Evaluation of the Compatibility of Crude Oil Blends and Its Impact on

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Evaluation of the Compatibility of Blends of Crude Oils and its Impact on Fouling Propensity Estrella Rogel, Kyle Hench, Toni Miao, Eddy Lee, and Ghaz Dickakian Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02030 • Publication Date (Web): 10 Aug 2018 Downloaded from http://pubs.acs.org on August 13, 2018

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Evaluation of the Compatibility of Blends of Crude Oils and its Impact on Fouling Propensity. Rogel, E.1; Hench, K.1; Miao, T.1; Lee, E.1; Dickakian, G.2 1. Chevron Energy Technology Company, 100 Chevron Way, Richmond, California 94802, United States 2. Fouling and Coking Technology Incorporated, 1911 Pleasant Creek Drive, Kingwood, Texas 77345, United States Abstract The effect of the blending of crude oils on fouling propensity was investigated to determine the extent that compatibility of crude oils influences fouling. In this study, fouling testing was performed for a series of crude oil blends with diverse compatibility as measured using a titration method. After testing, fouling deposits were recovered and analyzed. For the studied blends, the results show that compatibility is the driving force for fouling propensity. Asphaltene and inorganic contents also play a significant, but secondary role in the fouling of these samples. Filtration and temperature effects were also evaluated for some of the blends. As expected, temperature decreases fouling propensity, while filtration has a low impact indicating that, for the studied blends, fouling is temperature dependent and not driven by the presence of suspended inorganic particles. A correlation is proposed to describe the fouling propensity based on compatibility measurements of the blends at room temperature. The analysis of the deposits revealed correlations between deposit composition and blend characteristics. In particular, soot formation during fouling seems to be related to micro carbon residue in the blends. It was also found that infrared measurements can be used to estimate the formation of carbonaceous materials in the deposits. The presence of long paraffins in the deposit could have a large impact on fouling propensity and might indicate that the interaction between paraffins and asphaltenes persists even at high temperatures. The results demonstrate that compatibility 1 ACS Paragon Plus Environment

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measurements using titration techniques can be successfully used to minimize fouling by optimizing concentrations of the components of a specific blend. Introduction In a refinery, problems associated with deposition of asphaltenes or “asphaltene-like” molecules can occur in a series of different processes. For instance, separation processes such as deasphalting and distillation can modify the characteristics of the fluids and induce precipitation. Additionally, processes of conversion such as visbreaking, hydrocracking, steam cracking, etc. produce changes that can lead to asphaltene precipitation. Furthermore, in some cases, such as steam cracking, there are not asphaltenes in the feed, but synthetic highly insoluble asphaltenes are formed.1 One of the likeliest places to observe the consequences of asphaltene precipitation is at the pre-heat trains of the crude oil distillation unit. Fouling of heat exchangers in this unit is primarily attributed to asphaltene precipitation. In this case, asphaltene precipitation is related to the use of incompatible crude oils.2-3 The economic consequences of crude oil fouling in refinery preheat trains are significant and affect directly refining margins and revenue losses.4 In fact, earlier studies indicated that around 50% of the total fouling-related costs for the whole refinery originates in the pre-heat trains of the crude distillation unit.5 An excellent review of the economic consequences, operational difficulties, environmental impact and safety hazards of crude oil fouling can be found elsewhere.6 Two or more crude oils form a compatible blend at a fixed composition if asphaltenes do not precipitate. Asphaltene precipitation in blends of crude oils depends on two main variables: the 2 ACS Paragon Plus Environment

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solvent power of maltenes and asphaltene solubility characteristics. Both characteristics can be measured using well-known titration techniques that relay on considering that crude oils and their blends behave as regular solutions.7,8 Usually, these measurements are carried out at temperatures close to room temperature. The results are used to optimize blending of feedstocks to prevent asphaltene precipitation and, therefore, to avoid accelerated fouling in pre-train heat exchangers. Other measurements that have frequently been used to estimate compatibility are based on composition.8,9 Two of these methods are the asphaltene-resins ratio and the Colloidal Instability Index (CII= (Saturates + Asphaltenes)/(Aromatics + Resins)). Both are based on the weight percentages obtained from Saturates, Aromatics, Resins, and Asphaltenes (SARA) analysis and assume that crude oils are colloidal systems in which some components act as asphaltene stabilizers (resins and aromatics) while others contribute to their destabilization (saturates). The main weakness of these methods is that they only consider composition, not chemical characteristics of the components. It is a basic assumption in all these methods that components coming from different crude oils behave similarly. This creates a degree of doubt in the meaning of the results. For instance, in the case of the CII, values larger than 0.9 indicate unstable asphaltenes while values lower than 0.7 indicate an oil with stable asphaltenes. The stability of samples with values between 0.7 and 0.9 is uncertain.9 Recent studies10,11 also pointed out to SARA results being inconsistent with the precipitation behavior of crude oils. Based on practical experience; there is a clear link between compatibility as measured using titration techniques and fouling in heat exchangers.12-15 Blending effects on fouling have been investigated for crude oils regarding their compatibility.16 In particular, it seems that the initial fouling rate could be correlated with the colloidal instability index (CII) and that the addition of a crude oil with high solvent power can significantly decrease fouling even if added in small 3 ACS Paragon Plus Environment

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amounts.17 Besides compatibility, other characteristics of the crude oils are also linked to fouling and can be acting synergistically with asphaltene insolubility to accelerate fouling. Thanks to these studies, it is known that sulfur content in the crude oils18-19 produces deposits which are enriched in iron sulfide. Also, the metal content in the form of insoluble inorganics can play a significant role in deposition.18 Another relevant study indicates that basic nitrogen can be associated with the inhibition of fouling in heat exchangers.20 Crude oils with total base numbers (TBN) lower than 100 ppm show high fouling, while crude oils having a TBN higher than 200 ppm are related to low fouling. Based on these previous findings, a good characterization of the crude oil is key to the understanding of the causes of accelerated fouling in pre-train heat exchangers.20,21 On the other hand, understanding how blending can affect fouling has become particularly pertinent as more nonconventional crude oils (i.e., synthetic and shale oils) are processed in refineries. In particular, the great variability of these crude oils can be worrisome, regarding its effects on compatibility upon blending. A better understanding of how this variability can affect fouling is needed to address these concerns. In this work, we analyze this issue by preparing blends of crude oils with widely different compatibilities and by evaluating their fouling and the characteristics of their deposits. In particular, we want to explore how blends with similar compatibilities might exhibit vastly different fouling propensities. Another key aspect is the evaluation of deposit characteristics and their link to blend properties. This type of characterization is important since real deposits from refineries are usually complex mixtures of hydrocarbons, coke-like materials, and minerals such as iron sulfide. Comparative examination of blends and their deposits can shed light on the foulants originally present in crude oils and their deposition mechanisms. 4 ACS Paragon Plus Environment

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Our primary goal is to establish a direct link between the characteristics of feedstocks or blends of crude oils and their fouling propensity with a particular focus on the effect of compatibility in the formation of fouling deposits. Experimental Section Samples. Six crude oils from different origins were used in this study. Crude oils F, P, and B are medium or heavy crude oils with relative high asphaltene content. Crude oils R and L are light paraffinic crude oils. BB is a crude oil known to produce fouling problems in refineries. The main chemical and physical properties of the crude oils are shown in Table 1. Asphaltene content was determined by the in-line filtration method.22 Sulfur, vanadium, and nickel by inductively coupled plasma-atomic emission spectrometry (ICP-AES). Ashes were determining by Thermogravimetric Analysis (TGA). Twelve blends were prepared based on crude oils F, P, B, L, and R according to Table 2. Crude Oil BB was tested by itself. Preparation of Blends. Blends were prepared by adding the lighter component to the heavier one and then they were shaken during 15 minutes using a paint shaker. Fouling testing and compatibility measurements were carried out without delay as soon as blends were prepared to avoid kinetic effects. As crude oil BB showed floc under the opyical microscope, it was heated and homogenized using the paint shaker for 15 minutes.

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Table 1. Crude Oil Characteristics Crude API Asphaltene Oil

Content

Sulfur

Nitrogen

Vanadium

Nickel

Ash

(wt. %)

(ppm)

(ppm)

(ppm)

(wt. %)

(wt. %) F

24.8

6.17

3.25

2750

122.0

46.5

0.00

P

21.4

6.71

3.42

4700

96.8

24.8

0.00

B

14.9

5.14

2.41

3920

108.1

37.6

0.00

L

41.9

0.42

0.13

725

1.4

0.5

0.21

R

42.5

0.84

0.21

328

0.6

0.1

1.42

BB

34.9

0.97

0.31

829

4.4

2.7

1.07

Compatibility Testing. The evaluation of the compatibility or stability of crude oils comprises the titration of three solutions with different concentrations of the material in toluene. Each of these solutions is then titrated with n-heptane at a constant delivery rate. An optical device is used to monitor the titration continuously. In this case, the change in percent transmittance (%T) of the detected radiation is followed as the titration proceeds. At the beginning of each titration, transmittance increases due to dilution with precipitant. At the flocculation onset point, the formation of asphaltene particles causes an immediate decrease in transmittance due to light scattering. The maximum corresponds to the flocculation onset.


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Based on the flocculation onsets for the three solutions, the Heithaus compatibility parameters23 are calculated: Pa is the peptizability of the asphaltenes, Po is the solvent power of the maltenes and P represents the overall compatibility of the system and is calculated according to: P=Po/(1-Pa)

(1)

P should be larger than 1 for the system to be stable. Ra= 1-Pa is called solvent requirement.P is calculated as: P=Po/Ra

(2)

Repeatability of this titration method was evaluated based on 25 measurements of a crude oil ( 33.2 oAPI with an asphaltene content of 1.33 wt. %) obtained during three months. The testing yielded the following standard deviations: Pa (0.006), Po(0.02) and P (0.06) corresponding to errors of 2.7 %, 4.0 %, 2.4 % respectively. Fouling Testing and Recovery of Deposits. In these tests, the blend is circulated through an electrical heated annular probe at a constant flow rate. The annular probe consists of a heater tube and a shell. The heater tube is composed of carbon steel. A detailed scheme of the instrument and methodology can be found elsewhere.24 During the testing, the fluid is constantly stirred to avoid sedimentation of particulates. It is also kept at high pressure (725 psi) under a nitrogen atmosphere to avoid vaporization of light components. The fluid is pumped at a constant rate (3 cm3/min) through the probe. In these tests, the fluid does not recirculate and goes directly into a waste container after leaving the heating section. During the experiments, the heater tube temperature is set at 343oC or 288oC


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and held constant for 2 h. These temperatures were selected to reflect the hot end of the preheat train in refineries and the low temperature end, respectively. Estimation of the Reynolds number indicates laminar flow regime. Although the conditions of the testing do not reflect real conditions on heat exchangers, they allow for fast screening of the fouling propensity that can be related to the fouling in refineries.25 After testing, the whole test unit is allowed to cool down to room temperature. During the experiment, as the fluid passes through the heating section, a deposit might accumulate. The formation of a deposit decreases the heat flow to the fluid and, therefore, the outlet temperature of the fluid decreases. The decrease of the outlet temperature as a function of the time can be used to describe the extent of the fouling:25 ∆T = Toutlet, t – Toutlet, 0

(3)

Toutlet, t and Toutlet, 0 are the outlet temperatures of the fluid at time t and at t=0 when the tube is clean. ∆T is also called fouling propensity and has been used to characterize the potential of crude oils to foul heat exchangers.13 Based on repeated testing, the fouling propensity shows an average error of 5o C. The deposits were recovered at the end of each fouling test. After the unit cools down, the heater tube is carefully removed from the exchanger assembly and gently washed with a jet of toluene until the washing is colorless. Then, the deposit is washed with a jet of heptane to remove the toluene and, the heater tube is dried in a vacuum oven at full vacuum at 180oF for one hour. Finally, the deposit is gently scraped with a plastic spatula. A similar procedure to recover deposits has been reported before.25


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Characterization of Deposits. Carbon, hydrogen, and nitrogen (CHN) analysis of deposits was carried out with a Carlo Erba model 1108 analyzer. Sulfur was determined by combustion analysis with infrared detection. Wax distributions were obtained by High Temperature Gas Chromatography (HTGC) using a J&W Scientific DB-1HT 30m x 0.320mm ID x 10 µm film thickness column. A Hewlett Packard/Agilent 6890 with a 7673 autosampler and Flame Ionization Detector (FID) system was used. Samples were dissolved in HPLC grade CS2 and injected on-column (1.0 µl). Data were collected and analyzed using an Agilent EZChrom Chromatographic Data System. Ash content was determined by Thermogravimetric Analysis. In the test, 15 mg of sample are placed in a Platinum pan and then onto a balance beam into a furnace. The weight of the sample is monitored as a function of the sample temperature. The sample is run from room temperature to 550°C under N2 (100 mL/min), 550–900°C under air (100ml/min) with 10°C/min heating rate. Ash content is defined as the remaining sample at 900°C. Infrared spectra were obtained in a Varian 7000e FT-IR infrared spectrophotometer. Transmission measurements were carried out on a Diamond Anvil Cell (DAC), which is used to compress the sample for transmission analysis. Spectra were measured from 4000 cm-1 to approximately 400 cm-1 using a Deuterated Triglycine Sulfate (DTGS) detector as the average of 32 scans acquired at 4 cm-1. We used the infrared spectra results as a measurement of the formation of carbonaceous material. We propose a ratio between the height of the baseline at 2000 cm-1 and the height of the signal at 2900 cm-1 that we called “Carbonization Index”.


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RESULTS AND DISCUSSION Fouling Propensity and Compatibility. Table 2 lists the characteristics of the blends used in the fouling testing. Compatibility measurements are shown in this table and comprise a wide range of stabilities regarding asphaltene precipitation. According to the P-values presented in this table, all the samples are stable (P>1.0) at the conditions of the compatibility test. However, as it will be shown in this section, many of these blends produce significant fouling at 343 oC. Previous work26 using crude oil blends has shown that fouling of heat exchangers by asphaltenes could be caused by mixtures that are nearly incompatible with P values between 1.0 and 1.4. This behavior was attributed to the increase in the tendency of asphaltenes to adsorb on heated metal surfaces as the solvent power of the blend decreases. On the other hand, an increase in temperature has two contrary effects:27 in the absence of specific intermolecular forces, two fluids mix more easily at high temperatures because of the (negative) contribution of the entropy of mixing to the Gibbs energy of mixing increases with temperature, favoring mixing. Secondly, an increase in temperature also reduces liquid density, and that reduction decreases solvent power. Additionally, at 343oC chemical reactions such as cracking take place and this also reduces solvent power and decreases asphaltene solubility.28 The cited temperature effects contribute to the decrease of solubility of asphaltenes at high temperatures resulting in a high fouling propensity than expected based on P-values measured at room temperature. However, results in the next sections will show that P-values can be used to qualitatively estimate fouling at high temperatures for specific blends. Besides the blends shown in this table, Crude Oil BB was also tested. This sample is composed of a blend of crude oils known to have produced accelerated fouling in at least one refinery. Figure 1 shows its fouling propensity as a function of time. The experiment was 10 ACS Paragon Plus Environment

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stopped just after 1 hour due to high fouling. According to the criteria developed by Brons and Rudy,12 ∆T (eq. 2) indicates that this particular crude oil has a large probability of cause fouling problems in heat exchangers, which coincides with observations from the refinery. Crude Oil BB contains a small asphaltene content as shown in Table 1. However, this crude oil has a lower solvent power. Its solvent power (Po=0.25) is just a fourth of the standard solvent power of toluene (Po=1), indicating a highly paraffinic character. P- value is 0.98, and therefore, this crude oil is unstable.

Table 2. Blend composition and compatibility (P) determined by titration. Blends Crude Oils

F

#1

#2

#3

25

50

75

P

#4

#5

#6

#7

#8

#9

20

40

50

25

50

75

B L R

75

50

25

75 80

60

50

50

#10

#11

#12

20

40

50

80

60

50

25

Properties API Gravity Asphaltene1 Content (wt. %) Compatibility (P) Micro Carbon Residue (wt. %)

37.3 33.0 29.1 25.8 30.4 31.7 36.9 31.8 26.2 36.3 31.5 28.4 1.69 2.89 4.11 1.54 2.51 3.06 1.81 3.09 4.34 1.27 1.90 2.23 1.57 1.82 2.28 1.01 1.31 1.54 1.36 2.11 2.75 2.77 2.56 2.29

3.13 4.98 6.83 2.36 4.31 5.29 3.50 5.72 7.93 2.27 4.12 5.05


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1.

Determined by in-line filtration method. 70 60 50

∆ T (oC)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40 30 20 10 0 0

10

20

30 40 Time (min)

50

60

70

Figure 1. Fouling propensity of Crude Oil BB as a function of time at 343 oC. Figure 2a shows the fouling propensity as a function of time of the blends prepared using crude oil R (343 oC). This figure indicates that the fouling propensities for both sets of blends are very different and seem to depend on the second component. The P/R blends show a high fouling propensity while the fouling propensity of the B/R blends is very low. This behavior is in agreement with P values shown in Table 2. The relationship between fouling propensity at 1 h and corresponding P-values is shown in Figure 2b. This plot suggests that fouling propensity increases exponentially as P-values decrease and approach the instability boundary (P=1). Regarding how asphaltene solubility characteristics can affect fouling, the critical solubility parameter of the asphaltenes in the blend B/R (21.8 MPa0.5) is smaller than the critical solubility parameter of asphaltenes in P/R (22.2 MPa0.5). This comparison shows that B/R asphaltenes are easier to solubilize in hydrocarbons than P/R asphaltenes. Critical solubility parameters were calculated according to Andersen using compatibility measurements.8


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100 20/80 P/R

a)

90

40/60 P/R 80

50/50 P/R 20/80 B/R

70

40/60 B/R

∆ T (oC)

60

50/50 B/R

50 40 30 20 10 0 0

20

40

60 80 Time (min)

100

120

140

100

b)

90 80 70 ∆ T at 1 h (oC)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 50 40 30 20 10 0 1.00

1.50

2.00 P

2.50

3.00

Figure 2. a) Fouling Propensity at 343 oC for blends containing crude oil R. b) Fouling propensity at 1 h for blends containing crude oil R (343 oC). Figure 3a shows the fouling propensity as a function of time of the blends prepared using crude oil L while a plot of the fouling propensity at 1 h as a function of P values is shown in Figure 3b. For these blends, although fouling propensity increases as P value decreases, there is a significant dispersion in the data as Figure 3b shows. In particular, two data points seem to be outliers in this plot. Both of them correspond to the blends F/L. Figure 3a shows that for the blends F/L have a nonproportional behavior with respect the content of the paraffinic crude oil L. This means that the 50/50 F/L blend has a larger fouling propensity than 25/75 F/L. Intuitively, one might expect that the larger the amount of paraffinic crude oil L, the lower the P-value and the larger the fouling propensity. Although P-values follow this intuitive rule, this is not the case for the fouling propensity of blends F/L as shown in Figure 3a.


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Figure 3. a) Fouling Propensity at 343 oC for blends containing crude oil L. Arrows indicate the blends with nonproportional behavior with respect to the content of the crude oil L b) Fouling propensity at 1 h for blends containing crude oil R (343 oC). In general, fouling propensity increases as compatibility P decreases in both sets of samples. However, comparison of Figures 2b and 3b indicates that many of the blends containing crude oil L have larger fouling propensities than blends containing crude oil R even though they have similar P values. These results indicate that compatibility measurements can be used to minimize fouling by optimizing concentrations of the components of a specific blend, but they cannot be used to compare blends consistent of different components since other factors such as inorganics, and sulfur can play a significant role.


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According to equation 1, P represents the comparison between the solvent power of the maltenes in the system and the minimal solvent power the asphaltenes required to be soluble in the maltenes. This value has to be understood as an indication of the probability of precipitation to occur. As P decreases, more fouling is expected. However, fouling can also be caused or enhanced by the presence of other foulants (clay, dirt, sand, or olefins) or fouling precursors (olefins, sulfur compounds, etc.), In any case, it is possible to write equations that take into consideration different foulants or fouling precursors in the feeds. Several mathematical correlations have been developed using this principle that includes a compatibility term as well as terms for other foulants or fouling precursors such as metal content, sulfur, basic nitrogen content, etc.14 All of these components have been related to fouling of crude oils in several studies.29 For the studied blends, we found that the best equation to describe the fouling propensity was: ∆T= 18.684 log[ (ASP/ASH)(Ra/(Ra+Po-2(RaPo)0.5)] – 12.565

(4)

In this equation, Ra and Po are the solvent requirement of asphaltenes and the solvent power of the maltenes, respectively, ASP is the asphaltene content, and ASH is the ash content. Figure 4 shows the comparison between experimental fouling propensities and the ones calculated using equation (4). Analysis of the residuals indicates that the average residual value is 13oC which is around three times the error of fouling propensity measurements. A plot of the residuals can be found in the Supplementary Material. The random distribution of the residuals indicates a good fit of the data.30 Another interesting aspect regarding this equation is the presence of the ash content in the denominator of the equation which seems to indicate that the presence of inorganics (ashes) help in reducing fouling propensity. A plausible explanation to this phenomenon can be found in the role of nanoparticles in the inhibition of formation damage 15 ACS Paragon Plus Environment

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by asphaltenes in oil reservoirs.31-32 In these studies, nanoparticles of different types are used to delay the agglomeration and precipitation of asphaltenes at typical reservoir temperatures. Based on filtration experiments that will be described in a next section, it is reasonable to assume that the inorganics present in some of the studied blends have sizes lower than 1 µm, so these particles are in the nanoscale region. Correlations between initial fouling rates and compatibility measurements have been reported before and have shown to follow a nonlineal correlation.14

Figure 4. Comparison of the experimental fouling propensity and calculated values using equation (4). Fouling testing at 343 oC. Effect of Temperature on Fouling Propensity. Figure 5a shows the effect of temperature (343 oC and 288 oC) on blends containing crude oil R. The decrease in temperature significantly decreases the fouling propensity of the 20/80 P/R blend, around 70 %. In contrast, the fouling propensity of blend 40/60 P/R and 40/60 B/R decreases an average of 53 and 42 % respectively. Figure 5b shows the effects of temperature on fouling propensity for samples containing crude oil L. As occurred for the blends containing crude oil R, fouling propensity decreases more for the blend with the largest propensity, almost 80 %, in comparison with 60 % for the less fouling 16 ACS Paragon Plus Environment

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blend. Also, there is an inversion in fouling propensity at low temperature in comparison with the fouling propensity at high temperature. This new order in the fouling propensity of the samples correlates with what it is expected based on the increased content of the paraffinic oil L. This is a significant change in comparison with the results obtained at 343oC. Also, the significant changes in fouling propensity for some of the samples as the temperature decreases is another indication that the fouling is not driven by the presence of suspended inorganic particulates, but the product of processes that are temperature dependent and that generate insoluble material as the temperature increases.29

Figure 5. Effect of temperature on fouling propensity a) Blends containing crude oil R. b) Blends containing crude oil L.


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The correlation between fouling propensity at 1 h and P values similar to equation 4 was found at 288oC. Figure 6 shows this correlation: ∆T= 5.1762 log[ (ASP/ASH)(Ra/(Ra+Po-2(RaPo)0.5)] – 0.4303

(5)

Analysis of the residuals indicates that the average residual value is around 1oC, considerably lower than the average error in the measuerments of fouling propensity. Distribution of residuals can be seen in the Supplementary Material. In both equations (4, 5) the main contributor to the fouling propensity is the term corresponding to the solubility characteristics of the asphaltenes and maltenes determined by titration. Asphaltene and ash content play a minor role in this correlation indicating that the driving force for the observed fouling in this set of samples was the decrease in solubility of the asphaltenes as a consequence of blending. Additional terms including sulfur, nitrogen, or metal content do not improve the correlation in a significant way and, therefore, they are not considered here.

Figure 6. Comparison of the experimental fouling propensity and calculated values using equation (5). Fouling testing at 288 oC.


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Effect of Filtration on Fouling Propensity. The effect of filtration on fouling propensity was carried out based on previous work that pointed out a major effect of the suspended particle concentration in fouling.29,33 Experimental evidence indicated that the decrease in the concentration of suspended particles correlated with the decrease of fouling.33 In fact, elimination of the particulates using a 1 micron filter decreases fouling rate in a crude oil to barely detectable levels.29 In this previous work, it was found that the fouling showed a relatively weak temperature dependence at low temperatures and attributed the fouling to suspended particulate matter such as corrosion products, silt, etc. In the present study, two blends were filtered using a 1 micron filter at 120 oC before performing the fouling test. Figure 7 shows the effect of filtration for the two blends: 20/80 P/R and 20/80 B/R. In the case of the first blend that has high fouling propensity, the decrease is 20 %, while in the low fouling 20/80 B/R the differences are within the error of the fouling test. The relatively low impact of filtration in comparison to previous results29,33 seems to indicate that for these blends, particles larger than 1 micron are not the main driving force in the fouling or that the samples do not contain particles larger than 1 micron. In the case of asphaltene particles formed as a consequence of low solvent power, the size of asphaltene particles should decrease as the temperature increases as a consequence of the increase of the solubility. As the filtration was carried out at high temperature, there may be no large asphaltene particles or relatively fewer particles in the samples. Studies of Furrial crude oil using small-angle X-ray scattering (SAXS), a known unstable crude oil with severe flocculation problems34 showed a decrease in particle size as temperature increases.35 Studies of asphaltene solutions at different temperatures using small-angle neutron scattering (SANS) indicated that the size of the asphaltene particles decreases from room temperature to very high temperatures 19 ACS Paragon Plus Environment

Energy & Fuels

(290oC), a phenomenon consistent with a continuous disaggregation process.36 In these studies, results indicate that sizes of asphaltene particles at high temperatures are on the order of nanometers. Another plausible explanation for the lack of effect of the filtration on fouling can be found in the kinetics of flocculation. As the blends were filtered as soon as they were prepared, it is possible that there was not enough time for the asphaltene particles to reach the size required to be retained in the filter. Several studies have shown that the appearance of asphaltene flocs larger than 1 micron can take hours and, in some cases, even months.37

100 90 80

Fouling Propensity (∆ ∆ T/oC)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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70

20/80 P/R before filtration

60

20/80 P/R after filtration

50

20/80 B/R before filtration

40

20/80 B/R after filtration

30 20 10 0 0

20

40

60 80 Time (min)

100

120

140

Figure 7. Effect of filtration on blends containing crude oil R. Analysis of Deposits. The deposits obtained from the fouling tests at 343oC were recovered and analyzed. Table 3 shows the elemental composition of these deposits. The H/C ratios varied from 0.78 to 1.38, indicating deposits of different characteristics. Large H/C ratios indicate deposits that contain lighter materials, while low H/C ratios indicate enrichment in carbonaceous materials that are deficient in hydrogen content.


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On the other hand, the results from Table 3 indicate that some of the deposits should contain a large amount of inorganic materials. In these cases, the decrease in carbon and hydrogen content seems to be related to the presence of low amounts of crude oils R and L. This correlation seems to be opposite to what it is expected since R and L contain large amounts of ashes. In comparison, the other crude oils present in the blends contain undetectable amounts of ash. This finding points out to a mechanism where low compatibility leads to large fouling propensity with deposits enriched in hydrocarbon components, while high compatible blends produce deposits depleted in hydrocarbon components and enriched in inorganic ones. Table 3. Elemental Analysis of Fouling Deposits obtained at 343 oC Sample

Carbon

Hydrogen

Nitrogen

H/C

(wt. %)

(wt. %)

(wt. %)1

molar ratio

BB

75.16

8.49

--

1.36

20/80 B/R

83.27

6.11

--

0.88

40/60 B/R

15.89

1.33

--

1.00

20/80 P//R

82.85

9.5

--

1.38

40/ 60 P//R

70.74

4.63

1.47

0.79

50/50 F/L

71.01

4.64

1.16

0.78

75/25 F/L

34.91

2.52

--

0.87

25/75 P/L

77.7

7.09

1.38

1.09

75/25 P/L

35.28

2.57

--

0.87

1

Non reported nitrogen indicates nitrogen content lower than 1 wt.%, outside of the limit of

detection.


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In agreement with the elemental analysis, FTIR plots of the recovered deposits show different characteristics depending on the sample. Two examples of extreme characteristics are shown in Figure 8. The first one corresponding to the 20/80 P/R blend with a fouling propensity of 68 oC at 1 h: its FTIR spectrum shows two intense bands between 2800 and 3000 cm-1 due to CH3 and CH2 asymmetric and symmetric C-H stretches of alkyl chains, characteristic of large alkyl chains. It also does not show the characteristic shift of the baseline when carbonaceous material is present. In contrast, the spectrum for the blend 40/60 B/R (low fouling propensity 7 o

C at 1 h) is featureless. This spectrum also shows a high baseline that is indicative of

carbonized material confirming the nature of the fouling deposits. Carbonaceous material such as soot causes a shift in the baseline of the spectrum due to absorption and scattering of light. In general, as the amount of crude oil with a large asphaltene content increases in the blends, there is a larger displacement of the baseline in the IR of the corresponding deposit. This displacement correlates with asphaltene content in the blend indicating that asphaltenes are the main precursors of the carbonaceous material. Since there are no other spectral features in the region around 2000 cm-1, this area has been used to assess the level of soot in samples of used lubricant oils.38 In this work, we will use this measurement for the first time in solid samples to evaluate the level of carbonaceous material in the deposits. We propose a ratio between the height of the baseline at 2000 cm-1 and the height of the signal at 2900 cm-1 (Carbonization Index). This measurement correlates with the hydrogen content of the deposit as shown in Figure 9. Furthermore, the carbonization index also correlates with the microcarbon residue (MCR) of the blends as shown in Figure 10. The larger the MCR, the higher the baseline of the spectrum. This correlation is not unexpected as


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the MCR has been used extensively as a measurement to predict coke formation.39 On the other hand, the carbonization index does not correlate with fouling propensity.

Absorbance

a)

3900

3400

2900

2400

1900

1400

900

400

1400

900

400

Wavelength (cm-1)

b)

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3900

3400

2900

2400

1900

Wavelength (cm-1)

Figure 8. FTIR spectra of fouling deposits obtained at 343 oC. a) Blend 20/80 P/R b) Blend 40/60 B/R.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 9. Carbonization Index as a function of Hydrogen Content for deposits obtained at 343oC.

Figure 10. Carbonization Index determined for deposits (343 oC) as a function of microcarbon residue (MCR) of the blends. It is well known that alkanes precipitate asphaltenes and paraffinic crude oils induce precipitation when blending with regular crude oils. The asphaltene precipitation effect is the most important factor to consider regarding fouling propensity in the studied blends as shown in


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a previous section. Another factor worth to explore is the presence of high molecular weight waxy materials in these blends. Figure 11 shows a comparison of the distribution of N-paraffins in the blend 20/80 P /R and its corresponding fouling deposit obtained at 343 oC. This figure indicates that the paraffins in this deposit are enriched in high molecular weight paraffins in comparison with the original blend. It is unexpected to find waxes in this deposit considering that under the conditions of the tests, these waxes should be in liquid phase since their melting points are lower than the temperature of the tests and their solubility increases with temperature (i.e., melting point of C40 is 80.2 oC).40 The existence of these molecules in the deposit should contribute greatly to the fouling propensity as these compounds have low thermal conductivity41 and can account partially for the loss of heat transfer in some of the samples that have the highest fouling propensity.

7 6 Blend 20/80 P/R 5 Fouling Deposit Content (wt%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

4 3 2 1 0 8-

9

11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41+ N-Paraffins

Figure 11. Comparison of the N-paraffin distributions in a fouling deposit and its parent blend.


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Previous studies have shown that waxes co-precipitate with asphaltenes42-45although the mechanism of co-precipitation is not clear. Some authors considered that the co-precipitation of waxes with asphaltenes occurs because one species precipitate and the other becomes entrained in the precipitating solid.45 Nevertheless, there is evidence that paraffins might have an effect on asphaltene structuring in petroleum.46

On the other hand, a comparative analysis of the

molecular distribution of waxes isolated from the crude oil and deposits found that in three of the four crude oils analyzed, the waxes that coprecipitate with asphaltenes have, in average, larger molecular sizes than the ones present in the parent crude oil. For one of the crude oils, both wax distributions were similar.42 Calorimetric tests indicate that n-alkanes can be partially immobilized in the protective shell formed by aliphatic lateral chains of asphaltenes. The immobilized alkanes facilitate the nucleation and subsequent deposition of waxes.47 These different aspects have been discussed in the literature for at least two decades, but the discussion has been limited to processes that occur close to ambient temperature. It can be supposed that for highly paraffinic blends, asphaltene precipitation at the conditions of the fouling tests can lead to the coprecipitation of waxes. However, there is no previous evidence in the literature of studies at high temperature that support this theory. In general, for the samples studied the mechanism of fouling should be similar to one described previously for unstable samples.25 In this mechanism, asphaltenes are transported to the hot surface of the heated tube. Other materials such as inorganic components or waxes become entrained. Asphaltenes and some of the entrained materials undergo thorough chemical changes due to thermal effects. In particular, asphaltenes seem to be the main source of soot formation.


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CONCLUSIONS Fouling tests of several crude oil blends indicate that the main driving force for fouling was incompatibility. The blend of highly paraffinic crude oils with medium and heavy ones decreases asphaltene solubility in the fluids and, as a consequence, increases fouling propensity. For the studied blends, asphaltene and ash content contribute to fouling in a minor scale. A correlation between fouling propensity and these parameters was developed and used to describe fouling propensity at two different temperatures. Filtration of two blends before performing fouling tests shows little or no effect on the fouling propensity. On the other hand, decreasing the temperature of the test produces a significant decrease in fouling propensity. Both results support the relevant role that compatibility of blends has on the fouling behavior. Analysis of the deposits indicates that the formation of carbonaceous material found in the deposits is linked to the MCR of the blends. Evidence of paraffin presence in deposits was also found. As these compounds have low thermal conductivity, they can contribute significantly to fouling propensity. It has also shown that infrared measurements can be used to estimate the formation of carbonaceous materials in the deposits. Finally, compatibility measurements using titration techniques can be successfully used to minimize fouling by optimizing concentrations of the components of a specific blend, but they


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

cannot be used to compare blends consistent of different components since other factors (i.e., inorganics, sulfur content) can play a significant role in fouling.

Acknowledgement The authors want to thank Chevron ETC for the support to carry out this work. REFERENCES 1. Cabrales-Navarro, F. A.; Pereira-Almao, P. Catalytic Steam Cracking of a Deasphalted Vacuum Residue Using a Ni/K Ultradispersed Catalyst. Energy Fuels 2017, 31(3),31213131. 2. Dickakian, G.; Seay, S. Asphaltene precipitation primary crude exchanger fouling mechanism. Oil Gas J. 1988, 86 (10), 47-50. 3. van den Berg, F. G. A.; Kapusta S. D.; Ooms, A. C.; Smith, A. J. Fouling and Compatibility of Crudes as Basis for a New Crude Selection Strategy. Pet. Sci. Technol. 2003, 21 (3-4), 557-568. 4. Macchietto, S.; Hewitt, G. F.; Coletti, F.; Crittenden, B. D.; Dugwell, D. R.; Galindo, A.; Jackson, G.; Kandiyoti, R.; Kazarian, S. G.; Luckham, P. F.; Matar, O. K. Fouling in crude oil preheat trains: a systematic solution to an old problem. Heat Transfer Eng. 2011, 32(3-4):197-215. 5. Van Nostrand, W.L.; Leach, S.H.; Haluska, J.L. Economic penalties associated with the fouling of refinery heat transfer equipment” Fouling Heat Transfer Equip. 1981, 619-43.


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6. Coletti, F.; Joshi, H. M.; Macchietto, S.; Hewitt, G. F. Introduction. In Crude Oil Fouling. Deposit Characterization, Measurements, and Modeling; Coletti, F., Hewitt, G., F. Eds.; Gulf Professional Publisher, United States, 2015; pp 1-22. 7. Wiehe, I. A.; Kennedy, R. J. The oil compatibility model and crude oil incompatibility. Energy Fuels 2000, 14(1):56-59. 8. Andersen, S. I. Flocculation onset titration of petroleum asphaltenes. Energy Fuels 1999, 13(2), 315-22. 9. Asomaning, S. Test methods for determining asphaltene stability in crude oils. Pet. Sci. Technol. 2003, 21(3-4), 581-90. 10. Guzmán, R.; Ancheyta, J.; Trejo, F.; Rodríguez, S.; Methods for determining asphaltene stability in crude oils. Fuel 2017, 188, 530-543. 11. Santos, D.; Filho, E. B.; Dourado, R. S.; Amaral, M.; Filipakis, S.; Oliveira L. M.; Guimarães, R. C.; Santos, A. F.; Borges, G. R.; Franceschi, E.; Dariva, C. Study of Asphaltene Precipitation in Crude Oils at Desalter Conditions by Near-Infrared Spectroscopy. Energy Fuels 2017, 31(5), 5031-5036. 12. Brons, G.; Rudy, T. M. Fouling of whole crudes on heated surfaces. In Proceedings of the 4th International Conference Heat Exchanger Fouling and Cleaning—Challenges and Opportunities Davos, Switzerland., 2001, pp. 8-13. 13. Wiehe, I. A. Process Chemistry of Petroleum Macromolecules. CRC Press, New York, 2008. pp. 223. 14. Hong, E.; Watkinson, A. P. Precipitation and Fouling in Heavy Oil–Diluent Blends, Heat Transfer Eng. 2011, 30(10-11), 786-793.


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31. Franco C. A.; Nassar, N. N.; Ruiz, M. A.; Pereira-Almao, P.; Cortés, F. B. Nanoparticles for inhibition of asphaltenes damage: adsorption study and displacement test on porous media. Energy Fuels 2013, 27(6), 2899-2907. 32. Betancur S.; Carmona, J. C.; Nassar, N. N.; Franco, C. A.; Cortés, F. B. Role of particle size and surface acidity of silica gel nanoparticles in inhibition of formation damage by asphaltene in oil reservoirs. Ind. Eng. Chem. Res. 2016 ,55(21), 6122-6132. 33. Saleh, Z. S.; Sheikholeslami, R.; Watkinson, A. P. Fouling characteristics of a light Australian crude oil. Heat Transfer Eng. 2005, 26(1), 15-22. 34. Goncalves, S.; Castillo, J.; Fernandez, A.; Hung, J. Absorbance and fluorescence spectroscopy on the aggregation behavior of asphaltene–toluene solutions. Fuel. 2004 83(13):1823-8. 35. Sheu, E. Y.; Acevedo, S. Effect of pressure and temperature on colloidal structure of furrial crude oil. Energy Fuels. 2001, 15(3), 702-707. 36. Espinat, D.; Fenistein, D.; Barre, L.; Frot, D.; Briolant, Y. Effects of temperature and pressure on asphaltenes agglomeration in toluene. A light, X-ray, and neutron scattering investigation. Energy Fuels. 2004, 18(5), 1243-1249. 37. Maqbool, T.; Balgoa, A. T.; Fogler, H. S. Revisiting asphaltene precipitation from crude oils: A case of neglected kinetic effects. Energy Fuels. 2009, 23(7), 3681-3686. 38. Van De Voort F. R.; Sedman J.; Cocciardi R. A.; Pinchuk, D. FTIR condition monitoring of in-service lubricants: ongoing developments and future perspectives. Tribol. Trans. 2006, 49(3), 410-418. 39. Meyers, R. A. Handbook of petroleum refining processes Mc Graw-Hill, Second Edition, Canada, 1997, pp.1237.


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40. Lourdin, D.; Roux, A.H.; Grolier, J.-P.E.; Buisine, J.-M. Thermobarometric and differential scanning calorimetric study of the polymorphism of some even and odd paraffins (C26, C27, C40, C60). Thermochim. Acta 1992, 204(1), 99-110. 41. Vélez C.; Khayet M.; De Zárate J. O. Temperature-dependent thermal properties of solid/liquid phase change even-numbered n-alkanes: n-Hexadecane, n-octadecane and neicosane. Applied Energy 2015 143, 383-394. 42. Ganeeva1, Y. M.; Yusupova, T. N.; Romanov, G. V.; Gubaidullin, A. T.; Samigullina, A. I. The composition and thermal properties of waxes in oil asphaltenes J Therm Anal Calorim. 2015, 122(3), 1365-1373. 43. Garcia, M. D. Crude oil wax crystallization. The effect of heavy n-paraffins and flocculated asphaltenes. Energy Fuels 2000, 14(5), 1043-1048. 44. Garcia, M. D.; Carbognani, L. Asphaltene− paraffin structural interactions. Effect on crude oil stability. Energy Fuels 2001, 15(5), 1021-1027. 45. Yang, X.; Kilpatrick, P. Asphaltenes and waxes do not interact synergistically and coprecipitate in solid organic deposits. Energy Fuels 2005, 19(4), 1360-1376. 46. Stachowiak, C.; Viguie, J.-R.; Grolier, J.-P. E.; Rogalski, M. Effect of n-alkanes on asphaltene structuring in petroleum oils. Langmuir 2005, 21(11), 4824-4829. 47. Mahmoud, R.; Gierycz, P.; Solimando R.; Rogalski, R. Calorimetric probing of nalkane− petroleum asphaltene interactions. Energy Fuels 2005, 19(6), 2474-2479.


Evaluation of the Compatibility of Crude Oil Blends and Its Impact on

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