Experimental

3.2.1 Chemicals

A sample of marine heavy fuel oil was collected from the port of Sydney, New South Wales, Australia. All other chemicals used for the experiment were standard laboratory chemicals. Their details are as follows;

• Adsorbent: - Silica gel grade 922, mesh size 200-325 (AJAX Chemical Ltd.).

• Solvents :-

o n-pentane (UNILAB, LR grade) o Toluene (BDH, AR grade)

o Methanol (MERCK, AR grade) and o Chloroform (UNILAB, LR grade).

All solvents were purified by fractional distillation prior to their use. Normal-pentane was distilled by a single plate conventional distillation apparatus at 37 oC. Toluene was similarly also distilled and collected at 110 oC. This precaution was taken to avoid contamination of the original sample by high boiling point impurities in the solvent, which could be concentrated on evaporation of the solvents after fractions were collected. Gas chromatography was used to check the quality of the solvents before and after fractional distillation.

3.2.2 Procedure

A flowchart of the current experimental procedure used for HFO separation is shown in Figure 3-1.

Figure 3-1: Flowchart of the experimental procedure of separation of HFO into different fractions.

3.2.2.1 Precipitation of Asphaltenes

As shown in Figure 3-1, the asphaltenes fraction was initially separated from the heavy fuel oil because it can cause adverse effects on the adsorbent by becoming irreversibly adsorbed. Asphaltenes are also known as ‘bad actors’ in refineries as they promote coke and sludge formation as well as causing catalyst deactivation during the processes.

As used in the literature [139, 145], a 40:1 ratio of solvent to sample was used. An accurately weighed 2.557 g sample of heavy fuel oil was mixed with 100 mL of purified n-pentane. This mixture (heavy fuel oil + n-pentane) was stirred for 24 hours at room temperature and allowed to settle for approximately 3 hours. The precipitated asphaltene fraction was separated from the mixture by vacuum filtration, the most common method reported for the separation of asphaltenes from maltenes. For filtration, a 0.2 micron, 47 mm size Nylon filter paper and a laboratory scale Buchner vacuum filtration apparatus, employing 150 mmHg vacuum at room temperature was used. Precipitated asphaltenes were collected on the filter paper and the solvent was allowed to evaporate from the filter paper at room temperature in a vacuum dessicator for approximately 24 hours. Further, the small amount of asphaltenes which was deposited on the surface of the glass vessel during stirring and which could not be removed by washing with n-pentane was collected by dissolving in chloroform. The chloroform was also allowed to evaporate off for at least 24 hours. Finally, the dried asphaltenes which were collected on the filter paper and also via

chloroform were collected and weighed separately. The total weight of asphaltenes in heavy fuel oil is given by the summation of the weights collected on both the filter paper and by chloroform dissolution.

3.2.2.2 Separation of Maltenes into Saturates, Aromatics and Resins

Maltenes in pentane solution were isolated from asphaltenes by vacuum filtration and collected as a filtrate as described above. During vacuum filtration some solvent loss occurred due to the low boiling point of n-pentane (37 oC). Additional solvent was added to make up the volume of the solution of maltenes to 50 mL. From this solution of maltenes, 10 mL (20% by vol) was taken and mixed with 15-20 g silica gel, which had been previously activated by heating for 24 hours at 115 oC. Precautions were taken to ensure uniform mixing of the solution of maltenes with the silica gel by using a vacuum rotary evaporator to remove the solvent during the mixing of silica gel and maltenes, over a period of approximately 3 hours. This uniform mixing technique was found very efficient and useful later during the experiment in terms of column development for the separation of the fractions.

A glass column with an internal diameter of 2 cm and a height of 35 cm was used for the sequential elution solvent chromatography (SESC). This column was filled with slurry of pre activated silica gel (grade 1) in pentane up to 23 cm, while the top 5 cm of the column was filled with the mixture of maltenes and silica gel mixture prepared earlier.

Once the column had been prepared, separation was commenced. The saturates fraction was eluted by passing 125 mL of n-pentane through the column and the eluate was collected in 10 mL test tubes in sequence. All test tubes were later examined by gas chromatography to ensure the elution of saturates was complete. Afterwards, only those test tubes which contained significant amounts of saturates were mixed to form a composite solution of saturates in n-pentane. As expected, it was found that of all the test tubes, only the middle set of test tubes in the sequence contained appreciable amount of saturates. Some initial and final tubes were free from saturates. This proved that the amount of solvent used was sufficient to elute all saturates from the maltenes. In this way,

saturates were removed from the maltenes and the next step was to separate the aromatics and resins.

Aromatics were eluted similarly to saturates, using 125 mL of toluene as eluent and the eluted fractions were collected in 10 mL test tubes. The presence of aromatics in all test tubes was detected by gas chromatographic analysis. Thereafter, a composite sample of aromatics in toluene was prepared. Once saturates and aromatics had been removed, it proved a difficult task to remove the entire resins fraction from the column due to their polar characteristics similar to asphaltenes.

Initially a mixture of 110 mL of toluene and 15 mL (12% by vol) of methanol was used to elute the resins fraction. However, this was not found to be effective in completely removing the resins from the column. Hence, a second volume of 40 mL of toluene and 10 mL (20% by vol) of methanol mixture was applied, which eluted most of the resins from the column. Finally a 25 mL toluene and 25 mL (50% by vol) methanol mixture was applied to ensure that all the resins had been removed. In this way, all resins were eluted and a composite sample was prepared. Thus, all three fractions of maltenes (saturates, aromatics and resins) were separated by the sequential elution solvent chromatography and collected in different containers. The next task was to analyse all the separated fractions.

After the separation of the three fractions, the solvents were removed by employing a rotary evaporator operating at around 150 mmHg vacuum and 90 oC. However, this was found to be incapable of removing all the toluene from the aromatics and resins fractions. Consequently, a vacuum oven was used to remove the last traces of toluene from the aromatics and resins fractions. The vacuum oven was operated for 4 hours at 50 oC and 10 mmHg. Finally, the dried saturates, aromatics and resin were collected and weighed accurately. The amounts and yields of these fractions are discussed in a later section of this chapter.

3.2.2.3 Gas Chromatography

After the separation of heavy fuel oil into four fractions, gas chromatography (GC) was performed on the light fractions to characterise the fractions. Prior to GC analysis the separated saturates fraction was mixed with pentane and a 25 mL solution was prepared. Similarly, a 10 mL solution of the separated aromatics in toluene was prepared. GC analysis was conducted on a HEWLETT PACKARD-5890 gas chromatography instrument equipped with a 30m length, 0.25 mm id, 0.5 micron thickness BPX5 column, and flame ionisation detector (FID). The GC column temperature was programmed in the following manner: 40 oC for the first 4 minutes, then from 40 oC to 300 oC at a rate of 7 oC /min and finally the temperature was held constant at 300 oC for 15 minutes. During GC analysis injection volumes of 2 microlitre of saturates and aromatics fractions were employed using a hot needle injection technique. The results of GC analysis are discussed later in section 3.3.2.

3.2.2.4 Mass Spectrometry

To understand the chemistry of fuel it is important to know the molecular weight distribution of its fractions. The mean molecular weight helps to assign the physical and chemical properties correlations of hydrocarbons. In the present work it is very important to know the mean molecular weight of each fraction and its range, to fit the Γ-distribution function parameters which are the basic requirement of any continuous thermodynamics modelling. Mass spectrometric (MS) analysis was conducted on a KRATOS CONCEPT ISQ high resolution magnetic sector mass spectrometer. Mass spectrometric analysis was performed to try to determine the mean molecular weights and the molecular weight ranges of the separated SARA fractions of heavy fuel oil. Electron ionisation at 70 eV was employed with a Desorption Chemical Ionisation (DCI) probe heated from 40 oC to 500

o

C at 2 oC/sec. Each fraction (Saturates, Aromatics, Resins and Asphaltenes) was scanned between charge/mass ratio (m/z) 50 to 1800 molecular weight ranges at 1 second per decade. The accelerating voltage and source temperature used for this analysis was 4 kV and 200 oC respectively. All data of this analysis were acquired in “raw” profile mode. Prior to analysis the instrument was calibrated between m/z 50 to 1300. The results obtained by this analysis are discussed in a later section (Section 3.3.4) of this chapter.