Dilution rate

Continuous processes may be used for the production of yeast biomass. Raw Hquid feed is added continuously to the fermentor and an equal volume of fermentor Hquid is removed to harvest the yeast ceUs. These may be a single homogeneous fermentation in a stirred fermentor or two fermentors in series. Growth rates are high a typical dilution rate in the production of C. utilis on sulfite waste Hquor is 0.25, ie, one-fourth of the fermentor volume is harvested hourly. 

Candida utilis is grown on sulfite waste Hquor in Western Europe and North America, on sugar cane molasses in Cuba and Taiwan and on ceUulose acid hydrolysates in Eastern Europe and the former Soviet Union. C. ///i/if utilizes hexoses, pentoses, and many organic acids. Sulfite Hquor from hardwoods contains 2—3% fermentable sugars of which 20% are hexoses and 80% pentoses in softwood Hquors the proportions are reversed. The SO2 must be stripped out to allow yeast growth, which is carried out in large, highly-aerated fermentors. Eor continuous fermentations, carried out at pH 4 and 30°C, the dilution rate is 0.27—0.30 (34). 

The nutrition and the product mix can be advantageously manipulated as functions of dilution rate. A serious problem, however, is... 

From these equations, the behavior of X and S as functions of dilution rate can be plotted as in Fig. 24-22. 

The interesting features are (1) X goes to zero and S reaches as D approaches [1 (2) S is not a function of when D is less than (3) the maintenance coefficient is very important at low dilution rate but has httle effect afterwards and (4) is never so high that [L can be reached, thus washout always occurs before [L and is a function of S. ... 

The dilution rate D is equal to 1/t, where t = u/V, and is equal to the mean residence time of the tank. Equation 11-63 shows that controlling the dilution rate D (Le., the flowrate into the reactor) can control the specific cellular growth rate of a microbial species p. 

A eonsequenee of applying Equations 11-68 and 11-69 respeetively in deseribing the ehemostat, is that the maximum dilution rate is limited to a value that is slightly less than the maximum speeifie growth rate Pmax- This is referred to as the eritieal dilution rate D. ... 

The dilution rate at whieh washout oeeurs ean be determined by setting C, = 0 in Equation 11-69. This beeomes... 

The mierobial growth rate (and henee the dilution rate) for maximum produetion (or eonsumption) must be below the maximum speeifie growth rate (and henee the eritieal dilution rate). Therefore, Equation 11-73 is given by... 

Eigure 11-20 shows produetion rate, eell eoneentration, and substrate eoneentration as funetions of dilution rate. Erom Equation 11-74, the maximum produetion rate ean be determined. 

Fig ure 11-20. Cell concentration and production rate as a function of dilution rate. (Source Fogler [19].)... 

Dilution rate A measure of the rate at whieh the existing medium in eontinuous eultivation of organisms is replaeed with fresh medium. 

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. 

Design arrangements for reduced-temperature secondary systems (sometimes referred to as injection circuits) include fixed provisions for minimum dilution rates. Conventional system-balancing devices with three-port automatic modulating valves to regulate mixed water temperatures and, hence, heat output are used. Automatic safety controls must prevent excessive temperatures occurring in the coil circuits, as floor fabrics or finishes could be damaged very rapidly. 

D = dilution rate (flow rate/culture volume units of h1) x = steady state product concentration (units kg m 3). 

We can see that for type 1 processes, high growth rate is obligately linked to a high rate of product formation. Indeed, this is the case for all products produced by a fermentative mode of metabolism, eg ethanol, lactic add, acetone. Chemostat studies have shown that for most aerobic processes when growth is limited by some nutrient other than the carbon source, the yield of product decreases with increase in spedfic growth rate (p or D p = dilution rate (D) in chemostat culture). Conversely, both the biomass yield and the spedfic rate of substrate utilisation (qs g substrate g biomass-1 h-1) increase with spedfic growth rate. 

The relationships between spedfic rate of substrate consumption and dilution rate and between yield coeffidents and dilution rates are shown in Figure 32. 

Figure 3.2 Theoretical relationships for (a) qs against dilution rate and for (b) Yp/S and Yx/s against dilution rate. The micro-organism is grown aerobically in a nitrogen limited chemostat culture.

J""J Can you think of a disadvantage of operating a process at low dilution rate ... 

Aim3 aerobic bioprocess was operated in a continuous mode with nitrogen as the growth limiting nutrient. The steady state biomass concentration (x), the biomass yield coefficient (Yx/S) and the product yield coefficient (Yp/t) were determined at a low and at a high dilution rate (D). 

Which of the two dilution rates should the process be operated at (Hint compare productivities for the product). Note that biomass productivity = D.x). 

The observed values of Y /, at the different dilution rates can also be used to determine Y values. This is achieved by defining Yx/S and Y in terms of their respective... 

A bacterium was grown as a glucose-limited chemostat culture and steady state growth yield (Yx/J was measured at different dilution rates. 

In practice, carbon limited chemostat cultures are used to estimate the P/O quotient These conditions are used because they favour the most efficient conversion of the carbon substrate into cellular material, ie the highest efficiency of energy conservation. The steady state respiration rate (qo,) is measured as a function of dilution rate (specific growth rate) and Yq can be obtained from the reciprocal of the slope of the plot. qo, is also known as the metabolic quotient for oxygen or the specific rate of oxygen consumption. 

Studies show that the production of 1kg dry biomass requires 2.0 kg sugar, 0.7 kg oxygen, 0.1 kg ammonia, with the liberation of 12,300 k Joules heat. A typical continuous fermentation operates at a dilution rate (D) = 0.2 h 1, with sugar concentration of 3% (w/v) in the incoming medium. With a fermentor of 50 m3 capacity and 90% utilisation of carbohydrate [ie 0.3% (w/v) sugar in the outgoing medium] what would be ... 

Productivity. The continuous system operates such that the biomass concentration remains constant (specific growth rate = dilution rate). Productivity is therefore given by the biomass concentration x the dilution rate. 

Compare a) the minimum OTR and b) heat evolution rate of a continuous fermentation system based on n-alkanes operating at a dilution rate of 0.2 h 1 and a biomass concentration of 13.5 kg m 3, to a similar system based on carbohydrate. You may need to look back a few pages to get the relevant information concerning carbohydrate utilisation (Section 4.7). 

False. Batch cultures can convert high proportions of substrates, as growth can be allowed to proceed until substrates are exhausted. In continuous cultures substrates are never fully converted, as medium is continuously removed. In fact, residual substrate concentration increases as the dilution rate increases, until virtually all of the medium remains unused. Continuous cultures usually recycle the medium after biomass removal to increase the efficiency of substrate conversion. 

Continuous cultures can be ru n at different dilution rates. An optimum rate must be chosen, taking account of output, substrate utilisation and aeration and cooling requirement. We will work this out for ourselves later on. 

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