I n d . E n g . C h e m . Res. 1989, 28, 814-824
814
Temperature Effects on the Performance of Thin-Film Composite, Aromatic Polyamide Membranes Hussein Mehdizadeh and James M. Dickson" D e p a r t m e n t of Chemical Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7
Peter K. Eriksson FilmTec Corporation, 7200 Ohms Lane, Minneapolis, Minnesota 55435
T h e effects of temperature on the performance of four thin-film composite, aromatic polyamide commercial membranes (FilmTec, FT30 membranes) have been investigated experimentally. The operating conditions cover a temperature range of 5-60 "C and a pressure range of 350-7000 kPa, and the feed solution used is a 2000 ppm aqueous solution of sodium chloride. In general, it has been found that separation is independent of temperature a t higher pressures. However, for lower pressures, separation decreases, passes through a minimum, and increases with increasing temperature. T h e permeation flux increases significantly with temperature. The compaction of the membranes causes the pure water permeability coefficient, A , to vary over the entire pressure range. Compaction is more pronounced as the temperature is increased. The A values, extrapolated to zero pressure, varied with temperature and were well represented by an Arrhenius plot. The apparent activation energies for water flux were found to vary with membrane type and temperature range but were independent of membrane sample. The higher separation membranes and lower temperature range had the higher activation energies.
Membrane separation processes are playing an increasing role in many applications, such as water desalination, industrial and municipal wastewater treatment, biomedical engineering, food processing, dairy industry, pulp and paper industry, electroplating waste treatment, gas separation, and others (Sourirajan, 1977). These processes are attractive since they are simple, they can be applied to a variety of different problems, high fluxes and separations are possible, there is no phase change required, which reduces the energy requirements, and they operate isothermally so that no heating or cooling is required. This paper is concerned with the effects of temperature on the performance of thin-film composite, reverse-osmosis membranes. It has been widely noted that temperature has significant effects on the performance of reverse-osmosis membranes (Lonsdale et al., 1965; Govindan and Sourirajan, 1966; Agrawal and Sourirajan, 1969; Ohya and Taniguchi, 1974; Burghoff and Pusch, 1976; Saltonstall, 1976; Brandon and Samfield, 1978; Cadotte et al., 1980; Kimura and Nomura, 1981; Chen et al., 1983; Dale and Okos, 1983; Kurihara et al., 1983). FilmTec Corporation has also noticed that, in the field, temperature has a large effect on the flux and separation of FT30 polyamide membranes (Cadotte et ai., 1980). In order to properly evaluate these effects, i t is important to have data that have been collected under carefully controlled laboratory conditions for comparison. Such an experimental plan has been undertaken for two FilmTec Corporation FT30 thin-film composite membranes: the SW30HR and the BW30. SW3OHR is a high-rejection membrane used for seawater desalination, and BW30 is used for brackish water desalination (Cadotte, 1985). These two membranes are chemically similar but have different performance; the exact difference between these membranes is proprietary. Two samples of each membrane have been used for a total of four membrane samples. This paper investigates the
* To whom correspondence
should be addressed.
0888-5885/89/2628-0S14$01.50/0
effects of temperature on the polyamide membranes, experimentally, and a future paper will consider modeling of these effects. The fluxes of solvent and solute have been expressed, by previous researchers, in terms of an Arrhenius-type equation (see, for example, Saltonstall (1976)), N i= A iexp(-Ei/RT) where E, is the apparent activation energy for transport of species i through the membrane. The magnitude of this energy for solvent (water) appears to vary from 19250 kJ/kmol for membranes with little salt rejection to nearly 25 120 kJ/kmol for membranes with salt rejections in the order of 99% for cellulose acetate membranes (Saltonstall, 1976). These values are independent of pressure, solute identity, and solute concentration. A t the same time, the corresponding values of the energy for solute (sodium chloride) transport varied from 20 100 to more than 29 300 kJ/kinol and were always greater than the apparent activation energies of water. These values, stated above, imply that the diffusion through the membrane is hindered. A hindered diffusion process could be the consequence of molecular steric effects, interactions between solute, solvent, and membrane, and/or some form of flow obstruction. The apparent activation energies have been obtained for the polyamide membranes and are discussed in the Results and Discussion section. This idea, that solute and solvent have to gain a minimum energy (apparent activation energy) in order to pass through the membrane, has been obtained by analogy with the idea of activation energy in chemical reactions. Therefore, eq 1states a mechanism for transport of solute and solvent through membrane in a phenomenological manner similar to the transport of material from the reactant state to the product state in chemical reactions. T o determine the performance of each membrane, the Kimura-Sourirajan analysis (KSA) has been employed (Kimura and Sourirajan, 1967; Sourirajan, 1970). h c 0 1989 American Chemical Society
Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989 815 cording to this model, the solvent and solute fluxes are given by
NB = A [ A P - (az- a3)]
where A is the solvent permeability coefficient, DAMKIr is the solute transport parameter, CA2is the solute concentration just outside the membrane (at the feed side) which is usually higher than the feed concentration, CAI, and CASis the permeate concentration. The pure solvent flux, Np, can be written from eq 2 as
Alr
Rnnutl‘
*lY
Calroller
Np = AAP (4) A mass-transfer coefficient, k , which characterizes the boundary layer at the feed side of the membrane, can be obtained from the film theory (Kimura and Sourirajan, 1967) as
F-I? R.lrlpr.tlon Cyd.
Iml.1.d Ea.
(2)
Figure 1. Schematic diagram of the reverse-osmosis testing equipment. Ol O”1.”””””.’””’””
100. L
eo. 0 - +D BO.
70.
10.
40.
30.
-
10.
-
IO.
60.
so.
-
70.
b
- 0 - +D -
eo.
= 1500 k P a = 4000 k P o = 7 0 0 0 kPo
-
so.
80.
= i s o o kPa = 4 0 0 0 kPa = 7000 kPa 0
20. -
I
40.
3
10.
.-
IO.
-
100. l “ “ “ “ ‘ “ l . . . . . . . . l r . l l . . l . . l r ,
80.
-
r 3 0
70.
-
b
+
= 500 kPa = 1500 kPo = 4000 kPo =7000 kPo
0
80.
W
-
50.
0
x
4
40.
3 30. 10. IO.
-
-
+
1
Figure 2. Mass-transfer coefficients versus temperature for the (a) SW30-1, (b) SW30-2, (c) BW30-1, and (d) BW30-2 membranes. The reference k values (at 25 “ C ) are 15.35 X lo”, 22.49 X lo”, 32.84 X lo”, and 27.65 X 10” m/s for the SW30-1, SW30-2, BW30-1, and BW30-2 membranes, respectively.
816
Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989
t
I . , . . . .
0.9 0.0
I
I
, . . . ( . . . .
...
,
l
, . . . ( . , . . ,
= W30-1 = W30-2 = Bw30-1
0 I3 X
+
BW30-2
i
f
0
=
-0- -0,
-o,
SW30-1
= SW30-2
0.70 110
Run No 120
I30
150
1kO
160
170
180
Figure 4. Standard experiments: pure water flux versus run number for all the membranes. The solid line is the average flux measured up to run 152, and the dashed line is for the higher run numbers.
Run No. (a)
0.90
0.80
0.75
A theoretical separation, f ', based on the boundary layer concentration, C A 2 , can be calculated as
1
f' =
CA2
- CA3 CA2
-
which represents the theoretical separation that should be obtained in the absence of concentration polarization. In eq 1, the effect of temperature on solute and solvent flux was modeled by an Arrhenius-type equation. A theoretically more satisfying approach is to describe the effect of temperature on the model parameters. For example, the parameters of the Kimura-Sourirajan analysis, A and D m K / r , have been expressed as a function of temperature as (Sourirajan, 1970)
-
+X
-
= BW30-1 =BW30-2
-:
AqB =
0 7 0 " " " " ~ " ~ " " ' ~ ' ' ' ' 1 ' ' ' ' ~
constant
(
kK = D + K ) 7
By mass balance, the solute and solvent fluxes are related to the permeate concentration by = 'NA
+ NB
mAl
- mA3 mAl
(7)
and for moderately dilute solutions, molalities can be replaced by molar concentrations as
f=
- cA3 CAI
for cellulose acetate membranes (referred to as CA-NRC-18 by Sourirajan (1970)), for 0.5-2.0 M sodium chloride solutions, and at 5-36 "C. Similarly, Connell and Dickson (1988) correlated the temperature dependencies of the parameters in the finely porous model using Arrhenius equations for the separation of toluene from water with cellulose acetate membranes. In general, the effects of temperature on any parameter, U, can be modeled by an Arrhenius equation normalized about temperature Trefas (Connell and Dickson, 1988) (12)
where E is the apparent activation energy associated with parameter U. Equation 12 can be rewritten as
The separation, f, is defined as
f=
e0.005(T-Tmr) ref
u = urefe-(E/R)((l/r)-(l/T,d)
N A
CA3
(10)
In U = In Uref- E( R T
L,
(13)
Tref
which implies that In U varies linearly with 1/T. For example, eq 11 can be rearranged to the same form as eq 12 and 13. From a generalized mass-transfer correlation, the variation of k with temperature is predicted by (Sourirajan,
Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989 817
.. .,. .. .,...., ..,.
4.5
E
2
-
03-
0
= l - S C
-
x
=l
0.1
N
0.1
1
- 1 = 25 C
25 C
= T = 45
Y
-
0.1
X
a
EL
c
0.4 -
0.1
Y al
s 0
SO0
1000
1500
0
1000
Operating Pressure , kPa
1.5,
(4 . . . . , . . . . , .
1000
woo
coo0
Operating Pressure, kPa
(b1 , ,
.,.
s.1 S.
'"F
Y
-1
25 C
1
s.5 5. 4.5
4.
3.5 3. 2.5 2.
1.5 1.
0.5
0.
Operating Pressure, kPa
2000
4000
1000
IWO
Operating Pressure, kPa
(d)
(c)
Figure 5. Pure water flux versus operating pressure with temperature as a parameter for the (a) SW30-1 membrane, low-pressure range; (b) SW30-1 membrane, full-pressure range; (c) BW30-2 membrane, low-pressure range; and (d) BW30-2 membrane, full-pressure range. The straight lines are NP = AOAF'.
1970; Connell and Dickson, 1988)
An additional effect of temperature is the increased plastic creep observed at higher temperatures (Sourirajan, 1970; Merten et al., 1968). Therefore, a t higher temperatures, there is both increased flux and increased compaction. These two effects must both be considered in analyzing data a t different temperatures. In this paper, values of the osmotic pressure as a function of concentration and temperature for NaC1-water were taken from the literature (see Appendix I1 of Sourirajan (1970)).
Experimental Section Four FT30 polyamide membranes have been examined; two SW30HR (seawater, high-rejection) and two BW30 (brackish water) membranes, manufactured by FilmTec
Corporation. The membranes, which are called SW30-1, SW30-2, BW30-1, and BW30-2, were cut from flat sheets and compacted for 16 h at 25 "C and 8500 kPa. The solute used in all 79 experiments was pure sodium chloride, and the solvent was deionized and distilled water. The effective surface area of each membrane was 15.08 cm2. System Parameters. The independent variables which were set for an experiment were the solute, solute concentration, feed flow rate, pressure (values reported are gauge pressures), and temperature. The dependent variables which were measured are the permeate concentration, pure solvent (water) permeation flux, and permeation flux of the solution. The pH of feed and permeate solutions were measured for some experiments. The range of each operating variable and the experimental error are pressure temperature
350-7000 kPa 5-60 "C
*0.2% f0.2 "C for each cell 10.25 O C over all cells
feed flow rate aqueous feed concn
1000 mL/min 2000 ppm
*1.0% 11.0%
818 Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989
530.
410. cj
a A
430.
LI)
.
N
E
310.
c
0
E
330.
2 m * 210. 0 e
X
130.
U 180. 130. 80.
0
-
I
" '
1000
::::Ia1,.". ,
' 1000
'
'
s
5000
4000
3000
'
I
a
6000
+
1000
' 1000
-'P.s
Operating Pressure, k P a
5.6
,
,
* SW30-1
.,
5.7
=, S W 3 0 - 2
5.1
en
5.9
.
,I 6.
T
(a) = 15 = 20 = 25 = 30
-
c c C C
c
35
= 40 C = 15 c
-
m
c
50
= 60 C
z
-"
Y
m N
E
3
E
= BW30-1 = W30-2
-28.
2
01-
0
-30
r(
X
4.5
5.6
5.7
5.1
5.9
6.
en T
U
(6 1 "
1000
I
Figure 7. Effect of temperature on the compaction coefficient, m: (a) SW30 and (b) BW30 membranes.
n
v '
2000
I
"
3000
'
I
4000
'
I
'
3000
y
.
6000
7000
8000
Operating Pressure, kPa
(b) Figure 6. Effect of pressure and temperature on compaction as represented by the decrease in pure water permeability coefficient, A . Back-extrapolation to zero pressure using linear least squares gives Ao, the pure water permeability a t zero pressure. (a) SW30-1 and (b) BW30-2 membranes.
Reverse-Osmosis Experiments. The following series of experiments were designed to evaluate the performance of the thin-film composite aromatic polyamide membranes at different temperatures and pressures. Experiments that were more likely to damage the membranes were done last. Therefore, the earlier results could still be compared even if the membranes were damaged later on. The order of experiments in each section is randomized to reduce the interference of any systematic change in the membrane on the observed results.
Experiments with pure water feed have been performed before and after each NaC1-H20 experiment. Some NaCl experiments, designated as "standard experiments", have been repeated to monitor any membrane changes. The standard experiment was operated at 25 "C, 1500 kPa, and 2000 ppm NaCl solution. The experimental plan was as follows: (i) Membrane compaction a t 25 "C, 8500 kPa, and 2000 ppm aqueous NaCl solutions for 16 hours (i.e., until the permeate flux and concentration become stable). (ii) Standard experiment. (iii) Experiments at 25 "C and a t each of the following pressures (randomized): 350,500, 1500,4000, and 7000 kPa. (iv) Standard experiment. (v) Experiments at 5000, 10000, and 15000 ppm, 25 "C, and 1500 kPa. (vi) Experiments at pressures in the range 350-7000 kPa and a t temperatures in the range 5-25 "C. After each set of experiments at a fixed temperature, the standard experiment is repeated. (vii) Experiments at several pressures in the range 350-7000 kPa and at temperatures of 30,35,
Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989 819 and 40 "C. Standard experiment is repeated after each set of experiments at a fixed temperature. (viii) Step vi repeated at 45, 50, and 60 "C, followed by standard experiment after each set of experiments. A single experiment consists of measuring the pure water flux at the appropriate conditions, switching to NaC1-H20 solution, and after steady state is achieved, measuring the concentration of the feed, CAI, the concentration of the permeate, CA3, and the solution flux, nT. Finally the pure water flux is again measured. The reported np value is the average pure water flux. Reverse-Osmosis Equipment. The reverse-osmosis testing system, as shown in Figure 1, consists of a feed reservoir, a diaphragm metering pump, an accumulator, high/low-pressure protector, six radial-flow test cells, a pressure gauge, a pressure regulator, and a temperaturecontrolling section which consists of a refrigeration system, a heating system, and a WEST Model 2070 microprocessor-based temperature controller. The feed solution is heated or cooled in a heat exchanger before entering the flow cells. Most of the equipment is contained in an insulated chamber (as illustrated in Figure 1) which is outfitted with heat exchangers for heating and cooling the system. The system allows independent control of temperature, pressure, feed flow rate, and feed concentration. The concentration of the solute (sodium chloride) in water solutions was measured by a YSI Model 31 conductivity bridge (with a Beckman pipet conductivity cell) and using a calibration curve which correlates conductances (pf2-l) to concentrations (parts per million) at 25 "C temperature. Samples were warmed or cooled to 25 f 0.1 "C before analysis.
0
h
= SW30-1 = 5W30-2 = BW30-1 = BW30-2
-17. 2.15
3.25
1 --x103,
T
3.5
3.75
3 s
3 75
K-'
Results and Discussion In this section, the data obtained are presented and interpreted with attention to the effects of temperature on performance. Analysis of the Raw Data. The raw data for all 79 experiments are processed by a computer program in the following manner: (i) obtain molar fluxes, NA, NB,and NT, from experimental mass fluxes and the permeate concentration, and then eq 4 gives A from the pure water flux, N p ; (ii) obtain C A 2 from eq 2 using C A ~A, , and N B from step i and the known relationship between concentration and osmotic pressure; (iii) calculate the mass-transfer coefficient, k , from eq 5 using N T , CAI,CA2,and c A 3 from steps i and ii. The effect of temperature on separation is considered qualitatively in this paper. The analysis of the effect of temperature on solute transport will be considered in detail in another paper. Determination of the Mass-Transfer Coefficients. For relatively low concentration feed solutions, the difference between pure water flux, N p ,and solution flux, NT, is small, which leads to a relatively large error in determining r2and hence CA2 from eq 2. Subsequently, there is a larger error in calculating k from eq 5. Therefore, even small experimental errors in the measurements of N p ,NT, CAI,or CA3can have a large effect on the k values, hence the wide variations of these values for the experiments at the same temperature as can be seen in Figure 2. Nonetheless, the k values at 25 "C and 1500 kPa were averaged and used as krefin eq 14. The k values calculated by the procedure in step iii above and predicted by eq 14 are plotted in Figure 2. Given the expected error in calculating k for low-concentration experiments, the agreement between trends of the data and eq 14 is good.
70 ? IS
3 7s
Figure 8. Arrhenius plots: (a) In Ao versus 1 / T for all membranes (by eq 13); (b) In ( l / v ) versus 1 / T for water. The slopes of the lines change at about 313 K.
To check the krefvalues used above, a few experiments at 25 "C, 1500 kPa, and concentrations of 5000,lO 000, and 15000 ppm were performed. A t high concentrations, the differences between N p and NT become larger, giving a more accurate value of CA2 and therefore a more accurate estimate of k . These k values for each cell were reasonably constant and agreed with krefby a maximum deviation of 14%. The k values predited by eq 14 are used to recalculate CA2(from eq 5) and f ' . The effect of using the corrected mass-transfer coefficients on f ' is small (less than 3%). Results for the Standard Experiment. The results obtained for the standard experiments at different times are presented in Figures 3 and 4. Run number is used to approximate time. Run number represents the order in which the experiments were performed, with each run taking about 1 work day, and the system was shut off
820 Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989
I
0 9s
f'
.
. .=**=+=e &!+L+L;' -.: -a-x-*
0
0
0
x - ~ ~ x . - y . # ~ x - x
0
-
085-
0
.El
'+ : o
015:
=P =P =P =P =P
-
0 95
-
085-
-
O I -
0
0
0 8 -
: o
..
= 350 k.Pa = 500 kPa = 1500 kPa = 4000 kPa = 7000 kPa
:
: o .El x + : o
01s:
=P =P =P =P =P
= 350 kPo = 500 kPa = 1500 kPa = 4000 kPa = 7000 kPo
-
07
BW30-2
overnight between runs. Figure 3a illustrates how separation, f , varies with time (as represented by run number) for the SW30-1 and SW30-2 membranes. The separation remains essentially constant over the time of testing. The average separation for each membrane is presented as the dashed and solid line for the SW30-1 and SW30-2 membranes, respectively. The values are 97.7% and 96.7%, respectively. Similar results are presented for the BW30 membranes in Figure 3b. The separation for the BW30-1 membrane is constant (96.4%) as a function of time. However, the separation for the BW30-2 membrane oscillates above and below the results for the BW30-1 membrane but has about the same separation as the BW30-I membrane, on average (Le., 96.4%). Perhaps the BW30-2 membrane sample has some surface defects or is otherwise not completely representative of BW30 membranes. By comparison, the SW30 membranes have higher separations than the BW30 membranes, as expected. The variation of the pure water flux for all four membranes as a function of run number is presented in Figure
4. Up to run 152, the flux for each of the membranes is constant. After run 152, the flux for each membrane decreases slowly over the remaining experiments. The change in the flux corresponds to the first run performed a t 40 "C. This result indicates that some progressive change in the membrane permeability occurs at 40 O C and higher. Although different membrane materials were used, these results are coincidently consistent with those observed by others (Kimura and Nomura, 1981; Merten et al., 1968). The BW30 membranes show a larger decrease in flux than the SW30 membranes, which is typical that higher flux membranes are more susceptible to changes in permeability. By comparison, the BW30 membranes have higher permeation fluxes (up to about 100% more) than the SW30 membranes, as expected. The cause of the change in permeability above 40 "C is discussed later. The small differences in flux and separation between the SW30-1 and SW30-2 can be expected for membrane samples cut from different places on the membrane sheet. Similar behavior is observed for the two BW30 membranes.
Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989 821 I.
.
~
.
l
.
l
.
1
.
1
'
Y
I'
0.05
t
0 I3 X
0.8
0
1000
2000
JOOO
4000
-1
-
0.1
-
0.05
-
>'
2s c = T = 60 C
=T
5000
SOOO
7000
(000
0.0
. 0
I
'
IO00
I
. T =
X
=T = T = 60 C
*
.
I
SO00
2000
-
5 c 25 c
0
4000
.
SOOO
I
'
*
8000
7000
SOOO
Operating Pressure , kPa (a)
1.
,
l
' 0
I
'
l
.
1
.
1
'
0.95
1
'
n
x
Q X
-
