Once the process design stage ends, the equipment design begins. This stage of design converts process requirements into actual hardware.
Selection Guide for Tray Column
Tray tower is the mostly preferred for high solid concentrated feed slurries and for large diameter columns.
The factors discussed below also influences the choice between trays & packing. As these are guidelines for selection of trays or packing for a particular service, it is recommended to analyze each design case on its own merit for selection.
|
Sr. No. |
System Favoring Tray Column |
System Favoring Packed Column |
|
01 |
Solid handling |
Vacuum system |
|
02 |
High liquid rate feed composition and
temperature |
Low pressure drop application, Revamps- The pressure drop reduction |
|
03 |
Large diameter columns | Small diameter columns< 900 mm |
|
04 |
Performance prediction is easy | Corrosive system Or Foaming system |
Types of Tray
The particular tray selection and its design can materially affect the performance of a given distillation, absorption, or stripping system. Each tray should be designed so as to give as efficient a contact between the vapors and liquid as possible, within reasonable economic limits.
A] SCM Valve Tray
Valve trays are perforated sheet metal decks on which round, liftable valves are mounted. The vapors flows through valves which are installed parallel to the outlet weir.
Vapour flows lifts the caps, thus self-creating a flow area for the passage of vapour. The lifting cap directs the vapour to flow horizontally into the liquid, thus providing better mixing than is possible in sieve trays. Valve trays combine high capacity and excellent efficiency with a wide operating range.
Valve trays are also classified as,
- Fix Valve
- Float Valve
Fixed Valve Trays Floating Valve Trays Advantages:
- Excellent liquid/ vapors contacting.
- Higher capacity.
- Higher flexibility than sieve trays.
- Can handle higher loadings.
- Low-pressure drop than bubble cap.
B] SCM Sieve Tray
Sieve trays are flat perforated plate in which vapour rises through small holes which punched downwardly in tray floor, & bubbles through liquid in fairly uniform manner. They have comparable capacity as valve trays. Vapour passes straight upward through the liquid on the plate. The arrangement, number of holes and size of the holes are design parameters.
Pressure drop depends upon the direction of holes punched. The smaller hole diameter results in lower pressure drop and vice versa. This occurs due to ratio of hole diameter to the tray thickness. Number of holes and hole diameter is depending upon the vapor flow rate required. Sieve trays can have lowest turndown ratio.
Advantages:
- Simple construction Low entrainment,
- low cost Low maintenance cost
- Low fouling tendency
Disadvantages:
- Less-flexible to varying loads than
C) SCM Bubble Cap Tray
Riser or chimney fitted over each hole which covers the entire riser area. The cap is mounted so that there is a space between riser and cap to allow the passage for vapour. Vapour rises through the chimney and is directed downward by the cap. In this type of trays, gas strikes the cap and dispersed back to the liquid. After specific pressure achieved by the vapors, the cap floats and it allows vapors to flow. Finally, vapours discharges through slots in the cap, and bubbled through the liquid on the tray.
This type of tray allows mixing vapor with liquid. The size of the cap creates the hydraulic gradient across the tray deck and a high vapor side pressure drop. So, it makes it comparatively costlier.
Applications:
- Works well in high turndown ratio.
Advantages:
- It prevents liquid weeping.
- Tray deck is leak-proof.
Disadvantages:
- Only can operate at low vapor and liquid rates. (Less than 2 GPM per foot of average of flow width).
- It’s expensive.
Selection Of Tray Tower
This chart compares type of trays which are widely used in the industry.
| Tray Type | Capacity | Efficiency | Pressure Drop | Entrainment | Turndown Ratio | Cost |
| Bubble Cap | Medium-High | Medium- High | High | Approx. 3 times than sieve tray |
Excellent |
Double than sieve tray |
| Valve | High-Very High | High | Medium-High | Medium | 4-10 | 50% more than sieve tray |
| Sieve | High | High | Medium | Medium | 2.1 | Cheapest |