Chemical Reaction Fouling - A Review
Chemical Reaction Fouling - A Review
Chemical Reaction Fouling - A Review
Address correspondence to Professor A. P. Watkinson, Department of Chemical Engineering, The University of British Columbia, Vancouver,
BC V6T 1Z4, Canada.
0894-1777/97//$17.00
PII S0894-1777(96)00138-0
362
a)
Bulk Liquid
bulk reaction
precipitation of insoluble B
ed by mass transfer
mass
transfer
surface reaction
A
( adhesion
B
~I~
~'--C
Thermal Boundary
Layer
Heat Transfer Surface
b)
Chemical Reaction
A,
O,
Indene
C,H,
Peroxy
Radicals
RO,-
O,
B,
Heat C,
--- Polyperoxides
- Oxygenated
A
(C,H,OO).
Deposits,
Figure 1. (a) General multistep chemical reaction fouling mechanism. (b) Application of mechanism to
model solutions of indene.
established for most fouling species. Autoxidation was the
mechanism of indene fouling for solutions with calculated
oxygen contents as low as 1.8 ppm [11].
This paper focuses on a survey of the more recent
organic fluid fouling and related literature, based on the
classification in Refs. 2 and 3. Autoxidation fouling is
interpreted by using known chemistry. The residuum processing literature suggests some approaches for dealing
with fouling under nonoxidative conditions, for which the
chemical reactions causing fouling have not been elucidated or incorporated into fouling models.
AUTOXIDATION
The autoxidation of hydrocarbons has been identified as
the main source of unwanted deposits in reviews of fuel
storage stability [12], in the formation of unwanted g u m s
in jet fuel feed lines, and in many cases of heat exchanger
fouling in the temperature range from ambient to 300C
[2]. Deposition in oxygenated hydrocarbon systems above
250-300C is dominated by thermal condensation and
cracking reactions. Autoxidation, or the autocatalytic oxidation of hydrocarbons, consists of a complex set of free
radical reactions, and recent work has been the source of
significant insights into the fouling problem in these systems.
Table 1 is a summary of recent investigations of autoxidation fouling [13-35]. Earlier studies, such as those by
Taylor and coworkers on jet fuel deposition under vaporizing conditions, are discussed in Refs. 2 and 3. Most studies
Reference
Test Fluid
Jet A fuel
[15] Marteney
and
Spadiccini
(1986)
[16, 17] Morris
et al.
(1988, 1989)
[18] Morris and
Mushrush
(1991)
[19] Wilson and
Watkinson
(1992)
[20] Asomaning
and
Watkinson
(1992)
[21] Jones et al.
(1992)
Hexadecane
S additives
Jet fuels,
additives,
model solutions
Indene in
different solvents
Alkenes in
kerosene
3 Jet A fuels,
hexadecane
3 Jet A fuels
aerated,
deaerated
additives
DF2, kerosene
Indene in
kerosene
Jet-A fuels
Styrene/heptane
sulfur species
Apparatus
(Measurement
Method)
Temperature
Range
Flow
Velocity
OtherAna~ysis
Methods
Tubular heater,
constant heat flux
( A Tw~al,mass
deposition)
Tubular heater,
constant heat flux,
metal wafer inserts
(A Two1, mass
deposition)
Tubular heater,
constant heat flux
(A Tw~al, mass
deposition)
Adapted JFTOT
unit, constant
heat flux
(mass deposition)
150-538C
6-30 m / s
127-357C
0.07 m / s
Re(in) = 60
152-600C
Re = 400,
3000,
21,000
Oxygen analysis
Annular heater,
constant heat flux
(thermal resistance)
Annular heater,
constant heat flux
(thermal resistance)
T~ua = 180240C
Tbulk = 80C
Tsua = 150200C
Tbulk = 80C-
Re = 300012,000
Chemical analysis
of hydroperoxides,
indene, deposit
Reaction flasks
(mass deposition on
discs)
Isothermal heated
tube, optical cell
(absorbance,
scattering)
Reaction flasks
(mass deposition
on discs)
Isothermal tube
(mass deposition)
185C
Tbulk ~ Twan
< 625C
Re = 30011,000
Isothermal tube;
Twall- Tbulk
varied
(mass deposition)
Annular heater,
constant heat flux
(thermal resistance)
Isothermal tube
(mass deposition)
140-350C
Re = 10007000
Tbulk = 80C
T~ua = 137216C
185C
155-255C
Re = 500017,000
Annular
heater
Scanning electron
microscopy
of deposits
< 502C
Re = 11,000
Gums*
Slow
~"= 4-13.5
min
160-200C
Tsua = 180190C
Tbulk = 100C
Optical analysis
Gums*; TGA/MS**
of deposits
Slow
~-= 1-25
min
0.9-2.4 m / s
Oxygen, methane
analysis
Filtration
Chemical analysis
of indene, ROOH,
deposit, and gum
Oxygen analysis
Gums*
(continued)
364
Apparatus
(Measurement
Method)
Temperature
Range
Flow
Velocity
Other Analysis
Methods
Reference
Test Fluid
[28] Heneghan
et al.
(1995)
Jet A fuels,
Oxygen conc.
varied,
additives
Jet fuels,
additives
Isothermal tube
(mass deposition)
270-335C
r < 6s
Oxygen analysis
Isothermal tube
(mass deposition)
185C
Slow
Oxygen analysis
Gums*
Indene in
kerosene and
lube oil,
oxygen varied
Indene in
kerosene and
lube oil,
antioxidants
Annular heater,
constant heat flux
(thermal resistance)
Tbulk = 85C
Tsurf = 188C
Re = 11,000
Chemical analysis
of indene, ROOH,
deposit, and gum
Annular heater,
constant heat flux
(thermal resistance)
Tbulk =
80-100C
Tsurf = 180240C
Re = 30006500
Chemical analysis
of indene, ROOH,
deposit, and gum
Indene in lube
oil
Tubular heater,
constant heat flux
(mass deposition,
pressure drop, and
thermal resistance)
Tbulk = 100C
Tsurf = 180225C
Re = 300014,000
Scanning electron
microscopy of
deposits
Chemical analysis
of indene, ROOH,
deposit, and gum
* Gums: classification of gum and filtration products based on solubilities; r-residence time in tubular section.
** TGA/MS: thermogravimetric analysis/mass spectroscopy.
detailed review is given in Ref. 37. The following abbreviated kinetic scheme shows the complex nature of the
reactions leading to deposit formation.
Initiation
R H Initiator R"
R O O H --* R O - + H O "
2 R O O H ---, R O 2 + R O " + H 2 0
(la)
(lb)
(lc)
Propagation
R . + 0 2 --) R O 2
RO 2 + RH
abstraction
(2)
R O O H + R - (hydroperoxide)
(3a)
R O 2 + R H addition
......... : R O O R " (--- R' .)(polyperoxide)
(3b)
R O . + R H ~ radicals, products
(7)
Termination
R . + R . --, products
(4)
R" + R O 2 ~ products
(5)
R O 2 + R O 2 ~ products
(6)
d[RH]
d[O2]
dt
at
~ t
(k3a + k3b)[RH] v ~
(8)
366
Edwards and Zabarnick [52] investigated the phenomenon whereby several jet fuel fouling studies of slow
flow through a tubular reactor showed a decrease in
deposition at wall or film temperatures of 370C. This
work indicated that the change in deposition was linked
to bulk fluid reaction effects and not to an increase in
the solubility of precursors in the fuel as it became supercritical.
The mode of heat transfer also has an important effect
on the amount of deposition observed in autoxidation
fouling. Increased deposition from hexadecane and
kerosene under vaporizing conditions was reported
[53-55]. Oufer and Knudsen [35] studied fouling from
oxygenated and deaerated model solutions of styrene in
heptane under subcooled boiling conditions. Although
autoxidation was not always the fouling mechanism involved, the reported dependencies on temperature and
flow rate were significantly different from those observed
under conditions of sensible heat transfer. Chin and
Lefebvre [25] reviewed the effect of fuel pressure in jet
fuel fouling. Earlier studies had shown that deposition
decreased with increasing fuel pressure [56], whereas most
other workers reported that pressure had no effect above
a threshold value. These effects were linked to vaporization phenomena.
M E C H A N I S M S A N D M O D E L S OF
AUTOXIDATION FOULING
Autoxidation fouling includes the formation of precursors
and the transport of these precursors to the surface,
Reference
Test Fluid
Temperature
Effect
Velocity Effect
Comments
Jet A fuel
E = 42 k J / m o l
rate ot Re 65
RP-1, JP-7,
propanes
JP-S and aromatic
rich blends
Thermal resistance
maximum at (flow
rate)- 1
Little effect of
flow rate > 250C
Thermal resistance
Jet A fuel
Maximum in
thermal resistance
as Tsurf increases
E = 42 k J / m o l
T~u~f < 250C;
E = 167 k J / m o l
Tsu~f > 250C
E = 200 k J / m o l
(est.)
DF2, kerosene
Increase with
increasing Tbulk
and Tsurf - Tbulk
Rate
Jet A fuel
Fouling resistance
Bulk reaction effects
E = 150 k J / m o l
(oxygen kinetics)
E = 39 k J / m o l
E = 128 k J / m o l
(oxygen kinetics)
E = 82-85 k J / m o l
Fouling resistance
E = 76.4 k J / m o l
Rate cx Re -n
2>n>l
Rate ct Re -n
2>n>l
Indene in kerosene
Jet A fuels
ot R e '65
JFFOT breakthrough
temperature = 252C
Bulk reaction effects
Fouling resistance
185C
+ 1.8s
[] 1.0
80
&
O 0.125
0.75
O.S
0.10
uJ
60
o_
t,.)
~,
N
o
a
,,o
40
J-
eeeo IE.~'lb
~ D -..x
l,m e o e e
20
0
0 ~
STRESS
DURATION
10
12
(rain)
Because the importance of oxygen has long been recognized in a qualitative manner, most processing of organic
fluids is done with maximal exclusion of oxygen. Under
such conditions, reactions leading to fouling are primarily
thermal decomposition (thermolysis, pyrolysis, cracking,
etc.) or vinyl-type polymerization. Styrene polymerization,
which has often been used as a model chemical system
[48], continues to be studied (Table 3). Oufer and Knudsen [35, 51] reported experimental data and a model for
polymerization fouling under flow boiling conditions. Epstein [57] used the styrene polymerization data of Crittenden [48] to verify a mathematical model for chemical
reaction fouling. Thus, for both polymerization and autoxidation, recent work related the reaction to the fouling.
For thermal decomposition, this link is yet to be established.
Table 3 also lists recent studies on fouling of crude oils
and other undefined petroleum mixtures [60-65]. For
crude oils, fouling can be caused by contaminants such as
inorganic chemicals, sediments, and corrosion products or
by constituents of the oil itself. In none of these papers
are the chemical reactions (other than autoxidation) that
lead to the fouling deposit discussed. In some cases,
reactions and kinetic rate constants are mentioned, but no
details of the types of reactions are given.
The effect of feedstock composition is perhaps the most
significant of all the variables in chemical reaction fouling.
Studies by Dickakian [60, 61, 67], are reportedly based on
hundreds of different petroleum streams; however, results
are not published in a form that other researchers can
readily duplicate and are not generally in the public
368
Test Fluid
Apparatus
Tempera ture
Range (C)
Velocity
(m / s)
Other
[60] Dickakian
(1989)
Crude oils
Annular TFT*
Tb 275
[61] Dickakian
(1990)
FCC streams,
oils,
asphaltenes
Crude oil
TFT unit
Ts 510-593
T b 350-382
Refinery preheat
train
T~ 165-260
Tb to 250
1.1-2.1
[63] Crittenden
et al. (1993)
[64] Haquet et al.
(1995)
Tubular heater
~ 197-218
Tb 140
T b 230-270
0.5
(Re 6800)
Gas oils
Tb 40-320
0.2-0.7
Styrene/heptane
[62] Crittenden
et al. (1992)
Isobutane in N 2
P = 0.1 MPa
Refinery
exchanger and
inserts
S and T, plate
exchanger**
Coupon in
quench stream
(mass deposition)
Annular
Scanning electron
microscopy,
deposit aging
Deposit
analysis,
P 35-58 atm
Deposit
thickness and
analysis
P 15 bar,
Inserts
Turbotal insert
24-day tests
Tb 350-500
Surface effects
T~ 180-190
T b 100
0.9-2.4
Subcooled
boiling,
sulfur species
0 05
Velocity-
1.2
ft/s
0.04
0.03
14
0.02
//."
~t
= o.oi
g
Ib4
.t"i,-
0.00
200
250
. . . .
300
FILm
. . . .
. . . .
3.50
400
. . . .
450
. . . .
500
Temperatu.re,
(10)
d R f / d t = a Re/3- e x p ( - E / R T f ) - Y~'.
Foullng
2 0 0
1 0 0
'
10
'
20
'
50
'
40
N / m
50
2
proach entails costs that must be offset by potential savmgs. Costs of fouling have been reviewed by Bott [5].
Numerous types of antifoulant additives have been described [4, 77]: antioxidants, metal deactivators (MDAs),
dispersants, detergents, size limiters, and coke suppressants. Antioxidants interrupt the formation of fouling precursors either by converting hydroperoxides into stable
products or by scavenging peroxy radicals. Metal deactivators reduce the initiation properties of metal ions, whereas
detergents and dispersants prevent fouling precursors from
generating permanent deposits. There is a considerable
body of literature concerning the mitigation of autoxidation during fuel storage but little characteristic of the
higher temperatures found in many heat exchangers and
jet fuel feed systems. Additives that perform well in lowtemperature tests do not automatically function well at
higher temperatures.
Studies of the effects of a commercial antioxidant, BHT
(dibutylhydroxytoluene), on heat exchanger fouling from
model solutions of indene [33] showed that the antioxidant
inhibited the onset of autoxidation. There was no appreciable change in fouling mechanism or rate after the
antioxidant had been exhausted. Similar delays in the
onset of fouling in jet fuels were reported [29] when an
antioxidant (BHT) and an MDA were used. Other types of
jet fuel additives did not change the rate of reaction of
oxygen significantly but reduced deposition considerably
[24, 29]. These results are consistent with the different
additive mechanisms.
Increased rather than reduced deposition was observed
at high BHT concentration [33]. The increase in fouling
was attributed to reaction of the additive at the surface.
The surface temperature in this work (240C) was markedly
higher than the "ceiling" temperature for BHT antioxidation. Similar problems were found with a proprietary
phenol additive in jet fuel fouling [28]. Antioxidants, which
modify the reaction by sacrificial action, should not be
used beyond their known effectiveness limits. The polar
nature of antioxidants can lead to insoluble oxidation
products. Zabarnick and Grinstead [36] reported similar
results for BHT and other additives in fuel stability tests
at 140C. They also noted that different trends in additive
performance were evident in flowing tests at higher temperatures. The effectiveness of four different additives
(antioxidant, MDA, dispersant, detergent-dispersant mixture) was examined for three different jet fuels [31]. The
antioxidant, BHT, and MDA were not effective at the
temperatures used, which were above the antioxidant ceiling temperature. These compounds delayed the onset of
fouling by inhibiting the bulk reaction, as reported in Ref.
33. Heneghan et al. [28], however, found that metal deactivators gave promising reduction in deposition from other
jet fuels. Jones and Balster [29] found that dispersants,
which reduce particulate agglomeration in the bulk fluid,
gave the greatest reduction in deposition. The discussion
of fouling mechanisms suggests that dispersants would be
less effective in cases where deposition was controlled by
surface reaction and attachment. Mitigation of organic
fouling thus requires a reliable understanding of the fouling mechanism involved. Selection of additives is likely to
remain feedstock specific but should consider both the
feedstock composition and whether bulk or surface reaction/attachment effects are important. Testing of additive
packages is likely to remain an integral part of additive
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selection. Tam [78] illustrated the effect of some commercial antifoulants on the deposition from a fuel oil doped
with a heavy oil rich in asphaltenes. It is clear from Fig. 6
that such commercial antifoulants can be effective, reducing fouling from a significant rate to essentially zero.
However, the mechanism by which the antifoulants work
requires further research.
C O N C L U D I N G REMARKS
The classification of organic fluid fouling into situations
caused primarily by autoxidation, polymerization, and
thermal decomposition clearly has its value; however, the
complex case of asphaltene fouling does not yet fit well
within this structure. Much progress has been made in
autoxidation fouling, in part because of the supporting
studies on fuel stability and jet fuel utilization. Polymerization fouling has been narrowly focused on styrene
reactions, and trends should be verified in other systems.
II
I
Improved semiempirical mathematical models are becoming available. With regard to thermal decomposition-induced fouling, the process has been formulated as an
incompatibility or a phase separation problem, but little
progress has been made in fouling studies to determine
reaction and solubility limits. For higher-temperature conditions, the research related to the chemistry of petroleum
residue processing appears to offer some explanations of
observed fouling behavior. This extensive research could
be profitably extended to provide a route to better understanding of chemical reaction fouling in organic fluids at
moderate temperatures. The use of inserts and of chemical additives remains a major focus of the control of
fouling in industrial situations; an understanding of both
mitigation methods would benefit from further fundamental research.
Ongoing support to A.P.W. by the Natural Sciences and Engineering
Research Council of Canada is appreciated.
372
NOMENCLATURE
s t o i c h i o m e t r i c p a r a m e t e r s in r e a c t i o n s (12) and
(13)
A + a s p h a l t e n e c o n t e n t o f feed, w t . %
A* a s p h a l t e n e c o r e c o n c e n t r a t i o n , w t . %
E activation energy, k J / m o l
H + p e n t a n e soluble f r a c t i o n of feed, w t . %
H* side chains s t r i p p e d f r o m asphaltenes, w t . %
ki r e a c t i o n rate constant, step i
R init initiation rate
[RH] c o n c e n t r a t i o n of h y d r o c a r b o n , R H , m o l / L
R.
h y d r o c a r b o n radical
ROOH
hydroperoxide
Re fouling resistance, m 2 K / ( k W )
Tsurf t e m p e r a t u r e at surface or d e p o s i t - f l u i d
interface, C, K
Tbulk t e m p e r a t u r e o f bulk fluid, C, K
Tf film t e m p e r a t u r e , C, K
TI t o l u e n e insoluble fraction, w t . %
t time, h
V volatile fraction w t . %
a, m, n, y
Greek Symbols
o~,/3, 3, p a r a m e t e r s in Eq. (10)
T s h e a r stress
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