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

Abstract

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

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

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

the range 15-80 μm from oil-in-water mixtures. The DAF tank used in this investigation was a

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

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

and oil-in-water measuring methods were used to estimate theη. Dimensional analysis concluded

that the η in this experiment is a function of eight other dimensionless groups and the

experimental data has been subjected to multivariable linear regression. The resulting correlation

was found to have a root mean square error of 6.0%, but predict η outside the range zero and

one. An alternative mathematical formulation was devised that cannot predict η outside the

* Corresponding author: Tel.: +441483686943

E-mail address: a.mohamadradzi@surrey.ac.uk

1

range. Regression of the data by this formulation, which had the same number of adjustable

parameters as the linear regression, was successful with a lower root mean square error of 5.5%.

Keywords: Dissolved air floatation (DAF); droplet size distribution; oil droplet removal;

wastewater treatment; separation process; multivariable regression.

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 to 100 μm. DAF has been applied in many

industries such as in the preparation of raw water by water companies at wastewater treatment

plants (Edzwald and Haarhoff, 2012; Hague, 2003), to remove particles in the mining and

mineral processing industries (Al-Thyabat and Al-Zoubi, 2012; Rodrigues and Rubio, 2007), for

pre-treatment in the desalination process (Haarhoff and Edzwald, 2013), for cleaning up animal

waste in the agricultural industries (Creamer et al., 2010), for waste treatment in food-processing

plants (Yoo and Hsieh, 2010), and in crude 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

(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

generated in this DAF unit have a diameter of between 40 and 65 μm (Hague, 2003).

2

The flotation method is divided into three different flow schemes, which are, full-flow

pressure flotation, split-flow pressure flotation, and recycled-flow pressure flotation (Al-

Shamrani et al., 2002b; Edzwald and Haarhoff, 2012; Hanafy and Nabih, 2007; Zouboulis and

Avranas, 2000). The DAF unit used in these investigations does not follow any of 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 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 to be mixed with air under pressure (Hague, 2003). Fig. 1.1

shows this version of full flow pressure DAF. No pre-treatment is applied to the influent flowing

into the DAF tank.

Fig. 1.1 Full flow pressure DAF

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

3

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(S¿¿o)¿ (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=γ wa−γ ow−γ oa Eq. 1

Here,γwa,γow, and γoa 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

4

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 to 80 µm

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 ρo of 900 kg

m-3, kinematic viscosity ν 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 densityρoof 810 kg m-3, kinematic viscosity ν 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.

5

2.1.1 Experimental Apparatus and Procedures

The DAF unit built in the University of Surrey is illustrated in Fig. 2.2 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 μm. 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.

6

Fig. 2.2 Schematic diagram of DAF rig

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. 2.3.

Fig. 2.3 Dimensioned side view of a scale model DAF tank, TK-03

7

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 TK-05. 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.

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. 2.4 (a) and (b) show 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 both oils obtained linear lines of

best fit from the wavelength of 400 nm and is mathematically described in Eq. 2 for vegetable oil

8

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.

0 10 20 30 40 50 60 70 80 90 1000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

(a) Veg. oil - hexane calibration curve

300nm 400nm Linear (400nm) Linear (400nm) 500nm

Coil (%)

Absorb

ance

0 10 20 30 40 50 60 70 80 90 1000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5(b) Lamp oil - hexane calibration curve

300nm 400nm Linear (400nm) 500nm

Coil (%)

Absorb

ance

Fig. 2.4. (a) Vegetable oil- hexane and (b) lamp oil - hexane calibrations

C veg. oil=a−0.05980.00934 Eq. 2

C lamp.oil=a−0.04460.00813 Eq. 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

minutes 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

9

obtained was transformed to oil concentration Coilusing Eq. 2 or Eq. 3, depending on the type of

oil. Finally, the oil droplet removal efficiency from the FastHEX method (ηFH) was estimated

using Eq. 4. a¿ and aout are the absorbance reading obtained from the UV-Vis of the inlet and

outlet samples, respectively. The value of k i, 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).

ηFH=a¿−aout

a¿−k iEq. 4

2.1.3 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 μm in 5 μm 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 the Appendix A and the experiments

found to be satisfactory.

Finally, the oil droplet removal efficiency from the droplet counting method ηCC was then

calculated using Eq. 5.

10

ηCC=∑

¿ni V o ,i−∑

outniV o ,i

∑¿

ni V o,i

Eq. 5

ni and V o , i are the number of oil droplets obtained from the Coulter Counter and volume of oil

droplet, both at specific diameter, d i 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 tensiometer manufactured by KRŰSS. The surface

tension of water-air 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 3:1.

11

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 ηFH 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 ηFHwere 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 ηFH decreased with increasing inlet oil concentration

(See Fig. 3.5) 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).

12

0.00 0.05 0.10 0.15 0.20 0.250

20

40

60

80

100

No DAF, Lamp oil+ Tap water

No DAF, Veg oil + Salty water

No DAF, Lamp oil+ Salty water

3 barg, Lamp oil + Salty water

3 barg, Lamp oil + Tap water

3 barg, Veg oil + Tap water

4 barg, Lamp oil + Salty water

4 barg, Veg oil + Salty water

4 barg, Lamp oil + Tap water

Coil(in) (%)

𝜂FH (%

)

Fig. 3.5. Removal efficiency of oil droplets from oil-in-water mixture measured by

FastHEX method (First published by Aliff Radzuan et al., (2014)

The increase in ηFHwith 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 surface

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 ηFH to deteriorate. The oil droplets that were not

successfully attached flowed to the effluent tank.

13

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. 3.6 shows that applying DAF at 4 barg resulted in ηFH 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 lowerηFH, which was 74.3%, and ηFH became lower at 35.2% when DAF was

not used. The ηFH 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.

14

0 1 2 3 4 50

20

40

60

80

100

Lamp oil + Salty water Veg. oil + Tap water

P (barg)

𝜂FH (%

)

Fig. 3.6 Effect of the air saturator pressure on the removal efficiency

The bubbles produced at any pressure in the DAF tank resulted in better ηFH 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 than those at 3 barg, which in turn were much better than No DAF results.

Fig. 3.6 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. 3.5, which show a greater gradient at 3 barg than the

equivalent set at 4 barg.

15

3.1.3 The Effect of Temperature

The temperature of the continuous phase taken from the TK-02 and TK-06 was either at the

room temperature or 35 °C. Fig. 3.7 shows that the ηFHincreased slightly with increased

temperature for the different types of oil and saturator pressure. The highest ηFH (%)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 ηFHwere 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, ηFHwas 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 ηFH 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% 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%.

16

0.00 0.05 0.10 0.15 0.20 0.250

20

40

60

80

100

4 barg, Veg. oil + Salty water + 35 °C 4 barg, Veg. oil + Salty water No DAF, Lamp oil + Tap water + 35 °CNo DAF, Lamp oil + Tap water

Coil(in) (%)

𝜂FH (%

)

Fig. 3.7. Effect of temperature on the removal efficiency

Experiments conducted at the higher temperature obtained slightly better ηFHthan 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 Error: Reference source not found 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 mm2s-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.

17

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, ηFH 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. 3.8 show that all the experiments conducted with

salty water obtained a better ηFHcompared to the corresponding experiment that used tap water.

0.00 0.05 0.10 0.15 0.20 0.250

20

40

60

80

100

Lamp oil + Tap water Lamp oil + Salty water Veg. oil + Tap water Veg. oil + Salty water

Coil(in) (%)

𝜂FH (%

)

Fig. 3.8. Effect of salinity and type of oil on the removal efficiency by FastHEX method

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 Jr and Shell Development Co, 1980) and adding salt into the

18

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 μm. The

addition of salt increased ηFH because it increases the So as shown in Table 3:1.

3.1.5 The Effect of Type of Oil

Based on the terminal rise velocity, the ηFHin 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 ηFH than lamp oil (Refer to Fig. 3.8). The ηFH 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 Section 3.2.4) 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 3:1. The Soof lamp oil mixed with tap water were lower than Soof vegetable oil with tap

water. Higher So values were obtained when salty water was used. These values are supported by

19

the observation that the interaction between vegetable oil and tap/salty water caused the Du

Nouy ring to become more hydrophilic.

Table 3:1. The surface/interfacial tension and spreading coefficient for lamp oil and

vegetable oil with tap and salty water at 20 ±1 °C

Tap water Salty water

PropertyLamp oil Vegetable oil Lamp oil Vegetable oil

mN m-1 mN m-1 mN m-1 mN m-1

Water-air γwa 68.2 68.6 68.9 69.2

Water-oil γow 39.7 14.0 35.1 9.40

Oil-air γoa 26.3 34.3 26.2 33.3

Spreading Coefficient So 2.20 20.3 7.60 26.5

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. 3.9 (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

20

efficiencies of 31.4% and 30% for Coil(in) between 0.10% and 0.20%. The results are shown in

Fig. 3.9 (b).

0.00 0.05 0.10 0.15 0.20 0.250

20

40

60

80

100

(a) No DAF, Lamp oil + Tap water

FH CC

Coil(in) (%)

𝜂(%)

0.00 0.05 0.10 0.15 0.20 0.250

20

40

60

80

100

(b) 4 barg, Lamp oil + Tap water

FH CC

Coil(in) (%) 𝜂(%)

Fig. 3.9 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

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. 3.10 (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. 3.10 (b)

21

0.05 0.10 0.15 0.20 0.250

20

40

60

80

100

(a) 3 barg Lamp oil + Tap water

FH CC

Coil(in) (%)

𝜂(%)

0.05 0.10 0.15 0.20 0.250

20

40

60

80

100

(b) 3 barg Veg oil + Tap water

FH CC

Coil(in) (%)

𝜂(%)Fig. 3.10 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

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. 3.11.

0.00 0.05 0.10 0.15 0.20 0.250

20

40

60

80

100

4 barg, Veg. oil + Tap water

FH CC

Coil(in) (%)

E (%

)

22

Fig. 3.11 The comparison of oil-in-water (FH) and droplet counting (CC) methods on the

vegetable oil + tap water at 4 barg

A parity plot shown in Fig. 3.12, 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. 3.12 a) 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. 3.12 (b).

0 20 40 60 80 1000

20

40

60

80

100

f(x) = 1.04538104773258 x

(a)

𝜂CC (%)

𝜂FH (%)

20 40 60 800

20

40

60

80f(x) = 0.995600997840556 x

(b)

𝜂CC (%)

𝜂FH (%)

Fig. 3.12. Parity plot of the oil droplet removal efficiencies between oil-in-water and

droplet counting methods. Figure (a) shows the original best-fit line while (b) is the new

best-fit line after multiplying η CC by 1.05

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

23

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 ηCC was shown in Table 3:2.

Table 3:2 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)

Oil Droplet Removal Efficiencies (%)Veg. oil Veg. oil Lamp oil Lamp oil0.10% 0.20% 0.10% 0.20%

10 76.6 51.4 37.2 32.815 71.1 51.8 36.1 30.320 64.8 45.3 31.7 28.925 37.5 22.6 27.2 25.430 15.3 14.1 10.9 11.7

Then, they were plotted against the flow rate of the mixture in Fig. 3.13. 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 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.

24

5 10 15 20 25 30 350

20

40

60

80

100

Veg. oil 0.1% Veg. oil 0.2% Lamp oil 0.1 % Lamp oil 0.2%

Veg. oil 0.1% R Veg.oil 0.2% R Lamp oil 0.1% R Lamp oil 0.2% R

Mixture flow rate into DAF tank (L/min)

𝜂CC (%

)

Fig. 3.13. The effect of the mixture flow rates into the DAF tank on ηCCfor vegetable oil and

lamp oil. The symbols represent the experimental data and the lines are the trend from the

linear correlation of multiple regressions

A worse ηCC 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 ηCC.

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. 3.14. The Sauter

mean diameter at inlet of experiment with (a) 4 barg and (b) No DAF were 30.1 μm and 31.1 μm

25

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.

15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60 60-65 65-70 70-75 75-8002468

101214161820

(a) 4 barg

Inlet Outlet

do Range μm

Vf (%)

15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60 60-65 65-70 70-75 75-8002468

101214161820

(b) No DAF

Inlet Outlet

do Range

Vf (%)

Fig. 3.14. Droplet size distribution of the lamp oil at (a) 4 barg (b) No DAF, + tap water

d32=do, v

3

do, a2 Eq. 6

The average oil droplet removal efficiencies obtained from the Coulter Counter (η¿¿ 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 theηcc.

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.

26

Fig. 3.15 shows the ηcc 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 µm and 65-70 µm. This produced negative ηcc for the No DAF case. The ηcc

differences for the oil droplets between 45-50 μm and 60-65 μm 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 μm and above. Lower ηccdifferences especially below 45 μm mean that the effect of

DAF was not effective below at that particular oil droplet diameter.

15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60 60-65 65-70-40

-20

0

20

40

60

80

100

(a) Average CC𝜂

4 barg No DAF

do Range (μm)

𝜂CC (%)

15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60 60-65 65-700

20

40

60

80

100

(b) Difference CC𝜂

do Range (μm)

𝜂CC (%)

Fig. 3.15. (a) Average and (b) difference of ηccfor lamp oil at 4 barg and No DAF with Coil(in)

at 0.2%

27

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 V f

were reported. Fig. 3.16 shows the oil droplet size distribution for the vegetable oil conducted

under air saturator pressure of 4 barg. The average ηCCwith Coil(in) of approximately 0.20% and

0.10% were found to be 45% and 65% respectively. The similar set of experiments analysed with

ηFH obtained 52% for Coil(in) 0.20% and 77% for Coil(in) 0.10%.

15-2020-2525-3030-3535-4040-4545-5050-5555-6060-6565-7070-7575-80

02468

10121416182022

(a) Coil(in) = 0.2%

Inlet Outlet

do Range (μm)

Vf (%)

15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60 60-65 65-70 70-75 75-8002468

10121416182022

(b) Coil(in) = 0.1%

Inlet Outlet

do Range (μm)

Vf (%)

Fig. 3.16. Droplet size distributions of the veg. oil at 4 barg, Coil(in) at (a) 0.2% and (b) 0.1%

No obvious coalescence was noticed in the vegetable oil experiments, as can be seen in Fig.

3.17. The ηccdifferences obtained after 45-50 μm for vegetable oil are small. The experiments

demonstrated that a lower inlet oil concentration produced better oil ηCCthan at a higher inlet oil

concentration. The reasons of this occurrence have been discussed in Section 3.1.1. Fig. 3.17

suggests that DAF is effective at lower droplet sizes for vegetable oil than with lamp oil.

28

15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60 60-65 65-700

20

40

60

80

100

(a) Average CC𝜂

0.10% 0.20%

do Range (μm)

𝜂cc (%)

15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60 60-65 65-700

20

40

60

80

100

(b) Difference CC𝜂

do Range (μm)

𝜂CC (%)Fig. 3.17. (a) Average and (b) difference of ηccfor vegetable oil Coil(in) at 0.2% and 0.1%

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 (π) method. This involved the identification of the relevant

experimental parameters, which are two dimensionless parameters which are, oil droplet removal

efficiency (η), flow rate ratio of oil and continuous phase into static mixer Coil(¿), and 10

dimensional parameters which are, dynamic viscosity of continuous phaseμw, weight of salt Sw,

spreading coefficient So, air saturator pressure P, Sauter mean diameter d32, density of

continuous phase ρc, density of oil ρo, 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

29

other length scales to do with the design of the tank. The repeating variables used in this analysis

areρc,Q, and L. Three fundamental dimensions are mass (M), length (L), and time (T).

The analysis concluded that the oil droplet removal efficiency, η 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(C ¿¿oil(¿))¿, weight fraction of salt in water

continuous phase (W ¿¿ s) ,¿ ratio of saturator and atmospheric pressure( Pr ) , spreading

coefficient ratio over surface tension of water-air(S¿¿ r )¿, ratio of oil and continuous phase

density(ρr), Reynolds number¿) 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 tankd32 / L. L was chose as the

denominator as the length of the tank associates with the flow direction of the mixture (inlet to

outlet) and residence time of the mixture in the DAF tank.

η=f ¿) Eq. 7

ℜ= Q× ρL × μw

Eq. 8

Fr=g× L5

Q2 Eq. 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. 3.13).

30

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.

η=ϕ+α Coil(¿)+β W s+σ P r+ψ Sr +ξ ρr +φRe+δFr+Ω d32/ L Eq. 10

The value of each coefficient calculated from the multiple variable regression was found to be

ϕ= 2.22×10-1, α= -1.03, β= 5.74, σ= 3.72×10-2, ψ= 8.79×10-1, ξ= 1.40×10-1, φ= 9.18×10-5 δ=

4.06×10-10, and Ω= -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 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 to 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. 3.18, 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 mathematical form of Eq. 10 to prevent it from

predicting efficiencies lying outside the range zero to one.

31

0.00 0.10 0.20 0.30 0.40-0.2

0.0

0.2

0.4

0.6

0.8

1.0

No DAF, Lamp oil + Tap water

Linear correlation Experiment

Coil(in) (%)

𝜂CC

Fig. 3.18. Linear correlation obtained from the multiple regression, which predict the oil

droplet removal efficiencies less than zero for Coil(in) > 0.32%,

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 ϕ= 2.30×101, α= -2.61, β= -8.37, σ= 9.78×10-2, ψ= 1.20×101, ξ

= -2.91×101, φ= -4.96×10-5 δ= 3.05×10-9, and Ω= -8.37×103 are used in Eq. 11. However, this

correlation was not predicted oil droplet removal efficiency, instead in the term of tanh−12 [ η−1 ].

tanh−12 [ η−1 ]=ϕ+α Coil(¿)+β W s+σ Pr +ψ Sr+ξ ρr+φRe+δFr+Ωd32

LEq. 11

Fig. 3.19 shows the linear correlations based on the IHT values for the same experimental

conditions used in Fig. 3.18.

32

0.00 0.10 0.20 0.30 0.40-1.5

-1.0

-0.5

0.0

0.5

1.0

No DAF, Lamp oil+ Tap water

IHT Regression

Coil(in) (%)

tanh

-12(

-1)𝜂

Fig. 3.19. Correlation using the inverse hyperbolic tangent

To obtain the hopefully better prediction of the removal efficiency, the transformation was

inverted leading to Eq. 12. Fig. 3.20 shows the hyperbolic tangent predictions for the same

experimental data as is used in Fig. 3.19. The predictions of η must lie in the range of 0 to 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 tanh. Therefore, they can only be referred from the linear correlation.

η=12 [1+tanh(ϕ+α Coil (¿ )+β W s+σ Pr+ψ Sr+ξ ρr+φRe+δFr+Ω

d32

L ) ] Eq. 12

33

0.00 0.10 0.20 0.30 0.400.0

0.2

0.4

0.6

0.8

1.0No DAF, Lamp oil+ Tap water

Transformation Experiment

Coil(in) (%)

𝜂CC

Fig. 3.20. Hyperbolic tangent transformation that predicts efficiency in a range of 0-100%

Parity plots (See Fig. 3.21) 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 to 1. These two benefits

arguably justify the additional complexity of the mathematical form used. The correlation strictly

only applies to DAF tanks geometrically similar to the one used in our experiments.

34

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

f(x) = 1.02586248664676 x

(a) Linear

Epredicted

Eexper

imenta

l

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

f(x) = 0.999051187407721 x

(b) Transformation

Epredicted

Eexper

imenta

l

Fig. 3.21 Parity plot for (a) linear correlation and (b) hyperbolic transformation of the

experimental and predicted Ewith lines of best fit added

4 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 (ηCC andηFH) 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.

35

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.

A dimensional analysis was conducted using Buckingham’s Pi (π) method with the oil droplet

removal efficiencies is a function ofCoil,W s, Pr, Sr,ρr, ℜ, F r, 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%.

5 Notation

Abbreviations DescriptionDAF Dissolved air flotation Veg. oil Vegetable oilUV-Vis Ultraviolet Visible spectroscopyEquipment DescriptionC-01 Saturator pressure columnFI-01 to FI-03 Flow indicatorF-01 FilterP-01 to P-02 Flexible vane pumpP-03 Air driven pumpSMV-02 Static mixerTK-01 Oil feed tankTK-02 Water feed tankTK-03 Dissolved air flotation tankTK-04 Water sump tankTK-05 Effluent tankTK-06 Salty water tankV-01 to V-09 ValvesV-10 Pressure regulatorV-04 a/b/c Air saturated water inlet nozzles

36

Symbol Description Unit

a Absorbance value -ain Absorbance value for inlet oil as measured by UV Spectrometer -aout Absorbance value for outlet oil as measured by UV Spectrometer -Clamp.oil Concentration of lamp oil (volume fraction) -Coil Oil concentration (volume fraction) -Coil(in) Ratio of inlet oil and continuous phase flow rates -Cveg.oil Concentration of vegetable oil (volume fraction) -do,a Surface area of oil droplet m2

do,v Volume of oil droplet m3

d32 Sauter mean diameter (Defined in Eq. 6) mdo Oil droplet diameter mFr Froude number of the mixture in the DAF tank (Defined in Eq. 9) -g Gravity acceleration m s-2

Hc Cuvette height mki Constant value for oil-hexane calibration -L Length mn Number of oil droplets obtained from Coulter Counter -P Saturator gauge pressure PascalPr Ratio of saturator and atmospheric pressure -Q Volumetric flow rate m3 s-1

Re Reynolds number of the mixture in the DAF tank (Defined in Eq. 8) -So Spreading coefficient (Defined in Eq. 1) N m-1

Sr Spreading coefficient ratio of oil and continuous phase -Sw Weight of salt to mix with 1 L of water kgto Time taken for oil droplet to be on the surface of the cuvette su Horizontal velocity of the fluid m s-1

utTerminal rise velocity (Defined in Error: Reference source not found) m s-1

v Kinematic viscosity m2 s-1

Vf Volume fraction of oil droplet in the mixture -Ws Weight fraction of salt in water continuous phase -Greek Letters Description Unitɤ Surface tension N m-1

ɤoa Oil-air surface tension N m-1

ɤow Oil-water interfacial tension N m-1

ɤwa Water-air surface tension N m-1

μw Dynamic viscosity of water N s m-2

ρ Density kg m-3

ρo Oil density kg m-3

ρc Continuous phase density kg m-3

37

ηcc Oil droplet removal efficiency from oil-in-water measuring method -ηFH Oil droplet removal efficiency by FastHEX method -

Acknowledgements

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).

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7 Appendix

7.1 Appendix A

The terminal rise velocity for both types of oil was calculated using Eq. A 1, which is valid

for laminar flow. For instance, for a 60 µm of droplet diameter, the terminal rise velocities for

vegetable oil and lamp oil were estimated to be 1.96x10-4 m s-1 and 3.72x10-4 m s-1 respectively.

41

ut=g (ρc− ρo)do

2

18 µwEq. A 1

Here, ut, g, ρc,ρo, do, µw 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 (t ¿¿o)¿ for the vegetable oil droplets to rise to the surface in the cuvette, was

calculated by considering the height of the cuvette(H ¿¿c )¿, which is of 0.05 m, using Eq. A 2.

t o=H c

ut (oil)Eq. A 2

It took approximately two and a half minutes to carry out the analysis for all range of sizes.

Based on the Error: Reference source not found and Error: Reference source not found, 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 μm of lamp oil and vegetable oil took 76 s and 144 s to

reach the surface respectively while 15 μm 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 to 15 μm on 5 μm intervals) before the droplets reach the surface of the

cuvette.

7.2 Appendix B

Table Appendix B 1 and Table Appendix B 2 show the results obtained from 12 sets of oil-

in-water measuring method (ηFH) and 16 sets of droplet counting method (ηCC) analyses. The

42

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).

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Table Appendix B 1 The full set of results from oil-in-water measuring method

  Q to DAF tank (L/min P (barg) Type of oil Continuous

Phase Coil (%) η (%)

FastHEX Set 1 20 0 Lamp oil Tap water0.06 34.70.12 25.00.16 21.1

FastHEX Set 2 20 4 Lamp oil Tap water0.06 49.10.11 34.40.16 27.1

FastHEX Set 3 20 4 Lamp oil Salty water0.06 80.80.11 69.10.16 61.9

FastHEX Set 4 20 3 Lamp oil Salty water0.06 74.30.11 60.20.15 48.4

FastHex Set 5 20 0 Veg. oil Salty water0.04 63.10.12 55.50.22 44.0

FastHex Set 6 20 4 Veg. oil Salty water0.04 95.80.12 91.30.22 86.3

FastHEX Set 7 20 4 Veg. oil Tap water0.04 75.00.12 63.60.22 52.2

FastHEX Set 8 20 0 Lamp oil Salty water0.06 56.00.12 45.00.17 37.0

FastHEX Set 9 20 4 Veg. oil Salty water0.05 95.60.12 93.10.24 88.7

FastHEX Set 10 20 0 Lamp oil Tap water0.07 36.80.12 27.80.19 19.1

FastHEX Set 11 20 3 Lamp oil Tap water0.12 28.00.21 21.9

FastHEX Set 12 20 3 Veg. oil Tap water 0.12 56.4

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0.22 39.0

Table Appendix B 2 The full set of results from droplet counting method

  Q to DAF tank (L/min

P (barg) Type of oil Continuou

s Phase Coil (%) η (%)

Coulter Counter Set 1 20 4 Lamp oil Tap water

0.20 20.60.20 25.70.10 28.40.10 28.9

Coulter Counter Set 2 20 0 Lamp oil Tap water

0.10 28.60.10 26.40.20 20.50.20 21.7

Coulter Counter Set 3 20 4 Veg. oil Tap water

0.20 46.90.20 49.30.10 64.80.10 65.6

Coulter Counter Set 4 20 4 Lamp oil Tap water

0.10 33.60.10 32.70.20 30.90.20 29.8

Coulter Counter Set 5 20 0 Lamp oil Tap water

0.20 16.20.20 17.90.10 22.20.10 22.2

Coulter Counter Set 6 10 4 Veg. oil Tap water

0.10 76.80.10 83.10.20 55.70.20 54.0

Coulter Counter Set 7 15 4 Veg. oil Tap water

0.10 74.50.10 74.70.20 55.40.20 53.4

Coulter Counter Set 8 20 4 Veg. oil Tap water

0.20 48.90.20 46.30.10 69.10.10 67.0

Coulter Counter 10 4 Lamp oil Tap water 0.10 38.9

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Set 90.10 39.20.20 34.50.20 34.4

Coulter Counter Set 10 15 4 Lamp oil Tap water

0.10 37.90.10 37.90.20 33.70.20 30.0

Coulter Counter Set 11 25 4 Lamp oil Tap water

0.05 34.30.05 31.80.10 28.60.10 28.60.20 24.30.20 28.7

Coulter Counter Set 12 25 0 Lamp oil Tap water

0.10 14.80.10 14.80.20 13.00.20 13.5

Coulter Counter Set 13 20 3 Lamp oil Tap water

0.10 29.80.10 31.20.20 23.50.20 23.5

Coulter Counter Set 14 20 3 Veg. oil Tap water

0.10 54.80.10 57.40.20 37.50.20 40.4

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