Optimal reflux rate

The next step is to optimize reflux rate for the column. As shown in Figure 13.8b, the horizontal gap between the vertical axis and CGCC pinch point is the scope for reflux improvement. The CGCC will move closer to the vertical axis when the reflux ratio is reduced. The reflux rate optimization must be considered before other thermal... 

Optimal reflux rate in operation depends on the operating margin, which is defined as the difference of product sales minus feed cost and energy cost. When energy cost is too high, it could drive the operation toward lower reflux rate and vice versa for the case of lower energy cost. 

Urmila M. Diwekar, R.K. Malik, and K.P. Madhavan. Optimal reflux rate policy determination for multicomponent batch distillation columns. Comp. Chem. Eng., 11 629-637, 1987. 

Figure 4.3 Optimizing feed preheat duty at a constant reflux rate.

Table 5.2 summarises the results for two cases (i) constant vapour boil-up rate, (ii) variable vapour boilup rate. The initial and final time optimal reflux ratio values are shown in Table 5.2 for both cases. The optimal reflux ratios between these two points follow according to Equation P.13 for each case. See details in the original reference (Robinson, 1969). 

Case Boilup Rate Optimal Reflux Ratio at Minimum... 

Two binary mixtures are being processed in a batch distillation column with 15 plates and vapour boilup rate of 250 moles/hr following the operation sequence given in Figure 7.7. The amount of distillate, batch time and profit of the operation are shown in Table 7.6 (base case). The optimal reflux ratio profiles are shown in Figure 7.8. It is desired to simultaneously optimise the design (number of plates) and operation (reflux ratio and batch time) for this multiple separation duties. The column operates with the same boil up rate as the base case and the sales values of different products are given in Table 7.6. 

Referring to Figure 8.2 and given a batch charge (BO, xB0)> a desired amount of distillate DI of specified purity x D1 and final bottom product B2 of specified purity x b2 Mujtaba (1989) determined the amount and composition of the off-cut (Rl, x R1) and the reflux rate policy r(t) which minimised the overall distillation time. In this formulation instead of optimising Rl, xR1) the mixed charge to the reboiler (Bc, xBC) was optimised and at the end of the solution the optimal (Rl, x RI) was evaluated from the overall balance around the mixer in Figure 8.2. The dynamic optimisation problem is formulated as ... 

Since the column is operating in full charge mode the constraints given by Equations 10.2 and 10.5 must be satisfied for the first and second intervals respectively. A number of cases were studied using different maximum allowable values for the solvent feed rate (E ) in the first interval. These give values for for the first interval. For each case, Table 10.8 shows the optimal reflux ratio, the solvent feed rate, the lengths of the time intervals and the productivity. The case 1 represents the base case where no solvent was introduced into the column. 

For Tasks 1 and 3 two time intervals are used. For Task 2 only one time interval is used. Within each interval the reflux ratio, the solvent feed rate and the length of the interval are optimised. Vapour liquid equilibrium is calculated using UNIQUAC model. A number of cases were studied for different amount of initial feed charge (Bo) to the reboiler. For each case, the optimal reflux and solvent feed profiles for each Task, percentage of Acetone and solvent recovered and the overall profit of the operation are shown in Table 10.10. 

Figure 3.4 Reflux rate optimization for an existing column, (Reprinted by permission, Copyright Instrument Society of America, 1978, from P. R. Latour, instrumentation Technology. July 1978.)...

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. 

It is essential to avoid flooding and dumping, which could severely affect fractionation and thus energy efficiency. Fractionation efficiency can be monitored by column internal V/F ratios. Desired V/F can be achieved jointly by optimizing feed heater outlet temperature, fractionation stripping steam, and overhead reflux rate together with pump-around heat duties, which are used to control excess heat in the column. 

Heater Outlet Temperature This temperature can affect the lift of diesel out of UCO. A too low heater outlet temperature would cause a slump of diesel into UCO, which degrades the value of diesel into fuel oil. Too high of a heater outlet temperature will cause unnecessarily high reflux rate at the expense of extra heater duty. The heater outlet temperature is mainly a function of the hydrocarbon partial pressure in the flash zone because the distillation cut point is a function of the diesel distillation specification and fractionation efficiency. An optimal heater outlet temperature could be determined by the fractionation overflash, which measures the internal reflux rate. 

Pump-Around Many fractionation towers have pump-arounds to remove excess heat in the key sections of the tower. The effect of increasing pump-around rate is reduced internal reflux rate in the trays above the pump-around, but increased internal reflux rate below the pump-around. Thus, change in pumparound duty affects fractionation. On the other hand, pump-around rates and return temperature have effects on heat recovery via the heat exchanger network. It is not straightforward in optimizing pump-around duties and temperamres since the effects on both fractionation and heat recovery can only be assessed in a simulation model. An APC application incorporated with process simulation should be able to handle this optimization. 

However in operation, optimal reflux ratio is determined based on the trade-off product value and energy cost. When the reflux ratio increases, the separation improves at the expense of increased reboiling duty (Figure 14.5). As a result, top product rate decreases while the bottom product rate increases. As shown in Figure 14.5, the cost of reboiling duty presents a linear relationship with reflux ratio but the product rate is nonlinear and presents a different trend as reboiling duty. 

Because minimum values of these parameters were determined before at the stage of calculation of the mode of minimum reflux (see Section 6.8), design their values should be chosen reasoning from economic considerations taking into account energy and capital expenditures. This choice is similar to that of optimal reflux excess coefficient for two-section columns. Along with that, the equaUty of vapor flow rates in the second and third columns in the cross-section of output of side product is taken into consideration. 

The overall control strategy depends on the reflux policy selected and can vary from simple manual control to elaborate systems designed to control the reflux based on some optimization scheme. Aside from the overall control strategy, the primary controllers most commonly used include the following the reboiler steam rate controls the pressure drop across the column, the reflux rate controls the temperature at some point in the column, the condenser cooling water rate controls the condensate temperature, and the product rate controls the liquid level in the accumulator. 

Solvent oil is a by-product of the platinum reforming plant of petrochemical factories. The target of optimization is to increase the recovery of solvent oil. There are six affecting factors for the recoveiy of solvent oil reflux rate (X]), flow rate of the first side line (X2), pressure at tower top (X3), temperature of reflux (X4), temperature at solvent oil tower bottom (X5), temperature of the 35th-tower plate of solvent oil tower (Xe). Some data from the industrial records are listed in Table 14.2. 

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]. 

Reactive distillation involves additional degrees of freedom (Mujtaba and Macchietto, 1997). If the controllable parameters remaining to be specified, namely (1) one heat input, and (2) the flow rate of the product (or the reflux ratio), are determined via optimization, all of the values of Vh Lk, Tk, xi h and yik and the enthalpies can be calculated. More than 2 degrees of freedom can be introduced by eliminating some of the prespecified parameters values. 

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