Silica Fouling in Direct Contact Membrane Distillation - American

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Silica fouling in Direct Contact Membrane Distillation Jack L. Gilron, Yitzhak Ladizansky, and Eli Korin Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie400265b • Publication Date (Web): 06 May 2013 Downloaded from http://pubs.acs.org on May 13, 2013

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Industrial & Engineering Chemistry Research

Silica fouling in Direct Contact Membrane Distillation Jack Gilron*1, Yitzhak Ladizansky2, and Eli Korin2 1 Zuckerberg Institute for Water Research in Blaustein Institutes for Desert Research 2 Dept. of Chemical Engineering Ben-Gurion University of the Negev, Beer Sheva, Israel ABSTRACT Recent studies of membrane distillation have shown varying effects on flux when sparingly soluble salts of calcium precipitate.

Another common sparingly soluble

mineral often found in brackish water feeds is silica. Because of its normal solubility silica is stable to higher concentrations at higher temperature than at room temperature and this should allow further water recovery and volume reduction of brackish water RO brines by membrane distillation. We report here on a series of experiments conducted with hollow fiber and flat sheet DCMD modules with silica solutions with saturation indices in the range 1.5 to 2.2 at temperatures of 60-75 °C.

We found that unlike previous work on calcium salt

scalants in cross-flow hollow fiber modules, in the same type of modules silica fouling consistently caused significant flux decline after 2-7 hours effective induction time, with declines of up to 70%.

In flat sheet modules, the induction time for 400

mg/L silica solutions at 75 °C was 4-7 h independent of initial flux ranging from 1550 kg/m2-h, implying very different supersaturations at the membrane surface. SEM studies of silica fouled membranes support a proposed model that the fouling occurs by deposition of colloidal silica in the pore mouths leading to partial wetting followed by aggravated temperature and concentration polarization leading to a cascade of precipitation that then blocks the pore to vapor transport.

1

*Corresponding Author: [email protected]

1 ACS Paragon Plus Environment


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1. Introduction:

Membrane distillation (MD) is a unit operation in which water vapor is driven across an interface stabilized by a hydrophobic porous membrane which transmits vapor but not liquid. The driving force is provided by the vapor pressure gradient between the feed and distillate side which is maintained by a temperature gradient (direct contact MD – DCMD, Air Gap MD - AGMD) or by a directly imposed partial pressure gradient (sweeping gas MD -SGMD, vacuum MD - VMD). MD has been closely examined as a process to make use of waste heat in treating brines arising from other desalination processes

1-3

and brines produced in the extraction of liquid petroleum

and natural gas 4. It has the advantage over traditional thermal methods of having a smaller footprint and being resistant to the corrosion often encountered in traditional thermal evaporators. One of the issues that does arise, however, is the extent to which MD will be less or more resistant to the scaling from the sparingly soluble salts often found in highly concentrated brines that are supersaturated under the conditions of operation. For membrane in hollow fiber configuration, some studies 5, 6 have shown that DCMD modules operated in cross flow mode, are surprisingly resilient to scaling from calcium carbonate and calcium sulfate.

Other studies seem to indicate that

calcium carbonate can cause flux decline under certain circumstances 7-9 where other studies on flat sheet membrane show gypsum to be problematic 10. One of the most ubiquitous minerals in brines from desalination and petroleum production is silica. It differs from calcium carbonate in displaying normal solubility meaning that more of it can be retained at higher temperatures as opposed to calcium carbonate. However, temperature polarization at the brine-side membrane surface in


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MD means that the highest silica supersaturation will be at the membrane surface potentially encouraging surface deposition of silica. The present study looked at the performance of direct contact MD in hollow fiber and flat sheet configurations on BWRO concentrates and synthetic solutions, in an effort to reveal the phenomenology and mechanisms of silica fouling in such systems. Based on the phenomena of silica fouling observed in these systems, the authors present a mechanistic model that qualitatively accounts for these observations. 2. Material and Methods: The MD membranes used in these experiments were from both hollow fiber and flat sheet geometry. The hollow fiber cross-flow modules had membranes from fluorosilicone coated polypropylene and had brine flowing on the shell side perpendicular to the fiber axis while the cold distillate stream was fed in the lumen (see Table 1 herein, and module description in 5). In addition a flat sheet module was employed to test PVDF membranes with 0.8 µm nominal pore size supplied by GVS, Italy. The geometry of the flat cell was brine and distillate flow channels 22 mm wide by 116 mm long by 1 mm high. The flow channels were formed by silicon rubber gaskets (see Figure 1), and the cell was operated in countercurrent mode. Brine flow recycle rates were varied between 30 and 55 L/h in both modules. In the case of flat sheet modules the 50 L/min flow rate allowed a 0.63 m/s linear velocity parallel to the membrane corresponding to a Reynold’s number of 1663 at the temperature at which the flat sheet was run (75 °C).

Contact angle measurements of the flat sheet membrane were carried out with a SpectraPhysics OCA goniometer with automatic dispensing of water drops. The



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measurements were carried out in sessile drop mode in triplicate using the internal goniometer software to analyze images and extract the contact angle.

1/4 in. ports for brine and distillate stream 1 mm gasket defining channel height

Figure 1: Flat sheet test cell showing geometry of flow channel (left) and cross-section of the flow cell (right)

Flat sheet membrane samples were examined with a high-resolution scanning electric microscope (JEOL JSM 7400 FEM) with EDX attachment for element analysis of imaged surfaces. All samples were coated with 5 nm Au before analysis.

Cross

sections of the membranes were prepared originally by freeze fracture under liquid nitrogen, but this was not effective with the nonwoven backing of the PVDF membrane. Subsequent cross section was prepared by embedding the membrane in EPON epoxy resin and micro-toming to get membrane cross-sections. The experimental flow system is displayed in Figure 2. The brine flow loop was composed of two reservoirs (2 and 3) that could be readily switched between DI and synthetic silica solutions via three-way valves (6). Volume of brine used in the experiments was 5 L. These solutions were maintained at their operating temperature with hotplates (5) that were modulated by temperature controllers (4) (Eurotherm). A brine pump (P1-03) (Touton Pumps) was used to circulate the brine through the MD 4 ACS Paragon Plus Environment

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module (1). The distillate loop contained a recirculation pump (P1-02) which circulated the distillate from a reservoir (8) through the MD membrane module (1) and back with the reservoir having an overflow of excess permeate to a permeate collection vessel (9) on a balance (Gibbertini AR4000AR, 4 kg capacity, 0.02 g resolution) connected to a computer (12) to register the change in weight with time. The distillate reservoir was thermostatted by an immersed heat exchange coil fed from a chiller (7). Inlet and outlet temperatures on the brine and distillate streams were measured with thermocouples whose output was recorded on a multichannel datalogger (11) (Almemo 2390-5), that downloaded to the computer (12).

Figure 2: (1) AMT cross-flow test cell., (2) Brine reservoir (3) Brine volume makeup solution (DI water) also used for generating baseline experiment (4) Temperature controller (5) Hotplates, (6) three way valve for switching between silica solution and DI as feed (7) Chiller (8) distillate reservoir with overflow (9) distillate collection vessel (10) balance connected to computer (12), TI – Process stream Temperature indicators connected to temperature data logger (11). FIC – Flow indicators on process streams.



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Preliminary DCMD experiments using the cross-flow hollow fiber module were carried out on a concentrate brine from a batch RO concentration experiment on brackish water from the Mashabe Sadeh well field in which the well water was concentrated six-fold with a 4-inch spiral BW30 element (Dow, USA). The initial concentrations of the brine from this well water were Ca2+=1240 mg/L, SO42-=2340 mg/L, HCO3--=128 mg/L, SiO2=116 mg/L , P=1.4 mg/L. Synthetic silica solutions were prepared by dissolving water glass (sodium silicate, Sigma-Aldrich, USA) and adjusting the pH with HCl to neutral pH (6.5 – 7.5). Magnesium and calcium chloride were also added (Frutarom, Israel) since calcium and magnesium were found in the RO concentrate and also because magnesium and calcium are known to catalyze the polymerization of silica11. The saturation index of the synthetic silica solutions at the different operating temperatures was calculated using OLI thermodynamic software (OLI Systems, Morristown, N.J.). Silica concentrations were determined by the ammonium molybdate method. Calcium, magnesium and sodium were determined by ICP and sulfate and chloride where determined by turbidimety and argentometry respectively. Solids phases were identified by XRD. After each silica experiment the module was rinsed with distilled water and 1% sodium carbonate solution (pH 11.6) to dissolve any deposited silica.


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3. RESULTS 3.1 DCMD on BWRO concentrate Figure 3a shows the specific flux versus time curve for operation of the hollow fiber AMT module on BWRO concentrate. The nominal specific water flux was calculated by dividing the water vapor flux by the vapor pressure difference between bulk brine and distillate streams based on their temperatures using the Antoine equation to calculate vapor pressures. It can be seen that there is a moderate drop in specific flux on reaching volume concentration factor (VCF) of 1.6 on the BWRO brine, but that at constant VCF the flux is stable. The flux begins to decline again when the concentration run is resumed and at VCF=1.8 there is an even bigger drop. This turned out to be due to blocking of an inline filter resulting in low cross-flow velocities and enhanced concentration polarization. On analyzing the starting RO concentrate and the final DCMD brine (Figure 3b), it was seen that calcium, sulfate, phosphate, inorganic carbonate and silica all dropped significantly. Phosphate was present due to remaining antiscalant in the RO concentrate. It should also be noted that calcium only dropped from in the DCMD volume concentration factor (VCF) 1.6 to VCF 1.8 even though the total inorganic carbon (TIC) dropped significantly between VCF 1 and VCF 1.6. This can be explained by the transport of CO2 across the membrane during DCMD 6. Since calcium carbonate and calcium sulfate had not been seen to cause significant flux decline in previous work, further experiments were focused on the effects of sparingly soluble system of silica alone. 3.2 DCMD on synthetic silica solutions A series of experiments with silica in the presence of magnesium and calcium ion were run at inlet brine temperatures between 60 and 75 °C and silica concentrations



ranging from 200 to 400 mg/L as SiO2 was run on the hollow fiber cross-flow modules. 50

78

VCF=1.8

45 40

74

70

C

start conc. run

30 25 20

66

VCF=1.6

Tb,in

2

km, kg/m -h-bar

35

15 10

62

5 0

58 0

5

10

15

20

25

30

Time, h

A

Fraction of species remaining in solution

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1.2

5 h, VCF=1 21 h, VCF=1.62 27 h, VCF=1.8

1

0.8

0.6

0.4

0.2

0 2+ Ca2+ Ca

TIC

-

FF-

2SO4-2 SO4

P

Ba2+ Ba 2+

SiO2 SiO 2

B Figure 3: DCMD of a BWRO concentrate (VCF=6) of a brackish well water from Mashabe Sade A) Nominal transmembrane water vapor permeability and bulk brine temperature B) Fraction of brine species remaining as a function of DCMD volume concentration factor (VCF), TIC refers to total inorganic carbon.

The ratio of silica (as SiO2) to calcium to magnesium concentrations (all expressed in mg/L) in these experiments was 400:300:125 respectively. Figure 4 provides a plot


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of the saturation index (SI) of silica as a function of concentration and temperature as calculated by the OLI software with the conditions of the experiments conducted plotted on the graph. Reynold’s numbers for these runs ranged from 35-58 as calculated based on the external fiber diameter as in He et al.5 The first two experiments were conducted as concentration runs whereas experiments 3-6 were run at constant composition by continuously adding makeup water to replace the amount of distillate removed. Induction times were determined by when a decrease in flux occurred. This is illustrated in Figure 5. Examining Figure 5, one also can conclude that temperature polarization played a role in reducing the induction time by reducing surface temperatures and thereby increasing the silica SI at the membrane surface. While both experiments 5 and 6 had identical brine inlet temperatures and bulk silica concentrations, the experiment with the lowest brine recycle rate (and highest temperature polarization) had the shortest induction time.



SiO2, mg/L 0

120

240

360

480

600

4

3.5 55

SI65 C = 0.2722 [SiO2]

3

720 4

45

SI55 C = 0.3162 [SiO2]

3.5

Expt 3

SI75 C = 0.2364 [SiO2]

3

65

2.5

2.5

75 °C

SI

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2

Expt 2

2

Expt 4

1.5

1.5 Expts. 5,6

1

1 Expt 1

0.5

0.5 0

0 0

2

4

6 [SiO2], mM

8

10

12

Figure 4: Plot of bulk supersaturation index as function of temperature and silica content of the brine solution, along with placement of the silica scaling experiments in the cross-flow hollow fiber module on the same plot. The equations in the upper left-hand corner of the graph provide the linear correlation between SI at the temperature of the subscript and the silica concentration in mM. SI values for experiments correspond to brine inlet conditions.


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A

B

SiO2

τ = 4.1-5.67hr

Figure 5: Silica scaling experiment in AMT cross-flow HF module with 400 mg/L silica, 300 mg/L calcium ion and 125 mg/L magnesium ion. A) run with 30 L/h brine recycle B) run with 55 L/h brine recycle.

The results of all the experiments run on synthetic silica solutions are shown in table 2. In none of these experiments was a significant increase in permeate conductivity 11 ACS Paragon Plus Environment


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detected. As can be seen, at 200 mg/L of silica with bulk brine SI running between 0.87 and 1.5 no induction time is seen. On the other hand, in all the experiments where the SI exceeded 1.5, significant flux decline occurred (0.4

Silica Fouling in Direct Contact Membrane Distillation - American

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