Contribution to Design and Scale-Up of Packed Columns

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Verbindungen zeigten unter den gewalten Bedingungen entweder keine oder aber eine nur schwach ausgepragte antioxidative Wirkung, so daB praktisch nur wenig Aussicht besteht, durch diese Vorstufen der Maillard-Reaktion, wie sie im Rahmen des Trocknungsprozessesin Lebensmitteln gebildet werden konnen, eine antioxidative Wirkung zum Schutze von Lipiden zu erzielen. Literatur 1 W Grosch, Z. Lebensmittel-Unters. u. -Forsch. 157, 70 [1975]. 2 P. Schieberle u. W Grosch,J. Amer. Oil Chemists' SOC. 58, 602[1981]. K. E. Peers et al.,J. Sci. Food Agric. 35, 813 [1984]. 4 C. M. Houlihan, J. Amer. Oil Chemists' SOC. 61, 1036 [1984]. . H. P. KauJinann,Die Ernihrungsindustrie 120 [1973]. 5 6 A. Seher u. D. Loschner, Fette Seifen Anstrichmittel 88, 1 [19861. 7 R. Yamauchi et al., Agric. Biol. Chem. 46, 2997 [1982]. 8 R. S. Farag et al., Can. Inst. Food Sci. Technol.J. 15,174 [1982]. 9 K. Eichner, Prog. Fd. Nutr. Sci. 5, 441 [1981].3

H. Lingnert, SIK Rapport No. 458, 45 [1979]. L. B. Rockland, Analyt. Chem. 32, 1375 [1960]. 12 J. E. Hoke, Agr. Food Chem. 1, 928 [1953]. 13 K. H n , G. Miiller u. H. Pauhen, Liebigs Ann. Chem. 703,202 s1011

[196$ W Butte, J. Chromatogr. 261, 261 [1983]. 15 E. SchuZte u. K. Weber, Fat Sci. Technol. 91, 181 [1989]. 16 W Grosch, Z. Lebensmittel-Unters. u. -Forsch. 160, 371 [1976]. 17 S S. Chng et al., J. Food Sci. 42, 1102 [1977]. . 18 J. C. Uhl, Dissertation Universit;it Miinster, 1989. 19 A. Golan-Goldhirsch,Z. Lebensmittel-Unters. u. -Forsch. 182,29 [1986]. 20 M. Tokati u. M. M r t ,Agric. Biol. Chem. 49, 3545 [1985]. oial4

D anks agung Die Untersuchungen wurden im Rahmen der industriellen Gemeinschaftsforschung aus Mitteln des Bundesministers fiir Wirtschaft (BMWi)gefordert. Hierfiir sei an dieser Stelle verbindlich gedankt. Eingegangen am 10. Mai 1990.

Contribution to Design and Scale-Up of Packed Columns*By R B i l l e t * * Institute for Tharmo- and Fluid Dynamics, Ruhr-Universiq, Bochum, Federal Republic o Germany fThe demand for high accuracy in packed column design for countercurrent thermal separation processes, such as rectification, absorption and desorption, has been stepped up research in scaling of packed column efficiency in response to the growing application of packings for various systems in the chemical and allied industries. The investigations confmed the suitability of relationships for describing the performance, the validity of which was proved with differing systems and packings. This equations takes due account of geometric,hydrodynamic and system-relatedparameters that govern mass transfer and the fluiddynamics in packed columns.

Berechnung und Beitrag zur Auslegung von Fiillktirperkolonnen Die Forderung nach hoher Genauigkeit bei der Auslegung von Fullkorperkolonnen fiir Gegenstrom-Trennprozessewie Rektifiation, Absorption und Desorption fiihrte zu einer verstirkten Forschung auf dem Gebiete der Beschreibung der Wirksamkeit dieser Apparate bei ihrem steigenden Einsatz f i r Prozesse der chemischen und verwandten Industrien. Es werden Berechnungsmodelle zur Beschreibung der Leistungsfihigkeit von Fiillkorperkolonnen vorgestellh die gleichermal3en konstruktiven, hydrodynamischen und stomichen Parametem Rechnung tragen, und ihre Gultigkeit und Eignung fiir die praktische Anwendung wird durch Ergebnisse experimenteller Untersuchungen mit verschiedenen Stoffsystemen bestiitigt.

In response to the growing application of packings for various systems in the chemical and allied industries the demand for high accuracy in packed column design for countercurrent thermal separation processes, such as rectification, absorption and desorption, has been stepped up research in scaling of packed columns on the basis of fundamental studies of the fluid dynamics and mass transfer. These investigations confirmed the suitability of theoretically developed relationships, the validity of which was proved with differing systems and packings. The equations take due account of geometric, hydrodynamic and systemrelated parameters that govern the fluidity behaviour and mass transfer. Fundamental relationships The correlation, defining the specific column volume v,

per unit efficiency and per unit gas throughput according to Eqn. (11, with nJH being the number of theoretical stages per unit packing height and u, the vapour or gas load, is a criterion for the investment costs of the packings applied in the column. The relationship expressed by Eqn. (2) between nJH and the height equivalent to an overall transfer unit in the gas or vapour phase HTUOv is valid for the separation of mixtures with mass transfer predominantly in the gas or vapour phase. The stripping factor I. is fixed by the vapour/ liquid ratio U Vand the slope myxof the equilibrium curve according to Eqn. (3).

"I;-.- x-1 1HHTU,,

Inh

(2I(31

* Summary of papers presented at 38'h Canadian Chem. Eng.Confer., Edmonto, AlbertaXanada, 1988; at XIIP Interamerican Congress of Chem. Eng. PACHEC, Acapulco/Mexico, 1988; at Intern. Confer., on Chem. Eng. and Petroleum Technology, New Delhi, India, 1989. ** Author's address: Prof. Dr.-Ing. R B l e , Ruhr-University ilt Bochum, Institute for Thermo- and Fluid Dynamics, Department of Thermal Separation Processes, P. 0.Box 10 21 48, D-4630 Bochum.Fat Sci. Technol

1 IV

In the light of the HTU/NTU model, Eqn. (4) applies for the height HTUov of an overall transfer unit on the gasside.I41

Its evaluation requires the knowledge of the volumetric361

92. Jahrgang

Nr. 9

1990

mass transfer coefficients pv . a,,h in gas or vapour phase and PL * a+ in liquid phase which are principally dependent on gas load uv respectively on liquid load uL. Therefore the efficiency of a packed column and consequently its specific column volume are closely related to the fluid dynamics under operating conditions. Based on a theoretical model, recently developed by the author, a description of the fluiddynamics and mass transfer takes into account the great diversity in the geometry, structure and materials for packings in industrial columns, which normally entails differences in the fluiddynamics under operation conditions and thus in the packing

performance. The model correlations were checked against the results of comprehensive and systematic experimental studies performed in the Chair for Thermal Separation Processes in the Ruhr-University and figures taken from the literature. The number of measurements performed in the fluiddynamic tests was 500; in the tests to determine mass transfer on the liquid side, 1050; and in the tests to determine mass transfer on the gas side 1100. The measurements were made on about 60 packings of different designs and dimensions, some of which are shown in Fig. 1 as examples for dumped packings and in Fig. 2 as representatives for regular packings.

Packing main size 50 mm metalCMR No. 1.5

ceramic-

plastic Pall ring

Pall ring

Hake t te

N = 19350 a = 120E

= 0.978

N = 6400 a = 120 E = 0.78Hiflow ring

N = 6700 a = 110 6 = 0.92 Hiflow ring

N = 12400 a = 135 E = 0.93

Hiflow ring

DINPAC

N = 5000 a = 97.3 E = 0.973VSP ring

N = 4950 a = 86.7 E = 0.815Intalox saddle

N = 6400 a = 112 E = 0.93~ ~~

N = 29000 a = 135 E =0.92ENVI PAC

Norpac ring

N = 7800 a = 104 E = 0.98

N = 9300

a = 120 E = 0.77~

N = 7300 a = 90 E = 0.952E

N = 6800 a = 98 E = 0.961

Dimensions: N [ 1 /m3 I , a [ m 2/m3 I ,~~~

Im3 /m3 1Fat Sci. Technol. 92. Jahrgang Nr. 9

Fig. 1. Examples for fillings of dumped packed columns, investigated in this work

362

1990

Mellapak 250Y

Montz C1-300

a=250, ~ = 0 . 9 6Montz packing B1 - 300

a=300, &=0.90 Sulzer BX

a=300, ~ = 0 . 9 3 Ralupak 250Y

a=500, &=0.90 Gempak A2T

a=250, ~ = 0 . 9 6 3

a=202, E = 0.978E

-

Dimensions a [m2/m3 1 ,

[m3/m3 1

Fig. 2. Examples for regular packings, investigated in this work

M o d e l l i n g of c o u n t e r c u r r e n t f l o w i n p a c k ings The void space in the bed can be simplified regarded as a network of numerous channels, having the void fraction E and the specific area a of the packing, through which the liquid passes downwards in the form of a film or in lamell flow. The gas flows upwards through the bed, countercurrent to the liquid. Under stationary conditions, the force of gravity and the shear forces in the liquid film of the densityFat Sci. Technol. 92. Jahrgang XI.$11990

pL and the dynamic viscosity qL are assumed to be in equilibrium at a point representing any given thickness s in a coaxial layer within the liquid film, and the fnctional force exerted by the vapour of the density pv acts at the surface of the film. Operating magnitudes are the liquid load uL and the gas velocity u which effects also the liquid hold, up h,. The solution of the differential equation describing this flow model gives rise to Eqn. (5) where the index n and the flow factor 4 depend on the flow pattern.

363

151

dynamic viscosities qLand qv of the phases, and a factor C , that characterizes the geometry and surface area of the packing.

Eqn. (5) can be quahtatively described in accordance with the experimental investigations by the diagrams of Fig. 3, which is selfexplanatory and which shows the conditions, under which loading and flooding occur.Vapour velocily uvh,=

191All that is required for its application is a knowledge of the packing constants C,, which has to be determined by experiment. The index n, is a constant for the system. Below the phase inversion point, corresponding to (Wq . (pV/p3O.5 < 0.4, it is given by n, = -0.326. Above the phase inversion point, it has a value of n, = -0.723. If the g s velocity is higher than u =uv,s, the liquid holda . , up increases rapidly above the value at the loading point, i. e. hL > hL, until the flood point is reached, i. e. h, = hL,n. At this point, all the liquid is theoretically forced upwards by the stream of gas in the bed. According to the model the gas velocity at the flood point is given by Eqn. (10)

f(UY)u,.cmrl loading line llmding line

-c-

r -

a

--

sI

I

"1 = f ( h l ) u " m s t

oa

loading point flwding paint

Flooding condilions : Hold-uph,

%*odh, dhl

-

uV.FI

du, -= 0 -

U1.J

-

hl.FI

and the liquid hold-up at the flood point is defined by Eqn. (11), which mathematically satisfies the boundary conditions in Fig. 3.(11 I

2 Bza 8

2

r

v

uv = f(~L'Ul,,,",t00

>

loading point flooding point

Hold -UP hl

Fif 3. Qualitative describing of the fluiddynamic in packed co umns, operated in countercurrent phase flow of gas or vapourliquid systems Loading can be defined as the load at which the frictional force exerted by the gas prevents downflow of the inner coaxial layer of liquid in a flow channel, i.e. the load at which the liquid velocity becomes zero. Thus the gas velocity uv,s at the loading point is an important design criterion. It can be derived from the equations in the mathematical model resulting in Eqn. (6)161

These equations are theoretically derived correlations, which can be used for practical estimation of the flooding conditions. The application demands a knowledge of the flow factor in, which .is related to the main parameters by Eqn. (12). Below (Wq . (pv/pJJ.5 S 0.4, nn has an average value of nR = -0.194; and above this point, nF1 -0.708. =

and in Eqn. (7)(71

which describes the liquid hold-up h,, at the loading point. If the flow of the liquid film is laminar and vertically downwards the following values apply to C , and n: C,= 12" and n = 1/3. Finally Eqn. (8) is derived by combining Eqn. (6) and Eqn. (7).

The flow factor tsin Eqn. (8)depends according to Eqn. (9) on the phase ratio W e the densities pL and pv and the 364

This flow model was the subject of comprehensive experimental studies to determine whether is could be applied in general to all kinds of packed columns with countercurrent liquid/* or vapour systems. The correlation between all the measurements and the theoretically derived relationships for the loading and flood points was very good. The liquid loads varied between uL = 4.88 and 144m3/m2h; the densities of the gases or vapours, between p, = 0.3 and 1.37 kg/m3; the dynamic viscosities, between qv = 7.14 . and 18.19 * 10-6kg/m . s; the liquid densities, between pL = 750 and 1026kg/m3; and the dynamic viscosities between qL = 0.36 . 10-3 and 92.6 10-3 kg/m * s. Detailed figures on the evaluation of the model parameters are dealt with in Tables 1 to 2. They allow application of Eqns. (6) to (12). The Reynolds number for film flow in the liquid phase is defined by Eqn. (13). The evaluation of the experimental results has also revealed that if this Rqrnolds number is higher than 10, Eqn. (14) will apply for the relationship between the index n in Eqn. (7)and the number of packings N, their area a per m3 and their main dimension d. For regular packings n was found to be n = 213.Fat Sci. Technol. 92..Jahrgdng Nr. 91990

Table 1 Constantsfor hold-up, loading and flooding conditions Dumped Packings Pall ring Ralu ring Hiflow ring Hiflow ring; Super NOR PAC ring Raschig ring VSP ring, No. 2 Glitsch, CMR ring Envipac ring, No. 2 Dinpac ring, No. 1 Intalox saddle Intalox saddle; grid Hackette Material Metal Plastic Ceramic Plastic Metal Plastic Ceramic Plastic Plastic Ceramic Metal Metal Plastic Plastic Plastic Plastic Plastic Size d50 rnm 50 rnm 50 mrn 5 0 mm 50 rnrn 50 rnrn 50 mm 50 mm 5 0 mm 50 rnm 50 rnm 38 mm 60 mm 4.5 mm 50 mrn 50 mm 4.5 mm

Ch51.70 (i7.95 70.2981.71 73.03

CS2.7252.816 2.84(j 2.843

C,,,1.580

1.757 l.!)l:31.812 1.62ti 1,871 l.6!)4 1.7W 1.786 1.574 1.M!) 1.841 1.864 1.w 11.548

2.7022.8!/4 2.81!) 2.86ti 2.!).5!)

143.96

2.48294.17 121.M 115.502.806

2.6!)7 2.!)87

2.!U!) 2.382 2.67559.!) 1

1.657

Table 2 Constantsfor hold-up, loading and flooding conditions Regular Packings Ralu pack YC Montz packing Mellapak Y Gempak A2 T-304 Material Metal Metal Plastic Plastic Metal Size 250 mz/rn:{ B1-100 mz//m.' B1-200 rnVm' B1-300 mVm1 CZ-1100 m2/m+ 250 m2/m,'2012 rn2/m''

Ch97.0.552.3Y

CS 3.178 3.08!)3 . 1 16

cI 12.558 1.911 2.33!) 2.4641.973

34.8043.!)7 (j0.40

3.098 2.6533.1Fi7

2.!)8(i

2.464 2.0!)!)

t L=

hL .I .- 1 r

"1

M o d e l l i n g of m a s s t r a n s f e r i n p a c k i n g s A relatively new model not only describes the systems investigated quite accurately but is also just as valid for conventional as for modem packing. This model thus allows mass transfer to be described in terms of the packing geometry and the physical magnitudes that influence the gas-liquid system. The relationships derived from it can be applied to all countercurrent-flow columns, regardless whether the beds of packing have been dumped at random or arranged in a geometric pattern. As a rule, the direction in which the two phases flow through the channels in a bed of packing changes continuously. Thus the path followed by the one phase can be imagined to be split up into zones of mass transfer and mixing zones. In other words, after the phases have flowed through a mass transfer zone, the areas of contact are renewed. The average duration of contact of the phases T~ respectively 'cV in the individual zones of mass transfer in an element of length 1 is comparatively short in , packed columns. It is governed by the effective void fraction e of the packing and the liquid hold-up h, under operating conditions. At a liquid load uL, tL can be described by Eqn. (15) and if the vapour load is u tv is , given by Eqn. (16).Fat SCI. Technol. 92. Jahrgang Nr. 91990

Hence it assumed that mass transfer follows the law of instationary diffusion valid for brief periods of contact between the phases and described by the Eqn. (17), formulated by Higbie,

P:&.(

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