Pressure-swing distillation

320

Chapter 7. Pressure-swing distillation In the field of special distillation processes, pressure-swing distillation (PSD) is the one able to be utilized to separate the mixture with close boiling point or forming azeotrope, but no new additive added. So PSD is an environment-friendly process. However, the application of PSD is very limited, the well-known system almost only involving tetrahydrofuran (THF) / water. The reason may be that for most of the systems the change of x-y curve at different pressures isn't so distinct. This chapter tries to present a wide aspect of PSD in operation principle, operation mode and conceptual design. 1. INTRODUCTION

1.1. Separation principle The separation principle of pressure-swing distillation (PSD) is based on the fact that a simple change in pressure can alter relative volatility of the mixture with close boiling point or forming azeotrope. In some cases, this results in a significant change in the azeotropic composition or enlarging the relative volatility of close boiling point components, which allows the recovery of feed mixture without adding a separating agent. Therefore, an outstanding advantage of PSD is that it belongs to environment-friendly process. However, PSD is particularly challenging for the separation of homogeneous azeotropes. Table 1 lists some azeotropes that have the potential to be separated by PSD technique.

1.2. Operation modes There are three types of operation modes for PSD, i.e. continuous operation, batch operation and semi-continuous operation [4]. In continuous operation, the separation is performed using two columns maintained at different pressures. The distillates, which approach the azeotropic compositions at high and low pressure, are cycled between the two columns. In batch operation, only a single column is used for the separation. It is supposed that component A is the ultimate product in the mixture consisting of components A and B. The column is initially charged with the feed mixture (F) and operates at pressure Pl, as shown in Fig. 1. Component B is removed from the bottom of the tower, while at the top the mixture D1 approaching the azeotropic composition. Then, the column is recharged with the mixture D1 and operates at pressure P2. Thus, component A is obtained from the bottom, while at the top the mixture D2 approaching the azeotropic composition. Only can this process be carried out on the condition that the composition of the feed mixture xf is in the range of Xazeo2 and 1. To provide high recovery, this cycle may be repeated many times.

321

Table 1 E x a m p l e s o f PSD binary azeotropes, adapted from the references [ 1-3] No.

Components

1

carbon dioxide - ethylene

2

hydrochloric a c i d - water

3

w a t e r - acetonitrile

4

water - ethanol

5

w a t e r - acrylic acid

6

w a t e r - acetone

7

w a t e r - propylene oxide

8

w a t e r - methyl acetate

9

w a t e r - propionic acid

10

w a t e r - 2-methoxyethanol

11

w a t e r - 2-butanone (methyl ethyl ketone (MEK))

12

w a t e r - tetrahydrofuran (THF)

13

Carbon tetrachloride - ethanol

14

carbon tetrachloride - ethyl acetate

15

carbon tetrachloride - ethyl acetate

16

carbon tetrachloride - benzene

17

methanol-acetone

18

methanol - 2-butanone (MEK)

19

methanol - methyl propyl ketone

20

methanol - methyl acetate

21

methanol - ethyl acetate

22

methanol - benzene

23

methanol - dichloromethane

24

m e t h y l a m i n e - trimethylamine

25

ethanol - dioxane

26

ethanol - benzene

27

ethanol - heptane

28

dimethylamine - trimethylamine

29

2-propanol - benzene

30

propanol - benzene

31

Propanol - cyclohexane

32

2-butanone (MEK) - benzene

33

2-butanone (MEK) - cyclohexane

34

isobutyl alcohol - benzene

35

benzene - cyclohexane

36

Benzene - hexane

37

phenol - butyl acetate

38

aniline - octane

322

J

W'" j 0 (A)

Xazeol(D1)

Xazeo2(D2)

XF (F)

1 (B)

Fig. 1. T-x-y diagram at different pressures P1 and P2. In semi-continuous operation, only a single distillation column is involved. But the column operates continuously and periodically. It isn't emptied or recharged. Liquid levers are maintained on the trays or packing, stream is fed continuously to the reboiler, and cooling water is fed continuously to the condenser. The advantage is lower investment costs and shorter downtimes when the mixture to be separated is changed. An optimal-control algorithm has been programmed by Phimister and Seider [4]. Simulation results for the dehydration of tetrahydrofuran indicate that the column achieves production rates near 89% of the maximum throughput of a single column in the continuous process, which shows superior performance when compared with batch operation. So, undoubtedly, among the three types of operation modes, the mode of continuous operation is more desirable in most cases. 2. D E S I G N OF PSD

2.1. Column sequence A main problem in PSD is how to arrange the column sequence at pressures P1 and P2. It can be seen from Fig. 1 that if first at pressure P1 and then at pressure P2, the composition of the feed mixture xF is in the range of Xazeo2and 1; inversely, if first at pressure P2 and then at pressure P l, the composition of the feed mixture xv is in the range of 0 and Xazeol. But if the composition of the feed mixture XF doesn't fall into this desirable range, extra stream should be replenished to make into a new mixture and change the original composition.

323

67 kmol h-I (0.33 H20, 0.67 THF)

["-'~

ll7kmolh-l(0.19H20,0.81THF)[...~

,,I v

Rectifying section

Rectifying section

100 kmol h l (0.5 H20, 0.5 THF) Stripping section

Stripping section

Pressure P2 = 10 bar

Pressure Pi = 1bar 50 kmol hl (H20)

q-

(a)

1

50 kmol h l (THF)

239.3 kmol h -1 (0.19 H20, 0.81 THF)

-"'~

289.3 kmol h"l (0.33 H20, 0.67 THF)

1"

I

r |

100 kmol h" (0.5 H20, 0.5 THF)

_--, Pressure P2 = 10bar

-

,--, Pressure P1 = l bar

50 kmol h-I (THF)

F ~

50 kmol hl (H20)

(b)

Fig. 2. Column sequence for the dehydration of THF (a) first at pressure P1 and then at pressure P2, adapted from the reference [4]" (b) first at pressure P2 and then at pressure P1.

324

For example, a typical PSD process, involving the dehydration of tetrahydrofuran (THF), is illustrated in Fig. 2, where the material balance is marked [4, 5]. It is shown that the cycled flowrates when first at pressure P2 and then at pressure P~ are much larger than those when first at pressure P l and then at pressure P2. This means that the column sequence of first at pressure P l and then at pressure P2 is more economical. Yet PSD isn't restricted to binary mixtures only. It can be extended to separate multi-component mixtures containing distillation boundaries. The presence of a boundary in a ternary mixture means that the three pure components can't be separated without the addition of an entrainer. However, if the boundary (boundaries) can be shifted by adjusting the pressure, a pressure-swing sequence can be used, as shown in Fig. 3. 2.2. Column number The minimum number of columns required for a given separation (but not necessarily the optimum, especially for dilute feeds) can be calculated from [ 1, 6] N oI=N I,+N B-1

(1)

where N,~,z is the minimum number of columns required, N p

is the number of pure

component products; N/~ is equal to the number of boundaries crossed for PSD, but this doesn't include boundaries that disappear as the pressure changes. For extractive distillation, N j~ is equal to unity. For non-azeotropic system, this reduces to the familiar equality N :,,z = N p - 1 .

For example, in Fig. 3 N/, - 3

and N B - 1 because the distillation boundary @ is

crossed once by material balance line D2DIB: as the pressure is Pl in column 2. Therefore, N~,,z - 3 + 1 - 1 - 3 and at least three columns are needed. The process of extractive distillation for separating ternary mixture into its three constituent pure components is also illustrated in Fig. 4, in which also three columns are needed. That means N1~ = 1 for extractive distillation. Consequently, it is unlikely that PSD will be advantageous when more than one boundary is crossed because the minimum number of columns required increases. Noted that the azeotropic point for homogeneous azeotropes may vanish as the pressure decreases to some extent, and thus the column number is reduced ( N z~ -- 0). In principle, two pure components can be obtained at this pressure by ordinary distillation. However, this phenomenon isn't exploited commercially because in most cases the relative volatility remains close to 1.0, which results in a big reflux ratio and a large number of equilibrium stages.

325

Pure B Bs Material balance line I I - - II II

Distillation boundaries @ and @ at cited pressure Azeotrope

Il

• i

,\ i !

•

I Ill i

,,,

,

3

i

i

gl

B2 Pressure P1

Pure C

Pressure P2

Pure A

D3I r-y, D1

D2

FI(A+B+C)

LI----~ Pure C

I

Pressure P2

T--~ Pure A Pressure P~

# Pure B Pressure P2

(b) Fig. 3. PSD for separating ternary mixture into its three constituent pure components: (a) material balance lines; (b) column sequence; adapted from the reference [1 ].

326

r-¢ A+B+C

S+B+C

S+C

-p s Fig. 4. Extractive distillation for separating ternary mixture into its three constituent pure components. 1.00

0.80

0.60

0.40

0.20

O.OOf 0.00

0.20

0.40

0.60

0.80

1.00

Xl

Fig. 5. x-y phase diagram of the system of ethanol (1) / water (2); x and y represents the mole fraction in the liquid and vapor phases, respectively; O - at pressure 50 mmHg; A - at pressure 760mmHg; the data come from the reference [8].

327

As can be seen from Fig. 5, the VLE curve at pressure 50 mmHg is very close to diagonal at high ethanol concentration, although in this case the binary azeotrope is eliminated. Black [7] has indicated that a vacuum distillation carried out at 65 mmHg to break the ethanol / water azeotrope and produce anhydrous ethanol overhead isn't economically competitive with other azeotrope-breaking schemes. For this reason, it is thought that PSD takes advantage of a change in azeotropic composition with pressure, but maybe isn't perfect in getting high-purity of product. The topic on energy optimization of PSD system has been studied by Hamad and Dunn [9]. It is claimed that using global energy optimization strategies can significantly reduce the energy consumption as opposed to local energy optimization strategies for the minimum-boiling homogeneous azeotropic system of THF / water. REFERENCES

[1] J.R Knapp and M.F. Doherty, Ind. Eng. Chem. Res., 31 (1992) 346-357. [2] T.C. Frank, Chem. Eng. Prog., No. 4 (1997) 52-63. [3] L.H. Horsley and R.F. Gould (eds.), Advances in Chemistry 116, American Chemical Society, Washington, 1973. [4] J.R. Phimister and W.D. Seider, Ind. Eng. Chem. Res., 39 (2000) 122-130. [5] S.I. Abu-Eishah and W.L. Luyben, Ind. Eng. Chem. Res., 24 (1985) 132-140. [6] J.R. Knight, Synthesis and Design of Homogeneous Azeotropic Distillation Process, Ph.D. Dissertation, The University of Massachusetts at Amherst, 1991. [7] C. Black, Chem. Eng. Prog., No. 9 (1980) 78-85. [8] J. Gmehling and U. Onken (eds.), Vapor-liquid Equilibrium Data Collection (Aqueous-organic Systems), Dechema, Frankfurt, 1977. [9] A. Hamad and R.F. Dunn, Ind. Eng. Chem. Res., 41 (2002) 6082-6093.