7 replies to this topic

Why sieve trays are more efficient than bubble cap trays.

In case of sudden plant shutdown all fuild fuild in sieve column comes down. what should the precaution take to avoid the losses & how to take again startup for the plant?

[djack77494]

Why sieve trays are more efficient than bubble cap trays.

In case of sudden plant shutdown all fuild fuild in sieve column comes down. what should the precaution take to avoid the losses & how to take again startup for the plant?

I'm not sure where you've gotten the impression that sieve trays are more efficient than bubble cap trays. If you refer to the degree that an actual tray approaches true vapor-liquid equilibrium (a theoretical tray), then I believe the opposite is more true. Sieve trays could probably be claimed as being more efficient in terms of utilization of the tray's cross sectional area however. You can pack more openings per area in a sieve tray than a bubble cap tray, and you can tolerate greater vapor (and probably liquid as well) traffic in a sieve tray. As you note, sieve trays possess the characteristic of being self-draining, which might be viewed as undesirable. When you shutdown the column, the liquid will all drain into the sump. This is in contrast to bubble cap trays which may retain their liquid inventory over an extended period of time if provisions are not made for self drainage.

Dear Dijack,

As per "Chapter no. 6 of Distillation design of Henry Kister" i can say that sieve trays are more efficient than Bubble cap trays in terms of capacity, efficiency, cost, maintenance, fouling and erosion.

But as you rightly said the problem with sieve tray is self draining. Can have drain tank in the bottom of column. Can anybody comment on this?

[Andree]

I agree with Djack: as far as I know the bubble cap trays are more efficient that sieve trays - the efficiency is expressed as degree of approaching the liquid/vapor equilibrium, and this parameter is better for bubble cap trays comparing to sieve ones... of course optimum coice should take into account also other operational parameters as pressure drop, throughput and also consideration on cost and maintenance, reliablility etc... These are probably mostly in favour to sieve trays. And this is what probably author of the cited reference could have in mind as "efficiency"

[fallah]

But as you rightly said the problem with sieve tray is self draining. Can have drain tank in the bottom of column. Can anybody comment on this?

I think in the case of sieve tray, the volume of tower bottom shall be as large as possible tollerating all liquid existing in every moment of tower operation.

Regards

[djack77494]

I think in the case of sieve tray, the volume of tower bottom shall be as large as possible tollerating all liquid existing in every moment of tower operation.

I agree with fallah. The column's sump should be designed to handle the full liquid inventory in the column so as to avoid potential problems in the event of an unexpected shutdown.

Andree has correctly elaborated on what I intended when comparing the efficiencies of bubble cap vs. sieve trays. In most other ways, such as economically, efficient utilization of space, throughput for a given diameter, etc., sieve trays would outperform bubble cap trays.
Doug

[engprocess]

BUBBLE CAP TRAYS
Bubble cap assemblies serve to disperse the vapor on the tray and to maintain a minimum level of liquid on the tray. A few of the many kinds that have been used are shown in Fig. 13.31, together with their mode of action on a tray. The most used sizes are 4 or 6 in. diameter round caps, although for special services (such as cryogenic distillation) smaller sizes (down to 1 in.) have been used. Because of their greater cost, weight, higher pressure drop, and creation of a hydraulic gradient across the tray, bubble caps are rarely used these days. Since they maintain a positive seal on the tray, they are sometimes used for low liquid flow situations and for reactive distillations involving reactions in the liquid phase. The allowable vapor velocity for bubble cap trays is about the same as for sieve trays, and Figure 13.32( may be used. The most complete published source of design and performance prediction of these trays is given by W. L. Bolles (in Smith, 1963). Example 13.15 shows the calculation of tray diameter. A factor that is of concern with bubble cap trays is the development of a liquid gradient from inlet to outlet which results in corresponding variation in vapor flow across the cross section and usually to degradation of the efficiency. With other kinds of trays this effect rarely is serious. Data and procedures for analysis of this behavior are summarized by Bolles (in Smith, 1963, Chap. 14). There also are formulas and a numerical example of the design of all features of bubble cap trays. Although, as mentioned, new installations of such trays are infrequent, many older ones still are in operation and may need to be studied for changed conditions.
SIEVE TRAYS
A liquid level is maintained with an overflow weir while the vapor comes up through the perforated floor at sufficient velocity to keep most of the liquid from weeping through. Hole sizes may range from 1/8 to 1 in., but are mostly ¼–1/2 in. Hole area as a percentage of the active cross section is 5–15%, commonly 10%. The precise choice of these measurements is based on considerations of pressure drop, entrainment, weeping, and mass transfer efficiency. The range of conditions over which tray operation is satisfactory and the kinds of malfunctions that can occur are indicated roughly in Figure 13.32(a) and the behavior is shown schematically on Figure 13.31©.
The required tower diameter depends primarily on the vapor rate and density and the tray spacing, with a possibly overriding restriction of accommodating sufficient weir length to keep the gpm/in. of weir below about 8. Figure 13.32( is a correlation for the flooding velocity. Allowable velocity usually is taken as 80% of the flooding value. Corrections are indicated with the figure for the fractional hole area other than 10% and for surface tension other than 20 dyn/cm. Moreover, the correction for the kind of operation given with Figure 13.33 for valve trays is applicable to sieve trays. Weir heights of 2 in. are fairly standard and weir lengths about 75% of the tray diameter. For normal conditions downcomers are sized so that the depth of liquid in them is less than 50% and the residence time more than 3 sec. For foaming and foam-stable systems, the residence time may be two to three times this value. The topic of tray efficiency is covered in detail in Section 13.6, but here it can be stated that they are 80–90% in the vicinity of F= uv√vapor density for mixtures similar to water with alcohols and to C6 _ C7 hydrocarbons.
A detailed design of a tray includes specification of these items:
1. Hole dia, area, pitch and pattern.
2. Blanking of holes for less than eventual load.
3. Downcomer type, size, clearance, and weir height.
4. Tray thickness and material.
5. Pressure drop.
6. Turndown ratio before weeping begins.
7. Liquid gradient.
VALVE TRAYS
The openings in valve trays are covered with liftable caps that adjust themselves to vapor flow. Illustrations of two kinds of valves are in Figure 13.31(. The caps rest about 0.1 in. above the floor and rise to a maximum clearance of 0.32 in. The commonest hole diameter is 1.5 in. but sizes to 6 in. are available. Spacing of the standard diameter is 3–6 in. With 3 in. spacing, the number of valves is 12–14/sqft of free area. Some of the tray cross section is taken up by the downcomer, by supports, and by some of the central manway structure.
In spite of their apparent complexity of construction in comparison with sieve trays, they usually are less expensive than sieve trays because of their larger holes and thicker plates which need less
Formulas and procedures for calculation of detailed tray specifications may be obtained from the manufacturers of valve trays.

[Zauberberg]

What is your concern if there is no liquid level in trays when column is shut down? All common trays are made in such way they cannot hold liquid if there is no upward force of vapors, and there is no single reason for maintaining liquid level on the tray when fractionation process is stopped - except in the case when you need sufficient surge volume of liquid for maintaining tower heat balance until the whole unit is shut down. This applies for accumulator trays provided for circulating reflux draw-off, or if you are feeding downstream unit from one particular tray in the tower.

Sieve trays are more efficient for mass transfer compared to bubble-cap trays, which are usually employed in applications with low liquid rates. They are more resistant to weeping and are able to provide higher liquid inventory on the tray (important for TEG absorbers, for example).

Self-draining of trays is very important design and operational parameter, because it makes provision for maintenance. Even seal-welded collector trays (the ones which should have as less leakage in operation as possible) are made with drain holes for that purpose. Imagine that you have to shut down the tower and prepare it for inspection - the liquid which cannot be drained or steamed-out from the tower creates a potential hazard during maintenance activities. Because of this fact, all trays should be designed for self draining. The number and size of drain points (holes) is dependent on acceptable leakage rate during normal operation, and the size of these self-draining holes is usually such it doesn't affect regular tray operation and performance.

Best regards,