Development of tartaric esters as bifunctional additives of methanol-gasoline
© Zhang et al.; licensee Chemistry Central Ltd. 2014
Received: 26 July 2013
Accepted: 21 March 2014
Published: 15 April 2014
Methanol has become an alternative fuel for gasoline, which is facing a rapidly rising world demand with a limited oil supply. Methanol-gasoline has been used in China, but phase stability and vapor lock still need to be resolved in methanol-gasoline applications. In this paper, a series of tartaric esters were synthesized and used as phase stabilizers and saturation vapor pressure depressors for methanol-gasoline.
The results showed that the phase stabilities of tartaric esters for methanol-gasoline depend on the length of the alkoxy group. Several tartaric esters were found to be effective in various gasoline-methanol blends, and the tartaric esters display high capacity to depress the saturation vapor pressure of methanol-gasoline.
According to the results, it can be concluded that the tartaric esters have great potential to be bifunctional gasoline-methanol additives.
Rapidly rising world demand, together with the limited oil supply, means developing clean and alternative fuels increasingly draws worldwide attention . Among the alternative fuels, methanol displays fine combustion properties similar to gasoline and has such advantages as high octane number, low emissions, antiknock, rich resource, and mature technology, so it can be used as an alternative fuel for gasoline . In recent years, extensive research of the low percentage methanol-gasoline has been carried out, and it has been applied in Shanxi, Sichuan, Zhejiang, Inner Mongolia, Shaanxi, Xinjiang and other provinces of China gradually . However, there are several problems needing to resolve in methanol-gasoline application, in which the phase stability is the first and most important one. One of the popular solutions is to add phase stabilizer to reduce alcohol-oil interfacial tension [4, 5], such as ethers, ketones, esters, fatty alcohols, aliphatic hydrocarbons, fatty acids, non-ionic surfactants, acetal/ketones, biodiesels and amidines [6–8]. Secondly, the low boil point of methanol leads to high possibility of vapor lock by raising the vapor pressure of methanol-gasoline [9–11]. The current solution for vapor lock is to add pressure depressor, such as aliphatic ketones, lynn classes, fatty aldehydes, fatty ethers, acetals/ketals, etc. At present, few researches have researched bifunctional additives with the functions of improving the phase stability as well as depressing the effective vapor pressure of the methanol-gasoline mixtures. In this work, a series of tartaric esters was synthesized and screened in the methanol-gasoline as a bifunctional additive for the phase stability and vapor depressor.
Tartaric esters were synthesized by two methods with high yield. The purified tartaric esters were evaluated as phase stabilizer and saturation vapor pressure depressor of methanol-gasoline. The results show that the efficiency depends on the length of the tartaric esters’ alkoxy group. With the dosage of 0.1%, all tartaric esters can depress the saturation vapor pressure lower than that of gasoline, and decyl tartaric is the most effective one.
Materials and methods
All solvents were AR grade and purchased from Xi’an Chemical Agent Co, and the 93# gasoline is commercially available. The phase stabilizing and pressure reducing tests were carried out on DFY-cryostat instrument (Xi'an Yuhui Instrument Co.Ltd.) and DSL-080 vapor pressure detector (Dalian the Ceon Electronic Equipment Co.Ltd.).
Synthesis of tartaric esters
The short-chain alcohols can co-dissolved with water, so the produced water can not be separated from the reactant, while long-chain alcohol can carry the produced water out under reflux. So different methods were used for the synthesis of tartaric esters.
Method A : Tartaric acid (23 g), methanol (18 mL), and p-toluenesulfonic acid (TsOH) (0.6 g) were added in a 250 ml flask. Refluxing for 10 h, the mixture was cooled to room temperature. Methanol and methyl tartaric were distillated respectively. The synthesis of ethyl tartaric and propyl tartaric is similar to method above.
Method B [13, 14]: Tartaric acid (0.15 mol), n-butanol (0.45 mol), cyclohexane (30 mL), p-toluenesulfonic acid (0.5 g) were added in a 250 ml flask equipped with a water separator. Refluxing for 5 h, the mixture was cooled to room temperature, cyclohexane, n-butanol and n-butyl tartaric were separated by vacuum distillation. The synthesis of amyl tartaric, hexyl tartaric, hepyl tartaric, octyl tartaric, decyl tartaric is similar to method above.
Phase stability test
The fuel blends were prepared by blending 15, 30, 50 and 65 vol. % of methanol with base gasoline, and the gasoline blends were assigned as M15, M30, M50 and M65. The phase stabilizing tests were carried out according to Chinese National standards of GB 8017–87, GB/T 23799–2009, DB61/T 352–2004 and DB51/T 448–2004. First the test tube full of methanol-gasoline with different ratios was placed in a cryostat, and then the temperature was adjusted from 40°C to −25°C. At each degree, the tube was taken out and was shaken for two to three seconds, and the phase separation temperature was determined as the solution becomes cloudy [15, 16]. The tests were repeated until the separation temperature does not change for 3 times.
Vapor pressure test
Effect of tartaric esters on vapor pressure of methanol-gasoline was investigated according to Chinese standards of GB 8017–87. The methanol-gasoline was poured into the vapor pressure detector and put into the water bath of 37.8°C. The methanol-gasoline was intensive mixed by taking the detector from the water bath every 5 min and reversing violently. The operation was repeated until the pressure does not change for 3 times.
Results and discussion
Synthesis of tartaric esters
The results of the synthesis of tartaric esters
Tartaric acid : alcohol
Effect of tartaric ester on the phase stability of methanol-gasoline
The effect of tartaric ester on the evaporation of methanol-gasoline
Tartaric esters were synthesized and screened for their performances of phase stabilizing in M15, M30, M50 and M65 and pressure reducing in M15. The results show that the length of alkoxy group of tartaric esters effects on the phase stability of methanol-gasoline significantly. The phase stability of tartaric esters for the methanol-gasoline system with long length is more potent than that with short length. All of the synthesized esters are potent to depress the saturation vapor pressure of methanol-gasoline. With the dosage of 0.1%, all tartaric esters can depress the saturation vapor pressure lower than that of gasoline, and decyl tartaric is the most effective one.
Financial support from National Science Foundation of China (21306149), Scientific Research Program Funded by Shaanxi Provincial Education Department (2013JK0646).
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