摘要
生物柴油是一种清洁、可再生的液态燃料,然而生物柴油普遍存在点火性能差、燃烧功率低等缺点。以生物柴油为萃取剂开展丁醇萃取发酵,萃取有10g／L以上丁醇的生物柴油可作为“高品质改良型”的生物柴油而被直接使用,大幅降低了丁醇精馏回收过程的能耗。但是,萃取发酵会产生大量的萃余液(发酵废液),萃余液中仍含有8g／L左右的丁醇。废液直接排放既严重污染环境,又浪费了废液中宝贵的发酵产品和水资源。
本论文旨在提高丁醇萃取发酵中萃余液的回用率,以最为节水、节能的方式生产“高品质改良型”生物柴油。与此同时,以最为经济和节能的方式回收萃余液中的大部残余丁醇,提高目标产物-丁醇的得率,实现以生物柴油为萃取剂的丁醇萃取发酵联产“高品质改良型”的生物柴油和纯丁醇溶剂,以及产品多样性的目标。研究的主要结果如下：
(1)利用正辛醇(正辛醇／萃余液体积比0.2：1)萃取回收萃余液中的残余丁醇,萃余液中56%的丁醇可被浓缩回收,正辛醇中的丁醇浓度达到25.7g／L,有利于丁醇蒸馏精制的进行。此条件下,主目标产物-丁醇的得率提高了44%。
(2)活性炭作为色素类物质的高效吸附剂,可将萃余液中绝大部分发酵抑制性物质-类黑精去除。采用3%(w／v)活性炭吸附处理萃余液,可将一次性萃余液回用率从50%提高至100%,丁醇发酵性能不受影响。
(3)以生物柴油为萃取剂的丁醇萃取发酵中,利用活性炭吸附法处理萃余液,反复全回用14次,萃取发酵性能稳定,生物柴油中丁醇浓度稳定在10g／L以上,达到“高品质改良型”生物柴油的丁醇含量标准。(4)利用7L静态厌氧发酵罐,进一步验证了节水节能式丁醇萃取发酵联产“高品质改良型”生物柴油和丁醇溶剂的可行性。在此操作模式下,目标丁醇得率达到20%,每升发酵液产0.97L“改良型”生物柴油,产纯丁醇溶剂5g左右。
Biodiesel is a kind of clean and renewable liquid fuel. The existing biodiesels have a couple of disadvantages such as poor ignition performance and lower combustion power. Using biodiesels as extractant for acetone-butanol (AB) extractive fermentation, the biodiesels extracting more than 10g/L butanol could be used as "properties improved" biodiesel directly. This method would greatly reduce the energy consumed in distilling and recovering the fermentative solvents. However, AB extractive fermentation with biodiesels as the extractant also produces a large amount of waste supernatant which still contains about 8 g/L butanol. Direct disposal of the waste supernatant not only causes severe environmental pollution but also wastes valuable fermentation products and water resource.
This study aimed at increasing waste supernatant utilization ratio in butanol extractive fermentation to simultaneously and stably produce "properties improved" biodiesel and pure butanol in an energy-saving operation mode, as well as realizing the target of products diversity. The main conclusion could be summarized as follows:
(1) A small amount of n-octanol could be used to recover butanol remaining in the residual supernatant (octanol/supernatant:0.2:1). Under this condition, more than 56%of the remaining butanol could be recovered and butanol in n-octanol could be concentrated to a level of 25.7 g/L for easy and energy-saving purification, and the total butanol yield increased 44%.
(2) The waste supernatant was pre-treated by 3%(w/v) activated carbon to adhere the fermentative inhibitor-melanoidin accumulated in the waste supernatant. After the pre-treatment, the maximal recycle ratio of waste supernatant could be increased from 50% to 100% and the fermentation performance did not deteriorate as compared with that of using fresh water for medium preparation.
(3) With biodiesel as the extractant for butanol extractive fermentation, fully recycling fermentation waste supernatant could at least continued for 14 runs without any fermentation performance deterioration. In each fermentation run, butanol concentration in bio-diesel stabilized over 10g/L, which would potentially ensure the quality of "properties improved" bio-diesel.
(4) The proposed water & energy-saving butanol extractive fermentation system to simultaneously produce "properties improved" biodiesel and pure butanol was testified in a 7L static and anaerobic bioreactor. In this case, the "aimed" butanol yield increased to a level of 20%,0.97L "properties improved" biodiesel and about 5g pure butanol could simultaneously produced per one liter fermentation broth.
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