Bubble caps trays

In this type of tray, each opening assembly consists of a cap, or inverted cup, with slots at its base, fixed above an opening. The opening consists of a hole and a riser through which vapor rises from the tray below. Its flow is reversed downward by the cap, after which the vapor flows down around the riser and bubbles out through the slots and into the liquid. Bubble cap diameters are usually about three to four inches. Because of the liquid seal created by the riser, bubble caps can operate at wide ranges of vapor and liquid flows with little loss in tray efficiency. 

The first continuous distillation tower built was the Patent Still used in Britain to produce Scotch whiskey, in 1835. The patent still is to this day employed to make apple brandy in southern England. The original still, and the one 1 saw in England in 1992, had ordinary bubble-cap trays (except downpipes instead of downcomers were used). The major advantage of a bubble-cap tray is that the tray deck is leakproof. As shown in Fig. 4.5, the riser inside the cap is above the top of the outlet weir. This creates a mechanical seal on the tray deck, which prevents hquid weeping, regardless of the vapor flow. 

Bubble-cap trays maybe operated over a far wider range of vapor flows, without loss of tray efficiency. It is the author s experience that 

There really is no proper answer to this question. It is quite likely that the archaic, massively thick, bolted-up, cast-iron bubble-cap or tunnel-cap tray was the best tray ever built. However, compared to a modern valve tray, bubble-cap trays 

But in the natural gas fields, where modern design techniques have been slow to penetrate, bubble-cap trays are still widely employed to dehydrate and sweeten natural gas in remote locations. 

The problem we have been discussing—loss of tray efficiency due to low vapor velocity—is commonly called turndown. It is the opposite of flooding, which is indicated by loss of tray efficiency at high vapor velocity. To discriminate between flooding and weeping trays, we 

Bubble-cap trays maybe operated over a far wider range of vapor flows, without loss of tray efficiency. It is the author s experience that bubble-cap trays fractionate better in commercial service than do perforated (valve or sieve) trays. Why, then, are bubble-cap trays rarely used in a modern distillation  

However, the largest operating cost for many process units is the energy supplied to the reboilers. We should therefore avoid high reflux rates, and try to achieve the best efficiency point for distillation tower trays at a minimum vapor flow. This is best done by designing and installing the tray decks and outlet weirs as level as possible. 

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Fig. 27. Computed and experimental Hquid temperature profiles in an ammonia absorber with 5 bubble cap trays (107). Water was used as a solvent.

The lye boHer is usuaHy steam heated but may be direct-fired. Separation efficiency may be iacreased by adding a tower section with bubble-cap trays. To permit the bicarbonate content of the solution to buHd up, many plants are designed to recirculate the lye over the absorber tower with only 20—25% of the solution flowing over this tower passiag through the boHer. Several absorbers may also be used ia series to iacrease absorptioa efficieacies. 

The peripheral stiffening zone (tray ring) is generally 25 to 50 mm (1 to 2 in) wide and occupies 2 to 5 percent of the cross section, the fraction decreasing with increase in plate diameter. Peripheiy waste (Fig. 14-28) occurs primarily with bubble-cap trays and results from the inabihty to fit the cap layout to the circular form of the plate. Valves and perforations can be located close to the wall and little dead area results. Typical values of the fraction of the total cross-sectional area available for vapor dispersion and contact with the liquid for cross-flow plates with a chord weir equal to 75 percent of the column diameter are given in Table 14-6. 

Fair s empirical correlation for sieve and bubble-cap trays shown in Fig. 14-26 is similar. Note that Fig. 14-26 incorporates a velocity dependence (velocity) above 90 percent of flood for high-density systems. The correlation implicitly considers the tray design factors such as the open area, tray spacing, and hole diameter through the impact of these factors on percent of flood. 

Here are two quick approximation methods for bubble cap tray column diameter. 

Available in metal only, compared more with tray type performance than other packing materials. About same HETP as Spraypak for available data. Used In towers 24 inches and larger. Shows some performance advantage over bubble cap trays up to 75 psia in fractionation service, but reduced advantages above this pressure or in vacuum service. 

A common type of distillation contacting device used in refinery applications is the sieve tray. In the early 50 s and for many years before, the bubble cap tray was the mainstay of the distillation field. A sieve tray consists of a flat plate with regularly spaced holes, normally 1/2 to 1 inch in diameter. Liquid flows horizontally across the tray and into a channel, called a downcomer, which leads to the tray below. The sieve tray exhibits good capacity, excellent efficiency, low pressure drop, and good flexibility i.e., it will operate quite efficiently at tower loadings which are 1/2 to 1/3 of design values. 

Trays are generally divided into four categories (1) sieve trays, (2 ) val ve tray s. (3) bubble cap trays, and (4) high capacity/high efficiency trays. 

At low vapor rates, valve trays will weep. Bubble cap trays cannot weep (unless they are damaged). For this reason, it is generally assumed that bubble cap trays have nearly an infinite turndown ratio. This is true in absorption processes (e.g., glycol dehydration), in which it is more important to contact the vapor with liquid than the liquid with vapor. However, this is not true of distillation processes (e.g., stabilization), in which it is more important to contact the liquid with the vapor. 

As vapor rates decrease, the tray activity also decreases. There eventually comes a point at which some of the active devices (valves or bubble caps) become inactive. Liquid passing these inactive devices gets very little contact with vapor. At very low vapor rates, the vapor activity will concentrate only in certain sections of the tray (or, in the limit, one bubble cap or one valve). At this point, it is possible that liquid may flow across the entire active area without ever contacting a significant amount ot vapor. This will result in very low tray efficiencies for a distillation process. Nothing can be done with a bubble cap tray to compensate for this. 

Fnr stripping service, as in a glycol or amine contactor (see Chapt 7 a bubble cap trays are the most common. In recent years, there has b growing movement toward crimped sheet structured packing. Improved vapor and liquid distributor design in conjunction with struc-... 

In Table 8-2 Proctor [178] compares efficiencies of sieve and bubble cap trays (plates). He concludes that the sieve design provides a 15% improvement in plate efficiencies. To fully evaluate the actual efficiencies in any particular system, the physical properties, mechanical details of the trays, and flow rates must be considered. See Reference 2 also. 

Entrainment Only about one-third that of bubble cap trays. 

Capacity Quite similar to sieve tray, as high or higher than bubble cap tray from 50% up to 100% design rate (varies with system and design criteria). Performance at specification quality falls off at lower rates. 

Figure 8-65. Slip-type cartridge assembly for bubble cap trays in small column, 1-ft-10 5<6-in. I.D. Used by permission, Glitsch, Inc.

Figure 8-63. Bubble cap tray schematic—dynamic operation.

In addition nearly all of the major tray manufacturers can and do design bubble cap trays as well as the other t)pes on request for comparison with competitive types of trays. 

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