Batch distillation operating profiles

The batch distillation operation can be schematically represented as a State Task Network (STN). A state (denoted by a circle) represents a specified material, and a task (rectangular box) represents the operational task (distillation) which transforms the input state(s) into the output state(s) (Kondili et al., 1988 Mujtaba and Macchietto, 1993). For example, Figure 3.1 shows a single distillation task producing a main-cut 1 (Di) and a bottom residue product (Bj) from an initial charge (B0). States are characterized by the amount and composition of the mixture residing in them. Tasks are characterized by operational attributes such as then-duration, the reflux ratio profile used during the task, etc. 

Using the constant reflux operation strategy outlined in section 3.3.2 Greaves et al. (2001) carried out few experiments using different values of Rexp and different batch time. The accumulated distillate composition profiles as functions of batch time and distillate holdup are shown in Figures 3.13 and 3.14 respectively. Figure 3.15 shows the instant distillate composition profile for Rexp = 2. 

Seader and Henley (1998) considered the separation of a ternary mixture in a batch distillation column with B0 = 100 moles, xB0 = = <0.33, 0.33, 0.34> molefraction, relative volatility a= <2.0, 1.5, 1.0>, theoretical plates N = 3, reflux ratio R = 10 and vapour boilup ratio V = 110 kmol/hr. The column operation was simulated using the short-cut model of Sundaram and Evans (1993a). The results in terms of reboiler holdup (Bj), reboiler composition profile (xBI), accumulated distillate composition profile (xa), minimum number of plates (Nmin) and minimum... 

For single separation duty, Mujtaba and Macchietto (1993) proposed a method, based on extensions of the techniques of Mujtaba (1989) and Mujtaba and Macchietto (1988, 1989, 1991, 1992), to determine the optimal multiperiod operation policies for binary and general multicomponent batch distillation of a given feed mixture, with several main-cuts and off-cuts. A two level dynamic optimisation formulation was presented so as to maximise a general profit function for the multiperiod operation, subject to general constraints. The solution of this problem determines the optimal amount of each main and off cut, the optimal duration of each distillation task and the optimal reflux ratio profiles during each production period. The outer level optimisation maximises the profit function by... 

The results are summarised in Table 6.5. The accumulated and instant distillate composition profiles are shown in Figure 6.8. This operation requires a longer batch time and higher reflux ratios and gives more than 20% less profit compared to the results for case 2 (Table 6.4). This clearly shows the benefit of producing an intermediate off-cut for this specific separation problem. 

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. 

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. 

EXAMPLES To demonstrate the effect of the holdup specifications on the steady state solution of a batch distillation column at total reflux (a column operating at total reflux of type 2 D — 0, B = 0, F = 0), Examples 10-2 and 10-3 are presented in Table 10-6. The temperature profiles given in Table 10-7 were found by solving Examples 10-2 and 10-3 by use of the calcula-tional procedure described above.. 

The column shown in figure Id can be visualised as an inverted batch distillation column placed on top of a regular batch column, the two being connected at the withdrawal stage. Hence, feasibility studies for the regular and inverted batch distillation columns may be applied to the novel process provided that the concentration of the withdrawal tray lies on the column s profile. Therefore, it is possible to obtain pure intermediate-boiling product b from an infinite column operated at infinite reflux ratios, only if the distillate and sump vessels contain the binary mixtures a-b (light-intermediate boilers) and b-c (intermediate-heavy boilers), respectively. 

Optimal control of a batch distillation column consists in the determination of the suitable reflux policy with respect to a particular objective function (e.g. profit) and set of constraints. In the purpose of the present work, the optimisation problem is defined with an operating time objective function and purity constraints set on the recovery ratio (90%) and on the propylene glycol final purity (80% molar). Different basis fimctions have been adopted for the control vector parameterisation of the problem piecewise constant and linear, hyperbolic tangent function. Optimal reflux profiles are determined with the final conditions of the previous optimal reactions as initial conditions. The optimal profiles of the resultant distillations are presented on figure 2. 

The column liquid composition profile at the end of each operating step can be seen in Figure 13.7. Notiee that there is no methanol in the upper rectifying section at the end of Step 1 (time = 3.17 h). This is due to the continuous feeding of the entrainer into the column to push the methanol to appear only in the lower extractive section of the column. At the end of Step 2 (time = 5.34 h), the column composition profile has moved from the acetone comer toward the methanol comer. The top Uquid composition is close to the methanol comer at the end of Step 3 (time = 8.48 h). At this time, there is almost no acetone inside the column, so the separation is just like regular batch distillation. At the end of Step 4 (time = 12.26 h), the methanol pmity in the P2 product tank can no longer be maintained at its specification. At the same time, the bottom product has already satisfied the water purity specification. The column is shut down and the bottom product is collected. 

In the latter part of this chapter, a heteroazeotropic batch distillation system for acetic acid dehydration has been investigated. Unlike the continuous system where isobutyl acetate was found to be a good entrainer for the purpose of acetic acid dehydration, this entrainer is not suitable in batch operation. The main reason is that the column composition profile in batch operation is not fixed but time varying. Therefore, the closeness of the normal boiling-point temperatures between isobutyl acetate and acetic acid cause failure of the batch operation. On the other hand, the column composition profile is fixed in the continuous operation and is designed to avoid this two-component edge. 

The dominance of distiHation-based methods for the separation of Hquid mixtures makes a number of points about RCM and DRD significant. Residue curves trace the Hquid-phase composition of a simple single-stage batch stiHpot as a function of time. Residue curves also approximate the Hquid composition profiles in continuous staged or packed distillation columns operating at infinite reflux and reboil ratios, and are also indicative of many aspects of the behavior of continuous columns operating at practical reflux ratios (12). 

The minimum batch times for the individual cuts and for the whole multiperiod operation are presented in Table 8.8 together with the optimal amount of recycle and its composition for each cut. The percentage time savings using recycle policies are also shown for the individual cuts and also for the whole operation. Figure 8.18 shows the accumulated distillate and composition profile with and without recycle case for the operation. These also show the optimal reflux ratio profiles. Please see Mujtaba (1989) for the solution statistics for this example problem. 

Mujtaba and Macchietto (1997) solved a series of optimisation problems (PI) for different but fixed batch time tf (between 5 and 30 hrs) and for two given product purities, x d = 0.70 and x D - 0.80. Reflux ratio level was optimised over the batch time of operation. Figure 9.2 shows the typical plots of accumulated distillate and reboiler composition profiles for t f- 15 hrs and x D = 0.80, with the reflux ratio being optimised. 

The maximum conversion, the corresponding amount of product, optimal constant reflux ratio and heat load profiles for different batch times are shown in Figures 9.3-9.6. The maximum conversion profile achieved under total reflux operation (where no product is withdrawn) is also shown in Figure 9.3. The latter approximates the conversion which would be achieved in the absence of distillation. Note that if there is a large column holdup, the conversion under total reflux will not approximate the conversion achieved in the absence of distillation. 

If one were to conduct batch boiling with an initial charge composition located in region 1, the initial vapors produced would be rich in acetone, while the liquid would become richer in benzene. Operation in region 2 would result in a benzene-rich liquid as well however, the vapor produced will be more concentrated in chloroform. Furthermore, it should be noted that the nature of the profiles, and hence the distillation regions formed, are dependent on the system and type of azeotrope(s) present (either minimum or maximum boiling). 

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