Efficiency, bubble plates

Nonisothermal Gas Absorption. The computation of nonisothermal gas absorption processes is difficult because of all the interactions involved as described for packed columns. A computer is normally required for the enormous number of plate calculations necessary to estabUsh the correct concentration and temperature profiles through the tower. Suitable algorithms have been developed (46,105) and nonisothermal gas absorption in plate columns has been studied experimentally and the measured profiles compared to the calculated results (47,106). Figure 27 shows a typical Hquid temperature profile observed in an adiabatic bubble plate absorber (107). The close agreement between the calculated and observed profiles was obtained without adjusting parameters. The plate efficiencies required for the calculations were measured independendy on a single exact copy of the bubble cap plates installed in the five-tray absorber. 

Rate of Mass Transfer in Bubble Plates. The Murphree vapor efficiency, much like the height of a transfer unit in packed absorbers, characterizes the rate of mass transfer in the equipment. The value of the efficiency depends on a large number of parameters not normally known, and its prediction is therefore difficult and involved. Correlations have led to widely used empirical relationships, which can be used for rough estimates (109,110). The most fundamental approach for tray efficiency estimation, however, summarizing intensive research on this topic, may be found in reference 111. 

Much of this work was carried out using a special distilling column called a bubble-plate column (Fig. 141). Each plate really does act like a distilling flask with a very efficient column, and one distillation is really carried out on one physical plate. To calculate the number of plates (separation steps, or distillations) for a bubble-plate column, you just count them ... 

Geddes, R. L. Trans. Am. Inst. Chem. Eng. 42 (1946) 79. Local efficiencies of bubble plate fractionators. 

An equimola mixture of benzene and toluene is to be separated in a bubble-plate tower at the rate of 100 kg mol/h at 1 atm pressure. The overhead product must contain at least 98 mole percent benzene. The feed is saturated liquid. A tower is available containing 24 plates. Feed may be introduced either on the eleventh or the seventeenth plate from the top. The maximum vaporization capacity of the reboiler is 120 kg mol/h. The plates are about 50 percent efficient. How many moles per hour of overhead product can be obtained from this tower ... 

At near atmospheric pressures, oxidation and absorption rates are slow, and earlier atmospheric and low-pressure plants used between five and ten very large towers. Maximum strengths attainable were in the range of 42% to 52% HNO3. To reduce capital investment and to obtain increased operating efficiencies, absorption under pressure was developed, using various types of equipment, e.g., cascade coolers, packed columns, spray towers, and columns incorporatir bubble plates, sieves, and such special devices as the Kuhlman tray. 

In practice the packed tower has been losing out relative to the bubble tower. The development of corrosion-resistant alloys and of bubble-plate columns made of ceramic, glass, and plastic has made it possible to rectify corrosive mixtures in such units. The development of efficient laboratory bubble-plate columns as small as 1 in. in diameter has made it possible to carry out such distillation in the laboratory, and experience has indicated that these columns give data that are... 

The various perforated or bar-type plates have hi capacities (see curve 2y Fig. 16-13) and appear to produce the same efficiencies as bubble or Kaskade plates. Of even greater importance, they are mechanically very simple compared with bubble plates. In several designs, no downcomers whatever are provided. Turbogrid trays fail to function properly if the actual load is under about 40 per cent of the design load. 

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. 

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. 

The method for estimating point efficiency, outhned here, is not the only approach available for sieve plates, and more mechanistic methods are under development. For example, Prado and Fair [Ind. Eng. Chem. Re.s., 29, 1031 (1990)] have proposed a method whereby bubbling and jetting are taken into account however the method has not been vahdated tor nonaqueous systems. Chen and Chuang [Ind. Eng. Chem. Re.s., 32, 701 (1993)] have proposed a more mechanistic model for predicting point efficiency, but it needs evaluation against a commercial scale distillation data bank. One can expect more development in this area of plate efficiency prediction. 

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