Design of Experiments for determining the parameters...

Design of Experiments for determining the parameters affecting the behavior of the wheat straw

adsorbent

Kalliopi Chatzizaharia, Sofia Papadaki, Christina Tsiodra, Dimitrios Economides, Dimitrios

Hatziavramidis* School of Chemical Engineering

National Technical University of Athens Athens, Greece

*[email protected]

Dimitrios Sidiras Department of Industrial Management and Technology

University of Piraeus Piraeus, Greece

Abstract—The use of wheat straw, a lignocellulosic material, as an adsorbent of hydrocarbons in water has been investigated. Previous studies showed that this material can be utilized as an efficient adsorbent when its lignin content does not exceed 20% on a dry basis. This can be partly explained by the fact that lignin on the surface contains charged functional groups which may attract charged oil droplets. A 3- level Box-Behnken design, with factors the maleic acid (MC4H4O4) concentration and the temperature and time of wheat straw treatment, was performed to determine the effect of the parameters affecting the behavior of the treated wheat straw as an adsorbent. Several analytical methods such as Mercury Porosimetry, Thermogravinetric Analysis (TGA), Total Organic Carbon (TOC) and Scanning Electron Microscope (SEM) were utilized to characterize the material and its adsorbancy potential.

Keywords—wheat straw; adsorbancy; Box-Behnen design; Mercury porosimetry; TGA; TOC; SEM

I. INTRODUCTION Oil is one of the major resources of energy and commodity

chemicals in the industrialized world at present. Oil spills are major environmental disasters, resulting in contaminated water that could not be used into municipal water supply either for industry or for irrigation and of course with an extended negative economic impact on tourism and fisheries. Chemical dispersion, in situ burning, mechanical containment and oil sorption by adsorbents are generally accepted cleanup methods to combat oil pollution [1]–[3]. Adsorbent materials are an attractive choice for oil spills clean up because of the possibility of collection and complete removal of oil from the oil spill site. The addition of adsorbents to oil spill areas facilitates a change from liquid to semi-solid phase and once this change is achieved, the removal of the oil by removal of the absorbent becomes much easier [4]. Some properties of good adsorbent materials include hydrophobicity and oleophilicity, high uptake capacity, high rate of uptake, retention over time, oil recovery from adsorbents, and the reusability and biodegradability of the adsorbents [5]. In an

effort to devise new methods for containment/mitigation of oil spills, agricultural waste, such as wheat straw, a readily available, inexpensive material with low oil sorption capacity and hydrophobicity has been examined [6], [7].

This lignocellulosic material consists of three main components: cellulose, hemicellulose and lignin, with an average composition range of 40–50, 20–35 and 15–35%, respectively. Cellulose microfibers give wood cells their superior mechanical properties while hemicellulose and lignin act as a binder for cellulose crystallites. For the preparation of an adsorbent, the raw material is subjected to heat treatment or pyrolysis, which removes organic matter from the raw material resulting in an increase of material pore volume and surface area available for adsorption. The adsorbent capability depends on its structural and physicochemical features. The surface energy of a lignin film is similar to that of cellulose, but the contact angle of water for lignin is higher than that for cellulose, i.e., lignin is more hydrophobic or less hydrophilic than cellulose. The lignin surface contains charged functional groups that may attract charged oil droplets.

Straw-like adsorbing materials are capable of holding oil as a result of two processes: adsorption and absorption. The adsorption capacity depends primarily on the chemical structure of straw tissue that is in direct contact with oil, while the absorption capacity is a function of structure of straw stalks in bundles, distances between stalks, and cross-sections of each stalk and leaf. Due to high oil adsorption by straw, oil is mostly held by capillary forces on straw tissue and interior parts of stalk and by oil bridges between stalks. Absorption of oil depends on the surface properties of straw.

In the present work, thermo-hydrolyzed wheat straw treated under several conditions with maleic acid has been examined as hydrocarbon adsorbent. In previous studies, untreated and auto-hydrolyzed wheat straw were examined as hydrocarbon adsorbents with the use of several analytical method, such as Mercury Porosimetry, Thermogravinetric Analysis (TGA), Total Organic Carbon (TOC), Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectrometry (FTIR), X-

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Ray Diffraction Analysis (XRD) and Atomic Absorption Spectrometry (AAS). The first four methods were found to be the most critical for the evaluation of the efficiency of the treated wheat straw as adsorbent [8].

II. MATERIALS AND METHODS

A. Design of Experiments- Factorial experimental design A Design of Experiments (DOE) methodology was

followed in order to establish the effects of experimental conditions on the properties of the adsorbent samples. The factors (MC4H4O4 concentration, temperature and reaction time) are varied simultaneously in a controlled manner rather than one at a time, thus allowing exploration not only of the effects of the factors but also their interactions on the responses. Experimental design consists of a series of runs or tests, in which the input variables or factors are varied simultaneously in a controlled manner and the output variables or responses, are measured and evaluated for each run.

Response surface methods are used to construct models that describe the relationship between responses and factors. A 3- level Box-Behnken design was chosen, with three center points to check for curvature in the response surface. Box-Behnken designs have treatment combinations that are at the midpoints of the edges of the experimental space and require at least three factors. These designs allow efficient estimation of the first- and second-order coefficients and because they often have fewer design points, they can be less expensive to run than central composite designs with the same number of factors [9].

B. Wheat straw treatment method The wheat straw used in these experiments was harvested

in 2012, at the Kapareli village near the city of Thiva in the area of Kopaida in central Greece. This particular area was selected because of the abundance of raw material for full scale industrial applications [10]. The acid hydrolysis process took place in a 3.75L batch PARR 4843. The conditions chosen for the hydrolysis process were 0-50 min (without including the preheating time), 0.01M, 0.05M and 0.09M maleic acid concentrations. The acid hydrolysis process was performed at a liquid-to-solid radio of 20:1, which resulted in a 100 g of wheat straw added to a 2000ml volume of liquid [11]. The design table from Minitab software is presented in Table 2, where “Std. Order” shows what the order of the runs in the experiment would be if it was done in standard order and “Run Order” shows what the order of the runs in the experiment would be in random order. “Pt. Type” refers to the type of point in the designed experiment, where 0 is the center point and 2 the edge midpoint. The experimental runs were conducted under relatively homogeneous conditions (“Block”=1).

TABLE I. DOE CRITICAL FACTOR AND THEIR LEVELS

Factors Factor levels Units -1 0 1 MC4H4O4 Concentration, MC 0.01 0.05 0.09 M Temperature, Temp 140 160 180 °C Reaction time, t 0 25 50 min

TABLE II. DOE TABLE AND EXPERIMENTAL CONDITIONS

Std. Order

Run Order / Sample

Pt. Type Blocks MC Temp t

2 1 2 1 0.09 140 25

7 2 2 1 0.01 160 50

4 3 2 1 0.09 180 25

6 4 2 1 0.09 160 0

5 5 2 1 0.01 160 0

8 6 2 1 0.09 160 50

10 7 2 1 0.05 180 0

3 8 2 1 0.01 180 25

11 9 2 1 0.05 140 50

13 10 0 1 0.05 160 25

12 11 2 1 0.05 180 50

1 12 2 1 0.01 140 25

14 13 0 1 0.05 160 25

15 14 0 1 0.05 160 25

9 15 2 1 0.05 140 0

C. Scanning Electron Microscopy (SEM) The morphology of the treated wheat straw fibers was

determined from dried and milled samples, mounted on metal stubs, using a scanning electron microscope (SEM). Images were taken at 30 kV by a FEI Quanta 200 microscope, with a LFD detector and magnifications that ranged from 300 to 4000.

D. Mercury Porosimetry An idea of the adsorptive capacity of the modified wheat

straw samples can be obtained with mercury porosimetry which detects meso- and macro-pore sizes (2 nm–100 µm). Measurements were obtained using a Pascal 440 Porosimeter (Thermo Electron). Pressures between 0.01Torr and 200MPa can be developed which allows pores of 100µm to 3.75nm radius to be measured. In order to obtain reproducible results, it was necessary to obtain good vacuum before filling the sample cell with mercury at 0.01Torr for 30min [12].

E. Total Organic Carbon (TOC) The treated, dried and milled wheat straw is mounted on

metal stubs and is heated to 105oC in order to remove the moisture. The sample is placed in a crucible which is heated to 550oC for 2 hours [13]. The carbon content (% C) was calculated from the ash content (% ash) using the following equation [14]:

%C=((100-ash)*100)/1.8 (1)

F. Thermogravimetric Analysis (TGA) Thermogravimetric measurements were performed with a

Mettler-Toledo TGA/DSC 1 HT Integrated Thermal Gravimetric Analyzer with high-purity nitrogen as a carrier gas and a flow rate of 40 mL/min. About 5-12 mg samples were placed in an alumina pan of 70µL each time. The samples were heated from room temperature to 800 °C at a heating rate of 10 °C/min. The calculated thermogravimetic rate data were automatically recorded by the Integrated Thermal Gravimetric Analyzer system of Mettler-Toledo, using a STARe Thermal


Analysis Software. The thermogravimetric experiment was used to obtain the parameters of chemical dynamics and the influence of any error was reduced as much as possible. The average size of all samples was below 0.5 mm, so that the weight loss of the samples was controlled by reaction kinetics.

III. RESULTS AND DISCUSSION

A. Scanning Electron Microscopy (SEM) SEM images of the hydrothermally pretreated in acidic

environment wheat straw are presented in Fig. 1. Initially, the most apparent effect of the hydrothermal pretreatment apart from a color change from yellow into dark brown is the partial defibrillation or separation of individual fibers and cell types of the wheat straw. Although the pretreated material is quite heterogeneous and contains larger pieces (up to about 1 cm) that are easily recognized as straw, a significant fraction consists of cells that are either completely or partially separated from each other. All individual fibers seem to be intact despite the pretreatment, rather than being broken or otherwise disrupted.

The surface is covered with 'debris', possibly fractions of the middle lamellae, and a thin layer of deposits that seems to be covering the whole surface. Though, it was not possible to identify any primary or secondary wall cellulose microfibrils. Instead, an uneven surface of spherical and globular shapes was seen, with diameter approximately 20-100 nm, characteristic of lignin deposits as reported in the literature [15]–[17]. The overall structure of the individual fibers seems to show large structural changes such as rupture of fibers or a visible increase of porosity, which are believed to be associated with thermal pretreatments. Holes and cracks in the fibers indicate that the accessibility of the internal parts of the cell wall matrix has been improved due to structural dislocations. The primary and secondary cell walls appeared to be fully intact, except for the pits and simple perforations that already exist in certain cell types [18], [19]. Despite these observations of a substrate where the skeletal structure is intact and the crystallinity of the cellulose does not appear to have been lowered, the hydrothermally pretreated straw has been shown to be easily digestible by enzymes [20]. Consequently, the effectiveness of pretreatment must be related to hemicellulose removal and lignin relocation. This is in spite the fact that lignin is not removed by the pretreatment and that lignin is known to be responsible for unproductive adsorption of celluloses [21], [22]. It is well known that lignin encases the cellulose in the cell-wall matrix, hindering celluloses from reaching cellulose fibrils. We hypothesize that the migration of lignin to the outer surface exposes internal cellulose surfaces. More investigations are needed in order to confirm this. Selig et al (2007) have also observed the formation and migration of spherical lignin deposits onto the surface of fibres as a result of pretreatment, also suggesting that the deposited lignin can have a negative impact on the enzymatic cellulose hydrolysis. It is possible, however, that the surface lignin layer is easily removed by simple mechanical forces through mixing, due to lignin being less strongly bound to carbohydrate polymers compared with its native linkages. Furthermore, we theorize that the re-located lignin has exposed cellulose inside the cell wall, thus increasing the enzyme accessibility.


Fig. 1. SEM images for the samples of the modified wheat straw

B. Mercury porosimetry results The porosimetry parameters results are graphically

presented in Fig. 2. The total porosity ranges from 67.76 to 81.16 %, depending on the treatment conditions. The lowest porosimetry results are noted with treatment conditions of 0.09M maleic acid, 180°C temperature and 25 min reaction time (sample 3). The maximum porosity values are shown in samples 1, 5, 9 and 15, where various hydrolysis conditions are observed. The specific surface area is an indicator of adsorbance capability and adsorbent pore size. High specific surface area is associated with high adsorbance capability, favoring large organic molecules as in samples 2, 5, 12 and 15, and small size, favoring small molecules such as light diesel fractions. A more complete understanding of the effect of the experimental conditions on the porosimetry results can be achieved with a DOE analysis.


Fig. 2. Mercury porosimetry results of porosity, total specific area and bulk density for modified wheat straw

C. Total Organic Carbon (TOC) results According to previous studies, the conversion to CO2 of all

carbon forms in samples could be measured directly or indirectly and then converted to TOC, depending on the existence of inorganic carbonates. The methods and means of measuring TOC are destructive. Previous studies also show that the losses of organic compounds depend on different parameters such as oxidation, sample drying, and microbial degradation [23]. The TOC results for the samples are presented in Fig. 3.

Fig. 3. TOC average for the samples of the modified wheat straw

A previous study showed that the Total Organic Carbon (TOC) percentage in untreated wheat straw was 51.71% [8]. Table 3 presents the TOC reduction percentage of acid-hydrolyzed and treated wheat straw. The highest losses of organic carbon, 41.43% and 40.13% were measured after oxidation with 0.01M maleic acid (the minimum concentration) at 160 oC and 140 oC as well as in 0 minutes and 25minutes, respectively. The minimum reduction of TOC (23.06%) was achieved using 0.05M maleic acid concentration and the highest values of hydrolysis parameters. Fig. 2 shows TOC of hydrolyzed wheat straw using maleic acid compared with TOC in untreated wheat straw. As previously mentioned, the highest reduction of total carbon was achieved in samples 5, 12 and 13, which show high porosity as well.

D. Thermogravimetric Analysis (TGA) results The TGA profiles of the wheat straw samples under

pyrolysis and combustion conditions underscore the impact of the differing straw composition on its thermochemical conversion (Fig. 4). The pyrolysis profiles, in particular, demonstrate an increase in decomposition onset temperature, peak temperature and maximum rate of decomposition, indicative of a lower reactivity. In previous studies, it has been observed that the separation of pyrolysis decomposition into two distinct peaks upon water washing, with decomposition bands remaining separate and shifting to lower temperatures, following addition of KCl to washed straw. The combustion profiles display a broad volatile combustion peak, with a peak occurring at lower temperatures than under pyrolysis conditions, as observed by researchers studying other biomass materials. The profiles are typical of lignocellulosic materials,

showing the onset of an initial volatilization stage around 220 oC followed by a char oxidation stage at higher temperatures.

Fig. 4. TGA results for the samples of the modified wheat straw

E. Design of Experiments (DOE) Analysis for treated wheat straw In the DOE analysis, coded units were used in order to

compare the impact of various factors on responses. The experimental results of the analysis performed are presented in Table 3. Regression analysis and Analysis of Variance show a good stability and inherent variability of the center point responses. Porosity is found to be a strong function of the treatment temperature (Temp), with statistical significance p=0.002, total specific surface area (Surf. Area) is a strong function of Temp, maleic acid concentration (MC), reaction time (t), and quadratic terms and interactions, with statistical significance p<0.05, and bulk density (Bulk Density) is a strong function of MC, with statistical significance p=0.037, and Temp, with statistical significance p=0.001. Also, TOC is a strong function of MC and Temp, and TGA Mass loss (Mass loss) of MC and t, with statistical significance p<0.05. There are no factors with significant effect on TGA parameter “Peak Temp.”. The generated models (in uncoded-physical units) are:

Porosity = -0.21*Temp + 107.58 (2)

Surf. Area = -1008.75*MC + 1.42*Temp - 0.40*t - 0.01*Temp2 + 0.004*t2 + 5.33*MC*Temp + 2.68*MC*t + 48.00 (3)

Bulk Density = -0.38*MC + 0.02*Temp + 0.01 (4)


TOC = -94.66*MC - 0.17*Temp + 61.52 (5)

Mass loss = -182.72*MC - 0.35*t + 4.93*MC*t + 85.60 (6)

TABLE III. DOE EXPERIMENTAL RESULTS FOR POROSIMETRY, TOC AND TGA

Run Porosity (%)

Tot. specific surface

area (m2/g)

Bulk Density (g/cm3)

TOC (%)

TGA Mass loss (%)

Peak Temp.

(ºC)

1 77.844 13.347 0.266 29.19 72.65 320.8

2 75.211 22.053 0.307 28.45 67.93 360.3

3 67.755 17.814 0.342 24.12 65.50 312.6

4 70.642 21.899 0.289 27.73 72.73 301.7

5 76.748 32.500 0.317 41.43 87.35 349.0

6 73.543 22.186 0.282 25.24 73.02 360.8

7 71.57 19.122 0.317 25.98 73.27 318.5

8 68.512 18.227 0.377 26.56 71.39 340.5

9 78.784 22.263 0.257 29.43 74.70 356.3

10 74.597 21.752 0.311 28.23 76.47 369.4

11 70.948 17.379 0.361 23.06 71.95 344.4

12 74.44 30.821 0.299 40.13 76.60 345.5

13 73.832 20.246 0.274 36.34 71.06 315.4

14 76.525 20.360 0.293 28.99 76.47 369.4

15 81.158 25.622 0.260 28.22 75.43 356.6

The resulting surface and contour plots of the response variables are presented below in Fig. 5 and 6. The surface plot helps the visualization of the response surface and shows how a response variable relates to two factors based on model equations. On the other hand, contour plots show combinations of factors on a plane that produce same-value responses. Moreover, the interaction plots of the response variables are presented. An interaction occurs when the effect of one factor is dependent upon a second factor and these plots are used to compare the relative strength of the effects across factors.

Porosity Surf. Area

Bulk density

Fig. 5. Surface, contour and interaction plots of Porosity, Surf. Area and Bulk density vs the acid hydrolysis conditions

TOC Mass loss

Fig. 6. Surface, contour and interaction plots of TOC and Mass loss vs the acid hydrolysis conditions

65

70

75

0.000.05

75

80

180

160

1400.10

porosity

Temp

MC

72.0

73.5

75.0

0.000.05

75.0

76.5

00.10

40

20

0

40

porosity

t

MC

70

75

140160

80

0180

40

20

0

40

porosity

t

Temp

MC 0.05Temp 160t 25

Hold Values

15

20

25

0.000.000.05

0.00

25

30

1400.10

160

180

160

surf area

Temp

MC

20

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28

0.000.000.05

0.00

28

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00.10

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surf area

t

MC

15

20

140160

25

0180

40

20

40

surf area

t

Temp

MC 0.05Temp 160t 25

Hold Values

Temp*MC

0.080.060.040.02

176

168

160

152

144

t*MC

0.080.060.040.02

48

36

24

12

0

t*Temp

176168160152144

48

36

24

12

0

MC 0.05Temp 160t 25

Hold Values

> – – – – – – < 68

68 7070 7272 7474 7676 7878 80

80

porosity

Temp*MC

0.080.060.040.02

176

168

160

152

144

t*MC

0.080.060.040.02

48

36

24

12

0

t*Temp

176168160152144

48

36

24

12

0

MC 0.05Temp 160t 25

Hold Values

> – – – < 15

15 2020 2525 30

30

surf area

180160140 50250

80

75

70

80

75

70

MC

Temp

t

0.01 Corner0.05 Center0.09 Corner

MC Point Type

140 Corner160 Center180 Corner

Temp Point Type

Interaction Plot for porosityData Means

180160140 50250

32

24

16

32

24

16

MC

Temp

t


MC Point Type


Temp Point Type

Interaction Plot for Surf AreaData Means

0.25

0.30

0.000.000.05

0.000.05

0.35

1400.10

160

140

180

160

bulk density

Temp

MC

0.28

0.30

0.000.000.05

0.000.05

0.32

00.10

40

20

0

40

bulk density

t

MC

0.25

0.30

140140160

140160

0.35

40

20

0180

bulk density

t

Temp

MC 0.05Temp 160t 25

Hold Values

Temp*MC

0.080.060.040.02

176

168

160

152

144

t*MC

0.080.060.040.02

48

36

24

12

0

t*Temp

176168160152144

48

36

24

12

0

MC 0.05Temp 160t 25

Hold Values

> – – – – – < 0.26

0.26 0.280.28 0.300.30 0.320.32 0.340.34 0.36

0.36

bulk density

180160140 50250

0.35

0.30

0.25

0.35

0.30

0.25

MC

Temp

t


MC Point Type


Temp Point Type

Interaction Plot for Bulk densityData Means

25

30

35

0.000.05

35

40

1400.10

160

180

160

TOC

Temp

MC

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30

35

0.000.05

35

40

00.10

40

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40

TOC

t

MC

20

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28

140160

28

32

40

20

0180

TOC

t

Temp

MC 0.05Temp 160t 25

Hold Values

65

70

0.000.05

75

180

160

1400.10

Mass loss

Temp

MC

70

75

80

0.000.000.05

0.000.05

80

85

00.10

40

20

0

40

Mass loss

t

MC

70.072.5

75.0

140160

75.077.5

0180

40

20

0

40

Mass loss

t

Temp

MC 0.05Temp 160t 25

Hold Values

Temp*MC

0.080.060.040.02

176

168

160

152

144

t*MC

0.080.060.040.02

48

36

24

12

0

t*Temp

176168160152144

48

36

24

12

0

MC 0.05Temp 160t 25

Hold Values

> – – – – – – < 21

21 2424 2727 3030 3333 3636 39

39

TOC

Temp*MC

0.080.060.040.02

176

168

160

152

144

t*MC

0.080.060.040.02

48

36

24

12

0

t*Temp

176168160152144

48

36

24

12

0

MC 0.05Temp 160t 25

Hold Values

> – – – < 70

70 7575 8080 85

85

lossMass

180160140 50250

40

32

24

40

32

24

MC

Temp

t


MC Point Type


Temp Point Type

Interaction Plot for TOCData Means

180160140 5025090

80

70

90

80

70

MC

Temp

t


MC Point Type


MC Point Type


Temp Point Type


IV. CONCLUSIONS In the present work, the effect of the acid hydrolysis

conditions on the adsorbent capacity of wheat straw is investigated. A DOE is designed and executed, with input factors the maleic acid concentration (MC), the hydrolysis time (t) and temperature (Temp), and response the adsorbent capacity.

Several analytical techniques are used to enhance our understanding of the influence of various factors on adsorbent capacity. From previous research [8] SEM, mercury porosimetry, TOC and TGA are found to be the most critical in this evaluation. SEM images show interwoven cellulose microfibrils, partially imbedded in non-cellulosic polymers in the untreated straw and partially separate individual fibers in pretreated straw.-to. The pretreatment leaves a surface layer of debris and re-deposited cell-wall polymers on the individual fibers. The response variables for the DOE analysis are porosity, surface area, bulk density, TOC and TGA mass loss of the wheat straw samples. The models formulated from the DOE for all responses contain linear and interaction terms of the various factors. The model for the surface area, in addition to linear and interaction terms, contain quadratic terms as well. The porosimetry variables and TOC results are strongly affected by maleic acid concentration and temperature. Maleic acid concentration and reaction time are critical factors for TGA mass loss. Optimal hydrolysis conditions for pretreatment of wheat straw leading to maximum adsorptive capacity of the straw can be identified.

ACKNOWLEDGMENT This research has been co-financed by the European Union

(European Social Fund – ESF) and Greek national funds through the Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework (NSRF) - Research Funding Program: THALES. Investing in knowledge society through the European Social Fund. - Project: THALIS – University Of Piraeus – Development Of New Material From Waste Biomass For Hydrocarbons Adsorption In Aquatic Environments.

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Design of Experiments for determining the parameters...

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