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1. Introduction

“Produced Water” (PW) incurred dur-ing oil production and gas extraction con-sists of a complex mixture of dispersed and molecularly dissolved oil, other dis-solved organic components, suspended solid matter, dissolved chemicals used and possibly heavy metals and naturally occurring radioactive substances. PW con-stitutes the biggest process water volume which occurs in the oil and gas industry and is diffi cult to treat on account of its composition. For its introduction into the environment, different purity require-ments must be considered, which mainly depend on whether the water is introduced onshore or offshore. Due to the current environmental legislation, PW can only be initiated into the environment if the residual oil concentrations are in the low ppm range. Hence, the development of effi cient and effective cleaning processes and methods is a decisive prerequisite for the environmental compatibility of corresponding crude oil and natural gas production techniques and for their eco-nomic potential.

The cleaning of PW is usually carried out in multi-stage processes. In the fi rst stage, a signifi cant reduction of dispersed hydrocarbons and suspended solid matter is carried out, for example, through sed-imentation processes in the earth gravity fi eld or in the centrifugal fi eld. In order to achieve the permissible limit values for the existing contamination in PW before intro-duction to the environment, downstream membrane processes, such as microfi ltra-tion (MF) and ultrafi ltration (UF) process-es, are necessary in further stages.

Here, the ceramic hollow fi bre mem-brane (CHFM) examined in this study (Mann+Hummel) represents a new gen-eration of an inorganic membrane, which combines the advantages of a ceramic membrane material with the geometry of hollow fi bre membranes. In this study, the fi ltration performance of the CHFM is examined during the separation of oil/water mixtures at a low trans-membrane pressure (TMP) of 0.5 bar. Here, the resulting permeate fl ow and the residual concentration of oil and organic compo-nents (TC, total carbon) are respectively measured. The fouling behaviour of the CHFM during the oil/water separation is recorded depending on the operating conditions and the composition of the feed stream to be cleaned, and the effi ciency of physical and chemically supported back-wash processes is described. In all inves-tigations, a CHFM is used that has a pore size of d90 = 40 nm.

The oil/water mixture to be separated is produced on one hand as a defi ned model system (OMS, oily model system) with 30

ppm to 200 ppm initial oil concentration in the water and on the other hand is avail-able as real PW from the dewatering of crude oil tanks (TDPW, tank dewatering produced water), with an oil concentration of between 1,000 ppm and more than 5,000 ppm. During the examinations, a separation effi ciency was measured of > 99.5% with respect to the oil reten-tion and of between 61% and 94% with respect to the total carbon concentration. Through mechanical backwashing of the membrane, up to 80% of the initial per-meate fl ow rate could be achieved during fi ltration operation. With chemically sup-ported cleaning, the initial permeability of the new membrane was nearly achieved. Overall, the investigated membrane sys-tem also demonstrated a stable operating performance in experimental periods of several days.

Hereinafter, the most important back-ground on the creation, composition and purifi cation of PW is initially summarised.

2. Challenges of PW

2.1 PW and its managementPW is defi ned as wastewater that has

reached the surface by washing out the deposits during crude oil production and natural gas extraction. In view of industri-al applications, PW constitutes one of the biggest (waste) material fl ows that gener-ally occur in processes of crude oil pro-duction and natural gas extraction. Here, the PW is often mixed with spring water that occurs naturally in rock strata [1]. At the moment, two measures are primarily used for the disposal of PW: reinjection

Ceramic hollow fi bre membrane technology for the treatment of oil-fi eld produced water M. Ebrahimi, St. Kerker, S. Daume, F. Ehlen, I. Unger, St. Schütz, P. Czermak*

Hereinafter, we report about the development and application of an innovative ceramic ultrafi ltration hollow fi bre

membrane for oil/water separation during the treatment of “produced water” that results during oil production and gas

extraction. Produced water is a complex mixture of dispersed and molecularly dissolved oil, suspended solid matter

and other dissolved substances. It constitutes the biggest process water volume which occurs in the oil and gas

industry and is diffi cult to treat on account of its composition. For its introduction into the environment, different purity

requirements must be considered which mainly depend on whether the water is introduced onshore or offshore. Here,

the residual oil concentration must be in the low ppm range. Hence, the development of effi cient and effective cleaning

processes and methods is a decisive prerequisite for the environmental compatibility of corresponding crude oil and

natural gas production techniques and for their economic potential.

The ceramic hollow fi bre membrane examined in this study (Mann+Hummel GmbH, Ludwigsburg, Germany) represents

a new generation of an inorganic membrane which combines the advantages of a ceramic membrane material with the

geometry of hollow fi bre membranes.

* Dipl.-Ing. Mehrdad Ebrahimi a, b

Dipl.-Ing. Steffen Kerker a, b B. Sc. Sven Daume a, Dr.-Ing. Frank Ehlen c, M. Sc. Ina Unger c, Prof. Dr.-Ing. Steffen Schütz c, Prof. Dr.-Ing. Peter Czermak a, d

* Reference author: mehrdad.ebrahimi@kmub.thm.de, info@ehc-memtec.de

a Institute of Bioprocess Engineering and Pharmaceutical Technology – IBPT, University of Applied Sciences Mittelhessen, Gießen, Germany

b ehc-memtec UG, Giessen, Germanyc MANN+HUMMEL GMBH, Ludwigsburg, Germanyd Dept. of Chemical Engineering, Kansas State

University, Manhattan KS, USA

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F & S International Edition No. 15/2015 37

into the raw material deposit itself and treatment for further use and/or introduc-tion into the environment. Currently, more than 60% of the PW is introduced back into the deposits. The reinjection as well as the treatment of PW primarily require the separation of crude oil from other organic components and suspended solid matter [2].

The composition of PW and its physical properties vary signifi cantly depending on the geographical location of its origin and depending on the extracted raw material [3]. Produced Water often contains differ-ent volumes of dispersed crude oil, addi-tional undissolved and dissolved organic components, process chemicals, corrosion products, heavy metals, inorganic salts and naturally occurring radioactive com-ponents.

During crude oil production, three bar-rels of PW per barrel of crude oil occur on average [2, 4]. This ratio increases with increasing age of a deposit and, in older oil wells, reaches values of 7-10 barrels of PW per barrel of extracted crude oil [5]. Alzahrani et al. [6] estimate that, with worldwide crude oil extraction of 72,000,000 barrels/day, at least about 216,000,000 barrels/day of PW will result. On account of this huge volume of PW, the manner of handling it is of great sig-nifi cance for the public and for legislation [7]. The potential for the introduction of PW into the environment or for its sensible recycling is being extensively discussed [8].

The statutory requirements for the puri-ty of PW during introduction into the environment are very restrictive. Table 1 shows an overview of the permissible

residual crude oil concentrations during onshore and offshore introduction of PW in different world regions [9].

On account of these specifi cations, it is necessary to develop and/or improve innovative technologies for the purifi ca-tion of PW, not only in order to fulfi l the increasing legal requirements with regard to environmental protection, but also in order to increase the economic effi ciency of the purifi cation processes and in order thereby to take advantage of a possible new source of water [10].

2.2 PW treatment

The objective of the purifi cation of PW is the removal of crude oil, other organic components and suspended solid mat-ter and desalination. Usually multi-stage purifi cation processes are used in order to achieve the required low crude oil concen-tration. Customary process technologies, which are used as the fi rst and second stage during cleaning of PW are sand fi l-tration, sedimentation, fl otation, as well as apparatus hydrocyclones and separators, in order to signifi cantly reduce the content of dispersed hydrocarbons and suspended solid matter [11, 12].

Membrane processes are usually used in the third treatment stage of PW, in order to achieve low residual crude oil concen-tration and to treat the PW for subsequent desalination through reverse osmosis. Within the scope of these technologies, a great deal of research effort has been made for the treatment of PW by means of microfi ltration (MF), ultrafi ltration (UF), nanofi ltration (NF), reverse osmosis (RO), membrane distillation (MD) and/or corre-sponding combinations of these processes

[6, 13, 14]. Ahmadun et al. [15] give an overview of the different aspects of mem-brane technology in the treatment of PW. In a number of publications, the authors of this paper have previously described the possibilities offered by membrane tech-nology in the treatment of PW [16-20].

During the treatment of PW the main technical challenges that are coupled with the application of membrane processes are the achievement of high fl ow rates, high purifi cation effi ciency and low fouling ten-dency of the membrane during operation, as well as chemical and thermal stability of the membranes used. In the treatment of PW, there are often conditions that pre-clude the use of conventional polymeric membrane materials, on account of the chemical composition or high temperature of the feed stream to be purifi ed. In this context, using tubular or rotary ceramic membranes was suggested in order to treat PW.

Ceramic hollow fi bre membranes like they were used in this study are a new generation of inorganic membranes com-bining the advantages of an inorganic membrane material with hollow fi bre geometry [21, 22]. In comparison with ceramic tubular membranes, they have a higher specifi c fi lter area with respect to the volume of a corresponding membrane module [23].

3. Overview on the experiments

carried out

In this study, tests were carried out for the effi cient fi ltration of TDPW, and OMS by using an innovative ceramic ultrafi ltration hollow fi bre membrane

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38 F & S International Edition No. 15/2015

Highlights 2014

(Mann+Hummel) with a pore size of d 90 = 40 nm. Such a ceramic hollow fi bre membrane is shown in Fig. 1.

Within the scope of the test series, the infl uence of the cross-fl ow velocity (CFV) and the crude oil concentration in the feed stream on the membrane performance was examined. Other test series were used for the analysis of the cleanability of the mem-brane during fi ltration and the study of different physical and chemical cleaning procedures for the guarantee of a stable long term operation. In all examinations the TMP was 0.5 bar and the process temperature was 40 °C. The clean water permeability of the membrane used was determined with trans-membrane pres-sures of 0.25 bar to 1 bar.

4. Ceramic hollow fi bre membranes (CHFM)

The contents of this section are the description of CHFM, as well as their production and characterisation.

4.1 Typical features of ceramic membranesThe most important material and application characteristics of

ceramic membranes are:- High chemical and thermal stability, which allows the fi ltration

of acids, alkalis, solvents and hot fl uids, as well as the cleaning of the membranes using chemical cleaning agents

- High mechanical stability during fi ltration of abrasive fl uid ingredients

- Low fouling and adsorption tendency for organic molecules- Very high membrane purity after production through the required

sinter processCeramic hollow fi bre membranes, in particular, have other

advantages compared with ceramic membranes of other geome-tries (ceramic tubular and disk membranes):- High packing density (high ratio of the membrane surface to the

fi lter volume)- Defi ned fl ow conditions, in particular with in-out cross-fl ow

fi ltration- Low cost of materials in view of the fi lter area- Sintering process with low energy costs and short sintering

time, on account of the low wall thickness of the hollow fi bre membranesThe CHFM that was used for the examinations described below

was made by a fi bre spinning process in which, simultaneously with the spinning process, a phase inversion is carried out from the liquid spinning mass to the solid structure of the hollow fi bre membrane.

4.2 Fibre spinning and phase inversionThe starting materials for the preparation of the ceramic hollow

fi bre membrane by means of a fi bre spinning process are a ceramic powder, a polymer powder, a corresponding solvent system and a number of additives. These components are homogeneously mixed into a viscous spinning mass. The fi nished spinning mass is guided through a two-component spinning nozzle into an aqueous precipitation bath. Here, the solid structure of the hollow fi bre membrane forms from the liquid spinning mass through phase inversion. Here, in detail, the solvent in the spinning mass is displaced through the water of the precipitation bath. Because the polymer component in the spinning mass is dissolvable only in the solvent, but not in water, a solid structure develops in the form of the hollow fi bre membrane.

The form of the hollow fi bre is generated through the cross-sec-tional area in the nozzle mouth of the two-component spinning nozzle shown in Fig. 2. The external annular cross-section is permeated by the spinning mass. In the internal annular cross-sec-tion, an aqueous central fl uid fl ows, which has the same or similar composition as the precipitation bath. The central fl uid comes into

Fig. 3: Scanning electron microscope image of the CHFM which was used in the experiments: ceramic microfi ltration layer as a carrier structure and on it the functional ultrafi ltration layer as an active layer.

Fig. 2: Mouth cross-section of a two-component spinneret having an annular cross-section (spinning mass) and a circular cross-section (central fl uid)

spinning mass

annular cross-section

central fl uid

(water)

Fig. 1: Cross-section of a CHFM

Tab. 1: Allowed residual crude oil concentrations for onshore and offshore introduction of Produced Water into the environment.

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contact with the spinning mass only in the nozzle mouth, so that precipitation of the spinning mass within the nozzle is excluded. During the spinning process, the nozzle mouth is guided either at a short distance above the precipitation bath (spinning with an air gap), or it is immersed directly in the precipitation bath.

The so-called green fi bre originating during spinning shows fea-tures of a polymer hollow fi bre membrane. The ceramic particles from the spinning solution are enclosed in the polymer phase. The green fi bre is washed after the spinning in order to remove solvent residues and afterwards it is sintered at high temperatures. During the sintering process, the polymer is burned completely out of the fi bre structure and the ceramic particles combine with each other via sintering necks. At the end of the sintering process, there is a pure ceramic hollow fi bre membrane. The dimensions of this hol-low fi bre membrane are smaller than the dimensions of the green fi bre due to thermal shrinkage during sintering.

4.3 Specifi c design of the ceramic hollow fi bre membrane used

The CHFM which was used during examinations consists of two layers. The fi rst layer is a ceramic microfi ltration carrier structure with open pores and with a low trans-membrane pressure during operation, which results from the above-described spinning process during phase inversion. This carrier structure gives the necessary mechanical stability to the membrane. After the sinter-ing, this carrier structure is coated with a functional ceramic ultra-fi ltration layer as a separation-active layer on the feed side of the membrane. Because the membrane is permeated in all studies from the inside to the outside, the functional coating is also done on the internal luminal surface of the membrane. Both layers consist of Al2O3 aluminium oxide.

With this coating process, the membrane is fl ushed with a ceramic suspension that adheres in a thin layer on the inner surface of the carrier structure. After this coating step, the ceramic hollow fi bre membrane is sintered a second time, in order to obtain a stable connection between the ceramic coating and the ceramic carrier structure.

With the examined hollow fi bre membrane, the separation active layer has a thickness of only few micrometers. This mem-brane structure allows a low trans-membrane pressure during operation and a high permeate fl ow rate through the membrane from the inside to the outside, since the pressure drop in the open structure of the ceramic carrier layer compared to the pressure drop in the active ceramic ultrafi ltration layer can be almost completely neglected.

Figure 3 shows the active ultrafi ltration layer and the carrier structure in an image of a scanning electron microscope.

5. Filter modules, test set-up, test parameters and

substance systems

All fi ltration experiments were carried out with different lots of the same ceramic hollow fi bre membrane from Mann+Hummel. The data of the membrane used and the fi lter modules are sum-marised in Table 2. Figure 4 shows a membrane bundle as it is installed in a fi lter module.

5.1 Experiment set-up and test parametersThe experiment set-up according to Fig. 5 comprises a centrifu-

gal pump for fl uid pumping, a fi lter module with CHFM, as well as the line routings for the feed, the permeate and the retentate stream. The maximum operating pressure of the plant is 3 bar and the maximum operating temperature is 80 °C. The backwashing of the membrane can be carried out with a maximum pressure of 10 bar.

During the experiments, the CFV was varied between 1.5 m·s -1 and 2.5 m·s-1. The tests were carried out alternatively in the Fed-Batch mode or with full recycling of the permeate and retentate (Total Recycle Mode). The purely physical backwashing is done in different, but regular time intervals by the fact that permeate is pressed from the outside to the inside through the membrane against the fi ltration direction. The mean TMP was determined through measurement of the static pressure before and after the membrane module in defi ned time intervals. All fi ltration exper-iments were carried out with a fl uid temperature of 40 °C and with a low TMP of 0.5 bar. The initial crude oil concentration in

Fig. 4. Image of a fi lter module with ceramic hollow fi bre membranes, which was used in these examinations.

Tab. 2: Features of the CHFM and the fi lter modules examined.

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TDPW amounted to 1,000 to 5,200 ppm, with OMS at 35 ppm. The permeate fl ow rate was determined with an electronic scale (DS 36K0.2, core) that was read with the data acquisition soft-ware LabVIEW (National Instruments, Munich, Germany). The membrane was chemically cleaned after each experiment with 1% P3 Ultrasil-14 cleaning solution (Henkel, Duesseldorf, Germany). After the cleaning, the clean water fl ow rate of the membrane was measured.

An overview of the effect of different test parameters during separation of crude oil/water mixtures with ceramic membranes can be found in earlier works of the authors [16-20].

5.2 Analysis of the crude oil/water mixture

The continuous on-line measurement of the crude oil con-centration was made until the concentration range of one ppm with an on-line measurement system, which was developed for industrial applications (DECKMA HAMBURG GmbH, Hamburg, Germany). With a multi-range conductivity meter (HI 9033, Hanna Instruments, Kehl am Rhein, Germany), the electric conductivity of the feed and the permeate streams was determined. Samples from the feed stream, the permeate stream and the retentate stream were characterised with regard to their total carbon concentration (TC) with a TC measuring instrument (Shimadzu, Duisburg, Germany). The off-line measurement of the TC concentration and the dispersed crude oil is done with a fl uorescence-based meas-urement system (TD500D, Nordantec, Bremerhaven, Germany).

5.3 Characterisation of the crude oil/water mixtures

Oily water samples from TDPW were made available by Deutsche BP AG, Ölraffi nerie Emsland, Lingen. The OMS were prepared through pre-emulsifi cation of water with crude oil (Oilfi eld Bramberg) with a rotor-stator- homogeniser and subsequently with a high pressure homogeniser (Emulsifl ex C5, Avestin, Mannheim, Germany) with an operating pressure of 450 bar.

The fi nal concentration of the dispersed crude oil in the OMS systems was set through dilution with demineralised water. A summary of the physical properties of TDPW and OMS is shown in Table 3.

Figure 6 shows the representative drop size distributions in the feed stream of OMS (red curve), as well as in the feed stream of TDPW (blue curve). The drop size distributions were determined with a laser diffraction spectroscope (Mastersizer S, Malvern Instruments Ltd., Herrenberg, Germany).

6. Results

Hereinafter the test results are presented for the membrane fl ow rate, the separation performance and the backwashing ability of the investigated ceramic ultrafi ltration hollow fi bre membrane. Here, the CFV, the crude oil concentration in the feed stream, as well as the process parameters for the membrane cleaning during opera-tion were varied.

During the examinations carried out, the cross-fl ow velocity was between 1.5 m·s-1 and 2.5 m ·s-1, from which corre-sponding Reynolds numbers result for the fl ow in the hollow fi bre lumen of between 2,900 and 4,800, which indicates a turbu-lent fl ow. The Reynolds number is defi ned as follows:

with the density ρ of the liquid, the mean fl ow velocity v in the hollow fi bre lumen,

the inner diameter d of the CHFM and the dynamic viscosity μ of the liquid.

The membrane fouling that occurs with all tests is due to the formation of a top layer on the membrane surface, or blocking of the membrane pores by particle adsorption, or through sterical-ly-induced particle retention, or usually due to the combination of the named effects. Because membrane fouling leads to a reduction of the fi ltration effi ciency, a variation of the operating conditions during cross-fl ow fi ltration can be a purposeful approach, in order to analyse the mode of action of individual parameters of infl uence on the fouling behaviour of membranes and on the permeate fl ow rate [24, 25].

Tab. 3: Physical properties of the TDPW and OMS at 40 °C.

Fig. 6. Representative drop size distributions (relative volumetric frequency) of the crude oil drops in the feed stream of OMS (red curve) and TDPW (blue curve).

Fig. 5. Systems fl ow chart of the membrane fi lter system used in the laboratory, Fed-Batch mode and also operating with full recycling of permeate and retentate are possible.

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In the following Figures, the temporal course of the normalised fl ux performance is shown for different operating conditions. The normalised fl ux performance is defi ned as the ratio of the per-meate fl ow rate at time t relative to the permeate fl ow rate at the beginning of the respective test. For the time t = 0, the normalised permeate fl ow rate hence has the value 1. Here, the drop of the fl ow rate performance over time is a measure of the membrane fouling that occurs.

6.1 Infl uence of different initial oil concentration Figure 7 shows the temporal course of the normalised permeate

fl ow rate for the ceramic ultrafi ltration hollow fi bre membrane used during treatment of different oily waste waters (TDPW with 1,000 ppm crude oil concentration and OMS with 35 ppm crude oil concentration), at equal process parameters (CFV = 2.5 m·s -1), without backwashing in the ongoing fi ltration process.

The permeate fl ow rate in the fi rst process period (I) has a clear drop over time. Here, the fouling potential depends on the features of the feed solution and the process parameters, as well as on the surface features of the CHFM used. In the second period (II), there follows a variable length period with a diminished fl ow rate drop. In the third period (III), the permeate fl ows are almost stationary.

The results in Figure 7 also show that the normalised fl ow rate performance during OMS fi ltration drops over time more than during TDPW fi ltration, with a several times higher oil concentra-tion in the feed stream. On account of the fouling that increasingly occurs during TDPW fi ltration, an approximately stationary state is reached after a shorter time than during OMS fi ltration.

The stronger drop occurring in this representation of the stand-ardised fl ow performance during fi ltration of OMS with the low oil concentration of 35 ppm is due to the absolute value of the permeate fl ow rate at the beginning of OMS fi ltration being clearly higher than during TDPW fi ltration.

6.2 Infl uence of CFV variationIn Fig. 8, the temporal course of the normalised permeate

fl ow rate is shown during OMS fi ltration for two different cross-fl ow velocities (1.6 m·s-1 and 2.5 m·s-1).

The ceramic hollow fi bre membrane has a permeate fl ow rate at the beginning of OMS fi ltration of 190 l/(m 2h) with a cross-fl ow velocity of 2.5 m·s -1 and achieves a stable operating mode with a fl ow rate of 100 l/(m2h) within 3.5 hours test duration. During this test, the permeate fl ow rate has a drop of 43% within the ranges (I) and (II) in comparison to the initial fl ow rate. In this case, a mean permeate fl ow rate of about 124 l/(m 2h) was achieved after a test duration of fi ve hours in total.

Figure 8 also clearly shows the infl uence of a reduced cross-fl ow velocity of 1.6 m·s-1 on the membrane performance. The operating ranges (II) and (III) are shifted compared with their position with

a cross-fl ow velocity of 2.5 m·s -1. After a fi ltration duration of 4.5 hours, within the ranges (I) and (II), a clear drop of the initial fl ow rate value by 76% can be detected. During the whole test duration of fi ve hours, the mean permeate fl ow rate amounted to 97 l/(m2h).

6.3 Infl uence of fi ltration time

To record the behaviour of the ceramic hollow fi bre membrane for a fi ltration period of several days, the long-term tests for TDPW fi ltration in total recycle mode, as well as in fed-batch Mode, were carried out with and without backwashing. The cross-fl ow velocity in these tests was 2.0 m·s -1, and the trans-membrane pressure was 0.5 bar.

Fig. 8: Normalised permeate fl ow rate depending on the test time during OMS fi ltration (oil concentration 35 ppm), cross-fl ow velocities 1.6 m·s-1 and 2.5 m·s-1.

Fig. 7: Normalised permeate fl ow rate depending on the test time during TDPW fi ltration (oil concentration 1,000 ppm) and OMS fi ltration (oil concentration 35 ppm), cross-fl ow velocity 2.5 m·s-1

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The temporal profi les of the permeate fl ow rate for two differ-ent, very high oil concentrations in the TDPW (TDPW1 with 5,200 ppm and TDPW2 with 2,100 ppm) over two trial runs, each with 30 hours duration, are shown in Fig. 9.

The occurring permeate fl ow rates are clearly dependent here on the feed properties (composition and concentration of oil and organic components in the feed). The results show that the continuous increase of membrane fouling reduces the membrane permeability.

A comparison of the different membrane fl ow rates during TDPW1 and TDPW2 fi ltration, with the same operating parame-ters and with a test duration of about 17 hours (1020 min), showed that a higher average permeate fl ow rate with lower initial oil con-centration occurs with TDPW2. During the TDPW2 fi ltration, with an initial oil concentration of 2,100 ppm, the fl ow rate dropped in the Fed-Batch Mode within two hours from the initial value of 140 l/(m2h) to 90 l/(m2h). After subsequent switching to Total Recycle Mode, a roughly stable state appeared with a still only slightly dropping membrane fl ow rate during the next 7 hours fi ltration time. After 17 hours fi ltration time, the fl ow rate reached a value of 80 l/(m2h) and remained approximately constant up to the end of the test. The fl ow rate performance was high enough for the whole test time so that no backwashing was necessary.

Fig. 9 also shows a representative example of the results that were achieved during the TDPW1 fi ltration, with an initial oil concentra-tion of 5,200 ppm. With this test, the permeability of the membrane decreased continuously during the fi rst four hours, from 143 /(m2h) to 85l/(m2h), because of membrane fouling and dropped during the next twelve hours to a value of about 45 l/(m 2h). Hence, during the TDPW1 fi ltration, with an initial oil concentration of 5,200 ppm, a purely mechanical backwashing was carried out after about 17 hours, in order to increase the fl ow rate again. In the process, permeate was pressed from the outside of the membrane into the inside of the hollow fi bre during a backwashing time of 10 s. In this case, the backwash pressure was 3.5 bar. From Fig. 9 it is

Fig. 9: Normalised permeate fl ow rate depending on the test time during TDPW fi ltration with different oil concentrations (TDPW1: 5,200 ppm, TDPW2: 2,100 ppm) for a test duration of 30 h, cross-fl ow velocity 2.0 m/s; with backwashing (for TDPW1) and without backwashing (for TDPW2).

Fig. 10: Normalised permeate fl ow rate depending on the test time during TDPW fi ltration (oil concentration 5,200 ppm) for a test duration of nine days, cross-fl ow velocity 2.0 m·s-1.

Tab. 4: Summary of the separation effi ciencies with regard to oil and TC with six different experiments.

TRM: Total-Recycle Mode; FBM: Fed-batch Mode; LT: Long-term (nine days); MT: Mean-term (18 hours); TDPW: Tank dewatering produced water; OMS: Oily model system

Fig. 11: Pure water fl ow rates depending on the trans-membrane pressure for new and for chemically purifi ed ceramic hollow fi bre membranes after the fi ltration of oil/water mixtures for a fi ltration time of eight hours (on the left) and 70 hours (on the right).

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clear that through this simple mechanical backwashing, 75% of the original fl ow rate was achieved.

Backwashing can always be applied for the optimisation of the operating condi-tions for cross-fl ow fi lters, in order to con-trol the membrane fouling and/or reduce it [26]. The effective monitoring of the membrane fouling with MF and with UF processes also depends on the kind and on the effectiveness of backwashing. Different experiments have shown that an increase of the backwash frequency and the duration of the backwashing signifi -cantly reduces the membrane fouling [27]. Hence, the effectiveness of a quick back-washing for the increase of the membrane fl ow rate during cross-fl ow ultrafi ltration of Produced Water was shown in this study.

6.4 Oil and TC separation performance

Besides the permeate fl ow rate, the retention property of the membrane com-pared with crude oil and organic compo-nents (TC) is another important parameter that characterises the membrane perfor-mance. The separation effi ciency of the examined ceramic hollow fi bre membrane during six different tests is summarised in Table 4. With every examination, a very high oil retention of more than 99.5% appeared, both with OMS and also with TDPW, regardless of the initial oil con-centration in the feed stream. During OMS fi ltration, with an initial TC concentration between 50 and 100 ppm, a TC retention of 90-95% was achieved. The separation effi ciencies for TC during TDPW fi ltration were between 61% and 94%.

6.5 Effectiveness of backwashing

For the determination of the effective-ness of the backwashing, a long-term test for TDPW ultrafi ltration was carried out with a CHFM with a high oil concentra-tion in the feed of 5,200 ppm. The test duration was nine days with the inclusion of backwash cycles. The standardised per-meate fl ow rate depending on the test time is shown in Fig. 10.

With this test, the permeate fl ow rate decreased from 149 l/(m 2h) initially to 107 l/(m2h) after one hour of operating time. In another 15 hours, the permeate fl ow rate decreased to 46 l/(m 2h). In order to limit the minimum permeate fl ow rate to not less than 30% of the initial fl ux, two different backwash strategies were applied with different backwash frequencies.

In the fi rst backwash mode (I), back-washing with permeate is done every 18 hours for 10 s with a trans-membrane pressure of 3.5 bar, and every three hours

in the second backwash mode (II) under the same conditions. Immediately after backwashing, the permeate fl ow rate in the fi rst backwash mode increased to a value of 118 l/(m 2h), which corresponds to an increase of the permeate fl ow rate to 80% of the initial value. In the second backwash mode, a stationary fl ow rate of about 40 l/(m2h) occurred by the end of the experiment. The results of these examina-tions show that quick backfl ushing may be temporarily effective in fi ltration of PW with high oil concentrations, but that this effectiveness decreases with an increase in operating time.

6.6 Membrane chemical cleaning

The chemical cleaning of membranes is an integral process step in the operation of MF and UF systems in wastewater treatment and it has a signifi cant infl uence on membrane performance. In this study, the effectiveness of the chemical cleaning steps carried out was examined in a series of fi ltration experiments. For this purpose, the pure water fl ow rate of the new mem-brane with distilled water was measured before each fi ltration test at room tempera-ture and at different trans-membrane pres-sures. Accordingly, chemical cleaning of the membrane was carried out after each fi ltration of a crude oil/water mixture and afterwards the pure water fl ow rate was determined once more. During chemical cleaning, the membrane was rinsed with the alkaline surfactant solution described above.

In Fig. 11, representative results are shown for the cleaning effectiveness of the CHFM used. The respective pure water fl ow rate is applied depending on the trans-membrane pressure. The left dia-gram shows the pure water fl ow rates of a new membrane in comparison to a chemically purifi ed membrane, which was used for eight hours for the fi ltration of oil/water mixtures. In the right diagram, the membrane was in operation for more than 70 hours under the same boundary conditions.

After the fi ltration test and the mem-brane cleaning, a pure water fl ow rate of 94% was achieved for the new membrane, which was in use for eight hours, with respect to the pure water fl ow rate of the new membrane (left diagram in Fig. 11). With an operating time of 70 hours, the pure water fl ow rate of the membrane after chemical cleaning was 83% of the pure water fl ow rate of the new membrane (right diagram in Fig. 11). This shows that permanent fouling increases during longer operating times. Overall, the results of all the experiments show that the effective-ness of chemical cleaning for the exam-

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• hydrophilic low fouling membrane material• double asymmetric hollow fiber membranes• effective retention of particles and bacteria • high and stable permeate performance• flexible flushing modes • easy pre-treatment• high productivity • compact installation• high flow rates at low pressure utilization

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ined ceramic hollow fi bre membranes was between 70% and 100%, depending on the operating conditions of the oil/water separation (test duration, CFV and oil concentration in the feed stream).

7. Conclusions

Produced water (PW) as wastewater, which reaches the surface during crude oil extraction and natural gas extraction, constitutes the biggest wastewater volume that occurs in the crude oil and natural gas industries and is diffi cult to treat on account of contaminations contained in it and the often high oil concentration.

In this study, tests were presented for the effi cient treatment of PW, on one hand with the use of oil/water mixtures from crude oil tank dewatering (TDPW) and on the other hand on the basis of oil/water model systems (OMS). In this, a new innovative ultrafi ltration membrane was used, consisting of a ceramic hollow fi bre with a pore diameter of d 90 = 40 nm. During experiments, the infl uence of the cross-fl ow velocity and the oil concen-tration in the feed stream was examined with regard to the permeate fl ow rate and the purifi cation performance with regard to oil and organic components (TC). The trans-membrane pressure for all fi ltration tests was 0.5 bar.

During treatment of TDPW and OMS, separation effi ciencies were achieved with regard to the oil of more than 99.5% and with regard to organic matters of between 61% and 94%. Here, the cross-fl ow veloc-ities were between 1.5 m·s -1 and 2.5 m·s -1. The ceramic hollow fi bre membrane used combines the advantages of an inorganic membrane material with the geometry of hollow fi bre membranes and hence allows the application of a compact fi lter system. This allows a reduction of the space requirements and the weight of the required fi lter facilities with onshore and offshore plants under realistic indus-trial operating conditions. The presented membrane design also leads to a low trans-membrane pressure during operation and to high permeate fl ow rates because, with the presented double-layered mem-brane, the pressure drop across the open-pore carrier structure can be neglected in comparison to the pressure drop across the active ultrafi ltration separation layer.

The examinations have shown that the ceramic hollow fi bre membrane used is also a robust solution for the treatment of wastewater that is heavily polluted with oil. Filtration with ceramic hollow fi bre membranes can be used as an effec-tive technology for the separation of oil

and organic matters from produced water. Because the examined ceramic hollow fi bre membrane can be exposed to high crude oil concentrations in the feed stream of several thousand ppm, the potential is shown for combining several process stages in the present methods for the puri-fi cation of Produced Water into one stage through the use of membrane fi ltration with ceramic hollow fi bre membranes.

In this study, the effectiveness was shown of a purely mechanical and a chem-ically supported cleaning of the membrane by fast backwash cycles. The increase of membrane permeability after cleaning was determined in TDPW fi ltration processes with high oil concentrations. The results have shown that with chemical cleaning, pure water fl ow rates of 70% to 100% of a new membrane are achieved.

To optimise the fi ltration of produced water, other examinations are in prepa-ration. Here, the objective is the devel-opment of the optimum combination of important process parameters.

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