LECTURE EIGHT054410 PLANT DESIGN 054410 Plant Design LECTURE 8: REBOILER CIRCUIT DESIGN Daniel R. Lewin Department of Chemical Engineering Technion, Haifa, Israel Ref: Kern, R. “Thermosyphon Reboiler Piping Simplified,” Hydrocarbon Processing, Dec 1968, 47, No. 12, pp. 118-122 8-1 PLANT DESIGN - Daniel R. Lewin Reboiler Circuit Design Lecture Objectives After this lecture, you should: Understand the physics behind a thermosyphon. Be familiar with the four main types of reboiler arrangements in use, and their advantages and disadvantages. Be able to perform sizing calculations for a thermosyphon. Be able to select and design the appropriate reboiler circuit for a given application. 8-2 PLANT DESIGN - Daniel R. Lewin Reboiler Circuit Design 1 Daniel R. Lewin, Technion of aerated water in riser tube ∆P = diff.71 psig 8-4 PLANT DESIGN .Daniel R.061 ) 2.31 = 4.G.LECTURE EIGHT 054410 PLANT DESIGN How reboilers work “Almost as many towers flood because of reboiler problems as because of tray problems. of fluid in riser (in this case 1. or natural circulation.0) HRT = hgt of aerated water in riser tube (ft) DRT = S. Lewin Reboiler Circuit Design How reboilers work The driving force to promote flow through this reboiler is the density difference between the fluid in the reboiler feed line and the froth-filled reboiler return line. pressure between A and B (psi) 8-3 PLANT DESIGN .” Theory of thermosyphon. For the example on the right: ∆P = (20 ft ) ( 0.31 HRW = hgt of water above base (ft) DRW = S. can be illustrated by the airlift pump.6) − (15 ft ) ( 0. Lewin.Daniel R.G. Technion . Lewin Reboiler Circuit Design 2 Daniel R. ∆P = (HRW ) (DRW ) − (HRT )(DRT ) 2. Daniel R. 8-5 PLANT DESIGN .7 psig is consumed in overcoming frictional losses.Daniel R.7 psig. Lewin Kettle or gravity-fed reboilers Reboiler Circuit Design 3 Daniel R. the reboiler draw-off pan overflows. If they are more than the 4. the inlet line does not run liquid full. outlet line and nozzles. Lewin Reboiler Circuit Design Four Types Considered Once-through thermosyphon reboilers Circulating thermosyphon reboilers Forced-circulation reboilers 8-6 PLANT DESIGN . Lewin. Technion .LECTURE EIGHT 054410 PLANT DESIGN How reboilers work The developed ∆P of 4. reboiler. due to flow in the inlet line.7 psig. If these frictional losses are less than 4. and the flow to the reboiler is reduced until the friction losses drop to the available thermosyphon force. All the bottoms product comes from the liquid portion of the reboiler effluent. Lewin Reboiler Circuit Design Once-through Thermosyphon Reboiler A once-through thermosyphon reboiler can be equipped with a vertical baffle.LECTURE EIGHT 054410 PLANT DESIGN Once-through Thermosyphon Reboiler The following statements characterize the operation of a once-through thermosyphon reboiler: All the liquid from the bottom tray flows to the reboiler. 8-8 PLANT DESIGN .Daniel R. Lewin Reboiler Circuit Design 4 Daniel R. Lewin. Technion . is the same as the tower bottoms temp. The reboiler return liquid goes only to the hot side of the tower bottoms. None of the liquid from the bottom of the tower flows to the reboiler. Putting the reboiler return liquid to the colder side is poor engineering practice. The reboiler outlet temp. 8-7 PLANT DESIGN .Daniel R. None of the liquid from the bottom tray flows to the bottom of the tower. Can overcome large ∆Ps in the reboiler circuit. Disadvantages: Wastes energy. and the higher ∆P needs to be overcome. Some of the liquid from the reboiler outlet will always circulate back into the reboiler feed. (b) if a number of distinct heat sources supply the reboiler duty. Technion . Lewin Figure 5. 2.Daniel R. Main usages: (a) If the reboiler is a furnace. Tower bottoms product temperature and composition is the same as the temperature and composition of the feed to the reboiler. Advantages: 1. where loss of flow will lead to tube damage. Careful calculation of circuit ∆P is not critical.6 Reboiler Circuit Design 5 Daniel R. is always higher than tower bottoms temperature. Some of the liquid from the bottoms tray ends up as bottoms product. 8-9 PLANT DESIGN .LECTURE EIGHT 054410 PLANT DESIGN Circulating Thermosyphon Reboiler In this type of thermosyphon reboiler circuit: The reboiler outlet temp.10 PLANT DESIGN . but equipped with a pump to impose circulation. Lewin Reboiler Circuit Design Forced-circulation Reboiler Similar to a “once-through” design. 8 . Lewin.Daniel R. 11 Reboiler Circuit Design Kettle Reboiler 8 . Lewin Reboiler Circuit Design 6 Daniel R. Lewin 8 . It is partially vaporized.12 PLANT DESIGN . This liquid is the bottoms product.Daniel R. Lewin.Daniel R. Technion . which is set high enough to keep the tubes submerged. The domed top section of the reboiler separates the vapor and the liquid.LECTURE EIGHT 054410 PLANT DESIGN Kettle Reboiler In this type of thermosyphon reboiler circuit: Liquid flows from the column sump to the bottom of the kettle’s shell. PLANT DESIGN . The vapor flows back into the tower via the riser. The liquid overflows the baffle. 5 psi ∆pd − downcomer ∆P [psi] ⎫ ⎬ ∆pd + ∆pr = 0. Thus.25-0. The shell-side exchanger pressure drop.13 Once-through Reboiler Design Natural circulation is maintained if ∆P (driving force) ≤ ∆p (frictional losses) Driving force for circulation.Daniel R. Lewin Reboiler Circuit Design 8 . Sump level is not controlled! PLANT DESIGN . including the effect of the baffle height. Lewin. The vapor-line ∆P.14 PLANT DESIGN . ∆p = ∆pd + ∆pe + ∆pr ρ2 H2 ρ1 H1 ∆pe − reboiler ∆P [psi] − usually 0.LECTURE EIGHT 054410 PLANT DESIGN Kettle Reboiler The level in the tower sump is the sum of the following: Nozzle exit losses. increase in duty will lead to an increase in sump level.Daniel R. Liquid feed-line ∆P. including the vapor outlet nozzle loss. Technion . Note: Pressure in reboiler is always higher than tower pressure.1-1 psi/100 ft by design ∆pr − riser ∆P [psi] ⎭ 8 . Lewin Reboiler Circuit Design 7 Daniel R. P1 − P2 = ∆P = (1 144 ) ( ρ1H1 − ρ2H2 ) Pi − pressure [psi] ρi − density [lb/ft3 ] H1 − head [ft] Introducing a safety factor of 2: ∆P = (1 288 ) ( ρ1H1 − ρ2H2 ) Friction Losses. Kern (1968) recommends a head difference of 3 ft: ∆H = 3.15 PLANT DESIGN . the minimum downcomer nozzle elevation is limited to: 288∆p + ρ2H3 H1 ≥ ρ1 − ρ2 8 .LECTURE EIGHT 054410 PLANT DESIGN Once-through Reboiler Design Horizontal Reboilers: The minimum downcomer nozzle elevation above a horizontal reboiler centerline is: 288∆p − ( ∆H ) ρ2 H1 ≥ ρ1 − ρ2 where ∆H = H1 – H2. Technion .Daniel R.16 PLANT DESIGN . the driving force must be at least equal to the frictional losses: ∆P ≤ ∆p = ∆pd + ∆pe + ∆pr Here H2 = H1 + H3. Lewin Reboiler Circuit Design Circulating Reboiler Design Note that draw-off is from the bottom here! Horizontal Reboilers: ∆P = (1 288 ) ( ρ1H1 − ρ2H2 ) As before. Lewin. Lewin Reboiler Circuit Design 8 Daniel R. Thus. In which case: 288∆p − 3ρ2 ρ1 − ρ2 The density of fluid in riser is: ρ2 H2 ρ1 H1 H1 ≥ ρ2 = W WL ρL +WV ρV Wi − mass flows [lb/hr] 8 .Daniel R. H1′ = H 4 and H3 = H2 and: ∆P = (1 288 ) H1′ ( ρ1 − ρ3 ) − ρ2H2 and H1′ = 8 .Daniel R.17 PLANT DESIGN .Daniel R. ∆P = (1 288 ) ρ1H1′′ − ρ2H2 − ρ3H 4 Following Kern’s recommendation: ( ) H1′′ = 3 + H2 + H 4 The minimum draw-off nozzle elevation is: 288∆p − ρ2 (H 4 + 3) + ρ3H 4 ρ1 − ρ2 H1′′ ≥ 8 . Lewin ( ) 288∆p + ρ2H3 ρ1 − ρ2 Reboiler Circuit Design Once-through Reboiler Design Vertical Reboilers (top draw-off): Here. since H1′ + H3 = H2 + H 4 : 288∆p + ρ2 (H 4 − H3 ) + ρ3H 4 ρ1 − ρ2 H1' ρ3 H1′ ≥ The vertical reboiler should be flooded. The maximum elevation of the top tube-sheet should not be higher than the minimum liquid level in the tower.LECTURE EIGHT 054410 PLANT DESIGN Circulating Reboiler Sizing Vertical Reboilers (bottom draw-off): Conservative estimate of the exchanger H3 fluid density: ρ3 = ( ρ1 + ρ2 ) 2 Thus. Technion .18 PLANT DESIGN . ∆P = (1 288 ) ρ1H1′ − ρ2H2 − ρ3H 4 ( ) ∆P ≤ ∆p = ∆pd + ∆pe + ∆pr However. Lewin. Lewin Reboiler Circuit Design 9 Daniel R. thus at minimum. Lewin. The amount of incoming feed a thermosyphon reboiler can vaporize is typically between 30-40%. Others have achieve levels as high as 45-50% in special circumstances. Both phenomena limit the amount of heat that can be transferred to a boiling fluid to an upper limit – which leads to the upper limitation in vaporization. For some installations. There are two reason for this: (a) critical heat flux limitations. (b) vaporization blanketing. Technion .LECTURE EIGHT 054410 PLANT DESIGN Fraction of Fluid Evaporated Source: Andrew Sloley. Lewin Reboiler Circuit Design Critical Heat Flux in Boiling Duty 8 . Lewin Reboiler Circuit Design 10 Daniel R.20 PLANT DESIGN . it can be as low as 20-25%.19 PLANT DESIGN . Distillation Group Inc.Daniel R. 8 . For any given exchanger the limit depends upon the construction details and the system involved.Daniel R. 06 × 13) = 1.16 ft in actual design Reboiler Circuit Design PLANT DESIGN .06 ρ1 − ρ2 = 13 ft --.7 × 16 − 4.22 Minimum draw-off nozzle elevation: 288∆p − 3ρ2 288 × 1.750 lb/hr ρL = 36. Lewin 11 Daniel R.Daniel R.Daniel R.46 − 3 × 4. Lewin.19 0.39 0.86 psi Frictions losses: ∆p (psi) ∆p (psi) 8” 10” 0.850 lb/hr ρ1 = 36.7 lb/ft3 Riser: (30% vapor) Liquid: 130.19 0.06 H1 = = 36.7 − 4.95 0.32 = 4.35 2.LECTURE EIGHT 054410 PLANT DESIGN Example Calculation Downcomer: Liquid: 186. Lewin Reboiler Circuit Design Example Calculation Available driving force: ∆P = (1 288 ) ( ρ1H1 − ρ2H2 ) = (1 288 ) (36.71 ∆pr nozzles ∆pe ∆p 8 .7 lb/ft3 vapor: 56.46 ∆pd nozzles 8” 8” 0.13 0.10 0.21 PLANT DESIGN .100 lb/hr ρV = 1.35 1.43 0.7 + 30 1. Technion .06 lb/ft3 8 .10 1.32 lb/ft3 100 ρ2 = 70 30. Lewin. 8 . Distillation Group Inc. Technion .LECTURE EIGHT 054410 PLANT DESIGN Selection of Reboiler Type Source: Andrew Sloley. and their advantages and disadvantages.Daniel R. all these factors reduce to economics. Be familiar with the four main types of reboiler arrangements in use. In the end. Be able to perform sizing calculations for a thermosyphon. Lewin Reboiler Circuit Design 12 Daniel R. No one-size fits all selection exists. Many factors influence reboiler type selection. Be able to select and design the appropriate reboiler circuit for a given application. Every plant will weight the trade-off between these factors differently. Major factors include: o Plot space available o Total duty required o Fraction of tower liquid traffic vaporized o Fouling tendency o Temperature approach available o Temperature approach required 8 . Lewin Reboiler Circuit Design Summary After reviewing this lecture. you should: Understand the physics behind a thermosyphon.Daniel R.24 PLANT DESIGN .23 PLANT DESIGN .