Bioreactors interfacial area

The mass transfer, KL-a for a continuous stirred tank bioreactor can be correlated by power input per unit volume, bubble size, which reflects the interfacial area and superficial gas velocity.3 6 The general form of the correlations for evaluating KL-a is defined as a polynomial equation given by (3.6.1). 

Gas-Liquid Mass Transfer. Gas-liquid mass transfer within the three-phase fluidized bed bioreactor is dependent on the interfacial area available for mass transfer, a the gas-liquid mass transfer coefficient, kx, and the driving force that results from the concentration difference between the bulk liquid and the bulk gas. The latter can be easily controlled by varying the inlet gas concentration. Because estimations of the interfacial area available for mass transfer depends on somewhat challenging measurements of bubble size and bubble size distribution, much of the research on increasing mass transfer rates has concentrated on increasing the overall mass transfer coefficient, kxa, though several studies look at the influence of various process conditions on the individual parameters. Typical values of kxa reported in the literature are listed in Table 19. 

A recent option is that found in wave bioreactors, where the generation of waves increases oxygen transfer by augmenting both the interfacial area (a) and the global transfer coefficient kL (Singh, 1999). Therefore, these reactors are already available commercially at volumes up to 500 L. 

The main parameters having an influence on the performance of the process are the absorber volume, the interfacial area, the flow rate and the total concentration of Fe complexes in the absorber-inlet liquid stream, and the performance of the bioreactor reflected in the Damkohler numbers Da] and Da2. We start by building a base-case simulation, where v = 0.03m3/s, C,otai = 40mol/m3, and the large values Daj = Da2 = 100 ensure that the regeneration step does constrain the performance. 

Sizing of the absorption column started from a base case that assumed complete recovery of FeEDTA2- in the bioreactor. Then, sensitivity studies provided the values of the G/L interfacial area and of the absorber volume giving maximum performance. The values of the two Damkohler numbers characterizing reactor performance were found after relaxing the assumption of complete FeEDTA2-recovery. Finally, the specification of NO concentration in the purified gases was checked, for different feed conditions. 

Chapter 12 handles the design of a Biochemical Process for NOx Removal from flue gases. The process involves absorption and reaction steps. The analysis of the process kinetics shows that both large G/L interfacial area and small liquid fraction favor the absorption selectivity. Consequently, a spray tower is employed as the main process unit for which a detailed model is built. Model analysis reveals reasonable assumptions, which are the starting point of an analytical model. Then, the values of the critical parameters of the coupled absorber-bioreactor system are found. Sensitivity studies allow providing sufficient overdesign that ensures the purity of the outlet gas stream when faced with uncertain design parameters or with variability of the input stream. 

Simple transmission measurements with inexpensive components were made to estimate the local specific interfacial area of a suspended phase (i. e. of gas bubbles) in a bioreactor [473]. 

The reciprocating motion of the sieve plate generates vortices in the biosuspension. Each vortex region represents the elementary volume of the bioreactor. When the gas dispersion element moves upwards, the biosuspension is forced to pass through the holes of the sieve plates From each hole, a jet of biosuspension flows downward into the space between two sieve plates. The jet reverses direction as the element reverses direction. Very effective dispersive action is due to the periodic generation of bubbles, which renews the larger interfacial area on each reversal of direction. The important design characteristics of this reactor are summarized in Table XXV. 

The gas-liquid interfacial area (a) is a fundamental parameter in designing bioreactors because the knowledge of this parameter is required to calculate individual gas-liquid mass transfer rates (Vasquez et al., 2000). The interfacial area is a challenge to quantify because it is influenced by the bioreactor geometry and operating conditions, as well as the physical and chemical properties of the gas-liquid system. In some cases, the interfacial area is estimated by assuming a uniform bubble diameter and measuring the overall gas holdup e. In this case, the gas-liquid interfacial area is estimated from Chisti (1989) ... 

The bioreactor geometry effects in an ELALR can be quite complex and dynamic. As the area ratio increases, the liquid circulation velocity decreases. Hence, the gas-phase circulation time decreases and gas holdup increases. The increase in gas holdup leads to an increase in the interfacial area. Some bubble dynamics are reflected in the growth, but, due to the lower bubble-bubble interactions in ELALRs, the increase is fairly continuous, but at a relatively slow rate. For example, Joshi et al. (1990) showed that by increasing the area ratio from 0.25 to 1.0 using a 10-m ELALR at 0.3kW/m yielded a negligible increase... 

The term ki a is comprised of ki (gas mass transfer coefficient), which is influenced by the medium composition, while a represents the interfacial area of all bubbles in the bioreactor, which is influenced by aeration, coalescence, and... 

In a bioreactor, one is interested in the transfer per unit of volume of reactor, called Kia or the volumetric mass-transfer coefficient, a is the interfacial surface area per unit of volume of liquid. In a perfectly mixed tank, C has identical values at any point and C depends on the conditions in the gas phase at the outlet of the reactor. Several authors [60] consider that a better estimate of the driving force is given by the logarithmic mean concentration difference between the entry and the exit of gas. 

The area ratio effects on the liquid-phase mass transfer coefficient are more difficult to predict. Area ratio effects are usually studied by keeping the bioreactor volume equal, which requires the effective bioreactor height to be adjusted. As the height is increased, the interfacial solute gas concentration increases as well, which decreases the gas solubility and, in turn, the liquid-phase mass transfer coefficient. In addition, an increase in the area ratio decreases the liquid circulation rate, which increases gas holdup, but may decrease surface renewal. The greater height also raises the pressure drop and power consumption, which increases surface renewal and the liquid-phase mass transfer coefficient. The extent of these effects is dependent on the operational scale and power level, and it is hard to predict which will dominate. 

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