Trays, sieve illustration

Illustration 6.4 Estimate the efficiency of the sieve tray of Illustration 6.3. 

Figure 8-123 illustrates a typical sieve tray capacity chart. Entrainment by jet flooding or limitation by downcomer flooding are two of the main capacity limiting factors. The liquid backup in the downcomer must balance the pressure drop across the tray, with the process balance [209]. 

Figure 8-124 illustrates a typical pressure drop diagram for a sieve tray. Note that the figure is for liquid flowing... 

Figures 8-133-136 illtistrate the correlation of the data with the proposed model and resulting design procedttre. Additional illustrations accompany the reference. For Figure 8-133 the Cy and Cl parameters are plotted. For sieve trays, the actual hole velocities are used where for the Type-T valve tray the hole velocities are calctilated based on the maximum open area, A y. ...

The calculated entrainment values may be as good or better than measured values [183]. Figure 8-139 illustrates comparison of entrainment between bubble cap and sieve trays. Fair [183] concludes that for vacuum to moderate pressure applications, sieve trays are advantageous from an entrainment-flooding stand-point. 

Figure 8-140. Studies of sieve tray and bubble cap tray flooding (24-in. tray spacing). (Note that the references listed on the illustrations in Figure 8-140 are from the original source, while Ref. 185 Is from this text.) Used by pennission. Fair, J. R., Petro/Chem Engineer, Sept. (1961) p. 45, reproduced courtesy Petroleum Engineer International Dallas, Texas.

Pulsing means that either the whole liquid content of a sieve tray column is continually pushed up and down by a piston that moves to and fro, or the whole plate package is moved up and down [3]. Figure 9.5 illustrates the two extractor constructions schematically. They show about the same efficiency... 

F igure 14-36 illustrates the pressure drop of a typical moving valve tray as a function of gas velocity. At low velocities, all valves are closed. Gas rises through the crevices between the valves and the tray deck, with increasing pressure drop as the gas velocity rises. Once point A, the closed balance point (CBP), is reached, some valves begin to open. Upon further increase in gas velocity, more valves open until point B, the open balance point (OBP), is reached. Between points A and B, gas flow area increases with gas velocity, keeping pressure drop constant. Further increases in gas v ocity increase pressure drop similar to that in a sieve tray. 

Because best results are normally obtained with full crossflow plate operation for a sieve tray, units may be designed with a lift valve over the hole in the plate or over a riser from the plate so that the rising gas lifts this valve to allow the vapors to be passed horizontally into the liquid as is illustrated in Fig. 16-1. This liquid cannot easily flow back down the holes in the plate when the gas flow is low because the valve tends to close with the reduced gas flow. 

Examples 3 and 4 presented in the following illustrate methods for estimating pressure drop with bubble-cap contactors and with sieve-tray contactors. The examples also give information as to typical design conditions for the two types of contactors. 

Example 14.3 below illustrates the calculation of the tower diameter for a sieve tray. 

Valve trays. Figure 6.216 illustrates the dry pressure drop of a t31>ical valve tray as a function of vapor velocity. At low vapor velodties, all valves are dosed (i.e., seated on the tray deck). Vapor rises through the crevices between the valves and the tray deck, and fiiction losses through these crevices constitute the dry pressure drop. Once the closed balance point (CBP) is reached, there is sufficient force in Uie rising vapor to open some valves, A further increase in vapor velodty opens more valves. Since vapor flow area increases as valves open, pressure drop remains constant until all valves open. This occurs at the open balance point (OBP). Further increases of vapor velodty cause the dry pressure drop to escalate in a similar manner to a sieve tray. When two weights of valves are used in alternate rows on the tray, a similar behavior applies to each valve type. The result is the pressure drop-vapor velocity relationship in Fig. 6.19c,... 

Efficiencies can be scaled up from laboratory data taken with an Oldershaw column (a laboratory-scale sieve-tray column) tFair et al.. 1983 Kister. 1990T The overall efficiency measured in the Oldershaw column is often very close to the point efficiency measured in the large commercial column. This is illustrated in Figure 10-15. where the vapor velocity has been normalized with respect to the fraction of flooding IFair et al 19831. The point efficiency can be converted to Murphree and overall efficiencies once a model for the flow pattern on the tray has been adopted (see section 16.6T... 

Figure 12.16 Illustration of possible types of slurry bubble column reactors, (a) Simple bubble column, (b) cascade bubble column with sieve trays, (c) packed bubble column, (d) multishaft bubble column, and (e) bubble column with static mixers [61].

Figure 18—13 illustrates how sagging tray decks can aggravate the problem of low vapor flows. Also, the preceding discussion is almost as applicable to valve trays as to sieve trays. Valve trays most certainly leak at low vapor rales regardless of any vendor claims to the contrary. 

The distillation column illustrated in Figure 19.3 is used to separate benzene and toluene. It contains 35 sieve trays, with the feed on tray 18. The relevant flows are given in Table 19.1. Your assignment is to recommend changes in the tower operation to handle a 50% reduction in feed. Overhead composition must be maintained at 0.996 mole fraction benzene. Cooling water is used in the condenser, entering at 30°C and exiting at 45°C. Medium-pressure steam (185°C, 1135 kPa) is used in the reboiler. 

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