Note: Descriptions are shown in the official language in which they were submitted.
21~6234
DYNAMIC FOULING TEST PROCEDURE
Backqround of the Invention
1. Field of the Invention
The present invention relates to procedures for
carrying out fouling tests in the field, and more
particularly to on-line monitoring of fouling and on-line
measurement of anti-foulant performance.
2. Description of the Prior Art
Fouling is a problem associated with certain fluids
in a variety of common situations. For example, fouling
is a major problem encountered in the treatment of
various hydrocarbon charge stocks in oil refineries,
where the stocks are converted into valuable heating and
transportation fuels and petrochemical feedstocks. The
fouling is manifested as deposits that are formed
frequently on the metal surfaces of the processing
equipment and tend naturally to decrease the efficiency
of the intermediate processing operations. The results
of fouling appear in the form of heat transfer loss,
pressure drop, loss in throughput rate and an increase in
corrosion of the equipment.
Thus, techniques have been developed in an effort to
decrease fouling, many of which involve the application
of chemical additives that inhibit fouling. However,
such additives inhibit the fouling to varying degrees of
efficacy and variations among fouling situations mean
that antifoulants suitable in one situation are
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unacceptable in another. Thus, an additive of ideal
characteristics, including that of efficacy, and that has
universal application has not been developed. Moreover,
efficacy in a particular situation depends on a large
number of variables making predictions impossible or at
least impractical.
As a result, a trial and error approach typically is
taken to determine whether a particular proposed
anti-foulant would be suitable in the situation of
o concern. However, because of the dangers of failures
leading to continued fouling, of damaging equipment, of
contamination of the system to be treated, of resulting
shut-downs and of the need to dismantle the system to
determine efficacy by inspection of the equipment, it is
desirable to conduct such trials in a manner other than
by subjecting the system to be treated to the experiment.
U.S. Patent No. 4,383,438 to Eaton describes
apparatus and procedures for carrying out static fouling
tests in a laboratory environment. The fouling tests of
that patent are designed to simulate a heat exchanger
surface such as a particular section of a heated
exchanger tube exposed to a fouling liquid medium.
These tests represented a significant advancement
over the prior art techniques for testing fouling
conditions and the efficacy of antifoulants. However,
the tests still depend on approximation of actual field
conditions and so are not as reliable as desired in
predicting fouling tendency in the field.
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In the field, hydrocarbon fluids that exhibit
fouling tendencies flow through pipelines, heat
exchangers and the like and encounter a variety of
conditions that may not be able to be discovered (at
least not without disrupting the system) or duplicated in
a laboratory test such as that of the Eaton patent.
Moreover, the fluids flow at significant velocities.
Fiuid velocity has been found to be an important factor
in fouling, particularly in the effect its shear forces
have on deposits. Thus, static fouling tests are
particularly unreliable in predicting fouling in dynamic
field operations because they cannot duplicate
satisfactorily the velocity of fluid flow in the field.
Moreover, inaccuracy is inherent in static tests since
the metal surface is exposed to a set volume of fluid
rather than the continuous flow, high fluid volume
encountered by field surfaces. Continuous flow in the
lab is impractical because of the volume of fluid that
would be required for a continuous flow that may be
carried out over a testing period that can last weeks.
The technique of the Eaton patent employs a stirring
rotor to approximate the effect of the fluid velocity,
but still relies on a simulation of the true flow of
velocity and other field conditions, as well as on a set
volume of fluid rather than a continuous flow. Other
techniques feature continuous flow, but sacrifice the
fluid velocity to minimize fluid use or loss.
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Accordingly, dynamic tests are needed for monitoring
and measuring fouling tendencies under more realistic
field conditions that can take into account both fluid
velocity and continuous flow without substantial fluid
loss.
SummarY of the Invention
The present invention, therefore, is directed to a
novel method for dynamic fouling test process.
According to the process, a portion of a flowing fluid is
diverted into a plurality of test streams. An additive
to be tested is added to at least one of the test
streams. A test stream to which the additive was added
is directed to maintain contact with a first heated metal
surface for a time period sufficient to allow fouling to
accumulate on the metal surface. The heated metal
surface has means for heating the surface. A second test
stream is directed to maintain contact with a second
heated metal surface for a second time period, preferably
about the same time period as the first stream maintained
contact with the first heated surface. The second heated
metal surface has means for heating thé second metal
surface. The accumulation of fouling on the two metal
surfaces is measured and the accumulation on the first
metal surface is compared to the accumulation on the
second metal surface.
The present invention is also directed to a novel
apparatus for dynamic testing of the foulin~ tendency of
a flowing fluid. The apparatus comprises a slip stream
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conduit for divergence of a slip stream from a flow of
fluid to be tested, a plurality of test stream conduits
in fluid communication with the slip stream conduit and
means for dividing a flow of fluid in the slip stream
conduit into the plurality of test stream conduits, means
for introduction of an additive into at least one of the
test stream conduits, and a heated metal surface in each
test stream conduit located so that the heated metal
surface may be exposed to a fluid flowing through the
lo test stream conduit for a time period sufficient to allow
fouling to accumulate on the metal surface, the metal
surface having means for heating the surface.
Among the several advantages of this invention, may
be noted the provision of a method for carrying out
fouling tests in the field; the provision of such method
that permits on-line monitoring of fouling and on-line
measurement of anti-foulant performance; the provision of
such method that permits measurement of fouling tendency
in the field under more realistic conditions; the
provision of such method that permits measurement of
fouling tendency under field-like high velocity,
continuous flow conditions without substantial fluid
loss; and the provision of apparatus useful in carrying
out such method.
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Brief DescriPtion of the Drawings
Fig. 1 is a schematic representation of a typical
refinery unit side stream adapted with apparatus for
carrying out the test procedure of the invention; and
S Fig. 2 is a view of the testing device of the
invention shown in place in a conduit of the invention,
the conduit being shown in cross-section.
Detailed Description of the Preferred Embodiments
In accordance with the present invention, it has
1~ been discovered that by diverting a slip stream from a
flow of fluid, such as from refinery unit side streams,
to at least two test streams running in parallel and
including heated metal surfaces, injecting an additive to
one of the test streams and comparing the fouling on the
heated metal surface from that stream to the fouling on a
similar heated metal surface in a simllar stream to which
the additive has been injected under different conditions
or to which no additive or a second additive has been
injected, a more realistic test of anti-foulant efficacy
can be achieved than with standard static fouling test
methods. The test is carried out on the actual fluid of
concern. And, by contrast with such prior art static
tests, this new dynamic test procedure permits continual
renewal of the fluid being tested and takes into account
the fluid velocity and so reflects actual field
conditions more accurately without significant fluid
loss.
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Moreover, the new test procedure can be controlled
to simulate a variety of conditions and to test the
effects of various parameters. Thus, the time of
operation, the heat transfer rate, the heat exchanger
S skin temperature, the additive type, the additive
injection rate and the operating mode (i.e., operation at
constant heat transfer rate or constant skin temperature)
can be selected. In fact, for instance, for fluid flow
directed to a series of heat exchangers as may be
encountered in a refinery unit side stream, by adjusting
the skin temperature, any heat exchanger in the series
can be simulated for fouling rate studies. Not only
that, but by returning the test streams to the flow of
fluid after comparing the fouling, fluid loss is
eliminated.
Referriny now to Fig. 1, a typical refinery unit
side stream adapted with apparatus for carrying out the
test procedure of this invention is illustrated.
Although the test procedure and apparatus of this
invention may be applied to any flow of fluid, the
particular stream of Fig. 1 is shown for purposes of
explanation and understanding of a typical environment
for this invention. This invention is especially well
suited to testing fouling of heat exchangers and so to
application to fluid flow to heat exchangers, the
invention may be adapted by the same methods to a wide
variety of fluid streams.
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While the present test procedure may be employed for
any of a wide variety of media having a tendency to foul
metal surfaces such as heat exchanger surfaces, it is
particularly well suited for testing of hydrocarbon
fluids, especially crude oil. Such hydrocarbon fluids
contain hydrocarbons, but also various other components
or impurities investing in the fluid its tendency to foul
metal surfaces.
In the particular stream illustrated in Fig. 1,
crude oil from a desalter 5 is directed along a major
conduit 10 and pumped by a pump 12 to a series of heat
exchangers 1~, 16, 18, 20 and 22 to a furnace 24. An
antifoulant or other chemical additive may be added to
the stream by way of pump 26 at injection point P.
As shown in Fig. 1, a small slip stream may be
diverted from the stream flowing through conduit 10 and
studied with apparatus of this invention, which may be
described as a refinery exchanger fouling simulator. In
particular, gate valves 28 and 30 may be opened to create
a slip stream. The slip stream flows through valve 28
and then is divided into two or more sub-streams running
in parallel. Fig. 1 shows two substreams identified as
~a~ and "b", with each element associated with the
sub-streams to be discussed below designated ~a~ and "b",
respectively, to indicate association with the
correspollding sub-stream. Thus, for example, a pump
designated 32 would be identified as 32a in a sub-stream
~a~ and as 32b in sub-stream ~b~. Each sub-stream is
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carried by a conduit of, for example, three-quarter or
one inch in diameter, permitting a flow rate as desired,
such as about five to eight gallons per minute. However,
other diameters and flow rates may be employed as
appropriate for the system. Flow rate also can be varied
by changing the pipe diameter or by changing the pump
design or by adding additional piping to divert flow.
The substream temperature is the same as that of the flow
from which is diverted; for instance, about 250F
lo (about lZ0C). Alternatively, the substreams may be
equipped with a heat exchanger for varying the
temperature, such as from about 250F (120C) to
about 580F (300C).
For at least one of the sub-streams, a pump 32 is
- 15 provided to permit injection at point C of a chemical
additive to be tested. At least one of the sub-streams
may be left untreated as a control or comparison to the
treated sub-stream or sub-streams. Alternatively, all
sub-streams may be treated, with the additive type or
additive injection rate varied, or all sub-streams may be
treated equally simply to show a favorable comparison of
fouling tendency.
The sub-streams also may be adapted to account for
the residence times and mixing of the tested additives in
the stream 10. Thus, in the on-line additive treatment
of stream 10 via injection by pump 26, the design of the
refinery piping system (i.e., the length, diameter and
layout of the piping) and the location of additive
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injection causes the additive to remain in and mix with
the hydrocarbon in stream 10 for some amount of residence
time. The residence time varies from refinery unit to
refinery unit`and from refinery to refinery. The present
invention takes the residence time and mixing into
account.
In order to simulate the residence time of the
additive in the main stream 10, the residence time of the
additive in the substream may be increased such as by
increasing the diameter of the conduit carrying the
treated substream so that the conduit has an increased
diameter for a selected length, and then reducing the
diameter back to the original (or some other desired)
diameter. This section is designate in Fig. 1 by the box
marked ~R~. Thus, a wide variety of residence times may
be accommodated by varying the flow rate, the degree of
increase in diameter of the substream conduit and the
length of the section of the conduit having increased
diameter. Illustrative, but not limiting, of typical
residence times that may be desired is the range of about
one-half to about three minutes, such as about forty-five
to fifty seconds.
The effect of mixing of additive with the
hydrocarbon stream lO may be simulated in the substreams
by the inclusion of one or more static, in-line mixing
elements, the number of such mixing elements being
dependent on the degree of mixing to be simulated. Such
mixing elements are designated in Fig. 1 as 33. In
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addition, pulsation dampening equipment can be included
on the additive injection pumps 32, if so desired, to
eliminate the effect of pulsed additive injection.
Injection quills also can be included as needed to match
the system being simulated.
The pressure of the sub-stream then is increased by
pump 34 and the sub-stream is directed to a testing
device 36. The testing device includes a heat exchanger
which provides a heated metal surface against which the
fluid flows and on which fouling deposits may accumulate.
The heated metal surface may be a heated probe as
described in U.S. Patent No. 4,383,438 to Eaton. Fig. 2 illu-
strates the testing device, with the conduit shown in cross-section.
testing device, with the conduit shown in cross-section.
The sub-stream flows through conduit 38 in the direction
of arrow 42 to a T-shaped section, with the branch of the
T-shaped section opposite the flowing stream capped with
the probe 44 so that the fluid flows into the end 46 of
the probe and is routed away to flow in the direction of
arrow 48, at right angle to arrow 42. The probe 44 is
affixed to the T-section by means of fiange 52 and is
configured as described in the noted patent to Eaton.
Therefore, the probe comprises a metallic housing 54
enclosing an electrical resistance heating element and a
thermocouple in contact with the inner wall of
housing 54. The heating element and thermocouple are not
shown in Fig. 2, but are shown and discussed in the Eaton
patent. The metallic housing 54 most desirably should be
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of the same metal as the surfaces for which fouling is a
concern in the system. Thus, if the fouling tendency of
steel heat exchangers in the system is to be studied, the
metallic housing 54 would be of the same type of steel as
the heat exchangers.
As noted, the Eaton probe is just one possible heat
exchanger that may be used. In other versions of the
heat exchanger or metal surface, the fluid may contact
the surface by flowing across the surface (i.e., it may
flow parallel to and along the surface), by striking the
surface obliquely or by striking the surface head on.
Possible embodiments of the heated metal surface include,
for example, an electrically heated tube through which
the fluid flows inside as opposed to outside the tube may
be used. Or, a shell-in-tube heat exchanger, with a
heating oil and heater added to provide a hot fluid for
the shell side may be used with the fluid passing through
the tubes. It is simply necessary that a heated metal
surface be provided for exposure to the fluid flow. The
flowing fluid maintains contact with the metal surface
(that is, it flows along or against itj for a desired
period of time to evaluate the fouling tendency of the
fluid.
If the Eaton device is used, electrical leads 56
and 58 extend from the resistance heating element and the
thermocouple, respectively, to the outside of the probe
and conduit. Electrical leads 56 connect with a constant
current power source shown in the Eaton patent, but not
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in Fig. 2 herein. Electrical lead 58 connects with a
temperature gauge, which in turn preferably is connected
to a temperature recording device. The gauge and
recording device are shown in the Eaton patent, but not
in Fig. 2 herein. Alternatively, other heating and
temperature measurement techniques may be employed; for
example, a control panel with measuring devices, and a
different type of power source and recordlng and
controlling devices may be used.
Thus, the temperature of the surface of the metallic
housing of the Eaton probe (i.e., the skin temperature)
may be adjusted to the temperature of the surfaces for
which fouling is a concern. If another type of metal
surface is used instead of the Eaton probe, temperature
may be controlled in a similar manner or by standard
techniques for the heat exchanger employed for the metal
surface.
If desired, a heat exchanger may be placed in the
line before the metal surface as noted above to adjust
the temperature of the flowing fluid as well. Means for
measuring the temperature of the fluid flowing to the
metal surface should be provided in addition to the means
for measuring the temperature of the metal surface.
Thus, whereas the apparatus of Eaton required a
vessel and rotor in an effort to simulate field
conditions, the present device is placed in the actual
field setting and exposed to field conditions, and the
vessel and rotor are unnecessary. However, the metal
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surface is equipped with a control cabinet to provide for
control of the heated probe voltages using either the
heated metal surface temperature or the delivered wattage
as a control parameter. The control cabinet should also
contain a datalogging means to record the performance for
analysis. By measuring and comparing the fouling rates
on the probes, antifoulant performance can be determined.
According to this technique, therefore, any of the
time of operation, the additive type, the additive
injection rate, the operating mode (that is, for example,
running at constant heat transfer rate or constant skin
temperature), the heat transfer rate of the heat
exchanger simulated and the skin temperature of the heat
exchanger simulated (the latter two be interdependent),
may be control and varied and any of the other variables
and resultant fouling rate can be measured. As a result,
the dependency of fouling rate of a particular variable
of concern can be studied.
After flowing past the metal surface, the substreams
may be reunited at the T juncture 60 and directed via
conduit 62, through gate valve 30 and back to the main
conduit 10, eliminating waste of fluid otherwise
encountered during standard tests. Between runs, the
probe may be cleaned by first closing gate valves 28
and 30 and then opening flange 52 to remove the metal
surface. The metal surface may then be cleaned as
discussed in the Eaton patent.
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In view of the above, it will be seen that the
several advantages of the invention are achieved and
other advantageous results attained.
As various changes could be made in the above
methods and compositions withou~ departing from the scope
of the invention, it is intended that all matter
contained in the above description shall be interpreted
as illustrative and not in a limiting sense.
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