Driving Force Analysis (DFA)
1. PurposeDriving Force Analysis (DFA) is a tool that allows the user to build a qualitative model of the process, enabling rigorous analysis of systems with competing rate processes. Driving Force Analysis helps identify systematically:
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2. Information requirements
| Chemical processes | Physical processes |
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Rate processes (mechanisms, etc, from Transformation Map) Influence of:
Rate of reaction (qualitative is sufficient, but capture best quality of information available) Heat of reaction Equilibria Catalysis
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Rate processes (from Transformation Map)
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3. Procedure
3.1 Develop a Driving Force Table
| 1. The table has one column for each rate process identified in the Transformation Map. |
| 2. The table has one row for each influencing factor. |
| 3. It can be helpful to colour desired rate processes in green and undesired rate processes in red. To aid analysis of the table, it can help to put competing rate processes in adjacent columns. |
| 4. Populate the table by asking what would happen to the rate of the process being considered if the influencing factor was increased (e.g. impact of increase in concentration, temperature, etc). Use question marks to record unknown information, or in conjunction with other symbols where these represent an instinctive response. |
| 5. Note best information on rates and heats of transformations. Again, use question marks when appropriate. |
3.2 Driving Force Table symbols
| Description / example |
Suggested symbol |
| This driving force drives the rate process forwards, with normal (first order) dependency | + |
| This driving force drives the rate process forwards, with stronger (second order) dependency |
++ |
| This driving force drives the rate process forwards, with a weaker (fractional order) dependency | (+) |
| This driving force makes the forward rate process slower, or drives the reverse rate process | - -- (-) |
| This rate process needs this to be present, but the value of the driving force has no effect on the rate (zeroth order) | 0 |
| The rate reaches an asymptote as the value of this driving force rises (e.g. effect of pH near the pKa for protonatable species) |  |
| The rate goes through a maximum or a minimum as the value of this driving force rises |  |
| The effect of this driving force is unknown or uncertain | ? |
| This species is the product of this rate process |
P |
| The rate of this process depends on equipment design and operation (e.g. mass transfer processes) | Adj |
3.3 Analyse the Driving Force Table
Step 1: Look for differences between desired and undesired columns
- Effect of reagent concentrations
- Effect of other driving forces
- Relative rates
- Heat, pressure, etc
Focus on key branch points between desired and undesired rate processes.
Step 2: Record differences as potential operating strategies
- Keep PhCOOEt concentration high to favour [R1]
- Remove Ph2CO as it forms to avoid [R2]
- Exclude H2O to avoid [R3]
- Exclude O2 to avoid [R4]
Keep T down to avoid [R2]
Step 3: Consider how the operating strategies could be realised
- How can these conditions be delivered?
- Which strategy/ies offer the biggest benefit?
Step 4: Define proof of concept experiments
| Operating Strategy | Possible Process Concept |
| Keep PhCOOEt concentration high to favour [R1] | Use fed batch with PhMgBr added to PhCOOEt in the reactor |
| Remove Ph2CO as it forms to avoid [R2] | Probably not possible |
| Exclude H2O to avoid [R3] | Dry solvent and process materials; take care to exclude moisture |
| Exclude O2 to avoid [R4] | Use more thorough N2 inerting than usual |
| Keep T down to avoid [R2] | Experimentation needed to identify appropriate T |
3.4 DFA for multiphase processes
The same procedure above should be followed. However the Driving Force Table should be partitioned by phase, and all inter-phase mass transfer processes should be shown, each mass transfer process as a separate column. Note that the rate of mass transfer is affected by the choice of equipment and equipment operating parameters, and the rate for mass transfer processes should therefore be shown as "variable".
| 1. Capture information on desired and undesired rate processes in the major phase |
| 2. Insert column(s) for the mass transfer rate process(es) between two phases and populate with information |
| 3. Capture information on desired and undesired rate processes in the next phase |