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Separation of water-in-heavy oil emulsions using porous particles in a coalescence column
Accepted Manuscript Separation of Water-in-Heavy Oil Emulsions Using Porous Particles in a Coalescence Column Yajun Li, Houjian Gong, Mingzhe Dong, Yanghong Liu PII: DOI: Reference:
S1383-5866(16)30179-4 http://dx.doi.org/10.1016/j.seppur.2016.04.004 SEPPUR 12946
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
21 September 2015 26 February 2016 3 April 2016
Please cite this article as: Y. Li, H. Gong, M. Dong, Y. Liu, Separation of Water-in-Heavy Oil Emulsions Using Porous Particles in a Coalescence Column, Separation and Purification Technology (2016), doi: http://dx.doi.org/ 10.1016/j.seppur.2016.04.004
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Separation of Water-in-Heavy Oil Emulsions Using Porous Particles in a Coalescence Column Yajun Lia, Houjian Gonga, Mingzhe Dongb , Yanghong Liub a
College of Petroleum Engineering, China University of Petroleum (East China), Qingdao, Shandong Province, China 266580
b
Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, AB, Canada T2N 1N4
Abstract In this study, the principle of capillarity and the mechanism of a wetting film in porous media are applied in designing coalescence media. Water-wet porous particles are used for the first time in a coalescing column to enhance the separation of water from water-in-heavy oil emulsions. Experimental results show that this type of particles can remain water-wet in an oil environment and can significantly enhance the coalescence of water droplets in water-in-heavy oil emulsions. The coalescing column test results show that the flow of the emulsion through the 10 cm coalescing column reduced water content from 44.37% to 21.54% at 80 oC, without using a demulsifier. The coalescing column can further reduce the water content beyond what was reached in gravity separation using a high dosage of demulsifier. At a fixed temperature of 80 oC, when the dosage of the selected demulsifier changed from 50 to 100 to 150 ppm, water content was reduced to 10.49%, 1.32%, and 0.64%, respectively, with the use of a 10 cm coalescing column. These results indicate that the effect of adding a coalescing column to water separation from water-in-heavy oil emulsions is significant, as compared to using only a demulsifier in gravity separation. More importantly, flow through the coalescing column could reduce the water content in the heavy oil to a very low level (<1.0%) and, at the same time, reduce the consumption of demulsifier. Keywords: Separation; emulsion; porous particle; coalescence column; wettability; demulsifier.
Corresponding author Tel: +1 403 210 7642. E-mail: [email protected]
1. Introduction Crude oil is generally produced with water in the form of emulsions—dispersions of water droplets in oil. Emulsions are difficult to treat and cause a number of operational problems, such as the surging in separation equipment in gas-oil separating plants, the production of offspec crude oil, and the creation of high pressure drops in flow lines. Emulsions have to be treated to remove the dispersed water and associated inorganic salts1. The water-in-crude-oil emulsion of heavy oil is generally a thermodynamically unstable system; however, additives can be used to provide the necessary kinetic stability. The stability of water-in-crude-oil emulsions depends, in part, on the irreversible adsorption of asphaltenes, which are naphthenic acid and clays at the oil-water interface2, 3. Substantial treatments are required to break water-in-oil emulsions and to accelerate water separation. Along with gravity separation, heating, and centrifuge and hydro cyclone separation, chemical treatment with demulsifiers remains the most common process for breaking emulsions, although this technique is not always competitively effective
4-6
.
Furthermore, there has been much research discussing how to apply a coalescer to bring together dispersed droplets in oil-in-water emulsions7, 8. Coalescence is an effective process used to reduce high levels of oil and grease in an appropriate particle size range in oil-in-water (O/W) emulsion separation. The efficiency of oil removal from water largely depends on the properties of the filter medium, so the filter medium is the key criterion in a coalescence process for oily wastewater treatment
9-11
. Many
types of packing materials have been tested. These fall into the categories of fixed media, granular packing, and fiber packing. The materials range from high-tech oleophilic plastic fibres to such exotic packing materials as granulated black walnut shells. Some of the more common packing materials tested include glass, fibreglass, peat, coal, sand, and polyethylene fibres 12. Although hundreds of papers have been published regarding the use of porous media to coalesce dispersed droplets of oil-in-water emulsions, very few appear to be applicable to real-world problems. A significant portion of the literature deals with oil-in-water emulsions and, typically, with dilute oil concentrations. Many studies on water-in-oil emulsions employed synthetic emulsions, and often use simple oils such as kerosene, diesel, or mineral oils, rather than actual crude oils. The few studies that have examined resolving oilfieldderived emulsions include very early studies on light crude oil emulsions
13
. Madia et al.
14
concluded that oleophilic packing best coalesces oil droplets. Other studies have shown that 1
neutral wettability packings yield the greatest success
15
. Still others claim that packings
composed of “mixed media”—oleophilic and hydrophilic materials in the same bed—have a symbiotically enhanced success
16, 17
. These studies over the course of a decade showed that
the packing should have the same wettability as the dispersed phase. Of the materials tested— red rock from three different locations, fibreglass, polypropylene, coal, sand, lava rock, carbon granules, zorball (a commercial oil-absorbing mineral), and crusher dust—the most effective was a certain red rock which can maintain its water-wet character 13. The primary objective of this research is to use porous particles in the coalescence column for separating water from water-in-heavy oil emulsions. A series of experiments have been conducted to demonstrate that the dispersed phase can coalesce on the same wetting material. The key point is to determine the long term effect of a coalescence material on breaking the emulsions by employing a new coalescer column design to remove water and basic sediment from produced heavy oil emulsions at a laboratory scale.
2.Experimental 2.1 Materials In this research, Daqing heavy oil (Daqing, China) was used in all separation tests. Table 1 provides the properties of the heavy oil. The density of this crude oil is 920.8 kg/m3 at 20oC and is 900.6 kg/m3 at 50oC it. On the matter of density, the greater the density of the crude oil, the smaller the density differences between the oil and water, which causes difficulties in the gravitational settlement of heavy oil dehydration. When the temperature is at 50 oC, the viscosity of the crude oil is 235.6 mPa·s. The materials used for coalescer packing are obtained by rubblizing tiles made of clay to a size range of 4 to 10 mesh. The demulsifier, DMO 8601, is a commercial product provided by the Baker Hughes Company. It is a synergistic blend of oxyalkylated phenolic resins, alkylphenols and sulfonates in a mixture of solvents of aromatics and alcohols. Table 1 Properties of Daqing Heavy Oil Sample Density @
Density @
Viscosity @
Wax
20oC, kg/m3
50oC, kg/m3
50oC, mPas
content, %
920.8
900.6
235.6
29.7
2
Colloid, %
28.70
Asphaltene content, %
0.24
The coalescing material plays a key role in enhancing the separation of water drops from W/O emulsions. After considering numerous materials (hard, soft, and porous, etc.), the particles of porous materials were obtained by rubblizing tiles made of clay. The pore size range is between 2 and 100 μm with the mean pore diameter of 26 μm. The selected porous particles can satisfy the porosity and the wettability requirements for the packing material of the coalescence column. These particles were highly porous, with fine pores connected and open to the surfaces of the particles. The procedures to treat the coalescing porous particles are as follows: (1) Crush the tiles made of clay to obtain 4 to 10 mesh (diameter: 2-5mm) packing particles. (2) Rinse the particles by using water to remove fines on the surface of the particles. (3) Dry the porous materials by putting them in an oven at 100℃ for 24 hours. (4) Immerse the particles in pure water for 24 hours to make sure the particles are fully saturated with water.
2.2 Coalescence Flow Tests The experimental set-up for separating water-in-heavy oil emulsions is shown in Figure 1A. The set-up consists mainly of an oilfield emulsion tank, a coalescence column, a pump, a settling tank, and a water bath. Figure 1B shows the processing of a water-in-oil emulsion through the coalescing and settling units. The photo and schematic of the coalescing column are presented in Figure 1C. The pre-saturated porous particles are tightly packed in the holder to form the coalescing column. Two distributors are used at inlet and outlet ends of column to ensure a uniform flow of the emulsion through the porous particle bed. The emulsion is injected into the coalescence column from its bottom as shown in Figure 1C. The water droplets of the emulsion coalesce to form larger droplets when the emulsion flows through the bed. After the mixture of the oil and water is transferred to the settling unit, oil and water then separate by gravity.
3
Oil and water flow Oil and water
Oil outlet Porous particle pack
Oil
Water outlet
Distributor
Water
Emulsion pumped in
Coalescence column
(A)
Oil and water settling unit
(B)
(C) Figure 1 (A) Schematic of experimental set-up. (B) Schematic of coalescence column and oil and water settling unit. (C) The coalesce column model The procedure of the coalescence flow test includes the following steps: (1) Pack the coalescence column by adding water saturated porous particles into the bed holder, which is filled with water. (2) Connect the pump, emulsion tank, coalescence column, and settling unit. (3) Set the temperature of the water bath to warm up the emulsion, coalescence column, and settling unit to a desired test temperature. (4) Pump 500 mL of the emulsion, at 1.0 cm3/min, to flow through the coalescence column. (5) Keep the collected oil and water in the settling unit, at the same temperature, for 4 hours to allow water droplets to settle by gravity. (6) Take an oil sample from the oil layer in the settling unit for determining water content of the oil.
2.3 Water Content Analysis The water content in the oil after separation was analyzed using the Dean-Stark method18. 4
In the Dean-Stark analysis for determining water content in oil, the oil sample was mixed with the solvent (toluene) in the flask. The mixture was heated to vaporize the solvent and water. The vaporized solvent and water condensed in the condenser and were collected in the burette. From the water volume collected in the graduated tube (at the bottom) and the initial oil sample volume and the known densities, the water content in the oil was determined.
3. Results and Discussion 3.1 Wettability of the Coalescing Material When the dried particles of the materials were immersed in pure water, the immersed materials yielded fine bubbles and the bubbling persisted for a long time. Figures 2A and B show the bubbling phenomenon of the immersed materials and the dry particles. It was also observed that the surfaces of the pores of the materials were strongly water-wet. When the particles were immersed in water, water could therefore spontaneously imbibe into the pores and push air out. In addition, the materials were subsequently further tested for the ability of remaining strongly water-wet after being in contact with heavy oil. When the water saturated particles of the materials were first immersed in heavy oil for 24 hours, then put in contact with water again, the surfaces of the particles remained clear, as shown in Figure 2C; that is, after extended contact with heavy oil, the surfaces of the particles remained water-wet and were therefore not contaminated by the heavy oil. Figure 2D shows the same type of particles after first being in contact with heavy oil, then subsequently being in contact with water. There were oil stains on the surfaces of some particles, because the heavy oil apparently contaminated some areas of the surfaces when the dry particles came into contact with it. Figures 2E and 2F show the photos of the same particles depicted in Figures 2C and 2D, respectively, after sitting for a one week duration. Through the above two groups of experiments, it was proven that the experimental materials, after a preliminary water wet process guarantees water-wetness, and after stirring with heavy oil, could still maintain water-wetness. But, without a preliminary water-wet process, after mixing the dry materials and heavy oil together, the materials do not retain perfect water-wet characteristics, since there was partial contamination by the heavy oil. Therefore, the water-wet material was convincingly selected as the appropriate coalescing material needed for water separation from water-in-heavy oil emulsions.
5
(A)
(B)
(C)
(E)
(D)
(F)
Figure 2 Wettability of the coalescing material at different conditions: (A) the dried particles of the materials immersed in water; (B) the dried particles of the materials; (C) the wetted materials first immersed in heavy oil for 24 hours, then put in contact with water again; (D) the dried materials first immersed in heavy oil for 24 hours, then put in contact with water again; (E) and (F) are the photos of the same particles shown in (C) and (D), respectively, after sitting for a one week duration. In order to demonstrate that having fine pores inside the water-wet material is extremely important in maintaining water wetness in an oil-water system, a comparison test was conducted and is shown in Figure 3. When the water saturated porous material (left) and glass shards (right) were first in contact with water (Figure 3A) and then with heavy oil (Figure 3B), the porous material particles remained clean (note the particles near the wall of the bottle), but the glass shards were completely enshrouded by the heavy oil. After both were subsequently immersed in heavy oil for 24 hours, then put in contact with water again, the surfaces of the porous material particles were clear, but the glass shards were still enshrouded by the heavy 6
oil as shown in Figure 3C. This test demonstrated that strongly water-wet, non-porous materials like glass can be easily contaminated by a crude oil and can become oil wet in an oil-water system. Although a porous material is less water wet compared to glass, it can remain water wet in an oil-water system because of the water film on the surface of the particle that is connected with and maintained by the water in the fine pores within the particles.
(A)
(B)
(C) Figure 3 Comparison of the abilities of porous material and glass shards to maintain waterwetness after contacting crude oil: (A) the material and glass shards in water; (B) the material and glass shards subsequently in heavy oil; (C) the material and glass shards in an oil-water system. The water films on the particle surfaces, formed by wettability and capillary pressure of the porous materials and its inner pores, maintain the water wetness of the coalescing materials and thus have the coalescing function whenever water drops come to contact them. Therefore, the porous materials are reusable once they are saturated with water. In addition, with an
7
optimized injection flow rate, the designed separation devices can be used continuously to dispose W/O emulsion.
3.2 Effects of Temperature and Demulsifier on Separation of Water from W/O Emulsions without a Coalescence Column In order to determine the effect of temperature on the separation of water from water-inheavy oil emulsions, several tests were designed without employing either a demulsifier or a coalescing column. Separation tests by gravity, conducted under three different temperatures (60oC, 70oC, and 80oC) were carried out to see how much water could be removed without using other measures. The experimental results are plotted in Figure 4A. The results reveal that, under the same conditions, as the temperature increases, the water content of the waterin-oil emulsions decreases. The research results echo the proposition that heating an emulsion enhances its breaking or separation19. The results showed that the water content of the oil sample dropped from the original value of 48.37% to 48.08% at 60oC, to 46.95% at 70oC, and to 44.37% at 80oC. As can be seen in Figure 4A, the relationship between water content and temperature tends to be a downward curve, though not necessarily linear. This means that, if the temperature is further increased above 80oC, more water can be removed by gravity separation. However, this is likely not a practical way to reach a very low water content, such as ~1%; simply adding heat to the emulsion in order to meet the requirement of sale to a refinery would generally become prohibitively expensive. Demulsifier is usually used to accelerate the separation of emulsions. Therefore, the effect of demulsifier is also investigated and the experimental results are plotted in Figure 4B. As can be seen, the water content of the oil samples dropped significantly from 44.37% to 16.98% when 50 ppm demulsifier was added at 80oC. The water content then deceased from 16.98% to 4.54% as the emulsifier concentration was increased from 50 ppm to 300 ppm. The use of demulsifier can significantly improve the separation efficiency. The reduction of water content in the emulsion decreases with demulsifier concentration. The results indicate that the use of demulsifiers leads to effective emulsion resolutions. Figure 4B reveals that, at the same temperature, when the demulsifier concentration increases, the water content of the oil emulsion decreases. This is because temperature affects the physical properties of water, oil, interfacial films, and surfactant solubilities in both of the two 8
phases. As the temperature increases, the viscosity of the emulsion decreases significantly, promoting small water droplets to coalesce more readily at a lower viscosity. The results also show that, although the water content decreases with increasing demulsifier concentration, the efficiency of demulsifier decreases with concentration. The trend of the curve in Figure 4B also indicates that, at 80oC, it is will be difficult to reduce the water content to about 1.0% by simply adding more demulsifier. Adding too much demulsifier is not economic but may also create some problems to the refinery process due to the
50
50
48
40
Water Content， %
Water Content， %
introduction of surfactant to the crude oil.
46
44
42
40
20
60
70
30
20
10
0
80
0
Treating Temperature， C
50
100
150
200
250
300
350
Demulsifier Concentration， ppm
(A)
(B)
Figure 4 The variations of water content in heavy oil after separation: (A) Water content versus temperature without a demulsifier or a coalescing column; (B) Water content versus demulsifier concentration at 80oC.
3.3 Coalescing Enhancement of Water Separation from W/O Emulsions The results of the previous section showed that the addition of demulsifiers to an emulsion can significantly improve the coalescence of oil droplets. The results also showed that there was still more than a tolerable amount of water in the oil—4.54%—even when 300 ppm emulsifier was used at 80oC. Therefore, in addition to heating and the use of demulsifier, a coalescing column packed with water-wet porous particles was used to promote coalescence of water droplets in water-in-heavy oil emulsions. In total, nine coalescing column tests were carried out to investigate the enhancement of water separation from a water-in-heavy oil emulsion by using the water-wet porous particles as the coalescing media. In these tests, three demulsifier concentrations (50, 100, and 150 ppm) were applied and three temperatures (60, 9
70, and 80oC) were investigated. The coalescing column had a diameter of 4.0 cm and a height of 10.0 cm. The emulsion was pumped through the coalescing column at a flow rate of 1 cm3/min. The demulsifier was mixed with the emulsion in the emulsion tank before entering the coalescing column. The experimental results of the above nine tests are plotted in Figure 5. The experimental results reveal that, at the same emulsifier concentration, the water content of the water-in-heavy oil emulsions rapidly decreased when the temperature increased. This is because temperature affects the physical properties of water, oil, interfacial films, and surfactant solubilities in both of the two phases. With the temperature increase, the viscosity of the heavy oil decreased significantly and small water droplets coalesced easily inside the resultant low viscosity oil. The experimental results in Figure 5 also show that, at the same temperature, the water content of the oil emulsions rapidly decreased with an increase in the demulsifier concentration. All of the above results indicate that the temperature and demulsifier effects can be enhanced significantly when an emulsion flows through the coalescing column. This implies that incorporating the coalescing column in heavy oil fields should be able to reduce temperatures and demulsifier consumption and shorten the settling time. 25 50 ppm 100 ppm 150 ppm
Water Content， %
20 15 10 5 0 60
70
80
Temperature， C
Figure 5 Water content versus the temperature and demulsifier concentration in a coalescing column. In order to compare the effect of a coalescing column on the separation efficiency, the experimental data was re-plotted in Figure 6 for the specific test condition of 100 ppm demulsifier concentration. The results show that the flow of the emulsion through the 10 cm
10
coalescing column reduced water content from 21.45% to 13.20% at 60 oC, from 12.56% to 8.13% at 70oC, and from 12.43% to 1.32% at 80oC. The effect of adding a coalescing column to water separation is significant, as compared to just using a demulsifier in gravity separation. 30 With column Without column
Water Content， %
25 20 15 10 5 0 60
70
Temperature， C
80
Figure 6 Comparison of gravity separation and coalescing column enhanced gravity separation. Figure 7 shows the comparison of water separation results by gravity separation and coalescing column enhanced gravity separation at 80 oC with different demulsifier concentrations. In the coalescing column enhanced gravity separation tests, demulsifier concentration was changed from 1 to 150 ppm, at which point the water content had been reduced to less than 1.0%. In all tests, settling time duration was four hours. The results show that flow of the emulsion through the 10 cm coalescing column reduced water content from 44.37% to 21.54% (51% reduction) at 80oC, without using demulsifier. This result indicates that the coalescing column is effective in helping water droplets coalesce. The results also demonstrate that the coalescing column can reduce the water content beyond what was reached in gravity separation when a high dosage of demulsifier is employed. However, without using a coalescing column, at a fixed temperature of 80oC, and with a settling time of four hours, when the dosage of the selected demulsifier changed from 50 to 100 to 150 ppm, water content was reduced from 44.37% (no demulsifier) to 16.96%, 12.43%, and 9.59%, respectively, in the gravity separation. With the use of a 10 cm coalescing column, at the same separation temperature, the same settling time, and using the same three demulsifier dosages, water content was reduced from 21.54% (no demulsifier) to 10.49%, 1.32% and 0.64%, respectively, in the gravity separation. Thus, employing the coalescing column reduced water 11
content in the heavy oil by 38%, 89%, and 93%, compared to the water contents reached in the gravity separation without using the column with demulsifier dosages of 50, 100, and 150 ppm, respectively. These results again indicate that the effect of adding a coalescing column to water separation is significant, as compared to just using a demulsifier in gravity separation. More importantly, flow through the coalescing column could reduce the water content in the heavy oil to a very low level (<1.0%) and, at the same time, reduce the consumption of demulsifier. 50 Without column With column
Water Content， %
40
30
20
10
0 0
50
100
150
200
250
300
Demulsifier Concentration， ppm
Figure 7 Comparison of water separation results by gravity separation and coalescing column enhanced gravity separation at 80oC with different demulsifier concentrations.
3.4 Effect of Coalescing Column Height on Water Separation from Waterin-heavy Oil Emulsion The effect of the coalescing column height on water separation from water-in-heavy oil emulsion was also investigated. Three additional tests were conducted using a 20 cm coalescing column at three different temperatures (60, 70, and 80oC). In all three tests, 100 ppm demulsifier was used. The flow rate and settling time were the same as in the previous tests. The experimental results of the three 20 cm column tests, along with the results from the 10 cm column, for comparison, are provided in Figure 8. The results reveal that, under the same test conditions (temperature, demulsifier concentration, flow rate, and settling time), when the coalescing column height was increased from 10 cm to 20 cm, the water content of the oil emulsions decreased very slightly, as shown in Figure 8. That means that, under the test conditions used in this study, the 10 cm height was sufficient for the coalescing column.
12
However, this does not mean that 10 cm is the optimal height of the coalescing column. In field applications, more work is needed to find the optimal thickness for a particular coalescing column. 25 10 cm column 20 cm column
Water Content， %
20
15
10
5
0 60
70
Temperature， C
80
Figure 8 Water content versus coalescing column height.
3.5 Mechanism of Coalescence Using Water-wet Porous Particles The enhancement of separation of water from the water-in-heavy oil emulsions using the coalescence column can also be explained based on the settling equation (Stokes’ law)20:
(1) where Vt is the velocity of a settling water droplet (cm/s), g is the acceleration of gravity (981 cm/s2),
is the diameter of the water droplet (cm), w is water density (g/cm3), o is oil
density (g/cm3), and o is oil viscosity (mPa.s). The above equation shows the relationship of the settling velocity with respect to the density difference between the dispersed phase (water) and the continuous phase (oil), viscosity of the continuous phase, and the size of droplets in the dispersed phase (water). It can be seen from Equation (1) that the settling velocity is proportional to the square of the diameter of the water droplet. After the emulsion goes through the coalescence column, the water in the treated emulsion will be present in larger drops and will settle much faster than the original droplets. For example, if d increases by a factor of 10, Vt will increase by a factor of 100. If some water droplets in the original
13
emulsion are very small, they will not be able to settle down from the settling tank within a certain period of time. In the treatment of water-in-heavy oil emulsions, the purpose of using coalescing media is to promote coalescence of the water droplets within the emulsions. It is expected that a large surface area upon which water droplets can collect must be provided. It has been shown that the collection of water droplets onto the surface of the coalescing media depends on the equilibrium contact angle (in some cases, the dynamic contact angle) of the dispersed water droplets on the media surface
21
. If the equilibrium contact angle of the dispersed phase
droplets in the presence of a continuous phase is less than ~30°, the wetting character of the media surface favors the formation of a thin film of the dispersed phase onto the media surface. Subsequently, the dispersed phase droplets coalesce onto this thin film to form a liquid pool. On the other hand, if the equilibrium contact angle of the dispersed phase droplets in the presence of a continuous phase is greater than ~140°, there is neither film formation nor droplet attachment onto the media surface, meaning that there is no coalescence nor accumulation of the dispersed phase onto the surface of the coalescing media. Therefore, an appropriate coalescing material should be chosen for a particular dispersion system in order to get high coalescence efficiency, and the coalescing media also must provide a large surface area upon which the dispersed droplets can collide and accumulate. As demonstrated in Section 3.1 of this paper, in a packed coalescer, the wetting behavior of the water phase in the packing material is important in determining the performance of the coalescer. The water saturated porous particles must remain water wet after being in contact with the heavy crude oil for a long time. This is because: 1) the material of the porous particles is water wet, and 2) the capillary forces of the water/oil interface keep the water filling the fine pores up to the surfaces of the particles. In this case, the porous particles are covered by a thin layer of water (i.e., a water film). The contact angle of water on this type of surface is definitely zero, which means that the surface is completely, perfectly wetting. Therefore, if water droplets come into contact with the water film, the droplets should coalesce onto the water film. How does a water droplet in the water-in-heavy oil emulsion coalesce onto to the water film which covers the porous particle in a coalescing column? The cartoon in Figure 9 depicts the process as follows. The surfaces of the particles are covered by the water film and would be in contact with the continuous oil phase at the beginning of the coalescence process. The
14
water droplets are entrained in the oil phase and flow through the narrow space between the particles, as shown in Figure 9A, where a droplet moves toward the particles and the water films. Eventually, the depicted water droplet comes into contact with the surfaces of the particles, as shown in Figure 9B. At this point, the water droplet is not in direct contact with any water film because there is an oil film between them. Due to the drag force offered by the continuous oil phase, the water droplet is forced to flow through the narrow gap between the two particles, and the oil film between water droplet and water film on the particle surface gets increasingly thinner, and is forced to drain out at last, as shown in Figure 9C. This results in the spreading or coalescence of the water droplet onto the surfaces of the particles, as shown in Figure 9D. As emulsion flows through the coalescing column, more water droplets that approach the particles coalesce onto the water film already formed. The coalesced dispersed water accumulates at the downstream end of the channel between the particles to form a lump of water (Figure 9E) and subsequently detaches and flows away as a larger drop (Figure 9F). The size of the blob detached from the rear of the particle will be controlled by the space between the particles, the flow rate, and the oil-water interfacial tension. From the explanation of the mechanism of coalescence using water-wet porous particles above, smaller diameter of particles and the space between the particles could increase the specific surface area of the coalescence column, and then increase contact opportunities for water droplets with water films on the surface of porous particle, which will improve the coalescence proportion. However, flow resistance for the emulsion going through the coalescence column will increase to some extent. Therefore, appropriate diameter of particles and the space between the particles could improve the coalescence efficiency and reduce the flow resistance. Even better, the mean space between the particles should be approximate equal or less than half of the mean water droplet diameter.
(A)
(B) 15
(C)
(D)
(E)
(F)
Figure 9 Mechanism of water separation in water-in-heavy oil emulsion in a coalescing column: (A) Water droplet approaching the water-wet porous particles; (B) Forced drainage of oil between the water droplet and the particles; (C) Oil film between water droplet and water film on the particle surface getting increasingly thinner and disappeared at last; (D) Coalescence of water droplet and water film; (E) Water lump formed on the surface of particle; (F) Water lump detaching and flowing away. As depicted in Figure 9, the reason why coalescence works effectively in the breakup of emulsions lies in the tendency that, during coalescence, water droplets fuse, unite, or coalesce together to form a large, and later on, an even larger drop. This process of coalescence remains, most likely, irreversible and inevitably leads to a decrease in the number of water droplets and, eventually, to complete demulsification. This pronounced decrease in the number of water droplets illustrates the differences in scenarios with and without coalescence columns. After going through the coalescence column, some of the small water droplets 16
become larger ones and can then be removed from the oil in the settling tank. This means that the coalescence column can not only improve the settling velocity, but can also reduce the water content in the oil after a certain period of settling time. In the separation of water-in-oil emulsions, using a certain amount of demulsifier is to treat the adsorbed chemicals on oil-water interfaces of the emulsions and thus to enhance the coalescence of water drops (The experimental results were shown in Figure 7). For water-inheavy oil emulsion systems, paraffin, resins and asphaltenes in heavy oil can adsorb on the oil–water interfaces to form rigid films which prevent water-droplets from coalescing and consequently lead to stable emulsions. The demulsifier can weaken the formed interfacial films and speed up the coalescence of small water droplets upon contact each other, resulting in a coarse W/O emulsion. The coarse emulsion is further separated by coalescing with the water films surrounding the porous particles of the coalescence column when the emulsion flows through it.
4 Conclusions In this paper, the principle of capillarity and wetting film phenomena in porous media were applied to the coalescing material for W/O emulsion treatment. The utilized coalescing material has proved successful in enhancing water separation from water-in-heavy oil emulsions. This material (particles) for packing the coalescing column is both water-wet and porous (containing many fine pores within), and saturated by water prior to contacting heavy oil. It was shown that these particles will remain water-wet in oil because the capillary forces keep the water phase networked between the water in the fine pores inside the particles and the water films on the surfaces of the particles. The coalescing column is effective in promoting water droplets to coalesce, as compared to using only a demulsifier in gravity separation. Acknowledgements The authors gratefully acknowledge financial support from the Natural Sciences and Engineering Council (NSERC) of Canada as well as the Natural Science Foundation of China (Grant No. 51274225 and 51204198) for this research.
17
References (1)Abdel-Khalek, N.; Omar, A.; Barakat, Y. Relationship of structure to properties of some anionic surfactants as collectors in the flotation process. 1. Effect of chain length. J. Chem. Eng. Data 1999, 44 (1), 133-137. (2)Yarranton, H.; Sztukowski, D.; Urrutia, P. Effect of interfacial rheology on model emulsion coalescence: I. Interfacial rheology. J. Colloid Interface Sci. 2007, 310 (1), 246-252. (3)Tian, G.; Lei, Q.; Lui, W.; Li, F. Colloid chemistry in petroleum industry in China. J. Dispersion Sci. Technol. 2000, 21 (4), 433-448. (4)Al-Sabagh, A.; Badawi, A.; Noor El-Den, M. Breaking water-in-crude oil emulsions by novel demulsifiers based on maleic anhydride–oleic acid adduct. Pet. Sci. Technol. 2002, 20 (9-10), 887-914. (5)Pradilla, D.; Simon, S. C.; Sjöblom, J. Mixed interfaces of asphaltenes and model demulsifiers part II: study of desorption mechanisms at liquid-liquid interfaces. Energy Fuels 2015. (6)Sil a .
orges
.
lanco
.
ond n, M.; Salager, J.-L.; Pereira, J. C. Breaking of
water-in-crude oil emulsions. 5. Effect of acid-alkaline additives on the performance of chemical demulsifiers. Energy Fuels 2014, 28 (6), 3587-3593. (7)Buzzacchi, M.; Schmiedel, P.; von Rybinski, W. Dynamic surface tension of surfactant systems and its relation to foam formation and liquid film drainage on solid surfaces. Colloids Surf., A 2006, 273 (1), 47-54. (8)Zhang, L.; Zhu, T.; Sun, Y.; Jiang, B. Experimental study of precision-woven fabrics for oil-in-water emulsion coalescence: operating conditions and oil saturation. J. Dispersion Sci. Technol. 2015, 36 (2), 182-189. (9)Yang, B.; Chang, Q. Wettability studies of filter media using capillary rise test. Sep. Purif. Technol. 2008, 60 (3), 335-340. (10)Zhou, Y. B.; Chen, L.; Hu, X. M.; Lu, J. Modified resin coalescer for oil-in-water emulsion treatment: effect of operating conditions on oil removal performance. Ind. Eng. Chem. Res. 2008, 48 (3), 1660-1664. (11)Srinivasan, A.; Viraraghavan, T.; Ng, K. Coalescence/filtration of an oil-in-water emulsion in an immobilized mucor rouxii biomass bed. Sep. Sci. Technol. 2012, 47 (16), 2241-2249. (12)Viraraghavan, T.; Mathavan, G., Oil removal using peat filters. Sio: 1989; Vol. 40.
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(13)Renouf, G.; Kurucz, L.; Soveran, D. In Produced fluids separation using a coalescer column, Canadian International Petroleum Conference, Petroleum Society of Canada: 2005. (14)Madia, J. R.; Fruh, S. M.; Miller, C. A.; Beerbower, A. Granular packed bed coalescer: influence of packing wettability on coalescence. Environ. Sci. Technol. 1976, 10 (10), 10441046. (15)Moses, S.; Ng, K. A visual study of the breakdown of emulsions in porous coalescers. Chem. Eng. Sci. 1985, 40 (12), 2339-2350. (16)Sokolovic, S.; Secerov-Sokolovic, R.; Sevic, S. Two-stage coalescer for oil/water separation. Water Sci. Technol. 1992, 26 (9-11), 2073-2076. (17)Davies, G.; AFZAL, M.; Ali, F.; JEFFREYS, G. Coalescence in droplet dispersion bandsunusual surface effect. Chemical Engineer-London 1972, (266), 392. (18)Dean, E.; Stark, D. A convenient method for the determination of water in petroleum and other organic emulsions. Industrial & Engineering Chemistry 1920, 12 (5), 486-490. (19)Kokal, S.; Tang, T.; Schramm, L.; Sayegh, S. Electrokinetic and adsorption properties of asphaltenes. Colloids Surf., A 1995, 94 (2), 253-265. (20)Bond, W. LXXXII. Bubbles and drops and Stokes' law. The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science 1927, 4 (24), 889-898. (21)Basu, S. A study on effect of wetting on mechanism of coalescence in a model coalescer. J. Colloid Interface Sci. 1993, 159 (1), 68-76.
19
The
principle of capillarity and the mechanism of a wetting film in porous media are applied in
designing coalescence media by using water-wet porous particles in a coalescing column to enhance the separation of water from water-in-heavy oil emulsions. The
water-saturated porous particles can remain water-wet in an oil environment and can
significantly enhance the coalescence of water droplets in water-in-heavy oil emulsions. The
study provides a new method for the separation of water-in-heavy oil emulsions.
20
S1383-5866(16)30179-4 http://dx.doi.org/10.1016/j.seppur.2016.04.004 SEPPUR 12946
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
21 September 2015 26 February 2016 3 April 2016
Please cite this article as: Y. Li, H. Gong, M. Dong, Y. Liu, Separation of Water-in-Heavy Oil Emulsions Using Porous Particles in a Coalescence Column, Separation and Purification Technology (2016), doi: http://dx.doi.org/ 10.1016/j.seppur.2016.04.004
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Separation of Water-in-Heavy Oil Emulsions Using Porous Particles in a Coalescence Column Yajun Lia, Houjian Gonga, Mingzhe Dongb , Yanghong Liub a
College of Petroleum Engineering, China University of Petroleum (East China), Qingdao, Shandong Province, China 266580
b
Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, AB, Canada T2N 1N4
Abstract In this study, the principle of capillarity and the mechanism of a wetting film in porous media are applied in designing coalescence media. Water-wet porous particles are used for the first time in a coalescing column to enhance the separation of water from water-in-heavy oil emulsions. Experimental results show that this type of particles can remain water-wet in an oil environment and can significantly enhance the coalescence of water droplets in water-in-heavy oil emulsions. The coalescing column test results show that the flow of the emulsion through the 10 cm coalescing column reduced water content from 44.37% to 21.54% at 80 oC, without using a demulsifier. The coalescing column can further reduce the water content beyond what was reached in gravity separation using a high dosage of demulsifier. At a fixed temperature of 80 oC, when the dosage of the selected demulsifier changed from 50 to 100 to 150 ppm, water content was reduced to 10.49%, 1.32%, and 0.64%, respectively, with the use of a 10 cm coalescing column. These results indicate that the effect of adding a coalescing column to water separation from water-in-heavy oil emulsions is significant, as compared to using only a demulsifier in gravity separation. More importantly, flow through the coalescing column could reduce the water content in the heavy oil to a very low level (<1.0%) and, at the same time, reduce the consumption of demulsifier. Keywords: Separation; emulsion; porous particle; coalescence column; wettability; demulsifier.
Corresponding author Tel: +1 403 210 7642. E-mail: [email protected]
1. Introduction Crude oil is generally produced with water in the form of emulsions—dispersions of water droplets in oil. Emulsions are difficult to treat and cause a number of operational problems, such as the surging in separation equipment in gas-oil separating plants, the production of offspec crude oil, and the creation of high pressure drops in flow lines. Emulsions have to be treated to remove the dispersed water and associated inorganic salts1. The water-in-crude-oil emulsion of heavy oil is generally a thermodynamically unstable system; however, additives can be used to provide the necessary kinetic stability. The stability of water-in-crude-oil emulsions depends, in part, on the irreversible adsorption of asphaltenes, which are naphthenic acid and clays at the oil-water interface2, 3. Substantial treatments are required to break water-in-oil emulsions and to accelerate water separation. Along with gravity separation, heating, and centrifuge and hydro cyclone separation, chemical treatment with demulsifiers remains the most common process for breaking emulsions, although this technique is not always competitively effective
4-6
.
Furthermore, there has been much research discussing how to apply a coalescer to bring together dispersed droplets in oil-in-water emulsions7, 8. Coalescence is an effective process used to reduce high levels of oil and grease in an appropriate particle size range in oil-in-water (O/W) emulsion separation. The efficiency of oil removal from water largely depends on the properties of the filter medium, so the filter medium is the key criterion in a coalescence process for oily wastewater treatment
9-11
. Many
types of packing materials have been tested. These fall into the categories of fixed media, granular packing, and fiber packing. The materials range from high-tech oleophilic plastic fibres to such exotic packing materials as granulated black walnut shells. Some of the more common packing materials tested include glass, fibreglass, peat, coal, sand, and polyethylene fibres 12. Although hundreds of papers have been published regarding the use of porous media to coalesce dispersed droplets of oil-in-water emulsions, very few appear to be applicable to real-world problems. A significant portion of the literature deals with oil-in-water emulsions and, typically, with dilute oil concentrations. Many studies on water-in-oil emulsions employed synthetic emulsions, and often use simple oils such as kerosene, diesel, or mineral oils, rather than actual crude oils. The few studies that have examined resolving oilfieldderived emulsions include very early studies on light crude oil emulsions
13
. Madia et al.
14
concluded that oleophilic packing best coalesces oil droplets. Other studies have shown that 1
neutral wettability packings yield the greatest success
15
. Still others claim that packings
composed of “mixed media”—oleophilic and hydrophilic materials in the same bed—have a symbiotically enhanced success
16, 17
. These studies over the course of a decade showed that
the packing should have the same wettability as the dispersed phase. Of the materials tested— red rock from three different locations, fibreglass, polypropylene, coal, sand, lava rock, carbon granules, zorball (a commercial oil-absorbing mineral), and crusher dust—the most effective was a certain red rock which can maintain its water-wet character 13. The primary objective of this research is to use porous particles in the coalescence column for separating water from water-in-heavy oil emulsions. A series of experiments have been conducted to demonstrate that the dispersed phase can coalesce on the same wetting material. The key point is to determine the long term effect of a coalescence material on breaking the emulsions by employing a new coalescer column design to remove water and basic sediment from produced heavy oil emulsions at a laboratory scale.
2.Experimental 2.1 Materials In this research, Daqing heavy oil (Daqing, China) was used in all separation tests. Table 1 provides the properties of the heavy oil. The density of this crude oil is 920.8 kg/m3 at 20oC and is 900.6 kg/m3 at 50oC it. On the matter of density, the greater the density of the crude oil, the smaller the density differences between the oil and water, which causes difficulties in the gravitational settlement of heavy oil dehydration. When the temperature is at 50 oC, the viscosity of the crude oil is 235.6 mPa·s. The materials used for coalescer packing are obtained by rubblizing tiles made of clay to a size range of 4 to 10 mesh. The demulsifier, DMO 8601, is a commercial product provided by the Baker Hughes Company. It is a synergistic blend of oxyalkylated phenolic resins, alkylphenols and sulfonates in a mixture of solvents of aromatics and alcohols. Table 1 Properties of Daqing Heavy Oil Sample Density @
Density @
Viscosity @
Wax
20oC, kg/m3
50oC, kg/m3
50oC, mPas
content, %
920.8
900.6
235.6
29.7
2
Colloid, %
28.70
Asphaltene content, %
0.24
The coalescing material plays a key role in enhancing the separation of water drops from W/O emulsions. After considering numerous materials (hard, soft, and porous, etc.), the particles of porous materials were obtained by rubblizing tiles made of clay. The pore size range is between 2 and 100 μm with the mean pore diameter of 26 μm. The selected porous particles can satisfy the porosity and the wettability requirements for the packing material of the coalescence column. These particles were highly porous, with fine pores connected and open to the surfaces of the particles. The procedures to treat the coalescing porous particles are as follows: (1) Crush the tiles made of clay to obtain 4 to 10 mesh (diameter: 2-5mm) packing particles. (2) Rinse the particles by using water to remove fines on the surface of the particles. (3) Dry the porous materials by putting them in an oven at 100℃ for 24 hours. (4) Immerse the particles in pure water for 24 hours to make sure the particles are fully saturated with water.
2.2 Coalescence Flow Tests The experimental set-up for separating water-in-heavy oil emulsions is shown in Figure 1A. The set-up consists mainly of an oilfield emulsion tank, a coalescence column, a pump, a settling tank, and a water bath. Figure 1B shows the processing of a water-in-oil emulsion through the coalescing and settling units. The photo and schematic of the coalescing column are presented in Figure 1C. The pre-saturated porous particles are tightly packed in the holder to form the coalescing column. Two distributors are used at inlet and outlet ends of column to ensure a uniform flow of the emulsion through the porous particle bed. The emulsion is injected into the coalescence column from its bottom as shown in Figure 1C. The water droplets of the emulsion coalesce to form larger droplets when the emulsion flows through the bed. After the mixture of the oil and water is transferred to the settling unit, oil and water then separate by gravity.
3
Oil and water flow Oil and water
Oil outlet Porous particle pack
Oil
Water outlet
Distributor
Water
Emulsion pumped in
Coalescence column
(A)
Oil and water settling unit
(B)
(C) Figure 1 (A) Schematic of experimental set-up. (B) Schematic of coalescence column and oil and water settling unit. (C) The coalesce column model The procedure of the coalescence flow test includes the following steps: (1) Pack the coalescence column by adding water saturated porous particles into the bed holder, which is filled with water. (2) Connect the pump, emulsion tank, coalescence column, and settling unit. (3) Set the temperature of the water bath to warm up the emulsion, coalescence column, and settling unit to a desired test temperature. (4) Pump 500 mL of the emulsion, at 1.0 cm3/min, to flow through the coalescence column. (5) Keep the collected oil and water in the settling unit, at the same temperature, for 4 hours to allow water droplets to settle by gravity. (6) Take an oil sample from the oil layer in the settling unit for determining water content of the oil.
2.3 Water Content Analysis The water content in the oil after separation was analyzed using the Dean-Stark method18. 4
In the Dean-Stark analysis for determining water content in oil, the oil sample was mixed with the solvent (toluene) in the flask. The mixture was heated to vaporize the solvent and water. The vaporized solvent and water condensed in the condenser and were collected in the burette. From the water volume collected in the graduated tube (at the bottom) and the initial oil sample volume and the known densities, the water content in the oil was determined.
3. Results and Discussion 3.1 Wettability of the Coalescing Material When the dried particles of the materials were immersed in pure water, the immersed materials yielded fine bubbles and the bubbling persisted for a long time. Figures 2A and B show the bubbling phenomenon of the immersed materials and the dry particles. It was also observed that the surfaces of the pores of the materials were strongly water-wet. When the particles were immersed in water, water could therefore spontaneously imbibe into the pores and push air out. In addition, the materials were subsequently further tested for the ability of remaining strongly water-wet after being in contact with heavy oil. When the water saturated particles of the materials were first immersed in heavy oil for 24 hours, then put in contact with water again, the surfaces of the particles remained clear, as shown in Figure 2C; that is, after extended contact with heavy oil, the surfaces of the particles remained water-wet and were therefore not contaminated by the heavy oil. Figure 2D shows the same type of particles after first being in contact with heavy oil, then subsequently being in contact with water. There were oil stains on the surfaces of some particles, because the heavy oil apparently contaminated some areas of the surfaces when the dry particles came into contact with it. Figures 2E and 2F show the photos of the same particles depicted in Figures 2C and 2D, respectively, after sitting for a one week duration. Through the above two groups of experiments, it was proven that the experimental materials, after a preliminary water wet process guarantees water-wetness, and after stirring with heavy oil, could still maintain water-wetness. But, without a preliminary water-wet process, after mixing the dry materials and heavy oil together, the materials do not retain perfect water-wet characteristics, since there was partial contamination by the heavy oil. Therefore, the water-wet material was convincingly selected as the appropriate coalescing material needed for water separation from water-in-heavy oil emulsions.
5
(A)
(B)
(C)
(E)
(D)
(F)
Figure 2 Wettability of the coalescing material at different conditions: (A) the dried particles of the materials immersed in water; (B) the dried particles of the materials; (C) the wetted materials first immersed in heavy oil for 24 hours, then put in contact with water again; (D) the dried materials first immersed in heavy oil for 24 hours, then put in contact with water again; (E) and (F) are the photos of the same particles shown in (C) and (D), respectively, after sitting for a one week duration. In order to demonstrate that having fine pores inside the water-wet material is extremely important in maintaining water wetness in an oil-water system, a comparison test was conducted and is shown in Figure 3. When the water saturated porous material (left) and glass shards (right) were first in contact with water (Figure 3A) and then with heavy oil (Figure 3B), the porous material particles remained clean (note the particles near the wall of the bottle), but the glass shards were completely enshrouded by the heavy oil. After both were subsequently immersed in heavy oil for 24 hours, then put in contact with water again, the surfaces of the porous material particles were clear, but the glass shards were still enshrouded by the heavy 6
oil as shown in Figure 3C. This test demonstrated that strongly water-wet, non-porous materials like glass can be easily contaminated by a crude oil and can become oil wet in an oil-water system. Although a porous material is less water wet compared to glass, it can remain water wet in an oil-water system because of the water film on the surface of the particle that is connected with and maintained by the water in the fine pores within the particles.
(A)
(B)
(C) Figure 3 Comparison of the abilities of porous material and glass shards to maintain waterwetness after contacting crude oil: (A) the material and glass shards in water; (B) the material and glass shards subsequently in heavy oil; (C) the material and glass shards in an oil-water system. The water films on the particle surfaces, formed by wettability and capillary pressure of the porous materials and its inner pores, maintain the water wetness of the coalescing materials and thus have the coalescing function whenever water drops come to contact them. Therefore, the porous materials are reusable once they are saturated with water. In addition, with an
7
optimized injection flow rate, the designed separation devices can be used continuously to dispose W/O emulsion.
3.2 Effects of Temperature and Demulsifier on Separation of Water from W/O Emulsions without a Coalescence Column In order to determine the effect of temperature on the separation of water from water-inheavy oil emulsions, several tests were designed without employing either a demulsifier or a coalescing column. Separation tests by gravity, conducted under three different temperatures (60oC, 70oC, and 80oC) were carried out to see how much water could be removed without using other measures. The experimental results are plotted in Figure 4A. The results reveal that, under the same conditions, as the temperature increases, the water content of the waterin-oil emulsions decreases. The research results echo the proposition that heating an emulsion enhances its breaking or separation19. The results showed that the water content of the oil sample dropped from the original value of 48.37% to 48.08% at 60oC, to 46.95% at 70oC, and to 44.37% at 80oC. As can be seen in Figure 4A, the relationship between water content and temperature tends to be a downward curve, though not necessarily linear. This means that, if the temperature is further increased above 80oC, more water can be removed by gravity separation. However, this is likely not a practical way to reach a very low water content, such as ~1%; simply adding heat to the emulsion in order to meet the requirement of sale to a refinery would generally become prohibitively expensive. Demulsifier is usually used to accelerate the separation of emulsions. Therefore, the effect of demulsifier is also investigated and the experimental results are plotted in Figure 4B. As can be seen, the water content of the oil samples dropped significantly from 44.37% to 16.98% when 50 ppm demulsifier was added at 80oC. The water content then deceased from 16.98% to 4.54% as the emulsifier concentration was increased from 50 ppm to 300 ppm. The use of demulsifier can significantly improve the separation efficiency. The reduction of water content in the emulsion decreases with demulsifier concentration. The results indicate that the use of demulsifiers leads to effective emulsion resolutions. Figure 4B reveals that, at the same temperature, when the demulsifier concentration increases, the water content of the oil emulsion decreases. This is because temperature affects the physical properties of water, oil, interfacial films, and surfactant solubilities in both of the two 8
phases. As the temperature increases, the viscosity of the emulsion decreases significantly, promoting small water droplets to coalesce more readily at a lower viscosity. The results also show that, although the water content decreases with increasing demulsifier concentration, the efficiency of demulsifier decreases with concentration. The trend of the curve in Figure 4B also indicates that, at 80oC, it is will be difficult to reduce the water content to about 1.0% by simply adding more demulsifier. Adding too much demulsifier is not economic but may also create some problems to the refinery process due to the
50
50
48
40
Water Content， %
Water Content， %
introduction of surfactant to the crude oil.
46
44
42
40
20
60
70
30
20
10
0
80
0
Treating Temperature， C
50
100
150
200
250
300
350
Demulsifier Concentration， ppm
(A)
(B)
Figure 4 The variations of water content in heavy oil after separation: (A) Water content versus temperature without a demulsifier or a coalescing column; (B) Water content versus demulsifier concentration at 80oC.
3.3 Coalescing Enhancement of Water Separation from W/O Emulsions The results of the previous section showed that the addition of demulsifiers to an emulsion can significantly improve the coalescence of oil droplets. The results also showed that there was still more than a tolerable amount of water in the oil—4.54%—even when 300 ppm emulsifier was used at 80oC. Therefore, in addition to heating and the use of demulsifier, a coalescing column packed with water-wet porous particles was used to promote coalescence of water droplets in water-in-heavy oil emulsions. In total, nine coalescing column tests were carried out to investigate the enhancement of water separation from a water-in-heavy oil emulsion by using the water-wet porous particles as the coalescing media. In these tests, three demulsifier concentrations (50, 100, and 150 ppm) were applied and three temperatures (60, 9
70, and 80oC) were investigated. The coalescing column had a diameter of 4.0 cm and a height of 10.0 cm. The emulsion was pumped through the coalescing column at a flow rate of 1 cm3/min. The demulsifier was mixed with the emulsion in the emulsion tank before entering the coalescing column. The experimental results of the above nine tests are plotted in Figure 5. The experimental results reveal that, at the same emulsifier concentration, the water content of the water-in-heavy oil emulsions rapidly decreased when the temperature increased. This is because temperature affects the physical properties of water, oil, interfacial films, and surfactant solubilities in both of the two phases. With the temperature increase, the viscosity of the heavy oil decreased significantly and small water droplets coalesced easily inside the resultant low viscosity oil. The experimental results in Figure 5 also show that, at the same temperature, the water content of the oil emulsions rapidly decreased with an increase in the demulsifier concentration. All of the above results indicate that the temperature and demulsifier effects can be enhanced significantly when an emulsion flows through the coalescing column. This implies that incorporating the coalescing column in heavy oil fields should be able to reduce temperatures and demulsifier consumption and shorten the settling time. 25 50 ppm 100 ppm 150 ppm
Water Content， %
20 15 10 5 0 60
70
80
Temperature， C
Figure 5 Water content versus the temperature and demulsifier concentration in a coalescing column. In order to compare the effect of a coalescing column on the separation efficiency, the experimental data was re-plotted in Figure 6 for the specific test condition of 100 ppm demulsifier concentration. The results show that the flow of the emulsion through the 10 cm
10
coalescing column reduced water content from 21.45% to 13.20% at 60 oC, from 12.56% to 8.13% at 70oC, and from 12.43% to 1.32% at 80oC. The effect of adding a coalescing column to water separation is significant, as compared to just using a demulsifier in gravity separation. 30 With column Without column
Water Content， %
25 20 15 10 5 0 60
70
Temperature， C
80
Figure 6 Comparison of gravity separation and coalescing column enhanced gravity separation. Figure 7 shows the comparison of water separation results by gravity separation and coalescing column enhanced gravity separation at 80 oC with different demulsifier concentrations. In the coalescing column enhanced gravity separation tests, demulsifier concentration was changed from 1 to 150 ppm, at which point the water content had been reduced to less than 1.0%. In all tests, settling time duration was four hours. The results show that flow of the emulsion through the 10 cm coalescing column reduced water content from 44.37% to 21.54% (51% reduction) at 80oC, without using demulsifier. This result indicates that the coalescing column is effective in helping water droplets coalesce. The results also demonstrate that the coalescing column can reduce the water content beyond what was reached in gravity separation when a high dosage of demulsifier is employed. However, without using a coalescing column, at a fixed temperature of 80oC, and with a settling time of four hours, when the dosage of the selected demulsifier changed from 50 to 100 to 150 ppm, water content was reduced from 44.37% (no demulsifier) to 16.96%, 12.43%, and 9.59%, respectively, in the gravity separation. With the use of a 10 cm coalescing column, at the same separation temperature, the same settling time, and using the same three demulsifier dosages, water content was reduced from 21.54% (no demulsifier) to 10.49%, 1.32% and 0.64%, respectively, in the gravity separation. Thus, employing the coalescing column reduced water 11
content in the heavy oil by 38%, 89%, and 93%, compared to the water contents reached in the gravity separation without using the column with demulsifier dosages of 50, 100, and 150 ppm, respectively. These results again indicate that the effect of adding a coalescing column to water separation is significant, as compared to just using a demulsifier in gravity separation. More importantly, flow through the coalescing column could reduce the water content in the heavy oil to a very low level (<1.0%) and, at the same time, reduce the consumption of demulsifier. 50 Without column With column
Water Content， %
40
30
20
10
0 0
50
100
150
200
250
300
Demulsifier Concentration， ppm
Figure 7 Comparison of water separation results by gravity separation and coalescing column enhanced gravity separation at 80oC with different demulsifier concentrations.
3.4 Effect of Coalescing Column Height on Water Separation from Waterin-heavy Oil Emulsion The effect of the coalescing column height on water separation from water-in-heavy oil emulsion was also investigated. Three additional tests were conducted using a 20 cm coalescing column at three different temperatures (60, 70, and 80oC). In all three tests, 100 ppm demulsifier was used. The flow rate and settling time were the same as in the previous tests. The experimental results of the three 20 cm column tests, along with the results from the 10 cm column, for comparison, are provided in Figure 8. The results reveal that, under the same test conditions (temperature, demulsifier concentration, flow rate, and settling time), when the coalescing column height was increased from 10 cm to 20 cm, the water content of the oil emulsions decreased very slightly, as shown in Figure 8. That means that, under the test conditions used in this study, the 10 cm height was sufficient for the coalescing column.
12
However, this does not mean that 10 cm is the optimal height of the coalescing column. In field applications, more work is needed to find the optimal thickness for a particular coalescing column. 25 10 cm column 20 cm column
Water Content， %
20
15
10
5
0 60
70
Temperature， C
80
Figure 8 Water content versus coalescing column height.
3.5 Mechanism of Coalescence Using Water-wet Porous Particles The enhancement of separation of water from the water-in-heavy oil emulsions using the coalescence column can also be explained based on the settling equation (Stokes’ law)20:
(1) where Vt is the velocity of a settling water droplet (cm/s), g is the acceleration of gravity (981 cm/s2),
is the diameter of the water droplet (cm), w is water density (g/cm3), o is oil
density (g/cm3), and o is oil viscosity (mPa.s). The above equation shows the relationship of the settling velocity with respect to the density difference between the dispersed phase (water) and the continuous phase (oil), viscosity of the continuous phase, and the size of droplets in the dispersed phase (water). It can be seen from Equation (1) that the settling velocity is proportional to the square of the diameter of the water droplet. After the emulsion goes through the coalescence column, the water in the treated emulsion will be present in larger drops and will settle much faster than the original droplets. For example, if d increases by a factor of 10, Vt will increase by a factor of 100. If some water droplets in the original
13
emulsion are very small, they will not be able to settle down from the settling tank within a certain period of time. In the treatment of water-in-heavy oil emulsions, the purpose of using coalescing media is to promote coalescence of the water droplets within the emulsions. It is expected that a large surface area upon which water droplets can collect must be provided. It has been shown that the collection of water droplets onto the surface of the coalescing media depends on the equilibrium contact angle (in some cases, the dynamic contact angle) of the dispersed water droplets on the media surface
21
. If the equilibrium contact angle of the dispersed phase
droplets in the presence of a continuous phase is less than ~30°, the wetting character of the media surface favors the formation of a thin film of the dispersed phase onto the media surface. Subsequently, the dispersed phase droplets coalesce onto this thin film to form a liquid pool. On the other hand, if the equilibrium contact angle of the dispersed phase droplets in the presence of a continuous phase is greater than ~140°, there is neither film formation nor droplet attachment onto the media surface, meaning that there is no coalescence nor accumulation of the dispersed phase onto the surface of the coalescing media. Therefore, an appropriate coalescing material should be chosen for a particular dispersion system in order to get high coalescence efficiency, and the coalescing media also must provide a large surface area upon which the dispersed droplets can collide and accumulate. As demonstrated in Section 3.1 of this paper, in a packed coalescer, the wetting behavior of the water phase in the packing material is important in determining the performance of the coalescer. The water saturated porous particles must remain water wet after being in contact with the heavy crude oil for a long time. This is because: 1) the material of the porous particles is water wet, and 2) the capillary forces of the water/oil interface keep the water filling the fine pores up to the surfaces of the particles. In this case, the porous particles are covered by a thin layer of water (i.e., a water film). The contact angle of water on this type of surface is definitely zero, which means that the surface is completely, perfectly wetting. Therefore, if water droplets come into contact with the water film, the droplets should coalesce onto the water film. How does a water droplet in the water-in-heavy oil emulsion coalesce onto to the water film which covers the porous particle in a coalescing column? The cartoon in Figure 9 depicts the process as follows. The surfaces of the particles are covered by the water film and would be in contact with the continuous oil phase at the beginning of the coalescence process. The
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water droplets are entrained in the oil phase and flow through the narrow space between the particles, as shown in Figure 9A, where a droplet moves toward the particles and the water films. Eventually, the depicted water droplet comes into contact with the surfaces of the particles, as shown in Figure 9B. At this point, the water droplet is not in direct contact with any water film because there is an oil film between them. Due to the drag force offered by the continuous oil phase, the water droplet is forced to flow through the narrow gap between the two particles, and the oil film between water droplet and water film on the particle surface gets increasingly thinner, and is forced to drain out at last, as shown in Figure 9C. This results in the spreading or coalescence of the water droplet onto the surfaces of the particles, as shown in Figure 9D. As emulsion flows through the coalescing column, more water droplets that approach the particles coalesce onto the water film already formed. The coalesced dispersed water accumulates at the downstream end of the channel between the particles to form a lump of water (Figure 9E) and subsequently detaches and flows away as a larger drop (Figure 9F). The size of the blob detached from the rear of the particle will be controlled by the space between the particles, the flow rate, and the oil-water interfacial tension. From the explanation of the mechanism of coalescence using water-wet porous particles above, smaller diameter of particles and the space between the particles could increase the specific surface area of the coalescence column, and then increase contact opportunities for water droplets with water films on the surface of porous particle, which will improve the coalescence proportion. However, flow resistance for the emulsion going through the coalescence column will increase to some extent. Therefore, appropriate diameter of particles and the space between the particles could improve the coalescence efficiency and reduce the flow resistance. Even better, the mean space between the particles should be approximate equal or less than half of the mean water droplet diameter.
(A)
(B) 15
(C)
(D)
(E)
(F)
Figure 9 Mechanism of water separation in water-in-heavy oil emulsion in a coalescing column: (A) Water droplet approaching the water-wet porous particles; (B) Forced drainage of oil between the water droplet and the particles; (C) Oil film between water droplet and water film on the particle surface getting increasingly thinner and disappeared at last; (D) Coalescence of water droplet and water film; (E) Water lump formed on the surface of particle; (F) Water lump detaching and flowing away. As depicted in Figure 9, the reason why coalescence works effectively in the breakup of emulsions lies in the tendency that, during coalescence, water droplets fuse, unite, or coalesce together to form a large, and later on, an even larger drop. This process of coalescence remains, most likely, irreversible and inevitably leads to a decrease in the number of water droplets and, eventually, to complete demulsification. This pronounced decrease in the number of water droplets illustrates the differences in scenarios with and without coalescence columns. After going through the coalescence column, some of the small water droplets 16
become larger ones and can then be removed from the oil in the settling tank. This means that the coalescence column can not only improve the settling velocity, but can also reduce the water content in the oil after a certain period of settling time. In the separation of water-in-oil emulsions, using a certain amount of demulsifier is to treat the adsorbed chemicals on oil-water interfaces of the emulsions and thus to enhance the coalescence of water drops (The experimental results were shown in Figure 7). For water-inheavy oil emulsion systems, paraffin, resins and asphaltenes in heavy oil can adsorb on the oil–water interfaces to form rigid films which prevent water-droplets from coalescing and consequently lead to stable emulsions. The demulsifier can weaken the formed interfacial films and speed up the coalescence of small water droplets upon contact each other, resulting in a coarse W/O emulsion. The coarse emulsion is further separated by coalescing with the water films surrounding the porous particles of the coalescence column when the emulsion flows through it.
4 Conclusions In this paper, the principle of capillarity and wetting film phenomena in porous media were applied to the coalescing material for W/O emulsion treatment. The utilized coalescing material has proved successful in enhancing water separation from water-in-heavy oil emulsions. This material (particles) for packing the coalescing column is both water-wet and porous (containing many fine pores within), and saturated by water prior to contacting heavy oil. It was shown that these particles will remain water-wet in oil because the capillary forces keep the water phase networked between the water in the fine pores inside the particles and the water films on the surfaces of the particles. The coalescing column is effective in promoting water droplets to coalesce, as compared to using only a demulsifier in gravity separation. Acknowledgements The authors gratefully acknowledge financial support from the Natural Sciences and Engineering Council (NSERC) of Canada as well as the Natural Science Foundation of China (Grant No. 51274225 and 51204198) for this research.
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References (1)Abdel-Khalek, N.; Omar, A.; Barakat, Y. Relationship of structure to properties of some anionic surfactants as collectors in the flotation process. 1. Effect of chain length. J. Chem. Eng. Data 1999, 44 (1), 133-137. (2)Yarranton, H.; Sztukowski, D.; Urrutia, P. Effect of interfacial rheology on model emulsion coalescence: I. Interfacial rheology. J. Colloid Interface Sci. 2007, 310 (1), 246-252. (3)Tian, G.; Lei, Q.; Lui, W.; Li, F. Colloid chemistry in petroleum industry in China. J. Dispersion Sci. Technol. 2000, 21 (4), 433-448. (4)Al-Sabagh, A.; Badawi, A.; Noor El-Den, M. Breaking water-in-crude oil emulsions by novel demulsifiers based on maleic anhydride–oleic acid adduct. Pet. Sci. Technol. 2002, 20 (9-10), 887-914. (5)Pradilla, D.; Simon, S. C.; Sjöblom, J. Mixed interfaces of asphaltenes and model demulsifiers part II: study of desorption mechanisms at liquid-liquid interfaces. Energy Fuels 2015. (6)Sil a .
orges
.
lanco
.
ond n, M.; Salager, J.-L.; Pereira, J. C. Breaking of
water-in-crude oil emulsions. 5. Effect of acid-alkaline additives on the performance of chemical demulsifiers. Energy Fuels 2014, 28 (6), 3587-3593. (7)Buzzacchi, M.; Schmiedel, P.; von Rybinski, W. Dynamic surface tension of surfactant systems and its relation to foam formation and liquid film drainage on solid surfaces. Colloids Surf., A 2006, 273 (1), 47-54. (8)Zhang, L.; Zhu, T.; Sun, Y.; Jiang, B. Experimental study of precision-woven fabrics for oil-in-water emulsion coalescence: operating conditions and oil saturation. J. Dispersion Sci. Technol. 2015, 36 (2), 182-189. (9)Yang, B.; Chang, Q. Wettability studies of filter media using capillary rise test. Sep. Purif. Technol. 2008, 60 (3), 335-340. (10)Zhou, Y. B.; Chen, L.; Hu, X. M.; Lu, J. Modified resin coalescer for oil-in-water emulsion treatment: effect of operating conditions on oil removal performance. Ind. Eng. Chem. Res. 2008, 48 (3), 1660-1664. (11)Srinivasan, A.; Viraraghavan, T.; Ng, K. Coalescence/filtration of an oil-in-water emulsion in an immobilized mucor rouxii biomass bed. Sep. Sci. Technol. 2012, 47 (16), 2241-2249. (12)Viraraghavan, T.; Mathavan, G., Oil removal using peat filters. Sio: 1989; Vol. 40.
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(13)Renouf, G.; Kurucz, L.; Soveran, D. In Produced fluids separation using a coalescer column, Canadian International Petroleum Conference, Petroleum Society of Canada: 2005. (14)Madia, J. R.; Fruh, S. M.; Miller, C. A.; Beerbower, A. Granular packed bed coalescer: influence of packing wettability on coalescence. Environ. Sci. Technol. 1976, 10 (10), 10441046. (15)Moses, S.; Ng, K. A visual study of the breakdown of emulsions in porous coalescers. Chem. Eng. Sci. 1985, 40 (12), 2339-2350. (16)Sokolovic, S.; Secerov-Sokolovic, R.; Sevic, S. Two-stage coalescer for oil/water separation. Water Sci. Technol. 1992, 26 (9-11), 2073-2076. (17)Davies, G.; AFZAL, M.; Ali, F.; JEFFREYS, G. Coalescence in droplet dispersion bandsunusual surface effect. Chemical Engineer-London 1972, (266), 392. (18)Dean, E.; Stark, D. A convenient method for the determination of water in petroleum and other organic emulsions. Industrial & Engineering Chemistry 1920, 12 (5), 486-490. (19)Kokal, S.; Tang, T.; Schramm, L.; Sayegh, S. Electrokinetic and adsorption properties of asphaltenes. Colloids Surf., A 1995, 94 (2), 253-265. (20)Bond, W. LXXXII. Bubbles and drops and Stokes' law. The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science 1927, 4 (24), 889-898. (21)Basu, S. A study on effect of wetting on mechanism of coalescence in a model coalescer. J. Colloid Interface Sci. 1993, 159 (1), 68-76.
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The
principle of capillarity and the mechanism of a wetting film in porous media are applied in
designing coalescence media by using water-wet porous particles in a coalescing column to enhance the separation of water from water-in-heavy oil emulsions. The
water-saturated porous particles can remain water-wet in an oil environment and can
significantly enhance the coalescence of water droplets in water-in-heavy oil emulsions. The
study provides a new method for the separation of water-in-heavy oil emulsions.
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