Optimum reflux rate

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. 

Figure 1.10 illustrates this point, from plant test data obtained in a Texas refinery. Point A is called the incipient flood point, that point in the towers operation at which either an increase or a decrease in the reflux rate results in a loss of separation efficiency. You might call this the optimum reflux rate, that would be an alternate description of the incipient flood point, neglecting the energy cost of the reboiler steam. 

Figure 3.5 illustrates this relationship. Point A is the incipient flood point. In this case, the incipient flood point is defined as that operating pressure that maximizes the temperature difference across the tower at a particular reflux rate. How, then, do we select the optimum tower pressure, to obtain the best efficiency point for the trays Answer—look at the temperature profile across the column. 

The steady state optimisation problem is solved for different feed flow rates. The maximum achievable distillate rate, optimum reflux ratio (internal), total amount of distillate, pass time and recovery of key component (e.g. component 1) for the first pass are summarised in Table 11.2. For any pass p, the pass time (tp, hr), total amount of distillate (Dp, kmol) and recovery of key component (Rep) are calculated using ... 

Application. Previous methods (Secs. 3.2.1 to 3.2.6) produce a design. They take product compositions and deliver the number of stages, reflux, and optimum feed stage. The Smith-Brinkley method rates a column using the reverse sequence of steps. It takes the number of stages, reflux ratio, and actual feed location, and yields the product compositions. 

A section drawn through Figure 7 at a constant isobutane recycle rate is shown in Figure 8, for which the abscissa is now reflux ratio (reflux/isobutane recycle). The optimum corresponds to the value of reboiler duty giving the maximum profit for the specified recycle rate. As the reboiler duty (or reflux rate) decreases, the profit drops sharply. No reflux corresponds to the operation of the deisobutanizer as an "isostripper". Figure ff shows that "isostripper" operation significantly reduces unit profitability. 

Both modes usually are conducted with constant vaporization rate at an optimum value for the particular type of column construction. Figure 13.9 represents these modes on McCabe-Thiele diagrams. Small scale distillations often are controlled manually, but an automatic control scheme is shown in Figure 13.9(c). Constant overhead composition can be assured by control of temperature or directly of composition at the top of the column. Constant reflux is assured by flow control on that stream. Sometimes there is an advantage in operating at several different reflux rates at different times during the process, particularly with multicomponent mixtures as on Figure 13.10. 

Distillate composition Bottoms composition Feed tale Feed composition Feed enthalpy Desiga/minimum reflux ratio Optimum feed stage Pressure Number of stages Feed stage Reflux ratio Distillate rate... 

In order to keep the reboilers down to a reasonable size, the column has to be heated either with steam at a pressure of not less than 3.5 to 5 bar or wiA waste heat at a temperature level above 140-150°C. Unlike the prerun column, the pressurized column is a genuine distillation column as the overhead product has to meet the purity requirements of US Grade AA methanol. The reflux rate, the number of trays and the heat input can be varied within certain limits, and the most favourable design of the column and its economical operation have to be established by optimizing calculations. A column with the above-mentioned number of trays reaches its operating optimum with a reflux ratio of approximately 3.0 and a heat input of about 2.0 GJ per ton of total methanol produced. As the overhead product ftom the pressurized column is used to heat the atmospheric column, either of the two coliunns has to be used to distill some 50 % of the total methanol produced, except for slight differences in the reflux ratio. 

The conclusion is that operating targets should be a function of energy costs rather than a fixed number even with fixed composition limits. It is common to observe separation columns operating at reflux rate that are 50% higher than the optimum. For the debutanizer column operation discussed here, such an operation could cost operating margins in excess of 500,000 per year. 

FIGURE 14.6. Optimum reflux rate depends on energy price. (From White (2012), reprinted with permission by AIChE.)... 

A practical isotope separation plant can operate at neither minimum reflux (where the separation is zero, but the rate of production is high), nor at minimum number of stages (where the rate of production is zero, but the separation is high). A compromise is required. Since optimum reflux varies with stage number it is customary to employ tapered cascades for isotope separation. This results in marked savings in material hold-up, and in plant size and investment. 

The same kinetic case considered in the previous section is used in the reactor. The reactor effluent F (with composition z) is fed to a stripping column that sends the lighter component A out the top as a recycle stream and the heavier component B out the bottom as the final product stream. A stripping column is used, as opposed to a full column with reflux, because previous studies have shown that a stripper is the optimum economic design. Since the overhead is not a product stream, it does not have to have a high purity. The vapor going overhead is condensed and pumped back to the reactor at a molar flowrate D and composition xD. The liquid rate in the stripper is F and the vapor rate is D. 

Here the feed rate is maximised while the reflux ratio is optimised. The bottom product composition imposes an additional constraint to the problem. The results are summarised in Table 11.8 which gives the maximum feed rate, minimum batch time, optimum reflux ratio, and total number of batches for each mixture and total yearly profit. 

Policy 2, the instantaneous holdup and composition profiles in each vessel are shown in Figure 11.13. The optimum reflux flow rates and the instantaneous QR(t) profiles are shown in Figure 11.14. 

Fig. 11.14. Optimum Reflux Flow Rates and Instant Reboiler Duty Profiles of Policy 2. [Furlonge et al., 1999]j...

Each catalyst is evaluated catalytically under its optimum conditions, these conditions differ for die HMS-based catalysts and the amorphous silica-based systems. The temperature column in table 3 indicates the reflux temperature of the solvent in use for AMPS this is cyclohexane whereas for the HMS-based catalysts it is toluene. For AMPS it is found that the higher boiling point of toluene confers no advantage upon the rate of reaction - indeed the reaction proceeds more slowly in toluene when catalysed by AMPS. 

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