Reflux ratio in distillation

The ratio of liquid to gas flow will be fixed by the reflux ratio in distillation and in gas absorption will be selected to give the required separation with the most economic use of solvent. 

Is the reflux ratio in distillation column optimized Increase in reflux ratio increases the product purity. However, this causes a larger pressure drop in the column, and increases the reboiler temperature and the reboiler heat duty. Hence, there is a delicate balance among these factors, and often the reflux ratio has to be continuously adjusted to meet the change in feed composition and other day-to-day variations. 

As indicated in Fig. 11-7, the optimum reflux ratio occurs at the point where the sum of fixed charges and operating costs is a minimum. As a rough approximation, the optimum reflux mho usually falls in the range of 1.1 to 1.3 times the minimum reflux ratio. The following example illustrates the general method for determining the optimum reflux ratio in distillation operations. 

There are other mles-of-thumb based on economic experience, which the reader will recognize, such as the optimum reflux ratio in distillation and the optimum liquid to gas ratio in gas absorption. You may also specify recoveries of key conq)onents or their concentrations in an exit stream for separators. When we use any of these rules, the assim tion is that the calculated separator size will be of reasonable cost, approximating the optimum-size separator. Similarly, for chemical reactors we may specify conversion of a desirable con jound, its exit composition or an approach temperature difference. For chemical reactors, the approach temperature difference is the difference between the actual temperature and the chemical-equilibrium ten5)erature. Again, we assume that a reactor that approximates the optimum-size reactor will result when using this rule. 

You should also recall that there exists a minimum value of the reflux ratio for any given combination of product and feed specifications. This minimum value of R translates into a minimum allowable L/V. There is a similar minimum L/V for gas absorption. To find the minimum reflux ratio in distillation, we needed to plot an operating line and an equilibrium curve. 

Analogous to the minimum reflux ratio in distillation, there exists a minimum L/G ratio in absorption. Figure 6.3 compares the operating line to the equilibrium line for absorption. Because the slope of the operating line is L/G, reducing the liquid mass-separating agent flowrate decreases the slope of the line. Compare lines aA and ab for a separation in which the gas enters with concentration of solute which must be removed to Ya and the liquid... 

Similar kinds of constraints involve the reflux ratio in distillation, which must exceed the minimum value for the required separation. If the distillation tower pressure is adjusted, the minimum reflux ratio will change and the actual ratio must be maintained above the minimum value. Even when optimization is not performed, the decision variable values must be selected to avoid violating the inequality constraints. In some cases, the violations can be detected when examining the simulation results. In other cases, the imit subroutines are unable to solve the equations as, for example, when the reflux ratio is adjusted to a value below the minimum value for a specified split of the key components. 

Ratio control is a simple form of feedforward control that is commonly employed in controlling reactor feed compositions and in blending operations. It is also used to control the fuel-to-air ratio in heaters and boilers and to control the reflux ratio in distillation columns. The flow rate of one stream is used to provide the set point for another stream so that that ratio of the two streams is kept constant even if the flow of the first stream varies. Alternatively, the actual ratio between two flows can be used as the input to a controller. 

While process design and equipment specification are usually performed prior to the implementation of the process, optimization of operating conditions is carried out monthly, weekly, daily, hourly, or even eveiy minute. Optimization of plant operations determines the set points for each unit at the temperatures, pressures, and flow rates that are the best in some sense. For example, the selection of the percentage of excess air in a process heater is quite critical and involves a balance on the fuel-air ratio to assure complete combustion and at the same time make the maximum use of the Heating potential of the fuel. Typical day-to-day optimization in a plant minimizes steam consumption or cooling water consumption, optimizes the reflux ratio in a distillation column, or allocates raw materials on an economic basis [Latour, Hydro Proc., 58(6), 73, 1979, and Hydro. Proc., 58(7), 219, 1979]. 

Colburn, A. P. (1941) Trans. Am. Inst. Chem. Eng. 37, 805. The calculation of minimum reflux ratio in the distillation of multicomponent mixtures. 

In this example we illustrate the application of a one-dimensional search technique from Chapter 5 to a problem posed by Martin and coworkers (1981) of obtaining the optimal reflux ratio in a distillation column. 

Thermodynamics and mass transfer. Operating pressure, number of plates and reflux ratio in the distillation column temperature profile in the column equilibrium conditions in the reactor... 

The catalytic distillation reactor is in effect a multistage reactor, where each tray achieves equilibririm at its temperature and composition, with the temperature being lower at the top, where there is more B and higher at the bottom, where there is more C. With a high reflux ratio in the condenser to return any or C to the column, one can attain essentially complete reaction and separation of the products from each other. 

This criterion is similar to using a reflux ratio of 1.2 times the minimum reflux ratio in a full distillation column. It provides a reasonable compromise between the number of trays and the vapor boilup required. The slope of the resulting operating line is the liquid-to-vapor ratio F/D ... 

Note the Equation 3.3b shows that the reflux setting (f exp) in the cyclic operation can be considered as the average external reflux ratio. In other words, the Rexp is the ratio of the average reflux rate to the average distillate rate over a period of At. 

One or more Inequality Constraints (e.g. lower and uppers bounds of operating variables, such as temperature in a reactor, reflux ratio in a distillation column)... 

Table 4.6 in Chapter 4 presents the simulation results for a quaternary batch distillation. The amount of product and the composition of key component of each cut were used by Mujtaba (1989) to formulate and solve a minimum time problem for each cut. Optimal reflux ratio in each operation step is obtained independently of other step with the final state of each step being the initial state of the next step. 

In this separation, there are 4 distillation tasks (NT-4), producing 3 main product states MP= D1, D2, Bf) and 2 off-cut states OP= Rl, R2 from a feed mixture EF= FO. There are a total of 9 possible outer decision variables. Of these, the key component purities of the main-cuts and of the final bottom product are set to the values given by Nad and Spiegel (1987). Additional specification of the recovery of component 1 in Task 2 results in a total of 5 decision variables to be optimised in the outer level optimisation problem. The detailed dynamic model (Type IV-CMH) of Mujtaba and Macchietto (1993) was used here with non-ideal thermodynamics described by the Soave-Redlich-Kwong (SRK) equation of state. Two time intervals for the reflux ratio in Tasks 1 and 3 and 1 interval for Tasks 2 and 4 are used. This gives a total of 12 (6 reflux levels and 6 switching times) inner loop optimisation variables to be optimised. The input data, problem specifications and cost coefficients are given in Table 7.1. 

Z. Colburn, A. P., The Calculation of Minimum Reflux Ratio in the Distillation of Multicomponent. Mixtures, Tra7is. AIChE, Vol. 37, p. 805, 1941. 

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