Removal of Coke Particles from Oil Contaminated Marun

ARTICLE pubs.acs.org/IECR

Removal of Coke Particles from Oil Contaminated Marun Petrochemical Wastewater Using PVDF Microfiltration Membrane Sayed Siavash Madaeni,*,† Vahid Vatanpour,† Hossein Ahmadi Monfared,† Ahmad Arabi Shamsabadi,‡ Kaveh Majdian,‡ and Saeed Laki‡ † ‡

Membrane Research Center, Department of Chemical Engineering, Razi University, Kermanshah 67149, Iran Marun Petrochemical Co., Mahshahr, Iran ABSTRACT: In this paper, an experimental study of a polymeric PVDF microfiltration membrane is presented for complete removal of coke particles from the industrial wastewater of Marun Petrochemical Company. The operation parameters such as applied pressure, cross-flow velocity, and feed temperature were investigated. The effect of coke concentration in filtration of the synthetic feed showed that coke particles did not transport from the membrane in all concentrations and sequence filtrations. In addition, 100% rejection of coke particles was kept constant by changing the operation parameters or using different cleaning agents due to suitable selection of pore size and polymeric materials of the applied membrane. At higher applied pressures, the higher flux is obtained due to the higher driving force toward the membrane. Feed flux was enhanced by increasing the cross-flow velocity explained by raising mass transfer coefficient in the concentration boundary layer and producing more shear stress on the membrane surface. Three different types of cleaning agents (HCl, NaOH, and NaOCl) were used for feed flux recovery of the fouled membranes. Flux recovery results and surface SEM images of the cleaned membranes indicated that most of the foulants were successfully removed using NaClO as chemical cleaner. The sequence of cleaning agent effect on flux recovery was NaOCl > NaOH > HCl. Hypochlorite solution due to having oxidizing properties in addition to alkali properties resulted in a higher cleaning efficiency compared to NaOH.

1. INTRODUCTION Oily wastewaters are one of the major pollutants of the environment that have been generated from the emission of a variety of industrial oily wastewaters from different sources such as refineries, petrochemical plants, and transportation. This water has to been purified so that it can be reused to save water resources and to protect the environment.1 Several methods such as gravity separation and skimming, dissolved air flotation, de-emulsification, coagulation, and flocculation have been applied to oily wastewater treatment, which have some disadvantages such as low efficiency, high operation costs, corrosion, and recontamination problems.2 All of these conventional methods have physical and chemical principles, and they cannot guarantee separation efficiency and effluent quality. These processes are costly because of the high consumption of chemicals and sometimes the nonreacted chemicals are found in the final wastewaters.3 Membrane based separation processes are becoming a promising technology. This technology has several advantages including high selectivity, easy separation, mild operation, small area requirement, continuous and automatic operation, economic and fast operation, and low running investment.4
6 Previous studies showed that microfiltration (MF) processes are compatible with environmental standards.7
12 However, membrane fouling that typically forms by inorganic and organic materials existence in the oily wastewaters is the major problem in using membranes for oily wastewater treatment.13,14 Cross-flow microfiltration is a solid
liquid separation process, which covers a wide range of colloids and particles sizes (0.1
10 μm). This process can remove colloids, microparticles, r 2011 American Chemical Society

micro-organisms, and macromolecules in the suspension effectively based on pressure driven.15,16 The effects of cross-flow velocity, transmembrane pressure, feed temperature, pore size of the membrane, and concentration of the suspended solids on the membrane efficiency were reported.17 This economic and efficient filtration method is widely used in the food industry, the beverage industry, biotechnology (e.g., separation of bacteria from substrate), water, and wastewater treatment.17 Poly(vinylidene fluoride) (PVDF) is a semicrystalline membrane material that is widely used to prepare microfiltration (MF), ultrafiltration (UF), pervaporation, and membrane distillation membranes. Also oil
water separation is another field that PVDF is widely used in.16,17 The popularity of this membrane comes from its excellent chemical resistance (tolerates various acids and alkalies), thermal stability, mechanical strength and flexibility, oxidation resistance, and exceptional hydrolytic stability. The thermal stability of PVDF is due to its crystalline phase, and its flexibility is a result of the amorphous phase of this polymer.18
20 In this work, coke particles with a concentration of about 0.01 wt % were separated using a PVDF microfiltration membrane from Marun Petrochemical wastewater containing about 1 wt % gasoline and other oily compounds. This wastewater resulted from the washing of cracking catalysts of the polyolefin system. The presence of coke particles in wastewater causes the Received: June 28, 2011 Accepted: September 12, 2011 Revised: September 2, 2011 Published: September 12, 2011 11712

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Table 1. Characteristics of the Marun Wastewater VOC COD (mg/L)

BOD (mg/L)

in 550 °C (mg/L)

Turbidity (NTU)

TSS (mg/L)

TDS (mg/L)

Coke content (wt.%)

Coke size (μm)

2210

225

17

251

27

265

0.1

2
20

Figure 1. Schematic diagram of the experimental setup.

destruction of guard and coalescer filters in the separation of oily compounds from water. The cross-flow microfiltration process due to continuous flow system and economical aspect is suitable for this separation. The effect of different processing conditions such as applied pressure, temperature, cross-flow velocity, and sequential filtration on the membrane efficiency was investigated. In addition, the influence of different washing agents on membrane regeneration after fouling was examined.

Table 2. Different Conditions for the Conducted Experiments Experiments Effect of pressure

Effect of velocity

2. EXPERIMENTAL SECTION 2.1. Materials. The polymeric microfiltration membrane applied in this paper was GVHP hydrophilic polyvinylidene fluoride (PVDF) with a thickness of 125 μm and nominal pore size of 0.22 μm obtained from Millipore Corporation. Hydrochloric acid (HCl, 32%), sodium hydroxide (NaOH), and sodium hypochlorite (HClO, 10
12%) were purchased from Merck (Germany). All chemical agents were analytical grade and prepared using distilled water. 2.2. Process Feed. The wastewater was obtained from the Marun Petrochemical Co., which resulted from the washing process of cracking catalysts of the polyolefin system and used as a feed in the cross-flow microfiltration process. Analysis of the feed is presented in Table 1. Most of the analyses were carried out according to the American Standard Test Methods (ASTM). Chemical oxygen demand (COD), biological oxygen demand (BOD), total dissolved solids (TDS), total suspended solids (TSS), volatile organic carbon (VOC), and turbidity analysis were performed on the feed solution. To show the effect of coke concentration on membrane performance, the synthetic feed was used by preparing a mixture of different concentrations of coke particles (size in range of 1
100 μm) and distilled water. The concentration of coke in the synthetic feed was 5
20 times more than that for the Marun feed.

Effect of temperature

Pressure (bar)

Cross velocity (m/s)

Temperature (°C)

0.5

0.28

20

1

0.28

20

1.5

0.28

20

1.5

0

20

1.5

0.096

20

1.5 1.5

0.28 0.28

20 20

1.5

0.28

35

1.5

0.28

50

2.3. Experimental Setup. The laboratory scale setup was used in cross-flow mode for microfiltration of contaminated wastewaters. Figure 1 presents a schematic view of the experimental setup. The feed was pumped from the feed tank toward the cell. The feed tank had a heater to heat the feed or to keep it at a constant temperature. In addition, a tubular heat exchanger was used to control the feed temperature. The inlet pressure and cross-flow velocity were controlled by the bypass and outlet valves. With respect to membrane operation, transmembrane pressure and permeability were continuously measured. The module was rectangular with a cross-sectional area of 0.99 cm2. The surface area of the membrane in contact with the feed was equal to 25 cm2. The membrane was placed on the top of a thin layer of porous rigid material as an external support to prevent deformation of the membrane. The system was designed in such a way that all important operating parameters in the MF process such as volumetric flow rate, temperature, and operating pressure could be easily controlled. 2.4. Operating Conditions. First, distillated water was circulated in the test loop at different pressures to obtain pure water 11713

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Figure 2. Influence of coke concentration on membrane performance. Figure 4. Effect of pressure on permeate flux of Marun feed (average of five replicates was depicted).

Figure 3. Effect of sequence of synthetic feed microfiltration on membrane flux.

flux. During this time of stabilization of the membrane, stable pure water flux was relatively observed. The Marun feed was used as received and passed through the membrane at transmembrane pressures of 0.5, 1, and 1.5 bar at three different cross-flow velocities of 0, 0.096, and 0.28 m/s. The permeate flux was measured during different feed temperatures of 20, 35, and 50 °C. A new membrane was employed for each trial. The different conditions for the conducted experiment are listed in Table 2. After each set of experiments with the feed, the system was rinsed with water and then pure water flux was measured again under the conditions of initial testing until the steady state was attained. The membrane fouling tendency was measured from the difference between the steady state pure water fluxes. 2.5. SEM Images of Cleaned Membranes. In the selection of the best cleaning agent, scanning electron microscopy (SEM) images of the membrane surfaces after washing with different agents were obtained by Philips-XL30 and Cambridge scanning electron microscopes (SEM). The membrane samples were sliced into small pieces and cleaned with filter paper. Next, the samples were glued and gold sputtered to produce electric conductivity. The SEM images were taken under very high vacuum conditions at 17 kV. 2.6. Membrane Cleaning. Chemical cleaning was performed using 0.01 M HCl, NaOH, or NaClO separately at room temperature after the MF system was operated for 2 h. Before and after fouling, the pure water flux of the membrane was calculated by the measurement of the permeate weight. The water permeation flux was measured after 60 min. Prior to the cleaning, a membrane fouled by the Marun feed at a temperature

of 20 ( 1 °C, pressure of 1.5 bar, and cross-flow velocity of 2.8 m/s was fed into the system and feed permeate was measured. This flux was named the initial feed flux (Jf,i). The fouling operation was performed for all experiments after 120 min to generate similar fouling conditions. First membranes were rinsed with clean and hot water for 10 min to remove unbounded substances from the membrane surface and next were cleaned with different cleaning agents at a temperature of 20 ( 1 °C within 10 min. The membrane was washed again with water to remove any chemical agent within 5 min, and then the flux of the feed was determined; this flux is named the feed flux after chemical cleaning (Jf,c). Flux recovery (FR) has been used to demonstrate the cleaning efficiency:21 ! Jf , c FRð%Þ ¼  100 ð1Þ Jf , i

3. RESULTS AND DISCUSSIONS 3.1. Effect of Coke Concentration of Synthetic Feed on Membrane Performance. The main purpose of this study is the

separation of coke particles from oil contaminated wastewater. In this order, the synthetic feed with a high coke concentration (5
20 times more than Marun feed) was filtered to show the ability of the suggested microfiltration system and applied PVDF membrane to remove these coke particles. The feed temperature and applied pressure were 25 ( 1 °C and 0.5 bar, respectively. Figure 2 represents the influence of coke concentration on the flux of the membrane. At the beginning time, the membrane flux rapidly decreased due to precipitation of coke particles and formation of a cake layer on the membrane surface, which block the pores of the membrane. After about 40 min, the flux approximately reached a steady state amount. By increasing the coke amount, the flux is reduced, however, not by a great margin. The main importance of these experiments was that, for all concentrations, no coke particles passed from the membrane. Although the color of the synthetic feed in 0.2 wt % coke was darkly black, the turbidity and color of the permeate were the same as those for pure water, indicating the absence of coke particles in the permeate. These results show that the pore size of the selected PVDF membrane is suitable for this separation. Microfiltration of the synthetic feed was repeated four times with the used membranes, which were washed off-line with 11714

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Figure 6. Comparison of effect of cross-flow velocity and filtration sequence on treatment of Marun feed (120 min filtration time and cleaned with hypochlorite between each microfiltration). Figure 5. Effect of cross-flow velocity on membrane flux of Marun feed (1.5 bar, 20 °C, average of five replicates was reported).

distilled water for 10 min between each filtration. Figure 3 shows the results of the membrane flux after 150 min of filtration. As shown, repeating the filtration process causes a slight reduction in membrane flux. Also, these results show that fouling of membranes by coke particles is highly reversible and coke particles simply detach from the membrane surface by water washing. The rejection of coke particles after four sequences of filtration at all concentrations was 100%. 3.2. Effect of Pressure on Marun Feed Filtration. Figure 4 displays permeate flux data against different transmembrane pressure values (0.5, 1, and 1.5 bar) for the Marun feed with a fixed temperature of 20 °C. As expected, according to the solution diffusion model,22 permeate flux becomes larger as pressure increases. A slight decrease in flux was observed at 0.5 and 1 bar, and the steady state flux was reached within 30 min. But at 1.5 bar, the flux decline was more rapid (Figure 4) and the steady state flux was obtained after about 80 min. It must be mentioned that a higher initial and final flux were observed at a higher system pressure during this study but a faster decline in flux was also observed. From literature, the higher flux is due to the higher driving force toward the membrane at higher pressures as explained by Darcy’s Law.23 On the other hand, it is known that as transmembrane pressure increases, concentration polarization is greater and the components on the membrane surface increase,24 lead to an increase in reversible and irreversible fouling with pressure. The increasing pressure makes the oil droplets more compact on the membrane surface, and so, they block the membrane pores.13,25 Comparison of the permeate flux of the synthetic and Marun feed at 0.5 bar shows the flux of the Marun feed is considerably lower than that of the synthetic feed. It indicates that the oily compounds and not coke particles are effective fouling factor. 3.3. Effect of Cross-Flow Velocity. The time courses of filtration rates in cross-flow microfiltration under different cross-flow velocities are shown in Figure 5. The filtration pressure and the temperature are kept at 1.5 bar and 20 °C, respectively. In all cases, the flux decreased slightly with time. The plateau level was reached after 60 min of filtration. Increasing the cross-flow velocity increased steady state fluxes at the same transmembrane pressures. At dead end filtration (cross-flow velocity equals to 0 m/s), there was no turbulency or even applied shear. Therefore, the cake/gel layer of fouling can be created easily. Therefore, maximum fouling was observed and the permeate flux reduced significantly.

Figure 7. Effects of feed temperature on permeate flux (1.5 bar, 0.28 m/s).

Figure 8. Effect of various cleaning agents on feed flux recovery.

Increasing the cross-flow velocity improves the mass transfer coefficient in the concentration boundary layer and also increases the extent of mixing over the membrane surface.26 In addition, a high cross-flow velocity can produce more shear stress on the membrane surface,27 which can cause the fouling agent to detach from the membrane surface. Therefore, both of these phenomena reduce the membrane fouling and produce higher permeate flux. The increase of the shear stress results in a higher convection of the fluid toward the membrane and less formation of the deposited layer on the membrane surface. Considering that higher flow rates lead to more power consumption for pumping, the choice of very high flow rates is not economically feasible. Therefore, a 0.28 m/s flow rate was determined to be the best flow rate, and higher flow rates were not tested. 11715

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Figure 9. Surface SEM images of the PVDF membranes after cleaning with different solutions. (a) H2O, (b) HCl, (c) NaOH, and (d) NaOCl.

Figure 6 shows the effect of the cross-flow velocity on membrane flux after three sequences of filtration. After each filtration (120 min), the membrane was washed with hypochlorite solution (1 wt %). As can be seen, the flux was decreased by repeating the microfiltration. However, the flux of the higher cross-flow velocity (0.28 m/s) was relatively high even after three steps of

filtration. Comparison of the flux of membranes with cross-flow velocities of 0.096 and 0.28 m/s after three sequences of filtration indicated that a high flow rate caused the flux to reduce by about 38% of the initial amount, whereas the flux reduction for 0.096 m/s was about 49%. As a result, a high cross-flow rate can significantly affect membrane performance. 11716

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Industrial & Engineering Chemistry Research 3.4. Effect of Temperature. To investigate the influence of temperature on membrane performance, three different temperatures (25, 35, and 50 °C) were used. The effect of temperature on the flux of the GVHP PVDF membrane is shown in Figure 7. Temperature has an important effect on flux, which increases with temperature. This is due to the reduction in solvent viscosity, the increase of the solvent diffusion coefficient in the membranes, and polymer chain mobility.28 However, at higher temperatures, the flux decline is greater than at low temperatures. From Figure 7, it is apparent that the flux decreased more rapidly with the increase of temperature. It is reported that the rejection of solutes and particles can be decreased with increased temperature due to an increase in solute diffusion at higher temperatures.29 Because the membrane structure may expand, particles can easily pass through the membrane at higher temperatures.30 However, the used PVDF membrane did not pass any coke particles even at higher temperatures. The results obtained in these experiments indicated temperature has an important effect on the performance of the PVDF membrane. As a conclusion, it can be said that the GVHP PVDF membrane showed the best stability and durability against temperature for the filtration of the Marun feed. It should be mentioned that rejection of coke particles under various conditions such as different pressures, cross-flow velocities, and temperatures was 100% even after several sequences of filtration. 3.5. Chemical Cleaning of Fouled Membranes during Marun Feed Filtration. Fouling is the phrase applied to explain the loss of yield of a membrane device as it becomes chemically or physically changed by the process feed often by a minor component or contaminants.31 The influence of concentration polarization and fouling is always a reduction separation performance. The procedures to improve the performance can be classified into four categories: (a) pretreatment of feed solution, (b) adjustment or tailoring of membrane properties, (c) membrane cleaning, and (d) improvement of operating conditions.32 Although all the above methods decline fouling to some extent, cleaning methods will always be employed in practice. Chemical cleaning is the most important method for reducing fouling with a number of chemicals being used separately or in combination. Choosing the best materials depends on feed composition and precipitated layers on the membrane surface, which in most cases is selected by trial and error.33 The aim of this portion of the study was to find optimum cleaning agents and conditions to cleaning the used GVHP PVDF membranes fouled by the Marun feed. The effect of various chemicals, i.e. HCl as an acidic cleaning agent, NaOH as an alkali cleaner, and NaOCl as an alkali oxidizing agent, on feed flux recovery (FR) of the fouled membranes was investigated. Figure 8 shows the effect of various cleaning agents on feed flux recovery. Sodium hypochlorite solution (0.01 M) presented the highest flux recovery of 83.1%. The other agent, NaOH, showed a moderate effect (FR of 75.3%), and hydrochloric acid had the lowest flux recovery of 57.5%. This behavior can be attributed to the cleaning agent’s characteristics and foulant properties. High COD and BOD of the feed (Table 1) show that the most common component of wastewater is organic compounds containing dissolved organic compounds (including hydrocarbons, suspended oil, and oil emulsions). It is reported that acidic solutions are weak washing agents for the removal of organic fouling and are

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Figure 10. Comparison of different cleaning agents on recovery of membrane flux (1.5 bar, 20 °C, each flux after 120 min filtration).

often used for the removal of a metal oxide formed layer and inorganic precipitates on the membrane surface, especially for CaCO3, while alkaline cleaning is used to remove adsorbed organics.34 Therefore, hydrochloric acid cannot be a suitable cleaner for fouling resulting from organic and oily materials. Alkaline solutions clean organic-fouled membranes by hydrolysis and solubilization.35 These materials cause soapification of organic materials and dissolution of the foulants, which are deposited on the membrane surface, and then removes them. However, a hypochlorite solution due to having oxidizing properties in addition to alkali properties results in higher cleaning efficiency. Figure 9 shows the surface SEM of fouled membranes after cleaning with used washing agents. As shown in Figure 9a, when water was used as a cleaner, the foulants were not suitably removed from the surface of the membrane. By cleaning the fouled membranes with HCl and NaOH, the accumulation of foulants was relatively reduced and a smoother surface was achieved using NaOCl. These images indicate the best removal of participate foulants from the membrane surface was performed by a hypochlorite agent. Figure 10 compares the efficiency of various cleaning agents on recovery of the permeate flux of the Marun feed in three sequences of filtration. By repeating filtration, the flux of membranes was decreased. This reduction in flux was higher when hydrochloric acid was used. The results indicated that the NaOCl is the best cleaning agent for washing PVDF membranes fouled with oil contaminated wastewater even after three sequences of filtration. Coke determination results of permeate did not show the presence of any coke particles after sequence filtration of the Marun feed and using different cleaning agents. Therefore, the used washing agents do no have a destructive effect on the membrane structure. This indicates the ability of a used PVDF membrane to be a suitable membrane for Marun wastewater treatments.

4. CONCLUSION A polymeric PVDF microfiltration membrane was successfully used for the complete removal of coke particles from the industrial wastewater of the Marun Petrochemical Co. The effect of coke concentration on the synthetic feed showed that, at all 11717

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Industrial & Engineering Chemistry Research concentrations, no coke particles passed from the membrane and the pore size of the selected PVDF membrane was suitable for this separation. Also, the rejection of coke was 100% even with increased pressure, cross-flow velocity, and feed temperature. Comparison of the permeate flux of the synthetic and Marun feed indicated that oily compounds, not coke particles, are the effective fouling factor. The feed flux was improved by increasing the cross-flow velocity due to the increasing mass transfer coefficient in the concentration boundary layer and producing more shear stress on the membrane surface. With increasing feed temperature, the flux was increased because of the reduction in solvent viscosity, increase of solvent diffusion coefficient in the membranes, and polymer chain mobility. According to the organic nature of foulants of feed, hydrochloric acid as a cleaning agent showed low cleaning efficiency while NaClO and NaOH as alkaline agents showed good cleaning results. Comparison of SEM images of the cleaned membrane and feed flux recovery indicated that most of the foulants were successfully removed using NaClO as a chemical cleaning agent. A hypochlorite solution due to having oxidizing properties in addition to alkali properties results in a higher cleaning efficiency than NaOH.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: +98 831 4274530. Fax: +98 831 4274542. E-mail: smadaeni@ yahoo.com.

’ ACKNOWLEDGMENT The authors would like to acknowledge the financial support of Marun Petrochemical Company. ’ REFERENCES (1) Mueller, J.; Cen, Y.; Davis, R. H. Crossflow Microfiltration of Oily Water. J. Membr. Sci. 1997, 129, 221. (2) Yan, L.; Hong, S.; Li, M. L.; Li, Y. S. Application of the Al2O3
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