Glucose production costs

Economic Model Parameters and Glucose Production Costs for Batch and Semibatch Saccharification Processes... 

Fig. 7. Glucose production cost as a function of cellulase purchase price for three process configurations batch, semibatch, and semibatch with additional cellulase. Vertical dashed lines are cellulase purchase prices in 1999 (26.6 /kg) and in 2002 (12.2tf/kg).

The main question is whether synthesis of PHA in plants can succeed in bringing the cost of the polymer down to the range of 0.5 -1 US /kg. Bacterial production of PHA typically relies on a carbon source, such as sucrose or glucose, which is produced from photosynthesis and extracted from plants. Synthesis of PHA directly in plants would, therefore, represent a saving in terms of the number of intermediary steps linking C02 fixation to PHA production. Furthermore, starch is one of the cheapest plant commodity product on the market, at about 0.25 US /kg [86]. It is, thus, likely that the production cost of PHA in plants will be substantially cheaper than bacterial fermentation. The final cost of producing PHA in plants will depend on a number of factors. 

Fig. 3. Cost comparison of different routes to 1,3-propanediol [74], Production costs for the chemical processes and glucose route are based on estimations from ChemSystems. Estimations for the glycerol processes are based on US market prices of raw materials in 1998 and a production scale of 65,000 t/a. The energy costs for the glucose process are assumed to be the same as the fed-batch glycerol process...

By-products of starch refining and HFCS prodnction are significant and rednce HFCS production costs by 30 5% (Table 4.2). This is because the com is 70% starch on a dry weight basis, it also contains 10% protein, 4.5% fat and 2.7% erode fibre. In addition much of the glucose syrup produced is fermented to produce ethanol for fuel use. 

Starch costs are the main element in the production costs for HFCSs. Because starch is significantly cheaper than sucrose, HFCS production using glucose isomerase is more economical than by the hydrolysis of sucrose by inverlase. 

HFCS is such an important industrial product that other enzymes involved in its manufacture were also subjected to mutagenesis to increase yield and stability so as to cut production costs subsequently. Glycoamylase (GA) is the second of the three-enzyme series amylase-glycoamylase-glucose isomerase required to convert... 

Four major carboxylic acids, acetic, formic, glyceric, and lactic, are formed under mild alkaline oxidation conditions from xylose (Rahardja et al, 1994). At 40-60°C, lactic acid is formed from dilute aqueous solutions of xylose in the largest mass yield, about 50%, under continuous oxidation conditions. This is about 83% of the maximum theoretical yield of 60%. Under batch conditions, the yield of lactic acid from xylose is about 23%, while the yield from glucose is about 41%. These data indicate a commercial alkaline oxidation process for lactic acid from pentoses and hexoses is feasible, but it should be realized that the recovery of carboxylic acids having high water solubilities from dilute aqueous solutions can add signihcantly to production cost (Busche, 1984). 

It has been accounted that, on a production scale of PHB of 100,000 tons per year, the production costs will decrease from US 4.91 to US 3.72 kg , if hydrolysed com starch (US 0.22 kg ) is chosen as the carbon source instead of glucose (US 0.5 kg ) [33]. But this is still far beyond the cost for conventional polymers, which in 1995 was less than US 1 [32]. Lee et al. estimated that PHB and mcI-PHA can be produced at a cost of approximately US 2 kg [36]. The precondition therefore would be attaining high productivity and the use of inexpensive carbon sources. Among such substrates, molasses [37], starch [38], whey from the dairy industry [37-42], surplus glycerol from biodiesel production [39, 43], xylose [44, 45], and plant oils [46] are available. 

Product quality of PHAs is very much dependent on the polyester composition (see 3.2.2.1). In 1987, Byrom found that poly-(3HB-co-3HV) can be produced in large-scale fed-batch culture by supplementing the nutritional medium of a glucose-utilising mutant of Alcaligenes eutrophus (today known as Cupriavidus necator) with propionic acid (precursor for 3HV formation) [50]. Later it was shown that the utilisation of valeric acid instead of propionic acid results in a higher proportion of 3HV units [51]. The improvements in product quality of co- and terpolyesters, however, results in an increase of the production costs of the polymer because of the high price of the precursors. 

Xylose is the major sugar present in the hemicellulose fraction of agricultural residues, such as wheat straw. Because the raw material cost is greater than one-third of the overall ethanol production cost [5], fermentation of xylose together with glucose is needed to improve the economics of any lignocellulosic-based bioethanol process. 

In direct microbial conversion of lignocellulosic biomass into ethanol that could simplify the ethanol production process from these materials and reduce ethanol production costs, Clostridium thermocellum, a thermoanaerobe was used for enzyme production, hydrolysis and glucose fermentation (755). Cofermentation with C thermosaccharolyticum simultaneously converted the hemicellulosic sugars to ethanol. However, the formations of by-products such as acetic acid and low ethanol tolerance are some drawbacks of the process. Neurospora crassa produces extracellular cellulase and xylanase and has the ability to ferment cellulose to ethanol 139). 

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