As a second generation biofuel, butanol has attracted increasing attention during the last decade. Biobutanol can be produced through traditional ABE fermentation. However, fermentative butanol production is not yet economically competitive with petrochemical process, mainly due to high substrate cost, low product yield and concentration and high recovery cost. Many efforts have been made to improve fermentative butanol production.
Typical batch ABE fermentation usually gives a final butanol titer of 12-14 g/L. Butanol recovery from this dilute solution by distillation is very energy-intensive. Many alternative separation techniques have been developed. Among them, adsorption is a promising technique for its simple operation. In order to selectively recover butanol and release the product inhibition effect, four commercial materials were identified as potential adsorbents for butanol separation. These four adsorbents, including activated carbon Norit ROW 0.8, zeolite CBV901, polymeric resin Dowex Optipore L-493 and SD-2, showed high specific loading and adsorbent-aqueous partitioning coefficients for butanol. Adsorption isotherms and their regressions with Langmiur model were further studied for these adsorbents, which provided the theoretical basis for predicting the amount of butanol adsorbed on these adsorbents. In batch fermentation with in situ adsorption without pH control, activated carbon showed the best performance with 21.9 g/L total butanol production, and 71.3 g/L glucose consumption. The total butanol production with activated carbon increased by 87.2%, 51.0%, 44.1% and 90.4%, respectively, compared to the control (without adsorbent), L-493, SD-2 and CBV901. The integration of adsorption by activated carbon, with both free and immobilized cell fermentation, was demonstrated to be successful. The control free cell fermentation produced 18.3 g/L butanol in 54 h with a butanol productivity of 0.34 g/L/h, while free cell fermentation with adsorption produced over 31.6 g/L butanol in 106 h with a butanol productivity of over 0.30 g/L/h, offering a over 70% increase in butanol titer. The control immobilized cell fermentation produced 16.4 g/L butanol in 47 h with a butanol productivity of 0.35 g/L/h, while immobilized cell fermentation with adsorption produced ~54.6 g/L butanol in 122 h with a butanol productivity of ~0.45 g/L/h, an increase of ~30% and ~200% in butanol productivity and butanol titer, respectively, compared to the control experiments. Furthermore, ~150 g/L of butanol in the condensate could be recovered from desorption of adsorbents, which was easily concentrated to ~640 g/L after simple and naturally occurring phase separation. Therefore, based on the estimation on energy consumption of other separation technology (typically greater than 10 kJ/g), our highly-designed in situ product recovery (ISPR) process with activated carbon only required ~4.8 kJ/g butanol, with greater energy saving, showing its potential economical value for product recovery and integration with butanol fermentation to simultaneously remove inhibitory products.
In order to reduce the substrate cost, butanol has been produced from lignocellulosic biomass and thus called second generation biofuel. There are many different lignocellulosic biomass that can be explored for this purpose. Efforts have also been devoted to improve butanol production. Besides metabolic engineering, butanol production can be boosted by external driving forces which can redirect the electron and carbon flow towards butanol synthesis. In this work, engineered mutant strain Clostridium tyrobutyricum overexpressing adhE2 and ack knock out mutant was used. When provided with external driving forces, butanol production with high yields (greater than 0.30 g/g) was achieved in bioreactor. Fed-batch butanol fermentation from different carbon sources in a fibrous-bed bioreactor integrated with gas stripping was studied. Final butanol titer, yield and productivity of 12.1 g/L, 0.25 g/g and 0.12 g/L/h, respectively, were obtained from soybean hull hydrolysate (SHH). A glucose and xylose mixture mimicking sugar composition in SHH was also used to produce butanol, which resulted in a total butanol production of 24.7 g/L. This work was the first study to produce butanol from soybean hull hydrolysate integrated with gas stripping. This study demonstrated the feasibility of butanol fermentation from soybean hull and sugarcane bagasse hydrolysate integrated with butanol recovery by gas stripping.
Besides previously mentioned adsorption and gas stripping, pervaporation is also an effective way for butanol recovery from dilute solutions with high selectivity. In this work, high performance polydimethylsiloxane (PDMS) membranes and zeolite filled PDMS mixed matrix membranes (MMMs) were developed to recover butanol from model solutions. The effects of membrane filler zeolite, feed butanol concentration, and operating (feed) temperature on pervaporation performance of PDMS membranes and PDMS MMMs was studied. With the feed solution of 1.5 wt% butanol at 47 C, the PDMS MMM filled with 40 wt% zeolite was found to have the highest butanol separation factor of 77 with a butanol and total flux of 62 and 118 g/m2/h, respectively. For both PDMS membranes and PDMS MMMs, the separation factor can be further increased by elevating operating temperature, and permeation fluxes can be further boosted by reducing membrane thickness, increasing feed butanol concentration, and/or elevating operating temperature. The apparent activation energies of butanol permeation in PDMS membrane and zeolite filled (40 wt%) PDMS MMM were evaluated and found to be 34.3 and 44.2 kJ/mol, respectively. Compared to other studies reported in the literature, this work demonstrated higher butanol separation performance by pervaporation and showed huge energy saving compared to traditional distillation.
This project demonstrated efficient butanol recovery by adsorption, gas stripping and pervaporation from both model solution and fermentation broth. In situ product recovery largely improved fermentative butanol production.