Filtration

- Conventional filtration for particle size > 1 micron

- Slurry = solid particles + liquid suspension
- Liquid passes through filter medium while solid particles are retained on it
- Batch here, batch for small scale, continuous for large scale
- Filter cake acts as filter medium as time progresses (through pores of particles)
- Const pressure (rate of filtrate decreases) vs const rate (pressure applied increases)
- Can control feed flowrate to control pressure
- Filtration is removal of solid particles from a fluid by passing fluid through a filtering medium or
septum on which the solids are deposited
- Pressure based classification: a) above atm pressure on upstream side of filter medium (plate
and frame) b) a vacuum on the downstream side (rotary vacuum filter)
- Filters: a) cake (particles deposit on filter medium) b) clarifying (filter medium traps particles) c)
crossflow (suspension flows parallel to filter medium or membrane; less pressure drop)
- Requirements to be met by filter media: a) must retain solids to be filtered, giving a reasonably
clear filtrate b) must not plug or blind c) must be resistant chemically and strong enough to
physically withstand the process conditions (especially under pressure or vacuum conditions) d)
must permit the cake formed to discharge cleanly and completely e) must not be prohibitively
expensive
- Common filters medium is canvas cloth which is used here
- Corrosive liquids require filter mediums such as woolen cloth, metal cloth of Monel or stainless
steel, glass cloth, or paper (resistant to corrosion)
- Synthetic fabrics like nylon, polypropylene, other polymers are also resistant chemically
- If cake formed has very low porosity (very small voids formed or it forms very thick and
compressed layers), filtrate obtained very less. Therefore, here filter aid added to loosen the
cake so that porosity increases, and filtrate can pass through it
- Factors affecting rate of filtration: a) pressure drop across cake and filter media b) resistance of
cake c) resistance of filter media d) area of filtering surface (directly proportional) e) viscosity of
filtrate
- Filter medium resistance (only resistance in clarifying filters), normally important during early
stages of filtration
- Cake resistance is zero at start and increases with time as filtration proceeds
- Flux = dv/dt = driving force/ resistance = delta P / Rc+Rm, where delta P = delta Pc + delta Pm
- Pressure drop from comparison with ergun equation
- Alpha unit = m/kg
- Rm unit = 1/m
- Both in rage of 1010 to 1011
- t/v vs v is straight line

Plate and Frame Filter Press

- Const pressure operation

- Discontinuous pressure filter
- Bypass valve used to control flowrate from feed pump, feed goes to inlet line to plates and frames
tightly packed, filtrate coming out of filter press collected in outlet line, outlet pressure gauge to
measure the pressure, filtrate collected in filtrate tanks
- Batch filtration equipment
- Panel has two switches: agitator and feed pump
- Slurry tank stores 5% caco3
- Pump and bypass valve to control flowrate
- Pump pushes slurry from feed tank to plates and frames
- Canvas cloth put on plate, acting as filter medium
- Plate has ribbed surface
- Put frame in middle of two plates
- 3 plates and frames, last one in reverse so that no flow out of the system
- Holes in plates, frames, and canvas cloth, all three should be aligned since feed will go through
them
- Rotate to get proper fitting
- Filtrate collected through slot pipes
- Pressure may fluctuate (increase) due to cake formation during the process, so control it using valve
- Tight packing required so that no leaks so that liquid doesn’t flow out (due to high pressure
operation)
- Constant pressure 0.5 kg/cm2
- Initially not 0.5 so wait till it reaches that pressure, note that time taken (t s) and filtrate volume
required (Vs)
- Bypass valve open = pressure decrease, close = increase
- Gauge used to indicate pressure
- Cake formed on canvas cloth
- Stop when 10L collected
- Open system, scrap cake off the canvas cloth, dry it for 15 mins, measure weight of both wet cake
(before dying) and dry cake
- Thickness of cake = frame thickness
- Prepare different solution for varying conc of caco3
- Slope of kc/2 and intercept of 1/q0
- Alpha vs slurry conc and Rm vs slurry conc