Topic 2.4

Question 1

Define filtration

Answer 1

The separation of solids from a solid-liquid mixture (slurry) in a liquid by means of a porous medium or screen which retains the solids and allows the liquid to pass

Question 2

3 Types of filtration + describe

Answer 2

1. Cake filtration
   - proportion of solids in the slurry is large
   - solids deposited on filter medium in the form of cake
   - cake thickness progressively increases, leading to progressive increase in resistance to flow
   - Nutsch filter
2. Deep bed filtration
   - pores of the filter medium are much larger than the particles to be removed
   - particles will penetrate a considerable depth before being captured
   - proportion of solids in fluid is small compared to cake filtration
   - Deep bed up-flow filter
3. Cross flow filtration
   - feed flow is under pressure and tangential to the filter surface at high velocity
   - useful for removinng particles in micron or sub micron size
   - accumulated solids continuously swept away by force of the flow, thus minimizing build-up of solids on filter surface

Question 3

List factors affecting rate of filtration (5)

Answer 3

1. pressure drop across filter
2. filter surface area
3. viscosity of filtrate
4. resistance of filter cake
5. resistance of filter medium and inital layers of cake

Question 4

Using eqn, describe the 5 factors affecting rate of filtration

Answer 4

Pressure drop:
- rate of flow of filtration dV/dt is proportional to pressure difference across filter medium and filter cake
- pressure drop acheived by:
- 1. Gravity -> maintaining a head of slurry above filter medium / pressure developed depends on slurry density
- 2. vacuum -> connecting filtrate receiver to vacuum pump and cr4eating pressure difference across filter
- 3. pressure -> pump slurry into filter under pressure

Total CSA of filter cake (A):
- dV/dt is proportional to area of filter
- area can be increased by using larger filters
- in rotary drum filter, continuous removal of filter cake will give infinite area for filtration

Viscosity of filtrate (u):
- dV/dt is inversely proportional to viscosity of fluid
- decrease visocity = increase dV/dt

Resistance of filter cake, (r) specific cake resistance:
- dV/dt inversely proportional to r (resistance of both fitler medium and filter cake)
- specific surface area (S) of particles, and porosity (inverted 3) which is affecred by rigidity or compressability of cake, could affect r

Filter cake thickness (L):
- dV/dt inversely proportional to cake thickness
- preliminary decantation may be useful to decrease amount of solids

Question 5

Filtration theory: 2 modes of operating batch filter?

Answer 5

1. keeping pressure constant: rate of flow will progressively drop (amt. of cake will build up)
2. keeping flow rate of filtrate exiting constant: pressure must be gradually increased (to due gradual increase in cake thickness)

Question 6

Initial stages of formation of cake importance reasons?

Answer 6

1. rate of flow is greatest at the start of process since resistance is minimum -> no cake formed
2. high initial rates of filtration can result in plugging of pores of filter cloth and cause very high resistance to flow
3. orientation of particles in the initial layers can greatly influence the structure of the entire filter cake -> influence filtration rate

Question 7

Blocking filtration?

Answer 7

There are 2 extreme manners in which cake is formed:
1. penetration of pores by the particles, or
2. (desired) shielding of pores by particles forming bridges -> preferred as particles entering pore can increase resistance through filter mediam

One of the most important factors affecting tendency for blocking is concentration of particles
- the greater the concentration, the smaller the average distance beween particles, the lesser tendency for particles to be drawn into streamlines directed towards open pores
- particles in concentrated slurries tend to distribute themselves fairly evenly over the filter surface and form bridges
- slurries of higher concentrations give rise to cake of lower resistance than those from dilute suspensions

(refer to slides diagram)

Question 8

What is the function of filtration medium + explain?

Answer 8

• Main function is to act as support for the filter cake, while the inital layer of cake provides the true filter
• filter medium should be mechanically strong, resistant to corrosive action of fluid, and offer minimum resistance to filtrate flow
• woven filter materials commonly used, but during its selection, need to consider ease of cake removal from filter surface

Question 9

Filter medium GMP requiremennts?

Answer 9

1. Polypropylene, or 100% cellulose material (non-shedding type)
2. High mechanical strength, and chemical resistance
3. Abole to withstand sanitization temperatures of 80degC during CIP

Question 10

Pre-coating purposes? (3)

Answer 10

1. when clean filter cloth is needed: interface between the pre-coat and cloth can separate readily as cake discharges
2. when clear filtrate is required immediately
3. increase solid concentration of the slurry or porosity of filter cake
   - add filter aids into slurry
   - if slurry forms very dense cake with not much porosity, add filter aids

Question 11

Typical cake filtration process?

Answer 11

1. Slurry feed and filtration
2. Cake wash
3. Cake discharge

Question 12

Explain washing filter cake

Answer 12

Washing filter cake
- remove entrained mother liquor
- usually w/ purified water
- may use solvent with high vapour pressure and inert to cake to improve drying productivity
- channeling (cracks in cake formed, wash liq. will enter crack instead of going throuogh cake to displace mother liquor) can be a problem -> in complete washing, worst with compressible cakes, can be minimized by using a smaller pressure difference for washing than filtration

2 stages:
1. displacement washing - filtrate displaced from cake surfcae by washing fluid, 90% of filtrate removed
2. diffusion washing - solvent diffused into wash liquid from less accessible voids

Question 13

What is delayed cake filtration?

Answer 13

• resistance of filter cake progressively increases
• under constant pressure filtration, rate of filtration drops
• if build up of solids can be reduced, then effective cake thickness will be less and rate of flow of filtrate will increase

Question 14

How to prevent delayed cake filtration?

Answer 14

• use of moving blades in filter equipment to limit cake thickness
• thickness limited to clearance between blades and filter medium
• filtrate flows through cake at a constant rate but solides are retained in the slurry
• solid concentration in filter will increase until particles are in permanent physical contact with one another
• resistance to flow increases causing filtrate flow to drop
• slow blade speeds, cake thickness not constant but at high blade speeds, cake thickness remains almost constant
• care must be taken not to mess up cake or filtration

Question 15

GMP design for filters?

Answer 15

1. Filter media usually sintered stainless steel (2um) or polypropylene or filter cloth made of 100% cellulose (non shedding)
2. All product-wetted and interior parts should be polished stainless rstell with rounded corners to eliminate product holdup (up tp 0.45Ra)
3. All seals materials should be inert to process
4. CIP system should be available
5. If drying needed, clean, dry N2 should be used (0.22um filter)
6. Purified water used for filter cake washing

Question 16

Factors for considering filtration equipment?

Answer 16

• density, viscosity of fluid
• nature of solid particles, size, shape, packing characteristics
• solids concentration in slurry (high desired)
• valuable product in solid, liquid or both
• necessity for wqashing filtered solids
• any pretreatment required
• feed liquor temperature

Question 17

Describe rotary pressure drum filters + adv and disadv

Answer 17

Filtration cycle:
1. Feed zone: slurry enters under pressure to form filter cake
2. Washing zone: cake formed is washed with suitable solvent/water to remove impurities. Multiple wash zones can be installed
3. Drying zone: Clean dry N2 at 3 barg injected onto filtered washed cake to remove moisture
4. Discharge zone: Dried cake discharged continuously into downstream dryer for complete drying. Back purge N2 gas or a knife used to dislodge cake from filter media surface
5. Cloth wash zone: Filter media washed or residual cake on cloth surface (heels) -> ready for next filtration cycle

Adv:
- low moisgture content typically below 20%
- suitable for high output/production rates
- stainless steel construction -> easy to clean and prevent contamination from batch to batch
- can be CIP -> less labour intensive and less downtime between batches

Disadv:
- cloth/media washing is difficult, needs good CIP design system
- cake formation not visible due to gas tight design, hence inspection of cake formation during operation is not possible
- high maintenance cost

Question 17

Describe Horizontal plate filters + adv and disadv

Answer 17

Operation sequence:
1. Pre-coat with filter aid
2. Filtration under pressure forms the cake on the plates and filtrate flows down through circumferential holes on the central hollow shaft
3. Heel removal done once filtration cycle is completed by introducing clean dry air to push down slurry heel to bottom of the plate stack. hell slurry is sucked back to slurry tank by means of a special dip pipe
4. Cake washing, reslurry wash occurs by adding PW and rotating the shaft. PW discharged from hollow shaft with application of pressure. This is followed by heel removal
5. Cake drying continues by blowing N2 under pressure
6. Cake discharge, pressure is released and cake is discharged either by rotating or vibrating plates stack

Adv:
- Cake wash efficient due to re-slurry wash
- Produces high filtrate clarity, good for polishing applications
- plates provide good support for cake -> used for applications with thick and heavy cakes
- can be readily jacketed for applications when hot or cold temperatures to be preserved

Disadv:
- high headroom required for dismantling entire plate stack
- sealing is complex -> need to withstand internal pressure and side forces imposed by mechanical drive
- emptying of vessel in between cake filtration, washing and drying requires close monitoring of pressure inside vessel to ensure cake holds on to plates

Question 18

Describe Nutsche filters + adv and disadv

Answer 18

4 major components:
1. vessel
2. filter floor, cloth, woven mesh screen, or sintered metal plate medium
3. re-slurry arms for cake washing and smoothing
4. cake discharge mechanism

Operational sequence:
1. Filtration - filter charged with slurry and pressure applied to displace filtrate leaving cake retained over the filter medium
2. Cake washing - spray ring or connections on top introduce wash liquid over the cake. this displaces the mother liquor with wash liquid + an adv is the ability to smoothen cake’s surface prior to applying spray wash to ensure entire bed washed evenly (remove cracks to prevent channeling -> efficient washing)
3. Cake re-pulping/re-slurry - wash cake by re-pulping yields most efficient product purity. cake re-suspended in paddle arms for thorough mixing with wash solution. rotating arms move slowly downwards and shave the bed gradually layer by layer until entire cake enters slurry
4. Pressure drying - air or gas purges cake until moisture is reduced to asymptoic level. cake is smoothened by reversing rotation of paddle arms to obtain minimum moisture and seal cracks in cake so air or gas cannot bypass the bed
5. Vacuum drying - further reduction in cake moisture is obtained under vacuum and slowly rotating and lowering paddle arms to scrape and de-lump cake. The Nutsche filter, vessel, filter floor and paddles can be heated to enhance drying (final form of API crystals can be obtained)
6. Cake discharge - cake discharge valve opens, paddle arms are rotated and lowered to convey dry cake towards center. on some filters, the cloth or wovem mesh screen may be backwashed with water to dislodge and remove any cake residue

Adv:
- the vessel, filter floor and paddles can be heated when process conditions require elevated temperatures
- smoothening avoids wash liquid, air or gas purge from bypassing cake that tends to crack
- cake wash efficient due to re-slurry wash
- minimum floor space required

Disadv:
- not suitable when cakes are slow to form
- not suitable when cake is sticky and does not part readily from filter medium
- not suitable when product deteriorates during long downtime (heel expected)

Question 19

How do you address product stability of heel between batches?

Answer 19

Cool contents in between batches. (increase shelf life of heel)

Question 20

Describe centrifugal filtration + adv and disadv

Answer 20

Centrifugal forces provide alternate method of generating very high pressure gradients across filter cloth.
Filtration rates are substantially enhanced by the application of centrifugal forces.

Operational sequence:
1. centrifugal bown inerted with n1
2. slurry is charged as bowl rotates at speed of 800-1200rpm
3. once all slurry charged, cake is washed with purified water
4. after washing complete, spinning continues at lower speed until cake is dry
5. filtration cake discharged

Adv:
- suitable for sticky material separations
- minimize residual heels (non vertical ones)
- able to CIP
- high production rates achievable

Disadv:
- extensive auxiliary equipment required
- high energy requirements
- high cost and flame proof machines/motors required
- good shaft seals required for high speed rotating shafts
- high maintenance costs