CONCEPTUAL DESIGN OF SIMPLE DISTILLATION COLUMNS Goals o Determine feasibility of proposed separation Estimate tray and reflux requirements Estimate column diameter and height 9/2799 Simple distillation design
9/27/99 Simple Distillation Design 1 CONCEPTUAL DESIGN OF SIMPLE DISTILLATION COLUMNS Goals: u Determine feasibility of proposed separation u Estimate tray and reflux requirements u Estimate column diameter and height
DEFINITION OF SIMPLE DISTILLATION o a distillation column with a single feed and no side streams o Vapor-liquid equilibrium E Ideal raoult's Law) E Nearly ideal (modified raoult s Law Ey No azeotropes o Modest heat effects- no substantial variations in vapor and liquid traffic above and below the feed 9/2799 Simple distillation design
9/27/99 Simple Distillation Design 2 DEFINITION OF SIMPLE DISTILLATION u A distillation column with a single feed and no side streams u Vapor-liquid equilibrium ð Ideal (Raoult’s Law) ð Nearly ideal (modified Raoult’s Law) ð No azeotropes u Modest heat effects - no substantial variations in vapor and liquid traffic above and below the feed
REQUIREMENTS FOR A FEASIBLE DESIGN o Achieve product specifications E Purity 今 fractional recovery o Reboil and condense using availability utilities o Column diameter and height should be within fabricatable limits Diameter 7 E Height 35 m 9/2799 Simple distillation design
9/27/99 Simple Distillation Design 3 REQUIREMENTS FOR A FEASIBLE DESIGN u Achieve product specifications ð Purity ð Fractional recovery u Reboil and condense using availability utilities u Column diameter and height should be within fabricatable limits ð Diameter < 7 m ð Height < 35 m
Distillation Column design procedure Determine Calculate Column Specs and Constraints Minimum Reflux Requirement Underwood) Choose reflux rate stimate overal Set column and estimate Theoretical tray Tray Efficiency Pressure Requirement (cOnnell, Gilliland Murphree) Calculate vapor Calculate Actual and liquid OK? Trays Required Traffie and column VB,,LB,LT Height (HOu Calculate Calculate Column Mininum tray Diameter Above& below Requirement Feed (Fenske Nmin< 30? Dma<7m?与 Simulate and Size auxiliary Equipment e 9/2799 Simple distillation design
9/27/99 Simple Distillation Design 4 Determine Column Specs and Constraints Set Column Operating Pressure Calculate Mininum Tray Requirement (Fenske) OK? Modify No Nmin< 30? Yes No Calculate Minimum Reflux Requirement (Underwood) Choose Reflux Rate and Estimate Theoretical Tray Requirement (Gilliland) Calculate Vapor and Liquid Traffic VB , VT , LB , LT Calculate Column Diameter Above & Below Feed (Fair) Dmax < 7 m? Yes No Yes Estimate Overall Tray Efficiency (O'Connell, Murphree) Calculate Actual Trays Required and Column Height (HCOL) HCOL < 30 m? Simulate and Size Auxiliary Equipment Yes No Distillation Column Design Procedure
PRELIMINARY MATERIAL BALANCE o Calculate distribution of key components based on separation specifications Assume that all components lighter than the light key distribute entirely to the distillate o Assume that all components heavier than the heavy key distribute entirely to the bottoms o Calculate the distribution of components with volatilities between the keys by assuming infinite reflux and using the fenske equation 9/2799 Simple distillation design 5
9/27/99 Simple Distillation Design 5 PRELIMINARY MATERIAL BALANCE u Calculate distribution of key components based on separation specifications u Assume that all components lighter than the light key distribute entirely to the distillate u Assume that all components heavier than the heavy key distribute entirely to the bottoms u Calculate the distribution of components with volatilities between the keys by assuming infinite reflux and using the Fenske equation
DETERMINE THE OPERATING PRESSURE o Determine the bubble point temperature of the distillate Tbb=Tcw+△Tmin o Calculate the bubble point pressure ePbb=∑Paui(Tbub)xli Do same for the bottoms- determine tbbub and Pbbub In order to be able to condense the overhead vapor we must have pcol> pdbub And to reboil the bottoms we need Pbbub Pcol So, choose Pcol such that Pdbub Pcol Pbbub o Inot, either choose utilities with a wider temperature range or relax the separation specs 9/2799 Simple distillation design
9/27/99 Simple Distillation Design 6 DETERMINE THE OPERATING PRESSURE u Determine the bubble point temperature of the distillate ð Tdbub = Tcw + D Tmin u Calculate the bubble point pressure ð Pdbub = å Psat,i(Tdbub) xdi u Do same for the bottoms- determine Tbbub and Pbbub u In order to be able to condense the overhead vapor we must have Pcol > Pdbub u And to reboil the bottoms, we need Pbbub < Pcol u So, choose Pcol such that Pdbub < Pcol < Pbbub u If not, either choose utilities with a wider temperature range or relax the separation specs
DETERMINE THE MINIMUM TRAY REQUIREMENT Nmin o Calculate Nmin using the fenske equation As a rule of thumb, the column height should not exceed 30 m Normal tray spacing is 2 ft 0.6m Thus the maximum number of trays we can have is about 50 trays As another rule of thumb the number theoretical trays should be about 1.5 Nmin to 2.0 Nmin. So, if Nmin>30, the column height guideline will probably be exceeded if so, either E adjust the separation specs(if possible) accept the additional height requirement,or go to a different separation, e.g. extractive distillate 9/2799 Simple distillation design
9/27/99 Simple Distillation Design 7 DETERMINE THE MINIMUM TRAY REQUIREMENT Nmin u Calculate Nmin using the Fenske Equation u As a rule of thumb, the column height should not exceed 30 m. Normal tray spacing is 2 ft or 0.6 m. Thus the maximum number of trays we can have is about 50 trays u As another rule of thumb, the number theoretical trays should be about 1.5 Nmin to 2.0 Nmin. So, if Nmin > 30, the column height guideline will probably be exceeded u If so, either ð adjust the separation specs (if possible) ð accept the additional height requirement, or ð go to a different separation, e.g. extractive distillation
ESTIMATE THEORETICAL TRAY REQUIREMENT o Calculate Rmin(Underwood equation) Choose an operating reflux ratio R i Rule of thumb: 1.05 Rmin Estimate the reflux ratio(Gilliland correlation 9/2799 Simple distillation design
9/27/99 Simple Distillation Design 8 ESTIMATE THEORETICAL TRAY REQUIREMENT u Calculate Rmin (Underwood Equation) u Choose an operating reflux ratio R ð Rule of thumb: 1.05 Rmin < R < 1.1 Rmin ð Estimate theoretical tray requirement (Gilliland correlation) u Or choose the number of theoretical trays Nt ð Rule of thumb: 1.5 Nmin < Nt < 2.0 Nmin ð Estimate the reflux ratio (Gilliland correlation)
