US20140338867A1

US20140338867A1 - Shell and tube heat exchanger with improved anti-fouling properties

Info

Publication number: US20140338867A1
Application number: US14/361,114
Authority: US
Inventors: Tobias Svensson
Original assignee: Alfa Laval Corporate AB
Current assignee: Alfa Laval Corporate AB
Priority date: 2011-11-28
Filing date: 2012-11-28
Publication date: 2014-11-20
Also published as: KR20140106604A; EP2786085A1; CA2857248A1; WO2013081536A1; CN104067082A

Abstract

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. At least a part of the shell and tube heat exchanger is provided with a coating comprising silicon oxide, SiO_x.

Description

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.
US20080073063 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. 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, SiO_x, having an atomic ratio of O/Si >1, a content of carbon ≧10 atomic % and a coating layer thickness of about 1-30 μm, 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 μm, preferably 2-20 μm,
According to yet another aspect of the invention the coating comprising silicon oxide, SiO_x, has an atomic ratio of O/Si ≧1.5-3, preferably O/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 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.
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. With 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, SiO_x, having an atomic ratio of O/Si >1, preferably an atomic ratio of O/Si ≧1.5-3, and most preferably O/Si ≧2-2.5. A preferred silicon oxide is silica, SiO₂. 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 C═O, C—O, C—O—C, C—N, N—C—O, N—C═O, 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.
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 a metal oxide film of the surface 9. The coating bulk that follows said interface is the 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.
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 μm thick. The thickness of the coated film is important for the use in a shell and tube heat exchanger . A film thickness below 1 μm 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 μm. Thus, the film thickness of the silicon oxide sol containing composition is 1-30 μm, preferably 1.5-25 μm, preferably 2-20 μm, preferably 2-15 μm, preferably 2-10 μm and preferably 3-10 μm.
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% HNO₃in H₂O, 1% NaOH in H₂O 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 angle	H2O: 102-103°	H2O: 102-103°
measurements
Coating	4-10 μm	2-4 μm
thickness
Stability	1.2% HNO3 in H2O: 1½ h at 75° C.	1.2% HNO3 in H2O: 1½ h at 75° C.
	1% NaOH in H2O: 3 h at 85° C.	1% NaOH in H2O: 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 1½ 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
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 μm 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 1½ 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 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 Ultra^DLDx-ray photoelectron spectrometer. The samples were analyzed using a monochromatic Al x-ray source. The analysis area was below 1 mm².
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 O/Si.

TABLE 2

Sample	O/Si	C	O	Si	N

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, O 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 O/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 O/Si of 2.0 for the bulk pure silica SiO₂.
Inspection of Tubes after Operation
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 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 SiO₂(density 2.6 g/cm³) then the amount of coating per tube is about 20 g.

TABLE 3

	Average		Fouling
Surface	fouling* (g)	STDEV	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.
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.

Claims

1. 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, wherein

said shell and tube heat exchanger is provided with a coating comprising:

silicon oxide, SiO_x, having an atomic ratio of O/Si>1, a content of carbon ≧10 atomic % and a coating layer thickness of about 1-30 μm, which coating was prepared by sol-gel processing and applied to at least a part of the shell and tube heat exchanger surfaces.

2. A shell and tube heat exchanger according to clam 1, wherein the layer thickness of said coating on the surfaces is 1.5-25 μm, preferably 2-20 μm, more preferably 2-15 μm, even more preferably 2-10 μm, and most preferably 3-10 μm.

3. A shell and tube heat exchanger according to claim 1, wherein the coating comprising:

silicon oxide, SiO_x, has an atomic ratio of O/Si ≧1.5-3, preferably O/Si ≧2-2.5.

4. A shell and tube heat exchanger according to claim 1, wherein the composition has a content of carbon ≧20-60 atomic %, preferably 30-40 atomic %.