(587e) Bio-Butanol Separation By Adsorption: Adsorbent Screening and Breakthrough Experiments

Depletion of oil resources combined with the continuous rising of oil prices, political instability in oil-producing countries and environmental challenges are some of the reasons that have motivated significant interest for producing alternative biofuels from renewable and sustainable resources. Butanol, considered as one of the best renewable alternatives for gasoline, has attracted significant attention in recent years since it has several advantages over other biofuels. Higher energy density, better solubility in existing hydrocarbon fuels, net heat of combustion (NHOC) closer to the one for gasoline, lower vapor pressure and lower corrosiveness are some of the advantages which make butanol easier to handle, transfer and work with, compared to other renewable fuels.

The microorganisms used in biobutanol fermentation process are mostly anaerobic solventogenic Clostridia including Clostridium acetobutylicum and Clostridium beijerinkii. The fermentation process is commonly referred to as Acetone-Butanol-Ethanol (ABE) fermentation since these components are the main products in the fermentation broth in a typical ratio of 3:6:1 (wt%). Butanol concentration in the broth is less than 2 wt% and is usually in the order of 1 wt%.

Given the very low concentration of butanol and the presence of other components in the fermentation broth, even at very low concentrations, the major challenge remains to develop a cost-effective technique to separate and recover butanol as the final product. Distillation is the traditional recovery technique used for butanol separation and it is still the technique that is widely used for this purpose. The energy requirement for butanol separation by steam stripping distillation (24.2 MJ/kg) is comparable to the butanol NHOC (36 MJ/kg). Thus, finding another technique with lower energy demand is very important to make the biobutanol production process economically viable.

Other techniques used for butanol separation from fermentation broths include liquid-liquid extraction, gas stripping, perstraction, pervaporation and adsorption. According to different investigations, the energy requirement for adsorption is the lowest compared to other techniques.

An important number of studies have been carried out to separate and recover butanol from model solutions and aqueous mixtures. A wide range of materials such as activated carbon, polymeric resins, polyvinylpyridine (PVP), and zeolites are commonly used as butanol adsorbents. According to the results of these studies, zeolites and activated carbons have the highest adsorption capacity for butanol separation from dilute solutions. However, only a few studies were performed using different adsorbents to investigate the adsorption rate and the effect of other components present in fermentation broths.

The studies carried out in the literature for different adsorbents tested for butanol separation using model solutions were mainly concerned with their adsorption capacity and less attention was devoted to other significant factors influencing the adsorbent selection. In the present study, different types of activated carbons (AC F600 and AC F400 with particles sizes 0.55-0.75 and 1 mm, respectively) and zeolites (ZSM-5, NaY and silicalite with different SiO₂/Al₂O₃ ratios of 80 and 1.8 and more than 1000, respectively) were tested. The adsorption capacity and the rate of adsorption were measured for these adsorbents. In addition, the effect of the presence of other ABE broth components on butanol adsorption was also investigated.

Kinetic experiments were carried out using butanol-water solutions with an initial butanol concentration of approximately 10 g/L to investigate the rate of butanol adsorption for each adsorbent. The time constant (the time in which the butanol concentration decreased by 63.2 percent of its total change) was used to compare the rate of adsorption for different adsorbents. Results revealed that activated carbons F-400 and F-600 had much faster adsorption rates than the other adsorbents studied. By fitting the pseudo-second order equation (given by Eq. 1) to the experimental data and determining the time constant for all adsorbents, it was found that AC F-400 and F-600 had the time constants of 8.2 and 11.3 min, respectively, whereas this constant was much higher for other adsorbents (39.2, 71.5 and 109.3 min for Silicalite, ZSM-% and NaY, respectively).

Adsorption equilibrium isotherms were used to compare the capacity of different adsorbents for butanol adsorption. The results showed that activated carbon F-400 had the highest butanol adsorption capacity in comparison to other adsorbents. For example, at a 10 g/L butanol concentration at equilibrium, the butanol adsorption capacity for AC F-400 was 258 mg/g, while it was 149, 139, 107 and 103 mg/g for AC F-600, ZSM-5, NaY, and silicalite, respectively.

To investigate the adsorbent selectivity toward butanol, compared to other components present in the ABE broths, the isotherm experiments with binary solutions of water and each of the other components (acetone, ethanol, glucose, xylose, acetic acid and butyric acid) were performed with AC F-400. Results clearly showed that AC F-400 had in most cases a much higher selectivity toward butanol compared to other components. For example, at a concentration of 10 g/L of each component in the aqueous solutions at equilibrium, the adsorption capacities for butanol, butyric acid, acetone, acetic acid, glucose, xylose and ethanol were 294, 256, 115, 100, 96, 88 and 55 mg/g, respectively.

To further investigate the effect of the presence of other ABE fermentation broth components on butanol adsorption capacity, experiments were performed to study the effect of each component separately with AC F-400 as the adsorbent. Results showed that there is a slight decrease of adsorption capacity at low equilibrium concentrations of butanol but at concentrations higher than 5 g/L butanol, acetone has no effect on butanol adsorption by AC F-400. It was observed that the presence of ethanol does not affect butanol adsorption over the whole range of butanol equilibrium concentrations. The same effects were observed in the presence of glucose and even at 10 g/L initial concentration of these sugars for all butanol concentrations. However, the presence of acids (acetic acid and butyric acid) led to a decrease in butanol adsorption capacity and this effect was more significant for the presence of butyric acid. The presence of 1 g/L initial acetic acid decreased the butanol adsorption capacity by 19% and this effect became more significant when 5 g/L of acid was added to butanol-water solutions (26.5 % decrease). In the presence of butyric acid, the adsorption capacity decreased by 19.5 and 28.8% in the presence of 1 and 5 g/L butyric acid, respectively.

To design a specific butanol adsorption process it is important to understand the thermodynamic and kinetic characteristics of the adsorbent and adsorbates. Breakthrough experiments allow observing the dynamic behavior of the adsorbent and the feed solution components during the adsorption process. In breakthrough experiments, the curves of the effluent concentration of each component versus time show the detailed behavior of each compound present in the ABE broth. The breakthrough experimental results for ABE model solution (butanol 12.7, ethanol 2.2, acetone 6.1, acetic acid 4.8, butyric acid 4.9, glucose 5.3 and xylose 4.2 g/L) showed that butanol is preferentially adsorbed and the other components that were also adsorbed downstream of the butanol adsorption zone were later displaced by butanol and butyric acid during the breakthrough experiments. It was observed that ethanol, glucose and xylose are the first components to break through the adsorption bed as they can be displaced by acetic acid and acetone which are in turn displaced by butanol and butyric acid.

The results of the various experiments clearly showed that AC F-400 is one of the best adsorbents for butanol separation. This adsorbent has the highest affinity for butanol as well as a very high adsorption capacity and adsorption rate.

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