Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02857248 2014-05-26
WO 2013/081536 PCT/SE2012/051309
SHELL AND TUBE HEAT EXCHANGER
WITH IMPROVED ANTI-FOULING PROPERTIES
Technical Field
The present invention refers generally to shell and tube heat exchangers
allowing a heat transfer between two fluids at different temperature for
various
purposes. Specifically, the invention relates to a shell and tube heat
exchanger which
has been coated for improving anti-fouling properties and has in some
embodiments
been given predetermined, structural properties for ensuring that the coating
remains
on the shell and tube heat exchanger when it is used.
Background Art
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 surfaces. The
deposits
arise e.g. from the fluids in the equipment, microbial growth and/or dirt.
Shell and tube heat exchangers 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. Depending e.g. on the fluids used
the
heat exchanger 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 may both be
time
consuming and costly. Also, the process to which the shell and tube heat
exchanger is
connected to may have to be shut down during said cleaning.
The shell and tube heat exchangers are made of metals which have a high
surface free energy that results in most liquids easily wetting the surfaces.
Also, when heat exchanger surfaces are produced the forming operation of the
metal increases the surface roughness which often is associated with faster
build up of
fouling deposits.
GB2428604 discloses provision of a coating on shell and tube heat exchangers
to reduce fouling.
U520080073063 discloses a shell and tube heat exchanger coated with a low
surface energy material to reduce fouling.
It would be desirable to find new ways to ensure less fouling of heat
exchangers
and their surfaces in order to keep the heat exchangers running for longer
time periods.
CA 02857248 2014-05-26
WO 2013/081536 PCT/SE2012/051309
2
Also, a reduced shut down time for processes where shell and tube heat
exchanger 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 shell and tube heat
exchanger
with the heat exchanging fluids. Furthermore, cracks in the coating may occur
due to
torque and tension forces acting in the shell and tube heat exchanger in
applications
under high pressures.
Summary of the invention
It is an object of the present invention to provide improved surfaces for a
shell
and tube heat exchanger, which show a reduced fouling of the surfaces when in
use in
a shell and tube heat exchanger. Another object is to achieve surfaces for a
shell and
tube heat exchanger having antifouling properties which are wear resistant in
abrasive
environments and have high resistance against formation of cracks.
This object is achieved by a shell and tube heat exchanger comprising a shell
having an inlet end-cap attached to a first end of the shell, wherein an
outlet end-cap is
attached to a second end of the shell and a tube bundle being housed within
the shell,
said tube bundle including a plurality of parallel-spaced tubes that traverse
the interior
of shell from a first end to a second end of the tube bundle, and wherein a
plurality of
baffles are arranged within the shell supporting the parallel-spaced tubes of
the tube
bundle. The shell and tube heat exchanger is provided with a coating
comprising silicon
oxide, Si0,, having an atomic ratio of 0/Si > 1, a content of carbon 10
atomic% and a
coating layer thickness of about 1-30 pm, which coating was prepared by sol-
gel
processing and applied to at least a part of the shell and tube heat exchanger
surfaces.
According to another aspect of the invention the layer thickness of said
coating
on the shell and tube heat exchanger is 5-30 pm, preferably 2-20 pm,
According to yet another aspect of the invention the coating comprising
silicon
oxide, Si0),, has an atomic ratio of 0/Si 1.5-3, preferably 0/Si 2-2.5.
According to still another aspect of the invention the composition has a
content
of carbon 20-60 atomic%, preferably 30-40 atomic%.
The shell and tube heat exchanger is advantageous in that fouling of the
surfaces is reduced significantly. By applying a coating composition
comprising sol-gel
material with organosilicon compounds to the shell and tube heat exchanger
surfaces
both the surface free energy and roughness is lowered, leading to reduction of
fouling
CA 02857248 2014-05-26
WO 2013/081536 PCT/SE2012/051309
3
and easy cleaning of shell and tube heat exchanger surfaces. Moreover, the sol-
gel
coated shell and tube heat exchanger surfaces of the invention exhibit an
excellent
wear resistance and have a flexibility that reduces the risk of cracks
appearing in the
coating. Furthermore, by the shell and tube heat exchanger according to the
invention
it is possible to reduce the overall dimensions of the heat exchanger while
the heat
transfer capacity of the shell and tube heat exchanger is maintained.
Brief Description of the Drawings
Further objects, features and advantages of the invention will appear from the
following detailed description of different embodiments of the invention with
reference
to the accompanying schematic drawings, in which
Fig. 1 is a schematic drawing of a shell and tube heat exchanger according to
the invention,
Fig. 2 is a schematic cross section of a surface of a shell and tube heat
exchanger having an anti fouling coating according to the invention.
Detailed description of the invention
FIG. 1 is a side elevation of a shell and tube heat exchanger 1 arranged in
accordance with a preferred embodiment of the invention. The shell and tube
heat
exchanger 1 includes a shell 2 having an inlet end-cap 3 attached to a first
end 4 of the
shell 2. An outlet end-cap 5 is attached to a second end 6 of the shell 2.
A cutaway portion of shell 2 reveals a tube bundle 7 housed within the shell
2.
The tube bundle 7 includes a plurality of parallel-spaced tubes 8 that
traverse the
interior of shell 2 from a first end to a second end of the tube bundle. A
plurality of
baffles 11 are arranged within the shell 2 and support the parallel-spaced
tubes 8 of the
tube bundle 7.
In operation, a first heat exchange fluid, such as a flue gas, or hot medium
carrying waste heat, or the like, is introduced to shell 2 through an inlet.
The first heat
exchange fluid traverses the shell 2 through a pathway created by the baffles
and exits
the shell 2 through an outlet. A second heat exchange fluid, to be heated
within heat
exchanger 1 enters inlet end-cap 3 through an inlet. The second heat exchange
fluid
enters tube bundle 7 and is passed through parallel-spaced tubes 8, while
being
heated by the first heat exchange fluid passing through the shell side of heat
exchanger 1. The second heat exchange fluid eventually passes from tube bundle
7 to
outlet end-cap 5 and exits heat exchanger 1 through an outlet tube.
CA 02857248 2014-05-26
WO 2013/081536 PCT/SE2012/051309
4
The coating used according to the present invention may be referred to as a
non-stick coating and makes it easy to clean the surfaces of a fouled shell
and tube
heat exchanger. The coated surfaces according to the present invention show a
better
heat transfer over time compared to conventional shell and tube heat exchanger
surfaces since the latter ones gets fouled much quicker and thus decrease the
heat
transfer performance to a larger extent. The coating of the surfaces also
results in a
much more even surface thus resulting in better flow characteristics. Also the
pressure
drop is reduced over time for a shell and tube heat exchanger according to the
present
invention in comparison with conventional shell and tube heat exchangers,
since the
buildup of impurities, microorganisms and other substances is not as
pronounced.
The coated shell and tube heat exchanger according to the present invention
may easily be cleaned just using high pressure washing with water. VVith a
surface
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 the surfaces of a shell and tube heat
exchanger is coated with a composition comprising organosilicon compounds
using a
sol-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, Si0,, 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
preferred silicon oxide is silica, 5i02. The siliconoxide forms a three
dimensional
network having excellent adhesion to the surfaces.
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 functional
groups such as found in hydrocarbon chains or aromatic groups, e g 0=0, 0-0, 0-
0-
C, C-N, N-C-0, N-C=0, etc. Preferably the carbon content is 10 atomic%,
preferably
20-60 atomic%, and most preferably 30-40 atomic%. The hydrocarbons impart
flexibility and resilience to the coating. The hydrocarbon chains are
hydrophobic and
oleophobic which results in the non-stick properties of the coating.
CA 02857248 2014-05-26
WO 2013/081536 PCT/SE2012/051309
In Fig 2 is shown a schematic drawing of a surface 9 for a shell and tube heat
exchanger provided with a siliconoxide sol gel coating 10. Between the surface
9 itself
and the siliconoxide layer is an interface 11 between the coating siloxane and
and a
metal oxide film of the surface 9. The coating bulk that follows said
interface is the
5 siloxane network 12 with organic linker chains and voids that impart
flexibility to the
coating. The outermost layer is a functional surface 13, i e a
hydrophobic/oleophobic
surface for fouling reduction.
By the combination of a durable and yet flexible coating, a surface for a
shell
and tube 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 order
to avoid cracking of the coating when the surfaces move in relation to each
other.
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 one side of
the shell and
tube heat exchanger surface is to be coated. Alternatively, all surfaces of at
least one
side of a surface which during use in a shell and tube heat exchanger would be
in
contact with a fluid are coated. Also, at least one side of a shell and tube
heat
exchanger surface may be entirely coated. Alternatively, both sides of the
tube 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.
In another embodiment the method comprises a pretreatment of at least the
surfaces on the heat exchanger tubes 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 tube. 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 tube with said
sol. The
coating is preferably subjected to heat using conventional heating apparatus,
such as
e.g. ovens.
CA 02857248 2014-05-26
WO 2013/081536 PCT/SE2012/051309
6
The composition comprising SiOx is applied to a surface to be used in a shell
and tube heat exchanger. The application of the composition is done by means
of sol-
gel processing. The resulting film of said composition on the surface is
preferably
between 1 and 30 pm thick. The thickness of the coated film is important for
the use in
a shell and tube heat exchanger. A film thickness below 1 pm is considered
being not
enough wear resistant since the surfaces in a shell and tube 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
surfaces
influences the heat transfer and thus the performance of the shell and tube
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 surfaces 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.
From the description above follows that, although various embodiments of the
invention have been described and shown, the invention is not restricted
thereto, but
may also be embodied in other ways within the scope of the subject-matter
defined in
the following claims.
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/butylacetate.
Phase A
The analysis documents the properties of coatings concerning substrate wetting
and adhesion, contact angle, coating thickness and stability towards 1.2 %
HNO3 in
H20, 1 % NaOH in H20 and crude oil. The results are summarized below in Table
1.
CA 02857248 2014-05-26
WO 2013/081536 PCT/SE2012/051309
7
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 angle H20: 102-103 H20: 102-103
measurements
Coating 4-10 pm 2-4 pm
thickness
Stability 1.2% HNO3 in H20: 1% h at 75 C 1.2% HNO3 in H20: 1% h at 75 C
1% NaOH in H20: 3 h at 85 C 1 % NaOH in H20: 2 h at 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. 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 1% hour at 75 C and more than 24 hours at room temperature.
CA 02857248 2014-05-26
WO 2013/081536 PCT/SE2012/051309
8
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
Coating of shell and tube heat exchanger surfaces
Coat 1 and Coat 2 were applied to a tube bundle. All tubes underwent
pre-treatment which consisted of:
1. Submerging in liquid nitrogen (- 196 C)
2. Treatment with acidic and alkaline solutions to remove fouling
3. High pressure washing of the tubes with water
4. Assembly of the tube bundle for pressure testing
5. Disassembly of the tube bundle. Tubes left to dry before application
This pre-treatment was completed the day before Coat 1 and Coat 2 were
applied to the tubes. Consequently, this procedure did not follow the
recommended
approach as outlined in Phase A. As the tubes have been left to dry at ambient
temperature, some tubes were still wet. 15 tubes were treated with Coat 1 and
the
remaining 15 tubes with Coat 2 by spray coating. The heat exchanger tubes were
coated on both sides. The final film thickness was measured to be 2-4 pm and
the
coating was applied on both sides of the tubes. Curing/drying was performed at
elevated temperatures of 200 C or 160 C respectively for 1% hour in an on-site
oven.
Upon completion the coated heat exchangers were weighed and coating thickness
was
measured. It was observed that some tubes had some coating imperfections and
small
defects.
The heat exchanger tubes were then assembled with the remaining untreated
tubes. The coated tubes were placed respectively in the front, middle and end
of the
assembled. The evaluation of the coated tubes was performed after more than
seven
months of operation.
Phase C
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 known as
ESCA
(Electron Spectroscopy for Chemical Analysis). The XPS method provides
quantitative
CA 02857248 2014-05-26
WO 2013/081536
PCT/SE2012/051309
9
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 UltraDLD 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.
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.
CA 02857248 2014-05-26
WO 2013/081536 PCT/SE2012/051309
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
5i02.
Inspection of tubes after operation
5 The term fouling is used to describe the deposits formed on the tubes
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.
The visual inspection revealed that the tubes with the coating designated Coat
1
was covered with the least amount of fouling on the crude oil facing tube
side. Also, the
10 other coating system designated Coat 2 had a reduced amount of fouling
on the crude
oil facing tube side compared to the bare titanium surface but to a lesser
extent then
Coat 1
By subtracting the average weight of a clean tube from the weight recorded for
the individual fouled tubes 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. If the coating is estimated to
be pure 5i02
(density 2.6 g/cm3) then the amount of coating per tube 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 tubes were more easily removed
compared to the fouling adhering to the bare titanium surface, see Table 4.
The
difference in cleaning requirements was tested by manually wiping of the tubes
with a
tissue and by high pressure water cleaning. Just wiping the tubes with a
tissue showed
that the fouling was very easily removed from the coated tubes, contrary to
the
uncoated tubes. 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.
CA 02857248 2014-05-26
WO 2013/081536
PCT/SE2012/051309
11
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
View very little fouling reduced fouling fouling
significant
compared and widespread
Wipe very easy to very easy to fouling was not
with remove fouling remove fouling removed
tissue
High the tubes 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 tube 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 tubes were washed by high pressure
water,
which removed almost all fouling. No coating delimitation or failure was
observed for
either coating system.
