CONTINUOUS SEPARATION OF ARGON-HELIUM GAS MIXTURES IN A GLOW DISCHARGE JAMES
E. F L I N N ' A N D
ROBERT H. P R I C E
Department of Chemical Engineering, Cniwrsity of Cincinnati, Cincinnati, Ohio
In a novel electrical method of separating a gas mixture, argon-helium mixtures a t very low flow rates are separated continuously by passing them through a glow discharge device, where the argon is selectively ionized and transported toward the cathode. At a current level of 270 ma. equivalent argon ion flows on the order of 1 (mm. Hg)(cc./sec.) a t room temperature are obtainable. Increasing the flow rate in the range
studied decreases the concentration of both exit streams, while increasing the net quantity of argon separated. This is due to a reduction in the back-diffusion of neutral argon toward the anode. Pressures between l and 10 mm. of Hg, currents up to 270 ma., and flow rates of about 25, 51, and 76 (mm. Hg) (cc./sec.) are investigated for 5.43, 21.30, and 53.74% argon mixtures. Ion currents estimated from the experimental data are compared with theory.
a pure rare gas is made the conducting medium in a static direct current electrical discharge device, such as a glow discharge (4): positive ions of the gas are formed and, because of the electrical field present, drift toward the cathode end of the device. By replacing the pure gas with a binary mixture, the components of which differ in ionization potential, the component having the lower ionization potential can be separated: since ions of this component tend to predominate in the discharge. The degree of separation attainable in a static glo\v discharge has been measured for mixtures of argon, helium. and neon by Skaupy ( g ) , Skaupy and Bobek ( 7 0 ) , Matveeva ( R ) , and Frish and hlatveeva (5). This phenomenon is not limited to rare gas mixtures. as the early observations of Baly ( 7 ) indicate. HoLvever, which component of molecular gases will separate to the cathode cannot be predicted on the basis of ionization potential alone, inasmuch as dissociation and reaction bet\veen dissociation products can occur. In this study various argon-helium mixtures were passed continuously to the center of the positive column or luminous portion of a glow discharge and the argon concentration of the anode and cathode exit streams \vas determined for system pressures bet\veen 1 and 10 mm. of Hg: flow rates of 25.5. 51.0, and 76.5 (mm. Hg)(cc. /set.), and currents up to 270 ma. The degree of separation \vas ascertained by measuring the mole fraction of argon in both the cathode and anode exit streams. The concentration of argon in these streams was determined by a thermal conductivity method. From the data it was possible to estimate the magnitude of the ion flow in the discharge for comparison \vith values calculated from theory. HES
Experimental
Apparatus. The discharge device used is shown in Figure 1 . The gas mixtures to be separated entered the center of a 77-cm. length of 8-mm.-I.D. borosilicate tubing. connected a t each end to an electrode chamber in which tubular aluminum electrodes were mounted in a n upright position on tungsten wires protruding into the chambers. These wires provided thr connection to the external direct current power supply. 1
Present address, Battelle Memorial Institute, Columbus, Ohio.
The two electrode chambers were provided with side connections through which the separated mixtures flowed. O n leaving the electrode chambers the two exit streams passed through a 150-cc. sample bulb and then through a Teflon-glass needle valve to the vacuum pump. These needle valves were used to adjust the system pressure and the ratio of the anode and cathode flow to the inlet flow. The inlet gas flow to the system \.vas measured by means of a calibrated capillary tube. The system pressure was monitored continuously by means of a direct-reading Dubrovin vacuum gage having a range of 0.2 to 20 mm. of Hg and a sensitivity nine times that of a n ordinary mercury manometer. 'This gage was attached a t the anode end of the system, as indicated in Figure 1 , The gas samples obtained from the two exit streams \\.ere analyzed \vith a microthermal Conductivity gage similar to one described by Grew and Ibbs (6). The measuring and reference cells of this gage were made by mounting a tungsten filament from a 60-\vatt light bulb axially in a length of 1.9-mm.I.D. borosilicate capillary tubing. 4 sample \vas admitted to the cell by way of a side connection. The measuring cell was mounted atop a McLeod gage having a bulb volume of about 300 cc. The assembled gage is sho\vn in Figure 2. The sample to be analyzed was admitted to the 300-cc. bulb a t about 0.1 mm. of Hg and then compressed by raising the mercury level to a reference pressure of 50.0mm. of Hg. The measuring and reference cells were immersed in a water bath maintained a t 35' C. and formed thvo parallel legs of a h'heatstone bridge circuit. Current to the bridge was from a 12-volt storage battery. Procedure. The gas mixtures to be separated were obtained premixed in high pressure cylinders. Three mixtures, having compositions of 5.43. 21.30. and 53.747, argon. were investigated Two criteria had to be met before the data on the separation achieved were considered acceptable: The system had to be a t steady state, and the molar ratio of the cathode (or anode) flow to the inlet gas flow had to be 0.5. Steady state was considered to have been achieved when t\vo or more sets of samples, each comprising one anode and one cathode sample taken a t 15- to 20-minute intervals, did not differ significantly. The way in which the inlet flow was divided into two equal molar flow streams can be illustrated by reference to a material balance on the system. The flow quantity, Q (mm. Hg) (cc./sec.), used consistently in these experiments is defined in terms of the perfect gas law as
Q
=
nR,T = PV
(1) Hence, Q is proportional to the moles per second of gas flo\ving a t a given temperature level. A total mass balance around VOL. 5
NO. 1
JANUARY
1966
75
IOcm.
~
.._
7 0 cm, 35cm,
1 6mm.l.D
vacuum
9 inlet gas
to Dubrovin gouQe exit g a s
o n o d e chomber
cothode chomber
-7
35 mm I.D. tungsten wire Figure 1
Details of glow discharge device
respectively. Combining Equations 2 and 3 to eliminate Q a , one can obtain the molar ratio of the cathode to inlet flow in terms of the inlet and exit concentrations as
SAMPLE
r-"
THERMOMETERk
Therefore, to achieve the second of the two criteria mentioned it was necessary to measure the argon concentration of both exit streams with the system at steady state, and to use Equation 4 to calculate the ratio Qc/'Q. If the calculated ratio was not 0.5 within the limits of experimental error, a slight adjustment in the two outlet needle valves would be made and new samples taken after allowing some time for steady state again to be achieved. Usually, two or three trials were required to obtain the desired ratio, but this was not a great problem, as steady state was achieved quickly after a disturbance to the system. The current levels generally were set a t 90, 180, and 270 ma. Currents higher than 270 ma. caused the cathode electrode to begin melting because of excessive heating. The gas temperature in the heated discharge was roughly determined prior to many of the runs by measuring the initial rapid rise in the system pressure that occurred when the discharge was initiated at a specified current level, flow rate, and pressure. The gas temperature was then calculated from the pressure rise by assuming that the system consisted of a heated and unheated volume, and that the total moles of gas in the system before and after ignition did not change in the short time interval required for the measurement. Results
U
Figure 2. Microthermal conductivity cells mounted atop McLeod gage
the system, assuming that all the flow streams enter and leave at the same temperature, is
Q
=
Qa
+
Qc
(2)
where Q, Qa, and Qc are the inlet, anode exit, and cathode exit flow rates. An argon mass balance is xoQ = x,Qa
f xcQc
(3)
where x o 3 x,! and x c refer to the argon concentration in mole fraction of the inlet. anode exit, and cathode exit streams, 76
I&EC
PROCESS DESIGN A N D DEVELOPMENT
The experimental data are plotted as the concentration difference in mole fraction of argon between the cathode and anode exit streams os. current level for the various flow rates and pressures investigated, as shown in Figures 3 to 5 for the three gas mixtures. For each mixture the concentration difference increases with the current and decreases with increasing flow rate. The effect of changing pressure and gas composition is more easily summarized by reference to Figure 6, wherein, for a current level of 270 ma. and a flow rate of about 51 (mm. Hg)(cc./sec.), the concentration difference is plotted against pressure for each mixture. Below about 6 mm. of Hg the concentration difference is greatest for the 21.30% mixture; above 6 mm. of Hg the 5,43TG mixture gives the greater difference. Actually the 21 ,30Y0 mixture shows very little pressure dependence. The 53,747, mixture likewise appears pressure dependent only above about 5 mm. of Hg. The over-all dependence of pressure and gas concentration on the separation for the three mixtures is small relative to the effect of current and flow rate.
-
D
x
Y
-
-
.04 -
I
.06
-
__-e
I
50
0
Figure
100
I50 200 Current - m a .
2 50
300
Figure 5.
3. Concentration difference vs. discharge current
I
50
0
-
I
x
.OB -
0
X
I
I 250
-
I
300
xo't 1
2mmHg I mm.Hg
5.43%
2130%
.os
I
X"
-
- --
53.74x
.04
-
.02o .O3tL-----
.04
I 200
5 3 . 7 4 7 0 argon mixture
xu
.06
I I I50 Currant - m a . '
Concentration difference vs. discharge current
5 . 4 3 % argon mixture
.IO
I
100
-
-
-
.-.- 7 6 . 4 "
'*
,I
I
2
s
4
3
Figure 6.
6
r
s
8
IO
- mm.Hg.
P,
Effect of pressure on concentration difference 270 ma., and 51 (mm. Hg)(cc./rec.)
1.0
0
Figure 4.
50
100
150 Currant
- ma.
200
250
.8 -
300
Concentration difference vs. discharge current 2 1.30% argon mixture
0'
*
l
~
l
~
[
~ ,/'
/
cm?-mm.Hg/rec. ---- 25.5 51.3 _.
*I
(I
/
II
76.4
,/.;.'
-
\ 0
./
,/'
./
-
./. /' '
;
,'
u
-
./''
By replotting the data in terms of the quantity Q ( x C - x,)/4 us. current, curves like that shown in Figure 7 for the 21.30% mixture are obtained. Q ( x c - x,)/4 represents the net quan-
tity of argon transported to the cathode (or anode) per unit time. Figure 7 shows that increasing the flow rate actually increases the net quantity of argon separated, even though the concentration difference across the discharge is reduced by the increased flow. The average gas temperatures in the discharge increased with current, as the values of Table I indicate. The dependence of the gas temperature on pressure and flow rate could not be determined with any significance by the measuring technique described. VOL. 5
NO. 1
JANUARY
1 9 6 6
77
l
Table I. Average Gas Temperature in Discharge Current, Temp.,
Table II.
%
K. 340 390 450
Ma.
Argon
O
90
180 270
5 43 21 30 53 74
Typical values of the voltage drop across the electrodes are given in Table I1 as a function of percentage argon in the inlet mixture. pressure, and current. T h e flow rate did not affect the voltage across the discharge to any measurable extent. T h e data trends in the table are in accord with earlier static discharge studies. Discussion
I n a self-sustaining glow discharge of the type used ions are continually being generated in the body of the gas between the two electrodes. T h e ions move both radially toward the walls by a process known as ambipolar diffusion ( 7 ) , and longitudinally toward the cathode because of the potential drop along the discharge length. Upon arriving a t the wall a n ion is neutralized by recombination with a n electron there. At low pressure (1 to 10 mm. of Hg) the number of ions generated in a unit length of the positive column equals the number of ions neutralized at the wall of the discharge tube. Thus a n atom of the gas being ionized is ionized numerous times in the course of being transported to the cathode. T h e separation that occurs in a gas mixture in a static glow discharge is due for the most part to the net result of this ion transport toward the cathode and the back-diffusion of neutral atoms due to the established concentration gradient. T h e magnitude of the ion transport toward the cathode is a function of the discharge tube radius, gas pressure, current level, and gas concentration, among other things. I n a flow system there is the added complication of the bulk gas flow. At the flow rates and pressures of these experiments the bulk gas velocity is small relative to the ion and electron velocities in the discharge, as the calculated data of Table I11 indicate. Consequently, the bulk gas flow can be expected to have little or no effect on the ion current in the discharge. Model. T o facilitate the discussion of the experimental results a model of the separation system is given in Figure 8. T h e gas mixture of composition xo enters the system a t z = 0 a t room temperature T,, and flow rate Q. This inlet flow divides into two streams toward the anode and cathode. In the system the gas temperature is T,. At the dividing point z = 0 the bulk gas flow is essentially zero, but a n argon ion current, Qi’,a t the system temperature is moving toward the cathode. This ion current can be reduced to a n equivalent value at room temperature, Qi,by the expression Qi
= Qt’(TJTs)
(5)
Also a t the dividing point neutral argon atoms are diffusing toward the anode a t a rate Q d ’ (or Q d a t room temperature), as a result of the concentration gradient at that point. A mass balance around either the anode or cathode half of the system of Figure 8 will show that the net quantity of argon separated, for the condition of Qc;Q = 0.5, can be expressed as
Potential Drop across Electrodes in Volts 1 .Mm. Hg__ 5 .Mm. _ _ Hg ~ ~ ~_ 780 mu. 270 ma. 180 mu. 270 mu.
1025 1100 825
78
I & E C P R O C E S S DESIGN AND DEVELOPMENT
1000 750
1500 1500
1425 1300
950
850
estimate the ion current. Qi,Lvhich is causing the observed separation. Interpretation of Results. T h e increase in the concentration difference shown in Figures 3 to 5 with an increase in the discharge current is to be expected, since in the positive column of a glow discharge the positive ion concentration remains approximately equal to the electron concentration. Increasing the electron current through the discharge then increases proportionally the ion current Q i toward the cathode. T h e data, however. indicate that as the current increases to higher values the separation departs somewhat from a linear relationrhip. This is due in part to the increased back-diffusion of neutral argon which occurs at the higher gas temperature (Table I), and consequently a t the lower gas density that prevails in the discharge as the current is increased. This departure decreases slightly as the flow rate and pressure increase. Figures 3 to 5 also indicate that as the gas flow to the discharge is increased the concentration difference between the electrodes decreases. This ivould be expected unless in some way the ion current were increased by the increase in gas flow. Figure 7 on the other hand shows the importance of diffusion of neutral argon in the system; for as the flow rate is increased the net quantity of argon transported increases. If the ion current is assumed to be relatively unaffected by the gas flow: then the increase in argon transported can be explained in terms of a reduction in the back-diffusion of neutral argon toward the anode as a result of the lowered concentration gradients along the discharge length. In terms of the model of Figure 8> the diffusion flow of argon toward the anode (reduced to room temperature) a t the inlet point z = 0 can be written as
so that. with a constant ion flow, Qi,a reduction in dt, dz at the inlet would increase the net argon transported, since Q d in Equation 6 would be decreased. T h e diffusivity, D.of Equation 7 for a gas mixture is inversely proportional to pressure, so that Q d is independent of the system pressure. Furthermore, a t low pressure D is independent of the gas composition. T h e effects of pressure and argon concentration on the concentration difference noted in Figure 6 are not so easily explainable. At a pressure of 1.5 mm. of H g the results are qualitatively comparable with Matveeva’s (8) studies in a static glow discharge. in that the concentration difference increases as the argon percentage is increased to 21.30y0 from 5.4397,.
r
L
-zy J.
L
1
Ts
Q,
anode section
Lvhere Q iand Q d are the ion and diffusion flows a t conditions existing a t the inlet point z = 0. This result can be used to
1100
Q,xo,T,
cathode section
1’0
Figure 8.
Model of separation system
Qc
lXC
_
Table 111.
Calculated Particle Velocities in a Glow Discharge” Magnitude, Type Velocity Cm ./Sec. Bulk gas in discharge 11.5 Ion, longitudinal drift 3.000 Ion, radial drift 3,400 Argon atoms, thermal 53,000 Electrons. thermal 107 a Calculatpdfor discharge radius of 0.4 cm.; gas pressure of 10 mm. Hg; gas temperature of 450’ K.; 53.747~argon mixture; and inletjow rate of 77 ( m m . Hg)(cc./rec.) at 300’ K .
Table IV.
Ion Current, Qt ( M m . H g ) ( C c . / S e c . )
yo A r 5.43
21.30
53.74
T h e theory for the positive column of a glow discharge operating with a gas mixture predicts a decrease in the ion current with increasing pressure and percentage argon. This is the case when the assumption is made that only A r + and H e + atomic ions constitute the ion current, and that these ions are formed by direct electron impact. The authors believe that the pressure and concentration effects observed in these experiments, and in other static discharge studies. are due in the main to a number of pressureand concentration-dependent ion formation and loss reactions such occurring in the positive column of the discharge. T\VO possible reactions involve the formation of molecular argon and helium ions in the positive column by the HornbeckMolnar and three-body impact processes (7). T h e HornbeckMolnar mechanism involves the formation of a molecular rare gas ion by collision of a neutral atom with a n atom excited to a high lying state: symbolized by
Ion Currents Estimated from Experimental Data Pa, M m . Hg
Q =
Q=
Q =
1.0 2.0 5.0 10.0 1.0 5.0 10.0 1.0 5.0 10.0
0.684 0.84 0.96
0.706 0.86 0.97 0.98 1.13 1.22 1.18 1.50 1.60 1.16
..
25.5
31.0
...
1.11 1.13 ... 1.41 1.37 , . .
Qt aL.
76.5
... ...
... ... 1.19 ... ...
1.32 ...
as the concentration profile for the cathode section. result for the anode section is
.
0.695 0.850 0.965 0.980 1.12 1.18 1.18 1.45 1.43 1.16
A similar
As a consequence of the model, the concentration gradients a t z = 0. obtainable from these two expressions, differ somewhat. Since Q i is small relative to Q in the expressions. an arithmetic average of the t\vo evaluated gradients can be used :
R + e = R + + e
R*
+ R = RZ+ + e
(8)
This reaction is pressure-dependent below a critical pressuse which is such that the time between collisions of the neutral R atoms is on the order of the lifetime of the excited R+ atom. In the positive column of a glow discharge many excited gas atoms are formed along with the ions by electron iwpact, so that this mechanism is of interest. The three-body impact process can be written as
R+
+ 2R
=
R2+
+R
(3)
This reaction is obviously pressure-dependent, and in addition, depends upon the concentration of atomic ions previously formed by electron impact or some other process. The formation of a molecular ion of argon would result in two atoms of argon being transported toward the cathode. This latter reaction for the argon-helium system can be shown to be the more important above a pressure of about 3 mm. of Hg. Both reactions are concrntration-dependent when occurring in a gas mixture. Ion Currents Estimated from Experimental Data. Using Equation 6. values for the ion flow, Q r , can be estimated from the experimental data, providing that the diffusion gradient a t the inlet point can be calculated for the determination of Qd. This can be done by considering the system of Figure 8 as consisting of two sections, a n anode and a cathode section. In the cathode section the bulk gas flow is in the same direction as the ion flow and both oppose the diffusion of neutral argon. In the anode section the ion flow opposes both the bulk gas and tha diffusion flows. If one makes a mass balance on a differential segment in the cathode section, assuming that the ion current is constant along the discharge, and uses the boundary conditions that a t 2 = 0. x = Y,. and a t t = L. x = x c . one obtains
Substituting Equation 1 2 into Equation 7 : and the result into Equation 6, one obtains after rearrangement
since for equal molar exit stream flows ( x , - x,)
= (xo
- xu)
= (x,
- xu)/2
(14)
Equation 1 3 can be solved by trial and error, using the available experimental data, to obtain values of Q I . T h e results of such a calculation for a discharge current of 270 ma, are given in Table IV. The values in Table I\’ indicate that a t 270 ma. the equivalent argon ion current, Q t , observed experimentally is on the order of 1 (mm. Hg)(cc.;sec.). The Q t values increase with the percent argon in the inlet mixture, and with pressure in the case of the 5.43y0 argon mixture, but do not change much with flow rate. The last column in the table is the average of the preceding three. Figure 9 shows the shape of the concentration profiles calculated from Equations 10 and 11 for the experimental data obtained with the 2 1 . 3 0 7 , argon mixture a t 270 ma., 5.0 mm. of Hg, and inlet flow rates of 25.5, 5 1 . 3 , and 76.4 (mm. Hg) (cc. Isec.). At the higher flow rate the gradient at z = 0 is close to zero, so that a further increase in flow should give little or no increase in the net quantity of argon separated, and a reduction in the anode and cathode concentrations. Comparison with Theory. Theoretical values of Q iwere calculated in a development presented elsewhere ( 3 ) and are compared with the experimentally determined values in Table
V. In making the calculations for Table \’ it \vas necessary to make assumptions regarding the type of ions constituting the VOL. 5
NO. 1
JANUARY
1966
79
I" .--
/ 10
-.Ol
/
20
-
30
40
Z cm.
- .02
C
- .03
0
- .04 -.OS
A
Figure
9.
Calculated concentration profiles for various flow rates 21.30% argon mixture a t 270 ma., and 5 rnm. Hg (Mm. Hg)(Cc./Sec.) A. 25.5 B. 51.3 C. 76.4
discharge current and their formation reactions. T h e usual and simplest assumption in the absence of experimental observations is that only Ar + and H e ions are formed by electron impact. T h e values of Q r calculated in this case (column 3 of Table V) were found to be of the correct order of magnitude only for the 5.4376 mixture. T h e results also show the wrong pressure and concentration dependence. Some recent rate constant data of Dahler, Franklin, Munson, and Field ( 2 ) made it possible to extend the calculations to include the effect of molecular ion formation by the HornbeckMolnar and three-body impact processes described. The results shown in column 4. Table V, indicate only a slight improvement a t the higher pressures and argon concentrations, but are still not satisfactory. This lack of agreement +
Table V.
Ion Currents from Theory Compared with Experimental Values Ion Current. Q , ,_( M m . Hg)(Cc./Spc.) ~ _ _ ~.. -__ ~
P8,
Vc AT 5 43
21 30 53 74
M m . HZ
1 0 2 0 5 10 1 5
0 0 0 0 10 0 1. o 5.0
10.0
80
l&EC
A r + only 0 804 0 780 0 706 0 641
0 344 0 254 0 229 0.121
0.085 0.075
A r z + and Ar
810 783 657 538 358 397 444 0.140 0.187 0 0 0 0 0 0 0
0.170
Expti. 0 695 0 850 0 965 0 980 1 12 1 18 1 18 1,45 1.43 1.16
PROCESS DESIGN AND DEVELOPMENT
between the experimental and calculated values of Table \. is probably due in part to an inadequate theory, and in part to a lack of knowledge as to the type and relative numbers of ions which constitute the discharge current in the case of a gas mixture, Flow Ratio and Discharge Length. With the foregoing picture of the separation mechanism one can anticipate the effect of changing the ratio of the cathode floiv rate to the inlet flow rate: and of increasing or decreasing the length of the discharge tube. Increasing the ratio Qcl Q ? all other conditions remaining fixed, would reduce thc concmtration difference betwern the cathode and the inlet, but increase the ne1 quantity of argon transported and, hence, removed at the cathode per unit time. At the same time the concentration difference betxvern the anode and the inlrt \vould increasr. Decreasing the ratio Q c . Q Lvould have the opposite effect--i.e.. giving a higher cathode concentration but less argon transported due to an increase in the back-diffusion toward the anode. These results were observed expcrimen tally lvhile adjusting thP s).strni to give data representative of a Qc Q ratio of 0.5. Increasing the discharge length would increase the concentration difference between the two rlectrodes as well as the net quantity of argon separated. This can be Seen by examination of Equation 1 2 or 13. First of all Q iis not in general a function of the discharge length, but rather of the discharge radius. I n Equation 1 2 increasing L \\-auld tend to reduce the gradient dx/'dz a t the inlet point. and hcnce Q d . Therefore. a larger concentration difference between the cathode (or anode) and the inlet point could be maintained with the same or smaller gradient a t the inlet. Actually. Frish and Matvceva (5)
= = = =
cathode exit stream concentration, mole fraction argon anode exit stream concentration, mole fraction argon inlet gas stream concentration, mole fraction argon distance along discharge, cm.
investigated the effect of‘ changing the discharge length in a static dixharge, and found that the separation in terms of the conccritration ditrerence between the t\vo electrodes was increased.
xc xa
Nomenclature
literature Cited
discharge tube cross section? sq. cm. diftusion coefficient? s q cm.;sec. I, = rciuivalent discharge length between inlet point and anode (or cathode), cm. Py = sbsteni prrwire, mm. Ilg Q = inlrt gas fiow rate. (rnm. Hg)(cc.,!sec.) Q,, = diffusion flo\\,: (mm. Hg)(cc. Xr/’sec.)! a t room temperature 7‘, Q , z ’ = diffusion floiv a t system temperature T A 0, = equivalent ion current at room temperature, (mm. Hg) (cc. .4r ‘ w c . ) Q 2 ’=- cq\iivalent ion currcnt at system temperature R = neiitral rare gas atom K * = excited rare gas atom R.’ = atomic rare gas ion R, = molecular rare gas ion 7’, = room temperature. O K . 7.. = s)btt‘m temperature, O E;.
(1) Baly, E. C. C , London, Edinburgh Dublin Phil. .Wag. 35, Ser. 5,
A
= =
D
+
X,
2
200 (1893). (2) Dahler, J. S., Franklin, J. I>.,Munson, M. S. B., Field, F. H., J . Chem. Phys. 36(12), 3332 (1962). (3) Flinn, J. E., doctoral dissertation, University of Cincinnati, 1965 ; in press, University of Michigan microfilm. (4) Francis, G., “Handbuch der Physik,” Vol. XXI, pp. 53-203 (in English), Springer-Verlag, Berlin, 1956. (5) Frish, S. E., Matveeva, N. A,, Soviet Phys. 3(5): 971 (1958). (6) Grew: K. E., Ibbs, T. L., “Thermal Diffusion in Gases.” Cambridge University Press, Loridon, 1952. (7) Lo&, L. B.: “Basic Processes of Gaseous Electronics,” p. 107, UniLersity of California Press, Berkeley and Los Angeles, 1961. (8) Matveeva, N. 4.,Bull. Acad. Sa., USSR, Phys. Ser. 23, 1009 (1959). (9) Skaupy, F., Verh. deut. Physik. Ges. 18, 230-2 (1916). (10) Skaupy, F., Bobek, F., 2. Tech. Phys. 6,284 (1925). RECEIVED for review February 23, 1965 ACCEPTED July 19, 1965
HYDROLYSIS OF CELLULOSE ESTERS C A R L J . M A L M , R . E. G L E G G , J A N E T T. S A L Z E R , D. F. INGERICK, AND LEO J. T A N G H E Cellulose Technology Division, Eustman Kodak Go., Rochester, N . Y .
To understand the factors that influence the breakdown of cellulose acetate, propionate, a n d butyrate during hydrolysis, studies w e r e m a d e with triesters in solution in aliphatic acid-water solutions with sulfuric acid catalyst. An increase in water Concentration caused a d e c r e a s e in the rates of hydrolysis of the cellulose butyrate a n d propionate but not the a c e t a t e ; it also caused a d e c r e a s e in the r a t e of viscosity reduction, the amount of viscosity reduction for a given d e g r e e of hydrolysis, a n d the acidity a s measured b y the acidity function, H,. T h e effect of the acyl group in the ester a n d in the solvent w a s investigated independently und together. With different esters in acetic acid-water solutions the r a t e of hydrolysis and of viscosity reduction d e c r e a s e d in the order: acetyl, propionyl, n-butyryl; with the s a m e ester a n d different acyl groups in the solvent, the o r d e r is reversed, even when t h e results are corrected for re-esterification. T h e s a m e effect w a s previously found for the viscosity reduction of triesters in aliphatic acid-anhydride mixtures. The relative rates of hydrolysis a n d viscosity reduction of cellulose a c e t a t e in acetic acid-water, cellulose propionate in propionic acid-water, a n d cellulose butyrate in butyric acid-water d e p e n d on the concentration of the w a t e r in the solution. (3: 4>6) described the various factors that P influence . bvork the viscosity reduction of cellulose esters during KEVIOL‘S
the process of esterification. T h e present experiments extend this work to include factors influencing viscosity reduction during the hydrolysis of cellulose esters. I n the dope process for cellulose esterification the first product is a triester which sometimes is hydrolyzed to improve its solubility ; commercial cellulose esters are hydrolyzed to less than one hydroxyl group per dnhydroglucose unit, most to less than 0.5. ‘The viscosity reduction during hydrolysis rriust be minimized because the major viscosity rediiction has already occurred during esterification. Cellulose triesters were studied in aliphatic acid-water solutions containing sulfuric acid calalyst. Measurements were made of the increase in hydroxyl groups and the decrease in viscosity \vith time, under the influence of such variables as trrnperature, water concentration, and acyl group in the solvent
and ester. T h e results help to define the conditions best suited to the hydrolysis of cellulose acetate in acetic acid-water, cellulose propionate in propionic acid-water, and cellulose butyrate in butyric acid-water. Acidity function, H,, was measured in the various solvents a t different levels of water concentration in a n attempt to find correlatioris Lyith rates of hydiolysis and viscosity reduction. Materials a n d Methods Cellulose Esters. ‘I’he experiments were done with a n acetate, propionate, and butyrate that were essentially triesters (Table I) made from cotton linters. Hydrolysis. T h e hydrolysis was studied in acid-water solutions containing 2 to 207, water. T h e esters were dried a t 110’ C. for 2 hours before dissolving in the solvent. LVhen the ester was completely dissolved (by warming), the temperature was adjusted, the catalyst added with vigorous stirring, and the sample placed in a constant temperature bath. VOL. 5
NO. 1
JANUARY
1966
81
