Homogeneous Catalysis

3 Mass Transfer Effects on Liquid-Phase

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Chemical Reaction Rates A D O L F W . GESSNER Foster Wheeler Corp., Livingston, N . J.

The rate of liquid-phase chemical reactions involving transfer of reactants from another phase depends on the homogeneous liquid-phase kinetics, physical mass transfer rates of reactants, and their thermodynamic equilibria at the phase boundaries. The interaction among these phenomena produces four distinct types of behavior depending on chemical reaction velocity. These will be examined in this paper.

"\T industrially important liquid-phase chemical reactions require * * * * the transfer of one or several reactants into a reactive liquid phase from a gas, from another liquid phase of limited miscibility with the reactive liquid, or from a solid. Reaction products accumulate in the liquid or leave by vaporization, migrate to another liquid phase, or precipi tate as solids. The rate of chemical reaction in such systems is determined not only by the homogeneous liquid-phase reaction kinetics but also by the physical transfer rates of reactants and their thermodynamic equilibria at the phase boundaries. In reversible reactions, the chemical equilibrium constant and the rate of removal of reaction products are also important. a n v

The reactive liquid phase may be brought into contact with gases and immiscible liquids in spray columns, packed columns, bubble trays, sparged columns or sparged mechanically agitated tanks, pipeline re actors, and, in the laboratory, in shaken flasks, liquid jets and falling film apparatus. Owing to the relatively slow motion of immiscible liquids rela tive to one another, centrifugal action is sometimes used in liquid-liquid systems. Solid-liquid contact is achieved by suspension of small solid particles in the liquid, which often is agitated mechanically to prevent settling of the solids. In laboratory studies, solids cast into flat slabs or mounted on rotating cylinders are sometimes used. 35 Luberoff; Homogeneous Catalysis Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

36

HOMOGENEOUS CATALYSIS

The factors of primary importance in such chemical reaction systems


are:

(a) The interfacial area between the reactive liquid phase and the phases supplying reactants and removing products. (b) The contact time of the reactive liquid phase with these other phases and the degree of backmixing of the phases in the reaction vessel. ( c ) The thermodynamic equilibria of reactants and products at the phase boundaries. ( d ) Physical mass transfer rates between phases. (e) Mass transfer rates with chemical reaction. The first four will be examined only superficially in this work in order to permit a somewhat more detailed examination of the last topic—mass transfer rates with chemical reaction.

Interfacial Area The interfacial area between the reactive liquid phase and the other phases is determined by the equipment configuration and the fluid flow rates and properties—density, viscosity, and surface tension. Interfacial area is usually given the symbol a and is specified on the basis of a unit volume. Its dimensions are thus area/volume or length" . The interfacial area is known accurately only in some systems used in laboratory studies: falling laminar films, laminar cylindrical jets, undis turbed gas-liquid and liquid-liquid interfaces, and solid castings of known dimensions immersed in liquids. In all reactor systems used industrially such as packed towers, spray towers, and bubble trays, the interfacial area is relatively difficult to determine. Photographic, gamma-ray, light scatter ing and chemical methods have been used to determine a in bubble dispersions (5, 6, 7, 8, 10, 42). For an average bubble diameter d a superficial gas velocity u and a bubble rise velocity u , 1

Jh

R(J

n

a = Suz /{d u ) G

n

B

(1)

In packed columns, the total packing surface is known (Table I ) , but the portion of the packing which is wetted by the liquid and thus available for mass transfer is widely variable, ranging from 5 to 70% of the packing surface, depending on the superficial liquid velocity through the packing, the packing dimensions, and the type of liquid distributor used (17, 30, 36,38, 42). Liquid Contact Time and Backmixing The choice of reactor type is usually dictated by the liquid residence time required to attain the desired degree of reactant conversion and

Luberoff; Homogeneous Catalysis Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

3.

GESSNER Table I.

Total Surface Area of Some Column Packings Nominal Size, inch

Packing


37

Mass Transfer Effects (33)

Total Surface Area (ft* )

(cmr ) 1

1

Metal Raschig Rings

i 1 2 3

118-128 57-63 31 21

3.9-4.2 1.9-2.1 1.0 0.7

Ceramic Intalox Saddles

i 1 2

190 78 36

6.2 2.6 1.2

Metal Pall Rings

1 2

66 37

2.2 1.2

Table II.

Characteristics of Some Liquid-Phase Reactors Approximate Upper Limit of:

Equipment

Phases Interfacial Contacted Area cm.' 1

Spray Tower Packed Column

G/L G/L

0.01 3

Bubble Tray Tower

G/L

8 sq. cm./cc. of froth

G/L S/L, L / L , G/L G/L, L/L, S/L

5 10 2 10

Sparged Column Agitated Vessel Pipeline

Liquid Residence Time, sec. 12 sec/ft. of packed depth 15 sec./ft. of liquid path

Degree of Liquid Backmixing plug flow plug flow

unlimited unlimited

partly backmixed on each tray backmixed backmixed

unlimited

plug flow

by considerations of temperature and pressure control, susceptibility to plugging, safety, and cost. The average liquid residence time depends on the liquid feed rate and the reactor volume, but the residence time dis tribution and the average reactant concentration depend in addition on the degree of liquid mixing in the reactor. Mixing highly converted mate rial with fresh feed lowers the average reactant concentration and thus the reaction rate. Table II summarizes some characteristics of common reactor types. Interphase Equilibria of Reactants and Products Reactants and products on both sides of phase boundaries are as sumed to be at physical—not chemical—thermodynamic equilibrium.


38


This assumption is supported by many experimental studies. The con centrations of a component in two phases at equilibrium are related by equilibrium ratios, which remain approximately constant over a sub stantial concentration range. The distinction between physical and chemical equilibrium is im portant. For example, when chlorine is absorbed into water, it first enters the water as dissolved chlorine and then undergoes a relatively slow chemical reaction with water to form HOC1, H , and CI". Two equilibrium ratios may be written—one based on total chlorine in the liquid [ C l + J H O C 1 + JC1"], and the other based on dissolved C l only. It is the latter ratio which controls the mass transfer rate. As another example, when carbon dioxide is absorbed into alkaline aqueous solutions, it first dissolves as C 0 and then reacts with O H " to form bicarbonate ion. The equilibrium ratio controlling the mass transfer rate is PC02/ [ C 0 ] . This ratio is inde pendent of p H and is affected only by changes in the ionic strength of the solution. The interphase equilibria of the reaction products are important only for reversible chemical reactions.


+

2

2

2

2

Mass Transfer Without Chemical Reaction The mass transfer rate of a component, A , across a phase boundary has been found to be proportional to the concentration difference in A across a hypothetical fluid film between the bulk of each fluid phase and the phase boundary. Thus, N = k(c -c ) A

A

M

(2)

y

where N = rate of flow of reactant A across interface, gram-moles/sec./ sq. cm. c — concentration of A in the bulk of the phase, gram-moles/cc. CAI = concentration of A at the phase boundary, gram-moles/cc. k = mass transfer coefficient, cm./sec. A

A

This equation applies to both gases and liquids. In gases, the concen tration is often expressed in terms of partial pressure or mole fraction, and the units of k are changed accordingly. For the transfer of A from a gas into a liquid, we may write N A = M P A - P A I ) = k (c L

M

- c) A

(3)

At the phase boundary, it is assumed that equilibrium exists between dissolved and gaseous A . Thus, PAI = H c , A

A1

(4)

where H is the Henrys law constant for A , expressed in atm-cc./grammole. Combining Equations 3 and 4 results in Equation 5. A


3.

GESSNER

Mass Transfer Effects

39

p /H —c HA/ A by the definition A

Aiy

L

N Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 1, 2016 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/ba-1968-0070.ch003

A

(gram moles/sec./sq. cm.) = k c h

Al

(10)

It is convenient for comparison to define the mass transfer rate in the absence of chemical reaction under otherwise identical conditions by ^A=k °c L

(11)

Aly

where the superscript ο denotes "no chemical reaction," and the rate of reaction per unit volume by R = N a.

(12)

A

Four distinct types of behavior have been observed. These w i l l be identified as Regimes I, II, III, and IV.

DISTANCE

REACTION ZONE

Figure 2. Concentration profile across a gas-liquid inter face with very rapid reaction (Regime I). Reactants A and Β diffuse into a narrow reaction zone in the liquid film. N =k °c [1 + O c /(nO c )]. A

L

Ai

B

BL

A

Ai

Regime I—Very Fast Reactions If A and Β react very rapidly, the liquid film at the phase boundary is depleted rapidly of reactants A and B, and the reaction can proceed only as fast as the reactants can diffuse into the surface film from both


42


sides—A from the phase boundary and Β from the bulk of the liquid. A semiquantitative plot of the concentration profiles in and around the fluid film is shown in Figure 2. The dashed line indicates the approximate concentration gradient across the fluid film in the absence of chemical reaction. It will be noted that with chemical reaction, the concentration gradient of A is steeper. Thus, N should be greater than N ° and A

A


ΝA/NA'=

> 1

Hatta (19) first derived an expression for k /k of a completely stagnant fluid film and found L

1 +

O

L

(13)

based on the assumption

ia^

(14)

Of course, the assumption of a stagnant fluid film with an abrupt transi tion to the bulk of the liquid is not very plausible, but Equation 14 fits many experimental data well. Two other theories for predicting k /ki" for very fast reactions lead to similar results. Higbie's "penetration theory" (19), which is based on unsteady state diffusion into a laminar fluid stream, leads to the expres sion ( I I ) : L

This differs from Equation 14 only by the factor a / D A / D , which is often near unity. A derivation based on mass transfer in a turbulent boundary layer results in the expression ( 13, 24, 27, 31 ) : b

^-(• ^)(sr +

Again, the difference between this expression and Equation 14 is un important. W e may now define the reaction rate in Regime I as the rate of con sumption of A per unit volume ^ « = V «

A

I

( l + ^ ) - « i

W

The chemical reaction rate constant does not appear in this expression. Increasing the rate constant by adding catalysts thus has no accelerating effect. Examples of reactions in Regime I are given in Table IV. Régime II—Moderately Fast Reactions A totally different situation arises with moderately fast chemical reactions in which the surface film is not appreciably depleted of reactant


3.

Table IV.

Reaction Systems in Regime I Apparatus

Reference

packed column disc column packed tower valve tray solid slab in stirred tank solid pieces on rotating cylinder falling film falling film

1,2, 36 30 36 16 24 31 25 12

Component A Component Β O H " (NaOH) FeCl NH OH

CO., Cl 2

co

2

4

2


Benzoic acid

co co

2 2

43


GESSNER

O H " (NaOH) O H " (NaOH) MEA Ρ

c

c

1

y

ι

1

1

GAS [ FILM

GAS

BL

LIQUID /

A

L

^ °

DISTANCE

Figure 3. Concentration profile across a gas-liquid interface with moderately fast reaction (Regime II). Reactant A is partly consumed hy chemical reaction in the liquid film N

A

=

[2ΚΌ οβ ,/( Α

ΒΙ

α

+ l)V'*C '* Al

B. Reactant A diffuses across the surface film but at the same time par tially disappears because of reaction with B. The concentration of re actant Β is substantially constant throughout the surface film. Assuming the surface film to be stagnant, one can show that for a chemical reaction rate expression: -dc /dO = Kc *c e, A

A

B

(18) (19)

and the rate of consumption of A per unit volume is (20)


44


For the frequently encountered case a = β = 1, this reduces to *

L

=

VKDA7 ~

(21)

B

and NA Ri have been omitted. Figures 5 and 7 may be used to find the rate in the transition regions. m


50


The type of reaction vessel best suited to a given reaction system depends in part on whether or not it is in Regime I V . To maximize the rate in Regime IV, the volume fraction of reactive liquid phase t>L should be maximized. Maximizing the interfacial area a has no effect. A stirredtank reactor or sparged column with low gas holdup would be most suit able because v is near its maximum. To maximize the rate in Regimes I, II, and III, on the other hand, requires maximizing the interfacial area of the reactive liquid phase. This is best done in a sparged column with high gas holdup or a packed or tray column. Obviously, many other factors enter into the selection of the reactor type.


L

Nomenclature Roman Letters a c CAI CL B

= = "= =

1

D = d = dj, = g = Fr = H = Κ — k = k = B

G

k = k° = L

L

I = η = ρ = R = R i , Ru, R Re Sc Sh u u USG t>L, v B

G

= = = = = = =

interfacial area per unit volume, cm." concentration, gram-mole/cc. concentration of reactant A at phase boundary, gram-mole/cc. concentration of reactant Β in bulk of liquid phase, gram-mole /cc. diffusivity, sq. cm./sec. average bubble diameter, cm. average particle diameter, cm. gravitational acceleration, 981 cm./sq. sec. Froude number, u /(gl) Henry s law constant, p\/c atm.-cc./gram-mole chemical reaction rate constant, (gram-mole/cc.) "«"^ sec." mass transfer coefficient, cm./sec. mass transfer coefficient for gas film, cm./sec, or gram-mole/ sec./sq. cm./atm. mass transfer coefficient for liquid film, cm./sec. mass transfer coefficient of liquid film in absence of chemical reaction, cm./sec. a characteristic length dimension, cm. stoichiometric coefficient in chemical reaction A -f- nB —> products pressure, atm. rate of consumption of reactant A per unit volume, gram-mole/ cc./sec. , Riv = rate expressions for the four regimes, gram-mole/cc./ sec. Reynolds number, (pul/μ) Schmidt number, ( μ/pD ) Sherwood number, ( k I/O ) a characteristic velocity, cm./sec. bubble rise velocity, cm./sec. superficial gas velocity, cm./sec. volume fraction of liquid and gas

m

2

h

(1


)

1

3.

GESSNER


51

Greek Letters


α β θ μ ρ

«= = = = =

exponent of c in rate expression exponent in c in rate expression time, sec. viscosity, gram/cm./sec. density, gram/cc. A B

Superscripts and Subscripts A = reactant A Β = reactant B ; bubble, in u G = gas i = interface L = liquid S G = superficial gas velocity, u ο = no chemical reaction I, II, III, I V = numerals referring to the four reaction regimes B

8G

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52


(28) Roberts, D., Danckwerts, P. V., Chem. Eng. Sci. 17, 961 (1962). (29) Sharma, R., Danckwerts, P. V., Chem. Eng. Sci. 18, 729 (1963). (30) Sherwood, T. K., Pigford, R. L., "Absorption and Extraction," McGrawHill, New York, 1952. (31) Sherwood, T. K., Ryan, J. M., Chem. Eng. Sci. 11, 81 (1959). (32) Sideman, S., Hortascu, O., Fulton, J. W., Ind. Eng. Chem. 58, 32 (1966). (33) U . S. Stoneware Co., Bulletin TP54, "Tower Packings," Akron, Ohio, 1957. (34) Van Krevelen, D . W., Hoftijzer, P. J., Van Hooren, C. J., Rec. trav. chim. 66, 513 (1947). (35) Van Krevelen, D . W., Hoftijzer, P. J., Rec. trav. chim. 67, 563 (1948). (36) Van Krevelen, D . W . , Hoftijzer, P. J., Chem. Eng. Progr. 44, 529 (1948). (37) Vivian, J. E., Whitney, R. P., Chem. Eng. Progr. 43, 691 (1947). (38) Weisman, J., Bonilla, C. F., Ind. Eng. Chem. 42, 1099 (1950). (39) West, F. B., Gilbert, W . D . , Shimizu, T., Ind. Eng. Chem. 44, 2470 (1952). (40) Whitman, W. G., Chem. Met. Eng. 29 (4), (1923). (41) Yoshida, F., Ikeda, Α., Imakawa, S., Miura, Y., Ind. Eng. Chem. 52, 435 (1960). (42) Yoshida, F., Miura, Y., A. I. Ch. Ε. J. 9, 331 (1963). (43) Yoshida, F., Miura, Y., Ind. Eng. Chem. Process Design Develop. 2, 263 (1963). (44) Yoshida, F., Akita, Κ., Α. I. Ch. Ε. J. 11, 9 (1965). RECEIVED November 14, 1966.


Homogeneous Catalysis

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