Sieves efficiency

However, if one uses aggressive sieve motion on friable particles, it may result in particle attrition during the sieving process and result in the underestimation of particle size. Thus, a balance between sieve efficiency and the particle properties should be considered when selecting a method of analysis. 

Similarly, 2,3,5-trimethylphenol (239) was converted into the quinone 240 ca 80%) using H3PMoi204o"" or titanium substituted aluminophosphate (TiAPO-5) molecular sieves" . Efficient oxidation of phenols to the corresponding quinones has also been effected with H2O2-V-HMS (vanadium-containing mesoporous molecular sieves)" . 

A special case of acceptance efficiency is the one known as screen or sieve efficiency E, which assumes a perfectly formed screen so that the underflow is free from particles larger than the equivalent screen diameter x. According to this criterion. Equation 10.23 can be reexpressed as... 

Pulsed Columns. The efficiency of sieve-plate or packed columns is increased by the appHcation of sinusoidal pulsation to the contents of the column. The weU-distributed turbulence promotes dispersion and mass transfer while tending to reduce axial dispersion in comparison with the unpulsed column. This leads to a substantial reduction in HETS or HTU values. 

The pulsed-plate column is typically fitted with hori2ontal perforated plates or sieve plates which occupy the entire cross section of the column. The total free area of the plate is about 20—25%. The columns ate generally operated at frequencies of 1.5 to 4 H2 with ampHtudes 0.63 to 2.5 cm. The energy dissipated by the pulsations increases both the turbulence and the interfacial areas and greatly improves the mass-transfer efficiency compared to that of an unpulsed column. Pulsed-plate columns in diameters of up to 1.0 m or mote ate widely used in the nuclear industry (139,140). 

The efficiency ranking of desiccants in drying air is not always the same as that observed in drying other materials. Other materials may interact with the desiccants to reduce drying effectiveness. From a study of the efficiency of some 25 desiccants for drying several families of laboratory solvents and reagents it was concluded that molecular sieves are the desiccants of choice in most cases (9—17). 

D. R. Burfield,/ Org. Chem. 49, 3852—3854 (1984). Part 9, efficiency of dryiag 1,4-dioxane, water-saturated diethylether, and acetonitrile with CaSO, CaCl2, and molecular sieves 3A and 4A. 

Research. Much of the research on commercial-size distiUation equipment is being done by Fractionation Research, Inc. (FRI), a nonprofit, industry-sponsored, research corporation. The industrial sponsors are fabricators, designers, and constmctors, or users of distiUation equipment. PubHcations include Hquid mixing on sieve plates (91), bubble cap plate efficiency (92), and sieve plate efficiency (93,94). A motion picture of downcomer performance is also avaUable (95). References 96 and 97 cover the Hterature from 1967 to 1990. 

Example 8 Calculation of Rate-Based Distillation The separation of 655 lb mol/h of a bubble-point mixture of 16 mol % toluene, 9.5 mol % methanol, 53.3 mol % styrene, and 21.2 mol % ethylbenzene is to be earned out in a 9.84-ft diameter sieve-tray column having 40 sieve trays with 2-inch high weirs and on 24-inch tray spacing. The column is equipped with a total condenser and a partial reboiler. The feed wiU enter the column on the 21st tray from the top, where the column pressure will be 93 kPa, The bottom-tray pressure is 101 kPa and the top-tray pressure is 86 kPa. The distillate rate wiU be set at 167 lb mol/h in an attempt to obtain a sharp separation between toluene-methanol, which will tend to accumulate in the distillate, and styrene and ethylbenzene. A reflux ratio of 4.8 wiU be used. Plug flow of vapor and complete mixing of liquid wiU be assumed on each tray. K values will be computed from the UNIFAC activity-coefficient method and the Chan-Fair correlation will be used to estimate mass-transfer coefficients. Predict, with a rate-based model, the separation that will be achieved and back-calciilate from the computed tray compositions, the component vapor-phase Miirphree-tray efficiencies. 

Minimum allowable capacity of a column is determined by the need for effective dispersion and contacting of the phases. The types of plates differ in their ability to permit Tow flows of gas and liquid. A cross-flow sieve plate can operate at reduced gas flow down to a point where liquid drains through the perforations and gas dispersion is inadequate for good efficiency. Valve plates can be operated at veiy... 

If design shows a condition above the appropriate curve of Fig. 14-27, weeping will not be deleterious to plate periormance as measured by a drop in plate efficiency (as in Fig. 14-24 for the sieve plate). 

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. 

Direct Scale-Up of Laboratory Distillation Ljficiency Measurements It has been found by Fair, Null, and Bolles [Ind. Eng. Chem. Process Des. Dev., 22, 53 (1983)] that efficiency measurements in 25- and 50-mm (1- and 2-in-) diameter laboratory Oldersbaw columns closely approach tbe point efficiencies [Eq. (14-129)] measured in large sieve-plate columns. A representative comparison of scales of operation is shown in Fig. 14-37. Note that in order to achieve agreement between efficiencies it is necessaiy to ensure that (1) tbe systems being distilled are tbe same, (2) comparison is made at tbe same relative approach to tbe flood point, (3) operation is at total reflux, and (4) a standard Oldersbaw device (a small perforated-plate column with downcomers) is used in tbe laboratoiy experimentation. Fair et al. made careful comparisons for several systems, utibzing as large-scale information tbe published efficiency studies of Fractionation Research, Inc. 

FIG. 14-37 Overall column efficiency of 25-mm Oldersbaw column compared with point efficiency of 1,22-m-diameter-sieve sieve-plate column of Fractionation Research, Inc, System = cyclohexane-n-heptane, [(Fair, Null, and Bolles, Ind, Eng, Chem, Process Des, Dev, 22, 53 (i.982),]... 

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