Note: Descriptions are shown in the official language in which they were submitted.
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REDUCTION OF FOULING IN HEAT EXCHANGERS
FIELD OF THE INVENTION
[0001] 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
[0002J 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.
[0003J 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
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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.
[00041 US Patent 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.
[0005J US Patent 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
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cost to the design of what would otherwise be a relatively inexpensive device
which
normally contains no moving parts.
[0006] In practical terms, the reduction of scale or deposit formation by the
use
of inechanically 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 US
Patent No.
4,645,542 to Scharton, US Patent No. 4,655,846 to Scharton and US Patent 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 US Patent 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, US Patent 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. US
Patent 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.
[0007] 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
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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.
[0008] 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.
[0009] 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 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.
[0010] There is a need to reduce and/or eliminate fouling in heat exchangers,
which is presently not addressed by the prior art.
SUMMARY OF TI3E INVENTION
[0011] 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
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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.
[0012] 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 a'pplying 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.
[0013] 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/m2). 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
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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.
[00141 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.
[0015] 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
[0016] The invention will now be described in conjunction with the
accompanying drawings in which:
Figure 1 is a partial cross-sectional view of a shell and tube heat exchanger
in
accordance with an embodiment of the present invention;
Figure 2 is a cross-sectional view of a heat exchanger tube of Figure 1
illustrating the coating in accordance with the present invention;
Figure 3 is a schematic view of a heat exchanger in accordance with another
embodiment of the present invention;
Figure 4 is a simplified equipment schematic of a test rig for demonstrating
the
application of the present invention to a heat exchanger; and
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Figure 5 is a graphical representation of the results achieved in the testing
reported below.
DETAILED DESCRIPTION OF THE INVENTION
[0017] The present invention will now be described in greater detail in
connection with the attached figures. Figure 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.
[0018] 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
Figure 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.
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[0019] 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.
[0020] 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
Figure 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.
[0021] 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 US Patent Serial
No.
1 I/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. -
100221 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
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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
particulaxly useful.
100231 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.
[0024] 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
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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.
[0025] As shown in Figure 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 10-6 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
10-2 to about the normal flow rate and with frequencies in the upper half of
the range,
from about 10-6 to 0.1 of the normal flow rate through the exchanger.
100261 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 US Patent Serial 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 inechanical device that induces
tube
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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 Figure
1. The vibration producing device may be placed at various locations on or
near the
heat exchange unit as lorig as there is a mechanical link to the tubes.
[0027] 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 1000Hz), 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.
[0028] 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 wheri
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.
[0029] 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
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phase and vapor deposition methods are therefore likely to comment themselves
for this
purpose.
[0030] A preferred class of coatings are the low surface energy coatings
described in co-pending US Patent Application 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 C and which has a surface energy lower
than
50 millijoule/ma. 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 C to 500 C to clean said metal surface of any carbonaceous residues and
then
contacting the metal surface with the requisite organometallic compound.
[0031] 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 0 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.
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[0032] The organometallic compounds used to form the preferred low energy
coatings described in Application Serial 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.
[0033] 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
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.
[0034] 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/m2), preferably between 21 to 45 mJ/m2. 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 C to 400 C for a crude pre-heat
exchanger
train. This in turn provides for a weak interaction of foulants and corrosive
species
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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 Serial No.
60/751985,
"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.
[0035) 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 to 160 , preferably
110 to
150 , with angles of at least 130 giving best results.
[0036] 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.
[0037] 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 C to
500 C, preferably 300 C to 400 C for a time sufficient to remove the desired
deposits,
particularly carbonaceous deposits. The heating typically occurs at
atmospheric
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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.
[00381 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.
[0039] 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
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16
surface when working with stainless steel alloys. The combination of
electropolishing
and passivation is also a useful method for achieving this effect.
[0040] 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 C.
The
upper temperature range for coating is a function of the stability of the
particular
organometallic used for coating.
[0041] The thickness of the organometallic coating ranges from I 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.
[0042] A coated heat exchanger tube 34 is illustrated in cross-section in
Figure
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.
[0043] Operating temperatures for the metal surfaces coated with
organometallic molecules according to the invention should be maintained below
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17
450 C, preferably below 400 C, more preferably below 350 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.
[0044] 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/m2. '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.
[0045] To determine the effect of fluid pulsation on fouling, a pulsation flow
unit was added to an AlcorTM HLPS-400 Liquid Process Simulator. The resulting
test
rig is shown in Figure 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.
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18
[0046] 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 ConstametricTM 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.
[00471 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 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
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19
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 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.
(00481 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#luid
pulsation of 0.083 ml/sec in both the forward and reverse flow directions.
[0049] 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 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 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.
[0050] 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
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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.
[0051] Figure 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 Figure 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.
[00521 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.
