Pressure swing adsorption process for ethanol dehydration
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In recent years the ever increasing price of crude oil directed the research and development to the possibilities of employing biomass materials for fuel production. In particular, attention has been focused on bio-ethanol. A major challenge in the production of ethanol is the high cost associated with the separation of ethanol from the large excess of water. Distillation cannot remove water completely due to the presence of an azeotrope. The Pressure Swing Adsorption (PSA) succeeded on the industrial scale as an energy efficient alternative. The goal of this work was to perform a thorough study of the current PSA process, understand the dynamics of the process, the effect of operating parameters on the performance and identify possible improvements towards a more efficient operation. A general purpose package for the simulation of cyclic PSA process was developed. The dispersed plug flow model included the variation of axial velocity, heat effects and linear driving force mass transfer rate. The operation of a column could be isothermal, adiabatic or non-isothermal non-adiabatic. The system of partial differential equations was solved via method of lines using a stiff equation integration package. Capability of our package was tested using data available in the open literature. It was observed that heat effects play an important role in the process and only a proper start-up procedure can lead to an efficient operation. General effects of process variables were investigated as well. Reliable adsorption/desorption equilibrium and kinetic data are required for reasonable description of an industrial process. Pilot scale fixed bed adsorber apparatus was designed and constructed for this purpose. Water and ethanol adsorption was investigated separately. Equilibrium studies have shown that water data can be satisfactorily described by the Langmuir isotherm. Very little ethanol adsorption was observed and it was concluded that ethanol co-adsorption can be neglected for the PSA process modeling. Water kinetic data was measured using breakthrough experiments. A wide range of experimental conditions was covered for later design purposes. Column pressure, temperature, flow rate, pellet size and adsorbate concentration were changed accordingly so the effect of one operating parameter could be studied at a time. Both macropore and micropore diffusion inside the zeolite pellet was identified as a relevant mass transfer mechanism. Experimentally observed trends were used to formulate the adsorption/desorption kinetics model. Next, mathematical model for laboratory adsorption bed was developed and tested. Detailed heat transfer model was a necessity since the bed dynamics was affected by the heat transfer in the bed wall. The model was used to analyze the experimental data with previously formulated kinetic model as an input. The adsorption/desorption kinetic model for PSA design purposes was fully defined. A PSA simulator utilizing experimental equilibrium and kinetic data was used in the parametric study to investigate the effects of the feed stream temperature, the purge flow rate, the adsorption pressure, the feed water concentration and the cycle time for the 2-bed and 3-bed PSA process, respectively. It was found that the 3-bed cycle was largely overdesigned. Hence, the 3-bed process was used as the starting point for the design of a new PSA process working with the feed stream containing 80 wt% of ethanol (instead of 92 wt%). This modification makes it possible to use only one distillation column that can be operated with a lower reflux ratio. A PSA scenario with 80w% ethanol feed is attractive especially for the cellulosic ethanol plants using the gasification technology where the distillation towers could be eliminated completely.