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Determining the Effect of Plasticizers on Cellulose Triacetate for CO2/CH4 SeparationMin Wei, Benjamin Lam, Haiqing LinDepartment of Chemical and Biological Engineering, University at Buffalo (SUNY), Buffalo, NY 14260, USA

Project Overview Cellulose Triacetate and Crystallinity Doping with TEC

Background on CO2 Removal from Natural Gas

Motivation A wide utilization of commercial cellulose triacetate (CTA) for natural gas sweetening has been limited by its high crystallinity, resulting in low CO2 permeability.

ObjectiveDevelop advanced cellulose triacetate membranes for CO2/CH4 separation by doping small molecular size of plasticizers

Approach Preparation of a series of CTA doped with triethyl citrate (TEC) and triethyl 2-acetylcitrate (TEAC) to reduce crystallinity and increase CO2/CH4

selectivity.

Conventional separation by amine absorption/stripping technology• Cost intensive for absorbent

replacement• Large footprint requirement• High equipment corrosion rate

Membrane technology:• Economical and reliable• Compact and easy to scale-up• Energy-efficient

Molecular Transport in Polymers Polymeric membrane

High pressure

(up-stream)

Low pressure (down- stream)

Fick’s law

P: permeabilityD: diffusion coefficientS: solubility coefficient

Wijmans and Baker, J. Membr. Sci., 107, 1 (1995)Molecular simulation performed by Dr. Xiaoyan Wang

Diffusivity selectivity Solubility selectivity

Cellulose triacetate (CTA) :• Workhorse membrane material for

industrial CO2/CH4 separation• Good chemical resistance to higher

hydrocarbons contaminants in natural gas feeds

• Semi-crystalline with crystallinity as high as 37%, which significantly reduces gas permeability

Demonstration of crystal formation in polyethylene dramatically reducing CO2 permeability

Approach: Doping with small molecular size of plasticizers

Description Mass of CTA (g)

Mass of additive (g)

Mass of CH2Cl2

(g)100:0 5.0 0.0 95.090:10 4.5 0.5 95.080:20 34.0 1.0 95.060:40 3.0 2.0 95.050:50 2.5 2.5 95.0

Doping with small molecular size of plasticizers• Reduce CTA crystallinity• Enhance gas permeability• Enhance CO2/ CH4 selectivity

Doping with TEAC

Conclusion and Future Work

FTIR-ATR Analysis

TGA Analysis XRD Analysis

FTIR-ATR Analysis

XRD AnalysisTGA Analysis

Constant Volume/Variable Pressure System

• FTIR indicated blending of plasticizers into CTA polymer.

• TGA confirmed the actual amount of plasticizers in the membrane.

• XRD indicated decrease in CTA crystallinity with increasing weight ratio of plasticizers.

• Future Work: Membrane transport properties for permeability and solubility in doped membranes.

TEC

TEAC

• Absorbance curve of polymer blends are similar to CTA

• Increasing amount of plasticizer indicated decrease in membrane crytallinity

• Absorbance curve of polymer blends are similar to CTA

• Actual ratio of polymer blend confirmed through the weight

90:10 CTA:TEAC

5 10 15 20 25 30 352-Theta (°)

Rel

ativ

e In

tens

ity

Pure CTA

50:50 CTA:TEAC

80:20 CTA:TEAC

60:40 CTA:TEAC

1000 1200 1400 1600Wavenumber (cm-1)

Abs

orba

nce

(a.u

.)

Pure TEAC

50:50 CTA:TEAC

80:20 CTA:TEAC

Pure CTA

1371 cm-1

CH3 bending

1211 cm-1 C-O stretching

Pure CTA

6040 CTA:TEC8020 CTA:TEC

9010 CTA:TEC