Continuous process for biodiesel production from palm fatty acid distillate (PFAD) using helical static mixers as reactors

Accepted Manuscript Continuous process for biodiesel production from palm fatty acid distillate (PFAD) using helical static mixers as reactors

Krit Somnuk, Natthapon Soysuwan, Gumpon Prateepchaikul PII:

S0960-1481(18)30836-X

DOI:

10.1016/j.renene.2018.07.039

Reference:

RENE 10313

To appear in:

Renewable Energy

Received Date:

04 November 2017

Accepted Date:

08 July 2018

Please cite this article as: Krit Somnuk, Natthapon Soysuwan, Gumpon Prateepchaikul, Continuous process for biodiesel production from palm fatty acid distillate (PFAD) using helical static mixers as reactors, Renewable Energy (2018), doi: 10.1016/j.renene.2018.07.039

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Continuous process for biodiesel production from palm fatty acid distillate

2

(PFAD) using helical static mixers as reactors

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Krit Somnuk*, Natthapon Soysuwan, Gumpon Prateepchaikul

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Department of Mechanical Engineering, Faculty of Engineering, Prince of Songkla

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University, Hat Yai, Songkhla, Thailand, 90112

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*Corresponding author: [email protected]

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Abstract

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Three-step continuous process for producing biodiesel from palm fatty acid distillate

27 28

(PFAD) was optimized by response surface methodology (RSM). PFAD has high content of

29

free fatty acids (FFA) and is not suited for human consumption: normally it is used in soap or

30

in animal feed. The key parts of the three-step continuous process took place in helical static

31

mixers (HSMs) used as continuous reactors. The three-step process was optimized by RSM

32

with 5 levels for each of three factors in central composite design (CCD). First step was

33

esterification, then second step was esterification, and third was transesterification. Methyl

34

ester purities of 71.01 wt.% from first step, 95.94 wt.% from second step, and 99.96 wt.%

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from third step were achieved with total chemical consumption of (115.1 wt.% MeOH, 13.5

36

wt.% H2SO4, and 5.0 g/L KOH), and total residence time 147 sec in the 3 HSMs. In

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continuous processing the maximum yields were 109.5 wt.% first-esterified oil, 117.0 wt.%

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second-esterified oil, and 129.0 wt.% crude biodiesel, and purified biodiesel 86.4 wt.%, in the

39

separated phases from first, second, and third steps, and after purification, respectively. Ester

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purity from three-step process meets the standard specifications for commercial biodiesel in

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Thailand, US, and Europe.

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Keywords: continuous process; palm fatty acid distillate; helical static mixer; high free fatty

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acids.

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Nomenclature

49

ANOVA

analysis of variance

50

CH3OK

potassium methoxide

51

CJCO

Jatropha curcas seed oil

52

CPO

crude palm oil

53

CSTR

continuous stirred tank reactor

54

DG

diglyceride

55

DOE

design of experiment

56

DOF

degrees of freedom

57

FFA

free fatty acid

58

GC–FID

gas chromatograph–flame ionization detector

59

HSM

helical static mixer

60

H2SO4

sulfuric acid

61

k

number of variables

62

KOH

potassium hydroxide

63

ME

methyl ester

64

MeOH

methanol

65

MG

monoglyceride

66

MS

mean square

67

NaOH

sodium hydroxide

68

PFAD

palm fatty acid distillate

69

p-value

indicator of statistical significance.

70

RPO

refined palm oil

71

RSM

response surface methodology

72

R2

coefficient of determination

3

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R2adjusted

adjusted coefficient of determination

74

SM

static mixer

75

SS

sum of squares

76

TG

triglyceride

77

TLC/FID

thin layer chromatograph with flame ionization detector

78

vol.%

percentage by volume

79

wt.%

percentage by weight

80

Y

response variable

81

αx

axial point

82



coefficient

83 84 85 86 87 88 89 90 91 92 93 94 95 96 97

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1. Introduction

99 100

Biodiesel as renewable energy source can be used in diesel engines without any major

101

modifications [1]. PFAD is a by-product of low market value from the physical refining of

102

crude palm oil (CPO) to edible grade refined palm oil (RPO) [2]. The PFAD is not of human

103

edible grade, and it is normally used in making soap or animal feed. The biodiesel conversion

104

of PFAD with high free fatty acid (FFA) content was done in three continuous process steps.

105

To produce biodiesel from high FFA oil, esterification has been frequently used to convert

106

the FFA content in oil to esters [3,4]. However, excess alcohol and catalyst loading must be

107

used in the acid-catalyzed esterification to obtain high purity and yield of biodiesel from high

108

FFA [5,6]. In single step esterification, most of the miscible water in the reaction mixture is

109

continuously produced by the esterification reaction [7]. The generated wastewater hinders the

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extent of esterification, and the methanol (MeOH) and sulfuric acid (H2SO4) are diluted by

111

the generated wastewater [8,9]. In a three-step process, the generated wastewater was removed

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after the esterification reactions in first step, so it did not hinder the acid-catalyzed

113

esterification in the second step. In particular, the acid content in the second-esterified oil did

114

not hinder the transesterification in the third step where it could have caused saponification.

115

Saponification would reduce the conversion to ester, cause low catalytic activity, and increase

116

viscosity of the biodiesel [10,11].

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Processing in multiple steps helped solve these problems. For instance, Berchmans and

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Hirata [12] studied biodiesel production from high FFA content crude Jatropha curcas seed

119

oil (CJCO). Acid-catalyzed esterification was used in the first step to reduce the FFA content

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in CJCO to less than 1%. The first step was operated at 0.60 wt.% methanol-to-oil ratio, with 1

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wt.% H2SO4, and 60 min reaction time at 50oC. After the reaction, the mixture was allowed to

122

settle for 120 min. For biodiesel production base-catalyzed transesterification was used as the

123

second step to convert the first-esterified oil to methyl ester. As a result, 90% yield of methyl

124

ester was achieved at 0.24 wt.% methanol-to-oil and 1.4 wt.% NaOH, with 120 min reaction

125

time at 65oC [12]. These findings regarding the two-step process are in agreement with the

126

data reported by Chen et al. [13]. They studied biodiesel production from high FFA algae oil,

127

which was converted to biodiesel by esterification and transesterification. Before start-up of

128

the two-step process, Dinoflagellate oil was degummed by stirring with 1% phosphoric acid

129

and 10% water, and 60 min stirring time at 85oC, to remove phospholipids and non-lipid

130

impurities. The results showed that acid value of Dinoflagellate oil was reduced from 17-46

131

mg KOH/g to less than 2 mg KOH/g by the degumming process under the optimal conditions

132

of 30% MeOH, 1% H2SO4, and 120 min reaction time. In biodiesel production the low FFA

133

oil from first step was converted to biodiesel by transesterification. The results showed the

134

highest yield of 90.1% methyl ester achieved under the optimal conditions of 12:1 molar ratio

135

of alcohol-to-oil, 2% KOH, and 30 min reaction time at 65oC [13]. Regarding the application

136

of coiled tubular reactor in the continuous process, Nan et al. [14] optimized the biodiesel

137

production from microalgae oil by using a non-catalytic transesterification process in

138

supercritical methanol and ethanol. A 10-m coiled tubular reactor served as a continuous

139

reactor and the effects of various parameters on biodiesel conversion were investigated:

140

alcohol-to-oil molar ratio, water content, residence time, temperature, and pressure. The

141

optimal yields of methyl and ethyl esters were 90.8% and 87.8%, respectively [14]. Kurt et al.

142

[15] compared the liquid-liquid extraction efficiencies between a micro-coiled flow inverter, a

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micro helical coiled tube, and a straight capillary tube. They reported that the extraction

144

efficiency of helically coiled micro tubes and micro-coiled flow inverters is 20% better than

145

with straight capillary tubes [15]. This is because flow in helically coiled tubes enhances

146

transport phenomena such as heat and mass transfer [16,17].

147

7

To the best of our knowledge, continuous double-esterification reactions of high FFA

148

feedstock, followed by continuous transesterification to produce biodiesel, using helical static

149

mixers (HSM) in each step, has not previously been reported. Regarding the three steps of

150

continuous processing (first esterification, second esterification, and third transesterification),

151

the raw materials and chemical reactants of each step were reacted in HSM to accelerate the

152

reactions by mixing. Thus, a key element in the three-step continuous processing was the

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HSM used as continuous reactors. The mixtures flowed through static mixer elements inserted

154

into a curved tube. As advantages of HSM, it requires less space for installation than a straight

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tube reactor or a continuous tubular reactor. Moreover, there are several advantages that in-

156

line mixing has over continuous stirred tank reactor (CSTR), such as lower capital cost, lower

157

maintenance costs, and shorter reaction times.

158 159

2. Materials and methods

160 161

2.1 Materials and methods

162 163 164

2.1.1 Materials

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In the first step esterification, PFAD was used as the raw material. PFAD is a light

166

yellow solid wax at 30oC, with phase change from wax to liquid at 43oC (Fig. 1). The PFAD

167

was purchased from a large refinery facility performing palm oil extraction in eastern

168

Thailand. It was heated until molten to obtain homogeneous mixing with methanol, on

169

preparing the reactants for the first step esterification. The PFAD contained free fatty acids

170

(90.61 wt.% FFA), triglyceride (1.31 wt.% TG), diglyceride (2.33 wt.% DG), monoglyceride

171

(4.79 wt.% MG), and methyl ester (0.96 wt.% ME). The mean molecular weight of PFAD

172

was 263.7 g/mol. The density and viscosity of PFAD at 50oC were 0.869 kg/L (using

173

hydrometer method), and 12.68 cSt (using Julabo MD–16G Visco Bath), respectively. All

174

chemical reactants were of commercial grade: 99.7% MeOH and 98% H2SO4 were used for

175

the acid-catalyzed esterification in the first and the second step. The commercial grade

176

chemical reactants 99.7% MeOH and 98% KOH were used for the base-catalyzed

177

transesterification in the third step. The MeOH, H2SO4, and KOH were commercial grade and

178

purchased from P.General Trading Ltd., Part, Bangkok, Thailand. The weight percentages of

179

FFA, ME, TG, DG, and MG were analyzed by a thin layer chromatograph with flame

180

ionization detector (TLC/FID, model: IATROSCAN MK-65; Mishubishi Kagahu Latron

181

Inc.; Tokyo, Japan) [18]. Analysis used the following chemical standards: tripalmitin,

182

palmitic acid, methyl palmitate (sourced from Nacala Tesque); 1,3-distearin; DL palmitin

183

(mono palmitin) (sourced from Sigma Aldrich); and 1,2-di-stearin 99%, (sourced from

184

Research Plus) [18].

185

Regarding the analytical instruments and methods to be used in the quality control of

186

commercial biodiesel standard, the physical properties were determined for density using the

187

hydrometer method under the ASTM D1298-12b method [19], and viscosity, using the Julabo

188

MD–16G Visco Bath (Julabo Labortechnik GmbH; Seelbach; Germany) under the ASTM

189

D445-17a method [20]. The compositions: methyl ester, linolenic acid ester, MG, DG, TG,

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free glycerin, and total glycerin of commercial biodiesel standards were analyzed using a gas

191

chromatograph–flame ionization detector (GC–FID, GC 6890; Agilent Technologies; USA).

192

The EN 14103 [21] method is reference method for measuring the methyl ester, linolenic acid

193

ester, and the percentages of MG, DG, TG, free glycerin, and total glycerin were determined

194

according to the EN 14105 [22]. The instrument for determination of flash point using a

195

semi–automatic Pensky–Martens (Walter Herzog GmbH; Germany) under the ASTM D93-

196

16a [23]. The carbon residue was evaluated by ASTM D4530-15 [24] method using a

197

gravimetric method and high temperature furnace under the condition was 500±5oC at 15oC.

198

The water content was analyzed according to EN ISO 12937 [25] method using a Karl

199

Fischer coulometric titrator (DL39 Karl Fischer; Mettler–Toledo Instrument. Inc.;

200

Greifensee; Switzerland). The gravimetric method was used to determine the total

201

contamination under the condition was 0.8 m filter membrane and dry at 90±5oC at 30oC

202

(EN 12662 method [26]). Copper strip corrosion was tested according to ASTM D130-04

203

[27], using a Herzog HZ9011 instrument. The acid value is measured by a titration technique

204

as the mg of KOH required to neutralize the acids in one gram of the sample using the

205

Potentiometric Titrators 794 Basic Titrino (Metrohm; Switzerland) by following the ASTM

206

D664-09 [28]. The EN 14111 [29] is a reference method for measuring the iodine value in

207

biodiesel using a Wijs method, and GC–FID (GC 6850; Agilent Technologies; USA) was

208

used to investigate the methanol content under the standard test of EN 14110 [30]. The cetane

209

number was evaluated using a Cetane Rating Unit (Waukesha CFR F5; Waukesha County;

210

Wisconsin; USA) by following the standard test conditions (ASTM D613-18) [31]. The

211

phosphorus content was measured by the inductively coupled plasma optical emission

212

spectrometry (ICP-OES; Perkin-Elmer) according to the EN 14107 [32].

213 214

2.1.2 Reactions

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In the first step of the process, most of the FFA in PFAD was directly converted by

216 217

esterification to ester, and in the second step the residual FFA in the partly esterified oil from

218

first step was further esterified. The remaining FFA in second-esterified oil must be at

219

acceptable below 1 wt.% level after the second step. The acid-catalyzed esterification reaction

220

in the first and second steps is shown in Eq. (1) [33]. An advantage of this reaction is the high

221

yield of methyl ester in biodiesel production from high FFA content oil. Finally, the partial

222

glycerides in the second-esterified oil were converted to high purity methyl ester by

223

transesterification in the third step. This base-catalyzed transesterification is shown in Eq. (2)

224

[34].

225

Acid - catalyst

226

FFA  Alcohol        Ester  Water

(1)

Base - catalyst

227

Triglycerides  Alcohol        Glycerol  Ester

(2)

228 229

2.1.3 Equipment

230 231

The raw materials and chemical reactants of each continuous processing step were

232

reacted in a helical static mixer (HSM). The static mixer (SM) is a mixing device without any

233

moving parts, and static mixing elements were inserted into a curved tube to form a helical

234

static mixer (HSM). Regarding the configuration of the mixing elements, each element was

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180o twisted with a 1.5 of L/D (length to diameter ratio) and each element was 90 o connected

236

by spot welding. The dimensions of the mixing element were 10 mm in diameter, 15 mm in

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length, and 1 mm in thickness. After putting together each mixing element, they were

238

inserted into the empty tube to form a so-called twisted-ribbon of straight static mixers (SM).

239

The straight SM reactor was rolled by a pipe coils rolling machine until the helical static

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mixer (HSM) had 150 mm diameter coil of the 10 mm diameter curved tube. Both first step

241

and second step esterification used 10 m long HSM, and 1 m long HSM was installed for the

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third step transesterification to mix the second-esterified oil and (potassium methoxide)

243

CH3OK. Fig. 2 shows a schematic diagram of the three-step continuous process to produce

244

methyl ester from PFAD, with first esterification, second esterification, and third

245

transesterification. Three HSM units were used to blend the reaction mixture for FFA

246

reduction in the continuous esterifications of the first step and the second step, and for methyl

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ester production in continuous transesterification of the third step. Fig. 3 is a photo of the

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three-step continuous production process of biodiesel from PFAD using HSM.

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2.1.4 Procedures

251 252

Referring to Fig. 2, in preparation of the first step continuous esterification 25 L of

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PFAD in the tank (T2) was preheated by a band heater (HT1) and an immersion heater (HT2)

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to maintain its temperature at 50oC. Subsequently, MeOH in tank (T1) and PFAD in tank (T2)

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were added into the mixing tank (T3) and blended by a circulating pump (P1) until the

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mixture appeared homogeneous. The mixture of PFAD and MeOH in tank (T3) was

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continuously fed into the helical static mixer (HSM1) of the first esterification by the digital

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dosing pump (P2, Grundfos alldos model: DME 48-3). The H2SO4 in tank (T4) was also

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continuously fed into HSM1 by a digital dosing pump (P3, Grundfos alldos model: DDA 7.5–

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16). The first step esterification started when H2SO4 was added at the inlet of HSM1. The

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mixture of PFAD, MeOH, and H2SO4 flowed through the HSM1 to its outlet port and into the

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first step separator (SP1) to separate the first-esterified oil and generated wastewater

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continuously. The unit for continuous wastewater separation was specially designed for this

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gravity separation. After separation the first-esterified oil overflowed from first step separator

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(SP1) to first-esterified oil tank (T5).

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For the second step esterification processing, the temperature of first-esterified oil was

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maintained at 60oC by an immersion heater (HT2). The first-esterified oil in tank (T5) and

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MeOH in tank (T6) were continuously fed into the helical static mixer (HSM2) using two

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digital dosing pumps (P5 and P6, Grundfos alldos model: DDA 30-4). The H2SO4 in tank (T7)

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was fed into HSM2 by a digital dosing pump (P7, Grundfos alldos model: DDC 6–10). The

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second-esterified oil and wastewater from HSM2 were continuously separated by the second

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step separator (SP2), similarly as in the first step.

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After the second separation, the second-esterified oil in tank (T8) was maintained at

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60oC temperature by an immersion heater (HT2). Subsequently, the acid content in oil was

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checked before second-esterified oil was used as raw material in the third step continuous

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transesterification. For checking the acid content (due to FFA and residual sulfuric acid) in the

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second-esterified oil, KOH as base catalyst was dissolved in distilled water to prepare the

278

titrant solution [34]. The titration measured the acid content (consisting of residual H2SO4 and

279

FFAs) of the second-esterified oil by determining the KOH amount required to neutralize the

280

second-esterified oil. Thus, the second-esterified oil was titrated with KOH solution until the

281

solution reached the end point. The KOH amount required to neutralize oil is expressed in

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grams per liter. In the third step continuous transesterification, KOH loading for

283

neutralization and the catalyst were blended with MeOH in tank (T9) to prepare the solution

284

of potassium methoxide (CH3OK), by using a circulating pump (P11).

285

For the third step transesterification, the second-esterified oil in tank (T8) and CH3OK

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solution in tank (T9) were continuously fed into the helical static mixer (HSM3) of third step

287

using two digital dosing pumps (P9, and P10, Grundfos alldos model: DDA 30-4, and model:

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DMS 12-38, respectively). After transesterification reaction, the lighter crude biodiesel phase

289

and glycerol phase were continuously separated by the third step separator (SP3). After this

290

third separation the crude biodiesel overflowed from SP3 into the crude biodiesel tank (T10).

291

Regarding the sampling method, approximately 30 mL samples were taken at the sampling

292

ports along the length of the HSM. The samples from each continuous process step were

293

quickly cooled with 0oC water to stop the reaction. All the samples were then washed by

294

warm water to remove residual impurities. The percentages of FFA, ME, TG, DG, and MG in

295

the purified biodiesel were analyzed using the TLC/FID analyzer.

296 297

2.1.6 Experimental design

298 299

To optimize the methyl ester purity from each continuous processing step, RSM was

300

used with a 5-level and 3-factor central composite design (CCD). To obtain the rotatability of

301

a design, each independent variable in the CCD has 5 factor levels coded as: -α, -1, 0, +1, +α.

302

This ensures constant variance at points that are equidistant from the center point (0), and

303

therefore provides equal range in any direction, in the space of manipulated variables. The

304

axial point (αx) of rotatable CCD depends on the number of variables (k). For the axial point

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305

of three-step continuous processing, three independent variables were studied in the

306

experiment, so the number of variables (k) is equal to 3. Thus, the experiments were designed

307

for 5 levels of the independent variables coded as -1.682, -1, 0, +1, and +1.682, as determined

308

by Eq. (3) [35].

309 310

 x  4 2k

(3)

311 312

where αx is the axial point for rotatability, and k is the number of variables.

313 314

The actual values of the independent variables for each coded factor level are shown in

315

Table 1, and the design matrix is included in Table 2 and Table 3. The methyl ester purity

316

after each continuous processing step was fitted by a second order polynomial, using multiple

317

regression analysis. The general form of the second order polynomial model is shown in Eq.

318

(4) [36].

319 320

k

k

i 1

i 1

Y  β0   βi xi   βii xi2    βij xi x j   i  1 j i 1

(4)

321 322

where Y is the response variable (purity of methyl ester, wt.%); xi and xj are the uncoded

323

independent variables (methanol, catalyst, and length of HSM); β0 , βi , βii , and βij are the

324

intercept, linear, quadratic and interaction term coefficients, respectively; k is the number of

325

variables, and ε is the fitting error.

326

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Both first and second processing steps had three independent variables, namely

328

methanol (M1 and M2, for the first and second steps), sulfuric acid as acid catalyst (C1 and

329

C2), and length of HSM (L1 and L2). The dependent variables ME1, ME2, and ME3 are the

330

methyl ester purities of first, second, and third step, which were to be maximized based on

331

the predictive models. Also the third step of base-catalyzed transesterification had three

332

independent variables: methanol (M3), KOH as base catalyst (C3), and length of HSM (L3), to

333

be optimized for highest methyl ester purity. The optimizing conditions for continuous methyl

334

ester production from PFAD were analyzed by the TLC/FID method. The Solver in Microsoft

335

Excel (an add-in tool) was used to fit the models of first, second, and third steps of

336

continuous processing.

337 338

3. Results and discussion

339 340

3.1 Experimental results

341 342

Table 2 shows the eighteen experimental conditions and results from esterifications in

343

the first and the second steps of continuous processing, while those from the third step

344

continuous transesterification are in Table 3. It was found that most of the FFA in PFAD can

345

be converted to ester (ranging from 3.22 to 80.64 wt.% of ester) in the first step. The second

346

step esterification can produce methyl ester conversions from 89.41 to 96.38 wt.%. However,

347

the remaining TG, DG, and MG in this oil were detected by TLC/FID. Thus, the purity of

348

methyl ester from the second step does not meet the ester purity standard of Thailand or of

349

Europe. Therefore these partial glycerides had to be converted by the final third step

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continuous transesterification to achieve high purity of the ester. Biodiesel products with

351

98.72 to 99.83 wt.% methyl ester can be achieved, as shown in Table 3.

16

352 353

3.2 Response surface models and statistical analyses

354 355

3.2.1 Response surface models

356 357

The response surface methodology (RSM) was applied to maximize the purity of

358

methyl ester from each processing step of the continuous biodiesel production process, using

359

model fitting by multiple regression. The predictive models were multivariate second order

360

polynomials. The significant terms in these fitted models are shown in Eqs. (5-7). The

361

goodness-of-fit was assessed from the coefficient of determination (R2) and the adjusted

362

coefficient of determination (R2adjusted), listed in Table 4. The table also shows the p-values for

363

individual terms that were significant, and were therefore kept in the respective model.

364 365

ME1 =  + M1 + C1 + L1 + C12 + L12

(5)

366

ME2 =  + M2 + C2 + L2 + M22 + C22 + L22

(6)

367

ME3 =  + M3 + C3 + L3 + M3C3 + M32 + C32 + L32

(7)

368 369

where ME1 is methyl ester purity (wt.%), M1 is methanol (vol.%), C1 is sulfuric acid

370

(vol.%), and L1 is length of HSM (m), in the first step; ME2 is methyl ester purity (wt.%), M2

371

is methanol (vol.%), C2 is sulfuric acid (vol.%), and L2 is length of HSM (m), in the second

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step; and ME3 is methyl ester purity (wt.%), M3 is methanol (vol.%), C3 is KOH loading

373

(g/L), and L3 is length of HSM (m), in the third step. The  are fixed coefficients.

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374 375

3.2.2 Statistical analysis of response surface models

376 377

The p-values were used to assess statistical significance of individual model terms. A

378

p-value less than 0.05 indicates statistical significance, while a p-value exceeding 0.05

379

indicates insignificance at 95% confidence level [35]. According to Table 4, the linear terms

380

were highly significant in all models, with the smallest p-values. In both first step and third

381

step of the continuous processing, the lowest p-values or highest significances were found for

382

the terms M1 and M3, in Eq. (5) and Eq. (7), respectively. Thus, the methanol content

383

was strongly influential in both the first and the third steps for producing methyl ester using

384

HSM. Regarding the continuous second step esterification, the smallest p-value in Eq. (6)

385

was found for the term of L2, the linear dependence on the length of HSM or the residence

386

time in HSM. Therefore, the length of HSM is the most significant variable for the second

387

step. In the second step, the methanol content affecting through the term M2 had nearly

388

similar significance level. Regarding the sign of coefficients (), a positive sign indicates

389

increasing methyl ester purity, while a negative sign denotes negative influence of the

390

independent variable (methanol, catalyst, or length of HSM) on producing methyl ester. The

391

predictive models were analyzed to determine significances of the models by analysis of

392

variance (ANOVA), with results summarized in Table 5. F-test was used to check the model

393

significance. The calculated F0 for the model must be larger than the critical value (Fcrit) for a

394

given significance level. The calculated F0 value is defined as the mean square of regression

395

divided by the mean square of residual. The regression mean square (MSR) and the residual

396

mean square or mean squared error (MSE) were calculated by dividing the regression sum of

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397

squares (SSR) and the residual sum of squares (SSE) by the degrees of freedom (DOF),

398

respectively. With the number of coefficients (i) not counting , the DOF can be calculated

399

as n-1-i, where n is the number of experiments. The critical value (Fcrit) is defined as the

400

formula Fi,n-1-i, where  is the axial point for rotatability. The critical value can be accessed

401

in the F-distribution table at 95% confidence ( = 0.05). The results of F-test evaluation are

402

reported in Table 5, and the F0 for all models exceeded the Fcrit. As a result, the three

403

predictive models statistically significantly inform about the purity of methyl esters.

404 405

3.2.3 Response surface plots

406 407

Fig. 4 shows the relationships between the dependent and the independent variables

408

(length of HSM, MeOH, and H2SO4) as contour plots. These figures reflect the first (Figs.

409

4A, 4B, and Fig. 4C), second (Figs. 4D, 4E, and Fig. 4F), and third processing steps (Figs.

410

4G, 4H, and Fig. 4I), with the purity of methyl ester as the dependent variable.

411 412

3.2.4 Optimum conditions of continuous methyl ester production from PFAD

413

414

The models in Eqs. (5–7) were fitted to determine the optimum conditions for

415

continuous methyl ester production from PFAD. Each continuous processing step was

416

optimized for maximum purity of the esters, and the optimal conditions are shown in Table 6.

417

In the first step (Eq. 5), the maximal 86.85 wt.% ester purity was obtained with 64.4 wt.%

418

MeOH, 16.0 wt.% H2SO4, and 7 m length of HSM (approximately 81 sec residence time in

419

the continuous reactor). However, the excess methanol content was carefully considered,

420

since the main cost of biodiesel production is the methanol. The first-esterified oil will

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421

solidify to wax at room temperature (30oC), but no wax appears if the ester purity exceeds 70

422

wt.%. Wax formation in the first-esterified oil could block the continuous reactor, the

423

collection tank, or the continuous separator in biodiesel production form PFAD.

424

Consequently, 70 wt.% methyl ester purity was substituted into Eq. 5 for the dependent

425

variable (ME1), and the length of HSM, MeOH, and H2SO4 were solved for by Excel Solver.

426

This gave the recommended conditions of the first step: 41.3 wt.% MeOH, 12.0 wt.% H2SO4,

427

and 5 m length of HSM, expected to give 70 wt.% purity of ester by model prediction. In the

428

second step according to the model (Eq. 6), 96.10 wt.% methyl ester purity is maximal at the

429

optimal conditions 60.7 wt.% MeOH, 15.3 wt.% H2SO4, and 7 m length of HSM

430

(approximately 81 sec residence time in the continuous reactor). However, again the high

431

methanol consumption would be excessively costly, so 97.5 wt.% methyl ester purity was

432

required as value of the dependent variable (ME2). This gave the recommended conditions in

433

the second step: 53.4 wt.% MeOH, 16.8 wt.% H2SO4, and 7 m length of HSM that gave 97.50

434

wt.% methyl ester purity as model prediction. The recommended conditions decreased the

435

methanol consumption in the second step by approximately 12% from that at the optimal

436

conditions. Moreover, the FFA content in the second-esterified oil can be reduced to less than

437

1 wt.% under the recommended conditions, and this is an acceptable FFA level in oil to be

438

used in the biodiesel production by base-catalyzed transesterification. Finally, the third step

439

transesterification had model-based 99.96 wt.% maximal purity of methyl ester under the

440

optimal conditions: 12.3 wt.% MeOH, 5.0 g/L KOH, and 0.7 m length of HSM

441

(approximately 8 sec residence time in the continuous reactor). The recommended conditions

442

were tested experimentally, as described in the section on verification of methyl ester quality.

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20

443 444

3.3 Verification, yield and residual methanol

445 446

Table 6 shows the model-based optimal conditions, recommended conditions, and

447

residence times in the HSMs in continuous biodiesel production from PFAD. To verify the

448

three recommended operating conditions, TLC/FID was used to analyze the ester purity. Ester

449

purities of 71.01 wt.%, 95.94 wt.%, and 99.96 wt.% were achieved in actual experiments at 25

450

L/hr PFAD flow rate. These purities of esters are close to the model-predicted ester purities as

451

shown in Table 7. The yields 109.5 wt.% first-esterified oil, 117.0 wt.% second-esterified oil,

452

129.0 wt.% crude biodiesel, and 86.4 wt.% purified biodiesel, were achieved. The yields are

453

relative to 100 wt.% of initial PFAD. The yields of first-esterified oil, second-esterified oil,

454

and crude biodiesel are over 100 wt.%. Because the residual methanol and generated

455

wastewater diluted in these products before purification processing. Regarding the multi-step

456

biodiesel production process, the residual methanol in the products of each continuous

457

processing step will be used to react with the reaction mixture in the next process. Thus, these

458

yields are over 100 wt.% when compared with 100 wt.% of initial PFAD. The high yields of

459

the first-esterified oil, second-esterified oil, and crude biodiesel were attained with short

460

residence times in the HSMs (less than 58 sec, 81 sec, and 8 sec in the first, second, and third

461

steps). On average 86.4 wt.% yield of purified biodiesel from the continuous processing was

462

obtained after washing the biodiesel, with 100 wt.% referring to the weight basis of PFAD.

463

The crude biodiesel was washed to remove the residual catalyst and methanol. To achieve

464

high purity and yield of ester in biodiesel production from PFAD, excess methanol was used

465

in the three-step process to drive forward both esterification and transesterification reactions.

466

The unreacted methanol was distributed between the intermediate and final products, the

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467

waste waters, and glycerol. The residual methanol was analyzed by GC following EN 14110

468

standard test method [30], with the results shown in Table 7. The methanol contents in the

469

generated waste water were high at 17.9 wt.% and 42.2 wt.%, in the first and second steps,

470

respectively. Therefore, recovery of the residual methanol from generated wastewater would

471

be necessary in industrial scale operation. However, the residual methanol at 4.9 wt.% and at

472

12.5 wt.% in the first- and second-esterified oils, respectively, need not be recovered as it can

473

be used by the next reactions. In the third step of continuous processing, residual methanol

474

levels of 13.9 wt.% and 0.7 wt.% were present in the crude biodiesel and the glycerol The

475

physical properties of purified biodiesel from PFAD in comparison to the commercial

476

biodiesel standards of Thailand, the USA and Europe are given in Table 8.

477 478

3.4 Time and electricity consumptions in three-step continuous processing

479 480

3.4.1 Time consumption in the three-step continuous process

481 482

The total time consumption for the three-step continuous process was approximately

483

182 min (in the case of empty HSMs and continuous separators) from the start-up of

484

processing. In the first step, the residence times in the HSM and the continuous separator

485

were approximately 58 sec and 90 min, respectively. In the second step, the residence times

486

in HSM and continuous separator were approximately 81 sec and 60 min, respectively.

487

Notice that the phase separations after esterification in the first and second steps are the most

488

time consuming unit operations. The generated wastewater from each step consisted of water,

489

sulfuric acid, and methanol. To ensure that the generated wastewater can be treated by gravity

490

separation, 50 L and 35 L first and second separators were specifically designed to separate

491

these wastewaters from the continuous process. Finally, the smallest residence time was in

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22

492

the HSM of the third step, at only 8 sec. For the separation of crude biodiesel and glycerol

493

phases by gravity, the settling time in the separating funnel was observed. It took

494

approximately 30 min to completely separate the crude biodiesel and glycerol phases. Thus,

495

approximately 35 L third continuous separator was designed, to ensure that the glycerol phase

496

can be separated by gravity at the total flow rate of the mixture (second-esterified oil,

497

CH3OK). Fig. 5 shows the reaction conditions, the products, and the time consumption of the

498

methyl ester conversion in three-step continuous processing from PFAD using HSM under

499

the recommended conditions at 25 L/hr of PFAD.

500 501

3.4.2 Electricity consumption in the three-step continuous process

502 503

The average electricity consumption for the total process was measured by an electric

504

power meter, and is shown in Table 9. In the start-up 25 L of PFAD was preheated and

505

maintained at 50oC for 35 min by a band heater, which is an immersion heater. Subsequently,

506

PFAD was blended with MeOH and the mix was preheated to 50oC within 60 min by a

507

circulating pump, as preheating of PFAD and MeOH for the first actual step in the continuous

508

processing. Also in preparation for the start-up CH3OK solution was prepared by mixing with

509

a circulating pump for 10 min. The total electricity consumption of the start-up phase was

510

1.465 kW h. During the first process step the temperature of first-esterified oil was

511

maintained at 60oC. The PFAD mix with MeOH and H2SO4 was continuously fed into the 5-

512

m HSM by two digital dosing pumps. The total electricity consumption in the first step was

513

0.292 kW h. In the continuous second step, the first-esterified oil, MeOH, and H2SO4 flowed

514

continuously into the 7m-HSM, driven by three dosing pumps, and the total electricity

515

consumption in the second step was 0.445 kW h. In the final step of processing, the

516

temperature of second-esterified oil was maintained at 60oC, and two digital dosing pumps

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23

517

fed the second-esterified oil and CH3OK solution. The total electricity used in the third step

518

was 0.323 kW h. The overall total energy consumption of the process was 1.060 kW h for

519

producing approximately 22.4 L of purified biodiesel from 25 L of PFAD (excluding the

520

electricity consumption of startup, and of washing to purify the crude biodiesel). The average

521

energy consumption for crude biodiesel production was 0.0473 kW h/L.

522 523

4. Conclusions

524 525

A three-step continuous process for producing methyl ester from palm fatty acid

526

distillate (PFAD) was tested experimentally. Methanol in the presence of sulfuric acid

527

(H2SO4) as catalyst for the FFA reduction in the first step was followed by acid-catalyzed

528

esterification as the second step and by the third step in the presence of potassium hydroxide

529

(KOH) as base catalyst. The reactions in the three steps were performed in helical static

530

mixers (HSM). The experimentally achieved ester purities were 71.01 wt.% from the first

531

step, 95.94 wt.% from the second step, and 99.96 wt.% in the biodiesel from the third step.

532

The maximum yields were 109.5 wt.% first-esterified oil, 117.0 wt.% second-esterified oil,

533

129.0 wt.% crude biodiesel, and 86.4 wt.% purified biodiesel, following the phase separation

534

in each step, and after the final purification. The average energy consumption to produce

535

crude biodiesel was 0.0473 kW h/L. Moreover, the composition of biodiesel from PFAD was

536

analyzed to compare with the specifications of commercial biodiesel. It was found that the

537

methyl ester meets the standard specifications (B100) for commercial biodiesel in Thailand,

538

the USA, and the EU.

539 540

Acknowledgments

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541

This work was supported by the Energy Policy and Planning Office of Thailand

542 543

(EPPO), and by Prince of Songkla University, Grant No. ENG570563S.

544 545

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surface methodology. Energ Convers Manage 2016;115:178–90.

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650 651 652 653 654 655 656 657 658

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(A)

(B)

Fig. 1. PFAD was used as the raw material in the first step esterification, (A) PFAD at 30oC, and (B) PFAD at 43oC.

ACCEPTED MANUSCRIPT

Fig. 2. Schematic diagram of a three-step continuous process for biodiesel production from PFAD, using helical static mixers as reactors. (T1 and T6: MeOH tank, T2: PFAD tank, T3: mixing tank of PFAD and MeOH, T4 and T7: H2SO4 tank, SP1: first step separator, T5: firstesterified oil tank, SP2: second-esterified separator, T8: second-esterified oil tank, T9: CH3OK tank, SP3: third step separator, T10: crude biodiesel tank, P1: circulating pump of mixed PFAD and MeOH, P2: dosing pump of mixed PFAD and MeOH, P3 and P7: dosing pump of H2SO4, P4: circulating pump of first-esterified oil, P5: dosing pump of firstesterified oil, P6: dosing pump of MeOH, P8: circulating pump of second-esterified oil, P9: dosing pump of second-esterified oil, P10: dosing pump of CH3OK, P11: circulating pump of CH3OK, M: stirrer, HT1: band heater, HT2: immersion heater, SM: static mixer, HSM1, HSM2, HSM3: helical static mixers in the first, second, and third steps of continuous processing, respectively).

ACCEPTED MANUSCRIPT

Fig. 3. Three-step continuous process for biodiesel production from PFAD, using helical static mixers as reactors.

ACCEPTED MANUSCRIPT

(A)

(B)

(C)

(D)

(E)

(F)

(G)

(H)

(I)

Fig. 4. Contour plots of the fitted models for the three steps of continuous processing. In the first and second steps (A, D) show effects of length of HSM and MeOH, (B, E) show effects of length of HSM and H2SO4, and (C, F) show effects of H2SO4 and MeOH on purity of the

ACCEPTED MANUSCRIPT ester. For third step, (G) effects of length of HSM and MeOH, (H) effects of length of HSM and KOH, and (I) effects of KOH and MeOH, on the purity of ester.

under the recommended conditions at 25 L/hr of PFAD.

Fig 5. Methyl ester conversion in three-step continuous processing of PFAD using HSM

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT • Three-step continuous process for producing biodiesel from PFAD was optimized. • Helical static mixer was a key element in the three-step continuous processing. • Total residence time 147 s was obtained in the 3 units of helical static mixer. • Maximum 96.65 wt.% purity of biodiesel from three-step continuous processing.

ACCEPTED MANUSCRIPT Table 1 Translation table for factor levels in the experimental design for response surface methodology. Process Independent variable Coded level -1.682 -1 0 +1 First step M1 : Methanol (vol.%) 19.8 30.0 45.0 60.0 C1 : Sulfuric acid (vol.%) 0 2 5 8 L1 : Length of HSM (m) 0 2 5 8 Second step M2 : Methanol (vol.%) 19.8 30.0 45.0 60.0 C2 : Sulfuric acid (vol.%) 0 2 5 8 L2 : Length of HSM (m) 0 2 5 8 M3 : Methanol (vol.%) 2.6 6.0 11.0 16.0 Third step C3 : KOH (g/L) 0.1 1.5 3.5 5.5 L3 : Length of HSM (m) 0.0 0.2 0.5 0.8

+1.682 70.2 10 10 70.2 10 10 19.4 6.9 1.0

ACCEPTED MANUSCRIPT Table 2 Experimental design matrix along with results from the first and second steps of continuous processing. Methanol, Sulfuric acid, Length of HSM, Methyl ester, Methyl ester, Run M1 and M2 L1 and L2 C1 and C2 ME1 ME2 (vol.%) (vol.%) (m) (wt.%) (wt.%) 1 45.0 5 5 69.31 94.84 2 45.0 5 5 69.43 94.86 3 45.0 5 5 69.52 95.00 4 45.0 5 5 70.39 95.03 5 45.0 5 10 68.12 94.46 6 45.0 5 0 20.12 91.10 7 45.0 10 5 77.58 95.23 8 45.0 0 5 3.22 91.62 9 60.0 8 2 78.24 93.71 10 60.0 2 2 55.41 93.42 11 60.0 8 8 80.64 95.92 12 60.0 2 8 65.31 95.80 13 30.0 8 2 45.04 91.97 14 30.0 2 2 35.17 91.39 15 30.0 8 8 45.84 93.30 16 30.0 2 8 40.64 92.32 17 19.8 5 5 40.67 89.41 18 70.2 5 5 72.57 96.38 Note: For the first step esterification: M1, C1, L1, and ME1 are MeOH, H2SO4, length of helical static mixer, and purity of ester, respectively. The fitted model of ME1 is shown in Eq. 5. For the second step esterification: M2, C2, L2, and ME2 are MeOH, H2SO4, length of helical static mixer, and purity of ester, respectively. The fitted model of ME2 is shown in Eq. 6.

ACCEPTED MANUSCRIPT Table 3 Experimental design matrix and results from the third step of continuous processing. Methanol, KOH, Length of HSM, Methyl ester, Run M3 L3 C3 ME3 (vol.%) (g/L) (m) (wt.%) 1 11.0 3.5 0.5 99.76 2 11.0 3.5 0.5 99.72 3 11.0 3.5 0.5 99.73 4 11.0 3.5 0.5 99.76 5 11.0 3.5 1.0 99.76 6 11.0 3.5 0.0 99.29 7 11.0 6.9 0.5 99.83 8 11.0 0.1 0.5 98.97 9 16.0 5.5 0.2 99.50 10 16.0 1.5 0.2 99.34 11 16.0 5.5 0.8 99.81 12 16.0 1.5 0.8 99.56 13 6.0 5.5 0.2 99.49 14 6.0 1.5 0.2 98.72 15 6.0 5.5 0.8 99.55 16 6.0 1.5 0.8 98.93 17 2.6 3.5 0.5 98.83 18 19.4 3.5 0.5 99.75 Note: For the third step esterification: M3, C3, L3, and ME3 are MeOH, KOH, length of helical static mixer, and purity of ester, respectively. The fitted model of ME3 is shown in Eq. 7.

ACCEPTED MANUSCRIPT Table 4 Coefficients in the fitted response surface models. Eq. (5) Eq. (6) Eq. (7) Coefficient Value p-value Value p-value Value p-value -43.02 0.0264530 79.362 0.0000000 96.765 0.0000000  0.813 0.0034983 0.343 0.0026650 0.233 0.0000004  12.91 0.0074244 0.654 0.0273598 0.469 0.0000011  9.492 0.0358455 1.021 0.0021868 1.301 0.0002451  -0.012 0.0005941  -0.003 0.0204313 -0.007 0.0000096  -0.856 0.0464904 -0.046 0.0910936 -0.031 0.0000962  -0.707 0.0916852 -0.071 0.0146689 -0.913 0.0023339  R2 0.769 0.898 0.978 R2adjusted 0.672 0.842 0.962 2 2 Note: R is coefficient of determination, R adjusted is adjusted coefficient of determination, and p-value is an indicator of statistical significance.

ACCEPTED MANUSCRIPT Table 5 ANOVA for each response surface model representing the three-step continuous process. Source SS MS F0 Fcrit DOF Eq. (5) for first step Regression 6166.7 1233.3 7.968 3.10 (F0.05,5,12) 5 Residual (Error) 1857.5 154.79 12 Lack-of-Fit Error 1856.7 206.31 9 0.736 0.245 3 Pure Error 8024.2 17 Total Eq. (6) for second step Regression 58.77 9.795 16.07 3.09 (F0.05,6,11) 6 Residual (Error) 6.706 0.610 11 6.678 0.835 8 Lack-of-Fit Error Pure Error 0.02799 0.00933 3 Total 65.47 17 Eq. (7) for third step Regression 2.219 0.317 62.70 3.13 (F0.05,7,10) 7 Residual (Error) 0.05057 0.00506 10 0.04917 0.00702 7 Lack-of-Fit Error Pure Error 0.00139 0.000465 3 Total 2.270 17 Note: DOF is degrees of freedom, SS is sum of squares, and MS is mean square.

ACCEPTED MANUSCRIPT Table 6 Model-based optimal conditions, selected appropriate conditions, and residence times observed in continuous biodiesel production from PFAD. Condition Condition Optimized Recommended First step (esterification) 70.2 vol.% 64.4 wt.% 45.1 vol.% 41.3 wt.% Methanol 7.6 vol.% 16.0 wt.% 5.7 vol.% 12.0 wt.% Sulfuric acid Length of helical static mixer 7m 5m Residence time in HSM ≈ 81 s ≈ 58 s Second step (esterification) 64.8 vol.% 60.7 wt.% 57.0 vol.% 53.4 wt.% Methanol 7.1 vol.% 15.3 wt.% 7.8 vol.% 16.8 wt.% Sulfuric acid Length of helical static mixer 7m 7m Residence time in HSM ≈ 81 s ≈ 81 s Third step (transesterification) Methanol 13.0 vol.% 12.3 wt.% 13.0 vol.% 12.3 wt.% KOH 5.0 g/L 5.0 g/L Length of helical static mixer 0.7 0.7 Residence time in HSM ≈8 ≈8 Total 148 vol.% 137.4 wt.% 115.1 vol.% 107 wt.% Methanol consumption 14.7 vol.% 31.3 wt.% 13.5 vol.% 28.8 wt.% Sulfuric acid consumption KOH consumption 5.0 g/L 5.0 g/L Total length of helical static mixer 14.7 m 12.7 m Total residence time in HSMs ≈ 170 s ≈ 147 s Note: The percentage concentration: vol.% is the volume of chemical reactant per volume of raw material of each step, wt.% is the weight of chemical reactant per weight of raw material of each step, The densities of PFAD (at 50oC), methanol (at 30oC), sulfuric acid (at 30oC) were 0.869, 0.797, and 1.830 kg/L, respectively. The densities of firstesterified oil, second-esterified oil, and crude biodiesel at 60oC were 0.850, 0.847, and 0.840 kg/L, respectively.

ACCEPTED MANUSCRIPT Table 7 Compositions, yields, and residual methanol in the first- and second-esterified oils and in biodiesel from the three-step continuous processing, as analyzed by TLC/FID. Composition(a), yield(b), and residual methanol wt.% First step esterification Compositions of first-esterified oil(a) Free fatty acid 26.43 Methyl ester 71.01 Triglyceride 0.00 Diglyceride 1.17 Monoglyceride 1.39 (b) Yield First-esterified oil 109.5 First-waste water 43.8 Residual methanol Residual methanol in the first-esterified oil 4.6 Residual methanol in the first-waste water 22.1 Second step esterification Compositions of second esterified oil(a) Free fatty acid 0.95 Methyl ester 95.94 Triglyceride 0.00 Diglyceride 0.98 Monoglyceride 2.13 (b) Yield Second-esterified oil 117.0 Second-waste water 75.5 Residual methanol Residual methanol in the second esterified oil 11.7 Residual methanol in the second-waste water 47.9 Third step transesterification Compositions of purified biodiesel(a) Free fatty acid 0.00 Methyl ester 99.96 Triglyceride 0.00 Diglyceride 0.00 Monoglyceride 0.04 (b) Yield Crude biodiesel 129.0 Glycerol 2.3 Residual methanol Residual methanol in the crude biodiesel 12.9 Residual methanol in the glycerol 0.7 Purification Yield(b) Purified biodiesel (wt.%) 86.4 Note: (a) Results of actual experiment for each step. (b) Yield of each processing step (wt.%) = the weight of product (g) / the weight of initial PFAD (g) [37]. The yields are relative to 100 wt.% of initial PFAD.

Table 8 The properties of purified biodiesel from PFAD with comparison to requirements in commercial biodiesel standards of Thailand, the US, and the Europe Biodiesel standard Property Results Method THA [38] US [39] Europe [39] (ASTM, EN) (ASTM) (EN) Methyl ester (wt.%) 96.65 EN 14103 [21] 96.5 min 96.5 min Linolenic acid ester (wt.%) 0.277 EN 14103 [21] 12.0 max 12.0 max Density at 15 °C (kg/m3) 874 ASTM D1298 [19] 860-900 860-900 Viscosity at 40 °C (cSt) 4.27 ASTM D445 [20] 3.5-5.0 1.9-6.0 3.5-5.0 Flash point (°C) 167 ASTM D93 [23] 120 min 93 min 101 min Carbon residue (wt.%) < 0.1 ASTM D4530 [24] 0.3 max 0.05 max 0.3 max Water and sediment (vol.%) 0.045 EN ISO 12937 [25] 0.05 max 0.05 max 0.05 max Total contamination (mg/kg) 1.1 EN 12662 [26] 24 max 24 max Copper strip corrosion < No.1 ASTM D130 [27] No.1 max No.3 max No.1 max Acid value (mgKOH/g) 0.47 ASTM D664 [28] 0.50 max 0.50 max 0.50 max Iodine value (g Iodine/100 g) 50.03 EN 14111 [29] 120 max 120 max Methanol (wt.%) < 0.01 EN 14110 [30] 0.2 max 0.2 max 0.2 max Monoglyceride (wt.%) 0.109 EN 14105 [22] 0.7 max 0.8 max Diglyceride (wt.%) 0.038 EN 14105 [22] 0.2 max 0.2 max Triglyceride (wt.%) 0 EN 14105 [22] 0.2 max 0.2 max Free glycerin (wt.%) 0 EN 14105 [22] 0.02 max 0.02 max 0.02 max Total glycerin (wt.%) 0.038 EN 14105 [22] 0.25 max 0.24 max 0.25 max Cetane number 68.1 ASTM D613 [31] 51 min 47 min 51 min Phosphorus (wt.%) 0.0000722 EN 14107 [32] 0.001 max 0.001 max 0.0004 max

ACCEPTED MANUSCRIPT Table 9 Average electricity consumption in the continuous process. Continuous processing step First step for acid-catalyzed esterification - 25 L of PFAD was preheated to 50oC within 35 min - PFAD mixing with MeOH and preheated to 50oC within 60 min - Maintain the temperature of PFAD mixing with MeOH at 50oC - Two dosing pumps: PFAD mixing with MeOH and H2SO4 Total average electricity of first step Second step for acid-catalyzed esterification - Maintain the temperature of first-esterified oil at 60oC - Three dosing pumps: first-esterified oil, MeOH, and H2SO4 Total average electricity of second step Third step for base-catalyzed transesterification - Maintain the temperature of second esterified oil at 60oC - Preparing CH3OK by circulating pump within 10 min - Two dosing pumps: second esterified oil, and CH3OK Total average electricity of third step Total average electricity

Electricity (kW h) Startup Process 0.56 0.90 1.46

0.246 0.046 0.292

0

0.438 0.007 0.445

0.005 0.005 1.465

0.319 0.004 0.323 1.060