Drying Butanol Using Biosorbents in a Pressure Swing Adsorption Process

A significant challenge in large scale industrial production of butanol is its low product titer. Butanol needs to be purified to higher than 99% purity in order to be used for fuel applications. The focus of this study is to selectively remove water from butanol-water vapor to achieve fuel grade butanol in a pressure swing adsorption (PSA) system using biosorbents developed from agricultural byproduct canola meal (CM). CM was characterized by Fourier Transform Infrared Spectroscopy (FTIR) that CM contains polar groups such as hydroxyls, carboxyls, and amines in cellulose, hemi-cellulose and protein that have the potential for water adsorption. Physico-chemical characterizations were also done to understand the major composition, elemental make-up, devolatilization characteristics and particle size distribution of the CM used. The results demonstrated that biosorbent based on CM was able to successfully dry lower grade butanol and generate fuel grade butanol of over 99 v/v%. Five operating parameters were studied at two different levels to get the optimum process conditions for butanol drying, including temperature (95 and 111°C); pressure (135 and 201 kPa); feed butanol concentration (55 and 95 v/v %); feed flow rate (1.5 and 3 mL min-1) and particle size of adsorbent (0.425-1.18 mm and 4.7 mm pellets). Orthogonal array design (OAD) tool was used to design experiments and to evaluate the effects of these parameters. The performance of butanol dehydration was evaluated using five indices - water uptake; butanol uptake; water selectivity; butanol recovery; and maximum effluent butanol concentration in the effluent. The results demonstrated that feed butanol concentration, temperature and pressure were found to be the most significant factors overall, affecting most of the indices. The effects of individual operating parameters on each butanol dehydration index were determined and a set of optimum operating conditions were proposed by the range analysis of the orthogonal array design at 111oC, 135 kPa, feed butanol concentration of 55 v/v%, feed butanol-water liquid flowrate of 3 mL/min and biosorbent particle size of 0.43-1.18 mm. The experiments conducted at the above mentioned optimum conditions resulted in water uptake of 0.48 g/g-ads, water selectivity of 5.4, butanol recovery of 90%, and the maximum butanol concentration in the effluent being over 99 v/v% , which are better than that obtained at any other conditions investigated in this work. The Dubinin–Polanyi model based on adsorption potential theory displayed a goodness of fit to the water adsorption isotherm data with a r2 value of 0.95 and average relative error of just 3.5%. The mean free energy determined from the model was 0.02 kJ/mol indicated the adsorption is physical. Thermodynamic parameters were also evaluated which revealed that the water adsorption is exothermic and spontaneous. Water saturated adsorbent was regenerated at 110°C under vacuum and reusability was studied. The contribution of two major components of CM namely cellulose and protein were also examined for their capability to selectively remove water from butanol. The results showed both of them were able to dry water, however cellulose was found to have a higher water uptake and water selectivity than protein, indicating that it plays a major role in drying butanol. In order to compare the performance of CM on drying of butanol with other biomaterials, adsorption experiments were done using corn meal as adsorbent, which is one of the most common starch based biosorbents for ethanol drying. The results demonstrated that canola meal had a higher water uptake and water selectivity than corn meal. Use of CM over corn meal adsorbent is also desirable so as to avoid placing pressure on food consumption. In addition, drying of butanol using other cellulose based biosorbents such as oat hull was also explored. Oat hull demonstrated a potential to adsorb water and dehydrate butanol, which requires further in-depth investigation