Бурение грунтовых зондов, установка энергетических колодцев

Ethanol dehydration is accomplished with synthetic zeolite molecu­lar sieves, which are aluminosilicates. These adsorbents have open structures through which small molecules can diffuse, and small enough molecules pass through the pores and are adsorbed or entrapped, while larger molecules pass through without adsorbtion. Zeolite molecular sieves with a pore diameter of 3A are commonly used in the dehydration of ethanol since they can entrap water mol­ecules which have a diameter of 2.5 A. Ethanol molecules with a diameter of 4 A cannot enter the pores and therefore flow around the material. Molecular sieves can absorb water up to 22% of its own weight. The zeolite bed can be regenerated essentially an unlimited number of times by drying it with a blast of hot carbon dioxide.

Al-Asheh et al. have studied the breakthrough time and average outlet water content for the adsorption of water vapor on 3, 4, 5 A

types of molecular sieves at different inlet water contents of 5-12 wt% of an ethanol-water system, and their results are summarized in Table 15.2. These results clearly show that 3 A molecular sieve has the highest absorption capacity for water [3].

A number of researchers have studied the adsorption kinetics and pressure swing adsorption methods for dehydration of rectify­ing column ethanol to fuel grade ethanol [5, 6, 4, 7-9].

Generally, two beds of adsorbent are used to make the process continuous, and the dehydration process can be explained as fol­lows. Consider the first column packed with freshly activated molecular sieve. As ethanol-water vapors first enter the bed, water is diffused and adsorbed within the pores of the adsorbent struc­ture in a thin layer. As more ethanol enters the column, it passes through this layer to a slightly lower level where another incre­mental amount of water is absorbed. This continues until a point is reached where all possible water adsorption from ethanol solution is accomplished. Transfer of water from the vapor of ethanol-water solution to the molecular sieve occurs through a zone where water (adsorbate) content is reduced from its inlet to its outlet concentra­tion. This finite length of bed where the adsorbate transfer occurs is known as the mass transfer zone. While the active bed is under pressure carrying dehydration, the regeneration bed is under vac­uum. The shift of operation (swing) from one bed to another can be controlled with the help of control valves and automation.

The bed temperature is critical in regeneration. Bed temperatures in the 175-260°C range are usually employed for type 3 A molecu­lar sieves. This lower range minimizes polymerization of olefins on the molecular sieve surfaces when such materials are present in the gas. Slow heat up is recommended since most olefinic materials will be removed at minimum temperatures; 4 A and 5 A molecular sieves require temperatures in the 200-315°C range. After regen­eration, a cooling period is necessary to reduce the molecular sieve temperature to within 15-20°C of the temperature of the stream to be processed. This is most conveniently done by using the same gas stream as for heating, but with no heat input. For optimum regen­eration, gas flow should be countercurrent to adsorption during the heat-up cycle and concurrent during the cooling.

Jeong and coworkers have studied the production of anhydrous ethanol using various pressure swing adsorption (PSA) processes in a pilot plant [4]. In this research, anhydrous ethanol was pro­duced through different processes such as two-bed, multi-tube bed, two-step, and three-bed for analysis and comparison of each pro­cess. A representative sample of their results from two-bed type and multi-tube bed type processes are shown in Table 15.3. Through this study, two-bed process and multi-tube bed process were both shown to produce 99.5 wt% anhydrous ethanol from 87.0 wt% eth­anol. However, the multi-tube bed process showed lower energy consumption. The two-step bed process has the advantage of being able to produce anhydrous ethanol from input ethanol concentra­tion as low as 83.1 wt%. Lastly, the three-bed process allowed for longer regeneration time, making the process very stable and with higher yield due to less lost time in cycle switching [4].

In a recent study Yamamoto and coworkers compared adsorp­tion characteristics of five zeolites for dehydration of ethanol by evaluating diffusivity of water in porous structure [10]. For this study they used five species of commercial zeolites from Tosoh

Corp (LTA-Na, LTA-K, LTACa, FAU-Na and MOR-Na) with differ­ent frameworks, and different exchanged cation species. In their experiments equilibrium adsorption was measured using zeolite powder with a grain size of 75-100 pm after out gassing at 573 K. On the other hand, a packed bed breakthrough curve (BTC) was obtained using particles (containing 20 wt% of binder prepared from natural clay) with a grain size of 150-250 pm packed in a bed. Adsorption isotherm of water vapor on zeolites, differential heat of the adsorption of water vapor, the liquid-phase adsorption isotherm of water in ethanol and packed bed breakthrough curves (BTC) for the adsorption of water in ethanol were studied in this comprehensive investigation. As a result, they confirmed that an LTA or FAU zeolite exchanged with a monovalent cation species, such as a sodium cation or a potassium cation, showed a strong affinity to water in ethanol. They also found that the Langmuir model explained the liquid-phase adsorption of water in etha­nol on the zeolite more accurately than the Freundlich model. Using the constants determined from the Langmuir isotherm,

c Adsorbed amount of water determined from a packed bed BTC; in estimation of qH2O, BTC, the weight of the binder is excluded (not included in the net weight of the zeolite).

they calculated the BTC for a zeolite packed bed as regards to the dehydration of ethanol. The intra-particle diffusion coefficient of water in the zeolite particles was also estimated by fitting the cal­culated BTC to the experimental result. Dehydration performance of the zeolites examined by Yamamoto and coworkers are shown in Table 15.4.

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