Asphaltenes Aqueous Conversion to Humic and Fulvic Analogs via

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Asphaltenes Aqueous Conversion to Humic and Fulvic Analogs Via Oxy-Cracking Maryam Ashtari, Lante A Carbognani Ortega, and Pedro Rafael Pereira-Almao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00613 • Publication Date (Web): 01 Jun 2016 Downloaded from http://pubs.acs.org on June 3, 2016

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Energy & Fuels

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Asphaltenes Aqueous Conversion to Humic and Fulvic Analogs Via Oxy-Cracking 4 5 6

Maryam Ashtari,a,* Lante Carbognani,a Pedro Pereira-Almao a 7 9

8 a Department of Chemical & Petroleum Engineering, Schulich School of Engineering University of Calgary, Calgary, Canada, T2N1N4

10 1 13

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Abstract 14 16

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Asphaltenes surplus production in northern Alberta facilities as a result of heavy oil upgrading 18

17

incentivizes searching for alternative processes to coking or gasification, with less environmental 19

footprint and/or lower costs. Oxy cracking of asphaltenes in liquid phase under alkali conditions 21

20

could be a new approach to creating higher value products. In the present study, asphaltene 23

particles/drops were solubilized in water with the ultimate intention of reducing their molecular 25

24

2

weight, converting them to analogs of humic substances with potential applications as soil co26

fertilizers or other industrial applications. For evaluating the feasibility of solubilizing asphaltene 28

27

particles by oxy-cracking, experiments were done in a batch reactor to study the parameters 30

29

maximizing the solubilization towards liquid products: temperature, residence time, pressure, base 32

effect, asphaltenes mass, stirring speed. The results showed that temperature and residence time 3

31

are the two parameters affecting reaction the most, being the optimum values 180-210 °C and 1-2 35

34

hours, respectively. The intermediate product of asphaltenes oxy-cracking reaction is Water 37

36

Soluble Asphaltenes (WSA), and the final product is CO2. The water solubilized products were 39

found to contain organic carboxylic, carbonyl, phenolic and sulfonic functions, responsible for 41

40

38

their dissolution in water. At less severe conditions, WSA had characteristics similar to humic 42

analogs; but with increasing reaction severity, products aromaticity increased and lower molecular 4

43

weight components (fulvic analogs) were formed. 45 46 47 48 49 50 51 52 54

53 *

Corresponding Author. Tel:+14032109590.

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Email address: [email protected] 57 58 59 60 ACS Paragon Plus Environment

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Introduction 4 6

5

Asphaltenes are operationally defined as alkane insoluble and aromatic soluble components of oils. 7 8

Thy are the heaviest (high molecular weight), most polar and most complex compounds in crude 9 1

10

oil.1-4 Asphaltenes structures consist of fused polycondensed aromatic and naphthenic cores 13

12

connected via alicyclic and alkyl chains, and containing heteroatomic species such as oxygen, 14 15

nitrogen, sulfur, and traces of metal elements such as nickel and vanadium.5 The asphaltenes 16 18

17

solubility classification covers a broad range of molecular structures, and as a result, different 20

19

molecular weights have been proposed for them. Their average molecular size appears to be 21 23

2

between 300-1400 g/mol.6 24 26

25

Asphaltenes flocculation and deposition cause formation of coke-like stable precursors, which are 28

27

responsible for a variety of problems in oil production, transportation, and downstream in 29 31

30

refineries. They could also accumulate on equipment, create operational problems, and safety 3

32

hazards. Asphaltenes precipitation is rendered by changes in pressure, temperature, chemical 35

34

composition of crude oil, and diluent addition to crude oil.7-8 Because of the presence of 36 38

37

asphaltenes in both upstream and downstream facilities, and problems associated with their 40

39

existence, dealing with their separation and trying to extract valuable products from them by 41 42

further processing is of significant relevance. 43 4 46

45

The most common technology for processing asphaltenes rich streams is delayed coking. Although 48

47

the process could produce light distillates via thermal cracking, the by-product involves a 49 50

significant amount of undesirable coke, which is contaminated by high percentages of sulfur and 51 53

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metals. Disposal of the produced coke is a matter of concern, creating many environmental issues.9 54 5 56 57 58

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Further problems derived from coking processing were shown by a study performed at a refinery 5

4

in Canada, showing that coker and vacuum units were responsible for over 17% of total emissions 6 8

7

of alkanes and heavier hydrocarbons, over 42% of methane emissions and 25% of benzene 10

9

emissions.10 1 12

From the preceding problems related to asphaltenes and their significant percentages in heavy oil 13 15

14

(around 15% in bitumens)11 with about twice as much levels after coking processing, new 17

16

upgrading technologies are desirable focusing the production of high-value products such as 18 19

petrochemicals, while reducing environmental pollutants. 20 2

21

Exploring new pathways for upgrading asphaltenes pitch in search of potentially valuable 24

23

products, has led to examining the feasibility of oxy-cracking in liquid phase i.e., alkaline water. 25 27

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Oxy-cracking reaction under mild operating conditions solubilizes asphaltenes in water giving 29

28

origin to smaller molecular weight compounds than the parent asphaltenes which can be 31

30

considered as analogs of humic substances based on determined properties. 32 34

3

Humic substances are organic materials naturally produced by oxidative decomposition of 36

35

complex organic molecules.12 These materials play a critical role in the global carbon cycle. 37 38

Because of their large molecular weight carbon structures, these components could help in 39 41

40

transferring nutrients to plants and increasing water retention in soils. Furthermore, they could 43

42

have other industrial applications such as additives for controlling the settling rate of concrete, 4 45

dyes for leather preparation, agents in the woodworking industry, additives for increasing 46 48

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mechanical strength of ceramics, and also medical and environmental applications.13 50

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Humic substances have been classified based on their solubility in water at different pH, into three 51 53

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separate fractions: fulvic acids (alkali and acid soluble), humic acids (alkali soluble and acid 54 5 56 57 58


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insoluble), and humin (alkali and acid insoluble)13-16. The elemental analysis of different fulvic 5

4

acids and humic acids shows that their major components are C, H, O, N, and S, and the structure 6 7

of humic substances comprise aromatic rings attached to aliphatic chains.13,14 10

9

8

Past researches addressed the production of humic substances from coal and lignites 17-22. Treating 1 12

coal with 5% and 10% H2O2 in a water recirculating bath for 2 hrs at 70 °C was reported. The 13 15

14

mixture was filtered, washed and dried and then treated with 10 ml KOH at 70 °C for 2 hrs. The 17

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supernatant was filtered and washed with distilled water and then acidified with H2SO4 to 18 19

precipitate humic acids. The experiments were repeated using other oxidizing agents like KMNO4 20 2

21

and HNO3. The results showed that HNO3 produced the best yield in humic acids production with 24

23

a wide variety of acidic functional groups in comparison with other oxidizing agents. 17 The 25 27

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production of humic acids using nitric acid was studied by others; however, nitric acid was found 29

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expensive compared to the obtained products and the process required long reaction times for coal 31

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particles larger than 100 μm.18 32 34

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The production of humic substances from coal was investigated industrially using oxidative 36

35

processes. In one patent 37

19

, coal conversion to humic acids by pre-oxidizing the coal and then

39

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treating with the aqueous solution of formaldehyde and alkali bisulfate or with an aqueous solution 41

40

of ammonia and formaldehyde with the addition of sulfur dioxide at 100 °C to form water soluble 42 43

products was reported. The water-solubilized sulfur-methylated product was applicable as tanning 4 46

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agent, additive to control flow properties of drilling muds, fertilizer component, and graphite 48

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production. 49 50

In another patent 20, coal was bio-oxidized in water using thermophilic aerobic cultures for about 51 53

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48 hours at a temperature of 60 °C to produce humic acid, alcohols, methane, and light fatty acids. 54 5 56 57 58


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The bacteria concentration was 1-20 wt% of the mixture of coal, water, and bacteria. The 5

4

concentration of coal was between 0.01 to 50 wt% with up to 95% of the coal converted to humic 6 8

7

acid, other products were methane, light alcohols, and fatty acids. Although coal bio-oxidation was 10

9

proposed in several patents, this process is expensive and also produces significant amounts of 1 12

metabolic by-products. 13 15

14

In set of patents and articles18,21,22, humic acid was produced using dry oxidation of bituminous or 17

16

sub-bituminous coals or lignites. The process was carried out in fluidized bed reactors at 18 19

temperatures between 150°C to 300°C, pressure between 1.1 to 10 atm, and contact time between 20 2

21

30-600 mins. According to these works, coal oxidation was evidenced to occur over aliphatic 23 24

structures the most. 25 26 28

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The objective of the present work was to maximize the amount of water solubilized asphaltenes 30

29

(WSA) during oxy-cracking reaction in alkaline water. The effects of several parameters such as 32

31

temperature, pressure, type of base, ratios of water and asphaltenes, and mixing rate on maximizing 3 35

34

product were tested, and the optimum operating conditions for having maximum solubilized 37

36

product were obtained. Gas and liquid products were characterized using gas chromatography 38 39

(GC), Total Organic Carbon analysis (TOC), Fourier Transform Infra-Red (FTIR) and Ultraviolet40 42

41

visible (UV-visible) spectroscopies, metal analysis and elemental analysis to find the nature of 4

43

formed compounds, particularly those solubilized in water. The solubilized products and insoluble 45 46

solid remaining from the reaction were further separated, and compared with humic and fulvic 47 49

48

acids, and humins derived from nature. Finally, a feasible reaction pathway was proposed for 51

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asphaltenes oxy-cracking. 52 53 54 5 56 57 58


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2. Experimental: 5

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2.1. Materials 6 8

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The studied asphaltenes were extracted from Athabasca bitumen vacuum residue (ABVR) 10

9

following IP.143 standard,23 using industrial grade pentane as a precipitant solvent. NaOH solution 1 12

(5N), was used as solubilization enhancer. HNO3 (70%, ACS reagent), ICP cobalt standard 13 15

14

solution (993 ppm Co 1 wt. % HNO3), Toluene (ACS reagent > 99.9 %), HCl, (37%, ACS reagent), 17

16

Fluka humic acid (53680 Humic acid) all of them from Sigma-Aldrich (Oakville, Ontario, Canada) 18 19

were used as received. Fluka humic acid was selected because of its availability, also South 20 2

21

American natural humic acid and its potassium salt were analyzed, providing similar results to the 24

23

commercial sample (not reported in this article). Deionized water was used as reaction medium. 25 27

26

Oxygen, 99.993 % purity (Praxair, Canada), was used as the reactant gas. 28 30

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2.2. Experimental procedures and Apparatus 31 3

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Experiments were carried out using one batch stainless steel 4598 Parr reactor (Parr Instrument 35

34

Company, Moline, Il, USA). The reactor depth was 4.6 inch with the inside diameter of 1.3 inches, 36 37

and its volume was 100 mL. In all the experiments the asphaltenes mass was 0.5 g, the water mass 38 40

39

varied spanning the range of 2.5-20 gr. As a result of highly acidic media derived from oxidation 42

41

reactions, 1 mL 5N NaOH solution was added to neutralize the final liquid product. Also, a glass 43 4

liner was used to prevent damage to the reactor walls. 45 47

46

As oxidizing gas, pure oxygen was used. Temperature, mixing rate and pressure were controlled 49

48

using the Parr reactor controller. In each experiment, it takes about 40 min for the feed to reach 50 52

51

the set point temperature (180-250 °C). Afterwards, the reaction was carried out for the selected 54

53

reaction time (0-5hrs). 5 56 57 58


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Several parameters for optimization of oxy-cracking process were investigated, such as 5

4

temperature, pressure, residence time, mixing rate, base type, and ratio of water to asphaltenes. 6 8

7

After the reaction, the reactor was cooled down, and gas chromatography analysis was performed 10

9

on the produced gases. The liquid effluents were filtered and elemental analysis and TOC analysis 1 12

carried out on the remaining dried solid and liquid product respectively. 13 14 16

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For finding the optimum operating conditions for maximizing water solubilized products, 18

17

conversion and selectivity to WSA and CO2 were calculated using TOC and GC measurements. 20

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Small amounts of methane and ethylene were also produced during the reactions which were 21 23

2

considered for calculation of the carbon amounts in gas products. As the asphaltenes have 25

24

inextricable diversity, conversion and selectivity to products were calculated based on carbon mass 26 27

(mg) in reactants and products in the reaction phases, i.e., in liquid, in gases and in solid. 28 30

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The carbon input to the reactions comes from the elemental analysis from the initial feedstock: 31 3

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Carbon amount before reaction= asphaltenes mass 34

carbon % in feed

36

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Equations (1-3) show how carbon conversion and selectivity to products could be calculated. Keys 37 38

for identifying fractions are organic components in the liquid phase, which comprise: 39 41

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product B: WSA 43

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product C: CO2 in gas phase + CO2 in liquid phase (carbonates) 4 46

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𝒕𝒐𝒕𝒂𝒍 𝒄𝒐𝒏𝒗𝒆𝒓𝒔𝒊𝒐𝒏 = 47

(𝑶𝑪 (𝑾𝑺𝑨)+𝑰𝑪(𝑾𝑺𝑨)+𝑪𝑮 )×𝟏𝟎𝟎 𝑪𝑭

(1)

48 49 50 51

𝑺𝒆𝒍𝒆𝒄𝒕𝒊𝒗𝒊𝒕𝒚 𝒕𝒐 𝒑𝒓𝒐𝒅𝒖𝒄𝒕 𝑩 = 53

52

𝑶𝑪(𝑾𝑺𝑨) 𝑶𝑪(𝑾𝑺𝑨)+𝑰𝑪(𝑾𝑺𝑨)+𝑪𝑮

54 5 56 57 58


(2)

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1 3

2 𝑰𝑪(𝑾𝑺𝑨)+𝑪

𝑮 𝑺𝒆𝒍𝒆𝒄𝒕𝒊𝒗𝒊𝒕𝒚 𝒕𝒐 𝒑𝒓𝒐𝒅𝒖𝒄𝒕 𝑪 = 𝑶𝑪(𝑾𝑺𝑨)+𝑰𝑪(𝑾𝑺𝑨)+𝑪

5

4

𝑮

(3)

7

6

𝐶𝑂2 8

𝐺𝑎𝑠

9

𝐶𝐺 =

𝑃𝑉 𝑅𝑇

× 12

Other gas contents were determined small, thus neglected.

1

10

Liquid

TOC

12 13 14 15 17

16

Where OC(WSA) is organic carbon in WSA, IC(WSA) is inorganic carbon in WSA, CG is carbon 18 19

content in gas product and CF is carbon amount in feedstock. 20 21 2 23

2.3. Analytical methods 24 25 26 28

27

Several characterization techniques were performed on parent asphaltenes, WSA and remaining 30

29

asphaltenes after the oxy-cracking reaction. 31 3

32

2.3.1. Elemental and Metal Analysis. For analyzing C, H, N contents a combustion method using 35

34

a Perkin Elmer 2400 CHN analyzer (Waltham, Massachusetts, USA) was followed. S and N 36 38

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contents for organic materials were determined with an Antek 9000 system (Houston, TX, USA) 40

39

by running toluene solutions (10% wt/vol.). Calibration was performed with Accustandard IS41 42

17368 (N) and Accustandard SCO-500x (S) standards. Calibration for aqueous solutions was 43 45

4

achieved with thioacetamide. The relative standard deviations for measurements were: 0.7% 47

46

(carbon), 5% (hydrogen, sulfur and nitrogen). 48 49

Microwave assisted acid digestion experiments were carried out in a commercial unit model 50 52

51

MARS 6 from CEM Corporation (Matthews, NC, USA), provided with UltraPrep vessels of 100 54

53

ml capacity and MARSXpress DuoTemp controller. The system was operated at a frequency of 5 56 57 58


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2.45 GHz at 100% of full power (maximum of 1600 W). WSA sample (0.1g-0.5000 g weighted to 5

4

the nearest 0.1 mg) was placed in the vessel, and 1 ppm cobalt standard, (micro-pipetting 25.2 μL, 6 8

7

25.1 mg, from 993 ppm Co in 1 wt. % HNO3) was added. Finally, nitric acid 70% was added with 10

9

a manual siphon pump (10.5 mL). After finishing the digestion, the sample was cooled down to 1 12

around 25 °C, the carousel was transferred to a fume hood, the holder was removed, and caps 13 15

14

slowly loosen taking care that vessel cap holes face towards inside of fume hood while releasing 17

16

the brown NOx gas mixture. Metal concentrations in the samples were determined by ICP–AES 18 19

using an IRIS Intrepid II XDL, from Thermo-Instruments Canada, Inc. (Ontario, Canada). The 20 2

21

relative standard deviation for metal analysis was 10%. 24

23

The elemental analysis for nC5 asphaltenes derived from Athabasca bitumen vacuum residue 25 27

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was: C: 81.18 wt%, H: 8.25 wt%, N: 1.24 wt%, S: 8.00wt%. For the whole vacuum residue 29

28

(ABVR) was: C: 82.30 wt%, H: 9.72 wt%, N: 0.62 wt%, S: 5.31 wt%, V: 265 mg/kg, Ni: 115 31

30

mg/kg. 32 34

3

2.3.2. Total Organic Carbon Analyzer. A TOC-Vcp equipment from Shimadzu (Tokio, Japan) 36

35

was used to determine the content of the aqueous soluble organic and inorganic carbon present in 37 38

the WSA samples. All the measurements were performed in triplicate using the average of 39 41

40

measurements for calculations. The relative standard deviation for measurements was 5%. 43

42

20 mL of filtered WSA samples were placed in standard vials. Total carbon (TC) was automatically 4 45

determined with the program installed in the equipment, which then contemplate the addition of 46 48

47

acid to force evolution of CO2, measuring again the remaining organic soluble material which 50

49

represents TOC. All the steps are controlled by the software provided with the system. CO2 51 52 53 54 5 56 57 58


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produced by combustion of carbon containing materials is detected with an infra-red detector. 5

4

Details about the use of TOC-Vcp equipment is available elsewhere.24 6 8

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2.3.3. pH meter. pH was measured using a five easy Mettler Toledo pH meter (Mississauga, 10

9

Canada). The relative standard deviation for the analysis was 2%. 1 13

12

2.3.4. Gas Chromatography. Compositional analysis of the reaction gas phase was carried out 14 16

15

with a gas chromatograph SRI Model 8610C, (Menlo Park, CA, USA) fitted with a Thermal 18

17

Conductivity Detector (TCD) and two packed columns connected in parallel, a Molecular Sieve 19 20

13X and a Hayasep-D from SRI Instruments. 21 2 24

23

The amount of CO2, O2, CH4, and other low molecular weight hydrocarbons (less than seven 26

25

carbon atoms) in the gas effluents from the reaction, were measured five times at the end of each 28

27

test, and the average of measurements was used for calculations. The relative standard deviation 29 31

30

for analysis was 5 %. 32 34

3

2.3.5. UV-Visible. The analogs of fulvic and humic acids, and water-solubilized asphaltenes, were 36

35

analyzed using Evolution 260 Bio UV-Visible spectrometer (Thermo Scientific, Ontario, Canada) 37 39

38

to find E4/E6 ratio absorbance (400/600 nm wavelengths). Deionized water was used for preparing 41

40

the analyzed sample solutions. The relative standard deviation for analysis was 5%. 42 4

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2.3.6. FTIR Analysis. FTIR spectra in diffuse reflectance mode were recorded on a Nicolet 6700 45 46

spectrometer (Thermo Scientific, Waltham, MA USA) with a spectral resolution of 8 cm-1, over 47 49

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the range of 4000-500 cm-1. The background (pure KBr) was collected every two hours, being all 51

50

spectra baseline corrected. Water solubilized asphaltenes, humic, humin and fulvic analogs were 52 53 54 5 56 57 58


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brought to dryness using a rotary evaporator and a vacuum oven, to avoid water contribution to 5

4

the IR spectra. 6 7 8

2.3.7. Humic/Fulvic analogs separation. For separating humic and fulvic analogs from WSA, 9 1

10

HCl (37%) was added dropwise to WSA to decrease the pH of the product to 9). However, at higher temperatures (200-230°C), more acidic 36 38

37

functions were produced, and the pH decreased from neutralization reactions. Elemental analysis 40

39

of WSA between 190 °C-200 °C showed no significant change in comparison with the feed (H/C: 41 43

42

1.22, N/C:0.01), as the severity of reaction was not high (below 35 % conversion). 4 46

45

Residence time effects were also evaluated by varying this parameter from 30 min to 5 hours under 48

47

a constant temperature of 180 °C (results are shown in Fig. 3.). The reason for choosing 180°C as 49 51

50

operating temperature was to screen better the effect of residence time in oxy-cracking, as at higher 53

52

temperatures, the conversion was high even at short reaction times. By increasing time, conversion 54 5 56 57 58


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of asphaltenes to CO2 and WSA increased, but selectivity to WSA decreased and reached a plateau after 3 hours. The results showed that the optimum residence time for high selectivity and conversion to WSA was 2-2.5 hours. Therefore, for having more selectivity to WSA, it was found to be more appropriate to work at reaction times within the preceding time window. The effect of residence time on selectivity to WSA was similar to temperature effect. The selectivity to WSA changed between 80-30% from 0.5-5 hrs residence times, while temperatures varying in the range of 180 °C-250 °C changed selectivity to WSA from 80 % to 30 % (see Fig. 1). However, higher conversions were attainable at high temperature (˷ 100% conversion at 250 °C) and CO2 ramped up to 70% while its amount was below 50% at the longer tested residence times (at 180 °C). The pH of WSA samples was investigated as a function of residence times. According to results shown in Fig. 4, with increasing residence time, pH decreased. A possible explanation for these results is that, with increasing the residence time, more acidic compounds were produced. Another finding was that by increasing residence time beyond 2 hours, the hydrogen to carbon ratio decreased from 1.21 to 1.06, ascribed to the fact that oxygen cracks more asphaltene molecules and detaches the aliphatic moieties from their structures. These findings confirm that by increasing the severity of the oxy-cracking reaction, alkyl moieties disappear, being substituted with oxygen functions (evidence in this direction is provided by FTIR, discussed later in this article). From the preceding, the water solubilized sample is expected to increase the aromatic character under longer reaction times.

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3.2. Effect of Pressure on asphaltenes oxy-cracking reaction: 4 6

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For investigating the pressure effect over the production of WSA, pressure was changed between 7 8

340-1000 psi, at a constant temperature of 180 °C, and residence time of 0.5 hr. Again, lower set9 1

10

up values for temperature and residence time were chosen to screen better the effect of pressure 12 13

on selectivity and conversion. 14 15 17

16

One fundamental aspect for the subcritical liquid water oxy-cracking is guaranteeing that water is 19

18

in the liquid form. For having water in liquid form at temperatures spanning between 180-250 °C, 20 21

working pressure should be higher than the equilibrium water vapor pressure; this pressure is at 2 24

23

least 145 psi for 180 °C and 580 psi at 250 °C.27 Pressure effects on oxy-cracking selectivity and 25 26

conversion are shown in Fig. 5. 27 28 30

29

The experiments showed that the selectivity to WSA did not change significantly with increasing 32

31

the pressure; however, the conversion increased, reaching desired values (~85%). The production 34

3

of CO2 was kept at a reasonable low and constant value (26-28%). Thus, it is believed that optimum 35 37

36

reaction conditions for maximizing WSA production were identified. However, a trade-off should 39

38

be determined by weighting economics. It is not economical to work at high pressures, such as 40 41

1000 psi since the capital cost of the process would increase by the required thickness of reactors. 42 43 45

4

3.3. The Base effect on asphaltenes oxy-cracking reaction conversion and selectivity: 46 48

47

The effect of different bases (NaOH, KOH, and NH4OH) on the WSA production was examined. 50

49

In each experiment, 1 mL 5N base was added to 20 mL water and 0.5 gr asphaltenes. The results 51 53

52

are shown in Fig. 6. Initially, NaOH was added to asphaltenes and water to prevent corrosion 54 5 56 57 58


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produced by the high acidity generated during the oxy-cracking process; but further experiments 5

4

were conducted to determine whether or not metal cations from different hydroxides influence the 6 8

7

outcome of the reaction. Obtained results showed that addition of NaOH enhanced the asphaltenes 10

9

solubilization (The results will be discussed in further details in an ensuing article). These results 12

1

were consistent with other researchers’ findings, showing that addition of pH adjusters could 13 15

14

increase carbon incorporation into the water phase.26, 17

16

28

The results showed that the best

performance was determined for KOH. Adding this base increased the asphaltenes conversion, as 18 19

well as selectivity to WSA, more than NaOH and NH4OH at similar pH. NH4OH was found to 20 2

21

enhance WSA solubility the least. From these findings, it is suggested that metals from basic 24

23

hydroxides can have catalytic effects on WSA production beyond the mere neutralization or 25 27

26

saponification equilibrium displacement. Nevertheless, further research should be carried out to 29

28

confirm these aspects. 30 32

31

3.4. Effect of Asphaltenes Mass on oxy-cracking reaction: 3 35

34

Experiments were carried out at the 180 °C to screen the effect of asphaltenes mass on the oxygen 36 38

37

consumption, selectivity, and conversion. Asphaltenes weights between 0.5-4 gr were used for half 39 40

an hour of reaction at 500 psi. The water amount was constant (20 gr), and NaOH increased 41 43

42

proportionally to the increased asphaltene mass. 45

4

In Fig. 7A, the amount of consumed oxygen is shown against the mass of asphaltenes used. This 46 47

figure shows that the amount of consumed oxygen increased with the amount of used asphaltenes, 48 50

49

even though it is not in direct proportion to the mass. As the oxygen consumption did not increase 52

51

proportionally with asphaltene mass, two different regimes for the reaction could be observed. For 53 5

54

asphaltenes mass equal or below 1 g, chemical kinetics is controlling the rate of reaction, but for 56 57 58


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1 3

2

asphaltenes mass greater than 1 g, the oxygen consumption did not increase proportionally with 5

4

asphaltene mass, which indicates internal diffusion for asphaltene particles is controlling the rate 6 8

7

of the reaction. In fact, for asphaltenes mass greater than 1, asphaltenes could create aggregates 10

9

inside the reactor thus increasing mass transfer resistance and then control the reaction rate. Figure 1 12

7B shows a decline of asphaltenes conversion to WSA with increasing asphaltenes mass. The 13 15

14

selectivity to WSA was found to decrease very little with increasing asphaltenes mass, plotted as 17

16

the ratio of water/asphaltene. Results could indicate oxygen mass transfer limitation for water to 18 19

asphaltenes ratio below 20. So, for having a higher conversion, reactors with higher volumes 20 2

21

should be used for operation with higher water/asphaltenes ratios, which is another factor subject 24

23

to tradeoffs because large reactors are not attractive options. 25 27

26

3.5. Effect of stirring speed on asphaltenes oxy-cracking: 29

28

Another important factor for reactions is mixing rate. In the present study, mixing speeds were 31

30

varied between 100-1000 rpm. Varying stirring rate had two purposes: one of them was having 32 34

3

uniform contents in the reactor, and the other one was enhancing the oxygen transfer from the gas 36

35

phase to the liquid phase by minimizing the gas-liquid mass transfer resistance. Therefore, a higher 38

37

mixing rate would result advantageous for aqueous phase oxidation.27 39 41

40

The results are shown in Fig. 8. At a mixing speed of 500 and 1000 rpm, produced CO2 values 43

42

were very similar, suggesting that mixing speeds between 500 and 1000 rpm (and probably even 4 45

higher mixing speeds) have no effect on reaction progress and conversion. In other words, there is 46 48

47

no effect of mass transfer when using 500 and 1000 rpm. This finding implies that well-mixed 50

49

reactor content was maintained. The Reynolds numbers (Re) in different mixing rates are shown 51 52

in Table 1 (Re should be more than 10000 in stirred vessels to keep the turbulent regime29). 53 54 5 56 57 58


Page 17 of 47

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1 3

2

According to Re results, the system is in turbulent region and the gas-liquid mass transfer 5

4

resistance has been minimized. However, the situation became different when stirring speed was 6 8

7

decreased to 250 rpm. At this mixing rate, asphaltenes conversion to CO2 and WSA reduced. Fig. 10

9

8 illustrates the mass transfer controlling region. All the experiments were carried out at 180 °C, 1 12

residence time of 1 hr, and pressure of 500 psi. Thus, for avoiding mass transfer resistance the 13 15

14

mixing rates should be kept >= 500 rpm. 17

16

For calculating Re, Equation 4 was used. In this equation, density and viscosity of water at 180 °C 18 20

19

and 500 psi were used. D is the impeller diameter which was 2 cm in these experiments. 21 2

𝑅𝑒 = 24

23

𝜌𝑁𝐷2

(4)

𝜇

25 26 27 29

28 30

With 𝜌 = 888.6 31

𝑘𝑔 𝑚3

𝜇 = 0.00015 𝑃𝑎. 𝑠

32 3 34 36

35

Shear effects might also influence asphaltenes oxy-cracking reactions; however, study of these 38

37

require impellers with different symmetries and further experiments which were not contemplated 40

39

in the present study. 41 43

42

3.6. Summary of Asphaltene oxy-cracking optimization results 45

4

Based on the preceding experimental results, the following important optimum parameters were 46 47

identified. 48 50

49

1) Temperature and residence time were found to be the most important factors for optimizing 52

51

oxy-cracking reactions. For having a closer look to temperature and residence time combined 53 54

effects on selectivity and conversion of asphaltene oxy-cracking, these parameters were 5 56 57 58


Energy & Fuels

Page 18 of 47

1 2 3

investigated at temperatures between 180-225 °C and residence times of 0-2.5 hrs. The results 4 5

of conversion and selectivity as a function of temperature and residence time are respectively 6 7 8

shown in Figs 9A, B. Based on the results plotted on Fig. 9, for having desired conversion 9 10

(above 75%), and selectivity (above 64%), reaction times between 1-2 hr and temperatures 1 12

between 180-210 °C would be preferred. 13 15

14

2) Pressure increment could increase reaction conversion, but it does not affect selectivity 16 17

considerably. 18 20

19

3) To avoid mass transfer limitations in asphaltene oxy-cracking, mixing rates above 500 rpm 21 2

and water to asphaltene ratio equal or greater than 20 would be preferred. 23 24

3.7. Insights into WSA structures for products obtained under different reaction severities 25 27

26

3.7.1. FTIR investigation of WSAs produced at various temperatures 29

28

Fig. 10 shows how water solubilized product structures changed when these were produced at 30 32

31

different reaction temperatures. The outstanding features of the three studied cases are: 1) the 34

3

strong –OH bands (˷ 3500 cm-1), 2) carboxyl anions (doublet at ˷ 1450 and 1610 cm-1). 35 36

Hydrocarbon bands are barely visible (alkyls at ˷ 2850-2950 cm -1) and aromatics presence is not 37 39

38

evident; in the later case, the C=C stretch band (1600 cm-1) can be present, however overlapping 41

40

the carboxyl anion band. Aldehydes (˷ 2500 cm -1), carbonyls (1750 cm -1, probably esters), 42 4

43

sulfones (˷ 1150 cm -1) and sulfonics (˷ 900 cm -1) were all contributing in lower proportions to 45 46

the products derived from the higher tested temperatures (210 °C and 220 °C). The disappearance 47 49

48

of alkyl bands in the product obtained at a lower temperature (2550-2950 cm -1 bands) may be due 51

50

to partial oxidation without rupture of the alkyl chains. The carboxylic acid C=O band (˷ 1710 cm 52 54

53 5

-1

) is not clearly observed, i.e. seems to overlap with the carboxyl anion, appearing as a shoulder.

56 57 58


Page 19 of 47

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2

All the findings indicate that carboxyl anions and hydroxylated species were found to contribute 5

4

the most to produced WSAs. 6 7 8

3.7.2. Metal distributions for WSA, feed and remaining solids 9 10 12

1

Fig. 11 shows metal analysis carried out for the feed, remaining solid after reaction (humin 14

13

analogs) and water solubilized liquid products for the reactions performed at 190 °C and 210 °C. 15 17

16

The primary metals present in the feed are nickel, iron, and vanadium. By oxy-cracking asphaltene 19

18

particles, the heavy metals like nickel and vanadium remained within non-solubilized solids 21

20

(humin analogs) and did not transfer to WSA. So, the water solubilized product is practically free 2 24

23

of the initially present two toxic metals (V/Ni). Noticeably, more iron content was observed in the 26

25

humin analogs. The reason for higher iron contents in remaining solids could be the high acidic 27 28

reaction environment existing at the end of oxy-cracking reactions leading to corrosion of the 29 31

30

reactor parts in contact with the reaction medium (impeller, thermowell). 32 34

3

The humin analogs include sodium which could be a sign of formation of salts (phenolates, 35 36

carboxylates) or sodium carbonate trapped inside the solids. The solubility of sodium carbonate 37 39

38

decreases with rising temperature (sodium carbonate solubility in water is only 20.7-25 gr/ 100 ml 41

40

at 180-225 °C 30). The previous findings could indicate the formation of mineral layers around the 42 4

43

remaining asphaltenes particles. Non soluble solids (humins) were found to concentrate elemental 46

45

nitrogen (N) when compared to the feedstock asphaltenes (respective N/C ratios of 0.02 and 0.01). 47 49

48

WSA include mostly sodium, presumably in the form of sodium carboxylates and sulfonates, as 50 52

51

suggested by FTIR (Fig. 10). These WSA fractions were also observed to concentrate elemental 54

53

N when compared to the original asphaltenes (see Fig. 4). 5 56 57 58


Energy & Fuels

Page 20 of 47

1 3

2

3.8. Humin, fulvic and humic analogs product distributions obtained under different 5

4

reaction severities 6 8

7

A solubility criterion was pursued in the present work for separating asphaltenes water solubilized 10

9

fractions into analogs of humic fractions, described by other researchers both related to soil or coal 1 12

materials and natural products. 13 15

14

By addition of HCl (37 %) to WSA bringing the pH to humic > fulvic analogs. 12 14

13

In all cases, aromatic functionalities were observed to survive the oxy-cracking processing, 15 16

particularly for the humin analogs. 17 18 20

19

3) The same oxygenated functionalities were found to increase for all the studied fractions (humin 2

21

/ humic / fulvic analogs): -OH/ C=O (acids), aldehydes C=O, esters/ lactones C=O, sulfones 23 24

O=S=O. Only for the fulvic fractions, the phenolic -OH bands were observed. 25 26 28

27

4) Carboxylic acids plus carboxyl anions were observed present in studied WSA samples. For 30

29

HA/FA fractions isolated following the described separation protocol, i.e., filtration carried out 31 32

under pH ˷ 1, only the carboxylic (protonated) functions were detectable via FTIR. 3 34 36

35

3.10. Insights into the spanned range of molecular weights of WSA and separated fractions 38

37

using UV-visible spectroscopy 39 41

40

For having insights into the average molecular weights of WSA and separated fractions, samples 42 4

43

produced under different severities were analyzed using Uv-Visible spectrophotometry (Uv-Vis). 46

45

Uv-Vis is one standard technique used in soil science for evaluating molecular weights of humic 47 48

substances. Molecular weights are correlated with the E4/E6 ratio, determined as the ratio of 49 51

50

sample absorption at 400 and 600 nm.33 According to Chen et. al., the E4/E6 ratio of humic 53

52

materials is inversely proportional to their molecular weight. They also found that E4/E6 ratios 54 5 56 57 58


Page 27 of 47

Energy & Fuels

1 3

2

were not concentration dependent, however, were found to depend on the carbon and oxygen 5

4

contents, pH and amount of -COOH groups.40 E4/E6 ratios for natural humic acids have been 6 7

reported to be in the 3-5 range and fulvic acids in the 5-8 range.41 9

8

1

10

Fig. 18 shows the E4/E6 ratios for WSA products and the corresponding extracted humic and 13

12

fulvic analogs obtained under increasing carbon conversion conditions. The results revealed that 14 16

15

for WSA samples obtained with carbon conversions lower than 92%, the E4/E6 ratios felt below 18

17

5, which is characteristic of natural humic acids or, put into other terms, large components. For 20

19

carbon conversions >92%, smaller components, possibly analogs of fulvic materials were 21 23

2

observed, spanning E4/E6 ratios from 6-12. Separated fulvic analogs were found to have very large 25

24

E4/E6 ratios up to 36. These higher E4/E6 ratios obtained beyond typical values for natural fulvic 26 27

analogs and lower E4/E6 ratios for natural humic analogs (

Asphaltenes Aqueous Conversion to Humic and Fulvic Analogs via

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