Review of Solvents Based on Biomass for Mitigation of Wax Paraffin in Indonesian Oilfield

: This paper presents a review of the expectations and challenges of using biomass in the prevention and slowing of paraffin wax deposition that takes place during the crude oil production process. The inhibition of the deposition process involves the use of solvents from biomass that are generally available around the crude oil production field. The processes used to scale down the precipitation of wax include mixing crude oil with the manufacturer ʹ s solvent composed of toluene and xylene. The goal is to assess solvents sourced from biomass that are capable to slow down the wax deposition process. Wax appearance temperature is an important characteristic to evaluate the possible wax precipitation of a given fluid. Wax precipitation can be reduced by using some chemical additives, often called the pour point depressant. This additive is expected to be produced from local biomass which can compete with solvents currently produced on the market.


Introduction
The decline in crude oil production is a persistent challenge in the oil industry. The rate of production decreases over time, often caused by damage located in both the rock formation and the surface facilities. Close to 85% of those damages are resultant from oil wax congealing at the wellbore, in production string, and all through the surface pipelines [1]. The majority of crude oil and its products contain large amounts of oil wax called paraffin. Paraffin is a mixture of hydrocarbons formed from linear or normal chains, which are mainly composed of 20 to 40 carbon atoms [2]. Paraffin wax deposition causes equipment failure, upstream and downstream flow congestion, and loss of production, transportation capacity, and storage [3]. Due to the deposition of paraffin wax, plenty of wells are shut-in, countless pipelines blocked, transport vessels taken out of services, and refinery equipment closed at certain times, all resulting in a loss of income [1]. Once formed, the paraffin in which the liquid is transported is deposited on the wall of the pipes.
Paraffin deposition in hydrocarbon production systems is a major challenge for the development of oil and gas fields in cold environments or in deep waters [4][5][6] where nucleation and adhesion of the wax to the wall of the pipe occur [7]. Paraffin deposition in oil production is a critical problem that is difficult to control in terms of productivity and production operating costs. It begins from the reservoir then progresses to surface facilities [4]. A common method used to mitigate the problem is increasing the temperature of deposit wax. The problem must be handled as early as possible to prevent the decrease in oil production due to solids blocking the flow in the pipe. The primary target of heating the wax is to reduce the viscosity of the fluids. This can also be achieved by the use of some additives. Solvents generated from biomass can act as wax inhibitors.
Research on the application of biomass has mainly focused on the pharmaceutical industry, fuel reserves, and others [8]. However, there has been little discussion about the application of biomass to the crude oil in below surface conditions or near the wellbore. This study systematically reviews the expectations and challenges of using biomass in the prevention and slowing of paraffin wax deposition that occurs during the crude oil production process. Firstly, the review discusses the availability of raw materials in which the types and the area distribution of each material are shown. Secondly, this paper assesses the bioethanol production process consisting of three steps: pretreatment, hydrolysis, and fermentation. Thirdly, this paper examines the problems that occur due to the appearance of paraffin wax and how the solvent is frequently used in the oil industry. Biomass is a renewable energy source that has received attention; various types of biomass have been used such as coffee grounds [9], tea waste, and palm shells [10,11]. Bio-solvents are solvents that can be produced easily and economically where the production process consists of several stages: hydrolysis, fermentation, and the purification process through distillation [12][13][14][15][16][17][18]. One type of biosolvent that can be produced from biomass is bioethanol [19][20][21]. The raw materials for bioethanol can be sourced from: (1) biomass starch such as corn, cassava, and sorghum [12]; (2) sugar sources, such as cane crops, sugar cane juice, coconut sap, and gum [22,23]; and (3) source of cellulose including rice husks, bagasse, and corncobs [24,25]. In general, biomass basically consists of cellulose, hemicellulose, and lignin [19,26,27] which the availability of its raw materials is a great potential to be utilized.

Availability of Raw Materials
The use of waste in waste to energy (WTE) technology involves a treatment process to recover energy in the form of heat, electricity, or transportation fuel from waste sources [28]. WTE is used to create various types of products or semi-finished products from various types of waste such as palm oil waste [29,30], coffee grounds [9], and tea pulp, and even the use of nonorganic waste such as plastic waste [10].
Lignocellulosic biomass, such as agricultural residues (corn stovers and wheat straw), wood, and energy crops, is an attractive material for bioethanol fuel production since it is the most abundant renewable resource on earth [31,32]. Lignocellulosic biomass could produce up to 442 billion liters per year of bioethanol [33]. Thus, the total potential bioethanol production from crop residues and waste is 491 billion liters per year, about 16 times higher than the current world bioethanol production. Rice straw is one of the most abundant lignocellulose waste materials in the world, with an annual production of about 731 million tons, which is distributed in Africa (20.9 million tons), Asia (667.6 million tons), Europe (3.9 million tons), the Americas (37.2 million tons), and Oceania (1.7 million tons) [10]. Food waste from the rest of the food production is used as animal feed. Food waste is composed of complex compounds lignin, hemicellulose, and cellulose (lignocellulose), and the compound has the potential to be biologically converted into other compounds so that they can be used as a solvent [34].

Bioethanol Production Process
Bioethanol is made from lignocellulosic biomass found in waste, where this raw material is available in many environments and is expected to increase the value of the waste [35]. Bioethanol is produced through a process of pretreatment, hydrolysis, and fermentation [36][37][38][39]. The bioethanol production process is shown in Figure 1.

Pretreatment
Pretreatment is an important step to break down the carbohydrates in which the process aim is to reduce the size of the materials used [31,41] and unlock the cellulose by releasing the lignin in the cellulose [42][43][44]. Commonly used methods for pretreatment include physicochemical methods (ozonolysis, steam treatment, wet oxidation, and ammonia fiber explosion), chemical methods (acid hydrolysis, alkaline hydrolysis, and organic solvent treatment) [41,[45][46][47], electrical methods (such as pulsed electric field treatment), biological methods (using bacteria or fungi producing ligninolytic enzymes), or a combination of these [32,48].
The hydrolysis yield is determined as the amount of extracted reducing sugars per total amount of theoretical glucose in the biomass according to the following equation [68,69]: For a total conversion, the glucose theoretically obtained is the same as the amount of cellulose in biomass waste. Total reducing sugars are obtained using the method as follows [70]: Extracted reducing sugar mg Concentration mg/mL x Volume Fermentation yield (Y) is expressed as the obtained ethanol after the distillation process per the amount of extracted reducing sugars: Fermentation efficiency corresponds to the ratio of the produced ethanol over the theoretically produced ethanol [71], as follows: The bioethanol yield can be obtained using the following equation [29]: Finally, the physical properties of the obtained residue after acid hydrolysis were tested. This analysis included moisture, calorific value, and elemental, inorganic compounds, and immediate analysis according to the standards [68].

Fermentation
The fermentation process consists of the conversion of glucose obtained from hydrolysis to bioethanol by microorganism media such as yeast, bacteria, and fungi [62,72,73]. Saccharomyces cerevisiae was used for fermentation assays [74][75][76]. This fermentation process lasts several days depending on the number of yeast cell culture starters used. The larger the number of starters, the faster the fermentation process. Besides producing ethanol, fermentation produces also other substances, including water. One measure of the quality of bioethanol is its water content. The lower the water content, the better the quality of the bioethanol [19,77].

Wax Problems and Use of Solvents in the Oil Industry
Most of the commercial wax inhibitors tested can reduce the deposition of paraffin with low molecular weight (≤C34), but have little effect on high molecular weight paraffinic wax (C35-C44) [82]. Wax deposition in oil production flow mainly consists of C7 paraffin. Paraffin deposition is controlled by temperature, which is why heat application is preferable to manage the paraffin deposition [83,84]. When the temperature in a system decreases, paraffin in the liquid phase starts precipitating out of the solution as a solid. Wax deposition forms on the wall of the pipe where the temperature gradient is at its highest [3,85]. Pressure-temperature relationship of wax appearance in both reservoir fluid and stock tank oil (STO) condition is shown by Figure 2. The tendency and deposition rate can be adequately predicted by calculating the rate of diffusion of wax molecules onto the wall with the following equation: (6) where m is the mass of deposit (kg), ρ is the density of wax (kg/m 3 ), Dm is the molecular diffusion constant (m 2 /s), A is the deposition area (m 2 ), C is the concentration of wax (%), and r is the radial position (m) [86].
The radial concentration gradient can be easily calculated if it is broken down into two components by applying the chain rule as shown in the following equation: where T is temperature. The concentration gradient can be calculated from the wax concentration predicted by the thermodynamic model for the temperature range [3,86].
Additional chemicals considerably affect the process of paraffin wax deposition, where measurements show that wax deposition is rather high for all crude oil [87]. No correlation exists between the temperature and levels of total wax of crude oil [4]. The deposition of paraffin or wax can be called a depositor of carbonaceous material, which is insoluble or dispersed by crude oil under normal conditions [88,89].
The effectiveness of the inhibitor must be tested for each application. Tests must be conducted under all possible operating conditions. Chemical inhibitors for prevention of wax include: (1) Thermodynamic wax inhibitor (TWI)-pressing cloud point, reducing viscosity and pour point, requires a high volume; (2) Depressant/pour point-modifies the wax crystal structure, reduces viscosity, and produces stress, but does not reduce the rate of wax deposition; (3) Dispersants/surfactants coat wax crystals to prevent wax growth and change the wetting characteristics to minimize wax adhesion to the pipe wall or other crystals [3,90]. Wax characteristics are outlined in Table 2.

Pour Point Depressants (PPDs)
Pour point depressants (PPDs) are a solution in heavy or extra-heavy crude oils where the main problem is flowability, especially from offshore to ground facilities [92,93]. A crystal candle modifier, also called a PPD, is a chemical that determines the solubility of wax, adsorption, and nucleation. The modification of wax crystallization may help depress the crude oilʹs pour point, viscosity, and yield stress appreciably [94].
Examining the effect any different PPDs on the viscosity, pouring point, and temperature of the candle selection proved that PPD substances are able to reduce the wax crystals, changing the growth characteristics and surface of the crystal at metal surfaces that are attached like pipe walls [96], as shown in Figure 3. Studies were able to reduce crude oil from 1000 cP to 10 cP at 10 °C-20 °C in all of these additives, which increased the flow of crude oil. The following effect on the viscosity of crude oil was successfully modeled by regulating lower melting temperatures for wax molecules in the range of C21 to C45 in the presence of additives that increase flow [95]. Several studies have been conducted in Mexico to test the rheological properties of light and heavy oils by measuring pouring depressant performance using a combination of vinyl acetate, styrene, and n-butyl acrylate, and viscosity was reduced at 25 °C and 50 °C [97]. In other studies, the viscosity in heavy crude oil dropped at 25 °C and in light crude oil at 313 K due to the depressant pour point [92]. Figure 4 shows the effect of concentration on the depressant pour point. The pour point can be reduced at 11 °C and a dose of 400 ppm. At doses above 400 ppm, no further reduction in pour point occurs, which causes it to be uneconomical [94].
The pour point of pure crude oil is calculated by the following equation: Pour point reduction (ΔP) = PPpure -PPadditive (8) where PPpure is the pour point of the pure crude oil and PPadditive is the pour point of the crude oil containing additives [98].

Rheological Studies
The depressant pour point mixed into crude oil at 400 ppm is depicted in Figure 5. At temperatures above 55 °C, the viscosity of crude oil decreases and remains constant at subsequent temperatures [99], meaning that at this temperature, the crude oil flow benefits because it can flow without depressant pour points [100,101]. When the temperature decreases, the viscosity of crude oil increases, thereby increasing the viscosity of the oil, which is then accompanied by deposition [102,103]. Then, at temperatures below 50 °C, the wax appearance temperature (WAT) is higher without the addition of pour point depressant [104]. The addition of pour point depressant was proven to reduce the viscosity of crude oil so that it can increase the flow of low-temperature crude oil [105].

Evaluation of Paraffin Inhibition Efficiency
WAT Determination (ASTM Standard D5773-04, 2005) WAT is also called the cloud point [7,106]. Lately, some researchers have shown the advantage of calculating wax disappearance temperature (WDT) compared to WAT. WDT is the temperature at which the last wax crystal melts. The WDT is thermodynamically more precise because crystallization requires some saturation [107]. The efficiency of paraffin inhibition (total PIE) for total wax deposition can be calculated as [88]: where Wn(C11-C44) is the total amount of paraffin deposits without inhibitors (mg) and Wi(C11-C44) is the amount of paraffin deposit with added inhibitors (mg) [4,82]. The effects of commercial wax inhibitors on ∆WAT and inhibitor efficiency in wax deposition is shown by Table 3 below.

Discussion
Solvent treatments of wax deposition are often the most successful remediation methods but are also the most costly. Therefore, solvent remediation methods are usually reserved for applications where hot oil or hot water methods have shown little success. When solvents contact the wax, the deposits are dissolved until the solvents are saturated. If they are not removed after saturation is reached, the waxes may precipitate, resulting in a situation more severe than prior to treatment [3].

Production of Solvents for the Oil Industry
The process of producing a solvent from biomass waste certainly has the potential for development, especially with the increasing amount of waste produced by people today. Very little of that waste can be recycled back into a valuable product, which is a complex problem for the industrial world that must be addressed. This is especially true in the oil industry, as the chemical treatment needed to overcome this problem is enabling the development of the world of industry by optimizing petroleum production [108].

Solvent Application in the Oil Industry
One important challenge in crude oil production and fuel handling is the risk of wax deposition. This deposition is of particular concern in the production of crude oil from deep-water fields where the liquid produced can cool to near the surrounding water temperature, which is 4 °C or less. The paraffin deposits that can occur in these long transportation routes can endanger these projects due to partial or even complete blockages of these pipes [82,89].
Paraffin hydrocarbon liquids, including crude oil and condensate, form a paraffin solid phase when the temperature drops below the liquid cloud point [4]. Cast points are indicators of temperature where oil will solidify into a gel. At the pour point, the fluid still flows under gravity; at the next lower measurement temperature (lower than 3 °C), the fluid has already formed a gel and does not flow. In the ambient temperature, where the fluid is at or below the temperature of the pour point, special pouring strategies are needed to maintain fluid flow capability. So, the ambient temperature condition can be analyzed in general so that the analysis is conservative. The extreme 100-year minimum seabed temperature is used along the pipeline route [3].

Conclusions
In this study, we examined the feasibility of bio-solvent application to the inhibition of the deposition of wax that forms in oil production facilities. Generally, we found that the bioethanol production process, used to create a solvent, shows potential, especially bio-solvents which are created from biomass in the surrounding environment. The ability of bioethanol to act as a suitable solvent to prevent the deposition of paraffin wax must be determined. This study has been one of the first attempts to thoroughly assess this wax problem. The process of assessment for a wax problem in an oilfield can be summarized as follows: obtain a good sample; determine the cloud point or WAT based on solid-liquid equilibrium; determine the rheology including the viscosity, pour point, and gel strength; the crude oil composition must be quantified, including standard oil composition; wax deposition rates must be identified, including flow loop, wax melting point, interfacial tension; and the use of wax inhibitors from biomass or organic waste should be considered. Using a wax paraffin solvent produced from biomass is an intriguing challenge that could be explored in further research.

TWI
Thermodynamic wax inhibitor WAT Wax appearance temperature WDT Wax disappearance temperature WTE Waste to energy PIE Paraffin inhibition efficiency