Leiknes_subbed.qxp 30/3/09 09:21 Page 108
ater
Cleaning Efficiency of Biodegradable Products on
Fouled Ceramic Membranes by Analogue Produced Water
a report by
Sumihar HD Silalahi and TorOve Leiknes
Department of Hydraulic and Environmental Engineering, Norwegian University of Science and Technology
HSE – Produced W
In 2001, the Norwegian Ministry of Petroleum and Energy (OED) and 0.5µm fouled by analogue PW were tested. Two alkaline (Ultrasil
initiated the Oil and Gas in the 21st Century (OG21) programme in 115 and Derquim+) and two acidic (Ultrasil 73 and SurfactronCD50)
order to bring the oil and gas industry together under a common, commercial biodegradable products were evaluated. Cleaning efficiency
permanent national strategy to “generate new technology and after membrane cleaning was determined by assessing flux recovery
knowledge to ensure the profitable, environment-friendly development compared with the permeability of non-fouled membranes.
of the resources on the Norwegian Continental Shelf (NCS), and to
enhance the industry’s international competitiveness by producing Experimental Section
attractive new technology products and system solutions”. One of
the technology targets of OG21 is to develop and qualify technology Apparatus
and equipment for handling produced water (PW), with specific A bench-scale filtration unit with three parallel tubular α-Al
2
O
3
ceramic
objectives related to reducing environmental impact and developing membranes (provided by ECO-CERAMICS) was used, as shown in
technology for water treatment and water quality monitoring. Current Figure 1. Each line contained a membrane with a nominal pore size of
regulations for installations on the NCS have set a limit of 30mg/l of oil 0.1, 0.2 or 0.5µm. The membrane properties were as follows: 0.8cm
in water when PW is discharged to the sea.
1
inner diameter, 1.1cm outer diameter and 34cm length, giving an
effective membrane area of 85cm
2
for each tube. The experimental
Membrane technology is an optional process that can remove colloidal set-up enabled direct comparison of cleaning efficiency as a function
particles and small-droplet-size oil emulsions from water, and therefore of membrane pore size. A peristaltic pump was connected to
has the potential to meet future discharge standards and regulations. the permeate stream to give a constant permeate flow. Both retentate
Studies have reported that treatment of oily wastewater (0.1–10% oil and permeate were circulated back to the feed tank. A data acquisition
content) by microfiltration (MF) or ultrafiltration (UF) produces water with unit was used for continuous measurement of the transmembrane
fewer than 10–100 parts per million (ppm) of oil.
2,3
Both polymeric and pressure (TMP), temperature and pH.
inorganic membranes have been applied; however, in recent years there
has been growing interest in ceramic membranes for this application. The Preparation of Analogue Produced Water Feed
advantages of ceramic membranes in treating oily wastewater are The analogue feed was made using a crude oil with the characteristics
commonly stated as less sensitivity to both polar and chlorinated solvents, shown in Table 1 and diluted with salt water. The dispersed oil was
as well as to high oil fractions, compared with organic membranes. prepared by mixing oil and surfactant in an Ultraturrax homogeniser at
a mixing rate of 2,000rpm for four minutes. A non-ionic surfactant,
A major drawback in all membrane processes is the inherent fouling SERDOX, at a concentration of 200ppm was chosen for this study. The
phenomenon found in all systems.
2,3
During operation, membrane fouling feed was continuously stirred and thermoregulated in the feed storage
occurs, causing a progressive decrease of flux and potentially a loss of tank (see Figure 1). The pH was kept at 4 and adjusted using HCl 1N at
separation efficiency. Inevitably, fouling reaches a certain point where a temperature of 25±1ºC. The oil concentration was set at 350ppm
extended cleaning is necessary to regain the original permeability of the with a salinity of 3.5%, and solid particles of kaolin were added at
membrane. Different cleaning protocols and strategies can be employed a concentration of 50ppm. A scaling and corrosion inhibitor at a
depending on the separation process and the type of membrane used. concentration of 10ppm was added to the solution to make the final
Chemically enhanced cleaning of fouled membranes will obviously composition of the PW analogue. The resulting oil droplet size
produce a waste stream that needs to be managed. Regulations on the distribution was measured by optical sensing and visually analysed
NCS include a strict code for types and quantities of chemicals that are using a Jorin virtually imaged phased array (ViPA). The oil droplet
used offshore, which also applies to those used for membrane cleaning. characteristics were modelled to mimic PW effluents from a
The use of biodegradable cleaning agents is an interesting option that hydrocyclone with oil drops <15µm.
allows compliance with regulations and avoids the formation of secondary
pollutant streams during off-shore operations. Optimal cleaning of a Simulation Membrane Fouling
membrane will depend on the cleaning agent used and how it is applied. The membranes were fouled with the analogue PW for two hours.
Choosing an agent and protocol will be a function of the membrane Experiments were carried out with a crossflow velocity of 1m/s. The
material, foulant characteristics and cleaning conditions, i.e. constant fluxes for 0.1, 0.2 and 0.5µm membranes were 80, 140 and
concentration, temperature, pH, pressure, flow and time.
3–6
175 l/m
2
/hour (LMH), respectively. Different fluxes were applied to obtain
comparable fouling rates (i.e. TMP development) for each membrane
In our study, different types of cleaning agent and cleaning protocol/ pore size. The TMP for each membrane pore size was monitored during
strategy were evaluated for efficient flux recovery of α-Al
2
O
3
tubular the filtration to observe the progression of the fouling process to confirm
ceramic membranes. MF membranes with nominal pore sizes of 0.1, 0.2 that a comparable and reproducible fouling was generated.
108
© TOUCH BRIEFINGS 2009
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