U.S. patent application number 11/802617 was filed with the patent office on 2008-03-27 for reduction of fouling in heat exchangers.
This patent application is currently assigned to ExxonMobil Research and Engineering Company. Invention is credited to Glen B. Brons, LeRoy R. Clavenna, Ian A. Cody, Steve Colgrove, Ashley E. Cooper, Hugh L. Huffman, Julio D. Lobo, George A. Lutz, Limin Song, H. Alan Wolf, Mohsen S. Yeganeh.
Application Number | 20080073063 11/802617 |
Document ID | / |
Family ID | 38805675 |
Filed Date | 2008-03-27 |
United States Patent
Application |
20080073063 |
Kind Code |
A1 |
Clavenna; LeRoy R. ; et
al. |
March 27, 2008 |
Reduction of fouling in heat exchangers
Abstract
A method for reducing the formation of deposits on the inner
walls of a tubular heat exchanger through which a petroleum-based
liquid is flowing comprises applying one of fluid pressure
pulsations to the liquid flowing through the tubes of the exchanger
and vibration to the heat exchanger to effect a reduction of the
viscous boundary layer adjacent the inner walls of the tubular heat
exchange surfaces. Reduction of the viscous boundary layer at the
tube walls not only reduces the incidence of fouling with its
consequential beneficial effect on equipment life but it also has
the desirable effect of promoting heat transfer from the tube wall
to the liquid in the tubes. Fouling and corrosion are further
reduced by the use of a coating on the inner wall surfaces of the
exchanger tubes.
Inventors: |
Clavenna; LeRoy R.; (Baton
Rouge, LA) ; Cody; Ian A.; (Annandale, NJ) ;
Cooper; Ashley E.; (Baton Rouge, LA) ; Colgrove;
Steve; (Baton Rouge, LA) ; Huffman; Hugh L.;
(Lexington, KY) ; Lobo; Julio D.; (Alexandria,
VA) ; Song; Limin; (West Windsor, NJ) ; Wolf;
H. Alan; (Morristown, NJ) ; Brons; Glen B.;
(Phillipsburg, NJ) ; Lutz; George A.; (Brick,
NJ) ; Yeganeh; Mohsen S.; (Hillsborough, NJ) |
Correspondence
Address: |
ExxonMobil Research & Engineering Company
P.O. Box 900
1545 Route 22 East
Annandale
NJ
08801-0900
US
|
Assignee: |
ExxonMobil Research and Engineering
Company
Annandale
NJ
|
Family ID: |
38805675 |
Appl. No.: |
11/802617 |
Filed: |
May 24, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60815845 |
Jun 23, 2006 |
|
|
|
Current U.S.
Class: |
165/84 ;
165/133 |
Current CPC
Class: |
B01J 2219/0231 20130101;
B01J 2219/00247 20130101; B01J 2219/0245 20130101; F28G 7/00
20130101; B01J 2219/00094 20130101; B01J 19/02 20130101; F28F 19/02
20130101; B01J 19/185 20130101; F28F 2245/04 20130101; C10G 75/00
20130101; B01J 19/1825 20130101; F28D 2021/0059 20130101 |
Class at
Publication: |
165/084 ;
165/133 |
International
Class: |
F28D 11/06 20060101
F28D011/06; F28F 19/02 20060101 F28F019/02 |
Claims
1. A method for reducing the formation of deposits on walls of
tubes in a heat exchanger through which a petroleum-based liquid is
flowing, comprising: applying one of fluid pressure pulsations to
the liquid flowing through the tubes of the exchanger and vibration
to the heat exchanger, wherein the wall contacting the liquid
having an adherent fouling resistant coating having a surface
energy of not more than 50 mJ/m.sup.2.
2. A method according to claim 1, wherein mechanical vibration is
applied to the heat exchanger.
3. A method according to claim 1, wherein fluid pulsation
pulsations area applied to the liquid flowing through the
tubes.
4. A method according to claim 3, wherein the fluid pressure
pulsations are applied at a frequency in the range of 0.1 Hz to 20
kHz.
5. A method according to claim 3, wherein the amplitude of the
pulsations as measured by the incremental flow rate through the
heat exchange tubes is in the range from about 10.sup.-6 of the
normal heat exchanger flow rate to about an order of the normal
heat exchanger flow rate.
6. A method according to claim 5, wherein the pulsations are
applied at a frequency in the range from 0.1 to 10 kHz and the
amplitude of the pulsation as measured by the incremental flow rate
through the heat exchange tubes is in the range of about 10.sup.-2
of the normal heat exchanger flow rate to about the order of the
normal heat exchanger flow rate.
7. A method according to claim 5, wherein the pulsations are
applied at a frequency in the range from 10 to 20 kHz and the
amplitude of the pulsation as measured by the incremental flow rate
through the heat exchange tubes is in the range of about 10.sup.-6
of the normal heat exchanger flow rate to about 0.1 of the order of
the normal heat exchanger flow rate.
8. A method according to claim 1, wherein the adherent fouling
resistant coating on the wall of the tubes comprises: a layer of
organometallic molecules having a thickness of 1 to 10 molecular
layers, wherein the layer will not undergo substantial
decomposition at temperatures up to 450.degree. C., and wherein the
layer has a surface energy lower than 50 millijoule/m.sup.2.
9. A method according to claim 8, wherein the layer of
organometallic molecules is deposited on 80 to 100% of the inner
wall surface.
10. A method according to claim 8, wherein the metal in the
organometallic compound is silicon.
11. A method according to claim 8, wherein the organo moiety in the
organometallic compound is a hydrocarbyl group from 1 to 30 carbon
atoms.
12. A method according to claim 11, wherein the hydrocarbyl group
is aliphatic or aromatic and is substituted with at least one
functional group.
13. A method according to claim 8, wherein the organometallic
compound is an alkoxy silane, silane, silazone or phenyl
siloxane.
14. A method according to claim 13, wherein the organometallic
compound is hexamethyldisiloxane.
15. A method according to claim 8, wherein the surface energy of
the coating is between 18 and 50 mJ/m.sup.2.
16. A method according to claim 8, wherein the coating has a
surface with a water contact angle from 95.degree. to
160.degree..
17. A method according to claim 16, wherein the water contact angle
is from 110.degree. to 150.degree..
18. A method according to claim 16, wherein the water contact angle
is from 130.degree. to 160.degree..
19. A method according to claim 8, wherein mechanical vibration is
applied to the heat exchanger.
20. A method according to claim 8, wherein fluid pressure
pulsations are applied to the liquid flowing through the tubes.
21. A method according to claim 20, wherein the fluid pressure
pulsations are applied at a frequency in the range of 0.1 Hz to 20
kHz.
22. A method according to claim 20, wherein the amplitude of the
pulsations as measured by the incremental flow rate through the
heat exchange tubes is in the range from about 10.sup.-6 of the
normal heat exchanger flow rate to about an order of the normal
heat exchanger flow rate.
23. A method according to claim 22, wherein the pulsations are
applied at a frequency in the range from 0.1 to 10 kHz and the
amplitude of the pulsation as measured by the incremental flow rate
through the heat exchange tubes is in the range of about 10.sup.-2
of the normal heat exchanger flow rate to about the order of the
normal heat exchanger flow rate.
24. A method according to claim 22, wherein the pulsations are
applied at a frequency in the range from 10 to 20 kHz and the
amplitude of the pulsation as measured by the incremental flow rate
through the heat exchange tubes is in the range of about 10.sup.-6
of the normal heat exchanger flow rate to about 0.1 of the order of
the normal heat exchanger flow rate.
25. A heat exchanger for effecting heat exchange between a
petroleum-based liquid and a heat exchange medium, comprising: a
housing having an interior; a plurality of heat exchange tubes
having a hollow interior passage for the passage of one of the
petroleum-based liquid and the heat exchange medium therethrough,
wherein each tube having an interior surface and an exterior
surface; an adherent fouling resistant coating on one at least one
of the interior surface and the exterior surface, wherein the
coating having a surface energy of not more than 50 mJ/m.sup.2; and
one of fluid pressure pulsation generating device and a vibration
generating device, wherein the fluid pressure pulsation generating
device applying fluid pressure pulsations to the petroleum-based
liquid, wherein the vibration generating device applying mechanical
vibration to housing and the plurality of tubes.
26. The heat exchanger according to claim 25, wherein the heat
exchanger includes a fluid pressure pulsation generating device,
wherein the device generating fluid pressure pulsations are
generated at a frequency in the range of 0.1 Hz to 20 kHz.
27. The heat exchanger according to claim 25, wherein the adherent
fouling resistant coating comprises: a layer of organometallic
molecules having a thickness of 1 to 10 molecular layers, wherein
the layer will not undergo substantial decomposition at
temperatures up to 450.degree. C., and wherein the layer has a
surface energy lower than 50 millijoule/m.sup.2.
28. The heat exchanger according to claim 26, wherein the layer of
organometallic molecules is deposited on 80 to 100% of the inner
wall surface.
29. The heat exchanger according to claim 26, wherein the metal in
the organometallic compound is silicon.
30. The heat exchanger according to claim 26, wherein the organo
moiety in the organometallic compound is a hydrocarbyl group from 1
to 30 carbon atoms.
31. The heat exchanger according to claim 30, wherein the
hydrocarbyl group is aliphatic or aromatic and is substituted with
at least one functional group.
32. The heat exchanger according to claim 26, wherein the
organometallic compound is an alkoxy silane, silane, silazone or
phenyl siloxane.
33. The heat exchanger according to claim 26, wherein the
organometallic compound is hexamethyldisiloxane.
34. The heat exchanger according to claim 25, wherein the surface
energy of the coating is between 18 and 50 mJ/m.sup.2.
35. The heat exchanger according to claim 25, wherein the coating
has a surface with a water contact angle from 95.degree. to
160.degree..
36. The heat exchanger according to claim 35, wherein the water
contact angle is from 110.degree. to 150.degree..
37. The heat exchanger according to claim 35, wherein the water
contact angle is from 130.degree. to 160.degree..
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application relates and claims priority from U.S.
Provisional Patent Application No. 60/815,845, filed on Jun. 23,
2006, entitled "Reduction of Fouling in Heat Exchangers."
[0002] This application also relates to co-pending U.S. patent
application Ser. No. 11/304,874, which describes a method of
applying low surface energy coatings to heat exchanger surfaces to
reduce corrosion and the incidence of fouling on the surfaces,
co-pending U.S. Patent Application Ser. No. 60/751,985, which
describes a corrosion resistant material for reduced fouling in
heat exchangers and methods for making coatings of such materials,
and co-pending U.S. patent application Ser. No. 11/436,802, which
describes a method of reducing fouling in heat exchangers by
applying a mechanical force to the heat exchanger to induce
vibration in the heat exchanger, which causes a shear motion in the
liquid flowing within the heat exchanger.
FIELD OF THE INVENTION
[0003] This invention relates to the reduction of fouling in heat
exchangers. This invention also relates to a process and an
apparatus for preventing the deposition of solid matter on the
internal walls of heat exchangers.
BACKGROUND OF THE INVENTION
[0004] Heat exchangers are well known and widely used in the
chemical process industries and in petroleum refining. Heat
exchangers have a tendency to become fouled by deposits of solid
material, necessitating occasional removal from service for
cleaning. Fouling in heat exchangers used for petroleum type
streams can result from a number of mechanisms including chemical
reactions, corrosion, deposit of insoluble materials, and deposit
of materials made insoluble by the temperature difference between
the fluid and heat exchange wall. Two important fouling mechanisms
are chemical reactions and the deposition of insoluble materials.
In both fouling mechanisms, the reduction of the viscous sub-layer
(or boundary layer) close to the wall can mitigate the fouling
rate. In the case of chemical reactions, the high temperature at
the surface of the heat transfer wall activates molecules to form
precursors for the fouling residue. If these precursors are not
swept out of the relatively stagnant wall region they will
associate together and deposit on the wall. A reduction of the
boundary layer reduces the thickness of the stagnant region and
hence the amount of precursors available to form a fouling residue.
In the case of the deposition of insoluble materials, a reduction
in the boundary layer increases the shear near the wall and hence
exerts a greater force on the insoluble particle near the wall to
overcome the particle's attractive forces to the wall and hence
reducing its probability of deposition and incorporation into the
fouling residue.
[0005] When the walls of a heat exchanger become coated with
deposits, a number of difficulties ensue: (i) the heat transfer
rate between the tube wall and the material in the tube diminishes;
(ii) temperature regulation deteriorates, (iii) overheating often
develops in the tubing, leading to shortened equipment life; (iv)
shut-downs and cleaning cycles are necessary, and the longer the
exchanger tubing, the more expensive and difficult is the cleaning
job; (v) damage to the exchanger or ancillary equipment results
when reactor tubes become plugged and relief valves burst. Fouling
costs petroleum refineries significant amounts of money each year
due to lost efficiencies, lost throughput, and waste of energy. The
current method used to maintain heat exchanger efficiency is to
periodically bring the heat exchanger out of service to clean it by
chemical or mechanical methods. This can be costly and labor
intensive. This adds significantly to the maintenance cost of the
equipment and often requires replacement of the major components.
This downtime and the costs of unexpected/unplanned shutdowns also
add to the costs associated with fouling.
[0006] U.S. Pat. No. 4,271,007 to Souhrada, in dealing with deposit
formation in tubular reactors, mentions a number of methods for
preventing deposit formation including the control of reaction
conditions, adjustment of the feed rate of any catalyst to avoid
rapid reaction and consequent overheating, the addition of
inhibiting chemicals, the use of liquid curtains or oil films to
prevent solid materials from contacting the reactor walls, and
recycling a portion of the product from a reactor to the inlet, to
increase the linear flow rate in the reactor and maintain turbulent
flow conditions. Mention is also made that deposits may be removed
by mechanical means including high pressure water or steam jets, by
solvents or by chemical reaction. All these procedures, however,
require removing part or all of the reactor from service for the
cleaning cycle and the same would apply equally to heat exchanger
service with their concomitant losses in equipment utilization
rates as well as an undesirable labor burden.
[0007] U.S. Pat. No. 3,183,967 to Mettenleiter proposes reducing
the formation of sediment and scaling on the walls of heat
exchanger tubing by mounting the tubing header resiliently or
flexibly at one or both ends of the tube bundle and applying
vibration at predetermined intervals to repel solids from the walls
of the tubes. Arrangements of this type are, however, mechanically
complicated and add significant cost to the design of what would
otherwise be a relatively inexpensive device which normally
contains no moving parts.
[0008] In practical terms, the reduction of scale or deposit
formation by the use of mechanically applied vibration, as
described in Mettenleiter, by the use of flexible mounted tube
bundles with mechanical shakers or rappers has not achieved any
significant acceptance with heat exchangers. A different approach
using fluid pressure pulsations to clean fouled heat exchanger
surfaces has been described in U.S. Pat. No. 4,645,542 to Scharton,
U.S. Pat. No. 4,655,846 to Scharton and U.S. Pat. No. 5,674,323 to
Garcia but all these proposals have the marked disadvantage of
requiring the equipment to be taken out of service and subjected to
the cleaning procedure. The same is true of the sonic cleaning
method described in U.S. Pat. No. 4,461,651 to Hall. The use of
flow oscillations in a liquid reactant flowing through a reactor
such as a polymerization or cracking reactor for checking the
deposition of solids on the reactor walls in described in Sourhada
and in a similar vein, U.S. Pat. No. 3,819,740 to Saburo Hori
proposes the use of an ultrasonic wave generator for inhibiting the
accumulation of coke deposits on the walls of thermal cracking
reactors. U.S. Pat. No. 5,287,915 to Liu describes a method of
removing deposits from the walls of heat exchangers used for
cooling hot gases, e.g. in the production of synthesis gas, by
forming the heat exchanger tube into a moveable configuration such
as a coil which can then be vibrated or shaken by the use of
electrodynamic, hydraulic or mechanical means. One possibility
referred to is to use the water hammer effect to vibrate a coil
type exchanger, creating the water hammer by sudden changes in the
flow rate of the coolant in the tube.
[0009] There have been prior attempts at using coatings on the
surfaces of heat exchanges to reduce corrosion. These attempts are
not effective in reducing fouling. One, for example, intended for
forming a protective surface film functions by depositing a layer
of silica resulting from oxidative decomposition of an alkoxy
silane in the vapor phase on the metal surface. Another approach is
to passivate a reactor surface subject to coking by coating the
reactor surface with a layer that is from several microns to
several millimeters thick of a ceramic material deposited by
thermal decomposition of a silicon containing precursor in the
vapor phase. Both approaches result in a surface oxide with
relatively high surface energy that can attract unwanted deposits
of the surface. While these coatings can have some value in
preventing corrosion, they have proved to be ineffective in
reducing fouling.
[0010] Other coatings are based on polymeric materials such as
polyethylene and polyvinylfluoride with low surface energy such as
the coatings used to inhibit biofouling in aqueous environments at
ambient conditions. These polymeric coatings generally cannot
withstand higher temperature conditions typical of refinery
operations and are not effective to reduce hydrocarbon fouling
adequately.
[0011] The typical coatings for industrial conduits are generally
in the micron to millimeter range in thickness. This is usually to
ensure good surface coverage as well as provide a protective layer
of sufficient thickness to be robust during operating conditions.
Coatings of such thickness may, however, limit heat transfer.
Treatments with silicate sols, or paints rich in silicon or
aluminum typically produce relatively thick surfaces (micron to
millimeter) that can provide a physical boundary that protects the
underlying metal from corrosion. However, such treatments will not
have low surface energies if the surface terminates in an
oxide/hydroxide surface layer. The use of silanes for chemical
vapor deposition is also known but with the intent to diffuse Si,
C, H and other elements into the metal surface using high
temperatures (e.g. 600.degree. C.); the result is that the surface,
though non-metallic, can still have a high surface energy and will
not reject potential foulants. Thus, conventional treatments tend
to be inadequate either because they are too thick for good heat
transfer or, alternatively, do not adequately resist fouling.
[0012] There is a need to reduce and/or eliminate fouling in heat
exchangers, which is presently not addressed by the prior art.
SUMMARY OF THE INVENTION
[0013] It is an aspect of the present invention to combine
pulsation or vibration, which reduces the amount of available
foulants, with surface treatment, which reduces the probability of
the foulant adhering to the surface. The resulting combination
achieves a reduction of fouling that is greater than either method
when used separately. This can result in significant cost savings
because of the extended time period between cleaning of the heat
exchanger and the overall increased heat exchanger efficiency and,
in so doing, can minimize or prevent fouling of heater tubes which
will increase run lengths between turnarounds, avoid unplanned
shutdowns, avoid replacement of process tubes, improve overall
operations reliability and reduce the cost of decoking.
[0014] According to the present invention, the method for reducing
the formation of deposits on the walls of a heat exchanger through
which a petroleum-based liquid is flowing, comprises applying one
of fluid pressure pulsations and vibration to the liquid flowing
through the exchanger to effect a reduction of the viscous boundary
layer adjacent the walls of the heat exchange surface. The walls of
the heat exchange surfaces are coated with a low surface energy
material to which the expected deposits are non-adherent so that
the possibility of fouling is reduced further to an extent that is
not achievable by either expedient on its own.
[0015] The present invention therefore provides an improvement to a
heat exchanger which is used for effecting heat exchange between a
petroleum-based liquid and a heat exchange medium which flows on an
opposite side of a heat exchange surface to the liquid. It is an
aspect of the present invention to reduce fouling in the exchanger
on the side of the heat exchange surface in contact with the liquid
by applying fluid pressure pulsations to the petroleum liquid
flowing through the exchanger or vibration to the heat exchange
unit to effect shear motion in the petroleum liquid flowing through
the exchanger to effect a reduction of the viscous boundary layer
adjacent the walls of the heat exchange surface in contact with the
liquid so as to reduce the incidence of fouling and promote heat
transfer from the wall to the liquid. The wall of the heat exchange
surface, e.g. the inner wall of the tube, which in contact with the
liquid is selected as one which has an adherent, fouling resistant
coating having a low surface energy (e.g., not more than 50
mJ/m.sup.2). The combination of pulsation or vibration with a low
surface energy fouling resistant coating is effective in reducing
fouling, which improves heat transfer. The particles that cause
fouling are less likely to adhere to the low energy surface due to
lower adhesion strengths. The use of pulsation or vibration creates
oscillating shear stresses adjacent the walls of the exchanger and
is effective in removing the foulant particles from the wall of the
exchanger surfaces. The oscillating shear stresses act to tear or
pull the loosely adhered particles from the surfaces.
[0016] The principles of the present invention can be applied to
new heat exchangers or to retrofit an existing heat exchanger by
connecting a fluid pressure pulsator or vibration producing device
to side of the exchanger used for the petroleum liquid; again, the
heat exchange surface walls in contact with the petroleum liquid
are low-surface energy walls since these have been found to be the
most effective in reducing fouling. As described below, a number of
different fluid pulsator types may be used although
positive-displacement reciprocating pumps and diaphragm pumps will
be found to be efficacious for this purpose. Alternatively, it is
also contemplated that a vibration producing device can be
connected to the heat exchange unit to induce vibration in the heat
exchange unit to affect shear motion in the petroleum liquid
flowing through the heat exchange unit.
[0017] Reduction of the viscous boundary layer at the tube walls
not only reduces the incidence of fouling with its consequential
beneficial effect on equipment life but it also has the desirable
effect of promoting heat transfer from the tube wall to the liquid
in the tubes.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The invention will now be described in conjunction with the
accompanying drawings in which:
[0019] FIG. 1 is a partial cross-sectional view of a shell and tube
heat exchanger in accordance with an embodiment of the present
invention;
[0020] FIG. 2 is a cross-sectional view of a heat exchanger tube of
FIG. 1 illustrating the coating in accordance with the present
invention;
[0021] FIG. 3 is a schematic view of a heat exchanger in accordance
with another embodiment of the present invention;
[0022] FIG. 4 is a simplified equipment schematic of a test rig for
demonstrating the application of the present invention to a heat
exchanger; and
[0023] FIG. 5 is a graphical representation of the results achieved
in the testing reported below.
DETAILED DESCRIPTION OF THE INVENTION
[0024] The present invention winnow be described in greater detail
in connection with the attached figures. FIG. 1 is a tube-in-shell
heat exchanger 30, which is located upstream from a furnace (not
shown) and employs the principles of the present invention. The
tube-in-shell heat exchanger 30 disclosed herein illustrates one
application of the present invention to reduce sulfidation or
sulfidic corrosion and depositional fouling in refinery and
petrochemical applications. The tube-in-shell exchanger 30 is just
one heat transfer component falling under the scope of the
corrosion reduction and fouling mitigation measures in accordance
with the present invention. The principles of the present invention
are intended to be used in other heat exchangers including but not
limited to spiral heat exchangers, tube-in-tube heat exchangers and
plate-and-frame heat exchangers having at least one heat transfer
element. The principles of the present invention are intended to be
employed in other heat transfer components including furnaces,
furnace tubes and other heat transfer components which may be prone
to petroleum and/or vacuum residual fouling.
[0025] The heat exchanger 30 is used to pre-heat crude oil in a
refinery operation prior to entry into the furnace. The heat
exchanger 30 includes a housing or shell 31, which surrounds and
forms a hollow interior 32. A bundle 33 of heat exchanger tubes 34
is located within the hollow interior 32, as shown in FIG. 1. The
bundle 33 includes a plurality of tubes 34. The tubes 34 may be
arranged in a triangular configuration or a rectangular
configuration. Other tube arrangements are contemplated and
considered to be well within the scope of the present invention.
Each tube 34 has a generally hollow interior 35 such that the crude
oil to be heated flows there-through. The heating or warming fluid
(e.g., vacuum residual stream) flows through the hollow interior 32
to pre-heat the crude oil stream as the stream flows through the
hollow interior 35 towards the furnace. Alternatively, it is
contemplated that the crude oil may flow through the hollow
interior 32 of the housing 31.
[0026] Most heat exchangers are constructed from carbon steel for
reasons of cost possibly with tubes of copper, copper alloys, brass
(including muntz metal), cupronickel, stainless steel, admiralty
metal, aluminum or bronze (including aluminum bronzes and nickel
aluminum bronzes) but in principle, the present invention may be
applied to exchangers regardless of the construction material
although suitable choice of non-adherent coating material will be
required appropriate to the selected construction material.
[0027] The principal advantage of the present invention is that it
can be readily adapted for use in existing heat exchange equipment.
No mechanical modifications to the actual heat exchange unit are
required since the pressure pulsations may be applied to the liquid
flow stream in the exchanger tubes 34 by an external mechanism. For
example, a pulsation device 40 may be coupled to the liquid inlet
conduit 36 or liquid outlet conduit 37 (or both) on the tube side
of the exchanger 30, as shown in FIG. 3. As described below in
reference to the test rig used to validate the performance of the
invention, the fluid pulsation device 40 may be connected across
the exchanger, to provide pressure-balance positive pulsation on
one side of the exchanger with a negative pulsation on the other
side so as not create undesirable pressure excursions within the
exchanger itself. Alternatively, a single pulsator may be used,
normally on the inlet side, with a valve on the outlet side, if
necessary, to relieve any excess pressures created by the
pulses.
[0028] Like pressure pulsations, no internal modifications to the
heat exchange unit are required when vibration is applied to the
heat exchange unit. External units 50 may be connected to the heat
exchange unit to induce vibration creating the shear motion in the
liquid flow stream, as disclosed in co-pending U.S. patent Ser. No.
11/436,802, entitled "Mitigation of In-Tube Fouling in Heat
Exchangers Using Controlled Mechanical Vibration", the disclosure
of which is specifically incorporated herein by reference.
[0029] The present invention is applicable to use in heat
exchangers operating with a wide variety of liquids on the tube
side of the exchanger where a tendency to form deposits in the
tubes is a potential source of trouble, for example, water-based
liquids including emulsions and unstable solutions and oily liquids
such as petroleum-based liquids, e.g. crude oil, reduced crudes,
heavy refinery streams such as delayed coker feed, coker heavy gas
oil, visbreaker feed, vacuum gas oil, aromatic extract, and the
like. It is with petroleum feeds that the present invention is
particularly useful.
[0030] The pulsation device 40 will comprise any means for applying
liquid pressure pulsations to the tube side liquid. In the simplest
concept, the device may comprise a reciprocating pump type
mechanism with a cylinder connected to the inlet/outlet conduits of
the exchanger and a reciprocating piston in the cylinder to vary
the internal volume of the cylinder. As the piston moves within the
cylinder, the liquid will alternately be drawn into the cylinder
and then expelled from it, creating pulsations in the conduit to
which the device is connected. The use of a double-acting pump of
this kind with one side connected to the inlet conduit and the
other connected to the outlet conduit is particularly desirable
since it will create the desired pressure pulsations in the tubes
regardless of the pressure drop occurring in the exchanger tube
bundle. Variation in the frequency of the pulsations may be
afforded by variations in the reciprocation speed of the piston and
any desired variations in pulsation amplitude may be provided by
the use of a variable displacement pump, e.g. a variable
displacement piston pump, swashplate (stationary plate) pump and
its variations such as the wobble plate (rotary plate) pump or bent
axis pump.
[0031] Other types of pumps may also be used as the pulsation
device including diaphragm pumps and these may be practically
attractive since they offer the potential for activation of the
diaphragm by electrical, pneumatic or direct mechanical means with
the movement of the diaphragm controlled to provide the desired
frequency and amplitude (by control of the extent of diaphragm
movement). Other types may also be used but gear pumps and related
types such as the helical rotor and multi screw pumps which give a
relatively smooth (non pulsating) fluid flow are less preferred in
view of the objective of introducing pulsations which disrupt the
formation of the troublesome boundary layer. Other types which do
produce flow pulsations such as the lobe pump, the vane pump and
the similar radial piston pump, are normally less preferred
although they may be able to produce sufficient pulsation for the
desired purpose. Given the objective is to induce pulsations, other
types of pulsator may be used, for example, a flow interrupter
which periodically interrupts the liquid flow on the tube side.
Pulsators of this type may include, for example, siren type, rotary
vane pulsators in which the flow interruption is caused by the
repeated opening and closing of liquid flow passages in a
stator/rotor pair, each of which has radial flow openings which
coincide with rotation of the moving rotor member. The rotor may
suitably be given impetus by the use of vanes at an angle to the
direction of liquid flow, e.g. by making radial cuts in the rotor
disc and bending tabs away from the plane of the disc to form the
vanes. Another type is the reed valve type with spring metal vanes
which cover apertures in a disc and which are opened temporarily by
the pressure of the fluid in the tube, followed by a period when
the vane snaps closed until fluid pressure once more forces the
vane open.
[0032] As shown in FIG. 3, the pulsation device 40 is preferably
located close to the exchanger in order to ensure that the
pulsations are efficiently transferred to the liquid flow in the
tube bundle, that is, the pulsations are not degraded by passage
through intervening devices such as valves. Normally, the frequency
of the liquid pulsations will be in the range of 0.1 Hz to 20 kHz.
The amplitude of the pulsation as measured by the incremental flow
rate through the heat exchange tubes could range from about the
order of the normal heat exchanger flow rate at the lower end of
the range of pulsation frequencies to less than 10.sup.-6 of the
normal heat exchanger flow rate at the higher frequencies; because
of pressure drop limitations in the heat exchanger operation and/or
dissipation of higher frequencies in the fluid, the upper limit of
the pulse amplitude will decrease with increased frequency. Thus,
for example, in the lower half of this frequency range, the
amplitude of the pulsations could be from about 10.sup.-2 to about
the normal flow rate and with frequencies in the upper half of the
range, from about 10.sup.-6 to 0.1 of the normal flow rate through
the exchanger.
[0033] Alternatively, a vibration producing device 50 may be used
instead of the liquid pulsation device, described above. The
vibration producing device may be of the kind disclosed in
co-pending U.S. patent Ser. No. 11/436,802. The vibration producing
device may be externally connected to the heat exchange unit to
impart controlled vibrational energy to the tubes of the bundle.
The vibration producing device 50 can take the form of any type of
mechanical device that induces tube vibration while maintaining
structural integrity of the heat exchanger. Any device capable of
generating sufficient dynamic force at selected frequencies would
be suitable. The vibration producing device can be single device,
such as an impact hammer or electromagnetic shaker, or an array of
devices, such as hammers, shakers or piezoelectric stacks. An array
of devices 50 can be spatially distributed to generate the desired
dynamic signal to achieve an optimal vibrational frequency, as
shown in FIG. 1. The vibration producing device may be placed at
various locations on or near the heat exchange unit as long as
there is a mechanical link to the tubes.
[0034] Sufficient vibration energy can be transferred from the
tubes of the heat exchanger at vibration modes. There are low and
high frequency vibration modes of tubes. For low frequency modes
(typically below 1000 Hz), axial excitation is more efficient at
transmitting vibration energy, while at high frequency modes,
transverse excitation is more efficient. The density of the
vibration modes is higher at a high frequency range than at a low
frequency range (typically below 1000 Hz), and vibration energy
transfer efficiency is also higher in the high frequency range.
Further, displacement of tube vibration is very small at high
frequency (>1000 Hz) and insignificant for potential damage to
the tubes.
[0035] The use of vibration or pulsation reduces fouling in the
heat exchanger by creating oscillating shear stresses adjacent the
walls of the exchanger that reduce the boundary layers adjacent the
exchanger surfaces. These oscillating shear stresses when combined
with a low surface energy fouling resistant coating are effective
in reducing fouling because the fouling particles can be removed
from the heat exchanger surfaces, The oscillating shear stresses
act to tear or pull the loosely adhered particles from the
surfaces.
[0036] As noted above, many coatings of the conventional type, for
example, epoxies, tend to be relatively thick with a consequent
adverse effect on heat transfer and for this reason, coating
methods which result in a relatively thin thickness of coating,
desirably no more than 10 molecular layers thick, are preferred
provided that the necessary characteristics of corrosion and
fouling resistance are achieved. The gas phase and vapor deposition
methods are therefore likely to comment themselves for this
purpose.
[0037] A preferred class of coatings are the low surface energy
coatings described in co-pending U.S. patent application Ser. No.
11/304,874, to which reference is made for a description of such
coatings, their properties and methods of applying them to heat
exchanger surfaces. These coatings are constituted by a layer of
organometallic molecules which is 1 to 10 molecular layers thick
which will not undergo substantial decomposition at temperatures up
to 450.degree. C. and which has a surface energy lower than 50
millijoule/m.sup.2. The coatings are applied by contacting the
metal surface with an organometallic compound capable of bonding to
the metallic surface to form the desired surface layer. It is
preferred to prepare the metal surface prior to treatment by
heating the metal surface in an oxygen-containing atmosphere at
temperatures of from 100.degree. C. to 500.degree. C. to clean said
metal surface of any carbonaceous residues and then contacting the
metal surface with the requisite organometallic compound.
[0038] Alternative low energy surface coatings which inhibit the
deposition of fouling deposits are the coatings produced by the
processes known as Hollow Cathode Plasma Immersion Ion Processing
or the plasma-assisted chemical vapor deposition (PACVD) process,
both of which are commercially available processes (from the
Bekaert Company) which produce coatings referred to as
"Diamond-like Coatings". Diamond-like Coatings are amorphous carbon
based coatings with a high hardness and a low coefficient of
friction. Their composition and structure results in excellent wear
resistance and non-sticking characteristics. These coatings are
thin, chemically inert and have a low surface roughness. They can
be tailored to have a wide range of electrical resistivity with a
standard thickness between 0.002 and 0.04 mm. Compositionally, the
carbon Diamond-like Coatings are a mixture of sp2 and sp3 bonded
carbon atoms with a hydrogen concentration between 0-80%. These
coatings provide high hardness and abrasion resistance
characteristics. Diamond-like Composite Coatings comprising C, H,
Si and O are also available. Phosphate and phosphite ester
coatings, and fluorinated surface coatings which may be applied in
thin, adherent layers, which can also be produced by commercially
available processes are also potentially applicable.
[0039] The organometallic compounds used to form the preferred low
energy coatings described in application Ser. No. 11/304,874 are
those which are capable of bonding to a metal surface and which
will not decompose at the temperature to which the metallic surface
is exposed. Most organometallics used in the prior art to protect
metallic surfaces are employed as precursors and are converted to
oxides which function as the protective coating. In the case of the
preferred low energy coatings, the organometallic compound, not its
oxide, functions as the protective coating. Thus, the
organometallic coating functions as a chemical protective layer in
the monolayer range as compared to a physical barrier provided by
thicker coatings.
[0040] In the organometallic compounds used as coating materials,
the metallo components of the organometallic compounds are selected
from Groups 4 to 15 based on the IUPAC format for the Periodic
Table having Groups 1 to 18, and are preferably selected from Group
14, more preferably silicon and tin, especially silicon. The organo
components of the organometallic compounds are hydrocarbyl groups
having from 1 to 30 carbon atoms, preferably from 1 to 20 carbon
atoms, more preferably 1 to 10 carbon atoms. The hydrocarbyl groups
may be aliphatic or aromatic groups which aliphatic or aromatic
groups may be substituted with functional groups such as oxygen,
halogen, hydroxy and the like. Preferred hydrocarbyl groups include
methyl, ethyl, methoxy, ethoxy and phenyl. Preferred organometallic
compounds include alkysilanes, alkoxysilanes, silanes, silazanes
and alkyl and phenyl siloxanes. Especially preferred compounds
include alkyl- or alkoxysilanes having from 1 to 20 alkyl or alkoxy
groups, especially tetraalkoxy compounds such as
tetraethoxy-silane, alkylsilanes having from 1 to 6 alkyl groups,
especially hexamethyl-disiloxane.
[0041] The organometallic coating on the metallic surface should
preferably have a low energy surface, that is, a surface free
energy lower than 50 milliJoules/square meter (mJ/m.sup.2),
preferably between 21 to 45 mJ/m.sup.2. The low surface energy of
the layer ensures a low interfacial energy at, for example, the
interface between crude oil and the coated layers, even at the
higher temperature conditions found in typical heat exchangers,
e.g., 200.degree. C. to 400.degree. C. for a crude pre-heat
exchanger train. This in turn provides for a weak interaction of
foulants and corrosive species with the surface resulting in a
reduction in fouling and corrosion rate. Thermally conductive,
adherent, corrosion-resistant layers such as the ones produced by
electropolishing and as described below, can be used as substrates
for the superficial molecular layer-thick coating. Reference is
made to Application Ser. No. 60/751,985, "Corrosion Resistant
Material For Reduced Fouling, A Heat Exchanger Having Reduced
Fouling And A Method For Reducing Heat Exchanger Fouling In A
Refinery") for a more detailed description of these methods.
[0042] The surface free energy of the coating can be determined by
measuring the water contact angle. Similarly, the extent of the
surface modification by the organometallic coating can be measured
using water contact angles. This test measures the contact angle of
water in contact with the modified metal surface. An example of a
test procedure for measuring water contact angles is ASTM D-5725.
High water contact angles imply high hydrophobicity and good
coverage of the underlying metal (or metal oxide/sulfide) surface
by the organometallic coating. For the modified metal surfaces,
measured water contact angles are between 95.degree. to
160.degree., preferably 110.degree. to 150.degree., with angles of
at least 130.degree. giving best results.
[0043] The amount of covering of the organometallic coating layer
ranges from greater than 25% with, of course, correspondingly
better resistance to corrosion and fouling as the covering of the
metal surface approaches 100% of the metal surface. Thus, from 50
to 100% is preferably covered, more preferably from 80 to 100% and
for optimal results, 100% or as close to 100% as possible.
[0044] The metal surface to be protected is preferably clean of
carbonaceous deposits such as coke. This is important in continuous
processes in which a feed is heated while in contact with a metal
surface such as pipes used in refinery and chemical plant service,
heat exchangers and furnace tubes. After standard initial cleaning
with a light cycle oil, other light oil or other solvent and high
pressure water jetting or high pressure steam cleaning, the metal
surface is preferably cleaned by heating in the presence of an
oxygen-containing gas, preferably air, at temperatures of from
200.degree. C. to 500.degree. C., preferably 300.degree. C. to
400.degree. C. for a time sufficient to remove the desired
deposits, particularly carbonaceous deposits. The heating typically
occurs at atmospheric pressure although higher pressures are
acceptable. If salts are present, a water wash may be used to
remove salts. The cleaned metal surface may also be treated with a
solution of metal salt to enhance corrosion resistance as well as
the effectiveness of the organometallic coating process. For
example, a carbon steel surface may be first treated with a
chromium salt solution to form a chromium-rich surface layer.
Chromium-enhanced surface layers may be produced by
electro-polishing the tube in a solution containing chromic acid.
This is effective to increase the chromium concentration at the
surface when the chromium content in the substrate steel is less
than about 15 wt. %. The electropolishing technique is also
particularly advantageous because if is capable of producing a
surface with a surface roughness of less than 1000 nm, preferably
less than 500 nm and more preferably less than 250 nm, which, when
given a superficial organometallic coating is well able to resist
fouling deposits and corrosion. The chromium-enriched layer may
also be formed using various other techniques including
electroplating chromium onto another alloy such as a carbon steel,
thermal spray coating, laser deposition, sputtering, physical vapor
deposition, chemical vapor deposition, plasma powder welding
overlay, cladding, and diffusion bonding. It is also possible to
choose a high chromium alloy such as 316 stainless steel. The
chromium-enhanced layer may be mechanically polished and/or
electro-polished as described above to obtain a uniform surface
roughness within the desired range.
[0045] The chromium-enriched layer may be given a superficial oxide
layer prior to deposition of the organometallic coating. The oxide
layer will typically include an oxide species whose own composition
will be dependent on the metallurgy of the substrate; thus, with
ferrous metal tubes, the oxide layer may typically be expected to
include one or more of magnetite, iron-chromium spinels and
chromium oxides.
[0046] Various other techniques may also be used for the generation
of chromium-enriched surface layers on the exchanger tubes
including, but not limited to electroplating, thermal spray
coating, laser deposition, sputtering, physical vapor deposition,
chemical vapor deposition, plasma powder welding overlay, cladding,
and diffusion bonding. Passivation, that is treatment of metals
with dilute nitric or citric acid, for example, can also be used to
increase the concentration of chromium at the surface when working
with stainless steel alloys. The combination of electropolishing
and passivation is also a useful method for achieving this
effect.
[0047] The organometallic coating can be formed on the cleaned and
heated metal surface by exposing the heated metal to the selected
organometallic compound in the gaseous phase, liquid phase or mixed
liquid-vapor phase. The organometallic compound may, for example,
be sparged into the vapor state using a carrier gas such as
nitrogen or it may be mixed with a carrier liquid such as
cyclohexane, xylene, water carbon tetrachloride, chloroform, fuel
oil, lube boiling range hydrocarbon, crude oil and the like as a
dilute solution, e.g., up to 5 vol. %. The organometallic coating
process should preferably take place in the absence of an
oxygen-containing gas. The temperature of the coating process may
suitably range from ambient to 500.degree. C. The upper temperature
range for coating is a function of the stability of the particular
organometallic used for coating.
[0048] The thickness of the organometallic coating ranges from 1 to
10 molecular layers thick, preferably 1 to 3 molecular layers
thick, more preferably a monolayer thick. The thickness of the
molecular layer may be controlled by the deposition process, e.g.,
by controlling the time of exposure of the metal surface to the
organometallic compound and controlling the pressure under which
the coating is applied.
[0049] A coated heat exchanger tube 34 is illustrated in
cross-section in FIG. 3. In this case, the coating 5 is located on
the inside of the tube 34 consistent with the possibility that
fouling is expected on that side with a non-fouling medium on the
shell side. If, however, fouling were to be expected on the shell
side, the coating could be applied to that side or even to both
sides if necessary. The coating 5 extends over the inner surface of
tube 34 to provide resistance to fouling but in this case, this
resistance is enhanced by the use of the fluid pulsation technique
to provide superior anti-fouling performance.
[0050] Operating temperatures for the metal surfaces coated with
organometallic molecules according to the invention should be
maintained below 450.degree. C., preferably below 400.degree. C.,
more preferably below 350.degree. C. Some decomposition of
organometallic coating may occur depending on the nature of the
organometallic employed as coating and the operating temperature
employed. For example, phenyl silanes as coating agent can be
stable at higher temperatures and may be used in more severe
service than alkyl silanes. By "substantial decomposition" of the
layer of organometallic molecules is meant that the organometallic
molecules in the coating (covering) layer are reduced to less than
25% coverage of the metal surface.
[0051] The behavior of the present organometallic coatings is
believed to be at least in part a function of the organo moiety. It
appears that the organo moiety minimizes the interaction energy
with both polar and non-polar hydrocarbons and mitigates fouling
and corrosion in this manner. Minimizing corrosion can be linked
with minimizing fouling. For example, corrosion tends to increase
the metal surface area creating a trap for foulants. Ordinary
ceramic coatings of metals surfaces rely on a physical barrier to
mitigate corrosion. However, ceramic coatings will not be as
effective as organometallics because their surface energies may
still be high, i.e., greater than 100 mJ/m.sup.2. The same
reasoning applies to oxide coatings used to provide a physical
barrier. Thus, metals, particularly steels and ferrous metal
alloys, can be provided with a low surface energy monolayer or near
monolayer of organometallic coating that resists both corrosion and
fouling deposits in refineries and chemical plants.
[0052] To determine the effect of fluid pulsation on fouling, a
pulsation flow unit was added to an Alcor.TM. HLPS-400 Liquid
Process Simulator. The resulting test rig is shown in FIG. 4. The
Alcor HLPS-400 Hot Liquid Process Simulator is a laboratory tool
for predicting heat exchanger performance and the fouling
tendencies of specific process fluids. The Alcor HPLS operates in
the laminar flow regime at accelerated fouling conditions compared
to commercial heat exchangers which typically operate a high
turbulent flow regime at much lower fouling rate but in spite of
these differences, the Alcor HLPS has proven to be an excellent
tool for predicting the relative fouling tendencies of fluids in
commercial heat exchangers.
[0053] For this fouling study a crude oil was run in the Alcor
HLPS. The basic Alcor HPLS consists of a crude sample reservoir 10,
a heat exchanger test section 11 and a constant displacement pump
12 located downstream of the test section. The test oil is pumped
in a closed cycle from reservoir 10 through line 13 to test section
11. A branch line 15 passes from line 13 to one side of a the
positively driven, double acting, reciprocating displacement pump
12 which was a dual head Constametric.TM. HPLC metering pump
modified by removing the check valves from each pump head and
closing the inlet to the pump heads. In a similar manner, branch
line 16 extends from oil return line 17 to the other side of pump
12. Circulating pump 18 of the constant displacement type maintains
the oil in circulation during the test run in a closed cycle from
the reservoir, through the test section and then back to the
reservoir. Test section 11 comprises a cylindrical tube casing 20
which surrounds a centrally located test coupon in the form of a
rod 21 with a narrowed center section 25 within the tubular casing.
The tubular casing has a liquid inlet port 22 connected to feed
line 13 and an outlet port 23 connected to return line 17. Liquid
seal between the coupon and the casing is provided by gaskets at
each end with end caps 24 where the coupon exits the casing; the
gaskets also insulate the coupon electrically from the outer
casing. The coupon may be heated electrically by means of
electrical connections (not shown) at its two ends, connected to a
controller to supply current at various amperages depending on the
degree of heating required. In this test program, the coupon is
used as a surrogate for a heat exchanger tube.
[0054] The test run time was for three hours with additional
fifteen minute periods for each heat-up and cool-down. Tests were
carried out by charging a reservoir with 800 ml of the test crude.
The crude in the reservoir and lines to and from the test heat
exchanger were heated to 150.degree. C. To prevent vaporization of
the test fluid, the system was pressurized to approximately 3450
kPag (500 psig) with nitrogen. The fluid from the reservoir was
pulled through the test heat exchanger by the downstream constant
displacement circulation pump at a flow rate of 3 ml/min. which
returned the fluid to the top of the reservoir. A piston was placed
in the reservoir to separate the fresh crude from the used, heat
exchanged crude. In the test heat exchanger, the fluid flows up
through the annulus formed by a vertically positioned heated test
coupon rod. The test section of the heated rod is about 3.20 mm in
outside diameter and 60 mm long. The outer shell of the test heat
exchanger has a 5.10 mm inside diameter forming a 0.95 mm annular
space for flow. The temperature of the heated rod is controlled by
a thermocouple located inside the heated rod test section, set to
275.degree. C. in this study. The temperatures of the fluid to the
inlet and from the outlet of the heat exchanger were recorded over
the duration of the test. As deposits or fouling material builds up
on the surface of the heated rod, the outlet temperature of the
fluid from the heat exchanger decreases. This decrease results from
the insulating nature of the deposit on the rod. The decrease in
outlet temperature gives a measure of the fouling tendency of the
fluid on the rod surface as well as of the heat transfer efficiency
of the unit.
[0055] During operation the volume inside each head of the metering
pump was varied by 0.10 ml; the changes in volume of the pump heads
are approximately 180 degrees out of phase. In the pulsation tests,
the pump speed was adjusted to give a fluid pulsation of 0.083
ml/sec in both the forward and reverse flow directions.
[0056] The coating used in this study was obtained by treating the
Alcor rods with HMDSO (hexamethyldisiloxane). The Alcor rods used
in this study were 1018 carbon steel. Because the new rods are
coated with light oil, the rods were cleaned sequentially with
toluene, iso-propyl alcohol (IPA), water, and then IPA to dry the
rod. The clean rods were loaded in a horizontal, controlled
atmosphere, alumina tube furnace. In the HMDSO treatment, the rods
were first air oxidized at 350.degree. C. for one hour, followed by
purging the tube furnace with nitrogen for ten minutes, and then
treating the rods with HMDSO vapor at 350.degree. C. for one hour.
The HMDSO vapor is carried to the furnace by bubbling nitrogen
through a room temperature reservoir of HMDSO liquid. The Alcor
rods are cooled under the HMDSO vapor before removal from the tube
furnace. The effectiveness of the coating can be gauged the
increase of the water contact angle. The clean rods had an average
water contact angle of 81 degrees. After the HMDSO treatment, the
rods made an average water contact angle of 129 degrees.
[0057] To examine the effect of liquid pulsation plus HMDSO
treatment the following series of runs were made in the test unit:
the base case with non-coated rods and no pulsation, the pulsation
only case with the +/-0.083 ml/sec pulsation rate, the HMDSO
treatment only case, and the pulsation plus HMDSO treatment
case.
[0058] FIG. 5 shows the change in fouling obtained in the Alcor
runs as depicted by decrease in outlet temperature. The average of
two Alcor runs is shown for each case. The data curves in FIG. 5
show that the temperature decrease for the pulsation plus HMDSO
treatment case is smaller compared to the base case indicating a
lower fouling rate. The temperature decrease for the pulsation plus
HMDSO treatment case is also smaller than either the pulsation only
case or the HMDSO treatment case. This indicates a synergetic
effect on reducing fouling when pulsation is combined with surface
treatment. The starting temperature for the runs having pulsation
are also shifted to higher temperature due to the enhanced heat
transfer offered by the pulsation.
[0059] It will be apparent to those skilled in the art that various
modifications and/or variations may be made without departing from
the scope of the present invention. While the present invention has
been described in the context of the heat exchanger in a refinery
operation, the present invention is not intended to be so limited;
rather, it is contemplated that the surface coatings and vibration
and/or pulsation disclosed herein may be used in other portions of
a refinery operation where fouling may be of a concern. Thus, it is
intended that the present invention covers the modifications and
variations of the method herein, provided they come within the
scope of the appended claims and their equivalents.
* * * * *