Gang
Bai
a,
Qinzhen
Fan
*b,
Jianfeng
Sun
c,
Lihua
Cheng
b and
Xi-Ming
Song
*a
aLiaoning Key Laboratory for Green Synthesis and Preparative Chemistry of Advanced Materials, College of Chemistry, Liaoning University, Shenyang, 110036, China. E-mail: songlab@lnu.edu.cn; Fax: +86-24-62202378; Tel: +86-24-62202380
bCollege of Chemical Engineering, Guangdong University of Petrochemical Technology, Maoming 525000, P. R. China. E-mail: fanqinzhen@163.com; Fax: +86-0668-2873904; Tel: +86-13790909217
cFushun Research Institute of Petroleum and Petrochemical, Fushun 113001, P. R. China
First published on 25th September 2019
A novel forced separation method based on driving force vacuum sweating was used to prepare high melting point paraffin with high phase-change enthalpies. The effects of the vacuum pressure and final separation temperature on the forced separation of the paraffin components were investigated. The research results showed that the optimal vacuum pressure for forced separation was 80.0 kPa. The performance of forced separation was improved with the increase in the final temperature. Increasing the final temperature increased the driving force of the separation of solid-state components and liquid components during sweating, which improved the product yield, shortened the production cycle, and reduced the oil content. The phase changes exhibited by the separation products were tested at 70 °C under optimal conditions. The raw materials and forced separation products were analyzed through Fourier transform infrared spectroscopy analysis (FT-IR), gas chromatography analysis (GC), differential scanning calorimetry analysis (DSC), and X-ray diffraction analysis (XRD). The results of these analyses showed that as the forced separation temperature was increased, the carbon atom number distribution range of the products narrowed, and the content of n-paraffin was drastically improved. The content of n-paraffin in the final fraction obtained through the forced separation of paraffin was 89.8% with a phase-transition temperature of 69.74 °C and a phase-transition enthalpy of 214.71 J g−1. A significant solid–solid phase transition peak was observed in the final fraction obtained through the forced separation of paraffin, which verified that paraffin was an excellent phase-change material for energy storage.
However, there were many problems with the reported paraffin products, including a wide carbon atom distribution and low n-paraffin composition. Therefore, paraffin had difficulty in satisfying the demands of phase-change materials due to its high enthalpy during the solid–liquid phase transition, which limited its applications in thermal energy storage and industry applications.
The main methods used for paraffin production include molecular sieve dewaxing,11 urea dewaxing,12 molecular distillation,13 the solvent method,14 and sweating.15,16 Molecular sieve dewaxing and urea dewaxing are unsuitable for the production of PCMs with high melting points. Distillation, which involves separating different hydrocarbon substances in accordance with their different boiling points, requires a high number of tower plates for precise separation with increased pressure drops. Distillation is unsuitable for the production of paraffin due to the hydrocarbons forming solid paraffins and an oily fraction with higher average molecular weight petroleum distillates.
The solvent method, which separates normal paraffin and other hydrocarbon substances on the basis of their different solubilities in certain solvents, is unsuitable for the production of paraffin with high enthalpy performance because it has a negligible effect on narrowing the range of the carbon atom number distribution. The sweating method, which separates and purifies the various components in the raw materials on the basis of their different melting points, is a technically simple and environmentally friendly process that can be conducted without the addition of a solvent. It is significantly affected by the molecular weights and structures at the melting points of paraffin, which increase with the molecular weight. However, for the same molecular weight, the melting point of n-paraffin is higher than that of the branched paraffin associated with low melting points. Therefore, the sweating method could simultaneously enrich the n-paraffin content and narrow the range of the carbon atom number distribution, which makes it suitable for the production of PCMs. However, due to the higher content of branched alkanes in the waxes with high melting points and higher viscosity, a microcrystalline crystallization behavior is exhibited in the sweating method, which hampers the separation of the oil from the solid.
This study aimed to resolve the difficulties encountered in the complete separation of n-paraffin with high melting points from liquid oil with low melting points in conventional sweating. In this paper, a novel method of forced separation was studied to overcome the disadvantages of the low product yield and long operation period in the process of conventional sweating for the preparation of excellent phase-change paraffin. In this new method, the driving force of separation was increased by the formation of negative pressure, which improved the separation rate, enhanced the separation efficiency, and shortened the production cycle. The research results could provide both a practical foundation and theoretical support for the production of phase-change paraffin with high melting points, which could also be applied in related fields.
IBP | 10% | 30% | 50% | 70% | 90% | 95% |
---|---|---|---|---|---|---|
246 °C | 394 °C | 407 °C | 416 °C | 422 °C | 434 °C | 451 °C |
Paraffin wax | Value |
---|---|
Melting point (°C) | 64.3 |
Oil content (%) | 0.21 |
Saybolt color (Grade) | +30 |
Light stability (Grade) | 4 |
Needle penetration (1/10 mm at 25 °C) | 15 |
Kinematic viscosity (mm2 s−1 at 100 °C) | 5.421 |
Order (Grade) | 1 |
In this work, gravity was ignored, and the liquid was set to flow in a vertical tube with a tube radius of R and height of h during sweating. The upper pressure was P1, the lower pressure was P2, and P1 > P2, as shown in Fig. 2.
A cylindrical fluid with a tube coaxial radius of R was taken to calculate the pressure difference (ΔF) and the friction (f) from the following equations:
ΔF = (P1 − P2)πr2 | (1) |
![]() | (2) |
As the two forces were equal in a stable fluid, the following equation is obtained:
![]() | (3) |
![]() | (4) |
Given that r = R, v = 0, the flow rate was v when the radius was r, and we can get the following equation through the integration of both sides:
![]() | (5) |
A tubular flow layer with a radius of r and a thickness of dr was taken for analysis, and the cross-sectional area of the flow layer was dS = 2πrdr, therefore we could get the following equation:
![]() | (6) |
The general flow eqn (6) can be expressed as:
![]() | (7) |
Only considering the effect of gravity, the equation of the pressure difference could be rewritten as:
ΔF = mg = πr2ρgh | (8) |
As ΔF = f, the following equation is obtained:
![]() | (9) |
![]() | (10) |
Through integration, eqn (10) can be rewritten as:
![]() | (11) |
dS = 2πrdr | (12) |
![]() | (13) |
The performance of the volume of paraffin is calculated by the following equation:
![]() | (14) |
The equation for the volume of paraffin that flows out per unit time during the vacuum sweating is obtained by simultaneously integrating eqn (7) and (14)
![]() | (15) |
The separating efficiency (SE) is defined as the following:
![]() | (16) |
Eqn (15) shows that ΔP can be easily adjusted during vacuum sweating, and large ΔP values reflect that high volumes of liquid flow out per unit time. In conventional sweating, P1 = P2, that is, ΔP = 0. The formation of negative pressure in the lower part of the paraffin layer during sweating can increase ΔP, that is, increasing the driving force of the flow out of the liquid component during vacuum sweating improves the efficiency and the effect of the sweating (Fig. 3).
(1) The lower space of the dish plate of sweat device is filled with water. (2) Liquid raw material is then loaded into the sweating device after being heated to the melting point. (3) The raw material is cooled to a pre-set temperature at a rate of 4.0 ± 0.5 °C h−1. (4) The insulating water is released. The raw material is heated to the pre-set sweating final temperature at a warming rate of 1.0 ± 0.5 °C h−1. (5) The product is collected by continuously increasing the temperature to melt and remove the wax product. The vacuum system is activated during sweating, and the vacuum pressure is maintained in the buffer bottle to form a pressure difference up and down the paraffin layer during the forced separation sweating process.
As shown in Table 3, the vacuum pressure had little influence on the melting point of wax, but it could significantly reduce the oil content, which decreased with the increase in the vacuum degree. The maximum yield was obtained under a vacuum pressure of 80.0 kPa, but the yield decreased as the vacuum pressure further increased; this phenomenon could be explained by the fact that cracks were produced on the uneven wax layer by the excessive vacuum, which led to a lower product yield and higher melting point in the product due to the excessive local gas flow rate. Therefore, the vacuum pressure of 80.0 kPa was considered optimal in this experiment.
Item | I | V | VII | VIII | IX | X | XI | XII |
---|---|---|---|---|---|---|---|---|
a T -start and T-end are the start and end of the sweating temperatures, respectively. b Conventional sweating. | ||||||||
T -start/°C | 40 | 40 | 40 | 40 | 40 | 40 | 40 | 40 |
T -end/°C | 46 | 46 | 48 | 48 | 50 | 50 | 52 | 52 |
Vacuum degree/kPa | 0b | 80.0 | 0b | 80.0 | 0b | 80.0 | 0b | 80.0 |
Yield/% | 48.9 | 52.6 | 45.3 | 48.3 | 42.8 | 46.0 | 39.9 | 42.4 |
Oil content/% | 6.5 | 2.75 | 4.05 | 1.49 | 3.90 | 0.95 | 2.75 | 0.53 |
Melting point/°C | 53.8 | 53.9 | 54.6 | 54.5 | 54.6 | 54.8 | 55.2 | 55.2 |
As shown in Table 4, the oil content and the yield of the separated fractions decreased with the finial sweating temperatures for both conventional sweating and forced separation sweating, whereas the melting point increased with the temperature. Of particular concern was the wax oil content observed in experiment X for vacuum sweating, which decreased to less than 1%, whereas that observed in experiment XII significantly decreased to 0.53%. However, the wax oil content observed in experiment XI for conventional sweating remained high at 2.75% at the same final sweating temperature as experiment XII. Therefore, it was beneficial to increase the final sweating temperature for the separation of the paraffin fractions.
The comparative analysis of conventional sweating and forced separation sweating under different vacuum pressures and at different final sweating temperatures revealed that the vacuum sweating improved the yield, shortened the production cycle, and reduced the oil content, which could give a beneficial improvement for ordinary sweating. Compared with the conventional sweating, the driving force of the separation of solid-state components from liquid components could be increased by more than 30 times during the early stage of the vacuum sweating, and even by hundreds of times in the final stages with the vacuum pressure of 80.0 kPa and wax layer height of 30 cm. The forced separation method, through the improvement of conventional sweating, could discharge a large amount of oil as soon as possible in the early stage of sweating, which greatly reduced the amount of wax components dissolved in the oil, leading to the yield of vacuum sweating being higher than in conventional sweating. As shown in Table 4, compared with the conventional sweating method, the yield of the product through vacuum sweating at 80 kPa increased by 12.7% from 39.9% to 52.6% at the same production specification of 2.75% oil content for the slack wax from Daqing crude oil; whereas the final temperature of the reaction was reduced by 6 °C, and the reaction time was reduced by 50%. In the later period of sweating, it was difficult for the conventional sweating to discharge the small amount of oil adsorbed on the wax crystallization surface only through the action of gravity. However, vacuum sweating could improve the separation by forcing air through the wax layer to entrap the residual oil, thus effectively improving the separation effect.
Initial temperature | Sample A | Sample B | Sample C | Sample D | Sample E | |
---|---|---|---|---|---|---|
Temperature/°C | 58.0 | <62.0 | 62.0–64.9 | 64.9–68.1 | 68.1–69.5 | >69.5 |
Based on the previous experiments, the optimal conditions were as follows: vacuum pressure of 80.0 kPa, sweating starting temperature of 60.0 °C, and heating rate of 1 °C h−1. Tests on forced sweating separation were performed under the conditions shown in Table 5. The results are shown in Table 6.
Paraffin | Sample A | Sample B | Sample C | Sample D | Sample E | |
---|---|---|---|---|---|---|
Yield/% | 98.4 | 36.5 | 24.3 | 10.7 | 10.8 | 16.1 |
Melting point/°C | 64.3 | 60.0 | 64.0 | 66.4 | 67.6 | 69.3 |
Oil content/% | 0.46 | 0.55 | 0.24 | 0.17 | 0.11 | 0.07 |
As was known, the paraffin waxes of petroleum were crystalline below their melting point, with the size of the crystals decreasing with the increasing boiling point, which led to more oil being retained in the paraffin wax with high melting point. As is shown in Table 6, the melting point of the forced separation fraction increased significantly from 64.3 °C to 69.3 °C with the increase in the final separation temperature, while the oil content dramatically decreased from 0.46% to 0.07% with the separation efficiency improving to 84.78%. However, this was unsuitable for the production of wax products with melting points of approximately 70 °C through the conventional sweating method, which could separate the soap wax and paraffin with low melting points of 40 °C to 60 °C. The solid components (wax with a high melting point) and liquid components (wax with oil and low melting points) were difficult to completely separate through the conventional sweating process despite them being in different phases. It could be inferred from this that the range of carbon atom distribution and the content of normal paraffin in the final wax component during the late stage of conventional sweating were independent of the yield of wax products with melting points of approximately 70 °C.
Table 7 shows that the peak temperature of each fraction obtained through the forced separation of paraffin increased from 60.0 °C to 69.3 °C, consequently, the latent heat increased from 155.44 J g−1 to 214.71 J g−1 compared with 186.05 J g−1 for the raw paraffin, whereas the temperature difference T-width decreased dramatically from 21.84 °C to 5.88 °C compared with 11.38 °C of the raw paraffin. This indicated that high enthalpy paraffin with a melting point of 70 °C could be obtained through forced sweating separation. The final fraction obtained through the forced separation of paraffin exhibited better performance as a PCM than the raw paraffin, with a peak temperature of 69.74 °C, latent heat of 214.71 J g−1, and T-width of 5.88 °C, which was improved by 5.88 °C of the peak temperature, increased by 15.4% of the latent heat, and narrowed by 5.3 °C of the T-width. As the n-paraffin content increased, a significant peak as a well-defined transition point for the solid–solid phase transition was displayed at a temperature of 57.6 °C in the fraction obtained through the forced separation.
Paraffin | Sample A | Sample B | Sample C | Sample D | Sample E | |
---|---|---|---|---|---|---|
a T -start, T-peak, and T-end are the start, peak, and end of the solid–liquid transition temperatures, respectively. T-width is the difference between T-start and T-end; ΔH is the energy of solid–liquid transition. | ||||||
Yield, % | 98.4 | 36.5 | 24.3 | 10.7 | 10.8 | 16.1 |
Melting point, °C | 64.30 | 60.0 | 64.0 | 66.4 | 67.6 | 69.3 |
T -start, °C | 55.88 | 41.06 | 54.47 | 59.58 | 62.19 | 65.01 |
T -peak, °C | 63.86 | 55.95 | 63.15 | 65.60 | 67.17 | 69.74 |
T -end, °C | 67.26 | 62.90 | 65.73 | 67.86 | 68.77 | 70.89 |
T -width, °C | 11.38 | 21.84 | 11.26 | 8.28 | 6.58 | 5.88 |
ΔH, J g−1 | 186.05 | 155.44 | 181.26 | 197.17 | 209.28 | 214.71 |
![]() | ||
Fig. 7 Comparisons of the stroke and relative stroke with temperature increasing between paraffin and its final fraction obtained through forced separation for use as a thermostat. |
As shown in Fig. 7A, the transport paths of the final fraction sample E was 8.75 mm in the temperature range 70 °C to 80 °C; meanwhile the stroke was only 0.84 mm at 72 °C, which thus could be used for paraffin actuation. However, the transport path of the commercial paraffin (melting point of 64 °C), manufactured by PetroChina Fushun Petrochemical Company, was less than 8 mm even in the temperature range from 54 °C to 74 °C, which would not satisfy the engineering specification for an auto engine wax thermostat. This could be explained by the fact that commercial paraffin contains a small amount of oil, iso-paraffin, and small molecule n-alkanes, which easily result in shorter transport paths and a wider temperature range. As shown in Fig. 7B, the maximum relative stroke (max ΔL%) of the final fraction sample E was 26.5% (at 78 °C), compared with 9.6% (at 68 °C) for commercial paraffin. The temperature difference between the temperate of the maximum relative stroke and the temperature of the melting point was significantly increased to 8.7 °C, compared with the 4.1 °C of commercial paraffin wax. Therefore, forced separation based on vacuum sweating could narrow the carbon atom distribution, improve the driving force, and increase the separation effect.
(2) Compared with the conventional sweating method, the yield of the product through vacuum sweating at 80 KP increased by 12.7% from 39.9% to 52.6% at the same production specification of 2.75% oil content for the slack wax from Daqing crude oil.
(3) The separation efficiency for the final fraction of forced separation products was improved by 84.78% with an oil content of 0.07% through the vacuum sweating method without any chemical reaction, which could produce phase-change paraffin with a melting point of 70 °C.
(4) Forced separation through sweating drastically increased the content of normal paraffin and narrowed the distribution range of the carbon atom number distribution. The latent heat of the phase changes was considerably improved, and the temperature difference between the start and end of the phase changes was reduced in the final fraction obtained through the forced separation of paraffin, which showed better performance than commercial paraffin when used as a PCM.
This journal is © The Royal Society of Chemistry 2019 |