Heterogeneous Acid-Catalyzed Biodiesel Production ...

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Heterogeneous Acid-Catalyzed Biodiesel Production from Crude Tall Oil: A Low-Grade and Less Expensive Feedstock a

ab

Ntandoyenkosi Malusi Mkhize , B. Bruce Sithole a

& Mbuyu Germain Ntunka

a

Department of Chemical Engineering, University of KwaZulu-Natal, Durban, South Africa

b

Forestry and Forest Products Centre, Natural Resources and the Environment, Council for Scientific and Industrial Research (CSIR), Durban, South Africa Published online: 17 Jun 2015.

Click for updates To cite this article: Ntandoyenkosi Malusi Mkhize, B. Bruce Sithole & Mbuyu Germain Ntunka (2015) Heterogeneous AcidCatalyzed Biodiesel Production from Crude Tall Oil: A Low-Grade and Less Expensive Feedstock, Journal of Wood Chemistry and Technology, 35:5, 374-385, DOI: 10.1080/02773813.2014.984079 To link to this article: http://dx.doi.org/10.1080/02773813.2014.984079

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Journal of Wood Chemistry and Technology, 35:374–385, 2015 C Taylor & Francis Group, LLC Copyright ISSN: 0277-3813 print / 1532-2319 online DOI: 10.1080/02773813.2014.984079

HETEROGENEOUS ACID-CATALYZED BIODIESEL PRODUCTION FROM CRUDE TALL OIL: A LOW-GRADE AND LESS EXPENSIVE FEEDSTOCK Ntandoyenkosi Malusi Mkhize,1 B. Bruce Sithole,1,2 and Mbuyu Germain Ntunka1 1

Department of Chemical Engineering, University of KwaZulu-Natal, Durban, South Africa Forestry and Forest Products Centre, Natural Resources and the Environment, Council for Scientific and Industrial Research (CSIR), Durban, South Africa

Downloaded by [CSIR Information Services] at 02:27 09 September 2015

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The present study indicates that solid acid catalysis of crude tall oil (CTO) over a WO3 /ZrO2 catalyst is effective in converting the CTO fatty acids components into biodiesel in high yield. Preparation of the catalyst by an impregnation method was selected and WO3 activity was best at a loading mass fraction of 5% to ZrO2 support and activation at 500◦ C for five hours under air at atmospheric pressure. Optimal reaction conditions were reaction temperature at 250◦ C; methanol to CTO molar ratio at 10; reaction time four hours, catalyst mass fraction of 3%; and stirring intensity at 625 rpm. The conversion at optimal reaction conditions was 70%. The catalyst was highly active at temperatures higher than 200◦ C. The biodiesel produced met some, but not all, the diesel quality parameters stipulated by standard specifications such as ASTM D6751 and EN14214. KEYWORDS. Biodiesel, catalyst, crude tall oil, heterogeneous

INTRODUCTION

aration, and purification, and it produces large amounts of waste streams that require proper intensive treatment and disposal techniques.[8,9] Thus, it is desirable to reduce the cost of biodiesel synthesis in order for the technology to be economical and competitive with petroleum-derived diesel. This can be achieved in two practical ways. Firstly, by utilizing less-expensive feedstocks such as non-edible oils, industrial by-products, used edible oils. and animal fats. Secondly, by eliminating or reducing downstream processing stages, which are expensive to operate and which contribute to unwanted waste production.[10] However, this is not practical with the current industrial homogeneous alkali catalysis process. Thus, there is a need for processes which can utilize low-grade and less-expensive feedstocks that are characterized by high water and free fatty acids contents.[11] Also, a

The most common and currently used biodiesel synthesis process, homogeneous alkali catalysis, requires high-grade feedstocks, such as edible vegetable oils.[1–5] Edible feedstocks, such as refined soybean oil, sunflower oil, and rapeseed oil, are characterised by high glycerides content, traces of free fatty acids, and negligible water content.[1,6] These feedstocks are expensive and they compete with the food market. It is expensive to synthesize biodiesel from low-grade feedstocks and biodiesel production from these feedstocks cannot compete with petroleum-derived diesel fuels at their current prices.[7] Moreover, a homogeneous alkali-catalyzed process needs several energyintensive downstream processes, such as catalyst neutralization and separation, product sep-

Address correspondence to B. Bruce Sithole, Forestry and Forest Products Centre, Natural Resources and the Environment, Council for Scientific and Industrial Research (CSIR), 359 Mazisi Kunene Avenue, Bulwer, Durban, 4001, South Africa. E-mail: [email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lwct. 374

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better process depends on the elimination or reduction of the number of downstream stages. Thus, heterogeneous/solid acid catalysis is ideal as it can simultaneously spark catalysis transesterification and esterification processes.[10,12] In replacing expensive feedstocks with the low-grade and less-expensive feedstocks, the impediment faced with homogeneous alkali catalysis is the consumption of the catalyst by the free fatty acids present in high concentrations in these feedstocks. Free fatty acids consume alkali catalysts to form soaps via a saponification process.[11] Homogeneous acid catalysis is also not suitable for low-grade oils containing high levels of water, which can deactivate the catalyst.[13,14] However, homogeneous acid catalysis reacts 4,000-fold slower than homogeneous alkali catalysis.[15] In the presence of water, the acid catalyst promotes hydroesterification of glycerides. Both acid and base homogeneous catalysts require neutralization and the catalysts are not recoverable.[14] Heterogeneous catalysis has been researched for biodiesel synthesis from low-grade feedstocks such as used cooking oil.[13,14,16] The aim was to eliminate or reduce the downstream processes such as catalyst neutralization, washing processes, and separation processes. It was concluded that heterogeneous catalysts can be recovered, reused, and regenerated, which can reduce the cost of biodiesel production. However, the presence of free fatty acids still promotes saponification, even with a solid base catalyst.[17] Consequently, the soaps need to be separated from the products, thus adding to the number of downstream separation processes. The interesting low-grade feedstock for biodiesel production is crude tall oil (CTO), a by-product of alkaline or kraft pulping of coniferous woods. It is a liquid that separates from the spent pulping liquor remaining after the kraft pulping process. It is a viscous, dark, and odorous liquid. It is formed by phase separation from the aqueous phase of black liquor after partial evaporation of water.[18] In the alkaline or kraft pulping process, softwoods are cooked under determined pressure and temperature with sodium sulphide and sodium hy-

droxide. Wood fatty acids and resin acids are converted into sodium salts during the cooking process.[19] Thus, tall oil chemically consists (by mass fraction) of 20–50% fatty acids, 40–60% resin acids, and 10–15% unsaponifiable compounds. On average, crude tall oil output is 20–30 kg/ton wood.[20] This study will focus on determining the optimal reaction condition of biodiesel from CTO over heterogeneous acid catalysis. This catalysis process is conducted under critical conditions of methanol. Heterogeneous acid catalysts are recoverable, reusable, and can be regenerated.[8,21] Moreover, downstream processes can be eliminated or significantly reduced as heterogeneous catalysts are easily separated from reaction products. Little has been reported on the investigation of heterogeneous/solid catalysis for the production of biodiesel from CTO. Thus, the current study is aimed at investigating biodiesel production from CTO.

MATERIALS AND METHODS Crude Tall Oil and Biodiesel Characterization CTO was collected from a South African kraft mill that pulps pine (Pinus radiata). Moreover, CTO characteristic typical values were available from the supplier and were mostly based on published research work. CTO was characterized for moisture content, acid value, resin acid content, saponification value, unsaponifiable matter, crude tall oil components, crude tall oil density, crude tall oil kinematic viscosity, and crude tall oil higher heating value (HHV). Moisture content was determined by volumetric titration using a Karl Fischer Moisture Titrator MKS-500 model. Potentiometric titration technique was used to estimate crude tall oil acid value, resin acids value, and saponification value in accordance with the Technical Association of the Pulp and Paper Industry (TAPPI) standard methods. The composition of the CTO was determined by GC/MS using a Shimadzu GC/MS-QP2010PLUS instrument. The crude tall oil density was estimated using

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a 50 μl micropipette and a microbalance. Kinematic viscosity measurements were determined using a Brookfield RVDV-II+ viscometer. CTO was analyzed for carbon, hydrogen, nitrogen, and sulphur (CHNS) elemental analysis using a CHNS analyzer. Oxygen content from the oil samples was estimated as the balance from the amount of the other elements present. The ultimate analysis results of C, H, N, and O content were used to estimate the HHV using a correlation equation.[6] HHV = 33.5[C] + 142.3[H] − 15.4[O] −14.5[N] × 10−2

(1)

Catalyst Preparation and Characterization Catalyst Preparation. The tungsten zirconia (WO3 /ZrO2 ) catalyst was prepared using a suspension impregnation method, as described by Park and colleagues.[3] The reagents tungsten oxide (WO3 ) and ammonium metatungstate (NH4 )10 W12 O41 .5H2 O were obtained from Sigma Aldrich. To incorporate the mass fraction of 5% and 15% tungsten oxide (WO3 ), a predetermined quantity of ammonium metatungstate (NH4 )10 W12 O41 .5H2 O was dissolved in excess water and finely powdered, oven-dried hydrous zirconia (ZrO2 ) support was added to the solution, which was then refluxed at 110◦ C for 2 h. The excess water was evaporated on a water bath with continuous stirring and the resulting sample was oven-dried at 110◦ C for 12 h. Then, the dry samples were calcined at 500◦ C for 5 h in an air atmosphere, and finally stored in dry nitrogen atmosphere.[3] Catalyst Characterisation. The catalyst was characterized by various techniques, including Brunauer-Emmett-Teller (BET), X-ray powder diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) analyses. The surface area, pore volume, and pore diameter of the catalyst were estimated by BET technique using the Micromeritics TriStar II 3020 (Micromeritics, Norcross, USA). The catalyst samples for BET analysis were prepared by incorporation of the WO3 at 5 and 15% into

the ZrO2 support. Also prepared were pure ZrO2 and pure WO3 samples (controls) for BET analysis. After calcination at 500◦ C for five hours, three samples—pure ZrO2 , 5 and 10% WO3 /ZrO2 —were vacuum prepared by first degassing while heating initially at 51◦ C for 30 minutes. The temperature was ramped up to 90◦ C for an hour and continued with degassing. Finally, the temperature was increased to 200◦ C and then the samples were allowed to degas overnight. After the sample degassing was complete, the surface area, pore volume, and pore diameter were determined by measurement of UHP N2 at −196◦ C adsorption using a Micrometrics TriStar II 3020. A similar procedure was repeated to analyze 15 and 20% WO3 ZrO2 and pure WO3 samples. The XRD patterns of the catalyst were determined from XRD analysis. These patterns are used to certify structural property of WO3 loading through confirmation of crystalline WO3 and ZrO2 peaks.[3] The analysis covered a starting position at 2 θ = 3◦ to an end position at 2 θ = 90◦ . Measurement temperature was 25◦ C; anode material was cobalt and generator settings were 45 mA and 40kW. Fourier transform infrared spectroscopy (FTIR) was used to acquire the transmission spectra of the crude tall oil samples in the region between 4,000–400 cm−1. KBR disks of the samples were prepared and analyzed on a Perkin Elmer Spectrum RX I FTIR (Perkin Elmer, Midrand, South Africa) instrument. Biodiesel Production Biodiesel production experiments were conducted on a 600 mL autoclave reactor Parr 4563–4842 Mini Bench Top Reactor (Parr Instrument Company, Moline, USA). The reagents were CTO, HPLC-grade methanol, and tungsten zirconia (WO3 /ZrO2 ) catalyst. The experimental design consisted of five factors: reaction temperature, reaction time, methanol to alcohol molar ratio, catalyst amount, and stirring intensity. The two fractional factorial design 55-1 was used to investigate the effects of the factors on the biodiesel yield.[22] The reactor

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TABLE 1. Experimental conditions investigated at two different ranges Initial range Parameter

Low value

High value

Low value

High value

65 1 1 1 500

65 4 5 3 1200

180 1 15 1 625

250 4 30 3 1250

Reaction temperature (◦ C) Reaction time (hr) Methanol to CTO molar ratio Catalyst amount (weight%) Stirring intensity (rpm)


Final range

pressure for all of the experiments depended on the methanol/CTO molar ratio. The analysis of the fraction factorial design resulted in the improvement of the experimental conditions for reaction temperature, methanol to oil molar ratio, and stirring intensity, while the reaction time and catalyst amount remained unchanged (see Table 1). The mixture was transferred into a Buchner funnel and filtered through a 0.05 μm filter paper to remove the solid catalyst. The liquid mixture was then transferred into a pear-shaped separating funnel and allowed to separate overnight. The mixture separated into two phases: a top phase (biodiesel) that was lighter in color was separated from the bottom darker phase. Methanol and water were removed from the two phases by rotary evaporation. The two phases were then stored at ambient conditions for further analysis, such as fuel properties, compositional, proximate and elemental analysis.

RESULTS AND DISCUSSION Crude Tall Oil Characterization Results for CTO are shown in Table 2. The major components were palmitic (15%), linoleic (26%), oleic (58%), and stearic (1%) fatty acids. Trace amounts of myristic, linolenic, and behenic fatty acids were also identified. As expected, CTO also contained significant amounts of resin acid compounds, such as abietic, palustric, and pimaric acids, but these are not of interest in the production of biodiesel. The results in Table 2 also list unsaponifiable compounds, such as beta- and gamma-sitosterol, cholesterols, and stigmasterol. These compounds also

are not of interest in the production of biodiesel. Moreover, other unsaponifiable compounds, such as long-chained alcohols, were detected but at lower concentrations. A more detailed listing of the components found in the CTO is shown in Table 3. Figure 1 shows a typical chromatogram of the crude tall oil sample with margaric acid as an internal standard compound. The compounds that were identified are shown in Table 3. Characterisation of the Catalyst BET surface area analysis showed that pure ZrO2 has the largest surface area, pore volume, and pore diameter compared to both 5 and 10 wt.% WO3 loading. However, in spite TABLE 2. Properties and composition of crude tall oil Parameter

Value

Acid value (mgKOH/gCTO) Saponification value (mgKOH/gCTO) Kinematic viscosity at 40◦ C (cP) Kinematic viscosity at 20◦ C (cP) Density at 25◦ C (g/ml) Unsaponifiable matter (% water-free basis) Resin acids (% water-free basis) Fatty acids (% water-free basis) Moisture content (% mass fraction) Fatty acids Palmitic acid Linoleic acid Oleic acid Stearic acid Resin acids and unsaponifiables Abietic acid Palustric acid Pimaric acid beta- and gamma-Sitosterol Cholesterol Stigmasterol

139.2 48.2 344.1 826.0 0.847 16 47 37