Photo-degradation Effect on Naphtha Octane Number by Using UV Radiation

ةصلاخلا  ةعشلال ضرعت) ةفلتخم رابتخا فورظ تحت ةيجسفنبلا قوف ةعشلاا ىلا اهضيرعت مت ثيح رابتخا دوقوك ثحبلا اذه يف اثفنلا مادختسا مت رعت ، طقف زفحملا لماعلاو ديربتلا دوجوب ةعشلال ضرعت ، زفحملا لماعلاو ديربتلا دوجوب ةعشلال ضرعت ، ديربتلا دوجوب ةعشلال ض لاا ريسكت ةيلمع عبتت مت ثيح ( دسكؤملا لماعلاو زفحملا لماعلا دوجوب ةعشلال ضرعت ، دسكؤملا لماعلاو للحتلا تلاعافت نمض رصاو يئوضلا رصاوا رسكت ظحولف ، لا قوف ةعشلاا صاصتما ةجيتن ةديدع ةيلاعلا ةقاطلا تاذ ةيجسفنب وه للحتلا اذهل ةعقوتملا جئاتنلا نمو ، . اثفنلل يناتكولاا ددعلا ريغت

hydrocarbon fuels with long straight chain molecules have low octane number while fuels with short chain, branched and cyclic molecules have high octane number.
Naphtha is one of hydrocarbon fuels which is produced from crude oil by several methods such as: fractionation of distillates or even crude petroleum, hydrogenation of distillates, solvent extraction, alkylation processes and polymerization of unsaturated olefinic compounds or may be a combination of them. [2] It is a volatile and flammable fuel with specific gravity of (0.694) at 15Cº and boiling range about (30°C -200°C) . It consists of a complex mixture of more than a hundred hydrocarbon compounds with carbon atoms range between (C 4 -C 15 ). [3] When naphtha is exposed to UV radiation it will undergo a Photolysis (Photo-dissociation) process [4]. This process causes changes in chemical composition of naphtha and then in its octane number which depends on nature of the contained compounds in naphtha.
There were attempts of subjecting catalysts to varied sources of radiation to enhance their activity. some of these attempts were concentrated on the study of UV rays on the catalyst activity in conversion of unsaturated hydrocarbons. They found that if a catalytic reaction occurs in the presence of UV radiation , substantially complete conversion of reactants is obtained. Thus, they proved that the UV radiation exposure enhances the catalyst activity [5].
Two advantages for using of UV rays with hydrocarbon compounds, one of them is cracking which relates to hydrocarbon bonds and the other is enhancing of the catalyst activity which relates to catalysts in photo-catalysis process [6].  [7], used UV rays to improve the Octane number for Al-Dura product pool (70% Reformate + 30% Light Naphtha) in photo-degradation process in the presence of ZnO catalyst, with different exposure times . They raised the octane number by (5 degrees) at (8 hours) exposure.
The main aim of the present work is to study the changes in naphtha octane number under UV radiation exposure based on idea of C-C and C-H bonds breaking and re-structuring.

Sample Preparation and Test Procedure
Naphtha samples of 40 L for UV Reactor Unit or (500 ml) for UV Cell Unit are prepared. Each sample is mixed with catalyst for 20 min before UV exposure. The Octane number of the sample is measured either in CFR engine or SHATOX Octane meter, GCMS and FTIR analysis are also conducted for sample before exposure. The temperature of the sample is recorded initially and during the test. Also the pressure inside the UV Reactor tank is recorded initially and during the test. The sample is then exposed to UV rays for a specified period under various conditions. Octane measurement, GCMS and FTIR analysis are conducted after exposure to investigate the changes in the molecular structure caused by the UV exposure.

Gas Chromatography Mass Spectrometry (GCMS) Analysis
It is an apparatus used in the separation, identification and quantification of complex mixtures. The determination of these compounds is very difficult by the standard MS library. Therefore, the retention time was used as an index for the GC qualitative analysis [10]. The changes in the main effective compounds (i.e. compounds that affect the octane number) is determined by comparing the GCMS results before and after UV exposure. The octane number change may be interpreted according to the qualitative and quantitative changes in these compounds.

Fourier Transform Infrared Spectroscopy (FTIR) Analysis
This apparatus provides crucial information about the molecular structure of organic and inorganic components [11]. The FTIR technique is based on the absorption of IR radiation which occurs when photons transfer to sample molecules. These molecules are excited to a higher energy states [12] causing vibrations of molecular bonds (i.e. bending, stretching, rocking, twisting, wagging and out-of- -400 cm -1 ) in the IR region of the electromagnetic spectrum.

Results and Discussion
The results are divided into five groups according to the tests conditions:

1.Naphtha Exposed to UV Rays Only
The results of these tests are shown in table 1 & fig.8 .These results show a fluctuating response in the change of naphtha RON when exposed to UV rays only. It decreases to a minimum value of 51.3 after one hour exposure (initial RON 54.5) and then increases to 56 after two hours exposure and then decreases. This fluctuating in Octane number may be attributed to the effect of photolysis (Photodissociation) process.
It is known that the bonds of the branched compounds (iso-paraffin) are weaker than those in the straight chain compounds (n-paraffin) because the branched compound molecules are more compact with less surface area. This means that the intermolecular attractive forces of the branched compounds are smaller. Therefore, the broken bonds due to UV exposure are more in the branched compounds than in the straight chain compounds. In other words, the branches are broken and may be converted into small straight chain compounds. On the other hand, some of the UV energy may be used to break the bonds of straight chain molecules and converted them to branched molecules. As it is known that the RON of straight chain compounds (n-paraffins) is less than that of branched compounds (iso-paraffins), as shown in fig. (9) the net effect of these two restructuring processes may cause the fluctuation in Octane number changes.

2.Naphtha Exposed to UV Rays with Cooling
The results of these tests are shown in table (2) & fig. (10). These results indicate that no change occurs in the Octane number when naphtha is exposed to UV rays with the existence of cooling. An important fact may be concluded in these tests, that the cooling inhabits the Photolysis activity.
In other words, during the UV exposure with temperature rise process the interaction between photons and fuel molecules may occur causing the crack of the bonds of that molecules, whereas the cooling process inhibits that interaction and protects the molecules against the UV exposure effect.

3.Naphtha Exposed to UV Rays with Cooling and Catalyst
The results of these tests are shown in table (3)  When a catalyst is added to the fuel and exposed to UV rays the photo-catalysis process will occur (i.e. the acceleration of photoreaction in the presence of a catalyst) . In this process electrons of catalyst molecules are excited by UV photons and transferred from the valence band to the conduction band, leaving positive holes in the valence band, see fig. (12). The ultimate goal of the process is to have a reaction between the excited electrons with an oxidant to produce a reduced product, and also a reaction between the generated holes with a reductant to produce an oxidized product [13]. If there is no capturer for the electrons in conduction band (active electrons), as in this test, they return quickly to their valence bands and react with the holes (Recombination process).
Titanium dioxide (TiO 2 ) and Zinc oxide (ZnO) are used as a suitable catalysts . They are both semiconductors each of them have a band gap energy of (3.2 eV) [14]. The fluctuation in the RON change that is exhibited in these results may be attributed to the recombination process which occurs in the absence of capturer. Therefore only the photolysis effect will be existed.

4.Naphtha Exposed to UV Rays with Cooling , Catalyst and Oxidant O 2
Oxygen is known to act as a capturer of electrons that exist in the conduction band to generate radicals which may lead to production of high RON compounds. Air is used as a source of oxygen in this work in the presence of catalyst to improve RON of naphtha. Two tests are carried out with air flow rate of

5.Naphtha Exposed to UV Rays with Catalyst and Oxidant O 2
To investigate the effect of cooling on the photo-catalysis process samples with ZnO catalyst and oxidant are exposed to UV rays in the absence of cooling. The oxidant (which oxygen) flow rate value decreases again. In these tests the oxidant reacts with the excited electrons in conduction band generating radicals (reduced product) in photo-catalysis process. These radicals may lead to production of high RON compounds.

GCMS Tests
Due to the large number of compounds contained in naphtha, it is difficult to follow all changes in the chemical composition of the sample. It is known that the aromatic compounds have high RON values in comparison with the other hydrocarbons groups [15] as shown in fig. (9). Therefore, the GCMS test  At frequency of (2175.5 cm -1 ), (C ≡ C) bond from (alkynes) functional group is existed before UV exposure, but it is not apparent after the exposure.
At frequency of (2229.17 cm -1 ), (C ≡ C) bond from (alkynes) functional group appears after the exposure.
This test is also carried out on the same RON increasing sample that mentioned in the GCMS test. Figs. (22 & 24) show the results of the FTIR test before and after UV exposure respectively. The results exhibited the following changes.
At frequency of (2175.5 cm-1), bond from (alkynes) functional group and at frequency of (1350.86 cm-1), bond from (alkanes) functional group are existed before UV exposure, but they are eliminated after the exposure.
At frequency of (1404.94 cm-1), bond from (aromatics) functional group and at frequency of (706.14), bond from (aromatics) functional group are created after the exposure.

Conclusions
The following conclusions can be drawn: