10 synthesis of reaction separation system lec 10 homogenous separation
- 1. Lecture 10 : SYNTHESIS OF SEPARATION SYSTEM – HOMOGENOUS MIXTURE SEPARATIONAfter separating the heterogenous mixture, attempt can then be made to separate the homogenous mixture produced from the reaction. This is a harder separation !Remember from your earlier courses in separation !We need to create or add an extra phase in order to perform the separation. Most common example : Distillation, Liquid Extraction, Super Critical Fluid Extraction etc...Some guidelines to consider !There used to be a preferred rule long practised by industry wherebydistillation was given the first priority as chosen method for separating homogenous mixture. Now, it is used as a benchmark against other methods.1In sequencing the separator for homogenous mixture, a guide whichcan be used is to develop/design the sequence which consumes the leastenergy for the required separation.2
- 2. To illustrate the guidelines ….First choice !What circumstances ? Discuss !ABSORBERADSORBERif cannot performseparation oreconomically unattractiveExtraction11Liquid-LiquidSolid-LiquidDISTILLATIONMEMBRANE2Sequence preferred should bethe sequence that has highestenergy efficiency.2HOW ?Eg. For distillation, assess the minimum vapour ffow required for the entire separation
- 3. 1Circumstances not favouringdistillation1. Separation of low molecular wt. materials2. Separation of high molecular wt. heat sensitive material.3. Separation of components with low concentration.4. Separation of classes of components
- 4. 2Sequencing using minimum vapour flowUse short cut method to determine minimum reflux ratio for specified separationUNDERWOOD EQUATIONComparison is based on a consistent basis : infinite no. of stagesCalculate minimum vapour flow for each required separation (as specified) in every alternative sequence. Go for the sequence with the least total minimum vapour flow.DISTILLATIONDISTILLATIONDISTILLATIONDISTILLATIONDISTILLATIONDISTILLATIONcalculate minimum vapour flowfor each column !Use theUnderwood EquationWe will look at this in more detail later …
- 5. Short Cut Design Method for Distillation ColumnStages of Calculation for the Short Cut Method.i. Calculation of component distribution using Hengstebeck Method.Log (di/bi)Log (di/bi) for LKdiLog (di/bi) for HKbiaLLKaLKaHKaHHKLog (aij)
- 6. Stages of Calculation for the Short Cut Method.ii. Calculation of Minimum Number of Stages using Fenske Equation.Minimum No of Stages (Total Reflux)Geometric average of the relative volatility between LK to HK at the top and bottom of column.Another form of Fenske Equation
- 7. Stages of Calculation for the Short Cut Method.iii. Calculation of Minimum Reflux Ratio using Underwood Equation.The equation is used to determine the root q which is then used to solve the minimum reflux ratio equation. q – feed qualityai,j – relative volatility with reference to the heavy key jxi,F – feed composition , x i,D– Distillate composition
- 8. Stages of Calculation for the Short Cut Method.iv. Determination of Feed LocationKirkbride EquationZH and ZL – mole fraction of heavy and light key respectively in feed.Ratio of the no. of stages in rectifying to stripping section.XB,L and XD,H – mole fraction of light key in bottom product and heavy key in top product respectively.B and D – molar flow of bottom product and distillate.
- 9. Stages of Calculation for the Short Cut Method.v. Determination of Actual No of Stages based on Set Reflux Ratio (Gilliland Correlation)The correlation was originally represented in graphical form before an empirical was developed.Based on the calculated Rmin from Underwood equation, the X value is determined based on a set Reflux Ratio R.𝑋= 𝑅 −𝑅𝑚𝑖𝑛𝑅+1 Rmin is determined from Underwood Eqn.Then a Y value is determined from the X value using the correlation below;𝑌=0.2788− 1.3154 𝑋+0.4114 𝑋0.2910+0.8628ln𝑋 + 0.9020ln𝑋+ 1𝑋 The Y value obtained is then used to determined the number of stages N corresponding to the Reflux Ratio R using the equation below based on the Nmin determined from Fenske Eqn.𝑌= 𝑁 −𝑁𝑚𝑖𝑛𝑁+1
- 10. Stages of Calculation for the Short Cut Method.The correlation allows for the determination of corresponding No. of Stages N required as the Reflux Ratio R is varied from the minimum reflux ratio.No. of StagesxxxxxxxxxxxReflux RatioThis will allow for the capital energy trade off to be investigated. (Discuss how it is being done?)
- 11. Example for Short Cut method calculationGiven the separation specified below;The separation cut is between n-butane and i-pentaneDetermine the minimum number of stages and minimum reflux ratio.
- 12. SolutionCalculate the minimum no of stages using Fenske eqn.
- 13. SolutionCalculate the minimum Reflux Ratio using Underwood Eqn. (Assume sat. liq and therefore q = 1)i. Determine the root of Underwood equation q value.a i . ziComponenta i . zia i - qq = 1.5q = 1.3q = 1.350.0710.3551.0-0.4-0.4620.5640.0680.3120.769-0.571-0.600-0.0220.0680.300.714-0.667-0.667-0.252Propanei-butanen-butanei-pentanen-pentane0.250.390.500.200.30Close enough, take q value = 1.35Sum
- 14. Solutionii. Using the calculated root of the Underwood equation q , determine Rmin.a i . xDiComponentxDia i - qq = 1.350.150.691.66-0.06-0.022.42Propanei-butanen-butanei-pentanen-pentane0.110.330.530.020.01 = Rmin + 1SumTherefore Rmin = 1.42
- 15. Example on Distillation SequencingGiven the mixture to be separated;Each component is to be separated with a specification of 99 %. Assume that the non key component will not distribute but end up either at the top or bottom depending on their relative volatility compared to the key component.Determine the best sequence(s).
- 16. Example on Distillation SequencingThe possible sequences are :D/EC/DEC/DCD/EB/CDED/EB/CBC/DEA/BCDEB/CBC/DBCD/EB/CDC/DC/DED/EA/BAB/CDECD/EC/DABCDEAB/CA/BABC/DED/EA/BCB/CD/EABCD/EAB/CDC/DA/BA/BCDC/DB/CDB/CBC/DA/BCB/CABC/DAB/CA/B
- 17. Example on Distillation SequencingFor each of the column, we need to calculate the minimum vapour flow using Underwood Eqn….D/EC/DEC/DCD/EB/CDED/EB/CBC/DEA/BCDEB/CBC/DBCD/EB/CDC/DC/DED/EA/BAB/CDECD/EC/DABCDEAB/CA/BABC/DED/EA/BCB/CD/EABCD/EAB/CDC/DA/BA/BCDC/DB/CDB/CThe minimum vapour flow for each column in the sequence is added up to give the total minimum vapour flow for the sequence.BC/DA/BCB/CABC/DAB/CA/B
- 18. Example on Distillation SequencingCalculation done for some of the sequences…ABC/DEAB/CA/BD/EAABCBABCDEBCMixed SequenceDCDES Vmin = 5584 kmol/hrE
- 19. Example on Distillation SequencingCalculation done for some of the sequences…A/BCDEBC/DEB/CD/EABABCDEBCCMixed SequenceDDES Vmin = 5670 kmol/hrE
- 20. Example on Distillation SequencingCalculation done for some of the sequences…ABCD/EAB/CDA/BC/DABABCDMixed SequenceACABCDECDS Vmin = 5106 kmol/hrBED
- 21. Example on Distillation SequencingCalculation done for some of the sequences…ABCD/EABC/DAB/CA/BCDABCAIndirect SequenceABCDCS Vmin = 5188 kmol/hrABCDEDBE
- 22. In Summary…..For homogenous mixture separation, distillation has always been preferred by industry due to their established position over other methods and their versatility /flexibility in addressing range of feed mixture.As most reactor effluent mixture consist of many components, the separation for the individual components has to be done is a sequence. The sequence of these separators can be determined using established methods such as the short cut method (Fenske-Underwood Equation) for distillation column