Spiral wound element for separation

An improved spiral wound element for separations is disclosed wherein the improvement comprises using as the feed/retentate space one or more layers of a material having an open cross-sectional area in the range 30 to 70% and as the permeate spacer material at least three layers of material two of which are fine and have an open cross-sectional area of about 10 to 50% surrounding a coarse layer having an open cross-sectional area of about 50 to 90%.

BACKGROUND OF THE INVENTION 
Summary of the Invention 
An improved spiral wound membrane element comprising layers of membrane 
material fluid tight sealed along 3 edges enclosing a permeate spacer 
creating at least one permeate envelope upon which a feed/retentate spacer 
layer is laid, along at least one membrane face, the entire multi layer 
arrangement being wound around a hollow central mandrel (which may be 
closed at one end) and to which the permeate envelope is in fluid 
communication through the fourth unsealed edge, creating a spiral wound 
module element is disclosed which is useful in separation processes 
wherein a pressure gradient is maintained across the membrane from a feed 
side to a permeate side, the improvement comprising using as the 
feed/retentate spacer at least one layer of a material having an open 
cross-sectional area of at least 30-70% and using as the permeate spacer 
at least three layers of material characterized in that the outer layers 
are a fine material having an open cross-sectional area of about 10 to 50% 
and a coarse layer having an open cross-sectional area of about 50 to 90% 
interposed between the aforesaid fine outer layers and wherein the fine 
outer layers are in interface contact with the membrane layers enclosing 
the permeate spacer. 
DESCRIPTION OF THE RELATED ART 
Spiral wound elements contain permeate and retentate spacers as a routine 
matter of standard element design, see, e.g. U.S. Pat. No. 3,417,870. 
Various attempts have been made to improve the spacer materials. Thus. 
U.S. Pat. No. 4,861,487 describes a low pressure drop spacer composed of 
generally parallel elongated filaments positioned generally parallel to 
the flow direction of the feed stream and wherein the elongated filaments 
are connected by shorter bridge filaments which are placed at an angle to 
the flow of the feed stream to provide for a low pressure drop. European 
Patent Application 89305966.7 (publication number 347174) describes a 
spiral wound membrane cartridge wherein the feed spacer material having a 
plurality of parallel ribs extending in an axial direction, interconnected 
by a matrix of smaller filaments generally perpendicular to the parallel 
ribs which results in a reduction to the flow resistance. W091/11249 
describes a spiral wound element which utilizes a divided central mandrel 
and a permeate region which employs a high density porous spacer flanked 
on two sides with low density porous spacers. 
U.S. Pat. No. 5,069,793 describes a spiral wound element for use in 
pervaporation designed to produce maximum permeate flow throughput per 
element volume. This is achieved by use of a permeate spacer selected to 
take advantage of the fact that the total permeate flow throughput from a 
module passes through a maximum as the resistance to vapor transport of 
the permeate spacer material is progressively decreased. The capability of 
the permeate spacer material to transport permeating vapor from the 
membrane surface to the permeate collection pipe is expressed as a 
normalized conductivity, or permeate vapor flow, per unit pressure drop in 
the permeate channel, per unit transmembrane flux. The permeate channel is 
defined to use a spacer material such that permeate flow throughput is 
60-90% of the maximum possible value. The permeate spacer can be a sheet 
of material having a cross-sectional thickener which varies, giving from 
relatively thin at the far edge to thick at the edge adjacent the central 
mandrel. Alternatively the spacer can be made of multiple layers of the 
same or different spacer material. 
DESCRIPTION OF THE INVENTION 
In a spiral wound membrane separation element comprising a hollow central 
mandrel (which may be closed at one end) around which are wound multiple 
layers of membrane, feed spacers and permeate spacers wherein layers of 
membrane surround a permeate spacer, said membrane layers being fluid 
tight sealed along 3 edges producing a permeate envelope leaf, wherein 
multiple permeate envelope leaves are attached along their fourth unsealed 
edge in fluid communication with the interior of the hollow central 
mandrel, and a layer of feed/retentate spacer material extends along the 
outer surfaces of each permeate envelope leaf, the spiral winding of 
multiple permeate envelope leaves/feed-retentate spacers being wrapped 
with an outer wrap layer to prevent unwinding and the end of the winding 
being capped by an anti-telescoping device attached at the downstream end 
to prevent telescopic displacement of the spiral wound layers during use, 
the improvement comprising using as the feed/retentate spacer material at 
least one layer of a material having an open cross-sectional area of at 
least about 30 to 70%, preferably 30 to 50%, preferably using two layers 
of material placed between adjacent permeate envelope leaves, which 
feed/retentate spacer material can be of either the same or different 
material and of the same or different cross-sectional area, preferably 
such feed/retentate spacer material being insulated from the membrane 
surface by an interposed layer of chemically and thermally inert woven or 
non-woven fabric about 1 to 15 mils thick having a weight of about 0.5 to 
10 oz/sq. yard and a Frazier air permeability in the range 0.5 to 1000 
cfm/sq. foot at 1/2 inch water pressure, (example of a non-woven material 
being Nomex), such that upon winding the multiple permeate envelope leaves 
and interposed multiple feed/retentate spacer layer one obtains two layer 
of feed/retentate spacer material between adjacent permeate envelope 
leaves in the winding, and using as the permeate spacer three or more 
layers of material, the outer layers which are in contact with the 
membrane (i.e. with the membrane surface per se or with the integral 
backing of the membrane if the membrane is cast on a backing, this backing 
not being counted as one of the spacer layers) being a fine material 
having an open cross-sectional area of at least about 10 to 50%, 
preferably at least about 10 to 30% and interposed between the fine outer 
layers will be coarse layers having an open cross-sectional area of at 
least about 50 to 90%, preferably about 60 to 90%. The multilayer permeate 
spacer comprises at least 3 layers but may comprise 3 to up to 7 layers 
alternating fine and coarse provided that the outer layers in contact with 
the membrane are fine material layers. It is preferred that an odd number 
of layers be used to minimize intermeshing but an even number of layers 
can also be employed in which case it is preferred that the layers be of 
materials of different mesh size so as to prevent or minimize intermeshing 
of the layers. The limit on the number of layers used in fabricating the 
permeate spacer layer in each permeate envelope leaf, the thickness of 
each leaf, the length of each leaf, the number of leaves attached to the 
central mandrel and the number and thickness of the feed/retentate spacer 
between adjacent permeate envelope leaves will be set as a compromise 
among competing factors including the ability to ultimately wind the 
assembly around the central mandrel, the pressure drop along the length of 
each permeate envelope leaf as well as across the feed spacer and the 
membrane surface area obtainable in each spiral wound module. 
If too many layers are used or if the layers used are too thick it will 
become difficult to wind the spiral wound element. Also, obviously too 
thick a permeate envelope will negatively impact on the total membrane 
surface area available in the final spiral wound element of a given 
diameter. 
The preferred number of permeate spacer layers is 3 to 5. 
The multi-layer permeate spacer may be sized slightly smaller in its 
dimensions than the membrane layer surrounding it so that the spacer does 
not intrude into the area between the membranes at the three edges along 
which the membrane layers are fluid tight sealed. Intrusion of permeate 
spacer into this area interferes with the ability to effectively seal the 
membrane edges to create the permeate envelope. 
The preferred number of feed/retentate spacer layers is 2. In preparing the 
element it has been found that interposing a layer of chemically and 
thermally stable woven or non-woven material about 1 to 15 mils thick 
weighing about 0.5 to 10 oz/sq. yard and having a Frazier air permeability 
in the range 0.5 to 1000 cfm/sq. ft at 1/2 inch water pressure interposed 
between the feed/retentate spacer material and the surface of the membrane 
and the permeate spacer material and the surface of the membrane improves 
membrane element long term performance and improves the vacuum tightness 
of the resulting spiral wound package. This interposing layer acts as a 
shield between the membrane surface and the feed/retentate spacer layer 
and/or permeate spacer. For low temperature applications polyethylene, 
polypropylene, nylon, etc. felt can be used as the shield. For high 
temperature applications the choice is more limited, with polyamide (e.g. 
Nomex which is a blend of high temperature nylon and polyester), teflon, 
fiberglass or mixtures thereof being suitable candidates. 
This/these shield layers are not included in the count of permeate spacer 
layer or feed/retentate spacer layers. When used on the feed side the 
shield layer protects the membrane from being punctured by the feed 
spacer; when used on the permeate side it protects the membrane from the 
support mesh/permeate spacer layers. 
The layer of feed/retentate material extended on the surface of at least 
one face of the permeate envelope is substantially equivalent in its 
dimensions in term of length and width to the permeate envelope. 
Adhesives are used in preparing the spiral wound element. Different 
adhesives for different types of applications and environments have been 
identified and are described in U.S. Pat. No. 4,464,494 and U.S. Pat. No. 
4,582,726 incorporated herein by reference. Various other adhesives such 
as high temperature epoxy (e.g. Tra-bond 2125 from Tra-Con or Duralco 
4400, 4525, 4700, 4703 from Cortronics Corp.) or non-epoxy adhesives (e.g. 
alumina/zirconia/ceramic adhesives such as Resbond 903 HP, 904 Zirconia, 
904 Quartz and 906 Magnesia from Cortronics Corp.) may also be used. 
The spiral wound module wrapped in its outer wrap and fitted with the 
anti-telescoping device can be inserted into a pressure vessel having an 
internal diameter equal to the exterior diameter of the module, and long 
enough to hold from one to any number of modules in series, said pressure 
vessel being fitted with feed entrance/retentate exit means and separate 
manifold means for recovering permeate from the open end of the hollow 
central mandrel. Alternatively, multiple modules can be installed in 
parallel within a single containment vessel as described in U.S. Pat. No. 
4,083,780. 
In producing the spiral wrapped modules of the present invention having 
feed/retentate and permeate spacers as described various materials of 
construction can be used to meet the required spacer characteristics. 
The feed/retentate spacer can be a woven mesh material or a non-woven mesh 
material, e.g., a first layer of parallel spaced apart filaments covered 
by a second layer of parallel spaced apart filaments laying perpendicular 
or diagonally to the first layer wherein the filaments of the first and 
second layer are attached to each other at their points of contact, such a 
material hereinafter referred to as non-interwoven filament material. 
When using such mesh materials as the feed/retentate spacer the spacer will 
comprise a single layer or multiple layers of material fat least one of 
which is 16 to 80 mesh, preferably 16 to 60 mesh, more preferably 20 to 60 
mesh and between 10 to 30 mils thick preferably 17.25 mils thick. For ease 
of fabrication it is preferred that 2 layers of material be used as the 
feed/retentate spacer both layers being preferably made of the same 
material. Use can be made of different mesh sizes to prevent intermeshing. 
If more than 3 layers are used, the layer in contact with the faces of the 
membranes would be a finer material and the layer between these face 
contacting layers would be a coarser material within the aforesaid limits, 
e.g.. the fine layers could have a 50 to 80 mesh while the coarser layer 
could have a 20 to 50 mesh. The material(s) used will be such as to 
provide a feed spacer having an open cross-sectional area of at least 30 
to 70%, preferably about 30 to 50%. As previously stated, it is preferred 
that the membrane and the feed/retentate spacer be separated from direct 
contact by an insulating layer of chemically and thermally inert woven or 
non-woven fabric such as Nomex. 
A problem encountered when using multiple elements in series is that the 
feed/retentate flow rate through the end elements is low since a 
significant portion of the feed would have permeated across the membrane 
in the first few elements. This results in low feed velocity through the 
end elements and the performance of these elements is compromised. The low 
velocity through the end elements is aggravated when a relatively high 
open area aluminum (30 mesh--0.01" wire diameter--49% open area) screen is 
used as the feed spacer in the spiral wound element design. This screen 
gives low feed-to-retentate pressure drop, which is an important 
consideration with six elements in series. With this element design, the 
overall pressure drop across the elements and with two intermediate heat 
exchangers used to reheat the feed is expected to be less than 15 psi. 
The feed velocity through the elements can be increased by using a lower 
open area screen as the feed spacer. An example of such a material would 
be 50 mesh--0.0090" wire diameter stainless steel screen which has an open 
area of 30%. When two layers of this screen are used as the feed spacer, 
the feed-to-retentate pressure drop is 4.6 psi at 10 kg/min feed rate. 
This represents a significant increase in pressure drop versus the 
previously identified design with a single layer of 30 mesh aluminum feed 
spacer. Although the 2.times.50 mesh screen design is excellent for 
increasing the feed velocity thus creating turbulence, a disadvantage is 
that the overall pressure drop when using multiple elements, e.g.. with 
six elements in series and two intermediate heat exchangers, is well over 
40 psi if this element design were used. Since it is necessary to maintain 
at least 10 psi pressure on the retentate, it would then be necessary to 
operate the lead element at over 50 psi inlet pressure. This is not 
acceptable since this would exceed the maximum tolerable pressure of 
pervaporation spiral wound elements which is around 40 psi. 
In a preferred pervaporation process the elements are staged by using 
increasingly higher pressure drop feed spacers in order to get high feed 
velocity through the end elements. With this staged pervaporation process, 
a relatively high open area feed spacer such as 30 mesh aluminum would be 
used for the first four elements while a relatively low open area feed 
spacer such as 2.times.50 mesh stainless steel would be used for the last 
two elements. With this combination, the overall pressure drop for the 
system is expected to be less than 25 psi, which would be acceptable. More 
importantly it can also be expected that the performance of the end 
elements to be greatly improved since the feed velocity would be high. 
Another example would be to use 30 mesh aluminum as the feed spacer for the 
first two elements, 40 mesh aluminum feed spacer for the second two 
elements, and 2.times.50 mesh stainless steel feed spacer for the last two 
elements. Needless to say, there are numerous other ways to stage the 
elements with increasingly higher pressure drop feed spacers in order to 
achieve the desired high velocity through the end elements. 
This use of increasing pressure drop feed/retentate spacers in the down 
stream elements of multiple spiral wound elements in series should also be 
useful for reverse osmosis and ultrafiltration wherein the performance is 
especially sensitive to feed velocity. 
The permeate spacer material used can also be selected from the aforesaid 
woven or non-interwoven filament materials. As previously stated the 
spacer comprises an assembly of three or more layers, alternating fine and 
coarse material. The fine material which supports the membrane and 
prevents intrusion into the permeate spacer can be a woven or 
non-interwoven filament material having at least a 50 mesh or finer, 
preferably 60 to 300 mesh, more preferably 60 to 150 mesh, still more 
preferably 80 to 120 mesh, most preferably 100 to 120 mesh and about 3 to 
15 mils thick. The coarse material can also be a woven or non-interwoven 
filament material having less than a 80 mesh, preferably less than 50 
mesh, more preferably less than 35 mesh, most preferably less than 20 
mesh, and from 10 to 30 mils thick preferably from 17-25 mils thick, it 
being understood that in practice the fine material used will have a finer 
mesh than the coarse material used. Likewise when using fine mesh material 
in the 200-300 mesh range it is preferred that the coarse layer be in the 
30 to 80 mesh range when the element is to be used at elevated temperature 
and pressure. 
The permeate spacer can comprise 3 or more layers. When 3 layers are 
employed two layers of fine material (support layers) are used in contact 
with the membrane layer and a coarse layer is interposed between the two 
fine layers. If 4 layers of spacer material are used, the two outer layers 
in contact with the membrane are still the fine material support layer and 
coarse material constitute the two inner layers interposed between the two 
fine outer layers. Care should be taken when using this 4 layer embodiment 
to insure that the two coarse layers which are in contact with each other 
either have different cross-sectional profiles or of the same 
cross-sectional profile are out of register one with the other to insure 
that the coarse materials do not intermesh with each other which if that 
happened would result in a substantial reduction in the open 
cross-sectional area of the materials available of permeate flow. In such 
an intermesh situation flow would be inhibited and an undesired pressure 
drop across the permeate spacer resulting in reduced flux would be 
encountered. If five layers are employed they would be arranged in a 
fine/coarse/fine/coarse/fine sequence wherein the three fine layers could 
be the same or different materials of the same or different fine 
cross-sectional area, within the previous definition of fine material; 
likewise the two coarse layers could be the same or different materials of 
the same or different coarse cross-sectional area, again within the 
previous definition of coarse material. 
The spacer materials can be made from any plastic or metal, e.g., 
polyester, polysulfone, polyester, nylon, teflon, etc., or fiber glass, or 
stainless steel, aluminum or brass etc. In general any material which will 
be chemically inert and thermally stable in the intended environment of 
use of the final element can be employed as a material of construction. It 
is preferred however that the spacer material be made of metal, e.g. 
aluminum or stainless steel and more preferably that it be steel 
especially in the case of the fine mesh material. In order to insure 
optimum operability of the final element the spacer should be capable of 
preventing membrane intrusion into the permeate space under the pressures 
employed. This ability to prevent membrane intrusion has been correlated 
to spacer stiffness. An available measure of stiffness is the tensile 
modulus of elasticity. The stiffness of a number of common spacer 
materials is presented below: 
______________________________________ 
Polyester 2-3 .times. 10.sup.5 lbs/sq. inch 
Aluminum 10 .times. 10.sup.6 lbs/sq. inch 
Stainless steel 28 .times. 10.sup.6 lbs/sq. inch 
______________________________________ 
Thus, in the present invention the fine permeate spacer support material, 
in addition to having the recited open cross sectional area, has a 
stiffness of at least about 2-3.times.10.sup.5 lbs/sq. inch, preferably at 
least about 10.times.10.sup.6 lbs/sq. inch, most preferably at least about 
28.times.10.sup.6 lbs/sq. inch and higher, the fine permeate spacer 
support material being most preferably stainless steel. 
The stiffer material provides better support which eliminates or minimizes 
intrusion which in turn minimizes permeate pressure drop. This is 
especially true at operating conditions (i.e. in permeate at 
140.degree.+C.). This is to be compared to the performance of a less stiff 
material such as polyester. Because of the low stiffness factor of 
polyester, both the membrane and the polyester support are pushed into the 
permeate spacer channels, especially at higher temperatures and/or 
pressures. Thus if one considers material stiffness with mesh size, a 
finer material of greater stiffness can be used with a more coarse layer 
than can a similar fine mesh material of lesser stiffness. 
For example while a 200 mesh aluminum support may work satisfactorily with 
an 80 mesh coarse layer, the 200 mesh aluminum support would not be 
satisfactory with a 17.50 mesh coarse layer. However, a 200 mesh stainless 
steel support would be satisfactory with a 30-80 mesh layer because of its 
greater stiffness. Specific selections of materials within the aforesaid 
recitations are left to the practitioner to make with consideration being 
paid to the temperature and pressure of element application and the design 
or target permeate pressure drop across the element. 
When the element is to be used for pervaporation it is preferred that the 
fine material used as permeate spacer support be in the 60 to 150 mesh, 
preferably 80 to 120 mesh range 5 to 15 mils thick and be of stainless 
steel while the coarse material has a mesh size of less than 50 mesh and 
is 15 to 30 mils thick. 
In addition to the woven or non-interwoven filament materials previously 
described, the spacer materials having the necessary cross-sectional areas 
can be materials which exhibit no mesh but rather are spaced apart ribs 
running in parallel on a thin solid support sheet. Such sheets can be 
fabricated by casting or extruding with the aforesaid ribs cast or 
extruded as integral parts of the sheet. Alternatively individual 
filaments can be deposited on a pre-existing sheet. The channels defined 
by the spaces between the parallel ribs or filaments and the height of the 
ribs or filaments would provide the cross-sectional areas falling within 
the aforesaid definitions. Use of such materials would require that the 
sheets be oriented in the permeate envelope such that the channels would 
be aligned in the direction of permeate flow in the envelope into the 
hollow central mandrel. 
By the practice of the present invention performance of the spiral wound 
element in terms of both flux and selectivity is nearly identical to that 
of the membrane when used by itself, uninfluenced by any hydrodynamic 
effects introduced by spacer materials. 
The present invention is especially useful in the separation of aromatics 
from non-aromatics, such as in heavy cat naphtha separation, intermediate 
cat naphtha separation, light cat naphtha separation etc. 
Membranes which are useful in such separations include polyurea urethane 
disclosed and claimed in U.S. Pat. No. 4,914,064, polyurethane imides 
disclosed and claimed in U.S. Pat. No. 4,929.358, polyester imides 
disclosed and claimed in U.S. Pat. No. 4,944,880, isocyanurate crosslinked 
polyurethane membranes, disclosed and claimed in U.S. Pat. No. 4,983,338 
and U.S. Pat. No. 4,929,357, polyester membranes disclosed and claimed in 
U.S. Pat. No. 4,976,868, preferably the polyester imides of U.S. Pat. No. 
4,944,880 and U.S. Pat. No. 4,990,275, all of which are incorporated 
herein by reference. Polyacrylate membranes may also be used. Acrylic acid 
ester homopolymers or their copolymers with each other or with acrylic 
acid can be formed into membranes. The acrylic acid monomer units can be 
in free-acid form or partly or totally neutralized with metal or 
alkylammonium ions. The membranes can be covalently or ionically 
crosslinked. 
It has been found that membranes, such as the polyester imide membranes 
which when used in aromatics/non-aromatics separation processes such as 
heavy cat naphtha separation lose performance overtime due to the build up 
of a corrosion deposit layer (e.g. iron sulfide) on the membrane can be 
restored to their original performance levels by soaking the membrane in a 
gasoline dispersant/detergent, which is a surface active material having a 
molecular weight in the range from 500 to 3000. The dispersant/detergent 
has a backbone which can be polybutene or polypropylene, bearing with 
functional groups comprising ether amines, hydrocarbonyl amines, 
hydrocarbonyl amides or mixtures thereof. As example of a useable membrane 
regeneration detergent/dispersant is CS-3 Kerofluid available from BASF. 
It is also important in membrane separation processes especially 
pervaporation processes that the membranes be defect free. The presence of 
holes in membranes can significantly decrease membrane selectivity 
performance. It has been found that micro defects in pervaporation 
membranes can be identified before module or element assembly by brushing 
the surface of the membrane with a liquid such as heptane and pulling a 
vacuum or just brushing the surface of the membrane with a water, 
isopropyl alcohol (IPA) mixture (e.g. 50/50 by weight). Heptane or IPA 
passing through the defects wet the backing of the membrane upon which the 
membrane is cast resulting in translucent spots thus identifying the 
defect which can be patched by applying glue over the defect area.

The present invention is illustrated in the following non-limiting 
examples. 
EXAMPLE 1 
An element wherein the feed/retentate and permeate spacers consisted of 14 
mesh polyester (the permeate spacer being separated from the membrane by 
layers of Tricot 8846 polyester serving as membrane support) was compared 
with an improved element within the scope of the present invention wherein 
the 14 mesh polyester permeate spacer was sandwiched between layers of 80 
mesh stainless steel as support, (no Tricot 8846 polyester support layer 
being employed) and wherein the feed/retentate spacer was a 33 mesh 
Teflon. Tricot 8846 is available from Hornwood Inc. of Maryland. It is a 
woven fabric 4 mils thick having wales of 48 strands per inch and courses 
of 58 stands per inch. This fabric is coated with epoxy having a resin 
pick-up of 16%. 
The two elements were evaluated for the separation of heavy cat naphtha at 
100.degree. C. and 10 mbars permeate pressure using a polyurea/urethane 
membrane. 
The membrane was prepared as follows: 
A solution containing a polyurea-urethane polymer is prepared. Four point 
five six (4.56) grams (0.00228 moles) of polyethylene adipate (MW--2000), 
2.66 grams (0.00532 moles) of 500 MW polyethylene adipate and 3.81 grams 
(0.0152 moles) of 4,4'diphenylmethane diisocyanate are added to a 250 ml 
flask equipped with a stirrer and drying tube. The temperature is raised 
to 90.degree. C. and held for 2 hours with stirring to produce an 
isocyanate-end capped prepolymer. Twenty grams of dimethylformamide is 
added to this prepolymer and the mixture is stirred until clear. One point 
five grams (0.0076 moles) of 4,4'diamino-diphenylmethane is dissolved in 
ten grams of dimethylformamide and then added as a chain extender to the 
prepolymer solution. This mixture was then allowed to react at room 
temperature (approx. 22.degree. C.) for 20 minutes. This solution was 
diluted to 5 wt % such that the solution contained a 60/40 wt % blend of 
dimethylformamide/acetone. The solution was allowed to stand for one week. 
The viscosity of the aged solution was approximately 35 cps. After this 
period of time one wt % Zonyl FSN (Dupont) fluorosurfactant was added to 
the aged solution. A microporous teflon membrane (K-150 from Gore) with 
nominal 0.1 micron pores, 75% porosity cast on a non-woven 
Nomex/polyethylene terephthalate backing, the combination being 4 mils 
thick was coated with the polymer solution in a continuous operation. The 
coating was dried in an oven heated to 60.degree. C. This technique 
produced a composite membrane with a polyurea/urethane layer between 3 to 
4 microns in thickness. 
The results of the evaluation are presented in Table 1. 
TABLE 1 
______________________________________ 
PERFORMANCE DIFFERENCE BETWEEN 
PERVAPORATION ELEMENT DESIGNS 
______________________________________ 
Spiral Wound Element Design 
Permeate Spacer support/ 
Yes/Tricot 8846 
Yes/80 mesh 
Mesh Size Stainless Steel 
Permeate Spacer Mesh Size 
14 14 
Feed Spacer Mesh Size 
14 33 
Element Performance (1) 
Selectivity, Delta RON 
8.1 10.2 
Flux, Kg/M2-Day 31 48 
______________________________________ 
(1) At 100.degree. C. and 10 mbars permeate pressure. 
As can be seen the table, with the spiral wound design using polyester 
tricot spacer support, a selectivity of 8.1 was achieved. In contrast, 
with the spiral wound design using stainless steel spacer support, a 
selectivity of 10.2 was achieved. In aromatics/saturates separation, 
selectivity is measured by the octane difference between the permeate and 
the feed. The flux also improved significantly. It can be expected that 
the performance difference between the packages will be even greater at 
higher temperatures since flux would be significantly higher. 
After the evaluation of element performance using heavy cat naphtha was 
completed, studies directly measuring the flow characteristics of the 
permeate spacers were made. For these studies the outer wrap layer around 
the element was removed and the permeate envelope of one leaf was 
carefully opened to the atmosphere along the sealed edge furthest removed 
from and parallel with the hollow central mandrel. Various vacuum levels 
were then drawn on the hollow central mandrel and the resulting air flow 
rates through the permeate spacer were measured. The data are presented in 
Table 1B. 
TABLE 1B 
______________________________________ 
Comparison of Flow Characteristics of 
Permeate Spacer Designs 
vacuum level in 
central mandrel 
measured flow rate, 1/min (air at STP) 
(i.e. pressure 
element element 
drop in permeate 
with polyester 
with stainless 
spacer), mm Hg 
tricot support layer 
steel support layer 
______________________________________ 
4.0 4.0 
7.6 6.8 
14.7 11.3 
16.2 25.1 
27.9 11.9 
32.1 36.8 
46.0 46.4 
50.8 30.3 
54.0 19.3 
76.2 38.5 
127. 53.5 
140. 62.3 
______________________________________ 
The data show that for a given pressure drop, the air flow through element 
with the stainless steel support layer is significantly greater than for 
the element with the polyester Tricot 8846 support layer. A spacer flow 
resistance parameter can be calculated from a linear regression of the 
data. For the element, employing the polyester Tricot layer the parameter 
is 2.30 mm Hg/(1/min of STP air) while for the element employing the 
stainless steel layer it is 0.96 mm Hg/(1/min of STP air). A low value of 
the flow resistance parameter is desirable since it indicates reduced 
permeate pressure drop and thus that lower average permeate absolute 
pressure exists. For pervaporation, lower permeate absolute pressure is 
associated with higher selectivity and higher flux. 
EXAMPLE 2 
A separate example was performed to determine the effect of feed/retentate 
spacer open cross-sectional area and thickness on permeator performance. 
Four elements were fabricated. Each element used the same polyester-imide 
membrane material, which was made by first endcapping one part of 2000 
molecular weight polyethylene adipate (PEA) with two parts of pyrometallic 
dianhydride (PMDA) and then reacting one part of the endcapped polymer 
with methylene dianiline (MDA) to form a polyamic acid. The polyamic acid 
was then coated onto a 0.1 micron teflon sheet as previously described in 
Example 1. The viscosity of the polyamic acid was in the range of 90-150 
cps at room temperature. After the polyamic acid was deposited onto the 
teflon sheet, the polyamic acid was cured at 260.degree. C. for 7.25 
minutes. The permeate spacer used consisted of five layers of screens, 120 
mesh stainless steel/17 mesh aluminum/120 mesh stainless steel/17 mesh 
aluminum/120 mesh stainless steel. 
The four elements were evaluated on heavy cat naphtha at 140.degree. C. and 
10 mbar permeate pressure. 
Feed spacer of various open cross-sectional area (mesh) and thickness were 
used. The results are presented in Table 2. 
TABLE 2 
______________________________________ 
EFFECT OF FEED SER ON 
PERVAPORATION ELEMENT PERFORMANCE 
______________________________________ 
Feed Spacer 
Screen Size, Mesh 
67 33 18 14 
Thickness, Mils 
10 21 17 33 
Element Selectivity 
Delta RON 8.9 10.2 11.8 8.1 
______________________________________ 
It is seen that with thick/coarse spacer (33 mils/14 mesh) the element 
performance is low because feed velocity is relatively low at a given flow 
rate. Conversely with a thin/fine spacer (10 mils/67 mesh) element because 
of high pressure drop the performance, while improved over that achieved 
with the thick/coarse spacer is not as high as obtained with a spacer of 
more moderate thickness/and a coarseness between fine and coarse. 
The feed spacer in permeation elements, therefore, advantageously ranges 
from 16 to 80 mesh, preferably 16 to 60 mesh, more preferably 20 to 60 
mesh and from 10 to 30 mils thick, preferably 17-25 mils thick. 
EXAMPLE 3 
To further illustrate the effectiveness of the improved element design, 
several elements were fabricated and tested for the pervaporative 
separation of heavy cat naphtha. All test elements employed the same 
membrane material, the polyester imide of Example 2 cast on the same 
teflon membrane backing as previously described. 
Four sample elements were fabricated, A, B, C and D. 
Element A employed single layers of 14 mesh polyester as both the feed 
spacer and permeate spacer. When used as the permeate spacer it was 
isolated from the membrane surfaces by two layers of polyester felt spacer 
support (Tricot 8846), one layer on each side of the permeate spacer, to 
prevent damage to the membrane layers by the coarse permeate spacer during 
element fabrication. 
Elements B, C and D used multiple layers of permeate spacer material which 
were isolated from the membrane surfaces by intervening layers of Nomex, 
and either single or multiple layers of feed spacer material between 
adjacent permeate envelope leaves, in all cases the feed spacer material 
being a finer material having a mesh greater than 20. 
Table 3 presents the details of the feed and permeate spacers used in each 
of the four membrane elements and indicated the selectivity of each 
element in terms of delta MON (motor octane number) of the resulting 
permeate. 
The elements were all tested in a recirculating pilot plant with heavy cat 
naphtha at 140.degree. C., 15 psi feed pressure and 10 mbars permeate 
pressure. The effectiveness of each element was assessed by measuring the 
difference in the motor octane number (delta MON) between the permeate and 
the feed. Flux was not compared because the HCN used in the tests had been 
exposed to oxygen which negatively effects the flux performance of the 
membranes. 
When used by itself in a flat circular test cell without any feed or 
permeate spacers the membrane supported by a fine sintered porous metal 
support, under the same conditions exhibited a selectivity in terms of 
.DELTA.MON of about 12.9. 
As can be seen from Table 3, Element A exhibited a selectivity in terms of 
.DELTA.MON of 9.9, which is 3.0 MON lower than the membrane by itself. 
In comparison preferred elements B, C and D of the present invention using 
stiffer spacer material and finer/stiffer feed/retentate spacer material 
exhibited selectivity in terms of .DELTA.MON ranging from 11.7 to 12.4. in 
all cases an element efficiency of over 90%. 
It is expected that if the membrane was placed directly on the 14 mesh 
polyester permeate spacer without any fine mesh support intervening layer 
between the membrane and the 14 mesh polyester spacer, the membrane would 
fully embed into the permeate spacer, resulting in severely degraded 
performance, or be punctured by the spacer, resulting in inoperability. 
TABLE 3 
______________________________________ 
PERFORMANCE OF PERVAPORATION ELEMENT 
FEED PERMEATE 
SER SER SELEC- 
ELEMENT ARRANGE- ARRANGE- TIVITY, 
NO. MENT MENT DELTA MON 
______________________________________ 
A 14 MESH PE PE FELT 9.9 
(Tricot 8846) 
14 MESH PE 
PE FELT 
Nomex 
B 33 MESH AL 120 MESH SS 11.7 
33 MESH AL 17 MESH AL 
17 MESH AL 
120 MESH SS 
Nomex 
Nomex 
C 33 MESH AL 100 MESH SS 11.7 
17 MESH AL 
100 MESH SS 
17 MESH AL 
100 MESH SS 
Nomex 
Nomex 
D 50 MESH SS 100 MESH SS 12.4 
50 MESH SS 17 MESH AL 
100 MESH SS 
17 MESH AL 
100 MESH SS 
Nomex 
______________________________________ 
(1) Delta MON Selectivity of PEI Membrane alone is 12.9 
(2) PE: Polyester 
EXAMPLE 4 
A number of spiral wound element packages were prepared to evaluate the 
effect of putting a non-woven shield layer between the membrane and the 
feed/retentate spacer layers. Each element used the same membrane as 
described in Example 2. The results are presented below: 
______________________________________ 
Element I II 
______________________________________ 
Feed Spacer Arrangement Nomex 
50 mesh SS 50 mesh SS 
50 mesh SS 50 mesh SS 
Nomex 
Permeate Spacer 
Nomex Nomex 
Arrangement 100 mesh SS 100 mesh SS 
17 mesh AL 17 mesh AL 
100 mesh SS 100 mesh SS 
17 mesh AL 17 mesh AL 
100 mesh SS 100 mesh SS 
Nomex Nomex 
Vacuum Drop Time (min) 
4 21 
______________________________________ 
The vacuum drop time is a measure of the tightness of the spiral wound 
element package. In this test a 29" Hg vacuum is pulled on the element. 
The vacuum pump is then turned off. The pressure inside the element rises. 
The vacuum drop time is the time which it took the element to go from 29" 
to 22" Hg vacuum. The longer the vacuum drop time, the tighter is the 
element. 
Elements II and III were evaluated for vacuum drop time and also for the 
pervaporative separation of heavy cat naphtha. The pervaporation test was 
conducted on heavy cat naphtha initially at 140.degree. C. for 2 to 10 
days and then at 150.degree. C. for 20 to 21 days and 10 mm Hg vacuum 
pressure at a flow rate of 1300 lbs/hour. The results from the 150.degree. 
C. runs are reported in detail below. 
______________________________________ 
Element III II 
______________________________________ 
Feed Spacer Arrangement Nomex 
30 mesh AL 50 mesh SS 
50 mesh SS 
Nomex 
Permeate Spacer 
Nomex Nomex 
Arrangement 100 mesh SS 100 mesh SS 
17 mesh AL 17 mesh AL 
100 mesh SS 100 mesh SS 
17 mesh AL 17 mesh AL 
100 mesh SS 100 mesh SS 
Nomex Nomex 
Vacuum Drop Time (min.) 
4 21 
Days on Oil at 150.degree. C. 
20 21 
Initial Flux (kg/m.sup.2 /day) 
288 229 
Flux drop per day (%) 
-0.27* 0.24* 
Initial Permeate RONC 
100.5 100.4 
Permeate RONC Drop 
-0.087 -0.018* 
per day (%) 
______________________________________ 
*Not Statistically Significant 
(RONC Research Octane Number Clear) 
As can be seen from the table above, at 150.degree. C. high temperature 
operation element III with no Nomex showed higher initial performance, 
higher flux at constant selectivity as compared to element II which has 
Nomex on the feed side. However, element III showed significant loss in 
selectivity with days on oil whereas element II showed no statistically 
significant loss in selectivity with days on oil. The flux stabilities of 
both elements were satisfactory. 
In the above elements the 50 mesh SS is 11 mils thick, the 100 mesh SS is 9 
mils thick, the 17 mesh AL is 23 mils thick, the 30 mesh AL is 24 mils 
thick. The Nomex layer is a non woven fabric from Veratek Inc. The Nomex 
is identified as Nomex 1019 and is composed of a mixture of polyamide and 
polyester. It is 4.6 mils thick, has a weight of 3 oz/sq. yard and a 
Frazier air permeability of 2.5 cfm/ft.sup.2 at 1/2 inch water pressure. 
The elements were assembled using Tra-bond 2125 adhesive using 9 parts 
resin to 1 part catalyst as per manufacture recommendations. A diluent was 
added to make it less viscous. The diluent was Santicizer 160 plasticizer 
from Monsanto which is butyl benzyl phthalate used at 10% diluent 90% 
Tra-bond 2125. No surface treatment was needed for the permeate spacers 
although the central tube was wiped with B.F. Goodrich A-934-BY primer to 
remove any grease or dirt.