Monitoring and Control of an Adsorption System Using Electrical

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Monitor and Control of an Adsorption System using Electrical Properties of the Adsorbent for Organic Compound Abatement Ming-Ming Hu, Hamidreza Emamipour, David L. Johnsen, Mark J. Rood, Linhua Song, and Zailong Zhang Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 31 May 2017 Downloaded from http://pubs.acs.org on June 1, 2017

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Environmental Science & Technology

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Monitor and Control of an Adsorption System using Electrical

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Properties of the Adsorbent for Organic Compound Abatement

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Ming-Ming Hu†,‡, Hamidreza Emamipour‡, David L. Johnsen‡, Mark J. Rood*,‡,δ, Linhua

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Song*,§, Zailong Zhang§

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†

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266580, China

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‡

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61801, USA

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δ

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§

College of Chemical Engineering, China University of Petroleum (East China), Qingdao

Department of Civil and Environmental Engineering, University of Illinois, Urbana, Illinois

Guest Professor, China University of Petroleum (East China), Qingdao, 266580, China College of Science, China University of Petroleum (East China), Qingdao 266580, China

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ABSTRACT

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Adsorption systems typically need gas and temperature sensors to monitor their

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adsorption/regeneration cycles to separate gases from gas streams. Activated carbon fiber cloth

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(ACFC)-electrothermal swing adsorption (ESA) is an adsorption system that has the potential to

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be controlled with the electrical properties of the adsorbent and is studied here to monitor and

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control the adsorption/regeneration cycles without the use of gas and temperature sensors and to

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predict breakthrough before it occurs. ACFC’s electrical resistance was characterized based on

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amount of adsorbed organic gas/vapor and adsorbent temperature. These relationships were then

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used to develop control logic to monitor and control ESA cycles based on measured resistance

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and applied power values. Continuous sets of adsorption and regeneration cycles were performed

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sequentially based entirely on remote electrical measurements achieving ≥ 95 % capture

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efficiency at inlet concentrations of 2,000 and 4,000 ppmv for isobutane, acetone and toluene in

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dry and elevated relative humidity gas streams demonstrating a novel cyclic ESA system that

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does not require gas or temperature sensors. This contribution is important because it: reduces

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cost and simplifies the system, predicts breakthrough before its occurrence, and reduces

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emissions to the atmosphere.

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Introduction

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Volatile organic compounds (VOCs) are regulated as both indoor and outdoor air pollutants.

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Depending on the composition and exposure time, indoor VOC emissions cause eye, nose, and

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throat irritation, headaches, damage to the liver, kidney, and central nervous system, memory

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impairment, and cancer.1 Outdoor VOC emissions can cause photochemical smog2 and

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contribute to the formation of particulate matter which is an environmental issue influencing air

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quality, regional and global climate, and human health.3 VOCs are widely used to produce

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building materials, home and personal care products, and can be emitted into the atmosphere,

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often in dilute concentrations (< 5000 ppmv).4, 5 VOC control methods are classified as

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destructive, such as biofiltration and thermal oxidation,6 and recovery, such as absorption,7

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condensation,8 and adsorption.9

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Activated carbon fiber cloth (ACFC) is an effective adsorbent for VOC adsorption10 and recovery11, 12. ACFC (ACC 5092 including ACFC-10, ACFC-15, and ACFC-20 that vary based


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on the extent of activation) has faster mass and heat transfer kinetics and larger adsorption

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capacities than granular activated carbon due to its large BET surface area (810-1322 m2/g),

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microporosity (92.2%-95.8%), and micropore volume (0.39-0.62 cm3/g), and its high purity (0

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ash content).13, 14 The electrical resistivity of ACFC also makes it a promising adsorbent which

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can be electrothermally regenerated using the Joule effect by direct application of electrical

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voltage.5, 15 The trade-off of ACFC-15 is its cost of $100-500/kg16 adsorbent compared to

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98% adsorption efficiency for VOCs with a boiling point as low as -11.7 °C19 and

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concentrations from 73 to 10,000 ppmv.20,21, 22, 23 ESA was also assessed for biogas purification

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by removing undesirable VOCs.24

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Hydrocarbon sensors are used to measure the concentrations of organic compounds

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downstream of adsorption systems to determine when breakthrough occurs to end an adsorption

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cycle. Monitoring the adsorbent’s electrical resistance is a potential alternative to hydrocarbon

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sensors to determine when to end an adsorption cycle, to simplify the system and reduce cost.



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The electrical resistance of the adsorbent depends on the amount of a specific adsorbate

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adsorbed25, the type of adsorbent26, and the adsorbent’s temperature27. For example, electrical

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resistivity of ACFC samples depends on porosity and surface functional groups, and has been

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shown to decrease by 5% to 10% after an isobutane adsorption cycle.25

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Regeneration of ACFC typically involves electrothermal heating to a specified cartridge

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temperature and then cooling to a specified temperature before initiation of the adsorption cycle.

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The ACFC’s cartridge temperature has been determined using thermocouple5, infrared 24, and

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fluoroptic28 temperature sensors. Recently, electrical resistance-feedback was used to determine

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ACFC temperature and was evaluated as proof of concept to control regeneration heating cycles

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at 2000 ppmv isobutane inlet concentration for dry conditions.16 Alternatively, mass and energy

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balances can be used to model the ACFC cartridge temperature during regeneration heating

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based on power applied to the cartridge and select properties of system components.11

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This study provides for the first time automated control of adsorption/regeneration cycles

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based on electrical properties of the adsorbent. Effects of three adsorbates ranging in boiling

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points from -11.7 to 110.8 ºC and adsorbate concentrations ranging from 2000 to 4000 ppmv

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were used to document the performance of ACFC-ESA system at dry RH (< 1%) and elevated

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RH (80%) conditions. This method can also predict breakthrough before its occurrence to reduce

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emissions to the atmosphere during adsorption cycles. These contributions are important

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because they describe how to automatically operate an ESA system without using hydrocarbon

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and temperature sensors to simplify and reduce cost of the system, and to predict breakthrough

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before its occurrence to increase adsorbate recovery and reduce its emissions to the atmosphere.

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Experimental Apparatus and Methods


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Adsorption/Regeneration Apparatus and Methods: The bench-scale ACFC-ESA system

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used here to capture organic gas/vapor is similar to a previous system,27 but described here

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briefly for clarity and to describe revisions used for this research. The ACFC-ESA system

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consists of an organic gas/vapor generator; water vapor generator; an RH control system; an

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adsorption/regeneration vessel with electrothermal regeneration capability; RH, electrical

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resistance, hydrocarbon and temperature sensors; and a data acquisition and control system

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(Figure 1). All of the tests were completed at ambient indoor temperature (22-26 °C) at ambient

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pressure.

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Figure 1. Schematic of bench-scale ACFC-ESA system

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Adsorption cycles occurred by generating an inlet air stream at 50 SLPM that was filtered with

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a high efficiency particle air filter and then dried with silica gel. Water vapor was added to this

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air stream with a custom tubular membrane-based stainless steel humidifier.29 RH was measured

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with a capacitive-based RH sensor. Isobutane (Aeropres Corporation, purity > 98.4%), acetone

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(Sigma-Aldrich, purity ≥ 99.5%), or toluene (Macron, purity ≥ 99.5%) were separately injected 5 ACS Paragon Plus Environment


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into the air stream using a compressed gas cylinder or a syringe pump. Gas flow rates were

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controlled by mass flow controllers and reported at standard conditions of 298 K and 1 atm.

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Evaporation of acetone or toluene and their mixing with air was improved by heating the gas

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stream at the liquid’s injection point.11 The bypass for the vessel was used to achieve steady-state

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inlet gas conditions before passing the gas stream into the vessel to begin an adsorption cycle.

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The gas stream containing air and a specified organic compound at RH < 1% or 80% passed into

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a vessel during an adsorption cycle until the breakthrough setpoint was achieved defined as the

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outlet concentration achieving 10% of the inlet organic concentration.

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Adsorption and regeneration cycles were performed within the same vertical Pyrex vessel,

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which provided 1.43 L of internal volume and contained an annular cartridge of 112.7 g of

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ACFC-15 (ACC5092-15, Kynol). ACFC-15 has large equilibrium adsorption capacities from

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342 to 460 mg/g ACFC for organic compounds with a wide range of equilibrium vapor pressures

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from 2.9 to 320 kPa due to its large BET surface area, micropore volume and percent

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microporosity of 1322 m2/g, 0.62 cm3/g and 94.4%, respectively. Bulk carbon, hydrogen,

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oxygen, nitrogen, and ash contents of ACFC-15 are 93.6%, 3.5%, 2.6%, 0.3%, and 0% by mass,

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respectively.16 The gas stream entered into the bottom of the vessel and passed through the

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ACFC cartridge from the outside to the inside of the cartridge during adsorption cycles. An

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annular ACFC cartridge was constructed by wrapping a rectangular sheet of ACFC with length

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of 25 cm and width of 160 cm around a 1.9 cm outer diameter stainless steel annular electrode

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located at one end, and a 1.9 cm diameter stainless steel solid electrode located at the other end

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of the ACFC. The ACFC cartridge had 25 layers and was clamped to the electrodes with

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stainless steel hose clamps resulting in an effective cartridge length of 22 cm for the current to

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flow, which is the distance between the ends of the electrodes and their hose clamps. Electrical


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resistance of the ACFC cartridge with or without adsorbates was measured with a multimeter

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(Keithley, Model 2000) with its probes across the length of the ACFC and its fittings, which

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were clamped on each electrode with stainless steel hose clamps. Temperature of the ACFC was

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measured with 0.16 cm diameter Type K thermocouples. Concentration of downstream organic

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compounds was measured with a photoionization detector (PID). The PID was calibrated with

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the select organic compounds at the average outlet RH during each adsorption cycle.

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A regeneration cycle included inert gas purging, heating and cooling. Regeneration cycles

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occurred after each adsorption cycle by passing N2 (99.99% purity) at 5 SLPM for 3 min into the

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top of the Pyrex vessel, passing from the inside to the outside of the ACFC prior to heating, to

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purge O2 from the vessel to prevent ignition. The flow of N2 was reduced to 0.5 SLPM after

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purging O2 from the vessel. Electrical power was applied to the ACFC cartridge by clamping

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wires to each electrode with stainless steel hose clamp. The ACFC was regenerated by heating

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for 3 min with a ramp from 0 to 400 W, heated for 2.5 min with a ramp from 0 to 110 W, and

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then kept at constant power of 110 W for 4.5 min. Heating the ACFC with an initial setpoint of

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400 W heated the ACFC to 130 ºC within 3 min. Heating the ACFC with the second setpoint of

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110 W heated and then sustained the ACFC at 150 ºC for 7 min. The power setpoint for heating

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ACFC were initially determined using energy and mass balances for isobutane at dry RH

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conditions.11, 16 Direct current voltage and current applied to the ACFC were measured with true

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root mean square voltage and current transducers, respectively. After 10 min of Joule heating,

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power turned off and the ACFC’s resistance was measured as it cooled. Cooling during the end

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of the regeneration cycle was terminated when the ACFC achieved its 80 °C setpoint to start the

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next adsorption cycle.



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The slope of ACFC’s resistance (SR, ohm/s) was used to terminate the adsorption and

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regeneration cycles when SR values corresponded to a 10% breakthrough value of the organic

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compound and an ACFC temperature of 80 ºC, respectively. The SR values, based on resistance-

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feedback, were calculated as the change in resistance (R) with respect to time (t) using 1 min

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boxcar averages of dR/dt (equation 1): R − R dR = (1) dt t − t

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where R = cartridge resistance (ohm) at time = ti (sec), t = cartridge resistance (ohm) at

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time = ti (sec) – 120 sec. t = 0 if i-120 < 0. The 120 s resistance slope averaging time was

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selected to dampen noise in the electrical resistance measurements.

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Control and data acquisition of the ACFC-ESA system were fully automated. The control

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system controlled the valves and power applied to the ACFC. The control and data acquisition

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system used LabVIEW software (Version 2013 SP1).

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Performance Evaluation of Automated Adsorption/Regeneration Cycles: Criteria used to

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evaluate the performance of the ACFC-ESA system for automated adsorption/regeneration

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cycles were: 1) the ability of using SR values to terminate adsorption cycles at a specified

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breakthrough concentration, predict breakthrough before it occurs, and terminate regeneration

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cycles at a specified ACFC temperature; and 2) the impact of inlet organic adsorbate

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composition and concentration, and inlet RH on adsorption cycle breakthrough concentration, 3)

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capture capacity, and 4) capture efficiency.

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Control of automated adsorption cycles: Automated control to terminate adsorption cycles

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used an SR setpoint, which depends on amount and type of adsorbate(s) and the type of

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adsorbent, instead of measuring outlet organic gas compound concentration. Concentration


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downstream of the adsorption vessel was also measured (but not used for control) to evaluate the

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ability of SR values to determine when to terminate an adsorption cycle.

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Control of Automated Regeneration Cycles: Automated control to terminate regeneration

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cycles used an SR setpoint, which depends on adsorbent temperature. Temperature of the ACFC

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was also measured (but not used for control) to evaluate the ability of resistance measurements to

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determine when to terminate regeneration cycles based on ACFC temperature.

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Capture capacity: Capture capacity of ACFC was determined with a series of three automated

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adsorption/regeneration cycles. The mass ratio of adsorbed organic compound to ACFC (qadsorbed,

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(g organic adsorbed/g ACFC)) was calculated based on equation 2: t=tf Ptot Qg Mw yin yout qadsorbed = dt RTm 1-yin 1-yout t=0

(2)

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where Ptot = 1 atm, Qg = dry air flow rate (SLPM), Mw = molecular weight of the adsorbate

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(g/mol), R = ideal gas constant ((L·atm)/(K·mol)), T = 298 K, m = mass of adsorbent (112.7 g), t

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= time (min), tf = final adsorption cycle time (min), yin = mole fraction of the adsorbate in the

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inlet gas stream, and yout = mole fraction of the adsorbate in the outlet gas stream.

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Capture Efficiency: Capture efficiency for a specific adsorption cycle describes the percent of

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organic adsorbate adsorbed relative to the amount fed into the vessel. Capture efficiency was

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calculated based on equation 3: Capture efficiency=

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q adsorbed ×100 Ptot Qg Mw yin RTm 1-y tf in

(3)

Determination of Automated Control of Adsorption and Regeneration Cycles Based on

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Electrical Properties of the Adsorbent: SR setpoint values were determined by first

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performing tests that were not controlled based on resistance feedback and recording the SR

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values when: 1) the breakthrough concentration was 10% of the inlet concentration at the end of 9 ACS Paragon Plus Environment


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an adsorption cycle and 2) when ACFC temperature was 80 °C at the end of a regeneration cycle,

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which includes cooling before the adsorption cycle. Three automated adsorption/regeneration

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cycles were then controlled based on these SR values to evaluate reproducibility of the control of

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adsorption/regeneration cycles based on electrical properties of the adsorbent.

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Adsorbate Composition, Concentration, and RH Tested with ACFC-ESA System:

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Adsorption/regeneration cycles were performed for inlet gas streams containing adsorbates in air

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streams at inlet concentrations of 2000 and 4000 ppmv and RH of < 1% and 80% to investigate

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the effects of these adsorbates on the performance of the system. Isobutane, acetone and toluene

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were the organic adsorbates tested with the ACFC-ESA system (Table 1). These adsorbates were

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chosen because of their wide range of liquid densities (248 to 866 kg/m3), boiling points (-

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11.7 °C to 110.8 °C), solubilities in water (0.08 g/L to ∞), and equilibrium vapor pressures (2.9

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kPa to 302 kPa). Tests considering adsorption of organic compounds and water vapor have

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occurred due to the ubiquitous presence of water vapor in gas streams and water’s ability to

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cause competitive adsorption for organic adsorbates with activated carbon when RH is above

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60% as illustrated by water vapor isotherm on ACFC.14 Adsorption capacity for isobutane,

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acetone and benzene were decreased by 38%, 35% and 32% when RH was increased to >80%,

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respectively.10, 30 Table 1. Properties of Organic Adsorbates31

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Properties Class Molecular formula Molecular mass (g/mol) Density (kg/m3) Boiling point (°C) Auto ignition point (°C) Solubility in water (g/L) Vapor pressure at 20 °C (kPa)

Isobutane Alkane i-C4H10 58.12 248 (at 21 °C) -11.7 465 0.08 (at 20 °C )

Acetone Ketone C3H6O 58.08 791 (at 20 °C) 56.5 460 ∞

Toluene Aromatic C7H8 92.14 867 (at 20 °C) 110.8 535 0.50 (at 16 °C )

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24.5

2.9


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RESULTS AND DISCUSSION Overall structure of the monitored parameters describing the automated

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adsorption/regeneration cycles for toluene, isobutane and acetone are shown in Figures 2, S1,

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and S2, respectively, at dry and humid RH conditions. Tabulated results for these parameters are

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provided in Table S1. The parameters described in these figures are ACFC resistance, SR, ACFC

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temperature, outlet organic gas compound concentration, and outlet RH. All parameters are

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described continuously except for SR, outlet organic concentration, and RH. SR values are not

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reported during the heating sections of regeneration cycles because heating was controlled based

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on power-feedback. Outlet organic concentration is only reported for adsorption cycles because

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outlet concentrations of desorbed organic adsorbates during regeneration cycles were out of the

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PID’s measurement range. Ambient RH was measured and recorded during regeneration cycles

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instead of the vessel’s outlet gas stream because water concentration was out of range of the RH

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sensor’s measurement range during regeneration cycles.

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Initial regeneration of ACFC occurred to prepare the ACFC for the following cyclic

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adsorption and regeneration cycles. The figures demonstrate a high degree of reproducibility of

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curve structure for all cycles for all parameters describing adsorption and regeneration cycles.

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All vertical axes have the same range of values, while the horizontal axes range from 0 to either

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200 or 300 minutes depending on the adsorbates’ composition and concentration, caused by

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different affinity of the organic compounds to the ACFC and the potential for competitive

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adsorption caused by the water vapor. More detailed and quantitative description of these

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parameters are described in the following sections.

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Figure 2. Dependence of outlet RH, resistance, SR, adsorbent temperature and outlet

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concentration during adsorption and regeneration cycles of toluene: Conditions of the inlet

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gas stream were (a) 2000 ppmv, RH < 1%/100, (b) 4000 ppmv, RH < 1%/100, (c) 2000 ppmv,

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RH = 80%/100, and (d) 4000 ppmv, RH = 80%/100.

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Automated Control of Adsorption Cycles for ACFC-ESA System Based on Electrical Properties of the Adsorbent Electrical resistance and the resulting SR values of the ACFC changed with temperature and

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amount of adsorbed adsorbate(s) during an adsorption cycle (Figures 2, S1, and S2, and Table

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S1). There were three stages for SR values during adsorption cycles. Stage 1 occurred when

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cooling dominated SR values from 80 ºC to 40 ºC due to residual elevated temperature from the

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proceeding regeneration cycle. SR values increased to a maximum value of (17.50±2.56) ×10-4 12 ACS Paragon Plus Environment

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ohm/s and then decreased due to the reduced rate of increasing resistance. During stage 2,

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simultaneous cooling and adsorption caused the opposite trend of resistance, which caused SR

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values to decrease to zero. During stage 3, the adsorbent cooled to near steady-state conditions

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while adsorption continued resulting in SR values to decrease from zero to a minimum value of

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(-6.86±0.57) ×10-4 ohm/s and then increase to its specified SR setpoint value as adsorption

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approached equilibrium. For all isobutane, acetone and toluene tests, the SR values used to

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terminate the adsorption cycles were (-0.50±0.37) ×10-4 ohm/s, (-0.19±0.12) ×10-4 ohm/s and (-

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0.68±0.50) ×10-4 ohm/s, which corresponded to a 3.2±2.2%, 7.6±6.3% and 4.5±2.9%

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breakthrough values compared to the inlet concentrations, respectively. The resistance of the

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ACFC with isobutane, acetone, and toluene during adsorption tests started at 2.20±0.05 ohm and

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ended at 2.05±0.11 ohm. This trend is attributed to the increase of electron hopping between the

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ACFC’s nanographite domains. Adsorbates in the micropores have attraction to the nanographite

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domains, which decreases the spacing between the domains. The decreased spacing increases

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electron hopping between the ACFC graphite domains resulting in decreased resistance.25, 32

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Acetone has the largest SR value followed by isobutane and then toluene, which is in contrast to

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their affinity for ACFC. Affinity coefficients decrease from toluene to isobutane and then to

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acetone. 33, 34 Lower affinity of adsorbate for ACFC causes earlier occurrence of

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adsorption/desorption equilibrium resulting in a larger SR value (lower rate of decrease of

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resistance due to adsorbate adsorption). The breakthrough value for all tests is 5.1±4.6%, which

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show control accuracy of 51±46% compared to 10% breakthrough.

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The mean and standard deviations of breakthrough values for all breakthrough tests are

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presented in Figure 3a. All tests with inlet organic gas streams of 2000 ppmv are below 10%

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except for acetone. The breakthrough results with inlet organic gas streams of 4000 ppmv 13 ACS Paragon Plus Environment


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demonstrate improved results with all breakthrough values < 10% of the inlet concentrations.

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ACFC had a faster rate of adsorption for tests with inlet organic gas streams of 4000 ppmv,

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which resulted in more rapid, larger, and stable increases of SR values during breakthrough.

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Toluene tests with inlet concentration of 4000 ppmv and 80% RH are exemplified here (Figure

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2d). During the first adsorption cycle, SR values had a minimum value of (-7.36±0.24) ×10-4

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ohm/s. As adsorption continues, the SR value kept increasing until the SR reached the setpoint of

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-1.33×10-4 ohm/s resulting in termination of the adsorption cycle.

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Figure 3. Outlet concentration of adsorbates (vertical bars) (a) and capture capacity (b) for

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organic adsorbates at breakthrough during adsorption cycles. Outlet concentration of

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adsorbates (c) and capture capacity (d) for organic adsorbates before breakthrough during


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adsorption cycles to predict breakthrough. Error bars represent standard deviations above

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and below the mean. Some error bars in (c) are too small to see.

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High resolution SR and breakthrough values beginning with adsorption cycles and

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corresponding breakthrough curves for toluene, isobutane, and acetone are shown in Figures 4,

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S3, and S4, respectively. These figures correspond to Figures 2, S1, and S2, respectively. These

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results also demonstrate a high degree of reproducibility of curve structure for all cycles while

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using SR values to control adsorption cycles. As an adsorption cycle began, the SR values

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rapidly increased due to cooling of the ACFC from residual elevated temperature at the end of

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the preceding regeneration cycle. Breakthrough occurred when the SR values increased to (-

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0.31±0.12) ×10-4 ohm/s and (-1.57±0.60) ×10-4 ohm/s for all inlet gas streams of 2000 ppmv and

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4000 ppmv, respectively, due to adsorption of adsorbate(s) (Figure 3b and Table S1).

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Figure 4. Dependence of SR and outlet concentration during adsorption and regeneration

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cycles of toluene: Conditions of the inlet gas stream were (a) 2000 ppmv, RH < 1%, (b) 4000

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ppmv, RH < 1%, (c) 2000 ppmv, RH = 80%, and (d) 4000 ppmv, RH = 80%.

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Breakthrough was also predicted to occur before its occurrence when SR values increased to

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specific values for each organic adsorbate, regardless of the inlet organic concentrations or inlet

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RH values (Figures 4, S3 and S4, and Table 2). The SR values used to predict breakthrough

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before its occurrence for all isobutane, acetone and toluene tests were (-1.23±0.84) ×10-4 ohm/s,

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(-0.54±0.34) ×10-4 ohm/s and (1.10±0.77) ×10-4 ohm/s, which corresponded to percent of inlet

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concentration at the outlet of the adsorber of 0.20±0.11%, 0.25±0.07% and 0±0% (Figure 3c),

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respectively. The standard deviations for the SR values are caused by the range of inlet organic

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and water adsorbate concentrations for each composition of adsorbate. Use of SR to predict

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breakthrough before its occurrence shows better performance for higher compared to lower inlet

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organic concentrations because higher inlet concentration resulted in a faster rate of adsorption

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which resulted in more rapid, larger, and stable increases of SR values, as was the case when

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using SR setpoint to determine breakthrough at 10% of the inlet concentration. By using the SR

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value to predict the occurrence of breakthrough, the adsorption cycle can be terminated before

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breakthrough resulting in less emissions to the atmosphere compared to using hydrocarbon

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sensors to monitor the concentrations of organic compounds as the gas streams exits the vessel.

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The high resolution SR and outlet concentration plots describe a new and effective method to

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automatically control adsorption cycles using SR setpoint, how the SR values can be used to

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predict breakthrough before its occurrence, and demonstrate that hydrocarbon sensors are not

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needed at the outlet of the vessel for gas detection.

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Table 2. Conditions during Tests and Criteria for Evaluation of Performance of ACFC-

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ESA System by Using SR Values to Predict Breakthrough Before its Occurrence (Mean ±

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Standard Deviation) and SR Values before Breakthrough

Adsorbate

Isobutane

Acetone

Inlet RH (%)

Inlet concentration (ppmv)

Capture capacity (g adsorbate/g ACFC)

Capture efficiency (mass %)

Outlet concentration (ppmv)

Breakthrough percent (volume %)

SR before breakthrough (×10-4 ohm/s)

Monitoring and Control of an Adsorption System Using Electrical

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