solvent sublation

Journal of Petroleum Science and Engineering 80 (2012) 26–31

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Treatment of oilfield produced water by dissolved air precipitation/solvent sublation Fatemeh Bayati, Jalal Shayegan, Abolfazl Noorjahan ⁎ Department of Chemical Engineering and Petroleum, Sharif University of Technology, Azadi Ave., Tehran, Iran

a r t i c l e

i n f o

Article history: Received 4 February 2009 Accepted 9 October 2011 Available online 20 October 2011 Keywords: dissolved air precipitation solvent sublation produced water TOG BTEX

a b s t r a c t Dissolved Air Precipitation/Solvent Sublation (DAP/SS) was used for treatment of simulated and real oilfield produced water to generate very fine bubbles which are necessary for effective separation. In this method micro bubbles produced by saturation of air in a pressurized packed column were released in an atmospheric column leading the bubbles to raise resulting trapped contaminants in the Gibbs layer around them to be removed by a layer of immiscible solvent at the top of column. The method was conducted to solutions including Benzene, Toluene and Chlorobenzene (ClB) as part of BTEX contaminants in produced water, mixture of them as simulated produced water and real oilfield produced water. Obtained results for Removal Efficiency (RE) has been compared with a mathematical model of separation process to determine the ability of the model to predict RE and the important operating parameters. Effect of pressure in packed column (4, 5, 6, and 7 bar), salinity of samples and bubble size as important parameters on RE were examined. The results indicated positive effect of pressure and salinity on RE via reducing bubble size and increasing air flow rate in column. Furthermore the method was applied to a mixture of components to analyze effect of co-existence of components on RE. Presence of more than one component in the solution led to slower removal of contaminants due to limitation on number of bubbles. Also effect of DAP/SS system for TOG removal of produced water was investigated where RE of 70% achieved. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Produced water is one of the major by products of oil and gas exploitation which is produced in large amounts as waste stream up to 80% (McCormack et al., 2001). This large amount of by product is whether reused by injecting into the well or discharged to environment, water or soil (Lu et al., 2006). Depending on geological conditions and field position, produced water may have complex composition including: organic or inorganic ingredients like: salts, metals, dispersed oil, phenols, organic acids, dissolved hydrocarbons like: Benzene, Toluene, Chlorobenzene and Ethyl Benzene (BTEX), Poly Aromatic Hydrocarbons (PAHs) like: naphthalene, phenanthrene and dibenzothiophene (NPD) and their C1–C3 alkyl homologues and also some compounds which may be added to it during oil separation process (Røe Utvik, 1999). In order to discharge produce water to environment, the concentration of components must be less than the allowable levels of organic contaminants in the water. So, it is necessary to remove the excess contaminants using an appropriate method. Disposal of produced water with existing technologies is a costly and difficult procedure. According to Bauman (Thoma et al., 1999) the cost of treatment of produced water in Gulf of Mexico is estimated at $1.5 per bbl equivalent of oil produces. It is also estimated by Otto and ⁎ Corresponding author. E-mail addresses: [email protected] (F. Bayati), [email protected] (J. Shayegan). 0920-4105/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.petrol.2011.10.001

Arnold (Thoma et al., 1999) that reaching EPA produced water discharge requirements is not easily obtained by 60% of offshore platforms in Gulf of Mexico, especially when it comes to removal of dissolved oil components which may exceed concentration of dispersed phase. Thus, it is necessary to improve technologies which help in meeting the required environmental regulations at lower cost. Physical–chemical, mechanical and biological treatments are the conventional methods used for treatment of produced water. Gravitational separation, centrifugation, coagulation, flotation, filtration and adsorption combined with biological treatment are the conventional methods used for removal of free and emulsified oil and suspended solids and also BOD removal from produced water (Mota et al., 2008). More advance methods such as advanced oxidation process (Mota et al., 2008) or bubble column reactor (Smith et al., 1996a,b), (Smith et al., 1996a,b), (Smith and Valsaraj, 1997) and solvent sublation (Thoma et al., 1999) are required for the removal of toxic dissolved hydrophobic organic compounds (Mota et al., 2008). Solvent sublation with simultaneous advantages of separation and preconcentration was first introduced by Sebba (1960) for the removal of hydrophobic organic compounds from water (Lu et al., 2006). In this method dissolved hydrophobic organic compounds are carried by the bubble surfaces to the solvent layer at the top of the column. Comparing to ion flotation and liquid–liquid extraction, solvent sublation has many advantages. Simple operation, high efficiency, low amount of organic solvent, high concentration coefficient and direct analysis of organic phase are some of the superiorities of solvent

F. Bayati et al. / Journal of Petroleum Science and Engineering 80 (2012) 26–31

sublation (Bi et al., 2010). The immiscible oil layer on top of the column specifies solvent sublation from conventional bubble separation methods. It enhances treatment by reducing possibility of redispersion of components in the aqueous phase because of bubble bursting at the top of the column in bubble aeration columns. In addition, both volatile and non-volatile compounds can be removed while release of volatile compounds to atmosphere will also be reduced by being trapped in the organic solvent layer at top of the column. Despite all the advantages, solvent sublation needs improvement in theory and application (Bi et al., 2010). The physical processes involved in the separation of aqueous contaminants include Henry's partitioning between the bulk phases and interfacial partitioning at the air–water interface (the surface of the bubble). This interfacial partitioning is particularly important for compounds which have both low Henry's constants and low solubility (Bryson and Valsaraj, 2001). The hydrophobic compounds will collect on the surface of the bubble by diffusion through the boundary layer surrounding the air bubble because of their natural tendency to concentrate at the air–water interface and reducing their energy level (Valsaraj et al., 1991). Solvent sublation introduces additional removal mechanisms for dissolved compounds associated with the presence of the immiscible layer on top of the water column. Two mechanisms effect a liquid–liquid extraction. Axial mixing of the water column continually contacts new materials with the immiscible layer, and mass transfer out of the water will occur while the capacity of the immiscible layer is not exceeded. In addition, the rising bubbles will carry a sheath of water into the immiscible layer, and this phase mixing will also give rise to contaminant removal. Hence comparing to solvent extraction, solvent sublation has higher efficiency because of the increase in concentration of the hydrophobic compounds reaching the solvent layer (Thoma et al., 1999), (Lu et al., 2006). A mathematical model for the removal of dissolved compounds in wastewater has been published for a semi-batch (see Fig. 1) operation of a sublation column. This model by Valsaraj et al. (1991) is reproduced below:    β  −αt 1− 1−e α  1 3 2 β¼ πr c k1 þ Q a H c þ di K ow V o R       Qa 3 3 πr 2c k1 Vw Qa 3 þ H c þ K a þ di þ 1þ H c þ di α¼ Vw Vw K ow V o K ow V o R R R RE ¼ 1−

Cw ¼ C wi 

ð1Þ where RE is Removal Efficiency, Cwi is the initial aqueous contaminant concentration (mg L − 1), Cw is the contaminant concentration(mg l − 1) in different time of operation, t is treatment time (s), Qa is the gas flow rate (mL min − 1), R is the bubble radius (m), Kow

27

is the solvent (Octanol or Mineral Oil)–water partition coefficient (dimensionless), Hc is Henry's constant (dimensionless), V0 is the immiscible solvent layer volume (L), Vw is the water volume (L), Ka is the interfacial partitioning coefficient (m), di is the thickness of the water layer carried into the solvent layer (m) and kl is the solvent layer–water interfacial mass transfer coefficient (m s − 1) and rc is the column radius (m). From the above model and consideration of the separation mechanisms, it is clear that volatile compounds will be readily removed, and that the removal is favored for compounds with high partitioning to the air–water interface and the immiscible layer. This is the basis of observed difference between removal rates of different components in the simulated produced water and is discussed later. As mentioned above non-volatile compound are collected at the bubble–water interface. So to maximize the efficiency of the process one should maximize the bubble surface area which is only possible with reducing the bubbles diameter. So for production of smaller size bubbles Thoma et al. (1999) proposed Dissolved Air Precipitation (DAP) method. In this method, air is dissolved in water in a pressurized vessel (saturator) and then releasing the flow in atmospheric pressure through a precipitation valve generates very tiny bubbles with diameter in the range of 10–100 μm? while conventional bubble generation methods produce 5–10 times larger bubbles. Therefore in this study combination of DAP with Solvent Sublation has been investigated to analyze the capability of this method in treatment of the simulated and real oil field produced water. 2. Experiments 2.1. Chemicals The chemicals used in this study consist: Benzene, Toluene, Chlorobenzene, Octane and Decane were used as received from the MERCK Company with purity of >99.8%. They are representative of highly and moderately volatile species in simulated produced water. Methanol (99.8%), White Mineral oil (99%), Octanol (99.7%), NaCl (99+ %) and H2SO4were used as received from Fisher Scientific Company. 2.2. Produced water The produced water used for this research project was product of, Desalination unit of Asmari wastewater treatment facility at Karoon field, Ahvaz, Iran. 2.3. Apparatus The Dissolved Air Precipitation/Solvent Sublation (DAP/SS) system was similar to one prepared earlier by Thoma et al. (1999). An atmospheric column 65 cm × 176.5 cm 2 with one saturated air inlet and one water outlet conjunct to the pump (a chlorinator dosing pump from SALEM AB Co.) and the saturator, 35 cm × 314 cm2, half of which was filled with ceramic berl saddle for providing a large air– water contact area. The schematic of the system has been presented in Fig. 1. To analyze the TPH/TOG (Total Petroleum Hydrocarbon/ Total Oil–Grease) of produced water a CVH50 TPH/TOG Analyzer with 16 mm cell from Wilks Company has been used. 2.4. Preparation of samples

Fig. 1. A schematic of the experimental system.

For Toluene, Benzene and Chlorobenzene as they are insoluble in water, 1 g of each component was dissolved in 10 mL of Methanol, and then the solution was added to 10 L tap water. For simulated Produced Water, 1 g of Benzene, 1 g of Toluene and 1 g of Chlorobenzene were dissolved in 10 mL Methanol, and then the solution was added to 10 L tap water. As Šijački et al. (2010) reported on the effects of

F. Bayati et al. / Journal of Petroleum Science and Engineering 80 (2012) 26–31

alcohol addition (from Methanol to n-Octanol) on the main hydrodynamic characteristics of a bubble column, the addition of only a small amount of normal aliphatic alcohols influenced the gas hold-up and liquid velocity, as the presence of alcohols led to a decrease of surface tension. 2.5. Bubble generation While using DAP system, the primary components responsible for microbubble generation are the air–water saturator and the precipitation valve. Fig. 1 presents a schematic of the experimental system. Water is recirculated from the bottom of the column through a 12stage centrifugal pump (Teel Inc.) into the saturator and then through the precipitation valve in the sublation column. The 20 cm diameter/ 35 cm tall saturator was packed with 17 cm ceramic berl saddle packing to provide a large air–water contact area, and in normal operation the liquid level was held at one third of the column height. Liquid recirculation rate 35 L/h was used, and saturator pressures ranged from 4 to 7 bar. 2.6. Bubble size Experimental runs were performed in which the effect of the saturator pressure and salinity on the size of bubbles precipitated was determined. For this tests different concentration of NaCl solution, 0.5, 1, 2 and 4 wt.%, were prepared and used to generate bubble at different pressure ranged from 4 to 7 bars. After filling the system with solution the pump was turned on and the system valves were opened with air pressure applied to the saturator. After the saturator reached a steady state operation, it was operated for 5–7 min, allowing the bubble cloud to fill the column completely (Fig. 2). At this time, the column was isolated from the pump and saturator by closing both the depressurization valve and the recycle stream valve and turning off the pump. The bubble swarm rise velocity was found by measuring the time required for 54 cm (total height is 65 cm) of the column to clear. For bubbles in the diameter range generated by this system, Stoke's law applied in Eq. (2) is adequate for estimation of the bubble diameter (Jamialahmadi et al., 1994):   ð4:5Hϑw Þ 0:5 R ¼ 0:5   t g ðρw −ρa Þ

ð2Þ

where R is bubble radius (n), ϑw is water viscosity (Pa.s), ρw is water density (kg/m3), ρa is air density (kg/m3), H is considered height (54 cm), t is rising time (s) and g is gravitational acceleration (m/s2) (Fig. 3). 2.7. Sublation runs The column was filled with 10 L prepared sample. Then the pump was turned on until the saturator was filled with 5 L sample and then

15 cm

Fig. 2. Bubble releasing in atmospheric column.

70

Bubble diameter (micron)

28

60

50 Tap Water Salt Solution 0.5 %

40

Salt Solution 1% Salt Solution 2% Salt Solution 4%

30

20

Produced Water 19%

3

4

5

6

7

8

Pressure (bar) Fig. 3. Effect of the saturator pressure and salt concentration on the bubble size.

was kept fixed at this operating level. Then 2 cm solvent was added on the top of the water in the atmospheric column. The compressor was turned on until the pressure would reach the desired pressure. Then the precipitation valve was opened and the bubbles were released in the column. The opening of the valve was adjusted to hold the column water level constant. Samples for GC analysis were collected from a tee in the pump recycle line near the bottom of the bubble column. 2.8. Analysis Water samples from operational runs (40 mL) were extracted with 1 mL of HPLC-grade hexane and 0.5 μL of the extract was directly injected on the GC. The GC analysis was done on a 50m ×0.201mm ×0.5μm column (J&W). The conditions were: An initial temperature of 40 °C for 2 min, increasing temperature to 150 °C at 25 °C/min and 1 min in the final temperature of 150 °C. An inlet temperature of 200 °C and an FID detector temperature of 220 °C was used through the run. For analysis of produced water samples, 20 mL samples were acidified to pH b 2 using H2SO4and extracted with 20 mL Tetrachloroethtlene for 2 min then TOG (total Oil and Grease; including all oil normal hydrocarbons like aromatics, alkanes, alcohols, aldehydes, esters and carboxylic acides) of the extracted sample was read by TPH/ TOG (Total Petroleum Hydrocarbon/Total Oil–Grease)Analyzer. 3. Results and discussion 3.1. Analysis of bubble size As Bryson and Valsaraj (2001)) outlined in their work, two of the mechanisms of solute transport between the water and the organic solvent in sublation are dependent on the average gas bubble size and gas/water interfacial area in the column. The gas/water interfacial area av is determined primarily by the bubble size, R and gas phase hold-up, ɛg within the column: av ¼

6 g R

ð3Þ

The term ɛg represents the ratio of the overall gas volume to the total fluid volume at given gas and liquid flow rates. As mentioned before, one of the advantages of DAP over other bubble generation methods is the smaller bubble size. In this system bubble size is highly affected by vessel pressure, precipitation valve type and size and salt concentration. To identify the correlation between these parameters, the procedure described at Section 2.6 and Eq. (2) has been used to determine bubble average size and the results can be seen in Fig. 4

F. Bayati et al. / Journal of Petroleum Science and Engineering 80 (2012) 26–31

using a fixed valve. As expected, results show positive effect of increasing pressure and salt concentration to reduce the size of the bubbles. The effect of salt can be described by increasing surface tension which its direct result is increase in resistance against creation of larger surfaces, resulting smaller bubbles. Although this result is in contrast with Bryson and Valsaraj (2001)'s findings where they found in salt water solutions, increasing pressure resulted in a decrease in the median bubble diameter, while for fresh water the bubble diameter increased slightly. The only reason is the different between bubble generation methods. Also a sample of oilfield produced water which contains high concentration of salt was investigated. An interesting point about this produced water is that the bubble size produced in this water (salinity 19 wt.%) has decreased significantly because of high concentration of salt but rate of bubble size reduction by increasing pressure has reduced comparing to waters with lower salt concentration. In addition we found that by increasing the saturator pressure, the rate of precipitated air delivery has been increased. This is the consequence of this fact that at higher pressure, the amount of air dissolved is greater. This results show the capability of this method to treat produced water even where the salt concentration of the water is relatively high.

3.2. Efficiency of removing alkane compounds Fig. 4 presents the results of experimental runs in which Octane and Decane contamination was treated. The data show the ability of the method in removing normal alkane (i.e. 95% removal of the dissolved Octane in a period of 35 min) with high partitioning coefficient and Henry's constant. The modeled removal shown in Fig. 4 is based on a combination of measured, estimated and literature values for the parameters. As can be seen a priori predictions of the model comparing to different experimental values, overestimate the observed results. A number of factors may influence this; including incomplete mixing at the initial condition and polydisperse bubble size distribution (relatively few larger bubbles can significantly reduce the net surface area per volume of air throughput). Nevertheless, the prediction of the model is reasonably supported by the experimental observations. In Fig. 4 there is a significant discrepancy between the observed Decane removal and that predicted from the model. The primary reason for this is that the model is based on the removal of dissolved constituents, but in this case the Decane was spiked at approximately 1000 times its solubility, and must have existed as a micro disperse phase. The 70% removal over a 30 min period demonstrates that the micro bubbles are effective at simultaneously removing dissolved Octane (below its solubility) and a disperse phase.

3.3. Removing aromatics In Fig. 5 the results of runs using 100 mg L− 1 of a series of aromatic compounds including Toluene and Chlorobenzene (ClB) is presented. Based on the physical properties for these compounds presented in Table 1, it is not surprising that the removal efficiency and rate are much poorer than for the alkane compounds presented in previous section. The important parameters are much smaller for the aromatics than the alkanes. In addition, both the model and the data show the trend expected as compounds with higher Henry's constant and partition coefficient are more easily removed. Also as we expected, it is clear that the model predicts the observations with reasonable accuracy in the case of dissolved compound. In a set of experiments for Chlorobenzene and Toluene, effect of DAP/SS on RE has been investigated and the results are compared to the mathematical model presented by Valsaraj (Valsaraj et al., 1991) in Fig. 5. 3.4. Effect of pressure on RE Fig. 6 presents results of experiments conducted on studying effect of pressure on RE in case of Toluene and ClB. As discussed before, by increasing saturator pressure the size of the bubbles decreases which will result in improving RE of solute due to increase in the Gibbs layer area (liquid around the bubbles containing active sites which can take contaminant molecules) around the bubbles. The larger the Gibbs area available, the larger amount of hydrophobic solute entrapped and finally more solute sublated to the organic solvent phase, which is Mineral oil in this case. Furthermore increasing pressure while the liquid recycle rate is constant causes more air dissolved in the water and thus more gas flow in the column. Hence the contact area available for solute will be enhanced by increasing number of bubbles as well as reducing their size. Now as the pressure affect the bubble size, and air flow rate we can introduce the effect of pressure via these two parameters into Eq. (1) which need some calibration and extra measurements. It is worth to note that, as you have seen in Section 3.1, in the case of produced water the effect of bubble size can be neglected and only the gas flow rate should be considered. 3.5. Treatment of simulated produced water In another experiment Simulated Produced Water (SPW) containing similar concentrations of Benzene, Toluene and Chlorobenzene (as part of BTEX present in most of produced waters) has been treated. Fig. 7 shows the results of applying DAP/SS operation to this mixture. At the initial stages of the operation, higher volatility of Toluene causes faster removal of Toluene compared to ClB and Benzene. Also, comparing Benzene and ClB Removal Efficiency indicates that at initial stages of operation, higher Henry Constant of Benzene will result

1

1

0.9

0.9

0.8

Removal Efficiency

Removal Efficiency

29

0.7 0.6 0.5 Octane Thoma et. al. 1998 Decane Thoma et. al. 1998 Octane This work Decane This work Octane Model Decane Model

0.4 0.3 0.2 0.1 10

20

Time (min) Fig. 4. Removal efficiency of alkanes.

30

0.7 0.6 0.5 0.4 Toluene Experimental CLB Experimental Toluene Calculated CLB Calculated

0.3 0.2 0.1

0 0

0.8

40

0 0

20

40

60

80

100

120

Time (min) Fig. 5. Removal Efficiency of organics by DAP/SS and comparing the results with model.

30

F. Bayati et al. / Journal of Petroleum Science and Engineering 80 (2012) 26–31

0.6

Octane Decane Benzene Toluene Chlorobenzene

Solubility (mg L− 1)

Henry's constant

Interfacial partition coefficient

Octanol–water partition coefficient

0.4* 0.022* 1780* 515* 100**

132* 7.5* 0.22* 0.27* 0.23**

111* 810* 0.096* 0.26* 0.16**

151,400* 4,900,000* 125* 400* 300**

Removal Efficiency

Table 1 Physicochemical properties of the chemical studied.

⁎From Thoma et al. {{263}}. **Estimation.

0.5 0.4 0.3

Toluene

0.2

Benzene CLB

0.1

Average

0 0

20

40

60

80

100

Time (min) in faster removal by gas stripping while gradually by decreasing the concentration of solution Removal Efficiency of ClB reaches that of Benzene (see Fig. 7). Comparing the results with single component solution, multi component solution shows less RE for each component in the same time respecting to the case of single component solution. As can be seen in Eq. (1), the model is not sensitive to the initial concentration of the contaminant, and the RE efficiency in the same period of time (as other parameters are constant) is the same for all initial concentration. But results contradict this functionality which can be described by thermodynamic limitation (Smith et al., 1996a,b) in the mixture with higher concentration and/or occupation of bubbles surface with co-existing contaminant and lower chance of adsorption for each component at same time due to limited number of bubbles. It seems that more tests are required to find the minimum gas flow rate where the RE is independent of initial concentration. This results shows that the expected treatment time for complex mixture like produced water should be much longer than pure component, so we increased the operation time, and sampling intervals as presented in the next section.

Fig. 7. Removal of dissolved aromatics from a simulated mixture of toluene, benzene and chlorobenzene.

in Eq. (1), indicates that TOG removal in the produced water follows the exponential functionality like below:   ð−bt Þ y ¼ a 1−e

ð4Þ

Which is similar to Eq. (1) when all the parameters are constant. The result of curve fitting for RE according to time has been summarized in Eq. (5) and Eq. (6). As can be seen in Fig. 8 and Fig. 9, the experimental results still follow the Valsaraj proposed model (Eq. (1)). The results indicate that the presented method can be suitable for treatment of oilfield produced water with high salt concentration.   −:01577t 2 for P ¼ 6 bar R ¼ 0:89 RE ¼ 0:6781 1−e

ð5Þ

  −0:01698t 2 for P ¼ 4 bar R ¼ 0:92 RE ¼ 0:5868 1−e

ð6Þ

3.6. Treatment of produced water Effect of DAP/SS on TOG removal of produced water was investigated. The experiments were conducted on 15 L produced water in two different pressures (4 bar and 6 bar). The results presented in Fig. 7 indicate TOG removal of 70% for vessel pressure of 6 bar and about 65% for 4 bars. It is similar to simulated samples where higher pressure increases removal rate. Also it is obvious that at early stages removal rate is faster while decreases gradually until reaches a final value. The reason is great reduction in concentration of contaminant which gets closer to their solubility (from disperse to dissolved) and also decreases the driving force of mass transfer. Comparing the trend of removal with the mathematical model for removal of soluble compounds and dispersed hydrophobic compounds

The interesting point in the RE values in Fig. 8 and Fig. 9, is the great difference between the values from different pressures. As indicated before, in the case of produced water the effect of pressure on bubble size in negligible and the only important parameter is the increase of gas flow in column. Analysis of solvent layer composition after treatment of simulated solutions showed that with increase of pressure (means increasing gas flow) less amount of solutes are entrapped in the solvent layer and most of them has escaped to air. The more the solute is volatile, the more escaped to the atmosphere. To improve the efficiency of solvent sublation, the height of the solvent layer should be increased to extend the retention time of the bubbles in the solvent layer to reduce the chance of solute compound escaping to the air.

1 0.9 0.8

0.8

Removal Efficiency

Removal Efficiency

0.9 0.7 0.6 0.5 Toluene-7 bar Toluene-5 bar CLB-6 bar CLB-4 bar Toluene-7 bar calculated Toluene-5 bar calculated

0.4 0.3 0.2 0.1

0.7 0.6 0.5 0.4 0.3

6 bar

0.2

Model 6 bar

0.1

0

0 0

20

40

60

80

100

Time (min) Fig. 6. Effect of pressure on removal of toluene and chlorobenzen.

120

0

50

100

150

200

250

Time (min) Fig. 8. TOG removal of produced water at 6 bar pressure.

300

F. Bayati et al. / Journal of Petroleum Science and Engineering 80 (2012) 26–31

0.7

0.4

time follows the model described by Valsaraj. This general exponential functionally was fitted to RE data and good agreement in different pressure achieved. As the bubble size is not changed with pressure in produced water the only reason for higher RE at higher pressure found to be the larger gas flow in the column.

0.3

Acknowledgements

0.6

Removal Efficiency

31

0.5

0.2

We gratefully thank the Research & Technology Directorate Of Iranian National Oil Company for supporting the project and providing produced water for the experiments.

4 bar

0.1

Model 4 bar

0 0

50

100

150

200

250

300

Time (min) Fig. 9. TOG removal of produced water at 4 bar pressure.

4. Conclusions DAP as readily produce microbubbles with diameters of less than 100 μ?m was applied in solvent sublation process to reduce the bubble size and increase the removal efficiency. It has been found that the pressure has significant impact on the bubble size in low salt concentration when the effect on the produced water with high salt concentration is negligible. Comparing the results from experiment with those of mathematical model showed an acceptable agreement while the effect of pressure can be included in the model via bubble diameter and gas flow in the column. In addition to the removal of dissolved aliphatic and aromatic contaminants, the process was effective in the simultaneous removal of a microdisperse Decane phase. The removal of alkanes with high partitioning coefficient and Henry's constant found to be much faster than aromatics compound like Toluene and CLB. But the comparison of the model with experimental data indicate that the model overestimates the alkane removal while has good agreement with aromatics data. We did observe that the RE for each compound in a simulated produced water sample is less than those in the case of pure compound due to limitation of number of bubbles. These results also as we expected showed that in the mixtures, compound with higher Henry's constants and interfacial partitioning coefficient will be removed faster. The results of applying DAP/SS system to TOG removal of real produced water samples acknowledge the ability of this method to efficient treatment of produced water where the efficiency of 70% can be achieved without lots of effort. In addition, the trend of RE during

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