Batch distillation examples

FIG. 13-106 Distillate -composition profile for the miilticomponent-batch-distillation example. 

A similar apparatus but of a larger size can be used for "industrial batch distillation Examples of batch stills are shown in Figs 293, 291 292, pp 384—86 of Riegel s book (Ref 1)... 

Seader and Henley (1998) provided a detailed account of the merits and demerits of different integration methods with typical conventional batch distillation examples. They have noted three particular issues with the integration methods applied in batch distillation calculations that require attention. These are the truncation error, stability and stiffness ratio. These will be briefly discussed in the following. 

Batch Distillation Example. As an illustration of the use of this equation, consider the experiment performed by Rayleigh. 1,010 g. of a 7.57 mol per cent solution of acetic acid in water was distilled until the stUl contained 254 g. whose com-... 

Example 10 Calculation of Multicomponent Batch Distillation A charge of 45.4 kg mol (100 Ih-mol) of 25 mole percent heuzeue, 50 mole percent monochlorohenzene (MCB), and 25 mole percent orthodichloro-henzene (DCB) is to he distilled in a hatch still consisting of a rehoiler, a column containing 10 theoretical stages, a total condenser, a reflux drum, and a distillate accumulator. Condenser-reflux drum and tray holdups are 0.0056 and... 

Open-loop behavior of multicomponent distillation may be studied by solving modifications of the multicomponent equations of Distefano [Am. Inst. Chem. Eng. J., 14, 190 (1968)] as presented in the subsection Batch Distillation. One frequent modification is to include an equation, such as the Francis weir formula, to relate liquid holdup on a tray to liquid flow rate leaving the tray. Applications to azeotropic-distillation towers are particularly interesting because, as discussed by and ihustrated in the Following example from Prokopalds and Seider... 

Example 8-15 Batch Distillation, Vapor Boil-up Rate for Fixed Trays (used by permission of Treybal [129] clarification added by this author)... 

Figure 8-38A. Graphical integration for boil-up rate of batch distillation for Example 8-15. Used by permission, Treybal, R. E., Cftem. Eng. Oct. 5 (1970), p. 95.

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,... 

In terms of downstream processes, the flow-rates, compositions, and so on, dictate the size and number of each unit operation for example, while a batch distillation may be used to separate a single feed into a number of different product streams, a continuous distillation train would in general require N columns for N different product streams. The fact that a high degree of modeling is used in the design of each MPI, results in the generally held belief that continuous processes... 

The general approach to the solution of unsteady-state problems is illustrated in Example 2.15. Batch distillation is a further example of an unsteady-state material balance (see Volume 2, Chapter 11). 

Other synonyms for steady state are time-invariant, static, or stationary. These terms refer to a process in which the values of the dependent variables remain constant with respect to time. Unsteady state processes are also called nonsteady state, transient, or dynamic and represent the situation when the process-dependent variables change with time. A typical example of an unsteady state process is the operation of a batch distillation column, which would exhibit a time-varying product composition. A transient model reduces to a steady state model when d/dt = 0. Most optimization problems treated in this book are based on steady state models. Optimization problems involving dynamic models usually pertain to optimal control or real-time optimization problems (see Chapter 16)... 

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. 

The simplest example of batch distillation is a single stage, differential distillation, starting with a still pot, initially full, heated at a constant rate. In this process the vapour formed on boiling the liquid is removed at once from the system. Since this vapour is richer in the more volatile component than the liquid, it follows that the liquid remaining becomes steadily weaker in this component, with the result that the composition of the product progressively alters. Thus, whilst the vapour formed over a short period is in equilibrium with the liquid, the total vapour formed is not in equilibrium with the residual liquid. At the end of the process the liquid which has not been vaporised is removed as the bottom product. The analysis of this process was first proposed by Rayleigh(24). 

Figure 11.37. Batch distillation-constant reflux ratio (Example 11.13)...

The model of a multicomponent batch distillation column was derived in Sec. 3.13. For a simulation example, let us consider a ternary mixture. Three products will be produced and two slop cuts may also be produced. Constant relative volatility, equimolal overflow, constant tray holdup, and ideal trays are assumed. 

Table 5,14 gives a digital computer FORTRAN program for this three-component batch distillation dynamic simulation. The specific example is a column with 20 trays and relative volatilities of 9, 3, and 1. The vapor flow rate is constant at 100 mol/h. 

Three products (PI, P2, aod P3) and two slop cuts (SI and S2) are produced. The average composition of the products are 95 mole percent. The PI product is mostly the lightest component (component 1). The P2 product is mostly intermediate component (number 2) with some impurities of both the light and the heavy components. The final product P3 is what is left in the still pot and on the trays. The times to produce the various products and slop cuts are given in the results shown in Table 5.14. The total time for the batch distillation in this example is 6.4 hours. 

The catalyst components are generally dissolved in methyl acetate which acts as both reactant and solvent. Other solvents may be used and in fact, upon several batch recycles where lower boiling products are distilled off, the solvent is an ethylidene diacetate-acetic acid mixture. Any water introduced in the reaction mixture will be consumed via ester and anhydride hydrolysis, therefore anhydrous conditions are warranted. Typical batch reaction examples are presented in Table 1. There is generally sufficient reactivity when carbon monoxide and hydrogen are present at 200-500 psi. Similar results were obtained from the pilot plant using a continuous stirred tank reactor (CSTR). The reaction can also be run continuously over a supported catalyst with a feed of methyl acetate, methyl iodide, CO, and hydrogen. 

Early refiners utilized simple batch distillation to prepare kerosenes and lubricating oils. As the demand for these materials expanded and new crude oils were found, certain desirable and undesirable characteristics became apparent. Crude oils were selected from which products possessing desirable characteristics could be distilled—for example, oxidation stability, low smoke tendency, low carbon-forming tendency, small viscosity change with change in temperature (high viscosity index), light color, and attractive appearance were more likely to be found in petroleum of the paraffinic or Pennsylvania type. 

The simplest example is the batch distillation, conducted in an apparatus such as used in the laboratories. It consists of a flask (or retort), connected to a condenser (cooled by water or air), and a receiving... 

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 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. 

A summary of several example cases illustrated in Mujtaba and Macchietto (1998) is presented below. Instead of carrying out the investigation in a pilot-plant batch distillation column, a rigorous mathematical model (Chapter 4) for a conventional column was developed and incorporated into the minimum time optimisation problem which was numerically solved. Further details on optimisation techniques are presented in later chapters. 

With an example of batch distillation, Seader and Henley showed that the time step needed in implicit Euler s method was 200 times of that needed for explicit Euler s method. 

Robinson (1969) considered the following example problem. A binary feed mixture with an initial amount of charge, B0 = 100 kmol and composition xB0 = <0.50, 0.50> molefraction, having constant relative volatility of 2.0 was to be processed in a batch distillation column with 8 theoretical stages. The aim was to produce 40 kmol of distillate product (D) with composition (xd) of 0.5 molefraction for component 1 in minimum time (tF) using optimal reflux ratio (/ ). 

Mujtaba and Macchietto (1994) presented an industrial case study in which dynamic optimisation method of Mujtaba and Macchietto (1993) is utilised for the development of the optimal operation of an entire batch distillation campaign where 100 batches of fresh charge have to be processed with secondary reprocessing of intermediate off-cuts. The process involved a complex separation of a five-component mixture of industrial interest, described using non-ideal thermodynamic models. In addition, the operation of the whole production campaign was subject to a number of resource constraints, for example -... 

For example, if all the reaction products are valuable and have lower boiling temperature than the reactants, then conventional batch distillation would be most suitable. As the reaction proceeds the products will be separated in different main-cuts in sequential order. Conversion and yield can be greatly improved in such cases. If only some of the reaction products have low boiling temperature, then a conventional batch column will only remove those products as distillation proceeds. To separate the rest of the products by conventional distillation would require the removal of unreacted reactants from the column first. 

Mujtaba and Macchietto (1992) summarises a list (Table 9.1) of reaction schemes with boiling points of the species involved. For each reaction scheme (with reactants shown on the left and products on the right), the right type of batch distillation column is indicated. The recommendation does not hold when the desired products are those shown on the left hand side of each reaction scheme. For example, acetic anhydride is produced by dehydration of acetic acid (Acetic Acid <=> Acetic Anhydride + Water). In such cases, the use of CBD will be favourable to remove the water or the use of IBD will be favourable to remove the anhydride. In both configurations the equilibrium will shift to the right and the productivity of anhydride will improve. Wajge and Reklaitis (1999) considered such reaction scheme in a CBD column. Further details are in section 9.9. 

The features and modelling issues of IBD columns with or without chemical reaction are presented in Chapter 2 and 4 respectively. Simulation of IBD columns without chemical reactions is also presented in Chapter 4. After Robinson and Gilliland (1950) had introduced IBD columns, Abrams et al. (1987), Mujtaba and Macchietto (1994) and Sorensen and Skogestad (1996) used such columns for batch distillation and compared their performances with conventional columns. While Mujtaba and Macchietto (1994) studied simultaneous chemical reaction and separation using IBD columns, Sorensen and Skogestad (1996) presented the most comprehensive study on IBD columns. Some examples from these works are presented below. 

Figure S.2 Batch distillation of benzene-toluene at constant reflux ratio, Example 5,3, ia-e) McCabe-Thiele diagram for progressively reducing still concentration to 0.13 mole fraction benzene.

Hgwe 5.3 (Continued) Batch distillation of benzene-toluene with Wriable reflux ratio, Example 5.4. (o-e) McCabe-Thiele diagram fcr progressively increasing reflux ratio to 13 1. 

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