The Production Of Biobutanol And Its Integrated Recovery Technology

Biobutanol is considered as an advanced biofuel with potential prospects due to its advantages of high energy content and high compatibility with gasoline. However, due to the severe butanol toxicity to cells, a low butanol concentration and butanol productivity were the main obsracles through traditional acetone-butanol-ethanol fermentation. ABE fermentation integrated with recovery process could effectively increase butanol productivity and reduce the cost of separation by continuously removing toxic products. This paper describes the application of pervaporation and gas stripping for butanol recovery and the specific contents are as follows:The carbon nanotubes (CNTs) filled polydimethylsiloxane (PDMS) hybrid membrane was fabricated to evaluate its potential for butanol recovery from acetone-butanol-ethanol (ABE) fermentation broth. Compared with the homogeneous PDMS membrane, the CNTs filled into the PDMS membrane were beneficial for the improvement of butanol recovery in butanol flux and separation factor. The CNTs acting as sorption-active sites with super hydrophobicity could give an alternative route for mass transport through the inner tubes or along the smooth surface. The maximum total flux and butanol separation factor reached up to 244.3 g/m and 32.9, respectively, when the PDMS membrane filled with 10 wt% CNTs was used to separate butanol from the butanol/water solution at 80℃. In addition, the butanol flux and separation factor increased dramatically with the increasing of temperature and butanol titer in feed, since the higher temperature produced more free volumes in polymer chains to facilitate butanol permeation. A similar increase was also observed when butanol titer in solution increased.The polydimethylsiloxane-polyvinylidene fluoride (PDMS/PVDF) composite membrane was studied for its pervaporation performance to removal of butanol from butanol/ABE solution, fermentation broth as well as incorporated with acetone-butanol-ethanol (ABE) fermentation. The total flux and butanol titer in permeate through the PDMS/PVDF membrane were up to 769.6 g/m2·h and 323.5 g/L at 80℃, respectively. The butanol flux and total flux increased with increasing the feed temperature as well as the feed butanol titer. The butanol separation factor and butanol titer in permeate decreased slightly in the presence of acetone and ethanol in the feed due to their preferential dissolution and competitive permeation through the membrane. In fed-batch fermentation incorporated with pervaporation, butanol titer and flux in permeate maintained at a steady level with the range of 139.9～154.0 g/L and 13.3～16.3 g/m2·h, respectively, which indicated that the PDMS/PVDF composite membrane has excellent pervaporation performance.Two-stage gas stripping coupled with acetone-butanol-ethanol fermentation in a fibrous bed bioreactor was established for energy-efficient butanol recovery. The impacts of process parameters including butanol concentration, temperature and cell density with feed, gas flow rate, and cooling temperature on the efficiency of the gas stripping system were studied. High butanol concentration, low cell density and cooling temperature increased butanol titer in the condensate. The butanol titer in the condensate increased when stripping temperature increased from 25 to 55℃, and decreased when temperature was above 55℃. The optimal gas flow rate was 1.6 L/min, above which more water was stripped off and the condensate was diluted. After process optimization,48.5 g/L butanol (73.3 g/L ABE) was produced in the fed-batch fermentation with in situ gas stripping due to the reduced butanol inhibition on cells. The condensate containing 147.2 g/L butanol (199.0 g/L ABE) was produced by the first-stage gas stripping, while a highly concentrated condensate containing 515.3 g/L butanol (671.1 g/L ABE) was obtained from the second-stage gas stripping, which effectively decreased the final separation cost.This paper shows that the adding of the PVDF porous membrane as support layer and carbon nanotubes mixed for the fabrication of PDMS composite membrane is effective to improve pervaporation performance. In situ butanol recovery technology with pervaporation and gas stripping has significant effect on relieving toxicity inhibition, improving final product concentration and reducing the separation cost. Therefore butanol fermentation integrated with product recovery technology has potential applications in improving the economics of butanol production.