# Reaction Block - Other

Navigation: Models ➔ Sub-Models ➔ Reaction Block (RB) ➔ Source, Sink, Heat Exchange

Reaction Format RB Data Section RB Sub Model (Model Theory)
Reaction Editor Text File Format Reaction Block
(RB) Summary
Individual Reactions Reaction Extents Source / Sink /
Heat Exchange
Solving Order -
Sequential or Simultaneous
Energy Balance Heat of Reaction / Heat of Dilution / Partial Pressures

## Reaction Source

• SysCAD allows the user to define a single Source per Reaction Block. (For how to add a source using reaction editor, please see Adding a New Source)
• The Source can contain any number of species.
• Note the Source species may or may not also be added as part of the feed.
• Reactions using the Source species should not use the extent 'Fraction' = source species, as this may have an infinite number of solutions.
• Water (H2O(l)) and steam (H2O(g)) are not allowed as reaction sources.
• NB Some units which allow reactions do not allow reactions with sources. SysCAD will give the error "Reaction block Sources not allowed - Removed" in the message window. The Precipitator3 and ShellandTube2 models do not allow sources.
• There are three options for Source species. They can act as only a Source of material, only as a Recycle species, or as both as required.

#### Source Only Species

• SysCAD will add the Minimum amount of the Source species to satisfy all of the reactions in which they are used.
• For energy balance calculations, the energy added to the system is calculated at Feed conditions unless the user sets the Source temperature and pressure.

Note: SysCAD will not add recycle species produced in later reactions, it will only add additional species to satisfy all reactions. This may lead to more source species being added then if source and recycle were allowed.

#### Recycle Only Species

• In some cases a species will be a reactant in an early reaction, but then will be a product in a later reaction.
• In these cases the user would often like the produced species to be available for the earlier reactions.
• This option tells the Reaction Block that the species can be 'recycled' to earlier reactions.

Note: SysCAD will not add species as a true source to satisfy all of the reactions in which they are used, it will only allow species to be recycled. If the later reactions do not produce enough of the species, then the earlier reactions will not meet their target extents.

#### Source and Recycle

• Sources can act as both a true source of material, or as a recycle species.
• Sources and Recycles act as described in the previous sections.
• In this case, the species can act as either a true Source, only as a Recycle, or as both, as is required to meet the target reaction extents.

### Example

Reaction 1: 4 FeSO4(aq) + O2(g) + 2 H2SO4(aq) -> 2 Fe2[SO4]3(aq) + 2 H2O(l)
Reaction 2: 3 Fe2[SO4]3(aq) + S(s) + 4 H2O(l) -> 6 FeSO4(aq) + 4 H2SO4(aq)

H2SO4(aq) is consumed in reaction 1 and is produced in reaction 2.


Depending on the relative amount of material in the feed and the desired reaction extents, the following scenarios are possible:

1. There is enough H2SO4(aq) in the feed to satisfy reaction 1, addition of H2SO4(aq) is NOT required
2. There is not enough H2SO4(aq) in the feed to satisfy reaction 1, but Reaction 2 will produce enough H2SO4(aq) to satisfy the demand required by Reaction 1. So Recycle is required.
3. There is not enough H2SO4(aq) in the feed to satisfy reaction 1, and Reaction 2 does not produce enough H2SO4(aq) to satisfy the demand required by Reaction 1, so Source and recycle is required.

The best option for this situation is to select the "Source and Recycle" option, as it will recycle the H2SO4(aq) in reaction 2, and top up if there is not enough H2SO4(aq) to satisfy both reactions.

NOTES:

• If H2SO4(aq) is NOT specified as a Source or Recycle species, then Reaction 1 will not reach its target extent in scenarios 2 and 3.
• If H2SO4(aq) is specified as a Recycle only species, then Reaction 1 will reach its target extent in scenario 2, but in for scenario 3.
• If H2SO4(aq) is specified as a Source species, then Reaction 1 will reach its target extent in all scenarios. However, it will not account for the recycle amount from reaction 2, so therefore will add more H2SO4(aq) than required.
• For source and/recycle to work, the reactions must be in sequential order solver mode (ie, not simultaneous).

## Reaction Sink

• SysCAD allows the user to define a single Sink per Reaction Block. (For how to add a sink using reaction editor, please see Adding a New Sink)
• The Sink can contain any number of species and is used to remove the species from the unit AFTER the reactions are complete.
• Note that the species does not need to participate in the reactions.
• For energy balance calculations, the energy removed from the system is calculated at Product conditions (but prior to any Heat Exchange).
• Water (H2O(l)) and steam (H2O(g)) are not allowed as reaction sinks.

## Reaction Heat Exchange

• The user may add a single Heat Exchange per Reaction Block. (For how to add a Heat Exchange using reaction editor, please see Adding Heat Exchange)
• This is used to model additional heat loss or gain from the reaction block.
• There must be at least one reaction in the file for Heat Exchange to work.
• The amount of heat added or subtracted is given as the Heat Gain.
• Heat added to the reaction block is given as a positive number.
• Heat removed from the reaction block is given as a negative number.
• If a Reaction Sink is specified, the Heat Exchange is applied after material is sent to the sink.

HINT: If you want to add energy into the unit but do not want any reactions to occur. Just define a dummy reaction and set the reaction extent to 0. Then

1. define the Heat Exchange using of the specified methods. OR
2. use the Override Product Temperature option.

The following methods are provided:

Method Description Example

#### FinalT

• The FinalT method allows the user to specify the Final Temperature from the Reaction Block.
• This method adds or removes heat from the reaction block in order to achieve the required product temperature.
• There is an option to set FinalT = Feed temperature. This ensures that there is no change in temperature across the reaction block.

#### Power

• The Power method allows the user to specify a fixed energy, or power, addition to or from the Reaction Block.
• A positive number will add energy and hence heat the unit.
• A negative number will remove energy and cool the unit.

#### Approach

• The Approach method allows the user to specify a fractional approach to a Target Temperature from the Reaction Block.
• The user specifies the follwoing:
• Target temperature to be approached,
• The fractional approach, and
• The basis for the calculation (Feed or Product).
• The final temperature is calculated using the following formula:
$\displaystyle{ ApproachT = \cfrac{(FinalT - BasisT)}{(TargetT - BasisT)} }$
where
ApproachT = fractional approach (user specified)
FinalT = final temperature out of reaction block (to be determined)
TargetT = target temperature (user specified)
BasisT = the basis temperature
• If the user has chosen a basis of Feed, then BasisT = Reaction Block Feed Temperature.
• If the user has chosen a basis of Product, then BasisT = Reaction Block Product Temperature if there was no heat exchange (ie. after the reactions have taken place). The default Basis is Product.
• The Target temperature may be specified as "EnvironmentT". The target temperature will then be equal to the environment temperature specified in the PlantModel. This method allows the user to estimate the amount of heat loss or heat gain to/from the environment.
For a Target of 50 °C, an Approach of 50% and Basis = Feed (assuming the feed temperature entering the reaction block is 30°C), then the Final Temperature is calculated as follows:
$\displaystyle{ ApproachT = \cfrac{(FinalT - FeedT)}{(TargetT - FeedT)}*100 \% }$
$\displaystyle{ 50\%= \cfrac{(FinalT - 30)}{(50- 30)}*100\% }$
FinalT = 40°C (ie. 50% of the way from 30 to 50°C)

If an Approach of 1 (100%) is used then the Target temperature will be achieved. In this case the FinalT method could instead be used.

#### Electrolysis

• The Electrolysis method can be used to simulate electrowinning cells.
• This method allows the user to specify a cell efficiency for the unit.
• If the cell efficiency is 100% sufficient heat is added to the reaction block to maintain the product temperature at feed conditions.
• If the user specified a cell efficiency of less than 100%, the excess energy will be used to heat the product.

Thus the Cell Efficiency is defined as follows:

Cell Efficiency (%) = (Energy needed to maintain feed T)/(Energy exchanged) * 100%

If a cell efficiency of 100% is used then the product temperature = feed temperature.

Cell efficiency of 90%, feed temperature is 100°C and the energy required to maintain the feed temperature is 22500 kJ.

Total energy added = 22500/0.9 = 25000. The extra 2500 kJ energy will heat up the cell further. Thus, the product temperature > feed temperature.