Reflux ratio control

The batch distillation column consisted of 3 internal plates, reboiler and a total condenser. The reboiler was charged with a fresh feed of 5 kmol with Benzene molefraction 0.6. The total column holdup was 4 % of the charge. Half the holdup was in the condenser and the rest was distributed over the plates. The vapour load to the condenser was 3 kmol/hr. The required product purities were x oi = 0.90 and x B2 = 0.15. The solution of Equations 8.1-8.4 therefore gives DJ = 3.0 kmol and B2 = 2 kmol. This problem is same as case 3 shown in Table 8.1. Three reflux ratio (control) intervals were used to achieve (Dl, x Di) and one control interval to achieve (B2, x B2). 

Many industrial users of batch distillation (Chen, 1998 Greaves, 2003) find it difficult to implement the optimum reflux ratio profiles, obtained using rigorous mathematical methods, in their pilot plants. This is due to the fact that most models for batch distillation available in the literature treat the reflux ratio as a continuous variable (either constant or variable) while most pilot plants use an on-off type (switch between total reflux and total distillate operation) reflux ratio controller. In Greaves et al. 2001) a relationship between the continuous reflux ratio used in a model and the discrete reflux ratio used in the pilot plant is developed. This allows easy comparison between the model and the plant on a common basis. 

RR-BR Reflux ratio controls distillate composition and boilup ratio controls bottoms composition. 

In the second case (manipulate distillate to control reflux ratio), the variability of the distillate flow would be greatly reduced. The reflux drum level controller, manipulating reflux flowrate, is made P-only to get slow changes in reflux flowrate, and this gives slow" changes in distillate flowrrate in the reflux-ratio control structure. 

Reflux Ratio. Although we are on the subject of setting up ratios, now is a good time to discuss installing a reflux ratio control configuration. There are two alternative schemes. 

The control of this system is fairly easy. The total flow is controlled by manipulating the fresh feed of B. The fresh feed of reactant A sets the production rate, and the set point of the total B flow controller is ratioed to the flow rate of A. The control scheme features reflux ratio control and temperature controllers in both columns. 

In addition to the energy balance or material balance options, there are various hybrid schemes - the most weU known of which is the simplified Ryskamp scheme (Reference 1). In this version of the scheme we add a reflux ratio controller to the energy balance scheme. 

Figures 11.7 and 11.8 gives responses to positive and negative 20% changes in vapor boilup, the throughput handle in this control structure. These disturbances are handled well by the two-temperature control structure. Stable base-level regulatory control is achieved. The increase in Vs results in increases in both fresh feeds, and the distillate and bottoms streams increase. Product purities xd(q and xb(d> are maintained fairly close to their desired values. Product purities drop slightly below their specifications for the increase in V5 but rise above specifications for the decrease in throughput. Reflux increases because of the reflux ratio control stmcture.

Fractional distillation. Fig. II, 60, 2 illustrates a set-up for fractional distillation wdth a Hempel-type column and cold finger, the latter to give manual control of the reflux ratio. Any other fractionating colunm, e.g., an all-glass Dufton or a Widmer column may, of course, be used. 

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. 

If the produc ts from a column are especially pure, even this configuration may produce excessive interaction between the composition loops. Then the composition of the less pure product should oe con-troUed by manipulating its own flow the composition of the remaining product should be controlled by manipulating reflux ratio if it is the distillate or boilup ratio if it is the bottom product. 

For a constant reflux ratio, the value can be almost any ratio however, this ratio affects the number of theoretical plates and, consequently, actual trays installed in the rectification section to achieve the desired separation. Control of batch distillation is examined in Reference 134. 

Improved results are also secured by the use of a short reflux condenser ( cold finger ), Fig. II, 56, 22, inserted into the top of the column head the simplest type is shown in Fig. II, 56, 23. The condenser permits control of the reflux ratio by adjusting the rate of flow of water through it. 

Ratio control can be used where it is desired to maintain two flows at a constant ratio for example, reactor feeds and distillation column reflux. A typical scheme for ratio control is shown in Figure 5.21 (see p. 233). 

Top temperatures are usually controlled by varying the reflux ratio, and bottom temperatures by varying the boil-up rate. If reliable on-line analysers are available they can be incorporated in the control loop, but more complex control equipment will be needed. 

In an operating column the effective reflux ratio will be increased by vapour condensed within the column due to heat leakage through the walls. With a well-lagged column the heat loss will be small and no allowance is normally made for this increased flow in design calculations. If a column is poorly insulated, changes in the internal reflux due to sudden changes in the external conditions, such as a sudden rain storm, can have a noticeable effect on the column operation and control. 

Alternatively, by careful control of the reflux ratio, it is possible to hold the composition of the distillate constant for a time until the required reflux ratio becomes intolerably large, as illustrated in Figure 14.9. A mass balance gives ... 

Operation at constant reflux ratio is better than operation with constant distillate composition for high-yield batch separations. However, operation with constant distillate composition might be necessary if high product purity is required. In fact, it is not necessary to operate in one of these two special cases of constant reflux ratio or constant distillate composition. Given the appropriate control scheme, the reflux ratio can be varied through the batch... 

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. 

Once a distillation column is in operation, the number of trays is fixed and very few degrees of freedom can be manipulated to minimize operating costs. The reflux ratio frequently is used to control the steady-state operating point. Figure El2.4a shows typical variable cost patterns as a function of the reflux ratio. The optimization of reflux ratio is particularly attractive for columns that operate with... 

Automatic control of distillate composition (xD) may also be affected by control of the reflux ratio, for example to maintain the distillate composition at constant set point (xDset). 

Batch distillation with continuous control of distillate composition via the regulation of reflux ratio is illustrated in the simulation example BSTILL. In this an initial total reflux condition, required to establish the initial concentration profile with the column, is represented in the simulation by a high initial value of R, which then changes to the controller equation for conditions of distillate removal. 

Continuous binary distillation is illustrated by the simulation example CON-STILL. Here the dynamic simulation example is seen as a valuable adjunct to steady state design calculations, since with MADONNA the most important column design parameters (total column plate number, feed plate location and reflux ratio) come under the direct control of the simulator as facilitated by the use of sliders. Provided that sufficient simulation time is allowed for the column conditions to reach steady state, the resultant steady state profiles of composition versus plate number are easily obtained. In this way, the effects of changes in reflux ratio or choice of the optimum plate location on the resultant steady state profiles become almost immediately apparent. 

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