mastouri2010

Petroleum Science and Technology, 28:1415–1426, 2010

Copyright © Taylor & Francis Group, LLC

ISSN: 1091-6466 print/1532-2459 online

DOI: 10.1080/10916461003681695

The Effect of Temperature and Impeller Speed on

Mechanically Induced Gas Flotation (IGF)

Performance in Separation of Oil from

Oilfield-Produced Water

R. MASTOURI,1 S. M. BORGHEI,2 F. NADIM,3 AND

E. ROAYAEI4

1Department of Environment and Energy, Science and Research Branch,

Islamic Azad University, Tehran, Iran2Department of Chemical and Petroleum Engineering, Sharif University of

Technology, Tehran, Iran3University of Connecticut, Department of Civil and Environmental

Engineering, Storrs, Connecticut4Vice President in Project Studies, EOR Research Institude, National Iranian

Oil Company, Tehran, Iran

Abstract The effect of temperature and impeller speed on the performance of in-duced gas flotation (IGF) systems for the removal of oil from produced water in

different ranges (5–300 g/L) of total dissolved solids (TDS) was investigated in apilot plant study. Furthermore, it was evaluated whether the IGF pilot plant effluent

could reach the 15 mg/L outlet oil content as required by Article VI of the KuwaitConvention for Persian Gulf region, before being discharged to the sea. The results

showed that oil removal efficiencies up to 90% could be reached at high temperature(80ıC) in just one single flotation cell without adding any chemicals. Flotation unit,

however, should be followed by at least one more flotation cell in series in order toguarantee the Kuwait Convention marine pollution discharge standard for the effluent

oil content.

Keywords impeller speed, induced gas flotation (IGF), produced water, temperature,TDS

1. Introduction

High-temperature effluents are generated in some special industrial fields such as the

dyeing industry, soda ash production plants using Solvay techniques, water desalination

plants using thermal processes, crude oil desalting plants, geothermal plants, and mineral

processing in treating ores and metal extraction. Although the application of several

energy-saving methods such as the pinch method has preserved and used the temperature

of effluent streams, but the discharge of high-temperature effluents can still be observed,

especially in oil-producing countries in the Middle East where the cost of heating energy

is low. One of the streams that was the subject of attention for the authors was an

Address correspondence to Reza Mastouri, Department of Environment and Energy, Scienceand Research Branch, Islamic Azad University, Unit 4, No. 15, Ahuramazda St., Alvand St., Tehran,Iran. E-mail: [email protected]

1415

1416 R. Mastouri et al.

Table 1

Characteristics of Kharg Island–produced water

at the outlet of the crude oil factory’s desalter

Specifications of produced water Value

Temperature, ıC 90–110

pH 7.02

Suspended solids, mg/L 100–450

TDS @ 180ıC, g/L 250–310

Oil content, mg/L 100–3000

oilfield-produced water discharge that was a source of pollution both for the Kharg

Island soil and the receptive waters in Persian Gulf south of Iran. Produced waters can

contain a wide variety of environmentally regulated components, such as salts, metals,

naturally occurring radioactive materials (NORMs), and oil (Thoma et al., 1999; Bader,

2007; Cakmakci et al., 2008). The temperature of the mentioned effluent stream reached

110ıC, in which no biological treatment could be effective. In some cases, the total

dissolved solids (TDSs) of the stream reached up to 310 mg/L. Therefore, systematic

pilot plant tests were performed to investigate the efficiency of induced gas flotation

(IGF) systems on the removal of oil from oily water/produced water streams at different

TDSs, temperatures, and impeller rotational speeds (N) and to investigate whether the

flotation cell could reach the 15 mg/L outlet oil content before being discharged to the sea

according to Article VI (Pollution from Land-Based Sources) of the Kuwait Convention,

which states that all littoral states should take appropriate measures to prevent discharges

of pollution into Persian Gulf waters. Unfortunately, the Persian Gulf States may not be

well equipped to effectively control the discharge of domestic and industrial wastewater

into the Gulf waters (Nadim et al., 2008). The general characteristics of produced water

and its accompanying oil are presented in Tables 1 and 2, respectively.

1.1. Gas Flotation Units in Oily Wastewater Treatment Process

Gas flotation units work by introducing small gas bubbles into the wastewater being

treated. The gas bubbles acquire a small electronic charge, opposite that of the oil droplets.

As the gas bubbles rise through the oily wastewater, oil adsorbs to the bubbles (Bradley,

1990; Arnold and Stewart, 2008). The adsorption mechanism consists of several steps:

� Approach of oil drops and gas bubbles

Table 2

Characteristics of Kharg Island crude oil

Specifications of crude oil Value

SGr @ 60/60ıF (ASTM D4052) 0.85

API gravity (ASTM D4052) 34

Kinematic viscosity @ 20ıC (cSt.) (ASTM D-445) 8.01

Separation of Oil from Oilfield-Produced Water 1417

� Drainage and rupture of the interstitial film

� Adsorption of gas bubbles to the oil drops

� Spreading of oil on the bubble surface

� Rise of the conglomerate to the surface so that it will be skimmed off (Moosai and

Dawe, 2003)

At oil production sites, there are two main technologies for gas flotation: dissolved

gas flotation (DGF) and induced gas flotation (IGF). The difference in the two technolo-

gies is in how the gas bubbles are generated. For DGF, gas bubbles are generated by

reducing the pressure on the wastewater to the point where the natural gases dissolved in

the stream will break out. In IGF, gas bubbles are mechanically generated or hydraulically

induced in the oily wastewater. The size of the gas bubbles in DGF range from 30 to

100 �m. In IGF the bubbles will be larger and usually range from 500 to 2,000 �m

(Bradley, 1990; Casaday, 1993; Berne and Cordonnier, 1995; Zlokarnik, 1998; Jameson,

1999; Rubio et al., 2002; Welz et al., 2007).

1.2. Induced Gas Flotation

An IGF unit is an accelerated gravitational separation technique in which gas bubbles

are induced into a water-phase stream either by the use of an eductor device or by

a vortex set up by a mechanical rotor (impeller). The liquid phase (oily wastewater)

usually contains immiscible liquid droplets (oil) or oily solid particles so that the gas

bubbles attach themselves to the droplets. The oil appears lighter because the density

difference between the oil agglomerate and water is increased; consequently, the oil rises

faster, enabling a more rapid and effective separation from the aqueous phase. The oil

droplets and oil-coated solids rise to the surface where they are trapped in the resulting

foam/scum and removed from the flotation chamber when the foam/scum is skimmed

off (Strickland, 1980; Kitchener, 1985; Arnold and Stewart, 1998; Moosai and Dawe,

2002; Movafaghian et al., 2004). The internals of flotation cells such as nozzles, rotors,

baffles, and foam/scum skimmers for these units are patented designs and still are not

well understood (Chabot et al., 1998; Arnold and Stewart, 2008).

According to the high temperature of the Kharg Island oil refinery, a series of

tests were made to investigate the performance of IGF systems with different TDSs,

temperatures, and impeller rotational speeds by applying a laboratory pilot plant IGF

unit. In this article, the results of the performance of an innovative IGF pilot plant for

the treatment of synthetic produced water resembling Kharg Island oil refinery/terminal-

produced water, which contains an average oil content of 150 mg/L and TDS of 5–

300 g/L, at different temperatures (20ıC–100ıC) and varying impeller speeds (N D 450,

900, 1,450, 2,000 rpm) is presented.

1.3. Previous Studies

Although a great deal of research has been carried out on the application of flotation cells

to oil recovery, little works has been done systematically on the effect of temperature on

flotation performance and the studies only focus on a limited temperature ranges.

Strickland (1980) studied the flotation of oily water in batch-dispersed gas flotation

cells in ambient temperatures of 49ıC and 60ıC. It was concluded that oil recovery

increased with temperature at least up to 60ıC, but no justification for this phenomenon

was presented.


Sylvester and Byeseda (1980) studied the separation of oil in induced air flotation

(IAF) and concluded that variation in the feed temperature in the range of 17ıC to 32ıC

did not have a significant effect on the removal efficiency. Shannon and Buisson (1980)

explored the impact of high temperatures on dissolved air flotation (DAF). The efficiency

of air dissolution, bubble size distributions, pumping costs, and solids removal were found

to be similar at temperatures ranging from 10ıC to 80ıC. The capability of DAF did not

deteriorate until severe bubble coalescence occurred at 80ıC and 350 kPa.

Bubble behavior is one of the most important parameters in the performance of

flotation cells and many researchers have focused on its role, especially in recent years

in which advanced instrumentations and analyzers for bubble investigation have been

introduced (Gorain et al., 1997; Wang et al., 2001; Ghosh, 2004; Laakkonen et al., 2005;

Schwarz and Alexander, 2006). However, although a significant amount of work has

already been done on bubble coalescence, its mechanism is still not completely understood

even for pure liquids (Craig, 2004; Grau and Heiskanen, 2005). Studies conducted on the

effect of liquid temperature on IGF performance are limited and inconclusive (Claudio

et al., 2006). Moreover, when the purpose is IGF efficiency investigation, the bubble

size and gas holdup cannot be studied individually, because the oil–bubble and oil–oil

attachment are also of importance (Strickland, 1980). Because of the above-mentioned

ambiguities, the accurate performance of flotation cells is still unresolved (Sanada et al.,

2005).

2. Experimental

2.1. Material

2.1.1. IGF Pilot Plant. Figure 1a shows a schematic diagram of the IGF pilot plant.

A Plexiglas cylindrical column, 280 mm in height and 260 mm in diameter, was used

as the flotation cell. Top of the cell could be fully dismantled from the body of the cell,

allowing complete cleanup of the cells after each test. The flotation cell was equipped

with a variable-speed motor (Bosch drill, GSB 13,600 W, coupled with a dimmer). The

motor speed varied from 450 to 2,000 rpm. The stirrer/rotor speed was measured by a

handheld digital laser tachometer (DT 2236B). A stainless steel flat-blade rotor (blade

diameter/tank diameter D 1/3) was installed at a certain height for optimum agitation. A

new stator was used to disperse the stream and gas bubbles. The novel combination of

this rotor and stator in the flotation cell, called the Mastour flotation cell, had very good

bubble generation and dispersion in the flotation cell. Gorain et al. (1995) applied four

different types of impellers, Pipsa, Chile-X, Dorr-Oliver, and OutoKumpo, and showed

that in spite of small differences in trends, the general performance of impellers in the

flotation process was independent of impeller type. Therefore, it is believed that the

performance of Mastour flotation cell in different conditions is not limited to its type.

Hydraulic retention time (HRT) for the IGF pilot was set at 1 min as suggested by

Movafaghian et al. (2004) and Arnold and Stewart (2008) and it was controlled and

calibrated by the emulsion preparation tank outlet valve.

2.1.2. Crude Oil. Crude oil from Dorood I oilfield, Kharg Island, was used for oil-in-

water emulsion preparation. The properties of the crude oil are specified in Table 2.

2.1.3. Produced Water. Tap water and table salt were used to prepare the saline water

resembling the TDS of the produced water.


Figure 1. IGF pilot setup: (a) IGF flotation cell; (b) feed gas; and (c) O-W emulsion preparation

tank.

2.1.4. Feed Gas. Pure nitrogen gas (N2) from an N2 capsule equipped with a regulator,

flow indicator (FI), and pressure indicator (PI) was considered for supplying nitrogen as

feed gas with a rate of 10 L/min. A pressure indicator on the top of the cell was installed

to control the positive pressure in the vapor space in the cell. The schematic of feed gas

is shown in Figure 1b.

2.2. Procedure

2.2.1. Oil–Water Emulsion Preparation. A schematic diagram for equipment setup used

for preparation of oil–water (O-W) emulsion is shown in Figure 1c. A polyethylene (PE)

mixing tank with the capacity of 85 L was used to store the O-W emulsion. It was scaled

in different levels/capacities for further volumetric determinations. A 2,000-W electric

heater was used to heat the water and a temperature indicator (TI) instrument ranging

from 0ıC to 120ıC was applied to monitor the temperature. One variable-speed mixer

equipped with a glass stirrer was used to emulsify oil and salt in water. When the water

in the preparation tank reached the intended temperature, the mixer started to work at

2,000 rpm so that the oil droplets were completely dispersed in water (Bing et al., 2007).

A specified amount of the previously weighed salt was added to the heated water. After

about 5 min, crude oil was gradually added to the tank and was allowed to mix for

30 min so that the oil droplets were completely dispersed in water.

The mixer speed was then reduced to 400 rpm. Although not measured, but according

to similar procedure, the mean diameter of oil droplets in this method would be 20 �m


and the maximum diameter of oil droplets would hardly exceed 100 �m (Bing et al.,

2007). The outlet valve was opened and the O-W emulsion was transferred to the flotation

cell with constant flow rate of 14 L/min. To reach a constant oil-in-water content of 150

˙ 5 mg/L in all of the tests, several samples were taken at the inlet of flotation cell

for calibration. In order to determine the performance of the IGF system in oil removal,

samples were taken at the inlet and outlet of the flotation unit in each running process.

Sampling points are shown in Figure 1a. The oil and grease measurements were made

according to the methods presented in the Standard Methods for the Examination of

Water and Wastewater (1992).

3. Results and Discussion

The results of all the outlet oil contents vs. temperature are presented in Figures 2a to

2g. Each figure illustrates four impeller speeds (N D 450, 900, 1,450, 2,000 rpm), inlet

oil content of 150 mg/L and constant TDS, and the performance of IGF due to outlet oil

content vs. the effect of temperature from 20ıC to 100ıC.

3.1. Effect of Temperature

A general review of Figures 2a to 2g shows that according to the test setups, the IGF

performance improves with increasing temperature up to a specific temperature and then

the oil removal efficiency decreases. For TDS of 5 g/L (Figure 2a), the best performances

are obtained at 50ıC, which are equivalent to 70, 66, 65, and 69 mg/L for N D 450, 900,

1,450, and 2,000 rpm, respectively, except for N D 1,450 rpm, in which the minimum

outlet oil content is achieved at two thermal points: T D 50ıC and T D 60ıC. At the TDS

of 50 g/L (Figure 2b), the optimum efficiencies are achieved at 60ıC, which are equivalent

to 44, 42, 39, and 42 mg/L for N D 450, 900, 1,450, and 2,000 rpm, respectively.

Regarding the TDS of 100 g/L (Figure 2c), it is concluded that the best performances

occur at the temperature of 60ıC in which the minimum outlet oil content reaches 40,

37, 35, and 35 mg/L during the impeller speeds of 450, 900, 1,450, and 2,000 rpm,

respectively. There is an exception in IGF performance at 70ıC and N D 450 rpm in

which the efficiency equals the conditions of T D 60ıC.

During the TDS of 150 g/L (Figure 2d), the best performances are obtained at 70ıC,

which are equivalent to 34, 30, 27, and 25 mg/L for N D 450, 900, 1,450, and 2,000 rpm,

respectively. From Figure 2e, it can be seen that at the TDS of 200 g/L, the best results

are performed at 70ıC, in which the outlet oil content reached 36, 35, 22, and 25 mg/L

and the impeller speeds are equivalent to 450, 900, 1,450, and 2,000 rpm, respectively.

There is an exception in IGF performance at temperature of 80ıC at N D 900 rpm in

which the efficiency equals the conditions of T D 70ıC.

From Figure 2f, it is evident that at TDS of 250 g/L, the best performances are

obtained at 80ıC, which are equivalent to 33, 30, 20, and 22 mg/L for N D 450, 900,

1,450, and 2,000 rpm, respectively. Regarding the TDS of 300 g/L (Figure 2g), it is

concluded that the best performances take place at the temperature of 80ıC in which the

minimum outlet oil content reaches 19, 15, 28, and 32 mg/L during the impeller speeds

of 450, 900, 1,450, and 2,000 rpm, respectively.

An overall view of Figures 2a to 2g indicates that during a constant TDS, at all four

impeller speeds (N D 450 to 2,000 rpm), the outlet oil content was reduced with increase

in temperature up to 50ıC–80ıC and then increased with the temperature increase up to

100ıC.


(a)

(b)

Figure 2. Effect of temperature and impeller speed variations on the IGF oil removal efficiency

with constant 150 mg/L inlet oil content at different TDSs. (continued)

It should be noted that temperature not only affects the liquid density and viscosity

but also impacts the oil–bubble coalescence. However, physical phenomena related to

flotation cells, such as motion, coalescence, breakup, etc., are generally very complex

and have not yet been clarified (Sanada et al., 2005). From the environmental point

of view, the global effect of temperature in IGF systems could be interpreted by oil

removal performance, which seems to diminish at temperatures higher than 50ıC–80ıC

according to the effluent TDS. The elevated TDS content raises the optimum maximum

temperatures for better oil flotation to higher degrees. The results agree with the findings

of Strickland (1980) and Arnold and Stewart (2008), who observed the positive effect

of temperature increase on the oil removal efficiency in flotation cells up to 60ıC, and


(c)

(d)

Figure 2. (Continued).

according to the authors’ review, this is the first time that the behavior of flotation cells

in elevated temperatures (higher than 60ıC) is investigated.

It was visually observed that at temperatures higher than 60ıC, water vapor appeared

in the gas space above the produced water and during the induction of the impeller, water

vapor and moisture were drawn into the liquid instead of nitrogen gas. This phenomenon

may also affect the gas bubble size, bubble formation, and oil–bubble coalescence.

3.2. Effect of Impeller Speed (N)

With the conditions of the tests described in the previous section and at constant TDS,

the trend of IGF performance is generally the same at different impeller speeds but many

exceptions make it impossible to suggest a direct relationship between impeller speed and

oil removal efficiency at different conditions for the executed tests. The minimum outlet


(e)

(f)


oil content (15 mg/L) was related to the impeller speed (N) of 900 rpm at 80ıC. During

the tests, the increase in the impeller speed (N) did not cause a significant change in

IGF oil removal performance and in many cases the effect was negative. From the visual

observations of the color of brownish milky liquid resulting from oil–bubble attachment

at different conditions, it could be concluded that an increase in the impeller speed

increases the gas induction to the liquid, as was explained by Girgin et al. (2006), but

not necessarily better performance in IGF oil removal, as was experienced by Strickland

(1980) and the present authors. An increase in impeller speed led to a decrease in both

bubble size and oil droplet size, which does not necessarily cause better attachment and

flotation. The smaller oil bubbles also may need more time to reach the surface of the

column and this could result in the presence of oil–bubble droplets in effluent stream and

a consequent increase in oil content measurements.


(g)


According to the conditions of these tests, in global, after reaching a minimum

impeller speed necessary for gas induction, the effect of temperature seems to be more

important than the impeller speed for oil removal in IGF systems.

3.3. Environmental Regulations and IGF Global Oil

Removal Efficiency

From an environmental point of view, it is important to check whether the selected

technology meets the regional environmental regulations or not. Because the tests in this

study were conducted under conditions resembling the Kharg Island–produced water, the

results of the IGF pilot should be compared with the Kuwait Convention, limiting the

oil content concentration in the discharge stream up to 15 mg/L. According to the inlet

oil content of 150 mg/L, the maximum IGF oil removal efficiency was at temperature of

80ıC at the TDS of 300 g/L and impeller speed of 900 rpm, which leads to 15 mg/L outlet

oil content and 90% oil removal efficiency. At temperature of 100ıC, the minimum oil

removal efficiency took place at N D 450 and 900 rpm where the outlet oil concentration

was measured as 84 mg/L, which was equivalent to 44% and the maximum oil removal

efficiency took place at N D 900 rpm with the oil removal efficiency equivalent to 80%

and outlet oil concentration of 42 mg/L. Therefore, if the goal is to use an IGF system for

the discussed specific produced water at high temperatures (50ıC or higher), without the

use of any chemicals, a single flotation cell is insufficient for all cases and at least a dual

IGF system should be applied to decrease the 150 mg/L inlet oil content to permissible

level of less than 15 mg/L.

4. Conclusions

� At very high TDS produced waters, and with an impeller speed of 450 to 2,000 rpm,

the oil removal efficiency increases with the increase in temperature when it is raised

from 20ıC to 50ıC–80ıC based on the effluent TDS, regardless of the impeller speed.

Oil removal efficiency is then diminished up to 100ıC. Many factors such as liquid

density, viscosity, surface tension, bubble coalescence, etc., may cause these behaviors.


The induction of water vapor and water moisture from the free space on top of the

liquid instead of blanket gas was also considered in this research.

� After reaching a minimum impeller speed necessary for gas induction, the effect of

higher impeller speed showed to be less effective in comparison with temperature in

the conditions of this research. In global, the trend of IGF performance is generally

the same at different impeller speeds.

� In order to meet the 15 mg/L regulation of the Kuwait Convention for onshore oily

wastewater discharges to Persian Gulf areas, for the Kharg Island–produced water

stream with the TDS of at least 250 g/L, an IGF system with two flotation cells in

series would be sufficient without the addition of any chemicals provided that the

temperature does not fall below 70ıC.

It is recommended that further studies investigate the effect of salinity and tempera-

ture on interfacial tension of oil–bubble and oil–water and their effect on IGF efficiency.

The systematic oil–bubble size determination in different cases is also suggested.

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mastouri2010

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