Ethanol, dehydration

For our second nonideal system, we look at a process that has extremely nonideal VLB behavior and has a complex flowsheet. The components involved are ethanol, water, and benzene. Ethanol and water at atmospheric pressure form a minimum-boiling homogeneous azeotrope at 351K of composition 90mol% ethanol. Much more complexity is introduced by the benzene/water system, which forms two liquid phases with partial miscibility. The flowsheet contains two distillation columns and a decanter. There are two recycle streams, which create very difficult convergence problems as we will see. So this complex example is a challenging simulation case. 

The origins of the example go back over a century when a process to produce high-purity ethanol was needed. Ethanol is widely produced in fermentation processes. A typical mixture from a fermentation process has very low ethanol concentrations (4-6 mol%). If this mixture is fed to a distillation column operating at atmospheric pressure, high-purity water can be produced out at the bottom, but the ethanol purity of the distillate caimot exceed 90 mol% because of the azeotrope. 

The first item to explore is the complex vapor-liquid-liquid equilibrium (VLLE) of this heterogeneous vapor-liquid-liquid system. 

At present, roughly 80% of the current global energy needs comes from fossil fuels. Besides, oil is used as a raw material for the production of several chemical products. Ethanol (C2H5OH), a natural product obtained from biomass, is, on the one hand, a renewable source of energy that would be an important factor for near-zero carbon dioxide (C02) emissions, on the other hand, it is the basis for a C2 chemistry, that is, a raw material for the production of different chemical products [19,21,137-147], Besides, ethanol is accessible, can be easily transported, biodegradable, has low toxicity, and can be transformed by catalytic reactions [137], 

Ethanol Dehydration in a Flow Reactor at Atmospheric Pressure 

Framework Type Channel System Pore Pore Diameter [A] 

The reported catalysts were prepared as follows the natural zeolite rock HC (see Table 4.1 in Chapter 4), the Na-ZSM-5 zeolite (Si/Al = 25) (provided by Costa-Novella and Nefiodov), and the Na-MOR (Si/Al = 5.5), K-LTL (Si/Al = 3.0), and the Na-Y (Si/Al = 2.1) zeolites provided by Union Carbide were exchanged with ammonium and then deammoniated by repeated exchange with a 3 M ammonium chloride solution at 373 K for 2h followed by calcinations at 673 K for 4h (see Section 9.3.1) to obtain the H-HEU, H-MFI, H-MOR, H-LTL, and H-FAU samples [21], 

From Table 9.4, it is evident that the natural and MFI zeolites have lower yields than the zeolites with larger pores (see Table 9.5) besides, the MFI zeolite has a higher selectivity to ether and the natural zeolite to ethylene [21], It is evident that the zeolite with higher selectivity to ethylene at a low temperature is the natural zeolite, and that at the higher temperature, the selectivity is toward the olefin for all zeolites [21], 

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Although most of the installed solvent dehydration systems have been for ethanol dehydration, dehydration of other solvents including 2-propanol, ethylene glycol, acetone, and methylene chloride, has been considered. 

CatalyticaHy Active Species. The most common catalyticaHy active materials are metals, metal oxides, and metal sulfides. OccasionaHy, these are used in pure form examples are Raney nickel, used for fat hydrogenation, and y-Al O, used for ethanol dehydration. More often the catalyticaHy active component is highly dispersed on the surface of a support and may constitute no more than about 1% of the total catalyst. The main reason for dispersing the catalytic species is the expense. The expensive material must be accessible to reactants, and this requires that most of the catalytic material be present at a surface. This is possible only if the material is dispersed as minute particles, as smaH as 1 nm in diameter and even less. It is not practical to use minute... 

Catalysis by Metal Oxides and Zeolites. Metal oxides are common catalyst supports and catalysts. Some metal oxides alone are industrial catalysts an example is the y-Al202 used for ethanol dehydration to give ethylene. But these simple oxides are the exception mixed metal oxides are more... 

FIG. 13-70 Three-column sequence for ethanol dehydration with cyclohexane (operating column C2 in the direct mode). 

FIG. 25-20 Ethanol dehydration using pervaporation membrane. (SOURCE Redrawn from Ref. 24.)... 

EFFECT OF ETHANOL DEHYDRATION ON THE REVERSAL OF FORMALDEHYDE CROSS-LINKS... 

EFFECT OF FIXATION AND ETHANOL DEHYDRATION ON PROTEIN STRUCTURE... 

Fowler CB, O Leary TJ, Mason JT. Modeling formalin fixation and histological processing with bovine ribonuclease A effects of ethanol dehydration on reversal of formaldehyde-induced cross-links. Lab. Invest. 2008 88 785-791. 

Figure 19.1 A schematic view of the common formaldehyde-induced modifications in proteins. Reactive methylol adducts are formed in the initial reaction between formaldehyde and cysteine or the amino groups of basic amino acid residues. The methylol adduct can subsequently undergo a dehydration reaction to form a Schiff s base. Adducted residues can undergo a second reaction to form methylene bridges or can convert to the ethoxymethyl adduct after the ethanol dehydration step.

As described in Section 3.2.3, the use of acidic supports such as A1203 favors the dehydration of ethanol to ethylene, which leads to a severe carbon deposition.66,76,78,85 Reactions with lower H20/ethanol ratio can also favor several side reactions mentioned above and result in carbon deposition on the catalyst surface. Possible strategies to reduce the carbon deposition include (i) neutralization of acidic sites responsible for ethanol dehydration to ethylene and/or modification of the support nature, including less acidic oxides or redox oxides, (ii) use of a feed containing higher H20/ethanol molar ratio, and (iii) addition of a small concentration of air or 02 in the feed. 

Membranes of PVA/PAcr.Ac blends evidence a selective permeability against different components of a liquid mixture. So, they may be used for the ethanol dehydration by pervaporation technique. 

White hexagonal crystals density 2.82 g/cm slightly soluble in cold water, solubility decreasing with temperature insoluble in ethanol dehydrates in the air at 400°C. 

Saito and Niiyama (241) investigated the transient behavior of ethanol dehydration catalyzed by Baj sPW O. When the ethanol feed was stopped after a steady state had been attained, ethylene continued to form for a prolonged period, whereas ether, formation decreased rapidly. This transient behavior, as well as the kinetics under stationary conditions, was well simulated with a model based on the assumption that the ethylene and ether are formed by unimolecular and bimolecular reactions in the bulk, respectively. 

The unusual pressure dependences of the rate and selectivity associated with the pseudoliquid that were observed for ethanol dehydration catalyzed by H3PWl204o are shown in Fig. 40 169, 243). The rates of ether and ethylene... 

The first application of pervaporation was the removal of water from an azeotropic mixture of water and ethanol. By definition, the evaporative separation term /3evap for an azeotropic mixture is 1 because, at the azeotropic concentration, the vapor and the liquid phases have the same composition. Thus, the 200- to 500-fold separation achieved by pervaporation membranes in ethanol dehydration is due entirely to the selectivity of the membrane, which is much more permeable to water than to ethanol. This ability to achieve a large separation where distillation fails is why pervaporation is also being considered for the separation of aromatic/aliphatic mixtures in oil refinery applications. The evaporation separation term in these closely boiling mixtures is again close to 1, but a substantial separation is achieved due to the greater permeability of the membrane to the aromatic components. 

Figure 9.11 Photograph of a 50-m2 GFT plate-and-frame module and an ethanol dehydration system fitted with this type of module. The module is contained in the large vacuum chamber on the left-hand side of the pervaporation system [44]...

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