Tower Design and SpecificationProcess Engineering

By Travis Abrams

Tower InternalsTwo routes to go: Trays or Packing

Which one to choose?Advantages of Packing• Good where low pressure drop is desired. For hydrocarbon systems :vapor density

and liquid-to-vapor density ratio where structured packing has few, if any, problems are: Vapor density less than 1.5 lb/ft3 (24 kg/m3).

• Liquid-to-vapor density ratio greater than 18.

• Packing can sometimes handle higher throughput for a revamp situation, depending on the liquid loading.

• Packing is more tolerant of foaming system, which is why it is used for TEG contactors.

• Packing is good for extremely corrosive systems because plastics and ceramics can be used.

Trays VS Packing

Trays

• Trays will most likely be less expensive for pressures up to 150 psig and they can later be debottlenecked with packing

• Trays should probably be used above 150 psig.

• Theoretical trays are easier to define than HETP of packing

Types of Trays2. Sieve Trays

And fixed valve trays

3. Bubble Cap Trays 4. Grid Trays

5. Duel flow trays

1. Valve Trays

Types of packing

• Random: Pall rings, intalox, -many to choose from!

• Structured-usually higher capacity for the same HETP than random packing. Also lower pressure drop than random packing

Tray Design

• Trays have two principal features

1. Active Area

Active Area

Inlet Weir

Outlet Weir

Sloped Downcomer

2. Downcomers

Weir Height

Downcomer Clearance

Tray Passes

• How many passes does a Tower Need?

• 1 pass through 5 pass

• With tower sizing programs use trial and error, changing the tray pass to get smallest tower diameter

Tower Sizing Programs

• Koch-Glitch• Sultzer• Old Nutter program available-same as

Sultzer• Old Norton program available, but it is the

same as Kock-Glitch.• Jaeger program has HETP calculations• FRI for sieve, bubble cap, duel flow in old

card reading fortran code

Relief Weirs

• Used to give better liquid distribution for segmental downcomers that are on the small side

Weir Baffles and Picket weir

• For low liquid/vapor ratios (spray regime)

3/4 of tray spacing

Feed Distributors

• Liquid Feed

• Always a symetrical design for equal distribution

• Pipe and Slot size in distributor should be 2-5 ft/sec for liquids and 20-30 ft/sec for vapor

• Mixed phase: add liquid and vapor sized areas together, avoid slug flow!

Distributor Placement

Reboiler Placement

Tray Spacing

• 18 to 24 inches is the normal range depends on hieght of toewr VS ID of tower economics.

• Manway or feed section is 30 to 36 inches.• Bottom downcomer seal is trayspacing plus 6”.• Top section is usually 4 feet from top tray to

tangent or seam.• Manways every 10-15 trays (depends on client

specification)

Packing Internals

Revamping a Trayed Diesel HDS Fractionator

11.00

5.00feet

5.50

23.25feet

2.00feet

2.00feet

3.00feet

3.00feet

2.00feet

10.00feet

2.00feet

NEW HIGH EFFICIENCYPACKING (STRUCTURED)

NEW DRAW TRAY ANDREDISTRIBUTOR

NEW HIGH EFFICIENCYPACKING (STRUCTURED)

NEWMANWAYAND NEWLEVELNOZZLES

NEWKEROSENEDRAW

EXISTING CHIMNEY TRAYAND DISTRIBUTOR WILLBE REUSED AS IS.

NEW 10" FEED NOZZLE NEW 10" FEED NOZZLE

NEW TOP DISTRIBUTORTRAY

HDS FRACTIONATOR

TRAY 19

TRAY 18

TRAY 17

7.25feet

Running the Koch program KG Tower

Kg-tower.lnk

Reboiler Placement

• Kettle Reboiler with bottom surge in kettle

• Kettle Reboiler with surge in bottom of tower

Thermosyphon reboilers

• Less than a theoretical stage • One theoretical stage

Once Through Reboiler

Drawing the Tower:The Process Sketch

• Helpful (but not essential) to draw to scale

• For mutipass trays-start the tray pass at the bottom to set the bottom tray layout

• Dimension Everything!

Top Section

Outlet Vapor

Vent Nozzle

Reflux Distributor

Temperature

Column Feed

Tower Middle

Bottom of Tower is larger because Of thermosyphon Reboiler arrangementAnd liquid hold up requirements

Tower Bottom