Study on the Effect of Modified Walnut Shell Filter on Oil Removal from Oilfield

Walnut shell filter materials has strong adsorption and contaminants interception ability, anti-oil immersion, high hardness, good wear resistance and stable chemical properties, its surface was easily polluted by the absorbed crude oil in oily sewage to reduce its filtering and backwashing effect. The surface modification of the walnut shell was the introduction of hydrophilic groups to enhance its hydrophilic oleophobic property; Used different ratio of sulfite to boil the walnut shell at a certain temperature to make a modification on the hydrophilicity of walnut shell surface, the modified filter materials with varying degree of surface modification was obtained. Measured the contact angle between walnut shell and water, the wetting performance was significantly improved, the contact angle was decreased from 95° to 36.75°, the surface properties were changed from lipophilicity to hydrophily. the micropores of surface become smaller, and it was conducive to the accumulation of emulsified oil beads; After filtration treatment of oily wastewater containing polymer, filter oil removal rate was over 78%, the oil removal rate of the modified filter materials was increased by 25% than that of the unmodified filter materials, and oil content in the filtered sewage was below 45 mg/L, the backwashing oil removal effect of modified filter materials increased from 17% to 87%, intercepted oil in the before process of filtration was more easier removed in the after process of backwashing to make the filtering performance regeneration of walnut shell filter materials. In the case of filtration treatment on the same amount of oily sewage its lifetime could be extended more than twice.


Introduction
Bed filtration was a widely used technique for oil removal in the oily sewage processing. The commonly used filters materials are quartz sand, walnut shell, fiber materials, microporous ceramics, film and so on. Natural walnut shell with strong adsorption capacity, anti-oil immersion, high hardness, good wear resistance, chemical stability and a low particle density can be easily gotten from a wide range of sources. it is easy to produce hydraulic rebound, and it need not too much backwashing strength, so it is easy to be regenerated and reused and widely used in oilfield sewage treatment. after the use of natural walnut shell filter materials for some time, the surface adsorption of oil beads to reduce the rate of oil removal easily leads to filter bed layer compaction and quick reduce of backwashing effect. Through the modification of the filter materials surface, the introduction of hydrophilic sulfonic acid groups changed the surface properties from the lipophilic hydrophobic into hydrophilic oleophobic, oil removal rate was greatly improved by sewage filtration, filter materials were easy to anti-flush and regenerate, and its service life was extended.

Experimental Procedure Experimental instruments and chemicals
Walnut shell particle size of 1.6~2.0 mm was washed with water to clear, then dried at 105°C in the study. The crude oil was SZ36-1 standard crude oil, simulated sewage was prepared according to the oil field water quality: polymer was 50 mg/L, salinity was 9374.13 mg/L, A high shear emulsifier at 12,000 rpm was used to prepare sewage sample of containing oil and polymer, Oil content referred to SY/T 0530-2011 "the determination of oil content in oil field sewage-spectrophotometric method". C2000D1 contact angle measuring instrument, Hitachi-SU8010 Scanning Electron Microscope, TU-1810PC UV spectrophotometer, AQUA fast II turbidity meter, WAW-1000B universal pressure testing machine, FLUKO-GF30 high-shear emulsifying machine, CH-II micro continuous sewage treatment device; self-made filter: No. 1 filter D=40 mm, H=400 mm; No. 2 filter D=60 mm, H=1500 mm.

Experimental methods
Modification of filter materials: Took clean and dry walnut shell filter materials 200 g in the three-necked reaction flask, added deionized water to the flask until all the filter materials immersed, mixed with a certain ratio of industrial Na 2 CO 3 , Na 2 SO 3 and NaHSO 3 solutions, and then placed in the oil bath pot, connected the mixing device and condensing device, set the oil bath temperature, started to boil the mixed solution to react for different time, poured out the filter materials when the reaction ended, rinsed with water to colorless, placed in a drying oven at the temperature of 105°C for 3 h to save. By changing the ratio of the solution to get different modified filter materials, the series of modified filter materials was formed [1][2][3][4].

Abstract
Walnut shell filter materials has strong adsorption and contaminants interception ability, anti-oil immersion, high hardness, good wear resistance and stable chemical properties, its surface was easily polluted by the absorbed crude oil in oily sewage to reduce its filtering and backwashing effect. The surface modification of the walnut shell was the introduction of hydrophilic groups to enhance its hydrophilic oleophobic property; Used different ratio of sulfite to boil the walnut shell at a certain temperature to make a modification on the hydrophilicity of walnut shell surface, the modified filter materials with varying degree of surface modification was obtained. Measured the contact angle between walnut shell and water, the wetting performance was significantly improved, the contact angle was decreased from 95° to 36.75°, the surface properties were changed from lipophilicity to hydrophily. the micropores of surface become smaller, and it was conducive to the accumulation of emulsified oil beads; After filtration treatment of oily wastewater containing polymer, filter oil removal rate was over 78%, the oil removal rate of the modified filter materials was increased by 25% than that of the unmodified filter materials, and oil content in the filtered sewage was below 45 mg/L, the backwashing oil removal effect of modified filter materials increased from 17% to 87%, intercepted oil in the before process of filtration was more easier removed in the after process of backwashing to make the filtering performance regeneration of walnut shell filter materials. In the case of filtration treatment on the same amount of oily sewage its lifetime could be extended more than twice.
bottom with a height of 80 mm (d=1.2 mm), and then the two filters was used in series. Filter materials filling scheme 3: No. 2 filter was filled with filter materials at the height of 1450 mm.

Results and Discussion
Analysis of surface morphology and mechanical properties of filter materials before and after modification Determination of surface contact angle of walnut shell: The surface of the walnut shell was subjected to surface hydrophobicity modification reaction at the reaction temperature according to the method of 1.2.1 using different proportions of the drugs. Get a series of surface modified filter materials a~f, measuring its surface and water contact angle, modified before the filter -0, the results were as follows: The contact angle was a significant measure of the relationship between matter and liquid wettability. θ=90° could be used as the boundary between wetting and non-wetting, when θ<90° was wettable, θ>90° was non-wettable [5][6][7][8][9]. From Table 1, it could be seen that the contact angle of the unmodified filter materials was 95°, indicating that the surface hydrophobicity was strong. The surface contact angle of the modified walnut shell was reduced to 36.8°~66.3°, which indicated that the surface hydrophilicity of the modified walnut shell had improved remarkably, and the hydrophilicity of c-filter materials was the best.

Changes in the surface morphology of the filter materials:
The surface morphology of the filter materials 0 and c was analyzed by SEM. The results were presented in Figure 1. From the surface scanning electron microscopy of the filter materials, before the modification, the microporosity of the filter materials was large, emulsified oil by filtration was more susceptible to clogging microporosity. The surface morphology of the walnut shell changed after the introduction of sulfonic acid, and the micropores became smaller.

Analysis of mechanical properties of filter materials before and after modification:
In the cylindrical diameter of 40 mm two round steel plate filled with a layer of walnut shell. The force of the universal pressure tester slowly increased at a rate of 0.1 kN/s in the detection of walnut shell deformation parameters, the stress-strain curve about before and after the modification of filter materials was shown in Figure  2. It could be seen from Figure 2, the destructive force increased to 50 kN at the speed of 0.1 kN/s, the damage strength of 39.8 MPa, before the modification of 0-filter materials deformation was 30.0 mm, modified c-filter materials deformation was 30.4 mm, When the strain was 60%, the deformability was similar, and the two kinds of materials have no powderiness, which indicated that the mechanical properties had no changes.
Filter bed resistance: According to the program 3, filled 0-filter materials and c-filter materials at height of 1450 mm, after different filter materials filled, resistance change of the filter bed was studied. The U-shaped differential pressure gauge was connected to the process to measure the pressure change value of the oil-containing wastewater containing the polymer during the filtration treatment. The device was a standard ± 10000 Pa inverted U-type device, the structure of the principle was shown in Figure 2 ( Table 2).
From the results of differential pressure measurement, the resistance in the filtration process was 2.60~2.63 kPa, which indicated that the pressure value of the filter bed changed little during the filtration process.

Filter materials and sewage oil interaction
Put 30 g of 0 filter materials and c filter materials in a 250 ml conical flask with a stoppered, and respectively took 150 mL water with the oil content of 86.60 mg/L into a conical flask, then oscillated and adsorbed for 1 hin a 60°C thermostat oscillator; poured out the oily water in the bottle to measure the oil content, the filter materials in the bottle was washed with 150 ml water at 60°C, poured out the water to measure oil content; And then took 150 ml oily water samples into the repeated adsorption experiments, the results was shown in Table 3.
As could be seen from Table 3, with the increase of the adsorption number, the two kinds of filter materials on the oil adsorption capacity showed a downward trend, but the interception dirt capability of modified c-filter materials had been improved by 6~15 than that of the modified 0-filter materials. The reason was that surface pores of after modified filter materials became smaller, emulsified oil was not easy to be adsorbed on the surface of the filter materials but gathered into oil beads, in the backwash, the oil beads were more easily removed from the filter materials surface, so the oil content in backwash water using c-filter materials was higher than 0-filter materials, the oil gathered in the microporous surface could be washed out by backwashing, after the third adsorption, oil removal rate of c-filter materials can still increase by 5% than the 0 filter materials.

Filter degreasing effect
According to the filter filling program 1, filters were filled with c-filter materials and 0-filter materials. The oil content of sewage was 180.39 mg/L, water temperature was 60°C, pump flow was 480 mL/min, the water sample was continuously pumped into the filter, adjusted the outlet valve on the lower part of the filter to make the filter materials   completely submerged in the process of running, There are 5 times totally, each time collected 3 L of filtered water, recorded the time required and analyzed the oil content. After the total filtration of 16 L water, the filter materials was anti-flushed with 2 L of 60°C water, collected 1 L backwash water samples, measured the oil content of back wash water, the oil content of 0-filter materials back wash water and c-filter materials back wash water was 5.32 mg/L and 59.09 mg/L respectively.
From the experimental results in Figure 3, the oil removal rate of two filter materials had been increased by 25~28%, the oil removal rate of c-filter materials arrived to 77.5%, the oil content of c-filter materials backwash water had increased by 91% than that of the 0-filter materials backwash water, indicating that intercepted oil of the modified filter materials layer was more easily removed by back washing to make the filtering perform an easier to be regenerated. In the filtration, the filtration process was divided into aggregation migration and filter materials adsorption, the emulsified oil in sewage would be subject to the effects of various forces. In the process of agglomeration and migration, it would be subject to gravity sedimentation, hydraulic collision and Brown diffusion. In the process of materials adsorption, it would be subject to contact condensation, electrostatic attraction, adsorption, molecular gravitation and polymerization [10][11][12][13], the filtration effect of filter materials is limited by these forces. after the modification of the surface properties, the surface of the filter materials made a reversal from the lipophilic into hydrophilic, and the original oil wet surface was changed into the water wet surface in the filtration. The contact angle between emulsified oil in the sewage and walnut shell had been changed. It transformed from immersion adsorption to oil beads particles adsorption, so that the contact angle of emulsified oil particles in its surface decreased, after agglomeration into the large oil beads, the adsorption of oil beads followed the law of chemical and chemical flotation dynamics (F=4πrσ), The greater the particle size of the oil beads, the stronger the adhesion. Large particles of oil would also occur coalescence, the strength of coalescence was positively related to the particle size of oil [14].
It could be seen from Figure 3, Average filtration time of the modified c filter materials was 25 s longer than that of the 0 filter materials, because in the filtration process, compared to the unmodified filter materials, In addition to Walnut shell intercepting emulsion oil to form the filter layer resistance and suspended particle resistance here, its surface became wet would be subjected to additional capillary resistance, therefore filtration rate was slow, and the filter time was longer. it also made the contact time of the oil in the water with the filter materials longer, made the effect of adsorption, interception and capture enhanced, so the interception ability of the c filter materials to the oil was improved.

The oil removal effect of increasing the height of the filter bed
Filled with filter materials c and 0 respectively, the oil content of the sewage was 216.21 mg/L, water temperature was 60°C, pump flow was 480 mL/min, the water pump input filter, each time collected 3 L of filtered water, recorded the time required and analyzed the oil content of the sewage for a period of 5 times totally, Calculated oil removal rate by filtration.
It could be seen from the oil removal rate in Figure 4 that when the filling height of the filter materials increased, the oil removal rate effect was obviously improved, and the oil removal rate of c-filter materials was increased by 34% or more compared with 0-filter materials. The results showed that the increase of filter bed height and diameter increased the residence time of the sewage in the filter to a certain extent, increased the contact area between the dirty oil and the filter materials in the sewage, increased the contact volume, and increased the oil removal rate.

Dynamic parallel circulation filter backwash life experiment
The parallel test was carried out according to the filter filling scheme 2, the total filled height of filter materials was 680 mm, two filters were respectively filled with the same height of 0 filter materials and c filter materials, the pump dual outlet were connected to two filters, the oil content of the sewage was 245.34 mg/L, pump flow was 400 mL/min, each time collected 3 L of filtered water, recorded the time required and analyzed the oil content; Then used 60°C water to backwash, collected 2 L backwash water to analyze its oil content, analyzed the effect of three cycles of filtration -backwash (Table 4 and Figure 5). From the filtration-backwash three cycles of experimental results, the backwash degreasing effect of c-filter materials increased from the initial 17% to 87% than that of 0-filter materials, indicating that intercepted oil by c-filter materials was more easily removed by backwashing to make the filtering performance regeneration; Oil removal rate increased by 20%. Three cycles, the oil content of c filter materials was always 50% lower than that of 0 filter materials. In the case of filtration treatment on the same amount of oily sewage, the lifetime of c filter materials could be extended more than twice.

Conclusion
1) The surface of the filter materials changed from lipophilicity to hydrophilicity and the contact angle with water reduced from 95° to 36.8°. When the filter bed height was 1450 mm, and the pressure was 2.63 kPa, the pressure change was in very small range.
2) The sewage containing 50 mg/L polymer and 150~230 mg/L oil was treated, the oil removal rate of c-filter materials stabilized at 78%~81%, it increased by 25~28% than unmodified filter materials, backwash oil removal effect increased from the initial 17% to 87%.
3) The mechanical strength of the filter materials before and after modification had not changed, surface micro-pores became smaller and its number increased, which was beneficial to the aggregation of emulsified oil droplets. oil droplets aggregated on the surface of micropores was easy to be removed from the surface of the filter materials by backwashing. The oil intercepted capacity increased by 6-15% compared with 0-filter materials; filter bed height and filter cross-sectional area increased, the contact area of oil and filter became larger, the contact volume increased, the contact time increased, oil removal rate increased.

4)
intercepted oil of c-filter materials after filtration was more easier removed by backwashing to make the filtering performance regeneration. The oil content in c-filter materials backwash water was always 52%higher than that in 0-filter materials. In the case of filtration treatment on the same amount of oily sewage, its lifetime could be extended more than twice.