A closed loop exponential feeding law: Invariance and global stability analysis

This article addresses the computation of invariant control laws [A. Fradkov, I. Miroshnik, V. Nikiforov, Nonlinear and Adaptive Control of Complex Systems, Kluwer, 1999] for fed-batch fermenters represented by two standard models. It will be shown how to derive partial state feedbacks that, assuming ideal conditions and perfect model, keep the specific growth rate μ constant provided the initial conditions are adequate. The invariant control law is the closed loop version of the exponential feeding already suggested in several references as shown later. The paper presents an analysis of invariance and a study of global stability within the framework of partial stability. That is, stability with respect to some of the state variables. This enables us to treat the case with Haldane-like or non-monotonous kinetics.     

Controlled feeding of lignocellulosic substrate enhances the performance of fed-batch enzymatic hydrolysis in a stirred tank reactor

High initial viscosity in the high-solids (>15% (w/v)) enzymatic hydrolysis of lignocellulose is problematic especially in stirred tank reactor concepts. One potential way to avoid the high viscosity is the fed-batch feeding of lignocellulosic material to the reactor. In the current study the hydrolysis of filter paper with final concentration of 19.1% (w/w) was evaluated with different fed-batch procedures. Feeding was based on visual observation, stepwise feeding and the power requirement of the stirrer motor. All the fed-batch procedures resulted in similar yields within 30 h (47–49%) which were higher than with the batch process in similar reactor (38%). However, the mixing behavior was superior in the power based feeding as the instantaneous power of the stirrer motor was kept lower (<10 W) than in other fed-batch procedures (>20 W). The power controlled procedure was further evaluated with different enzyme doses, tip speeds and the power levels of substrate feed. Further study showed that the power controlled feeding is applicable also to other hydrolysis and mixing conditions if power levels of substrate feed are set correctly. Higher (15 FPU/g) enzyme dose caused shorter feeding time (3.0 ± 0.5 h) and lower energy consumption during the feeding period (14 ± 3 Wh) compared with lower (5 FPU/g) enzyme dose (7.0 ± 1.3 h and 33 ± 5 Wh, respectively). The tip speed and the power level of substrate feed had fewer effect on these factors. The performance of the hydrolysis process can thus be enhanced by the substrate feed controlled by the power of the stirrer motor.     

Design study of an AnSBBR for hydrogen production by co-digestion of whey with glycerin: Interaction effects of organic load,cycle time and feed strategy

An anaerobic sequencing batch biofilm reactor (AnSBBR) treating a mixture of dairy industry wastewater and biodiesel production wastewater (co-digestion of whey with glycerin) was applied to hydrogen production. The influence of fed-batch and batch mode, cycle time and interactions effects between influent concentration and cycle time (2, 3 and 4 h) over the organic loading rate were assessed in order to obtain a sensitivity analysis for important operational variables to the reactor. It was possible to find an optimal cycle time of 3 h with an influent concentration of 7000 mgCOD L^?1 (molar productivity 129.0 molH₂ m^?3 d^?1 and yield 5.4 molH₂ kgCOD^?1). Reactor operation in fed-batch mode allowed higher hydrogen production rates. Increasing the influent concentration (with a constant cycle time) was better for the hydrogen production process than decreasing the cycle length (with a constant influent concentration), which means that these two parameters have different weights in the organic loading rate. The best operational conditions produce hydrogen via acetic, butyric and valeric acids similarly. The system is able to produce 1.3 kJ per gram of COD applied.     

Influence of inoculum cell density and carbon dioxide concentration on fed-batch cultivation of Nannochloropsis oculata

The marine microalgae Nannochloropsis oculata is a promising source of biofuel because of its high lipid content. For achieving high productivity of oil from microalgae, a high cell concentration before harvesting is beneficial. The present study investigated fed-batch cultures of N. oculata fed with vitamins and nutrient solutions and found that the biomass yield of N. oculata in the fed-batch culture was 1.25 times higher than that in batch culture. Fed-batch cultivation, especially at high illumination, decreased the inhibitory effect of high carbon dioxide (CO₂) concentration on the microalgal growth. The specific growth rate was directly proportional to the light intensity in the CO₂ environment. A light intensity of 40,000 Lux was able to achieve high specific growth rates in fed-batch cultivation at a CO₂ volume fraction of 2%–15%. The tolerance of N. oculata to CO₂ was enhanced by the daily feeding of nutrients in the fed-batch cultivation. At 2% CO₂, a final cell density of about OD₆₈₂ = 11.4 was achieved in the fed-batch culture in 30 days. Furthermore, a cell density of 14.4 g L⁻¹ was obtained by outdoor fed-batch cultivation in 27 days.     

Lipid production in Porphyridium cruentum grown under different culture conditions

Autotrophic growth of Porphyridium cruentum under 18:12 h and 12:12 h light:dark cycles showed the maximum cell concentration of 2.1 g-dry wt./L, whereas the specific growth rate, 0.042 (1/h), at 18:6 h is faster than that of 12:12 h, 0.031 (1/h), respectively. The highest lipid accumulation level, 19.3 (%, w/w), was achieved at 12:12 h cycle. Under dark cultivation condition with 10 g/L of glucose, the lipid accumulation in the cell was 10.9 (%, w/w), whereas the heterotrophic growth with glycerol as the carbon resource showed low level of cell concentration and lipid production, compared to that of glucose. The glucose was decided to be a suitable carbon resource for the heterotrophic growth of P. cruentum. The lipids from P. cruentum seemed be feasible for biodiesel production, because over 30% of the lipid was C₁₆–C_18:1. The cultivation time and temperature were important factors to increase the maximum cell concentration. Extending the cultivation time helps maintain the maximum cell concentration, and higher lipid accumulation was achieved at 25 °C, compared to 35 °C. The fed-batch cultures showed that, under the light condition, the specific production rate was slightly decreased to 0.4% lipid/g-dry wt./day at the later stage, whereas, under the dark condition, the specific production rate was maintained to be a maximum value of 1.1% lipid/g-dry wt./day, even in the later stage of cultivation. The results indicate that the heterotrophic or 12:12 h cyclic mixotrophic growth of P. cruentum could be used for the production of biodiesel in long-term fed-batch cultivation of P. cruentum.     

Optimal control of dilute acid pretreatment and enzymatic hydrolysis for processing lignocellulosic feedstock

Lignocellulosic feedstock is one of the potential renewable sources for producing ethanol for transportation. The process steps viz., acid pretreatment and enzymatic hydrolysis in bio-chemical process route are intended to produce fermentable sugars, which can be readily fermented for producing ethanol. However, the dilute acid pre-treatment and enzymatic hydrolysis process steps are found to be economically inefficient. The present work aims at optimizing these process steps for improving the process performance. Such optimization is expected to increase conversion, reduce energy or material requirement, thereby improving the economics. The kinetic models of acid pretreatment and enzymatic hydrolysis for lignocellulosic feedstock processing are adapted from literature. Subsequently, these kinetic models are augmented by associated mass and energy balances, to develop a batch reactor model and fed-batch reactor model for dilute acid pretreatment and enzymatic hydrolysis processes, respectively. Optimal control with Pontryagin's maximum principle has been implemented to determine the optimal time dependent profiles of heating and cooling fluid flow rates and operating temperatures for acid pretreatment and substrate feed rate profile for enzymatic hydrolysis to optimize the respective processes performance. Different objective functions such as maximizing concentration of desired product, minimizing the batch time, and maximizing profit have been considered. The simulation results yielded an increase of 6.7% and 8.8% in final concentration of desired product; 43% and 42.5% reduction in batch processing time for pretreatment and enzymatic hydrolysis processes, respectively. Finally, the simulation results have also provided optimal operating policies which have increased the profit of pretreatment by 124% and enzymatic hydrolysis by 150%, thereby improving the techno-economic feasibility for processing lignocellulosic feedstock.     

Production of Beta-Carotene from Synthetic Medium by Blakeslea trispora in Fed-batch Culture

The production of β-carotene from a synthetic medium by Blakeslea trispora in fed-batch culture was investigated. A maximum β-carotene concentration of 85.0 mg L^-1 with productivity of 0.16 mg L^-1 h^-1 and specific β-carotene production rate of 0.01 mg g^-1 h^-1 was obtained by feeding the cells constantly with olive oil, cottonseed oil, soybean oil, antioxidant, and low concentration of growth factors at feeding rate of 4.2 mL h^-1 from the beginning of the fermentation. In this case, the fed-batch culture supported high values of biomass dry weight (11.0 g L^-1) and sugar utilization (0.976 g g^-1). The morphology of the fungus was studied during growth in fed-batch fermentation system using an image analysis system. Zygospores are the morphological forms, which are responsible for the production of the pigment. The highest percentage of zygospores (11.44%) was correlated with the highest percentage of intracellular β-carotene (0.72%) in the total biomass dry weight. Moreover, high percentages of vacuolated hyphae, evacuated cells, and degenerated hyphae of the microorganism were observed. This was due to the formation of high amount of H₂O₂ by exposure of the microorganism to high dissolved oxygen concentration.     

Bioethanol production from wheat straw by the thermotolerant yeast Kluyveromyces marxianus CECT 10875 in a simultaneous saccharification and fermentation fed-batch process

Solid content in the simultaneous saccharification and fermentation (SSF) broth should be as high as possible in order to reach higher ethanol concentration. In this work, several feeding strategies for ethanol production from steam-exploded wheat straw by Kluyveromyces marxianus CECT 10875 have been studied with the aim of obtaining higher ethanol concentrations. Previous fermentability tests as well as SSF processes showed the difficulty of using the slurry for ethanol production under the studied conditions. Notwithstanding, fed-batch SSF processes with water-insoluble solids (WIS) fraction resulted in better configuration, reaching the highest ethanol concentration (36.2 g/L) with an initial WIS content of 10% (w/v) and 4% (w/v) of substrate addition at 12 h, which meant 20% more ethanol when compared with batch SSF.