Turndown sieve trays

Figure 8-71 A. Nutter MVO high performance fixed valve tray with 4 1 turndown ratio. Used in new installations and to replace sieve trays. Used by permission, Nutter Engineering, Harsco Corp., Bui. CN-4.

Trays are usually designed with F-factor from 0.25 to 2.0 for a turndown of 8 1. Pressure drop per theoretical stage falls between 3 and 8 mm Hg. Note that bubble cap trays are on the high side and sieve trays are on the lower end of the range. Varying tray spacing and system efficiency, the HETP for trays are usually between 24 in. and 48 in. [133]. The C-factor is the familiar Souders and Brown capacity equation. 

Packed beds also seem to have a better turndown capability than valve or sieve trays, at low vapor flows. On the other hand, many packed fractionators seem quite intolerant of reduced liquid or reflux flow rates. This is typically a sign of an improperly designed distributor in the packed fractionator. 

Sieve trays (Fig. 14-18a) are perforated plates. The velocity of upflowing gas keeps the liquid from descending through the perforations (weeping). At low gas velocities, liquid weeps through the perforations, bypassing part of the tray and reducing tray efficiency. Because of this, sieve trays have relatively poor turndown. 

Fixed valve and sieve trays prevail when fouling or corrosion is expected, or if turndown is unimportant. Valve trays prevail when high turndown is required. The energy saved, even during short turndown periods, usually justifies the small additional cost of the moving valve trays. 

Dual-Flow Trays These are sieve trays with no downcomers (Fig. 14-27b). Liquid continuously weeps through the holes, hence their low efficiency. At peak loads they are typically 5 to 10 percent less efficient than sieve or valve trays, but as the gas rate is reduced, the efficiency gap rapidly widens, giving poor turndown. The absence of downcomers gives dual-flow trays more area, and therefore greater capacity, less entrainment, and less pressure drop, than conventional trays. Their pressure drop is further reduced by their large fractional hole area (typically 18 to 30 percent of the tower area). However, this low pressure drop also renders dual-flow trays prone to gas and liquid maldistribution. 

The turndown of valve trays is much better than sieve trays, but not as good as bubble-cap trays. Bubble-cap trays are the moBt suitable to handle extremely low liquid rate applications (less than 2 gpm per foot of average flow width (10)]. 

Turndown About 2 1. Not generally suitable for operation under variable loads About 4-5 1. Some special designs achieve (or claim) 10 1 or more Excellent, better than valve trays. Good at extremely low liquid rates Low, even lower than sieve trays (10). Unsuitable for variable load operation... 

For most other services, either sieve or valve trays are the best choice, Sieve trays are at an advantage when the service is fouling, or corrosive, or when turndown is unimportant, while valve trays are preferred when turndown is important. With high energy costs, the energy saved during even short turndown periods usually justifies the relatively low cost difference between valve and sieve trays. This has made valve trays most popular. 

Sieve trays have a poor turndown (about 2 1). The minimum operating throughput in sieve trays is almost always restricted by excessive weeping, Turndown of sieve trays can be improved by... 

Using low fractional hole areas A fractional hole area reduction to about 5 percent of the bubbling area typically boosts sieve tray turndown to about 3 to 4 1 at the expense of a lower maximum capacity, i.e., of a larger column diameter. This technique is not recommended because traying the column with valve trays is normally a cheaper alternative. 

However, sieve trays are not recommended for the following conditions (1) when very low pressure drop (less than 2.5 mmHg, or 0.39 kPa) is required (2) when high turndown ratios are required at low pressure drop or (3) when very low liquid rates are required below either 0.25 gal/(min)(ft2) [0.6 m3/(h)(m2)] of active tray area, or 1 gal/(min)(ft) [0.75 m3/(h)(m)] of average flow-path width. 

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. 

Three types of trays dominate the market sieve, valves and bubble-cap. Numerous commercial shapes exist. Sieve trays are low-cost in purchasing and maintenance, and robust in operation, can be designed for high throughput, but are penalised by a relatively low turndown, of about 2 1. The resistance to fouling is good, the entrainment moderate. The pressure drop can be lowered below down to 3 to 4 mbar. The market share of sieve trays is about 25%. 

Valve trays keeps the feature of sieve trays, but have the important advantage of a higher vapour turndown of 4 to 5 or more. They are not suitable for separations where high probability of fouling exists. Valve trays dominate the market with about 70% of the applications, although they are 2-3 times more expensive than sieve trays. 

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. 

When vapor and liquid flow rates change appreciably from tray to tray, column diameter, tray spacing, or hole area can be varied to reduce column cost and insure stable operation at high efficiency. Variation of tray spacing is particularly applicable to columns with sieve trays because of their low turndown ratio. 

---