Stirrer speed

N = stirrer speed, rpm. Method assumes perfect liquid mixiug. 

The effeet of temperature satisfies the Arrhenius relationship where the applieable range is relatively small beeause of low and high temperature effeets. The effeet of extreme pH values is related to the nature of enzymatie proteins as polyvalent aeids and bases, with aeid and basie groups (hydrophilie) eoneentrated on the outside of the protein. Einally, meehanieal forees sueh as surfaee tension and shear ean affeet enzyme aetivity by disturbing the shape of the enzyme moleeules. Sinee the shape of the aetive site of the enzyme is eonstrueted to eoirespond to the shape of the substrate, small alteration in the strueture ean severely affeet enzyme aetivity. Reaetor s stirrer speed, flowrate, and foaming must be eontrolled to maintain the produetivity of the enzyme. Consequently, during experimental investigations of the kineties enzyme eatalyzed reaetions, temperature, shear, and pH are earefully eontrolled the last by use of buffered solutions. 

The minimum stirrer speed required to just suspend the partiele. v, A js, may be ealeulated by the semi-empirieal equation due to Zweitering (1958)... 

Similarly, the dependenee j of nueleation rate B on magma density Mt and stirrer speed N in MSMPR erystallizers given by... 

Where most values of i a 1, within the range 0.14-1.07. Additionally, a dependenee on stirrer speed k is observed being in the range 0 < /c < 7.8 (Garside and Shah, 1980). Thus the observed nueleation rates in MSMPR erystallizers depend on both magma density and stirrer speed. They are thus likely to have been due to seeondary rather than primary nueleation. Further evidenee suggests these effeets result from erystal/solution interaetions (see Chapter 5). 

This, and other similar equations, show that seale-up of seeondary nueleation kineties on the basis of eonstant speeifie power input, e, should at most produee only a modest inerease in nueleation rate. Constant tip speed or eonstant stirrer speed to just maintain the erystals in suspension should both result in a deerease in nueleation rate with inereasing seale of operation (Garside and Davey, 1980). 

The dependenee of the nueleation rate on stirrer speed is strongly influeneed by the feed point position and the feed eoneentration. The small influenee of the stirrer speed and lienee the power input on the nueleation rate is observed for the feed point positions in a zone of small turbulenee (od). For the feed point inside the draft tube (id) and a residenee time of 660 s, reaehes a maximum at... 

The nucleation rate is plotted versus the supersaturation for different stirrer speeds in a log-log diagram (Figure 6.21). The kinetic order n in the correlating equation... 

The expeeted inerease of the disruption rate with the stirrer speed (power input) was eonfirmed for all the experiments (Figure 6.23). 

For stirrer speeds of 4.2, 8.4, 16.7, 25 and 33.4Fiz, agglomeration kernels obtained in this study vary from 0.01 to 183 s . Unfortunately, no other measured data for agglomeration of calcium oxalate analysed using Smoluchowski s kernel were found in the literature. The corresponding values reported by Wojcik and Jones (1997) for calcium carbonate, however, cover a range from 0.4 to 16.8s-. ... 

The conventional scale-up criteria scale-up with constant stirrer speed , scale-up with constant tip speed and scale-up with constant specific energy input are all based on the assumption that only one mixing process is limiting. If, for example, the specific energy input is kept constant with scale-up, the same micromixing behaviour could be expected on different scales. The mesomixing time, however, will change with scale-up as a result, the kinetic rates and particle properties will be different and scale-up will fail. 

The usual type of glass paddle stirrer is also widely used in conjunction with an electric motor fitted with either a transformer-type, or a solid-state, speed controller. The stirrer may be either connected directly to the motor shaft or to a spindle actuated by a gear box which forms an integral part of the motor housing by these means, wide variation in stirrer speed can be achieved. 

ITie inside film coefficient was found to be l.l kW/nr K for a stirrer speed of 1.5 Hz and to be proportional to the two-thirds power of the speed of rotation. 

The main reasons for the damage to cells in a reactor are the apparent shear forces and the collision of microcarriers with themselves and with turbulent eddies. In the literature studies are mainly focused on suspension cells and there again on hybridoma cells. The work reported in the hterature can be divided into two fields studies dealing with the influence of various stirrer speeds on cell viability and those investigating the influence of defined shear forces on cells with a viscosimeter. 

The shear forces are mainly in the range of 1 to lONm. This exposure causes cell death between 20 and 80% depending on the exposure duration which is between a few seconds and several hours. Studies performed in a bioreactor have an exposure duration of several days. The results are partly contradictory. Tramper et al. [30] found a critical stress level of 1.5 Nm" for insect cells, whereas Oh et al. [31] could not show an influence on hybridoma cells even at high stirrer speed. This shows that each cell line reacts different and that there is a necessity for defined stress systems if the results is to be comparable. 

Concerning adherent cells there are few studies in the literature. Some of them deal with the influence of stirrer speed on microcarrier cultures. Most studies using defined forces are from medical research. These studies, as well as those with production cells, use different types of exposure systems based on the parallel plate theory. They investigate the influence of stress on cell morphology and viability which is most important for arteriosclerosis research. 

The presence of a gas in the suspension results in an increase of the stirrer speed required to establish the state of complete suspension. The propeller usually requires a higher speed than the turbine. Furthermore, a critical volume gas flow exists above which drastic sedimentation of particles occurs. Hence, homogenisation of the suspension requires an increase of the rotational speed and/or a decrease of the gas flow rate. The hydrodynamics of suspensions with a solid fraction exceeding 0.25-0.3 becomes very complex because such suspensions behave like non-Newtonian liquids. This produces problems in the scale-up of operations. Hydrodynamics, gas hold-up, mass-transfer coefficients, etc. have been widely studied and many correlations can be found in literature (see e.g. Shah, 1991). 

The influence of gassing rate and stirrer speed on an oxidation reaction, in an aerated batch reactor, are to be investigated. The outlet gas is assumed to be essentially air, which eliminates the need for a gas balance for the well-mixed gas phase. 

Vary stirrer speed N and aeration rate G and observe the response in dissolved oxygen Cq and product concentration Cp. 

The results in Figs. 5.19 and 5.20 show the influence of stirrer speed and aeration rate. 

Figure 5.19. Response of dissolved oxygen and product concentration for a change in stirrer speed from N = 500 to 1000 to 100 rpm.

The preparations were carried out as usual in a three necked flask fitted with a stainless steel stirrer, condenser, and nitrogen inlet (10,11). The solvents and dichlorldes were added by syringe. The sodium was added as a freshly cut block and, prior to the reaction, melted in the refluxing solvent and stirred to form sand. The overall surface area of the sodium could be controlled by the stirrer speed. When necessary the stirrer speed was measured by a tachometer. 

The oxidative dehydrogenation of ethanolamine to sodium glycinate in 6.2 M NaOH was investigated using unpromoted and chromia promoted skeletal copper catalysts at 433 K and 0.9 MPa. The reaction was first order in ethanolamine concentration and was independent of caustic concentration, stirrer speed and particle size. Unpromoted skeletal copper lost surface area and activity with repeated cycles but a small amount of chromia (ca. 0.4 wt%) resulted in enhanced activity and stability. 

Figure 5 Effect of stirrer speed during ethanolamine dehydrogenation over unpromoted skeletal copper under standard conditions for particles with different sizes.

In a typical experiment, 50 g of NaOH flakes were first dissolved in 120 g of water. The required amount of catalyst was then wet loaded into the caustic solution and 72 g of ethanolamine added. Before heating the autoclave was closed and the air inside purged out with N2. The autoclave was then heated with the set temperature (433 K) reached after about 80 minutes. This time is taken as zero in the plots that follow. The standard operating conditions used for catalyst evaluation unless otherwise stated were as follows temperature 433 K pressure 0.9 MPa ethanolamine concentration 2.9 M NaOH concentration 6.2 M stirrer speed 80 rpm catalyst 8 g with particle size 106-211 pm. 

Typical concentration-time relationship for experimental runs. Temperature, 26°C pressure, 746 mm of mercury hydrogen flow, 30.7 x 10 5 mole/sec catalyst weight, 0.975 g stirrer speed, 1100 rpm slope, 13.7 x 10 5 mole/liter/sec. 

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