01-11-2012, 05:53 PM
MICROFILTRATION AND ULTRAFILTRATION
MICROFILTRATION.ppt (Size: 543 KB / Downloads: 43)
INTRODUCTION
Membrane separation: feed streams raging from gases to colloids
Microfiltration (MF): retain colloidal particles of several micrometers
Gas separation membranes: molecules of 0.3nm, resolution in diameter 0.02nm
Effective separation diameters: ration of ~104
Ultrafiltration (UF): next larges pores after MF
UF and MF: similar, but very different historical background
Membrane mediated fractionation: separation of a stream into two fractions on the basis of molecular or particulate size – primary use of UF, significant application of MF
Both processes: size exclusion principle; developed for aqueous separations (MF also used in gas-phase filtration)
Hemodialysis (artificial kidney):
relatively young
similar membrane, but different driving force for mass transfer (pressure not concentration difference)
Historical
1846 – discovery of nitrocellulose,
1855 – cellulose nitrate membranes
Developed for decades (mostly in Germany)
Gertrude Mueller at the Hygiene Institute, University of Hamburg: the micro flora from a large volume of water could be deposited intact on a small disk of microfiltration membrane; by culturing the membrane and counting the colonies, rapid and accurate determinations of the safety of drinking water could be made
MF technology: 1950s
RO and UF came much later in time, neither developed from MF (UF derived from RO)
Crossflow Filtration
Equation (1.5): two limiting cases
First, in the absence of any filterable matter no deposit on or accumulation at the membrane Rc = 0, Rm - only resistance
-- "water flux“ case (term describing the inherent porosity of a new membrane)
flux is proportional to pressure and inversely proportional to viscosity, so these are corrected to a standard basis to give a "standard water flux", a measure of the inherent porosity of a membrane
Mass Transfer
Why is flux flow-dependent?
Crossflow operation:
fluid is flowing past the membrane at a velocity many orders of magnitude higher than the velocity through the membrane
fluid moving perpendicular to the membrane carries with it material accumulating at the surface of the membrane; but the velocity of the stream parallel to the membrane will tend to redisperse the accumulated material.
Concentration polarization: since retained species accumulate near the membrane surface, their concentration there will be higher than it is in the bulk.
Filtration equation: filtration rate is inversely related to the amount of material accumulated at the filter surface
Mass transfer equations: rate of material redispersed is a function of concentration difference between the membrane surface and the bulk
Turbulent Mass Transfer
The vast majority of commercial UF and MF crossflow devices operate in turbulent flow
Figure 1.4: how flow and mass transfer interrelate in turbulent flow.
transverse fluid velocity at the wall is always zero
boundary layer thickness is defined as the location where 99% of the "action" takes place
outside the dynamic boundary layer we can assume plug flow (uniform velocity)
outside the concentration boundary layer we can assume uniform solute concentration in the bulk
Crossflow device truly operates at steady state (some systems run for months at constant flux)
the rate of the arrival of retained material at the membrane is thus equal to the rate of redispersion of the material already there (rate out = -rate in)