Study On Adsorption Characteristics Of Ethanol Water Mixture On Potato Powder Adsorbent

Fuel ethanol is a kind of renewable fuel, which can be used as the additive of automobile fuel, and replace to the part of the mineral fuel, and reduce the pollution at the same time, and has been paid more attention widespread. With the high economic growth in china, petroleum consumption is rapidly increasing and imports of crude oil are also rising, 49.8% of domestic consumption has to rely on import. The increase in petroleum consumption is causing a serious air pollution problem. To deal with energy security and the air pollution problem, the Chinese government has strongly promoted the fuel ethanol.However, mail issues with fuel ethanol are high product cost and high energy consumption. The highest energy cost of ethanol is the separation of ethanol and water. The key point in ethanol dehydration is represented by the fact that ethanol forms with water an azeotropic mixture. This mixture is usually separated into its constituents by azeotropic distillation, a step which consumes 60%～80% of the overall energy required by the fermentative plant. Among the alternative techniques to dehydrate ethanol, adsorption dehydration system has good industrial application prospect in producing anhydrous alcohol.Adsorption with starchy materials was evaluated as an energy efficient technology for separation of the ethanol-water azeotrope. Potato powder was used as adsorbent for water removal in this study. The advantages of potato powder adsorbent as following, potato powder is cheap, the energy consumption is low, in addition, it can be used as the raw material of fermentation method to produce anhydrous alcohol.The physical and chemical properties of potato powder were investigated in this study, before and after adsorption, the adsorbents were examined with IR, X-RD and SEM. A bench test scale apparatus with a stainless steel column of 25mm inner diameter and of 900mm packed able height for fixed bed adsorption was designed to perform the experiments at constant temperature. The experiments were performed to measure water isotherms on potato powder, measure breakthrough curves with 99.5wt% ethanol as breakthrough point and temperature distribution curves for adsorption under various operation conditions of different levels of superficial velocity, bed temperature, bed depth, granularities adsorbent and inlet concentration. The analysis and the comparison of experimental data were also performed to attain series basic conclusions about the effect of operation conditions on adsorption capability of potato powder. Adsorption equilibrium models have been used to fit the experimental data for water adsorption isotherms on potato powder. BET model was used to calculate the specific surface area of water on potato powder. Inverse gas chromatography (IGC) was used to study the adsorption of water and ethanol on potato powder. Klinkenberg model and BP neural network model were used to analyze the breakthrough curves for different operation conditions.The main results of our research work are as follows: 1. Results from the adsorbent evaluate experiments indicated that starch content of potato powder is 71.91%, the ratio is approximately 23% amylose to 77% amylopectin in potato powder and bed voidage is 0.5 with small bed pressure drop,potato powder is suitable for adsorbent. adsorption did not create visible change, After adsorption, potato powder keep the granules structures, and conserve the size and shape.was shown by the IR spectrum, X-RD patterns and SEM micrographs. Under experiment conditions, potato powder is stable adsorbent. It operates and regeneration at relatively mild conditions, and is stable under cyclic use when regeneration is carried out with air at 90℃. In the event that capacity was lost over a period of time, the potato powder could still be used to make fermentation-derived ethanol.2. Results from the adsorption experiments indicated that the sensitive factors for the vapor adsorption process included the vapor superficial velocity, the temperature, granularities and bed depth. Under experiment conditions, the breakthrough time and production capacity of potato for alcohol-water systems decreases as temperture rise, vapor superficial velocity increases and the size of adsorbent increases, and increases as the bed depth increases. From 93% to 95% ethanol, potato powder is high performance. Distillation of fermentation strengh ethanol to a about 93% ethanol-water production below the azeotrope, followed by adsorption to remove the remaining water. the production capacity is about 21.51g H₂O/g adsorbent at 80℃and inlet ethanol concentration 94.5% (v/v). When both the vapor superficial velocity and the feed composition are constant, the adsorption bed depth would be proportional to the breakthrough time and production capacity, the relation obtained was t_b = 0.085h-33.333 and P = 0.113h - 49.445, respectively.3. Results from the adsorption equilibrium experiments indicated that isotherm of potato powder exhibited Type II (according to the Brunauer's classification). The performances of the models examined in this work for experimental data fitting have the following order: GAB model, Handerson model, Sircar model, Oswin model, Peleg model, Langmuir model, adsorption potential theory of Polanyi, Smith model, and Henry law. GAB model included temperature parameters, and its precision was better than other mathematical models. Based on water-potato powder isotherm data in the range 0.05<α_w<0.35, the specific surface area was estimated for water adsorption as 93.019m²/g potato powder.4. Results from the IGC experiments indicated that water was adsorbed more strongly than ethanol on potato powder at 70～140℃. In addition, it was found that lower temperatures were more favourable for the adsorption of both solutes. In contrast, the separation capacities of potato powder with smaller particle size, i.e. 140 mesh and 100～120 mesh, were greater. The 140 mesh potato powder exhibited the highest separation at 70～140℃. Heats of adsorption were calculated from retention data are in the range from -12.55 to-13.1 kJ/mol for ethanol and -24.88 to -45.79 kJ/mol for water.5. Results from the predict of breakthrough curves indicated that by fitting the experimental results of breakthrough curves to Klinkenberg model, the overall mass-transfer coefficients were estimated as 0.001413, 0.001637 and 0.001936 at 80℃, 84℃and 88℃, respectively. Klinkenberg model and BP neural network model were successfully used for the prediction of breakthrough curves at different temperatures, the models can be used to instruct adsorption system design and operate.