chemical reaction fouling _ a review

ELSEVIER

Chemical Reaction Fouling: A Review

A. P. Watkinson Department of Chemical Engineering, The University of British Columbia, Vancouver, British Columbia, Canada

D. I. Wilson Department of Chemical Engineering, University of Cambridge, Cambridge, United Kingdom

• Recent research on fouling of organic fluids is reviewed. Fouling in organic mixtures can be caused by numerous reactions, including autoxidation, polymerization and thermal decomposition. Understanding of autoxidation processes and their link to fouling has developed markedly in the past few years. By contrast, for fouling under non-oxidative conditions, the chemical and physical processes have not been adequately explored. © Elsevier Science Inc., 1997

Keywords: heat exchanger fouling, hydrocarbons, organic fluids, autoxidation, deposits

I N T R O D U C T I O N

Chemical reaction fouling generally involves the following multistep process:

Reactants ~ precursors ~ foulant (soluble) (insoluble)

Various possible steps in this process are shown in Fig. 1 [1-3]. In the simplest case, the fouling precursors enter the exchanger with the f luid--for example, from a feed t ank- -and then form the deposit by reaction on the wall. Alternately, the reactants enter the exchanger and the precursors and foulants form in the exchanger, either in the bulk fluid, in the thermal boundary layer, or on the wall. Hence, not only reaction, but also transport of reactants, soluble precursors, or insoluble foulant may be important. Analysis of chemical reaction fouling in a given system may entail

1. identification of the reactants and precursors; 2. determination of the kinetics of reactions that form

precursors; and 3. determination of whether the solid fouling phase is

initially formed in the bulk, in the thermal boundary layer, or on the heated surface.

When these factors are known, available mathematical models can be used to describe the deposition process quantitatively. In contrast with other types of fouling, removal processes are usually less significant in organic systems, whereas aging of deposits is particularly important. Furthermore, in contrast with particulate fouling or scaling in inorganic systems, identification of the fouling precursor may be a major task. Sprague [4] discusses the importance of this identification stage in dealing with refinery fouling situations. In organic fluid streams such as petroleum cuts, there may be a large number of possible reactants, precursors, and reactions forming them. The temperature field may well dictate which reactions occur

and where in the exchanger they occur. Hence generalized solutions to chemical reaction fouling problems are unlikely.

Chemical reaction fouling, primarily of organic fluids, was reviewed by Crittenden [1], and by Watkinson [2, 3]. In addition, a chapter in a recent book by Bott [5] deals with this subject. Chemical reaction fouling for organic fluids was attributed to three general classes of reactions: autoxidation, polymerization, and thermal decomposition [2]. In plant operations, other factors may be involved. Murphy and Campbell [6] discuss fouling in refinery heat exchangers under seven categories, of which the following four--inorganic salts, sediments, filterable solids, and corrosion products--arise from impurities. Three o thers - - oxidative polymerization, asphaltene precipitation, and coke formation--arise from chemical reactions of constituents of the oil. Reaction of constituents with the heat transfer surface can give rise to corrosion fouling, which is outside the scope of this review.

For deaerated systems, polymerization rather than autoxidation fouling may occur at moderate temperatures where thermal decomposition is unimportant. The relative rate of fouling under vinyl polymerization and autoxidative conditions depends on the compound involved. In jet fuel studies [7, 8], significantly lower deposition rates are reported for deoxygenated conditions; whereas, with styrene, a common species for reaction fouling studies, reaction is faster by vinyl polymerization than by autoxidation [9]. Monomers such as methylacrylic acid are stored under conditions of low but nonzero oxygen saturation to minimize polymerization by either reaction route [10]. This variation emphasizes a primary difficulty in studies of chemical reaction fouling--that there rarely exists a single mechanism common to all species present in a given hydrocarbon system over the range of operating conditions of interest. The limit of oxygen concentration below which autoxidation ceases to be dominant has not been

Address correspondence to Professor A. P. Watkinson, Department of Chemical Engineering, The University of British Columbia, Vancouver, BC V6T 1Z4, Canada.

Experimental Thermal and Fluid Science 1997; 14:361-374 © Elsevier Science Inc., 1997 655 Avenue of the Americas, New York, NY 10010

0894-1777/97//$17.00 PII S0894-1777(96)00138-0

362 A.P. Watkinson and D. I. Wilson

a)

A bulk reaction

m a s s

transfer

surface reaction

A f

Bulk Liquid

precipitation of insoluble B ed by mass transfer

( adhesion ~I~ Thermal Boundary Layer

B ~'--C

Heat Transfer Surface

b) Chemical Reaction

A, O, Indene C,H,

Peroxy O, B, Heat C, Radicals --- Polyperoxides - Oxygenated RO,- 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 eluci- dated 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 300°C [2]. Deposition in oxygenated hydrocarbon systems above 250-300°C 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

of jet fuel storage stability have used mass deposition measurements, whereas heat exchanger studies have featured thermal measurements, using constant heat fluxes to maintain constant deposit-fluid interface temperatures. Both thermal and mass deposition measurements have been employed in the studies of jet fuel deposition. Re- cently Zabarnick and Grinstead [36] used quartz crystal resonance techniques in fuel thermal stability tests to monitor in situ mass deposition against time. Generation of fouling resistance-time data, which can be used di- rectly in design and operation, was the main advantage of traditional thermal measurement methods. A significant feature of recent studies is the use of "model" solutions consisting of compounds of known reaction chemistry and tendency to form deposits and relatively inert solvents. These solutions are used to control reaction chemistry, minimize variations between experiments, and explore deposition mechanisms. Another characteristic is the greater use of analysis techniques to study deposit chemistry and morphology [13, 26, 34], solution chemistry [33], and oxygen concentrations [24]. Soluble gum and filter deposition assays used in fuel storage stability tests are now frequently used in flow system studies.

REACTION CHEMISTRY

Autoxidation can occur without causing fouling; the oc- currence of fouling is determined by the reaction chemistry of the hydrocarbons present. The chemistry of hydrocarbon autoxidation has been studied extensively and a

Chemical Reaction Fouling: A Review 363

Table 1. Recent Investigations of Autoxidation Related Fouling

Reference Test Fluid

Apparatus (Measurement

Method) Temperature

Range Flow Other Ana~ysis

Velocity Methods

[13] Roback et al. RP-1, JP-7 fuels (1983) and propane

[14] Szetela et al. Jet A fuel (1986)

[15] Marteney JP-5 fuel and and aromatic blends Spadiccini (1986)

[16, 17] Morris Jet fuels, et al. additives, (1988, 1989)

[18] Morris and model solutions Mushrush (1991)

[19] Wilson and Indene in Watkinson different solvents (1992)

[20] Asomaning Alkenes in and kerosene Watkinson (1992)

[21] Jones et al. 3 Jet A fuels, (1992) hexadecane

[22] Parker et al. JP4, air sat'd (1992)

[23] Jones and Balster (1992)

[24] Heneghan et al. (1993)

[25] Chin and Lefebvre (1993)

[26] Zhang et al. (1993)

[8, 27] Jones and Balster (1993, 1994)

[35] Oufer and Knudsen (1993)

Hexadecane S additives

3 Jet A fuels aerated, deaerated additives

DF2, kerosene

Indene in kerosene

Jet-A fuels

Styrene/heptane sulfur species

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)

Annular heater, constant heat flux (thermal resistance)


Reaction flasks (mass deposition on discs)

Isothermal heated tube, optical cell (absorbance, scattering)

Reaction flasks (mass deposition on discs)

Isothermal tube (mass deposition)

Isothermal tube; Twall- Tbulk varied (mass deposition)


Isothermal tube (mass deposition)

Annular heater

150-538°C

127-357°C

152-600°C

190 <Ts~a < 538°C

T~u a = 180- 240°C

Tbulk = 80°C Tsu a = 150-

200°C Tbulk = 80°C -

185°C Tbulk ~ Twa n

< 502°C

160-200°C

< 625°C

140-350°C

Tbulk = 80°C T~u a = 137- 216°C

185°C 155-255°C

Tsu a = 180- 190°C

Tbulk = 100°C

6-30 m / s Scanning electron microscopy of deposits

0.07 m / s Re(in) = 60

Re = 400, Oxygen analysis 3000, 21,000

Re = 3000- Chemical analysis 12,000 of hydroperoxides,

indene, deposit Re = 11,000

Slow ~" = 4-13.5 min

Gums*

Optical analysis

Gums*; TGA/MS** of deposits

Re = 300- Oxygen, methane 11,000 analysis

Filtration

Re = 1000- 7000

Re = 5000- Chemical analysis 17,000 of indene, ROOH,

deposit, and gum Slow Oxygen analysis

~-= 1-25 Gums* min

0.9-2.4 m / s

(continued)

364 A.P . Watkinson and D. I. Wilson

Table 1. Recent Investigations of Autoxidation Related Fouling

Reference Test Fluid

Apparatus (Measurement

Method) Temperature Flow Other Analysis

Range Velocity Methods

[28] Heneghan Jet A fuels, et al. Oxygen conc. (1995) varied,

additives [29] Jones and Jet fuels,

Balster additives (1995)

[30, 31] Jones et al. (1995)

[11] Asomaning Indene in et al. kerosene and (1995) lube oil,

oxygen varied Indene in

kerosene and lube oil,

antioxidants

[32] Wilson and Watkinson (1995)

[33] Wilson et al. (1995)

[34] Wilson and Watkinson (1996)

Isothermal tube 270-335°C r < 6 s (mass deposition)

Isothermal tube 185°C Slow (mass deposition)



Tbulk = 85°C Re = 11,000 Tsurf = 188°C

Oxygen analysis

Oxygen analysis Gums*

Chemical analysis of indene, ROOH, deposit, and gum

Tbulk = 80-- Re = 3000- Chemical analysis 100°C 6500 of indene, ROOH,

Tsurf = 180- deposit, and gum 240°C

Indene in lube Tubular heater, Tbulk = 100°C Re = 3000- Scanning electron oil constant heat flux Tsurf = 180- 14,000 microscopy of

(mass deposition, 225°C deposits pressure drop, and Chemical analysis thermal resistance) 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 abbrevi- ated kinetic scheme shows the complex nature of the reactions leading to deposit formation.

Initiation R H Initiator R" ( la )

R O O H --* R O - + HO" ( lb)

2 R O O H ---, RO 2 • + RO" + H 2 0 ( lc)

Propagation

R . + 0 2 --) R O 2 • (2)

RO 2 • + R H

RO 2 • + R H ......... :

abstraction R O O H + R - (hydroperoxide)

(3a)

addition R O O R " (--- R' .)(polyperoxide)

(3b)

R O . + R H ~ radicals, products (7)

Termination

R . + R . --, products (4)

R" + RO 2 • ~ products (5)

RO 2 • + RO 2 • ~ products (6)

Steps (1)-(6) are termed the basic autoxidation scheme (BAS) in the literature. Initiation in step (la) may be

caused by thermal decomposition of the hydrocarbon, RH, or by its reaction with chemical initiators, metal ions, or ultraviolet light. Under conditions of excess oxygen, the hydrocarbon radical, R . , is rapidly oxidized to the peroxy radical R O 2 . , by step (2). Propagation occurs through steps (3a), (3b), and (2), forming hydroperoxides a n d / o r polyperoxides. Termination is primarily by step (6) when the peroxy radical is the dominant radical present. When autoxidation consists of these steps alone, the reaction rate is given by

d[RH] d[O2] ~ t dt at (k3a + k3b)[RH] v ~ (8)

which is zeroth order in oxygen if initiation [step (la)] does not involve oxygen. The products of autoxidation are thus hydroperoxides, polyperoxides, and the carbonyls and other oxygenated products generated by step (6). The thermal decomposition of hydroperoxides produces further radicals [steps ( lb and (lc)], and the reaction is thus autocatalytic. Unimolecular decomposition [step (lb)] is more important at lower hydroperoxide concentrations, whereas bimolecular decomposition is more significant at higher concentrations. Equation (8) shows that the rate of autoxidation and product distribution will thus change as the hydroperoxide concentration increases if both decomposition steps can occur. This is one source of complexity in understanding autoxidation fouling behavior.


When oxygen is limited, the dominant radical species changes from the peroxy radical to a mixture of peroxy, alkoxy ( R O . ) and alkyl (vinyl) radicals (R-) . Propagation steps such as step (7) and termination steps such as steps (4) and (5) become more significant, with associated changes in reaction kinetics and product distributions. These effects are evident in jet fuel deposition studies [27] featuring the slow flow (~ 0.07-1.5 ml/min) of aerated jet fuel through small-diameter tubes under near isothermal conditions (155-255°C). The experimental apparatus was analogous to a plug flow reactor. The formation of gums and deposits was shown to be oxygen limited, and the disappearance of oxygen was found to change from zeroth-order to first-order kinetic behavior as the reaction proceeded. Heneghan et al. [28] also reported complex kinetics in their study of fouling from jet fuels at different feed oxygen levels.

Heat exchanger fouling was investigated by using model solutions of indene in kerosene at different oxygen concentrations [11]. The induction period observed before fouling occurred decreased as the oxygen concentration increased. Both the initial fouling rate and the kinetic parameters for the reaction of indene varied with the saturating oxygen pressure raised to the power 0.6-0.7 at oxygen saturation pressures below 8.6 kPa. The oxygen content of the deposits decreased with decreasing dissolved oxygen content of the fouling fluid. This work confirmed that autoxidation reactions control the generation of fouling precursors.

Oxygen analysis data [29] provided quantitative evidence that a significant proportion of reacted oxygen in jet fuel autoxfdation was obtained as nonfouling products. Investigations of fuel storage stability have shown that fuels that readily undergo autoxidation are not necessarily those that generate significant amounts of gum or fouling [38]. Taylor [39] investigated the relation between compound chemistry and fouling behavior by using model solutions of alkanes, alkenes, and naphthenes in dodecane. Compounds that tend to undergo addition to form polyperoxides [step (3b)] were found to foul more heavily; substituted alkenes are particularly prone to oxidative addition owing to structural effects. This result was confirmed [20] for heat exchangers by using model solutions of alkenes in kerosene. Indene was found to cause severe fouling in both these investigations; this compound was known to undergo addition polymerization to form polyperoxides [40] and was identified as a major source of fouling in operating equipment [41]. Fouling in model solutions of indene, hexadecene, and dicyclopentadiene in kerosene and lube oil was shown [32, 33] to be caused by the formation of polyperoxide gums. These gums, being polar in nature, exhibit solubility limits in aliphatic solvents and precipitate out of solution when the limit has been reached. Watkinson and coworkers [32, 33] used recirculating solutions in their heat exchanger studies and the fouling resistance-time data featured an induction period where no autoxidation occured, a linear fouling regime, and a regime where the fouling resistance increased rapidly. Chemical analyses confirmed and the transition between the two fouling regimes corresponded to the onset of precipitation of insoluble gum. The insoluble gum formed agglomerates in the bulk liquid that were rapidly deposited.

Attempts to model the reaction kinetics in autoxidative systems [42-44] have not been very successful. Modeling studies have used simplified kinetic schemes such as a two-step model:

hydrocarbon + 0 2 ~ soluble precursor, P

insoluble precursor/deposit, (9) where the soluble precursor, P, was linked to hydroperoxide concentrations [45] or soluble gum concentrations [42]. Norton and Drayer [46] obtained reasonable agreement between a three-step model and experimental data for the autoxidation of hexadec-l-ene. These simple models are not based on the kinetic scheme outlined in Eqs. (1)-(7) and are thus likely to be limited in application to other systems.

The tendency of a hydrocarbon fluid to cause fouling thus depends on its composition. Mayo and Lan [38] investigated gum formation and oxygen consumption in aerated mixtures of amines and aromatics in dodecane at 130°C and reported large variations in reactivity and yield with different dopants. Fouling behavior of a mixture of indene and dicyclopentadiene was very different from that of the individual components [33]. Co-oxidation of compounds that undergo autoxidation introduces reaction steps that inhibit step (3b), the addition of oxygen to form polyperoxides [47]. It is thus unlikely that a priori predic- tions of fouling behavior based on chemical composition will be feasible. Wilson and Watkinson [19] also investigated solvent by effects in heat exchanger fouling by using model solutions with indene as dopant. Solvents such as tetralin, which were not inert to autoxidation, interrupted the formation of polyperoxides and thus inhibited fouling. Fouling was observed in solvents that were relatively inert to autoxidation. The solubility limit of the polyperoxide gums was found to depend on the aromaticity and polarity of the solution.

Solubility plays an important role in fouling, because deposits often appear to be formed from particulates [26, 48, 49]. Mayo et al. [50] argued that the solubilizing properties of detergents are the primary source of the mitigation of fouling by these compounds. Roback et al. [13], reported dendritic deposit growth in propane fouling above 316°C, whereas kerosene-type fuels generated spherical agglomerates. Dendritic growths are consistent with coking mechanisms, which would be expected for propane at these temperatures. Reported particulate sizes vary with feedstock and reaction conditions, from 0.015 /xm [49] to 6-20 tzm [32]. Optical methods were used to analyze particulates formed in jet fuel fouling by Parker et al. [22], who found that the particulates formed during once-through reaction of jet fuels at 502°C were more numerous but significantly smaller (diameter < 0.06 tzm) than those formed during fuel stability tests at ambient temperature (> 0.1 txm). Analyses of deposits found in autoxidative fouling show that the deposit consists of oxygenated gums or complex mixtures of their products after aging [32]. Values of thermal conductivities of fouling deposits lie in the range 0.1-0.5 W / ( m K). A value of 0.2 W / (m K) was reported for indene polyperoxide deposits from mass and thermal resistance data [34].

The effects of heteroatomic species containing sulfur and nitrogen in jet fuels are determined by their reactivity (i.e., structure), concentration, and operating conditions [2, 12]. Research in this area is ongoing [23, 44, 51].

366 A.P . Watkinson and D. I. Wilson

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 370°C. 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 vaporiza- tion phenomena.

M E C H A N I S M S A N D MODELS OF A U T O X I D A T I O N F O U L I N G

Autoxidation fouling includes the formation of precursors and the transport of these precursors to the surface,

where they form deposits. Figure 1 indicates the stages of fouling from model solutions of indene. The relative importance of each stage varies, and the key to understanding the observed fouling behavior is whether fouling is governed by generation of precursors in the bulk fluid. Table 2 is a summary of reported temperature and velocity effects in studies of autoxidation fouling, where deposition is measured per unit surface area. Entries in Table 2 are marked if significant chemical reaction occurred in the bulk fluid.

When the concentration of precursors in the bulk fluid is small, deposition is controlled by the transport of reactants to the hot surface, reaction at the surface, and attachment. This is the case in most heat exchangers (except reboilers and condensers). Reaction rates increase with surface temperature, and attachment decreases with increasing flow velocity. Wilson and Watkinson [34] showed that autoxidation fouling rates observed in two different heat exchanger geometries were consistent with a reaction-attachment model based on that of Epstein [57]. Roback et al. [13] observed this type of behavior in testing jet fuels at high wall temperatures and high flow rates. The bulk fuel temperature was not reported in their work, but their account suggests that little bulk reaction would have occurred. The maxima observed in their data cannot be explained by simple arguments.

When the concentration of precursors in the bulk fluid is large, deposition' is controlled by precursor concentration, mass transfer, and attachment. This is the scenario when gum concentrations reach their solubility limits and in many of the jet fuel fouling studies in Table 1. A heat

Table 2. Temperature and Velocity Effects on Rate of Autoxidation Fouling

Temperature Reference Test Fluid Effect Velocity Effect Comments

[49] Vranos et al. Jet A fuel E = 42 kJ /mol rate ot Re °6°5 Bulk reaction effects (1981)

[13] Roback et al. RP-1, JP-7, Thermal resistance (1983) propanes

JP-S and aromatic rich blends

[15] Marteney and Spadiccini (1986)

[14] Szetela et al. Jet A fuel (1986)

Maximum in thermal resistance as Tsurf increases

E = 42 kJ /mol T~u~f < 250°C; E = 167 kJ /mol Tsu~f > 250°C

E = 200 kJ /mol (est.)

Thermal resistance maximum at (flow rate)- 1

Little effect of flow rate > 250°C

[25] Chin and Lefebvre DF2, kerosene Increase with Rate ot R e °'65

(1993) increasing Tbulk and Tsurf - Tbulk

[8] Jones and Balster (1993)

[26] Zhang et al. (1993) [28] Heneghan et al.

(1995) [33] Wilson et al.

(1995) [34] Wilson and

Watkinson (1996)

E = 150 kJ /mol (oxygen kinetics)

E = 39 kJ /mol E = 128 kJ /mol

(oxygen kinetics) E = 82-85 kJ /mol

E = 76.4 kJ /mol

Jet A fuel

Indene in kerosene Jet A fuels

Indene in lube oil

Indene in lube oil

Rate ct (flow rate)

Rate cx Re- l°4 Rate ct (flow rate)

Rate cx Re -n 2 > n > l

Rate ct Re -n 2 > n > l

JFFOT breakthrough temperature = 252°C Bulk reaction effects

Bulk reaction effects at high fluid

temperatures Non-Arrhenius

temperature dependency

Bulk reaction effects suspected

Bulk reaction effects

Fouling resistance Bulk reaction effects

Fouling resistance

Fouling resistance


exchanger or jet fuel system operating in this mode can be considered a tubular reactor; bulk reaction kinetics thus determine the distribution of deposits in the device. The jet fuel studies of slow flow rates in Table 2 show increasing, or weakly increasing, deposition rates with flow rate as the effects of mass transfer and attachment tend to cancel out for the very small particulates formed in jet fuel autoxidation.

An example of bulk reaction control is shown in Fig. 2 [8], which shows one set of deposition profiles for a jet fuel passing through a near-isothermal tube at 185°C. Figure 2 also shows the oxygen concentration in the bulk liquid. The mass of deposit increases along the tube as the bulk reaction generates more precursors, until the oxygen is exhausted. The concentration of bulk precursors then decreases owing to further reaction and deposition, giving a decreasing deposit profile. Katta et al. [42] modeled this system as a reactor but found that they were limited by an incomplete understanding of the reaction kinetics. They did not include any attachment considerations or fouling effects. Jones and Balster [8] were not able to explain their deposition results for different tube sizes by using a reactor model, because they did not consider attachment. Care must thus be exercised in extracting reaction kinetics from such data, because mass transfer and attachment effects can significantly affect deposition behavior.

The transition between surface and bulk reaction control is evident in jet fuel fouling studies [14, 15] at constant heat fluxes, which maintained constant bulk fluid temperatures. The data are more difficult to interpret

owing to the changing surface temperature profiles in the apparatus. Szetela et al. [14] found that deposition rates increased exponentially with initial wall temperature until the wall temperature approached the thermal stability limit of the fuel. Plots of deposit rate against wall temperature (or axial position) showed a maximum, correspond- ing to precursor exhaustion, when the bulk fluid exit temperature was greater than 420 K. The data prior to a maximum showed a reasonable correlation between deposition rate and surface temperature, which is consistent with surface reaction/attachment control. They found that the location of a maximum did not correlate with wall temperature and suggested that this phenomenon was related to the residence time of the fuel.

The transition between bulk fluid reaction control and surface-thermal boundary layer control has been discussed in other types of fouling. Fryer [58] described similar features in comparing silica fouling in geothermal brines and in food fouling, particularly in the fouling of milk heat exchangers at temperatures < 100°C. At these temperatures, milk fouling is caused by the denaturation of milk proteins (mainly fl-lactoglobulin) followed by protein aggregation and deposition. Protein denaturation occurs rapidly above a critical temperature (70-75°C), so surface and bulk control can occur in different parts of the same heat exchanger if the bulk fluid temperature exceeds the critical value. Simulations of milk fouling based on both bulk and surface processes show reasonable agreement with experimental data and predict features analogous to the maximum in deposition in Fig. 2 [59].

FOULING IN DEOXYGENATED SYSTEMS

o

uJ

o t,.)

Z o_ ~,

N o

a J -

80

60

40

20

1 8 5 ° C

+ 1.8s [] 1.0 O 0.125 a 0.75 u 0.10 • O.S

& DISSOLVED OXYGEN (%)

• e

,,o

%

eee• ~ . . x o IE.~'lb D -- • •l,m e o e e

0

0 ~ 0 2 4 6 8 10 12

S T R E S S D U R A T I O N (rain)

Figure 2. Deposition rate at 185°C on 0.125-in tubing at various flow rates (cm3/min) for fuel initially saturated with air at room temperature. Superimposed is dissolved oxygen versus stress duration. From Jones and Balster [8].

Because the importance of oxygen has long been recog- nized 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 Knud- sen [35, 51] reported experimental data and a model for polymerization fouling under flow boiling conditions. Ep- stein [57] used the styrene polymerization data of Critten- den [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


Table 3. Recent Investigations of Fouling in Nonoxygenated Systems

Tempera ture Velocity Reference Test Fluid Apparatus Range (°C) (m / s) Other

[60] Dickakian Crude oils Annular TFT* T b 275 Scanning electron (1989) microscopy,

deposit aging [61] Dickakian FCC streams, TFT unit T s 510-593 Deposit

(1990) oils, T b 350-382 analysis, asphaltenes P 35-58 atm

[62] Crittenden Crude oil Refinery preheat T~ 165-260 1.1-2.1 Deposit et al. (1992) train T b to 250 thickness and

analysis [63] Crittenden Crude oil and Tubular heater ~ 197-218 0.5 P 15 bar,

et al. (1993) residue T b 140 (Re 6800) Inserts [64] Haquet et al. Crude/ Refinery T b 230-270 Turbotal insert

(1995) residue exchanger and inserts

[65] Shibuya et al. Gas oils S and T, plate T b 40-320 0.2-0.7 24-day tests (1995) exchanger**

[66] Bach et al. Isobutane in N 2 Coupon in T b 350-500 Surface effects (1995) P = 0.1 MPa quench stream

(mass deposition) Styrene/heptane Annular [35, 51] Oufer T~ 180-190 0.9-2.4 Subcooled

and Knudsen T b 100 boiling, (1993, 1994) sulfur species

* TFT: thermal fouling test unit (details in ref. [60]). ** S and T: shell and tube

domain. Variations in fouling of crude oils are said to be caused by changes in composition and are related to asphaltene/oil incompatibility [67]. For FCC slurry, asphaltenes and coke particles are the main causes of organic fouling [61]. Crittenden's work [62] provides valuable data on fouling of crude oil in an industrial unit. Although the range of conditions explored is necessarily limited by processing constraints, consistent velocity and temperature effects are reported. Decreases in tube-side velocity and increases in temperature led to increased fouling. Inorganics make up some 15-39% of the deposits, whereas coke represents 25-37 wt.%, which suggests that a combination of the mechanisms reported by Murphy and Camp- bell [6] are responsible.

Ebert and Panchal [68] present a novel semiempirical analysis that suggests that a combination of low temperature and high shear stress will produce a threshold condi- tion such that the fouling fate will be essentially zero. Data from a previous crude oil coking study were re- gressed in the form

dRf /d t = a Re/3- e x p ( - E / R T f ) - Y~'. (10)

Hence, if ~- is sufficiently large, the deposition rate is zero. This equation was applied to conditions that gave linear fouling rate with time and should not be confused with the usual fouling rate being equal to the difference between deposition and removal terms. The right hand side terms are empirical expressions for two components of the deposition term. This analysis gives a good fit to the data (Fig. 3) and leads to a simple diagram to identify fouling and nonfouling conditions (Fig. 4). The approach appears to be very useful for designers, although the chemical reac-

0 05

0.04

0.03

14

0.02

~t

= o.oi g Ib4

0.00

Veloc i ty - 1 . 2 f t / s /

V e l o c l t 7 " 2 . 5 /

/ / . "

. t " i , -

, , , , t . . . . ! . . . . i . . . . ! . . . . I . . . .

200 250 300 3.50 400 450 500

F I L m T e m p e r a t u . r e ,

Figure 3. Comparison of experimental and fitted fouling rates of crude oil by Ebert and Panchal [68].

tions are not identified as such and the formulation lacks a rigorous explanation by the authors.

Haquet et al. [64] describe the use of Turbotal internal devices to limit fouling of crude oils. They attribute their effect to cleaning of the surface and the renewal of the thermal boundary layer, which reduces the difference in temperature between the wall and the bulk fluid, thus minimizing fouling. In terms of the approach of Ebert and Panchal [68], both the increased shear at the wall and

5OO


Foullng

• ~ 2 0 0

1 0 0 - - - ' . . . . ' , , - - ' - - • , ' . . . .

0 10 20 50 40 50

Wall Shear Stress, N / m 2

Figure 4. Threshold-film temperature concept of Ebert and Panchal [68].

reduction of the film temperature should have positive effects. Crittenden et al. [63, 69] report data for a crude oil containing added waxy residue and review the use of wire matrix inserts. It is claimed that more than a simple reduction in wall temperature is responsible for reducing fouling. Shibuya et al. [65] report gas oil fouling in plate and in shell and tube heat exchangers. Addition of cracked stocks was found to increase fouling in a gas oil. Under both once-through and recirculated flow conditions, maximum fouling appeared to occur at an intermediate temperature of about 200°C. At both 180°C and 260°C, fouling was reduced. A marked increase in fouling was noted when the gas oil was presaturated with oxygen. It was noted that recirculation of fluids, a common laboratory procedure for fouling studies, gave increased fouling and hence was judged to be more severe than once-through processing. Fouling was much reduced in the plate exchangers, although the lack of pressure drop data pre- cluded a conclusion of how this effect was influenced by increased shear stress.

Under nonoxidative conditions, the organic part of fouling in crude oils and other noncracked refinery cuts is claimed to be due largely to precipitation of asphaltenic constituents [6, 67, 70]. Asphaltenes are a solubility class, generally defined as being the benzene soluble-heptane insoluble fraction of a petroleum stream. Strausz et al. [71] presented a hypothetical structure of asphaltenes (Fig. 5) containing aromatic clusters and side chains, which was consistent with recent research. Fouling is triggered by precipitation of the asphaltenes due to an incompatibility between asphaltenes and the remainder of the oil [67]. The precipitated asphaltenes adhere to the hot surface of the exchanger and then carbonize into coke. The physical and chemical steps that lead to the incompatibility have not been discussed in the fouling literature, although generalized reaction schemes have been presented [70]:

oils ~ resins ~ asphaltenes ~ coke (11)

Recent literature on processing of residuum and heavy oils (see, e.g., Ref. 72) discusses reactions that lead to coke formation in such processes as visbreaking and hy- drotreating and perhaps can be applied to fouling of fired

Figure 5. Hypothetical asphaltene, structure by Strausz et al. [71]. A, B, and C represent larger aromatic clusters; the rest of the structural units are based on experimental data.

heaters and extrapolated to the lower temperatures of preheat trains. Wiehe [73] has developed a model of residuum thermolysis based on the formation of asphaltene cores, A*, stripped of their stabilizing side chains. He assumes that asphaltene cores can be formed either by reaction of asphaltenes originally in the feedstock, A ÷, or by conversion of other solvent fractions, H ÷, in a parallel set of reactions:

A + ~ A * + n i l * + ( 1 - m - n ) V

H + ~ aA* + (1 - a)V (12) The soluble asphaltene cores eventually reach their solubility limit, A*ax, and then the excess undergoes a very rapid reaction to precipitate liquid coke (toluene insolubles):

[A* - A*ax] ~ (1 - y ) T I + y H * (13)

This reaction is essentially a phase separation process, controlled by equilibrium. This model provides an explanation for the observations that it is not asphaltene content alone that dictates fouling and suggests a mechanism for the incompatibility phenomenon in heating petroleum streams. The similarity to the solubility limit in autoxidation fouling [32], is evident. The extent to which the precipitation process rather than the reaction process controls this type of fouling under moderate temperatures ( < 350°C), where severe coking does not occur, is yet to be clarified. Furthermore, the same mechanistic issues discussed under autoxidation fouling--namely, conditions for bulk, boundary layer, or wall deposition--have yet to be established. This is an area of active research [74].

As described above for autoxidation fouling, the use of model compounds could well simplify the complex chemical situation in heavy hydrocarbon mixtures and illustrate the connections between the reaction and the fouling process. Model compounds have been used extensively to explore reaction pathways of asphaltenes; see, for exam-


ple, Ref. 75. Fouling studies using this approach would be valuable.

Fouling in higher-temperature pyrolysis systems is at- tracting increased attention. Bach et al. [66] provide a survey of recent work in the light hydrocarbon pyrolysis area and outline additional mechanisms for deposit formation in transfer line exchangers. Mass deposition onto coupons placed downstream from the cracking zone of a laboratory isobutane cracking apparatus was followed with time. Fouling rates at 500°C were found strongly depen- dent on the material of the coupon, being negligible for quartz, low for incoloy and inconel, and significant for 15Mo3 steel. This was taken as evidence for the catalytic role of the surface in deposition during cooling in dry cracked gas atmospheres. Building on earlier work by Froment and others, Huntrods et al. [76] simulated an ethane pyrolysis quench system and compared the results with performance over time of an industrial transfer line exchanger. Two mechanisms of coke formation were considered important: (1) formation of polymers from precursors such as dienes and acetylene and (2) surface-cat- alyzed reaction between carbon and the metallic tube surface.

DEPOSIT AGING

Some effects of deposit aging have been determined for oxygenated gum deposits [32]. Thermogravimetric analysis (TGA) and pyrolysis studies showed that composition of deposits found on heat exchanger surfaces could be generated by the thermal degradation of the polyperoxide gums precipitated from the bulk solution. The effect of deposit aging on reaction chemistry has also been discussed in jet fuel fouling. Jones and Balster [29] attributed the changes in deposition profiles and reduction in deposition rates between their 6-h and 72-h tests to changes in reaction kinetics. They did not consider the changes in the temperature profile in the fluid caused by extended fouling. Marteney and Spadiccini [15] did not find any appreciable variation in deposition rate with test duration. Others [14, 24], however, observed an increase in average deposition rate when the test duration was increased. Further work is required in this area.

Dickakian [60] has shown how the fraction of coke in a deposit from crude oil increases over time, whereas the fraction of asphaltenes decreases. An initial deposit con- tained 30% asphaltenes and no coke. After 3 h of heating and further fouling, the coke content of the deposit was 60% and the asphaltene content about 14%. The thermal resistance of the deposit will change owing both to the chemical reactions and to the further deposition. This aging reaction must be understood if the mechanisms of fouling are to be inferred from deposits taken from industrial exchangers that may have been on stream for many months. As described above, a start has been made in the analysis of aging reactions from autoxidation fouling.

MITIGATION

Control of fouling by manipulation of process variables such as temperature and velocity or by the use of inserts is discussed above. Modifications of process chemistry or physicochemical properties of precursors through additives provide an additional mitigation strategy. Each ap-

proach entails costs that must be offset by potential sav- mgs. 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 suppres- sants. 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 low- temperature 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 (240°C) was markedly higher than the "ceiling" temperature for BHT antioxida- tion. 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 140°C. 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 reac- t ion/attachment effects are important. Testing of additive packages is likely to remain an integral part of additive

o.gn

0.15

~ O.lO

E o.05 v

iff 0.1~

40.05

,,,-,.,.. ~ " :..~,,,,-,,,,,.'.,:,~,..g~'~,,,,1,,.j,.

I$11L TM

• •

t ime (hrs)

oon

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~ 0 . 1 0 =r~= ' % d , = r ~ = =

~o.05 jZ,"

50 -0.05

t ime (hrs)

• P ET-4250 I = Base 1 I

I

Figure 6. Effect of commercial antifoulants on fouling from heavy oil-fuel oil blend. Antifoulant concentration, 100 ppm; oil velocity, 0.6 m/s; Tb, 85°C; Ts, o, 245°C. From Tam [78].

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. Polymer- ization fouling has been narrowly focused on styrene reactions, and trends should be verified in other systems.

Improved semiempirical mathematical models are becom- ing available. With regard to thermal decomposition-in- duced 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 fundamen- tal research.

Ongoing support to A.P.W. by the Natural Sciences and Engineering Research Council of Canada is appreciated.

372 A . P . Watkinson and D. I. Wilson

a, m, n, y

A +

A*

E

H + H*

ki

R init [RH]

R . R O O H

Re

Tsurf

Tbulk

Tf TI

t

V

o~,/3, 3, T

NOMENCLATURE s to ich iomet r ic p a r a m e t e r s in reac t ions (12) and (13)

aspha l tene con ten t o f feed, w t .%

aspha l t ene core concen t ra t ion , wt .%

act ivat ion energy, k J / m o l

pen t ane soluble f rac t ion of feed, w t .%

side chains s t r ipped f rom asphal tenes , wt .%

reac t ion ra te constant , s tep i

in i t ia t ion ra te

concen t r a t i on of hydrocarbon , R H , m o l / L

hydroca rbon radical

hydrope rox ide

foul ing resis tance, m 2 K / ( k W )

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

t e m p e r a t u r e o f bulk fluid, °C, K

film t empe ra tu r e , °C, K

to luene insoluble f ract ion, w t .%

t ime, h

vola t i le f rac t ion w t . %

Greek Symbols p a r a m e t e r s in Eq. (10)

shear stress

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Received April 17, 1996; revised September 22, 1996

chemical reaction fouling _ a review

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