Dissolved Air Flotation for Treating Oilfield Produced ...

3rd International Conference on Civil, Environment and Waste Management (CEWM-16) Sept. 12-14, 2016 Dubai (UAE)

Dissolved Air Flotation for Treating Oilfield Produced Water - Preliminary Results Maryam Al-Shukaili, Asila Al-Aisri, Maryam Al-Mamari, and Ahmed Al-Dallal 

100 mm and less) rise as rigid spheres. These small bubble sizes apply to DAF and so spherical shapes apply and are used in DAF models. Larger bubbles of about 1–10 mm such as occurs in dispersed air flotation have the shape of ellipsoids. Even larger bubbles (>10 mm) take the shape of spherical caps. Bubble size is an important property in DAF. It affects the performance of collisions and attachment of particles to bubbles and bubble rise velocity [5]. The rising velocity of a micro-bubble is less than that of larger bubbles. This ensures a longer residence time in the flotation column to allow more opportunity for collisions between bubbles and oil droplets. The released bubbles will carry solid or oil particles toward surface. To enhance the flotation, different types of polymers additives are used to attachment or entrapment of air bubbles to solid particles.

Abstract— Batch Dissolved air flotation (DAF) unit was applied for treatment of synthetic produced water of 3000 mg/L (O/W emulsion) prepared from crude oil using commercial cationic polyacrylamide polymers namely CPAM 30% CPAM 40% and C309. The efficiencies of various DAF treatments were evaluated by measuring the % turbidity removal using a turbidity meter. The % turbidity removal of C309 and CPAM 40% were relatively higher than CPAM 30%. The flotation velocity range measured for all types of polymers used in this study is 0.3-0.6 cm/s which is within the normal flotation velocity. An increase in % turbidity removal was observed when we increased the recycle ratio from 0.714 to 0.83 while increasing the recycle ratio to 1 will not affect the separation. A maximum separation was observed at a recycle pressure of 40 psig within the applied pressure range of 30-50 psig.

In this study, the effect of operating parameters such as; type of polymers, recycle pressure and recycle ratio on the oil separation efficiency characterized by % turbidity removal has been carried out using batch dissolved air flotation test apparatus.

Keywords—Dissolved air flotation, produced water, O/W emulsion, polymer, DAF, polyacrylamide, turbidity

I. INTRODUCTION II. EXPERIMENTAL PROCEDURE

Produced water is one of the major by-products of oil and gas exploitation that is produced in large amounts up to 80% of the waste stream [1]. Depending on geological conditions and field position, produced water may have complex composition including: organic or inorganic ingredients like: salts, metals, dispersed oils, phenols, organic acids, dissolved hydrocarbons like: Benzene, Toluene, Ethylbenzene and Xylene (BTEX). Produced water contains hydrocarbons in the form of dispersed oil droplets, which, under proper conditions, can be coalesced into a continuous hydrocarbon liquid phase and then separated from the aqueous phase using various separation devices [2]. One oilfield in the Nimr area of Oman produces 12 million gallons per day (MGD) of oilfield wastewater [3]. The produced water is treated initially using dissolved air flotation so that the oil content is down to about several hundred milligrams per liter. The shallow water disposal requirement in Oman specifies oil content allowable in disposal water is up to a maximum of 5.0 mg/L. Dissolved air flotation is a process which separate solids or oil from liquid by dissolving air under pressure into liquid. Microbubbles were then released from liquid when the pressure is reduced to atmosphere. In order to produce micro bubbles in the flotation tank, a saturator pressure of 400– 600 kPa are recommended [4]. In flotation, the shape of bubbles is set by their rise velocity and hence the bubble size (diameter). Small bubbles (sizes of several

A. Materials and Equipment The chemicals used in the experiments are crude oil, a commercial detergent, food grade salt and commercial polymers (cationic polyacrylamide). Table 1 shows the specifications of polymers used. Fig. 1 shows the DAF batch unit that had being constructed in the workshop. All the experiments were conducted in this unit. The flowsheet of the unit is also shown in Fig. 2. The analytical equipment used for measuring the separation efficiency (as % turbidity removal) is the turbidity meter supplied by LaMotte Company (0.00 - 4000 NTU). B. Synthetic Wastewater Preparation About 1.5 liters of synthetic produced water solution was prepared by weighing 9 grams of salt in a laboratory mass balance. Then 3 mL of crude oil were added followed by 0.45 mL of synthetic detergent. Tab water of 1.5 liters was then added to the container. The whole contents was then agitated with high shear homogenizer for no less than 5 minutes to insure that all the oil was dispersed into water. The resulted produced water contains approximately 3000 mg/L of crude oil. C. General Procedure The sealed cover of the pressure cell was first removed and the pressure cell was filled with a certain volume of tap water according to the recycle ratio in order to have a final total

Manuscript received July. 6, 2016. Ahmed Al-Dallal is currently an assiatant professor at Faculty of Engineering, Sohar University, Sohar, Oman. E-mail: [email protected]. Maryam Al-Shukaili, Asila Al-Aisri and Maryam Al-Mamari were graduated with BSc degree from Faculty of Engineering/Sohar University.

99


volume of about 1 liter and the cover was then sealed. The air compressor was turned on and the pinch valve was used to control the pressure of the cell at a certain value (30-50 psig) for

a detention time of 10 minutes. This time is more than enough to achieve saturation [6]. A 0.1 wt% polymer solution was prepared by dissolving 1 gm of polymer powder in a 1 litter volumetric flask. A wastewater sample was added to the graduated cylinder as well as the required amount of polymer before 30 seconds the detention time has elapsed. The polymer solution is then mixed with wastewater used for experiment. The whole content of the pressurized cell was then discharged into the graduated cylinder after the detention time has elapsed using the bottom valve. The air compressor was then turned off and the pinch valve was opened to release the pressure in the cell. The height of the oil-water interface was recorded with time. A pipet was used to take a sample from the clarified wastewater phase for turbidity measurement.

TABLE 1: THE SPECIFICATIONS OF THE POLYMERS USED

III. RESULTS AND DISCUSSION A. The Effect of Polymer Type The effect of polymer concentration on the % turbidity removal for three different commercial type polymers are shown in Fig. 3. CPAM 40% addition will affect remarkable clarification even with lower concentration (1 mg/L) while CPAM 30% affect clarification at a minimum polymer concentration of 3 mg/L. The other type of polymers, C309 affect remarkably clarification process at a minimum concentration of 8 mg/L. These observations had been inspected visually by preliminary tests. In general the separation efficiency as % turbidity removal of C309 and CPAM 40% are relatively higher than CPAM 30%. The general trend for the effect of concentration of polymer was examined by Gehr and Henry [7] who studied the effect of polymer dosage on effluent SS for thickening of wasted activated sludge derived from mainly domestic sewage using DAF. Below a dosage of about. 4 mg/g, there was no improvement in the solids concentration, whereas above 4 mg/g there was. Kuo and Goh [8] shows also the pronounced effect of chemical dosage (including polymer additions ) on effluent

Fig. 1:DAF unit contains: (1) pressure gauge (4 bar); (2) pressure cell; (3) valve; (4) pipe; (5) on/off valve; (6) air flow meter; (7) air diffuser; (8) pinch valve; (9) graduated cylinder (1000 ml); (10) contacting junction; (11) air compressor .

Fig. 3. Effect of polymer dosage on the % turbidity removal of synthetic produced water for three commercial polymers at recycle pressure of 40 psig and recycle ratio of 1. Fig. 2: Flowsheet of the DAF Unit. 100


concentrations using DAF. Increase in chemical dosage leads to lower concentrations of suspended solids and oil/grease in the effluent. Zouboulis & Avranas [9] found that the maximum removal of emulsified oil by flotation was around 50% and it was achieved by the addition of 2.5 mg /L polyacrylamide at pH around 4 for a concentration range of polyacrylamide 0.5 -5 mg/L. In our case, there is no remarkable change in the % Turbidity removal for each of the three cationic polyacrylamide polymers used in this study within the ranges of polymer concentration used. The effect of polymer concentration on the flotation velocity is shown in Fig. 4. CPAM 30% and CPAM 40% didn’t show any remarkable change of flotation velocity with increasing polymer concentration, while for C309 the flotation velocity was increased as the concentration of polymer increased. From these data it appears that not always the flotation velocity is related to % turbidity removal. Nardi et al. [10] reached a similar trend, where in some cases the flotation velocity has no effect on the remaining turbidity fraction, and in other cases the remaining turbidity fraction is decreased with increasing the chemicals (including polymers) dosage. That the change is related to the type of chemicals (including polymers) used. In general the flotation velocity in this study is 0.3 – 0.6 cm/s. Normal flotation velocity of 0.5 cm/s was also found by Wang et al. [11]. B. The Effect of Recycle Ratio The effect of recycle ratio on % turbidity removal for CPAM 30% is shown in Fig. 5. The recycle ratio can be defined as the flow ratio of clarified water recycled into the dissolved air tank to the flow of oily wastewater to be treated. Clarified water is recycled into the dissolved air tank, which has some advantages such as saving clean water, full use of reagents left in the recycled water. The clarification efficiency decreases when the recycle ratio is too small. In contrast the total volume of wastewater through the treatment clarifier will increase if the

should be decreased by all means as long as the quality of the treated wastewater can be ensured. In general, the air to solids ratio (A/S) at constant applied recycle pressure is related to the recycle ratio. Nardi et al. [10] study the effect of recycle ratio (10-50%) on the remaining turbidity fraction. A minimum turbidity fraction was reached at a recycle ratio of 40%. Dassey and Theegala [12] study the effect of recycle ratio (0-40%) for the treatment of pretreatment on poultry processing wastewater. A 40% recycle flow was required to carry over 97% of the solids to the surface. At 0% recycle ratio an equivalent amount of solids was found settled at the bottom.ie increasing with increasing of recycle ratio. Li et al [13] found an increase in the efficiency of flotation with the increase of the recycle ratio (5 - 40%) with different applied air pressure. To determine the optimum recycle ratio, Al-Shamrani et.al [14] applied a constant saturator pressure of 80 psi and recycle ratios in range of 2–20%. The residual turbidity was measured. Optimum conditions for separation are obtained with an air to oil ratio of 0.0075 corresponding to a recycle ratio of 10%. Zhenga et al. [15] observed an increase in the turbidity removal efficiency when the reflux ratio is increased from 15-35%. Cristóvão et al. [16] also study the effect of recycle ratio of 0.67, 1 and 1.5 where a maximum removal efficiency of TSS and oil and grease was at a recycle ratio of 0.67. In this study the effect of recycle ratio in the range 0.77-1 on the % Turbidity removal was studied for CPAM 30% polymer as shown in Fig. 5. From this figure it appears that there this some enhancement in % turbidity removal when we increased the recycle ratio from 0.714 to 0.83 while increasing the recycle ratio to 1 will not enhance the separation efficiency. C. The Effect of Recycle Pressure At a constant recycle ratio the air to solids ratio (A/S) is related to the applied recycle pressure. The effect of recycle pressure in the range 30-50 psig on % turbidity removal was studied at different recycle ratio (0.714-1) as shown in Fig. 5. There is a sign of optimum separation at a recycle pressure of 40 psig. This trend is accordance with work of Li et al. [13] who found a maximum oil separation at 43.5 psig for a pressure range 14.5 – 72.5 psig. However this can be attributed to that, the pressure has a limit that should not be exceeded or there will be negative effects on the separation process. This appears because the dissolved air in the water can neither dissipate its energy nor reach the equilibrium bubble size when the pressure is too high, which causes turbulent flow that perturbs the fluid in the column and destroys the floc [13]. Destroying the floc decreases the efficiency of flotation. Couto et al. [17] studied the effect of pressure (44.1 - 88.2 psig) for treatment of milk

Fig. 4- Effect of polymer concentration on floating velocity of synthetic produced water for three commercial polymers at recycle pressure of 40 psig and recycle ratio of 1.

recycle ratio is too high, which means that the recycle ratio

101

3rd International Conference on Civil, Environment and Waste Management (CEWM-16) Sept. 12-14, 2016 Dubai (UAE) [6]

[7] [8]

[9]

[10]

[11]

Fig. 5: The effect of recycle ratio on % turbidity removal of synthetic produced water for CPAM 30% polymer

[12]

industry effluent by dissolved air flotation. Maximum separation efficiency was reached at 58.5 psig. His results are related to the fact that pressures over 58.5 psig result in smaller bubble diameters, thus leading to greater separation efficiency. Also Zhenga et al [15] study the effect of pressure within the range of 43.5 to 130.5 psig for DAF process and they found an optimum oil removal efficiency at about 75.4 psia. In contrast, Al-Shamrani et.al [14] found an increase in clarification as the pressure is increased from 50 psig to 80 psig. Vasseghian et al. [18] also found an increase in clarification (in term of COD) when the pressure is increase from 29.4 psig to 73.5 psig.

[13]

[14]

[15]

[16]

IV. CONCLUSIONS

[17]

In this work, DAF was studied for the treatment of synthetic produced water using different commercial cationic polyacrylamide polymers additives namely, CPAM 30% CPAM 40% and C309. The following conclusions could be drawn: -The separation efficiency as % turbidity removal of C309 and CPAM 40% are relatively higher than CPAM 30%. -The flotation velocity range for all types of polymers used in this study is 0.3 - 0.6 cm/s which is within the normal flotation velocity. -An increase in % turbidity removal was observed when we increased the recycle ratio from 0.714 to 0.83 while increasing the recycle ratio to 1 will not enhance the separation. -An optimum separation was observed at a recycle pressure of 40 psig within the applied pressure range 30-50 psig.

[18]

REFERENCES [1]

[2] [3] [4]

[5]

P. McCormack, P. Jones, M. J. Hetheridge and S. J. Rowland, “Analysis of oilfield produced waters and production chemicals by electrospray ionisation multi-stage mass spectrometry,” Water Res., vol. 35, no. 15, pp. 3567-3578, Oct. 2001. M. Stewart and K. Arnold, “Emulsions and oil treating equipment selection, sizing and troubleshooting”, Gulf Professional Publishing, 2009. Y. He, Z.W. Jiang, “Technology review: Treating oilfield wastewater,” Filtr. Sep., vol. 45, no. 5, pp. 14–16, Jun. 2008. J. K. Edzwald, J. P. Walsh, G. S. Kaminski and H. J. Dunn. “Flocculation and air requirements for dissolved air flotation,” Journal of the American Water Works Association, vol. 84, no. 3, pp. 92-100, Mar. 1992. J. K. Edzwald, "Dissolved air flotation and me,." Water Res., vol. 44, no. 7, pp. 2077-2106, April 2010.

102

A. S. Moursy and S. E. Abo El-Ela, “Treatment of oily refinery wastes using a dissolved air flotation process,” Environ. Int., vol. 7, no. 4, pp. 267–270, 1982. R. Gehr and J. G. Henry, “Polymer dosage control in dissolved air flotation,” Environ. Eng., vol. 109, no. 2, pp. 448-465, April 1983. E. C‐H Kuo and M. K‐H Goh, “Sewage clarification by dissolved air flotation and chemically assisted sedimentation,” Environ. Technol., vol. 13, no. 12, pp. 1141-1151, 1992. A. I. Zouboulis, A. Avranas, “Treatment of oil-in-water emulsions by coagulation and dissolved-air flotation,” Colloids Surf. A, vol. 172, no. 1-3, pp. 153-161, Oct. 2000. I. R. D. Nardi, , T. P. Fuzi, and V. D. Nery, “Performance evaluation and operating strategies of dissolved-air flotation system treating poultry slaughterhouse wastewater,” Resour. Conserv. Recycl., vol. 52, no. 3, pp. 533-544, Jan. 2008. L. K. Wang, E. Fahey and Z. Wu, “Dissolved air flotation,” in Physicochemical Treatment Processes, L. K. Wang, N. C. Pereira and Y. T. Hung, eds., The Humana Press, Inc., Totowa, 2005. A. J. Dassey and C. S. Theegala, “Evaluating coagulation pretreatment on poultry processing wastewater for dissolved air flotation,” J. Environ. Sci. Health A Tox. Hazard Subst. Environ. Eng., vol. 47, no. 13, pp. 2069-2076, Aug. 2012. X. B. Li, J. T. Liu, Y. T. Wang, C. Y. Wang, , and X. H. Zhou, “Separation of oil from wastewater by column flotation”, J. China Univ. of Mining & Tech., vol. 17, no. 4, pp. 546-577, Dec. 2007. A. A. Al-Shamrani, A. James and H. Xiao, “Separation of oil from water by dissolved air flotation”, Colloids Surf. A, vol. 209, no. 1, pp. 15-26, Sep. 2002. T. Zheng, Q. Wang, Z. Shi, P. Huang, J. Li, J. Zhang and J. Wang, “Separation of pollutants from oil-containing restaurant wastewater by novel microbubble air flotation and traditional dissolved air flotation,” Sep. Sci. Technol., vol. 50, no. 16, pp. 2568-2577, July 2015. R .O. Cristóvão , C .M. Botelhoa, R. J. E. Martins, J .M. Loureiroa and R. A. R. Boaventura, “Primary treatment optimization of a fish fanning wastewater from a portuguese plant,” Water Resour. Ind., vol.6, pp. 51- 63, Aug. 2014. H. J. B. Couto, M. V. Melo and G. Massarani, “Treatment of milk industry effluent by dissolved air flotation,” Braz. J. Chem. Eng., vol. 21, no. 1, pp. 83–91, Jan. – Mar. 2004. Y. Vasseghian, M. Ahmadi, R. Gholami and S. Aghaali, “COD removal prediction of DAF unit refinery wastewater by using neuro- fuzzy systems (ANFIS),” J. Chem. Pet. Eng.-Univ. Tehran, vol. 47, no. 1, pp. 61- 70, June 2013.