Bubble cap and valve trays

The plates may be any of several types, including sieve, bubble-cap, and valve trays. Valve trays constitute multiple self-adjusting orifices that provide nearly constant gas pressure drop over considerable ranges of variation in gas flow. The gas pressure drop that can be taken across a single plate is necessarily limited, so that units designed for high contacting power must use multiple plates. 

When conparing tray designs the turndown ratio is inqjortant because it is a measure of the flexibihty of a column in dealing with a change in flow rate. The turndown ratio is defined as the ratio of the maximum to minimum operating flow rate. For bubble cap and valve trays, the turndown ratio is about ten whereas for sieve trays it is only about three. 

Figure 6.18 Flooding factor for sieve, bubble cap, and valve trays. From Ref. 52 with permission.

As shown in Fig. 2.4, a minimum vapor rate exists below which liquid may weep or dump through tray perforations or risers instead of flowing completely across the active area and into the downcomer to the tray below. Below this minimum, the degree of contacting of liquid with vapor is reduced, causing tray efficiency to decline. The ratio of the vapor rate at flooding to the minimum vapor rate is the turndown ratio, which is approximately 10 for bubble cap and valve trays but only about 3 for sieve trays. 

Bubble cap and valve trays have very complex geometrical stmctures that make the rigorous prediction of the orifice coefficients rather difficult. As such elements are built in great numbers, experimental data of orifice coefficients are often provided by the vendors. 

For the calculation of the dry pressure loss of bubble cap and valve trays a shghtly different approach is recommended. The orifice coefficients are not related to the velocity in the smallest geometrical open area but to the whole active area of the tray since the quality of tray design depends not only on the pressme loss of a single element but also on the number of elements arranged on the tray. The following definition implies both quantities ... 

Fig. 5.4-11 Correlation of the orifice coefficients of bubble cap and valve trays...

In addition. Figure 13-20 shows the relative effect of column capacities on efficiencies. As seen, the behavior for bubble cap and valve trays is not very sensitive to capacity, in contrast to sieve trays and packed columns. 

Adsorbers, distillation colunuis, and packed lowers are more complicated vessels and as a result, the potential exists for more serious hazards. These vessels are subject to tlie same potential haz. uds discussed previously in relation to leaks, corrosion, and stress. However, llicse separation columns contain a wide variety of internals or separation devices. Adsorbers or strippers usually contain packing, packing supports, liquid distributors, hold-down plates, and weirs. Depending on tlie physical and chemical properties of the fluids being passed tlirough tlie tower, potential liazards may result if incompatible materials are used for llie internals. Reactivity with llie metals used may cause undesirable reactions, which may lead to elevated temperatures and pressures and, ullinialely, to vessel rupture. Distillation columns may contain internals such as sieve trays, bubble caps, and valve plates, wliicli are also in conlacl with tlie... 

The performance analysis of these trays is quite similar to bubble caps, but more so to valve trays, because the tray has the same basic mechanical features. The difference being that bubble caps and valves are replaced by perforations or holes in the tray for entrance of the gas to the liquid on the tray. Figures 8-67A and 8-118 and 119 represent the general construction of a sieve tray. 

Specifying the need for a tray-type column, the type of tray must be determined. Sieve trays are considered most appropriate for this application. They offer a simple and inexpensive construction with low pressure drop (if the hydraulic design is adequate). Bubble cap and valve-type trays offer advantages in controlling liquid droplet entrainment, but pose significant difficulties for installation of cooling coils. 

Fig. 3.1. Also note that this nonconventional design has the downcomer outlet area as additional active tray area. This additional active area is the tray deck area under the downcomer having valves, bubble caps, or sieve holes that allow the gas to pass through under the liquid downcomer area of the next tray up. ICPD tray programs dealing with the design and rating of sieve, bubble cap, and valve-type trays allow this active area input. This is an option shown in Table 3.1, which is offered in the three tray design/rating computer programs given in this book.

As you can see in Tables 3.6 and 3.7, the valve, bubble cap, and sieve trays share many of the same input prompts in the tray design programs supplied on the accompanying CD. 

To this point we have covered general tray design for any type of tray— valve, bubble cap, or sieve type. Beginning here, we will review the design and rating of valve-type trays, followed by bubble cap and sieve tray design and rating. 

Valve trays (see Figure 12-19) are units in which holes are covered with movable caps whose rise varies with gas flow rate. The valve tray is a very widely used device because it represents a useful compromise between the bubble cap and sieve trays. 

The tray as described is known as a sieve tray and it has perforations of up to about 12 mm diameter, although there are several alternative arrangements for promoting mass transfer on the tray, such as valve units, bubble caps and other devices described in Section 11.10.1. In all cases the aim is to promote good mixing of vapour and liquid with a low drop in pressure across the tray. 

Valve trays. These may be regarded as a cross between a bubble-cap and a sieve tray. The construction is similar to that of cap types, although there are no risers and no slots. It may be noted that with most types of valve tray the opening may be varied by the vapour flow, so that the trays can operate over a wide range of flowrates. Because of their flexibility and price, valve trays are tending to replace bubble-cap trays. Figure 11.53 shows a typical tray. 

The valve tray, which may be regarded as intermediate between the bubble cap and the sieve tray, offers advantages over both. The important feature of the tray is that liftable... 

In plate columns the two phases are intensively mixed on each plate and separated between each plate (Fig. 6.7-5). For the distribution of the light phase through the liquid a lot of devices were developed. The simplest one is a perforated sieve tray, where the supercritical phase can pass through. To avoid weeping of the liquid through the holes different devices like bubble caps or valves (Fig. 6.7-6) were developed. 

The O Connell correlation was based on data for bubble-cap trays. For sieve and valve trays, its predictions are likely to be slightly conservative. 

The discussions in this chapter emphasize sieve and valve trays, as these trays Eire most frequently encountered in industrial practice. Several of the considerations also apply to other tray types (e.g., bubble-cap trays). Considerations unique to bubble-cap trays were excluded from this chapter. The infrequent application of this type of tray in modern distillation practice argnes against a detailed discus-... 

The bubble-cap tray was the workhorse of distillation before the 1960s. It was superseded by the sieve and valve trays. Presently, bubble-cap trays are specified only for special applications, while sieve and valve trays are the moat popular types. 

---