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Optimizing Microwave-Assisted Pyrolysis of Phosphoric Acid-Activated Biomass: Impact of Concentration on Heating Rate and Carbonization Time Elmar Villota, Hanwu Lei, Moriko Qian, Zixu Yang, Shiela Marie Villota, Gayatri Yadavalli, and yayun zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03669 • Publication Date (Web): 26 Nov 2017 Downloaded from http://pubs.acs.org on December 7, 2017

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ACS Sustainable Chemistry & Engineering

Optimizing Microwave-Assisted Pyrolysis of Phosphoric Acid-Activated Biomass: Impact of Concentration on Heating Rate and Carbonization Time Elmar M. Villota , Hanwu Lei *, Moriko Qian , Zixu Yang , Shiela Marie A. Villota , Yayun 1,2

1

1

1

2

Zhang ,, Gayatri Yadavalli 1

[1]

1

Department of Biological Systems Engineering, Washington State University, Richland, WA 99345-1671, USA

[2]

Department of Agricultural and Biosystems Engineering, Central Luzon State University, Science City of Muñoz, Nueva Ecija, Philippines

*Corresponding author. Tel.: +1 509 372 7628; fax +1 509 372 7690. E-mail: [email protected] KEYWORDS. Activated carbon, chemical activation, microwave heating, pyrolysis time

ABSTRACT. Recent studies suggest the use of microwave energy in activated carbon (AC) production emphasizing efficiency specifically in pyrolysis step as it can significantly reduce heating time compared to conventional furnaces. However, there is no documented effort on the effect of the activating chemical agent on microwave heating dynamics and its impact on pyrolysis

ACS Paragon Plus Environment

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time despite its importance on efficiency on both technical and economic aspects of the process. Elucidating the heating rate of H PO activated biomass under microwave energy is one of the 3

4

objectives of the research while the ultimate goal is to find practical H PO concentration for 3

4

chemical activation of biomass precursors in obtaining optimum AC yield and textural characteristics. It was found that excessive H PO has negative effect of slowing down pyrolysis 3

4

time as it promotes poor microwave absorption on the biomass-H PO complex. In addition, H PO 3

4

3

4

undergoing conversion to its anhydride, P O , requires relatively high activation energy and its 2

5

conversion may possibly cause extended pyrolysis time. Numerical optimization revealed chemical activation at 56.50% H PO and pyrolysis under 650 W microwave power is a rational 3

4

balance in terms of maximizing yield and surface area while minimizing activating agents and microwave energy. Under these conditions, pyrolysis time of around 30 min, yield at around 39.65 wt. %, and surface area of 826.38 m /g can be expected. However, surface area can be as high as 2

1726.5 m /g but will require 84.83% H PO on activation, about 803.75 W power and 48 minutes 2

3

4

carbonization time.

INTRODUCTION Activated carbon (AC) is desired for many sorptions and catalysis applications due to its high surface area and pore volume, and pore size distribution. In general, the parent material and the process of porosity development (e.g. activation method) dictate these properties. Biomass as a feedstock for AC production is very attractive because they are available and renewable. One great challenge, as in any biomass conversion technologies, is the balance of technical and economic efficiency thus numerous studies are being done with this motivation. Further, it is worth to note that available AC in the market are application-driven and is a product of rigorous research and development work . 1


2

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Activation methods are generally classified as physical and chemical based on the nature of its mechanism. Main differences between the two processes are the activating agent and carbonization temperature. Physical involves steam, carbon dioxide, or other air mixtures as oxidizing agents and is carried out at a range of 600–900 °C both for activation and carbonization operations. On the other hand, conventional activating agents for chemical activation consist of H PO , ZnCl , and 3

4

2

KOH. Chemical activation can be carried out at standard room conditions while carbonization step depends on the chemical agent but is relatively at lower temperature than physical activation . In 2–4

view of optimizing specific process parameters to obtain a specified degree and quality of AC textural properties, chemical activation is favored over physical activation because process variables of the physical activation process was found to show no perceivable influence on surface area and the pore size distribution . Mostly used chemical activation agents in recent researches 1,5

include ZnCl and H PO but because of advantages in technological and environmental concerns, 2

3

4

the latter gained more attention over the former. Main advantages of H PO over ZnCl are the ease 3

4

2

of recovery and minimal environmental impacts . 3,6,7

Pyrolysis of H PO impregnated wood can be reduced to two parallel and independent reactions: 3

4

(1) volatilization of water from wood and (2) H PO solution and carbonization of H PO activated 3

4

3

4

wood materials . Overall conversion can be divided further into six reactions: (i) reaction of H PO 10

3

4

and biomass with evolution of biomass-H PO (BP) complex and water; (ii) evaporation of water 3

4

in the complex; (iii) water desorption of H PO to evolution of water and phosphorus pentoxide 3

4

(P O ); (iv) volatilization of P O ; (v) conversion of wood- H PO complex into char and volatiles; 2

5

2

5

3

4

and (vi) degradation of char to gases . Table 1 summarizes reaction parameters as observed by 8,10

Hared et al. (2007) in their kinetic study of pine wood activated by 85% H PO . 8

3

4


3


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Table 1. Reaction parameters of pyrolysis of H PO activated pine wood 3

REACTIONS

4

TEMPERATURE

ACTIVATION

RANGE, °C (i)

Biomass + H) PO, → BP + H/ O

ENERGY, kJ/mol -

Room temperature

a

(ii)

H/ O liquid → H/ O(gas)

(iii)

H) PO, → (P/ O8 + 3H/ O)

Continuous heating, 104

71.5

(iv)

P/ O8 (𝑙𝑖𝑞𝑢𝑖𝑑) → P/ O8 (𝑔𝑎𝑠)

580–585

102.5

(v)

BP → 𝛼char + 1 − 𝛼 volatiles

240–400

32.8

(vi)

Char → gas

higher than 700

61.5

100–230

7

30.8 b

/

- initiates instantaneously upon contact - another study by Majerus et al. (2012) observed it at around 104 °C [a]

[b]

11

Recent studies employing microwave as mode of heating in carbonization step of AC production from biomass proved that heating time is significantly reduced compared to conventional furnaces. Namazi et al. (2016) produced chemically activated carbon out of biochar from pulp mill sludge with similar surface area in 2 hours under 600° C conventional heating and in 5 minutes under 1200 W microwave . A similar study produced comparable textural properties 12

of activated carbon from sewage sludge heated in 500 °C furnace in 15 minutes and in 980 W microwave in 12 minutes . Activated carbon derived from raw bamboo chemically activated by 13

H PO observed to have faster activation rate and higher yield under microwave heating . Most 14

3

4

researchers suggested the use of microwave in activated carbon production from biomass precursors emphasizing efficient use of energy and fast heating operation

15–25

.

Though researchers have a universal agreement that microwave-assisted heating is a promising way of reducing pyrolysis time, most investigations employ heating time as an independent


4

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variable, rather than a response variable. Also, there is no account of temperature history or heating rate to at least have an insight on the thermal profile and understand the degree of conversion with respect to processing time. To date, there is no documented effort to look at the effect of activating chemical agent on microwave heating and its impact on total pyrolysis time when they are crucial on efficiency of both technical and economic aspects of the process. The study aims to outline the influence of H PO concentration on average heating rates and resulting pyrolysis time under 3

4

varying microwave power levels of chemically activated Douglas fir. It is also sought to find meaningful relationship between the said variables with the motivation of obtaining a predictive insight on the optimized process. METHODOLOGY AC Production The overall process of activated carbon production can be divided by three major operations: (1) chemical activation; (2) microwave carbonization; and (3) washing. Fifty grams of Douglas fir pellets (Bear Mountain Forest Products Inc., Cascade Locks, OR, USA) was used in each run. Chemical activation was done by soaking 50 g of Douglas fir pellets in a respective activating chemical agent concentrations in a 500 ml beaker. The activating agent was prepared based on the required concentration of the run (38.79 wt.%, 45 wt.%, 60 wt.%, 75 wt.% and 81.21 wt.%) by adding appropriate amount of deionized water from stock of 85% (wt./wt.) aqueous solution H PO 3

4

(Alfa Aesar, USA). The samples were soaked for 24 hours and were kept covered with a paraffin film inside the fume hood during soaking. All impregnated samples were placed for 4 to 6 hours in convection oven at 105 °C then left in fume hood overnight. Carbonization entails heating activated precursors under inert environment to thermally degrade components of the biomass other than carbon. Heating was accomplished using Sineo MAS-II


5


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batch 2.45 GHz microwave oven (Shanghai Sineo Microwave Chemistry Technology Co., Ltd., Shanghai, China). Impregnated samples were placed in a 500 ml quartz three-neck flask and was positioned inside the microwave cavity (Figure 1). The setup was similar to the pyrolysis setup for AC production by other researchers

26–29

. Inlet neck is for purging nitrogen gas into the reactors while

outlet is for the volatile gases produced during the pyrolysis. The reactor was purged with nitrogen gas (99% purity) at 1000 mL/min for 30 minutes prior to microwave pyrolysis to create an inert atmosphere. In-line to the outlet manifold was a condenser system with 1.8 °C cooling water. Temperature was measured by an infrared thermometer inserted through the middle neck via dead end quarts tube projecting through the geometric center of the flask. System is controlled by thermostat such that microwave energy source is switched on until desired temperature has reached and will be maintained with minimal microwave energy. . The microwave pyrolysis system was set according to the power requirement of the experimental design (Table 2.) while heating temperature limit was set to 450° C aiming for higher carbon yield as suggested by Lim et al. (2010) . 30

Figure 1. Simplified schematic of the microwave-assisted carbonization setup


6

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Once samples reached the set temperature, it was held for five more minutes for further heating. Carbons collected from the carbonization step were collected from the flask and were subjected to washing. In a 500 ml beaker, 200 ml of 0.1 N hydrochloric acid solution was added in the carbon product and was continuously stirred in a magnetic stirrer for at least 4 hours to eliminate metals that were possibly present and may clog pores

20,26,31

. AC samples were then washed further at least

eight more times to obtain neutral pH of around 6–7. This endpoint is critical as crystalline P O , 2

5

basically dehydrated H PO is expected to be present in the product as the pyrolysis temperature 3

4

might not be able to vaporize it. AC samples were dried in oven for at least 36 hours at 105 °C and collected for characterization. Response Surface Analysis Response surface methodology (RSM) is an effective tool of estimating main effects and interaction up to their cubic degree. It is also a popular approach in optimizing process variables as well as identifying and troubleshooting process weaknesses. In this study, it was considered mainly to identify the degree of influence of the main factors, and their interaction if any, specifically for process optimization and not for modeling. A conventional 2 circumscribed central 2

composite design with 5 central points and 4 factorial and 4 axial (α=1.141) points leading to 13 runs (Table 2) was considered in the study. Factors include H PO concentration with central point 3

4

and step of 60% and 10% and microwave power with central point and step of 700 W and 100 W respectively. Response considered include heating rate, total pyrolysis time, and yield — the primary interest of any biomass thermochemical conversion process. Surface area was also considered as a response because it is a fundamental property of activated carbon for any application. Using these responses, an optimum H PO and microwave power for the pyrolysis 3

4


7


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combination can be outlined. All statistical work was done using Design Expert 7.0 (Stat-Ease Inc., Minneapolis 55413). AC Characterization Key structural properties of activated carbons that makes it suitable candidate for catalysis and adsorption applications are surface area, because it directly correlated with the reactivity of the media, and porosity (pore volume and its size distribution), as it primarily dictates the selectivity and capacity of the AC to be used to a certain application. Surface area and other textural properties were quantified through Micromeritics® Tristar II 3020. AC samples N sorption isotherms were 2

obtained at 77 K, from which, BET surface area, pore volume and average pore size were calculated. Pore size distributions are obtained from N isotherms produced by the physisorption 2

analysis following Barrett-Joyner-Halenda (BJH) method. Samples were degassed and pre-treated at 250 °C overnight to remove impurities prior to analysis. The Fourier Transform Infrared Spectroscopy (FTIR) spectra of raw Douglas fir and produced AC were obtained with an IR Prestige 21 spectrometer in the attenuated total reflection (ATR) mode (Shimadzu, Ge Crystal; software: IR Solution). The spectra were obtained at 8 cm

−1

resolution from 500 to 4500 cm using a combined 64 scans. No further pretreatments were done −1

to prior to FTIR analysis. The elemental analysis of biochar was performed at Biosystems Engineering Laboratory of Washington State University (Pullman, WA) while SEM micrographs were obtained through the help of Francheschi Microscopy and Imaging Center (Pullman, WA). RESULTS AND DISCUSSION Heating Characteristics of H PO -Activated Douglas fir Under Microwave Energy 3

4

Since microwave heating is mainly driven by dielectric properties of material being heated, and thermal degradation of wood, wood-H PO complex, and H PO derivative involve series of product 3

4

3

4


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evolution, the rate of heating and temperature history are principal factors to understand in microwave-heated pyrolysis. In this method microwave energy absorbed by the sample is responsible to cause volatilization of both water and organic gases, and to effect sensible increase in temperature of the carbonized and unconverted masses. It is known that presence of water causes higher microwave absorptivity as biomass alone is a poor absorber of microwave energy

32,33

. In

contrast, increasing concentration of H PO was found to cause drop in dielectric constant which 3

4

could result in poor absorption of energy from microwave source . 34

Temperature history (Figure 2a) suggests that lower concentration H PO heats up way faster than 3

4

others with higher concentration and reaches the desired temperature in about 9 minutes. This can be attributed to the amount of free water present, which absorbs substantial microwave energy to cause vaporization of water and heating up to provide energy for further conversion. Samples activated with higher concentrations H PO showed a different trend. Increasing H PO resulted in 3

4

3

4

prolonged heating to attain complete carbonization. Longest heating time to achieve carbonization occurred at highest concentration in the study, 81.21 % H PO , and lasted for about 58 minutes. As 3

4

mentioned, higher concentration of H PO can result in poor microwave energy absorption. 3

4

Vaporization of water from sample’s moisture and water evolve reaction 1 stretched from 100° C to around 220 °C due to poor absorption of microwave energy to heat. Comparing the rate in increase in temperature as depicted by the slope of the temperature history (Fig 2a) from 100 °C–220 °C across all concentrations, there was an evident decrease on the rate of temperature rise with respect to H PO concentration to which the increase in processing time is 3

4

mainly accounted. Poor microwave absorption and higher energy requirement for desorption of excess H PO is deemed to be primary culprit on high H PO concentration impregnation scenario. 3

4

3

4

At the later stage of the process however, at above 230 °C, it can be noted that the rate of increase


9


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in temperature across all concentration were very similar. At this stage, all water has been vaporized and excess H PO were evolved to P O but most importantly, char was starting to evolve 3

4

2

5

in the significant amount causing substantial increase in microwave energy absorption. Microwave heating is improved when substantial amount of char has evolved since carbon has good microwave energy absorbing capacity

35,36

. Similar rate in increase in temperature and conversion

rate at this stage imply that similar energy absorption and similar reactions undergoing was occurring across all treatments.

Figure 2. Temperature history (a) under microwave heating and DTG (b) of H3PO4-impregnated wood Looking at the derivative thermogravimetric (DTG) analysis (Figure 2b) of biomass impregnated with 38% H PO , two peaks were observed. The first peak at around 100 °C, and the 3

4

second at around 160 °C, both can be attributed to reaction (ii), vaporization of water. First peak is due to evaporation of water from the sample and water evolved from reaction (i), the second peak implies shifting from reaction (ii) to reaction (iii) as rate controlling reaction in the mechanism. The peak at 160 °C appeared to be higher, as it also involves conversion of the excess H PO and instantaneous evaporation of water evolved in the same conversion reaction. Also, it is 3

4


10

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believed that release of light gases (CO, CO , and CH ) at relatively lower temperature is high likely 2

4

for H PO impregnated wood . Width of the observed peaks in the DTG depicts the exhaustion of 8,37

3

4

substrates undergoing the reaction. Relatively narrow width for both peaks on the DTG imply fast exhaustion of substrate in the involved reaction. DTG for 81.21 % H PO involves only one but 3

4

wide distinct peak that occurred at around 120 °C. This phenomenon suggests overlapping of reaction (ii) and reaction (iii) as an overall result of slow heating due to high activation energy requirement. The peak was observed to have stretched into a wide range time and temperature implying reaction (iii) as the rate controlling mechanism and invoking the factor of substrate quantity. Resulting heating rate, pyrolysis time, AC yield, and BET surface area for all the treatments are summarized in Table 2. Lowest concentration activation completed carbonization in 14.38 minutes while the highest was prolonged up to almost an hour, 57.80 minutes. Intermediate concentrations, 45%, 60%, and 75% were processed up to around 20, 35, and 44 minutes to achieve carbonization respectively, implying pyrolysis time to attain carbonization increases with H PO concentration. 3

4

From these results, effectivity of the microwave heating was found to decrease with increasing concentration based on the premise discussed earlier. Efficiency of heating can raise a concern on economy of the process since energy is considered a prime commodity. Having this concern, 81.21% concentration pose a critical level to consider since it is evident that under this treatment, microwave heating seems to be very poor and may have effect on overall economy of the process. Table 2. Pyrolysis time, heating rate yield, and BET surface area experimental response RUN

1

SAMPLE CODE

9

H PO CONCENTRATION, % 3

4

60.00

MICROWAVE POWER, W

700.00

PYROLYSIS TIME, min

32.44

AVERAGE HEATING RATE, °C/min 15.56

YIELD, wt. %

BET SURFACE AREA, m /g 2

36.73

1283.10


11


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2

13

60.00

700.00

31.19

16.70

41.52

1359.60

3

8

60.00

841.42

26.17

20.91

40.46

1103.45

4

10

60.00

700.00

26.76

16.59

42.55

-

5

4

75.00

800.00

32.84

15.85

41.72

1157.18

6

7

60.00

558.58

38.19

13.02

39.77

991.22

7

3

45.00

800.00

20.12

29.11

32.73

405.36

8

6

81.21

700.00

57.80

5.72

33.55

1725.70

9

11

60.00

700.00

27.39

19.41

42.39

-

10

12

60.00

700.00

32.24

15.49

42.52

998.30

11

2

75.00

600.00

44.43

10.93

39.87

941.46

12

5

38.79

700.00

14.38

42.92

41.96

285.85

13

1

45.00

600.00

20.37

27.48

39.72

319.09

AC yield obtained was found to be similar and not statistically different across all concentrations. Similar results was documented on H PO impregnation of palm shells where high yield was 3

4

derived on activation temperature of 425 °C . Although there is no statistical difference among 38

the derived yield on all the concentrations, it is important to note that longer heating tends to reduce yield because of burning of char as noted by some researchers

16,21

. BET surface area was found to

increase with increasing concentration having highest value of 1725.7 at 81.21%. BET Results can be due to the extent of degree of interaction of biomass and H PO under reaction (i), which is 3

4

expected at higher concentrations. Similar trend was documented by Nahil and Williams . 39

Response surface analysis and optimization


12

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1 2 3 Results of the response surface analysis is summarized in Figure 2 and Table 3. It revealed 4 5 significant effect of H PO concentration on heating rate (Figure 3a), total pyrolysis time (Figure 6 7 3b), and BET surface area (Figure 3d). 8 9 Design-Expert® Software 10 Design-Expert® Software Heating rate 11 42.918 Pyrolysis_time 57.8 12 5.7234 Design-Expert® Software 14.3783 13 X1 = A: Phosphoric acid Pyrolysis_time X1 = A: Phosphoric acid (b) 57.8 14 X2 = B: MW Power X2 = B: MW Power 14.3783 15 X1 = A: Phosphoric acid 16 X2 = B: MW Power 17 Design-Expert® Software 18 rate (a)Heating 19 42.918 20 5.7234 21 X1 = A: Phosphoric acid X2 = B: MW Power 22 Microwav e Power, W Phosphoric acid. wt.% Microwav e Power, W Phosphoric acid. wt.% Design-Expert® Software 23 Design-Expert® Software BET Surface Area 24 Yield 1725.7 42.55 Microwav e Power, W Phosphoric ac 25 Design-Expert® Software 285.85 32.73 26 Surface Area (d)BET1725.7 X1 = A: Phosphoric acid X1 = A: Phosphoric acid 27 X2 = B: MW Power X2 = B: MW Power 285.85 28 X1 = A: Phosphoric acid X2 = B: MW Power 29 Design-Expert® Software 30 Microwav e Power, W Phosphoric acid. wt.% (c)Yield 31 42.55 32 32.73 33 X1 = A: Phosphoric acid X2 = B: MW Power 34 35 Microwav e Power, W Phosphoric acid, wt.% Microwav e Power, W Phosphoric acid, wt.% Microwav e Power, W Phosphoric acid, wt.% 36 37 38 39 40 Figure 3. Response surface plots of (a) average heating rate, (b) total pyrolysis time, (c) yield, and 41 42 Microwav e Power, W Phosphoric acid, wt.% (d) BET surface area 43 44 45 Analysis of variance of heating rate resulted significant effects on H PO concentration and on 46 47 48 its quadratic term implying a strong dependence of heating rate on H PO concentration. Analysis 49 50 on pyrolysis time and BET surface area further revealed sole significant contribution of H PO 51 52 concentration on the said responses. Linear response surface model was suggested for pyrolysis 53 54 time showing a linear increase in total pyrolysis time with H PO concentration. Although no 55 56 57 58 59 ACS Paragon Plus Environment 60 3

4

33

47

39

20

13.5

39

23

15

7

33

800.00

800.00

75.00

750.00

52.50 800.00

600.00

67.50

60.00

650.00

52.50

600.00

13.5

45.00

1450

7

800.00

750.00

700.00

35.175

650.00

600.00

32.7

1205

1450

960

800

60.00

715

52.50

45.00

470

700

960

715

470

800.00

75.00

40.125

1205

75.00

67.50

42.6

75

750.00

67.50

67.5

800.00

700.00

60.00

Yield, wt.%

750.00

700.00

37.65

600

45.00

20

40.125

650

67.50

60.00

650.00

45.00

750

75.00

700.00

60.00

BET Surface Area, m2/g

42.6

23

15

52.50


600.00

Heating rate, deg.C/min

26.5

650.00

31

750.00

67.50

700.00

Yield, wt.%

47

31

Pyrolysis time, min

Pyrolysis time, min

Heating rate, deg.C/min

26.5

750.00

650.00

52.50 37.65

52.5

600.00

45.00

75

60

67.5

700.00

60

650.00

45

52.5

600.00

45

35.175

32.7

800

75.00

750

67.50

700

60.00

650

52.50

600

45.00

3

3

4

4

3

3

4

4

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Page 14 of 26

significant impact was traced, microwave power can slightly reduce pyrolysis time with increasing power. Similar trend was observed on BET surface area response surface plots. Yield, express in percentage, on the other wasn’t affected by neither H PO concentration nor microwave power as 3

4

suggested by a relatively flat response surface plot (Figure 3c). Table 3. Summary of P-Values of the analyses of variance conducted on the response parameters SOURCES OF VARIANCE

HEATING RATE

PYROLYSIS TIME

YIELD

BET SURFACE AREA

A (H PO )

< 0.0001

0.0001

0.7860

0.0056

B (Microwave Power)

0.1025

0.0654

0.6821

0.5572

AB (Interaction)

0.6373

0.2641

0.2358

0.8127

A

2

0.0190

0.3262

0.2163

0.1520

B

2

0.9053

0.9624

0.6540

0.1943

Suggested Model

Quadratic

Linear

N/A

Linear

Model R Squared

0.9274

0.8617

N/A

0.8390

3

4

-

Generally, in any processing technologies, it is desired to obtain best product characteristic with minimal resources. In the perspective of process optimization, it can be observed that there is an apparent tradeoff in obtaining more efficient process and better product that is longer processing time and higher H PO concentration. Both of this factor impacts economy of the process so it is 3

4

aimed to find optimum balance between these response factors to obtain more efficient process. To find the optimized conditions, a numerical optimization technique was used wherein an objective function called desirability, D , having a range from 0 to 1 representing least to most 40


14

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desirable respectively, is estimated based on individual response optimization goals. It is mathematically expressed in Equation 1. N

N

N

𝐷 = (𝑑7O ∗ 𝑑/Q ∗ … 𝑑ST )

O UV

NV S WX7 𝑑W

=

O UV

Equation 1

Where d is individual desirability of response i, r is define as the relative importance of i

i

individual response i (ranges from 1 to 5), and n is the number of responses. Table 4 outlined the constraints and desired goals in the optimization analysis. Table 4. Summary of the numerical optimization study CASE A: ECONOMIC PRIORITY

CASE B: PRODUCT PRIORITY

RELATIVE IMPORTANCE

RANGE

H3PO4 concentration, %

2

45-85

Minimize

56.50

In range

84.97

Microwave Power, W

2

550-850

Minimize

650.00

In range

803.75

Pyrolysis time, min

3

14.3857.80

Minimize

29.86

In range

48.06


3

-

Maximize

826.38

Maximize

1726.51

Yield, wt. %

3

-

Maximize

39.65

Maximize

39.65

VARIABLES

Goal

Optimized

Value

Goal

Optimized Value

In the optimization study, two important perspectives were explored, one giving emphasis on the process economy and the other on product property. Constraints and goal are set based on process economy and product quality perspective; minimizing resources on production (A) and maximizing the quality regardless of the resources (B). Limits considered are based on the results


15


Page 16 of 26

of the technical evaluation while relative importance are set giving more emphasis on process (pyrolysis time) and product (yield and BET surface area) responses among others. Based on the results, case A achieved an overall desirability of 0.632 while case B obtained a higher 0.840. Desirability is merely a measure of the degree of how the set criteria were met, and case B obtained higher value because it basically entails fewer constraints compared to case A. Optimization revealed 56.50% H PO activation and pyrolyzed 650 W microwave power at 450 °C 3

4

is practically rational based on constraints emphasizing on maximizing process and product responses. Under this condition, pyrolysis time can be done within 30 minutes and expected BET surface area is around 826.38 m /g. On the other hand, without economic constraints, BET can be 2

as high as 1726.5 m /g but will require 84.83% H PO concentration, about 803.75 W microwave 2

3

4

power and 48 min carbonization time. AC textural properties Summary of the derived AC textural characterization with respect to varying H PO 3

4

concentrations is presented in Table 5. Total pore volume and micropore volume were provided from the physisorption analyses and mesopores volume were calculated by subtracting micropore volume from total pore volume. Average pore width on the other hand, were calculated based on BET equation. Results of the total micropore volume on derived AC revealed that less micropore was observed with increasing concentration. This depicts that the increase in surface area in higher concentration impregnation was due mostly to development of mesopores and is mainly attributed to the extent of reaction between activation agent and the precursor. Further, textural properties suggest that the derived AC were generally mesoporous as revealed by the calculated average pore width and pore size distribution plots (Figure 4). Average pore size further supports the mesoporosity of the AC obtained. Mean pore size ranges from 2.25 nm to 3.72 nm falling at the


16

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lower limit range under mesoporous classification. It has been reported that under optimum microwave power and heating time, AC derived has more micropores and mesopores at lower end limit and is more organized and homogenous as compared to conventional heating

13,18,41

.

Table 5. AC textural properties at different concentrations H PO CONCENTRATION, 3

4

BET SURFACE AREA, m /g 2

% wt.

MICROPORE VOLUME,

TOTAL VOLUME

MESOPORE VOLUME*

cm /g

cm /g

cm /g

3

3

AVERAGE PORE WIDTH, nm

3

38.79

285.85

0.079

0.160

0.081

2.24

60

1283.10

0.015

1.146

1.130

a

3.54

81.21

1725.7

0.006

1.539

1.532

3.72

Comparing the distribution curves revealed that more mesopores evolved on 60 % and 81.21 % concentrations as compared to 38.89%. A look on their N isotherms clearly showed this trend as 2

it is observed that with increasing concentration, isotherms behaves closely as a mesoporous media. Nitrogen isotherm from 81.21% clearly exhibits a Type IV isotherm of IUPAC classification while 38.79 % behaves more of a Type I (microporous media). Isotherm from 60% 42

concentration behaves more of a transition between the two. It can be noted from Table 5 that micropore contributes to about 50 % of the total pore volume at 38.79 % while in 60 % and 81.21 % mesopores contributes almost the totality of the pore volume.


17


Page 18 of 26

Figure 4. Pore size distribution and N sorption isotherm at varying concentrations 2

For further characterization, AC derived from the 60% H PO activation and is carbonized under 3

4

700 W power was considered to represent the conditions considered. Figure 5 presents FTIR spectra of raw Douglas fir and that AC derived from 60% H PO activation. Apparent dominance 3

4

of carbon bonds (C-C at 650 band), weak oxygen bonds and significant reduction of hydrogen bonds were obvious upon comparison between the two spectra. Hydroxyl group, O-H, traced at the stretch of 3500-3300 cm and C-H located at around 2910 cm were both absent in the AC -1

-1

suggesting efficient dehydration action of H PO on Douglas fir 3

4

24,37

. Elemental analysis (Table 6)

supports this result as there is significant increase of carbon by % wt. and decrease in hydrogen and oxygen at the same time.


18

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Figure 5. FTIR Spectra of raw Douglas fir and AC derived from 60% H PO activation 3

4

Peaks involving C=O esters at around 1740 band was weakly detected on AC while the stretch from aromatic ring, C=C bond at around 1610 is more defined. Evident O-H peaks at around 1410-1260 bands from the raw Douglas fir were noticeably weaker on AC, suggesting its degradation, which again can be attributed to the dehydrating agent. Peaks at bands ranging 1200 to 1050 involves mainly carbon-oxygen (C-O) bonds, which can either be aromatic ethers, esters, alcohols, phenols, or the P=O bonds present in phosphate esters

24,43

were also very weak at AC.

One traceable peak at 1160 band may be attributed to alcohols and phenols or some presence of P O , an anhydride of H PO which have evolved in the carbonization process. 2

5

3

4

Table 6. Elemental composition of raw Douglas fir and AC derived from 60% H PO activation 3

COMPOSITION

4

RAW DOUGLAS FIR

AC FROM 60% H PO CONCENTRATION

C

49.19

86.30

H

6.24

2.46

N

0.13

0.12

O (bal.)

44.44

11.12

3

4


19


Page 20 of 26

Scanning electron microscope (SEM) micrographs of the AC sample derived at 60% H PO 3

4

activation are shown in Figure 6. Two different magnifications offer good optical representations of the pores developed in the activation process. On both images mesopores are apparent and the high surface area nature of the AC can be noted.

Figure 6. SEM images of AC derived from 60% H PO activation 3

4

CONCLUSIONS Microwave heating advantage of volumetric and energy efficient heating is only as good as samples’ ability to absorb and convert it to heat. In pyrolysis of H PO -activated Douglas fir, 3

4

moisture of the biomass and water evolved from chemical activation process plays important role of converting microwave into thermal energy at the onset of the pyrolysis. Residual H PO has 3

4

negative effect of slowing down pyrolysis as it decreases microwave absorption capacity of the biomass-H PO complex. In addition, H PO undergoing conversion to its anhydride, P O , requires 3

4

3

4

2

5

additional energy and conversion time that may result in extension of the overall pyrolysis time. Optimal concentration of H PO during chemical activation therefore is important to efficiently 3

4

pyrolyze or carbonize biomass under microwave energy in a practical manner. In the foregoing setup, despite highest surface area being obtained at biomass activated at 81.21% H PO , a tradeoff 3

4

involving long processing time and high H PO concentration requirement may not be 3

4


20

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economically appealing. Optimization revealed 56.50% H PO activation pyrolyzed at under 650 3

4

W microwave power at 450 °C is practically rational based on economic-oriented criteria. Under this condition, carbonization time can be attained within 30 minutes and expected BET surface area is about 826.38 m /g. Without economic constraints, BET can be as high as 1726.5 m /g but 2

2

will require 84.83% H PO concentration, about 803.75 W microwave power and 48 min 3

4

carbonization time. Textural characteristics of AC derived from H PO activated Douglas fir were 3

4

generally mesoporous with pore diameter at the range of 2.35 to 3.72 nm. Pore size distributions and N sorption isotherms further supported this observation. FTIR analysis qualitatively showed 2

effective dehydrating action of H PO as evidenced by significant reduction of O-H bonds on 3

4

spectral signatures while physical and textural makeup such as surface area and porosity were apparent from the micrographs produced by SEM imaging.

Acknowledgment This study was supported by The Agriculture and Food Research Initiative of National Institute of Food and Agriculture, United States Department of Agriculture (Award Number: 2016-6702124533; Award Number: 2016-33610-25904), Philippines’ Department of Science and Technology – Engineering Research and Development for Technology, and Central Luzon State University. REFERENCES (1) Marsh, H.; Rodrigues-Reinoso, F. Activated Carbon; Elsevier Science & Technology Books, 2006; Vol. 5. (2)

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“For Table of Contents Use Only”

Heating profiles of biomass-phosphoric acid complex under microwave pyrolysis is studied to understand the factors that affect processing time and resulting product properties. Paper presented a new perspective in optimizing microwave-assisted activated carbon production from renewable precursors via chemical activation route.


26

Optimizing Microwave-Assisted Pyrolysis of ... - ACS Publications

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