| Dehydration and desalination of crude oil emulsions is one of the significant subjects of the oil refining technology. The purposes of crude oil pretreatment involve the following: alleviating the corrosion and incrustation of treatment equipments, lightening the hazard of impurities of crude oil on the catalyst in the following process and increasing the quality of crude oil products. The crude oil which accessing in distillation tower should meet the following requirements: water mass content no more than 0.3% and salt content 3.0mg·L-1. At present, the main way to dehydrate and desalt from crude oil emulsions is using the electric desalting equipments associated with demulsifying agents. If the radius of water particles in crude oil emulsions is larger, the demulsifying effect of electric desalination equipment is preferable. On the contrary, if the radius of water particles in crude oil emulsions is smaller, the effect of two-stage of electric desalination will not be gratifying. Therefore, the more effective methods should be found to dehydrate and desalt from such crude oil emulsions.The researches by domestic and foreign experts indicated that emulsion breaking by ultrasound is an effective demulsifying method which reinforcing dehydration and desalination of crude oil emulsions. Emulsion breaking by ultrasound standing wave field is depends on the "displacement effect" to make the oil and water phase separated. Because of the "displacement effect", the water particles move to the wave nodes or the antinodes continually, then collide and coalesce. The larger radii of water particles come into being. Because of density differences of crude oil and water particles, the water particles deposit and are separated from the oil phase by means of gravitation. The asymmetrical sound pressure at different axial directions of ultrasound standing wave leads to asymmetric distribution of the water particles, which make them collide easly that is available for emulsions breaking.The original water content of the first batch of crude oil was 1.40% by volume, which was much higher than that of the Oil Refinery permitted. In this work, dehydration from this batch of crude oil was studied in ultrasound standing wave field.Several single factors affecting demulsification of crude oil were discussed. The orthogonal experiment was designed and analyzed to study these factors. The influencing sequence is: sedimentation time>ultrasound intensity>irradiation time. The optimum conditions for crude oil dehydration in ultrasound standing wave field involve the following: crude oil temperature 70°C, demulsifying agent dosage 20u.g-g"1, acoustic intensity 0.23W-cm'2, irradiation time 5min, sedimentation time 120min. Under these conditions, the water content of crude oil is reduced from original value 1.40% to 0.07% by volume, and the dehydration ratio is 95.0%. In consideration of lightening corrosion rate and increasing service life of the distillation equipments, it is realistically significant that water content in crude oil is reduced much lower than that of the Oil Refinery permitted at the optimum experimental condition. At the same conditions (temperature, demulsifying agent dosage, etc), dehydration effect in ultrasound standing wave field is evidently superior to the single heat sedimentation as well as ultrasound reverberation field. For example, at the same demulsifying agent dosage (20ug-g"1) and. sedimentation time (90min), water content is reduced from 0.29% by volume in the single heat sedimentation to 0.16% in ultrasound standing wave field. Accordingly, the dehydration radio increases by 11.7%. At the same dehydration effect, the irradiation time is reduced from 30min in ultrasound reverberation field to 5min in ultrasound standing wave field.Concerning the second batch of crude oil, the original water content is 0.025%, and salt content is 31.93mg-L"'. And dehydration and desalination from this batch of crude oil was carried out in ultrasound standing wave field. Several single factors which affect demulsifying of crude oil emulsions were considered. These single factors are as follows: water injection rate, sedimentation time, NS-2003 demulsifying agent dosage, ultrasound intensity and irradiation time. The preferable conditions involve the following: crude oil temperature 80°C, water injection rate 5%, demulsifying agent dosage 20ug-g'1, sedimentation time 90min, ultrasound intensity 0.38W-cm'2 and irradiation time 5min. Under these experimental conditions, water content is decreased to 0.37% by volume, salt content 3.85mg-L"' and dehydrationratio is 92.6%, desalination ratio 87.9%. Compared with the single heat sedimentation at the same sedimentation time, dehydration ratio increases by 5.9% and desalination by 7.1% in ultrasound reverberation field, and increases by 9.4% and 17.1% correspondingly in ultrasound standing wave field. In despite of the experimental temperature in laboratory was markedly lower than in the Oil Refinery (about 135°C), water and salt content in crude oil can be closed to the object of dehydration and desalination requirements by means of ultrasound standing wave field associated with demulsifying agent. It could be predicted that if the ultrasound pretreatment equipments are added in desalination workshop section in the Oil Refinery, it could remarkably lighten the burden of the electric desalination equipments.Salt balance was also studied in this dissertation. It found that less water injection rate leads to higher salt content in crude oil emulsions if other experimental conditions are the same. The reason is that less water result in less contact chances of water and crude oil, rather than salt saturation in water phase.Two-stage of water injection process was studied in the final portion of the research work. With the synergistic effect of ultrasound standing wave field and two-stage of water injection, salt content of crude oil emulsions is less than S.OOmg-L'1, while water content is slightly higher than that of one-stage of water injection process. At the preferable experimental conditions of the two-stage of water injection process (crude oil emulsions temperature 80 °C, water injection content 5%, demulsifying agent dosage 20ug-g"1, ultrasound intensity of standing wave field 0.38W-cm"2, irradiation time 5min, sedimentation temperature 75 °C, sedimentation time 90min), water content is 0.45% by volume and salt content l.Hmg-L"1. Compared with one-stage water injection process, the desalination ratio is increased by 9.7%.
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