DECANTER Prepared By: Bhaumik Parikh (130420105006) Sunny Chauhan (130420105007) Rusabh Bhavsar (130420105008) Shyam Chudasama (130420105009) Bhavik Dobariya (130420105010).

What is Centrifugal Decanter ? A centrifuge is a device, which employs a high rotational speed to separate components of different densities. This becomes relevant in the majority of industrial jobs where solids, liquids and gases are merged into a single mixture and the separation of these different phases is necessary. A decanter centrifuge separates solid materials from liquids in slurry and therefore plays an important role in wastewater treatment, chemical, oil and food processing industries. There are several factors that affect the performance of a decanter centrifuge and some design heuristics to be followed which are dependent upon given applications.

Principle of Centrifuge Decanter "The solid-bowl scroll-discharge centrifuge now almost universally known as the decanter centrifuge has, indeed, become the workhorse of a wide range of liquid/solid separation activities. Its application to the dewatering of waste sledges has made it a most valuable tool in combating environmental pollution. This has made the decanter a well-known and widely appreciated piece of equipment.

Decanter Centrifuge Rotating Bowl Rotating scroll Clarified liquid http://www.sgconsulting.co.za/Products/Decanters/Decanters.htm

General Idea Feed added to spinning bowl Sedimentation of particles occurs in centrifugal field Flow is upwards at a particular rate which determines residence time in device Separation happens if sedimentation velocity is high enough for particle to reach side of bowl within residence time Large particles have higher settling velocities than small particles Both large and small are still particles, have small Reynolds no.s (<1) and obey Stokes’ Law Fig 1

The Centrifugal Decanter http://www.nzifst.org.nz/unitoperations/mechseparation4.htm

CONSTRUCTION It consists of a cylindrical metal bowl, usually mounted vertically, that rotates about its axis at high speed. If the bowl is now rotated , as in fig the heavy liquid forms a layer next to the inside wall of the bowl. And a layer of a light liquid forms a layer inside a heavy liquid. A cylindrical interface of radius ri separates the two layers. Since the force of gravity can be neglected in comparison with the much greater centrifugal force, this interface is vertical. It is called Neutral Zone.

WORKING In the Operation of the machine , the feed is admitted continuously near the bottom of the bowl. Light liquid discharge at point 2 through ports near the axis of the bowl ; heavy liquid passes under a ring, inward towards the axis of rotation and discharge over a dam point 1. If there is negligible frictional resistance to the flow of the liquids as they leave the bowl , the Position of liquid -liquid interface is established by a hydrostatics balance and the relative ‘Heights”(Radial Distance from the axis) of the overflow ports at 1 and 2.

pB =pressure at free surface of light liquid at rB Assume that the Heavy liquid of Density ϷA,overflows the dam at radius rA and the light liquid of density ϷB , leaves through ports at radius rB .then if both liquids rotates with the bowl and friction is negligible , the pressure difference in he light liquid between rB and ri must Equal that in the heavy liquids between rA and rB . Thus pi – pB = pi – pA …………………………..(1) Where pi = pressure at liquid liquid interface pB =pressure at free surface of light liquid at rB pA = pressure at free surface of heavy liquid at rA

Derivation : From equation (1) pi – pB = pi – pA Where pi = Pressure at liquid –liquid surface. pA =pressure at free surface of light liquid at radius rA. pB = pressure at free surface of heavy liquid at radius of rB. Now , by substituting all values ;) ri^2 =[ra^2 –rb^2(density of B/density of A)] 1 – (density of B/density of A)

Centrifugal Motion Centrifugal acceleration = rω2 ω is the angular velocity in rad/s r is the radius of rotation Centrifugal force = mrω2 m is the mass of the particle

Sigma Factor The capacity of a centrifuge is defined by Σ Q is the throughput (m3/s) at which all particles with a terminal velocity ≥ uT (m/s) are retained Σ has units of m2 and is equivalent to the cross sectional area of a thickener with the same capacity

Determine Σ The contents of a fermenter are discharged to a centrifuge Volume of material is 100 m3 Centrifugation time is 5 hrs Particle size is 3 m – all particles of this size are separated Density of solid phase 1090 kg/m3 Cell free liquid density 1025 kg/m3 Cell free liquid viscosity 0.005 Pa.s

Sigma Factor – Bowl Centrifuge For the bowl centrifuge: ω is the angular velocity (rad/s) R is the outer radius of the bowl (m) rc is the radius of the clarified discharge weir (m) H is the height of the bowl (m) r is the inner radius of the liquid in the bowl (m) g is the acceleration due to gravity (m/s2) NOTE : A long thin bowl gives good separation.

Bowl Centrifuge

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