STUDY OF THE TEMPERATURE AND ENTHALPY OF WAX CRYSTALLIZATION FROM MIDDLE DISTILLATE BY DSC

ABSTRACT

The temperature and enthalpy of the wax crystallization as well as of melting have been studied in the middle distillate (boiling range: 250–375°C) obtained from the indigenous Bombay-High (Off-Shore) crude oil by using a differential scanning calorimeter (DSC). In order to have better understanding of the gel formation processes the broad distillate fraction was fractionated into five narrow fractions of 25°C interval each. From these narrow subfractions the saturates were separated from aromatics by column chromatography, and from saturates the n-paraffins were separated from iso-and cyclo-paraffins by urea adduction, to obtain the n-paraffins concentrates (urea adductables)–-the wax- and the saturated solvent portion–-the UNA. The thermal behaviour of narrow subfractions alongwith their urea adductables and the solvent portions have been studied and the wax appearance temperature (WAT) thus measured has been compared with those obtained by optical microscopy and with the ASTM cloud point, wherever possible. To obtain a clearer picture of the solidification process, further study has been done by preparing synthetic blends of urea adductables in different concentrations in the respective aromatic and iso- and cyclo-paraffinic solvents (UNA) and studying the thermal behaviour of each blend. It is found that the variation in WAT with wax concentration as measured by DSC is identical with that measured by optical microscopy and the ASTM cloud point. However, DSC values are lower than microscopic values and higher than ASTM cloud point. The enthalpy of the blends with the same amount of wax in the aromatic and iso- and cyclo-parffinic solvents indicated that it is higher in the saturated solvent in comparison to aromatic solvent. This confirms the fact that in an aromatic solvent the solubility of the wax is greater, and hence a comparatively lower WAT. The results are further discussed.     

GEL FORMATION IN VACUUM GAS OIL. I. ROLE OF COMPOSITION

The compositional analysis of aromatics, saturates and urea non adductables obtained from vacuum gas oil (VGO) distillate (370-550°C) and its seven sub-fractions: (1) 370-400, (2) 400-425, (3) 425-450, (4) 450-475, (5) 475-500, (6) 500-525 and (7) 525-550°C, derived from both paraffinic and aromatic base crude oils have been done using thin layer chromatography/flame ionisation detection technique. The physico- chemical characteristics are measured using the ASTM/ standard test procedures and an attempt has been made to correlate the physico-chemical properties with the composition. The phenomenon of gel formation at low temperatures and the characteristics of the gel in these VGO sub-fractions have been studied by X-ray diffractometry and photomicrography. It is found that at low temperatures the gel formation takes place due to the development of a three dimensional network by the crystallizing n-paraffins, as well as by the agglomeration of molecules due to very high viscosity. Both processes proceed simultaneously and the concentration and composition of n-paraffins have been found to determine the dominance or otherwise of the crystallization process.     

Composition and Properties of Some Petroleum Waxes

Structural composition of paraffin waxes and soft wax fraction derived from microcrystalline wax were determined. Waxes were fractionated by multistage solvent crystallization at different temperatures. The n-alkane components of the waxes were separated by urea adduction. The average structural parameters of parent waxes, their fractions, and urea adductables were estimated by ¹h and ¹³C NMR spectroscopy. The thermal parameters viz. phase transition temperature and the associated energy during phase transitions were determined by using DSC and correlated with the penetration temperature behavior of waxes. The carbon number distribution determined by GC for these waxes and their n-alkane components were also correlated with physical properties and thermal parameters.     

Composition and Properties of Some Petroleum Waxes

Abstract

Structural composition of paraffin waxes and soft wax fraction derived from microcrystalline wax were determined. Waxes were fractionated by multistage solvent crystallization at different temperatures. The n-alkane components of the waxes were separated by urea adduction. The average structural parameters of parent waxes, their fractions, and urea adductables were estimated by ¹h and ¹³C NMR spectroscopy. The thermal parameters viz. phase transition temperature and the associated energy during phase transitions were determined by using DSC and correlated with the penetration temperature behavior of waxes. The carbon number distribution determined by GC for these waxes and their n-alkane components were also correlated with physical properties and thermal parameters.     

溶剂脱蜡过程中脱蜡助剂对蜡结晶的影响

采用质谱法（MS）、偏光显微镜、差示扫描量热法（DSC）和X射线衍射法（XRD）等分析手段,考察了传统工艺生产的减四线糠精油（N4）和加氢500N含蜡油（J500）在溶剂脱蜡过程中,脱蜡助剂A和助剂B对含蜡油的脱蜡过滤速度、脱蜡油收率、蜡晶颗粒度、析蜡点和蜡晶结晶度的影响。结果表明,N4和J500中添加助剂A和助剂B后,脱蜡过滤速度、脱蜡油收率和析蜡点均提高,N4蜡晶的宏观颗粒度和微观结晶度增大,J500蜡晶的宏观颗粒度和微观结晶度变化不大,但颗粒聚集度增大。在过滤速度和脱蜡油收率方面,以链烷烃为主要蜡组分的N4对非极性助剂A的感受性好于极性助剂B,以链烷烃和环烷烃为主要蜡组分的J500对极性助剂B的感受性好于非极性助剂A。     

Effect of Crude Oil Composition and Blending on Flowing Properties

The effect of crude oil n-paraffin molecular weight distribution on wax crystallization risk was studied. Sixteen highly paraffinic crude oils from Eastern Venezuela were characterized, in terms of hydrocarbon family distribution, by High Temperature Simulate Distillation (HTSD), Gel Permeation Chromatography (GPC), and wax content. n-Paraffin chain length was correlated with crude oil cloud point and pour point. It was demonstrated that high molecular weight linear paraffins are responsible for crude oil wax precipitation. A quantitative correlation between molecular weight distribution and crude oil flowing properties was also obtained. It was found that the wider the molecular weight distribution, the lower is the wax crystallization risk. Blends of different paraffinic crude oils were prepared and their flowing properties were evaluated in comparison with the original crudes. Cloud points below the mean value were obtained. In some cases, a synergistic effect was observed (cloud points below the minimum of the two crude oils). Blends of some of these crudes with condensates afforded improvements on crude oil flow and a reduction of wax crystallization tendency (cloud point). This phenomenon can be attributed to a combination of two factors: I. increases in C24-n-paraffins, and II. a wider molecular weight distribution.     

APPLICATION OF X-RAY CT IMAGING AS AN ALTERNATIVE TOOL FOR CLOUD POINT DETERMINATION

This study explains the use of X-Ray CT (computerized tomography') as an alternative tool for cloud point determination of crude oils and dark fuel oils by showing the results for artificially prepared transparent oils. The technique is fully computerized and data gathering and analysis are achieved by taking the advantage of the processing of the CT images. In the study, the cloud points of diesel oil samples containing 5%, 10% and 15 % additional wax were determined with this new technique. Results show that the cloud points determined with this technique and the standardized ASTM D-3117 method are very close. This encourages the use of the proposed technique to determine the cloud point of transparent distillate fuels and dark fuel oils and crude oils, whose cloud points can not be determined easily and accurately.     

Study of Phase Transition in Hard Microcrystalline Waxes and Wax Blends by Differential Scanning Calorimetry

Phase transition temperature and associated energies in hard high melting microcrystalline waxes and its various blend with paraffin wax (melting range from 60 to 97°C) have been determined by DSC in both heating and cooling mode. The dependence of these on the composition and properties of waxes have been analyzed. The solid liquid transition temperature obtained by DSC has been compared with ASTM drop melting point of these wax samples. The present study has demonstrated that DSC can be of great use in identifying whether the wax sample is blend of different waxes or not.     

APPLICATION OF X-RAY CT IMAGING AS AN ALTERNATIVE TOOL FOR CLOUD POINT DETERMINATION

ABSTRACT

This study explains the use of X-Ray CT (computerized tomography’) as an alternative tool for cloud point determination of crude oils and dark fuel oils by showing the results for artificially prepared transparent oils. The technique is fully computerized and data gathering and analysis are achieved by taking the advantage of the processing of the CT images. In the study, the cloud points of diesel oil samples containing 5%, 10% and 15 % additional wax were determined with this new technique. Results show that the cloud points determined with this technique and the standardized ASTM D-3117 method are very close. This encourages the use of the proposed technique to determine the cloud point of transparent distillate fuels and dark fuel oils and crude oils, whose cloud points can not be determined easily and accurately.     

USE OF DSC TO STUDY THE FLOW PROPERTIES OF HYDROCRACKED BASE STOCKS AND LUBRICATING OILS

The differential scanning calorimetry (DSC) technique was found useful in assessing the effect of different processing conditions on the wax content and the wax-type of hydrocracked base stocks. The DSC thermograms showed a wide crystallization peak, in the range from -40 to -20°C, indicating the presence of paraffinic wax molecules. The second, narrow crystallization peak at about -75°C, indicated the presence of lower melting isoparaffinic or branched-type wax molecules. The DSC technique was used to study the onset wax crystallization temperature and the wax formation in hydrocracked base stocks containing different chemistry pour point depressants. The fumarate-vinylacetate (FVA) pour point depressant was not as effective as polymethacrylate (PMA) type chemistry. A decrease in depressed pour points of hydrocracked base stocks was found to follow a decrease in the onset wax crystallization temperature. In engine oil formulations containing hydrocracked base stocks, a decrease in depressed pour point was also found to follow a decrease in the onset wax crystallization temperature. For pour point depressants used in lubricating oils, the mechanism of decreasing the onset wax crystallization was found to be more important than the mechanism of suppressing the wax formation.     

Rheology of Middle Distillate. I. Role of Composition

The rheological behavior of middle distillate (250-375°C) fraction, obtained from the waxy Bombay-High Off-shore crude oil, and its five narrow sub-fractions of 25°C interval each, i.e., Fr 1 (250-275°C), Fr 2 (275-300°C), Fr 3 (300-325°C), Fr 4 (325-350°C), and Fr 5 (350-375°C), have been studied below their ASTM pour point temperatures. The rheograms (the shear stress vs. rate of shear) of the fraction and sub-fractions, at various temperatures below their pour points, are recorded on a Haake RV-12 Co-axial Rota Viscometer fitted with a NV sensor and a temperature programmer (PG-20) and attached with a programmed heating/cooling system. From these rheograms the flow parameters like plastic viscosity, apparent viscosity, and yield stress are obtained and their variations with temperature and shear rate have been studied in terms of compositions of the fraction/sub-fractions. These results might be useful in devising methodology for overcoming the wax separation phenomenon from middle distillates at low temperatures.     

Evaluation of Effects of Selected Wax Inhibitors on Wax Appearance and Disappearance Temperatures

In this investigation, a light transmittance method was used to evaluate the wax appearance temperatures (WAT) and wax disappearance temperatures (WDT) of model paraffin compounds (n-C₂₄H₅₀ (C₂₄) and n-C₃₆H₇₄ (C₃₆)) in n-decane (C₁₀) solutions both with and without wax inhibitors. The change in WAT at different paraffin concentrations in the presence of an inhibitor behaves as though there is a constant amount of paraffin removed by the inhibitor. However, the amount of apparent paraffin reduction by an inhibitor (e.g. 160 g of C₂₄ by one gram of an inhibitor) indicates that the inhibition mechanism cannot easily be explained by a simple “sequestering” effect. Wax inhibitors that decrease the WAT tend to also increase the WDT. Most of the wax inhibitors tested at a dosage of 100 ppm did suppress the WAT of lower molecular weight paraffin (C₂₄) solutions, but had little or no effect for higher molecular weight paraffin (C₃₆) solutions. Side-chain length of polymethacrylate wax inhibitors is an important performance parameter. Of the three polymethacrylate wax inhibitors tested, the one with the longest alkyl side-chain (C₁₈) had the most effect on suppressing the WAT and increasing the WDT of the binary mixtures (n-C₁₀-n-C₂₄ solutions).     

MODELLING OF THE SOLVENT-DEOILING PROCESS OF WAXES BY CONTINUOUS THERMODYNAMICS

Industrial waxes as petroleum slack waxes are multicomponent mixtures of mainly paraffinic species. Usually, for application the oily components of the waxes are undesired. Therefore, separation processes are needed which, in some cases, base on a solvent-deoiling process. Modelling such a process is the aim of this paper. For this purpose the solid-liquid equilibrium of a wax-solvent system is considered. To take into account the polydispersity of the wax continuous thermodynamics is applied. We assume the wax to consist of n-alkanes and of non-n-alkanes including all the other types of paraffinic species. Corresponding to that, the composition of both types of alkanes is described by separate Gaussian distributions with respect to the number of carbon atoms. The model requires information about how the molar enthalpy and entropy of fusion for both kinds of alkanes depend on the number of carbon atoms. Restricting to the interesting range 20-50 of the number of carbon atoms we assume linear relations The lower values of the enthalpy and entropy of fusion for the non-n-alkanes compared to those of the n-alkanes are described by the so-called degree of isomerization and cyclization. This quantity correlates to the refraction index that is easy available. The model needs only one interaction parameter to fit. We show experimental and calculated results for three systems consisting of a slack wax and a solvent mixture of 1,2-dichloroethane and toluene. The model is able to describe reasonably the enrichment of n-alkanes for the deoiled wax and the opposite for the the oily phase.     

AN EVALUATION OF OIL PRODUCED FROM ASPHALT RID (UTAH) TAB SAND AS A FEEDSTOCK FOR THE PRODUCTION OF ASPHALT AND TURBINE FUELS

In this article an evaluation is made of the potential end uses of an oil produced from Asphalt Ridge tar sand by wet forward combustion. The oil is evaluated with respect to its potential to produce a specification-grade asphalt and aviation turbine fuels. To accomplish this the oil was vacuum distilled to produce a distillate and a residue. The distillation residue meets all of the ASTM D 3381 Table 1 specification tests for an AC-10 asphalt. However, the viscosity at 135°C (275°F) is low when compared with the more stringent D 3381 Table 2 requirements. This indicates that the residue has a higher temperature susceptibility than allowed for by Table 2. The residue also has an unusually low aging index. This indicates that it may not set properly. However, it may also mean that it may be resistant to rapid age hardening. The results from successive freeze-thaw cycling indicate that the residue, when coated on appropriate aggregates, is comparable to or better than some petroleum asphalts coated on the same aggregates. Freeze-thaw cycling to failure is an indirect measure of the resistance of an asphalt-aggregate mixture to moisture-induced loss of strength

The distillate of the thermally-produced oil which represents about 50 wt % of the whole oil was also evaluated as a feedstock for the production of transportation fuels. The chemical and physical properties of the distillate are improved with respect to those of the original bitumen and the thermally-produced oil. Combined gas chromatographic/mass spectral analysis of the neutral fraction from the distillate indicates it is composed of predominantly aromatic structures. The aromatic structures are primarily of the 2- and 3-     

Understanding wax appearance temperature in light crude oils by DSC,NMR, and HT-GC

The wax precipitated from oil mixtures primarily consists of C18 to C36 (macro crystalline waxes) or C30 to C60 (micro crystalline waxes), both made of aligned paraffinic and naphthenic molecules. Paraffins have significant impact on wax deposition characteristics as paraffin waxes react and respond easily to temperature changes due to its straight chain structure. In this paper, 12 light crudes of different origin were analyzed by DSC. Method was standardized for the determination of WAT in light crudes and these were found to have WAT ranging from 10 to 30°C. Wax content is proportional to enthalpy of crystallization (ΔH) and the ΔH ranges from 3 to 9 J/g. WAT and wax content do not follow the same trend which is understandable as these parameters depends not only on the concentration and molecular weight of the waxes but also on the chemical nature of both waxy and non waxy part of the crude oil. To understand the trend in WAT, these crudes were also analyzed for paraffinic carbon (Cp), Saturates and CH₂/CH₃ ratio by NMR spectroscopy and carbon number distribution by HT-GC. It was observed that WAT correlates fairly well with CH₂/CH₃ ratio which is in an indicator of normal paraffin content as WAT depends primarily on the nature / type of paraffins. The anomalies can be explained through carbon–wise n-paraffin content by HT-GC.     

尿素脱蜡制备低熔点相变蜡的工艺研究

通以减一线馏分油（简称减一线油）为原料进行尿素脱蜡工艺研究,得到尿素络合制备低熔点相变蜡的适宜工艺条件为：络合温度为25 ℃、尿素溶液加入量（w）为91%、尿素溶液组成为m（尿素）：m（异丙醇）：m（水）＝45∶35∶20、反应时间为60 min、洗油量（w）为76%。在此条件下得到的粗蜡收率为28.2%,熔点为29.6 ℃,正构烷烃质量分数为94.9%,脱蜡油凝点小于－60 ℃。对该粗蜡进行发汗后处理可以得到相变蜡,其熔点为31 ℃,焓值为201.9 kJ/kg     

The Effect of Tube Orientation and Pour Point Depressant on Paraffin Wax Deposition

Abstract

A flow loop was used to study wax deposition in the laboratory. Data are presented to demonstrate that the tube orientation and pour point depressants have significant effects on the paraffin wax deposition for oil from the Octh Louie, a Cretaceous formation in the Manderson Field, Wyoming. Changing the tube orientation from horizontal to vertical reduced the amount of wax deposited by 28% for a tube wall (T_wall) temperature of 42°F. The circulating oil temperature (T_bulk) was held constant at 76 ± 3°F. The amount of wax deposited was found to be a linear function of the driving force (T_bulk – T_wall) and the wax appearance temperature (WAT) was estimated by extrapolating this linear relationship to a point of zero wax deposition. The WAT thus obtained (94.5°F) varied considerably with the cloud point (62°F) obtained using the ASTM D2500 method. Change in viscosity with time has also been investigated. Addition of a specific pour point depressant (PPD) at a concentration of 5 mL/gallon reduced wax deposition by 52%; concentrations of 10 mL/gallon of oil reduced the wax by 61%. Changing the flow regime from laminar to turbulent also had a significant effect, reducing the wax deposition by up to 42%. Gas chromatography–mass spectrometry (GC-MS), gas chromatography–flame ionization detection (GC-FID) analyses and other relevant property data pertaining to the oil are also presented.     

Improvement of Egyptian vacuum distillates by urea dewaxing

The dewaxing of paraffinic lube stocks is an essential step in the production of lubricants to improve the operability of machines especially in winter. The present work deals with study of the urea dewaxing process of two types of Egyptian vacuum distillates. The effect of different compositions of methanol to water saturated with urea and yield of the oil, percent of wax, pour point, refractive index, viscosity, viscosity index and specific gravity of the oil produced from the two types of distillates (I and II) were evaluated. The operating conditions of the urea adduct formation with n-paraffins using methanol to water mixture achieved the best pour point at −3.88 °C from an initial temperature of 4.4 °C for distillate I at (25/75) methanol to water. At the same ratio of methanol to water the best specific gravity of oil produced changed from 0.865 to 0.867, with viscosity index of 80. Percent yield of 50% for oil and percent wax of 50% were obtained. Results for distillate II, of higher specific gravity, are comparatively higher than those for distillate I. Experiments were carried out at room temperature.     

COMPOSITIONAL EVALUATION OF ASPHALT BINDER RECYCLING AGENTS

The results presented here indicate that it is possible to produce a high quality binder from hardened recycled asphalt by blending with recycling agents. The properties are presented in terms of Superpave grade and the hardening susceptibility (HS) which is defined as the slope of log viscosity versus infrared measured carbonyl area.

First it is shown that the effect of blending aged binder with aromatic agents is to decrease the HS in rough proportion to the amount of aromatic added and this is only slightly dependent on the aromatic source. The presence of saturates in the agent increases the HS. Using an AC-10 asphalt as the softening agent produced much higher HS values. Adding increasing amounts of aromatic agents on the other hand worsens the Superpave performance grade span. The addition of saturates to the agents has little effect on the span but shifts the span to lower temperature. Though increasing amounts of aromatic agents tended to decrease the grade span, in most instances the span was equal to that produced by blending the aged binder with the AC-10. The hardening susceptibility of the aromatic blends was better than that of the AC-10 blends and better even than the original asphalt.     

COMPOSITIONAL EVALUATION OF ASPHALT BINDER RECYCLING AGENTS

ABSTRACT

The results presented here indicate that it is possible to produce a high quality binder from hardened recycled asphalt by blending with recycling agents. The properties are presented in terms of Superpave grade and the hardening susceptibility (HS) which is defined as the slope of log viscosity versus infrared measured carbonyl area.

First it is shown that the effect of blending aged binder with aromatic agents is to decrease the HS in rough proportion to the amount of aromatic added and this is only slightly dependent on the aromatic source. The presence of saturates in the agent increases the HS. Using an AC-10 asphalt as the softening agent produced much higher HS values. Adding increasing amounts of aromatic agents on the other hand worsens the Superpave performance grade span. The addition of saturates to the agents has little effect on the span but shifts the span to lower temperature. Though increasing amounts of aromatic agents tended to decrease the grade span, in most instances the span was equal to that produced by blending the aged binder with the AC-10. The hardening susceptibility of the aromatic blends was better than that of the AC-10 blends and better even than the original asphalt.