Article pubs.acs.org/jced
Determination of Hydrogen Solubility in Heavy Fractions of Crude Oils by a Modified Direct Method Shunfeng Ji, Zongxian Wang,* Aijun Guo, Ying Zhou, and Kun Chen State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum (East China), 66 Changjiang West Road, Qingdao Economic Development Zone, Qingdao Shandong 266580, China ABSTRACT: The solubility of hydrogen in Venezuelan heavy coking gas oil (HCGO), Karamay atmospheric residue (KRAR), Liaohe atmospheric residue (LHAR), and Venezuelan atmospheric residue (VNAR) was measured at temperatures in the range 150 °C to 350 °C at pressures up to 11 MPa by a modified direct method. The measurement accuracy and reliability were examined by determining hydrogen solubility in toluene and n-heptane at temperatures of 25 °C, 50 °C and 100 °C, and results are in a good agreement with the data reported. The order of hydrogen solubility in the four feedstocks at the same experimental condition is VNAR < LHAR < KRAR < HCGO. The impact of oil properties, such as aromatic ring number and alkyl chain length, on hydrogen solubility was discussed. The results indicate that the condensed aromatics play a key role in the fractions of high boiling points. The hydrogen dissolving capacity in different derived oils with the same boiling range can be tentatively determined via analyzing the H/C atomic ratio of the oils.
1. INTRODUCTION The prominence of heavy oil such as bitumen is gradually increasing in today’s crudes market. It is a challenge for refineries to efficiently upgrade heavy oils to meet the need for transportation fuels.1 Of all the upgrading processes, hydrogenation or hydrocracking is a good choice to process the heavy oils or heavy fractions (i.e., residues). The solubility of hydrogen in heavy oils at elevated pressures and temperatures is a key factor affecting the hydrogenating efficiency, and also plays an important role in designing and optimizing the operation of the processes.2,3 However, hydrogen solubility data in heavy oils or heavy fractions are scarcely reported compared with that in model compounds or light fractions.3,4 Thus, the primary aim of this study is to obtain the data of hydrogen solubility in heavy fractions. There are several methods to determine the solubility of gases in liquid. Some of them can be used to measure hydrogen solubility at elevated pressures and temperatures. One is denoted as the direct method,3 in which a known quantity of liquid was withdrawn after the dissolution equilibrium was established in an autoclave. Hydrogen solubility was calculated by measuring the quantity of hydrogen separated out from the liquid later. For example, Lal et al.5 measured solubility of hydrogen in Athabasca bitumen by determining the volume of hydrogen separated out from the liquid using a calibrated buret. Ronze et al.6 determined hydrogen solubility in straight run gasoil by analyzing with a gas chromatograph the gases that separated out. Another method is typically called the indirect method.3 A known amount of hydrogen was charged into an optical cell or view cell, and hydrogen solubility was then calculated from the quantity change of hydrogen at equilibrium using PVT data. Among the studies3,7−10 using the indirect method, Cai et al.3,7 measured hydrogen solubility in heavy oil © XXXX American Chemical Society
and bitumen cuts, and also discussed the influence of solid additives on hydrogen solubility. Besides the two methods, Ding et al.11 measured hydrogen solubility in coal liquid using two in situ hydrogen probes to determine the hydrogen partial pressure in the equilibrium cell. Liaw et al.12 determined the hydrogen partial pressure by a hydrogen meter. In general, the direct method is thought to be more accurate,3 but sampling creates a host of deviations in operations or calculations. A U-shaped calibrated buret was often used to measure the hydrogen volume during sampling in the direct method.4,5 However, height difference of the liquid levels in the calibrated buret makes the hydrogen compressed after sampling. Hydrogen volumes determined are thus relatively smaller. To eliminate this error, further analyses and calculations are needed. This paper proposes a modified direct method for determining hydrogen solubility in heavy fractions of crude oils. A combined connecting vessel, which consists of a surge flask, a volumetric cylinder, and an inverted U-pipe, was applied to determine the volume of hydrogen separated out from the liquid instead of the U-shaped calibrated buret. The liquid levels in the combined connecting vessel could be adjusted horizontally by an elevator platform. This modification could reduce the number of calculation parameters (such as the volume of inner system) and simplify the calculation process, making the measurement more convenient and accurate without using gas chromatography. With the use of the modified method, hydrogen solubility in different heavy fractions was determined, and the possible correlations between Received: August 12, 2013 Accepted: November 14, 2013
A
dx.doi.org/10.1021/je400729v | J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Figure 1. Schematic diagram of the apparatus, 1, gas cylinder; 2, stirrer motor; 3, thermocouple well; 4, stirrer; 5, magnedrive; 6, liquid sample line; 7, liquid sample flask; 8, surge flask; 9, combined connecting vessel; 10, elevator platform.
established when the pressure was constant. For heavy fractions, a time of repose was necessary before sampling to ensure the bubbles caused by stirring separated out entirely. Otherwise, the hydrogen solubility would be inflated. The volume of water in the volumetric cylinder could be determined by reading the calibration on the side of the volumetric cylinder during the time of repose. The whole system was airtight. By turning on or off the two needle valves in sequence, about 2 g of liquid saturated with hydrogen was sampled into a weighed sample flask. Hydrogen was then separated out from the liquid under the atmospheric pressure and room temperature, pushing the water of the surge flask into the volumetric cylinder. The liquid level of the volumetric cylinder rose. When the temperature in the sample flask was equal to the room temperature, the two liquid levels would be adjusted horizontally by regulating the elevator platform. Then the volume change of the water in the volumetric cylinder was the total volume of hydrogen and liquid sample. The deviation in reading the value changes was about ± 0.1 mL. One hypothesis was needed here: hydrogen was seen as an ideal gas under normal temperature and pressure. Ultimately, the hydrogen solubility was calculated using the liquid mass, liquid density, hydrogen volume, atmospheric pressure, and room temperature. The hydrogen solubility (defined as S) was expressed as mole number of hydrogen per kilogram of liquid solvent. 2.3. Determination of the Vapor Pressures of the Feedstocks.13,14 Vapor pressures were used to calculate the partial pressures of hydrogen. About 65 g of degassed liquid was loaded into the autoclave cell, and the cell was filled with hydrogen. Then, 0.1 MPa hydrogen was charged into the cell, and this initial pressure (p0) and room temperature (T0) were both recorded. The autoclave was heated at a heating rate of 3 °C·min−1 without stirring. Total pressures were recorded as the temperature increased. Two assumptions were made: the mass of hydrogen in the cell was constant during the heating process; gases in the cell were treated as ideal gases under these conditions. Thus, the vapor pressures (pv) of the feedstocks could be calculated by eq 1:
hydrogen solubility and the properties of heavy oils were discussed.
2. EXPERIMENTAL SECTION 2.1. Apparatus. The apparatus for determining gas solubility in liquids is shown schematically in Figure 1. Briefly, the apparatus consists of three parts: equilibrium system, sampling system, and measure system. The equilibrium system is a stainless steel autoclave with a two-blade magnetically driven stirrer. The inner volume of the autoclave is about 300 mL. The two-blade magnetically driven stirrer can stir the gas phase and the liquid phase simultaneously in order to decrease the dissolution equilibrium time and keep the equilibrium state steady. The autoclave can support tests with operating pressures up to 20 MPa and temperatures up to 450 °C. The pressure in the autoclave is determined by a calibrated digital pressure gauge whose accuracy is 0.001 MPa. The uncertainty of the pressure gauge is ± 1 kPa. The temperature is determined with a thermocouple inserted into a thermocouple well which is immersed into the liquid phase. The deviation of temperatures from the setting value is ± 0.5 °C. The sampling system is composed of a stainless steel line pipe immersed into the liquid phase, a liquid sample flask, and a surge flask. Approximately 2 g of liquid sample can be taken out for further analysis. The measure system is a combined connecting vessel made up of a surge flask and a volumetric cylinder, both of which contain some water (i.e., the dashed area of measure system in Figure 1). The surge flask and the volumetric cylinder are connected with an inverted U-pipe which is inserted into the water of both sides respectively. The both ends of the Upipe are horizontal in the water. Gases are not allowed in the Upipe. 2.2. Procedure. About 65 g of degassed liquid was loaded into the autoclave cell. Different with the available literatures,4,5 in which at least 250 g of liquid was used, less liquid in this study could shorten the equilibrium time. However, the minimum of the liquid loaded must be enough to ensure that the bottom stirring blade immerges in the liquid. Then the autoclave cell was filled with hydrogen. The autoclave was heated to the desired temperature, and hydrogen was then charged into the autoclave. Stirring was carried on at 300 rpm during the whole process except sampling. The pressure drop was due to the dissolution of hydrogen. Equilibrium was
pv = p −
T p T0 0
(1)
where p is the total pressure, and T is the desired temperature. B
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2.4. Materials. The hydrogen used in this paper has a purity of 99.999%. Toluene was an analytical grade reagent which was bought from Sinopharm Chemical Reagent Co., Ltd. Feedstocks included Venezuelan heavy coking gas oil (HCGO), Karamay atmospheric residue (KRAR), Liaohe atmospheric residue (LHAR), and Venezuelan atmospheric residue (VNAR). HCGO (BP range: 270 °C to 518 °C) was achieved from the Liaohe refinery. The three atmospheric residues were distilled from crude oils originated from the China Fengcheng oilfield, China Liaohe oilfield, and Venezuela Orinoco oilfield, respectively. The initial boiling points of the three atmospheric residues are all about 350 °C.
3. RESULTS AND DISCUSSION 3.1. Measurement Accuracy. The modified hydrogen solubility measurement is a direct method in which sampling is a necessary procedure. Sampling makes the determination intuitive by determining the quantity of hydrogen dissolved directly, but it contains a host of uncertainties at the same time. Thus, to ensure the accuracy of the measurement, it is vital that each procedure during sampling should be as accurate as possible to minimize these uncertainties. The pressure gauge and thermocouple were calibrated previously, and the air tightness of the autoclave was also tested before any formal experiment was performed. When the dissolution equilibrium in the autoclave was established, the stirring was stopped. A repose time was necessary for the escape of bubbles in the liquid before sampling. Otherwise, a serious deviation would generate in the tests. Generally speaking, the time of repose needed increases as the viscosity of the feedstock increases. 1.5 h was thus chosen as the time of repose for each measurement, based on the values determined in the case of VNAR (with the highest viscosity among the feedstocks) at 150 °C. In addition, visual error in adjusting the two liquid levels horizontally added ± 0.1 mL of the hydrogen volume to the uncertainty in the procedure. Further, hydrogen solubility in toluene at 100 °C at 6 MPa was measured four times, and the error range was controlled at ± 5 %. The overall error of the modified method was then assessed by comparing hydrogen solubility values in toluene and nheptane determined by this method with reliable data reported previously4,8 (as shown in Figure 2a,b). It is indicated that a good agreement is shown between two sets of the values. Deviations of the two sets of data are all within ± 5 %. In general, this modified direct method has proven to be acceptable for determining the hydrogen solubility in liquids. 3.2. Characterization of Feedstocks. Three atmospheric residues (VNAR, LHAR, and KRAR) and a gas oil (HCGO) were used as feedstocks, selected properties of which are shown in Table 1. As shown in Table 1, the viscosities of the residues are much higher than that of gas oil, especially for VNAR. Necessary repose time is needed to ensure that the bubbles caused by stirring in the liquid phase could separate out before sampling. The molecular weight of HCGO is much smaller than that of the three atmospheric residues, which may mean the ring number or the chain length of the hypothetical molecule in HCGO is smaller.15 The increasing H/C atomic ratio of the feedstocks is accompanied by the decreasing aromaticity (fA) of feedstocks in the order: VNAR > HCGO > LHAR > KRAR. 1 H NMR analysis was carried out to further characterize the feedstocks, and the average structural parameters were
Figure 2. Hydrogen solubility in (a) toluene and (b) n-heptane.
Table 1. Properties of the Four Feedstocks sample density at 20 °C (g/cm3) kinetic viscosity at 100 °C (mm2/s) molecular weight elemental analysis C (wt %) H (wt %) S (wt %) N (wt %) H/C atomic ratio nickel (ppmw) vanadium (ppmw) average structural parametersa aromaticity (%) paraffinic carbon rate (%) total number of rings number of aromatic rings
VNAR
LHAR
KRAR
HCGO
1.0297 3327
0.9821 673
0.9650 1382
0.9691 9
781.7
686.3
736.8
360.0
83.82 9.96 4.31 0.71 1.42 113.00 417.00
86.83 11.42 0.36 0.86 1.57 68.70 1.81
85.89 11.53 0.36 0.92 1.60 43.50 0.77
85.54 10.82 2.61 0.45 1.51 0.06 0.06
0.33 0.41 7.45 3.97
0.25 0.54 5.57 2.76
0.22 0.55 5.92 2.58
0.31 0.41 3.25 1.46
a
The average structural parameters were determined by the modified Brown-Ladner (B-L) method, based on 1H NMR.
calculated using the modified Brown-Ladner (B-L) method.15 The data are also listed in Table 1. It is shown that VNAR has the highest fA and aromatic ring number (RA), which indicates that VNAR contains more condensed aromatics than the other feedstocks.15 While HCGO has a high value of fA but the lowest C
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Table 2. Hydrogen Solubility in the Four Feedstocks VNAR
LHAR
KRAR
HCGO
T/°C
p/MPa
S/mol·kg−1
p/MPa
S/mol·kg−1
p/MPa
S/mol·kg−1
p/MPa
S/mol·kg−1
150
2.154 4.202 6.531 8.737 10.124 1.897 4.083 6.958 8.356 10.037 2.013 4.373 6.307 7.963 10.740 1.640 3.983 6.036 8.437 10.203 1.932 4.526 6.463 7.867 10.025
0.0802 0.1363 0.1682 0.2201 0.2315 0.1146 0.1756 0.2482 0.2635 0.3108 0.1525 0.2387 0.2824 0.3203 0.3940 0.1742 0.2736 0.3335 0.3983 0.4325 0.2033 0.3225 0.3811 0.4438 0.5012
1.233 4.023 5.849 8.053 9.324 2.106 4.213 5.295 8.486 10.039 1.707 3.987 5.828 8.016 10.833 1.824 4.326 6.257 8.017 9.430 1.713 4.357 6.484 9.684 10.377
0.0819 0.1583 0.2222 0.2730 0.3505 0.1441 0.2167 0.2282 0.3928 0.4435 0.1364 0.2465 0.3218 0.4012 0.5126 0.1759 0.2903 0.4049 0.4587 0.5195 0.2111 0.3612 0.4730 0.6369 0.6891
2.213 4.123 5.939 7.867 9.384 2.031 4.411 5.604 8.023 10.343 2.267 4.153 6.832 8.463 10.695 2.051 4.213 6.351 8.127 10.447 2.053 3.900 6.067 9.522 10.563
0.1524 0.2013 0.2765 0.2879 0.3612 0.1564 0.2401 0.2745 0.3325 0.4212 0.2146 0.3072 0.4046 0.4525 0.5058 0.2513 0.3297 0.4292 0.5307 0.6049 0.2747 0.3743 0.4813 0.6639 0.7279
1.860 4.125 6.036 7.100 9.484 1.912 3.894 6.102 7.111 9.652 1.952 4.012 5.864 6.998 9.584 1.756 3.842 5.892 7.000 9.471 1.984 4.124 6.956 8.198 9.984
0.0842 0.1451 0.2056 0.2431 0.3221 0.1044 0.2136 0.3332 0.3753 0.5270 0.1431 0.2838 0.4298 0.5130 0.7025 0.1458 0.3190 0.4892 0.5712 0.7864 0.2748 0.4338 0.6228 0.7242 0.8796
200
250
300
350
RA (1.46), which means that the most aromatic molecules in HCGO are the monoring structures.15 3.3. Hydrogen Solubility in Heavy Fractions. Hydrogen solubility in heavy fractions was measured at temperatures from 150 °C to 350 °C at pressures up to 11 MPa; 150 °C was chosen as the lower temperature limit because the viscosities of the oils below that temperature are too high to properly perform experiment, and the upper temperature limit was determined by their thermal stability. The hydrogen solubility data acquired in the study are listed in Table 2. As shown in Table 2, hydrogen solubility in VNAR increases as the temperature and pressure increase. And at a given temperature, the hydrogen solubility increases linearly as the pressure increases. The similar trends are also shown in the other cases of three feedstocks. In general, the values of hydrogen solubility of the feedstocks at the same experimental condition follow in the order: VNAR < LHAR < KRAR < HCGO, as shown in Figure 3 for example, in which the values were determined at 350 °C. Since the experimental operations and conditions are the same, the most probable factor that causes the differences of the hydrogen solubility in different oils is the properties of the feedstocks. Park et al.16,17 reported that the hydrogen solubility in paraffins increases as the length of chains increases and the hydrogen solubility in aromatic hydrocarbons decreases as the number of aromatic rings increases. In addition, hydrogen solubility in alkanes is also believed to be higher than that in aromatics.8,16,17 For HCGO, in which the most aromatic molecules are the monoring structures, the impact of condensed aromatics (RA of HCGO is only 1.46) on hydrogen solubility is lower, which may cause the higher hydrogen solubility in HCGO. For the three residues, RA increases remarkably which means the impact of condensed aromatics
Figure 3. Hydrogen solubility in the four feedstocks at 350 °C.
becomes greater, leading to the lower hydrogen solubility than that of HCGO. Moreover, the trend of hydrogen solubility in three residues (i.e., VNAR < LHAR < KRAR) is totally opposite to the order of RA of feedstocks, which may indicate that the hydrogen dissolving capacity of different oils with similar boiling range could be determined tentatively on the basis of RA. Further, higher RA normally means lower H/C atomic ratio. Thus, for the oils with the same boiling range, the H/C atomic ratio could be also taken as a convenient parameter to determine the hydrogen dissolving capacity. D
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tems at Elevated Temperatures and Pressures. Fuel 1991, 70, 771− 777. (13) Zhu, T.; Ling, K.; Shen, J.; Wang, Y.; Zhang, H. Solubilities of Hydrogen in Coal Liquefied Oil. Coal Convers. 2006, 29, 32−36. (14) Mu, X.; Ling, K.; Zhang, H. Study on Equilibrium Solubility of Hydrogen in Coal Tar. Coal Convers. 2009, 32, 50−56. (15) Liang, W. Petroleum Chemistry; China University of Petroleum Press: Dongying, China, 2009. (16) Park, J.; Robinson, R. L., Jr.; Gasem, K. A. M. Solubilities of Hydrogen in Heavy Normal Paraffins at Temperatures from 323.2 to 423.2 K and Pressures to 17.4 MPa. J. Chem. Eng. Data 1995, 40, 241− 244. (17) Park, J.; Robinson, R. L., Jr.; Gasem, K. A. M. Solubilities of Hydrogen in Aromatic Hydrocarbons from 323 to 433 K and Pressures to 21.7 MPa. J. Chem. Eng. Data 1996, 41, 70−73.
4. CONCLUSION A modified direct method, in which a combined connecting vessel consists of a surge flask and a volumetric cylinder is applied instead of the U-shaped calibrated buret in the conventional methods, for determining hydrogen solubility in liquids has been implemented successfully. The measurement accuracy and reliability were examined by determining hydrogen solubility in toluene and n-heptane at temperatures of 25 °C, 50 °C, and 100 °C; the results determined are in good agreement with data reported. This method was applied to measure hydrogen solubility in HCGO, VNAR, LHAR, and KRAR. The values of hydrogen solubility of the feedstocks at the same experimental condition follow in the order: VNAR < LHAR < KRAR < HCGO. Oil properties like condensed aromatics content have a big impact on hydrogen solubility. Through analyzing the H/C atomic ratio of the oils having the same boiling range, the hydrogen dissolving capacity could be tentatively determined.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: 086-0532-86981851. Fax: 086-0532-86981787. Funding
This work was supported by the China National Petroleum Corporation (CNPC) Foundation under the Grant “Research on Key Technology for Heavy Oil Processing” (W2008E-1503/ 01) and the Natural Science Foundation of Shandong Province, China (No. ZR2010BM008). Notes
The authors declare no competing financial interest.
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REFERENCES
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