Application of a polytetrafluoroethylene (PTFE) flat membrane for the treatment of pre-treated ASP flooding produced water in a Daqing oilfield

Abstract

ASP (alkaline/surfactant/polymer) flooding oilfield wastewater, which is commonly produced in oil extraction processes, contains high organic content, and waste crude oil, water resources and sewage need to be treated prior to re-injection. In this study, pre-treated ASP flooding oilfield water produced in Daqing, China was treated by a PTFE microfiltration membrane and the removal efficiency of the main pollutants in the oilfield-produced water was studied. The content of oil and suspended solids (SS) in the permeate obtained with the membranes was 1.15 mg L⁻¹ and 1.61 mg L⁻¹, respectively, which met the re-injected water quality standards in China. A microscopic analysis of fouling showed that the membrane pollutants were mainly composed of petroleum organic pollutants, and a small amount of SiO₂, CaCO₃, MgCO₃ and other inorganic pollutants was observed. Accordingly the operating conditions of operating flux, back-washing cycle, back-washing flux and gas-washing flux were investigated to decrease membrane fouling.

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1. Introduction

With crude oil prices increasing gradually and the necessity of stabilizing production levels from a maturing field base, the ASP (alkaline/surfactant/polymer) flooding process has seen renewed interest in recent years as a promising method of enhancing oil recovery (EOR).¹ During the process of oil extraction, produced water (oily wastewater) is generated after dehydration of the produced liquid, which has a potential environmental impact. For example, it hinders the water body reoxygenation process and decreases dissolved oxygen. In general, a high concentration of APAM remains in the wastewater with characteristics of high viscosity and oily content. In the Daqing oilfield in China, the amount of produced water is up to about 3 × 10⁸ tons. Therefore, the produced water requires proper treatment before re-injection.²

Membrane technology is a promising alternative for the treatment of oilfield-produced water compared to conventional physical and chemical methods, which are not efficient for treating stable oil/water (o/w) emulsions (size ≤ 20 μm), especially when the oil droplets are finely dispersed and the concentration is low.³ Membranes are thin films of inorganic materials or synthetic organics that selectively separate a fluid from its components.⁴ Depending on membrane pore size and hydrophilicity, membrane treatment processes include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO). Among these membrane treatment processes, use of the MF membrane is one of the most effective methods for oil wastewater treatment, mainly for produced water due to its high oil removal efficiency, low energy cost, and small space requirements, and it does not require chemical additives. Moreover, extensive studies have proven its efficiency.^5–15

As the core of membrane treatment processes, materials for constructing membranes are highly important for cost-effective application of membrane treatment processes. Key technical obstacles (flux degradation, low average flux rate, and uncertain membrane life) hinder the widespread use of membrane treatment processes for treating heavily oil-contaminated water.¹⁶ Moreover, membrane fouling is of great concern when membranes are used in oilfield-produced water treatment, which causes the flux efficiency to decline. Various types of membranes have been used to treat produced water, such as tubular UF modules,¹⁷ tubular PVDF-UF and polysulfone-MF,¹⁸ NF-thin-film composite polyamide and RO-thin-film composite polyamide, and so forth.^19–21

The polytetrafluoroethylene (PTFE) membrane is a new type of flat membrane. Compared to other membranes, it has the advantages of stable insulating performance, excellent thermostability, high mechanical strength, and excellent chemical resistance.²² Due to its ultralow surface energy and high hydrophobicity, the PTFE membrane is commonly used in membrane distillation^23–27 and proton exchange membrane fuel cells.^28–34 However, there are few studies on the treatment of oilfield wastewater by PTFE membranes. In this study, a membrane treatment process with a PTFE flat membrane was used to treat raw water from an oilfield in Daqing, China. Characteristics of removal of the main pollutants, the membrane fouling mechanism and the control strategy were also investigated.

2. Materials and methods

2.1. PTFE flat membrane module

Flat membranes manufactured by VALQUA Industries, Japan, were made of polytetrafluoroethylene (PTFE) on a support layer of polyethylene terephthalate (PET). According to the manufacturer, the working length and width were 290 mm and 180 mm, respectively, corresponding to a membrane area of 0.1 m² (two sides) with an average pore size of 0.1 μm. The service life of the flat membrane was estimated to be approximately 4 years, and accordingly the cost of treating the wastewater was 0.18 dollars per m³ during the whole life cycle of the PTFE membrane. The flat membrane module is shown in Fig. 1.

Fig. 1 Schematic diagram of the experimental apparatus.

2.2. Experimental setup

A schematic of the experimental apparatus installed at Water Agencies is shown in Fig. 1. The apparatus primarily consisted of a pressure transducer, a constant flow pump, a raw water tank and a data acquisition system. Raw water was derived from an oil extraction plant in Daqing that is the largest oilfield in China. The flat membrane module was vertically fixed in a membrane tank (0.018 m³), in the upper part of the raw water tank, the effective volume of which was 0.06125 m³. Effluent was pumped from the raw water tank by a constant flow pump, and was then stored in an effluent tank. A pressure transducer was used to monitor the transmembrane pressure (TMP), which was recorded by the data acquisition system. The microfiltration operating cycle was: 9 min of filtration, 1 min interval, 60 min backwashing cycle, 1 min of backwashing. The backwashing flux, microfiltration operating flux and intensity of the gas washing were 90 L m⁻² h⁻¹, 10 L m⁻² h⁻¹ and 10 m³ m⁻² h⁻¹, respectively. The microfiltration process was terminated to clean the fouled membrane when the TMP reached a predetermined value of 80 kPa. Specifically, the PTFE membrane was cleaned on days 8, 16, and 23 during the experiment.

2.3. Characteristics of raw water

The raw wastewater used for the tests was collected from a Daqing oil extraction plant. The raw wastewater in the tank was pretreated by coagulation, air floatation, an advanced oxidation process and sand filtration in sequence. The detailed characterization is shown in Table 1. As is seen from Table 1, the pretreated water contained a high content of anionic polyacrylamide (APAM) (683–790 mg L⁻¹) which was used as the oil displacement. The pretreated water also contained a large amount of organic matter and suspended solids (COD_cr 1262–1499 mg L⁻¹, TOC 986–1172 mg L⁻¹ and SS 71–109 mg L⁻¹) which might cause significant organic pollution both on the membrane surface and in the membrane pores. The pH of the pretreated water was in the range of 10.3–12.6, indicating a high alkalinity, and the temperature of the feed water was maintained between 38 °C and 40 °C with a thermostat.

Table 1 Composition and concentration of the pretreated water

Component	Concentration (mg L^−1)
Median particle diameter (μm)	1–10
Oil	5.43–6.73
Suspended solids	71–109
TOC	986–1172
APAM	683–756
Surface active agent	20–53
CODcr	1262–1499
Turbidity	15.2–18.9
Carbonate	1500–2500
Bicarbonate	2500–3500

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2.4. Analytical methods

The oil and APAM content was analyzed by a UV spectrophotometer (UV2550, SHIMADZU, Japan). Turbidity was measured by a turbidimeter (TURBO550, WTW, Germany). Total organic carbon (TOC) values were obtained with a TOC analyzer (TOC-VCPH, SHIMADZU, Japan). The concentration of chemical oxygen demand (COD) was measured by the conventional method of heavy potassium chromate. SS were measured by a weight method: 100 mL of the water samples was passed through a 0.45 μm microfilter, and the retentate was dried in an oven at temperatures of 103–105 °C until the retention weight did not change (≤0.4 mg).¹³ The median particle diameter was measured by a ZETA nanometer particle size analyzer (Nano S, Malvern, England).

A scanning electron microscope (SEM, HITACHI S4800 HSD, Japan) equipped with an energy dispersive X-ray spectrometer (EDX, KEVEN, USA) was used to identify the microstructures of the surface and cross-sections of the membranes. The membrane samples were stored in a well-ventilated location for a period of time and then coated with gold for 30 s. The images of the surface and cross-sections of the virginal and fouled membranes were obtained by SEM at various magnifications.

To analyze the surface morphology of the PTFE flat membranes, atomic force microscopy (AFM, Digital Instruments, Veeco, USA) was used. The membrane samples were scanned over a range of 10 μm × 10 μm. Nanoscope V5.30 software was used to analyze the obtained data.

A Fourier transform infrared spectrometer (FT-IR, spectrum One B PerkinElmer, USA) was used to analyze the chemical composition of the original and fouled membranes. All spectra were collected in the range of 4000–500 cm⁻¹ at room temperature with a resolution of 4.0 cm⁻¹.

3. Results and discussion

3.1. Performance of the membrane module

Oil removal is one of the most important targets during oilfield-produced water treatment. The average content and removal efficiency of oil are shown in Fig. 2. The variation of oil concentration in the influent stream was caused by the real production process of the oilfield in Daqing. The oil concentration in the pretreated wastewater was 5.43–6.73 mg L⁻¹, and it decreased to 1.15 mg L⁻¹ after the PTFE membrane treatment, with an average oil removal of 80.7%. This completely meets the requirements of the SY/T 5329-2012 standard (a criterion of China Petroleum Industry).³⁵ The results suggested that the PTFE membrane had an efficient oil removal which may be partly attributed to oil adsorption onto the surface of the membrane or in the membrane pores.¹¹ In addition, oil wrapped by high molecular mass APAM could be removed by PTFE membrane retention.

Fig. 2 Time evolution of the oil concentration during microfiltration by the PTFE membrane.

APAM is another ubiquitous contaminant in oilfield-produced wastewater. Given its high viscosity and large particle size, APAM removal by the PTFE membrane was efficient. The APAM concentration in the pretreated wastewater and after membrane effluent was 683–756 mg L⁻¹ and less than 30 mg L⁻¹, respectively (Fig. 3). More than 97.4% of APAM was removed. The high removal efficiency of APAM is attributed to the large molecular weight of APAM in raw water, and the APAM molecular size was increased by the interaction between the molecules because of the high viscosity.³⁶ However, during the oil extraction and pretreatment process, APAM could be easily fragmented into low molecular mass compounds; thus, polymers were found in the effluent after the membrane treatment process.

Fig. 3 Time evolution of the APAM concentration during microfiltration by the PTFE membrane.

Fig. 4 depicts the time course of the COD_cr and TOC concentrations during the experiment. In the pretreated wastewater, the average COD concentration reached 1262–1499 mg L⁻¹, and the percentage of COD removed by the PTFE membrane microfiltration process was 88%. The oil in the pretreated wastewater was partly dissolved, and these soluble oil compounds in water were difficult to remove. The TOC removal (83.1% on average) during the membrane treatment process was similar to that of COD. Oil is a mixture of hydrocarbons including naphthalene, phenanthrene, benzene, toluene, ethylbenzene and xylenes (BTEX), polyaromatic hydrocarbons (PAHs), dibenzothiophene (NPD) and phenols.⁴ Most of the hydrocarbons cannot dissolve in water; therefore, the oil was of the dispersed type. The high efficiency of COD removal attained in the membrane treatment process is due to the retention of dispersed oily material.³⁷ Some classes of organic compounds in the oil were removed to a certain extent, and many other organic substances contributed to the residual COD and TOC content in the effluent. Due to the relatively easy separation, it can be inferred that the majority of organics in the wastewater were substances with a high molecular weight. Moreover, enhanced removal for mid-sized and small molecular organics was enabled as the macromolecular organics, which first blocked the membrane pores, effectively made the membrane pore size smaller and led to an efficient interception of mid-sized and small molecular organics.³⁸

Fig. 4 Performance of the PTFE membrane with respect to COD_cr and TOC removal.

Moreover, the performance of the PTFE membrane for suspended solid (SS) and turbidity removal was also investigated in this study. The characteristics of the PTFE membrane-driven SS and turbidity removal are shown in Fig. 5. The turbidity of the pretreated wastewater ranged from 15.2–18.9 NTU, and it decreased to less than 2.5 NTU. Some small size particles could be adsorbed and wrapped by APAM and co-removed together with APAM. The results indicated an efficient removal of SS. The average SS concentrations in the pretreated wastewater and effluent were 71–109 mg L⁻¹ and 1.61 mg L⁻¹, respectively, with a removal percentage of 98.2%, which also met the standards of the B1 grade in the SY/T 5329-2012 standard (SS ≤ 2.0 mg L⁻¹).³⁵ Table 2 gives the median particle diameter of SS in pretreated wastewater and effluent. The median particle diameter of SS in the pretreated wastewater varied from 3.32–9.62 μm, and was not detectable in the effluent, suggesting a good performance of the PTFE membrane for SS removal. Similar results were also observed by Weschenfelder et al.³⁹ We inferred that the PTFE membrane has a strong mechanical sieving capacity and can retain particles and colloids that are bigger than the pore size.

Fig. 5 Variation of SS and turbidity removal during microfiltration.

Table 2 The median particle diameter of SS in the feed and effluent

Index	Feed	Effluent
Median particle diameter (μm)	3.32–9.62	Undetected

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3.2. Analysis of membrane fouling in the PTFE membrane treatment process

Membrane fouling and consequent flux reduction is a common problem in membrane systems. The characteristics of the PTFE membrane before and after treating the oilfield-produced water were compared. Fig. 6 shows the SEM images of the surface and a cross-section of the PTFE membrane before and after treatment. The pores and surface of the membrane were smooth on the original membrane (Fig. 6a). However, significant membrane fouling developed, many large size particles accumulated on the surface and almost all of the pores were blocked after treatment. As the pollutants gradually accumulated, the cake layer of the PTFE membrane surface became irregular and rough (Fig. 6b). Fig. 6c shows that a thin separating layer and fibrous supporting layer could be observed inside the original membrane. After long-term operation, a 2 to 3 μm-thick fouled layer had developed on the membrane surface (Fig. 6d), which was consistent with results reported by Zhang et al.⁴⁰ It can be inferred that the PTFE membrane was fouled due to both pore blocking and gel layer formation. Pore blocking occurred when a permeate drag force caused macromolecule extrusion deformation, and a dense gel layer developed as the microfiltration time was prolonged. These types of fouling mechanisms have been reported in many colloidal particle systems.^41,42

Fig. 6 SEM images of the surface of the original membrane (a) and fouled membrane (b) and a cross-section of the original membrane (c) and fouled membrane (d).

Three-dimensional AFM images were also compared between the original and fouled membranes (Fig. 7). In Fig. 7a and b, different shades in the graphs indicate the contour and appearance of the membrane surface. The surface was relatively compact, and the peak was abundant and distributed intensively in the original membrane. Bulk was distributed on the membrane surface, and the peak was sparse and significantly decreased in the fouled membrane relative to the original membrane due to the pollutants accumulated on the membrane surface. The cross-sections of the PTFE membrane were analyzed by AFM technology. The results were similar to those obtained in Fig. 6. The peaks and valleys crossing the sections of the PTFE membrane varied greatly as pollutants accumulated on the surface (Fig. 7c and d).

Fig. 7 AFM images of the original membrane (a) and fouled membrane (b) and section analyses of the original membrane (c) and fouled membrane (d).

To better understand and control membrane fouling in the PTFE membrane treatment process, the elements of pollutants that were accumulated on the membrane surface were analyzed (Table 3 and Fig. 8). Table 3 lists the average proportion of each element. Carbon (C) and fluorine (F) had weight percentages higher than 93% on the original membrane because they are the primary elements of the PTFE membrane, whereas various elements, including C, F, O, Mg, Cu, Al, Cl, Na, Si, Ca, K and Fe, were found on the polluted membrane. The main elements of the pollutants were oxygen (14.02%) and carbon (38%). The membrane fouling observed in this study can be categorized mainly as organic fouling due to the polymer and petroleum pollutants in the wastewater. The appearance of Cu, Si, Ca and Fe is attributed to the expansion of minerals from the rock during the process of oilfield wastewater production. Because the alkalinity in oilfield wastewater is relatively high, Si, Fe, Ca and Mg could exist in inorganic forms of SiO₂, Fe(OH)₃, CaCO₃ and MgCO₃.

Table 3 EDX analysis of the membrane surface of the virgin and fouled membranes

Elements	Mass percent on virgin membrane (%)	Mass percent on fouled membrane (%)
C	28.26	38
O	6.68	14.02
F	65.06	17.44
Cu	0.00	1.01
Na	0.00	2.11
Mg	0.00	0.37
Al	0.00	0.38
Si	0.00	9.54
Cl	0.00	0.86
K	0.00	0.59
Ca	0.00	2.12
Fe	0.00	13.55

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As illustrated in Fig. 8, FTIR-ATR analysis was employed to investigate membrane organic fouling in this study. The FTIR spectrum of the clean PTFE membrane shows three typical transmittance bands at 3336 cm⁻¹, 1206 cm⁻¹ and 1152 cm⁻¹. More abundant absorbance peaks appeared on the fouled PTFE membrane in addition to the original peaks (Fig. 7a and b). The absorbance peaks at 3284 cm⁻¹ and 1550 cm⁻¹ represent N–H bond (amides), indicating the existence of polyacrylamide on the polluted membrane surface. Moreover, the infrared spectrum of the composition of the foulant shows bands at 2923 cm⁻¹, 2853 cm⁻¹, 1641 cm⁻¹, 1203 cm⁻¹, 1144 cm⁻¹ and 1044 cm⁻¹. According to the FTIR standard spectra, the bands at 2923 cm⁻¹ and 1641 cm⁻¹ are assigned to the C–H stretching mode and C=C stretching mode of alkane, and the band at 1203 cm⁻¹ belongs to the C–O–C stretching mode of an aliphatic ether group, whereas the bands at 1144 cm⁻¹ and 1044 cm⁻¹ are due to the C–O stretching mode of alcohol and the C–O–C stretching mode of alicyclic ether, respectively. The results demonstrate that organic fouling is present on the membrane after 31 d of the microfiltration process, and the peaks observed in the FTIR spectra revealed that oil and APAM were the main pollutants retained on the membrane surface, which originate from pretreated wastewater.^43–45

Fig. 8 FT-IR spectra of the original membrane (a) and the fouled membrane (b).

3.3. Effects of operating conditions on membrane fouling

The operating conditions of the membrane treatment process strongly affected membrane fouling. In this study, we discuss the effects of membrane flux and the back-flushing cycle on membrane fouling. For the different membrane fluxes tested in Fig. 9a, the trans-membrane pressure (TMP) was significantly increasing at the beginning of the experiments; however, the rate of increase of TMP of membrane C (30 L m⁻² h⁻¹) was obviously larger than that of membranes A (10 L m⁻² h⁻¹) and B (20 L m⁻² h⁻¹). The TMP of membrane B and C reached 60 kPa at 5.5 h and 72 kPa at 2.5 h, respectively. The sharp increase in TMP during the first few hours was due to the accumulation of APAM, oil and SS on the membrane surface and the subsequent formation of a gel layer. During the steady state of the membrane treatment process, the TMPs of membranes A, B, and C were almost constant, and a dynamic equilibrium was developed inside the membrane, stabilizing the quality of the effluent. Moreover, slight membrane fouling occurred when the membrane flux was 10 L m⁻² h⁻¹; therefore, in the following experiments, the membrane flux was set to be 10 L m⁻² h⁻¹.

Fig. 9 Variation of TMP under different (a) operating fluxes, (b) backwashing cycles, (c) backwashing fluxes, (d) gas-washing fluxes.

During a long-term operating process, membrane fouling might become increasingly serious, which would deteriorate the effluent quality and place a heavy burden on the membrane. Although an aeration process can remove some pollutants on the membrane surface and relieve membrane fouling to some extent, it can not remove pollutants inside the membrane. Thus, it is essential to clean the membrane periodically to recover the membrane flux. Fig. 9b shows the time course of the membrane treatment process with different back-flushing cycles. The TMP increased most slowly, compared to other conditions, when the back-flushing cycle was 30 min, and after 24 h, the TMP was still below 60 kPa. With an increasing water flushing cycle, the TMP increased sharply. The TMP increased to 80 kPa when the back-flushing cycle was 180 min. APAM and o/w induce serious membrane fouling, and the formation of a cake layer on the membrane surface is the main mechanism of membrane fouling. A longer back-flushing cycle induces greater accumulation of APAM and o/w on the membrane surface and thus more serious membrane fouling. A shorter back-flushing cycle benefits membrane fouling control.

Membrane fouling induces flux reduction, and water backwashing was investigated as a cleaning strategy to maximize the flux recovery of the fouled membranes (Fig. 9c). The TMP increased rapidly and reached more than 70 kPa after 24 h when the water flux for backwashing was 30 L m⁻² h⁻¹. As the water flux increased, the growth rate of TMP gradually decreased because the pollutants inside the membrane were lifted by backwashing, and backwashing loosened and detached the foulant debris from the membrane surface.⁴⁶ Water fluxes of 90 L m⁻² h⁻¹ and 120 L m⁻² h⁻¹ showed no significant difference in effect on the membrane flux recovery, indicating that the optimum water flux for backwashing and membrane flux recovery was 90 L m⁻² h⁻¹ because the foulants on the membrane surface were loosely swept, and the particles (such as oil) were not washed away and could go through the pores or be adsorbed by the membrane internally.⁴⁷ Membrane fouling caused by oilfield wastewater was not completely reversible, and backwashing could only remove reversible membrane fouling. Nevertheless, increasing the water flux for backwashing significantly sharply decreased the water production rate during the MF process; therefore, both membrane fouling cleaning and the water production rate should be considered to optimize the water flux during backwashing.

Similarly, we also investigated gas washing for membrane cleaning (Fig. 9d). The TMP increased to 78 kPa after 24 h with a gas flux of 0 m³ m⁻² h⁻¹. The increasing trend of TMP showed little difference and the TMP increased to 70, 69 and 70 kPa after 24 h when the gas flux was 10, 20 and 30 m³ m⁻² h⁻¹, respectively, relatively lower than that at 0 m³ m⁻² h⁻¹. The results revealed that gas washing relieved membrane fouling well and recovered membrane flux, but the variance of gas flux weakly impacted the membrane cleaning efficiency because the large-sized compounds formed a cake layer or gel layer on the membrane surface, and heavy viscosity and strong force led to inconspicuous effects of gas washing on membrane cleaning.

4. Conclusion

The main objective of this study was to evaluate the application of a PTFE microfiltration membrane in the treatment of practical ASP flooding oilfield-produced water. The removal of the main pollutants in ASP flooding oilfield-produced water was studied. The quality of the effluent met the reinjected water quality standards in China. The fouling mechanism of the PTFE membrane used for microfiltration was analyzed, and APAM, oil and some inorganic pollutants, such as SiO₂, CaCO₃ and MgCO₃, were the main membrane pollutants. Moreover, the effects of the operating conditions on membrane fouling were discussed, and the results showed that a membrane flux of 10 L m⁻² h⁻¹, backwashing cycle of 60 min, backwashing flux of 90 L m⁻² h⁻¹ or gas washing flux of 10 m³ m⁻² h⁻¹ could effectively control membrane fouling.

Acknowledgements

This work was supported by the National Science Foundation (Grant No. 51578390) and the State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology) (No. 2016DX11).

Notes and references

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