Biomass separation pretreatment

Irrespective of the type of biomass used for ethanol production, the biomass needs to be pretreated to make the carbohydrates available for fermentation. However, which enzymes can be used depends on the source of the biomass. In addition, the biomass needs pretreatment before the enzymes are used. The first step of the pretreatment can be of a physical nature. Once the biomass is physically pretreated, the cellulose structures are open for enzyme action. In biomass from forests, the substance is mainly in the form of cellulose. Targeted enzymes are selective for the reaction of cellulose to glucose, and therefore there are no degradation byproducts, as occurs in acid conversion technology. There are at least three ways this can be performed. Firstly, in separate hydrolysis and fermentation, the pretreated biomass is treated with cellulase, which hydrolyzes the cellulose to glucose at 50 °C and pH 4.8. Secondly, in simultaneous fermentation and saccharification (SSF) the hydrolysis and fermentation take place in the same bioreactor. Thirdly,... 

The second step in the production of bioethanol involves the separation of the components (cellulose, hemicellulose, and lignin) of tiie biomass into its individual components. Pretreatment involves the utilization of the possible metiiods to obtain the separation. Pretreatment methods are subject to ongoing and intense research worldwide. Possible pretreatment methods can be classified as follows, although not all of them have been developed enough to be feasible for applications in large-scale processes (Taherzadeh and Karimi, 2008). 

More than 10 years operating experience with full-scale MBRs has shown that the use of membranes in the activated sludge process requires an adaptation of the overall process— consisting of (1) pretreatment, (2) aeration tank, and (3) filtration—to the specific strengths and weaknesses of membranes for biomass separation. As the specific measmes required may differ with the module design, the various options will be discussed in general in the following sections. 

Harvest the cellulase-containing transgenic plants while they are green, macerate them, and separate the solid to produce an enzyme concentrate, which can later be used in the enzymatic hydrolysis of pretreated biomass. Depending on the production level of this enzyme in transgenic plants, the need for externally added cellu-lases in the enzymatic hydrolysis step might be avoided or minimized. 

Fuel pretreatment is viewed as a way to iir rove combustion behavior and reduce a number of operating problems. Greater effort is needed on separation of undesirable constituents and con onents from biomass fuels. 

Because the hemicellulose fraction of biomass materials can be separated from lignin and cellulose by dilute acid treatment, cellulose becomes more reactive towards cellulase. Hemicellulose hydrolysis rates vary with acid concentration, temperature, and solid-to-liquid ratio. With most lignocellulosic materials, complete hemicellulose hydrolysis can be achieved in 5-10 min at 160°C or 30-60 min at 140 °C. Dilute acid hydrolysis forms the basis of many pretreatment processes for example, autohydrolysis and steam explosion are based on high-temperature dilute acid catalyzed hydrolysis of biomass. 

The pretreatment of any lignocellulosic biomass is cmcial before enzymatic hydrolysis. The objective of pretreatment is to decrease the crystallinity of cellulose which enhances the hydrolysis of cellulose by cellulases (17). Various pretreatment options are available to fractionate, solubilize, hydrolyze and separate cellulose, hemicellulose and lignin components (1,18-20). These include concentrated acid (27), dilute acid (22), SOj (25), alkali (24, 25), hydrogen peroxide (26), wet-oxidation (27), steam explosion (autohydrolysis) (28), ammonia fiber explosion (AFEX) (29), CO2 explosion (30), liquid hot water (31) and organic solvent treatments (52). In each option, the biomass is reduced in size and its physical structure is opened. Some methods of pretreatment of Lignocellulose is given in Table I. 

Since most industrial enzymes are produced extracellularly, only unit operations used for recovering these types of enzymes are considered. For intracellularly produced enzymes, additional steps must be included to harvest and open the cells (e.g., by a bead mill or high-pressure homogenizer). This complicates the process and increases the costs hence this type of processing is avoided, if possible. A recovery process typically consists of a pretreatment, followed by a primary separation of the enzyme from the biomass. Later, the enzyme is concentrated by removal of water, and unwanted impurities are removed in a purification step. Figure 27.4 illustrates an example of a simplified recovery process. 

In the primary separation, the pretreated liquid containing the enzyme is separated from the biomass and other insolnble materials by filtration [7] or centrifugation [8]. The two technologies nse different driving forces (pressure and gravity in case of filtration and centrifngation, respectively), bnt both of them can be nsed for bacteria as well as fnngi. 

A biorefinery is the integral upstream, midstream, and downstream processing of biomass into a range of produas. In the classification system lEA Bioenergy Task 42 (described in the next chapter) has differentiated between mechanical pretreatments (extraction, fractionation, separation), thermochemical conversions, chemical conversions, enzymatic conversions, and microbial (fermentation both aerobic, anaerobic) conversions. 

There are several examples describing the production of multiple compounds from biomass, or its derived components (Table 9.2). Dairy manure is a type of biomass that contains 12% hemicellulose, 22% cellulose, and 18% crude protein, representing a lai e potential source of carbohydrates as a carbon source and proteins as a nitrc en source. This substrate is uniquely adaptable and advant eous for animal manure refineries. The Rhizopus oryzae ATCC 20344 cell factory produced fumaric acid and chitin fiom the dairy manure. A liquid/ solid separation was used to obtain a nitrc en-rich liquid stream for fungal growth and subsequent chitin accumulation, while the manure s solid stream, which primarily consisted of carbohydrates in the form of cellulose and hemicellulose, could be converted by various pretreatment methods into monosaccharides for fumaric acid production. 

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