Robust membranes for concentration of black liquor

Sankar Nair1, Meisha Shofner2, Scott Sinquefield3

1School of Chemical & Biomolecular Engineering2School of Materials Science and Engineering

3Institute of Paper Science and TechnologyGeorgia Institute of Technology, Atlanta, GA

Motivation

• Currently, black liquor concentration (from 15% to 80% solids) is performed in large steam-heated multiple-effect evaporators

• Membrane separation has the potential to efficiently remove half of this water prior to evaporation

• With full deployment, the forest products/pulp and paper industry would save approximately 0.5 Quad of steam energy annually

• At a conservative cost of $2/M Btu, the value translates to $1 Billion annually

• Key challenge is that - unlike conventional nanofiltration (NF) and reverse-osmosis (RO) membranes for water purification/desalination applications - the membranes required here must be capable of withstanding:

- High pH (>12)- Temperature (80-90°C)- Different fouling species in black liquor

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Membrane Process Options

Two-stage: High-MW organic species and colloids are first rejected by a nanofiltration (NF) membrane, and small molecules and ions are then rejected by a reverse osmosis (RO) membrane

Single-stage: An RO membrane simply removes the desired amount of water to create a concentrated BL stream of about 30-35 wt% solids for the 3rd evaporator stage 3

Robust Membranes for Black Liquor Concentration

Two attractive classes of materials for BLC applications

(1) Zeolites: nanoporous crystalline aluminosilicates

- High water flux (due to pores of 0.5-0.8 nm size)

- Established membrane fabrication process on ceramic and metallic surfaces

- Current commercial application in alcohol dehydration from bioreactors

- Also emerging commercial application in desalination by RO

- Liquid phase separation: more tolerant to defects than gas separations

? Good resistance to fouling by organics at higher pH (negatively charged framework)

? Good resistance to inorganic salts and alkaline solutions

? Current cost: $100-500/m24

Nanoporous Zeolite Membranes in Commercial Applications

• Zeolite membranes are in commercial production for certain applications

• Primarily for removal of water from organics (avoiding azeotropic distillation)

• Use highly hydrophilic zeolite LTA

• Fluxes of water are high (> 1 kg/m2/h) and high selectivity (can be in the 1000s)

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Ceramic-Supported Zeolitic Membranes

• Ceramic (e.g., alumina) tubes

• Can be arranged into a shell-and-tube module

• Ceramic hollow fibers (~1 mm)• Produced by spinning• Can give denser packing

and more surface areaper unit volume

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Robust Membranes for Black Liquor Concentration

Two attractive classes of materials for BLC applications

(2) Carbon Molecular Sieve (CMS) Membranes

- Produced by controlled pyrolysis of polymer precursors (PAN, polyimides, others)

- High water selectivity (small pores < 0.5 nm): could be very useful in RO step

- High resistance to alkaline solutions and inorganic salts

? Membrane fabrication processes on ceramic and metallic surfaces, and in the form of hollow fibers

? CMS with larger, tunable pore sizes could be very useful in NF step

- Estimated cost: $50-100/m2

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Deliverables (24 months)

• Prototype CMS/Zeolite membranes with the correct range of pore size and microstructure thatsuccessfully perform black liquor concentration

• Optimized membranes with appropriate pore structure and surface microstructure to reducefouling/plugging by reactions with the retentate species

• Data set of extended measurements with both model streams and real black liquor streams todetermine the long-term resistance of the membranes to the caustic liquor

• Our target performance deliverables are: (1) greater than 95% rejection of salts, and(2) greater than 99% rejection of larger organic solutes

• Preliminary process calculations, with estimates of the production rates of water, salt solutions,and concentrated black liquor as well as energy consumption

• An economic analysis, including NPV and IRR, will be started early in the project and updatedthroughout the project

•If successful, the development would be at TRL-5 (i.e. ready to build a two-stage bench scaleunit under a hypothetical Phase II)

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Value of Deliverables

• With full market adoption, the industry could save up to 0.5 Quads of steam energyannually.

• Offset somewhat by the need for high pressure pumps

• If condensing steam turbines are used (at 15% eff), the excess steam couldgenerate 16MW.

• A further benefit is the production of clean water

• Evaporator scale formation is greatly reduced.

• Heat transfer is improved and washing cycles become easier.

•If commercial anti-foulant additives are used, their need/costs will be reduced.

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Highlights of Approach

• CMS membranes will be prepared by first forming polymeric (e.g., polyimide) membranes (either free-standing or on ceramic tubular surfaces), followed by controlled pyrolysis

• Zeolite FAU and LTA membranes will be prepared by hydrothermal growth processesfrom silica and alumina precursors in aqueous solutions

• Zeolite-CMS membranes will also be examined (overgrowth of zeolite layer on CMS or vice versa) as a potential way to further improve longevity characteristics

• A suite of characterization techniques will be employed to measure the structural, permeation, and mechanical properties: SEM, XRD, RO/NF, TGA-DSC, AFM, DMA

• ‘Post-mortem’ characterization of membranes after exposure to BL for extendedperiods

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Current Status

Task Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q8Validation of pyrolysis system with trial CMS membrane synthesis and comparison with literatureValidation of liquid-phase membrane separation (RO/NF) system with simulated feeds and comparison with literature

Synthesis of prototype CMS and ZCMS membranes and BL concentration experiments

Deliverable 1and YR1 report (end Q4) Detailed permeation measurements to assess effects of pH, temperature, and simulated feed compositionsOptimization of CMS/ZCMS membrane pore structure and surface characteristicsPermeation measurements with real BL feeds

Deliverables 2 and 3 (end Q7)Process mass and energy balance calculations using Kraft flowsheet and membrane properties obtained in deliverables 2 and 3

Economics, IIR estimateDeliverable 4 and 5, and Final report (end Q8)

• Received informal notice of funding in Feb 2014, project number and budget expected Mar 2014

• Currently in ‘Q0’ mode: conducting postdoc recruiting, expected to be complete Apr 2014

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Nair Group Research Overview - 2014 http://nair.chbe.gatech.eduScience and Engineering of Nanoporous Materials and Membranes

Inorganic and Hybrid Membranes

Angew Chem (2011, 2012), JACS (2009), JMS (2009), JPCC (2007,2008), Angew Chem (2007), Chem. Mater. (2004), Nature Mater (2003),US Pats. # 6983863 and #7087288US Pat. Appls. #61/115576, 12/460115, 61/210080

Fundamentals of Transport in Nanopores

JPC Letters (2013), JACS (2012), JACS (2010), JPC Letters (2009), ACS Nano (2009), JPCC (2008), US Pat. Appl. #61/288236

Nanomaterials Engineering

Nature Communications (2014), Nano Letters (2012), JACS (2011), ACS Nano (2010), ACS Nano (2008), US Pat. # 7835870, US. Pat. Appls. #20070099191, #61/315457

Thin Films/Membranes, Separations CO2 Capture, Biofuels, Fuel Cells Supports All Application Areas

Mesoporous silica membranes on polymer hollow fibers

MOF/polymer nanocomposite membranes

MOF membranes on polymer hollow fibers

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Shofner Group Research Overview - 2014

Processing-Structure-Property Relationships for Polymers and Composites

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http://shofnerlab.gatech.edu

Polymer Crystallization in Nanocomposites for

Microstructural Design

Cellulose Nanocrystal/Polymer

Composites

Paper Properties and Applications

Verma, Shofner, and Griffin, Physica Status Solidi B (in press)

Xu, Girouard, Schueneman, Shofner, and Meredith, Polymer (2013)

Kaur, Lee, Bucknall, and Shofner, ACSApplied Materials and Interfaces (2012)