Distillation Tray Design
2
V¿NGIN€€RING O O o FACTS AT YOUR FINGERTIPS Department Editor: Scott Jenkins I n a distillation column troy, vopor posses upward through liquid that is flowing acroS5 a horizontol perforated plate. Vapor passing through the perfo- rated plate forms a two-phase mixture with the liquid and enables moss transfer contocting. This mixture is typicolly quite turbulent. Troy design must oilow the turbulent liquid to foil oway From the rising vapor in the space above the tray, while also enabling the vopor bubbles to rise out of the falling liquid in the downcomer. The downcomer is usuolly o verticol plote thot enables the already contacted froth to travel down to the next troy without remixing with the up-fiowing vapor from the tray below. Downcomers Vapor flow j Î Î Î Î ' Liquid flow Side view of a simple tray arrangement Generolly, designing a column troy en- tails determining the minimum downcom- er oreo thot still allows vapor bubbles to rise through the liquid, selecting the number of downcomers, determining the octive area, and checking the flow path length to see if o person can pass through a tray manway. These foctors ore the primary drivers for determining overall tower size. Downcomer area is determined by the maximum recommended downcomer ve- locity. Divide the volumetric flow of liquid by the downcomer velocity to obtain the downcomer top area. Typically a curve of maximum downcomer velocity versus the density difference between liquid and vapor is consulted during this process. Maximum downcomer velocity guideline 0.45 .£ 0.4 £.•=0.35 p u " o |w 0.3 Z i "^s If 0.2 0.15 10 20 30 40 50 60 Delta-density (PL-PV), Ib/ft^ A downcomer is generally straight unless its areo exceeds 8% of the tower area. In that case, the downcomer is sloped such thot its bottom area is 60% of its top orea. Active area The octive orea of a distillation tower is where the vapor contacts the liquid to effect mass transfer. Above the active area, where the liquid falls away from the rising vopor, is the volume where the vapor can expand. Typically, the active area is colculated to be the tower cross- sectionol area minus the downcomer top and downcomer bottom area. The minimum active area (ft2) for nor- mal valve trays can be determined from the following relotionship, which is o modification of a commonly used correla- tion [ /] token at 82% of jet flood: Active area = V-Load / [T^^ (0.0762 - 0.00092(pV,))-0.0nW,] Where, V-Load =CFS^{pV/[pL-pV¡)°' TS = Tray spacing, in. p V = Vapor density, Ib/ft^ = Weir loading, gal/min per in. = Vapor volumetric flow, ft^/s The required octive oreo is dependant on the vapor density and weir loading. Note that the weir iooding need not be known at this point. Assume a weir load- ing value of 5 gol/min per in. intially. Typical troy spacings are 24 in. Tower area and diameter Bosed on the above oreos, the overall tower area and diameter con be deter- mined by the following: A, = A D/op ^Dbottom Where, A; = Tower area, ft^ ^Dtop = Downcomer area at top, ft^ Aoboimm = Downcomer orea a( bottom, ft' AA = Active Areo, ft^ D = Tower inner dia., ft Number of downcomers Once the tower diameter is determined, then the number of downcomers can be chosen. As o starting point, an initial design should use a single downcomer. The resulting weir length is calculated from a stondard chord-length calcula- tion, which is iterative for o given downcomer area. Where, By^. = Weir length of one downcomer, ft Distillation TVay Design A good place to stort the iterotive process is with a weir length 0.8 times the tower diameter. If the resulting weir Iooding is greoter than 12 gal/min per in,, then increase the number of troy passes to two. Recalculate the outlet weir length for each of the side downcomers of the column by using half the downcom- er area. Check the weir Iooding again (for the troy with side downcomers). If the weir loading continues to exceed 12 gal/min per in,, increase the number of troy passes to four. It is assumed that the two-pass tray with side downcomers has the shortest weir length. The simplest approoch to designing 4-pass trays is to assume equol bubbling area and make the side downcomers one- quarter of the total downcomer area, and make the center (and off<enter) downcom- ers one-holf of the total downcomer areo, Maintaining the resulting downcomer widths at 6 in. or more will allow o person to reach into the dawncomer for ¡nstollo- tion. In oddition, make sure the resulting troy-flow poth-length is 16 in. or greoter to enable a person to physicolly poss through the trays. These minimum size criterio moy increase the column diameter to above the previously calculated value. Other considerations Other criteria that need to be consid- ered are; downcomer backup, spray fluidization, and entrainment. In oddition, minimum load conditions need to be determined. The criteria for determining the low-end vapor ond liquid range are weeping, tray stability and dry-tray pres- sure drop. Reference 1. Glitsch Inc. "Ballast Tray Design Manual; Bui ietin No. 4900." 3rd Ed. Gtitsch Inc.. Dallas, Tex, 1974, Noie: Materiol for the June "Focts at Your Finger tips" was supplied by Dan Summers, tray technol- ogy manager, Sulzer Chemtech USA Inc
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Transcript of Distillation Tray Design
V¿NGIN€€RINGO
O
o
FACTS AT YOUR FINGERTIPSDepartment Editor: Scott Jenkins
In a distillation column troy, voporposses upward through liquid that isflowing acroS5 a horizontol perforated
plate. Vapor passing through the perfo-rated plate forms a two-phase mixturewith the liquid and enables moss transfercontocting. This mixture is typicolly quiteturbulent. Troy design must oilow theturbulent liquid to foil oway From therising vapor in the space above the tray,while also enabling the vopor bubblesto rise out of the falling liquid in thedowncomer. The downcomer is usuollyo verticol plote thot enables the alreadycontacted froth to travel down to the nexttroy without remixing with the up-fiowingvapor from the tray below.
Downcomers
Vapor flow
j Î Î Î Î' Liquid flow
Side view of a simple tray arrangement
Generolly, designing a column troy en-tails determining the minimum downcom-er oreo thot still allows vapor bubblesto rise through the liquid, selecting thenumber of downcomers, determiningthe octive area, and checking the flowpath length to see if o person can passthrough a tray manway. These foctorsore the primary drivers for determiningoverall tower size.
Downcomer area is determined by themaximum recommended downcomer ve-locity. Divide the volumetric flow of liquidby the downcomer velocity to obtain thedowncomer top area. Typically a curveof maximum downcomer velocity versusthe density difference between liquid andvapor is consulted during this process.
Maximum downcomer velocity guideline
0.45
. £ 0.4
£.•=0.35p u" o| w 0.3
Z i " sI f 0.2
0.1510 20 30 40 50 60
Delta-density (PL-PV), Ib/ft
A downcomer is generally straightunless its areo exceeds 8% of the tower
area. In that case, the downcomer issloped such thot its bottom area is 60%of its top orea.
Active areaThe octive orea of a distillation tower iswhere the vapor contacts the liquid toeffect mass transfer. Above the activearea, where the liquid falls away fromthe rising vopor, is the volume where thevapor can expand. Typically, the activearea is colculated to be the tower cross-sectionol area minus the downcomer topand downcomer bottom area.
The minimum active area (ft2) for nor-mal valve trays can be determined fromthe following relotionship, which is omodification of a commonly used correla-tion [ /] token at 82% of jet flood:
Active area = V-Load / [T^^ (0.0762 -0.00092(pV,))-0.0nW,]
Where,V-Load =CFS^{pV/[pL-pV¡)°'TS = Tray spacing, in.p V = Vapor density, Ib/ft^
= Weir loading, gal/min per in.= Vapor volumetric flow, ft^/s
The required octive oreo is dependanton the vapor density and weir loading.Note that the weir iooding need not beknown at this point. Assume a weir load-ing value of 5 gol/min per in. intially.Typical troy spacings are 24 in.
Tower area and diameterBosed on the above oreos, the overalltower area and diameter con be deter-mined by the following:
A, = AD/op ^Dbottom
Where,A; = Tower area, ft^Dtop = Downcomer area at top, ftAoboimm = Downcomer orea a( bottom, ft'AA = Active Areo, ftD = Tower inner dia., ft
Number of downcomersOnce the tower diameter is determined,then the number of downcomers can bechosen. As o starting point, an initialdesign should use a single downcomer.The resulting weir length is calculatedfrom a stondard chord-length calcula-tion, which is iterative for o givendowncomer area.
Where,
By^. = Weir length of one downcomer, ft
DistillationTVay Design
A good place to stort the iterotiveprocess is with a weir length 0.8 timesthe tower diameter. If the resulting weirIooding is greoter than 12 gal/min perin,, then increase the number of troypasses to two. Recalculate the outlet weirlength for each of the side downcomersof the column by using half the downcom-er area. Check the weir Iooding again(for the troy with side downcomers). Ifthe weir loading continues to exceed 12gal/min per in,, increase the number oftroy passes to four. It is assumed that thetwo-pass tray with side downcomers hasthe shortest weir length.
The simplest approoch to designing4-pass trays is to assume equol bubblingarea and make the side downcomers one-quarter of the total downcomer area, andmake the center (and off<enter) downcom-
ers one-holf of the total downcomer areo,Maintaining the resulting downcomer
widths at 6 in. or more will allow o personto reach into the dawncomer for ¡nstollo-tion. In oddition, make sure the resultingtroy-flow poth-length is 16 in. or greoterto enable a person to physicolly possthrough the trays. These minimum sizecriterio moy increase the column diameterto above the previously calculated value.
Other considerationsOther criteria that need to be consid-ered are; downcomer backup, sprayfluidization, and entrainment. In oddition,minimum load conditions need to bedetermined. The criteria for determiningthe low-end vapor ond liquid range areweeping, tray stability and dry-tray pres-sure drop.
Reference1. Glitsch Inc. "Ballast Tray Design Manual; Bui
ietin No. 4900." 3rd Ed. Gtitsch Inc.. Dallas,Tex, 1974,
Noie: Materiol for the June "Focts at Your Fingertips" was supplied by Dan Summers, tray technol-ogy manager, Sulzer Chemtech USA Inc
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