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December 2012, Volume 3, No.6Inte rnational Journal of Chemical and Environmental Engineering

Thermodynamic analysis of vapor- liquid

equilibrium for (water-acrylic acid) and (aceticacid-acrylic acid) systems at moderate pressure

A. Attiaa.b

, F. Muteleta*

, J.C Moise a

. R. Solimandoa*

, Med. R Jedayb

aUniversité de Lorraine, Laboratoire de Réactions et Génie des Procédés (UPR CNRS 3349),

1 rue Grandville, BP 20451 54001 Nancy, France.b

University of Gabes, National Engineering school, Research unit of Energetic and Environment UR/99, ,Street of’Omar El Khattab, 6029, Gabes, TUNISIA. a Fabrice MUTELET

*Corresponding author E-mail :[email protected]

Abstract: In this paper, experimental vapor-liquid equilibria data for two binary systems {water + acrylic acid} and {acetic acid + acrylic acid}

are determined at low pressure by ebulliometer apparatus. Our experimental data are also in good agreement with those published in

the literature. The vapor-liquid equilibria of binary systems are represented using NRTL-HOC and UNIQUAC-HOC models. A goodquantitative agreement was obtained with both models. It was found that the average deviation from the NRTL model is slightly

greater than those from the UNIQUAC model. Selected thermodynamic models can now represent with a good accuracy the relative

volatility.

Keywords: carboxylic acids, vapor-liquid equilibria, acrylic acid, NRTL-HOC, UNIQUAC-HOC, relative volatility.

1. Introduction

Acrylic acid and its esters are among the most versatilemono mers for providing performance properties to a wide

variety of polymers. An important part of acrylic acid isusual in the production of acrylate esters which include

methyl, ethyl, n-butyl, and 2-ethylhexyl acrylate, else is

purified into glacial acrylic acid and subsequently utilizedfor the production of poly-acrylic acid or copolymers

which find applications in super absorbents, detergent co-

builders, dispersants, flocculants , and thickeners. Acrylate

esters impart many desirable qualities to polymeric

materials, such as color stability and clarity when exposed

to light, heat and aging resistance, good weather ability,low temperature flexibility, and base resistance [1-3].

A current process for producing acrylic acid has been

practiced by catalytic oxidation of propylene or acrolein

in gaseous phase. Purification steps consist of collect,separate and recover of acrylic acid from impurities like

as acetic acid, water.

For the development of actual separation processes of

acrylic acid, there is a need for insight into the

fundamental phenomena of non ideal phase equilibria.

The determination of an adequate thermodynamic model

for wide range of temperature and pressure is necessary

for modeling and optimizing an industrial purification.

For these reasons, the evaluation and accuracy of the

experimental data reported in literature for different

systems is absolutely necessary. Interesting binary

systems such as {water-acrylic acid} and {acetic acid-

acrylic acid} are s tudied in this work. These s ystems were

limited reported in the literature. For e xample, the {water-acrylic acid} system has been studied only by Olson et

al.[4-8]. The {acetic acid-acrylic acid} has been

investigated by Chubarov et al [5], Linek et al. [9] and by

Trybula et al. [10]. Unfortunately, acids form particular

complexes that are due to hydrogen bonding interactions

[11-12]. Dimerization of carboxylic acid is as a result of

these specific chemical forces acting between molecules.

This paper evaluates the reliability of experimental points

in a large range of the interval of pressure and

temperature. Thermodynamic behavior for carboxylic

acids is characterized by complexity and non ideality of

vapor phase that cannot be neglected at low pressure [11-

13]. Hayden O’Connell [12] correlation was used tocompute fugacity coefficients of vapor phase. All

equilibrium data were correlated with NRTL [14] and

UNIQUAC [15] activity coefficient models which were

introduced to obtain adequate parameters in order to

represent the relative volatilities. All binary interaction

parameters of these models were taken from Aspen plus

commercial software.

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The rmodynamic analysis of vapor- liquid equilibrium for {water-acrylic acid} and {acetic acid- acrylic acid} systems at modera te pressure

387

2. Materials and Method

2.1. Materi als

In this work, all compounds were purchased from

commercial sources and used without any further

purification. Acry lic acid stabilized with 200 pp m 4-

methoxyphenol (CAS 79-10-7) and acetic acid (CAS 64-

19-7) were from Sigma Aldrich. The mole fraction

purities stated by the manufacturers were greater than0.99 and 0.998 for acrylic acid and acid acetic

respectively. Water was purified by a M illi-Q system with

resistivity lesser than 18Ω.cm.

2.2.

Apparatus and procedure

Ebulliometer is designed to accurately measure isobaric

vapor-liquid equilibria of pure component and mixtures

[16-23]. This apparatus (EEA/3000) is based on dynamic

recirculation of liquid and vapor phase allowing it to

reach thermodynamic equilibria. It is operated in a batch

mode at atmospheric and reduced pressure up 0.13 kPa. Adetailed diagram of this experimental system is presented

in figure 1.

The stainless steel/glass and PTFE design of the unitallows to study of a large range of products. The pressure

is fixed and constant by using a vacuum pump. The

accuracy of the pressure calibrator was ±0.01kPa.

Firstly, a mixture is introduced through loaded 150 cm3

feed flask (1) and passed into a buffering reserve equiped

by magnetic stirrer (5). Then, mixture pas sed directly into

a boiler (2) and is heated by mean of an electrical

resistance generating evaporation calibrated 0 to 100%

with maximum power 500 W. Adiabatic double envelope

equilibrium chambers (3) ensure a thermodynamic

equilibrium and provide a contact between two phases.

The temperature is measured using temperature probe

which is placed on the equilibrium chamber. Theuncertainty in the temperature measurement is estimated

to ± 0.01 K. when thermodynamic equilibria is reached.

Vapor phase is condensed in condenser (8) and the

dropped in the buffering reserve, while a liquid phase is

recycled directly in the buffering cell. Vapor phase

sampling and liquid phase sampling are taken from

sampling ports (6) and (7) and are analyzed by gas

chromatography. A vacuum pump (9), assures

measurement of VLE at low pressure. A trap of dry ice

(4) can also be used to protect the pump of any contact

with the fluid‘s s tudied.

Figure1.Experimental apparatus of VLE measurement

The gas chromatography (Agilent technology) was

equipped with a FID detector and the column used was

capillary column (10 m 0.53 mm 0.7 mm, Agilent)

packed with polyethyleneglycol. The carrier gas was

helium flowing at 20 mL3/min, and the operating

conditions were as follows: injector temperature, 543.15

K, detector temperature, 573.15 K. The initial temperature

of the oven was fired at 333.15 K during 4 min andgradient of 10K/min was impos ed up to 453.15K.

3. Results and Discussions

To test the performance of the equilibrium apparatus, we

measured vapor pressure of pure compound such as

ethanol. Table 1 shows that measured values and the

literature data [26] are in good agreement. In general an

average s tarted deviation lower than 0.23%.

Isobaric vapor liquid equilibria data of {water

(1) + acrylic acid (2)} and {acetic acid (1) + acrylic acid

(2)} measured in this work but also data taken for the

literature are presented in figures 2 and 3.Acetic acid and water vapor pressure data were found in

the DIPPR databank are listed in table 2. Acrylic acid

parameters are obtained by fitting experimental data [9,

25-28].

Table 1 . Vapor press ure of Ethanol

Table 2 . Parameter of the extended Antoin e equ ation ( l nP/kPa=

A+B/T+ClnT+DTE)

The equation that describes thermodynamic equilibrium between phases at temperature T and pressure P is:

( )exp( )

L S S S i i

i i i i i i

V P P Py P x

RT

(1)

Where i is the fugacity coefficient of component i in the

vapor phase, yi and xi are the molar fractions in the vapor

and liquid phases respectively, γi is the activity coefficient

of i with respect to the reference fugacity, is is the

fugacity coefficient of the pure saturated vapor of

component i, Psi is the saturated vapor pressure of

P/PaMeasuredT/K [26]

calculatedT/K Errors %

3199.70 283.15 283.75 0.21

5946.20 293.15 293.29 0.05

7972.70 298.15 298.11 0.01

10600.00 303.15 303.03 0.04

18000.00 313.15 312.91 0.0729500.00 323.15 322.91 0.07

46900.00 333.15 332.77 0.11

72300.00 343.15 342.36 0.23

Compound A B C D E

Acetic Acid 46.3622 -6304.5 -4.2985 8.88*10-18

2

Acrylic acid 40.7143 -6738.33 -3.22080 5.22*10-7

2

Water 66.7412 -7258 .20 -7.3037 4.16*10-6

2




388

component i at temperature T, and Vil is the mo lar

volume of component i is equal to the partial molar

volume of component i at these conditions. At low

pressure, the term ( )exp( )

L S

i iV P P

RT

is approximately

equal to unit thus equation (1) can be rewritten as :

ii

S

i

S

iii x P Py (2)

The vapor phase fugacity coefficients of acetic acid and

acrylic acid were obtained by using the virial equation ofstate truncated after the second term. The second virial

coefficients were determined by the Hayden and

O’Connell corre lation. The phys ical properties [29-30] of

the pure components required in calculation are presented

in table 3.

Table 3. Properties of pure compone nts use d in calcu lati ng th e

second virial coefficie nt

T c critical temperature, P c critical pressure. V c critical volume, wacentric factor, RD gyration radius, DM dipole moment.

Figu re 2. T – x (1)-y(1) diagrams of {water(1)-acryli c acid(2)} at

diffe rent pressures: (A) 101.325 kPa , (B) 50 kPa, (C) 6 .67 kPa,experimental values, ■[4], ▲ [5] and --- UNIQUAC.

Figu re 3. T – x (1) -y(1) diagrams of {acetic acid(1)-acrylic acid(2)} at

diffe rent pressures: (A) 101.325 kPa , 26.7 kPa, experi me nta lvalues, ■[9], ♦ [5] and --- UNIQUAC.

Activity coefficients data investigated are computed using

the NonRandom Two-Liquid equation (NTRL) proposed

by Renon and Prausnitz [14] and the UNIversal QUAs i-

Chemical (UNIQUAC) theory developed by Abrams and

Prausnitz [15].

For the NRTL Model, the activity coefficient γ i, for any

component i of the binary sys tem is given by:

)(ln

1

1

1

11

1

m

l

l li

m

r r rjrj

ji

m

jm

l

l ij

j ji

m

l

l li

m

j j ji ji

i

x

xG

xG

xG

x

xG

(3)

With )exp( ji ji jiG ,T

ba

ji

ji ji and

ij ji

Where g is an energy parameter characterizing the

interaction of species i and j,i

x is the mole fraction of

component i. the nonrandomness parameter. Although

can be treated as an adjustable parameter, in this study

was set equal to 0.3 according the literature [14].

For the UNIQUAC Model, the activity coefficient γ i, forany component i of the binary system is given by:

m

jm

k

kjk

ji j

ii jiiii

i

i

i

i

i qqql q z

x 1

1

lnln2

lnln

(4)

Where

m

j j j

ii

i

xr

xr

1

,

m

j j j

ii

i

xq

xq

1

)1()(2

j j j j r qr z

l andT

ba

ji

ji ji

Here, the lattice coordination number z is assumed to beequal to 10. r i and q i are respectively a relative volume

and surface area of the pure component i. Parameters r i

and qi are respectively relative to molecular Van derWaals volumes and molecular surface areas. They a re

calculated as the sum o f the group volume and group area

parameters R k and Qk :i

i k k

k

r R andi

i k k

k

q Q (5)

Where i

k the number of is groups of type k in molecule i.

The group parameters R k and Qk are obtained from van

der Waals group volumes and surface areas. Vk and Ak

taken from the UNIFAC group contributions [31]:

15.17

k

k

V R and

92.5 10

k k

AQ

(6)

The values of 15.17 and 2.5109 are respectively the

standard segment volume and standard segment area of a

methylene group [31].

The maximum Likelihood method and Britt-Luecke

algorithm were used [32-33]. The binary VLE problem

the maximum likelihood objective function is:

Acetic Acid Acrylic Acid Water

T c /K 591.95 615 647 .096

P c /kPa 5786 5660 22064

V c /m

3

.Kmol

-1

0.1797 0.208 0.05595w 0.2640 0.4665 0 .345

RD .10-10

/m 2.61 2.978 61.5

DM .10-30

/C.m 5.8 4.67 6.17




389

))()(( 2

,

,,

2

1 ,

,,

im

iest im N

i im

iest im

obj y

y y

T

T T F

(7)

Where N is the number of experimental and calculated

mole fractions of one phase and y designed a vapour mole

fraction estimated and measured.

From table 4to table 7, we present T- data for different

systems at different pressures.

Table 4. T- data for {water (1) -acrylic acid (2)} at 50 k Pa

Table 5. T- data for {water (1)-acryli c acid(2)} at 101, 325 kPa

Table 6. T- data for {acetic acid (1)-acrylic acid(2)} at 26.67 KPa

Table 7. T- data for {acetic acid (1)-acrylic acid(2)} at 100.44 kPa

The binary interaction parameters of NRTL and

UNIQUAC models were obtained using ASPEN PLUS

V7.2. The estimated NRTL and UNIQUAC binaryinteraction parameters for the systems studied here are

presented in table 8.

Table 8. Correlation Parameters for Activity Coefficients

All data show c that no extremum and hence no azeotropeexists for {water + acrylic acid} at 101.325 kPa [4].

The use of Hayden O’Connell second virial coefficient to

calculate vapor phase fugacity coefficients and

compositions is essential for systems containing organic

acids.

All the activity coefficient models provide a similar

correlation of experimental data. The value of average

means of vapor molar fraction Δy <0.033, temperature (ΔT/T)

<0.3% and the average absolute deviation of activity

coefficients Δγ1<0.09 and Δγ2<0.26 are listed in table 9.

T/K

354.29 1.01 -

354.36 1.04 3.66

354.595 1.18 2.21

356.10 1.29 1.76

358.35 1.25 1.58

359.50 1.23 1.55

361.32 1.36 1.37

362.75 1.32 1.36

366.53 1.39 1.23

381.22 1.45 1.07394.20 - 0.96

T/K

372.50 1.01 -

372.70 1.02 4.91

372.77 1.01 5.62

372.87 1.01 4.94

372.97 1.03 3.91

373.07 1.05 3.34

373.21 1.05 3.15

373.30 1.06 2.86

373.48 1.08 2.57

373.89 1.12 2.21

374.84 1.17 1.89

375.74 1.23 1.63

377.45 1.35 1.35

380.05 1.35 1.27

385.80 1.57 1.10

383.03 1.51 1.13

391.45 1.76 1.03

402.00 2.17 0.97

414.40 - 1.00

T/K

352.31 1.00 -

354.96 1.03 2.72

359.03 1.11 1.79

359.51 1.13 1.72

361.00 1.16 1.59

362.94 1.21 1.46

365.63 1.32 1.29

367.47 1.43 1.21

371.36 1.79 1.08

375.52 - 1.00

T/K 1 2

390.75 1.01 -

393.65 1.05 2.09

396.18 1.11 1.68

399.31 1.14 1.53

400.26 1.14 1.50

406.14 1.3684 1.17

407.08 1.4066 1.15

410.26 1.6985 1.05

414.00 - 1.01

aij a ji bij b ji

Water (1) +acrylic acid(2)

UNIQUAC -0.8068 2.0793 327.5566 -913.8214

NRTL 2.7911 -2.1478 -275.1986 665.3429

Acetic acid(1) + acrylic acid(2)

UNIQUAC -0.747 8 1.6879 310.8726 -864.3531 NRTL 1.7676 -3.4119 -575.837 1606.8865




390

All activity coefficients obtained are greater than 1. It

designed that mixtures have positive deviation (γ 1>1)

from Raoult’s law; the co mpounds dislike each other. The

attraction between identical molecules (A-A and B-B) is

stranger than between different molecules (A-B) this may

cause the formation of minimum-boiling azeotrope and

heterogeneity.

The VLE properties of binary systems studied in this

work a well represented with both models. Themeasurement of the degree of enrichment, or the ease of

separation, is the relative volatility between pair of

components.

Table 9. Averages deviations

Figure 4 and 5 illustrate evolution of relative volatilities.

Then, the tendency of acrylic acid to vaporize is greater

compared to water and acid acetic. In fact, volatility is

directly related to a substance's vapor pressure. At lower

vapor pressure, volatility increases slightly.

Figure 4. Relative volatility – x (1) diagrams o f {water (1)-acryl icacid(2)} at different pressures: (A) 101.325 kPa . (B) 50 kPa. (C)

6.67 kPa. experimental values. ■[7]. ▲ [5] and --- UNIQUAC.

Figu re 5. Relative vola til ity – x (1) diagrams of {acetic acid(1)-acrylic acid(2)} at different pressures : (A) 101.325 kPa . (B) 26.7

kPa . (C) 6.67 kPa, experimental values♦ [5],■[9],▲ [10] and ---

UNIQUAC.

4.

Conclusions

In this paper, measurement of the isobaric vapor – liquid

equilibrium of two interesting indus trial systems {water +

acrylic acid} and {acetic acid-acrylic acid} were carried

out. The NRTL and UNIQUAC models accurately

correlate the experimental data. A good quantitative

agreement was obtained with these models and the

average deviation from the NRTL model greater than

those from the UNIQUAC model.

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http://en.wikipedia.org/wiki/Vaporize

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http://en.wikipedia.org/wiki/Vaporize




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