480 Ind. Eng. Chem. Fundam. 1904, 23, 400-404

d3 = ((do - x,)~[(Yc~ + 2Ddid2 + C(~3do + ~ 2 4 + ciddl -

(do - ~,)~[2czdz + c@il + (do - xw)[(2cidz + c2dJ(di - ci) + (4 - ~Jc id i l - cidi(di - cJ21/

((do - X,)2[3C1 - (do - x,)(E +2Ddo + C X ~ ) ] ) (A21)

Thus the coefficients cl, c2, c3, do, dl, d2, and d3 are known in terms of the quantities x,(= co), a, and y. Representations A13 and A13a for x and y at any location in the countercurrent permeator may now be used to represent conditions a t the feed end

Xf = X , + CiRfl + c2Re2 + c3Re3 (A22)

yf = do + dlRfl + d2Re2 + d3Rf13 (A22a)

Nomenclature A = constant defined by eq A6 a,,, al, a2, a3 = coefficients in expansions 14 and A1 defined

B = constant defined by eq A6a bo, bl, b2, b3 = coefficients in expansions 14a and A2 defined

C = constant defined by eq A6b co, cl, c2, c3 = coefficients in expansions 15 and A13 defined

D = constant defined by eq A6c do, dl, d2, d3 = coefficients in expansions 15a and A13a defined

E = constant defined by eq A6d L = molar flow rate of high pressure gas at a given permeator

P = pressure of high-pressure gas p = pressure of low-pressure gas

by eq A3, A4, AS, and A10, respectively

by eq A5, AI, A9 and All , respectively

by eq A14, A15, A18 and A20, respectively

by eq A16, A17, A19, and A21, respectively

location

Q1, Qz = permeability coefficient of more permeable species 1 and the less permeable species 2, respectively

Rf = nondimensional membrane area for cocurrent permeator defined by eq 3

R, = nondimensional membrane area for countercurrent permeator defined by eq 9

S, ST = membrane area at a given permeator location and the total membrane area, respectively

V = molar flow of low pressure gas at a given permeator location

x , xf, x , = mole fraction of more permeable species 1 in high-pressure gas at any permeator location, feed entry, and residue exit locations, respectively

y , yf, yw = mole fraction of more permeable species 1 in low- pressure permeate gas at any permeator location, feed entry, and residue exit locations, respectively

Greek Letters (Y = ideal separation factor, defined by eq 4 y = pressure ratio, defined by eq 4 6 = thickness of nonporous membrane 0 = permeator cut fraction defined by eq 13b or 13c Subscripts f = pertaining to the permeator axial location where high

T = total value, used for membrane area w = pertaining to the permeator axial location where high

Literature Cited Antonson, C. R.; Gardner, R. J.; King, C. F.; KO, D. Y. Ind. Eng. Chem.

Process Des. Dev. 1977, 16, 473. Broad, R.; M.Eng. (Chemical) Thesis, Stevens Institute of Technology, Hobo-

ken, NJ, 1981. Hwang, S. T.; Kammermeyer, K. "Membranes in Separations"; Wiley: New

York, 1975. Maciean, D. L.; Graham, T. E. Chem. Eng. Feb 1980, 25, 54. Matson, S. L.; Lopez, J.; Quinn, J. A. Chem. Eng. Scl. 1983, 38(4), 503. Pan, C. Y.; Habgood, H. W. Ind . Eng. Chem. Fundam. 1974, 13, 323. Pan, C . Y.; Habgood, H. W. Can. J. Chem. Eng. 1978, 56, 210. Stern, S. A. In "Membrane Separation Processes", Meares, P.. Ed.; Eisevier:

Stern, S. A.; Walawender, W. P. Sep. Scl. 1989, 4, 129. Thorman, J. M.; Rhim, H.; Hwang, S. T. Chem. Eng. Sci. 1975, 3 0 , 751. Waiawender, W. P.; Stern, S. A. Sep. Sci. 1972, 7(5) , 553.

Received for review July 11, 1983 Revised manuscript received February 13, 1984

Accepted March 20, 1984

pressure feed enters

pressure feed exits

New York, 1976; Chapter 8.

Liquid Extraction of Furfural from Aqueous Solution

John R. Croker' and Ron 0. Bowrey School of Food Technology, University of New South Wales, Kensington, N.S. W., 2033, Australia

Three solvents were evaluated for the liquid-liquid extraction of furfural from aqueous solution. Complete ternary diagrams were prepared for each of the systems water-furfural-methyi isobutyl ketone, water-furfural-toluene, and water-furfurfal-isobutyl acetate by use of data obtained at 30 ' C . The data were analyzed to give equations for the equilibrium lines for each system and the conslstency of the tie-line data was confirmed using an Oth- mer-Toblas plot. The data indicate that toluene is the most effective solvent of those tried for the removal of furfural from aqueous solution, although isobutyl acetate may be preferred because of its low toxicity.

Introduction When furfural is formed at high temperature and

pressure (180-200 OC, 8-10 atm) and separated from aqueous solution by azeotropic distillation, as occurs in

* CSR Limited, Sugar Division, 55 Clarence St., Sydney, NSW 2000, Australia.

present furfural proce3ses, considerable product loss occurs due to thermal degradation of the furfural. Commercial processes operate with a yield of 30-35% of theoretical furfural (Jaeggle, 1975). As an alternative, a process has been suggested by the authors in which furfural is made at high temperature and pressure in a plug flow reactor with fast cooling of the contents a t the point of maximum

$3 1904 American Chemical Society 0196-4313/04/ 1023-0400$01.50/0

Ind. Eng. Chem. Fundam., Vol. 23, No. 4, 1984 481

furfural yield (Croker, 1983). Widiams and Dunlop (1948) give an expression for the rate constant of the first-order decomposition of furfural in acidic aqueous media of

(1) where k is the reaction rate constant, mi&, N is normality of acid in the solution, and T i s absolute temperature, K. At 180 "C in 0.10 N acid solution this rate constant is 0.0032 min-'. A cooling of the reaction mixture from 180 to 30 "C will reduce the rate of reaction to a negligible value. At this temperature it is proposed to remove the furfural from solution by liquid extraction and to separate the resulting furfural/solvent solution by vacuum distil- lation.

Liquid extraction of furfural from aqueous solutions was first evaluated by using ethyl acetate (nimble and Dunlop, 1940). This was proposed as a steam-saving alternative to azeotropic distillation. While giving a satisfactory separation, the high solubility of ethyl acetate in water (7.7% by mass at 30 "C) means that a plant using this process requires a comprehensive ester recovery unit.

Toluene and methyl isobutyl ketone (MIBK) have been investigated for this separation. Knight (1943) determined the distribution of furfural between toluene and water; however, his work did not include complete ternary equilibrium data. Knight evaluated the performance of a packed column extractor with this liquid system. Con- way and Philip (1953) determined complete equilibrium data for the system water-furfural-methyl isobutyl ketone at 25 "C. A pilot plant using MIBK was operated suc- cessfully by Wilson (1970) to remove furfural from dilute aqueous solution where it had been formed by the hy- drolysis of bagasse.

The authors have investigated three solvents for furfural extraction, each at 30 "C. This temperature was chosen as being more applicable to processing conditions than 20 or 25 "C due to the wide availability of cooling water a t temperatures greater than 20 "C, in the tropical areas where bagasse is available. Complete equilibrium diagrams have been determined for the ternary systems water- furfural-MIBK, water-furfural-toluene, and water- furfural-isobutyl acetate (IBA), using a modification of the method of Othmer, White and Trueger (1941).

MIBK was evaluated in order to test the method by comparison with the work of Conway and Philip which had been performed at 25 "C. Conway and Philip established the equilibrium curve by taking a mixture of two compo- nents, made by mixing known volumes of each component, and titrating this solution with the third component until the cloud (equilibrium) point was reached. Measurement of the volume of the third component together with the density of each component enabled the mass fraction of each component to be calculated. The variation in furfural concentration with solution density was determined for the raffinate and extract phases, and these data were used to establish tie-line terminal point positions. There is a possibility for inaccuracy in such a determination on the raffinate (water) side of the diagram due to the small variation in density with furfural concentration. This may account for the raffinate ends of the tie-lines of this system being located so close to each other as seen in Figure 1, which was prepared from the data of Conway and Philip (1953).

Experimental Method The procedure of Conway and Philips was modified by

weighing each component when it was added to the mix- ture. I t was believed that changes in density relative to furfural concentration are not as sensitive as refractive

k = N x 10(-4365/T+8.145)

100 Furfural

A

Water Mass p e r cent M I B K

Figure 1. Equilibrium diagram for water-furfural-methyl isobutyl ketone at 25 "C. Data of Conway and Philip (1953).

Table I. The System Water-Furfural-MIBK, 30 "C equilibrium data, mass %

water furfural MIBK

1.9 1.88 3.1 3.6 3.6 4.2 4.5 3.8 4.8 5.6 4.7 6.4 5.7

98.1 96.49 95.8 95.6 95.5 95.66 95.02 93.21 92.4 91.3

Extract Line 0.0 0.42

12.4 28.1 39.7 47.1 57.2 57.3 65.0 77.1 79.9 86.2 94.3

Raffinate Line 0.0 1.9 2.1 2.9 3.0 3.1 3.9 6.1 7.4 8.7

98.1 97.7 84.5 65.3 56,7 48.1 38.3 38.9 30.2 11.3 15.4

7.4 0.0

1.9 1.61 1.5 1.5 1.5 1.24 1.08 0.63 0.2 0.0

index variations. This work used refractive index changes along the equilibrium curves to fix the tie-line terminal points. The variation of refractive index with furfural concentration along the equilibrium line followed a straight line relationship in each system studied.

Materials Used. All chemicals (except furfural) used were 99% pure or better. Distilled water was used in all determinations. The furfural, which was dark brown when delivered due to autoxidation (Khol'kin, 1960), was dis- tilled under vacuum to yield a clear liquid, slightly greenish-yellow.

Results Experimentally determined equilibrium data for the

system water-furfural-MIBK at 30 "C are given in Table I. Data for the distribution of furfural between water and MIBK are given in Table 11. The equilibrium diagram for the system Watel-furfural-MIBK at 30 "C, determined by the method mentioned, is shown in Figure 2. The tie-lines were more spread out than those found by Conway and Philip, while the equilibrium curves had moved in slightly, indicating greater mutual solubility at the higher temperature. Note that systems of this type, with two

482

Table 11.

Ind. Eng. Chem. Fundam., Vol. 23, No. 4, 1984

Furfural Concentrations in Equilibrium Phases

* c . 0 -

n \ n

l

"

1.0 , 1

mass % furfural i /

water phase MIBK phase 0.64 13.5 2.25 27.6 4.75 53.1 5.39 68.4 6.93 76.7

M I BK

Figure 2. Equilibrium diagram for water-furfural-methyl isobutyl ketone at 30 "C. Data of Croker (1983).

pairs of nonconsolute liquids, have no plait point. It was observed that each of the equilibrium lines could

be correlated by a linear regression between solvent con- centration ( x axis) and furfural concentration (y axis). This occurred due to the low mutual solubilities of two of the liquid pairs and the consequent nearness of the equilibrium points to the boundaries of the system. From the data given in Table I, the extract (solvent rich) line in this system has the following equation where 1.2 is the regression coefficient for the correlation.

% furfural = 94.04 - 0.9606.(% MIBK); r2 = 0.9998 (2)

The raffinate (water rich) line has the equation

% furfural = 8.56 - 0.97454% MIBK); r2 = 0.9745 (3)

Tie lines were determined by separation of a mixture in the two-phase region into its two component phases and analyzing each phase with refractive index data. It can be demonstrated by a material balance that the initial mixture and the resultant separated phases are collinear (inverse lever arm rule). Consequently, the composition data of each of the tie-line terminal points were regressed along with the composition data of the relevant mixture point to check that all lay on the same straight line. Re- gression coefficients obtained here were of the order of 0.98 or better.

Tie-line data were plotted according to the method of Othmer and Tobias (1942). This method gives a straight line plot of the conjugate curve by plotting the relationship

log ((1 - b ) / b ) = m log ((1 - a ) / a ) + n (4)

where b is mass fraction of solvent in the solvent phase, a is mass fraction of water in the water phase, and m and n are constants. An Othmer-Tobias plot for this system is presented in Figure 3.

The system water-furfural-toluene was reevaluated because the literature gave distribution data only (Knight, 1943). These data are insufficient for design purposes.

! / I v ! I

0. 5 f ~ Y'=3.739 ~ SLOPE=2.858 1 R'2=0.9733

/ ! /

O / /

/

/ / O

0 0 -0.5 ,..

-1.0 -2. 0 -1.8 -1.6 -1. 4 -1. 2

log ( 1 - a ) / a

Figure 3. Other-Tobias plot of tie-line data for water-furfural- MIBK at 30 O C .

Table 111. The System Water-Furfural-Toluene, 30 "C

equilibrium data, mass %

water furfural toluene

0.1 1.5 0.7 1.2 0.9 0.9 2.2 1.6 2.0 2.2 3.0 4.5 4.1 5.7

97.96 97.94 97.17 94.46 91.8 91.5 91.3 91.3

0.0 12.7 17.4 25.1 29.4 40.1 55.9 56.6 62.6 61.8 78.6 80.1 87.0 94.3

Raffinate Line 1.8 1.8 2.6 5.4 8.2 8.5 8.7 8.7

99.9 85.8 81.9 73.7 69.7 59.0 41.9 41.8 35.4 30.0 18.4 15.4

8 .9 0.0

0.24 0.26 0.23 0.14 0.0 0.0 0.0 0.0

Table IV. Furfural Concentrations in Eauilibrium Phases mass % furfural

water phase toluene phase

1.97 12.29 3.42 34.7 4.71 55.1 6.04 69.1 7.11 78.0

Experimentally determined equilibrium data are given in Table 111. Table IV lists data for the distribution of furfural between water and toluene at 30 "C. These data were treated like those of the system water-furfural- MIBK. A regression analysis of the equilibrium lines re- sulted in the equations

% furfural = 95.65 - 0.95414% toluene); r2 = 0.9994 (5)

for the extract line and % furfural = 8.578 - 26.35.(% toluene); r2 = 0.9909 (6)

Ind. Eng. Chem. Fundam., Vol. 23, No. 4, 1984 483

Table V. The System Water-Furfural-Isobutyl Acetate, 30 "C

/

P/ / , , , , , ,

100 Fu r fu r01 m

WotPr Moss per cent Toluene

Figure 4. Equilibrium diagram for water-furfural-toluene at 30 OC.

Y ' =3. 705 SLOPE=2. 768

R'Z=O. 9998

/

P/ /

/

d /

/ /

/ d

/ /

/ /

/

equilibrium data, mass 7%

water furfural IBA 2.1 1.8 2.6 2.6 2.1 2.4 3.0 2.8 3.5 3.6 3.6 3.9 4.0 3.9 4.4 4.5 4.8 5.1 5.4

99.1 98.1 97.7 97.3 97.24 97.3 96.48 96.54 96.17 96.1 95.4 95.09 94.44 94.14 93.63 91.5 91.5 91.3 91.3

0.0 0.0

11.0 13.5 15.5 22.4 24.7 27.9 43.8 44.8 46.5 53.8 61.8 63.4 63.8 79.7 80.1 80.3 87.4

Raffinate Line 0.0 1.2 1.6 1.7 2.0 2.0 2.7 2.8 3.3 3.5 4.2 4.5 5.3 5.5 6.1 8.5 8.5 8.7 8.7

97.9 98.2 86.4 83.9 82.4 75.2 72.3 69.3 52.7 51.6 49.9 42.3 34.2 32.7 31.8 15.8 15.1 14.6

7.2

0.9 0.7 0.7 1.0 0.76 0.7 0.82 0.66 0.53 0.4 0.4 0.41 0.26 0.36 0.27 0.0 0.0 0.0 0.0

Table VI. Furfural Concentrations in Eauilibrium Phases mass % furfural

water phase IBA phase 1.6 12.6 2.4 18.1 2.5 21.4 4.3 35.8 4.9 40.0 6.3 61.4 7.7 73.9 7.8 78.4

100, F u r f u r a l

A\

ocetate

Figure 6. Equilibrium diagram for water-furfural-isobutyl acetate at 30 "C.

404 Ind. Eng. Chem. Fundam. 1984, 23, 484-489

Y'=3.319 1 SLOPEc2. 601 / . R"2=0.9533 ,4 /

/ /

I J.0- /

- P I /

o / / i

, / i / , , , , , ,

- i . d I -2.3 -1 .8 -1.6 - 1 . 4 -1 .2

Figure 7. Othmer-Tobias plot of tie-line data for water-furfural- isobutyl acetate a t 30 O C .

more toxic than either of the other two solvents studied (Sax, 1979); this may influence the final choice of solvent for such an extraction process.

In ternary liquid systems of low mutual solubility be- tween two pairs of liquids, as in the systems studied, it was

iog ( 1 - a ) / o

found that the equilibrium relationship could be satis- factorily described by a straight line plot of the compo- sition data. The Othmer-Tobias method of describing the conjugate curve was satisfactory for correlation of tie-line data in the systems studied.

Determination of equilibrium concentrations by re- fractive index measurements was satisfactory for the systems studied.

Registry No. Furfural, 98-01-1; toluene, 108-88-3; isobutyl acetate, 110-19-0; m e t h y l isobutyl ketone, 108-10-1.

Literature Cited Conway, J. 6.; Philip, J. 6. Ind. Eng. Chem. 1953, 45(5), 1083-5. Croker, J. R. MSc. Thesis, University of New South Wales, Kensington, Aus-

Jaeggie, W. Escber Wyss News 1975, (2). 38-51. Khol'kin, Yu. I . Zh. Priki. Khim. 1960, 33(4), 914-9. Knight, 0. S. Trans AIChE 1943, 39, 439-56. Othmer, D. F.; Tobias, P. E. Ind . Eng. Chem. 1942, 34(6), 693-6. Othmer, D. F.; White, R . E.; Trueger, E. Ind. Eng. Chem. 1941, 33(10),

Sax, J. I. "Dangerous Propertles of Industrial Materials", 5th ad.; Van Nost-

Trimble, F.; Dunlop, A. P. Ind. Eng. Chem., Anal. Ed. 1940, 72(12), 721-2. William, D. L.; Dunlop, A. P. Ind. Eng. Chem. 1948, 40(2), 239-41. Wilson, B. W. "Furfural Production from Bagasse", Proceedings Seminar on

Utilisation of Bagasse, Mackay, Australia, Nov 5-6, 1970; pp 48-64.

tralia, 1983.

1240-8.

rand-Relnhold: New York, 1979.

Receiued for review July 19, 1983 Accepted February 10, 1984

Incipient Fluidization Condition for a Tapered Fluidized Bed

Yan-Fu Shl, Y. S. Yu, and L. T. Fan'

Department of Chemical Engineering, Kansas State University, Manhattan, Kansas 66506

A model has been proposed for the condition of incipient fluidization in a tapered bed. The model is based on the balance between the forces, including the gravitational and fluid f r i i n a l forces, exerted on the fluidized particles and the total effective weight of the particles. Equations have been derived from the model for predicting the critical fluidizing velocity (onset velocity) in terms of the superflclal veloclty at the bottom of the fluidized bed and the maximum pressure drop through the tapered bed. A series of experiments was carried out in two-dimensional, tapered beds with different apex angles using water as the fluidizing medium and silica gel or sand as the fluidized particles. The experimental data indicate that the proposed model is valid and the derived equations are of practical use.

Introduction Straight cylindrical or columnar fluidized beds have been

employed extensively in the process industries. Recently, the use of tapered fluidized beds is beginning to receive much attention for biochemical reactions and biological treatment of waste water (see, e.g., Scott and Hancher, 1976; Pitt et al., 1981). Tapered fluidized beds have also been used successfully in chemical reactions, crystalliza- tion, and in other areas (see, e.g., Levey et al., 1960; Ishii, 1973; Golubkovich, 1975; Dighe et al., 1981).

Features of the tapered fluidized bed, especially its ad- vantages over the columnar fluidized bed, are discussed below.

Usually the size distribution of a particle system which can be employed in a columnar fluidized bed need be narrow. If the particle size distribution is too broad, small particles may be entrained and large particles may be defluidized, settling on the distributor. The cross-sectional area of the tapered fluidized bed is enlarged along the bed

0196-4313/84/1023-0484$01.50/0

height from the bottom to the top. Therefore, the velocity of the fluidizing medium is relatively high at the bottom, ensuring fluidization of the large particles, and it is rela- tively low at the top, preventing entrainment of the small particles. Therefore, we can operate the tapered fluidized bed with particles whose size distribution is wide. This feature is specially important for an operation in which the particle size changes (coal combustion, crystallization, microbial growth, etc.).

For an intensely exothermic reaction in the columnar fluidized bed the major fraction of heat is released near the distributor, creating a high-temperature zone and possibly destroying the distributor and sintering the particles. However, in the tapered fluidized bed, the ve- locity of the fluidizing medium at the bottom of the bed is fairly high. This gives rise to a low particle concentra- tion, thus resulting in a low reaction rate and reduced rate of heat release. Therefore, the generation of a high tem- perature zone near the distributor can be prevented.

0 1984 American Chemical Society