Effect of Beaded Activated Carbon Fluidization on Adsorption of

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Effect of Beaded Activated Carbon Fluidization on Adsorption of Volatile Organic Compounds Samineh Kamravaei, Pooya Shariaty, Masoud Jahandar Lashaki, John D. Atkinson, Zaher Hashisho, John H. Phillips, James E Anderson, and Mark Nichols Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04165 • Publication Date (Web): 16 Jan 2017 Downloaded from http://pubs.acs.org on January 22, 2017

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Industrial & Engineering Chemistry Research

Effect of Beaded Activated Carbon Fluidization on Adsorption of Volatile Organic Compounds Samineh Kamravaei 1, Pooya Shariaty 1, Masoud Jahandar Lashaki 1, John D. Atkinson 1, Zaher Hashisho 1*, John H. Phillips 2, James E. Anderson 3, Mark Nichols 3 1

University of Alberta, Department of Civil and Environmental Engineering, Edmonton, AB

T6G 2W2, Canada 2

Ford Motor Company, Environmental Quality Office, Dearborn, MI 48126 USA

3

Ford Motor Company, Research and Advanced Engineering, Dearborn, MI 48121 USA

* Corresponding author: Tel.: +1-780-492-0247; Fax: +1-780-492-0249; E-mail: [email protected]

ABSTRACT This research investigates the effect of adsorbent bed configuration on volatile organic compounds

(VOCs)

adsorption

with

beaded

activated

carbon

(BAC).

Five-cycle

adsorption/desorption tests using a single VOC (1,2,4-trimethylbenzene) and a mixture of nine VOCs were completed using fixed and fluidized bed adsorber configurations. Adsorption tests were completed with full loading of the adsorbent. All regeneration tests were completed in the fixed bed arrangement. The adsorption capacity of the BAC was not affected by the adsorption bed’s configuration in case of reaching full breakthrough. For the VOC mixture, however, 30% less heel buildup was observed for the fluidized bed configuration. Higher accumulation of heavy adsorbates was found in fixed bed as opposed to fluidized bed, explaining the higher heel buildup in the former. On the other hand, improved mass transfer was found across the entire fluidized bed as a result of better gas-particle contact. These results show that, besides the expected engineering advantages of a fluidized bed adsorption system (e.g., lower pressure drop), decreased heel buildup is an additional advantage when using fluidized bed adsorption in industrial settings.

1

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INTRODUCTION Automotive painting operations use large amounts of organic solvents in coating processes involving both solvent-based and waterborne coatings. Adsorption with activated carbon is widely used for volatile organic compounds (VOCs) control because of its high adsorption capacity and the possibility of adsorbate recovery and adsorbent reuse. One challenge for such systems is heel buildup, which is the accumulation of adsorbates that cannot be removed from the adsorbent using the prescribed process regeneration condition. Heel buildup during gas phase capture of VOCs could be attributed to non-desorbed physisorption, chemisorption, or adsorbate decomposition (coke formation).1-6 Of these, non-desorbed physisorption is easiest to remove. We previously showed, for example, that heel present after regeneration at 288 °C is removed by heating to 400 °C.2 While others (i.e. chemisorption and adsorbate decomposition) involve adsorbate/adsorbent interactions that change the nature of the adsorbate (i.e. bond formation between the adsorbate and the adsorbent and bond breaking).7 For adsorbent systems to be most effective, heel buildup should be minimized. However, regeneration strategies may be undesirable or insufficient for removing chemisorbed or strongly physisorbed compounds. To minimize energy use during regeneration and extend the lifetime of adsorbents, preventing heel buildup from occurring is preferred to the use of more aggressive regeneration methods for removing strongly adsorbed compounds. The choice of an adsorber configuration depends on several factors such as the adsorbent attrition resistance, cost, flow rate, and system complexity. The engineering advantages of fluidized bed adsorbers are well-established and include lower pressure drops, sharper breakthrough curves, and less formation of localized hotspots compared to fixed bed adsorption systems.8-10 Fluidized beds are especially applicable for treating large flow rates, consuming less energy, and improving mass and heat transfer.8, 11-15 However, there is a knowledge gap about the impact of a fluidized adsorbent bed on heel buildup of organic contaminants. For fixed bed adsorbers, competitive adsorption has been described as a contributor to heel development due to non-uniform distribution of heavy adsorbates.16 Since fluidized bed adsorbers evenly disperse the adsorbent throughout the adsorption cycle, there is potential for minimizing competitive adsorption prior to VOC breakthrough (i.e., competitive adsorption occurs from the beginning of adsorption in fixed bed, specifically at the inlet of the bed, but mostly after breakthrough in

2


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fluidized bed). Because of the advantages cited above, fluidized bed systems are relevant for large-scale industrial processes.17-19 Reduced heel accumulation would be another attractive feature for industrial applications, but heel buildup research has focused exclusively on fixed bed adsorption systems.2-4, 6, 20 As such, there is a clear need to investigate the impact of adsorbent fluidization on heel buildup. Many investigations have been completed to improve the performance of adsorption and desorption processes. However, there is a knowledge gap about how the configuration of the adsorbent bed influences heel buildup. This is important to understand because large scale industrial processes often use fluidized bed systems to minimize pressure drop, but heel buildup research has focused exclusively on fixed bed systems. This paper describes the impact of adsorption bed configuration on heel buildup when adsorbing gas-phase VOCs with beaded activated carbon. Cyclic adsorption/desorption experiments quantify cumulative heel development in fluidized bed adsorption systems, and compare the results to fixed bed adsorption systems that are regenerated in the same way. Industrially relevant adsorbate mixtures are used to simulate emissions from automotive painting booths. By comparing and contrasting the performance of fixed and fluidized bed adsorption configurations, this paper strives to increase our fundamental understanding of heel formation on activated carbon and to provide additional guidance for the selection and design of large-scale industrial adsorption processes.

METHODS Adsorbent and Adsorbates Microporous beaded activated carbon (BAC, G-70R; Kureha Corporation) was heated at 150 °C for 24 h to remove water and was then stored in a desiccator prior to use. The BAC has an average particle diameter of 0.71 mm, with 99.5 wt% between 0.60 mm and 0.84 mm.2, 16 BAC is ideal for fluidized bed systems because of its mechanical strength (low attrition) and fluidity.21-23 Adsorption

experiments

were

completed

with

a

single-component

(1,2,4-

trimethylbenzene) and a multi-component adsorbate stream. The multi-component stream consisted of nine organic compounds representative of industrial painting emissions and mixed

3



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in equal volume percentages (VOC mixture 1, Table 1). For select experiments, a modified equimolar mixture of eight VOCs was used for compatibility with the gas chromatography-mass spectrometry (GC-MS) instrument (VOC mixture 2, Table 1). Table 1 Composition of the VOC mixture 1 and modified VOC mixture 2 Chemical name n-butanol (99.9% Fisher scientific)

Chemical Structure H3C

OH

Boiling point (ºC)

Molecular weight (g/gmol)

Kinetic diameter (nm)

(%) in Mixture 1

(%) in Mixture 2

118

74.1

0.43 24

11.1

14.3

126

116.2

N/A*

11.1

14.3

151

114.2

0.525# 25

11.1

14.3

171

118.2

N/A

11.1

14.3

174

142.3

0.43 26

11.1

14.3

170

120.2

0.68 27

11.1

14.3

186

148.2

N/A

11.1

14.3

218

128.2

0.62 28

11.1

0

271

105.1

N/A

11.1

0

176

118.2

N/A

0

14.3

O

n-butylacetate (>99%, Acros Organics)

CH3

O

H3 C

O

2-heptanone (98%, Acros Organics)

H 3C

2-butoxyethanol (>99%, Acros Organics)

H3C

n-decane (99.5%, Fisher Scientific)

H3C

CH 3 OH

O

CH3 CH 3 CH 3

1,2,4-trimethylbenzene (98%, Acros Organics) CH 3

CH3

2,2-dimethylpropylbenzene (85%, Chemsampco)

H3C

CH3

naphthalene (>99%, Sigma-Aldrich) diethanolamine (>98%, Sigma-Aldrich) indan (95%, Sigma-Aldrich) * #

CH3 N HO

OH

N/A: Not available The kinetic diameter of 2-butanone is used as estimate of the kinetic diameter for 2-heptanone

4


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1


Adsorption and desorption processes

2

The experimental setup for the fixed and fluidized bed configurations consists of an

3

adsorption/desorption stainless steel tube (20 cm long, 1.44 cm inner diameter, and 1.91 cm outer

4

diameter) filled with 7.0 ± 0.2 g of dry BAC, an adsorbate generation system, a gas detection

5

system, a power application module, and a data acquisition and control system (DAC) (Figure 1).

6

Adsorption/desorption experiments were completed in five cycles with BAC full-loading

7

achieved in both configurations. While fully loading the adsorbent is not used for industrial

8

compliance with environmental regulations, it tends to highlight the effect of bed configuration

9

on competitive adsorption. Loading was considered complete when the adsorber inlet

10

concentration just equaled the effluent concentration. The adsorption temperature was

11

maintained at 25 °C during all adsorption experiments (using the same setup described below for

12

regeneration). Adsorption was stopped after 3 h for the single component and after 4 h for the

13

multi-component adsorbate streams. Desorption cycles were completed in a fixed bed

14

configuration, regardless of adsorption configuration. For the fixed bed configuration, glass wool

15

at the top and bottom of adsorbent bed secured the adsorbent in place, while for the fluidized

16

bed, glass wool was used at the bottom of the adsorbent bed and a stainless steel mesh screen

17

was used at the top of the adsorption tube to prevent adsorbent loss during fluidization. The

18

fluidized bed porosity was 0.78 and the superficial gas velocity was 1.02 m/s, which was

19

between the minimum fluidization velocity (0.07 m/s) and the terminal velocity (2.79 m/s),

20

calculated based on the literature.19 These values were achieved with a carrier gas (air) flow rate

21

of 10 standard liters per minute (SLPM, standard conditions at 1 atm and 25 °C).

Thermocouple

Data Acquisition and Control Fume Hood Photoionization/ Flame-ionization Detector

Air (10 SLPM)

Syringe Pump

Adsorber/ Desorber Adsorption Flow Regeneration Flow 3-Way Valve

N2 (1 SLPM)

22

5



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1 2

Figure 1 Schematic of the adsorption/desorption setup for fixed and fluidized bed adsorption tests

3

The adsorbate generation system used a syringe pump (New Era, NE-300) to inject

4

adsorbate into 10 SLPM of filtered compressed air. A compressed air filter (Norman Filter Co.)

5

removed water and hydrocarbons from the air stream upstream of the adsorbate injection. The

6

syringe pump injection rate was adjusted to maintain an inlet adsorbate concentration of 500

7

ppmv for all the experiments. The injection rate was calculated based on the ideal gas law, using

8

the adsorbates’ density and molecular weight.

9

Inlet and outlet VOC concentrations were determined intermittently with a

10

photoionization detector (PID, Minirae 2000, Rae Systems) for the single-component adsorbate

11

and with a flame-ionization detector (FID, Baseline Mocon, Series 9000) for the multi-

12

component adsorbate. The FID used 35 cm3/min of ultrahigh purity hydrogen and 175 cm3/min

13

of air as the combustion gas. The gas detectors were calibrated before each experiment and the

14

inlet adsorbate concentration had stabilized before the flow was directed towards the adsorbent.

15

Air was used for zero calibration and the 500 ppmv single and multi-component adsorbate stream

16

was used as the span point for its respective experiments.

17

Thermal regeneration was completed using heating (Omega) and insulation (Fisher

18

Scientific) tapes wrapped around the adsorption/desorption tube. A solid-state relay controlled

19

power application to the heating tape. A regeneration temperature of 288 °C was chosen to

20

simulate the high desorption temperature commonly used when treating high boiling point

21

compounds (boiling points ranged from 118 to 271 oC) typically present in paint solvents.23, 29

22

The regeneration temperature was measured with a 0.9 mm outer diameter, type K thermocouple

23

(Omega) in the center of the fixed BAC bed, controlled with a DAC system and data logger

24

(National Instruments, Compact DAQ) equipped with analog input and output modules. The data

25

logger was interfaced to the thermocouple and the solid state relay.

26

High purity (99.9984%) nitrogen (1 SLPM) purged oxygen from the bed and removed

27

desorbed compounds during fixed bed regeneration. At this flow rate the BAC did not fluidize in

28

the adsorption/desorption tube, but remained in the form of a packed bed. Desorption lasted 3 h

29

at a set-point temperature of 288 ºC and was followed by 50 min cooling. The desorption heating

30

time (3 h) includes approximately 35-40 min to reach the set-point temperature. Adsorption

6


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1

capacity was quantified using mass balances. The adsorption tube was weighed before and after

2

adsorption cycles to quantify adsorption capacity. Similarly, the cumulative heel buildup was

3

quantified by mass balance and thermo-gravimetric analysis. After the 5th desorption cycle, the

4

tube weight was compared to its initial weight before the first adsorption cycle to determine

5

cumulative heel build-up over five cycles.2 The thermo-gravimetric analysis is described in the

6

following section.

7

Thermo-gravimetric analysis

8

Thermal stability of the regenerated BAC samples was determined using thermo-

9

gravimetric analysis (TGA, Mettler Toledo, TGA/DSC 1). The temperature increased at 20

10

ºC/min from 30 ºC to 120 ºC in 50 cm3/min of N2, stabilized for 15 min, increased at 20 °C/min

11

to 288 ºC, stabilized for 30 min, increased at 20 ºC/min to 530 ºC, and finally stabilized for 30

12

min. TGA was used on BAC samples obtained after five cycles of mixture 1

13

adsorption/desorption to quantify heel buildup. It was also used on selected BAC samples

14

removed from the inlet, middle, or outlet of the adsorption bed after one cycle of mixture 1

15

adsorption/desorption. The fixed bed adsorber was divided (with glass wool) into three sections,

16

each containing one-third of the dry adsorbent. For the fluidized bed adsorber, after desorption,

17

BAC was manually removed from the inlet and outlet of the adsorber, after which BAC was

18

removed from the middle of the adsorber.

19

Micropore analysis

20

BAC samples obtained after five adsorption/desorption cycles were characterized using nitrogen

21

adsorption (iQ2MP, Quantachrome) at 77 K with relative pressures from 10-6 to 0.995. Samples

22

of 30 to 40 mg were degassed at 120 ºC for 5 h to remove moisture and organics before

23

performing the adsorption experiment. BET surface area and micropore volume were obtained

24

using relative pressure ranges of 0.01-0.07 (to avoid quasi-capillary condensation in micropores)

25

using the BET equation30, 31 and 0.2-0.4 using V-t method32, 33, respectively. Total pore volume

26

was measured at P/P0 = 0.95 to avoid interparticle condensation. Pore size distributions were

27

obtained using quenched solid density functional theory.34-36

7



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1

Gas chromatography-mass spectrometry (GC-MS) analysis

2

For the modified multi-component adsorbate stream (VOC mixture 2, Table 1),

3

adsorption experiments were performed for 390 min to achieve equilibration of all individual

4

VOCs. Full loading of the adsorbent occurred by 200 min similar to what was observed for VOC

5

mixture 1, but the test was continued for a longer duration to ensure and demonstrate no change

6

in the effluent concentration, likely leading to preferential adsorption of the higher molecular

7

weight compounds. The effluent stream during adsorption was characterized using GC-MS. A

8

modified adsorbate stream was used to avoid issues resulting from the high boiling points of

9

naphthalene and diethanolamine (218°C and 271 °C, respectively). Individual components had a

10

concentration of 62.5 ppmv and the total concentration was 500 ppmv. The GC-MS instrument,

11

calibration, and experimental methods are described elsewhere,16 but a brief overview is

12

provided here for clarity.

13

The GC-MS (Agilent Technologies model 7890A GC interfaced to 5975C inert MSD

14

with Triple-Axis Detector) was calibrated using the VOCs in the inlet gas stream (concentration

15

of each adsorbate was 62.5 ppmv). Effluent gas samples were collected every 15 min during

16

adsorption with 250 mL Tedlar bags (Saint Gobain Chemware) and injected immediately into the

17

instrument. The GC was equipped with a DB-1 advanced fused-silica capillary column that is 60

18

m long with a 0.32 mm diameter and 3 µm film thickness (Agilent J&W). The injected sample

19

was carried through the column using 2.6 mL/min of ultrapure helium. The injection volume was

20

1 mL and the split ratio was 10:1. The injection port temperature was 260 ºC and the oven

21

temperature was ramped at 20 °C/min from 80 to 260 ºC and held at 260 °C for 2 min, resulting

22

in a total analysis duration of 11 min.

23

RESULTS AND DISCUSSION

24

Breakthrough curves are used to assess changes in capacity during consecutive

25

adsorption cycles for the fixed and fluidized bed configurations. Consistent and reproducible

26

results were obtained from triplicate breakthrough tests. A sample of five consecutive adsorption

27

cycles using the single component (1,2,4-trimethylbenzene) (a, b) and multi-component (mixture

28

1) (c, d) VOC streams are shown in Figure 2. No VOC breakthrough was detected at least during

29

the first 90 mins of the experiment indicating negligible or no channeling in the tube.

8


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(b)600

1st cycle 2nd cycle 3rd cycle 4th cycle 5th cycle

500 400

Effluent Concentration (ppmv)


(a) 600

Fixed Bed Single Adsorbate

300 200 100


500 400

Fluidized Bed Single Adsorbate

300 200 100 0

0 0

(c) 600

50

100 150 200 Time (min)

0

250

(d) 600


500 400


Effluent Concentraion (ppmv)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fixed Bed Multi Adsorbate

300 200 100 0

50

100 150 Time (min)

200

250

200

250


500 400

Fluidized Bed Multi Adsorbate

300 200 100 0

0

50

100 150 Time (min)

200

250

0

50

100 150 Time (min)

1 2 3

Figure 2 Sample breakthrough curves for five adsorption cycles using different adsorption bed configurations and adsorbate streams: (a, b) single-component adsorbate (1,2,4trimethylbenzene) and (c, d) multi-component adsorbate (mixture 1)

4

For both adsorbate streams, breakthrough profiles were steeper (larger slope) for the

5

fluidized bed configuration, indicating more efficient mass transfer. This was quantified using a

6

throughput ratio (TPR), which is the ratio of breakthrough time (when effluent concentration is

7

1% of the influent) to the time when effluent concentration is 50% of the influent. Larger TPR

8

occurred for all the five cycles of the adsorption in fluidized bed for both single and multi-

9

component tests. For single-component tests, 1st cycle TPR was 0.74 ± 0.05 and 0.87 ± 0.04 for

10

triplicate experiments for fixed and fluidized bed configurations, respectively. These results are

11

consistent with the literature. Previous literature also found sharper breakthrough curves using

12

fluidized bed configurations when adsorbing methanol and isobutane on ZSM-5 (zeolite) and

9



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13, 15

1

toluene on a polymeric adsorbent, respectively.

2

component tests (Figure 2-c and Figure 2-d). For multi-component tests, the average 1st cycle

3

TPR for triplicate experiments were 0.73 ± 0.03 and 0.79 ± 0.01 for fixed and fluidized bed

4

configurations, respectively, based on breakthrough curves determined with the FID.

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A similar trend was obtained for multi-

5

Breakthrough times for the 1st adsorption cycle for the fixed and fluidized bed

6

configurations, respectively, were 101 ± 1 and 114 ± 1 min for single-component tests and 130 ±

7

6 and 144 ± 4 min for multi-component tests (Figure 2). These observed improvements for the

8

fluidized bed configuration (higher TPR and longer breakthrough times) are attributed to

9

improved mass transfer and mixing achieved by fluidizing the adsorbent. For single-component

10

adsorption of 1,2,4-trimethylbenzene, both adsorber configurations demonstrated negligible

11

change in breakthrough times during consecutive adsorption cycles, suggesting that adsorption is

12

reversible, adsorption capacity can be completely recovered with thermal regeneration, and heel

13

buildup is negligible.37 This is preferable for industrial adsorbents used in cyclic adsorption-

14

desorption systems, as it implies an extended adsorbent lifetime.38

15

The 5-cycle average adsorption capacity (in wt%) for 1,2,4-trimethylbenzene was 44.8 ±

16

0.3 for the fixed bed and 44.3 ± 0.6 for the fluidized bed, using mass balance on the adsorption

17

tube. Similar adsorption capacities were also obtained using the area above breakthrough curves.

18

The longer breakthrough time for the fluidized bed is negated by its higher TPR, resulting in the

19

same adsorption capacity as the fixed bed adsorber, as expected. The 5-cycle, cumulative heel

20

buildup for the single-component system was < 1 wt% for both adsorber configurations.

21

For the multi-component adsorption tests, the adsorption capacity decreased after each

22

adsorption/desorption cycle due to heel buildup. The average adsorption capacity for the first

23

cycle, determined using a mass balance, was 46.6% ± 1.0% and 48.6% ± 1.6% of the adsorbent

24

weight for fixed and fluidized bed configurations, respectively, which demonstrates similar

25

capacities obtained after fully loading the virgin adsorbent in both configurations. Unlike single

26

compound tests, the area above breakthrough curves were not consistent with mass balance

27

results, despite showing similar values for both configurations for the first adsorption cycle. This

28

is attributed to the FID’s low response factor for oxygenated compounds,39, 40 resulting in lower

29

concentration readings when using the FID, and hence larger areas above the breakthrough

30

curve. Therefore, the mass balance approach was identified as a more reliable technique in this

10


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1

work. However, breakthrough time and adsorption capacity consistently decreased with

2

adsorbent regeneration and reuse (Figure 2). The breakthrough during the 5th cycle occurred 29%

3

± 3% and 17% ± 1% earlier than during the 1st cycle for the fixed and fluidized beds,

4

respectively (Figure 3). This decrease demonstrates incomplete recovery of the adsorption

5

capacity during consecutive cycles2 and is more pronounced in the fixed bed configuration. The

6

adsorption capacities decreased to 39.8 ± 1.0 wt% and 43.6 ± 0.9 wt% in the 5th cycle for fixed

7

bed (14.6% decrease) and fluidized bed (10.2% decrease) configurations, respectively. The 5-

8

cycle cumulative heel buildup was 10.8 ± 0.1 wt% and 7.6 ± 0.1 wt % for fixed bed and fluidized

9

bed adsorption configurations, respectively, or 42% greater for the fixed bed configuration

10

(Figure 3) consistent with its greater reduction in breakthrough time and greater drop in

11

adsorption capacity. Thermo-gravimetric analyses also supported this conclusion (Figure 3). For

12

the fixed bed configuration, heel formation with a similar adsorbate mixture has been previously

13

attributed to competitive adsorption resulting from high molecular weight, bulky compounds

14

with high boiling points (that are difficult to desorb) displacing low molecular weight

15

compounds with low boiling points (that are easier to desorb).16, 41-44 This primarily occurs at the

16

fixed bed adsorber’s inlet. In contrast, fluidized bed configuration distributes the organics evenly

17

throughout the adsorbent bed, resulting in lower heel development. Even distribution of

18

adsorbates and correspondingly low heel buildup, therefore, appears to be a notable advantage of

19

fluidized bed configuration. 30

29.0 ± 3.0

Breakthrough Time Decrease (% Change from 1st to 5th Cycle) Adsorption Capacity Decrease (% Change from 1st to 5th Cycle)

25 Percent (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5-Cycle Heel Formation (wt%, Mass Balance) 5-Cycle Heel Formation (wt%, TGA)

20 15

17.0 ± 1.0 14.6 ± 1.0 10.8 ± 0.1

10

10.2 ± 1.3

8.1 ± 0.1

7.6 ± 0.1 5.8 ± 0.1

5 0

20

Fixed Bed

Fluidized Bed

11



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1 2 3 4

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Figure 3 Percent changes in breakthrough time and adsorption capacity for multicomponent adsorbate stream. Also, cumulative heel formation after five adsorptiondesorption cycles based on gravimetric mass balances and TGA (Results are based on triplicate experiments)

5

The average consumed energies for regenerating BAC loaded in fixed bed and fluidized

6

bed adsorption configurations are shown in Table 2. The temperature profile and energy

7

consumption were the same for all regeneration experiments completed after adsorption cycles

8

using each configuration. This indicates that the differences in heel buildup and changes to the

9

adsorption capacity and breakthrough time between the fluidized and fixed bed adsorption

10 11 12 13

configurations are not due to biased desorption conditions. Table 2 Average energy consumption for all the experiments during 3 h fixed bed desorption (standard deviation is based on all the desorption experiments completed after adsorption process for each configuration) Fixed bed adsorption configuration Fluidized bed adsorption configuration

Single-component 91.4 ± 6.8 MJ 92.9 ± 8.6 MJ

Multi-component 83.8 ± 7.1 MJ 84.0 ± 2.9 MJ

14

Pore size distributions of regenerated BACs used for 5-cycles of adsorption of multi-

15

component adsorbate mixture 1 are provided in Figure 4 with specific surface area, total pore

16

volume, and micropore volume of the samples. Decreases in each of these physical parameters

17

are noted for the used adsorbents in both adsorber configurations. The decrease is attributed to

18

heel buildup and pore blockage from bulky molecules (with high molecular weight and boiling

19

point) in the adsorbate, such as naphthalene. While all adsorbates tested here have kinetic

20

diameters < 8 Å 20, near the lower end of the BAC pore size distribution, molecules with similar

21

size to narrow micropores and high boiling points may be trapped due to superposition of wall

22

effects, resulting in blocking the pore or its entrance. This prevents nitrogen penetration and

23

adsorption during micropore surface analysis.2, 45

12


Page 13 of 24

BAC Sample

0.2

Surface area Total pore Micropore (m2/g) volume(cm3/g) volume(cm3/g)

Virgin BAC

1349

0.57

0.50

Full saturation (fluidized)

1045

0.45

0.37

Full saturation (fixed)

897

0.38

0.32

0.15 d(V) (cm3/Å/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.1

0.05

0 5

1 2 3

10

15 Pore width (Å)

20

Figure 4 Pore size distributions of virgin and regenerated BACs previously saturated with VOC mixture 1 for five adsorption/desorption cycles

4

Decreases in surface area, total pore volume, and micropore volume are larger for the

5

fixed bed BAC, consistent with higher heel development. In particular, the volume of narrow

6

micropores was lower for samples loaded in the fixed bed configuration than in the fluidized bed

7

configuration, confirming previous studies that suggested heel buildup occurs in pores with

8

widths closest to the adsorbate kinetic diameters.2, 16 Since desorption was completed under the

9

same conditions (Table 2), regardless of adsorption configuration, differences in the physical

10

properties of adsorbents can be entirely attributed to differences during adsorption.

11

It is again suggested that improved mixing (enhanced gas-solid mass transfer) in the

12

fluidized bed system allows for uniform adsorbate distribution across the BAC bed, minimizing

13

competitive adsorption and subsequent heel buildup. This hypothesis is assessed using TGA to

14

quantify heel build-up at different BAC locations (inlet, middle, outlet) in the fixed and fluidized

15

beds, for the multi-component adsorbate stream. The expectation is that heel build-up is

16

uniformly distributed throughout the fluidized bed, independent of location, but it is concentrated

17

in BAC at the adsorption tube inlet in the fixed bed configuration. Because fluidization provides

18

adsorbate mixing and better mass transfer between the gas and the adsorbent, it was expected

13



1

that the distribution of heavy adsorbates (more susceptible to heel formation) on the BAC bed

2

would be more uniform in the fluidized bed than in the fixed bed.8 For the single component

3

adsorbate, differences were negligible because competitive adsorption does not occur and heel

4

build-up is minimal (< 1%). 100 99 98 Weight (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 24

97 96 95 94 93 92 0

Fixed (Outlet)

Fluidized (Outlet)

Fixed (Middle)

Fluidized (Middle)

Fixed (Inlet)

Fluidized (Inlet)

100

200

300 400 Temperature (oC)

500

600

5 6 7

Figure 5 TGA results for regenerated BAC after one cycle adsorption of the VOC mixture from inlet, middle, and outlet of the adsorbers

8

For the multi-component tests, non-uniform adsorbate distribution was more prominent

9

in the fixed bed configuration. For the fixed bed, the amount of heel indicated by the TGA

10

analysis was 124 ± 18 % higher for BAC sampled from the inlet compared to the outlet of the

11

bed. It should be noted that the major portion of the weight loss for the inlet of the fixed bed

12

occurred after 288 °C associated with a greater accumulation of heavy adsorbates. A much more

13

uniform distribution was found for the fluidized bed, with only a 15 ± 2 % difference in heel for

14

BAC sampled from the inlet of the settled bed compared to the outlet. This result is consistent

15

with the literature, as Hamed 46 similarly showed uniform adsorbate (humidity) distribution on a

16

fluidized silica gel bed. The opposite was true in the fixed bed configuration, where adsorption

17

initially occurred at the adsorber inlet and then proceeded towards the adsorber outlet.12

18

A fixed bed configuration inhibits uniform adsorbate contact throughout the adsorbent

19

bed. That is to say, BAC near the fixed bed inlet is constantly in contact with the incoming

14


Page 15 of 24

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1

adsorbate, while BAC at the bed exit only is exposed to the adsorbate that breaks through. This

2

promotes adsorbate displacement on BAC near the adsorber’s inlet, as heavier compounds will

3

preferentially displace lighter compounds, explaining the larger mass losses observed in Figure 5

4

at the adsorber inlet.16 In fluidized bed adsorbers, however, mixing of the adsorbate and

5

adsorbent reduces the occurrence of uneven adsorption, simultaneously decreasing heel buildup.

6

These heavy molecules continue to accumulate through the 5-cycle adsorption experiments,

7

resulting in higher heel formation for the fixed bed adsorber than the fluidized bed adsorber.

8

Competitive adsorption and adsorption kinetics were verified and quantified (Figure 6)

9

by sampling the adsorbers’ effluent streams during multi-component adsorption experiments and

10

analyzing with GC-MS. As described earlier, the multi-component adsorbate for both bed

11

configurations was modified (mixture 2, Table 1) to accommodate the sampling and GC-MS

12

analysis method, but competitive adsorption trends were still evident. The previously observed

13

trends of later and sharper breakthrough for the fluidized bed were seen here as well, with

14

breakthrough starting after 85 min and 97 min for the fixed and fluidized bed configurations,

15

respectively.

16

n-butanol (boiling point of 118 ºC) and n-butylacetate (126 ºC), were the first compounds

17

detected for both bed configurations. These compounds have the lowest boiling points and small

18

kinetic diameters of the mixture, suggesting a low adsorption affinity for the BAC compared to

19

the heavier components in the mixture.16,

20

concentration values for these species to be higher than their inlet concentrations,41 as the

21

displaced VOC concentration adds to the inlet concentration. Once again, breakthrough curves,

22

even for individual adsorbate components, are sharper for the fluidized bed, indicating a shorter

23

mass transfer zone.47

43

Competitive adsorption causes the peak

24

Although breakthrough starts later in the fluidized bed, the lighter compounds’ peak

25

concentrations appear sooner and have higher maximum values than in the fixed bed. The

26

shallower slope of the breakthrough curve in the fixed bed can be attributed to adsorption non-

27

uniformity resulting in re-adsorption of lighter molecules in the upper part of the adsorber. This

28

phenomenon is primarily observed for the two lowest molecular weight compounds in the

29

mixture. For compounds larger than n-butylacetate, breakthrough curves for the fixed and

30

fluidized beds were similar. TGA showed higher heel formation in the fixed bed adsorber inlet, 15



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Page 16 of 24

1

attributed to competitive adsorption. In contrast, in the fixed bed, desorbed compounds from

2

competitive adsorption can be re-adsorbed (potentially multiple times) in the less saturated BAC

3

located closer to the adsorption tube outlet. BAC in the fluidized bed is more equally loaded

4

throughout the adsorption process, but competitive adsorption still occurs with heavier VOCs

5

replacing lighter VOCs over time until an equilibrium distribution of VOCs is reached.48, 49 Our

6

previous studies showed that heel buildup in the described adsorption system is primarily caused

7

by non-desorbed physisorption (< 10% contribution from chemisorption).16 In this work,

8

accumulation of heavy compounds at the fixed bed inlet results in pore blockage. These heavy

9

compounds remained non-desorbed at the prescribed regeneration conditions.

10

VOC components in the effluent gas (in order of increasing breakthrough time) were n-

11

butanol (B.P. 118 ºC), n-butylacetate (126 ºC), 2-heptanone (151 ºC), 2-butoxyethanol (171 ºC),

12

n-decane

13

dimethylpropylbenzene (186 ºC), for both configurations. Generally, VOC breakthrough times

14

followed the same order as boiling points, except for 1,2,4-trimethylbenzene.16,

15

compound’s longer breakthrough compared to 2-butoxyethanol and n-decane might be attributed

16

to its bulky structure (i.e., larger kinetic diameter, Table 1). An aromatic ring with three side

17

chains is notably different than the straight chain structures of n-decane and 2-butoxyethanol,

18

and desorption of bulkier molecules from micropores is more difficult due to increased diffusion

19

resistance.16 As previously mentioned, large heel buildup can occur in micropores5 because of

20

attractive forces between opposing walls, which increases adsorption energy and thus the

21

required energy consumption during regeneration.50 In this case, a temperature of 288 °C may

22

not suffice for complete desorption of heavy molecules accumulated at the inlet of the fixed bed.

23

Independent of adsorber configuration, it can be concluded that compounds with high boiling

24

points and/or high molecular weights have more adsorption affinity. For components with

25

similar boiling points, molecular weight, structure, and functionalities must be considered, again

26

independent of adsorber configuration.16, 42

(174

ºC),

1,2,4-trimethylbenzene

16

(170

ºC),


indan

(176

ºC),

and 44

2,2-

This

(a)

250

600 500

200

400 150 300 100 200 50

100

0

0 0

50

n-butanol (118°C) 2-butoxyethanol (171°C) indan (176°C) Total by FID

100

150

200 250 Time (min) n-butyl acetate (126°C) n-decane (174°C) 2,2-dimethylpropylbenzene (186°C)

17


300

350

400

2-heptanone (151°C) 1,2,4-trimethylbenzene (170°C) Total by GCMS

Total effluent concentration by FID & GC-MS (ppmv)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Individual analyte concentration in the effluent (ppmv)

Page 17 of 24

Page 18 of 24

(b) 600

250

500

200

400 150 300 100 200 50

100

0

0 0

50

n-butanol (118°C) 2-butoxyethanol (171°C) indan (176°C) Total by FID

100

150

200 250 Time (min) n-butyl acetate (126°C) n-decane (174°C) 2,2-dimethylpropylbenzene (186°C)

300

350

400

Total effluent concentration by FID & GC-MS (ppmv)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Individual analyte concentration in the effluent (ppmv)


2-heptanone (151°C) 1,2,4-trimethylbenzene (170°C) Total by GCMS

Figure 6 Effluent concentration during adsorption of VOC Mixture 2 on BAC in order of the components’ retention time in GC using (a) fixed bed and (b) fluidized bed configuration. The boiling points are shown by the components in the legends. The second axis on the right gives the total adsorbates concentration measured by GC-MS and FID in the effluent.

18


Page 19 of 24

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CONCLUSIONS This research compares the performance of a fixed bed and a fluidized bed configuration for VOC adsorption. Five-cycle, adsorption experiments were used to assess heel buildup in both adsorption configurations, with the same desorption methods used for both experiments. Thermo-gravimetric analysis, micropore analysis, and gas chromatography-mass spectrometry were used to support the mass balances and breakthrough curves resulting from adsorption cycling. Five-cycle heel formation is 30% lower for the fluidized bed configuration compared to the fixed bed configuration when a multi-component adsorbate was used. Furthermore, after 5 cycles, the fluidized bed displayed reductions in adsorption capacity and breakthrough time that were 41 % and 30 % less than the reductions displayed by the fixed bed. As expected, no major differences were noted for single adsorbate gas streams. More generally, it can be concluded that the adsorption bed configuration affects both adsorption kinetics and heel buildup. Differences in heel buildup are attributed to mixing in the fluidized bed causing uniform adsorbate distributions, minimizing competitive adsorption for the multi-component adsorbate but not affecting the single-component system. While desorber design and operating conditions are more critical in preventing heel buildup, this research shows that adsorber design and operating conditions can play a critical role in the long-term performance of a VOC abatement system. ACKNOWLEDGEMENTS We acknowledge financial support for this research from Ford Motor Company and the Natural Science and Engineering Research Council (NSERC) of Canada. We also acknowledge the support of infrastructure and instruments grants from Canada Foundation for Innovation (CFI), NSERC, and Alberta Advanced Education and Technology. While this article is believed to contain correct information, Ford Motor Company (Ford) does not expressly or impliedly warrant, nor assume any responsibility, for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, nor represent that its use would not infringe the rights of third parties. Reference to any commercial product or process does not constitute its endorsement. This article does not provide financial, safety, medical, consumer product, or public policy advice or recommendation. Readers should independently replicate all experiments, calculations, and results. The views and opinions

19



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expressed are of the authors and do not necessarily reflect those of Ford. This disclaimer may not be removed, altered, superseded or modified without prior Ford permission. REFERENCES: (1) Chiang, Y. C.; Chiang, P. C.; Huang, C. P. Effects of Pore Structure and Temperature on VOC Adsorption on Activated Carbon. Carbon 2001, 39, 523. (2) Lashaki, M. J.; Fayaz, M.; Wang, H.; Hashisho, Z.; Philips, J. H.; Anderson, J. E.; Nichols, M. Effect of Adsorption and Regeneration Temperature on Irreversible Adsorption of Organic Vapors on Beaded Activated Carbon. Environ. Sci. Technol. 2012, 46, 4083. (3) Niknaddaf, S.; Atkinson, J. D.; Shariaty, P.; Jahandar Lashaki, M.; Hashisho, Z.; Phillips, J. H.; Anderson, J. E.; Nichols, M. Heel Formation during Volatile Organic Compound Desorption From Activated Carbon Fiber Cloth. Carbon 2016, 96, 131. (4) Fayaz, M.; Shariaty, P.; Atkinson, J. D.; Hashisho, Z.; Phillips, J. H.; Anderson, J. E.; Nichols, M. Using Microwave Heating to Improve the Desorption Efficiency of High Molecular Weight VOC from Beaded Activated Carbon. Environ. Sci. Technol. 2015, 49, 4536. (5) Jahandar Lashaki, M.; Atkinson, J. D.; Hashisho, Z.; Phillips, J. H.; Anderson, J. E.; Nichols, M. The Role of Beaded Activated Carbon's Pore Size Distribution on Heel Formation during Cyclic Adsorption/Desorption of Organic Vapors. J. Hazard. Mater. 2016, 315, 42. (6) Jahandar Lashaki, M.; Atkinson, J. D.; Hashisho, Z.; Phillips, J. H.; Anderson, J. E.; Nichols, M.; Misovski, T. Effect of Desorption Purge Gas Oxygen Impurity on Irreversible Adsorption of Organic Vapors. Carbon 2016, 99, 310. (7) Liu, P. K. T.; Feltch, S. M.; Wagner, N. J. Thermal Desorption Behavior of Aliphatic and Aromatic Hydrocarbons Loaded on Activated Carbon. Ind. Eng. Chem. Res. 1987, 26, 1540. (8) Yazbek, W.; Pré, P.; Delebarre, A. Adsorption and Desorption of Volatile Organic Compounds in Fluidized Bed. J. Environ. Eng. 2006, 132, 442. (9) Sanders, R. E. Designs that Lacked Inherent Safety: Case Histories. J. Hazard. Mater. 2003, 104, 149. (10) Delage, F.; Pre, P.; Le Cloirec, P. Mass Transfer and Warming During Adsorption of High Concentrations of VOCs on An Activated Carbon Bed: Experimental and Theoretical Analysis. Environ. Sci. Technol. 2000, 34, 4816. (11) Danielsson, M. A.; Hudon, V. VOC Emission Control Using a Fluidized-bed Adsorption System. Met. Finish. 1994, 92, 89. (12) Hamed, A. M.; Abd El Rahman, W. R.; El-Eman, S. H. Experimental Study of the Transient Adsorption/Desorption Characteristics of Silica Gel Particles in Fluidized Bed. Energy 2010, 35, 2468. (13) Ng, Y. L.; Yan, R.; Tsen, L. T. S.; Yong, L. C.; Liu, M.; Liang, D. T. Volatile Organic Compound Adsorption in a Gas-Solid Fluidized Bed. Water Sci. Technol. 2004, 50, 233. (14) Reichhold, A.; Hofbauer, H. Internally Circulating Fluidized Bed for Continuous Adsorption and Desorption. Chem. Eng. Process. 1995, 34, 521. (15) Song, W.; Tondeur, D.; Luo, L.; Li, J. VOC Adsorption in Circulating Gas Fluidized Bed. Adsorption 2005, 11, 853. (16) Wang, H.; Jahandar Lashaki, M.; Fayaz, M.; Hashisho, Z.; Philips, J. H.; Anderson, J. E.; Nichols, M. Adsorption and Desorption of Mixtures of Organic Vapors on Beaded Activated Carbon. Environ. Sci. Technol. 2012, 46, 8341.

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List of figure captions

Figure 1 Schematic of the adsorption/desorption setup for fixed and fluidized bed adsorption tests ................................................................................................................................................. 6 Figure 2 Sample breakthrough curves for five adsorption cycles using different adsorption bed configurations and adsorbate streams: (a, b) single-component adsorbate (1,2,4trimethylbenzene) and (c, d) multi-component adsorbate (mixture 1) ........................................... 9 Figure 3 Percent changes in breakthrough time and adsorption capacity for multi-component adsorbate stream. Also, cumulative heel formation after five adsorption-desorption cycles based on gravimetric mass balances and TGA (Results are based on triplicate experiments) ............... 12 Figure 4 Pore size distributions of virgin and regenerated BACs previously saturated with VOC mixture 1 for five adsorption/desorption cycles ........................................................................... 13 Figure 5 TGA results for regenerated BAC after one cycle adsorption of the VOC mixture from inlet, middle, and outlet of the adsorbers ...................................................................................... 14 Figure 6 Effluent concentration during adsorption of VOC Mixture 2 on BAC in order of the components’ retention time in GC using (a) fixed bed and (b) fluidized bed configuration. The boiling points are shown by the components in the legends. The second axis on the right gives the total adsorbates concentration measured by GC-MS and FID in the effluent. ....................... 18

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Effect of Beaded Activated Carbon Fluidization on Adsorption of

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