Trays, sieve hydraulics

Trays operate within a hydraulic envelope. At excessively high vapor rates, liquid is carried upward from one tray to the next (essentially back mixing the liquid phase in the tower). For valve trays and sieve trays,. i capacity limit can be reached at low vapor rates when liquid falls through the rray floor rather than being forced across the active area into tlic downcomers. Because the liquid does not flow across the trays, it rass.scs contact with the vapor, and the separation efficiency drops dramatically. ... 

Koch, R. and Kuzniar, J. (1966) International Chem. Eng. 6 (Oct.) 618. Hydraulic calculations of a weir 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. 

Sieve trays are used throughout the absorption column, however two distinct hydraulic designs are required. The first sieve plate design is required for trays below the weak-acid feed point. Above the weak-acid feed point, the downcoming liquid flowrate is diminished. The vapour flowrate essentially remains constant throughout the column. Different vapour to liquid ratios above and below the weak-add feed point require a second hydraulic design to be considered. 

Correct the HHDS sieve tray hydraulic gradient with the beta factor (froth correction) ... 

For valve trays, the hydraulic gradient is somewhat larger than that of sieve trays, probably not by much (13). In a similar manner to sieve trays, it is often neglected in the pressure drop calculation (7-9,71,80). In cases of a long flow path of liquid, it should be checked using the Hughmark and O Connell correlation above. 

Pressure Drop. Since efficiency depends on hydraulic factors, a method will first be given for estimating pressure drop across a sieve tray. The total drop across the tray is taken as the sum of the drop through the holes plus the drop through the aerated mixture ... 

Steve Tray Capacity at Flooding The capacity of a sieve tray is determined by hydraulic mechanisms involved in flooding and is not... 

For pressures near atmospheric, bubble cap trays with the smaller caps are approximately equivalent in hydraulic capacity and separating efficiency to valve or sieve trays. 

This area of sieve tray froth hydraulics has been the snbject of considerable stndy. While the relationships thus far presented are not completely fundamental, they are simple to apply and are sufficiently reliable for most engineering purposes. 

The segments of the buildup are (a) the equivalent clear liquid bead on the tray h Ll (b) any hydraulic gradient A caused by resistance to liquid flow across the tray, which usually is not significant for sieve trays, (c) liquid head equivalent to pressure loss due to flow under the downcomer apron. A. and (d) total pressure loss across the tray above, necessarily included to maintain the dynamic pressure balance between point A (just above the floor of tray 3) and point B in the vapor space above tray 2. 

Figure 12-8 Friction factor used in the calculation of the hydraulic gradient, hg, for sieve trays with crossflow. [B. D. Smith, Design of Equilibrium Stage Processes, McGraw-Hill Book Company, New York, 1963, by courtesy McGraw-Hill Book Company.]...

The tray hydraulics supplies equations to calculate the liquid molar hold-up from geometry, as well as for coupling the pressure drop with vapour and liquid tray flows. The following relations hold for sieve trays. The liquid flow rate over a wire crest of length W and height is given by the formula (Francis) ... 

TRAYS allows the design of sieve-trays, sieve-valve, tunnel-valve, Baycr-valves, bubble trays, Vario-Flex-valve - including all hydraulics parameters and tray efficiency for absorption, desorption and rectification. 

Design a suitable sieve tray for the absorber of Prob. 8.4 and compute its hydraulics (which may be considered constant for aU the trays) and the number of real trays required, graphically and through Eq. (8.16). Take the surface tension as 30 dyn/cm. 

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