Solid-Liquid Separation: Centrifugation

SOLID LIQUID SEPARATION: CENTRIFUGATION Figure by MIT OCW. Centrifugation is Effective in Volume Reduction Characteristics of process streams in primary recovery •Volume at or near process maximum •Heterogeneous solid/liquid mixture •Particle size distribution •Typical particle sizes: •Bacteria 1 to 2 µ •Yeast 3 to 5 µ •Actinomyces 2 to 30 µ •Molds 4 to >30 µ •Microbial flocs 10 to >100 µ •Cell debris <1 µ •Solids content: 1-10% dry basis 5-50% wet basis •Time variable product and stream quality •Viscosity: 1 to >5,000 cp •Temperature: 4-40 C •Mechanically (shear) sensitive materials •Temperature and pH sensitive materials •Potentially toxic materials Centrifuge Equipment Process Needs •Cleaning in place (CIP) •Steam in place (SIP) •Automation for start-up and operation •Short process times •Flexibility in process stream properties •Temperature control •Containment to prevent entry of contaminants and release of solvents or toxic materials •Corrosion resistance •Small footprint •Fail safe system in case of off-balance •Continuous operation for long times •Interface with upstream and downstream operations OPERATING REGION FOR SOLID/LIQUID SEPARATION AND SOLUTE CONCENTRATION 10 1 0.1 0.01 0.001 dp2∆ρ Conventional Filtration High Speed Centrifugation Microfiltration Ultrafiltration Evaporation Size (microns) 100 Hyperfiltration 0.1 1.0 10 Concentration ( % dry matter) dp2∆P for filtration STOKE’S LAW CALCULATION OF CENTRIFUGE THROUGHPUT, Q Q = VS A d p ( ρp − ρs ) VS = gc 18µ 2 Q= Volumetric throughput (cm3/s) Vs=Sedimentation velocity (cm/s) dp=particle diameter (cm) ρp=particle density (g/cm3) ρs=solvent density (g/cm3) µ=solvent viscosity (g/cm-s) gc=gravitational constant (cm/s2) Implications of Stoke’s Law Causes for deviation •Non-spherical particles •Non-Newtonian rheology •Hindered settling •Non-uniform flow Opportunities to improve •Increase particle size •Increase density difference •Reduce viscosity •Increase gravitational force Continuous Separation in a Tank Inlet Outlet h l b Figure by MIT OCW. A = Surface area V = Volume Q = Throughput – independent of height & proportional to area Vlim = Sedimentation velocity T = Time V bhl ⎫ = Q Q ⎪ ⎪ h ⎪ t= ⎬⇒ ϑlim ⎪ ⎪ A = bl ⎪ ⎭ t= Q = ϑlim A Comparison of Sedimentation in a Tank and Centrifuge Feed Effluent Sediment Increase AREA to increase Q Figure by MIT OCW. Feed Effluent Sediment ω Figure by MIT OCW. Centrifugal Coefficient & Stoke’s Law C = centrifugal coefficient R – radius of rotor W = angular velocity D = rotor diameter N = rotor speed V = settling rate D = particle diameter S = particle density S1 = medium density N = medium viscosity g = gravitational constant Sedimentation Velocity Q = VlimA d ∆ρg v = 18µ 2 Tank lim lim Disc-Stack Separator d ∆ρ v = rω 18µ 2 lim lim What is different? rω2 2 AREA Q = VlimA dA = N2πr (cot a) dr ω a { { dr cot a dr r Figure by MIT OCW. The area of interest is the projected area perpendicular to the centrifugal force THROUGHPUT Q Q = VlimA ω a r1 Q=∫ r2 ϑlim ∆P 2 rω N 2πr cot adr 18η ⇔ ⎡ϑ ∆P ⎤ ⎡ 2π ⎤ Q = ⎢ lim ⎥ ⎢ ω 2 N cot a r12 − r22 ⎥ ⎦ ⎣18η ⎦ ⎣3 ( 1 r2 ) 2 r1 Figure by MIT OCW. Process Medium Separator Design Area Equivalent = Σ Throughput = Q= ⎡ϑ ∆P ⎤ ⎡ 2π ⎤ Q = ⎢ lim ⎥ ⎢ ω 2 N cot a r12 − r22 ⎥ ⎦ ⎣ 18η ⎦ ⎣ 3 ( ) ⎤ ⎡ϑ ∆P ⎤ ⎡ 2π 2 Q = ⎢ lim g ⎥ ⎢ ω N cot a r12 − r22 ⎥ 3g ⎣18η ⎦ ⎣ ⎦ ( 1 Sedimentation velocity for a particle Q Σ = Q Σ 2 1 ) Note the gravitational force constant 2 Σ = surface area required for sedimentation in a tank to achieve same result as centrifuge 2 Σ Q =Q Σ 2 1 2 1 1 Process Objectives CONCENTRATION Solid product in feed CLARIFICATION Dissolved product in feed Light phase – any solids are lost product Heavy phase – any liquid is an impurity Light phase – solids are impurities Heavy phase – any liquid is lost product Choosing the right separator Primary Selection – Solids content SEPARATOR TYPE Chamber bowl centrifuge Solid bowl centrifuge Self cleaning separator Nozzle bowl separator Centrifugal decanter 0 20 SOLIDS CONTENT 40 60 80 100