Pressure to Molarity or vice-versa is also available here. This conversion is done mainly for the purpose of re-initializing the kinetic parameters at an average temperature. Therefore, it is not an exact conversion. For example, if converting from partial pressure to molarity, the preexponential conversion depends on a (RT)^n factor, where R is the ideal gas constant; n is the sum of all orders for the rate equation(or site term), and T is the temperature. In this factor, T is substituted with the value of the average Temperature among all included sets and datapoints. Thus, the simulation of a given set will give same results after the change of Concentration basis units only if the set has the same temperature at all datapoints, and they are all equal to the average temperature. In general, this is guaranteed only for isothermal experimental sets.
  As for Concentration change from or to Mass Density, no conversion is done on any parameter or project data.
• The values in the Results tree are not modified, so you must re-run the project to get the numerical results with values in the new units.
  

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