batch extractive distillation as a hybrid process… chisa 2004 1 batch extractive distillation as a...

Batch Extractive Distillation as a Hybrid process… CHISA 2004Batch Extractive Distillation as a Hybrid process… CHISA 2004 11

BATCH EXTRACTIVE DISTILLATION AS A BATCH EXTRACTIVE DISTILLATION AS A HYBRID PROCESS:HYBRID PROCESS:

SEPARATION OF MINIMUM BOILING SEPARATION OF MINIMUM BOILING AZEOTROPESAZEOTROPES

Kotai B.Kotai B.11, P. Lang, P. Lang2 2 , G. Modla, G. Modla33

11MSc (PhD student), MSc (PhD student), 22PhD (associate professor), PhD (associate professor), 33MSc (chemical engineer)MSc (chemical engineer)

Budapest University of Technology and EconomicsBudapest University of Technology and Economics, , Department of Chemical and FoodDepartment of Chemical and Food EngineeringEngineering

H-1521 Budapest, Muegyetem rkp. 3-5, [email protected] Budapest, Muegyetem rkp. 3-5, [email protected]


The aim of the lecture: to study the batch hybrid extractive distillation for the separation of minimum boiling point azeotropes with feasibility studies and rigorous simulation.

OUTLINEOUTLINE:

I. IntroductionI. Introduction II. Feasibility studiesII. Feasibility studies II.1. Feasibility methodII.1. Feasibility method II.2. ResultsII.2. Results III. Rigorous simulationIII. Rigorous simulation III.1. Simulation methodIII.1. Simulation method III.2. ResultsIII.2. Results IV. ConclusionIV. Conclusion


I. INTRODUCTIONI. INTRODUCTION

Extractive distillation (ED):Efficient separation method, the binary (A-B) azeotropic mixture is separated by the aid of a heavy separating agent (solvent, E). The solvent does not form any azeotrope. The continuous ED is frequently applied in the industry.

Stichlmair and Fair (1998) considered the ED as a hybrid process:- absorption + distillation (separation of minimum azeotropes),- desorption + distillation (separation of maximum azeotropes).

The role of the solvent:It favourably changes the VLE conditions, it breaks the azeotrope.

The benefits of the batch distillation:- It is more flexible then the continuous distillation.- Several products can be obtained on a single column.- High product purity can be ensured.

The batch extractive distillation (BED) simultaneously provides the advantages of the batch and those of the extractive distillation.


Model of batch extractive distillation column (A/B separation)

Continuous E feeding (F > 0) is applied in both cases.

a. BED b. Hybrid process

BED:- two column sections: rectifying and extractive one,- significant reflux (R > 0),- solvent feeding and reflux are liquids close to the boiling point.

Hybrid process:- only one column section: absorption (extractive),- no external reflux (R = 0),- solvent feeding is highly subcooled liquid.

Condenser

Reboiler

Feeding of E

Condenser

Feeding of E

f=1

f

N

1

N

Reboiler

a. BED b. Hybrid process


Separation steps

BED:1. Start up under total reflux (R=) without solvent feeding (F=0).2. Purification of the distillate under R= and continuous feeding of E (F>0, optional).3. Production of A under continuous feeding of E (0<R<, F>0).4. Separation B/E (0<R<, F=0).

Hybrid process:- Separation A/B by absorption (R=0, F>0).- Separation B/E by distillation (0<R<, F=0).

In both cases mainly the A/B separation step is investigated.


II. FEASIBILITY STUDIESII. FEASIBILITY STUDIES

Simplifying assumptions: - negligible tray hold-up, - quasi-steady state, - constant molar overflow.

The feasibility method is based on the analysis of the still path on the map of possible column section profiles. The separation is instantaneously feasible if from the still liquid of actual composition (xs(t)) the distillate of specified composition can be produced.The condition of instantaneous feasibility: the actual point of the still path (xs(t)) can be connected - BED: with the prescribed distillate point (xD,sp) by a column profile consisting of an extractive and a rectifying profile.- hybrid process: with the feed stage liquid composition (x1) being in equilibrium with the prescribed distillate composition (y1 = xD,sp).

II.1. FEASIBILITY METHOD

In order to study the hybrid process we extended the feasibility method of Lelkes, Lang et al. by making possible the variation of the heat condition of the solvent feeding (q) and that of the reflux.


II.2. RESULTS OF THE FEASIBILITY STUDIES

The necessary but not sufficient condition of the feasibility of the hybrid process is

that the extractive profiles arrive at the AE edge. The A/E separation must be

provided by one theoretical stage (feed plate).

Hybrid process


The map of the extractive profiles (F/V = 1.31, xD= 0.99, 0.0005, 0.0095)Solvent feeding:

a. boiling point liquid (q=1) b. strongly subcooled liquid (q=1.32)

Prescribed purity A can not be produced even by highly subcooled liquid solvent under resonable F/V ratios since the profiles arrive at the BE edge from the whole triangle. The profiles arrive at the AE edge only in the case of very high F/V ratios.

• Acetone (A) – methanol (B) + water (E)

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

A (acetone)

B (methanol)

E (water)

q=1,0

Az

IFR

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1A (acetone)

B (methanol)

E (water)

q=1,32

Az

IFR


• Ethanol (A) – water (B) + ethylene glycol (E)

The boiling point of E is much higher (197.3°C) than that of A (78.3°C) and B (100°C).

The map of the extractive profiles (xD=0.99, 0.0005, 0.0095)Solvent feeding:

a. boiling point liquid (q=1) b. strongly subcooled liquid (q=1.32)

The results show the decisive role of q. In the case q=1 the profiles arrive at the BE edge from the whole triangle (infeasible region, IFR). In the second case a considerable part of the profiles (feasible region, FR) arrive at the AE edge and the separation can be feasible (with the aid of the feed plate since αA,E >> 1).

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

q=1

Az

B (water)

E (ethylene glycol) A (ethanol)

IFR

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

q=1.32

Az

B (water)

E (etylene glycol) A (ethanol)

IFR

FR


Simplifying assumptions:

- theoretical trays,- negligible vapour hold-up,- constant volume of liquid hold-up,- negligible fluid dynamic lags.

III. RIGOROUS SIMULATIONIII. RIGOROUS SIMULATION

III.1. SIMULATION METHOD

Solution method:Use of a professional simulator (ChemCAD 5.3: BATCHCOLUMN, DYNCOLUMN): quasi-steady state approximation, simultaneous correction method.

We tried to produce the recovery of A of the BED by an absorption step (under the product purity prescribed).


III.2. RESULTS OF THE RIGOROUS SIMULATION

BED

Input data:

Column data: 18 theoretical plates, the solvent is fed to plate 6 (f = 6), the liquid holdup: 70 cm3/plate.Operational parameters: P = 1.013 bar, R = 4, the heat duty of the reboiler: 1.5 kW.The quantity of (the binary, equimolar) charge: Uch=156.9 mol (9 dm3). The solvent is pure water (TF = 80°C).The prescribed purity of product: xD,A,av = 0.94. Stopping criterion of purification step: xD,B ≤ 0.006.

Results:The prescribed purity product was obtained with high recovery (ηA = 94 %) besides low B pollution (xD,B,av= 0.006) by one production step.

• Acetone (A) – methanol (B) + water (E)


Hybrid process

Input data (different from those of the BED):

- Solvent feed plate: 1.- Operational parameters: R = 0, TF = 20 °C.

Results:

The end of the start-up xD,A = xaz. At the beginning of the absorption step xD,A immediately decreases.

At low values of F the product is considerably polluted by both B and E.

A

B

E

The evolution of the distillate composition (F = 5 l/h)


At high values of F B can be withdrawn from the product which is considerably polluted by E.

The evolution of the distillate composition (F = 13 l/h)

The prescribed purity can not be produced by only one absorption step. For the separation A/E an additional distillation step is required. The new goal of the absorption step is the withdrawal of B (xD,B,av ≤ 0.006) and not the production of A in the prescribed purity.

A

B

E


The effect of the variation of F (hybrid process, acetone (A) – methanol (B) + water (E))

Without purification step

F, l/hr x D,A,av x D,B,av x D,E,av SD, mol Δt 3, h ηA, %

11 0.645 0.349 105.0 1.63 84.912 0.644 0.350 115.6 2.35 93.313 0.638 0.356 120.1 3.28 96.015 0.581 0.413 135.9 10.84 98.9

0.006

With purification step (Stopping criterion of purification step: xD,B ≤ 0.006)

F, l/hr x D,A,av x D,B,av x D,E,av SD, mol Δt 3, h ηA, %

5 0.708 0.285 26.4 0.22 23.48 0.654 0.340 48.7 0.49 39.912 0.612 0.382 124.2 2.71 95.314 0.582 0.412 134.5 5.94 98.115 0.517 0.477 153.2 13.15 99.3

0.006

On the increase of F the recovery of A and the water content of the distillate increase in both cases. Acceptable recovery can be produced only under high solvent consumption. The application of the purification step (F >0, R = ) slightly improves the recovery of A.

SF/Uch= 11.8


The influence of F on the average production rate (Dav)

0

10

20

30

40

50

60

70

80

90

5 7 9 11 13 15F, l/hr

Dav

, mo

l/hr

w ith purif icationstep

w ithout purif icationstep

On the increase of F the average production rate (Dav) falls considerably.The greater is the value of F, the greater part of the heat furnished in the still is consumed by the warming up of the solvent.

The effect of the number of stages

The increase of the number of stages doesn’t ameliorate the A/E separation.


The energy and solvent consumptions

In order to produce same quantity and purity acetone with the hybrid process we have to apply 5 times greater quantity of solvent (SF) and 1.4 times more energy (SQ) than in the case of BED. The BED is much more economical for the separation of mixture acetone (A) – methanol (B) by the aid of water (E).

The maximum volume of still liquid

During the hybrid process the volume of still liquid increase to an unreal extent.

SF, dm3SQF, kJ SQN+1, kJ SQ, kJ

BED 6.6 1655 17820 19475Hybrid p. 33.48 - 27436 27436

Uch, dm3 UN+1,max, dm3 UN+1,max/Uch, dm3

BED 9.0 10.0 1.1Hybrid p. 9.0 94.3 10.5

We compared energy and solvent consumptions and the maximum volume of the still liquid keeping constant the recovery of A.


BED

Input data:

Column data: 13 theoretical plates, the solvent is fed to plate 3 (f=3), the liquid holdup: 70 cm3/plate.Operational parameters: P = 1.013 bar, R = 2, the heat duty of the reboiler: 1.5 kW.Charge: Uch = 160 mol (8.62 dm3), xch = [0.9, 0.1, 0]. The solvent is pure ethylene glycol (TF = 158°C). The prescribed purity of product: xD,A,av= 0.98.

• Ethanol (A) – water (B) + ethylene glycol (E)


The effect of the variation of F

The influence of F on the recovery of A

Under moderate R and moderate number of rectifying plates (Nrect = 2) even with a low flow rate of E high recovery of A can be obtained. There is an optimum value of F where the recovery of A is the highest. On the increase of F the production rates falls and the duration of the production step is increased.

The application of a purification step did not improve the separation.

97.5

98.0

98.5

99.0

99.5

100.0

1 2 3 4 5 6 7 8

F, l/hr

Rec

ove

ry o

f A

, %


Hybrid process

Input data (different from those of the BED):

- Solvent feed plate: 1.- Operational parameters: R = 0, TF = 20 °C.

We try to obtain the recovery of the BED (ηA = 99.6 %) with one absorption step.

The evolution of the distillate composition (F = 7 l/h)(Ethanol (A) – water (B) + ethylene glycol (E))

The prescribed product purity can be reached at all only with high flow rate of E (Fmin).

A

B

E


The effect of F

The greater is the value of F - the greater part of the heat furnished in the still is consumed by the warming up of the solvent,- the lower the distillate flow rate (Fmax).

In the possible region of F the recovery of A and the composition of the product didn’t vary considerably. On the increase of F the duration of the production step strongly increased. When F was 10 l/hr after a certain time the distillate flow rate decreased to zero.

The application of a purification step did not improve the separation.

F, l/hr x D,A,av x D,B,av x D,E,av SD, mol Δt 3, hr ηA, %

7 0.004 0.016 146.1 3.43 99.48 0.004 0.016 146.3 5.48 99.69 0.004 0.016 146.2 13.52 99.5

10 0.002 0.018 122.4 7.82 83.3

0.980


Comparison of the BED and the hybrid process(Ethanol (A) – water (B) + ethylene glycol (E))

We compared energy and solvent consumptions and the maximum volume of the still liquid keeping constant the recovery of A.

The energy and solvent consumptions

SF, dm3SQF, kJ SQN+1, kJ SQ, kJ

BED 6.52 2744 17607 20351Hybrid 15.84 - 14256 14256

The maximum volume of still liquid

Uch, dm3 UN+1,max, dm3 UN+1,max/Uch, dm3

BED 8.62 8.62 (6.2*) 1.0Hybrid 8.62 24.6 2.9

*at the end of A production step.

In order to produce same quantity and purity ethanol with the hybrid process we have to apply 2.4 times greater quantity of solvent and 30 % less energy than in the case of BED. The volume of still liquid considerably increased during the hybrid process whilst it decreased during the BED.


The criteria of the efficient application of the hybrid process

The A/E separation must be very easy (αA,E >> 1).

The difference between the boiling (Tbp) and freezing point (Tfp) of the solvent must be very great (it can be subcooled to a great extent). The heat condition of E-feeding has a maximum.

If these criteria are satisfied, the hybrid process can be applied, but

- the reboiler temperature varies a lot during the A production step (78 oC 189 oC),- the B/E separation step usually requires the application of vacuum.

If these criteria are not satisfied

- B can be withdrawn from beside E under low energy consumption, but- an additional A/E separation step (rectification) must be inserted.

E

EfpEbpEmolp TTcq

)(

1 ,,,,max


IV. CONCLUSIONIV. CONCLUSION

We studied the batch hybrid extractive distillation for the separation of minimum boiling point azeotropes with feasibility and rigorous simulation calculations.

Our former feasibility method was extended for the hybrid process.

Comparing the hybrid process with the batch extractive distillation (BED) we concluded that:

- The BED process provides higher degree of freedom, greater flexibility and can produce better results than the hybrid process.

- In the case of BED the boiling point of the solvent can be much closer to those of the original components.

The criteria of the successful application of the hybrid process were determined.


THANK YOU

FOR YOUR ATTENTION

ACKNOWLEDGEMENTACKNOWLEDGEMENT

This work was supported by the Hungarian Scientific Research Foundation (“OTKA”, No: T-034659).

batch extractive distillation as a hybrid process… chisa 2004 1 batch extractive distillation as a...

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