Distillation Column Design 1

(Material developed based on Product and Process design principles:Synthesis, analysis and evaluation, 3e by Seader and Widagdo)

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Distillation column designMESH equationsRigorous analysis requires simultaneous solution of mathematical relationships for each stage.

MESH equations: (1) Material Balances, (2) Equilibrium Relationships, (3) Summation Relationships, and (4) Heat (energy) Balances.H83 PS1/H84 CFL

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ParametersEstimate of condenser and reboiler pressurePressure gradient in the column affects the equilibrium temperature and composition at each stageCondenser pressure will be selected to enable the cooling of the vapor with cooling waterReflux Ratio, RNumber of theoretical stages, NFeed tray locationH83 PS1/H84 CFL

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H83 PS1/H84 CFLColumn design stepsPreliminary steps:Calculate Pressure - Splitter operationCalculate R, N, Nf Short cut distillation

Set up distillation column: Use the values obtained from the previous steps to start simulation

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H83 PS1/H84 CFLDepropanizer example

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H83 PS1/H84 CFLSplitter setupPreliminary steps:Define the material stream- Select SRK EOS

Select splitter unit

Select field as your unit set

Install inlet, outlet and energy streams

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H83 PS1/H84 CFLSplitter setup

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H83 PS1/H84 CFLDefine splitsSplits are defined based on the distillate specifications

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H83 PS1/H84 CFLTo start with, assume the condenser pressure is the same as feed pressure and the distillate vapor fraction as 1.

Parameter tab

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H83 PS1/H84 CFLBottom pressure estimationUse SET unit to simulate a constant pressure dropThis is the variable we control

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H83 PS1/H84 CFLBottom pressure estimationSelect the pressure of the stream B and the source as stream D (since the pressure of D is used to calculate the pressure of B

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H83 PS1/H84 CFLBottom pressure estimationAssume a constant pressure drop

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H83 PS1/H84 CFLBottom pressure estimationThe system is now converged. However, we need to recheck the condenser and reboiler pressure with the consideration of relative volatility.

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Time for exercise!H83 PS1/H84 CFL

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H83 PS1/H84 CFLPerform a case study to find the dependence of pressure on relative volatility on the new stream. Here, we created a clone of the feed to work on the case study.Relative volatility

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H83 PS1/H84 CFLVolatility is defined at its bubble point. So, make the vapor fraction of feed as zero after deleting the temperature Relative volatilityCloned stream

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H83 PS1/H84 CFLDefine alpha using spreadsheet by importing K values for the key components

Spreadsheet

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H83 PS1/H84 CFLImport K values for the key componentsSpreadsheet

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H83 PS1/H84 CFLEstimate alpha valuesSpreadsheet

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H83 PS1/H84 CFLNow the case study can be performed using to study the effect f pressure on alphaCase study

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H83 PS1/H84 CFLSelect the dependent (alpha) and independent (pressure) variables.Case study

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H83 PS1/H84 CFLSelect the relative volatility and the pressure of stream 1-2 as the variablesCase study

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H83 PS1/H84 CFLSelect the relative volatility and the pressure of feed 2 as the variablesCase study

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H83 PS1/H84 CFLNow start a case study (alpha). Our objective is to analyze the dependence of relative volatility on pressure.Case study

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H83 PS1/H84 CFLDefine the range of pressure at which the analysis will be conducted. Then run and view resultsCase study

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H83 PS1/H84 CFLAlpha is inversely proportional to pressure. Since we target high alpha, select low pressureCase study

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Time for exercise!H83 PS1/H84 CFL

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H83 PS1/H84 CFLAlpha is inversely proportional to pressure. Since we target high alpha, select low pressureThe effect of temperature-pressure at condenser and reboiler should be consideredUse cooling water and steam to save the utility costTo enable reboiling with steam, the reboiler temperature should be lower than 366oF to allow efficient heat transfer when using high pressure steam at 150psia. To use cooling water in condenser, the condenser temperature should be around 100 -120oF.

Effect of pressure on temperature

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H83 PS1/H84 CFLEffect of pressure on temperatureAdd a new case study-Temperature

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H83 PS1/H84 CFLEffect of pressure on temperatureAdd the temperatures and pressures of streams B and D (Bs pressure is calculated by SET unit) and variablesSet the temperatures as dependent and pressure as independent variables.

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H83 PS1/H84 CFLEffect of pressure on temperatureSet the same range for pressure as in the previous case.

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H83 PS1/H84 CFLEffect of pressure on temperatureView the result by pressing result button.

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H83 PS1/H84 CFLEffect of pressure on temperatureSince the Alpha change is only by 0.2 in the pressure range of 205psia and 265psia, we neglect the effects of alpha and take the benefit of heat exchange instead. Close the window and return to the splitter page.

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H83 PS1/H84 CFLEffect of pressure on temperatureEven the temperature value at 300psia is well within the limit of not more that 366oF.At D-temperature, to fulfill the lower limit of 100oF, pressure must be about 205psia. D upper limit is 120oF which limit the pressure to 265psia. Since the pressure lies between 205psia and 265psia, higher temperature in condenser provides higher driving force and thus requires less area in condenser, an operating pressure of 250psia for condenser is selected.

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H83 PS1/H84 CFLCondenser and reboiler pressureCheck the results at the worksheet tab of splitter

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H83 PS1/H84 CFLCheck the results at the worksheet tab of splitterCondenser and reboiler pressure

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Time for exercise!H83 PS1/H84 CFL

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Short cut distillationUse the pressure estimated in splitter operation to get estimates on

1)Reflux Ratio, R2)Number of theoretical stages, N3)Feed tray locationH83 PS1/H84 CFL

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H83 PS1/H84 CFLBasic stepsThe top product must be in vapor form to avoid condensing ethane

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H83 PS1/H84 CFLDefine key componentsActual reflux=1.75* minimum reflux(in this case)

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H83 PS1/H84 CFLResults of short cut distillationNumber of trays = 16Feed entry tray: 8 or 9Verify the distillate composition with the target specification

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Time for exercise!H83 PS1/H84 CFL

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