- Email: [email protected]
Trends in design and performance of commercially available reverse osmosis modules
Desalination, 46 (1983)91-100
91
ElaevierSciencePubliahera B.V.,Amaterdam-Printedin TheNetherlands
TRENDS IN DESIGN AND PERFORMANCE OF COMMERCIALLY AVAILABLE REVERSE OSMOSIS MODULES
J.D. BIRKETT Arthur D. Little, Inc., Cambridge, MA (USA)
ABSTRACTS In an effort to rationalize the specifications supplied with commercially available reverse osmosis modules, the author compiled nominal performance data on 145 modules offered by 12 manufacturers. Because test conditions varied considerably, the production rates were recalculated in terms of capacity per day, per unit of pressure in excess of osmotic pressure, and also in terms of capacity per day, per unit excess pressure, per unit volume of the membrane module. This latter parameter, which may be termed "pragmatic flux", can be plotted versus percent salt rejection, module length, and module diameter for various families of modules to illustrate advantages and limitations of scaleup in different membrane systems. Dans un effort de rationaliser les specifications four&s avec les modules d'osmose inverse disponibles, l'auteur a rassemble les don&es de rendement de 145 modules offerts par 12 producteurs. Parce que les conditions d'essai ont varie considerablement, les taux de production ont et4 recalcul& en terme de capacite journaliere, par unite de pression en exces de la pression osmotique, et aussi en tenne de capacitg par jour d'unite de pression en exces, par unite de volume de la membrane du module. Ce dernier parametre peut etre nomme 'le flux pragmatique' et peut alors etre prepar: en fonction du pourcent de la rejection de sel, de la longueur du module et de son diametre pour plusieurs familles de membrane afin d'illustrer les avantages et les limites des augmentations de taille en differents systemes. Urn die Leistungsdaten von industriell angebotenen Umkehr Osmose Modulen auf einen Nenner au bringen, hat der Autor Angaben uber die Nennleistung von 145 Modulen von 12 Herstellem zusammengestellt. Da die Messbedingungen sehr Produktionsdaten umgerechnet auf unterschiedlich waren, wurden die Tagesleistung pro Druckeinheit iiber den osmotischen Druck sowie auf Tagesleistung pro Einheit fiberdruckund pro Volumeneinheit des Membran Moduls. Das letztere Leistungsmass, hier "pragmatic flux" genannt, wird in Abhsngigkeit vom R&khalteverm$gen fiirSalz sowie von L&rge and Durchmesser der Modulen f& verschiedene Modulfamilien in Kurven gezeigt. An Hand der Kurven werden Vorteile und Grenzen fGr Vergr&serung verschiedener Membransysteme ergrtert.
OOll-9164/83/$03.00 0 1983 E1sevierSciencePubliiersB.V.
92 TRENDS IN DESIGN AND PERFORMANCE OF COMMERCIALLY AVAILABLE REVERSE OSMOSIS MODULES
J.D. BIRKETT Arthur D. Little, Inc., Cambridge, MA (USA)
BACKGROUND Manufacturers of commercially available reverse osmosis membrane modules and their distributors generally distribute specification sheets describing the nominal performance of their units under certain operating conditions. However, due to variations in test protocols, such as percent product recovery, feed water concentration, and applied pressure, it is often difficult to make direct comparison of products. Therefore an informal procedure was adapted a number of months ago to maintain a current file of available modules with their performance reduced to a set of common conditions. This has not only made it much easier to see how new commercial offerings fit into the population of current modules, but has also made it possible to illustrate some of the variations in performance within a family of modules utilizing the same membrane material.
APPROACH Specification sheets .were obtained from 12 module manufacturers, located in the United States, Europe, and Japan. These represented the great majority of modules available during 1981 and 1982. No effort was made to include developmental modules nor test results from the technical literature. From this literature was recorded, for each module:
.
manufacturer;
.
module model number;
.
module configuration (spiral, hollow fiber, plate and frame);
.
membrane type (assymetric cellulose acetate (or blend), thin fiber components, assymetric polyamide, etc.);
.
nominal permeation rate;
.
test feed concentration;
.
test recovery rate;
.
percent salt rejection;
l
applied pressure;
.
module diameter (with or without pressure shell); and
.
module length (with or without pressure shell).
93
From the feed concentration and percent recovery, it is possible to calculate brine reject concentration and average concentration in the brine channel. From this it is possible to obtain the osmotic pressure (a? on the brine side. Net driving pressure (NDP) across the membrane is then the applied pressure minus7T .
(We may safely neglect the osmotic pressure on the permeate side as it
is usually very small.)
At first an effort was made to calculate the volume of the module with some accuracy but this was hampered by the fact that some modules have integral pressure shells, some are available in slightly varying lengths to fit various housings, and some have other constrictions or bulges which render precise volumes meaningless. Instead it was decided to classify modules into a limited number of size categories with nominal diameters of 5. 10. 15, 20 and 25 cm (2, 4, 6, 8 and 10 inch) and lengths of 30, 50, 60 and 100 cm (12. 20, 25 and 40 inch). The rated capacity of a module (nominal permeation rate) could then be reduced to the form:
(volume of water)
l
-1 -1 day .NDP
l
-1 (volume of module)
which has the metric units of:
m
3
l
-1 -1 -3 -1 -1 day a bar s m = day s bar
or English units of: -1
gallons a day
-3 . psi-1 . ft .
We have designated this the "pragmatic flux" of the module. This introduction of module volume rather than the more traditional surface area of membrane was made in order to adopt the perspective of the eventual end user or purchaser who does not necessarily concern himself with subtleties of surface mechanisms.
RESULTS An initial effort was made to plot pragmatic flux versus percent rejection for all modules of a given size (such as 10 cm X 100 cm) in order to demonstrate the logical inverse relationship between membrane "tightness" and permeate rate. However in the resulting graph such a relationship, while present, was not distinctly or usefully shown.
Therefore the figure was redrawn with
two changes. Firstly, the horizontal axis was changed to read 100-41rejection, or % salt passage, so that both axes read at zero in the lower left corner.
94
Secondly, groups of data points representing members of a family, i.e., the same generic membrane material and manufacturer, were connected by lines. The result was Figures 1 6 2 for 10 cm X 100 cm and 20 cm X 100 cm modules respectively.
Figure 1 contains a few inexplicable points but in general the trend is as expected. It is interesting to note also that in this plot, module families offering higher overall fluxes also show higher dependence of flux on rejection rate. In retrospect this is not surprising, although not anticipated a priori.
Figure 2, for 20 cm X 100 cm modules, is better behaved, presumably because we are seeing more of the influence of the membrane itself and less that of fittings, manifolds, end caps, etc.
Once again families with higher intrinsic
fluxes generally show higher dependence on reject rate.
Figure 3 illustrates the dependence of pragmatic flux upon the module length, for 10 cm nominal diameter modules.
It illustrates some initial im-
provement as modules are lengthened but suggests that there is little change and even sometime a decrease in performance between 68 and 100 cm.
This sup-
ports the premise that the principal motivation for producing longer membrane modules is not necessarily improved volumetric efficiency, but rather reduced module manufacturing costs per unit capacity as well as reduced costs in systems fabrication through reduced requirements for fittings and labor.
(This is
elaborated upon in Reference 1.)
In Figures 4 and 5, pragmatic flux is plotted versus module diameter for modules of 100 cm length. Here we can readily see that an increase in diameter leads to increased volumetric efficiency for increases from S-10 cm in all cases and for increase from lo-20 in cases of modules showing low intrinsic flux.
However for higher productivity systems, generally utilizing thin film
composite or hollow fiber membranes, there can often be a decrease in volumetric performance as the diameter is increased above 100 cm.
This is due at
least in part to higher pressure drops in the product water channels of large modules.
It is interesting to note that the two top pairs of data points in
Figure 5 both represent hollow fiber systems. That with the larger diameter fiber is not adversely affected by increasing module diameter whilst the other system with finer fibers is.
However different winding, packing, spacer, and
feed water inlet systems may also contribute to differences in performance and so it would be unwise to presume that differences in fiber diameters were the sole contributing factors, although the literature (Reference 2) suggests the
95
effect of such product-side pressure drops.
DISCUSSION This paper does not presume to suggest that certain membrane or module systems are necessarily inferior or superior to any others. It does however offer one method of displaying the performance of various modules in a consistent fashion which can then be used to indicate where a module or family of modules fits into the overall population of modules. When a module is seen to depart from the general pattern, this should be taken as an indication that the manufacturer may have made a conscious effort to improve or sacrifice performance in other areas (oxidation resistance, tolerance of pH extremes, thermal stability, etc.) in order to achieve satisfactory overall performance for a particular application.
REFERENCES 1 M.E. Mattson and M. Lew, Recent Advances in Reverse Osmosis and Electrodialysis Membrane Desalting Technology, Desalination, 41 (1982) 881-24. 2 W.T. Hanbury, A. Yeceer, M. Tzimopoulos, and C. Byabagambi, Pressure Drops Along the Bores of Hollow Fiber Membranes - Their Measurement, Production, and Effect on Fiber Bundle Performance. Proceedings of the International Congress on Desalination and Water Reuse, InternationalDesalination and Environmental Association, Manama. Bahrain, 1984, Vol. 1, pp. 301-318.
0
1, PRAGMATIC
I!
CM MODULES,
FIGURE
.
100 - x
100 - %
REJECTION
I
6
FLUX VERSUS
I
4
REJECTION
I
8
FOR 10 x
I
10
100
'0
'20
80
.oo
97
I
is
z T_ ISd
I
I
p
0 N
T_Ava xfllj
SNOllWF)
3IlWWSVtl,-j
I 0 II
\
GAL‘oNS*DAY;iFT;5PSI-1
PRAGMATIC FLUX
8
99
18-
"36
16"
"32
"28 I
12"
"24 ";",
10"
-20
-=~ 8"
-16
:>,, ,¢C
!
~.. ~ °
'=i CL
O _.t ,_J ,=¢
. iiiil
fill
-12
MODULE DIAMETER FIGURE 4~ PRAGMATIC FLUX VERSUS MODULE DIAMETER FOR CELLULOSE ACETATE (AND BLEND} MODULES 100 CM IN LENGTH
100
50 A /
40
30
20
10
_,-a
c’
0
40
Y5
80
(-----
FIBER
> MODULES
INCH CM
MODULE DIAMETER
FIGURE 5, PRAGMATIC FLUX VERSUS FOR HOLLOW
8
(---100
> AND THIN
CM IN
LENGTH
MODULE FILM
DIAMETER COMPOSITE
91
ElaevierSciencePubliahera B.V.,Amaterdam-Printedin TheNetherlands
TRENDS IN DESIGN AND PERFORMANCE OF COMMERCIALLY AVAILABLE REVERSE OSMOSIS MODULES
J.D. BIRKETT Arthur D. Little, Inc., Cambridge, MA (USA)
ABSTRACTS In an effort to rationalize the specifications supplied with commercially available reverse osmosis modules, the author compiled nominal performance data on 145 modules offered by 12 manufacturers. Because test conditions varied considerably, the production rates were recalculated in terms of capacity per day, per unit of pressure in excess of osmotic pressure, and also in terms of capacity per day, per unit excess pressure, per unit volume of the membrane module. This latter parameter, which may be termed "pragmatic flux", can be plotted versus percent salt rejection, module length, and module diameter for various families of modules to illustrate advantages and limitations of scaleup in different membrane systems. Dans un effort de rationaliser les specifications four&s avec les modules d'osmose inverse disponibles, l'auteur a rassemble les don&es de rendement de 145 modules offerts par 12 producteurs. Parce que les conditions d'essai ont varie considerablement, les taux de production ont et4 recalcul& en terme de capacite journaliere, par unite de pression en exces de la pression osmotique, et aussi en tenne de capacitg par jour d'unite de pression en exces, par unite de volume de la membrane du module. Ce dernier parametre peut etre nomme 'le flux pragmatique' et peut alors etre prepar: en fonction du pourcent de la rejection de sel, de la longueur du module et de son diametre pour plusieurs familles de membrane afin d'illustrer les avantages et les limites des augmentations de taille en differents systemes. Urn die Leistungsdaten von industriell angebotenen Umkehr Osmose Modulen auf einen Nenner au bringen, hat der Autor Angaben uber die Nennleistung von 145 Modulen von 12 Herstellem zusammengestellt. Da die Messbedingungen sehr Produktionsdaten umgerechnet auf unterschiedlich waren, wurden die Tagesleistung pro Druckeinheit iiber den osmotischen Druck sowie auf Tagesleistung pro Einheit fiberdruckund pro Volumeneinheit des Membran Moduls. Das letztere Leistungsmass, hier "pragmatic flux" genannt, wird in Abhsngigkeit vom R&khalteverm$gen fiirSalz sowie von L&rge and Durchmesser der Modulen f& verschiedene Modulfamilien in Kurven gezeigt. An Hand der Kurven werden Vorteile und Grenzen fGr Vergr&serung verschiedener Membransysteme ergrtert.
OOll-9164/83/$03.00 0 1983 E1sevierSciencePubliiersB.V.
92 TRENDS IN DESIGN AND PERFORMANCE OF COMMERCIALLY AVAILABLE REVERSE OSMOSIS MODULES
J.D. BIRKETT Arthur D. Little, Inc., Cambridge, MA (USA)
BACKGROUND Manufacturers of commercially available reverse osmosis membrane modules and their distributors generally distribute specification sheets describing the nominal performance of their units under certain operating conditions. However, due to variations in test protocols, such as percent product recovery, feed water concentration, and applied pressure, it is often difficult to make direct comparison of products. Therefore an informal procedure was adapted a number of months ago to maintain a current file of available modules with their performance reduced to a set of common conditions. This has not only made it much easier to see how new commercial offerings fit into the population of current modules, but has also made it possible to illustrate some of the variations in performance within a family of modules utilizing the same membrane material.
APPROACH Specification sheets .were obtained from 12 module manufacturers, located in the United States, Europe, and Japan. These represented the great majority of modules available during 1981 and 1982. No effort was made to include developmental modules nor test results from the technical literature. From this literature was recorded, for each module:
.
manufacturer;
.
module model number;
.
module configuration (spiral, hollow fiber, plate and frame);
.
membrane type (assymetric cellulose acetate (or blend), thin fiber components, assymetric polyamide, etc.);
.
nominal permeation rate;
.
test feed concentration;
.
test recovery rate;
.
percent salt rejection;
l
applied pressure;
.
module diameter (with or without pressure shell); and
.
module length (with or without pressure shell).
93
From the feed concentration and percent recovery, it is possible to calculate brine reject concentration and average concentration in the brine channel. From this it is possible to obtain the osmotic pressure (a? on the brine side. Net driving pressure (NDP) across the membrane is then the applied pressure minus7T .
(We may safely neglect the osmotic pressure on the permeate side as it
is usually very small.)
At first an effort was made to calculate the volume of the module with some accuracy but this was hampered by the fact that some modules have integral pressure shells, some are available in slightly varying lengths to fit various housings, and some have other constrictions or bulges which render precise volumes meaningless. Instead it was decided to classify modules into a limited number of size categories with nominal diameters of 5. 10. 15, 20 and 25 cm (2, 4, 6, 8 and 10 inch) and lengths of 30, 50, 60 and 100 cm (12. 20, 25 and 40 inch). The rated capacity of a module (nominal permeation rate) could then be reduced to the form:
(volume of water)
l
-1 -1 day .NDP
l
-1 (volume of module)
which has the metric units of:
m
3
l
-1 -1 -3 -1 -1 day a bar s m = day s bar
or English units of: -1
gallons a day
-3 . psi-1 . ft .
We have designated this the "pragmatic flux" of the module. This introduction of module volume rather than the more traditional surface area of membrane was made in order to adopt the perspective of the eventual end user or purchaser who does not necessarily concern himself with subtleties of surface mechanisms.
RESULTS An initial effort was made to plot pragmatic flux versus percent rejection for all modules of a given size (such as 10 cm X 100 cm) in order to demonstrate the logical inverse relationship between membrane "tightness" and permeate rate. However in the resulting graph such a relationship, while present, was not distinctly or usefully shown.
Therefore the figure was redrawn with
two changes. Firstly, the horizontal axis was changed to read 100-41rejection, or % salt passage, so that both axes read at zero in the lower left corner.
94
Secondly, groups of data points representing members of a family, i.e., the same generic membrane material and manufacturer, were connected by lines. The result was Figures 1 6 2 for 10 cm X 100 cm and 20 cm X 100 cm modules respectively.
Figure 1 contains a few inexplicable points but in general the trend is as expected. It is interesting to note also that in this plot, module families offering higher overall fluxes also show higher dependence of flux on rejection rate. In retrospect this is not surprising, although not anticipated a priori.
Figure 2, for 20 cm X 100 cm modules, is better behaved, presumably because we are seeing more of the influence of the membrane itself and less that of fittings, manifolds, end caps, etc.
Once again families with higher intrinsic
fluxes generally show higher dependence on reject rate.
Figure 3 illustrates the dependence of pragmatic flux upon the module length, for 10 cm nominal diameter modules.
It illustrates some initial im-
provement as modules are lengthened but suggests that there is little change and even sometime a decrease in performance between 68 and 100 cm.
This sup-
ports the premise that the principal motivation for producing longer membrane modules is not necessarily improved volumetric efficiency, but rather reduced module manufacturing costs per unit capacity as well as reduced costs in systems fabrication through reduced requirements for fittings and labor.
(This is
elaborated upon in Reference 1.)
In Figures 4 and 5, pragmatic flux is plotted versus module diameter for modules of 100 cm length. Here we can readily see that an increase in diameter leads to increased volumetric efficiency for increases from S-10 cm in all cases and for increase from lo-20 in cases of modules showing low intrinsic flux.
However for higher productivity systems, generally utilizing thin film
composite or hollow fiber membranes, there can often be a decrease in volumetric performance as the diameter is increased above 100 cm.
This is due at
least in part to higher pressure drops in the product water channels of large modules.
It is interesting to note that the two top pairs of data points in
Figure 5 both represent hollow fiber systems. That with the larger diameter fiber is not adversely affected by increasing module diameter whilst the other system with finer fibers is.
However different winding, packing, spacer, and
feed water inlet systems may also contribute to differences in performance and so it would be unwise to presume that differences in fiber diameters were the sole contributing factors, although the literature (Reference 2) suggests the
95
effect of such product-side pressure drops.
DISCUSSION This paper does not presume to suggest that certain membrane or module systems are necessarily inferior or superior to any others. It does however offer one method of displaying the performance of various modules in a consistent fashion which can then be used to indicate where a module or family of modules fits into the overall population of modules. When a module is seen to depart from the general pattern, this should be taken as an indication that the manufacturer may have made a conscious effort to improve or sacrifice performance in other areas (oxidation resistance, tolerance of pH extremes, thermal stability, etc.) in order to achieve satisfactory overall performance for a particular application.
REFERENCES 1 M.E. Mattson and M. Lew, Recent Advances in Reverse Osmosis and Electrodialysis Membrane Desalting Technology, Desalination, 41 (1982) 881-24. 2 W.T. Hanbury, A. Yeceer, M. Tzimopoulos, and C. Byabagambi, Pressure Drops Along the Bores of Hollow Fiber Membranes - Their Measurement, Production, and Effect on Fiber Bundle Performance. Proceedings of the International Congress on Desalination and Water Reuse, InternationalDesalination and Environmental Association, Manama. Bahrain, 1984, Vol. 1, pp. 301-318.
0
1, PRAGMATIC
I!
CM MODULES,
FIGURE
.
100 - x
100 - %
REJECTION
I
6
FLUX VERSUS
I
4
REJECTION
I
8
FOR 10 x
I
10
100
'0
'20
80
.oo
97
I
is
z T_ ISd
I
I
p
0 N
T_Ava xfllj
SNOllWF)
3IlWWSVtl,-j
I 0 II
\
GAL‘oNS*DAY;iFT;5PSI-1
PRAGMATIC FLUX
8
99
18-
"36
16"
"32
"28 I
12"
"24 ";",
10"
-20
-=~ 8"
-16
:>,, ,¢C
!
~.. ~ °
'=i CL
O _.t ,_J ,=¢
. iiiil
fill
-12
MODULE DIAMETER FIGURE 4~ PRAGMATIC FLUX VERSUS MODULE DIAMETER FOR CELLULOSE ACETATE (AND BLEND} MODULES 100 CM IN LENGTH
100
50 A /
40
30
20
10
_,-a
c’
0
40
Y5
80
(-----
FIBER
> MODULES
INCH CM
MODULE DIAMETER
FIGURE 5, PRAGMATIC FLUX VERSUS FOR HOLLOW
8
(---100
> AND THIN
CM IN
LENGTH
MODULE FILM
DIAMETER COMPOSITE