AJAE 1(2):54-65(2010)	ISSN:1836-9448

Flux behavioir and energy consumption of Ultrafiltration (UF) Process of milk

*1 Bahnasawy A.H and 2Shenana M.E.

1Agricultural Engineering Department, Moshtohor, Faculty of Agriculture, Benha University, PO Box 13736, Egypt

2Agricultural Engineering Department, Moshtohor, Faculty of Agriculture, Benha University, PO Box 13736, Egypt

*Corresponding author: bahnasawyadel@hotmail.com

Abstract

Flux decline and recovery of UF process used in concentrating milk at different operational pressures were studied. Filter medium resistance during filtration and cleaning was estimated from the experimental data. Energy consumption during filtration and cleaning processes were estimated. Regarding the flux behavior, retentate and permeate fluxes declined with time at different operational pressures during filtration, on contrary, flux during cleaning increased with time at all pressures studied. Filter medium resistance increased linearly with time of filtration at different operational pressures during concentration. It increased during the first 40 min of cleaning process and decreased with time to reach the minimum resistance at the full flux recovery. The total energy consumed for filtration ranged from 50.08 to 62.54 kJ/L of retentate, while it ranged from 18.18 to 21.65 kJ/L of permeate. The energy consumed for cleaning ranged from 87.08 to 107 kJ at different operational pressures.

Keywords:  Ultrafiltration, Permeate, Retentate, Flux decline, Fouling, Flux recovery.

Abbreviations

Am_Membrane area (m2); D_Effective diffusivity (m2/s); ks_Mass transfer coefficient (m/s); t_Filtration time (s) V_Filtration volume (m3); ν_Cross-flow velocity (m/s); ∆P_Transmembrane pressure (Pa); r_The molecular radius (m); Rm_Membrane medium resistance (m-1); μ_Filtrate viscosity (Pa.s); ρ_Density (kg/m3); α_Specific cake resistance (m-1); σ_Boundary layer thickness (m); dh_Hydraulic radius (m); T_Temperature (K); J_The permeate flux (L/min.m2); P1_Pressure at the inlet of the module (Pa); P2_Pressure at module outlet (Pa); Qcirc_Average circulation flow rate (m3 s−1); η_Pumping efficiency (%).

Introduction

Membrane technology constitutes an efficient and ecological process for the extraction (concentration, purification and fractionation) of valuable molecules from wastes or by-products in agro -food industry. Dairy applications probably account for the largest share of studies, particularly for the understanding under different processing conditions (i.e. pH, tempe-rature, ionic strength, etc.) of the interactions between individual components of milk or between these components and membranes (Gourley et al., 1995, and Daufin et al., 1998).

Membrane separation process for liquid systems is conventionally classified in terms of the size ranges of materials separated (microfiltration, 10 μm–0.1 μm; ultrafiltration, 0.1 μm–5 nm; nanofiltration, 5 nm–0.5 nm; reverse osmosis 0.5 nm). The ultra-filtration process has become particularly important for concentrating proteinaceous solutions. Ultrafiltr-ation performance is limited, however, due to the build up of the solutes at the membrane surface. This is the so-called concentration polarization effect (Bowen and Williams, 2007).

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The major problem in membrane separation process is decline in flux over time of operation. This flux decline is attributed to the fouling of membrane. Membrane fouling is affected by three major factors, namely, the membrane material properties, the feed characteristics and the operating parameters (Platt and. Nyström, 2007, Matzinos and Álvarez, 2002, Zhang, and Liu, 2003, Kazemimoghadam and Mohammadi, 2007, Juang and Lin, 2004, Rai et al., 2007).

Tong et al. (1988) studied the characteristics of proteinaceous foultants and flux decline during the early stages of whole milk ultrafiltraion, and they found that in early stages of milk UF, adsorption fouling is probably the primary mechanism of flux decline.
A pilot study was undertaken to evaluate the UF process in dead-end filtration compared to cross-flow filtration mode in terms of water loss, energy consumption (flux rate achievable). The study performed on surface water, i.e., high variable water quality, has shown that the membrane specific flux in cross-flow filtration seems to be more stable at high flux, i.e., 100 L/h.m2. However, results have shown that in dead-end filtration, a gain of about 0.3 kWh/m3 is realized on energetic balance. In this case, the energy consumption is around 0.2 kWh/m3 and represents only a small percentage of the operation and maintenance costs. However, maximum flux achievable needs to be tested for both operating modes in order to evaluate the capital cost in terms of membrane surface requirement (Glucina et al., 1998).

An energy analysis has demonstrated that the major energy consumption takes place in the thermal process and not in the mechanical pumping of the fluid. In addition, higher increments in permeate volume can be achieved by increasing transmembrane pressure, not temperature. The mathematical analysis evaluated of optimum values of the engineering parameters necessary to design and operate skim milk ultrafiltration units (Rinaldoni, et al., 2009).
Performances and energy consumption of pilot-scale spiral wound and hollow fiber modules were compared (Cheryan and Kuo, 1983). Exponential flux decay behavior was typical for both units. Flux was affected by pressure in the first 60 min of operation; for the spiral wound unit, flow rate had a beneficial effect only at pressures above 135 kPa. After 3 to 5 h of operation, however, flux became independent of pressure for both modules; flow rate affected the flux of the hollow fiber module but not of the spiral wound module. The average flux of the hollow fiber unit during concentration of whey was double that of the spiral unit, but energy required for recirculation within the hollow fiber unit was higher. Compared to tubular and plate-and-frame units, the spiral and

hollow fiber units consumed 10 to 100 times less energy per unit volume of permeate removed. Chabeaud et al. (2009) studied the performances of ultrafiltration membranes for fractionating a fish protein hydrolysate. They concluded that the cleaning efficiency of the membrane regeneration, at 10 bars, two cleaning procedures are necessary to recover 90% of the initial water permeation flux. The pressure increase makes the cleaning more difficult (only 76% of flux is recovered). Specific energy requirements are practically constant and equal to 0.9 MJ/kg of peptide recovered until a volume reduction factor (VRF) of 2.3 but they increase then significantly to reach 1.20 MJ/kg at VRF = 4.8.

In the present study, milk concentration using UF under different operational pressures as well as the flux behavior during filtration and cleaning processes were investigated. Also, Filter medium resistance and energy consumption during filtration and cleaning processes at different pressures were estimated.

Materials and methods

Milk supply

Fresh Cow's and Buffalo's milks (1:1) were obtained from the herds of the Faculty of Agriculture, Moshtohor, Benha University, Egypt. The total solids and fat contents of the mixed milk were 13.0 and 5.3%, respectively.

UF equipment description

The UF unit consists of 200L stainless steel storage tank, two screw pumps (feeding and circulation pumps), 2 pressure gages, Carbosep M2 membrane (91.Sc.37.5206.1.08 model, 01703 Miribel, Codex, France) it has the following specifications: 37 tube/module, total area of 0.84 m2, membrane length is 120 cm, has an external and internal diameters of 1.0 and 0.6 cm, respectively and empty weight is 8.8 kg, The details of schematic diagram of the experimental set up are presented in fig. 1. Membranes were cleaned and sanitized at different pressures and the cleaning efficiency was evaluated and the procedure of Bird and Bartlett, (2002) and Madaeni et al. (2001) was followed.

Determination of fouling

For fouling and performance studies, three batches, 160L each were used. A total recycled mode was used, retentate was returned to the feed tank and permeate was removed continuously to reach the desired milk concentration. Solid content was measu-red in the retentate and the process was stopped when

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UF Modules

Pressure

gauge

Temperature

gauge

Bypas

Permeate

Flow meter

Heat exchanger

Cooling

water

	Pressure		
CP	gauge		

FP

Retentate

Feed

tank

Fig 1. Schematic of the experimental setup.

the desired concentration was achieved (32 to 33%). Retentate and permeate flow rates were measured directly with a flow-meter (fixed in the UF equipment) placed in the flow stream. Readings were taken every 15 min. Solid not fat (SNF) in retentate were measured using a hand-held Brix refractometer (Agato, Brix 32, made in Japan). The operational pressures were selected as 3, 4, 5 and 6 bars. Pressure drop was kept on 2 bars for all processes under different conditions and time of process. The averages of the three batches were taken.

Filter Medium Resistance Determination

In UF of milk concentration, permeate flux declines with time due to membrane fouling, which very com-

plicated phenomenon is caused by many chemical and physical properties interactions. It is believed that membrane fouling is a dynamic process starting with pore blocking followed by continuous cake formation on the membrane surface. Pore blocking is a fast process observed at the beginning of UF for a clean membrane due to its high initial permeates flux. As UF process goes on, accumulation and deposition of particles on the membrane surface begin and gel layer is formed. The layer resistance becomes dominant after the initial stage. Ignoring the pore blocking resistance, the whole resistance can be considered as membrane resistance, deposited solute resistance and boundary layer resistance (Marshall et al., 1993). Therefore, for UF of milk concentration

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