Weirs height

FIG. 14-24 Performance of two crossflow plates operating at 0.13 bar pressure and total reflux. Test mixture etbylbenzene/styrene. Spacing between plates is 0.50 m, and outlet weir height is 38 mm. Ut = superficial vapor velocity, pc = vapor density. [Billet, Comad, and Giuhh, I. Chem. E. Symp. Ser. No. 32, 5, 111 (1969).]... 

Weir height is less than 15 percent of plate spacing... 

The value of weir height crest may be calculated from the Francis weir equation and its modifications for various weir types. For a segmental weir and for height in millimeters of clear liquid,... 

For sieve or valve plates, h = h , outlet weir height. For bubble-cap plates, h = height of static seal. Tbe original references present vaH-dations against laboratoiy and small-commercial-column data. Modifications of tbe efficiency equation for absorption-stripping are also included. 

Billet and cowodcers [Chem. Jng. Tech., 38, 825 (1966) Jnstn. Chem. Engrs. Symp. Set No. 32, 5, 111 (1969] used the ethylbenzene/ styrene system at 100 torr and a 0.8-m column with 500-mm plate spacing. Two weir heights were used, 19 and 38 mm. Operation was at... 

Testing of plates and other devices is carried out by Fractionation Research, Inc. for industrial sponsors. Some of the test data for sieve plates have been published for the cyclohexane//i-heptane and isobu-tane//i-butane systems. Representative data are shown in Fig. 14-43. These are taken from Sakata and Yanagi Jn.stn. Chem. Engis. Symp. See. No. 56, 3.2/21 (1979)] and Yanagi and Sakata [Jnd. Eng. Chem. Proc. Des. Devel, 21, 712 (1982)]. The column diameter was 1.2 m, tray spacing was 600 mm, and weir height was 50 mm. 

GPM = Column liquid loading, gal/min Hj,. = Downcomer backup, inches of liquid hi = Condensing side film coefficient, Btu/hrft °F H,(j = Head loss under downcomer, inches of liquid H v = Weir height, ins. 

FA = fractional free area h, = weir height, inches... 

Dynamic Slot Seol,hdi Outjet Weir Height l totic Slot Seal,h ... 

The adjustable weir feature of many tray designs allows a standard tray to be utilized in different services by readjusting the weir height as needed. The fixed portion of the weir should never be lower than the top of the slots of the bubble caps. Depending upon service, the adjustable weir... 

Referring to Figure 8-63, the weir height, h, must always be greater than the clearance under the downcomer, i.e., between bottom of downcomer and tray floor, hdci-Always avoid too low clearance as this can cause flooding of liquid in the downcomer. There are flow conditions... 

Some authors recommend clearance of if in. to in. less than the tray weir height, but always greater than if in. [190]. 

The bottom of the downcomer must be sealed below the operating liquid level on the tray. Due to tolerance in fabrication and tray level, it is customary to set the downcomer seal referenced to the weir height on the outlet side of the tray. Recommended seals, based on no inlet weir adjacent to the downcomer, and referenced as mentioned are given in Table 8-19. 

Inlet weir height above tray floor, 3 in. 

This may be calculated as recommended for bubble cap trays. Minimum weir height is 0.5-in., with 1-3 in. preferred. See Figure 8-67A. 

For stable tray operation, the hydraulic gradient should be less than one half the dry tray pressure drop. For conditions of high weir height and high Vq (Pv) the greater... 

Figure 8-137. Flooding capacity, sieve trays weir height is less than 15% of tray spacing low- to non-foaming system hole area at least 10% hole sizes Ms-in. to M-in. dia. surface tension = 20 dynes/cm. Used by permission, Fair, J. R., Petro/Chem. Engineer, Sept (1961), p. 46, reproduced courtesy of Petroleum Engineer International, Dallas, Texas.

From a consideration of contacting requirements, a tower 9.5 ft in diameter is selected. Other pertinent details are 24-in. tray spacing, 1-in. weir height, /e-in. dia. holes, 10% hole area (referred to active area) and 8.3 ft downcomer area. 

Intensifies as outlet weir height increases and as the liquid flow rate increases. 

Select as quite common Me-in. dia. holes, with hole area/tower area = 0.10, 14 U S Std. gauge stainless steel tray material, which is 0.078 in. thick, 2-in. weir height, and 24-in. tray spacing. 

Area, Hole Diameter, Hole Spacing Weir Height and Downcomer Area, Pet. Ref. V. 34, p. 114 (1935). 102. 

An optimal tray design, one that balances tray and downcomer area so that neither prematurely restricts capacity, and set weir height, weir geometry, clearance under the downcomer, and fractional hole area so as to maximize efficiency and capacity. 

Weir heights are 2 in., weir lengths about 75% of tray diameter, iiquid rate a maximum of about 8 gpm/in. of weir muitipass arrangements are used at high liquid rates. 

Ln(weir) is zero for the condition Mn< Mns. M is the mass of liquid on plate n, and Mns is the mass of liquid on the plate corresponding to the weir height or static liquid holdup on the plate. The rate of loss of liquid from the plate by weepage, however, will depend on the total mass of liquid on the plate... 

Efficiencies will be lower for vacuum distillations, as low weir heights are used to keep the pressure drop small (see Section 11.10.4). 

It is important to set close tolerances on the weir height, downcomer clearance, and plate flatness, to ensure an even flow of liquid across the plate. The tolerances specified will depend on the dimensions of the plate but will typically be about 3 mm. 

Make a trial plate layout downcomer area, active area, hole area, hole size, weir height (Sections 11.13.8 to 11.13.10). 

Weir height less than 15 per cent of the plate spacing. 

The height of the weir determines the volume of liquid on the plate and is an important factor in determining the plate efficiency (see Section 11.10.4). A high weir will increase the plate efficiency but at the expense of a higher plate pressure drop. For columns operating above atmospheric pressure the weir heights will normally be between 40 mm to 90 mm (1.5 to 3.5 in.) 40 to 50 mm is recommended. For vacuum operation lower weir heights are used to reduce the pressure drop 6 to 12 mm ( to in.) is recommended. 

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