Dispersion Performance of Methanol-Diesel Emulsified Fuel Prepared by High Gravity Technology

Jiao Weizhou; Li Jing; Liu Youzhi; Zhang Qiaoling; Liu Wenli; Xu Chengcheng; Guo Liang

(Shanxi Province Key Laboratory of Higee-Oriented Chemical Engineering, North University of China, Taiyuan 030051)

Dispersion Performance of Methanol-Diesel Emulsified Fuel Prepared by High Gravity Technology

Jiao Weizhou; Li Jing; Liu Youzhi; Zhang Qiaoling; Liu Wenli; Xu Chengcheng; Guo Liang

(Shanxi Province Key Laboratory of Higee-Oriented Chemical Engineering, North University of China, Taiyuan 030051)

A new continuous process for preparing methanol-diesel emulsified fuel with an Impinging Stream-Rotating Packed Bed is proposed. The droplet size of dispersed phase (methanol) of the emulsified fuel has a significant effect on the combustion of methanol-diesel emulsified fuel. In this paper, the methanol-diesel emulsified fuel uses diesel as the continuous phase and methanol as the dispersed phase. The Sauter mean diameter of the dispersed phase of methanol-diesel emulsified fuel was characterized with microphotography and arithmetic method. The experimental result showed that the Sauter mean diameter of the dispersed phase, which was decreased with the augmentation of the high gravity factor, liquid flow rate and emulsifier dosage, was inversely proportional to the methanol content. The Sauter mean diameter of the dispersed phase can be controlled and adjusted in the range of 12—40 μm through the change of operating conditions. The correlative expressions of the Sauter mean diameter of emulsified fuel were obtained and the calculated values agreed well with the experimental values.

emulsion; methanol-diesel blend; impinging stream-rotating packed bed; dispersion; Sauter mean diameter

1 Introduction

At present, diesel engines are still the most fuel-efficient internal combustion engines, and diesel fuel is the dominant fuel used by the commercial transportation, industrial and agricultural sectors[1-3]. However, over the last 20 years, there have been major social problems relating to the air pollution and greenhouse gas effects, which have been caused by the engine emissions and other contributors. The major pollutants emitted from diesel engines are particulate matter (PM), oxides of nitrogen (NOx), SO2, CO2, etc. These pollutants cause damage to the ozone layer, enhance the greenhouse gas effect, and produce acid rain. Therefore, improving the performance of engines and reducing their emissions could be helpful to the improvement of fuel economy and reduction of environmental pollution. As a result, reducing engine emissions is a major research aspect in engine development because of the increasing concerns on environmental protection and the stringent exhaust gas regulations.

The emulsification technique is one of the effective approaches to improving the fuel economy and reducing the pollutants emission from diesel engines. Emulsion is mainly composed of two different liquids, i.e. oil and water, one of which is distributed in the other in the form of liquid droplets. Because the boiling point of water is lower than that of oil, drops of water reach their boiling point first and swell rapidly through enveloping oil layers. The vaporization of water phase blows up the oil layer to form much smaller oil droplets, which can result in significant increase of the specific surface area of oil droplets. The increase in mixing extent and the increased contact surface between air and smaller fuel droplets would lead to a considerable improvement in burning rate and fuel efficiency. The water phase substances include methanol, ethanol, ethers, and butanol[4–7]. In China, coal reserves are abundant and coal-to-methanol technology is practicable because of low price of coal. Methanol is regarded as one of the promising alternative fuels or oxygen additives for diesel engines due to its advantages of low price and high oxygen content. As an oxygenated fuel, methanol has some attractive features, and some properties of methanol are listed in Table 1.

Table 1 Properties of Methanol and Diese

The performance of methanol-diesel emulsified fuel depends on two main factors, i.e., the emulsification equipment and surfactants. A lot of emulsification devices, such as the mechanical stirrer, the homogenizer and the colloid mill, are suitable for the preparation of emulsified fuel[8–11]. However, such pieces of equipment have several disadvantages, such as large volume, high energy consumption, and batch operation, which could hamper the development of the methanol–diesel emulsified fuel technology. The ultrasonic assisted emulsification[12-14]could be used as a effective method to prepare emulsion which is available only in the laboratory. To satisfy the requirements of the emulsification process, highly efficient continuous emulsification equipment must be developed. The rotating packed bed (RPB) with a high rotating speed is used to generate high centrifugal force for enhancing gas-liquid mass transfer rate[15-21]. A new Impinging Stream-Rotating Packed Bed (IS-RPB) was presented by Liu Youzhi, et al. for improving liquid/liquid masstransfer and mixing. The high gravity technology is disseminated from the gas/liquid to liquid/liquid fields featuring higher dispersion and turbulence, which has been applied in micromixing, extraction, liquid membrane separation, chemical reaction, and other domains[22-26].

In recent years, many studies[27-28]have referred to the optimized formulation of emulsifiers as well as the proper emulsification process. The effects of the optimized conditions on methanol–diesel emulsified fuel have been determined. However, only a few reports are available on the dispersion performance of methanol-diesel emulsified fuel. The Sauter mean diameter of the dispersed phase (methanol) of methanol-diesel emulsified fuel affects not only the stabilization and viscosity of emulsion, but also the combustion process in the diesel engine. Therefore, it is important to investigate the dispersion performance of methanol-diesel emulsified fuel. The more uniform and smaller the size of dispersed phase (methanol) droplets in emulsified fuel is, the better the micro-explosion and spray in combustion would be. In this paper, a novel route for preparing the continuous methanol–diesel emulsified fuel by using the IS-RPB equipment under high-gravity fields is proposed, and the effect of the emulsifier dosage, methanol content, high gravity factor, and liquid flow rate of diesel fuel on the Sauter mean diameters is discussed.

2 Experimental

2.1 Materials and equipment

Commercial diesel was purchased at gas stations in the city of Taiyuan, China. The industrial-grade anhydrous methanol with a purity of 99.8%, was obtained from the Tianjin Chemical Company in China. The surfactants are composed of the lipophilic and hydrophilic emulsifiers, which were non-ionic surfactants and made up of C, H, and O elements.

The methanol-diesel emulsified fuel was prepared by a novel continuous emulsification mixer—IS-RPB equipment. An optical electron microscope (BK-DM200/320, made by the Chongqing Optec Instrument Co., Ltd.) was used to observe the distribution and droplet-size of the dispersed phase of the methanol.

2.2 Principles of IS-RPB

The principle of IS-RPB is to create a high gravity environment via the action of centrifugal force, where the two streams flowing along the same axis in the opposite direction impinge forward on each other at a high speed, and its impinging spray surface enters the inner brim of RPB along the radial direction. As the result of such a collision, the worse impinging spray surface is further mixed by the aid of RPB. The predominant mixing characteristic of ISRPB device is that the end-effect in the inner radius of RPB offsets the fringe effect of IS; thus, the fluid is mixed for the second time. Therefore, the coupling between the inner diameter size of RPB and the impinging stream surface is important to avoid the brim effect of IS and to improve the mixing effect. In present work, the type of packing is made of stainless steel wire mesh with a diam-eter of 0.3 mm and a porosity of 0.96. The axial height of the bed is 3 cm. The inner and outer diameter of the rotor packing bed is 6 cm and 10 cm, respectively. The high gravity factor varies from 26.5 to 373.2. The schematic diagram of IS-RPB unit is presented in Figure. 1.

Figure 1 Schematic of the Rotating Packed Bed unit

2.3 Experimental details

Methanol-diesel emulsified fuel was prepared using anhydrous methanol and diesel with the aid of surfactants. The experimental setup is shown in Fig. 2. During the experiments, solutions 1 and 2 were prepared firstly. The anhydrous methanol and hydrophilic surfactant were mixed to form the solution 1. The solution 2 consisted of diesel fuel and lipophilic surfactant. Both solutions were pumped into the IS-RPB and mixed with each other. The flow rate of solution 2 varied from 30 L/h to 90 L/h. The temperature of solutions was controlled in a range of 20—23 ℃. The HLB (hydrophobic–lipophilic balance) value of the compound surfactants is maintained at 5.4 by adjusting the ratio of hydrophilic to lipophilic surfactants. The content of the compound surfactants added to the methanol-diesel mixture varied from 1 m% to 5 m%. The volume fraction of the methanol was 5%, 10%, 15%, 20% and 25%, respectively, whereas that of the diesel was 95%, 90%, 85%, 80% and 75%, respectively.

The droplet size of emulsified fuel in dispersed phase was characterized by the Sauter mean diametersd32determined by the optical electron microscope. The Sauter mean diametersd32is defined (see Eq. (1)[29]) as the ratio of the volume of the dispersed phase to its total surface area. The smaller the Sauter mean diameterd32, the better the dispersion of the methanol.

Figure 2 Schematic diagram of the experimental setup

3 Results and Discussion

3.1 Influence of high gravity factorβ

Effect of high gravity factor on the Sauter mean diameterd32of methanol-diesel emulsified fuel is presented in Figure 3 under the operating conditions comprising a liquid flow rate of 70 L/h, a methanol content of 10% and an emulsifier dosage of 3%. It can be seen from Figure 3 that the Sauter mean diameterd32decreased with an increasing high gravity factorβ, and decreased sharply to 16μm from 35 μm, when the high gravity factorβincreased from 26.5 to 208.1. This phenomenon was ascribed to the fact that the relative velocity among all kinds of liquid elements (e.g. droplet, thread, and film) and packing were affected more greatly by the rotational speed. Consequently, the frequency of the vigorous impingement and coalescencedispersion of the liquid elements will quicken, which would lead to smaller particle size and better dispersion quality. When the high gravity factor exceeded 208.1, the Sauter mean diameter of the dispersed phase tended to be flat, since it was the outcome of competition between two scenarios, viz., firstly, a favorable case of enhanced liquid/liquid contact and mixing resulted from the increasing high gravity factor and enhanced frequency for renewing liquid/liquid surface, and secondly, an unfavorable case of liquid/liquid contact and mixing arising from the reduced residence time. Hence an appropriate high gravity factorβwas specified at 208.1 under laboratory conditions, while the mean diameterd32of the dispersed phase was 16 μm.

Figure 3 Effect of high gravity factor on the dispersion performance

Figure 4 Dispersed phase droplet size in emulsified fuel determined by video-microscopy in the high gravity equipment

3.2 Influence of liquid flow rateVA

Figure 5 presents the dispersity characteristics as a function of liquid flow rate of dieselVAin a range of from 30 L/h to 90 L/h at a fixed high gravity factor of 208.1. The liquid flow rate of diesel has a significant influence on the Sauter mean diameter of emulsified fuel. The Sauter mean diameter decreased sharply and then gradually reached a plateau with the further increase of the liquid flow rate. This phenomenon may be ascribed to the following two reasons. One reason is that a higher liquid flow rate causes a higher injection velocity, and furthermore the larger relative velocity between liquids and the packing produces a more homogeneous mixing. Consequently, the effective mixing efficiency is enhanced as a result of increased liquid flow rate. On the other hand, when liquid flow rate of diesel exceeded 70 L/h, the coalescence-redispersion frequencies were in a dynamic equilibrium of the methanol and diesel in the high gravity field. As a result, the Sauter mean diameter of dispersed phase in the emulsified fuel was not a variable.

Under the operating condition involving a methanol content of 10%, an emulsifier dosage of 3%, a HLB value of 5.4 and a high gravity factor of 208.1, the Sauter mean diameters d32of the dispersed phase are 19 μm and 23 μm (see Figure 6 and 7) when the liquid flow rates are 90 L/h and 50 L/h, respectively.

Figure 5 Effect of liquid flow rate on the dispersion performance

Figure 6 Dispersed phase droplet size of emulsified fuel determined by video-microscopy in the high gravity equipment

Figure 7 Dispersed phase droplet size of emulsified fuel determined by video-microscopy in the high gravity equipment

3.3 Influence of methanol contentC

Figure 8 shows the relationship between the dispersed phase size of emulsified fuel and methanol content at aconstant high gravity factor of 208.1, an emulsifier dosage of 3%, and a HLB value 5.4. The results indicate that the Sauter mean diameter of dispersed phase increased with an increasing methanol content. This phenomenon is attributed to the fact that the methanol content increases in diesel. As shown in Figs. 9 and 10, the Sauter mean diametersd32of the dispersed phase are 12 μm and 22 μm, when the methanol content is 5% and 15%, respectively, with the system operating at a high gravity factor of 208.1, an emulsifier dosage of 3%, a HLB value of 5.4 and a liquid flow rate of 70 L/h .

Figure 8 Effect of methanol content on the dispersion performance

Figure 9 Dispersed phase droplet size of emulsified fuel determined by video-microscopy in the high gravity equipment

Figure 10 Dispersed phase droplet size of emulsified fuel determined by video-microscopy in the high gravity equipment

3.4 Influence of emulsifier dosageθ

Figure 11 displays the effect of emulsifier dosageθranging from 1% to 5% on the Sauter mean diametersd32of emulsified fuel. It indicates that the Sauter mean diameterd32decreases with the increase in the emulsifier dosageθ. The surfactants are composed of the lipophilic and the hydrophilic emulsifiers, which form a thin film interface between the diesel fuel and methanol. The compound surfactants could reduce surface tension between the diesel and methanol, activate the surfaces, and maximize their superficial contact areas to produce methanol-in-diesel emulsions. This is because the hydrophilic group in the surfactant could absorb the methanol molecule and the lipophilic group could absorb the diesel. The emulsifier as a continuous film phase could bind up the methanol species when the emulsifier dosage was further increased, which could lead to a reduction of dispersed phase size. The Sauter mean diameterd32of the dispersed phase is 33 μm (see Figure 12) under the operating conditions involving a liquid flow rate of 70 L/h, a methanol content of 15% , an emulsifier dosage of 1%, a HLB value 5.4 and a high gravity factor of 208.1.

Figure 11 Effect of emulsifier dosage on the dispersion performance

Figure 12 Dispersed phase droplet size of emulsified fuel determined by video-microscopy in the high gravity equipment

3.5 Comparison with other kinds of mixing equipment

Under the similar experimental conditions, the Sauter mean diametersd32of emulsified fuel prepared by high gravity equipment (IS-RPB) were compared with those prepared by the high speed stirrer. Figure13 illustrates the effects of the methanol content on dispersion performance obtained with different emulsification equipment. The results indicate that the methanol content has an obvious effect on the Sauter mean diameterd32of the dispersed phase in the emulsified fuel. Meanwhile, as shown in Figure 13, the Sauter mean diameterd32of emulsified fuel increases obviously with an increasing methanol content. At the same time, the Sauter mean diameterd32of emulsified fuel prepared by the high gravity equipment is smaller than that prepared by the high speed stirrer. This phenomenon may be ascribed to the different emulsification equipment structure. In the continuous ISRPB equipment, the force exerted on every fluid particle of methanol and diesel is homogeneous in high gravity fields. The methanol and diesel are emulsified effectively with the aid of the huge force produced by the speed in rotating packed bed. Consequently, the frequency of the vigorous impingement and coalescence-dispersion of the liquid elements will make the Sauter mean diameter d32of the dispersed phase smaller and more uniform (see Figure 16). In the intermittently operating high speed stirrer, the force exerted on methanol and diesel droplets is dissimilar to their different locations, which make the Sauter mean diameterd32of the dispersed phase irregular. This point has been verified by the difference in Sauter mean diameterd32of the emulsified fuel prepared respectively by the high gravity equipment and the high speed stirrer under the same operating conditions (Figure 14—16). The Sauter mean diameterd32of the dispersed phase is 14 μm (see Figure 14) in the high gravity equipment under the operating conditions involving a liquid flow rate of 70 L/h, a methanol content of 15%, an emulsifier dosage of 4%, a HLB value 5.4, and a high gravity factor of 208.1. As shown in Figure15, the Sauter mean diameterd32of the dispersed phase is 26 μm when the emulsification time is 2 min at a rotor speed of 5×2 800 r/min, a methanol content of 15% and an emulsifier dosage of 4%.

Figure 13 Effect of different kinds of emulsification equipment on the dispersion performance

Figure 14 Dispersed phase droplet size of emulsified fuel determined by video-microscopy of the high gravity equipment

Figure 15 Dispersed phase droplet size of emulsified fuel determined by video-microscopy of the high speed stirrer equipment

Figure 16 Effect of different kinds of emulsification equipment on the dispersion performance

3.6 Correlative expression of the Sauter mean diameterd32for the dispersed phase

The correlative expression of the Sauter mean diameter

d32of the dispersed phase is assumed to be comprised of the following parameters (see Eq. (2) ):

where A, a, b, c and d are the coefficients to be determined. The correlative expression of the Sauter mean diameterd32of the dispersed phase is obtained with the MATLAB program (see Eq. (3)). It can be seen from Figure17 that a comparison of experimental data with calculated values indicates to a relative error of within ±10%. This expression can be used at a high gravity factorβranging from 26.50 to 373.2, a liquid flow rateVAof diesel fuel equating to 30—90 L/h, an emulsifier dosageθof 1%—5%, and a methanol content of 5%—25%. The correlative expressions of the Sauter mean diameterd32was obtained and the calculated values agreed with the experimental values well, and its mean, minimum and maximal deviations were 4.2%, 0.2% and 8.9%, respectively.

Figure 17 Particle volume fraction and the distribution of the dispersed phase obtained in different kinds of emulsification equipment

4 Conclusions

(1) A novel process for preparing methanol-diesel emulsified fuel by high gravity impinging stream-rotating packed bed technology is proposed, which provides a continuous and efficient pathway to achieve the target.

(2) The Sauter mean diameter of the dispersed phase of methanol-diesel emulsified fuel decreased with the augmentation of the high gravity factor, emulsifier dosage and flow rate of the diesel, and decrease in the methanol content.

(3) The Sauter mean diameterd32of the dispersed phase can be controlled and adjusted in the range of 12—40 μm by changing the operating conditions. Under the operating conditions involving a high gravity factor of 208.1, a liquid diesel flow rate of 70 L/h, an emulsifier dosage of 3% and a methanol content of 15%, the Sauter mean diameter is 22 μm.

(4) The correlative expressions of the Sauter mean diameter of emulsified fuel were obtained and the calculated values agreed well with the experimental values, with its mean deviation being less than 10%.

Acknowledgement:This work was financially supported by the Natural Science Foundation of China (No.21206153, 21376229) and the Science and Technology Foundation of Province Shanxi of China (No.2010021007-2, 2012011008-2).

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Recieved date: 2013-10-09; Accepted date: 2014-02-07.

Dr. Jiao Weizhou, Telephone: +86-351-3921986; E-mail: jwz0306@126.com.