Modelling high pressure extraction processes

EuropeanSymposiumon ComputerAidedProcessEngineering- 10 S. Pierucci(Editor) 9 2000ElsevierScienceB.V.All rightsreserved.

577

Modelling High Pressure Extraction Processes Mojca Skerget, Zeljko Knez* University of Maribor, Faculty of Chemistry and Chemical Engineering, Smetanova 17, SI-2000 Maribor, Slovenia phone: +386/62/22 94 461, fax: +386/62/22 50 13, E-mail: [email protected]

The objective of this work was to study the semibatch flow extraction of oil and other active ingredients from some plant materials (Silybum marianum, pepper, paprika and cocoa) with supercritical fluids such as CO2 and n-propane at different operating conditions and to analyze the dynamic behaviour of the extraction runs by a mathematical model employed by Peker [1] and Goto [2]. 1. INTRODUCTION High pressure extraction process represents an alternative to conventional separation methods (steam distillation, extraction with organic solvents, molecular distillation,...), because of favourable properties of supercritical fluids (SCF) (solvent recovery, simple separation, favourable thermical conditions, solvent free products of high added value, etc.). Nowadays, extended research work on the application of SCF as solvents in the extraction and fractionation processes of essential oils and aromatic components from plant materials is carried out. SCF represent natural alternatives to chloro- and fluorocarbons and other ozonedepleting or smog-causing compounds. Their greatest success to date has been as replacements for chlorinated solvents in coffee decaffeination and spice-extraction processes [3]. For most supercritical applications, CO2 is typically employed. CO2 is ideal for applications in foods, beverages or pharmaceuticals because it is nontoxic, nonflammable, inexpensive and widely available [4]. For the design of high pressure extraction process, beside the knowledge of phase equilibria, the knowledge of mass-transfer rates is essential. The problem, which persists in dimensioning the SCF-processes, is that usually no physico-chemical and transport data of the investigated components are available in the literature and are difficult and timeconsuming task to measure experimentally. Therefore they are usually estimated with different group contribution or empirical methods. Further, in most cases conventional models for modelling phase equilibria and extraction rates in dependence of pressure does not fit the experimental points well because of extreme operating conditions. To solve the problems conventional models have been modified or new models have been developed. However, the problem has not been completely solved yet. *To whom correspondence shouldbe addressed.

578

2. M A T H E M A T I C A L M O D E L The model used by Peker [ 1] and Goto [2] is based on the following assumptions: 1. adsorption - desorption equilibrium of extractable component from solid tissue, 2. the diffusion of extractable component dissolved in supercritical fluid to the surface, 3. mass transfer through the external film into the bulk. The final expression for the commulative fraction of a solute extracted up to dimensionless time | is defined as [ 1,2]:

1~Ix- dO

F(|

i2ao

I

A exp(~-| = 1-a L a,

- 1)

exp(a ! _.O) - 11 a 2

(1)

J

and the final expression for dimensionless solute concentration x = -

C

in effluent is:

Co

x(o> = A[exp(a,-Ol- exp(a,.Ot]

(2)

where c is the concentration of a solute in a solvent, co is the initial concentration of a solute in material and: t 0 =(3) 1;

1(

)

(4)

4:C)

(5)

al = - ~ - b + ~ / b 2 4.C

a 2 =-~(- b - 4b 2

(1-cz).O A = LP r" + '~l - " "lJ> K 1 " Ja " a , -t a 2 )

b=

9 1 0 ( 1 - c~) + - - + ~ [3 + (1- ~).K ot ot

9

c = [13+(1-13).K].c~

(6)

(7)

(8)

F approaches unity at large values of time. a is the void fraction in bed and 13 the porosity of particle. The equilibrium adsorption coefficient K is defined by equation: % =K.ci

(9)

579 where: Cs is sorbed essential oil in the solid particle and ci is the concentration of essential oil in the pore of solid particle. For K<
(10)

where time x is the total bed volume divided by the volumetric flow rate of supercritical fluid, kp is the combined mass-transfer coefficient, given for the sphere by equation: 15.kf/R kp=

5+Bi

(11)

where kf is the external film mass-transfer coefficient and R is the sphere radius. The Biot number Bi is expressed in terms of the effective intraparticle diffusion coefficient De: Bi- kf-R De

(12)

Do =Dm'13 2

(13)

where DAB is the binary diffusion coefficient for essential oil in supercritical fluid. When Bi >> 5, intraparticle diffusion resistance would dominate over the external mass-transfer resistance [ 1].

2.1. Estimation of properties for theoretical analysis. The size of the solid particle was determined with the sieve analysis and the density of solid material was measured with helium pycnometer (multi volume pycnometer 1305, Micrometrics, USA). The bed void fraction ~ was 0.26 and the porosity of the particle [3 was calculated from solid and apparent density:J3 = 1 - p p / P s . The estimation for the initial concentration of extractable substance in the material co was obtained experimentally with the extraction run until all extractable substances were removed. The binary diffusion coefficients DAB were estimated with Takahashi method [5] in consideration of Fuller equation. For liquid propane at 40~ the binary diffusion coefficient was calculated with Wilke - Chang estimation method [5]. The external mass-transfer coefficients kf were calculated with the Wakao and Kaquei correlation [ 1,2,6]. For the calculation the FORTRAN was used and the adsorption equilibrium constant K was calculated with the regression of experimental data. 3. RESULTS Figures 1 and 2 show the comparison of experimental and calculated extraction curves and Table 1 presents average absolute relative deviation (AARD), calculated for each extraction run.

580

12]

1.2

t 40~ - - - 40~ A 40~ 40~

"

1 O

~0.8

9 9

~ 0.6

0.4

o

bar)-exp. bar-calc. bar-exp. bar-calc.

o.8

0.6 = o 0.2

150 150 400 400

bar-exp. ~'

- - 80~ i

0

..................

"~0.4 0.2

475 bar-calc.

!

!

!

10 20 30 kg CO 2 / kg material

0

t

40

i

0

I

I ........

10 20 30 kg C02 / kg material

a)

i

40

b) 65~ 9 100~ )r 100~ - - - 65~ 100~ ...... 100~

1.6 2"1.4 =o 1.2

480 bar-exp. 480bar-exp. 300 ba r-exp. 480 bar-calc. 480 bar-calc, 300 bar-calc.

! J [ [ l 1

;0.8

~ 0.6

~ 0.4 g 0.:2 0

~.~.~.;~.:~..~-" i'" ~ ,r'r--

1

0

i"l

I

f

20 40 60 kg C02 / kg material

80

c) Fig. 1. Kinetics of semicontinuous extraction of a) pepper, b) p a p r i k a [7] and c) c o c o a butter f r o m c o c o a w i t h d e n s e CO2.

I ~= 2 -] / ".~.~

[] A 9 0

i.e

25~ 40~ 60~ 80~

~-~--~-X X,~

=

--FI

_J ....... 2 5 ~

~1.5/I . . . . . 40~ /l - - 60~

~ 0.6

1 d[ - - 7 80~

[] 40~

60O exp

0.4

~0.5

[] 80~ 40~ ....... 60~ 80oC.calc.

8 0.2

0

0

0

20 40 60 kg CO 2 / kg material

a)

80

w

0

5 10 kg propane / kg material

b)

Fig. 2. Kinetics of semicontinuous extraction of seeds of Silybum marianum with a) dense CO2 at 200 bar and b) dense n-propane at 60 bar.

15

581 Table 1 Extraction conditions and estimated parameters. T (~

P (bar)

kf 105

Qv (l/h)

(m/s)

k (s ~)

DAB 109

(m2/s)

De 1011 (m2/s)

K

AARD

11.16 7.87 82.10

12.9 4.1 9.5

(%)

Carbon dioxide: Cocoa D = 0 . 0 1 6 7 m m , [3=0.2, c0=12% 65 100 100

480 480 300

34.02 38.36 46.82

0.750 1.661 3.177

2.165 4.770 9.049

1.32 2.85 5.24

Carbon dioxide: Paprika D = 0.165 mm, 13= 0.1, C0(aromaticcomponents) = 12.35%, 40 40

150 400

36.1 29.5

0.114 0.031

0.030 0.009

5.28 11.4 20.96

C0(coloringcomponents) "-

4.999 1.54

1.85%

4.999 1.54

18.77 11.85

11.9 13.0

39.08

5.70

3.8

73.2 106.4 183.5 249.4

175.49 176.56 375.92 849.52

8.4 8.1 5.9 17.7

110.9 88.0 229.5

16.41 15.87 60.42

2.1 9.9 18.3

Carbon dioxide: Pepper D = 0 . 2 5 m m , [3=0.3, co=6% 80

475

32.6

0.621

0.107

4.342

Carbon dioxide: Silybum mariannum D = 0.9 mm, [3 = 0.635, co = 23% 25 40 60 80

200 200 200 200

0.46 0.42 0.51 0.54

0.356 0.521 1.006 1.433

0.017 0.024 0.045 0.063

1.814 2.639 4.550 6.184

n-propane: Silybum mariannum D = 0.9 mm, [3 = 0.635, co = 23% 40 60 80

60 60 60

0.82 0.67 1.11

0.570 0.447 1.486

0.026 0.021 0.063

2.751 2.183 5.692

100 ~-~ ]yieldcalc - yieldexp ] AARD(%) = --N" i=z yieldexp Average absolute relative deviation (AARD), calculated for the extraction of pepper, paprika and cocoa with dense CO2 is in the range from 3.8% to 13%. In case of CO2 extraction of Silybum marianum, AARD is under 10% (from 2.3% to 8.4%) except at conditions 40~ 100 bar and 80~ 200 bar, where the yield of extraction is relatively low, adsorption constant K is high and AARD is 23.1% and 17.7%, respectively. In case of npropane, AARD is low at 40~ (between 1.8% and 3%) and with the temperature increase it varies between 1.8% and 20.2%. Due to higher errors observed for modelling extraction runs performed with n-propane at 60~ and 80~ it seems that Takahashi method (in consideration of Fuller equation) used for estimation of the binary diffusion coefficients DAB

582 is not adequate for propane gas extraction system. The errors when Wilke-Chang equation was used at 40~ are much lower. It can be concluded that the model approximates the experimental data well when CO2 is used for the extraction and when operating parameters are chosen so that the adsorption equilibrium constant is not too high and a desorption of the solute from the solid tissue is enabled. Errors could be the consequence of a fact that not only active ingredients were extracted in the process, but also some other components such as waxes, fats .... Therefore, the estimated initial concentrations presented in Table 1 are larger than the concentrations found in the literature. Table 1 presents the estimated mass transfer parameters. The adsorption equilibrium constant K changes with temperature and pressure. Generally, at constant pressure K decreases with the increase of temperature and at constant temperature K decreases with the increase of pressure. An exception can be observed for the extraction of Silybum marianum with CO2 at constant pressure 200 bar, where K increases with the increase of temperature. The values of K are generally lower when propane is used as a solvent for the extraction of

Silybum marianum. Binary diffusion coefficients in mixtures of SC gas and low volatile component calculated are in the range from 0.1 x 10-9 to 7.1 x 10-9 m 2/ s and combined mass-transfer coefficients vary from 0.001 to 0.083 s -1 for paprika, pepper and Silybum marianum extractions and are higher in the case of cocoa butter extraction from cocoa, where it varies from 2.2 to 9.0 s~.

REFERENCES H. Peker, M. P. Srinivasan, J.M. Smith and B. J. McCoy, AIChE J., 38,5(1992), 761-770. M. Goto, M. Sato and T. Hirose, J.Chem.Eng.Japan, 26,4(1993), 401-407. C. Chin, C. Crabb, G. Ondrey and T. Kamiya, Chem. Eng., October 1998, 32-41. 2;. Knez and A. Ris Some Novel Applications of Supercritical Fluids in Food Processing, Engineering & Food, Sheffield Academic Press, Part 2, (1997) pp I/5-I/8.; at ICEF 7, Sheffield, UK 5. R.C. Reid, J. M. Prausnitz and B. E. Poling, The Properties of Gases and Liquids. Fourth Edition, McGraw-Hill Inc., New York 1987, p.587. 6. G. Brunner, Ber.Bunsenges. Phys. Chem. , 88(1984), 887-891. 7. M. Skerget, 2;. Knez, Z. Novak and D. Bauman, Acta Alimentaria, 27,2(1998), 149-160.

1. 2. 3. 4.