www.myChemE.com Simple Distillation Model
Introduction This spreadsheet is a simple distillation that cna be used to investigate the effect of design parameters on distillation performance. The calculation method uses a number of simplifying assumptions and should not be used in detailed design The spreadsheet is split into the following sections - A "How to Use This Calculation" Worksheet - The Distillation Worksheet itself - marked "Calculation" - A Theory Worksheet which presents the equations used in the calculation. It is recommended that the user first reads the 'How to Use These Calculation' worksheet before starting a calculation.
Revision Rev. 1 Initial issue
22-Sep-11
Disclaimer: This calculation provides a simplified model of a distillation column for preliminary design purposes only. We cannot be held responsible for its use. As with all areas of process engineering, calculations should be checked by a competent engineer.
www.myChemE.com
Simple Distillation Model Revision 1 See 'How to use these Calculation' worksheet for notes on its use. Cells in Green, with bold Text, to be completed by user - blue cells give calculated output
Calculation Title: Press CRTL+Q to run model
Condenser Relative Volatility (Alpha)
2.4
Total number of Trays 10 Feed Feed Flow Feed Quality
Type of Condenser Partial Condenser
Reflux Ratio (Mole Basis) 0.5
Distillate Total Molar Flowrate 100.0 kmol/h Mole fraction Benzene 0.70 Mole fraction Toluene 0.30
100.0 kmol/h Vapour Feed
Feed Composition Component 1 Benzene Component 1 Mole Fraction 0.70 Component 2 Toluene Component 2 Mole Fraction 0.30
Feed Tray 6 ERROR Messages
Bottom Product Total Molar Flowrate 0.0 kmol/h Mole fraction Benzene 0.00 Mole fraction Toluene 1.00 Condenser
Disclaimer: This calculation is a simplified model of a distillation column for preliminary design. We cannot be held responsible for its use. As with all areas of process engineering, calculations should be checked by a competent engineer.
www.myChemE.com
Simplified Distillation Model
Revision 1
HOW TO USE THIS CALCULATION 1.0
INTRODUCTION
This spreadsheet is a simplified model of a distilation which can be used to study the impact of design parameters on distillation performance. It is for use as a preliminary design tool only and should not be used to for the detailed specification of a distillation column. The simplified model is based on a number of key assumptions 1 It is a binary system - i.e. there are only two components in the mixture. 2 The two components are ideal, with a constant relative volatility. There is a constant molar overflow in the column as the two components have the same molar latent heat of evaporation and there are no heat losses or heat of mixing. 3 The plates are 100% efficient - each plate can be considered as one theoretical plate. The assumptions underpinning the model are discussed in more detail in the Theory Worksheet.
2.0
How to use this spreadsheet
2.1
Colour Coding The following colour coding is used: Boxes shaded light green, with text in bold, require a user input. Boxes shaded light blue give a calculated output. An error message is displayed if the user attempts to overwrite the calculated value.
2.2
Calculation Title The spreadsheet leaves space to add a Calculation Title at the top. Although ths space is not strictly necessary, it helps describe the calculation - this can be invaluable if it is to be checked by another engineer.
2.3
Number of Trays The user enters the total number of trays (or plates) in the column. It should be noted that each tray is assumed to 100% efficient (see theory worksheet for details). The column must have at least one tray and a maximum of ten.
2.4
Feed Tray The user enters the position of the feed tray - i.e. the tray at which the feed enters the column. The trays are counted from the bottom. In other words Tray 1 is at the bottom just above the reboiler and Tray 2 is above Tray 1. So if the column is specified with a total of 6 trays and the feed tray is number 3, the column will 3 trays below the feed tray, followed by the feed tray and then a further 2 trays above the feed tray. The number of the feed tray cannot be greater than the total number of trays - the spreadsheet will display an error message if the user tries to do this.
2.5
Reflux Ratio The user must specify the Reflux Ratio on a molar basis. The Reflux Ratio is the liquid molar flow returning to the column divided by the distillate molar flow. As it is a fraction of the total distillate flow, it must have a value between zero and 1. If the reflux ratio is set to zero, there is no liquid returned to the top of the column. This will mean that the section of the column above the feed does not achieve any separation. Separation is maximised by setting the reflux ratio to 1, however this means that no distillate product is collected. See the Theory Worksheet for more information.
Disclaimer: This calculation is a simplified model of a distillation column for preliminary design. We cannot be held responsible for its use. As with all areas of process engineering, calculations should be checked by a competent engineer.
www.myChemE.com
Simplified Distillation Model 2.6
Revision 1
Condenser Type The user must specify the type of condenser. There are two options: Full Condenser or Partial Condenser. If the Full Condenser option is selected, all of the vapour leaving the top tray is condensed. If the Partial Condenser option is selected, the distillate product is retrieved as a vapour while the condensate is returned to the column as a liquid. Hence, a partial condenser provides an additional separation stage. The differencies between the two options are discussed in more detail in the Theory Worksheet.
2.6
Feed Information - Component 1 and 2 The user enters the names of components 1 and 2 (e.g. "Benzene" and "Toluene"). Like the calculation title, this information is not strictly necessary, however it makes the output easier to follow.
2.7
Relative Volatility (Alpha) The user must input the relative volatility (alpha) of the two components. For an ideal mixture, the relative volatility is the ratio of the two components' vapour pressures at a set temperature. For components with similar boiling points, their relative volatility will be close to one, making them difficult to separate by distillation. This can be studied using this spreadsheet by changing the value of alpha used. The concept of relative volatility is discussed in more detail in the Theory Worksheet and is also discussed via the following link: Relative Volatility
2.8
Feed Flowrate The user enters the total molar flowrate, in kmol/h. This is the molar flow for both component 1 and 2.
2.9
Mole Fraction of Component 1 The user inputs the mole fraction of Component 1 in the Feed. This must be a value between 0 and 1. As this is a binary system, the spreadsheet automatically calculates the mole fraction of Component 2 in the feed.
2.10 Feed Quality The user inputs the feed quality. There are three options: "Liquid Feed", "Vapour Feed" or "Mixed Feed". If "Liquid Feed" is selected, the feed is treated as a saturated liquid. If "Vapour Feed" is selected, the feed is treated as a saturated vapour. If "Mixed Feed" is selected, the feed is treated as 50% liquid and 50% vapour. Sub-cooled liquid feed or superheated vapour feeds are not possible.
2.11 Distillate Flowrate The user enters the Distillate (Top) Product total molar flowrate, in kmol/h. This is the molar flow for both component 1 and 2.
2.12 Distillate Mole Fractions The spreadsheet will calculate the component mole fractions in the distillate. This requires the use of a macro (see "Solving the Spreadsheet" below). If the spreadsheet has not converged, no mole fractions will be displaced.
2.13 Bottom Product Flowrate and Mole Fractions The spreadsheet calculates both the total bottom product flow rate and mole fractions. As with the distillate mole fractions, these require the use of a macro (Press CTRL+Q) to solve, If the spreadsheet has not converged, no mole fractions will be displaced.
3.0
SOLVING THE SPREADSHEET As noted above, the spreadsheet uses a macro to calculate the distillate and bottom product compositions. Therefore, to make this spreadsheet work, macros must not be disabled. The macro is activated by pressing the keys CTRL and Q simultaneously.
Disclaimer: This calculation is a simplified model of a distillation column for preliminary design. We cannot be held responsible for its use. As with all areas of process engineering, calculations should be checked by a competent engineer.
www.myChemE.com
Simplified Distillation Model 4.0
Revision 1
ERROR MESSAGES The spreadsheet can display a number of error messages. Many of the input cells are limited to certain values. For example, the component mole fractions must be between 0 and 1. If the user attempts to input a value outside the acceptable range, the spreadsheet will alert the user. When the user first inputs values into the spreadsheet, an error message will be displayed that the calculation has not converged. To converge the calculation, activate the macro (CTRL+Q). The spreadsheet will also alert the user if parts of the column have a vapour or liquid flow below zero. If this occurs, the user should adjust one, or more, of the following: feed quality, reflux ratio, distillate molar flowrate. Finally, the spreadsheet will warn the user if the Distillate flow is greater than the Feed flow - a situation which is clearly not feasible.
Disclaimer: This calculation is a simplified model of a distillation column for preliminary design. We cannot be held responsible for its use. As with all areas of process engineering, calculations should be checked by a competent engineer.
www.myChemE.com
Simplified Distillation Model
Revision 1
CALCULATION THEORY 1.0
INTRODUCTION
This spreadsheet is a simplified model of a distilation which can be used to study the impact of design parameters on distillation performance. It is for use as a preliminary design tool only and should not be used to for the detailed specification of a distillation column.
2.0
ASSUMPTIONS USED IN THE MODEL
To keep the model simple, a number of simplifications are made: Binary System The number of equations required to solve a distillation model increases dramatically as the number of components increases. Therefore, to minimise complexity, the model is based on a binary system (i.e. Only two components). Heat Balance To reduce the number of equations still further, the heat balance is ignored. This means that the model does not predict operating temperatures in the column. To allow the heat balance to be ignored, the following assumptions are made to the model: i. The latent heat of vaporisation for both components is the same. As a consequence, the vapour and liquid flows in both the rectifying and stripping sections of the column remain constant. ii. The feed flow is assumed to be at its boiling point - either saturated vapour, saturated liquid or a mix of vapour and liquid. Sub-cooled liquid or superheated vapour feeds are not permitted. iii There are no heat losses from the distillation column. Vapour / Liquid Equilibrium A simplified physical property model is used to calculate vapour-liquid equilibrium. The model assumes that the system has a constant relative volatility. Stage Efficiency Liquid entrainment in the vapour, weepage of liquid through the vapour perforations in the plate, as well as other imperfections, mean that plates in real distillation columns are not 100% efficient. These effects are ignored in the model.
3.0
CALCULATION OF THE OVERALL MASS BALANCE
The overall column mass balance is calculated by the model by splitting the column into two parts: i. The Rectifying (Top) Section of the Distillation Column - this is the part of the distillation column above the feed plate, including the condenser. ii. The Enriching (Bottom) Section of the Distillation Column - this is the part of the distillation column below the feed plate, including the reboiler.
3.1
Overall Mass Balance on the Rectifying Section
A sketch of the Rectifying Section is shown in Figure 1 below:
Disclaimer: This calculation is a simplified model of a distillation column for preliminary design. We cannot be held responsible for its use. As with all areas of process engineering, calculations should be checked by a competent engineer.
www.myChemE.com
Simplified Distillation Model
Revision 1
The reflux flow sets the liquid flow down the Rectifying Section of the Column.
Reflux Flow = L_Rectifying = D x R
Equation (1)
Where: Reflux Flow L_Rectifying D R
-
Reflux molar flow returning to the column from the condenser, kmol/h Liquid molar flow down the Rectifying Section of the Column, kmol/h Distillate molar flow leaving the column as product, kmol/h Molar Reflux Ratio (Dimensionless)
The vapour flow up the Rectifying Section of the column is set by the vapour flow up the stripping section plus the vapour fraction of the feed flow into the column.
V_Rectifying = F x (1 - Quality) + V_Stripping
Equation (2)
Where: V_Rectifying F Quality V_Stripping
-
Vapour molar flow down the Rectifying Section of the Column, kmol/h Feed molar flow into the Column, kmol/h Feed Quality (Dimensionless) Vapour molar flow down the Stripping Section of the Column, kmol/h
The feed quality is set by the user when they define the feed. If a liquid feed is specified, there is no feed contribution to the vapour flow in the rectifying section of the column (Quality = 1). If a vapour feed is specified, all the feed flows up the rectifying section (Quality =0). If a mixed feed is specified, half of the feed flows up the rectifying section of the column (Quality = 0.5).
3.2
Overall Mass Balance on the Stripping Section
A sketch of the Stripping Section is shown in Figure 2 below:
Disclaimer: This calculation is a simplified model of a distillation column for preliminary design. We cannot be held responsible for its use. As with all areas of process engineering, calculations should be checked by a competent engineer.
www.myChemE.com
Simplified Distillation Model
Revision 1
As the Feed flow and Distillate flow is set by the user, the Bottom product flow, B, (in kmol/h) can be calculated:
B= F-D
Equation (3)
The liquid flow up the Stripping Section of the column is set by the liquid flow down the rectifying section plus the liquid fraction of the feed flow into the column.
L_Stripping = F x Quality + L_Rectifying
Equation (4)
Where: L_Stripping - Liquid molar flow down the Stripping Section of the Column, kmol/h The vapour flow up the distillation column is the quantity of vapour generated by the reboiler. In the mass balance it is the difference between the liquid flow down the stripping section and the bottom product.
V_Stripping = L_Stripping - B 3.3
Equation (5)
Component Molar Balance
Having established the overall vapour and liquid flows within the column, the spreadsheet performs a tray by tray component molar balance. This is an iterative calculation - hence the need for a macro. As there are only two components, calculating the mole fraction of one component fixes the other. The following approach is followed for each stage
Disclaimer: This calculation is a simplified model of a distillation column for preliminary design. We cannot be held responsible for its use. As with all areas of process engineering, calculations should be checked by a competent engineer.
www.myChemE.com
Simplified Distillation Model
Revision 1
1. Guess the liquid mole fraction, x, of component 1 2. Calculate the vapour mole fraction, y, of component 2, using relatively volatility - see Equation 6.
y =
.x x]
Equation (6)
3. Perform a molar balance for component 1 over each the stage. This is a calculation of the molar flow entering and leaving the stage. For the feed tray, the molar balance considers the component feed flow onto the tray. 4 If the molar flows onto and off the tray do not balance, the macro re-adjusts the tray liquid mole fraction. The model is solved when the mole fractions on each stage is balanced.
Disclaimer: This calculation is a simplified model of a distillation column for preliminary design. We cannot be held responsible for its use. As with all areas of process engineering, calculations should be checked by a competent engineer.
