aeration and agitation 20 q

One of the correlations in the literature shows that kLa is proportional to three terms with each term raised to an exponent. What are these three terms?

Gross gassed power over the volume of the liquid, gas superficial velocity based on tank diameter, and impeller rotation speed

is the axial position of one or more impellers on the shaft in the tank important? explain

Yes, its important bc if the distance between 2 axial impellers is less than the diameter of the impeller itself the unaerated power number is significantly less, about 0.5

write in words the material balance for the uptake of oxygen and state assumptions

assume tank is in steady state, assume 2 main resistances and neglect gas, the balance is In = Accumulation

name the two basic types of impellers and explain how they agitate water in the tank. sketch two types of each.

axial, which moves the water vertically and perpendicular from the impeller due to its angled impeller blades, and radial which moves the water horizontally to push the water outward.

why does the use of nitrogen as a sparge gas decrease the concentration of dissolved oxygen?

because its an inert gas and it has a smaller molecular weight meaning that is can force the oxygen molecules up and out of the tank

the literature correlation shows that the ratio of gassed to ungassed power decreased with increasing flow number. why?

because when flow is increased the system density decreases so the impellers have an easier time moving through the fluid (move better through air, more flow means more air)

flux is directly proportional to a driving force and inversely proportional to a resistance. write the differential eq and explain each term

d(Co2)/dt = kLa(C*-C)

at a given temp which has a greater equilib concentration of dissolved O2? sea water, tap water, deionized water

deionized water has the greatest because it doesnt have salt or other minerals/vitamins taking up space in the molecules so it has more room to hold O2 molecules. then it would be tap and then salt water.

describe how dissolved oxygen probe works and how the torque meter works

dissolved oxygen probe is an optical sensor that measures the lifetime of luminescence to determine amount of O2. the torque meter is a torque sensor that converts torque into an electrical signal.

name 5 commercial unit ops that utilize aeration/agitation

fish hatcheries (aeration), CSTRs (agitation), fish tanks (areation), wastewater treatment, maintaining media homogenaity

4 ways to increase mass transfer of oxygen (or increase kLa and constant T)

increase impeller speed, increase gas flow rate, increase number of impellers, increase diameter of impellers

instrument used for measuring temp? how does it work and what 3 other types of measurement devices are used in the lab?

its a thermocouple that works by creating a voltage between 2 different metals that can be converted into a temperature. other devices: pyrometer, thermometer, thermistor

define what the reynolds number, power number, flow number and weber number are a ratio of

reynolds is a ratio of impeller diameter, speed and density to viscosity; power number is a ratio of ungassed power to the impeller speed and diameter; Flow number is a ratio of flow rate to impeller speed and diameter; Weber number is a ratio of impeller speed, diameter and density to surface tension.

using symbols define the following dimensionless numbers

reynolds number is a dimensionless number that helps to obtain...

how does a mass flow controller work? what are the units?

sending a small amount of flow to a sensor that measures overall change in flow; SLPM standard L per min

describe gas sparger used and name 2 other types

the gas sparger helps provide even and consistent gas flow, larger gas-liquid contact area and it is corrosion and temp resistant. other types are pneumatic spargers and air jet spargers

what is the purpose of baffles?

to prevent vortexes and to help create an uneven flow to promote more mixing and agitation

explain two film theory and draw a sketch

two films that are resistances to mass transfer, a liquid and a gas.

does the temp influence the saturated O2 concentration in water? explain. in ppm, what is the equilibrium concentration in water at 25 C?

yes it does because warmer water holds less O2, there is more room for the O2 to escape because of faster moving molecules. 8.3 ppm