Bioreactors in solid state fermentation technology: Design, applications and engineering aspects

In recent years, substantial credibility in employing Solid-State Fermentation (SSF) technique has been witnessed owing to its numerous advantages over submerged fermentation (SmF). In spite of enormous advantages, true potential of SSF technology has not been fully realized at industrial scale. The lack of rational and scalable bioreactor designs backed by mathematical models and automated control system that could successfully address heterogeneity with respect to heat and mass, and also operate aseptically, remains the prime reason for it. As a result, there still exists vast scope in SSF bioreactor research and development to facilitate broad spectrum of biotechnological applications. The present article reviews state-of-the-art in SSF technology with focus on bioreactors that have been employed for bioprocess applications, in particular, enzyme production. Based on the mode of operation, bioreactors are divided into four categories with emphasis on design features, effect of operating conditions on productivity, applications and limitations. Selected modeling studies developed over the years, have been revised and presented in problem specific manner in order to address the limitations. Some interesting designs including few recent ones that have been proposed and/or employed at pilot and industrial levels are discussed in more detail.

Aeration and mass transfer optimization in a rectangular airlift loop photobioreactor for the production of microalgae

Effects of superficial gas velocity and top clearance on gas holdup, liquid circulation velocity, mixing time, and mass transfer coefficient are investigated in a new airlift loop photobioreactor (PBR), and empirical models for its rational control and scale-up are proposed. In addition, the impact of top clearance on hydrodynamics, especially on the gas holdup in the internal airlift loop reactor, is clarified; a novel volume expansion technique is developed to determine the low gas holdup in the PBR. Moreover, a model strain of Chlorella vulgaris is cultivated in the PBR and the volumetric power is analyzed with a classic model, and then the aeration is optimized. It shows that the designed PBR, a cost-effective reactor, is promising for the mass cultivation of microalgae.

Evaluation of power consumption of paddle wheel in an open raceway pond

Open raceway ponds are widely adopted in microalgae cultivation. Paddle wheels consume the most part of power during the process of cultivation in open raceway ponds. The configuration of blades directly determines power consumption for paddle wheels. In this work, power consumption of four blades configurations was determined in a bench-scale open raceway pond of 2.2 m(2). The effect of blades configuration, the influence of filling levels from 5 to 15 cm and influence of rotational speeds from 7 to 15 r min(-1) on shaft power consumption (P(S)), fluid velocity (U(c)) and paddle wheel efficiency (η) was investigated. Results demonstrated that flat blades were the most efficient configuration. Higher culture depth led to larger U(c), more P(s) and larger η, especially when blades were not totally immerged in water. Under the same filling level and rotational speed, the value of P(S) decreases in the order: zigzagged, flat, forward-curved and back-curved, respectively. The zigzagged blades led to a larger U(c) at the culture depth of 5 cm, while flat and forward-curved blades drove a larger U(c) when culture depth was higher than 5 cm. The maximum value of η was 0.50 with flat blades at 11 r min(-1) and 15 cm of culture depth. Empirical correlations of non-dimensional numbers related to operation parameters and blades geometry for four paddle wheel blades were also proposed.