Multi-Effect Evaporation: Effects Tab

ContEvaporationEffects.jpg

This tab appears on the interface dialog of the Continuous Multi-Effect Evaporation operation.

Variable Definitions, Ranges and Default Values

The following table shows a brief description of the variables appearing in this tab; for a detailed description of these variables, see Multi-Effect Evaporation: Description of Terms The table also displays their default values and their generally acceptable range. Note that due to how the variables are used in the modeling equations, the range of acceptable values may be further reduced; for more details, see Multi-Effect Evaporation: Modeling Assumptions & Constraints.

Variable

Default Value

Range

 

○ Neglect Heat Of Dilution
Check this box if the effect of dilution on the enthalpy of a solution can be neglected.

Yes

Yes/No

○ Feed Enthalpy Correction (kcal/kg)
A correction of feed solution enthalpy used to account for the heat of dilution of that solution; defined as the difference between the value of feed solution enthalpy that accounts for heat of dilution and the value of feed solution enthalpy that is calculated if the heat of dilution is neglected; available only if the option ‘Neglect Heat of Dilution’ is not checked.

0.0

Positive

● Feed Enthalpy
The enthalpy of the feed solution. If the option “Neglect Heat of Dilution” is checked, this corresponds to the value of feed solution enthalpy that is calculated if the heat of dilution is neglected. If the above option is not checked, this corresponds to the value of feed solution enthalpy that accounts for heat of dilution. This is determined by adding the specified feed enthalpy correction to the feed solution enthalpy value that is calculated if the heat of dilution is neglected.

0.0

Positive

○ Liquor Temp. In Last Effect (oC)
The boiling point of liquor in the last effect (according to the flow of vapor) of a multi-effect evaporator. For forward feed, this is the same as the temperature of the final concentrated solution. For backward feed, it’s the boiling point of liquor at the pressure of vapor in the last effect.

40.0

Positive

○ Calculated For Steam Using Steam Calculator
Check this box in order to calculate the thermodynamic properties of the vapor phase (per effect) as if it is all steam using the built-in steam calculator.

No

Yes/No

● Mean Heat Transfer Area (m2)
The average of the heat transfer areas of all the effects that comprise a multi-effect evaporator unit.

0.0

Positive

◙ Specific Heat Duty (Kcal/kg)
The specific heat source of the heating fluid used in the steam chest of each effect. The heating fluid in the first effect may consist of heating agent and/or desuperheating agent and/or recompressed vapor. For all other effects, the heating fluid in one effect will consist of the vapor produced in the preceding effect.

0.0

Positive

○ Heat Transfer Coefficient (W/m2-K)
The overall heat transfer coefficient of each effect.

2,800.0

Positive

● Heat Transfer Area (m2)
The heat transfer area of each effect.

0.0

Positive

● Mean Temp. Difference (oC)
The temperature difference between the hot-side stream (heating fluid) and the cold-side stream (solution) for each effect.

0.0

Positive

● Liquor BP (oC)
The boiling point of liquor in each effect.

0.0

Positive

○ BPE (oC)
The boiling point elevation in each effect.

0.0

Positive

● Vapor BP (oC)
The boiling point of vapor in each effect.

0.0

Positive

◙ Pressure (bar)
The pressure of vapor in each effect.

1.01325

Positive

○ Relative Liquor Enthalpy (Kcal/kg)
A correction of liquor enthalpy used to account for the heat of dilution of that solution for each effect; defined as the difference between the value of liquor enthalpy that accounts for heat of dilution and the value of feed solution enthalpy that is calculated if the heat of dilution is neglected; available only if the option ‘Neglect Heat of Dilution’ is not checked.

0.0

Positive

○ Relative Vapor Enthalpy (Kcal/kg)
A correction of vapor enthalpy for each effect; defined as the difference between the value of vapor enthalpy and the value of feed solution enthalpy that is calculated if the heat of dilution is neglected; available only if the “Set by User” option is selected for the Vapor Enthalpy (Per Effect).

0.0

Positive

◙ Liquor Enthalpy (Kcal/kg)
The enthalpy of concentrated solution at the outlet of each effect. If the option “Neglect Heat of Dilution” is checked, this corresponds to the value of liquor enthalpy that is calculated if the heat of dilution is neglected. If the above option is not checked, this corresponds to the value of liquor enthalpy that accounts for heat of dilution. For each effect, this is determined by adding the corresponding relative liquor enthalpy to the feed solution enthalpy that is calculated if the heat of dilution is neglected.

0.0

Positive

◙ Vapor Enthalpy (Kcal/kg)
The enthalpy of vapor produced in each effect. If the vapor enthalpy is calculated, this corresponds to the value of vapor enthalpy that is calculated by the program. If the vapor enthalpy is set by user, then the vapor enthalpy for each effect is determined by adding the corresponding relative vapor enthalpy to the feed solution enthalpy that is calculated if the heat of dilution is neglected.

0.0

Positive

● Vapor Flow (kg/h)
The evaporation rate in an effect.

0.0

Positive

Symbol Key: ○ User-specified value (always input); ● Calculated value (always output); ◙ Sometimes input, sometimes output

Specification Choices / Comments

The following list describes the available specification choices in this tab; for more details on how these are implemented, see Multi-Effect Evaporation: Modeling Calculations.

•If you check the option ‘Neglect Heat Of Dilution’...

SuperPro Designer’s simulation engine will calculate the Feed Enthalpy and the Liquor Enthalpy for each effect the standard way; those values will not account for heat of dilution.

•If you do not check the option ‘Neglect Heat Of Dilution’...

You must specify the Feed Enthalpy Correction and Relative Liquor Enthalpy for each effect. Based on these values, the program will determine the Feed Enthalpy and the Liquor Enthalpy for each effect.

•If the Mean Temperature Difference (Per Effect) is ‘Set By User’...

You can set the Mean Temp. Difference for each effect in the corresponding column of the ‘Evaporation Data (Per Effect)’ table. Note that this option is only available if you specify two or more effects.

•If the Mean Temperature Difference (Per Effect) is ‘Calculated Based On Equal Heat Transfer Areas’...

SuperPro Designer’s simulation engine will calculate the Mean Temp. Difference for each effect such that the heat transfer areas of all effects are equal (within a certain degree of tolerance; a predefined relative tolerance of 10-11 is used). Note that this option is only available if you specify two or more effects.

•If you check the option ‘Calculated For Steam Using Steam Calculator’ for determining the properties of vapor (per effect)...

It is assumed that the thermodynamic properties (i.e., pressure, temperature and specific enthalpy) of vapor produced in each effect are essentially those of steam. In other words, the vapor produced in each effect is considered to be all steam. SuperPro Designer’s simulation engine will calculate, among others, the Pressure, Vapor Enthalpy and Specific Heat Duty for each effect, by employing the built-in steam calculator in order to determine the thermodynamic properies of steam at different conditions.

If this option is checked, the “Set by User’ options for the Vapor Enthalpy (per effect) and the Specific Heat Duty (per effect) will not be available.

•If you do not check the option ‘Calculated For Steam Using Steam Calculator’ for determining the properties of vapor (per effect)...

SuperPro Designer’s simulation engine will calculate the Pressure (per effect) by combining the ideal gas laws of Dalton and Raoult and the Antoine Equations of all pure components that are set as volatile.

If this option is checked, both the “Set by User’ and “Calculated” options will be available for the Vapor Enthalpy (per effect) and the Specific Heat Duty (per effect).

•If the Vapor Enthalpy (Per Effect) is ‘Set By User’...

You must specify the values of Relative Vapor Enthalpy for each effect. Based on these values, the program will determine the Vapor Enthalpy for each effect. Note that this option is only available if the option ‘Calculated for Steam Using Steam Calculator’ is not checked.

•If the Vapor Enthalpy (Per Effect) is ‘Calculated’...

SuperPro Designer’s simulation engine will calculate the Vapor Enthalpy for each effect the standard way. Note that this option is only available if the option ‘Calculated for Steam Using Steam Calculator’ is not checked.

•If the Specific Heat Duty (Per Effect>1) is ‘Set By User’...

The Specific Heat Duty values of all effects except the first one must be set by the user. Note that this option is only available if you specify two or more effects.and the option ‘Calculated for Steam Using Steam Calculator’ is not checked.

•If the Specific Heat Duty (Per Effect>1) is ‘Calculated’...

SuperPro Designer’s simulation engine will calculate the Specific Heat Duty values of all effects except the first one, using the standard way. Note that this option is only available if you specify two or more effects.and the option ‘Calculated for Steam Using Steam Calculator’ is not checked.

•To add, insert, or delete an effect...

Each row of the Evaporation Data Per Effect table corresponds to an effect. To add a new effect to the end of the table, click Add (Add01045.bmp) on the toolbar. To insert a new effect above an existing one, select the effect by clicking on its row number and click Insert (Insert01046.bmp) on the toolbar. To delete an effect, select the effect by clicking on its row number and click Delete (Delete01047.bmp) on the toolbar. Note that the Delete button is disabled if the table contains a single effect. You can add or insert a maximum of ten effects.