I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1460
encountered in continuous operation. It might be well to point out, however, that if somewhat higher conductivities are to be measured, a frequency of 60 cycles second can be applied. This modification leads to a considera le reduction in equipment, since the alternating current power line can then be used directly instead of the oscillator. Increased sensitivity to changes of specific resistance a t lower levels can be obtained by suitably shunting t,he voltmeter, thereby reducing the value of r in Equation 9.
98t 96-
a
Y Li F!
52
94-
92-
c 8
Y 2 9088
impurity is present, by xnea,ns of a measurement of specific resistance alone. One additional fact should be noted concerning the calibration points shown in Figure 3. Although a comparison of one set of samples (different sets are indicated by different symbols in the figure) with another wa.s in error as much as 1.50/, a t high purities, the accuracy of the calibration for any one set was more consistent. A change in purity was always accompanied by a change of meter reading in the correct direction. Thus changes in purity were indicated to an accuracy much better than 1 . 5 ~ o , and the indicated purity always varied as it should with changes in still temperature and reflux ra.te. It) is of interest tha,t. the conductance of methyl vinyl ether measured under flow conditions was consistently lower than that of an equivalent immobile sample. A t first this effect was ascribed t o a cleansing of the cell by the flowing liquid, but the findings could also be explained in wcordance with Plumley’s theory ( 5 ) . If favorable orientation in the electric field counteracts recombination of the ions formed by spontaneous dissociation one may expect a lawer conductivity in a flowing medium since the ions are being carried away by the moving liquid. ACK8OW LEDGMENT
-
\
A. 0
Vol. 43, No. 6
Figure 3.
The authors would like to thank H. Beller, new products mansger of the General Aniline Works Division and his associates a t the Graseelli plant, for their wholehearted cooperation ; they are also grateful to R. M. Fuoss who made several valuable suggest,ions on reading the manuscript. The chemical analyses of the methyl vinyl ether eamplrs vere performed by S. Siggis and R . 1,. Edsberg.
\
\
50 I00 150 200 250 RECORDER DEFLECTION (ARBITRARY SCALE1
3,
Recorder Scale Calibration
In the h a l installation provisions were made for continuous recording by modifying the output of the electronic voltmeter so t h a t it actuated a conventional temperature recorder. Figure 3 shows the correlation between the readings obtained on the recorder and the values for methyl vinyl ether content determined by chemical analyses of identical samples. The methods described by Siggia (8) and Siggia and Edsberg (9) were used for these determinations. As shown in Figure 3, in most cases the correlation was found to be quite good, Of course, since the same effect of decreasing the resistance may be obtained from many different kinds of impurity, it is impossible t o tell which
Enginnyring
,
LITERATURE CITED
Fuoss, R. M., J . Am. Chem. SOC.,55, 21 (1933). Jones, G., and Josephs, R. C., Ibid., 50, 1049 (1928). Onsager, L., J . Chem. Phys., 2, 599 (1934). Pao, C. S., Phys. Rel;.,64, 60 (1943). Plumley, H. J., Ibid.,59, 200 (1941). Rosenthal, R., Instruments, 23, 664 (1950). (7) Schildknecht, C. E., Zoss, A. O., and McKinley, C . , IND.ENO.
(1) (2) (3) (4) (5) (6)
CHEM.,39, 180 (1947). ( 8 ) Siggia, S., Anal. Chem., 19, 1025 (1947). (9) Siggia, S., and Edsberg, R. L., Anal. C h m . , 20,762 (1948). RECEIVEDOatober 23, 1950. Presented before the Gordon Research Conference on Instrumentation (A.A.A.S.), New London, N. H. (Auguet 1950)
ifisance of Liqui nts in Gas Absor
Process development I
P. V. DANCKWERTS UNIVERSITY D E P A R T M E N T OF C H E M I C A L E N G I N E E R I N G , T E N N I S C O U R T R O A D , C A M B R I D G E , E N G L A N D
T
HE conventional picture of the process of absorption of a gas into a n agitated liquid, under conditions such that (‘the liquid film is controlling,” is t h a t there exists a stagnant “film” of liquid at the interface, similar to the laminar sublayer formed when a fluid flows past a solid. The concentration of the bulk of the liquid beneath this film is kept uniform by turbulent mixing, while the surface concentration has a t all times the saturated value c“. The rate of absorption (per unit area) will then be given by the expression
R
=
D - (c*
XL
-
GO)
(1)
where R is the rate of absorption per unit area, c, is the bulk concentration of the solute, D is its diffusivity, and X L i? “the effective film thickness” (9). It seems doubtful, however, whether the conventional picture bearP a very close relationship to the actual mechanism of absorption. The conditions reauirrd to maintain a stagnant film at the
June 1951
*
U
INDUSTRIAL AND ENGINEERING CHEMISTRY
free surface of an agitated liquid appear to be lacking, and it seems more probable that turbulence extends to the surface and that there is no laminar boundary layer. I n particular, if we consider the liquid flowing over the packing in an absorption tower, it appears most unlikely that the surface layer of liquid maintains its identity throughout all the discontinuities of flow. The fictitious nature of the “liquid film” is probably widely suspected; nevertheless, it is constantly referred to as though it actually existed. This may be regarded, for many purposes, as a harmless and convenient usage, as measured absorption rates appear to conform to the expression
R = ~ L ( c *- eo)
(2)
1461
tested by experiment. It may well be found that the processes occurring in gas-absorption equipment are too complicated to be treated successfully by such simple methods as those presented here, but the author hopes that these may constitute a step in the right direction. ABSORPTION INTO A STAGNANT LIQUID
Consideration is limited in the first place to the case where the surface of the liquid is at all timea saturated with the solute gas, and there is no chemical reaction between solute and solvent. If the normal diffusion law is obeyed in the liquid, the rate of absorption (quantity per unit area per unit time) into a stagnant liquid of infinite depth is given by
where k ~the , liquid-film mass-transfer coefficient, is constant for a $(e) = ( C * - e o ) (3) pjven liquid and gas under given conditions. However, if the film model is in fact an unwhere 6’ is the time of exrealistic one, it may lead t o to the gas and the posure T h e present theory of gas absorption is based on a picerroneous results if it is used initial concentration is uniture of the absorption mechanism which is probably unas the basis for theories which formly equal to co (see disrealistic. For many practical purposes this is nnimporseek to relate k~ to the concussion of derivations of R tant, but the theory is likely to be misleading when used ditions of operation. below). to predict the behavior of systems for which no experiIt may be of value, thereI t can be seen that unless mental data are available. A more plausible picture of the fore, to show t h a t the proved the surface is renewed by absorption process is therefore suggested. usefulness of Equation 2 in stirring or by convection, the The usual assumption of a “stagnant film” of liquid at no way provides support for rate of absorption becomes the interface is abandoned. Instead it is supposed that the stagnant-film hypothesis, v e r y slow a f t e r a time. the surface is continually being replaced with fresh liquid, as this expression can be Stagnant water will normally and a theory of absorption rates is developed on this basis. derived from what is conabsorb in 1 hour a quantity It appears to agree generally with experience, but it must sidered t o be a more realistic of gas equivalent to a satube regarded as tentative until confirmed experimentally. picture of the processes occurrated layer about 2 mm. thick If the detailed features of the theory are confirmed, it ring during absorption into offers a powerful method of predicting the performance of (taking D sq. em. pcr an a g i t a t e d l i q u i d . T h e second). If the “depth of absorption equipment. Failing this, it may still provide treatmeDt is here extended to penetration” is arbitrarily desuggestions for a new method of approach to the subject. cases in which gas-film resistfined as the depth a t which the ance or surface resistance rise in concentration is l/loa plays a part. Provided no that a t the surface, it will be equal to 3.6 d E (see Equation chemical reactions occur, the expressions derived for the rate of ab27), with a value of about 6 mm. at 1 hour. (In the case, for sorption are similar in form to those based on the film theory, but instance, of carbon dioxide and water, $ will conform to Equation a different significance is attached t o the liquid-film coefficient. 3 only for small values of 8; thereafter convection currents will On the other hand, when the absorbed gas undergoes a reaction in arise because the density of a solution of carbon dioxide is greater solution, the two methods of treatment give different expressions than that of water, and the predicted values of $ will be exceeded.) for the relationship of the rate of absorption to such quantities as Under certain conditions Equation 3 can be applied as a close the liquid-film coefficient without reaction, the reaction-velocity approximation to: (a)liquid layers of restricted depth, and ( b ) constant, and the diffusivities of the components of the system.’ liquid moving parallel to the surface with a velocity that varies The expressions derived in this paper for the rate of a b s o r p with the depth. The necessary condition for ( a ) is that the time tion, R, under various conditions contain a quantity, 8 , which of exposure should be so short that the depth of penetration is relates to the rate of renewal of liquid surface. A complete deless than the depth of the liquid; for ( b ) it must be so short that velopment of the subject would require further discussion of the the depth of penetration is less than the depth at which the velocrelation of s to the hydrodynamics and geometry of the system. ity is appreciably different from that a t the surface. For the present s is regarded as a quantity which must be determined experimentally for any given system. The various expresABSORPTION INTO SURFACE O F TURBULENT LIQUID sions for R in terms of s are thus likely to be of use mainly in comWhen a liquid is in turbulent motion it is a mass of eddies paring rates of absorption in systems of equal 8. For instance, which incessantly change their conformation and position. suppose the rate of absorption of carbon dioxide into water is These eddies are pictured as continually exposing fresh surfaces to measured in a packed tower. With the same packing and flow the gas, while sweeping away and mixing into the bulk parts of rate, and with a liquid of virtually the same density and visthe surface which have been in contact with the gas for varying cosity, s will have the same value, and under these conditions it may prove possible to use the measurements on carbon dioxide lengths of time. The assumption will be made that during the and water to predict the rate of absorption of, say, sulfur dioxide time of exposure of any portion of the liquid it absorbs gas a t a into water or carbon dioxide into dilute alkali. (The possibility rate given by Equation 3, as in case ( b ) above. This is equivalent to assuming that the “scale of turbulence” is much greater of doing so depends in part on factors discussed under “Nonhomothan the “depth of penetration” of the solute diffusing from the geneous Distribution of Surface Ages.”) surface, so that relative motion of the liquid a t different levels No attempt has been made to compare the expressions derived here with published experimental measurements. I n spite of close beneath the surface may be disregarded. Velocity gradients no doubt exist beneath the surface, but while an increase in the enormous number of these, very few are suitable for an analyintensity of turbulence will accentuate these, it will a t the same sis of this sort, and in any case a knowledge of some of the essentime decrease the depth of penetration by shortening the period tial physical constants is at present lacking (particularly of ks,the “surface resistance” coefficient). for which any part of the liquid is exposed to the gas before being submerged once more. The theory must be regarded as speculative until it has been
-
. *
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1462
Consider a liquid which is maintained in turbulent motion by stirring a t a steady rate. The total area of the surface which is exposed to the gas will be taken as equal t o unit and the average rate of absorption is uniform over the area. &e motion of the liquid will continually replace with fresh surface those parts which are older, in the sense that they have been exposed for a finite length of time, The mean rate of production of fresh surface will be constant and equal to s, and the chance of a n element of surface being replaced within a given time is assumed to be independent of its age; hence the fractional rate of replacement of the elements belonging to any age group is equal to s. Let the area of surface comprising those elements having ages de) be +(B)d6. At steady state this does not between 0 and (0 vary with time. Hence in a short time interval equal to de the area entering the age group 0 . . , (8 4- de) from the age group (e - de) . . , 8 will be +(B)dB. This is also equal to the area in age de) , . e less the portion of this which is replaced by group (0 fresh surface in a time equal to de-i.e.,
+
.
-
. .
+@)dB
de)
Therefore
=+(e - de) dO(1 - sd8)
= +(0)
-
2
de
-
s+(e)de
a n cl Since also
J;;“
+dB
=
1
we have
9
(4)
= se-sff
For areas other than unity, +(@)deis equal to the fractiori of the area which at steady state is in the age group e . . , . (e de). ,$(e) is referred to here as the surface-age distribution function. The rate of absorption into those elements of surface having age e and combined-area s e - d d e is found from Equation 3 to be
+,
l/.s
(c” - c o ) s e - d de. Hence the mean rate of absorption per unit area of turbulent surface is
= (c*
-
c,)
dns
(5)
Comparing this with Equation 2 we see that identified with k ~ .
4%can
be
It has been tacitly assumed in the above calculation that immediately underlying all freshly formed surface is liquid with the same concentration, ca, of dissolved gas. I n identifying 4 /09 with kL, co is identified with the mean bulk concentration of the liquid; this implies t h a t the mean concentration does not change with time, and that the bulk of the liquid is of uniform concentration. The first condition is automatically fulfilled in a continuous process operating a t steady state. The extent t o vihich the second is fulfilled will depend on the depth and flow chnracteristics of the liquid. Consider a case in which a t steady state the mean concentration a t a level A , close to the surface, is C A , while t h a t a t a greater depth, B , is maintained a t the bulkor discharge concentration, co (the depth of A being comparable with the scale of turbulence). Solute is carried from A to B by eddy diffusion, and foi steady state (CX
-
ca)
4%=
(CA
-
c,)ka = (c*
-
Co)kL
or
where k . is~ the coefficient of mass transfer from A to B by eddy diffusion. s and k~ will be functions only of the physical properties and flow conditions of the liquid and the geometry of the vessel. Hence, provided these remain constant, we see from
Vol. 43, No. 6
Equation 6 t h a t for a series of solutes 1 / k ~should be linear in
l/dB. Further complications‘will be introduced if the solute undergocs chemical reaction in the liquid, or if there is a gas-film resistance or surface resistance. These are considered in connection with absorption in packed columns, and the expressions obtainrd are in general applicable to stirred liquids also. PACKED ABSORPTION TOWERS
Observation of the behavior of filaments of colored liquid suggests that the liquid running over the packing in a tower is i n some places flowing in a thin laminar film, while a t others it is in turbulent motion caused by discontinuities in the wetted surface. Most of the exposed surface appears to belong to liquid in laminar rather than turbulent flow-at least provided the rate of flow is not too great. K e consider first the case in which virtually all absorption it. assumed to take place into the surface of t’hose areas in which t,he liquid is in laminar flow. The system is assumed to be a t steady state. Consider a thin horizontal section of the packing. The section is supposed to have a volume great enough to offer a representative sample of flow conditions, but its height must be sufficiently small t’o allo~vthe bulk concentration of the liquid to be regarded as uniform. Where a stream of liquid in laminar flow encounters a discontinuity in the packing, tarbulent conditions prevail and the liquid is assumed to be thoroughly mixed before passing on to another laminar region. Fresh liquid surface, with a concentration equal (subject to local variations which will be ignored) to the mean concentration of the liquid stream, is thus being generated at. the top of these laminar regions and destroyed a t the same rate a t the points of discontinuiby. The time taken by the surface of the liquid to travel from the preceding point of discontinuit’y to some fixed point on the wetted surface of the p‘acking is termed the “age” of the surface at this point. It’is assumed t h a t the local l a t e of absorption of gas is the samc as for a stagnant liquid of infinite depth which has been exposetl to the gas for a time equal to the local age of the surface. A coniplete analysis of the assumption cannot he given here, but thc following considerations show t’hat it probably represents a reasonable approximation. A solution has been obtained (by graphical methods) for the rate of absorption of gas into a laminar layer of liquid flowing under the influence of gravity down a vertical wall. It is assumed that the parabolic velocity profile is fully developed, and that the surface is constantly saturated. The ratio of the amount of gas absorbed by an element. of moving iurface within a time e of its formation to the amount absorbed in time 0 by a,n equal area of surface of stagnant, infinitely deep 1DM where d is the liquid depends only on the quantity L = ypd4 thickness of t,he liquid layer and I is the distance which the element of surface moves in time e. The difference between the quantities absorbed in the two cases amounts to less than 575, provided L < 0.1. To give an idea of the significance of this condition, d will be given the rather low value of 0.01 em., with I = 5 em., and D = 10-6 sq. em. per second. L is then about, 0.05 (for water). Although this analysis ignores the fact that the parabolic velocity distribution is established only after a considerable distance of travel, and that the surfaces of the packing will not generally be vertical, it does indicate t h a t the assumption is likely to he justified under conditions of practical importance. While the discrepancy increases with the age of the surface, absorption into L L ~ l d e surfaces r” is relat,ively unimportant in determining the mean rate of absorption. The packing is supposed to be arranged in a completely random manner, so t h a t there is no correlation between the age of any element of moving surface and the probability of its being destroyed within a given time (the validity of this assumption is discussed under “hTonhomogeneous Distribution of Surface Ages”). Under these circumstances the age-distribution function +(e) is easily shown to be the same as t h a t already found for the stirred liquid (Equation 4)-namely, se -so. The surface-
June 1951
1463
INDUSTRIAL AND ENGINEERING CHEMISTRY
renewal factor, s, will in this case depend on the physical properties and rate of flow of the liquid and the geometry and size of the packing. If the rate of absorption into a stagnant liquid, nThich has been exposed t o the gas for a time e, is denoted by $(e), the mean rate of absorption per unit area of wetted surface in the section of packing will be
values of k ~ but , this requires confirmation. Further investigation may show the phenomenon to be one of considerable importance in determining rates of gas absorption. Until more information is available, it is not possible to say whether ICs is a function of the age of the surface, although this seems unlikely. Assuming it to be constant, we have by analogy with Equation 9
(7)
e* - eo 1 1
R =
7E+;
a
*
The function $ will take different forms according t o the behavior of the liquid and gas which are under consideration. I n if there is no gas-film resistance, and principle it may be found by solving the appropriate diffusion equations with the required boundary conditions (see discussion of derivation of expressions for R). However, the necessity for this can usually be avoided, and the mathematical treatment simplified, by making use of the f a c t that the expression for R/s, when there is gas-film resistance of appreciable magnitude. as can be seen from Equation 7, is the Laplace transform of $, SOLUTE DESTROYED BY FIRSFORDERREACTION WITH SOLVENT. The transform is considerably easier t o find than $ itself. Mean Absorption Rates for Various Types of System. The derivation of the expressions given here is discussed a t the end of this paper. SVRFACE CONSTANTLY SATURATED, NO CHEMICAL REACTION where T is the first-order reaction velocity constant. This may IN SOLUTION. be put in the form R = (c* - c,) (8)
d&
As in the analogous case of absorption into a turbulent liquid (when k E is large), fican be identified with kL. If the solvent and the flow conditions remain unchanged, k~ should be proportional to 4 0 for a series of solutes, provided they behave toward the solvent in the manner postulated. GAS-FIU RESISTANCE, No CHEMICAL REACTION. (9)
if fl is identified with k ~ the , hypothetical liquid-film coefficient for t h e absorption of the same gas in the same solvent ~ i t h out reaction. The value of s might be determined from measurements of k~ for a gas which did not react with the solvent. If there is a gas-film resistance t o absorption, Equation 12 hecomes
c*
R =
4
s
In deriving this expression it has been assumed t h a t the system obeys Henry’s law and t h a t the gas-film mass-transfer coefficient, kc, has a constant value independent of the local rate of absorption into the liquid. The latter is almost certainly an oversimplification, but the approximation is probably no more drastic than t h a t involved in the normal treatment of gas-film coefficients, and the result should give at least a general indication of the behavior to be expected from this type of system. It is interesting t o find t h a t Equation 9 is similar in form to t h a t derived by a very different path from the simple two-film theory. It is shown in the discussion of derivation of expressions for R t h a t the conventional rule for the addition of “resistances in series” can be applied t o all the types of systems here considered. SURFACE RESISTANCE,No CHEXICALREACTION.Even when there is no effective gas-film resistance t o absorption-for instance, when a pure gas is absorbed-there is some evidence that the gas and the liquid may not attain equilibrium a t the interface immediately on being brought into contact (4, 7 ) . Such a phenomenon may be explained by assuming that only a very small fraction of the gas molecules striking the surface enters the liquid, while the remainder is reflected (this hypothesis has been discussed more fully elsewhere, 4). An alternative explanation might be that there is a region in the immediate neighborhood of the liquid surface in which surface forces restrict the mobility of the solute molecules, so t h a t their diffusivity is much lower than the bulk of the liquid. In either case, the rate of absorption a t any time will be given by a n expression of the form (c* - c s ) k s , where cs (the concentration at the surface or immediately beneath it) is a function of the time of exposure. Only in the case of carbon dioxide and water is there any experimental evidence as to the value of k s ( 7 ) . The apparent value (ea. 0.05 em. per second) is in this case of the same order of magnitude as normal
- co 1
H
r;, +
(14)
d D m )
H / k c should be replaced by l / k s if only surface resistance is important, and by ( l / k s H / k c ) if both are of appreciable magnitude. The term involving co may be omitted when conditions are such t h a t the bulk concentration of unreacted absorbed gas is negligibly small. This will be the case if a t all points in the absorption tower AR e, In a simple case such as this it is, of course, easy if rC. is known to determine R from the expression
n
=
Jo’”
+(e).rC.(e).de
(19)
which is of general applicability. I n a dumped pac,king, even of elements of uniform size, may be expected t o approximate much more closely to the form of Equstion 4, since the linear rate of flow of the liquid will vary widely with the inclination of the surface, and there will be considerable variation in the distance of travel between discontinuities. It may be objected t h a t there is bound to be a finite upper limit to the age of any element of surface. However, this is not lilrely t.0 lead t o serious error in itself, provided the distribution of the “younger” parts of the surface does not depart too widely from t h a t given by Equation 4. This is because most of the absorption takes place into surface which has been exposed for only a short time, and the upper limit of the integral in Equation 7 can be given a finite value without greatly altering the value of the
Vol. 43, No. 6
integral. For instance, in the simplest type of system, in which there is n o surface or gas-film resistance or chemical reaction, the integral is (cf. Equation 5):
If the upper limit of this integral is changed to ec, it becomes on rearrangement d&erj which is within 10% of 1 / 5 8 if soc > 1.5. Tentative values of s, calculated from published values of k~ for carbon diovide arid water in packed towers (8) are of the order of magnitude 5 see.-’, indicating that less than one tenth of the observed absorption rate is due to surfaces older than 0.33 second. Nonuniform Value of s. If an appreciable fraction of the absorption in a pachcd column takes place into the surfacr of turbulrnt liquid a t the pointa of discontinuity, the form of ill again be modificd We may picture the effective area of surfacc a8 being made up of a number of areas, a, each having a burface-age distribution similar to Equation 4 but with its own value, sa, for 8 . This leads to the exprefision
aC,
If for either oi the reasons discussed above, thc function expressing the distribution of surface ages in an absorption syptem differs from se-@, and if its actual form isundetermined, systematic investigations or attempts to predict absorption ratrs in various types of system may be rendered difficult. I n the simplest case, in which there is no chemical reaction or gas-film or surface resistance, we find from Equations 8 and 19
for nonrandom packing, and from Equations 8 and 20
for nonuniform values of s. are
Similarly, the analogs of Equation 15
Equations 21 to 24 resemble Equations 8 and 15 in that they express R as the product of a term containing known (or independently measurable) physical constants, and another which depends only on flow conditions, etc., and which must be determined empirically. Thus, provided the latter term remains constant, there should be a simple and predictable relationship between the value of R and the values of the physical constants in the first term. However, if there is any gas-film or surface resistance or if a first- or second-order reaction takes place, we find that the reyultr ing expressions no longer have the relative simplicity which marks the equations referring to systems in which $ = 8e-&. For instance, Equation 9 (simple gas-film resistance) becomes
when
$
is undetermined (cf. Equation 29), and
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1951
when the value of s is not homogeneous. Thus even a system with constant flow conditions, and for which k a is known, will show no simple relationship between R and D . All t h a t can be said ie t h a t R/(c* - e,) will be a unique function of D for a series of solutes if H / k c is kept constant, and t h a t it will be a unique function of H/SGfor a series of systems in which D is constant (liquid-flow conditions assumed constant throughout). Such considerations indicate a basis for the correlation of experimental results, but are not of great practical valut-. Similar conclusions may be reached regarding systems of the types referred to in Equations 10 t o 14. dERlVATION OF EXPRESSlONS FOR R IN VARIOUS TYPES OF SYSTEM
Two methods may be used. One is to solve the appropriate partial differential equation to find +(e), the rate of absorption per unit area of surface into a stagnant liquid which has been exposed to the gas for time e. $(e) may then be multiplied by se-sO and integrated. as already explained, t o give R. This method is used in cases 1, 2, and 4 below. Alternatively, the Laplace transform may be applied t o the diffusion equation and the resulting ‘*subsidiary equation” solved for i ( ~s), , the Laplace transform of c ( r , e) ( 3 ) . R is equal to - D s ( d z / d x ) , = 0. The latter method, which is illustrated in cases 3 and 6 below, is generally considerably simpler than the former, but is applicable only when the‘ diffusion equation is linear. A general rule for the “addition of resistances” is derived below, so t h a t it is not necessary to consider the effect of an added gas-film or surface resistance for each type of system separately. 1.
1465
3. First-Order Reaction between Dissolved Gas and Solvent.
>
o > c < c ~ , x =~ . , e > o c = ca,x 0, e = c = C * , X = 0, e 0
C,L)&+r” ae ax
If conditions are such t h a t the concentration of unreacted absorbed gas in the bulk of the liquid is negligibly small (c, = 0), the solution of the above is, as has been shown elsewhere (6),
whence
This result could have been reached much more directly by the Laplace transform method, which will be illustrated by ita application t o the eneral case where eo 0. Multiplying eacx term in the differential equation by e -80, integrating with respect t o e between 0 and 03, and making use of the first boundary condition, TVC find ( 3 )
*
sC
- co =
dzE Ddx2
- rC
while the second and third boundary conditions become
where C, the Laplace transform of c, is defined by
No Reaction. Surface Constantly Saturated. C,
>
c
= c ~ , x =0 0 , e > o
e
= c,,, x 0, = C = C * , X = O , B > O
o The solution of the above is
The solution t o t,his is (1) c = co
+ (e* -
c,)er.fc
c o t
[A] de0
p * \ s )-
e =
2
whence
(r
~._le
-8
. -
+4
(33)
Now $(e), the rate of absorption at time e, is equal t o whence it can be seen from the definition of C that
bC - D (z), =
R (c*
=
-
co) d5s
C=G,
-D
(3 -
X>O,
= k(c*
c=c.,
- c),
x = m ,
whence
-
e-%(e)de
=
-sD
J m
e--s8
(2)
x = o dB
so that R is immediately found from Equation 33:
(34)
e=o x = 0, e
>0
e > O
The second condition states t h a t the rate of absorption is a t all times proportional to the difference between the surface concentration and the saturated concentration. k stands for k s or k G / H . The solution is known (.e) and may be put in the form $(e) = k(c* c,) = k(c* eo)ek20/Qwfc Lk (29)
-
sJ.;”
(28)
2. Surface or Gas-Film Resistance. This system illustrates the rule for the )addition of resistances given in Case 6. ,The differential equation is the same as in case 1, but the boundary conditions become L
=
J;]
4. Instantaneous Reaction between Absorbed Gas and Reagent in Solution. The method of solution for t h e stagnant liquid has been given (6). There is a steadily deepening zone beneath the surface which contains absorbed gas but no reagent. This is bounded by a plane a t which the concentration of both is zero, and beneath this the liquid contains reagent alone. The absorbed gas and the reagent obey the normal diffusion equations, but the boundary conditions are somewhat complicated. The solution is
(35) from which Equation 15 follows. 5. Second-Order Reaction between Absorbed Gas and Reagent in Solution. Attention will be confined t o the case in which the concentration of unreacted absorbed gas in the bulk of the liquid is negligibly small compared to c*. Putting c(x, e) = concentration of unreacted absorbed gas, ~ ’ ( 28) , = concentration of unreacted reagent (both in chemical
1466
INDUSTRIAL AND ENGINEERING CHEMISTRY
equivalents per unit volume), and T ’ = the second-order reaction velocity constant, we have for a stagnant liquid c = C*,X c = 0, x
=
Vol. 43, No. 6
where k is the appropriate mass-transfer coefficient. This transforms to
o,e > o
> 0, e = o c=O,x= m,8>0 C’ = e;, x > 0,e = o
The new solution for Cis
c’=c,I,x= m,e>O dc’ -=o,x=o,~>o dX
These equations are nonlinear and no solution has so far been found. However, Equations 31 and 32 can be used as approximations, under certain conditions, if r‘c: is substituted for r. Let us call the expressions for R and $(e) obtained in this way R1 and $ ~ ( e ) ,while the exact solutions of the above equations would lead to other expressions, Rz and &(e). Now it can be shown (6) that $2 3 $1 if r‘c*e < < 1. If r‘c*e is taken to be O.O5-i.e., e = O.OS/r’c*-the difference between and is less than 5%, hence the approximation
.JI;
0.05/r ’e*
=
e-s%(e)de
whence
(39)
O.O5/r ’ c *
s
JL
e-ao$,(B)dB
In other words, if R has the form
R
If in addition it is stipulated that
has an error less than 5%.
L0*05/T’c’ Jd” > 0.95s
e-+l(e)de
(36)
e-ao$l(e)de
sJdm
=s
e-%(e)de
But from Equations 31 and 32
Jd”
e-ao$,(e)de
=
Rz
Jo
,-
0.05(r’cd
= eyf
f w
- A ) B .\/B
R =
e*
-A
1
G5’i
I n cases such as Equation 15 this rule is ambiguous, and recourse must be had to the equation for E to determine the effect of a n added resistance.
r0.05/r’c* e--&$.l(e)de
(c*
( A and B being constants) when the surface is constantly saturated, then for the case where there is a resistance l / k at t h e surface
the following approximation can be used with an error of less than 10%: R1=
=
These arguments are easily extended to the case where there are two resistances a t the surface.
+ s) -
T’p*
CONCLUSION
JU
The value of the ratio is seen to depend both on c:/c* and on P’c:/s, but because interest centers on systems in which s and T’c.) are of comparable magnitude-i.e., diffusion and reaction play roughly equal parts in determining the absorption ratewe may put r‘c,/s = 1 for purposes of illustration. It is then found that c : / c * > 50 if Equation 36 is to be complied with. 6 . Rule for Addition of Resistances. I n all types of systems considered here the equation for C(x,s),the Laplace transform of the concentration a t x, has the following form: dzE _
- aE
dx2 -
D
b -
5
(37)
where a and b are constants. I n the case where there is no surface or gas-film resistance, so that the surface of the liquid is con-
stantly saturated with the gas, the boundary condition a t the surface becomes
whence
The considerations advanced in this paper raise two distinct questions, which can be answered only by experiment. 1. Is the surface-renewal mechanism here postulated closer t o the truth than the conventional picture of an undisturbed layer a t the surface of the liquid? 2. If so, does the distribution of surface ages in absorption equipment of practical interest approximate to Equation 41 It should be possible to shed some light on the hrst by qualitative observations on stirred and flowing liquids, without reference to absorption. If the answer should prove to be “yes,” then the conventional picture is misleading, and should not be used as the basis for theories of absorption although its terminology would probably be retained as a matter of convenience. As regards the second, careful measurements of rates of absorption, and of the relevant physical quantities, such as k s , would be required to determine thP answer; it mould be convenient if this should prove to be “yes,” because a precise mathematical treatment could then be employed, as described above. Should the answer to the second question prove to be “no,” the predictions of the theory lose a great deal of their precision and utility, but this in itself would not be a valid reason for preferring the “stagnant film” hypothesis as a basis for further theoretical developments. IVOM ENC LATURE
and
If, however, there is a constant resistance a t the surface, the boundary condition is changed to
A = area of wetted surface per unit volume of packing n = area of those parts of absorbing surface having value sa for s c = concentration of absorbed gas in liquid a t x, 8 c* = saturated concentration of gas in liquid co = initial or bulk concentration of gas in liquid e’ = concentration of reagent in solution C: = initial or bulk Concentration of reagent
E
= Laplace transform of c =
Lm-
D = diffusivity of absorbed gas
ce
@de
June 1951
. f
INDUSTRIAL AND ENGINEERING CHEMISTRY‘
D‘ = diffusivity of reagent in solution d = thickness of liquid layer on packing or other solid surface H = Henry’s law constant k = generalized mass-transfer coefficient kr, = liquid-film mass-transfer coefficient kc = gas-film mass-transfer coefficient (partial-pressure units) ks = mass-transfer coefficient for surface resistance (concentration units) kE = mass-transfer coefficient for eddy diffusion I = distance traveled in time e by element of surface of moving liquid R = mean rate of absorption per unit area of nonstagnant liquid r = velocity constant for first-order reaction between absorbed gas and liquid $”’ velocity constant for second-order reaction between absorbed gas and reagent in solution s = fractional rate of renewal of surface of liquid so = value of s for area a volume of liquid per unit volume of packing X L = “effective thickness of liquid film” x = distance beneath surface of liquid B = quantity defined by Equation 16 e = time for which a liquid surface has been exposed to gas, “age” of surface * = rate of absorption into unit area of surface of stagnant liquid
1467
4 = surface-age distribution function M = viscosity of liquid p = densityofliquid erf(z) =
1 - erfc(z)
=
e -Pdy (numerical values may
be found in tables) LITERATURE CITED
(1) Carslaw, H. S., and Jaeger, J. C., “Conduction of Heat in
Solids,” p. 43, Oxford University Press, 1947. (2)Ibid., p. 53. ( 3 ) Ibid., p. 240. (4) Danokwerts, P. V., Research, 2, 494 (1949). (5) Danokwerts, P. Trans. Faraday Soc., 46,300 (1950). (6)Ibid., p. 701. (7) Higbie, R., Trans. Am. Inst. Chem. Engrs., 31, 65 (1935). (8) Perry, J. H., “Chemical Engineer’s Handbook,” pp. 1179. 1184, New York, MoGraw-Hill Book Co., 1941. (9) Sherwood, T. K., “Absorption and Extraction,” p. 61, New York,
v.,
v =
McGraw-Hill Book Co., 1937. (10) Ibid., p. 196. (11) Ibid., p. 202. RECEIVED August 8,1950.
Air-Water Contact Operations in a Packed Column
Engrnyring Process development I
FUMITAKE YOSHIDA
AND
TATSUO TANAKA
DEPARTMENT OF CHEMICAL ENGINEERING, KYOTO UNIVERSITY, KYOTO, J A P A N
S
1
1MULTANEOUS interphase transfer of heat and water vapor between air and water flowing countercurrently in packed columns is of considerable engineering importance in humidifiers, dehumidifiers, and water coolers. T h e performance of this equipment used t o be expressed in terms of the over-all coefficient of heat or mass transfer, or in the over-all height of transfer unit (H.T.U.), the bulk-water temperature usually being assumed equal t o the interfacial temperature. However, this assumption is indisputably valid only in the case of constant water temperature humidification, in which the heat given up by air is wholly consumed for evaporating water and hence no heat transfer takes place across the water film. T h e present work was intended (I) t o study the effects of the gas and water rates on the true gas-film coefficients of heat and mass transfer from the constant water temperature data; (2) t o investigate how the water-film resistance, if any, was affected by the gas and water rates from the water-cooling data; (3) to study whether the correlations for the film coefficients obtained from the above tw8 operations were applicable to the dehumidification data. This work was undertaken before the similar study by McAdams and coworkers ( a ) was published. Some discrepancies exist between the results of the two investigations. EXPERIMENTAL
T h e schematic diagram of the apparatus is shown in Figure 1. The column (Figure 2) was 10 inches (25 om.) in inside diameter and was dumped-packed with 15-, 25-, or 35-mm. ceramic Rasohig rings t o a depth of 12.5 inches. It was necessary to
T h e work was undertaken to study the film coefficients of mass and heat transfer in the three kinds of air-water contact operations-i.e., constant water temperature humidification, water cooling, and dehumidification, in columns packed with ceramic Raschig rings. The results showed that liquid-film resistance was not negligible as compared with gas-film resistance and that the same empirical equations for gas- and liquid-film coefficients could practically correlate the performance of a packed column throughout the three operations. The gasfilm coefficients of heat and mass transfer were proportional to gas rate and to the 0.2 power of water rate, while the liquid-film coefficient of heat transfer was proportional to the 0.8 power of water rate. The ratio of eas-film coefficient of heat transfer to that of mass transfer nearly equaled the humid heat of air in the column. The correlations obtained should make it possible to design packed column-type air-water contact apparatus on a sounder basis.
make the packed section relatively short in order to obtain substantial driving potential at the top of t h e packin . Due care was taken in the column design t o minimize the e n 3 effects owing to the sprays above apd below the packed section. As shown in Figure 2, nineteen overflow pipes from t h e waterdistributing tray were extended down t o t h e top of the packing, thus eliminating the air-water contact above the packed section. Below the packing, there were a tray with air risers and an overflow ipe through which water was drawn out of the column. The c h a n c e between the water level on the tray and the bottom
