Azeotropic mixture Batch distillation

Chapter 13 will illustrate the operations of batch distillation utilizing two previously mentioned separation methods for azeotropic mixtures extractive distillation and heterogeneous azeotropic distillation. In the first part of Chapter 13, operation of batch extractive distillation is studied for separating acetone and methanol using water as the entrainer and separating IPA and water using DMSO as the entrainer. In the latter part of that chapter, a heteroazeo-tropic batch distillation system for acetic acid dehydration will be studied. 

Despite the advances in the thermodynamics for predicting azeotropic mixture, feasible distillation boundaries, and sequence of cuts, the azeotropic batch distillation system is still incipient in terms of design, optimization, and optimal control. 

Even though the simple distillation process has no practical use as a method for separating mixtures, simple distillation residue curve maps have extremely usehil appHcations. These maps can be used to test the consistency of experimental azeotropic data (16,17,19) to predict the order and content of the cuts in batch distillation (20—22) and, in continuous distillation, to determine whether a given mixture is separable by distillation, identify feasible entrainers/solvents, predict the attainable product compositions, quaHtatively predict the composition profile shape, and synthesize the corresponding distillation sequences (16,23—30). By identifying the limited separations achievable by distillation, residue curve maps are also usehil in synthesizing separation sequences combining distillation with other methods. 

Separation of n-hexane-ethyl acetate mixtures by azeotropic batch distillation with heterogeneous entrainers... 

Process synthesis and design of these non-conventional distillation processes proceed in two steps. The first step—process synthesis—is the selection of one or more candidate entrainers along with the computation of thermodynamic properties like residue curve maps that help assess many column features such as the adequate column configuration and the corresponding product cuts sequence. The second step—process design—involves the search for optimal values of batch distillation parameters such as the entrainer amount, reflux ratio, boiler duty and number of stages. The complexity of the second step depends on the solutions obtained at the previous level, because efficiency in azeotropic and extractive distillation is largely determined by the mixture thermodynamic properties that are closely linked to the nature of the entrainer. Hence, we have established a complete set of rules for the selection of feasible entrainers for the separation of non ideal mixtures... 

Selection of a suitable entrainer for the separation of w-hexane-ethyl acetate mixtures by heterogeneous azeotropic batch distillation... 

I. Rodriguez-Donis, V. Gerbaud, X. Joulia, Entrainer selection rules for the separation of azeotropic and close boiling temperature mixtures by homogeneous batch distillation, Ind. Eng. Chem. Res 40 (2001) 2729-2741. 

R. Diissel, J. Stichlmair, Separation of azeotropic mixtures by batch distillation using an entrainer, Comp. Chem. Eng. 19 (1995) si 13— si 18. 

The final dehydration of potassium acetate is carried out in reactor 6 (until the total elimination of moisture). For that purpose the washed and dried reactor is filled with toluene from batch box 2 then potassium acetate is sent at agitation through an overflow pipe. The potassium acetate has already been dried in vacuum drafts and weighed. The jacket of the reactor is filled with vapour (0.3 MPa), and cooler 5, with water. The dehydration is carried out by distilling the azeotropic mixture of toluene and water at 100-115 °C. The distilled mixture is sent into cooler 5 to condense. The condensate is collected in receptacle 7. The speed of the distillation of the azeotropic mixture is regulated by changing the vapour supply in the reactor jacket. After 200-250 1 of the azeotropic mixture have been distilled, toluene is returned from collector 7 into the reactor. The distilled water is collected in the lower part of collector 7, and the separated toluene is sent back into the reactor. 

Out of collector 13 the base salve solution of trimethylborate is sent into tank 16, where at 200 °C trimethylborate is distilled. The distilled fraction, which contains 88-90% of trimethylborate, is collected into collector 6 and sent into the tank of rectification tower 7 the base salve from tank 16 is sent through container 17 back into batch box 5. During rectification all methyl alcohol is separated in the form of azeotropic mixture with trimethylborate and collected in collector 19 trimethylborate remains in the tower tank. The azeotropic mixture is sent through collector 11 for repeated extraction into tower 12 the ready product, 98.5-99.5% trimethylborate, is sent from the tank of tower 7 into collector 18. 

However, there are cases where none of these batch distillation columns can be used economically to improve conversion or yield. Also, complications typically arise if there are any azeotropes present in the mixture. 

The initial distillate cut is the lightest and, as the distillation progresses, the liquid remaining in the reboiler becomes continuously richer in the heavier components, and subsequent distillate cuts become increasingly heavier. The residue remaining in the reboiler after the last distillate cut is the heaviest cut. A multicomponent feed mixture may be separated in one batch distillation column into a number of products with specified purities. Given the required number of trays and reflux ratio, a batch distillation column could, in principle, separate a normal feed mixture (one that is not reactive or azeotrope forming) into its pure constituents. 

Distillation is a method of separation based on the difference in composition between a liquid mixture and the vapor formed from it. The composition difference is due to differing effective vapor pressures, or volatilities, of the components of the liquid. When such a difference does not exist, as at an azeotropic point, separation by distillation is not possible. The most elementaiy form of the method is simpledistillation in which the liquid mixture is brought to boiling and the vapor formed is separated and condensed to form a product if the process is continuous, it is called flash distillation or an equilibrium flash, and if the feed mixture is available as an isolated batch of maleiial. the process is a form of batch distillation and the compositions of the collected vapor and residua] liquid are thus time dependent. 

Step-Ill The organic layer is transferred to a batch distillation still equipped with a packed tower. The mixture is distilled. As the water-n-butanol system forms a heterogeneous azeotrope, the condensate separates into two layers in the receiver. The organic layer is refluxed, and the water layer is drawn as distillate and discharged as effluent. 

Fig. 5.2 -43 Batch distillation of a ternary mixture with maximum azeotrope and boundary distillation line (A) residuum line (B) standard plot of product concentrations...

For on-site separation/purification of recovered solvent it is necessary to consider the number and complexity of distillations needed to obtain materials which are suitably pure for reuse. Where mixtures must be separated into individual solvents this can require several distillations, particularly where the solvents form azeotropes - this can significantly add to costs. The major costs associated with solvent purification are normally the capital required for distillation columns, energy and the additional staffing needs to oversee the operation. Where azeotropic distillations are required the cost of distillation columns can be greater than the capital cost of the recovery unit itself and staffing costs can be a significant variable cost (particularly if batch distillation is required). 

A6. Batch-by-Night, Inc. has developed a new sinple batch with reflux system fFigure 9-1 with some of stream D refluxed to the still pot) that they claim will outperform the normal sinple batch fFigure 9-11. Suppose you want to batch distill a mixture similar to methanol and water where there is no azeotrope and two liquid phases are not formed. Both systems are loaded with the same charge F moles with the same mole fraction Xp, and distillation is done to the same value... 

An automatic design method for batch extractive distillation, one of the most important techniques for separating low relative volatility or azeotropic mixtures, is presented. Example calculations are performed to the acetone-methanol mixture using water as entrainer. The NLP and MINLP problems are solved with applying GAMS D1COPT++. 

---