A Novel Continuous Multiphase Reactor for Chemically Processing

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Kinetics, Catalysis, and Reaction Engineering

A Novel Continuous Multiphase Reactor For Chemically Processing Polymer Fibers Eric John Hukkanen, Bryan Barton, Jasson Patton, David Schlader, Yiqun Zhang, Xiaohua Qiu, Lora Brehm, Bryan Haskins, Weijun Wang, Nicholas Horstman, Mark Spalding, Daniel A Hickman, and Christopher W Derstine Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00482 • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 21, 2018

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Industrial & Engineering Chemistry Research

A Novel Continuous Multiphase Reactor For Chemically Processing Polymer Fibers Eric J. Hukkanen,∗,†,‡ Bryan E. Barton,†,¶ Jasson T. Patton,† David R. Schlader,† Yiqun Zhang,† Xiaohua Qiu,† Lora Brehm,† Bryan Haskins,† Weijun Wang,†,§ Nicholas Horstman,† Mark A. Spalding,† Daniel A. Hickman,† and Christopher W. Derstine† †The Dow Chemical Company, Midland, MI 48674, United States ‡Current address: Dow Electronic Materials, 455 Forest Street, Marlborough, MA 01752 ¶Current address: Dow Electronic Materials, 451 Bellevue Road, Newark, DE 19713 §Current address: Eastman Chemical Company, 200 South Wilcox Drive, Kingsport, TN 37660 E-mail: [email protected] Abstract Chemical reactions on the surface or throughout a thermoplastic fiber present an interesting and challenging reaction engineering problem. This paper presents the design and implementation of a modular multiphase continuous stirred tank reactor for the sulfonation of polyethylene fibers. Polyethylene fiber tows, comprising of 10006000 filaments, are sulfonated in a series of stirred tank reactors. Fundamental reaction engineering principles are applied to address common batch-to-continuous reactor challenges, such as heat and mass transfer, material balances, and reaction kinetics. The introduction of polymer fibers to a continuous process also requires the solution of practical problems associated with fiber processing. Additional considerations are

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introduced for designing a reactor system for continuous processing of fiber using liquid with a batch reactor or CSTR contacting pattern in each vessel and containing the effluent gaseous reaction byproducts in a controlled vent stream. The overall sulfonation reaction stoichiometry was determined such that ≈1 mol of SO3 reacts per mol CH2 fed. Additionally, carbon sulfonation selectivity was determined to be 95-99%, depending on the sulfonation conditions.

Introduction Polyethylene has been demonstrated to be a suitable and alternative precursor material for the production of carbon fibers. 1–3 Researchers have demonstrated that carbon fibers can be produced by carbonizing sulfonated polyethylene fibers. 3–10 Our current understanding of sulfonation of polyethylene is as follows: Upon immersion of the fiber, sulfur trioxide (SO3 ) diffuses into the polymer matrix, where it reacts with the pendant hydrogen atoms on the carbon backbone primarily via oxidative dehydrogenation, generating a polyene structure with additional sulfonic acid groups present. With elevated temperature (100-180), the sulfonic acid groups undergo a cross-linking reaction eliminating SO2 and H2 O, resulting in a highly cross-linked fiber. This step is only partially completed in acid. The remaining sulfonic acid groups are eliminated during low-temperature heat treatment (i.e., 600), dehydrogenation occurs, along with some undesired carbon oxidation, resulting in a carbon matrix in the form of graphitic carbon. This is the form of the final product, known as carbon fiber. The proposed chemical details of the transformation of polyethylene fiber to carbon fiber are presented by Barton et al. 11 The proposed mechanism is represented in Figure 1. This proposed mechanism represents our expectations prior to the work described in this report. Several other researchers support the sulfonation reaction pathway shown in Figure 2. In these studies, intermediate structures contain sulfonic acid groups en route to dehydrogenation. In both cases, sulfur dioxide is a byproduct of the sulfonation process. There are 2


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SO3H

SO3

+ H2SO3

SO2 + H2O

Figure 1: Principle reaction mechanism for vinyl formation involving repetitive insertion and elimination events. SO3H

O

SO3H

SO3, H2SO4 O SO3H

n

OH

OH

SO3H

n

Figure 2: Simplified reaction mechanism for sulfonation of polyethylene using SO3 containing reagents.

some reports that do investigate the potential for additional reaction pathways. Cameron and Main have measured sulfur dioxide and carbon dioxide during sulfonation of polyethylene. 12,13 A simplified overall reaction was proposed that incorporated CO2 in the product stream. Idage et al. proposed a more sophisticated mechanism that produces CO2 and other functional groups (e.g., ketones, aldehydes, etc.). The proposed mechanisms and functional groups were supported with x-ray photoelectron spectroscopy. 14 A batch sulfonation reactor was initially designed to support early stage development, proof of concept, and screening of different process conditions, such as polymer fiber, reactor temperature, residence time, and applied tension. The typical batch experiment is as follows: two sections of polyethylene were tied with carbon fiber leads and positioned in the reactor. The red dots represent knots between the polyethylene and carbon fiber. See Figure 3. These knots also represented vulnerable points in the fiber, and the most probable failure point within the fiber tow. Ultimately, the tension applied to the polyethylene fiber was restricted by the knot stability. Once the acid was added, there was a heat up period to the desired reaction temperature. Following reaction at the desired residence time, there was also a cool down period. As a result, the polyethylene fiber experienced a non-isothermal temperature 3



history, as well as uncontrolled heating and cooling. With the small reactor design, there was no mixing and most likely external concentration gradients relative to the fiber. To be

Figure 3: Batch sulfonation reactor system.

clear, this only represents the first reactor, typically oleum or sulfonation solvent/phase. 15–17 One or two subsequent sulfuric acid treatments follow. In some cases, retying of knots are required, resulting in additional unnecessary fiber handling. Additionally, subsequent acid treatments undergo different applied tensions, temperatures, and residence times. A continuous sulfonation reactor was designed to enable production of high quality sulfonated polyethylene for carbon fiber production and characterization. 4,18 This work serves as an important step in understanding the overall polyethylene sulfonation reaction. Polyethylene-derived carbon fibers are stabilized by exhaustive sulfonation prior to carbonization. The overall reaction stoichiometry and sulfur usage are critical to process scale-up, reactor design, and process economics. Sulfur usage directly influences the process economics by affecting the size of a spent acid recovery unit or the toll manufacturing costs associated with spent acid recovery. This paper reports the design of a continuous multi-bath sulfonation system, analytical method development, method validation, and ap-

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plication of gas chromatography and elemental analysis to the sulfonation of polyethylene fibers for determining the overall reaction coefficients and sulfur usage.

Experimental Reagents and polymer Fuming sulfuric acid (Acros Organics Sulfuric acid, extra pure, fuming, 20-30% free SO3 ; CAS: 8014-95-7) and sulfuric acid (EMD, 95-98%, CAS: 7664939) were used as received. Fuming sulfuric acid, 1% (1FSA) and 6% (6FSA) free SO3 , were prepared internally by dilution of 20% oleum (20FSA) with concentrated sulfuric acid (95-98%) (SA). ASPUN 6850A is a linear low density polyethylene (LLDPE) from The Dow Chemical Company (density (g/cm3 ) = 0.955; melt mass flow rate (ASTM D 1238, g/10 min) = 30 (190◦ C/2.16 kg)).

Polymer fiber preparation Polyethylene fiber filaments were prepared by melt spinning the polyethylene resin using a Hills (Hills, Inc., West Melbourne, FL) Market Development Scale melt spin line, model DHP. The polyethylene filaments were subsequently stretched in the solid-state and combined to produce tows. Further details on the preparation of polyethylene fibers are reported by Spalding et al. 19 Properties of the polyethylene tows are reported in Table S1.

Continuous sulfonation reactor The basic apparatus used to sulfonate fibers under tension in a continuous fashion is shown in Figure 4. The reactor system consists of three sulfonation reactors and a continuous deionized water rinse. The process flow diagram for the continuous sulfonation process is provided in

Trademark of The Dow Chemical Company

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Figure S1. Additional reactor design considerations were implemented for gas-phase analysis. The reactor headspace is completely isolated from the fiber sub-surface entry and exit points using 3/4” glass dip tubes. A nitrogen flow sweeps the headspace and exits the reactor to the GC/scrubber. The true gas production rate was determined by scaling the measured effluent flow relative to the projected area of the isolated reactor headspace (excluding cross-sectional area of dip tubes) to true projected area of the entire reactor headspace. Each reactor has a flat bottom and drain valve. The flat bottom was chosen to facilitate mixing with a stir bar. Overhead stirring or recirculation loops were preferred, but the reactor internals did provide some baffling for better heat and mass transfer. Temperature was controlled by electrical heat tapes wrapped around the reactor. A 100 mm PTFE Schott flange lid was modified with appropriate fittings for fiber entry and exit, internal support structure, thermocouples, and purge lines. Temperature and N2 purge in each sulfonation reactor were controlled independently using a Camile Data Acquisition and Control System.

Figure 4: Photograph of continuous polyethylene fiber sulfonation reactor.

Two motors were selected to continuously feed polyethylene fiber through the sulfonation reactor at a constant rotational speed while maintaining a constant tension across the fiber. An Oriental Motor BX460A-200S / OPX-1A, Brushless DC Motor System (Oriental Motor U.S.A. Corp.) was selected and modified for the constant speed motor. The constant speed motor had a rotational speed range of 0.015-15 rpm (200:1 gear ratio). This rotational speed range was deemed sufficient for the the desired space time range of 5-140 min per reactor. An 6


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Oriental Motor 4TK10GN-AW2U / 4GN3.6SA Torque Motor (Oriental Motor U.S.A. Corp.) was implemented to provide the desired constant fiber tension. The operator adjusted the DC voltage, across a 1-5 volt range, to vary the applied torque. A hand-held digital tension meter, Checkline DTMX-500 (Electromatic Equipment Co., Inc.), measured on-line tension measurements at the inlet and outlet of the reactor system.

Sulfonation conditions Three sulfonation experiments were completed to validate the design of the continuous reactor system, analytical methods, and overall reaction stoichiometry. See Table S2. A large diameter polyethylene fiber tow was used in Experiment 1., based on number of filaments and total denier. The fiber tow used in Experiments 2 and 3 was a more highly-drawn polyethylene, resulting in smaller filament diameter and higher strength. Two linear feed rates were selected to provide a target reactor space time of 15 and 60 minutes per reactor. Space time was chosen, rather than residence time, as the fiber length undergoes shrinkage as the chemistry progresses. Space time in Experiments 1 and 2 was 60 minutes; space time in Experiment 3 was 15 minutes. Each reactor was charged with the appropriate oleum concentration or concentrated sulfuric acid, according to Table S2. Higher temperature in the first oleum reactor was possible by lowering the oleum concentration (Experiment 3). Temperature in the third reactor, R3 , was increased in 10 increments from 120-140 to evaluate impact of sulfonation conditions on evolved gases. Tension was controlled between the first and second reactor and after the final reactor (water rinse). Sulfonation experiments were designed to fully stabilize the polyethylene fiber. A fully stabilized fiber enables the determination of the overall reaction stoichiometry, in the absence of unreacted polyethylene. This further ensures that carbon fibers derived from such stabilized fibers do not contain defects (e.g., hollow fibers).

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Gas-phase analysis Gas-phase analysis of the sulfonation reactor head space was performed using a Siemens Maxum Process GC Edition II system. The GC contains a dual air-bath oven, three dualchannel electronic pressure controllers (EPC), one thermal conductivity thermistor detector block (six measurement TCDs), one flame ionization detector, two auxiliary pressure regulators, five Valco 10-port, 2-position valves, one Valco-6-port sample switching valve, and two Valco micrometering needle valves. Carbon monoxide, carbon dioxide, methane, ethane, ethylene, and sulfur dioxide responses were directly identified and calibrated with known calibration standards.

Extent of stabilization by sulfonation Extent of polyethylene stabilization by sulfonation was determined by thermogravimetric analyses (TGA). Polyethylene decomposes between 400-500 under pyrolysis conditions; negligible char remains >600. A fully stabilized polyethylene fiber will lose

A Novel Continuous Multiphase Reactor for Chemically Processing

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