Homoazeotropic Distillation of Maximum Azeotropes in a Batch Rectifier Homoazeotropic Distillation of Maximum Azeotropes in a Batch Rectifier with Continuous Entrainer Feeding: with Continuous Entrainer Feeding: I. Feasibility StudiesI. Feasibility Studies

P. Lang, G. Modla, B. Benadda1, Z. LelkesBudapest University of Technology and Economics, Hungary,

1INSA-Lyon LAEPSI, France

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SN

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B (chloroform)

E (benzene) A (acetone)

MW

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Objective: to study the performance of the batch extractive distillation (BED) for the separation of maximum azeotropes on the basis of feasibility calculations

Conclusion: by the continuous feeding of the solvent the feasibility region can be extended to a great extent and significantly greater recovery can be obtained.

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B (chloroform)boundary of BED (eb)

boundary of BD (R=inf)

boundary of BD (rb) (R=20)

ep

rp

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eb3rb3

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1. R=1002. R=203. R=10

I. PrinciplesFeasibility method

The method is based on the calculation of the still path (xS) and possible composition profiles (x) of the column.Simplifying assumptions:negligible tray hold-up, quasi-steady state in the column,constant molar overflow, boiling point liquid entrainer feeding and reflux.The differential equations used for the calculations:

x-profiles still-path

Feasibility means that starting from the charge of given composition (xch) under the given operating conditions (R, F/V) the specified distillate composition (xD,sp) can be obtained, that is, the still path can be connected with the point xD,sp by at least one possible column profile.

Batch Distillation (BD) and Batch Extractive Distillation (BED)

BD steps:•0. Addition of a great quantity of E to the charge.•1. Start-up (R=)•2. Production of A. •3. Separation B/E.

II. Feasibility studiesResidue-curve map

If R= the BD column profile follows a simpledistillation residue curve determined by theVLE-conditions.

BED steps:•0. Addition of a small quantity of E to the charge.•1. Start-up (R=, F=0).•2. Production of A under continuous feeding of E (R<, F>0)•3. Separation B/E.

Comparison of two solvents (benzene, toluene)

The rectifying boundaries show that the benzene is more selective. Its rectifying boundary reaches earlier the BE edge.By the extractive boundaries separating feasible and infeasible regions (eb) the feasibility region of benzene is greater. The two unstable separatrices between BE and AE edges (ebu) are nearer to the EB edge for the toluene. The toluene can be separated from A more easily in the rectifying section.

III. Influence of the operational parametersThe effect of R under F=constant

On the decrease of reflux ratio both the rectifying (rb) and the extractive boundaries (eb) get closer to vertex A, both feasibility regions decrease. There is no maximum reflux ratio.

The effect of R under SF=constant

On the increase of R the extractive boundary (eb) firstly removes from vertex A then above a certain R it approaches vertex A suggesting that there is an optimum reflux ratio (Ropt). The unstable separatrices (ebu) come closer to the AB edge.

The effect of F (R=constant)

The diminution of the feasibility region (at high F/V values) suggests that there is maximum flow rate of solvent (Fmax). On the increase of F SN gets further from vertex A. After meeting the rectifying and extractive boundaries it can leave the ternary diagram.

The limiting values of the parameters

The highly curved separatrix (between S and SN) divides the triangular diagram onto two main distillation regions. From a charge of azeotropic composition (saddle point S) pure A can be obtained by BD if we add a small quantity of E, so the still composition (xs) gets in the feasible region II. The boundary limits the recovery of A (A).

One part of the profiles (regions FR1 and FR2) arrive at the stable node SN located on the AE edge. These feasible profiles can reach a rectifying profile arriving at vertex A. A is limited by the boundary (eb) between feasible and infeasible (IFR1, IFR2) regions.

The extractive profile boundary (eb) is located further from vertex A in the region of moderate SF/Uch ratios. By continuous feeding of E greater A can be reached by BED under the same entrainer consumption (SF). The feasibility region of the BED is greater.

The map of extractive profiles

For the extractive section of a BED column the residue curve map is no more valid and the map of extractive profiles must be studied.

BD and BED boundaries

In order to compare the maximum recoveries of A we must compare the location of the boundaries of extractive (ep) and rectifying profiles (rp).

BD• minimum reflux ratio Rmin,BD.•minimum number of rectifying trays (Nr,min,BD).•minimum quantity of solvent (SF0,min) if R<•maximum quantity of solvent (SF0,max) if R<

BED•minimum reflux ratio (Rmin,BED < Rmin,BD) •minimum number of rectifying (Nr,min) and extractive trays (Ne,min)•minimum flow rate of solvent (Fmin)•maximum flow rate of solvent (Fmax)

Extractive section is necessary only if the BD is not feasible.

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boundary of infeasibleregion (eb)

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R=100F/V=1/4

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1. R=100, F/V=0.0512. R=20, F/V=0.253. R=9.5, F/V=0.54. R=4.25, F/V=1

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eb2 1. F/V=0.252. F/V=0.53. F/V=1

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