Dilution rate changes

P7-19 ( Lactic acid is produced by a LacUihaciUiK species cultured in a CSTR. To increase the cell concentration and production rate, most of the cells in the reactor outlet are recye ed to the CSTR.. such that the cell concentration in the product stream is 105 of cell coiieentraiion in the reactor. Find the optimum dilution rale that will maximize live rate of tactic acid production in the reactor. How does this optimum dilution rate change if the exit cell concentration fraction is changed [r, = (ap + i)C. 1... 

This complicated commensalistic system is shown to be less stable, with limit-cycle response occurring after dilution rate changes, as would be expected from a model with more parameters. Though virtually any feedback from the dependent to the independent species will cause some overshoot after a step change in D, the most pronounced oscillatory behavior is caused by feedback inhibition and feedforward activation. Limited agreement with experimental data was obtained even though the analysis was somewhat limited due to the complexity of the system and the large number of differential equations. Similar results were obtained by Sheintuch (1980), who examined the dynamics of commensalistic systems with self- and cross-inhibition. Multiplicity of steady states was observed as well as oscillatory states with these complex kinetics and in the case of a reactor with biomass recirculation. Stability and dynamics are summarized in a qualitative phase plane by this author. 

A dilution ventilation rate of at least 6 air changes per hour (ach) is recommended, with 12 or more ach recommended for new construction or renovation. This may not provide sufficient dilution to allow workers to enter without respiratory protection, but it is considered a feasible dilution rate that will reduce the risk of infection for those workers who must enter the room with respiratory protection. Dilution also reduces the contaminant concentration and therefore the risk when temporary leakage from the room occurs such as when doors are opened or closed. 

Neai the wash out, the reactor is very sensitive to variations of dilution rate D. A small change in D gives a relatively large shift in X and S. The rate of cell production per unit volume of reactor is DX. These quantities are shown in Figure 6.5, where there is a sharp maximum in the curve of DX. We can compute maximal cell rate by taking the derivative of DX with respect to D, then solving the equation. The derivative of DX with respect to D is defined as ... 

When a continuous culture is fed with substrate of concentration 1.00 g/1, the critical dilution rate for washout is 0.2857 h-1. This changes to 0.0983 h-1 if the same organism is used but the feed concentration is 3.00 g/1. Calculate the effluent substrate concentration when, in each case, the fermenter is operated at its maximum productivity. 

Fig. 13. The change of the steady-state concentrations as a function of the dilution rate without (curves 1) and with (e = 0.2, thus, E = 2.9 if M = 0) dispersed organic phase (curves 2)...

Next, an experiment was run in which 2.5 g/L of sodium butyrate was added to P2 medium to investigate whether it could be converted to butanol. A control experiment was run containing P2 medium. A separate control experiment was run before each experiment. This is essential because biomass accumulation in the reactor changes with time, thus affecting performance of the reactor (5). The reactor produced 4.77 g/L of total ABE, of which acetone, butanol, and ethanol were 1.51,3.14, and 0.12 g/L, respectively (Table 1). It resulted in a total ABE productivity of 1.53 g/(L-h) and a glucose utilization of 29.4% of that available in the feed of 59.1 g/L. The acid concentration in the effluent was 1.56 g/L. Following this, P2 medium was supplemented with sodium butyrate and the experiment was conducted at the same dilution rate. The reactor produced 1.55 g/L of acetone, 4.04 g/L of butanol, and 0.11 g/L of ethanol, for a total ABE concentration of 5.70 g/L, compared with 4.77 g/L in the control experiment. The productivity was 1.82 g/(L-h), compared with 1.53 g/(L-h) for the control experiment. These experiments suggested that butyrate was used by the culture to produce additional butanol. Note that 0.9 g/L of butanol was produced from 1.65 g/L of butyrate (2.5 g/L in feed, 0.85 g/L in effluent). The yield calculations do not include the amount of butyrate that was utilized by the culture. 

The present study on butanol fermentation has been focused primarily on the effects of pH and dilution rate (D) in continuous cultures of the mutant strain from C. acetobutylicum ATCC 55025. To overcome the problems of low productivity and yield of butanol, cell immobilization in a convoluted fibrous bed bioreactor (FBB) and feeding with dextrose and butyric acid as cosubstrates to produce butanol and reduce production of ancillary byproducts were used in the fermentation. By changing the dilution rate from 0.1 to 1.2 h 1 at pH 4.3 and varying the pH from 3.5 to 5.5 at the dilution rate of 0.6 hr1, the optimal conditions for high productivity and butanol yield were investigated. 

Ueno, Y., Haruta, S., Ishii, M., and Igarashi, Y. (2001). Changes in product formation and bacterial community by dilution rate on carbohydrate fermentation by methanogenic microflora in continuous flow stirred tank reactor. Appl. Microbiol. Biotechnol. 57, 65-73. 

Figure 9.16A shows a typical operational configuration of a continuous bioreactor culture. In Figure 9.16B, the cell concentration data of a real continuous culture, operated at three different dilution rates, are presented. The rectangles indicate the different steady states observed, whereas the arrows show the moment of change in dilution rate. Furthermore, a guide line has been included in Figure 9.16B to help interpretation of the cell concentration profile. 

This means that the dilution rate in the fermentor sets the growth rate of the biomass, and a change in the dilution rate will cause a change in the growth rate. 

P7-29a a CSTR is being operated at steady state. The cell growth follows the Monod growth law without inhibition. The exiting substrate and cell concentrations are measured as a function of the volumetric flow rate (represented as the dilution rate), and the results are shown below. Of course, measurements are not taken until steady state is achieved after each change in the flow rate. Neglect substrate consumption for maintenance and the death rate, and assume that is zero. For run 4, the entering substrate concentration was 50 g/dia and the volumetric flow rate of the substrate was 2 dmVs,... 

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