Oil/water separation technologies Where stable emulsions cannot be removed mechanically, the application of demulsifiers, coagulants and flocculants accelerates the separation process BERTHOLD OTZISK Kurita Europe
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mulsions can cause fouling and under-deposit corrosion problems in distillation columns, heat exchangers and reboilers. Commercial methods for breaking emulsions include settling, heating, distillation, centrifuging, electrical treatment, chemical treatment and ltration. These separation technoltechnologies can be used in combination to secure optimum results. Emulsions can be classied as oil-in-water and water-in-oil types. The type of emulsion can be deterdeter mined by adding a drop of emulsion into a beaker containing water and oil. If the emulsion is of the water-in-oil type, the drop diffuses through the oil but remains in water. The oil-in-water emulsion diffuses through the water, but not through the oil. Both types of emulsions can co-exist in crude oil side by side.
groups are demulsiers.
mainly
used
as
Typical applications in refineries Tank farm treatment Crude oil, intermediates and nished products are stored in the tank farm. It is the rst facility in a renery where free water can be removed by settling from the oil. Pumped crude oil from the well contains water in emulsied and free states. A crude oil emulsion consists of small globules of water surrounded by oil. Water is the internal phase and oil is the exter-
The nature of the emulsion changes from crude to crude, which can influence the performance of the emulsion breaker b reaker programme
Separation of water-in-oil emulsions In this type of emulsion, water is the internal dispersed or discontinuous phase, while oil is the external or continuous phase. Separation by the different gravity of the two phases is a very slow process, but nal phase, which can easily be can be accelerated by the assistance detected by microscope. With the of chemicals. The chemicals used help of gravity, small water dropare termed demulsiers, emulsion lets coalesce to form bigger breakers or wetting agents. These droplets. An adequate residence additives are surfactants, which time is essential for separation into migrate to the oil/water interface. two phases. The bigger droplets They adsorb on the oil lms nally settle down to be removed surrounding water droplets and by drainage. break the oil lms. Then, water Most of the time, emulsied droplets aggregate to form water water cannot be separated effecdrops large enough to gravitation- tively by gravity settling only, as ally separate them from the oil. the emulsion can separate into Non-ionic surfactants having both three phases: lipophilic and hydrophilic • Oil on the top
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Water at the bottom • Persistent emulsion in the middle or below the water layer. To break such a persistent emulemul sion, chemicals have to be applied. A number of demulsiers are commercially available with varying degrees of performance and selectivity. Generally, demulsiers are diluted with an organic solvent and injected into crude oils. The nature of the emulsion changes from crude to crude, which can inuence the performance of the emulsion breaker programme. This necessitates the evaluation of cost effectiveness and performance in breaking the emulsion. •
Crude oil desalting Crude oil fed from the tank farm to the crude distillation unit still contains water, salts, sludge and various kinds of impurities. This can cause corrosion, fouling, plugging and catalyst degradation in the downstream rening units. The main purpose of electrostatic desalting is therefore to remove impurities, such as inorganic microparticles, suspended solids and water-soluble contaminants, together with the water. The major variables and effects on the desalter operation are: • Wash water mixing • Wash water quality and rate • Desalting temperature • Electric eld • Retention time • Use of demulsiers. Wash water is added in front of the mixing valve to the crude oil to prepare a temporary emulsion. A key point of desalting is an appropriate mixing of crude oil with the
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wash water to obtain a forces. A cationic charged long-chain polysufcient desalting rate. mer neutralises the Heating lowers the viscosnegatively charged oil ity of crude oil. This droplets. The repellent promotes demulsication forces are weakened and and the formation of large oil droplets are brought water droplets from the together. This resolves emulsion. An electric eld is induced by AC or DC the emulsion of water current in the oil/water and oil. The emulmixture to improve water sion-breaking process coalescence. The electrical involves three steps: • Agglomeration eld imposes an electrical Figure 1 Evaluation of demulsifiers • Creaming charge on the small water droplets entrained in the temporary glasses. With the exception of the • Coalescence. Figure 2 illustrates the physical emulsion. The water droplets blank sample, 10 ppm of different coalesce into bigger droplets, which demulsiers were added into the process of oil-in-water emulsion can settle by gravity. Therefore, centrifuge glasses and thoroughly breaking. Agglomeration is the sufcient retention time in a mixed again. After 24 hours, the association of small dispersed desalter is required for efcient water content and salt content was phase droplets (clusters). Creaming water and oil separation. A suitable determined to nd the best is the concentration of the dispersed demulsier is commonly used to performing demulsier. In this phase. Coalescence is the drainage promote the separation of water case, Kurita EB-4110 and Kurita of the continuous phase. The oil and oil. The desalted crude oil is EB-4113 showed the highest desalt- droplets agglomerate by forming continuously fed from the desalter ing and dehydration efciency for bigger droplets and clusters, and vessel to the atmospheric crude this crude oil. EB-4110 is an oil- are collected at the surface. The distillation column. The desalter soluble demulsier, which is typi- addition of an emulsion breaker efuent water is discharged from cally injected into crude oil in front additive helps to accelerate the the desalter vessel to the wastewa- of the desalter mixing valve. separation process. These types of ter treatment facility. EB-4113 is a water-soluble demulsi- emulsion breakers are surface Figure 1 shows the laboratory er, which is typically injected into active components, which destabilise the dispersed phase. evaluation of demulsiers in desalter wash water. comparison with an untreated crude oil sample (blank). For the Separation of oil-in-water emulsions Typical applications in refineries and evaluation of a demulsier, the In aqueous systems, the hydrocar- petrochemical plants crude oil was mixed with 4 wt% bons generally carry a negative Ethylene production wash water and agitated with an charge at their surface. Often, they Ethylene is mainly produced by electric stirrer. This mixture was are steady dispersed into small steam cracking. This process transferred into several centrifuge droplets because of their repellent includes thermal cracking, cooling, compression and separation. Light liquid hydrocarbons (naphtha) and gases are converted mainly into unsaturated smaller molecules, which are separated by compression and distillation. The hot gases leaving the cracking furnaces are Creaming immediately quenched in oil Agglomeration quench and water quench columns. The purpose of the cooling is to prevent polymerisation and the formation of unwanted byproducts. The collected quench water is sepaEmulsion rated from heavy hydrocarbons in the oil/water separator. Often, the separated quench water still contains hydrocarbons, which are dispersed in the aqueous phase. Creaming and Demulsiers are usually applied Coalescence coalescence to improve the separation of hydrocarbons from the quench water. It Figure 2 Oil-in-water emulsion breaking process is mandatory to provide the correct
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amount of chemical to avoid overdosing of the demulsier, because at higher concentrations demulsiers have the tendency to act as emulsiers — instead of breaking the emulsion, they form a more stable emulsion with hydrocarbons. A simple beaker test is a good tool to check how much demulsier is required for the separation of hydrocarbons and water. Figure 3 shows the result of a laboratory evaluation to nd the best performing emulsion breaker for quench water. In this case, no demulsier was applied in front of the oil/water separator. The separated water still contained hydrocarbons, which were dispersed in the aqueous phase. It was not possible to remove these hydrocar bons in the oil/water separator. The target of the laboratory test was to select an emulsion breaker that signicantly improves the removal of hydrocarbons from water within 10 minutes’ residence time. In this case, the emulsion breaker added to bottle No. 3 showed the best performance. Within the dened time frame, an impressive hydrocarbon layer was formed, while other samples showed no effects or poor separation. The good results of the laboratory test were conrmed later in a eld trial. Wastewater treatment Water is used intensively in reneries and petrochemical processes, and during its use it becomes contaminated with hydrocarbons, increasing the biological (BOD) and chemical oxygen demand (COD) of the efuent water. Cooling water, process efuents, rain water and surface water are collected at the wastewater plant together with a very briny efuent stream from the desalting process. Typical pollutants are hydrogen sulphide, ammonia, cyanides, metals and suspended solids. Effective waste treatment technologies are required to comply with all legal requirements. The wastewater treatment methods are generally classied into three categories of mechanical, chemical and biological treatments, and a wastewater plant is typically designed in three steps:
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Figure 3 Quench water from oil/water separator
Mechanical separation • Chemical and biological treatment • Filtration and polishing. The main purpose of the primary treatment is the separation of oil and water. The removal of oil is a very important step to reduce the loading to the downstream treatment processes, because high oil levels are toxic for the microorganisms used in biological treatment. The wastewater typically contains oil in water emulsions, with oil dispersed in the continuous water phase. Common primary treatment units are: • API oil/water separator • Corrugated plate interceptor (CPI) • Dissolved air otation (DAF) • Induced air otation (IAF). The function of an API oil/water separator is based on the difference in specic gravity of oil and water. Suspended heavy particles settle to the bottom of the separator to be scraped by a rake into the sludge pit, which is discharged continuously. The oil rises to the top of the separator. The wastewater accumulates as a middle layer between the settled solids and the oil phase. It can be sent to a otation unit for further treatment. Substances such as oil or particles can be separated from water by otation. Mechanical otation and dissolved air otation are applied to increase the otating velocity of particles. Fine air bubbles are generated in water. The upward ow of the bubbles and the adhesion of bubbles with particles •
improves the efciency of the otation. The oating oil is skimmed continuously to be pumped into the slop oil system. Conclusions Oil/water emulsions appear in many areas of reneries and petrochemical plants, and can cause operational problems as well as infringement of environmental regulations. There is a variety of separation equipment available on the market to separate the oil phase from the water phase. In such cases, where stable emulsions are formed that cannot be removed mechanically, the application of demulsiers, coagulants and occulants accelerates the separation process and improves mechanical performance. This helps to full legal requirements, reduces corrosion and fouling risks, and results in reduced maintenance costs and higher equipment availability. Further reading 1 Kurita Handbook of Water Treatment , 2nd English Ed, Kurita Water Industries Ltd, Japan, 1999. 2 Ullmann´s Encyclopedia of Industrial Chemistry – The Ultimate Reference, Release 2012, 8th Ed, Wiley Online Library, Wiley-VCH. 3 Hartinger L, Handbuch der Abwasser-und Recyclingtechnik, 2nd Auflage, 1991, Hanser, Germany. Berthold Otzisk is a Consulting Engineer in the Technical Department of Kurita Europe GmbH, Viersen, Germany, where he focuses on refinery and petrochemical applications. Email:
[email protected]
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