Biobutanol Dehydration Using Oat Hull Based Biosorbent
Date
2020-12-17
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
0000-0003-0062-4553
Type
Thesis
Degree Level
Doctoral
Abstract
Biobutanol has gained increasing interest due to its application in fuel industry. Butanol has lower vapor pressure, higher energy content, and less corrosive character than ethanol, which makes butanol a superior fuel additive. Usually, biobutanol is produced by the acetone-butanol-ethanol (ABE) fermentation process from renewable feedstock. To obtain pure butanol from the fermentation broth, preliminary distillation is usually applied to remove acetone and ethanol from butanol. Then, the decantation and two-column distillation are used to separate butanol from heterogeneous azeotrope of butanol and water (butanol composition of 55.5 wt%). This leads to high energy consumption to break the butanol and water azeotrope. Therefore, efforts have been made to develop various approaches to separate water from the butanol-water azeotrope. Among these approaches, adsorption is known as an effective method for separation and purification due to its low cost, high efficiency, and easy operation.
With regard to the reasons mentioned above, the goal of this research is to develop an alternative technology of sorption to remove water directly from the butanol-water azeotrope vapor generated from the distillation in the biobutanol industry in to reduce or replace the downstream decantation and distillations in the biobutanol industry. This work also aims to investigate the isotherm, mechanisms, and kinetics aspects of water and butanol sorption by natural material based biosorbents.
The overall research is divided into 5 phases:
In Phase I, a biosorbent was developed from the oat hull, and the biosorbent was characterized by various methods. In addition, the water sorption on the biosorbent was visualized with the aid of microscope imaging. The oat hull based biosorbent was used to dehydrate butanol in a packed column in order to produce biofuel products. The sorption performance and the separation factors of water over butanol on the biosorbent were determined. Through the dehydration process developed for this work, high purity butanol products (95.3%, 97.1%, 98.1%, and 99.0%) were achieved from low-grade butanol-water mixtures (56.6%, 69.1%, 79.7%, and 90.3%). The highest water sorption capacity is 146±8 mg/g, and the highest separation factor of water over butanol is 2.94±0.13. The results indicate the oat hull based biosorbent has the capability for butanol dehydration. The effects of parameters were investigated, and the effects of temperature and feed concentration are significant.
In Phase II, the water sorption equilibrium data obtained from the single component system and butanol-water binary system was simulated by the Dubinin-Polanyi model. Besides, the equilibrium data of butanol sorption on the biosorbent was studied. The results indicate the large pore Dubinin-Polanyi model is able to describe water and butanol sorption equilibrium data. The approximate adsorption site energy distribution was calculated to further analyze the equilibrium data. The site energy distribution shows the maximum water sorption capacity is much higher than that of butanol in either the single or binary system. The weighted mean site energy for water sorption in the butanol-water binary system is 2378 J/mol, which is higher than that of butanol (2105 J/mol), showing water has a higher affinity on the biosorbent. The thermodynamic study provided evidence of the physical and exothermic nature of the sorption process. The dipole-dipole attraction may suggest the mechanism of water and butanol molecules sorption on the biosorbent.
To better understand the fundamentals of water and butanol sorption, Phase III investigated the sorption dynamics of pure butanol and pure water. The Klinkenberg model simulated the sorption breakthrough curves obtained in pure water or pure butanol single component sorption system satisfactorily. The mass transfer coefficient and mass transfer resistances were investigated from the modeling results. The sorption of pure water or butanol on the oat hull based biosorbent was controlled by mass transfer. The values of overall mass transfer resistance of pure water sorption are lower than those of butanol at the same conditions, which suggests that water has more favorable sorption performance than butanol.
In Phase IV, the dynamics of water sorption from butanol-water vapor mixtures in a fixed bed column were investigated. Water sorption breakthrough curves in the binary system were simulated by the Bohart-Adams model and Klinkenberg model. The Klinkenberg model is better for simulating the breakthrough curves. This implies that the water sorption mechanism and dynamics are more likely to follow the Klinkenberg model’s theory. The rate of water sorption was controlled by mass transfer resistance based on the estimated values of mass transfer resistances. The kinetically driven competitive sorption of butanol and water was discussed to explain the basis of the separation of the butanol-water mixture by the oat hull biosorbent. Compared to butanol, water has a more favorable sorption performance on the oat hull biosorbent. Thus, this biosorbent is able to dehydrate butanol solutions.
Phase V involved the reusability study of the oat hull based biosorbent. The biosorbent was regenerated and reused in the same column without being changed. It was demonstrated that the biosorbent derived from the oat hull used in this study was regenerated and reused successfully for more than 20 cycles with satisfactory performance and high stability. The oat hull based biosorbent used in this work is able to be used continually. The preliminary economic analysis shows the production of anhydrous biobutanol by pressure swing adsorption process using the oat hull based biosorbent is more economical with 29% lower utilities cost than using the traditional adsorbent molecular sieves.
The aforementioned studies demonstrated that the oat hull based biosorbent is able to dehydrate the water/butanol binary azeotrope (butanol composition of 55.5 wt%) and the feed of higher butanol concentrations of 56.6–90.3 wt.% through the dynamic sorption¬ process.
Description
Keywords
biobutanol, dehydration, biosorbent, oat hull
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Chemical and Biological Engineering
Program
Chemical Engineering