Evaporation Block (Evap)

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General Description

This uses a relatively simple method of representing Evaporation from a unit.

Please note that the Evaporation sub-model does not remove the evaporated species from the unit, they remain with the stream. If the unit has a vent stream connected, or a Splitter model configured to remove the gas, then the vapour will exit in the gas stream. Otherwise the vapours will exit with the slurry stream.

The first variable that the user can configure is 'Model'. The options are:

1. None: - There is no Evaporation from the unit.
2. Water - The user specifies Evaporation of water (H2O(l)) only.
3. MultiComponent - The user may specify the Evaporation of all species that are defined with both liquid and gaseous phases in the project, e.g. NH3(l) and NH3(g).

The following methods for specifying the Evaporation are available for both the 'Water' and 'MultiComponent' methods:

1. Fraction Evaporated: The user specifies the fraction of the species in the feed that is evaporated.
2. Mass Evaporated: The user specifies the mass of the species in the feed that is evaporated.

The following additional methods are available for the 'Water' method only (and if energy balance is enabled):

1. Surface Area (Holman): The model will calculate the amount of water evaporated based on the surface area, ambient temperature, relative humidity and wind speed. Please see Holman Surface Area Calculation for the equation used.
2. Surface Area (ASHRAE): The model will calculate the amount of water evaporated based on the surface area, ambient temperature, water temperature, relative humidity and wind speed. Please see ASHRAE Surface Area Calculation for the equation used.
3. Evaporation Energy: The user specifies the energy to be used to calculate evaporation. This is the total amount of energy that can be used to evaporate the water. If the constant temperature option is used with this method, then this is equivalent to the Energy Addition method described below.
4. Temperature Drop: The user may specify the required temperature drop, SysCAD then calculates the amount of water to be evaporated to achieve this.
5. Final Temp: The user may specify the final temperature of the material exiting from the Evaporation sub-model, SysCAD then calculates the amount of water to be evaporated to achieve this.
6. Energy Addition: The user specifies how much energy is added to the unit to cause Evaporation. SysCAD then calculates the amount of water to be evaporated based on the external energy added, the stream temperature will be maintained.

The user may force a constant temperature (ForceConstT) across the Evaporation sub-model for the following methods:

• Fraction Evaporated
• Mass Evaporated
• Surface Area
• Evaporation Energy

Model Theory

Energy Balance

• For the Fraction Evaporated, Mass Evaporated and both Surface Area methods with ForceConstT option disabled, the amount of evaporation is determined and then SysCAD performs an energy balance to determine the final temperature with no energy being added to the model.

• If the ForceConstT option is used, the feed temperature will be maintained and SysCAD will calculate the energy required to be added to the model to maintain the temperature. This energy is reported as the HeatFlow.

• If energy balance is switched off in the project then the feed temperature will be maintained.

• The Temperature and Energy methods (Evaporation Energy, Temperature Drop, Final Temperature and Energy Addition) are only available if the Model chosen is Water. For these methods, SysCAD uses the energy difference between the water and steam (Latent Heat of Vaporisation) to determine how much evaporation occurs (Note: none of these methods are available if energy balance is switched off in the project).

• For the Temperature methods, the final temperature will be as specified by the user.

• For the Evaporation Energy method with ForceConstT option disabled, the final temperature will be determined by energy balance.

• For the Evaporation Energy method with ForceConstT option enabled or Energy Addition method the energy specified is the energy added to the model (reported as HeatFlow) and the feed temperature is maintained.

Holman Surface Area Calculation

The amount of evaporation is calculated in mm/s using the following correlation:

[math]\displaystyle{ \mathbf{\mathit{E \; = \; \cfrac{[0.7*(3.2127 + 1.9224u) \; * \; (p_s - p_w)^{0.88}]}{86400}}} }[/math]

where

E = evaporation (mm/s)

u = daily wind movement (m/s)

p_s = saturation vapour pressure of water at the air temperature (kPa)

p_w = actual vapour pressure of water = ps * relative humidity of the air (kPa)

The Mass of water evaporated is calculated as follows:

[math]\displaystyle{ \mathbf{\mathit{M_e=\cfrac{E * Area * \rho}{1000}}} }[/math]

where

M_e = Mass of water evaporated (kg/s)

Area = User defined area of the unit (m^2)

[math]\displaystyle{ \rho }[/math] = Density of water in the unit, i.e. at the feed temperature (kg/m^3)

Application: NB this correlation is for evaporation from bodies of water and is not applicable to situations where the evaporating liquid temperature is significantly different than the free stream air temperature.

Reference: Holman J.P. Heat Transfer, SI Metric Edition, pp 595, McGraw-Hill 1989. Original reference: Kohler, M.A., Nordenson, T.J.and Fox, W.E. (1995) Evaporation from Pans and Lakes. US Department of Commerce and Research Paper 38.

ASHRAE Surface Area Calculation

The amount of evaporation is calculated in kg/s using the following correlation:

[math]\displaystyle{ \mathbf{\mathit{w_p=\cfrac{A}{Y} * (p_w - p_a) * (C_1 + C_2 * V)}} }[/math]

where

w_p = evaporation of water (kg/s)

A = user defined surface area of the unit (m²)

V = air velocity over water surface (m/s)

Y = Latent Heat at the surface water temperature (kJ/kg)

p_a = saturation vapour pressure of water at the air dew point (kPa)

p_w = saturation vapour pressure of water at the surface water temperature (kPa)

C₁ = 0.089 W/(m².Pa)

C₂ = 0.0782 W.s/(m³.Pa)

Reference: 1999 American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Applications Handbook.

Limits

• Limits are set on the maximum fraction evaporated and minimum product temperature.
• The maximum fraction evaporated (relevant for all methods except Fraction Evaporated) prevents the relevant species from being evaporated past a specified fraction independent of other user specified requirements. This prevents all the material being evaporated due an unforeseen circumstance such as a small feed flow.
• Similarly, the minimum product temperature prevents the relevant species from being evaporated if it was to result in the final product temperature being below the specified minimum (the default being the ambient temperature for the project). This prevents unrealistically low product temperatures being obtained due to large amounts of evaporation relative to the whole stream. This limit is not relevant if the Final Temperature method is used or the ForceConstT option is enabled.
• If either of these limits are reached the unit will display a condition warning unless the ShowLimitMsgs option is disabled.

Hints

• The implemented evaporation equation may only be suitable for certain operations. For example, the Holman Surface Area equation is insensitive to the temperature of the feed stream, so this method may be best for unit operations that are at, or close to, ambient temperature. Users should investigate the methods (by reading the references articles) to decide if they are suitable for their operation before accepting the results.
• If User-Defined Evaporation Calculation functions available, the user can set the method to Mass Evaporated (Fixed), and then use a PGM file to calculate and set the evaporation rate. For an example of how to implement custom evaporation correlation, please see Example Evaporation Correlation.

Data Sections

A description of the variables on the Evap tab page is given here.

Tag (Long/Short)	Input / Calc	Description/Calculated Variables / Options
Evap
Model	None	There is no Evaporation from the unit.
	Water	The user specifies Evaporation of water (H2O(l)) only.
	MultiComponent	The user may specify the Evaporation of all species that are defined with both liquid and gaseous phases in the project.
Requirements
On	Tick Box	This enables the Evaporation sub-model. If this is NOT ticked, then no evaporation occurs and the sub-model is ignored.
Method	Fraction Evaporated	The user specifies the fraction of the specified species - either Water only, or water and other species - in the feed that is evaporated.
	Mass Evaporated	The user specifies the mass of the specified species - either Water only, or water and other species - in the feed that is evaporated.
	Surface Area (Holman)	The user specifies the surface area of the unit, and they may also specify the ambient conditions, or use the default values in the project (Set at Plant Model - Environment). This is only available if the Model chosen is Water.
	Surface Area (ASHRAE)	The user specifies the surface area of the unit, and they may also specify the ambient conditions, or use the default values in the project (Set at Plant Model - Environment). This is only available if the Model chosen is Water.
	Evaporation Energy	The user specifies the energy of evaporation. This is the total amount of energy that can be used to evaporate the water. If the constant temperature option is used then this is equivalent to the Energy Addition method described below. This is only available if the Model chosen is Water.
	Temperature Drop	The user may specify the required temperature drop due to Evaporation. This is only available if the Model chosen is Water.
	Final Temperature	The user specifies the product temperature from the Evaporation sub-model. This is only available if the Model chosen is Water.
	Energy Addition	The user specifies how much energy is added to the unit to cause Evaporation. Since external energy is being added, the feed temperature will be maintained. This is only available if the Model chosen is Water.
ForceConstT	Tick Box	The user may force the sub-model to maintain a constant temperature, i.e. Product Temperature = Feed Temperature and the energy for evaporation is added to the unit. This option is only available if any of the following methods are selected: Fraction Evaporated, Mass Evaporated, Surface Area or Evaporation Energy.
FracReqd	Input	The Mass Fraction of the Feed flow of the required species that must be evaporated. Visible if Fraction Evaporated is the selected Method.
MassFlowReqd / QmReqd	Input	The Mass flow of the required species that must be evaporated. Visible if Mass Evaporated is the selected Method.
SurfaceArea	Input	The surface area of the unit. Visible if Surface Area is the selected Method.
UseAmbientData	Tick Box	Visible if Surface Area is the selected Method. If this is enabled then the project data for Air Temperature, Wind speed and Relative Humidity will be used. This data is set and viewed at Plant Model - Environment. If this is NOT enabled, then the user may override the data using the fields below.
AmbientTemp	Input	The temperature of the ambient air. This field is only visible if Surface Area is the selected Method and UseAmbientData is NOT enabled.
WindSpeed	Input	The ambient wind speed. This field is only visible if Surface Area is the selected Method and UseAmbientData is NOT enabled.
RelativeHumidity	Input	The relative humidity of the ambient air. This field is only visible if Surface Area is the selected Method and UseAmbientData is NOT enabled.
EvapEnergyReqd	Input	The energy used for evaporation. Visible if Evaporation Energy is the selected Method.
TempDropReqd / TDropReqd	Input	The temperature drop caused by evaporation. Visible if Temperature Drop is the selected Method.
FinalT_Reqd	Input	The product, or Final, temperature from the Evaporation sub-model. Visible if Final Temperature is the selected Method.
EnergyAdditionReqd	Input	The amount of energy added to the Evaporation sub-model. Visible if Energy Addition is the selected Method.
Limits
MaxFraction	Input	The maximum fraction of the relevant species in the feed stream that may be evaporated. This is not shown if Fraction Evaporated is the selected Method.
The TMin_Method field is not shown if the Method chosen is Final Temperature or the ForceConstT option is enabled.
TMinMethod	Ambient	The minimum allowed temperature from the Evaporation sub-model is the Ambient temperature. (This is specified in Plant Model - Environment Tab).
	User	The user specifies the minimum allowed temperature from the Evaporation sub-model.
TMin	Input	The user defined Minimum Temperature from the Evaporation sub-model. This is only visible if User is the TMin Method specified.
TMinUsed	Calc	The minimum allowed temperature used by the Evaporation sub-model. This field is not visible if the ForceConstT option is enabled.
ShowLimitMsgs	Tick Box	If this is disabled then no messages will be displayed by the sub-model, e.g. if a limit is encountered, then the message will not be displayed.
Results
State	Display	The state of the sub-model. If the configuration of the Evaporation sub-model is good and everything is working correctly, the State will be 'OK'. Otherwise, it will display any deviations from requirements, as shown to the left.
EvapMassFlow / EvapQm	Calc	The total mass of the species that has been evaporated.
EvapFrac	Calc	The fraction of feed material that has been evaporated.
Feed.MassFlow / Feed.Qm	Calc	The mass flow of feed material to the Evaporation sub-model.
Feed.Qm.Water(l) / Feed.Qm.H2O(l)	Calc	The mass flow of water in the feed to the Evaporation sub-model. This is only visible if the selected model is 'Water'.
Feed.Qm.Steam(g) / Feed.Qm.H2O(g)	Calc	The mass flow of water vapour (steam) in the feed to the Evaporation sub-model. This is only visible if the selected model is 'Water'.
Pressure / P	Calc	The operating pressure of the Evaporation sub-model.
Feed.Temperature / Feed.T	Calc	The Temperature of feed material to the Evaporation sub-model.
TempDrop / TDrop	Calc	The Temperature drop across the Evaporation sub-model.
Prod.Temperature / Prod.T	Calc	The product Temperature from the Evaporation sub-model.
Iterations	Calc	The number of iterations required to solve the Evaporation sub-model.
AmbientTempUsed	Calc	Only visible if Method chosen is Surface Area. The ambient temperature used in the calculations.
WindSpeedUsed	Calc	Only visible if Method chosen is Surface Area. The wind speed used in the calculations.
RelHumidityUsed	Calc	Only visible if Method chosen is Surface Area. The relative humidity used in the calculations.
WaterLatHtVap	Calc	The latent heat of vaporisation of water at the operating pressure of the sub-model and the product temperature. This is only visible if the selected model is 'Water'. Note: This calculation ignores partial pressures.
WaterSatT@P	Calc	The saturation temperature of water at the operating pressure of the sub-model. This is only visible if the selected model is 'Water'.
HeatFlow	Calc	The amount of energy added to the model. This is relevant if the ForceConstT option is used or if the Energy Addition method is used.