Energy & Fuels 2003, 17, 625-630
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Study on the Compatibility of High-Paraffin Crude Oil with Electric Desalting Demulsifiers Guiling Liu, Xinru Xu, and Jinsheng Gao* Research Institute of Petroleum Procession, East China University of Science and Technology, Meilong Road 130, 200237, Shanghai, P.R. China Received July 25, 2002. Revised Manuscript Received December 2, 2002
The effect of paraffin in crude oil on the electric desalting and dewatering process was studied in this paper. The test result manifested that the dynamic viscosity of crude oil increased with the increase of paraffinic content in crude oil. Paraffin in crude oil exerts a great influence on the electric desalting and dewatering process because paraffin leads to higher viscosity of crude oil and emulsion stability as well as inefficient dewatering, and hence decreases the desalting efficiency. Some kinds of demulsifier series were tested in the process of the electric desalting and dewatering. The triblock copolymer structure of PO-EO-PO for polyhydric alcohol is more effective with high-paraffin crude oil than the diblock of PO-EO. Diamine series polymerized with propylene oxide (PO) were the most compatible with this type of crude oil for dewatering, whereas, polyamine series polymerized with propylene oxide (PO) and ethylene oxide (EO) were the most compatible for desalting.
Introduction All crudes contain different amounts of salts, which are harmful in the refining process. For example, under the high temperature of 343 °C, such water-soluble salts as magnesium and calcium chloride easily hydrolyze and produce highly corrosive hydrogen chloride. Hydrogen sulfide and mercaptan from crude oil, as well as hydrogen chloride, are the major reason for equipment corrosion, especially of the trays and condenser on the top of the crude column.1 Besides, salts of carbonate, sulfate, and organic acids can precipitate and deposit on the surface of equipment, such as heat exchangers and furnaces, resulting in cracking furnace tube fouling, scaling, and coking, and also lowering the heat transfer rate.2 Additionally, after distillation, the majority of the salts are left in residual and heavy stocks, which degrade the end-product such as petroleum coke and bitumen. It is clear that alkali and alkaline earth metals in crudes have stronger alkalinity, which leads to acidic catalyst deactivation, especially to catalytic cracking of heavy oil. On the other hand, silicon and aluminum oxide will react with the catalyst at higher temperature and thus produce a eutectic. Calcium in crude can accumulate on the surface of the catalyst during the catalyst regeneration of catalytic and hydrogen cracking, etc. Besides, arsenic can cause catalyst poisoning in the reforming process.3-5 All these not only damage the structure of catalyst, but also worsen catalyst * Corresponding author. (1) Dong, C.; Li, X.; Xu, S. Analysis of Basic Corrosion in Petrochemical Equipment. Corrosion Protection 2000, 22 (2), 49-53, 63. (2) Taylor, S. E. Resolving Crude Oil Emulsions. Chem. Ind. 1992, 9, 770-773. (3) Zhao, T.; Qi, L.; Wang, X. Study of Salt Composition and the Desalting Effect in Some Crude Oils. Acta Petrolei Sinica (Petroleum Processing Section) 1988, 4 (3), 76-81.
regeneration. So it is of great importance for crude oil to be desalted. However, to achieve expected desalting efficiency, a small proportion of washing water will be completely mixed with crude oil in order to dissolve the salts and impurities in crude, then the oil/water emulsion will be separated. It means that the desalting process goes with the dewatering process. As well-known, the properties of crude from different areas are quite different and an evaluated demulsifier is usually not as effective with other different types of crude oil. Therefore these cause troubles in evaluating and synthesizing a demulsifier. As a result, it is essential to study the relation between crude properties and the demulsifier structure in order to find a compatible demulsifier among many different properties of crude. The properties of crude refer to density, viscosity, as well as the content of wax, asphaltene, sulfur, and acidity. In this paper, highparaffin crude oil was used in the desalting and dewatering process to investigate the compatibility of several demulsifiers with this type of crude, and to find the regularity of the desalting and dewatering of each demulsifier series and the appropriate demulsifier structure for crude oil. Experimental Section A certain ratio (wt %) of paraffin (60#), of which the content and carbon distribution is shown in Figure 1, was added into crude oil to get crude oils with different paraffin contents. Tables 1 and 2 list the properties of crude oil and different high-paraffin crude oils (1-8), respectively. (4) Lin, S. Petroleum Refining Engineering. Pet. Ind. Pub. 1988, 4045. (5) Petroleum Institute of East China. Petroleum Refining Engineering. Pet. Ind. Pub. 1981, 421.
10.1021/ef020166g CCC: $25.00 © 2003 American Chemical Society Published on Web 03/25/2003
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Liu et al. Table 1. Properties of Crude Oil
density g/cm3 (20 °C)
viscosity mm2/s (50 °C)
viscosity mm2/s (100 °C)
freezing point (°C)
acid value mgOH/g
sulfur content %
paraffin content %
colloid %
asphaltene %
salt content mg/L
0.8930
7.60
2.96
-1
0.12
9
5.58
22.97
2.28
8.85
Figure 2. Sketch diagram of the PDY-1 instrument for the electric desalting of crude oil. Figure 1. Content of n- and i-paraffins and the carbon distribution of paraffin (60#). Table 2. Properties of Different High-Paraffin Crude Oils (1-8) item
density d420
viscosity mm2/s (50 °C)
paraffinic content %
sample 1 sample 2 sample 3 sample 4 sample 5 sample 6 sample 7 sample 8
0.8930 0.8980 0.8976 0.8988 0.9012 0.9044 0.9192 0.9201
7.60 8.08 9.84 14.22 16.23 20.42 25.41 28.25
5.58 8.4 10.1 13.1 16.3 18.5 22.4 25.9
Measurement for the Content of n- and i-Paraffins and the Carbon Distribution of Paraffin (60#). Gas chromatography (GC) HP-690 is used to analyze the content of n- and i-paraffins and the carbon distribution of paraffin (60#) at higher temperature, including a capillary column, vaporizer, and detector used at 450 °C; helium carrier at 30 mL/min. The standard sample of C5-C60 from distillation is injected into the gas chromatography apparatus to qualitatively test the carbon numbers of n- and i-paraffins. Compared to the GC results of paraffin 60#, n- and i-paraffin content of paraffin 60# can be calculated by the method of the area normalization rule of correction factor. Process of Electric Desalting and Analysis of Salt Content in Crude Oil. The oil sample after preheating and stirring uniformly was delivered into the homogenizer, in which 5% (wt) wash water was also added, and then stirred at 9000 r/min for 1 min. After that, the emulsion and appropriate demulsifier were put into test bottles. Put The bottles that are equipped with electrodes were placed fixed in the oscillator and shaken for 1 min, then stored in a constanttemperature bath at 85 ( 2 °C for 10 min, electric field for 20 min, and allowed to settle for another 10 min (Figure 2). The volume of the separated water was recorded every 5 min. At last, the salt content in oil after desalting was analyzed. A PDY-1 instrument of electric dewatering was used in the experiment and a WC-2 microcoulometric detector of salt content was used to detect the salt content in crude oil. The principle of the detector is that crude oil mixed with polar solvent was heated to extract the salt, and then centrifuged. A small amount of extracted liquid was taken out by injector and delivered into the ethanoic acid electrolyte containing amounts of silver ion, so the chlorine ion of the
sample can react with the silver ion as follows:
Cl- + Ag+ f AgClV The consumed silver ion for the reaction will be supplied by electrode, so according to Faraday’s laws (of electrolysis), salt content of the sample will be obtained by measuring the changing of electric quantity for supplying silver ion. Measurement for Dynamic Viscosity of Crude Oil with Varying Paraffinic Content. The dynamic viscosity of oil sample was measured by an L-90 rheological instrument, which is equipped with mainframe, activator, constant-temperature bath, temperature indicator, and cylindric drum. Its principle is that, by rotating in the measured sample, the drum will get a viscous resistance which produces a counterforce to make the shell deflect of electrical machine and such a deflection gives a hairspring a moment of torque, which keeps a balance in viscous resistance, and so a certain value which is of direct proportion with the drum’s viscous resistance will be displayed on a dial. As a result, the measured shear stress can be obtained by multiplying the value showed above by the shear stress of the drum and thus dynamic viscosity is that the shear rate divides the corresponding shear stress. Synthesis of Demulsifier. In a 500 mL reactor of high pressure fitted with a condenser, mechanical stirrer, thermocouple, and manometer, a polyhydric alcohol, polyamine, phenolic resin, and polyethylene polyamine as an initiator, were polymerized with a certain amount of propylene oxide (PO) and ethylene oxide (EO) at a proper reactive condition. Using this method a series of block copolymers were made and used for the experiment.
Results and Discussion Effect of Paraffin on the Process of Electric Desalting and Dewatering for High-Paraffin Crude Oil. Paraffin is a waxy, saturated hydrocarbon obtained from petroleum fractions and is mainly made up of alkane compounds from C18H38 to C32H66, with a 3866 °C melting point. It contains normal alkanes as well as a small number of alkane hydrocarbons, cycloalkanes, and aromatic hydrocarbons.6 The crystal form of paraffin belongs to monoclinic and triclinic system and its grain is acerose.7 Paraffin 60# is composed of n(6) Elsharkawy, A. M.; Al-shhhaf, T. A.; Fahim, M. A. Wax deposition from Middle East crudes. Fuel 2000, 79, 1047-1055.
Compatibility of High-Paraffin Crude Oils with Demulsifiers
Figure 3. Paraffinic content in crude oil vs desalting efficiency.
Figure 4. Paraffinic content in crude oil added demulsifier B1 vs dewatering.
and i-paraffins from C22 to C33, containing n- and i-paraffins for 90.14% and 9.86%, respectively (Figure 1). For every high-paraffin crude oil, there is different paraffinic content, For instance, the paraffinic content of Panjing crude from Liaohe oilfield is around 10.35%;9 paraffinic content is 17.53% in crude oil from some areas of Dagang oilfield;10 and wax content in Daqing crude oil is about 26.6% (25% paraffinic content).9 Effect of Paraffin on Desalting and Dewatering Efficiency for High-Paraffin Crude Oil. Here desalting and dewatering efficiency (DDE) were used as the results of electric desalting and dewatering. Desalting efficiency (%) ) (So - Si)/So × 100 (mg/L) and dewatering efficiency (%) ) Wi/Wo × 100 (mL), in which So or Wo represents initial salt or water content in crude, respectively, whereas Si or Wi represents salt or water content after desalting, respectively. Figures 3 and 4 show the results of electric desalting and dewatering for crude oil with different paraffinic content. It is clear in Figures 3 and 4 that paraffin in crude oil affects the desalting and dewatering efficiency significantly; that is, the desalting efficiency of crude oil with lower paraffinic content (8.4%) is only 26.2%, while the increase of paraffinic content in crude oil makes the (7) Rønningsen, H. P.; Bjørndal, B.; Hansen, A. B.; Pedersen, W. B. Energy Fuels 1991, 5, 895-908. (8) Khan, Z. H.; buSeedo, F. A.; Al-Besharah, J.; Salman, M. Improvement of the quality of heavily weathered crude oils. Fuel 1995, 74 (9), 1375-1381. (9) Xu, G.; Zhou, H. The Rheological Property of Crude Oil from DaGang Oilfield. J. Jianghan Pet. Inst. 2000, 22 (9), 54-55. (10) Ahmed, N. S.; Nassar, A. M. J. Polym. Res. 2001, 8 (3), 191195.
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Figure 5. The rate of desalting and dewatering vs DDE.
desalting efficiency worse. For dewatering efficiency, crudes containing higher paraffin are less than 10%. As for the effect factors of the stability of the crude-oil emulsion, the composition of the dispersed phase (oil phase) will affect emulsion stability, sometimes even crucially.8 In general, at lower temperature, highmelting paraffin in oil easily forms many fine and netlike crystals, which make a stronger crystal barrier around the water drops. Such fine paraffinic crystals hinder the aggregation of the water drops. However, the salts dissolved in water still in crude oil because the added wash water cannot give a better separation from the crude phase. As a result, the emulsion becomes more stable and thus decreases thedesalting efficiency. In other words, the great quantity of paraffin in crude oil not only leads to serious emulsification but also the achieved desalting efficiency was far less than expected. Figures 3 and 4 also represent the desalting and dewatering efficiency (DDE) of 1-8 samples with added demulsifier B1 in a dosage of 50 ppm, respectively. Comparison with no addition of demulsifier B1, DDE of all crude oil have improved, but the desalting efficiency of demulsifier B1 obviously decreases when paraffinic content in crude oil is more than 13.1%. In particular, as paraffinic content in crude oil increases from 5.58% to 13%, desalting efficiency of demulsifier B1 decreases from 86% to 22-25%. The result indicates that polyvinyl ether demulsifier B1 is compatible only with low-paraffin crude oil. Effect of Paraffin on the Rate of Desalting and Dewatering for High-Paraffin Crude Oil. In some cases, the dewatering efficiency for high-paraffin crude oil is better, but desalting is worse during the electric desalting and dewatering process. For instance, sample 3 containing 10.1% paraffinic content was used to carry out the experiment, then the rate of dewatering vs desalting and dewatering efficiency (DDE) can be observed in Figure 5. The desalting and dewatering efficiency are 35.2% and 64%, respectively, at 30 min settlement time, while dewatering efficiency reaches 88% at 40 min settlement time, only 36.4% desalting efficiency, which is not as much as expected. This demonstrates that desalting efficiency is not in direct proportion to dewatering efficiency, namely, higher dewatering efficiency does not mean lower salt content in crude oil. The reason is that the desalting and dewatering process not only is a process of demulsification and settlement, but also includes oil-water mixing, wash-
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Figure 6. Dynamic viscosity vs paraffinic content in crude oil (shear rate r ) 406 s-1).
ing, salt extracting, demulsifying, water-drop agglomerating, separating of water and oil, and so forth. Maybe high-paraffin crude oil containing relatively less lipophilic substances such as asphalt and colloid as well as lower specific weight has a relatively rapid dewatering rate, but the salt transfer is a relatively slow process, that is to say, there exists a balance between desalting and dewatering efficiency in the electric desalting and dewatering process of crude oil, and the dewatering efficiency cannot match the dewatering efficiency when the requirement of the salt transfer from oil phase to aqueous phase cannot be met. For further understanding the effect of paraffin on DDE, dynamic viscosity vs paraffinic content in crude oil was studied, with the results shown in Figure 6. A comparatively rapid increase in dynamic viscosity with an increase in the paraffinic content, in particular, paraffinic content in crude oil is over 10%. The reason is that, under lower temperature, paraffin in crude oil can easily form fine crystals and net-like structure, which raise the viscosity of crude oil. Effect of Demulsifier Structure on Dewatering and Desalting Efficiency (DDE) for High-Paraffin Crude Oil. 1. Effect of Molecular Weight of Demulsifier on DDE. The average molecular weight of demulsifier produces a great impact on dewatering and desalting effects. Under the same initiator, the higher amounts of propylene oxide (PO), the more carbon amounts in lipophilic group, the higher average molecular weight of demulsifier, the less CMC (critical micelle concentration) of demulsifier and thus the better efficiency of dewatering and desalting of demulsifier. But when the amount of PO increases excessively, increasing with negative reaction producing PO autopolymer as well as many substances of small molecular weight, the range of molecular weight of demulsifier will become wider,4 which makes the amount ofeffective components of demulsifier decrease, and hence lead to a decrease in DDE. Several demulsifiers series synthesized by different initiators such as polyhydric alcohol, phenolic resin, phenolamine, diamine, and polyethylene polyamine were used to investigate DDE to high-paraffin crude oil. Sample 5 with 13.1% paraffinic content was chosen to carry out the experiment. The results can be seen in Figures 7-10. By contrast to what is observed in Figures 3 and 4, Figures 7-10 indicate that DDE of all demulsifiers series have improved, among which polyethylene polyamine series polymerized with PO get a
Liu et al.
Figure 7. Diamine, polyethylene polyamine, and polyhydric alcohol series vs desalting efficiency.
Figure 8. Diamine, polyethylene polyamine, and polyhydric alcohol series vs dewatering efficiency.
Figure 9. Phenolamine, phenolic resin series vs desalting efficiency.
better result in desalting efficiency than any other demulsifiers series, especially when the PO/initiator ratio is about 120 (Figure 7). There is better dewatering efficiency for diamine series, but its desalting efficiency is lower than that of polyethylene polyamine series. Both phenolamine and polyhydric alcohol series are not better in DDE. When it comes to phenolic resin series, they are the most incompatible with this type of oil for their desalting efficiencies are less than 20% (Figure 9). In conclusion, diamine series are more compatible with high-paraffin crude oil in dewatering efficiency than the other demulsifiers series, which are polymerized only with PO. 2. Comparison of DDE between Triblock of PO-EOPO and Diblock Copolymer Structure of PO-EO of Polyhydric Alcohol Series. The diblock demulsifier structure of PO-EO is copolymer, which is polymerized with initiator, PO, and EO, while the triblock demulsifier
Compatibility of High-Paraffin Crude Oils with Demulsifiers
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Figure 10. Phenolamine, phenolic resin series vs dewatering efficiency.
Figure 13. HLB of diamine series vs DDE.
Figure 11. Comparison between PO-EO-PO and PO-EO demulsifier in desalting efficiency.
Figure 14. HLB of polyethylene polyamine series vs DDE.
Figure 12. Comparison between PO-EO-PO and PO-EO demulsifier in dewatering efficiency.
structure of PO-EO-PO means that diblock demulsifier of PO-EO is polymerized further with PO. For polyhydric alcohol series, PO-EO-PO demulsifiers obtained by different weight ratio of PO/initiator were polymerized with the same weight ratio of EO and PO. In Figures 11 and 12, a comparison is made between PO-EO-PO and PO-EO demulsifiers of polyhydric alcohol series in DDE for high-paraffin crude oil. As it can be seen in Figures 11 and 12, PO-EO-PO demulsifiers are more compatible with high-paraffin crude oil than PO-EO demulsifiers. But both the desalting and dewatering effect of polyhydric alcohol demulsifiers of PO-EO-PO structure are less than those of polyamine and diamine series. As a result, the latter were used for the following experiment. 3. Effect of Demulsifier Polymerized with EO on DDE. In general, demulsifier polymerized with propylene
oxide (PO) and ethylene oxide (EO) will affect the hydrophile-lipophile property of demulsifier, which is expressed as HLB (hydrophile-lipophile balance number); that is, HLB ) [hydrophilic group/(lipophilic group + hydrophilic group)] × (100/5).11 The demulsifier from block polymerization with PO and EO is made up of a hydrophilic part (EO chain) and a lipophilic part (PO chain). The hydrophilic property of the demulsifier will increase with an increase of amounts of EO;12 however, when adsorbing at the oil/water interface in an emulsion, the demulsifier will keep balance in its hydrophile-lipophile property in order to permute the original emulsifier.13 For diamine series, demulsifiers polymerized with different PO/initiator ratios were polymerized with the same weight of EO further, and then conducted the experiment of desalting and dewatering. The results are shown in Figure 13. From Figure 13, diamine series condensed with EO have improved desalting efficiency. Corresponding to HLB, HLB for 5.7 of demulsifier has the highest desalting efficiency, but there are nearly no variations in dewatering efficiency; Polyethylene polyamine series with PO/initiator ratio of 120 were polymerized with different weight ratios of EO further, then different HLB of demulsifier vs DDE were shown in Figure 10. From the results of Figure 14, HLB of demulsifier for 4 gets the best desalting (11) Cooper, D. G., et al. J. Chem. Eng. 1980, 58 (5), 576. (12) Xu, X.; Zhan, M.; Zhang, Y. Study on Deep Desalting Demulsifiers for the Desalting and Dewatering of Daqing Crude oil. Acta Petrolei Sinica (Petroleum Processing Section) 1995, 11 (2), 57-62. (13) The Resolution of Emulsions, Including Crude Oil Emulsion, in Relation to HLB Behavior. In Emulsions, A Fundamental and Practical Approach. Averyard, R., Binks, B. P., Fletcher, P. D. I., Ye, X., Lu, J. R., Sjoblom, J., Eds.; Kluwer Academic Publisher: Amsterdam, The Netherlands, 1992; pp 97-110.
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efficiency, that is, desalting efficiency of demulsifier reaches 88%, in addition to dewatering efficiency for 86%. This is because polyethylene polyamine demulsifiers series can damage the paraffin crystals and its netlike structure, and effectively dissolve any fine crystals. This leads to the decrease of paraffinic content in crude oil and is beneficial to the aggregation of the water drops, settlement, and separation of the phases (water and crude oil phases). Eventually, the better desalting efficiency can be achieved. Conclusions The test result manifested that paraffin in crude oil exerts a great influence on the electric desalting and dewatering process because paraffin leads to higher viscosity of crude oil and emulsion stability as well as inefficient dewatering, and hence decreases the desalting efficiency. Desalting efficiency is not in direct proportion to dewatering efficiency, namely, higher dewatering efficiency does not mean lower salt content in crude oil.
Liu et al.
By the measurement of high-paraffin crude oil, it can be found that the dynamic viscosity of crude oil increases with the increase of paraffinic content; in particular, paraffinic content in crude oil is over 10%. Some kinds of demulsifier series were used in the process of the electric desalting. The results manifested that, to polyhydric alcohol, triblock demulsifier structure of PO-EO-PO is more compatible with high-paraffin crude oil than diblock one of PO-EO. Among several demulsifiers series, diamine series polymerized with PO were the most compatible with high-paraffin crude oil in dewatering, while polyamine series polymerized with PO and EO were the most compatible in desalting. Demulsifiers condensed with EO will improve hydrophile-lipophile property. HLB of diamine demulsifier for 5.7 has the highest desalting efficiency. HLB of polyethylene polyamine demulsifier for 4 gets the best desalting efficiency. EF020166G
