Comparison between two policy strategies for scheduling trucks in a biomass logistic system.

Approximately 75% of the cost to load, haul, and deliver a weekly supply of herbaceous biomass from temporary storage locations near the production fields to a bioprocessing plant (50Mg/h average capacity, 24/7 operation) is truck cost. The management policy that a bioprocessing plant uses to schedule trucks determines the maximum number of trucks required, and thereby, the total cost for the logistic system. Three land use rates corresponding to 50%, 45%, and 40% of existing pastureland within a 3.2-km radius of chosen satellite storage locations were used to establish a production base surrounding the plant location. Total area harvested was 25,500 ha, or about 2.1% of the total land area in the 7-county region studied. Assumed average yield was 8.3Mg/ha. Two different management policies, one based on travel time (Policy 1) and another based on the assignment of trucks to given sectors of the surrounding production base (Policy 2) were used to develop truck schedules. The logistic system was modeled as a discrete event simulation model, and the schedule was validated. The maximum number of trucks needed for the logistic system was 32, 33 and 34 for 50%, 45% and 40% land use rates, respectively. In Policy 1, the maximum number of loads accumulated in the at-plant inventory was 384 truckloads at 50% land use rate (maximum inventory corresponds to about 3 days of plant operation). In Policy 2, the maximum number of loads accumulated in the at-plant inventory was 330 truckloads at 50% land use rate. Total number of loader and unloader operating hours for both the policies was computed, and the loader and unloader utilization rates were 83.5% and 70.8%, respectively. The delivered cost (load, haul, and unload) varied from USD14.68 (Policy 1) to USD16.14 per Mg (Policy 2) for 15% w.b. moisture content biomass.

Scale-up of microbubble dispersion generator for aerobic fermentation.

A laboratory-scale microbubble dispersion (MBD) generator was shown to improve oxygen transfer to aerobic microorganisms when coupled to the conventional air-sparger. However, the process was not demonstrated on a large scale to prove its practical application. We investigated the scale-up of a spinning-disk MBD generator for the aerobic fermentation of Saccharomyces cerevisiae (baker's yeast). A 1-L spinning-disk MBD generator was used to supply air for 1- and 50-L working volume fermentation of baker's yeast. For the two levels investigated, the MBD generator maintained an adequate supply of surfactant-stabilized air microbubbles to the microorganisms at a relatively low agitation rate (150 rpm). There was a significant improvement in oxygen transfer to the microorganism relative to the conventional sparger. The volumetric mass transfer coefficient, kLa, for the MBD system at 150 rpm was 765 h(-1) compared to 937 h(-1) for the conventional sparger at 500 rpm. It is plausible to surmise that fermentation using larger working volumes may further improve the kLa values and the dissolved oxygen (DO) levels because of longer hold-up times and, consequently, improve cell growth. There was no statistically significant difference between the cell mass yield on substrate (0.43 g/g) under the MBD regime at an agitation rate of 150 rpm and that achieved for the conventional air-sparged system (0.53 g/g) at an agitation rate of 500 rpm. The total power consumption per unit volume of broth in the 50-L conventional air-sparged system was threefold that for the MBD unit for a similar product yield. Practical application of the MBD technology can be expected to reduce power consumption and therefore operating costs for aerobic fermentation.