Tray overall

Determine the effects of the physical properties of the system on column efficiency. Tray efficiency is a function of (1) physical properties of the system, such as viscosity, surface tension, relative volatility, and diffusivity (2) tray hydraulics, such as liquid height, hole size, fraction of tray area open, length of liquid flow path, and weir configuration and (3) degree of separation of the liquid and vapor streams leaving the tray. Overall column efficiency is based on the same factors, but will ordinarily be less than individual-tray efficiency. 

Use standard sieve trays. Overall tray efficiency is estimated to be 90% for operation at 85% of flooding. 

Performance data on some typical tray and compartment diyers are tabulated in Table 12-10. These indicate that an overall rate of evaporation of 0.0025 to 0.025 kg water/(s m") of tray area may be expected from tray and tray-truck diyers. The thermal efficiency of this type of diyer will vary from 20 to 50 percent, depending on the diying temperature used and the humidity of the exhaust air. In diying to very low moisture contents under temperature restrictions, the thermal efficiency may be in the order of 10 percent. The major operating cost for a tray diyer is the labor involved in loading and unloading the trays. About two labor-hours are required to load and unload a standard two-truck tray diyer. In addition, about one-third to one-fifth of a... 

This overall flow pattern in a distillation column provides countercurrent contacting of vapor and hquid streams on all the trays through the column. Vapor and liquid phases on a given tray approach thermal, pressure, and composition equilibriums to an extent dependent upon the efficiency of the contac ting tray. 

The lighter (lower-boiling) components tend to concentrate in the vapor phase, while the heavier (higher-boihng) components tend toward the liquid phase. The result is a vapor phase that becomes richer in hght components as it passes up the column and a liquid phase that Becomes richer in heavy components as it cascades downward. The overall separation achieved between the distillate and the bottoms depends primarily on the r elative volatilities of the components, the number of contacting trays, and the ratio of the liquid-phase flow rate to the vapor-phase flow rate. 

Tray Efficiencies in Plate Absorbers and Strippers Compn-tations of the nnmber of theoretical plates N assnme that the hqnia on each plate is completely mixed and that the vapor leaving the plate is in eqnihbrinm with the liqnid. In actnal practice a condition of complete eqnihbrinm cannot exist since interphase mass transfer reqnires a finite driving-force difference. This leads to the definition of an overall plate efficiency... 

Tbe best-established theoretical method for predicting E is that of tbe AlCbE [Buhhle-Tray Design Manual, American Institute of Chemical Engineers, New York, 1958). It is based on tbe sequential prediction of point efficiency, Murpbree efficiency, and overall column efficiency ... 

After actual theoretical trays are determined (see Actual reflux and theoretical stages) one needs to estimate the actual physical number of trays required in the distillation column. This is usually done by dividing the actual theoretical trays by the overall average fractional tray efficiency. Then a few extra trays are normally added for offload conditions, such as a change in feed composition. 

A tray spacing of 0.41 m may be employed. All columns are assumed to have an overall efficiency of 25%. The fixed cost of the column is obtained by multiplying the equipment cost by a factor of 5 to account for installation, instrumentation and other ancillary devices. 

Fenske Equation Overall Minimum Total Trays with Total Condenser... 

For a condition of overall total trays allowance is to be made for feed tray effect, then add one more theoretical tray to the total. As demonstrated in the tabulation to follow, allowance should be made for the reboiler and condenser. 

Note that these values for theoretical trays do contain corrections in overall efficiency, and hence are not the actual trays for the binary distillation column. Efficiencies generally run 50-60% for systems of this type which will yield a column of actual trays almost twice the theoretical at the operating reflux. 

Murphree [85] developed point and overall distillation tray efficiencies, which are examined in detail in Reference 2. The expressions are [59] ... 

Biddulph [90] emphasizes the importance of using point efficiencies rather than tray efficiencies or overall column efficiencies, due to the wide fluctuations that often exist. 

The reboiler is considered 100% efficient, and likewise any partial condenser, if used. Therefore the value Nq represents the theoretical trays or stages in the column proper, excluding the reboiler and partial condenser. Eq represents the overall tray efficiency for the system based upon actual test data of the same or similar systems, or from the plot of Figure 8-29, giving operating information preference (if reliable). 

Mols of distillate or overhead product, lb mols/hr or batch distillation, mols Mols component, i, in distillate Vaporization efficiency of steam distillation Overall column efficiency Overall tray efficiency Eqg = Murphree point efficiency, fraction Murphree plate/tray efficiency, = E ... 

Calculate fraction absorbed for each component, assuming a fixed overall tray efficiency for an... 

Unfortimately, the efficiencies for tray and overall column operation are incomplete and nullify to a certain extent some very high quality theoretical performance design. Tray efficiencies may be estimated by Figure 8-29 or Table 8-11. 

Absorption Efficiency, or fraction absorbed Overall tray efficiency, fraction Stripping efficiency, or fraction stripped Fraction of v + li absorbed by the liquid Fraction of loi stripped out of the liquid Mols individual components stripped per hour Total heat of absorption of absorbed components, thousand Btu/day... 

Figure 9-17. Overall comparison of capacity at flood for 24-in. tray spacing with random packing. Reproduced with permission of the American Institute of Chemical Engineers, Kister, H. Z., Larson, K. F., Yanagi, T., Chemical Engineering Progress, V. 90., No. 2 (1994) p. 23 all rights reserved.

The HETP (Height Equivalent to a Theoretical Plate (stage or plate)) is the tray spacing divided by the fractional overall tray efficiency [82]. The transfer unit concept has been useful for generalized correlations [89]. Because packed towers operate with continuously changing compositions through the packed height, the concept... 

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