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Batch cultivation of Azadirachta indica hairy roots was carried out in different liquid-phase bioreactor configurations (stirred-tank, bubble column, bubble column with polypropylene basket, and polyurethane foam disc as root supports) to investigate possible scale-up of the A. indica hairy root culture for in vitro production of the biopesticide azadirachtin. The hairy roots failed to grow in the conventional bioreactor designs (stirred tank and bubble column). However, modified bubble column reactor (with polyurethane foam as root support) configuration facilitated high-density culture of A. indica hairy roots with a biomass production of 9.2 g l-1dry weight and azadirachtin yield of 3.2 mg g-1 leading to a volumetric productivity of azadirachtin as 1.14 mg l-1 day-1. The antifeedant activity in the hairy roots was also evaluated by no choice feeding tests with known concentrations of the hairy root powder and its solvent extract separately on the desert locust Schistocerca gregaria. The hairy root powder and its solvent extract demonstrated a high level of antifeedant activity (with an antifeedant index of 97 % at a concentration of 2 % w/v and 83 % at a concentration of 0.05 % (w/v), respectively, in ethanol). � 2013 Springer Science+Business Media New York.

In this paper we analyze the behavior of a coupled reactor-separator system with reactant recycle. The reactor is represented by a continuously stirred tank reactor, and the separator is represented by a flash unit. The reactor is operated isothermally and sustains a first-order reaction A ? B. The individual units always possess a unique, stable, and feasible steady state. Surprisingly, even for the simple model system considered here, more complex patterns of behavior-involving infeasibility, multiple steady states, and limit cycles-can be observed when the recycle is closed. It is shown that the behavior crucially depends on the flow and the flash control strategy. Stability criteria are derived for different flow and flash control strategies. They depend on the operating conditions and on the basic physicochemical properties of the mixture. Potential sources for instability are systematically identified and illustrated.

In this work we investigate the behavior of a coupled reactor - separator system. The reactor considered is a continuous stirred tank reactor, and the coupling between the two units occurs via recycle of the reactant-rich stream (assumed to be the bottoms) from the downstream flash to the upstream reactor. We investigate two reactions: (i) an irreversible isothermal first-order reaction A ? B and (ii) an elementary cubic autocatalytic isothermal reaction of the form A + 2B ? 3B. The separator is modeled as a flash unit operating isothermally and isobarically. The feed to the flash is restricted to be a binary mixture. This uniquely fixes the composition of the two effluent streams, leaving the flash unit. The main focus in this work is to study the effect of delay arising from the transportation lag from the reactor to the separator. The behavior of the system is governed by delay differential equations. We want to study how the dynamic instability induced by delay crucially depends on the flow control strategy of the coupled reactor - separator system. It is shown that delay does not induce any instability if the fresh feed flow rate F0 is flow-controlled and the molar holdup is controlled using the reactor effluent flow rate F. In the second control strategy where the reactor effluent flow rate F is flow-controlled and the holdup is controlled using fresh feed flow rate F0, delay above a threshold value can induce instability for the two reaction systems considered. In the third control strategy, both F and F0 are flow-controlled and the holdup in the reactor is allowed to vary. Here again delay beyond a critical value can introduce a new region of instability for the isothermal first-order reaction, and it increases the region of dynamic instability for the cubic autocatalytic reaction. The main result of the paper is to demonstrate that some of the preferred control structures of Luyben that are stable can be rendered unstable by sufficiently large delay.