Mitigation of Asphaltene Deposit Formation via Chemical Additives: A Review

Abstract

The deposition of asphaltenes in the petroleum industry has been found to be a significant factor affecting the profitability of petroleum production and refining. For this reason, many efforts have been made to clarify the mechanism of deposition formation and to find measures to reduce its harmful impact on the efficiency of oil production and refining. Recent reports on the mechanism of deposit formation by asphaltenes suggest that it is a phase transition phenomenon. Many studies have shown that this process can be slowed by using chemical inhibitors. Different classes of chemical substances (non-polymeric, organic compounds, polymers, ionic liquids and nanomaterials) have been found to be capable of inhibiting asphaltene precipitation. This paper presents a comprehensive review of asphaltene deposition research and makes an attempt to decipher the convoluted asphaltene deposition phenomena and relate the chemistry of asphaltene inhibitors to the nature of treated petroleum oils. The choice of appropriate additives to mitigate asphaltene deposition in commercial oil and gas facilities requires comprehensive knowledge of chemistry of oils, asphaltenes, and the chemical substances, along with the appropriate laboratory techniques that best mimic the commercial operation conditions.

1. Introduction

The deposition of wax, hydrates, scales and asphaltenes is troublesome for both crude oil production and oil refining [1,2]. Among them, the deposition of asphaltenes has been identified as the most challenging, resulting in adverse effects on the profitability of production systems [1]. Asphaltene deposition causes formation damage and wellbore plugging, and in some extreme cases a wellbore can even be completely plugged [3]. Furthermore, in downstream processing, asphaltene precipitation leads to the plugging of flow facilities, the formation of solids in storage tanks and a reduction in the catalyst activity, and can cause coke formation in refineries [4]. This, in turn, results in the loss of billions of dollars in the oil industry, which is reflected in reduced production capacity, wells prematurely shutting and implications for management techniques [4,5]. The control of asphaltene deposition problems involves expensive corrective measures in both oil production and oil refining [1,4,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32]. For example, the cost of treatment of asphaltene deposits in the well can be as high as USD 3 M, while the financial loss due to lost production can be as high as USD 1.2 M/day, even with the scenario of a USD 30/bbl oil price [15,33]. In an ebullated bed vacuum residue hydrocracker, a day off for the cleaning of fouled equipment due to sediment formation from asphaltene aggregation and precipitation is equivalent to a loss of profit opportunity of about 600 KUSD/day [3,12,34,35,36,37,38,39,40,41].

Asphaltenes are known as the heaviest and most polar fraction of crude oils, defined by solubility criteria as soluble in aromatic solvents such as toluene and benzene and insoluble in light paraffinic solvents, including n-heptane, n-hexane and n-pentane. In reservoir conditions, asphaltenes are stable in the oil phase. Variations in the thermodynamic conditions of the oil mixture (i.e., temperature, pressure and composition of oil) can induce asphaltene instability, causing flocculation, precipitation and consequently deposition of asphaltene [4,32]. Flocculation refers to the process of the asphaltene molecules’ agglomeration, which subsequently leads to a solid-phase formation in the liquid phase, known as precipitation, and lastly the solid particulates are deposited onto a surface [31]. However, it should be noted that asphaltene precipitation is a necessary but not sufficient condition for asphaltene deposition [1,3]. Light crude oils are more prone to asphaltene precipitation in comparison with heavy oils with higher amounts of asphaltene, since the former have larger quantities of alkanes which act as asphaltene precipitants [4,42].

Asphaltene precipitation and deposition is a complex process that has been the subject of research for decades [33]. Many experimental procedures and modeling approaches have been reported to describe the kinetic behavior and thermodynamic states of asphaltenes at different thermodynamic and process conditions. Generally, the phase behavior of asphaltenes in crude oil is described by two main approaches, i.e., solution theory and colloid theory, which differ in their basic concept regarding the state of asphaltenes in crude oil [4]. In the colloidal model, asphaltenes are considered to be suspended in the oil mixture where resins act as a peptizing agent. This approach assumes that the precipitation of asphaltenes is an irreversible process, caused by a reduction in the stabilizing capability of the resins. In the solubility model approach, asphaltene molecules are considered to be soluble in the oil, forming a true solution, with the precipitation process considered to be a reversible process. Asphaltene precipitation is caused by a reduction in the solvent power of the oil due to thermodynamic changes which include pressure, composition and temperature changes [1,4,43]. A large number of studies have been dedicated to elucidating the mechanisms of deposit formation from asphaltene aggregation with the aim to find corrective measures to mitigate and prevent deposition from asphaltenes in the oil industry [1,4,5,15,27,28,29,30,32,44,45,46,47,48,49]. One of the promising strategies in that matter has been found to be treating the oils with chemical additives (also known as asphaltene inhibitors and dispersants) [50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68]. It has been reported that the additives can slow the process of aggregation of asphaltenes, inhibiting the deposit formation rate [69,70,71,72]. Currently, research into asphaltene deposit mitigation via chemical additives is gaining momentum and extensive research is being conducted to develop cost-effective inhibitors or mitigation agents of asphaltene deposition. A Scopus database search using the keywords “asphaltene” and “inhibitor” showed that over the last 10 years there has been a 70% increase in publications on the subject (Figure 1).

The methods employed to test the efficacy of the synthetic additives in their inhibition of deposit formation in oils have already been summarized [73,74,75]. To the best of our knowledge, no such overview of the chemistry of the additives used in many applications with distinct oils to define the relationship of oil chemistry to additive chemistry has been made so far. The aim of this study is to review the chemical nature of the additives employed to slow the process of sediment formation by asphaltene precipitation in different oils, searching for the relationship of oil chemistry to additive chemistry which gives the optimum performance. The current review is divided into several sections. The first section provides background information related to asphaltene characteristics and their tendency towards self-association, aggregate formation, precipitation and deposit formation. The second part critically reviews the chemistries of conventionally applied and newly developed additives with respect to their molecular structure, possible interactions with asphaltene molecule and their effectiveness with regard to the mitigation of asphaltene deposition. Lastly, the main conclusions and recommendations for future work are provided.

2. Review on Asphaltene Characteristics, Aggregation and Deposit Formation

Asphaltenes are dark brown to black friable solids that have no definite melting point and usually foam and swell on heating to leave a carbonaceous residue [76]. They consist of components which have polyaromatic molecules with heteroatoms and aliphatic chains attached to their polyaromatic cores [1]. They may also have metals (V, Ni, Fe) and vary in their chemical composition from one crude oil to the other. It has been shown that the hydrogen and carbon content of asphaltenes of different crude oils vary in the range of 82 ± 3% and 8.1 ± 0.7%, respectively [1]. A hydrogen/carbon ratio of 1.2 was registered for different crude oil asphaltene samples [75]. Heteroatoms also exist in various concentrations, with oxygen, sulfur and nitrogen having 0.3–15.3% [4,72], 0.3–15.8% [4,72] and 0.6–3.3%, respectively [4]. Sulfur is present in the form of heterocycles, sulfide or thiophene groups, while nitrogen atoms are present as pyrrole, pyridine and quinoline functional groups and oxygen as hydroxyl, carbonyl and carboxyl groups [33]. Table 1 exemplifies the contents of carbon, oxygen, sulfur and nitrogen and the molecular weight of asphaltene samples from different sources. The data in Table 1 show the large variations in the elemental composition of asphaltenes derived from different crude oil sources. It can easily be concluded that different characteristics of asphaltenes will require different characteristics of chemical additives to prevent deposition.

The word ‘asphaltenes’ is still vague, as not much can be said to be known clearly about the molecules of components building this crude oil fraction [1]. Irrespective of belonging to the n-alkane insolubility class, another complication in understanding the definition of asphaltenes is the fact that the experimental segregation of asphaltenes from the bulk petroleum phase hinges not only on their solubility but also on the accidence and size of the phase areas of the second phase, as explained in [77]. Depending on the morphology and size of the second phase area, some of the components which may be considered to pertain to the asphaltenes can go through the filter or remain on it, and in this way asphaltene fractions with different component composition can be obtained [77,78].

Their molecular weight and structure have been the subject of long-lasting debate among researchers [75,79,80,81,82,83,84,85,86,87,88]. The reported values of the molecular weights are in the order of a few hundred to several million [27]. This variation may be explained by the different types of asphaltenes found in crude oils, the tendency of asphaltenes to self-associate and the experimental methods employed to measure the molecular weights, i.e., vapor pressure osmometry (VPO), gel permeation chromatography (GPC), field ionization mass spectroscopy, laser desorption mass spectroscopy, matrix-assisted laser desorption ionization, electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry and atmospheric-pressure chemical ionization and fluorescence depolarization [15,27]. Methods such as VPO and GPC tend to produce much higher molecular weights compared to the newer techniques, probably because the results have been obtained from asphaltene aggregates rather than basic molecules. Some researchers based, on measurements with time-resolved fluorescence depolarization, state that asphaltene’s molecular weight is about 750 g/mol, that is, between 500 and 1000 g/mol, and that their main structure is “like your hand” or an “island” with seven fused aromatic rings forming the palm (core) and aliphatic chains (fingers) attached to the core (palm) [79,89,90,91,92,93,94,95]. On the other hand, Wiehe [16] ascribes to the opinion that the average molecular weight of asphaltenes is of the order of 3000 g/mol, measured by VPO at 130 °C and o-dichlorbenzene to avoid the determination of an associated molecular weight of asphaltenes. The application of VPO at temperatures of about 50–70 °C and toluene as a solvent report a molecular weight of associated asphaltenes that is typically higher [96]. Strausz et al. [80] mentioned that the highest molecular weight asphaltene fractions have a molecular weight > 17,000 g/mol. The higher molecular weight suggests the presence of “archipelago” structures in the asphaltenes. The availability of “archipelago” structures in the asphaltenes has been proved indirectly by pyrolysis studies [81,82], and directly by high-resolution Fourier transform–ion cyclotron resonance mass spectrometry (FT-ICR MS) with preliminary extrography fractionation [87]. The results reported by Chacón-Patiño et al. [87] confirmed that as molecular weight increases, the asphaltenes exhibited increased contributions of archipelago structural motifs. Thus, the asphaltenes consists of two main structural motifs designated as an “island” and an “archipelago”, and the dominance of each of them seems to depend on the origin of the oil sample [88].

Gray et al. [97] reported that the archipelago content of the asphaltenes from different origins was positively correlated with the yield of liquid products boiling below 524 °C, while the yield of coke solids increased with the fraction of island structures in the asphaltenes during thermal cracking and catalytic hydroconversion. Therefore, the concept of a predominant “island” structure existing in the asphaltenes regardless of their origin, postulated by the research group of Mullins [79,90,91,98,99] has been disproved, and it has been found that both structures coexist in both asphaltene and maltene fractions of petroleum fluids [83]. Concerning the true upper molecular weight boundary of asphaltenes, Chacón-Patiño et al. [86] conclude that it remains unknown, as inefficient/lacking ionization of species >1200 g/mol could also arise from their low volatility/ultra-high boiling points (>650 °C) when ultra-high-resolution mass spectrometry is utilized. They deduce that the disparities in molecular weight of asphaltenes discussed by Strausz [80], Wiehe [16,96] and the research group of Mullins [79,90,91,98,99] are caused by the aggregation affinity of asphaltenes [84]. Unlike the other petroleum components (maltenes), asphaltenes are prone to form aggregates in solution [5]. Similar to the molecular weight and the existence of “island” and “archipelago” structural motifs, the nature of aggregation, aggregate size distribution and dimensions of the nanoaggregates is still disputed, and the impact of this aggregation on phase behavior, physical properties and the processing of heavy petroleum fractions is not clear.

Table 1. Contents of carbon, oxygen, sulfur and nitrogen and the molecular weight of asphaltene samples from different sources.

No.	Asphaltene Type	%C	%H	%N	%S	%O	MW (Method)	Sa *	Reference
1	C7 Kuwait MG-130 asphaltenes	89.69	6.23	0.6	3.48		1692 (VPO)	0.444	[100]
2	C7 Kuwait MN-33 asphaltenes	88.4	8.27	0.35	2.13		1940 (VPO)	0.68	[100]
3	C7 Kuwait MG-144 asphaltenes	90.38	6.33	0.49	2.8		1752 (VPO)	0.45	[100]
4	C7 Kuwait MN-39 asphaltenes	88.77	6.28	0.41	4.54		1215 (VPO)	0.458	[100]
5	C5 Arab Light asphaltenes	84.23	7.76	0.75	6.3	0.96 a		0.668	[101]
6	C5 Arab Heavy asphaltenes	83.17	8.28	0.84	7.18	0.53 a		0.729	[101]
7	C5 Arab Medium asphaltenes	83.65	8.31	0.65	6.41	0.98 a		0.728	[101]
8	C5 Arab Berri asphaltenes	85.05	7.24	0.27	6.3	1.12 a		0.605	[101]
9	C7 Maya asphaltenes	82.54	8.46	1.11	7.1		5190 (VPO)	0.752	[102]
10	C7 Maya asphaltenes	81.62	7.26	1.46	8.46	1.02 a	5190 (VPO)	0.64	[103]
11	C7 Isthmus asphaltenes	83.99	7.3	1.35	6.48	0.79 a	3375 (VPO)	0.622	[103]
12	C7 Olmeca asphaltenes	87.16	7.38	1.34	3.48	0.64 a	2663 (VPO)	0.601	[103]
13	C5 Maya asphaltenes	81.23	8.11	1.32	8.25	0.97 a	3680 (VPO)	0.732	[103]
14	C5 Isthmus asphaltenes	83.9	8	1.33	6.06	0.71 a	2603 (VPO)	0.695	[103]
15	C5 Olmeca asphaltenes	86.94	7.91	1.33	3.2	0.62 a	1707 (VPO)	0.658	[103]
16	C6 A-dead oil asphaltenes	88.5	10.3	0.12	0.02	1.01		0.853	[65]
17	C6 F-dead oil asphaltenes	88.29	10.68	0.11	0.01	0.88		0.884	[65]
18	C7 PC asphaltenes	84.56	6.78	0.93	5.79	1.43 a	3100 (VPO)	0.558	[104]
19	C7 Iri asphaltenes	81.92	7.15	1.15	4.66	5.12 a	7400 (VPO)	0.625	[104]
20	C7 Karamay asphaltenes	84.44	8.49	1.75	0.82	4.06		0.737	[104]
21	C7 Lungu asphaltenes	83.99	7.31	1.41	4.38	2.74		0.623	[104]
22	C7 OL1-Furrial field	84.4	6.75	1.31	3.5	4.04 a	2100 (ND)	0.556	[105]
23	C7 OL2-Barua-Motatan field	84.9	8.6	1.31	4.5	0.69 a	3098 (ND)	0.743	[105]
24	C7 LO asphaltenes	83.6	6.95	1.06	4.64	2.6	3346 (VPO)	0.586	[66]
25	C7 M1-O asphaltenes	83.09	7.4	1.34	5.9	1.29	4550 (VPO)	0.641	[66]
26	C7 M2-O asphaltenes	82.78	7.2	1.28	6.91	1.4	3380 (VPO)	0.622	[66]
27	C7 Maya-type asphaltenes	85.07	7.21				818 (ND)	0.602	[106]
28	C7 Kuwait asphaltenes	79.65	8.31	0.77	7.48	3.79	1520 (VPO)	0.767	[107]
29	C7 El Furrial asphaltenes	85.5	6.9	1.73	3.4	2.50 a		0.563	[108]
30	C5 Cold Lake asphaltenes	79.9	7.5	1.3	7.6		2000 (GPC)	0.683	[109]
31	C5 Mobil oil asphaltenes	84	8	2.7			12170 (ND)	0.694	[110]
32	C5 Gulf of Mexico asphaltenes	85.17	8.63	1.24	1.14	2.38		0.743	[111]
33	C5 West of Africa asphaltenes	84.91	8.2	1.44	1.78	2.27		0.705	[111]
34	C5 North Sea asphaltenes	82.56	8.48	0.72	1.71	2.86		0.754	[111]
35	C5 Brazilian asphaltenes	85.67	8.15	1.89	1.36	1.86		0.693	[111]
36	C5 Gulf of Mexico asphaltenes	79.5	7.9	0.8	3.95	6.68		0.728	[111]
37	C7 Iran asphaltenes	74.56	6.74	0.85	6.86	10.99	1622 (ND)	0.653	[112]
38	C7 Iran asphaltenes	74.47	6.1	1	3.14	15.29 a		0.575	[113]
39	C7 Kuwaiti oil well Asphaltenes	84.61	5.79	1.08	4.15	3.17	815 (MALDI/MS)	0.432	[72]
40	C7 Asphaltenes	78.64	7.56	0.8	9.04	1.21		0.702	[114]
41	C7 Asphaltenes A1	75.2	8.14	0.78	15.78			0.796	[115]
42	C7 Asphaltenes A2	57.64	5.99	0.93	10.91	1.83		0.763	[115]
43	C7 Asphaltenes A3	64.75	7.2	0.84	13.38	4.36		0.817	[115]
	MIN	57.64	5.79	0.11	0.01	0.53	815	0.432
	MAX	90.38	10.68	2.7	15.78	15.29	12170	0.884

^a—Oxygen content calculated by difference; C₇—asphaltene fraction precipitated by n-heptane; C₆—asphaltene fraction precipitated by n-hexane; C₅—asphaltene fraction precipitated by n-pentane; VPO—vapor pressure osmometry; GPC—gel permeation chromatography; MALDI/MS—matrix-assisted laser desorption ionization mass spectrometry; ND—no data. *—Calculated Sa-value (the peptizability of an asphaltene according to ASTM D 7157) by Equations (1)–(4) extracted from the references [116,117,118].

Table 1. Contents of carbon, oxygen, sulfur and nitrogen and the molecular weight of asphaltene samples from different sources.

No.	Asphaltene Type	%C	%H	%N	%S	%O	MW (Method)	Sa *	Reference
1	C7 Kuwait MG-130 asphaltenes	89.69	6.23	0.6	3.48		1692 (VPO)	0.444	[100]
2	C7 Kuwait MN-33 asphaltenes	88.4	8.27	0.35	2.13		1940 (VPO)	0.68	[100]
3	C7 Kuwait MG-144 asphaltenes	90.38	6.33	0.49	2.8		1752 (VPO)	0.45	[100]
4	C7 Kuwait MN-39 asphaltenes	88.77	6.28	0.41	4.54		1215 (VPO)	0.458	[100]
5	C5 Arab Light asphaltenes	84.23	7.76	0.75	6.3	0.96 a		0.668	[101]
6	C5 Arab Heavy asphaltenes	83.17	8.28	0.84	7.18	0.53 a		0.729	[101]
7	C5 Arab Medium asphaltenes	83.65	8.31	0.65	6.41	0.98 a		0.728	[101]
8	C5 Arab Berri asphaltenes	85.05	7.24	0.27	6.3	1.12 a		0.605	[101]
9	C7 Maya asphaltenes	82.54	8.46	1.11	7.1		5190 (VPO)	0.752	[102]
10	C7 Maya asphaltenes	81.62	7.26	1.46	8.46	1.02 a	5190 (VPO)	0.64	[103]
11	C7 Isthmus asphaltenes	83.99	7.3	1.35	6.48	0.79 a	3375 (VPO)	0.622	[103]
12	C7 Olmeca asphaltenes	87.16	7.38	1.34	3.48	0.64 a	2663 (VPO)	0.601	[103]
13	C5 Maya asphaltenes	81.23	8.11	1.32	8.25	0.97 a	3680 (VPO)	0.732	[103]
14	C5 Isthmus asphaltenes	83.9	8	1.33	6.06	0.71 a	2603 (VPO)	0.695	[103]
15	C5 Olmeca asphaltenes	86.94	7.91	1.33	3.2	0.62 a	1707 (VPO)	0.658	[103]
16	C6 A-dead oil asphaltenes	88.5	10.3	0.12	0.02	1.01		0.853	[65]
17	C6 F-dead oil asphaltenes	88.29	10.68	0.11	0.01	0.88		0.884	[65]
18	C7 PC asphaltenes	84.56	6.78	0.93	5.79	1.43 a	3100 (VPO)	0.558	[104]
19	C7 Iri asphaltenes	81.92	7.15	1.15	4.66	5.12 a	7400 (VPO)	0.625	[104]
20	C7 Karamay asphaltenes	84.44	8.49	1.75	0.82	4.06		0.737	[104]
21	C7 Lungu asphaltenes	83.99	7.31	1.41	4.38	2.74		0.623	[104]
22	C7 OL1-Furrial field	84.4	6.75	1.31	3.5	4.04 a	2100 (ND)	0.556	[105]
23	C7 OL2-Barua-Motatan field	84.9	8.6	1.31	4.5	0.69 a	3098 (ND)	0.743	[105]
24	C7 LO asphaltenes	83.6	6.95	1.06	4.64	2.6	3346 (VPO)	0.586	[66]
25	C7 M1-O asphaltenes	83.09	7.4	1.34	5.9	1.29	4550 (VPO)	0.641	[66]
26	C7 M2-O asphaltenes	82.78	7.2	1.28	6.91	1.4	3380 (VPO)	0.622	[66]
27	C7 Maya-type asphaltenes	85.07	7.21				818 (ND)	0.602	[106]
28	C7 Kuwait asphaltenes	79.65	8.31	0.77	7.48	3.79	1520 (VPO)	0.767	[107]
29	C7 El Furrial asphaltenes	85.5	6.9	1.73	3.4	2.50 a		0.563	[108]
30	C5 Cold Lake asphaltenes	79.9	7.5	1.3	7.6		2000 (GPC)	0.683	[109]
31	C5 Mobil oil asphaltenes	84	8	2.7			12170 (ND)	0.694	[110]
32	C5 Gulf of Mexico asphaltenes	85.17	8.63	1.24	1.14	2.38		0.743	[111]
33	C5 West of Africa asphaltenes	84.91	8.2	1.44	1.78	2.27		0.705	[111]
34	C5 North Sea asphaltenes	82.56	8.48	0.72	1.71	2.86		0.754	[111]
35	C5 Brazilian asphaltenes	85.67	8.15	1.89	1.36	1.86		0.693	[111]
36	C5 Gulf of Mexico asphaltenes	79.5	7.9	0.8	3.95	6.68		0.728	[111]
37	C7 Iran asphaltenes	74.56	6.74	0.85	6.86	10.99	1622 (ND)	0.653	[112]
38	C7 Iran asphaltenes	74.47	6.1	1	3.14	15.29 a		0.575	[113]
39	C7 Kuwaiti oil well Asphaltenes	84.61	5.79	1.08	4.15	3.17	815 (MALDI/MS)	0.432	[72]
40	C7 Asphaltenes	78.64	7.56	0.8	9.04	1.21		0.702	[114]
41	C7 Asphaltenes A1	75.2	8.14	0.78	15.78			0.796	[115]
42	C7 Asphaltenes A2	57.64	5.99	0.93	10.91	1.83		0.763	[115]
43	C7 Asphaltenes A3	64.75	7.2	0.84	13.38	4.36		0.817	[115]
	MIN	57.64	5.79	0.11	0.01	0.53	815	0.432
	MAX	90.38	10.68	2.7	15.78	15.29	12170	0.884

^a—Oxygen content calculated by difference; C₇—asphaltene fraction precipitated by n-heptane; C₆—asphaltene fraction precipitated by n-hexane; C₅—asphaltene fraction precipitated by n-pentane; VPO—vapor pressure osmometry; GPC—gel permeation chromatography; MALDI/MS—matrix-assisted laser desorption ionization mass spectrometry; ND—no data. *—Calculated Sa-value (the peptizability of an asphaltene according to ASTM D 7157) by Equations (1)–(4) extracted from the references [116,117,118].

$$sp CCR, wt.\%={\displaystyle \frac{LN\left(1.8536\right)-LN\left(Asp\frac{H}{C}ratio\right)}{0.011}}\hspace{1em}\hspace{1em}{\mathrm{R}}^2=0.976$$

(1)

$$VR CCR, wt.\%=0.8312\times Asp CCR-13.8\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}{\mathrm{R}}^2=0.915$$

(2)

$$Sa=0.9393-0.0105\times VR CCR\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em} {\mathrm{R}}^2=0.896$$

(3)

where

Asp CCR = Conradson carbon content of asphaltene, wt.%;

Asp${\displaystyle \frac{H}{C}}$ratio = Atomic ratio ${\displaystyle \frac{H}{C}}$ of asphaltenes;

VR CCR = Conradson carbon content of petroleum vacuum residue, wt.%;

Sa = the peptizability of an asphaltene according to ASTM D 7157.

According to the Yen–Mullins model [95,99] the asphaltene molecule has a size of about 1.5 nm, while according to Yarranton et al. [119], its size is 2–3 nm [120]. The average size of asphaltene aggregates according to the Yen–Mullins model [95,99] is ~2 nm, while according to Gray and Yarranton [120] it is in the range of 5–20 nm depending on the experimental method and the concentration. Gray and Yarranton [120] determine aggregates as assemblies of two or more molecules formed by molecular association interactions. These aggregates have sizes in the range of 2–20 nm and are stably suspended in most crude oils and their heavy oil derivatives, as distinct from much larger domains of asphaltene material that form during flocculation or precipitation when they are no longer stable in the crude oil or solvent. The concentration at which the asphaltenes start to aggregate is not well defined. Association has been observed by spectroscopic methods at concentrations as low as 0.15 mg/L [121,122]. Thus, in typical crude oils, where the asphaltene concentration is over 1 wt.% (over 8000 mg/L), most asphaltenes capable of self-association are likely to be aggregated [120]. Discussing the two main proposed concepts for asphaltene aggregation, threshold aggregation and step-wise aggregation, Gray and Yarranton [120] conclude that the step-wise addition mechanism is the appropriate approach for modeling asphaltene aggregation. Multiple techniques have been used to detect the onset of asphaltene aggregation at as low as 25 mg/kg [120,123] and to determine the average size or molecular weight in crude oils, vacuum residues or solvent solutions [119], but most of these methods give no direct indication of the size distribution or maximum size of asphaltene aggregates [120]. Data from nanofiltration to eliminate asphaltenes from bitumen at 200 °C [124], small-angle X-ray scattering (SAXS) at 240 °C [125] and diffusion and adsorption on catalyst pellets at 250 °C [126] indicate that significant concentrations of nanoaggregates persist at surprisingly high temperatures [127]. All the studies with ultracentrifugation show that significant mass fractions of the asphaltenes are present as stable aggregates in solution [123]. Porto et al. [5] put two questions forward: (1) What forces are behind the association of asphaltene molecules? (2) And at what point does aggregation result in precipitation? The list of potential intermolecular forces that could contribute to nanoaggregation has been well defined and summarized [128,129]. They are itemized as follows: acid–base [130]; free radical pancake bonding [131]; charge transfer [132] (strong driver−electron sharing interactions); van der Waals interactions; Polar group–electrostatic dipole interactions [133]; hydrogen bonding [134] (medium drivers); parallel π − π interactions [135,136,137]; and T-shaped π − π interactions [138] (weak to strong depending on the size of the aromatic group and repulsion due to appended groups). When archipelago molecules are abundant in an asphaltene fraction, all these forces can combine to help stabilize an aggregate. This combination of forces can lead to stability at elevated temperatures and persistence of nanoaggregates at extremely low concentrations in solvents such as toluene. The experimental results of Chacón-Patiño et al. [139] suggest that components with multiple oxygen functional groups will give the strongest aggregation. The data from GPC and VPO suggest a maximum molecular weight of ca. 30,000 g/mol for asphaltene nanoaggregates in good solvents, while nanofiltration suggests a maximum size of ca. 100 nm in stable crude oil [127]. In both cases, the asphaltene monomers range from approximately 400 to 1500 g/mol. It is worth mentioning here that the range of asphaltene monomer molecular weight variation is reasonable to expect to be different for vacuum residues, which have distinct molecular weights. For example, in the study of van den Berg et al. [140], the molecular weight of vacuum residues, separated from 11 crude oils at the same cut point of 520 °C, varied between 677 and 1265 g/mol. Understandably, the molecular weight of the asphaltene fraction from the vacuum residue having molecular weight of 1265 g/mol should be expected to be bigger than that of the vacuum residue with a molecular weight of 677 g/mol.

Asphaltene precipitation begins with the instantaneous formation of particles of dimensions from 200 nm to 2 μm [125]. Vargas et al. [141] showed that asphaltenes are smaller than 300 nm when they first precipitate. These primary particles then flocculate to give porous particles with mean diameters up to 400 μm, depending on the concentration and shear forces [11,142,143]. Asphaltene precipitation can be modeled by the use of flocculation kinetics [11,142,143]. Detailed studies of asphaltene precipitation show that the kinetics of the primary particle formation are extremely fast, with a rapid increase in dimension from aggregated molecules ca. 5−20 nm to primary particles of diameters up to 400 μm in a fraction of one second [142]. This behavior can only be explained as a phase transition, not as an extended molecular aggregation process wherein dissolved molecules associate stepwise with the existing aggregates [127]. Like any multi-component separation process, some components appear almost exclusively in the petroleum maltene-rich phase, while others appear mainly in the asphaltene-rich phase and others in both phases [84]. Asphaltene flocculation and subsequent precipitation occlude alkane-soluble compounds inside nanoaggregate networks that are difficult to remove even through extended washing with alkane solvents. Those compounds typically reveal lower carbon numbers, double bond equivalents and levels of heteroatoms (i.e., N and O) compared to the bulk of the alkane-insoluble asphaltenes [127]. The asphaltene-rich phase is a solution of both aggregated and molecular species, whereas the n-heptane-soluble maltene fraction gives little evidence for aggregation based on vapor pressure osmometry, gel permeation chromatography and fluorescence spectroscopy [121,144,145,146]. Gray et al. [127] infer that the details of nanoaggregation have little impact on phase behavior.

The investigation of molecular weights of asphaltenes before and after catalytic hy-droconversion at 400−430 °C has indicated that the molecular weight of the aggregating asphaltenes dropped from 2000 g/mol in the feed to 650 g/mol after 80% conversion of the vacuum residue [147]. The fraction of asphaltenes that did not associate at all to form ag-gregates, designated as “neutrals”, increased from 0.06 to 0.23 after vacuum residue hydrocracking [127]. The modeled distribution of molecular weights of asphaltene aggregates of Athabasca asphaltenes before treatment and after hydroconversion from 56% to 80% conversion of vacuum residue showed a shift from a feed population dominated by aggregates of 2000−20,000 g/mol in the feed, comprising 3−30 molecules on average, to a population dominated by monomers and dimers in the processed product [147]. Aggregation in toluene is dramatically reduced, even though both the feed and the product asphaltenes were recovered by the same phase separation process [127]. This observation emphasizes the conclusion that significant aggregation is not a prerequisite for the phase separation of asphaltenes [127]. The reactions occurring during vacuum residue hydrocracking will drive a feed of Athabasca asphaltenes from a diverse mixture rich in archipelago structure and polar functional groups toward a much simpler assemblage of large aromatics and partly hydrogenated aromatics with short side chains [127]. However, the hydrocracked asphaltenes were found to be much more amenable to precipitation and deposit formation than the unadulterated asphaltenes [37,38,40].

Asphaltene precipitation can be considered a result of solubility changes provoked by alterations to composition, temperature and pressure [148]. Unlike the strong forces responsible for asphaltene nanoaggregate formation, asphaltene phase behavior is deemed to be driven by van der Waals dispersion forces among the aggregates [5,141]. The dispersion forces in asphaltenes and their aggregates are mainly due to the localized cloud of electrons from the polycyclic aromatic core and heteroatoms. That is why the more aromatic asphaltenes indicate a higher deposition tendency [147]. The hydrocracked asphaltenes are more aromatic than the unadulterated asphaltenes, as reported in [149], and although they have a lower tendency to aggregate, they have a much higher inclination to deposit formation [40,149,150]. In a polydisperse material like asphaltenes [151], which are a mixture of molecules with a broad distribution of sizes and molecular characteristics, the amount of the heaviest asphaltene fraction seems to have a significant impact on deposit formation. Therefore, asphaltene polydispersity plays a major role in determining asphaltene precipitation and deposition formation. Thus, the chemical additives, which could change the solubility of the least soluble subfraction of the asphaltene mixture, may alleviate the deposit formation issue.

Considering the very high complexity of the knottiest fraction of petroleum—asphaltenes—and the established conclusion from petroleum research that no two crude oils are the same [152], one can easily make the parallel that no two asphaltene fractions are the same, both by their nature and by their methods of separation, characterization and measurement. This can explain the great disputes about asphaltene characteristics (content, molecular weight, degree of aggregation, the mechanism of precipitation, etc.) [153,154]. Another issue in characterizing asphaltenes is obtaining consistent data from experimental measurements. For example, the data reported in [100] for three different asphaltenes, MG-130, MG-144 and MN-39 (see asphaltene data points Nos. 1, 3 and 4 in Table 1), indicated a ${\displaystyle \frac{H}{C}}$ atomic ratio of 0.83, 0.84 and 0.85, respectively, while the densities at 25 °C were communicated to be 1.20831, 1.22684 and 1.30993, respectively. It is well known that there is a strong negative correlation between atomic ${\displaystyle \frac{H}{C}}$ ratio and density [116], whereas the data shown above demonstrate the existence of a positive correlation, i.e., the higher the atomic ${\displaystyle \frac{H}{C}}$ ratio, the higher the density. This suggests that either the density measurements or the hydrogen and carbon contents are incorrect for the three asphaltenes mentioned above.

The data in Table 1 confirm the vast diversity in properties of asphaltenes derived from different oils and obtained by different methods. In addition, an attempt to search for the presence of statistically meaningful relationships between the different asphaltene traits from the data in Table 1 was unsuccessful. However, it has been reported that asphaltene hydrogen content, density and Conradson carbon content correlated well with their peptizability (Sa), measured by the dual solvent titration method ASTM D7157 [116,117,118].

Based on these relations and Equations (1)–(3), the Sa-value for the asphaltenes was calculated (Table 1) [116,117,118]. It is evident that Sa values vary between 0.432 and 0.884, demonstrating a significant difference in the asphaltene peptizability, and therefore one may expect different behavior of the distinct asphaltenes when treated with various additives.

3. Review of the Chemical Additives Used to Prevent Asphaltene Deposit Formation

The resin fraction in petroleum fluids has been postulated to act as a natural disper-sant to keep asphaltenes dispersed by approximately 4 nm in diameter particles [63]. Resins are inherent to crude oil and are identified as the most polar aromatic species in deasphalted oil, and similarly to asphaltenes are defined by solubility (soluble in nC₅₊ alkanes and insoluble in liquid propane [155]) and adsorption (adsorbents: fuller’s earth, alumina or silica, eluted with pyridine or a mixture of toluene and methanol [156]) criteria. Resins and asphaltenes both derive from the same parent molecules, and thus contain similar molecular features [157]. Nevertheless, compared to asphaltenes, resins have smaller chromophores and relatively longer aliphatic side chains, which increase their solubility in aliphatic solvents [42,157,158]. This in turn allows them to act as a link between the nonpolar saturates and polar asphaltenes of the crude oil [42]. Therefore, researchers have tried to utilize these chemistries by finding synthetic analogs of the natural resins. Moreover, the synthetic analogs they have created were found to be even more effective asphaltene stabilizers than the natural resins [63].

A commonly used method in industry to reduce asphaltene deposition is dilution with aromatic fractions obtained from a fluid catalytic cracking process [159,160,161]. Figure 2 illustrates how the addition of high-aromatic-fluid catalytic cracking slurry oil (80% aromatic content) to a hydrocracked hot middle pressure separator (HMPS) bottom product from a commercial ebullated bed H-Oil vacuum residue hydrocracker decreases the asphaltene content, a result due to asphaltene dissolution.

In

Table 2. Percentage reduction in asphaltenes due to the addition of FCC SLO to the H-Oil HMPS bottom product.

Asphaltenes	FCC SLO Added to the H-Oil HMPS Bottom Product
	10%	20%	30%
	Asphaltene Reduction, %
C5 asphaltenes	−48.6	−39.3	−35.5
C7 asphaltenes	−47.4	−46.6	−47.5

, the percentage of reduction of C₅- and C₇- asphaltenes calculated by Equation (4) is given. In order to account for the effect of dilution, the asphaltene content in the H-Oil HMPS bottom product without FCC SLO addition was multiplied by the weight fraction of the H-Oil HMPS bottom product in the blend.

$$\% Red.asph.={\displaystyle \frac{({Asph.}_{with FCC SLO}-{Asph.}_{without FCC SLO})}{{Asph.}_{without FCC SLO}}}\times 100$$

(4)

where $\% Red.asph.$—percentage of reduction in asphaltenes; ${Asph.}_{with FCC SLO}$—asphaltene content with FCC SLO addition, wt.%; and ${Asph.}_{without FCC SLO}$—asphaltene content without FCC SLO addition, wt.%.

Wiehe and Jermansen [63] showed that synthetic additives can completely stabilize asphaltenes in five different crude oils (Figure 3), and these dispersants can be even more effective asphaltene stabilizers than natural resins, which are presented in petroleum in a much higher concentration. In the same manner, Ovalles et al. [162] showed that the addition of 100 ppm additive to the hydrotreated product resulted in ~55% reduction in asphaltene content.

Chemical additives, depending on the mechanism by which they prevent the precipitation of asphaltenes, can be divided into asphaltene inhibitors and asphaltene dispersants [64,163,164]. Similar to asphaltenes and resins, which are not very well defined and where some overlapping may exist, the distinction between inhibitors and dispersants of asphaltenes is more or less vague. It is considered that asphaltene inhibitors deter the aggregation of asphaltene molecules and delay the onset of asphaltene flocculation, while asphaltene dispersants do not affect the flocculation point, but reduce the size of flocculated asphaltene particles and thus stabilize the asphaltenes within the crude oil [64]. However, it should be noted that asphaltene content is usually determined by physical separation from the bulk fluid after the addition of n-alkanes forming a secondary phase, and by definition only the material that can be physically separated using a filter can be counted towards the asphaltene content. The latter is dependent not only on asphaltene solubility but also on the morphology and size of the phase domains of the second phase [77]. In this concept, a “dispersant” is defined as an anti-agglomeration agent that enables the petroleum fluid to pass through a filter (due to the smaller size of the asphaltene molecules) and therefore is not counted as an asphaltene. On the other hand, an “inhibitor” must act on the bulk fluid and modify its tendency to phase segregate when n-alkanes are added. Inhibitors can also function as dispersants, but asphaltene dispersants do not generally function as inhibitors [64]. In the literature, the terms “inhibitor” and “dispersant” are not always clearly distinguished and are usually used interchangeably [164].

In general, asphaltene dispersants and inhibitors can be divided into non-polymeric organic compounds [63,165,166,167,168], polymers with different functional groups [169,170,171,172], ionic liquids [173,174,175,176], metal nanoparticles [177,178,179,180,181,182,183,184,185] and commercial inhibitors [65,186].

Table 3. Chemical compounds described in the literature that have been tested as inhibitors, dispersants and solvents.

Nr	Additive	Nr	Additive	Nr	Additive	Nr	Additive	Nr	Additive
1	sodium dodecylbenzene sulfonate [187,188,189]	21	phenol [190,191,192]	41	4-dodecylresorcinol [57,107,165,193,194]	61	propoxylated poly(dodecylphenol formaldehyde) [195]	81	dodecyl pyridinium chloride [196]
2	sodium octylbenzenesulfonate [189]	22	cresol [165,193,197,198]	42	sorbitan monooleate [199]	62	aromatic polyisobutylene succinimides [200]	82	1-dodecyl-3-methylimidazolium chloride [196]
3	sodium dodecyl sulfate [201]	23	ethylphenol [165,189,193]	43	PL (two functional groups of hydroxyls connected to a benzene) [191]	63	poly(vinyltoluene-co-alpha-methylstyrene) [56]	83	1-allyl-3-methylimidazolium oleate [202]
4	triethanolamine lauryl ether sulfate [203]	24	butylphenol [165,189]	44	hexylamine [197,204]	64	polyisobutylene succinimide [57]	84	1-allyl-3-methylimidazolium abeitate [202]
5	sodium lauryl ether sulfate [203]	25	hexylphenol [165,189,193,205]	45	dodecylamine [206,207]	65	polyisobutylene succinic ester [57]	85	1-allyl-3-methylimidazolium cardanoxy [202]
6	dioctyl sodium sulfosuccinate [208]	26	octylphenol [61,165,189,193,205,209,210]	46	hexadecylamine [198]	66	nonylphenol-formaldehyde resin (modified by polyamines) [57]	86	synthesized deep eutectic solvent [206,211]
7	acetic acid [197]	27	nonylphenol [66,105,107,165,167,191,193,198,199,207,212,213,214,215,216,217,218]	47	diethanoldodecylamine [199]	67	p-octylpyridinium chloride [189]	87	nonylphenol formaldehyde resin/Toluene [72]
8	isopropanol [197]	28	dodecylphenol [110,165,189,193,205,209,214,219,220,221]	48	coconut diethanol amide [203,222,223]	68	p-butylpyridinium chloride [189]	88	nonylphenol formaldehyde resin/Xylene [72]
9	caprylic acid [190,224,225]	29	secbutylphenol [165,189,193,219]	49	N-aryl amino-alcohols [106]	69	p-dodeylpyridinium chloride [189]	89	nonylphenol formaldehyde resin/Coconut oil [72]
10	oleic acid [190,225]	30	tertbutylphenol [219]	50	poly(maleic anhydride l-octadecene) [110]	70	p-dodeylpyridinium tetrafluoroborate [189]	90	nonylphenol formaldehyde resin/Andiroba oil [72]
11	lauric acid [206]	31	tertoctylphenol [165,193,219]	51	nonylphenolic resin [219,226]	71	p-dodeylpyridinium hexafluorophosphate [189]	91	mPVOH-1 (standard polymer) [227]
12	palmitic acid [223]	32	creosol [212,228,229]	52	dodecyl phenolic resin [110]	72	N-butylisoquinolinium chloride [189]	92	mPVOH-2 (non-ionic polymer) [227]
13	stearic acid [198]	33	heptyloxyphenol [165,193]	53	sulfonated polystyrene [230]	73	1-propylboronic acid-3-alkylimidazolium bromides [173]	93	mPVOH-1 (anionic polymer) [227]
14	benzoic acid [69,167,190,191,201]	34	methanol [197]	54	polycardanol [230]	74	1-propenyl-3-alkylimidazolium bromide [173]	94	dodecylbenzene sulfonic acid [67,69,186,192,211,220,222,231,232]
15	octylbenzoic acid [233]	35	1-octanol [204,207]	55	nonylphenol formaldehyde resins [221]	75	1-butyl-3-methylimidazolium bromide [61,173,234,235]	95	auric alcohol [220]
16	salicylic acid [69,167,201,231]	36	dodecanol [108]	56	polyolefin amide alkeneamine [54]	76	1-butyl-3-methylimidazolium chloride [61,173,235,236]	96	BZSS-012 inhibitor [65]
17	phthalic acid [167,191]	37	2-ethyl-1-hexanol [201]	57	alkylated phenol [54,208]	77	didecyldimethylammonium nitrate [173]	97	I-8 commercial inhibitor [237]
18	sebacic acid [224]	38	benzyl alcohol [198,201]	58	poly olefin esters [52]	78	1-butyl-3-methylimidazolium nitrate [236]	98	I-9 commercial inhibitor [237]
19	etyltrimethylammonium bromide [104,187,201]	39	Triton X-100 [198]	59	polyolefin alkeneamine [205,238,239,240]	79	1-methyl-1H-imidazol-3-ium-2-carboxybenzoate [236]	99	I-15 commercial inhibitor [237]
20	zwitterionic liquid [241,242]	40	ethoxylated nonylphenol [105,111,165,187,191,193,198,199,207,219,225]	60	poly(dodecylphenol formaldehyde) [195]	80	dodecyl thiazolium chloride [195]	100	4-hexylphenol [237]
101	4-octylphenol [237,243]	121	poly(styrene-co-octadecyl maleimide) [171]	141	tetrabutylammonium hexafluorophosphate [176]	161	vanadyl petroporphyrins [244,245,246]	181	nanoparticles of NiO supported on silica gel: SNi5; SNi15 [247]
102	4-dodecylphenol [237]	122	1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide [223]	142	cetyltrimethylammoniun bromide [176]	162	Ni petroporphyrins [246]	182	carbon nanotubes [248]
103	sodium dodecyl sulfonic acid [188]	123	1-butylpyridinium bis(trifluoromethylsulfonyl)imide [234]	143	tetrabutylphosphonium bromide [176]	163	ethylbenzene [249]	183	Fe3O4/Chitosan nanocomposite [250,251]
104	CA 2 inhibitor [186]	124	1-butyl-3-methyl-imidazolium tetrafluoroborate [234]	144	toluene [66,107,165,189,190,204,212,233,249,252,253,254]	164	nanoparticles of Al2O3; Fe3O4; NiO [185]	184	nanoparticles of Fe3O4; Fe3O4/SiO2; Fe3O4/TiO2; Fe3O4/ Chitosan [250]
105	CA 1 inhibitor [186]	125	PL5001 [255]	145	cyclohexane [197]	165	nanoparticles of SiO2; Al2O3; MgO; ZnO;TiO2; kaolin [256]	185	TiO2, ZrO2, CeO2 nanoparticles [177]
106	4-dodecylbenzenesulfonic acid/4-nonylphenol mixture [186]	126	multi-alkylated aromatic amides [257]	146	dicyclopentadiene [197]	166	nanoparticle of stainless steel (304L), iron, and aluminum [258]	186	Co3O4 nanoparticles modified by SiO2 [259]
107	trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylphentyl) phosphinate [67,70]	127	fluorine-containing tetrameric copolymer [260]	147	naphthalene [198,201,212,228,229]	167	nanoparticles of kaolin, CaCO3, TiO2, BaSO4, Fe3O4, FeS, SiO2 (hydrophilic/hydrophobic) [111]	187	nanoparticles of MgO, CaO, SiO2, Al2O3 [261]
108	trihexyl(tetradecyl)phosphonium bis(2-ethylhexyl)phosphate [70]	128	trioctylmethylammonium dodecylsulfate [262]	148	quinoline [212,229]	168	nanoparticles of NiO; Fe2O3; WO3; MgO; CaCO3; ZrO2 [181]	188	Co3O4 nanoparticles [180]
109	AUT Force 110 [263,264]	129	polyoctadecylacrylate [265]	149	indole (solid) [212,228,229]	169	nanoparticles of Co3O; Fe3O4; MgO; CaO; TiO2; NiO [182]	189	SiO2 nanoparticles from rice husks [266,267]
110	X2; Y1; Y2; Y3; Z1 inhibitors [192]	130	polyoctadecylacrylate-maleic anhydride [265]	150	phenanthrene [186,202]	170	nanoparticles of Slica gel, SNi5, SNi15, Zeolite, Al, AlNi5, AlNi15, PdNi/Al, [268]	190	polythiophene-coated Fe3O4 nanoparticles [115,269,270,271]
111	rhamnolipid [67]	131	polyoctadecylacrylate-maleic anhydride-aniline [265]	151	benzene [190,201,212,249,272]	171	nanoparticles of Slica gel: S11, S11A, S11N, S11B, S58, S240 [273]	191	DBSA nanoemulsion [114]
112	3-hexadecyl-1-methyl-1H-imidazol-3-ium tetrachloroferrate(III) [174]	132	polyoctadecyl-acrylate-maleic anhydride-naphthylamine [265]	152	m-xylene [190,191,197,212,228,249,274,275]	172	nanoparticle of SiO2 [111,227,273,276]	192	carbon nanoparticles [277]
113	1-ethyl-3-hexadecyl-1H-imidazol-3-ium tetrachloroferrate(III) [174]	133	esters of tannic acid and heptanoyl chloride [278]	153	1, 2, 4-trimethylbenzene [212]	173	nanoparticles of SiO2; Al2O3; MgO [276,279]	193	carboxylate-alumoxane nanoparticles [280]
114	1-butyl-3-hexadecyl-1H-imidazol-3-ium tetrachloroferrate(III) [174]	134	tetramethylammonium bromide [176]	154	decalin [212,228,229]	174	nanoparticles of TiO2, ZrO2, SiO2 [279]	194	oil/water nanoemulsions and oil/water nanoemulsions with DBSA [232]
115	tetrahydrofuran [248]	135	tetraethylammonium bromide [176]	155	tetralin [164,188,209,225]	175	nanoparticle of TiO2 [111,182,281,282,283]	195	nanoparticles of SiO2 and polyvinylpyrrolidone stabilizer [284,285]
116	furan [249]	136	tetrapropylammonium bromide [176]	156	pyridine [187,209]	176	nanoparticle of iron (II and III) oxide [111,178,179,181,182,185,250,286,287]	196	multi-walled carbon nanotubes-Fe3O4 nanoparticles [285]
117	furfural [249]	137	tetrabutylammonium fluoride [176]	157	benzothiophene [209,225,288]	177	nanoparticles of NiO; Fe2O3 [286,289]	197	nanoparticles of graphene oxide, TiO2, SiO2, MgO [62]
118	methyl acetate [249]	138	tetrabutylammonium chloeide [176]	158	nonylbenzene [162,190,290]	178	nanoparticles of MgO [181,182,276,291]
119	ethyl acetate [249]	139	tetrabutylammonium bromide [176]	159	p-xylene [241]	179	nanoparticles of Al2O3 [72,179,183,185,292,293,294,295]
120	ethyl propionate [249]	140	tetrabutylammonium iodide [176]	160	aromatic naphtha A 150 [244]	180	nanoparticles of NiO [181,182,185,286,296,297]

summarizes the chemical additives used as asphaltene inhibitors and dispersants in a variety of studies.

The process of inhibition of asphaltene precipitation by the addition of chemical additives is complex and not fully understood. It is suggested that the effectiveness of additives in stabilizing asphaltenes is controlled by the strength of their interaction with asphaltenes and their ability to form a stabilizing layer around asphaltene particles [178]. It is assumed that the additives interact with asphaltenes through π − π interactions, acid–base interactions, hydrogen bonds, metal ion complexation and dipole–dipole interactions [100,288,298]. The realization of these interactions and their strengths depend on the molecular structure of the additives used and the crude oil and asphaltene properties that they are applied to. Since asphaltenes have a wide range of properties (i.e., polarity, molecular weight, aromaticity, heteroatom content), the asphaltene inhibitors may not be universal to prevent asphaltene precipitation in different crude oils and heavy oil fractions [237]. That is why chemical additives are usually designed based on the asphaltene characteristics to have the best interactions and efficiencies [65].

The group of non-polymeric organic compounds includes aromatic hydrocarbons, alkyl phenols, organic acids, aromatic amines, alkyl amines, phosphoric esters, phosphonocarboxylic acids, etc. The addition of aromatic hydrocarbons to crude oil is known to delay the precipitation of asphaltenes due to π − π interactions between their aromatic rings and the asphaltene monomers, as exemplified in Figure 2 [299]. Clarke and Pruden [228] showed that toluene, p-xylene, naphthalene and phenanthrene are additives that are very effective in delaying the onset of asphaltene precipitation in bitumen, with phenanthrene (20% concentration) showing the best performance. The authors [228] explain that the better performance of phenanthrene is due to its high aromaticity and molecular weight, which gives this compound resin-like features. They also showed that the addition of chemicals with heteroatoms (quinoline, indole, benzothiophene) and hydrogen donor compounds (tetralin, decalin) has almost no effect in delaying the onset of asphaltene precipitation [228]. On the other hand, the presence of a polar group in the dispersant molecule is shown to increase its effectiveness [107,165,228]. Zahedi et al. [166] revealed that with the addition of dodecylbenzene sulphonic acid, containing the -SOOH group, to Iranian crude oil at a 5-fold lower concentration (2000 ppm optimal concentration) compared to toluene (10,000 ppm optimal concentration), a much greater reduction in precipitated asphaltenes is achieved.

Pereira et al. [108] explained the low efficiency of dodecanol (25 wt.%) compared to ethoxylated nonylphenol with the low polarity of dodecanol. In the same way, Zhou et al. [290] compared the performance of biodiesel with that of oleic acid in dispersing asphaltenes and concluded that the dispersion effect increased with an increasing polarity of the inhibitor, and this trend can be attributed to the stronger interactions of polar functional groups between the inhibitor and asphaltene. Investigating a series of alkylbenzene-derived amphiphiles as asphaltene stabilizers, Chang and Fogler [165] concluded that increasing the polarity of the head group enhances the attraction of amphiphiles to asphaltenes through acid–base interactions. Polar head has a different binding potential with asphaltene molecules. The comparison between alkylbenzene dispersants containing different polar groups (-COOH; -OH; -NH₃; -SOOH) shows that the alkylbenzene-dispersant-containing -SOOH group is the most effective in stabilizing asphaltenes [63,155]. Karambeigi et al. [167] investigated the effect of salicylic acid, phthalic acid, nonylphenol, phenanthrene, benzoic acid and IR95 additives on preventing the precipitation of asphaltenes in reservoirs. They concluded that compounds with -COOH functional groups reacted better with asphaltenes than others. The authors [167] also show that the best performance among the investigated non-commercial inhibitors was from the salicylic acid with a 34% reduction in asphaltene precipitation. It is explained that the additional hydroxyl group on the benzoic ring of the salicylic acid creates an effective and stable π − π interaction with the asphaltenes.

Östlund et al. [168] investigated the adsorption of different amphiphiles with varying functional groups onto asphaltenes regarding their solubilizing and stabilizing effect. They found that amphiphiles that contained alkaline functional groups such as -NH₂ had the lowest adsorption of the tested compounds, while compounds that contained -COOH functional groups were better adsorbed to asphaltenes than other subgroups. Furthermore, the sulfonic acid group containing amphiphilic molecules was noticed to have a profound effect on the stability of asphaltenes and these molecules adsorbed in a large extent to asphaltenes [168]. Kelland [64] suggested that the reason for the better performance of sulfonic and carboxylic acids compared to alkylamines is due to the greater probability of hydrogen bonding between the acid groups in the dispersant and the basic centers (nitrogen-containing groups) in the asphaltene molecule, since they are found in greater quantities in the asphaltene molecules compared to the acid centers (hydroxyl and carboxyl groups). The addition of a polar group to the chemical additive may be favorable; however, using too many polar groups or one group that is too polar can reduce the solubility of the additive in the oil [64].

The effectiveness of the dispersant in stabilizing asphaltenes depends not only on the presence of a polar group in its molecule, but also on the length of the alkyl chain. Dehaghani and Badizad [223] concluded that inhibition characteristics of an additive are driven by the synergy between the strength of the polar head and the tail length of chemicals, as well as inhibitor concentration in the oil phase. The effect of dispersant chain length was investigated using p-alkylphenols with chain lengths from C₁ to C₁₂, and it was found that the minimum alkyl chain length of p-alkylphenols to provide sufficient ability to stabilize asphaltenes was about 6 carbon atoms [165].

A too-long alkyl chain can lead to crystallization and precipitation of the inhibitor. Wiehe and Jermansen [63] proposed the use of branched, double-tailed dispersants with a total length distribution split between the two tails, which successfully suppresses crystallization and overcomes all side chain length limitations. The authors found that the dispersant possessing optimal properties, of the alkylbenzsulfonic acid type, contained a -SOOH group in its structure, two fused aromatic rings and a branched chain with a total length of at least 30 carbon atoms [63].

Dodecylbenzene sulfonic acid and nonylphenol are some of the most studied compounds as asphaltene dispersants, and have been used for many years in the field as asphaltene dispersants in both upstream and downstream processes [64]. Chang and Fogler [165] found that the effectiveness of the asphaltene dispersants on C₅-asphaltenes extracted from Mobil crude oil decreased in the following order: dodecylbenzene sulfonic acids (DBSA) > nonylphenol > p-[(hydroxyethoxy)ethoxyl-n-nonylbenzene > nonylbenzene. In the presence of 1–2 wt.% DBSA, asphaltenes can be totally solubilized in heptane solvent, while when using nonylphenol, a concentration of 7 wt.% is required to completely dissolve asphaltenes. In another study, Balestrin et al. [186] compared the action of DBSA, nonylphenol, a mixture of the two and commercial additives CA1 and CA2 as asphaltene inhibitors in Brazilian crude oil. The reported efficiency was as follows: DBSA < DBSA−nonylphenol (mixture) < nonylphenol < CA2 (phenolic polymer) ≪ CA1 (polymer solution in an aromatic medium containing an amino compound as the main active component). However, Goual and Firoozabadi [254] concluded that there is a threshold concentration at which DBSA can be used as an effective additive in reducing asphaltene precipitation. The authors demonstrated that at low concentrations of DBSA (<1 wt.%), the amount of precipitation increases with increasing amphiphile concentration, while at higher concentrations (>1 wt.%) a reverse trend is observed (as DBSA concentration increases, the amount of precipitation decreases).

The different performances of DBSA and nonylphenol are probably due to the different structural and compositional characteristics of the asphaltenes or the different components contained in the crude oil. Barcenas et al. [300] demonstrated that the same additive, depending on the type of oil to which it is applied, can act as an inhibitor or promoter of the asphaltene precipitation process. Rogel et al. [301] found that asphaltene stabilization with an inhibitor was affected by the composition of the crude oil. Through their research, the authors indicate that the activity of the inhibitor (DBSA) decreases as the base number of the crude oils increases. DBSA applied to the studied types of oil in a low concentration (5 vol%) showed negative activity in almost all cases, while at higher concentrations (10 vol%), the inhibitor showed activity in most cases. The authors suggest that this may indicate that at higher concentrations of the inhibitor, there is enough amphiphile to complete the neutralization of the basic functionalities and start acting as an asphaltene stabilizer. This means that these interactions also affect the concentration range of application of the dispersants. Smith et al. [302] showed that the specificity of the asphaltene inhibitor can be explained by acid–base-type interactions between the inhibitor and polar species in the crude oil or asphaltene fractions.

Wang et al. [104] investigated the ability of two ionic amphiphiles, DBSA and dodecyl trimethyl ammonium bromide (DTAB), to stabilize two types of asphaltenes (Karamay and Lungu). Karamay asphaltenes contain large amounts of carboxyl groups and calcium and are negatively charged, whereas Lungu asphaltenes are rich in nickel, vanadium and pyrrolic structures and are positively charged. DBSA had good ability to stabilize Lungu asphaltenes but had no effect on Karamay asphaltenes. In contrast, DTAB has good ability to disperse Karamay asphaltenes but has no obvious effect on Lungu asphaltenes. Authors concluded that the electric property of asphaltenes plays an important role in the interaction between asphaltenes and amphiphiles. The negatively charged asphaltenes tend to be dispersed by cationic amphiphiles, whereas the positively charged asphaltenes tend to be dispersed by anionic amphiphiles. Therefore, the action of the asphaltene dispersant depends both on its structure and on the characteristics of the asphaltenes and the type of oil in which they are contained. Horeh et al. [65] showed that large asphaltene aromatic sheet size, short peripheral chain length, high heteroatom content and density of hydroxyl groups may lead to higher self-association. Authors suggest that appropriate additives for this type of asphaltene molecule contain small polycyclic aromatic sheets, connected to long side chains with appropriate functional groups. On the other hand, for asphaltenes with small aromatic sheet size and low self-association tendency, even inhibitors with a linear structure would be effective [65]. Likewise, Amiri et al. [264] demonstrated that compatibility between the additive and asphaltene molecules is essential to achieve a good performance of the former.

Due to the structural similarity of polymers to resins and asphaltenes, they are considered to be compounds that would be effective in stabilizing asphaltenes. Moreover, the variety of functional groups that can be attached to the large polymer molecule allows for the adjustment of their inhibition properties [171,172]. Polymers can interact with asphaltenes through π − π stacking, hydrogen bonding, van der Waals forces and acid–base interactions [230]. Alkylphenol-aldehyde resin oligomers are a class of polymeric asphaltene inhibitor that have been investigated the most and are commonly used in the oil industry [169]. The efficiency of alkylphenol formaldehyde resin is attributed to the fact that the polar -OH group of the phenolic part of the resin forms hydrogen bonds with asphaltene molecules and the long alkyl radical of the resin provides good solubility in the hydrocarbon part [303]. Ovalles et al. [169] are focused on the synthesis and characterization of nonylphenol formaldehyde resins (NPFR) for preventing asphaltene precipitation in vacuum residue and hydroprocessed petroleum samples. It was found that the activity of NPFR as an asphaltene dispersant depended not only on the type of sample (asphaltenes, unadulterated or processed) but also on the temperature, molecular weight and concentration of the resin [169].

Polyisobutylene succinimides are reported to be particularly effective in reducing the precipitation of asphaltenes with high solubilizing ability and stabilizing effect [304]. The mechanism of action is explained by the adsorption of the polar part of their molecule on the surface of the asphaltenes and the formation of a coat with directed hydrocarbon radicals in the oil volume [304]. This coat prevents the coagulation of the particles and their contact with each other due to the emergence of electrostatic forces of repulsion between their charges of the same name [304]. Chávez-Miyauchi [200] synthesized four aromatic polyisobutylene succinimides and tested them (conc. 0.0–0.2 g/L) as asphaltene-dispersing viscosity reducers for heavy and extra-heavy crude oils. Two of the tested succinimides contain hydroxyl and boronic acid functional groups, respectively [200]. Aromatic succinimides containing these functional groups showed much higher efficiency as dispersants due to the action of these functional groups as hydrogen bond donors; thus, there are two types of interactions between succinimide and asphaltene: hydrogen bonding and π − π stacking. In the absence of hydroxyl and boronic acid functionality, aromatic succinimides interact with asphaltenes only through π − π stacking [200].

Zhu et al. [171] demonstrated that the effectiveness of a polymer depends on its molecular weight and the structure of the alkyl chain. Authors showed that a molecular weight of 3500 Da is the best for poly(styrene-co-octadecyl maleimide) (SNODMI), while higher or lower molecular weight has an adverse effect on its performance [171]. As the content of octadecyl branches of SNODMI increases, the stabilizing performance improves [171]. Abdullah and Al-Lohedan [278] showed that tannic acid esters’ dispersal abilities increased with an increasing number of alkyl chains. Cheng et al. [170] synthesized maleic anhydride-co-octadecene copolymers with different aromatic pendants (N-(3-aminopropyl)imidazole, aniline, 2-aminopyridine and 4-aminopyridine) and found that the precipitation onset point of asphaltenes increased the most with the copolymer containing 2-aminopyridine (50 ppm), while the copolymer containing N-(3-aminopropyl)imidazole showed the worst inhibition efficiency [170]. It is assumed that the pyridine group has a stronger adsorption capacity than the imidazole group. It has also been found that polymer concentration and grafting ratio are beneficial to inhibit the asphaltene precipitation [170].

Palermo and Lucas [172] investigated the performance of copolymers based on styrene-methacrylate and styrene-cinnamate (concentration 0.01, 0.025, 0.05 and 0.1 wt.%/v), with different degrees of sulfonation, as asphaltene stabilizers. The authors concluded that the behavior of the copolymer is related to the content of hydrocarbon chains and the molar mass [172]. Sulfonation of the copolymers increases their stabilizing effect, but an excess of sulfone groups leads to asphaltene flocculation [172].

In recent years, the interest of scientists has also been directed to the study of ionic liquids (ILs) as asphaltene dispersants [173,174,176,196,202,236,262]. Ionic liquids are molten salts (melting at or below 100 °C) consisting of organic cations associated with organic or inorganic anions [305,306]. These non-flammable and eco-friendly chemicals have low vapor pressure and are stable at elevated temperatures [67]. The anion and cation variety allows for changes of the ILs’ properties according to a specific application [306]. Hu et al. [189] reported that ILs that are based on an anion with a high negative charge density and a cation that has a sufficiently low positive charge density can effectively inhibit the precipitation of asphaltenes. It is suggested that the interaction between asphaltenes and ionic liquids occurs through π − π interactions between the cation and asphaltene and through hydrogen bonds [236]. Boukherissa et al. [173] demonstrated that ionic liquids (1-propyl boronic acid-3-alkylimidazolium bromides and 1-propenyl-3-alkylimidazolium bromides) may be successfully used to control the size of precipitated asphaltene aggregates. The boronic acid moiety, substituted in the lateral chain of an ionic liquid, bonds with asphaltenes and induces a significant reduction in the size of aggregates in flocculating solution. The smallest aggregates with a size of about 20 nm were obtained with 1-propyl boronic acid-3-hexadecylimidazolium bromide. This class of dispersants acts directly at the molecular level. A minimum length of eight carbons was necessary to obtain sterical stabilization of IL-asphaltene complexes. El-hoshoudy et al. [174] synthesized three ILs by mixing iron (III) chloride with different halogenated alkyl imidazolium salts. The experimental results show that 1-butyl-3-hexadecyl-1H-imidazol-3-ium tetrachloroferrate (III) [(-C4H9-IL) FeCl4⁻] is the most effective dispersant, owing to the increased length of the attached carbon chain. Ghanem et al. [175] evaluated a series of alkyl imidazolium-based acidic ionic liquids as asphaltene dispersants (1H-imidazol-3-ium 4-dodecylbenzenesulfunate (IL-0); 1-butyl-1H-imidazol-3-ium 4-dodecylbenzenesulfunate (IL-4); 1-decyl-1H-imidazol-3-ium 4-dodecylbenzenesulfunate (IL-10); and 1-hexadecyl-1H-imidazol-3-ium 4-dodecylbenzenesulfunate (IL-16)). The experimental studies indicate that IL-10 and IL-16 are the most effective dispersants, in relation to the long alkyl chain and the surface activity. Baghersaei et al. [176] investigated the interaction of ten commercial quaternary ammonium and phosphonium salts with asphaltene molecules. The results showed that the interaction of these quaternary salts is strongly influenced by the hydrophobic nature of the salt, and salts with more hydrophobic cations and anions, such as tetrabutylammonium iodide, tetrabutylammonium hexafluorophosphate and cetyltrimethylammonium bromide can better disperse asphaltene aggregations. The results also revealed that the role of anions such as iodide and hexafluorophosphate in the dispersion of asphaltene aggregates is more important than that of cations.

Emerging technologies, which garner attention related to the utilization of nanotechnology for mitigating asphaltene deposition as nanomaterials, offer exceptionally large and functional surface areas that exhibit an appropriate surface activity and selectivity toward asphaltene molecules [42,307,308,309,310]. Nanoparticles (NP) are defined as materials that possess structural dimensions within the range of 1–100 nm [311,312]. The high surface area enables nanoparticles to adsorb or react with a larger number of molecules, suggesting high effectiveness in mitigating asphaltene deposition. Farooq et al. [42] indicated the main factors (properties of metal oxides, asphaltene properties, external factors) influencing the adsorption of asphaltenes on nanoparticles. Various nanoparticles have been used for the adsorption of asphaltene molecules and the inhibition of their precipitation. Nassar et al. [182] investigated asphaltene adsorption on six different metal oxide nanoparticles, namely Fe₃O₄, Co₃O₄, TiO₂, MgO, CaO and NiO. The results obtained showed that the extent of adsorption and the quality of adsorption are not always related. These differences can be attributed to a different degree of interaction between the metal oxide surface and asphaltenes. The authors suggest that asphaltenes are adsorbed on the nanoparticles through polar interactions, mainly acid–base ones. Other authors suggest that asphaltenes are adsorbed onto the nanoparticles via physisorption and/or chemisorption [313,314,315]. Hosseinpour et al. [181] investigated the influence of the surface acidity and basicity of nanoparticles on the asphaltene adsorption capacity. In their study, three groups of NPs were considered: acidic (WO₃, NiO), amphoteric (Fe₃O₄, ZrO₂), and basic (MgO, CaCO₃) [181]. Among the studied NPs, NiO exhibits the highest capacity and affinity for asphaltene adsorption since it has the highest density of surface acid sites and net positive charge. Experimental results also show that basic asphaltenes are susceptible to adsorption, mostly onto acidic metal oxides’ surfaces [181]. The authors explain asphaltene adsorption onto the metal oxides by acid−base interactions and electrostatic attractions [181], as stated by Nassar et al. [182]. Shojaati et al. [185] investigated the effect of Fe₃O₄, NiO and Al₂O₃ nanoparticles on the onset point of asphaltene precipitation. The three types of metal oxide nanoparticles that they studied were effective in suppressing the onset and reducing the amount of asphaltenes precipitated. The best results were obtained for γ-Al₂O₃ nanoparticles with 0.1 wt.%. The authors concluded that the metal oxide nanoparticles with more Bronsted acid sites in their structures were more capable of having a polar interaction with asphaltenes, and consequently could suppress the precipitation process [185].

Due to their high surface area, nanoparticles are prone to self-aggregation. To overcome this, coated nanoparticles are applied. The coating structure can strongly affect the amount of asphaltene adsorption [184]. The functional groups of the coatings influence the adsorption capacity of the nanoparticles [184]. Wang et al. [259] investigated Co₃O₄ nanoparticles modified by SiO₂ film. The addition of SiO₂ film to the surface of Co₃O₄ nanoparticles significantly increases their specific surface area from 20.87 m²/g to 29.25 m²/g and inhibits their self-aggregation. Setoodeh et al. [184] investigated coated Fe₃O₄ nanoparticles as inhibitors of asphaltene precipitation. Graphene oxide–Fe₃O₄ nanoparticles showed the highest adsorption efficiency, followed by chitosan–Fe₃O₄, Fe₃O₄ and SiO₂-Fe₃O₄ nanoparticles. The results of the research by Setoodeh et al. [184] show that the amount of adsorption of asphaltenes strongly depends on the type of coatings applied. Bagherpour et al. [280] prepared two types of carboxylate–alumoxane nanoparticles (functionalized boehmite by methoxyacetic acid and functionalized pseudo-boehmite by methoxyacetic acid) for asphaltene adsorption. The results of their research show that both types of functionalized nanoparticles tend to adsorb asphaltene molecules and have a positive effect on delaying the precipitation of asphaltenes due to molecular interactions between the surface of the nanoparticles and asphaltene molecules. Lopez et al. [316] synthesized green nanocomposites based on the interaction of cardanol and SiO₂ nanoparticles. It was found that the adsorption of n-C₇ asphaltenes onto the surface of the nanocomposite leads to a decrease in the available asphaltenes in the bulk solution and to reductions in the aggregate size of up to 58.5%. Mohammadi et al. [279] investigated the inhibition of asphaltene precipitation by TiO₂, SiO₂ and ZrO₂ nanofluids. It was found that TiO₂-particles can effectively improve the stability of asphaltenes in pH = 4 medium (under acidic conditions) by forming hydrogen bonds, while the materials used in this experiment under alkaline conditions were unable to form hydrogen bonds; therefore, they were unable to prevent the precipitation of asphaltenes. Nanoparticles have been successfully applied in six oilfields in Colombia, and it has been shown that the asphaltene content in the produced oil of the well remained stable, which led to increased oil production [295,317,318].

Commercial inhibitors are usually a mixture of several chemicals where the name and the composition of the mixture are not disclosed [164]. Balestrin et al. [186] reported the use of two commercial inhibitors, CA1 and CA2. The CA1 inhibitor is a polymer solution in an aromatic medium containing an amino compound as the main active component, while inhibitor CA2 contains a phenolic polymer as the main active compound. Horeh et al. [65] investigated the effectiveness of a commercial oil-based asphaltene inhibitor, denominated BZSS-012. The active phase of the inhibitor is a mixture of functionalized polymers with an average molecular weight of 1180 Da. Other studies on the effectiveness of commercial inhibitors have also been reported [3,167,237].

Figure 4 summarizes the proportion of types of chemical additives used as asphaltene inhibitors and dispersants in the published literature.

However, it is hard to draw straightforward conclusions on the overall behavior of chemical additives since researchers estimate the additive performance via utilizing different approaches, techniques, methods and conditions for additive selection, research and development [42]. Enayat et al. [68] reviewed commonly used techniques to evaluate the performance of asphaltene inhibitors. The authors [68] concluded that most of the current additive evaluation techniques are based on their ability to disperse asphaltenes in the oil system. However, the authors emphasize that additives with good dispersion efficiency do not necessarily reduce asphaltene deposition. It is also pointed out that the good performance of an additive evaluated by the reviewed methods is not well corroborated by field success.

On the other hand, chemical additives are applied to asphaltene fractions, whose composition and properties depend on the method used in extracting and separating them. For example, n-heptane-precipitated asphaltenes have a higher degree of aromaticity (lower hydrogen-to-carbon atomic ratio) and higher proportions of heteroatoms than n-pentane asphaltenes. On the other hand, pressure-precipitated asphaltenes have fewer double bonds and more sulfur species than the alkane-separated asphaltenes. The asphaltenes precipitated in the lab are different from those deposited in the field [15]. Asphaltenes are also known to be different according to their source. All of this indicates that the selection of successful additives to slow asphaltene deposition in commercial oil and gas facilities is an extremely complex and intricate task. It seems that it cannot be solved without proper laboratory equipment that best mimics the commercial operation conditions and without a good knowledge of oil and additive chemistry [15].

The great specificity of the action of different chemical additives in the treatment of different oils and the use of the onset of asphaltene precipitation as a standardized metric of action is illustrated in Figure 5.

It is evident that while toluene demonstrated the lowest inhibition efficiency when treating crude oil (APO = 12.2% n-C₇) (Figure 5c) [203], the same additive was the best performer with the crude oil 1 (APO n-C₇ at 28.5 mL) (Figure 5a) [319]. The dodecyl benzene sulfonic acid (DDBSA) was 10 times more efficient than the toluene when treating the crude oil (APO = 12.2% n-C₇) (Figure 5c) [203] and 10 times less efficient than the toluene when treating crude oil 2 (APO CO₂ at 6.8 mL) (Figure 5b) [319]. All of this indicates that asphaltene precipitation behavior depends on both the type of crude oil and the nature of the additive.

The results reported by Wiehe and Jermansen [63], as exemplified in Figure 4, suggest that the performance of asphaltene dispersants is related to the solubility (peptizability) of asphaltenes, expressed by the toluene equivalence. Unfortunately, that was the only reference found in this review that allows for the derivation of a quantitative relation of an asphaltene characteristic to the performance of asphaltene dispersants. As observed from the data in Table 1, the solubility of asphaltenes expressed by Sa-value can vary considerably and a search for a relation similar to that when distinct asphaltenes are treated with various additives may help to find some quantitative dependence of efficiency of an additive on its characteristics and asphaltene solubility properties. Moreover, the peptizability measured by a dual solvent titration method as shown by Rogel et al. [320] affects deposit formation and fouling.

4. Conclusions

This review revealed that asphaltene precipitation is a phase transition process in which in a fraction of one-second particles of diameters up to 400 μm from aggregated molecules with diameters of ca. 5−20 nm are formed. The details of nanoaggregation are suggested to have little impact on phase behavior. In contrast to the strong forces responsible for asphaltene nanoaggregate formation, asphaltene phase transition is deemed to be driven by van der Waals dispersion forces among the aggregates. It was found that asphaltene polydispersity plays a major role in determining asphaltene precipitation and deposition formation, with the heaviest asphaltene fraction having a significant impact on deposit formation. Solvents solubilize asphaltenes by acting between asphaltene molecules and replacing the asphaltene–asphaltene bond with asphaltene–solvent π − π interactions. The efficiency of asphaltene precipitate dissolution also depends on the origin of the petroleum fluid. The role of dispersants in the petroleum industry is to increase the spatial repulsion between asphaltene aggregates by reducing their overall size and by changing the intermolecular interactions between asphaltene aggregates molecules.

The current review on chemical additives used for asphaltene deposition control led to the conclusion that additives can have different performances, i.e., act as inhibitors and dispersants, enhance aggregation, or can have no effect. Overall, the variables that affect additives’ performance can be summarized as follows: (i) crude oil properties, (ii) asphaltene characteristics (structure, composition and solubility), (iii) additive structure and functionalities, (iv) additive amount and (v) additive surface properties. The successful application of a chemical treatment program to decrease the asphaltene deposit formation rate in commercial oil facilities is highly dependent not only on the additive formulation, but also on the laboratory tests and equipment used to select the most appropriate chemical for the specific petroleum fluid and to monitor its performance during its use in the field.

5. Knowledge Gaps and Future Perspectives

The summarized investigations on asphaltene deposit formation mitigation by using chemical additives reveal the presence of great specificity for each individual case, which embarrasses to a great extent any generalization. Based on the relationship found between asphaltene dispersant performance and oil solubility characteristics, one may suggest that in the future research in this field characterization using a dual solvent titration method could provide a basis for the determination of some quantitative relationships between the different oils and the distinct chemicals employed as asphaltene dispersants. The lack of clear, standardized criteria for the evaluation of chemical additive performance makes it difficult to fully assess the true potential of the applied additives. Furthermore, most of the studies are laboratory-scale evaluations, which implies that the selection or the development of sophisticated laboratory methodology that best mimics the specific commercial facility operation in which the chemical additives are intended to be applied would be the main direction of future research in this field.