Generalised model for heteroazeotropic batch distillation with variable decanter hold-up

Abstract

OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible. This is an author-deposited version published in : http://oatao. hold-up. (2013) Separation and Purification Technology, vol. 115. pp. 9-19. ISSN 1383-5866 Any correspondance concerning this service should be sent to the repository administrator: staff-oatao@listes-diff.inp-toulouse.fr t r a c tA general model of batch heteroazeotropic distillation is proposed. Both liquid phases present in the decanter can be refluxed or withdrawn as distillate, their hold-up can be increased, decreased or kept constant, as well. By assuming maximal separation, that is, that the composition of the condensate always equals to that of the heteroazeotrope, the still path equation was derived. The still path directions are determined for all the 16 possible operational policies. It is possible to steer the still path in a desired direction by changing the operational parameters, which allows the recovery of a pure component in the still. The still path directions are validated by rigorous simulations for three policies not published yet using the mixture water -formaldehyde -propyl formate. From the 16 operational policies, 11 can be considered as useful in practice. To demonstrate the advantage of using a non-traditional policy, the separation of the mixture aniline -ethylene glycol -water was investigated, as well. By using a non-traditional operational policy with hold-up reduction in the decanter a higher purity of ethylene glycol was obtained in the still.

General Model for Studying the Feasibility of Heterogeneous Extractive Batch Distillation

“…The model of Hegely et al 6 was extended to batch heterogeneous extractive distillation (BHED), by taking into consideration continuous entrainer feeding. The possibility of refluxing or withdrawing both liquid phases as distillate was maintained, as well as the variability of the hold-up of the phases in the decanter.…”

“…In eq 13, besides the first two terms, which represent the removal of the E-rich and the E-lean phase from the column, respectively, a new term appeared, compared to eq 8, which describes the effect of the continuous feeding of the entrainer (that is, a change in the still composition toward the entrainer composition). As stated by Hegely et al, 6 the removal of either liquid phase from the column can be realized by either withdrawing it as distillate, or accumulating it in the decanter. In the case of total reflux (no distillate, constant decanter holdup) of a liquid phase (r R = 1 or r W = 1), the corresponding term disappears from eq 13.…”

“…In BHAD, r R and r W determine the still path direction, and V only influences the speed of distillation; 6 in BHED, the still path direction is also determined by the relative magnitude of F E and V. A higher F E / V ratio obviously diverts the still path in a more prominent way toward the entrainer composition. Hegely et al 6 distinguished 16 possible operational policies, depending on the values of r R and r W , categorized into four distinct intervals: zero (0), between zero and one (<1), one (1), and higher than one (>1). The continuous entrainer feeding does not change the classification of the operational policies, as the value of the entrainer flow rate cannot be divided into separate categories (assuming that it is higher than the minimum value necessary for process feasibility, and not so high as to render the decanter holdup homogeneous).…”

“…4,5 The possible directions of the still path of BHAD can be classified into eight still path zones (see Figure 3a. 6 ), covering in all possible directions. At least one operational policy belongs to each zone; that is, the still composition can be directed in any desired direction.…”

Separation of ethyl acetate–isooctane mixture by heteroazeotropic batch distillation

“…Simulation and experimental results have shown key differences between homogeneous and heterogeneous batch distillation and led to the publication of general feasibility rules (Rodriguez-Donis et al, 2001a,b;Skouras et al, 2005). Indeed, some advantages of heterogeneous batch processes compared to homogeneous systems are: (i) more suitable candidate entrainers and hence, more design alternatives for the separation of non-ideal mixtures, (ii) simplified distillation sequences thanks to the liquid-liquid phase split occurring in some parts of the column and in the decanter, (iii) the addition of a smaller amount of entrainer to the original mixture, (iv) the use of more flexible reflux policies through any combination of the entrainer-rich phase and the distillate-rich phase, (v) depending on the reflux policy, the typical unstable separatrices of residue curve map are not a limit anymore for the separation of components located in different basic distillation regions (Lang and Modla, 2006;Hegely et al, 2013).…”