Detoxification: a key step in the biorefinery process (1)

It has been recognized that hemicellulose from wood furnish can be pre-extracted using hot water, acid, alkali, and organic solvent and then converted into bioethanol by fermentation. Typically, the wood extracts from hot-water or organo-aqueous extraction consist of monomeric and oligomers of sugars (hexoses and pentoses), acetic acid, methanol, aromatic compounds, and other low molecular-weight (MW) extractable substances. However, the presence of various lignins derived components and phenolics interferes with the enzyme adsorption and function to the target substrate in the subsequent hydrolysis process, as well as potentially toxic to the micro-organisms for fermentation. In addition, small molecules such as acetic acid, furfural and hydroxyl methylfurfural (HMF) produced from these processes are also potent inhibitors for fermentation. Obviously, separation of the toxic products from the pre-extracted liquor mixture is a key step in the biorefinery process.

There are several methods which can be used for separation purpose, including striping, distillation, centrifugation, chemical separation, and filtration. Striping off volatile compounds can be performed right after the extraction stage but doesn’t remove the lignin derived compounds. Multiple fractional distillation is often proposed but this is an energy-intensive operation with high capital cost, especially for the separation of chemicals that can form azeotropes such as water and acetic acid. Chemical separation requires additional chemicals to bond the target compounds such that they can be separated more easily. However it is usually hard to achieve the desirable product stream with high purity for a multiple compound liquor mixture and this also requires additional chemical cost and recovery, which is not practical for a massive production. Centrifugation obviously can not work for the wood extracts or pulping spent liquor. Filtration is a simple physical method that can be easily performed with fair capital cost and most importantly, it can preserve the values of the separated streams. Typically, membrane filtration with specific pore size (molecular weight cut-off) can be used to separate different molecules in the extract liquor. But its captial and operation cost, regeneration are the trade-off.