Descripción de las propuestas

A Sterlitech CF042 laboratory scale, cross flow, membrane filtration unit was used. It was arranged with over 2.0 litres of feed storage. The feed stream was pumped from the feed storage tank to the feed inlet, figure 3.1. The feed inlet was located on the cell bottom. Flow continues through a manifold into the membrane cavity. Once in the cavity, the solution flows tangentially across the membrane surface. A portion of the solution permeates the membrane and flows through the permeate carrier, which is located on top of the cell. The permeate flows to the center of the cell body top, is collected in a manifold and then flows out the permeate outlet connection into permeate collection glassware, figure 3.2. The concentrate stream, which contains the material rejected by the membrane, continues sweeping over the membrane and collects in the manifold. The concentration then flows out the concentrate tube into the feed storage. The operational pressure was maintained at 45 bar in all runs as per Table 3.3 using a pressure regulating valve on the feed line which constant pressure feed to be maintained when the osmotic pressure increased as result of TDS increase in the recycled stream. The temperature of the water, after the main pump, was controlled to 15.5 degree by a cooling system shown in figure - 3.2.

A seawater membrane element (SW30) was sourced from DOW Chemical Company. The element was cut and accurately opened to disassemble the membrane itself from protective materials. Then the membrane sheets were cut from the permeate tube and then stored with sodium meta-bisulphate (0.1% concentration) and in plastic bags. The plastic bags were stored in the laboratory fridge at approximately 4˚C. During this time the plastic bags and sodium meta-bisulphate were replaced with the new bags each two- three weeks to keep the membrane samples in good condition. Any detected oxidised membrane samples were removed from the plastic bags and disposed. Prior to each RO

56 experiment, 4 x 8 cm of membrane sample was cut and installed in the base of the RO test cell. The new membrane was compressed for 1 hour with deionised water at the operational pressure of 45 bar prior to each RO experiment. To avoid any abrupt pressure or cross-flow variations during start-up, so as to prevent possible membrane damage, the feed flow was increased gradually over 30-60 second time frame. The specified cross-flow velocity was achieved gradually over 20-30 seconds.

Table 3.3 – RO operating parameters and maximum operation limits.

Parameter Operation Operation

limits

Effective Membrane Area 42 cm2 _-

Maximum feed turbidity (the feed was filtered with 0.45µm filter or similar UF filters)

< 1 NTU 1 NTU

Maximum pressure 45 bar 69 bar

Maximum operating temperature 15.5ºC 45ºC

pH range, continuous operation 3 - 11 2 - 11

Flow range 1 – 6 L/m2_{/h 3.6 m3/h}

Flux range 10 – 60 L/m2_{/h -}

The RO system, figure 3.1, includes a high pressure pump, frequency controller to vary the pump speed, cooler and feed tank storage. The system also included a check valve for pressure control allowing smooth operation and constant pressure over the period of the experiment. All experiments were conducted in a continuous flow pilot system with concentrate recycling and continuous removal of the permeate. A sharp decline in permeate flow rate denoted the solubility limits and scaling thresholds had been reached and this was confirmed by chemical analysis of the recycled concentrate stream. ROSA software was used to estimate the osmotic pressure increase across the experiments so that the required experimental pressure could be estimated. The required operating pressure was set at 45 bar as a result of these estimates. The experimental RO system was designed to enable unattended overnight operation with constant withdrawing of permeate and recycling of concentrate.

Analyses of flux decline results, caused by silica deposition and scaling processes, and based on examination of the decay in permeability at various silica concentrations

57 prevailing on the membrane surface. Reinforcing data for the on-set of silica fouling (polymerisation and precipitation) was gathered from dissolved silica concentrations (declined dissolved silica concentrations) in the recycling stream with data obtained from ICP spectroscopy and/or silica molybdenum method (Hatch) and later by scanning electron microscopy (SEM) membrane examination. The experimental silica concentration data was compared to the theoretical silica concentration in the concentrate recycling stream. The theoretical concentrations we calculated using the initial feed silica concentration and assuming 100% rejection of silica and no scaling. When the theoretical and experiment values were no longer consistent, with the experimental values being lower than the theoretical values, scaling was assumed to occur.

All experiments reported in this study were conducted at constant pressure and with subsequent flow rates in the range of 1.3 – 1.7 L/h (flow velocity ~ 2.5m/s) given by the Hydro-cell M-03 pump installed in the system. The feed flow to the RO unit was set by the constant pressure and resistance across the cell. The filtration times varied between 8 and 36 hours due to the large variation in osmotic pressure between experiments with low and high salinity that effected the operational flux, and because all experiments were run until the water recovery reached approximately >70%.

58 Figure 3.1 – RO system – experimental arrangement and main equipment used

59 During filtration significant scaling formation was sometimes observed from the operating conditions, when pressure dropped quickly and the experiment was terminated. The membrane was carefully removed from the RO unit for examination for any silica scale formation. SW30 high salt rejection commercial seawater RO membrane by DOW Film Tec was used throughout all the experiments, and characteristics of the 4” membrane that was dissected are listed in Table 3.4.

Table 3.4 – Dow FilmTec SW30HRLE membrane characteristics DOW Filmtec SW30HRLE Membrane permeability (L/m2_/hr/bar)

Stab. salt rejection for standard

feedwater conditions

(%)

Min salt rejection for standard feedwater conditions (%) Max feed flow rate (m3_/h) Seawater 1.16 – 1.32 99.7 99.4 2.5

All total dissolved solids (TDS), electrical conductivity (EC), temperature and pH values were measured with a Hach model H2500 combined conductivity-pH- temperature meter. Conversion of conductivity to TDS was undertaken using the internally calibrated conversion within the portable Hach meter based on standardised NaCl solutions. For quality control TDS measurement for selected samples was analysed using standard method described in section 3.4.

The total concentrations of cations (Na+_{, Mg}2+_{, Ca}2+_{, Sr}2+_{, Ba}2+_{, Al}3+_{) and silica were}

determined by inductively coupled plasma (ICP) spectroscopy (OES). Reactive silica concentrations were determined by the silico-molybdate method using a Hach spectrophotometer DR 2000/2010 as described in details in 3.4.3 of this chapter.

To perform total and dissolved silica analysis, samples were taken from the recycling stream storage (RO feed storage) at each 2.0 - 2.5% of permeate recovery, depending on the experimental conditions, and in particular the flux.

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Figure 3.2 – RO experimental apparatus at the Victoria University, Melbourne (Figure 3.1)

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