V¿NGIN€€RING O FACTS AT YOUR
FINGERTIPS
Distillation TVay Design
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 perforated 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
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 crosssectionol area minus the downcomer top and downcomer bottom area. The minimum active area (ft2) for normal valve trays can be determined from the following relotionship, which is o modification of a commonly used correlation [ /] token at 82% of jet flood: Active area = V-Load / [T^^ (0.0762 -
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 downcomer 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 onequarter of the total downcomer area, and make the center (and off
0.00092(pV,))-0.0nW,] Where, V-Load =CFS^{pV/[pL-pV¡)°'
Vapor flow
j ÎÎÎ Î
O
'
TS = Tray spacing, in. p V = Vapor density, Ib/ft^ = Weir loading, gal/min per in. = Vapor volumetric flow, ft^/s
Liquid flow
Side view of a simple tray arrangement
Generolly, designing a column troy entails determining the minimum downcomer 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 velocity. 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
o
0.3
Z i "^s I f 0.2
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 loading 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 determined by the following:
ers one-holf of the total downcomer areo, Maintaining the resulting downcomer widths at 6 in. or more will allow o person A, = A D/op to reach into the dawncomer for ¡nstollo^Dbottom tion. In oddition, make sure the resulting troy-flow poth-length is 16 in. or greoter Where, to enable a person to physicolly poss A; = Tower area, ft^ through the trays. These minimum size ^Dtop = Downcomer area at top, ft^ criterio moy increase the column diameter Aoboimm = Downcomer orea a( bottom, ft' to above the previously calculated value. AA = Active Areo, ft^ D = Tower inner dia., ft Other considerations Other criteria that need to be considNumber of downcomers ered are; downcomer backup, spray Once the tower diameter is determined, fluidization, and entrainment. In oddition, then the number of downcomers can be minimum load conditions need to be chosen. As o starting point, an initial determined. The criteria for determining design should use a single downcomer. the low-end vapor ond liquid range are The resulting weir length is calculated weeping, tray stability and dry-tray presfrom a stondard chord-length calculasure drop. tion, which is iterative for o given downcomer area. Reference
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
Where, By^. = Weir length of one downcomer, ft
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 technology manager, Sulzer Chemtech USA Inc
Copyright of Chemical Engineering is the property of Chemical Week Associates and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use.
