Marine fuel oil composition

Abstract

To provide a fuel oil composition comprising a sludge dispersant that not only suppresses agglomeration of potential sludge in marine fuel oil but also promotes redispersion of existing sludge and is less likely to precipitate in the marine fuel oil.SOLUTION: A fuel oil composition of the present invention comprises: a marine fuel oil (A) having a sulfur content of 0.50 mass% or less and a kinematic viscosity at 50°C of 10 mm2 / s to 180 mm2 / s; and a dispersant (B) consisting of (a) an amine represented by the following general formula (1) and (b) a neutralization salt of a monovalent carboxylic acid having 5 to 24 carbon atoms, where the molar ratio of (a) to (b) is amol%:bmol%=95:5 to 55:45, the dispersant (B) being contained at 0.01 to 10 mass%. In the formula, R1 is a hydrocarbon group having 5 to 22 carbon atoms; R2 and R3 are independently hydrogen or a hydrocarbon group having 1 to 3 carbon atoms; n is 0 or 1; and X is a straight-chain hydrocarbon group having 1 to 5 carbon atoms.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a marine fuel oil composition. [Background technology]

[0002] Fuel oils used in internal combustion engines, ships, aircraft, external combustion engines, etc., produce precipitates and deposits (sludge) due to asphaltene and other components during storage and use. Sludge causes various problems, such as clogging of fuel strainers and deposition at the bottom of fuel tanks. For this reason, dispersants that uniformly disperse sludge are used in these fuel oils. For example, Patent Document 1 proposes a compound consisting of an aliphatic polycarboxylic acid such as dimer acid and an aliphatic amine as a dispersant that is effective against sludge in gasoline.

[0003] Meanwhile, in recent years, in order to improve environmental issues, the International Maritime Organization (IMO) mandated in 2020 that the sulfur content of marine fuel oil be limited to 0.5% or less. The sulfur content, which will be reduced by this regulation, is a component that contributes to inhibiting the aggregation of sludge-causing substances (latent sludge) in marine fuel oil. Therefore, a reduction in the sulfur content reduces the dispersibility of latent sludge, which was previously dispersed in marine fuel oil. This may lead to the aggregation of latent sludge more than before, making it more likely to produce sludge (actual sludge) that can cause filter clogging and other problems. As fuel oil compositions containing sludge dispersants suitable for such low-sulfur marine fuels, for example, Patent Document 2 proposes a fuel oil composition containing calcium phenate, and Patent Document 3 proposes a fuel oil composition containing a phosphate ester. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-65848 [Patent Document 2] Japanese Patent Publication No. 2022-55583 [Patent Document 3] Japanese Patent Application Publication No. 2020-183459 Summary of the Invention [Problem to be solved by the invention]

[0005] Patent Documents 2 and 3 propose fuel oil compositions containing sludge dispersants suitable for low-sulfur marine fuels. However, the fuel oil compositions containing the sludge dispersants described in these documents only have the effect of suppressing the aggregation of potential sludge. Therefore, it is difficult to redisperse existing sludge using fuel oil compositions containing the sludge dispersants described in these documents. Furthermore, if the sludge dispersant precipitates at low temperatures, it will no longer be effective as a dispersant and the dispersant may clog filters, etc. Therefore, fuel oil compositions containing sludge dispersants are also required to be less likely to precipitate in the marine fuel oil composition.

[0006] An object of the present invention is to provide a marine fuel oil composition containing a sludge dispersant that not only inhibits the aggregation of potential sludge in marine fuel oil but also promotes the re-dispersion of existing sludge, and is less likely to precipitate in marine fuel oil. [Means for solving the problem]

[0007] The marine fuel oil composition that solves the above problem has a sulfur content of 0.50 mass% or less and a kinematic viscosity at 50°C of 10mm 2 / s~180mm 2 and (a) an amine represented by the following general formula (1) and (b) a neutralized salt of a monocarboxylic acid having 5 to 24 carbon atoms, wherein the molar ratio of (a) to (b) is a mol% :b mol% and a dispersant (B) having a ratio of 95:5 to 55:45, and the dispersant (B) is contained in an amount of 0.01 to 10 mass %. [ka] [In general formula (1), R 1 is a hydrocarbon group having 5 to 22 carbon atoms, and R 2and R 3 are independently hydrogen or a hydrocarbon group having 1 to 3 carbon atoms, n is 0 or 1, and X is a linear hydrocarbon group having 1 to 5 carbon atoms. [Effects of the Invention]

[0008] The marine fuel oil composition of the present invention not only inhibits the aggregation of potential sludge in fuel oil, but also disperses existing sludge. In addition, the sludge dispersant is less likely to precipitate. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the dispersant of the present invention and a marine fuel oil composition (hereinafter also referred to as "fuel oil composition") containing the dispersant of the present invention and a marine fuel oil will be described in detail.

[0010] In this specification, numerical ranges defined using the symbol "to" are inclusive of the numerical values ​​at both ends (upper and lower limits) of the symbol "to." For example, "2 to 10" represents a range from 2 to 10.

[0011] [Ship fuel oil (A)] Marine fuel oil (A) has a sulfur content of 0.50% by mass or less. There are no particular limitations on such fuel oils, and any fuel oil can be used as long as the sulfur content of the final product is 0.50% by mass or less. There are also no particular limitations on the method for adjusting the sulfur content. When using a fuel base oil with a high sulfur content, the sulfur content can be reduced by treating the base oil in a desulfurization unit, or by mixing it with another fuel base oil with a low sulfur content, thereby adjusting the sulfur content to 0.50% by mass or less.

[0012] If the sulfur content is more than 0.50% by mass, the increase in sulfur oxides in the exhaust gas will deteriorate environmental performance. From this perspective, the sulfur content is preferably 0.40% by mass or less, and more preferably 0.35% by mass or less. In the present invention, the sulfur content is measured according to JIS K 2541-4:2003 (Crude oil and petroleum products - Determination of sulfur content - Part 4: Radioactive excitation method).

[0013] Marine fuel oil (A) has a kinematic viscosity of 10mm at 50°C. 2 / s~180mm 2 / s. This is to ensure proper operation of the pump and flow meter that supplies fuel to the diesel engine, and to maintain the operability of the preheater that adjusts the fuel temperature so that the kinematic viscosity is appropriate when sprayed into the engine. From this perspective, the kinematic viscosity at 50°C is 15mm 2 / s~100mm 2 / s is preferred, 20 mm 2 / s~40mm 2 In the present invention, the kinematic viscosity is measured in accordance with JIS K 2283:2000 (Testing method for kinematic viscosity of crude oil and petroleum products).

[0014] Marine fuel oil (A) has a sulfur content of 0.50% by mass or less and a kinematic viscosity of 10mm at 50°C. 2 / s~180mm 2 The fuel base oil is not particularly limited as long as it has a viscosity of 1 / s, and can be appropriately selected depending on the purpose and conditions of use. For example, kerosene, light oil, and heavy oil can be used, and one or more types can be selected from these fuel base oils and used. Specific examples of fuel base oils are listed below, but the fuel base oil is not limited to these.

[0015] Kerosene that complies with JIS K2203-2009 standards Diesel oil that complies with JIS K2204-2007 standards (Special No. 1 Diesel Oil, No. 1 Diesel Oil, No. 2 Diesel Oil, No. 3 Diesel Oil, Special No. 3 Diesel Oil) (Common international names are GO (Gas Oil) and DO (Diesel Oil)) Marine Distillate Fuels (DMX, DMA, DFA, DMZ, DFZ, DMB, DFB) that meet ISO 8217-2017 standards (General international names are MDF (Marine Diesel Fuel) and MDO (Marine Diesel Oil)) Marine Residual Fuels that meet ISO 8217-2017 standards (RMA10, RMB30, RMD80, RME180, RMG180, RMG380, RMG500, RMG700, RMK380, RMK500, RMK700) (Common international names are MFO (Marine Fuel Oil), HFO (Heavy Fuel Oil), and RFO (Residual Fuel Oil)) Heavy oil conforming to JIS K2205-1991 standards (Type 1 No. 1, Type 1 No. 2, Type 2, Type 3 No. 1, Type 3 No. 2, Type 3 No. 3) (Common international names are MFO (Marine Fuel Oil), HFO (Heavy Fuel Oil), and RFO (Residual Fuel Oil)) Semi-finished products used in the production of the above light or heavy oils

[0016] Examples of the semi-finished products include straight run gas oil, atmospheric distillation residue, vacuum gas oil, vacuum distillation residue, desulfurized gas oil, desulfurized vacuum gas oil, direct desulfurized residue, cracked gas oil, cracked residue, etc. The meanings of these terms are as follows: Straight-run diesel: A diesel fraction obtained by distilling crude oil in an atmospheric distillation unit. Atmospheric distillation residue: Residual oil fraction obtained by distilling crude oil in an atmospheric distillation unit Vacuum gas oil: A gas oil fraction obtained by distilling atmospheric distillation residue in a vacuum distillation unit. Vacuum distillation residue: Residual oil fraction obtained by distilling atmospheric distillation residue in a vacuum distillation unit Desulfurized diesel: diesel fraction obtained by desulfurizing straight-run diesel Desulfurized vacuum gas oil: A gas oil fraction obtained by desulfurizing vacuum gas oil Direct desulfurized residual oil: Residual oil fraction obtained by desulfurizing atmospheric distillation residual oil Cracked diesel: A diesel fraction obtained by cracking vacuum diesel, desulfurized vacuum diesel, atmospheric distillation residue, or direct desulfurized residue in an FCC unit (catalytic cracking unit). Cracked residual oil: Residual oil fraction obtained by cracking vacuum gas oil, desulfurized vacuum gas oil, atmospheric distillation residual oil, or direct desulfurized residual oil in an FCC unit (catalytic cracking unit).

[0017] Incidentally, biofuel or FT synthetic fuel can also be used as the marine fuel oil (A) if necessary. Examples are shown below, but are not limited to these. Biofuels Animal and vegetable oils such as palm oil, rapeseed oil, soybean oil, sunflower oil, corn oil, sesame oil, coconut oil, whale oil, and waste cooking oil Fatty acid alkyl esters obtained by transesterification of animal and vegetable oils with alcohol (For example, when methanol is used as the alcohol, it becomes fatty acid methyl ester (FAME)). Hydrogenated vegetable oil (HVO) obtained by hydrorefining vegetable oil ·FT synthetic fuel Liquid fuel obtained by FT synthesis (Fischer-Tropsch synthesis) from CO and H2 (The raw materials are not particularly limited and may be CO2, water (H2O), natural gas, biomass, etc.)

[0018] In the present invention, since the effects of suppressing the generation of actual sludge and promoting the re-dispersion of actual sludge are significantly observed, it is preferable to use light oil (including Special No. 1 light oil, No. 1 light oil, No. 2 light oil, No. 3 light oil, Special No. 3 light oil, MDF, MDO, MFO, HFO, and RFO) or heavy oil (including Type 1 No. 1, Type 1 No. 2, Type 2, Type 3 No. 1, Type 3 No. 2, and Type 3 No. 3), and it is particularly preferable to use heavy oil.

[0019] The density of marine fuel oil (A) is 0.8800 to 0.9850 g / cm 3 The density is preferably 0.8800 g / cm 3 By setting the density at 0.9850 g / cm or more, it is possible to prevent a decrease in the total calorific value. 3 By setting the density as follows, it is possible to prevent a decrease in storage stability due to a decrease in sludge separation performance during treatment in a centrifugal purifier. From this viewpoint, the density of the marine fuel oil (A) is set to 0.8850 to 0.9600 g / cm. 3 More preferably, 0.8900 to 0.9400 g / cm 3In the present invention, the density of the marine fuel oil (A) is measured in accordance with JIS K 2249-1:2011 (Crude oil and petroleum products - Determination of density - Part 1: vibration method).

[0020] The carbon residue content of the marine fuel oil (A) is preferably 15% by mass or less. By keeping the carbon residue content at 15% by mass or less, it is possible to prevent deterioration of environmental performance due to an increase in soot and smoke in the exhaust gas. From this viewpoint, the carbon residue content of the marine fuel oil (A) is more preferably 10% by mass or less, and even more preferably 5% by mass or less. In the present invention, the carbon residue content of the marine fuel oil (A) is measured according to JIS K 2270-2:2009 (Crude oil and petroleum products - Determination of carbon residue content - Part 2: Micro method).

[0021] The acid value of the marine fuel oil (A) is preferably 1.0 mgKOH / g or less. By setting the acid value to 1.0 mgKOH / g or less, it is possible to prevent sludge formation and deterioration of storage stability due to fuel oil deterioration. From this viewpoint, the acid value of the marine fuel oil (A) is preferably 0.5 mgKOH / g or less, and more preferably 0.1 mgKOH / g or less. In the present invention, the acid value of the marine fuel oil (A) is measured according to JIS K 2501:2003 (Petroleum products and lubricants - Neutralization number test method).

[0022] The CCAI (Calculated Carbon Aromatic Index) of the marine fuel oil (A) is preferably 860 or less. By setting the CCAI to 860 or less, it is possible to prevent a decrease in combustion performance in a diesel engine. From this viewpoint, the CCAI is more preferably 845 or less, and even more preferably 830 or less. In the present invention, the CCAI of the marine fuel oil (A) is measured in accordance with ISO 8217:2017 (Annex F).

[0023] The water content of marine fuel oil (A) is preferably 0.50% by volume or less. By keeping the water content at 0.50% by volume or less, it is possible to prevent deterioration of storage stability due to the formation of sludge caused by the asphaltene component in the fuel oil and water forming an emulsion. From this viewpoint, the water content is preferably 0.10% by volume or less, and more preferably 0.05% by volume or less. In the present invention, the water content of marine fuel oil (A) is measured according to JIS K 2275-1:2015 (Crude oil and petroleum products - Determination of water content - Part 1: Distillation method).

[0024] [Dispersant (B)] The dispersant (B) is a neutralized salt of (a) an amine represented by the following general formula (1) and (b) a monocarboxylic acid.

[0025] [ka]

[0026] In formula (1), R 1 R represents a saturated or unsaturated hydrocarbon group having 5 to 22 carbon atoms, and may be either linear or branched. 1 Examples of the alkyl group include linear saturated hydrocarbon groups such as heptyl, octyl, lauryl, myristyl, palmityl, stearyl, and behenyl; branched saturated hydrocarbon groups such as isopentyl, isooctyl, 2-ethylhexyl, isononyl, isodecyl, isostearyl, 2-octyldodecyl, and 2-hexyldodecyl; and unsaturated hydrocarbon groups such as palmitoyl, oleyl, and linoleyl. By setting the carbon number to 5 or more, redispersibility in existing sludge can be sufficiently improved, and by setting it to 22 or less, precipitation can be made less likely to occur when added to fuel oil.

[0027] R 1From the viewpoint of improving the redispersibility of existing sludge, R is preferably a linear or branched saturated hydrocarbon group or a linear or branched unsaturated hydrocarbon group having 6 to 22 carbon atoms, more preferably a linear or branched saturated hydrocarbon group or a linear or branched unsaturated hydrocarbon group having 8 to 18 carbon atoms, and even more preferably a linear saturated hydrocarbon group having 12 to 18 carbon atoms. 1 is preferably a lauryl group, a myristyl group, a palmityl group, a stearyl group, or an oleyl group, and particularly preferably a stearyl group.

[0028] R 2 and R 3 R independently represent hydrogen or a hydrocarbon group having 1 to 3 carbon atoms, and the hydrocarbon group having 3 carbon atoms may be either linear or branched. 2 and R 3 By making is hydrogen or a hydrocarbon group having 1 to 3 carbon atoms, precipitation can be made less likely to occur when added to fuel oil.

[0029] R 2 and R 3 are independently preferably hydrogen or a methyl group having 1 carbon atom, and more preferably hydrogen.

[0030] In formula (1), n ​​represents 0 or 1, and preferably n is 0. By setting n to 0 or 1, the effects of inhibiting the coagulation of potential sludge and promoting the re-dispersion of existing sludge are sufficiently exhibited.

[0031] In formula (1), X represents a linear hydrocarbon group having 1 to 5 carbon atoms. Examples of X include a methyl group, an ethyl group, a propyl group, a butyl group, and a pentyl group. By making X a linear hydrocarbon group having 1 to 5 carbon atoms, the effects of inhibiting the aggregation of potential sludge and promoting the re-dispersion of existing sludge are sufficiently exhibited. X is preferably a saturated linear hydrocarbon group having 2 to 4 carbon atoms. X is preferably an ethyl group, a propyl group, or a butyl group, and particularly preferably a propyl group.

[0032] The (b) carboxylic acid is a monocarboxylic acid having 5 to 24 carbon atoms and may be either linear or branched. Examples of the (b) carboxylic acid include linear saturated carboxylic acids such as valeric acid, caproic acid, caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, and behenic acid; branched saturated carboxylic acids such as 2-ethylhexanoic acid, isononanoic acid, and isostearic acid; and linear unsaturated carboxylic acids such as oleic acid, linoleic acid, linolenic acid, and elaidic acid. Using a carboxylic acid having 5 or more carbon atoms effectively inhibits the aggregation of latent sludge and promotes the redispersion of existing sludge. Using a carboxylic acid having 24 or fewer carbon atoms effectively inhibits precipitation when added to fuel oil. Furthermore, the use of a monocarboxylic acid effectively inhibits the aggregation of latent sludge and promotes the redispersion of existing sludge.

[0033] From the viewpoint of improving the dispersibility of potential sludge and the redispersibility of actual sludge, the (b) carboxylic acid is preferably a linear or branched saturated carboxylic acid or a linear or branched unsaturated carboxylic acid having 7 to 24 carbon atoms, more preferably a linear or branched saturated carboxylic acid or a linear or branched unsaturated carboxylic acid having 7 to 19 carbon atoms, and even more preferably a linear or branched saturated carboxylic acid having 12 to 18 carbon atoms. For example, lauric acid, myristic acid, palmitic acid, and stearic acid are preferred, and stearic acid is particularly preferred.

[0034] Regarding the molar ratio of (a) amine and (b) carboxylic acid, from the viewpoint of enhancing the redispersibility of the actual sludge, a mol% :b mol% = 95:5 to 55:45. Preferably, a mol% :b mol% = 90:10 to 60:40, more preferably a mol% :b mol% =80:20~70:30.

[0035] With respect to (a) the total carbon number of the amine and (b) the total carbon number of the carboxylic acid, in order to enhance the redispersibility of the existing sludge, the value of the following formula (2) is preferably 0.6 or more and less than 5.0, and more preferably 0.8 or more and less than 2.0. [(a) total number of carbon atoms in amines] / [(b) total number of carbon atoms in carboxylic acids] Equation (2)

[0036] [Fuel oil composition] The fuel oil composition contains a marine fuel oil (A) and a dispersant (B).

[0037] The fuel oil composition contains a dispersant (B) in an amount of 0.01 to 10% by mass, preferably 0.05 to 5% by mass, and more preferably 0.1 to 2% by mass. The marine fuel oil (A) is preferably contained in an amount of 90 to 99.99% by mass, more preferably 95 to 99.95% by mass, and even more preferably 98 to 99.9% by mass. Increasing the content of dispersant (B) in the fuel oil composition more satisfactorily suppresses the aggregation of potential sludge and enhances the redispersibility of existing sludge. By not including an excessive amount of dispersant (B), precipitation is less likely to occur when added to fuel oil. [Example]

[0038] The present invention will be explained in more detail below with reference to examples and comparative examples.

[0039] [Synthesis of Dispersant 1] In a 500 mL four-neck flask equipped with a thermometer, nitrogen inlet tube, and stirrer, 105 g of aromatic solvent (Idemitsu Kosan Co., Ltd., Ipzol 150), 181 g (0.63 mol) of stearylamine (NOF Corporation, Nissan Amine ABT), and 64 g (0.21 mol) of stearic acid (NOF Corporation, NAA-180) were charged and mixed with stirring at 70°C for 1 hour to obtain Dispersant 1. Dispersants 2 to 10 were synthesized using the same procedure.

[0040] (a) Type of amine, (b) type of monocarboxylic acid used in the synthesis of dispersants 1 to 10, a mol% :bmol% The molar ratios and the values ​​of formula (2) are shown in Table 1.

[0041] [Table 1]

[0042] [Preparation of fuel oil composition A] A 1-L flask was charged with 697.9 g of the three types of heavy oil and 3 g (active component 2.1 g) of dispersant 1, and the mixture was stirred and mixed at 70°C for 30 minutes to prepare fuel oil composition A. Fuel oil compositions B to L were then prepared using the same procedure.

[0043] The properties of the three types of heavy oil used are as follows: ·Sulfur content: 0.310% by mass ·Kinematic viscosity at 50℃: 34.6mm 2 / s ·Density: 0.9268g / cm 3 ·Residual carbon content: 3.64% by mass Acid value: 0.04mgKOH / g CCAI:820 Moisture: 0.02% by volume

[0044] The compositions of fuel oil compositions A to L are shown in Table 2. The values ​​in Table 2 indicate mass %. [Table 2]

[0045] [Inhibition of coagulation of potential sludge] For each of fuel oil compositions A to L, the ability to inhibit coagulation of potential sludge was evaluated using the potential total sediment test described in ISO 10307-2. Specifically, each fuel oil composition was heated at 100°C for 24 hours, filtered, and the mass of sludge on the filter paper was measured. The sludge amount was evaluated according to the following criteria, with the amount of sludge before preparation of each fuel oil composition taken as 100%. The smaller the number, the better the dispersibility of potential sludge. ◎:0~30% by mass ○: 30~70% by mass ×:70~100% by mass

[0046] [Dispersibility of actual sludge] For each of fuel oil compositions A to L, the redispersibility of actual sludge was evaluated using the actual total sediment test described in ISO 10307-1. Specifically, each fuel oil composition was heated to 100°C and filtered, and the mass of sludge on the filter paper was measured. The evaluation was based on the amount of sludge when the amount of sludge before preparation of each fuel oil composition without additives was taken as 100%, and the evaluation was carried out according to the following criteria. The smaller the number, the better the redispersibility of actual sludge. ◎: 0~30% ○: 30~70% ×: 70~100%

[0047] [Precipitation property] 40 mL of each of the fuel oil compositions A to L was placed in a 50 mL screw tube, and each was left to stand at 25° C. for 24 hours, after which the presence or absence of white solid precipitation was visually confirmed. Evaluation was carried out according to the following criteria. ○: No precipitation ×: Precipitation

[0048] Table 3 shows the evaluation results of fuel oil compositions A to L.

[0049] [Table 3]

[0050] As shown in Examples 1 to 6 in Table 3, by blending the dispersant (B) into the fuel oil composition of the present invention, the amount of potential sludge and the amount of actual sludge can be reduced.

[0051] On the other hand, in Comparative Example 1, the amount of actual sludge increased because the content of amine in the dispersant was high.

[0052] In Comparative Example 2, the amine R 1 Since the carbon number is 4, the actual amount of sludge increased.

[0053] In Comparative Example 3, since dimer acid, which is a dicarboxylic acid, was contained instead of monocarboxylic acid, the amount of potential sludge and the amount of actual sludge increased.

[0054] In Comparative Example 4, the amount of actual sludge increased because the content of carboxylic acid was greater than that of amine.

[0055] In Comparative Example 5, R 2 The precipitation occurred because the ester group was a stearyl group instead of hydrogen or a hydrocarbon group having 1 to 3 carbon atoms.

[0056] In Comparative Example 6, the amount of potential sludge and the amount of actual sludge increased because the content of dispersant in the fuel oil composition was low. [Industrial Applicability]

[0057] The present invention can be suitably used as a low-sulfur marine fuel oil that reduces the adverse effects of potential sludge, actual sludge, and deposition of dispersants.

Claims

[Claim 1] The sulfur content is 0.50% by mass or less, and the kinematic viscosity at 50°C is 10 mm ２ / s~180mm ２ and (a) an amine represented by the following general formula (1) and (b) a neutralized salt of a monocarboxylic acid having 5 to 24 carbon atoms, wherein the molar ratio of (a) to (b) is a ｍｏｌ％ :b ｍｏｌ％ and a dispersant (B) having a ratio of 95:5 to 55:45, Contains 0.01 to 10% by mass of a dispersant (B), Marine fuel oil composition. 【Chemical 1】 [In general formula (1), R １ is a hydrocarbon group having 5 to 22 carbon atoms, and R ２ and R ３ are independently hydrogen or a hydrocarbon group having 1 to 3 carbon atoms, n is 0 or 1, and X is a linear hydrocarbon group having 1 to 5 carbon atoms.

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