Removal of fine oil droplets from oil-in-water mixtures by dissolved air flotation

chemical engineering research and design 1 1 5 ( 2 0 1 6 ) 19–33

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Removal of fine oil droplets from oil-in-water mixtures by dissolved air flotation M.R. Aliff Radzuan a,b,∗ , M.A. Abia-Biteo Belope a , R.B. Thorpe a a

Department of Chemical & Process Engineering, Faculty of Engineering and Physical Sciences, University of Surrey, GU2 7XH Guildford, UK b Universiti Kuala Lumpur, 50250 Kuala Lumpur, Malaysia

a r t i c l e

i n f o

a b s t r a c t

Article history:

Dissolved air flotation (DAF) is often used after a primary gravity separator to enhance the

Received 9 February 2016

quality of wastewater, so it can be released to streams, rivers or the sea. The main aim of the

Received in revised form 8 August

DAF experiments reported here was to measure the oil droplet removal efficiency () mostly

2016

in the range 15–80 ␮m from oil-in-water mixtures. The DAF tank used in this investigation

Accepted 9 September 2016

was a scale model of real DAF unit. Two kinds of oil, vegetable and mineral and two types of

Available online 16 September 2016

water, fresh and salty were used, and four other operating parameters were varied. A droplet

Keywords:

analysis concluded that the  in this experiment is a function of eight other dimensionless

counting and oil-in-water measuring methods were used to estimate the . Dimensional Dissolved air floatation (DAF)

groups and the experimental data has been subjected to multivariable linear regression.

Droplet size distribution

The resulting correlation was found to have a root mean square error of 6.0%, but predict 

Oil droplet removal

outside the range zero and one. An alternative mathematical formulation was devised that

Wastewater treatment

cannot predict  outside the range. Regression of the data by this formulation, which had

Separation process

the same number of adjustable parameters as the linear regression, was successful with a

Multivariable regression

lower root mean square error of 5.5%. © 2016 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1.

Introduction

Dissolved air flotation (DAF) is a separation technique used to treat water polluted with particles, droplets or microorganisms in a range of 10–100 ␮m. DAF has been applied in many industries such as in the

(Edzwald, 2010). The water that contains the dissolved air is then released from the nozzles located at the bottom of the contact zone of the DAF tank. The rapid pressure reduction in the DAF tank allows the formation of micro bubbles of various sizes. Most of the micro bubbles

preparation of raw water by water companies at wastewater treatment plants (Edzwald and Haarhoff, 2012; Hague, 2003), to remove parti-

generated in this DAF unit have a diameter of between 40 and 65 ␮m (Hague, 2003). The flotation method is divided into three different flow schemes,

cles in the mining and mineral processing industries (Al-Thyabat and Al-Zoubi, 2012; Rodrigues and Rubio, 2007), for pre-treatment in the

which are, full-flow pressure flotation, split-flow pressure flotation, and recycled-flow pressure flotation (Al-Shamrani et al., 2002b; Edzwald

desalination process (Haarhoff and Edzwald, 2013), for cleaning up ani-

and Haarhoff, 2012; Hanafy and Nabih, 2007; Zouboulis and Avranas,

mal waste in the agricultural industries (Creamer et al., 2010), for waste

2000). The DAF unit used in these investigations does not follow any of

treatment in food-processing plants (Yoo and Hsieh, 2010), and in crude

these schemes in order to prevent the saturator being damaged by the oil. However, the scheme used in these experiments is almost similar to Recycle-Flow Pressure DAF. The difference is that fresh air saturated

oil refineries (Moursy and Abo El-Ela, 1982). The water is saturated with air in a saturator packed column under a pressure of between 1 and 4 barg. Several researchers have suggested different ranges of saturator pressure, which are 4–5 barg (Zouboulis and Avranas, 2000), 4–6 barg (Al-Shamrani et al., 2002a) and 5 barg

water pressure instead of clarified water is flowing into the air saturator pressure column. Air saturated water pressure is also considered as a ‘recycle flow’ because the water is pumped into the saturator vessel

∗ Corresponding author at: Department of Chemical & Process Engineering, Faculty of Engineering and Physical Sciences, University of Surrey, GU2 7XH Guildford, UK. E-mail address: [email protected] (M.R. Aliff Radzuan). http://dx.doi.org/10.1016/j.cherd.2016.09.013 0263-8762/© 2016 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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chemical engineering research and design 1 1 5 ( 2 0 1 6 ) 19–33

Fig. 1 – Full flow pressure DAF. to be mixed with air under pressure (Hague, 2003). Fig. 1 shows this version of full flow pressure DAF. No pre-treatment is applied to the influent flowing into the DAF tank. The air bubbles and the oil droplets attach together to form agglomerates. The agglomerate-in-water has a higher average density difference than the oil droplet-in-water. This characteristic enhances the buoyant force and thereby the rise velocity of the oil. The flotation is successful once the oil droplets create a layer on the surface of DAF tank and the water sufficiently clarified. Contact of the air bubble with an oil droplet does not guarantee the successful rise of the oil droplet to the surface. The oil droplet needs to spread over the air bubble to form an agglomerate that is able to tolerate the drag and gravitational forces without breaking up as it moves. The spreading interaction between the air bubble and oil droplet is governed by the spreading coefficient (So ) (Refer Eq. (1)). This coefficient is the difference in the interfacial tensions acting on a single contact line between the three phases, air, oil and water. So = wa − ow − oa

(1)

Here, wa , ow , and oa are water–air surface tension, oil–water interfacial tension and oil–air surface tension respectively. In order for the oil to spread over the air bubbles, the So must be positive (Bassam, 1989; Moosai and Dawe, 2003; Oliveira et al., 1999). Negative So will make the oil to form a definite contact angle with the other two phases (Moosai and Dawe, 2003). There are several scientific studies that have discussed the performance of DAF units by measuring the oil droplet removal efficiency in terms of change in chemical oxygen demand (COD), biological oxygen demand (BOD), and total dissolved solids (TDS) (Al-Shamrani et al., 2002a; Galil and Wolf, 2001; Hanafy and Nabih, 2007; Karhu et al., 2014; Moosai and Dawe, 2003; Moursy and Abo El-Ela, 1982; Oliveira et al., 1999; Rattanapan et al., 2011; Zouboulis and Avranas, 2000). The oil droplet removal efficiency reported in this paper was measured either using an accurate oil-in-water analysis method (FastHEX) (Abia-Biteo Belope and Thorpe, 2007) or an oil droplet counting method using Coulter Counter. The oil droplet counting method was used because it can measure the number of oil droplets from the sampling outlet in any desired size range. The main aim of this research project was to investigate the performance of DAF to remove as many oil droplets, with a diameter in the range 15–80 ␮m from the oil-in-water mixtures because it was desired to focus on the use of DAF, as a polishing unit operation post gravity separation.

2.

Materials and methods

2.1.

Materials

The liquids used in the experimental study were Guildford, Surrey tap water, salty water, which was made by adding about 3.5% by concentration of NaCl (supplied by British Salt Limited) to mimic the average salinity of seawater and some produced waters, Vegetable oil (triglycerides) that was supplied by Tesco Supermarket, since it is relevant to the food

industry and lamp oil, a mineral oil similar to that on an oil production platform. The lamp oil was supplied by Lamplight Farms was the safest in terms of vapour pressure among other mineral oils considered. The vegetable oil has a clear pale yellow appearance, with density o of 900 kg m−3 , kinematic viscosity  of 5.71 × 10−5 m2 s−1 at 21 ◦ C and 3.96 × 10−5 m2 s−1 at 35 ◦ C. Lamp oil has a light yellow appearance with density o of 810 kg m−3 , kinematic viscosity  of 1.54 × 10−6 m2 s−1 at 21 ◦ C and 1.46 × 10−6 m2 s−1 at 35 ◦ C. The kinematic viscosity was measured using a capillary viscometer supplied by Poulten Selfe & Lee Ltd. Despite the possibility of a product such as supermarket vegetable oil varying in composition with time, the physical properties were measured several times and minimal variance observed.

2.1.1.

Experimental apparatus and procedures

The DAF unit built in the University of Surrey is illustrated in Fig. 2 and was used by Abia-Biteo Belope (2010); Aliff Radzuan et al. (2014). It consists of an oil feed tank (TK-01), a water feed tank (TK-02), and a flotation tank (TK-03). It was made of Perspex and had dimensions of 1.0 m in length, 0.32 m in width and 0.39 m height. The DAF unit was also equipped with a water sump tank (TK-04), an effluent tank (TK-05) and a salty water tank (TK-06). Two flexible vane pumps (P-01 and P-02) and an air driven pump (P-03) were installed. A static mixer (SMV-02) supplied by Sulzer Chemtech was used to break up the oil droplets to a diameter below 100 ␮m. A filter (F-01) was used to prevent tiny particles from entering TK-03 via three sintered metal nozzles V-04 a/b/c that are located at the bottom of contact zone of DAF tank. An air saturator pressure column (C-01), flow rate indicators (FI-01 to FI-03), several valves (V-01 to V-10) and a pressure regulator (V-11) were also installed. The oil was pumped through P-01 at different flow rates to mix with the tap water from TK-02. Experiments with salty water required V-06 to be fully closed and V-08 fully opened. The flow rates of tap water or salty water were controlled using V-06. They were set at 45 or 30 L/min before entering static mixer. The static mixer was made of 316-stainless steel with a 2 mm hydraulic mean diameter and a void fraction of 0.82. Then, the oil-in-water mixture was fed into TK-03, which is a scale model of DAF unit operated by Thames Water plc. (Hague, 2003). The actual model of this DAF tank is used to remove solid particles from the wastewater, while the investigation reported in this paper involves removing the oil droplets from oil-in-water mixtures. A side view of the scale model DAF tank with dimensions is shown in Fig. 3. The water stream used to produce micro-bubbles in the flotation tank was pumped from TK-04 using P-03 into C-01 that operated at a pressure between 1 and 4 barg. The water ran over packing in C-01 and became saturated with air. The air-saturated water was subsequently flowed through F-01 before being routed to injection valves V-04 a/b/c at the bottom of the contact zone. Sample points were placed in both the inlet and outlet pipelines to the flotation tank. Samples were collected at these points after steady state was achieved and were analysed using methods discussed in Sections 2.1.2 and 2.1.3. The clarified water stream was routed directly into TK05. Each run was conducted over eight minutes after which time the water feed tank was almost empty. The height differences in the feed tanks did not alter the flow rates to any significant extent.

chemical engineering research and design 1 1 5 ( 2 0 1 6 ) 19–33

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Fig. 2 – Schematic diagram of DAF rig.

Fig. 3 – Dimensioned side view of a scale model DAF tank, TK-03.

Fig. 4 – (a) Vegetable oil–hexane and (b) lamp oil–hexane calibrations.

2.1.2.

Oil-in-water measuring method (FastHEX)

Ultraviolet–visible spectrophotometry (UV–vis) was used to analyse the samples of influent and effluent from the DAF tank by measuring the intensity of fluorescent light (absorbance) that is generated by a known concentration of oil. The absorbance values were used to create a calibration curve, which was then applied to calculate the concentration of oils. This method is known as the FastHEX method and was used

for oil-in-water measurement in the Alba Field (Abia-Biteo Belope, 2010). Fig. 4(a) and (b) shows the calibration curves of vegetable oil – hexane and Lamp oil – hexane from the UV–vis at 300, 400 and 500 nm wavelengths. A 300 nm wavelength obtained almost horizontal line, which is not desirable. 500 nm wavelength shows a straight line but the absorbance values were very close to zero. The best absorbance response was produced at 400 nm wavelength. The calibration curve for

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chemical engineering research and design 1 1 5 ( 2 0 1 6 ) 19–33

Table 1 – The surface/interfacial tension and spreading coefficient for lamp oil and vegetable oil with tap and salty water at 20 ± 1 ◦ C. Property

Tap water

Water–air (␥wa ) Water–oil (␥ow ) Oil–air (␥oa ) Spreading coefficient (So )

Salty water

Lamp oil (mN m−1 )

Vegetable oil (mN m−1 )

Lamp oil (mN m−1 )

Vegetable oil (mN m−1 )

68.2 39.7 26.3 2.20

68.6 14.0 34.3 20.3

68.9 35.1 26.2 7.60

69.2 9.40 33.3 26.5

both oils obtained linear lines of best fit from the wavelength of 400 nm and is mathematically described in Eq. (2) for vegetable oil and Eq. (3) for lamp oil. These equations were used to estimate the concentration of veg. oil and lamp oil collected from the rig. Here, a is the absorbance value obtained from the UV–vis. Cveg.oil =

a − 0.0598 0.00934

Clamp oil =

a − 0.0446 0.00813

(2)

(3)

100 ml of sample was collected from the inlet and outlet of the DAF rig. Each sample was mixed with 10 ml of hexane in a 200 ml beaker. Then, the samples were shaken gently for 3 min and allowed to settle until a layer of hexane and dissolved oil completely developed at the top. 10 ml of the hexane–oil mixture in the beaker was transferred into a UV–vis cuvette using a pipette. The cuvette was located in the UV–vis spectrometer with another cuvette containing a blank sample (hexane) and the absorbance was measured. The value of absorbance obtained was transformed to oil concentration Coil using Eq. (2) or (3), depending on the type of oil. Finally, the oil droplet removal efficiency from the FastHEX method (FH ) was estimated using Eq. (4). ain and aout are the absorbance reading obtained from the UV–vis of the inlet and outlet samples, respectively. The value of ki , which is the intercept value of the oil–hexane calibration was obtained from the calibration curve at 400 nm, which values are 0.0446 for lamp oil and 0.0598 for vegetable oil (Aliff Radzuan et al., 2014). FH =

2.1.3.

ain − aout ain − ki

(4)

Droplet counting method

A Coulter Counter was used for the droplet counting method. It was used by Abia-Biteo Belope (2010); Aliff Radzuan et al. (2014) to count the number of oil droplets that passed through an aperture of which is selected based on the size range of interest. The analyses were started quickly after the samples were collected from the DAF rig to reduce the chances of oil droplets coalescence. The counts were taken from 80 to 15 ␮m in 5 ␮m intervals. It is beneficial to estimate the terminal rise velocity of the oil droplets so that the time taken for the oil droplet to reach the surface of the cuvette can be determined. This time should be longer than the time taken to conduct the count. This matter is discussed in Appendix A and the experiments found to be satisfactory. Finally, the oil droplet removal efficiency from the droplet counting method CC was then calculated using Eq. (5). CC =

in ni Vo,i − out ni Vo,i in ni Vo,i

(5)

ni and Vo, i are the number of oil droplets obtained from the Coulter Counter and volume of oil droplet, both at specific diameter, di respectively.

2.1.4.

Surface/interfacial tension measurement

The surface/interfacial tensions were measured using Du Nouy Ring method (Shaw, 1992). This was begun with removing the precipitates that stuck on the ring by burning it on the Bunsen burner. Then, the ring was hung inside the tensiome˝ The surface tension of water–air ter manufactured by KRUSS. was measured by fully immersing the ring in the bottle containing water and bringing it up to the surface. Next, the initial surface tension value was reset to zero and measurement was started. The second measurement involved with the interfacial tension of water–oil. 10 mL of lamp oil was poured onto the water in the same bottle without touching the ring. The ring was fully immersed in the water and brought up very close to the water–oil surface. The value was first reset to zero and measurement was then started. The same procedures as water–air surface tension were applied to measure the surface tension of oil–air. The same procedures were applied to measure lamp oil with salty water, vegetable oil with tap water and vegetable oil with salty water. All of the measurements were done three times and the average value was recorded. The results of these experiments are shown in Table 1.

3.

Results and discussion

3.1.

Oil-in-water measuring method (FastHEX)

3.1.1.

The effect of inlet oil concentration

The effects of the inlet oil concentration entering the flotation tank were measured by varying the oil flow rates into the static mixer, while the other variables were kept constant. Three different inlet oil concentrations were used for each set of conditions. The FH of the experiment 4 barg, vegetable oil + tap water were found to be 75.0%, 63.6% and 52.2% at Coil(in) of approximately 0.04%, 0.12% and 0.22%, respectively. A similar pattern of results was demonstrated for the experiment with lamp oil in which the FH were found to be 49.1%, 34.4% and 27.1% at Coil(in) of approximately 0.06%, 0.11%, and 0.16%, respectively. All of the experiments conducted showed that the FH decreased with increasing inlet oil concentration (see Fig. 5) and this trend is often linear or close to linear. The trends of the results obtained were similar to the experiments conducted by Hanafy and Nabih (2007). The increase in FH with decreasing Coil(in) maybe explained as follows. As the inlet oil concentration decreased, the quality of the collision and attachment were increased. This was because a lower number of oil droplets in the DAF tank reduced the chances of coalescence. Hence, a greater fraction of oil droplets were collected and formed a sludge layer on the sur-

chemical engineering research and design 1 1 5 ( 2 0 1 6 ) 19–33

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Fig. 5 – Removal efficiency of oil droplets from oil-in-water mixture measured by FastHEX method (first published by Aliff Radzuan et al. (2014)).

Fig. 6 – Effect of the air saturator pressure on the removal efficiency. face of the mixture. The small fraction of the oil droplets that did not attach to the air bubbles increased as Coil(in) increased and caused FH to deteriorate. The oil droplets that were not successfully attached flowed to the effluent tank.

3.1.2.

The effect of air saturator pressure

Due to the pressure limit of the laboratory air supply, which was 4 barg, only pressures of 3, 4, and 0 barg (No DAF) were applied to saturator in the flotation process. It has been identified that the bubble size stays the same when the pressure is increased above 3.5 barg (Han et al., 2002) although the number of bubbles would be larger. However, pressure greater than 5 barg seems to be rarely used for DAF, so the pressures of greatest interest have been covered in this investigation. Fig. 6 shows that applying DAF at 4 barg resulted in FH of 80.8%, when lamp oil was used as the dispersed phase and salty water as the continuous phase. DAF under a pressure of 3 barg presented a slightly lower FH , which was 74.3%, and FH became lower at 35.2% when DAF was not used. The FH of the experiments with the vegetable oil and tap water were found to be 32.6%, 39.0% and 52.2% for saturator pressures of 0, 3 and 4 barg, respectively. Abia-Biteo Belope (2010) also reported that the best removal efficiency can be obtained at 4 barg than at 3 or 0 barg for this DAF unit. The bubbles produced at any pressure in the DAF tank resulted in better FH compared to using gravity separation (0 barg/No DAF). This demonstrates the effectiveness of the DAF in separating dispersed oil droplets in the size range of interest. The results at 4 barg show a slightly better performance

Fig. 7 – Effect of temperature on the removal efficiency. than those at 3 barg, which in turn were much better than No DAF results. Fig. 6 shows a trend that, within experimental error, may be described as linear. The amount of air dissolved and then released was proportional to the gauge pressure as described in Henry’s law. This may give a better matching between the number of bubbles and the number of oil droplets, especially at a higher inlet oil concentration. This is a hypothesis that seems to be supported by the results shown in Fig. 5, which show a greater gradient at 3 barg than the equivalent set at 4 barg.

3.1.3.

The effect of temperature

The temperature of the continuous phase taken from the TK02 and TK-06 was either at the room temperature or 35 ◦ C. Fig. 7 shows that the FH increased slightly with increased temperature for the different types of oil and saturator pressure. The highest FH (%) value was obtained from the experiments on the 4 barg, vegetable oil + salty water at 35 ◦ C and Coil(in) 0.05%, which was found to be 95.6%. The FH were reduced to 93.1% and 88.7% as Coil(in) was increased between 0.1% and 0.2%, respectively. For the similar set of experiments but at the room temperature, FH was found to be 95.8%, 91.3% and 86.3%. A similar pattern of results was obtained for the experiments on lamp oil + tap water between the room temperature and 35 ◦ C, without the presence of DAF. Experiments at 35 ◦ C gave higher values of FH that were found to be 36.8%, 27.8% and 19.1% at Coil(in) of approximately 0.07%, 0.12% and 0.19%, while 34.7%

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chemical engineering research and design 1 1 5 ( 2 0 1 6 ) 19–33

25.0% and 21.1% were obtained for the experiments at room temperature and at Coil(in) of approximately 0.06%, 0.12% and 0.16%. Experiments conducted at the higher temperature obtained slightly better FH than the experiments conducted at room temperature. This maybe expected as the increase in temperature reduces the viscosity of the fluids being separated; therefore, the rise velocity of the oil droplets as stated in Eq. (A1) is increased (Abdel-Aal et al., 2003). The kinematic viscosities of the oils were measured and they decrease with increasing temperature as presented in Section 2.1. An obvious kinematic viscosity difference can be seen from the vegetable oil where the values are 57.6 and 39.6 mm2 s−1 between 20 ◦ C and 35 ◦ C, respectively. The kinematic viscosities of the tap water and salty water were reported and show the reduction of 28% and 27%, for the temperature between 20 ◦ C and 35 ◦ C (Sharqawy et al., 2011). However, the effect on efficiency is much less than 27%, which suggests that another temperature dependent factor must be operative, which is depressing efficiency.

3.1.4.

The effect of water salinity

The investigation was carried out using two different types of continuous phase: Guilford, Surrey tap water and salty water (water with NaCl added to mimic sea water or produced water). For the experiment of 4 barg, lamp oil + salty water and at lowest inlet oil concentration, FH was found to be 80.8%, compared to 49.1% for the experiment with tap water. This difference is similar to the results for vegetable oil, which obtained 95.8% and 75.0% for the runs with salty and tap water respectively. The results in Fig. 8 show that all the experiments conducted with salty water obtained a better FH compared to the corresponding experiment that used tap water. Possible reasons for this behaviour are salt alters the surface charges of the oil droplets, which encourages the attachment of the air bubbles to the oil droplets (Moosai and Dawe, 2003; Oliveira et al., 1999; Strickland and Shell Development Co., 1980) and adding salt into the water decreases the average bubble diameter by reducing the chances of bubble coalescence (Thoma et al., 1999). To maintain high collision efficiency, coalescence of the bubbles should be avoided so that they remain a good size to match the oil droplets, i.e. below 100 ␮m. The addition of

Fig. 8 – Effect of salinity and type of oil on the removal efficiency by FastHEX method.

salt increased FH because it increases the So as shown in Table 1.

3.1.5.

The effect of type of oil

Based on the terminal rise velocity, the FH in the lamp oil experiments was expected to be better than that of vegetable oil because lamp oil droplets have a higher rise velocity, mostly due to the bigger density difference between the lamp oil and water. Hanafy and Nabih (2007) found that best removal efficiency was obtained from the cotton oil that has lower specific gravity than car and corn oils. However, the experiments conducted showed contrasting results. The experiments with vegetable oil presented higher FH than lamp oil (refer to Fig. 8). The FH for the vegetable oil and lamp oil together with tap water were found to be 63.6% and 34.4%, respectively. Coalescence of oil droplets was one of the reasons of this occurrence as was a difference in inlet droplet size. The issue of droplet size will be addressed later in Section 3.2. Coalescence did not occur for any range of droplet sizes with vegetable oil (refer to Section3.2.4) and so this cannot be the explanation. Another reason is found in the fact that vegetable oil has a higher spreading coefficient, So than lamp oil. The measurement was conducted twice and the average is reported in Table 1. The So of lamp oil mixed with tap water were lower than So of vegetable oil with tap water. Higher So values were obtained when salty water was used. These values are supported by the observation that the interaction between vegetable oil and tap/salty water caused the Du Nouy ring to become more hydrophilic.

Fig. 9 – The comparison of oil-in-water (FH) and droplet counting method (CC) on the lamp oil + tap water between experiments with (a) No DAF and at (b) 4 barg.

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Fig. 10 – The comparison of oil-in-water (FH) and droplet counting method (CC) on (a) 3 barg, lamp oil + tap water and (b) 3 barg, vegetable oil + tap water.

3.2.

Droplet counting method (Coulter Counter)

3.2.1. Comparison between oil-in-water measuring and droplet counting technique Oil-in-water (FastHEX) and droplet counting (Coulter Counter) methods were used to measure the oil droplet removal efficiencies for the same or very similar conditions. Then both methods were compared to explore the agreement between them. The oil droplet removal efficiencies of the experiments with No DAF, lamp oil and tap water that analysed with FastHEX were found to be 25.0% and 21.1%, and the analysis with the Coulter Counter were found to be 24.8% and 21.9% for Coil(in) of approximately 0.10% and 0.20%, respectively. The results are shown in Fig. 9(a). The experiment involved 4 barg, lamp oil and tap water analysed with FastHEX were found to be 34.4% and 27.1%, and the analyses with Coulter Counter obtained oil droplet removal efficiencies of 31.4% and 30% for Coil(in) between 0.10% and 0.20%. The results are shown in Fig. 9(b). Experiments with the saturator pressure set at 3 barg using lamp oil and tap water gave removal efficiencies obtained from FastHEX analysis of 28.0% and 21.9%. The oil droplet removal efficiencies measured by the Coulter Counter were found to be 28.6% and 21.8 for Coil(in) of approximately 0.11% and 0.21%. The results are shown in Fig. 10(a). Experiments with the saturator pressure set at 3 barg using vegetable oil and tap water gave removal efficiencies obtained from Coulter Counter analysis of 53.5% and 37.1% while the analyses with FastHEX were found to be 56.9% and 37.3% for Coil(in) of approximately 0.10% and 0.20%. The results are shown in Fig. 10(b). The last comparison involved the experiments with the saturator pressure set 4 barg using vegetable oil and tap water. The oil droplet removal efficiencies obtained from Coulter Counter analysis were found to be 62.2% and 45.9% while the analysis with FastHEX gave 64.7% and 54.3% for Coil(in) of approximately 0.10% and 0.20%. The results are shown in Fig. 11. A parity plot shown in Fig. 12, was used to compare the experimental data obtained from the FastHex and Coulter Counter methods. It shows that efficiencies obtained by FastHex were approximately a factor of 1.05 (refer Fig. 12a) higher than those obtained using the Coulter Counter. To obtain a better parity, all the results from the Coulter Counter experiments were multiplied by 1.05. As a result, the best-fit line of the parity plot became y = 1.00. The result is shown in Fig. 12(b).

Fig. 11 – The comparison of oil-in-water (FH) and droplet counting (CC) methods on the vegetable oil + tap water at 4 barg. Table 2 – The oil droplet removal efficiencies based on the mixture flow rate into the DAF tank at Coil(in) of approximately 0.1% and 0.2%. Flow rate (L/min)

10 15 20 25 30

Oil droplet removal efficiencies (%) Veg. oil 0.10%

Veg. oil 0.20%

Lamp oil 0.10%

76.6 71.1 64.8 37.5 15.3

51.4 51.8 45.3 22.6 14.1

37.2 36.1 31.7 27.2 10.9

Lamp oil 0.20% 32.8 30.3 28.9 25.4 11.7

It is concluded that (1) when adjusted by a factor of 1.05, the Coulter Counter method values of removal efficiencies agree with the FastHex method and (2) the Coulter Counter method is a reasonable method to explore the behaviour of DAF with respect to the effect on oil droplets of different sizes.

3.2.2.

The effect of flow rate into the DAF tank

The flow rates of the mixture into the DAF tank were varied between 10 L/min and 30 L/min in 5 L/min intervals. The flow rate to the static mixer was kept at 30 L/min to maintain the same oil droplet size distribution. The experiments involved 4 barg, lamp oil + tap water and 4 barg, vegetable oil + tap water, at Coil(in) of approximately 0.10% and 0.20%, respectively. Each of the runs was done twice, and the average CC was shown in Table 2. Then, they were plotted against the flow rate of the mixture in Fig. 13. Also, plotted as linear are the predictions from the linear correlation of multiple regressions. This correlation will be discussed later in Section 3.3.2. The vegetable oil shows a steeper reduction of removal efficiency than the lamp oil and

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Fig. 12 – Parity plot of the oil droplet removal efficiencies between oil-in-water and droplet counting methods. (a) shows the original best-fit line while (b) is the new best-fit line after multiplying CC by 1.05.

Fig. 13 – The effect of the mixture flow rates into the DAF tank on CC for vegetable oil and lamp oil. The symbols represent the experimental data and the lines are the trend from the linear correlation of multiple regressions. deviated from the lines between 25 and 30 L/min. The value of efficiency apparently becomes independent of type of oil between 25 and 30 L/min. A worse CC was obtained at higher flow rate for example at 30 L/min. Varying the flow rates to the DAF tank influences the mean residence time of the oil droplets in the DAF tank. A shorter mean residence time of oil droplets in the DAF, reduces the chances of collision and attachment of air bubbles over oil droplets, therefore reduces the CC .

3.2.3.

Droplet size distribution of lamp oil at Coil(in) 0.2%

Each of the experiments was conducted twice to explore reproducibility, which was acceptable and the average values are those reported elsewhere in the paper. The lamp oil droplet size distributions of the inlet and the outlet of the DAF tank are shown in Fig. 14. The Sauter mean diameter at inlet of experiment with (a) 4 barg and (b) No DAF were 30.1 ␮m and 31.1 ␮m respectively, which is to be expected, as there was no difference in the two experiments upstream of the DAF tank. The Sauter mean diameter was calculated using Eq. (6). d0,v and d0,a which are the volume equivalent diameter of the oil droplets and the surface area equivalent diameter of oil droplets, respectively. d32 =

d3o,v d2o,a

(6)

The average oil droplet removal efficiencies obtained from the Coulter Counter (CC ) of run (a) with 4 barg and run (b) No DAF were found to be 29% and 16% respectively. This supports

the conclusion of Section 3.1.2, which the experiments with DAF enhance the CC . Analyses of the same set of experiments by FastHEX method obtained similar values of Ecc for 4 barg and No DAF, which were found to be 27.1% and 16.1%, respectively. Fig. 15 shows the CC between the experiment of lamp oil at 4 barg and without DAF. These values were estimated using Eq. (5). Coalescence of lamp oil droplets occurred especially in the ranges 60–65 ␮m and 65–70 ␮m. This produced negative CC for the No DAF case. The CC differences for the oil droplets between 45–50 ␮m and 60–65 ␮m were above 20%. This is because of they have a similar diameter range to the air bubbles. Therefore, the use of DAF was shown to enhance the oil droplet removal efficiencies from an oil-in-water mixture mostly in the range 45 ␮m and above. Lower CC differences especially below 45 ␮m mean that the effect of DAF was not effective below at that particular oil droplet diameter.

3.2.4. Droplet size distribution of vegetable oil at 0.2% and 0.1% inlet oil concentration These experiments were conducted twice to confirm reproducibility and the average of Vf were reported. Fig. 16 shows the oil droplet size distribution for the vegetable oil conducted under air saturator pressure of 4 barg. The average CC with Coil(in) of approximately 0.20% and 0.10% were found to be 45% and 65% respectively. The similar set of experiments analysed with FH obtained 52% for Coil(in) 0.20% and 77% for Coil(in) 0.10%. No obvious coalescence was noticed in the vegetable oil experiments, as can be seen in Fig. 17. The CC differences obtained after 45–50 ␮m for vegetable oil are small. The experiments demonstrated that a lower inlet oil concentration produced better oil CC than at a higher inlet oil concentration. The reasons of this occurrence have been discussed in Section 3.1.1. Fig. 17 suggests that DAF is effective at lower droplet sizes for vegetable oil than with lamp oil.

3.3.

Correlation of experimental data

3.3.1.

Dimensional analysis

The apparatus used in the experiment is a scale model of an industrial DAF tank. Therefore, dimensional analysis offers efficient consolidation of the data prior to correlation and was undertaken using Buckingham’s Pi (␲) method. This involved the identification of the relevant experimental parameters, which are two dimensionless parameters which are, oil droplet removal efficiency (), flow rate ratio of oil and

chemical engineering research and design 1 1 5 ( 2 0 1 6 ) 19–33

27

Fig. 14 – Droplet size distribution of the lamp oil at (a) 4 barg (b) No DAF, +tap water.

Fig. 15 – (a) Average and (b) difference of CC for lamp oil at 4 barg and No DAF with Coil(in) at 0.2%.

Fig. 16 – Droplet size distributions of the veg. oil at 4 barg, Coil(in) at (a) 0.2% and (b) 0.1%. continuous phase into static mixer Coil(in) , and 10 dimensional parameters which are dynamic viscosity of continuous phase w , weight of salt Sw , spreading coefficient So , air saturator pressure P, Sauter mean diameter d32 , density of continuous phase c , density of oil o , gravity acceleration g, mixture volumetric flow rate Q, and length of the tank L, which by the requirement for geometrical similarity is related to all other length scales to do with the design of the tank. The repeating variables used in this analysis are c , Q, and L. Three fundamental dimensions are mass (M), length (L), and time (T).

The analysis concluded that the oil droplet removal efficiency,  in this experiment is a function of eight other dimensionless groups (refer Eq. (7)), which are; ratio of oil and continuous phase volume flow rates into the static mixer (Coil(in) ), weight fraction of salt in water continuous phase (Ws ), ratio of saturator and atmospheric pressure (Pr ), spreading coefficient ratio over surface tension of water–air (Sr ), ratio of oil and continuous phase density (r ), Reynolds number (Re) as described in Eq. (8), Froude number (Fr) as described in Eq. (9) and ratio of Sauter mean diameter over the length of the DAF tank d32 /L. L was chose as the denominator as the length

28

chemical engineering research and design 1 1 5 ( 2 0 1 6 ) 19–33

Fig. 17 – (a) Average and (b) difference of CC for vegetable oil Coil(in) at 0.2% and 0.1%. of the tank associates with the flow direction of the mixture (inlet to outlet) and residence time of the mixture in the DAF tank.  = f (Coil(in) , Ws , Pr , Sr , r , Re, Fr, d32 /L)

(7)

Re =

Q × L × w

(8)

Fr =

g × L5 Q2

(9)

3.3.2.

Multiple regression

Not all the experimental data were included in the multiple regression. Those not used were the experimental data of vegetable oil at 25 L/min and 30 L/min, and lamp oil at 30 L/min. This was because the removal efficiencies from these experiments deviated from the linear trend of the data (refer Fig. 13). A linear correlation for overall oil droplet removal efficiencies was obtained by multi variable regression, using Analysis ToolPak option in Microsoft Excel® . It was decided to correlate the data by linear regression because the graphs suggest no great amount of curvature and was the simplest way to fit the data, having the minimum number of parameters to be determined and thus conforms to Ockham’s razor. Linear regression is expressed in mathematical form as in Eq. (10).  = ␾ + ˛Coil(in) + ˇWs + Pr + Sr + r + ϕRe + ıFr + d32 /L(10) The value of each coefficient calculated from the multiple variable regression was found to be = 2.22 × 10−1 , ˛ = −1.03,  = 3.72 × 10−2 , = 8.79 × 10−1 , = 1.40 × 10−1 , ˇ = 5.74, ϕ = 9.18 × 10−5 , ı = 4.06 × 10−10 , and  = −3.46 × 10−3 . Positive coefficient means that the dimensionless group is directly proportional while negative means indirectly proportional to the oil droplet removal efficiency. From this regression, it shows that water salinity has a greater influence on the oil droplet removal efficiency. This was related to the alteration of surface charge of the oil droplets and the enhancement of spreading coefficient as discussed in Section 3.1.4. All the lines based on Eq. (10) that were plotted on the graphs of experimental data predict efficiency in the range 0–1. However, Eq. (10) predicts efficiency less than zero if an experiment were conducted outside the range of Coil(in) tested. For example in Fig. 18, at Coil(in) of approximately 0.35%, the oil droplet removal efficiency is predicted to be less than zero (−0.03), which is unrealistic. There is nothing in the

Fig. 18 – Linear correlation obtained from the multiple regression, which predict the oil droplet removal efficiencies less than zero for Coil(in) > 0.32%. mathematical form of Eq. (10) to prevent it from predicting efficiencies lying outside the range zero to one. In order to force the correlation to always predict efficiency within the range zero to one, a mathematical transformation of a shifted inverse hyperbolic tangent (IHT) was applied to the efficiency data and the resulting data set subject to linear regression. This regression involved the same number of undetermined coefficients as the previous correlation. The new coefficients were calculated and found to be = 2.30 × 101 , ˛ = −2.61, ˇ = −8.37,  = 9.78 × 10−2 , = 1.20 × 101 , = −2.91 × 101 , ϕ = −4.96 × 10−5 , ı = 3.05 × 10−9 , and  = −8.37 × 103 are used in Eq. (11). However, this correlation was not predicted oil droplet removal efficiency, instead in the term of tanh−1 2 [ − 1].

tanh−1 2 [ − 1] = ␾ + ˛Coil(in) + ˇWs + Pr + Sr + r + ϕRe + ıFr + 

d32 L

(11)

Fig. 19 shows the linear correlations based on the IHT values for the same experimental conditions used in Fig. 18. To obtain the hopefully better prediction of the removal efficiency, the transformation was inverted leading to Eq. (12). Fig. 20 shows the hyperbolic tangent predictions for the same experimental data as is used in Fig. 19. The predictions of  must lie in the range of 0–1 at any Coil(in) . The value of the new coefficients did not represent the relationship or the influence of the dimensionless group to the oil droplet removal efficiency anymore because it was inverted with tan h. Therefore,

chemical engineering research and design 1 1 5 ( 2 0 1 6 ) 19–33

29

of the mathematical form used. The correlation strictly only applies to DAF tanks geometrically similar to the one used in our experiments.

4.

Fig. 19 – Correlation using the inverse hyperbolic tangent.

Conclusions

Several operational parameters were investigated to determine their effect on the oil droplet removal efficiency from an oil-in-water mixture by using DAF. All the parameters tested were demonstrated that DAF could provide a significant intensification to the separation performance of oil droplets from the oil-in-water mixtures. The experiments have revealed that a better oil droplet removal efficiency (CC and FH ) can be obtained; a) At a low inlet oil concentration b) At an optimum air saturator pressure (4 barg), which produced bubbles of the similar size as the oil droplets. c) At a higher temperature to decrease the viscosity of the mixture. d) With oil in salty water mixture; salty water encourages the attachment of oil droplets to air bubbles. e) With vegetable oil that has a higher and positive spreading coefficient than lamp oil, in which greatly influenced the oil droplet removal efficiency.

Fig. 20 – Hyperbolic tangent transformation that predicts efficiency in a range of 0–100%. they can only be referred from the linear correlation. =

 1 1 + tanh ␾ + ˛Coil(in) + ˇWs + Pr 2

+ Sr + r + ␸Re + ıFr + 

d32 L



(12)

Parity plots (see Fig. 21) were attempted to explore the goodness of fit. The hyperbolic tranfsormation does not suffer by comparison with the linear regression. This is supported by the root mean square error, which was found to be 6.0% for the linear correlation and 5.5% for the inverse hyperbolic tangent transformation. The hyperbolic tangent transformation method, as stated Eq. (12), is preferred as it has a lower root mean square of error than linear regression and the oil droplet removal efficiencies are guaranteed to be in the range 0–1. These two benefits arguably justify the additional complexity

A dimensional analysis was conducted using Buckingham’s Pi (␲) method with the oil droplet removal efficiencies is a function of Coil , Ws , Pr , Sr , r , Re, Fr, and d32 /L. An inverse hyperbolic tangent correlation of the experimental data has been successful, which can predict removal efficiency in the range of zero to one with a root mean square error of 5.5%.

Notation

Abbreviations DAF Dissolved air flotation Veg. oil Vegetable oil UV–vis Ultraviolet visible spectroscopy Equipment Saturator pressure column C-01 FI-01 to FI-03 Flow indicator Filter F-01 P-01 to P-02 Flexible vane pump

Fig. 21 – Parity plot for (a) linear correlation and (b) hyperbolic transformation of the experimental and predicted E with lines of best fit added.

30

chemical engineering research and design 1 1 5 ( 2 0 1 6 ) 19–33

P-03 Air driven pump SMV-02 Static mixer Oil feed tank TK-01 Water feed tank TK-02 TK-03 Dissolved air flotation tank TK-04 Water sump tank Effluent tank TK-05 TK-06 Salty water tank V-01 to V-09 Valves Pressure regulator V-10 V-04 a/b/c Air saturated water inlet nozzles Symbol a ain aout Clamp.oil Coil Coil(in) Cveg.oil do,a do,v d32 do Fr g Hc ki L n P Pr Q Re So Sr Sw to u ut v Vf Ws

Dynamic viscosity of water (N s m−2 ) Density (kg m−3 ) Oil density (kg m−3 ) Continuous phase density (kg m−3 ) Oil droplet removal efficiency from oil-in-water measuring method Oil droplet removal efficiency by FastHEX method

w  o c CC FH

Acknowledgements

Absorbance value Absorbance value for inlet oil as measured by UV spectrometer Absorbance value for outlet oil as measured by UV spectrometer Concentration of lamp oil (volume fraction) Oil concentration (volume fraction) Ratio of inlet oil and continuous phase flow rates Concentration of vegetable oil (volume fraction) Surface area of oil droplet (m2 ) Volume of oil droplet (m3 ) Sauter mean diameter (defined in Eq. (6)) (m) Oil droplet diameter (m) Froude number of the mixture in the DAF tank (defined in Eq. (9)) Gravity acceleration (m s−2 ) Cuvette height (m) Constant value for oil–hexane calibration Length (m) Number of oil droplets obtained from Coulter Counter Saturator gauge pressure (Pa) Ratio of saturator and atmospheric pressure Volumetric flow rate (m3 s−1 ) Reynolds number of the mixture in the DAF tank (defined in Eq. (8)) Spreading coefficient (defined in Eq. (1)) (N m−1 ) Spreading coefficient ratio of oil and continuous phase Weight of salt to mix with 1 L of water (kg) Time taken for oil droplet to be on the surface of the cuvette (s) Horizontal velocity of the fluid (m s−1 ) Terminal rise velocity (defined in Eq. (A1)) (m s−1 ) Kinematic viscosity (m2 s−1 ) Volume fraction of oil droplet in the mixture Weight fraction of salt in water continuous phase

We would like to acknowledge Professor Spencer Taylor and Miss Hiu Tung Chu from the BP Centre for Petroleum and Surface Chemistry for the spreading coefficient measurements. M.R. Aliff Radzuan acknowledges his PhD scholarship from Majlis Amanah Rakyat (MARA).

Appendix A. The terminal rise velocity for both types of oil was calculated using Eq. (A1), which is valid for laminar flow. For instance, for a 60 ␮m of droplet diameter, the terminal rise velocities for vegetable oil and lamp oil were estimated to be 1.96 × 10−4 m s−1 and 3.72 × 10−4 m s−1 respectively. ut =

g (c − o ) d2o 18␮w

(A1)

Here, ut , g, c , o , do , ␮w are terminal rise velocity, gravity acceleration, density of continuous phase, density of oil, diameter of oil droplet and dynamic viscosity of water respectively. The time taken (to ) for the vegetable oil droplets to rise to the surface in the cuvette, was calculated by considering the height of the cuvette (Hc ), which is of 0.05 m, using Eq. (A2). to =

Hc t(oil)

(A2)

It took approximately two and a half minutes to carry out the analysis for all range of sizes. Based on Eqs. (A1) and (A2), a bigger oil droplet has faster terminal rise velocity and a shorter time to form a layer on the surface of the cuvette than a smaller oil droplet. 80 ␮m of lamp oil and vegetable oil took 76 s and 144 s to reach the surface respectively while 15 ␮m of lamp oil and vegetable oil took 2150 s and 4085 s. Therefore, two and a half minutes were sufficient to complete the analyses by measuring larger oil droplets first (80–15 ␮m on 5 ␮m intervals) before the droplets reach the surface of the cuvette.

Appendix B. Tables B1 and B2 show the results obtained from 12 sets of oil-in-water measuring method (FH ) and 16 sets of droplet counting method (CC ) analyses. The experiments conducted at different (1) flow rate into DAF tank, (2) air saturator pressure, (3) type of oil, (4) type of continuous phase, and (5) inlet oil concentration Coil(in) .

Greek letters  Surface tension (N m−1 ) Oil–air surface tension (N m−1 )  oa  ow Oil–water interfacial tension (N m−1 ) Water–air surface tension (N m−1 )  wa

Table B1 – The full set of results from oil-in-water measuring method.

FastHEX Set 1

Q to DAF tank (L/min)

P (barg)

Type of oil

Continuous phase

Coil (%)

 (%)

20

0

Lamp oil

Tap water

0.06 0.12 0.16

34.7 25 21.1

31

chemical engineering research and design 1 1 5 ( 2 0 1 6 ) 19–33

– Table B1 (Continued) Q to DAF tank (L/min)

P (barg)

Type of oil

Continuous phase

Coil (%)

 (%)

FastHEX Set 2

20

4

Lamp oil

Tap water

0.06 0.11 0.16

49.1 34.4 27.1

FastHEX Set 3

20

4

Lamp oil

Salty water

0.06 0.11 0.16

80.8 69.1 61.9

FastHEX Set 4

20

3

Lamp oil

Salty water

0.06 0.11 0.15

74.3 60.2 48.4

FastHex Set 5

20

0

Veg. oil

Salty water

0.04 0.12 0.22

63.1 55.5 44

FastHex Set 6

20

4

Veg. oil

Salty water

0.04 0.12 0.22

95.8 91.3 86.3

FastHEX Set 7

20

4

Veg. oil

Tap water

0.04 0.12 0.22

75 63.6 52.2

FastHEX Set 8

20

0

Lamp oil

Salty water

0.06 0.12 0.17

56 45 37

FastHEX Set 9

20

4

Veg. oil

Salty water

0.05 0.12 0.24

95.6 93.1 88.7

FastHEX Set 10

20

0

Lamp oil

Tap water

0.07 0.12 0.19

36.8 27.8 19.1

FastHEX Set 11

20

3

Lamp oil

Tap water

0.12 0.21

28 21.9

FastHEX Set 12

20

3

Veg. oil

Tap water

0.12 0.22

56.4 39

Table B2 – The full set of results from droplet counting method. Q to DAF tank (L/min)

P (barg)

Type of oil

Continuous phase

Coil (%)

 (%)

Coulter Counter Set 1

20

4

Lamp oil

Tap water

0.2 0.2 0.1 0.1

20.6 25.7 28.4 28.9

Coulter Counter Set 2

20

0

Lamp oil

Tap water

0.1 0.1 0.2 0.2

28.6 26.4 20.5 21.7

Coulter Counter Set 3

20

4

Veg. oil

Tap water

0.2 0.2 0.1 0.1

46.9 49.3 64.8 65.6

Coulter Counter Set 4

20

4

Lamp oil

Tap water

0.1 0.1 0.2 0.2

33.6 32.7 30.9 29.8

Coulter Counter Set 5

20

0

Lamp oil

Tap water

0.2 0.2 0.1 0.1

16.2 17.9 22.2 22.2

Coulter Counter Set 6

10

4

Veg. oil

Tap water

0.1 0.1 0.2 0.2

76.8 83.1 55.7 54

32

chemical engineering research and design 1 1 5 ( 2 0 1 6 ) 19–33

– Table B2 (Continued) Q to DAF tank (L/min)

P (barg)

Type of oil

Continuous phase

Coil (%)

 (%)

Coulter Counter Set 7

15

4

Veg. oil

Tap water

0.1 0.1 0.2 0.2

74.5 74.7 55.4 53.4

Coulter Counter Set 8

20

4

Veg. oil

Tap water

0.2 0.2 0.1 0.1

48.9 46.3 69.1 67

Coulter Counter Set 9

10

4

Lamp oil

Tap water

0.1 0.1 0.2 0.2

38.9 39.2 34.5 34.4

Coulter Counter Set 10

15

4

Lamp oil

Tap water

0.1 0.1 0.2 0.2

37.9 37.9 33.7 30

Coulter Counter Set 11

25

4

Lamp oil

Tap water

0.05 0.05 0.1 0.1 0.2 0.2

34.3 31.8 28.6 28.6 24.3 28.7

Coulter Counter Set 12

25

0

Lamp oil

Tap water

0.1 0.1 0.2 0.2

14.8 14.8 13 13.5

Coulter Counter Set 13

20

3

Lamp oil

Tap water

0.1 0.1 0.2 0.2

29.8 31.2 23.5 23.5

Coulter Counter Set 14

20

3

Veg. oil

Tap water

0.1 0.1 0.2 0.2

54.8 57.4 37.5 40.4

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