Technical resources: Column Sizing

Fraction of the crosssectional area available for vapourliquid disengagement decreases when the downcomer area is increased. Thus, optimum design of the tray involves a balance between the tray area and the downcomer area (i.e. the capacity for the tray deck should match the capacity of the downcomer).

Tray operational efficiency:

1. Point Efficiency: Function of vapor flow through froth.

2. Crossflow Eff: Liquid flow path length

Froth height and flow path length are optimized to get max efficiency

Tray Efficiency: Inversely proportional to volatility and viscosity. Eff= 0.492 (alpha*mu)^-0.245

Process engineer provides preliminary tray diameter. Ultimately to be given and confirmed by tray vendor

Tray diameter is determined using correlations and is dependent on:

Tray spacing
Flooding
System Factor
Number of passes

KG tower give preliminary diameter with following inputs:

Vapor: flowrate, density, viscosity
Liquid: flowrate, density, viscosity, surface tension
System factor
Flooding
Tray spacing
Number of passes

Important Parameters:

Tray Spacing:

Set by maintenance requirement/support requirement. (crawling space, cleaning etc). Typically higher diameter column.
Lower tray spacing implies shorter column height => lower cost
Typical tray spacing: 12" and higher

DC Area:

The downcomer area at the top is sized such that the velocity of the ascending vapour bubbles exceeds the downflow velocity of the liquid.
Dependent on liquid residence time in DC. 3 seconds for non-foaming, 9 seconds for foaming
Min DC chord = 60% of column diameter. Minimum DC width, 5 in
Since the separation of the vapour-liquid mixture is complete at the bottom of the downcomer, a sloped downcomer can be used to maximize the active tray area. In this case, the downcomer area at the bottom should be about 60% of that at the top

DC Backup:

30-40% of clear liquid w.r.t tray spacing + weir height
Function of tray pressure drop DC clearance

DC Exit velocity:

Approx 1.5 m/s
Can be reduced by increasing DC clearance.

Weir Design: Outlet weirs are used to control the froth height on the tray.

Weir height 1 - 4" and the downcomer clearance, where the liquid is discharged from the bottom of the downcomer onto the tray below, should be 0.5 in (1.25 cm) smaller than the outlet weir height to ensure a positive downcomer seal

Flooding: typically 80-85% jet flooding.

Flooding is brought about by excessive vapour flow, causing liquid to be entrained in the vapour up the column or froth in DC reaching top of outlet weir.
Decrease in separation efficiency
Flooding increases pressure drop

System Factor: Derating of tray performance to account for foaming. Foaming decreases vapor liquid equilibrium/tray capacity.

Typical hydrocarbon separation: 0.85
Amine: 0.80
Caustic wash: 0.65
SWS: 0.60

Number of passes:

The number of flow passes is set to allow the tray to operate at a weir loading that does not result in excessive weir crest.
The optimum weir loading is 4-6 US gallons per minute and the maximum loading is about 20.
Minimum is better.
Need to increase to cater high liquid capacity.
Start with single pass tray; increase if required.
Need to have minimum flow path length (16") while increasing number of passes.
Short flow path lengths not efficient
Number of Passes, Minimum tower diameter (m) 2 passes, 1.5m. 4 passes, 3 m

Tray open area: 5 to 14%

Valve density: 10 to 14 valves per ft2

Types of trays: Sieve, Valve, Bubble cap

Sieve: simplest construction, low turndown upto 50%, lowest cost, low maintenance, low corrosion

Valve: high turndown upto 20%, moderate cost, maintenance, corrosion

Bubblecap: highest turndown, cost, corrosion and maintenance

Tray Pressure Drop:

Dry Tray DP
Pressure drop without including effect of liquid head. Indication of vapor velocity through tray valves.
Starting point is 2 inches of liquid.
Should be limited to 15% of tray spacing

Total Tray DP: 8-10 mm Hg (1.0-1.4 kPa)

Weir Load: Indication of tray liquid load