Ind. Eng. Chem. Res. 1988,27, 716-718
716
administered by the US.Department of Agriculture.
Literature Cited Beattie, J. L.; Cole, W. M. US Patent 4 591 631, 1986. Beinor, R. T.; Cole, W. M. US Patent 4623713, 1986. Belmares, H.: Jimenez, L. L.; Ortega, M. Znd. Eng. Chem. Prod. Res. Deu. 1980, I9(1), 107. Coran. A. Y. In Science and Technology of Rubber; Eirich, F. R., Ed.; Academic: New York, 1978; ppi91-338. Eagle, F. A. Rubber Chem. Technol. 1981,54(3), 662. Engler, C. R.; McIntyre, D. Dept. of Commerce Report EDA/RED84-43, Feb 1984; pp 7-12. Hall, G. E., Jr. US Patent 2713572, 1955. Kay, E. L.; Gutierrez, R. US Patent 4 542 191, 1985. McFadden, K.; Nelson, S. H. Dept. of Energy Report DOE/ER/ 30006-T1, Sep 1981; pp 78-79.
McLaughlin, S. P. Econ. Bot. 1985, 39(4), 473. Schloman, W. W., Jr.; Davis, J. A. US Patent 4621 118, 1986a. Schloman, W. W., Jr.; Davis, J. A. US Patent 4616068, 198613. Schloman, W. W., Jr.; Davis, J. A. US Patent 4 622 365, 1986c. Schloman, W. W., Jr.; Garrot, D. J., Jr.; Ray, D. T.; Bennett, D. J. J. Agric. Food Chem. 1986, 34(2), 177. Schloman, W. W., Jr.; Hively, R. A,; Krishen, A.; Andrews, A. M. J . Agric. Food Chem. 1983,31(4), 873. Studebaker, M. L.; Betty, J. R. In Science and Technology of Rubber; Eirich, F . R., Ed.; Academic: New York, 1978; pp 367-418. Weihe, D. L.; Nivert, J. J. In Proceedings of the Third International Guayule Conference, Pasadena, Calif., 1980; Gregg, E. C., Tipton, J. L., Huang, H. T., Eds.; Guayule Rubber Society: Riverside, CA, 1983; pp 115-125.
Received for review August 31, 1987 Accepted December 18, 1987
COMMUNICATIONS Concentration Profile Inversion in Distillation Rating Programs with Tray Efficiencies A rating program for a distillation column with a fixed number of trays and with tray efficiencies of less than 100% has been found to display an interesting phenomenon. An inversion in the liquid concentration profile above the feed tray can occur when the rating program is used to calculate the required reflux flow rate to keep product compositions constant as feed composition changes. This requires a modification in the criteria of the rating program to permit a one-tray decrease in liquid composition on the tray above the feed tray. Distillation column simulations are commonly used to analyze two types of problems: design and rating (Buckley et al., 1985). In the design problem, the number of trays required to achieve desired product purities is determined for a given value of the reflux ratio. In the rating problem, the performance of an existing column is analyzed. In this case, the number of trays is fixed, and a trial and error procedure must be used to determine required reflux flows or resulting compositions. In the design of a column, specification of an integer number of trays will generally result in one terminal product which is purer than the design specification. The last step in such a design procedure may require use of a rating program to "fine tune" the reflux flow to produce exactly the desired product concentration. Another common use of a rating program is the determination of steady-state relationships among design variables (e.g., effect of feed concentration changes on reflux flow and reboiler diity to maintain desired product purities). Such steady-state gains (open or closed loop) are important as an initial step in operability analysis. In this report we examine a rating problem in which the number of trays and terminal compositions are fixed. Some interesting results are observed when an attempt is made to calculate the required reflux flow and reboiler duty as the feed composition is changed. To illustrate this situation, the ethanol-water separation at atmospheric pressure is used. ,
Tray Efficiency of 100% First consider ideal trays with a base case feed composition of 20 mol % ethanol. All cases consider a saturated 0888-5885/88/2627-0716$01.50/0
Table I. Design Parameters E, % z NT 0.2 18 100 0.2 72 40 72 40 0.11
NF 2
5 5
RR 1.73 1.74 1.80
QR, 106Btu/h 11.20 11.26 6.30
liquid feed of 10o0 mol/h, distillate composition of 83 mol % ethanol, bottom composition of 1 mol % ethanol, and a partial reboiler. Other design parameters are specified in Table I. Design and rating programs were used to calculate the number of trays and exact reflux flow. Figure 1shows the vapor and liquid compositions in the bottom section of the column, assuming equimolal overflow and 100% tray efficiencies. According to Buckley et al. (1978), if a reflux is guessed that is too small, a premature switch from the stripping to the rectifying operating line can result. This leads to the unrealistic situation illustrated in Figure 2. Such a case will result in negative liquid compositionsand failure of the rating program. Buckley et al. suggest that the problem can be solved by making sure that x N F + ~> xNF, i.e., that composition increases as one moves from the feed tray up to the tray above the feed. If this criterion is not met, it means that the guessed value of reflux is too low and reflux should be increased.
Tray Efficiency of 40% Consider the case in which the tray efficiency is 40%. Of course, the total number of trays and the feed tray are not the same as in the 100% efficiency case. 0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 4, 1988 717 0.60
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nonoptimum feed tray (see Figure 4). For lower feed concentrations, an early switch to the rectifying operating line can result in a reversal in the liquid concentration profile on the tray immediately above the feed tray. Such a situation is shown in Figure 5 for a feed composition of 11 mol %. The switch to the rectifying operating line at tray 5 gives a liquid composition on tray 6 that is lower than that on tray 5. After this one-tray reversal, the liquid composition profile recovers its positive slope. This unusual phenomenon is only possible when tray efficiencies of less than 100% are considered. Note that the vapor compositions increase continuously as one moves up the column, despite the inversion in the liquid composition. McCabe-Thiele diagrams have been used up to this point to illustrate the effect graphically. Similar results have been found in a rigorous nonequimolal overflow simulation. Table I1 gives these results. Liquid composition drops from 8.93% on tray 5 to 5.78% on tray 6. These results illustrate that the mathematics are predicting that the liquid composition profile can show a one-tray inversion when tray efficiencies of less than 100% are considered. This reversal can be explained physically by recognizing that the feed is being introduced onto a very nonoptimum tray. The feed tray composition is higher than the composition that would occur on tray 5 if a more optimum (higher) feed tray were used. If the feed tray
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I n d . E n g . Chem. R e s . 1988,27, 718-721
718
Table 11. Rigorous Simulation (Feed Composition, 11 mol %
Ethanol) tray T,O F
206.88 203.18 198.44 194.17 190.93 188.70 193.24 192.94 192.12 190.43 187.59 184.22 181.42
0 1 2 3 4 5 6 7 8 9 10 11 12
n 0.0100 0.0191 0.0342 0.0528 0.0719 0.0893 0.0578 0.0594 0.0643 0.0755 0.0996 0.1428 0.2027
y 0.0417 0.0950 0.1613 0.2300 0.2920 0.3433 0.3452 0.3483 0.3555 0.3706 0.3973 0.4341 0.4738
L , mol/h 878.05 1232.95 1228.06 1224.37 1222.25 1221.34 207.88 206.83 206.79 206.73 206.80 207.36 208.59
V , mol/h 354.88 350.01 346.33 344.20 343.29 329.83 328.80 328.74 328.69 328.69 329.31 330.54 332.03
were moved up to tray 8, a lower reflux ratio could be used to make the same separation with the same number of total trays in the column. The operating line in this case would be closer to the equilibrium line and the liquid composition on tray 5 would be lower. The higher tray 5 liquid composition (that results when a nonoptimum feed tray is used) should give a high tray 5 vapor composition entering the rectifying section. But, tray 5 vapor composition will not be high if the tray efficiency is low. A component balance around the entire rectifying section shows that x 6 must decrease as y s decreases. Therefore, reducing tray efficiency reduces y5, which reduces x 6 . At some efficiency, x 6 can become less than x 5 . The appropriate criteria to use in a rating program with efficiencies of less than 100% are (1)x N F + ~> 0 and (2) xNF+2
’
xNF+l.
In these examples, Murphree tray efficiencies defined on the basis of the vapor phase have been used, and the tray-to-tray calculations have been performed from the bottom to the top of the column. A similar reversal in the vapor concentration profile could be observed if liquid tray efficiencies were used and the calculations were made from the top to the bottom. This reversal would occur when switching from the rectifying to the stripping operating line prematurely. Murphree tray efficiency has been studied in this paper because it is the most widely used efficiency. Similar results would be expected if other tray efficiencies were
utilized, such as those proposed by Holland, Hausen, and Standart (summarized by King (1971)). If overall column efficiencies were used, the phenomenon of concentration profile inversion will not be seen.
Conclusion The phenomenon of composition inversion can occur in distillation rating programs when tray efficiencies of less than 100% are used. The rating program must be modfied to permit a one-tray decrease in liquid composition on the tray above the feed tray. Nomenclature E = tray efficiency F = feed flow, mol/h L = liquid flow, mol/h N T = total number of trays N F = feed tray QR = reboiler duty, 106Btu/h RR = reflux ratio T = temperature, O F V = vapor flow, mol/h xb = bottoms composition xd = distillate composition x, = nth tray liquid composition XNF = liquid composition on feed tray y n = nth tray vapor composition z = feed composition Literature Cited Buckley, P. S.; Luyben, W. L.; Cox, R. K. Chem. Eng. Prog. 1978, June, 49. Buckley, P. S.; Luyben, W. L.; Shunta, J. P. Design of Distillation Column Control Systems; Instrument Society of America: New York, 1985. King, C. J. Separation Processes; McGraw-Hill: New York, 1971.
Cristian A. Muhrer, Michael A. Collura William L. Luyben* Chemical Process M o d e k n g and Control Research Center Department o f Chemical Engineering Lehigh University Bethlehem, Pennsylvania 18015 Received for review December 23, 1986 Revised manuscript received October 14, 1987 Accepted December 23, 1987
Wet Oxidation Catalyzed by Ruthenium Supported on Cerium( IV) Oxides The activity of precious metal catalysts in the wet oxidation of organic compounds was investigated. Ruthenium was the most active catalyst among the precious metals examined, and cerium(1V) oxide was the most effective support. T h e Ru/Ce catalyst rivaled homogeneous copper catalyst, which is used in the practical wastewater treatment, for the oxidation of n-propyl alcohol, n-butyl alcohol, phenol, acetamide, poly(propy1ene glycol), and acetic acid. In addition, it was especially effective for the oxidation of some compounds with high oxygen content such as poly(ethy1ene glycol), ethylene glycol, formaldehyde, and formic acid. The most general and widely used process for wastewater purification is biological treatment. However, it cannot be applied to the purification of highly contaminated wastewaters or wastewaters containing toxic materials to microorganisms. These wastewaters can be purified by wet oxidation. Organic pollutants in wastewaters are oxidized to carbon dioxide and water under oxygen pressure a t high temperatures, e.g., 423-573 K. The process 0888-5885/88/2627-0~18$01.50/0
has been applied successfully to the treatment of wastewaters discharged from petroleum and petrochemical industries (Tagashira et al., 1976) and to the treatment of pulp and paper mill wastes (Teletzke, 1964). However, the relatively severe operative conditions require high installation and running costs. Therefore, it is desirable to develop catalysts that can be used under milder conditions. The most practical and the most active catalyst is homo@ 1988 American Chemical Society
