Simulation of adsorption and separation of ethanol–water mixture with zeolite and carbon nanotube
Introduction
Ethanol is now viewed as a potential replacement of fossil fuel such as gasoline and diesel oil, which needs millions of years to reproduce. Bioethanol is the ethanol produced from agricultural waste through fermentation. It is a renewable source of energy and is added into the gasoline to be used in automobile engines. But the moisture in gasoline-containing ethanol must be very slight. It is difficult to obtain ethanol in high purity by normal separation process because of the azeotropic point of ethanol–water system, and the normal dehydration process is energy consuming. Thus ethanol dehydration under high concentration by non-distillation process is urgent. The separation of the aqueous ethanol mixture using adsorption technique has been intensively investigated [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Many researchers have a desire to find an excellent adsorbent and cut the cost of energy.
The material used for adsorption and separation of ethanol–water mixture is various, ranging from organic starch [1], [3], [7], [10] to inorganic zeolite [2], [5], [6], [8]. They all have pores on the nanoscale. The nanoscale pores are one of the main determinations for the separation performance of the material. The performance of nanopore would differ in size, architecture and chemical components. A well understanding of the performance of nanoscale pore would be benefit to a rational design of better separation material. But it is still very challenging to perform experiments to investigate the adsorption within nanopore in detail.
Molecule simulations are well suited to study the molecules in the nanopore. With careful design and analysis, the relationship between the performance of the pore and its structural and chemical properties can be well elucidated. Recently available atomic resolution structures permit one to study nanopore through molecular simulations, which can be used to determine the underlying mechanisms of selectivity and explain in detail the relationship between pore architecture and function. For instance, Striolo et al. [12], [13], [14] studied the adsorption of water in the carbon nanotube (CNT) with different size. They found that the adsorption/desorption behavior of the water are strongly based on the tube size. Shevade et al. [15] showed that the adsorption of methanol–water in the active carbon can be changed extraordinary if there are several hydrophilic sites. Our previous studies [16], [17], [18] showed that the change of carbon nanotube size and hydrophilic modification of the pore in the pore mouth has substantial influence on the behavior of water in the tube, and this influence varied with tube size. We have also studied the separation of alkane in zeolite. The results show that there is a critical pore size, which is 10-membered-ring for alkanes. If the channel sizes of zeolites are larger than this critical pore size, they will prefer i-butane to n-butane; otherwise prefer n-butane to i-butane [19]. Several groups have studied the separation of ethanol–water mixture in the polyatomic membrane and zeolites with molecular simulations and get more insight into the separation mechanism of these membranes [20], [21], [22], [23].
Here we use the carbon nanotube and zeolite to adsorb ethanol–water mixtures. We choose zeolites because of their wide application in industry. Previous studies show that the carbon nanotube has very special property [24]. The behavior of molecules in CNTs has been paid increasing attention recently. The properties of water inside CNTs have been studied extensively, such as the adsorption [12], [14], [25], [26], phase transition [27], diffusion [28], [29] and structure of hydrogen bond [16], [30], [31], [32]. The effect of the diameter of CNTs on the structure properties of water molecules inside was also investigated [30]. However, there are few studies about ethanol in CNT. The information about confined ethanol molecules is still far from the demand. People know little about the structure properties of confined ethanol molecules, so the issue of separation of ethanol–water mixture with CNT is very interesting. Furthermore, zeolite and CNT has different hydrophilic/hydrophobic property, they should have different separation performance to ethanol–water mixture. In this work, the ethanol–water mixture investigated ranges from the low mole fraction to the high mole fraction of water. Different zeolites and CNTs with different pore size are studied to investigate the performance of the pore on its size.
Section snippets
Potential models
Different potential models are available for water [33], [34], [35], [36]. The TIP3P, TIP4P and SPC, SPC/E models for water are used widely and are commonly applied in literatures for its adsorption and diffusion properties. However, no universal model has been developed for this molecule. The SPC/E model has been used extensively in the literature for a wide variety of different systems since it was first introduced. Ramachandran et al. [33] investigated the adsorption of water from the vapor
Ethanol–water in zeolite
From Fig. 1 we can see the adsorption amount of water is larger than that of ethanol in these four zeolites. With the mole fraction of water increasing, the adsorption amount of water increase and the adsorption amount of ethanol decrease slightly or is changeless (see Fig. 1c). From the snapshots of ethanol–water mixture molecules in zeolites (Fig. 2) we do not find the orientation adsorption of either component, namely each adsorbed molecule located randomly, it does not prefer any special
Conclusions
The zeolites prefer water to ethanol. The adsorption and separation performance of zeolite is determined by pore size and framework of zeolite. It could be used for ethanol dehydration.
The pore size of CNT55 is so small that only water can enter CNT55, which shows “sieving-effect”. If the desorption is not difficult to carry it would be the best dehydration material. Because CNTs are more hydrophobic than zeolite, the CNT66, CNT77, CNT88 and CNT99 are all prefer ethanol to water at low mole
Acknowledgements
The present work was supported by the Joint Research Fund for Young Scholars in Hong Kong and Abroad (no. 20428606), the National Natural Science Foundation of China (grant nos. 20246002 and 20236010), National High Technology Research and Development Program of China (no. 2003CB615700), the Key Science Foundation of Jiangsu Province, China (BK 2004215). The authors also acknowledge computer time provided by the College of Computer Engineering and Science, Shanghai University.
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