Received: 28 December 2022, Accepted: 23 February 2023, Published: 31 March 2023, Publisher: UTP Press, Creative Commons: CC BY 4.0
REVIEW ON POUR POINT DEPRESSANTS FOR WAXY CRUDE OIL ISSUES
Nurul Nizzam Zamri, Hazlina Husin*
Department of Petroleum Engineering, Universiti Teknologi PETRONAS, Malaysia
*Email: [email protected] ABSTRACT
Crude oil contains heavy paraffin molecules that cause the deposition of solid wax. Wax deposition is a critical problem in the petroleum industry, which results in various production problems. Pour Point Depressant (PPD) is a chemical additive that prevents wax deposition. PPD is employed to lower the pour point temperature allowing crude oil to flow at a significantly lower temperature. Several types of PPD include polymer, nano-composite and a natural organic compound. This review paper provides insights regarding different types of chemical additives to solve waxy crude oil issues. The effectiveness of these chemical additives is compared based on the changes in pour point value in degrees Celsius, as it is one of the physical properties of crude oil. Ultimately, this paper provides information regarding the performance of different PPDs in reducing pour point temperature to solve wax deposition issues.
Keywords: Flow assurance, paraffin wax, wax deposition, mechanism of wax deposition, chemical additives, pour point depressant
INTRODUCTION
Crude oil is a complex mixture of different hydrocarbons. The difference in the composition made the characteristics of crude oil vary depending on its origin [1]-[4]. Crude oil comprises saturates, asphaltenes, aromatic and resins. The saturates, a hydrocarbon with over 20 carbon chains, are known as paraffin wax (Figure 1). It is the main saturates that contribute to the formation and deposition of solid wax. Paraffinic wax has been a huge historical challenge for operators in the oil industry [5]-[8]. This challenge arises when the equilibrium is disturbed during the production or transportation of crude oil, which leads to alteration in pressure, temperature, and composition of production fluid. The wax deposition is a major flow assurance problem affecting the petroleum industry [9]-[10]. Flow assurance ensures an economical flow of hydrocarbon stream transport from the reservoir to its final storage destination, comprising hydrates,
wax deposition, asphaltenes, slugging, naphthenates, scales, corrosion, erosion, and erosion emulsion that involves the crude oil [6].
The crude oil temperature will gradually decrease along the production line in the crude oil production system, initiating the wax to precipitate from the oil. This situation will cause flow assurance issues, especially in restarting and shutdown of pipelines. One of the major flow assurance challenges is wax and paraffin and deposition, which occur in the inner walls of production and transportation pipelines and on the surface and downhole of the production equipment.
It introduces rheological problems which interfere with the oil production line and transportation and cause handling problems [5]-[6],[11]-[13]. In some cases, wax deposits have caused formation plugging during stimulation treatments [7],[13]-[14]. The wax deposition
process can eventually significantly reduce the internal diameter of the tubular, casing, perforations, pump strings, rods, and pipelines, in the worst case, cause complete blockage to the flow lines, leading to tremendous profit loss [5],[11]. The cost of production and transportation was increased due to handling the wax deposition problem. Furthermore, it creates more issues in regions where the surrounding temperature is seasonally very low [13].
This review paper covers three types of chemical additives to solve the waxy crude oil issue. The effectiveness of these chemical additives is compared based on the changes in pour point value in degrees Celsius, as it is one of the physical properties of crude oil. The chemical additives include polymer, nano- composite and a natural organic compound. Thus, this paper is expected to serve as a foundation for future research on developing more effective and efficient PPD.
WAXY CRUDE OIL
Crude oil, which contains a high wax content, will exhibit a high wax apparent temperature (WAT) at which the wax molecules will begin to precipitate if the temperature of the crude oil falls below this temperature [15]. As the temperature of crude oil decreases, the wax crystal starts to overlap, interlock and grow to create a crystalline net in the crude oil, which traps the hydrocarbon molecules hindering the flowability of oil. The temperature at which the crude oil cannot flow is known as the pour point [16]-[19].
The pour point is defined as the temperature at which the waxy crude oil loses its ability to flow [11]. Crude oil with a high proportion of long hydrocarbon chain usually has a high pour point temperature, making it more vulnerable to wax deposition issues [19]. The wax deposition in crude oil increases fluid viscosity, increasing pressure in production lines. Thus, additional energy is required to increase the pressure to mitigate the wax deposition problem [6].
Figure 1 Paraffin wax general structure [20]
Various treatment methods are developed to handle the wax deposition, which includes physical treatment (electrical, electromagnetic, magnetic, vibration, acoustic), mechanical (pigging, scraping), thermal (heating), chemical (dilution, inhibitor, surfactant), and biological treatments. The method is selected based on the nature of the wax, location, and cost.
[5],[18],[21]-[22]. Heating transportation technology is one of the effective methods. However, it consumes a large amount of energy, increasing the cost. In contrast, chemical additives are usually time-saving and cost- effective [15]. The utilisation of effective chemical additives has the capacity to produce a significant improvement in flow assurances compared to removal procedures through mechanical or thermal [14]. Vinyl acetate copolymer, acrylate copolymer and their derivatives 1'2 are the main additives which are utilised to enhance the flow assurance of a very waxy crude oil at significantly low temperatures [23].
CHEMICAL ADDITIVES
Chemical additives such as wax inhibitors are incorporated to control the wax deposition process and improve liquid flow. The example approaches the chemical wax deposition problem are the utilisation of wax inhibitors (wax crystal modifier), wax dispersant and PPDs flow improvers (FIs) [5],[13],[24]. PPDs flow improvers (FIs) as chemical additives were able to reduce the pour point of the crude oil, thus allowing the crude oil to flow even at a temperature below its natural WAT [13].
Wax inhibitors could ensure low fluid viscosity and prevent gelling. Wax inhibitors chemically modify the wax structure to reduce the possibility of forming a crystalline net. The presence of an inhibitor inside the crude oil hinders the agglomeration of wax crystals by preventing the formation of the crystalline net and the formation of tiny waxy crystals [19]. The wax inhibitor does not terminate the wax deposition completely;
instead, it decelerates the waxing process and allows the accumulated wax to flow into the production line [5].
Polymer inhibitors also worked as additives in flow assurance. Polymer inhibitors were constructed by having polar and non-polar molecules, where the polar molecules will hinder the wax crystallisation while the non-polar part joined the wax molecules and inhibitor to each other. The wax accumulation was restricted due to the presence of the polymer inhibitor [6]. The different type of polymer additives results in different
efficiency for different oils. This is due to the significant differences in the mechanism of the additives in crude oil and the different composition of the crude oils.
Based on various additive testing, comparing the efficiency of the polymers in different oils often results in inconsistent conclusions [22]. It is impractical to testify that one type of chemical can be used for all types of waxy crude oil due to the inconsistent variation of the crude oil composition from different sources. The additives will undergo in-depth laboratory tests and field experiments before being applied commercially [12].
Figure 2 Paraffin wax general structure Schematic presentation of wax inhibitor
influence on the wax crystal [25]
PPD are one of the most commonly used for crude oil pre-treatment to ensure the flowability of waxy crude oil at low temperatures, specifically below the crude oil Wax Appearance Temperature (WAT) [13],[18],[26].
PPDs are organic polymers with high molecular weight, which can modify the wax crystals and improve the rheological properties of waxy crude oil.
Homopolymers and copolymers of different monomers are an example of conventional PPDs [17],[27].
POUR POINT DEPRESSANT
PPDs is a wax crystal modifier through the co- crystallisation process to inhibit the crystallisation of wax molecules. The process leaves the polar moiety end tail to create steric hindrance, obstructing wax crystallisation [5]. The presence of PPDs in crude oil modifies the growth process and nucleation of the wax crystal, thus impairing the crystalline network structure formation [7],[11]. PPDs made it possible for crude oil to be able to flow at a lower temperature, hence avoiding flow assurance problems such as plugged
formation, wax deposition and asphaltene formation [13]. Various kinds of polymers are used as PPDs to control wax crystallisation [13]. Specific structures of acrylate/methacrylate polymers were recognised as effective pour point depressants for certain specific crude oil compositions [12].
The example types of commercially used PPDs are polymethacrylate (PMA) [28], poly alkyl acrylates [22], ethylene-vinyl acetate (EVA) [29], maleic anhydride polymers [30], poly alpha olefin (PAO) [31], nano- composites or nanohybrid PPDs [32] and also several bio-based additives [33]-[39] which produces significant results in the upstream industry.
PMA is extensively applied as PPD for crude oil as it has a long chain that matches the normal alkanes in the oil composition [17],[40] as shown in Figure 3.
Polymethacrylates are known as one of the main additives used in crude oil PPD. However, they produce lower efficiency than poly alkyl acrylates [41]. Alkyl acrylates with longer carbon chains (>C16) are widely used as PPDs for crude oil [22],[41]-[42].
Figure 3 General structure of polymethacrylate polymer (Source: Tetz and Jorgensen [43])
Poly (ethylene-co-vinyl acetate) (EVA) copolymers (Figure 4) produce a good wax inhibitor efficiency.
Machado et al. [27] found that pour point reduction with more than 26°C was observed by adding as low as 50 ppm of EVA polymer with 20% vinyl acetate content.
However, the copolymer depressant efficiency starts to drop if the concentration is increased due to the precipitation of the copolymer itself [12],[27].
Wax Crystal Agglomerating
Cold Surface
Wax Inhibitor
Depositing to Cold Surface
Interfering with Agglomeration Interfering with Deposition
Figure 4 General structure of Poly (ethylene-co-vinyl acetate) (EVA) copolymers [44]
In addition, polymers with comb-shaped like maleic anhydride copolymers are also reported as good PPD [45]. Soliman et al. [45] developed two different maleic anhydride copolymers, namely poly n-alkyl linoleate- co-succinic anhydride) as shown in Figure 5. Moreover, its esterified form compared to its performance as PPD on Egyptian waxy crude oil. Both copolymers can reduce the pour point by 18°C and 21°C, respectively.
The performance of these PPDs is remarkable since most polymeric additives are considered efficient if they can reduce the pour point up to 9ºC [45].
Figure 5 General structure of poly (alkyl esters of maleinized linoleic acid) [45]
Polymer or inorganic nano-composite, which comprises inorganic nanoparticles, has also become the centre of attention for scientific research [46]. The development of nano-composite or nanohybrid PPDs as chemical additives are growing and has shown significant improvements in terms of efficiency [47]. Nano- hybrid PPDs, which have good dispersion ability, high adsorption affinity and a large surface area, has opened a new way to enhance the flowability of hydrocarbon [32]. According to Wang et al. [48], nano-hybrid PPDs performed better as a pour point depressant than conventional PPDs. Li et al. [15]
developed a nano-hybrid PPD which combined nano- montmorillonite (MMT) and Ethylene Vinyl Acetate (EVA) copolymer through doping (Figure 6). The nano-hybrid copolymer can produce an additional reduction of pour point for the model waxy crude oil up to 12°C compared to the performance of pure EVA copolymer. However, it is costly to produce a higher
doping quantity of the nano-hybrid/nano-composite PPDs, and it will be not economical in certain cases [47].
0 5 10 15 20 25 30 35 40
0.00 0.05 0.10 0.50 0.20
Figure 6 Nano-hybrid copolymer performance comparison to Nano-MMT and EVA copolymer [15]
In recent years, various types of PPDs have been developed based on natural products such as canola oil [33], castor seed oil [34], jatropha seed oil [36], and rubber seed oil [35]. These bio-additives are developed to prevent the produced crude oil from being contaminated and prevent any possible effects towards the environment if conventional polymer additives are utilised [36]-[38]. They can also be combined with other chemicals to enhance the efficiency of the developed bio-additives [49]. Akinyemi et al. [34] developed a PPD consisting of a blend of canola oil and jatropha seed oil and discovered that it significantly impacts the pour point and wax deposition tendency. The bio-additive achieved a minimum of 79.1% Paraffin Inhibition Efficiency (PIE) [34]. In another study led by Chen et al. [33] and Akinyemi et al. [35], using jatropha seed oil in flow improvers achieved a pour point reduction of up to 17°C. The performance of these bio-additives is similar to that of conventional wax inhibitors, indicating that their application is relevant in the petroleum industry. Table 1 shows the effectiveness of three types of PPDs in reducing the pour point temperature. The highest pour point reduction for each type is 26°C for polymer [27], 32°C for nano-composite [15] and 30°C for natural organic compound [37].
Table 1 Performance of different types of PPDs in reducing pour point temperature
PPD Type
PPD Name Crude Oil Sample
ΔPP (°C)
Ref
Polymer
Octadecyl methacrylate
Brazilian
diesel oil 22 [16]
Poly (alkyl linoleate- co-tetra-esters)
Egyptian waxy c rude oil
18 [45]
EVA Albacora field,
Brazil 26 [27]
Tri-trithanolamine di Oleate
Western oil field of India
17 [50]
Zinc alkyl benzene sulfonate
Chunguang oil field,
China
16 [51]
Nano- composite
EVA (33)/nano- MMT
Modeloil which contains 25 wt% of paraffin
32 [15]
Poly (octadecyl acrylate-co-vinyl
dodecanoic-co- VTOP-BT)
Qarun’s waxy
crude, Egypt 30 [19]
PPDR-1%GO
Waxy crude from a western Indian
oil field
30 [47]
Poly (methyl methacrylate)- Graphene Oxide
Waxy crude oil, western Indian oil
fields
23 [52]
Poly (ethylene- vinyl alcohol)- graphene oxide
Waxy crude oil, western Indian oil
fields
12 [53]
Natural Organic Compound
Cashew Nut Shell Liquid
Western onshore Cambay basin,
India
30 [37]
Jatropha Seed Oil
& Castor Seed Oil
Western onshore Cambay basin,
India
17 [35]
Derivatised natural cashew nut shell
liquid
Niger-Delta waxy
crude oil 18 [38]
Borate ester of natural cashew nut
shell liquid
Niger-Delta waxy
crude oil 24 [54]
Shikakai fruit (Senegalia rugata)
extract
Indian Field waxy
crude oil 12 [55]
ACKNOWLEDGEMENT
The authors acknowledge the financial support throughout the project from YUTP grant 015LC0-383.
CONCLUSION
Paraffinic wax issues are inevitable in the petroleum industry. Today, no technique could completely remove a wax molecule from crude oil to prevent wax deposition. The current mitigation process would only hinder paraffin crystallisation. Various research has been conducted, which include nanoparticles and natural-based additives to ensure effectiveness in terms of cost and performance. Thus, this review provides insights regarding the performance of polymer, nano-composite and natural organic compound PPDs in reducing pour point temperature to solve wax deposition issues.
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