Fractionating columns minimum reflux ratio

Cost of Reboiler for Alkylation Unit Heat-Pump Fractionator if Fractionation Column Operation is Assumed to be at Minimum Reflux Ratio... 

Examples chosen for this category include the operations of vapor/liquid separation, heat transfer, and fluid flow. The Underwood method of estimating the minimum reflux ratio for a fractionation column multicomponent... 

When we approach the minimum reflux ratio, we will encounter a "pinch" somewhere in the column. At the pinch, we take an infinite number of steps without changing the mole fractions ... 

For multicomponent separations, it is often necessary to estimate the minimum reflux ratio of a fractionating column. A method developed for this purpose by Underwood [10] requires the solution of the equation... 

If an infinite or nearly infinite number of equilibrium stages is involved, a zone of constant composition must exist in the fractionating column. In this instance, there is no measurable change in the composition of liquid or vapor from stage to stage. Under these conditions, the reflux ratio can be defined as the minimum reflux ratio, R j, with... 

Between the lower and upper invariant zones, the mole fraction of both keys in the vapor phase decreases, and the ratio of light key to heavy key decreases. This region of the column serves to remove the light nonkey components from the liquid flowing down and the heavy nonkey component from the material that will flow up and form the distillate. The small amount of reverse fractionation shown for the key components is an interesting phenomenon that is often found in real columns operating at close to the minimum reflux ratio. 

A mixture of xylenes plus other aromatics is separated in a large fractionating column operating at atmospheric pressure. Calculate the minimum number of plates and the minimum reflux ratio for the conditions in Table 19.4. Use the Gilliland correlation to estimate the reflux ratio that will permit the separation to occur in 100 ideal stages. The relative volatilities are calculated for 18 psia and 150°C, the estimated conditions near the feed tray. 

Actually there is no sharp line of demarcation between these five sections, but this division serves as a useful picture for considering the case of the minimum reflux ratio. The feed to the fractionating column would be introduced on some plate in intermediate section 3, and the true criterion for the minimum reflux ratio should be based on matching the ratio of the concentrations of the key components above and below the feed plate under conditions such that a pinched-in section occurs both above and below the feed plate. For mixtures of normal volatility, a pinched-in region in only one section does not necessarily mean that an infinite number of plates would be required to perform the desired separation at the reflux ratio under consideration, since by relocating the feed plate, such as to shift the ratio of the concentrations of the key components at this plate, the section that was not limited could be made to do more separation and thereby relieve the load on the pinched-in section. In other words, for mixtures of normal volatilities the condition of the minimum reflux ratio is not determined by either the fractionation above or below the feed plate alone, but is determined such that the separation is limited both above and below the feed. The conditions in the intermediate feed section lead to... 

Shortcut Module The required input for the design mode consists of identification of the key components to be separated, specification of the fractional recoveries of each key conponent in the overhead product, the column pressure and pressure drop, and the ratio of actual to minimum reflux ratio to be used in the column. The simulator will estimate the number of theoretical plates required, the exit stream conditions (bottom and overhead products), optimum feed location, and the reboiler and condenser duties. 

The feed to a distillation column is at 405T and 100 psia enters at 1272 Ibmol/h with the following compositions in mole fraction Ethane 0.0148, propane 0.7315, z-butane 0.0681, -butane 0.1462, z-Pentane 0.0173, zz-pen-tane 0.015, zz-hexane 0.0071. HK in distillate is z -butane 0.02 mole fraction LK in bottom is propane 0.025 mole fraction for reflux ratios of 1.5. Calculate the minimum reflux ratio and column performance ... 

If the oxygen impurity associated with the overhead stream in the single column described in Problem 6.25 is to be reduced to a mole fraction of 1 %, what overhead condenser duty must be specified so that the column operates with an external reflux ratio that is 10 % greater than the minimum reflux ratio How many theoretical plates are required under these operating conditions ... 

For standard types of finite-stage contactor columns operated in the range of allowable velocities where the overall column efficiencies are essentially constant, O Connell has correlated efficiency data on the basis of liquid viscosity and relative volatility (or gas solubility). The results for fractionators and absorbers are presented in Fig. 16-9. This correlation is based, primarily, on experimental data obtained with bubble-cap columns having a liquid path of less than 5 ft and operated at a reflux ratio near the minimum value. Figure 16-9 is adequate for design estimates with most types of commercial equipment and... 

Samples containing Hght components must be debutanized in a preliminary step. In the method, a weighed sample of 1-10 hters is distilled in a fractionating column having an efficiency at total reflux of 14-18 theoretical plates. A reflux ratio of 5 1 is maintained throughout except at the minimum pressure of 2mmHg, where a ratio of 2 1 can be used. The mass and density of each fraction are used to prepare distillation curves either by mass or volume. This method is often referred to as the 15-5 method. 

A distillation column has a feed of 100 kmol/h. The feed is 10 mol% LNK, 55 mol% LK, and 35 mol% HK and is a saturated liquid. The reflux ratio is 1.2 times the minimum. We desire 99.5% recovery of the light key in the distillate. The mole fraction of the light key in the distillate should be 0.75. Use the FUG approach to estimate the number of ideal stages required and the optimal location of the feed stage. Equilibrium data ... 

Reflux ratio and minimum number of stages (trays) The design of a fractionating column could be looked upon as a capital cost versus an energy cost trade-off issue, which is basically a function of... 

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