Bioresource Technology 47 (1994) 131 - 134

PHYSICAL PROPERTIES OF TALLOW ESTER AND DIESEL FUEL BLENDS*

Y u s u f Al i & M. A. H a n n a

Department of Biological Systems Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska 68583-0726, USA

(Received 29 March 1993; revised version received and accepted 15 July 1993)

Abstract Physical properties of neat tallow methyl ester and etha- nol blends were determined to find the ester/ethanol blend which had the same viscosity as that of number 2 diesel fuel. Tallow ester and ethanol in a 65:35 (v/v) ratio gave the same viscosity as that of diesel fuel. This blend was mixed with number 2 diesel fuel in ratios varying from 0 to 100%, and viscosities, densities and cetane indexes of the blends were determined. Mixture viscosity was not affected by ester content. Viscosity of the mixture decreased as temperature increased. The cetane number of the tallow ester/ethanol/diesel blend was greater than that of pure number 2 diesel fuel. The cetane number of 56 for the tallow ester/ethanol blend was reduced to 46 for a blend of 25% tallow ester/etha- nol and 75% diesel fuel. It was concluded that tallow ester had potential as a diesel fuel substitute.

Key words: Transesterification, pyrolysis, micro- emulsion, coking, distillation, viscosity, density, cetane number, API gravity, tallow, diesel fuel, tallow ester.

INTRODUCTION

The downturn in petroleum-based energy costs has had a tendency to stifle research in the area of alterna- tive fuels. Nevertheless, vulnerability of foreign sup- plies and environmental concerns about emissions have rekindled interest in the development and use of non-petroleum based renewable fuels. Among the re- newable fuels being considered are various types of fats and oils derived from plant and animal sources. These fats and oils have potential as diesel engine fuels but there is a need for continued and concentrated research (Quick, 1980).

The primary problem in the direct use of vegetable oils in direct-injected engines is spray atomization because of their high viscosities. High viscosities of

*Journal Series Number 10299 of the University of Nebraska Agricultural Research Division.

Bioresource Technology 0960-8524/94/S07.00 © 1994 Elsevier Science Limited, England. Printed in Great Britain

vegetable oils lead to poor fuel atomization and ineffi- cient mixing with air in the combustion chamber, which contributes to incomplete burning, causing injector nozzle coking, deposits, ring sticking related to depo- sits and dilution of lubricating oil (Ryan et al., 1984). Tallow, on the other hand, is a solid at room tempera- ture, having a melting point temperature of 45°C. It cannot be used directly as a mobile fuel. Thus, tallow needs to be modified to keep it in liquid form at tem- peratures as low as possible.

The viscosity problem has always been approached in at least four ways: (1) transesterification, (2) dilution, (3) pyrolysis and (4) microemulsion (Bagby, 1987). Through transesterification, the oils and fats can be converted into glycerol and fatty methyl or ethyl esters; the latter having viscosities low enough to be used as diesel fuels, but still three to four times greater than number 2 diesel fuel. Dilution of vegetable oils with diesel fuel reduced viscosity to a great extent, but the resultant viscosity is still greater than diesel fuel in pro- portion to the dilution.

Limited work has been done on the pyrolysis of vegetable oil. Energy requirements and end products are of concern with this process. Microemulsification shows considerable promise for providing low viscosity fuel blends but this fuel has low cetane numbers and may contain unacceptable levels of glycerol as is the case with dilutions. The adaptability of tallow as a fuel for diesel engines can be improved by reducing its viscosity either by transesterification or by blending it with other fuels, or both. Therefore, the objective of his project was to determine the physical properties, including viscosity and cetane number, of tallow ester and blends with ethanol and number 2 diesel fuel.

131

METHODS

The beef tallow used in this study was obtained gratis from Excel Corp. of Schuyler, NE. The free fatty acid (FFA) content was determined by AACC method 58- 15 (AACC, 1983). The total FFA content of the raw tallow, which is indicative of tallow quality, was 1.67%. The tallow was then alkali refined using the method described by Swern (1979).

132 Yusuf Ali, M. A. Hanna

Transesterification reaction procedure A sample of 500 g (0.5814 mol) of tallow was placed in a 1000 ml flat-bottom flask equipped with a mechani- cal stirrer, thermometer and condenser. The tallow was stirred and heated to 60°C. In another beaker, 111"6 g (3"4884 tool) of methanol was mixed with 5"55 g of sodium hydroxide (1% by weight assuming 90% pure NaOH). A 6 : 1 molar ratio of methanol to tallow was used as suggested by Freedman et al. (1984) for better esterification. The mixture of methanol and NaOH was heated to 60°C and stirred until all of the NaOH was dissolved. This mixture was then added to the tallow, stirred rigorously, and further heated to 70-75°C. Heating and stirring continued for 2 h. The mixture was then allowed to cool to room temperature, and the ester and glycerol layers were separated in a separatory funnel. Excess methanol was removed from the ester by distillation. The ester was purified by washing three times with distilled water and drying under vacuum.

Viscosity measurement First, the viscosity of number 2 diesel was measured at 30°C. A Brookfield Synchro-Lectric LV viscometer with UL adapter was used to determine the viscosity. The temperature of the sample was maintained within +0-5°C with a constant temperature bath (Tamson, Neslab Instruments Inc.). Then, the viscosities of pure tallow ester and blends of tallow ester with 25, 50 and 75% ethanol were also measured at 30°C. A regression was performed to find the relationship between the viscosity and blend composition. From the regression, the optimum blend of tallow ester and ethanol which had the same viscosity as that of number 2 diesel fuel was determined.

A blend of 65:35 tallow ester/ethanol was prepared and this was mixed with diesel fuel in the ratio from 0 to 100% in increments of 10%. Eleven samples were prepared and the viscosity of each sample was deter- mined at temperatures ranging from 10 to 60°C in 10°C increments. The experimental design was a split plot with tallow ester, fuel ethanol and diesel fuel as the main units and temperature as the subunit. The experi- ment was not replicated but five viscosity measure- ments were made on each sample.

Density measurement The atmospheric liquid density of each blend was determined by measuring sample mass in a 10 ml pycnometer. This method gave an accuracy of approxi- mately + 0"0003 g ml-

Cetane number determination Samples of 65:35 tallow ester/ethanol mixture were 25, 50 and 75% diesel fuel blended and sent to Profes- sional Service Industries, Inc. (Omaha, NE) to find the calculated cetane index (CCI). Tests were not repli- cated. The CCI is based on API gravity and the mid- boiling point of the petroleum hydrocarbons, and gives numbers that correspond quite closely to the cetane number as determined by the engine testing method.

m ~4 n

E

oo _~2 >

0 0 : 1 ~ 1 ~ : 0

Vlscosily of # 2 Diesel Fuel @ 30 eC

20:80 40 :60 60:40 80:120

Tallow Ester to Ethanol Ratios (%)

Fig. I. Viscosity as a function of tallow ester to ethanol ratios (%).

The same procedure was used in this study to find the CCI of tallow ester/ethanol and diesel blends.

RESULTS AND DISCUSSION

Viscosity of ester/ethanol blends The effect of blending tallow ester with ethanol on vis- cosity is presented in Fig. 1. Regression analyses were performed to find the relationship between viscosity and ester/ethanol ratio. It was observed that a quad- ratic trend was significant (F= 230.91 Pr> F= 0-0001) and represented the relationship. The equation was

Y= 1-0777 - 0.013 211 X+ 0"000 56X 2

where Y= viscosity (mPa. s), and X = ethanol content of the blend (% v/v).

An ester/ethanol blend of 65 : 35 was determined to have the same viscosity as number 2 diesel fuel. This ratio was used to make subsequent blends with diesel fuel.

Effect of temperature and blend (diesel, ethanol and ester) on viscosity The statistical analysis, conducted using a split plot design with diesel and ester/ethanol blends as the main units and temperature as the subunit, showed that there was a significant interaction between blend and tem- perature (F= 23.21, Pr> F=0.0001). The combined effects of temperature and blend on viscosity are shown in Fig. 2. It can be observed from the figure that as temperature increased, the viscosity of the fuel blend decreased.

Density of blends The densities of various blends of number 2 diesel fuel and a blend of 65% tallow ester and 35% ethanol as measured by 10 ml pycnometer, are presented in Table 1. It is also important to note that the density of pure tallow ester was reduced from 0"866 g ml- 1 to 0"835 g m1-1 when blended with 35% (v/v) ethanol. There were no significant differences in the densities of the

Physical properties of tallow ester and diesel fuel blends 133

blended samples. All the densities were close to the density of pure diesel fuel.

Fuel propert ies Selected fuel properties of the blended fuels are pre- sented in Table 2. The initial boili n g p()i n t (I B P) of pure tallow ester was 210-5°C while thc IBP of number 2 diesel fuel was 202°C. The IBP of the blended fuels varied from 65 to 76°C. The low IBPs of the blended fuels were due to the presence of ethanol which had a boiling point of 78°C. Since the IBP of the blended fuel was greater than 37°C, cavitation and dual phases of fuel in the fuel pump should not be problematic. The final boiling points (FBPs) of the samples were between 306 and 326°C, which were lower than the FBP of number 2 diesel fuel of 361 °C.

The cetane index for all the samples was greater than the cetane index for number 2 diesel fuel which is generally in the in the range of 42-50 (ASTM, 1986). Regression analysis for variation of the cetane index with an ester/ethanol/diesel blend was performed and

65

60

X © @ 55

o

0

45

~ = 0 .~2 40 I I I I

0 : 1 0 0 2 0 : 8 0 4 0 : 6 0 6 0 : 4 0 8 0 : 2 0 1 0 0 : 0

#2 Diesel Fuel : 650 Tallow Ester-35% Ethanol Blend Ratios (v/v %)

Fig. 2. Viscosity as a function of temperature and tallow ester/ethanol: diesel blend.

it was found that the cetane index varied linearly with the blend of ester and diesel. The regression equation was

Y= 61.799 - 0.2008 X(R 2 = 0-982)

where Y is the cetane index and X is the diesel content of the blend (% v/v).

The cetane number of a blend of 13 :7 :80% ester:ethanol:diesel was verified by running it in a cetane engine. The cetane engine value was 48.9 as opposed to the CCI value of 48.17.

CONCLUSIONS

-- The viscosity of the tallow ester can be reduced to the viscosity of diesel fuel by blending it with 35% (v/v) ethanol.

- -Tal low ester/ethanol blend viscosity decreases with increasing temperature.

-- The fuel properties of all the blends were closely aligned with those of number 2 diesel fuel.

- - The density of tallow ester blended with ethanol was close to diesel fuel.

Table 1. Densities of tallow ester/ethanol and diesel fuel blends

Diesel: 65% tallow ester-35% Density ethanol (v/v) (g ml- i)

0:100 0"835 10:90 0-833 20:80 0"832 30:70 0-830 40:60 0"831 50:50 0"831 60:40 0"832 70:30 0"832 80:20 0"830 90:10 0"832

100:0 0-830

Table 2. Fuel properties of tallow ester and its blends with ethanol and diesel

Fuel type API gravity

Distillation Cetane index

IBP 50% 95% FBP (°C) (°C) (°C) (°C)

Ester 30'30 210.5 Ester:ethanol 36.20 75.5

(65:35 (v/v)) Ester: ethanol :diesel 36.20 65"5

(48.75: 26-25:25 (v/v)) Ester: ethanol :diesel 36.20 76-0

(32.5:17.5:50 (v/v)) Ester:ethanol :diesel 36.70 75.5

(16.25:8.75:75 (v/v)) Ester: ethanol: diesel 37.40 76.6

(13:7:80 (v/v)) Number 2 diesel fuel ~ 37"56 202.0

323-0 326"0 326"5 53'54 310-0 323-0 327'0 62'57

282"0 309-0 318"0 55"50

269"5 294-5 308"0 52"00

248"0 301"0 306"0 47-00

253"5 292-0 292"0 48.17

280-5 358"0 361"0 --

"Properties of number 2 diesel fuel reported by Sims (1985).

134 Yusuf Ali, M. A. Hanna

i

7

Fig. 3.

4

2

0

Cetane index as a function of number 2 diesel fuel to tallow ester ratio (%).

R E F E R E N C E S

AACC (1983). Method 58-15. In Approved Methods of the American Association of Cereal Chemists, 8th edn. AACC, St Paul, MI, USA.

ASTM (1986). ASTM D613: Ignition quality of diesel fuels by the cetane method. In Annual Book of ASTM Stand- ards, Vol. 05.04, Test methods for rating motors, diesel, and aviation fuels. ASTM, Philadelphia, PA.

Bagby, M. O. (1987). Vegetable oils for diesel fuel: opportu- nities for development. ASAE paper No. 87-1588. ASAE, St Joseph, MI.

Freedman, B., Pryde, E. H. & Mounts, T. L. (1984). Variables affecting the yields of fatty acid esters from transesterified vegetable oils. JAOCS, 61 (10), 1638-43.

Quick, G. R. (1980). Developments in use of vegetable oil as fuel for diesel engines. ASAE paper No. 80-1525. ASAE, St Joseph, MI.

Ryan, T. W., Dodge, L. G. & Callahan, T. J. (1984). The effects of vegetable oil properties on injection and com- bustion in two different diesel engines. JAOCS, 61 (10), 1610-19.

Sims, R. E. H. (1985). Tallow esters as an alternative diesel fuel. Trans. oftheASAE, 28 (3), 716-21.

Swern, D. (ed.) (1979). Bailey's Industrial Oil and Fat Pro- ducts, Vol. 1,4th edn. John Wiley & Sons, New York.