Note: Descriptions are shown in the official language in which they were submitted.
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HEAT EXCHANGER PLATES WITH ANTI-FOULING PROPERTIES
Field of the Invention
The present invention relates to a plate for a plate heat exchanger, a
plate pack, a plate heat exchanger and a method of producing a plate for a
plate heat exchanger for improving anti-fouling properties and facilitating
cleaning of plate heat exchangers.
Background
In many industrial processes fouling of heat transfer equipment is of
concern. In order to keep a satisfying performance of the equipment regular
service and cleaning it is necessary to remove build up of deposits on the
heat
transfer plates. The deposits arise e.g. from the fluids in the equipment,
microbial growth and/or dirt.
Plate heat exchangers (PHE) in use may over time get fouled which
leads to a decreased heat transfer and increased pressure drop, and thus leads
to an overall reduced performance of the heat exchanger. Thus heat
exchangers which are not permanently joined will eventually need to be opened
and cleaned. Depending e.g. on the fluids used in the heat exchanger plates
may be seriously fouled and difficult to clean, thus requiring strong
detergents
and/or powerful mechanical cleaning over a substantial time period in order to
restore the performance of the heat exchanger. The cleaning of PHE may both
be time consuming and costly. Also, the process to which the PHE normally is
connected to may have to be shut down during said cleaning of the PHE.
The plates of heat exchangers are made of metal sheets. The base
material, i.e. metals used, have a high surface free energy that results in
most
liquids easily wetting the surface of the sheets.
Also, when heat exchanger plates are produced the forming operation of
sheet metal increases the surface roughness which often is associated with
faster build up of fouling deposits.
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W02009034359 discloses provision of a coating to reduce biofouling of
surfaces in aquatic environments wherein the coating is applied by use of
Plasma Assisted Chemical Vapour Deposition (PACVD).
US20090123730 discloses a surface of a heat exchanger which is to be
soldered by means of a flux, and said surface is in addition to the flux also
provided with at least one more layer containing an additive. The additive is
reacted in order to modify the surface during soldering.
W02008119751 discloses production of a hydrophobic coating for
condensers wherein the coating comprises sol-gel materials based on e.g.
silicon oxide sol.
JP2000345355 relates to improving corrosion resistance and discloses a
film consisting of 55-99 wt% Si02 and 45-1wt% Zr02 which film is formed using
sol-gel processing.
US2006/0196644 discloses a heat exchanger provided with a hydrophilic
surface coating comprising a gel produced by sol-gel processing.
It would be desirable to find new ways to ensure less fouling of heat
exchangers and their plates in order to keep the heat exchangers running for
longer time periods as well as more easily cleaned heat exchangers and plates.
Also, a reduced shut down time for processes where PHEs are involved would
be desirable.
A problem encountered with presently known antifouling coatings is the
poor wear resistance of the coatings in applications with abrasive heat
exchanging media, e g sand or other particulate material which enters the PHE
with the heat exchanging fluids. Furthermore, cracks in the coating may occur
due to torque and tension forces acting on the plate packages in applications
under high pressures.
Summary of the invention
It is an object of the present invention to provide improved plates for a
PHE, which show a reduced fouling of the plates when in use in a PHE. Another
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object is to achieve plates for a PHE having an antifouling surface which are
wear resistant in abrasive environments and have high resistance against
formation of cracks.
This object is achieved by plates being coated with a sol-gel composition
which results in plates with reduced fouling of the plate when in use in a
heat
exchanger. By applying a coating composition comprising sol-gel material with
organosilicon compounds to the heat exchanger plate both the surface free
energy and roughness is lowered, leading to reduction of fouling and easy
cleaning of heat exchanger plates. Moreover, the sol-gel coated PHE plates of
the invention exhibit an excellent wear resistance and have a flexibility that
reduces the risk of cracks appearing in the coating.
The present invention also relates to a heat exchanger and a plate pack
for plate heat exchangers comprising a number of heat transfer plates of the
kinds as defined herein.
The present invention further relates to a method of producing a heat
transfer plate comprising the steps of:
a) forming a plate for a plate heat exchanger from a base material,
b) preparing a composition by means of sol-gel processing on at least a
part of the plate, which composition comprises organosilicon compounds,
c) drying and/or curing said composition to form a coating comprising
silicon oxide (SiOx).
Brief Description of the Drawings
Fig. 1 is a schematic drawing of the M20 PHE plate pack with the relative
position of the plates used in tests, both plates coated according to the
present
invention and conventional uncoated plates.
Fig. 2 shows pictures of plates according to the present invention and a
conventional plate after disassembly after operation for 7 months.
Fig. 3 is a schematic cross section of a plate for a plate heat exchanger
having an anti fouling coating according to the invention.
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Detailed description of the invention
The coating used according to the present invention may be referred to
as a non-stick coating and makes it easy to clean the plates of a fouled heat
exchanger. The coated plates according to the present invention show a better
heat transfer over time compared to conventional heat exchanger plates since
the latter ones gets fouled much quicker and thus decrease the heat transfer
performance to a larger extent. The coating of the plates also results in a
much
more even surface thus resulting in better flow characteristics. Also the
pressure drop is reduced over time for a plate heat exchanger according to the
present invention in comparison with conventional plate heat exchangers, since
the buildup of impurities, microorganisms and other substances is not as
pronounced.
The coated plates according to the present invention may easily be
cleaned just using high pressure washing with water. With a plate according to
the present invention there is no need for extensive time consuming mechanical
cleaning or cleaning using strong acids, bases or detergents, such as e.g.
NaOH and HNO3.
According to the present invention a plate for use in a plate heat
exchanger is coated with a composition comprising organosilicon compounds
using a sal-gel process. The organosilicon compounds are starting materials
used in the sol-gel process and are preferably silicon alkoxy compounds. In
the
sol-gel process a sol is converted into a gel to produce nano-materials.
Through
hydrolysis and condensation reactions a three-dimensional network of
interlayered molecules is produced in a liquid. Thermal processing stages
serve
to process the gel further into nano-materials or nanostructures resulting in
a
final coating. The coating comprising said nano-materials or nanostructures
mainly comprise silicon oxide, SiO., having an atomic ratio of 0/Si > 1,
preferably an atomic ratio of 0/Si ?_ 1.5-3, and most preferably 0/Si 2-2.5. A
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preferred silicon oxide is silica, Si02. The siliconoxide forms a three
dimensional
network having excellent adhesion to the plates.
The coating of the present invention further has a content of carbon such
as found in hydrocarbon chains. The hydrocarbons may or may not have
5 functional groups such as found in hydrocarbon chains or aromatic groups,
e g
C=0, C-0, C-O-C, C-N, N-C-0, N-C=O, etc. Preferably the carbon content is ?
atomic%, preferably? 20-60 atomic%, and most preferably? 30-40
atomic%. The hydrocarbons impart flexibility and resilience to the coating
which
is especially important in gasketed plate heat exchangers since the plates
move
10 during operation due to high pressures exerted on the plates in the
plate
package. The hydrocarbon chains are hydrophobic and oleophobic which
results in the non-stick properties of the coating.
In Fig 3 is shown a schematic drawing of a plate for a plate heat
exchanger provided with a siliconoxide sol gel coating. Between the plate 30
itself and the siliconoxide layer is an interface 32 between the coating
siloxane
and and a metal oxide film of the plate. The coating bulk that follows said
interface is the siloxane network 34 with organic linker chains and voids that
impart flexibility to the coating. The outermost layer is a functional surface
36, i
e a hydrophobic/oleophobic surface for fouling reduction.
By the combination of a durable and yet flexible coating, a plate for a
plate heat exchanger is achieved which has excellent non-stick properties and
also is wear and crack resistant. The flexibility of the coating is especially
important in plate heat exchangers provided with gaskets between the plates
since it is a well known problem that the plate package is not rigid resulting
in
that coatings on the plates tend to crack when the flexible plates move and
bend in relation to each other, a phenomenon called "snakeing".
In one embodiment of the present invention at least one sol comprising
organosilicon compounds is applied to the surface to be coated. The surface
may be wetted/coated with the sol in any suitable way. It is preferable for
the
surface coating to be applied by spraying, dipping or flooding. At least a
part of
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one side of the heat exchanger plate is to be coated. Alternatively, all
surfaces
of at least one side of a plate which during use in a PHE would be in contact
with a fluid are coated. Also, at least one side of a heat exchanger plate may
be
entirely coated. Alternatively, both sides of the plate may be coated. If both
sides are coated, they may be partly or fully coated, in any combination.
Naturally, more surfaces than the surfaces intended to be in contact with
fluid
may be coated. Preferably, all surfaces in contact with a fluid giving rise to
fouling are coated. Also the gaskets may be coated with the composition
according to the present invention. The coating composition is preferably only
applied on the surface of gaskets designated to be in contact with at least
one
fluid when in use in a PHE. In view of the above the coating composition
according to the present invention may be applied to bare PHE plates or PHE
plates with gaskets attached to them. When discussed in the present
application surfaces of plates and gaskets in contact with at least one fluid
is
intended to relate to surfaces in contact with fluid(s) within the heat
exchanger.
In another embodiment the method comprises a pretreatment of at least
the surfaces on the heat exchanger plates to be coated with at least one sol.
This pretreatment is also preferably carried out by means of dipping, flooding
or
spraying. The pretreatment is used to clean the surfaces to be coated in order
to obtain increased adhesion of the latter coating to the heat exchanger
plate.
Examples of such pretreatments are treatment with acetone and/or alkaline
solutions, e.g. caustic solution.
In another embodiment the method comprises thermal processing
stages, e.g. a drying operation may be carried out after a pretreatment and a
drying and/or curing operation is often necessary after the actual coating of
the
plate with said sol. The coating is preferably subjected to heat using
conventional heating apparatus, such as e.g. ovens.
The composition comprising SiOx is applied to a plate to be used in a
plate heat exchanger. The application of the composition is done by means of
sol-gel processing. The resulting film of said composition on the plate is
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preferably between 1 and 30 pm thick. The thickness of the coated film is
important for the use of the plate in a not permanently joined heat exchanger.
A
film thickness below 1 pm is considered being not enough wear resistant since
the plates in a plate heat exchanger in use are able to move slightly in
relation
to each other. This slight movement causes wear on the film and with time the
coating will become worn down. Also the thickness of the film has an upper
limit
since the application of substances on the heat transfer plates influences the
heat transfer and thus the performance of the plate heat exchanger. The upper
limit for the applied film is preferably 30 pm. Thus, the film thickness of
the
silicon oxide sol containing composition is 1-30 pm, preferably 1.5-25 pm,
preferably 2-20 pm, preferably 2-15 pm, preferably 2-10 pm and preferably 3-10
pm.
The base material for the plates may be chosen from several metals and
metal alloys. Preferably, the base material is chosen from titanium, nickel,
copper, any alloys of the before mentioned, stainless steel and/or carbon
steel.
However, titanium, any alloys of the before mentioned or stainless steel is
preferred.
Examples
In the search for prolonged operational time of off-shore equipment, tests
were conducted on low surface energy glass ceramic coatings.
Two low surface energy glass ceramic coatings Coat 1 and Coat 2 were tested
and the results are presented below. Coat 1 is a silan terminated polymer in
butyl acetate and Coat 2 is a polysiloxan-urethan resin in solvent
naphtha/butyl-
acetate.
Phase A
The analysis documents the properties of coatings concerning substrate
wetting and adhesion, contact angel, coating thickness and stability towards
1.2
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% HNO3 in H20, 1 % NaOH in H20 and crude oil. The results are summarized
below in Table 1.
Table 1
Coat 1 Coat 2
Substrate Excellent Excellent
wetting
Substrate Al: 0/0 Al: 0/0
adhesion
Stainless steel: 0/0 Stainless steel: 0/0
Ti: 0/0 (see below) Ti: 0/0 (see below)
Contact angel
H20: 102-103 H20: 102-103
measurements
Coating
thickness 4-10 pm 2-4 pm
Stability 1.2% HNO3 in H20: 1% hat 75 C 1.2% HNO3 in H20: 1% hat
75 C
1% NaOH in H20: 3 hat 85 C 1 % NaOH in H20: 2 hat 85 C
Crude oil: 6 months at RT Crude oil: 6 months at RT
Both coatings showed excellent wetting when spray coated onto either
stainless steel or titanium substrates.
Adhesion was determined by cross-cut/tape test according to DIN EN
ISO 2409. Rating is from 0 (excellent) to 5 (terrible). 0 or 1 is acceptable
while 2
to 5 is not. First digit indicates rating after cross cut (1 mm grid) and the
second
digit gives rating after tape has been applied and taken off again.
To obtain the best adhesion for Coat 1 and Coat 2 the substrates required pre-
treatment.
To obtain the best adhesion of Coat 1 on stainless steel the substrate
must be pre-treated. The substrate is submerged in an
alkaline cleaning detergent for 30 minutes. Afterwards the substrate is washed
with water and demineralized water and dried before Coat 1 is applied within
half an hour to achieve the optimal adhesion. Tests have shown the adhesion is
reduced if cleaning of the substrate is only carried out with acetone. Pre-
treatment is also necessary for stainless steel substrates coated with Coat 2.
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This coating displayed unaffected adhesion whether an alkaline detergent or
acetone was used as pre-treatment. If the pre-treatment step is neglected or
not
made correctly it will affect coating adhesion.
Both coatings showed good stability under acidic condition. The coatings
were stable for 11/2 hour at 75 C and more than 24 hours at room temperature.
Under alkaline conditions Coat 1 showed a better result than Coat 2.
Coat 1 could withstand the alkaline conditions for 3 hours at 85 C and Coat 2
for 2 hours at 85 C. Both coatings showed no decomposition or reduction in
oleophobic properties after being submerged for 6 months in crude oil at room
temperature.
Phase B
M3 heat exchanger plates with gaskets were partly coated and then
tested. To examine whether the partly coated gaskets would compromise the
operation of the PHE, these test included pressure tests. It was concluded
that
partly coating of gaskets with the coatings did not impact operation. The M3
plates were not operated with crude oil.
Phase C
Coating of PHE plates
Coat 1 and Coat 2 were applied to a total of 30 titanium M20 heat
exchange plates (measuring 175 x 62 cm) used in a crude oil cooler. All plates
underwent pre-treatment which consisted of:
1. Submerging in liquid nitrogen (- 196 C) to remove gaskets
2. Treatment with acidic and alkaline solutions to remove fouling
3. High pressure washing of the plates with water
4. Assembly of the PHE stack for pressure testing
5. Disassembly of the PHE stack. Plates left to dry before application
This pre-treatment was completed the day before Coat 1 and Coat 2
were applied to the plates. Consequently, this procedure did not follow the
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recommended approach as outlined in Phase A. As the plates have been left to
dry at ambient temperature, some plates were still wet. 15 plates were treated
with Coat 1 and the remaining 15 plates with Coat 2 by spray coating. The heat
exchanger plates were coated on both sides and then placed in a rack. As the
5 plates had glued gaskets on them, both the plates and the gaskets were
coated. The final film thickness was measured to be 2-4 pm and the coating
was applied on both sides of the plates. Curing/drying was performed at
elevated temperatures of 200 C or 160 C respectively for 11/2 hour in an on-
site oven. Upon completion the coated heat exchangers were weighed and
10 coating thickness was measured. It was observed that some plates had
some
coating imperfections and small defects.
All plates were stamped with a unique number for later identification.
The heat exchanger plates were then assembled with the remaining
untreated 319 plates. The coated prates were placed respectively in the front,
middle and end of the assembled unit and the position of the coated plates in
the PHE stack are shown in Fig 1. Fig 1 illustrates five plates coated with
Coat
1 at 10, five plates without coating at 12, five plates coated with Coat 2 at
14,
five plates coated with Coat 1 at 16, five plates without coating at 18, five
plates
coated with Coat 2 at 20, five plates coated with Coat 1 at 22, five plates
without coating at 24, five plates coated with Coat 2 at 20. The evaluation of
the
coated plates was performed after more than seven months of operation.
The plates that later, after termination of off-shore operation, were
chosen for detailed analysis were positioned at the left (plate no. 3 and 6),
middle (plate no. 12 and 17) and the right (plate no 22 and 29) positions in
Figure 1.
Phase D
Determination of content in coating by XPS analysis
Three different silicon oxide-coated Ti substrates were analyzed before
and after use by means of XPS (X-ray Photoelectron Spectroscopy), also
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known as ESCA (Electron Spectroscopy for Chemical Analysis). The XPS
method provides quantitative chemical information ¨ the chemical composition
expressed in atomic% - for the outermost 2-10 nm of surfaces.
The measuring principle is that a sample, placed in high vacuum, is
irradiated with well defined x-ray energy resulting in the emission of
photoelectrons. Only those from the outermost surface layers reach the
detector. By analyzing the kinetic energy of these photoelectrons, their
binding
energy can be calculated, thus giving their origin in relation to the element
and
the electron shell.
XPS provides quantitative data on both the elemental composition and
different chemical states of an element (different functional groups, chemical
bonding, oxidation state, etc). All elements except hydrogen and helium are
detected and the surface chemical composition obtained is expressed in
atomic%.
XPS spectra were recorded using a Kratos AXIS Ultract x-ray
photoelectron spectrometer. The samples were analyzed using a
monochromatic Al x-ray source. The analysis area was below 1mm2.
In the analysis wide spectra were run to detect elements present in the
surface layer. The relative surface compositions were obtained from
quantification of detail spectra run for each element.
The following three samples were XPS analyzed:
1. Siliconoxide (new) on Ti-plate ¨ coating on both sides.
2. Siliconoxide (used) on Ti-plate ¨ coating on one side
3. Siliconoxide on DIN 1.4401 stainless steel plate, coating on both
sides.
The analysis was performed in one position per sample, except for
sample 1, where two positions were analyzed. The results are summarized in
Table 2 showing the relative surface composition in atomic% and atomic ratio
0/Si.
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Table 2
Sample 0/Si C 0 Si
1 new (pt 1) 2.25 61.1 23.5 10.5 4.2
2 new (pt 2) 2.30 61.0 23.9 10.4 4.1
2 used 2.29 68.0 19.5 8.6 3.1
3 1.46 41.9 34.3 23.4 (0.2)*
*weak peak in detail spectra, signal close to noise level
As seen in Table 2 mainly C, 0 and Si were detected on the outermost
surfaces, i e 41.9-68.0 atomic% C, 19.5-34.3 atomic% 0 and 8.6-23.4 atomic%
Si.
Note that in the atomic ratios 0/Si, the total amount of oxygen is used.
This means that also oxygen in functional groups with carbon is included.
Otherwise for silica, from theory is expected a ratio 0/Si of 2.0 for the bulk
pure
silica Si02.
Inspection during operation
After four months of operation an off-shore pre-inspection by thermo-
imaging was performed. Thermo-image of the mid region of heat exchanger in
operation. The identity of the two coating systems was presumed from the
installation, but it was obvious that two sets of PHE plates show increased
heat
transfer compared to the rest of the PHE unit.
The inspection showed an elevation temperature at the coated plates.
The non-coated plates showed a lower operating temperature. The difference in
temperature is presumed due to reduced fouling, hence a higher crude oil flow
in the coated region which produces an elevated temperature.
Inspection of plates after operation
The term fouling is used to describe the deposits formed on the PHE
plates during operation. The fouling are residues and deposits formed by the
crude oil and consists of a waxy, organic part and a mineral/inorganic part.
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The visual inspection revealed that the plates with the coating designated
Coat 1 was covered with the least amount of fouling on the crude oil facing
plate side. Also, the other coating system designated Coat 2 had a reduced
amount of fouling on the crude oil facing plate side compared to the bare
titanium surface but to a lesser extent then Coat 1. The bare titanium plates
at
the end of the plate pack were completely covered in a thick layer of crude
oil
derived fouling.
Images taken off-shore during the disassembly (Fig 2) showed significant
reduction in fouling on both of the coated plates compared to the uncoated
plates. Figure 2 illustrates a heat exchanger plate immediately after off-
shore
disassembly. Coat 1 is showed at left and coat 2 is shown in the middle. The
uncoated plate is shown to the right.
By subtracting the average weight of a clean plate from the weight
recorded for the individual fouled plates the average amount of fouling per
surface type was calculated (table 3). Note, the weight of the coating was not
compensated for and so the real fouling reduction is slightly higher. For a
T20-
M plate the heat transfer surface is 0.85 m2 so for a plate with a 4 pm thick
coating on front and back the total volume is around 6.8 cm3. If the coating
is
estimated to be pure Si02 (density 2.6 g/cm3) then the amount of coating per
plate is about 20 g.
Table 3
Surface Average STDEV Fouling
fouling* (g) reduction (%)
Titanium 585 125 -
Coat 1 203 48 65
Coat 2 427 144 27
For both coating systems the fouling of the plates were more easily
removed compared to the fouling adhering to the bare titanium surface, see
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Table 4. The difference in cleaning requirements was tested by manually wiping
of the plates with a tissue and by high pressure water cleaning. Just wiping
the
plates with a tissue showed that the fouling was very easily removed from the
coated plates, contrary to the uncoated plates. By using water jet all fouling
except for one or two small patches could be removed from the Coat 1 coated
surface. On the Coat 2 coated surface some more fouling was present after
water jet cleaning. This fouling had the appearance of slightly burnt oil.
Some loss of coating was observed in the contact points but overall the
coated surface that had been in contact with the crude oil was in a good
condition.
On the sea water facing side both coatings had deteriorated and could
be peeled of quite easily.
Table 4
Coat 1 Coat 2 Non-coated
very little fouling reduced fouling fouling
significant
View compared and widespread
Wipe very easy to very easy to fouling was not
with remove fouling remove fouling removed
tissue
High the plates most of the fouling even after
attempts
pressure appeared as new was removed of manual removal
water of fouling, still a
washing considerable layer
remains
The coating tolerance to immersion in liquid nitrogen for gasket removal
was tested. One Coat 1 and one Coat 2 plate were treated in liquid nitrogen,
at
-196 C, to remove the rubber gaskets. The coatings did not appear do suffer
from the extreme temperature changes. Subsequently the plates were washed
by high pressure water, which removed almost all fouling. No coating
delimitation or failure was observed for either coating system.
