Variable reflux ratio, batch distillation

Batch with Constant Reflux Ratio, 48 Batch with Variable Reflux Rate Rectification, 50 Example 8-14 Batch Distillation, Constant Reflux Following the Procedure of Block, 51 Example 8-15 Vapor Boil-up Rate for Fixed Trays, 53 Example 8-16 Binary Batch Differential Distillation, 54 Example 8-17 Multicomponent Batch Distillation, 55 Steam Distillation, 57 Example 8-18 Multicomponent Steam Flash, 59 Example 8-18 Continuous Steam Flash Separation Process — Separation of Non-Volatile Component from Organics, 61 Example 8-20 Open Steam Stripping of Heavy Absorber Rich Oil of Light Hydrocarbon Content, 62 Distillation with Heat Balance,... 

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. 

The design of a new system for a specific separation involves determining a minimum reflux ratio and selecting a control protocol (fixed or variable reflux ratio) and an amount of time to be allowed for distilling a batch of some given size. 

Figure S.3 Batch distillation of benzene-toluene with variable reflux ratio, Example 5,4. ta-e) McCabe-Thiele diagram for progressively increasing reflux ratio to 13 1.

If a batch distillation is carried out at variable reflux ratio, it is possible u> maintain a constant purity of distillate and to do this until blocked by material balance limitations, A McCabe-Thiele diagram illustrating this method of operation is shown in Fig. 5.5-11. The value of xu is kept constant, and the slope of the operating line is varied in accordance with the overall material balance, The figure shows the conditions at time 0 and time t. Note that the three equilibrium stages include the stillpot. 

The batch distillation column can also be operated with variable reflux ratio to keep x constant. The operating Eq. 19-261 is still valid. Now the slope will vary, but the intersection with the y=x line will be constant at Xp,. The McCabe-Thiele diagram for this case is shown in Figure 9-7. This diagram relates Xs to Xp). Since Xp, is kept constant, the calculation procedure is somewhat different. 

Minimum Vapor Requirements for Batch Distillation. Consider the batch distillation of an equimolal mixture of A and B. The relative volatility, is constant at 2 and the average distillate is to be 96 mol per cent A. Calculate the minimum mols of vapor for 60 mol per cent recovery bf A in the distillate for both the constant reflux ratio and the variable reflux ratio cases. 

When a batch distillation is carried out by the variable reflux ratio method to give a constant value of xd, and the distillation is continued until the reflux ratio is essentially total reflux, the amount of holdup in the column at the end of the distillation can be easily calculated by using the y x line as the operating line. Such a procedure gives the composition of the liquid on each plate, and a correction can be applied for effect of, the holdup on the percentage yield of a given fraction. 

Hence the reflux ratio, the amount of distillate, and the bottoms composition can be related to the fractional distillation time. This is done in Example 13.4, which studies batch distillations at constant overhead composition and also finds the suitable constant reflux ratio that enables meeting required overhead and residue specifications. Although the variable reflux operation is slightly more difficult to control, this example shows that it is substantially more efficient thermally—the average reflux ratio is much lower—than the other type of operation. 

The optimal control of a process can be defined as a control sequence in time, which when applied to the process over a specified control interval, will cause it to operate in some optimal manner. The criterion for optimality is defined in terms of an objective function and constraints and the process is characterised by a dynamic model. The optimality criterion in batch distillation may have a number of forms, maximising a profit function, maximising the amount of product, minimising the batch time, etc. subject to any constraints on the system. The most common constraints in batch distillation are on the amount and on the purity of the product at the end of the process or at some intermediate point in time. The most common control variable of the process is the reflux ratio for a conventional column and reboil ratio for an inverted column and both for an MVC column. 

Discrete reflux ratio used in most pilot plant batch distillation columns, including those used in industrial R D Departments (Jenkins, 2000 Greaves, 2003), does not allow a direct implementation of the optimum reflux ratio (treated as a continuous variable) obtained using a model based technique (as presented in earlier chapters of this book). In Greaves et al. (2001), a relationship between the continuous and the discrete reflux ratio is developed. This allows easy communication between the model and the process and comparison on a common basis. 

We consider an industrial reactive semibatch distillation process. A trans-esterification of two esters and two alcohols takes place in the reboiler. A limited amount of educt alcohol is fed to the reboiler to increase the reaction rate. The product alcohol is distillated from the reboiler to shift the reaction in the product direction. In the main cut period the product alcohol is accumulated with a given purity specification. In the off-cut period, the reaction proceeds to the end of the batch and results in a mixture of the product ester and the educt alcohol in the reboiler. The composition of the educt ester is required to be smaller than a specified value, so that a difficult separation step can be avoided. The aim of the optimization is to minimize the batch time. The independent variables of the problem are the feed flow rate F of the educt alcohol and the reflux ratio Rv. The deterministic and stochastic nonlinear dynamic optimization problem can be formulated as follows... 

Optimizing batch distillation operations can have a significant economic impact, especially when the separation of high-value chemicals is involved. The control variable for optimizing a batch distillation product is the reflux ratio policy, i.e., the variation of the reflux ratio with time. As... 

If distillate flow is selected as the variable to be manipulated for product-quality control, reflux Is then dependent. In the continuous system, product quality was affected by both D/F and V/F. But here, F = 0, so It follows that product quality Is a function of the ratio of the remaining variables, that Is, D/V. In.a sense, a batch still is similar to the enriching section of a continuous tower, part of whose vapor flow is feed. If, in the... 

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