Reflux Optimum

Reflux - minimum reflux Optimum reflux - minimum reflux... 

Ratio, cash, 140-142 currenf 140-142 reflux, optimum, 371-376 turnover, 190-191... 

Unless otherwise stated, the usual simplifying assumptions of saturated liquid feed and reflux, optimum feed plate, no heat losses, steady state, and constant molar liquid and vapor flows apply to each of the following problems. Additional problems can be formulated readily from many of the problems in Chapter 10. 

Condenser and eboiler AT. The losses for AT are typically far greater than those for reflux beyond the minimum. The economic optimum for temperature differential is usually under 15°C, in contrast to the values of over 50°C often used in the past. This is probably the biggest opportunity for improvement in the practice of distillation. A specific example is the replacement of direct-fired reboilers with steam (qv) heat. 

T.eflux Tatio. Generally, the optimum reflux ratio is below 1.15 and often below 1.05 minimum. At this point, excess reflux is a minor contributor to column inefficiency. When designing for this tolerance, correct vapor—Hquid equiUbrium (VLE) and adequate controls are essential. 

Checking Against Optimum Design. This attempts to answer the question whether a balance needs to be as it is. The first thing to compare against is the best current practice. Information is available ia the Hterature (13) for large-volume chemicals such as NH, CH OH, urea, and ethylene. The second step is to look for obvious violations of good practice on iadividual pieces of equipment. Examples of violations are stack temperatures > 150° C process streams > 120° C, cooled by air or water process streams > 65° C, heated by steam t/ urbine 65% reflux ratio > 1.15 times minimum and excess air > 10% on clean fuels. 

Redux Ra.te, The optimum reflux rate for a distillation column depends on the value of energy, but is generally between 1.05 times and 1.25 times the reflux rate, which could be used with infinite trays. At this level, excess reflux is a secondary contributor to column inefficiency. However, when designing to this tolerance, correct vapor—Hquid equiUbrium data and adequate controls are essential. 

Optimum Reflux Ratio The general effecl of the operating reflux ratio on fixed costs, operating costs, and the sum of these is shown in Fig. 13-39. In ordinary situations, the minimum on the total-cost cui ve wih geueraUy occur at an operating reflux ratio of from 1.1 to 1.5 times the minimum R = Lv + i/D value, with the lower value corresponding to a value of the relative volatility close to 1. 

FIG. 13-39 Location of the optimum reflux for a given feed and speeiffed sep-aration. 

However, the total number of equilibrium stages N, N/N,n, or the external-reflux ratio can be substituted for one of these three specifications. It should be noted that the feed location is automatically specified as the optimum one this is assumed in the Underwood equations. The assumption of saturated reflux is also inherent in the Fenske and Underwood equations. An important limitation on the Underwood equations is the assumption of constant molar overflow. As discussed by Henley and Seader (op. cit.), this assumption can lead to a prediction of the minimum reflux that is considerably lower than the actual value. No such assumption is inherent in the Fenske equation. An exact calculational technique for minimum reflux is given by Tavana and Hansen [Jnd. E/ig. Chem. Process Des. Dev., 18, 154 (1979)]. A computer program for the FUG method is given by Chang [Hydrocarbon Process., 60(8), 79 (1980)]. The method is best applied to mixtures that form ideal or nearly ideal solutions. 

Optimization As stated previously, optimization studies should include the entire system. Such a study was made by Fair and BoUes [Chem. Eng., 75(9), 156 (1968)], using a hght-hydrocarbon system and with the objective of defining optimum reflux ratio. Coolants used were at —87, —40, and +30°C (—125, —40, and +85°F), corresponding to different pressures of operation and associated different condens-... 

Colburn (chemical engineering lecture notes, University of Delaware, 1943) proposed that the optimum reflux ratio is... 

Colburn relationship found that the optimum number of trays varies from 2 to 3 times the number at total reflux. Gilliland [Ind. Eng. Chem, . 32, 1220 (1940)] from the establishment of an empirical relationship between reflux ratio and theoretical trays based on a study of existing columns indicated that... 

The effect of utilities costs on optimum operation was noted by Kiguchi and Ridgway [Pet. Refiner,. 35(12), 179 (1956)], who indicated that in petroleum-distillation columns the optimum reflux ratio varies between 1.1 and 1.5 times the minimum reflux ratio. When refrigeration is involved, 1. IRmm < flopt < 1 is used in the condensers, 1.2Rrniii < fLpt < 1 -4Rrn... 

Figure 8-28 presents the usual determination of optimum or near optimum theoretical trays at actual reflux based on performance. It is not necessarily the point of least cost for all operating costs, febrication costs or types of trays. A cost study should be made to determine the merits of moving to one side or other of the so-called optimum point From the Figure 8-28 ...

St = Theoretical trays/stages at actual reflux, L/D, including reboiler and total condenser Sopt = Optimum stripping factor (SR)i = Separation factor... 

Mix the hydrogenated product with 20 ml thionyl chloride and 0.5 g di-methylformamide and reflux the resulting solution with agitation under a nitrogen atmosphere at 80-85°C. Optimum reaction time for C18 alkanesulfonate is 45 min longer times produce oxidized materials. 

Economically optimum reflux ratio is about 1.2 times the minimum reflux ratio Rm. 

The MUF resin formulation is built up from combination of certain amount of formalin, melamine and urea (in initial and post refluxing stages) and also sorbitol. Variation on the formulation gives different resin properties. The optimum resin properties give the optimum MUF resin formulation. From the properties analysis data, the optimum formulation is determined by using Mixture Experimental Design D-optimal criterion. The selective criteria... 

This will be possible for only a few practical design problems. The technique is illustrated in Example 1.1, and in the derivation of the formula for optimum pipe diameter in Chapter 5. The determination of the economic reflux ratio for a distillation column, which is discussed in Volume 2, Chapter 11, is an example of the use of a graphical procedure to find the optimum value. 

Note Above a reflux ratio of 4 there is little change in the number of stages required, and the optimum reflux ratio will be near this value. 

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