Mechanical Separations Quiz
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Questions and Answers

What happens to cream when whole milk is allowed to stand?

  • Cream rises to the top of the milk. (correct)
  • Cream remains mixed with the milk.
  • Cream rises to the bottom of the milk.
  • Cream dissolves in the milk.

What is the primary advantage of using a centrifuge over natural sedimentation?

  • Centrifuges produce less force.
  • Centrifuges don't require rotation.
  • Centrifuges operate faster and increase separation efficiency. (correct)
  • Centrifuges can separate solids and liquids instantly.

Which equation correctly describes the centrifugal force acting on a particle in circular motion?

  • Fc = mrω² (correct)
  • Fc = mrω
  • Fc = mv²/r (correct)
  • Fc = mg

At what point can the effects of gravity usually be neglected in the analysis of separation in industrial centrifuges?

<p>When the centrifugal force exceeds gravitational force. (D)</p> Signup and view all the answers

What does the variable 'N' represent in the equation for centrifugal force?

<p>The rotational speed in revolutions per minute (C)</p> Signup and view all the answers

How can the centrifugal acceleration be expressed for comparative purposes?

<p>As a multiple of 'g'. (C)</p> Signup and view all the answers

Using a centrifuge, what is the resultant centrifugal acceleration derived from a rotational speed of 2000 rev/min and a radius of 10cm?

<p>450g (B)</p> Signup and view all the answers

What will likely happen if the rotational speed of a centrifuge is increased significantly?

<p>The efficiency of separation will improve. (C)</p> Signup and view all the answers

What factor significantly influences the settling velocity of a particle according to Stokes' Law?

<p>Particle diameter (C)</p> Signup and view all the answers

What does the constant 's' represent in the empirical relationship for compressibility?

<p>Constant for the material's compressibility (B)</p> Signup and view all the answers

What is the condition for Stokes' Law to be applicable?

<p>The flow must be streamline (A)</p> Signup and view all the answers

What is the primary function of the mechanical support in filtration equipment?

<p>To provide structural support for the filter medium (A)</p> Signup and view all the answers

What is the value of the viscosity of air used in the calculations?

<p>1.8 x 10^-5 Nsm^-2 (D)</p> Signup and view all the answers

In a plate and frame filter press, where does the filter cake build up during filtration?

<p>On the upstream side of the cloth (A)</p> Signup and view all the answers

Which diameter of dust particle would settle faster in air, assuming other factors remain constant?

<p>100μm (D)</p> Signup and view all the answers

What is the Reynolds number for the 60μm particles calculated to be?

<p>0.56 (B)</p> Signup and view all the answers

What happens when the space between the frames of a plate and frame filter press becomes filled with cake?

<p>The press needs to be dismantled for cleaning (C)</p> Signup and view all the answers

Which of the following best describes the operation of a plate and frame filter press during the filtration cycle?

<p>Initially, the pressure drop is small and then increases (D)</p> Signup and view all the answers

What does the term 'free settling' imply in the context of sedimentation?

<p>The motion of particles is independent of one another (C)</p> Signup and view all the answers

What is a significant disadvantage of plate and frame filter presses?

<p>They are difficult to mechanize extensively (D)</p> Signup and view all the answers

In what scenario is it necessary to seek more detailed references instead of using Stokes' Law?

<p>When Reynolds number exceeds 2 (D)</p> Signup and view all the answers

What must be done occasionally to maintain the efficiency of a plate and frame filter press?

<p>Dismantle and clean the pressed cake (D)</p> Signup and view all the answers

What is the proportional relationship of settling velocity (vm) for different diameters of particles?

<p>vm is proportional to the square of the diameters (D)</p> Signup and view all the answers

In which situation might it be necessary to wash the filter cake?

<p>To remove traces of the solution from the filter medium (D)</p> Signup and view all the answers

What is terminal velocity?

<p>The maximum velocity reached by particles under constant force. (C)</p> Signup and view all the answers

Which factors influence terminal velocity?

<p>The shape and size of the particles. (A), The density of the fluid. (C)</p> Signup and view all the answers

What does the drag force on a particle depend on?

<p>The coefficient of drag and the velocity of the particle. (D)</p> Signup and view all the answers

What does the equation $Fs = Va (\rho_p - \rho_f)$ represent in particle motion?

<p>The force applied to a particle minus forces resisting it. (A)</p> Signup and view all the answers

What is the significance of the coefficient known as the drag coefficient (C)?

<p>It indicates how easily a particle can be affected by fluid resistance. (D)</p> Signup and view all the answers

Which of the following equations represents terminal velocity for spherical particles?

<p>$v_m = \frac{D^2a(\rho_p - \rho_f)}{18\mu}$ (C)</p> Signup and view all the answers

What role does sedimentation play in fluid dynamics?

<p>It separates particulate material from fluid streams using gravitational forces. (A)</p> Signup and view all the answers

How is the projected area (A) of a spherical particle defined?

<p>$A = \pi D^2/4$ (C)</p> Signup and view all the answers

What must be considered when performing sieve analysis on particles that differ greatly from spherical shape?

<p>It requires supplementation with a microscopical examination. (D)</p> Signup and view all the answers

What type of sieves are typically used in the flour-milling industry for particle separation?

<p>Continuous vibrating sieves with increasing apertures. (A)</p> Signup and view all the answers

What is critical for achieving effective separation with vibrating screens?

<p>The amplitude and frequency of vibrations must be optimized. (B)</p> Signup and view all the answers

Which industrial sieve type utilizes a horizontal cylinder for particle separation?

<p>Rotary screens where material tumbles inside. (A)</p> Signup and view all the answers

What does Stokes' Law help to determine in the context of particle separation?

<p>The velocity of particles relative to the fluid. (D)</p> Signup and view all the answers

In sieve analysis, what typically occurs to the finer particles as they progress along a continuous vibrating sieve?

<p>They are removed at each stage of the sieve. (B)</p> Signup and view all the answers

What is the primary purpose of using air classification below the sieves in certain industrial applications?

<p>To effectively remove lighter impurities like bran. (C)</p> Signup and view all the answers

What size range of particles can typically be screened industrially?

<p>Particles larger than 50μm. (A)</p> Signup and view all the answers

What is the primary function of the conical plates in liquid/liquid separation centrifuges?

<p>To provide smoother flow and improve separation (D)</p> Signup and view all the answers

What challenge do solids present in liquid/solid separation within a centrifuge?

<p>They create disturbances in the flow pattern (C)</p> Signup and view all the answers

How do nozzles function in the context of handling solids in a centrifuge?

<p>They release accumulated solids along with some heavy liquid (A)</p> Signup and view all the answers

What is the role of telescoping action in a centrifugal bowl?

<p>To move accumulated solids towards the outlet (A)</p> Signup and view all the answers

In a horizontal bowl centrifuge with scroll discharge, what happens to the liquid during operation?

<p>It is continuously discharged towards the center (C)</p> Signup and view all the answers

What is the recommended scroll speed relative to the bowl speed in a horizontal bowl centrifuge operating at 2000 rev/min?

<p>25 rev/min (D)</p> Signup and view all the answers

Why is it important to maintain differential speeds between the scroll and the bowl in centrifuges?

<p>To ensure effective separation of liquids and solids (D)</p> Signup and view all the answers

What potential issue arises from using stationary ploughs in a liquid/solid separation centrifuge?

<p>They may create disturbances in the flow patterns (B)</p> Signup and view all the answers

Flashcards

Terminal Velocity

The constant velocity reached by a particle falling through a fluid under the influence of gravity, after an initial acceleration phase.

Drag Force

The force that resists the motion of a particle through a fluid, due to friction.

Sedimentation

The process of separating particles from a fluid based on gravity's influence.

External force

The force causing the motion of a particle, such as gravity.

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Terminal Velocity Equation

vm = D2a(ρp - ρf) /18μ

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Drag Coefficient

A constant that relates drag force to particle properties, velocity and the fluid.

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Particle Density

The mass of a particle per unit volume.

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Fluid Viscosity

A measure of a fluid's resistance to flow.

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Stokes' Law

An equation that describes the settling velocity of spherical particles in a fluid under the influence of gravity.

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Sedimentation

A process used to separate particles from a fluid based on differences in their densities.

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Settling Velocity (vm)

The velocity at which a particle settles under gravity in a fluid.

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Reynolds Number (Re)

A dimensionless number that indicates whether fluid flow is laminar (smooth) or turbulent.

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Streamline flow

Smooth, orderly flow of fluid where layers don't mix.

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Particle Diameter (D)

The size of a particle, often measured in microns (µm).

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Stokes' Law Equation

vm = D^2 * g * (ρp - ρf) / 18μ

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Application of sedimentation (food industry)

Separating dirt, debris, crystals, dust, or product particles from raw materials, crystals, and air streams.

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Centrifugal Force

A force that acts outwards on a particle moving in a circular path. It's the force needed to keep the particle on the path

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Centrifugal Force Equation (10.5)

Fc = mrω² where Fc is the force, m is the mass, r is radius and ω is angular velocity

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Centrifugal Force Equation (10.6)

Fc = mv²/r where Fc is the force, m is the mass, v is velocity and r is radius

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Centrifugal Force Equation (10.7)

Fc = 0.011 mrN² where Fc is the force, m is the mass, r is radius and N is rotational speed (in rpm)

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Centrifugal Acceleration

The acceleration experienced by a particle in a rotating system due to the centrifugal force, it is proportional to r*N^2

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Centrifuge

Equipment used to separate substances of different densities by applying centrifugal force.

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g-force

A measure of centrifugal force relative to the force of gravity.

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Separation of Milk Components (example)

Whole milk separating into cream and skim milk due to gravity; Centrifuges are used for faster separation by applying centrifugal force (in comparison with gravity).

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Centrifuge Liquid/Liquid Separation

Conical plates in a centrifuge improve liquid/liquid separation by providing smoother flow and better separation.

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Liquid/Solid Separation Challenge

Solids in liquid centrifuges are harder to handle than liquids, as stationary ploughs disrupt the flow needed for separation.

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Centrifuge Solid Handling (Nozzles)

Nozzles on the centrifuge bowl can discharge accumulated solids and heavy liquid.

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Telescoping Bowl Method

Solids accumulate in telescoping bowl sections, moving towards the outlet.

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Horizontal Bowl Centrifuge

The horizontal bowl centrifuge uses a rotating scroll to continuously discharge solids, while liquid flows to the opposite side.

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Scroll Speed vs Bowl Speed

The scroll's speed relative to the bowl is crucial for proper operation and should be carefully controlled to avoid major disruptions.

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Centrifuge Output Streams

Centrifuges separate feed into three streams (solid, heavy liquid, light liquid), where solids often carry some heavy liquid.

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Centrifuge Separation Analogy

Centrifuge action is similar to gravity settling, using weirs and overflows to separate components like a settling tank but uses much higher centrifugal force.

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Sieve Analysis

A method for determining particle size distribution.

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Particle Size Distribution

The range of particle sizes and their abundances.

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Industrial Sieves

Tools for separating particles of different sizes, including rotary, vibrating, and multi-deck screens.

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Stokes' Law

Calculates settling velocity of spherical particles in a fluid.

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Settling Velocity

The speed at which a particle settles through a fluid due to gravity.

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Sieving Efficiency

Proportion of particles smaller than a size that pass through a sieve.

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Particle Separation

Separating particles based on forces like gravity or centrifugal force.

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Vibrating Screens

Screens that use vibrations to separate particles by size.

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Cake compressibility

The relationship between cake resistance (r) and pressure drop (ΔP) during filtration. It describes how cake resistance changes with pressure.

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Filtration Equipment

Equipment used to separate solids from liquids by forcing the liquid (filtrate) through a porous material (filter medium) while the solids (cake) remain behind.

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Plate and Frame Filter Press

A type of filtration equipment comprised of plates and frames, used to separate solids from liquids. Filter cake builds up sequentially.

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Filter cake

Solid material separated out during the filtration process

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Filtrate

The liquid that passes through the filter medium during filtration

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Constant rate filtration

Early stage of filtration where pressure drop across the filter is low and the filtration process happens at a consistent rate.

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Constant pressure filtration

Later stage of filtration when the cake builds up and the pressure drop is high, making the filtration process constant.

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Filtration cycle

A complete process of filtration,cake buildup, cleaning/dismantling, and preparation for a new round of filtration.

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Study Notes

Mechanical Separations

  • Mechanical separations are categorized into four groups: sedimentation, centrifugal separation, filtration, and sieving.
  • Sedimentation separates immiscible liquids or a liquid and a solid based on density differences using gravity. Centrifugal forces can often speed up the process.
  • Filtration separates a solid from a liquid by passing the mixture through a material with fine pores small enough to stop the solid particles, but allowing the liquid to pass.
  • Sieving is used to classify solid particles by using a barrier that allows only smaller particles to pass through.
  • Mechanical separation of particles from a fluid is influenced by forces such as gravity, centrifugal force, pressure forces, and restraining forces (sieving/filtration). Particle characteristics (size, shape, density) and fluid properties (viscosity, density) are crucial in the separating process.

Velocity of Particles Moving in a Fluid

  • Under constant force (e.g., gravity), particles in a fluid initially accelerate then reach a uniform velocity called terminal velocity.
  • Terminal velocity depends on particle size, density, shape, and fluid properties.
  • Two key forces act on particles moving in a fluid: the external force causing motion and a drag force due to fluid friction.
  • The net external force on a moving particle is the difference between the applied force and the resisting drag force.

Sedimentation

  • Sedimentation uses gravitational force to separate particles from fluids, often in food industries.
  • Particles settle at different rates based on density, size, and shape.
  • Stokes' Law is a fundamental equation used to calculate settling velocity of particles.

Flotation

  • Flotation is a technique used to separate fine particles, which might not settle effectively by gravity.
  • Flotation involves using air bubbles to lift the particles to the surface, changing their buoyancy.
  • Surface active agents can enhance flotation.

Centrifugal Separations

  • Centrifugal separators use rapid rotation to increase the separation rate of particles.
  • Centrifugal force is calculated using the formula Fc = (mv²)/r.
  • Heavier particles are forced outward in centrifuges due to centrifugal force, lighter particles move inward.
  • Centrifugal force greatly increases separation rate compared to relying just on gravity. This is used in dairy products for separating milk from cream.

Filtration

  • Filtration is a method for removing solids from a liquid based on the pore size.
  • Filter resistance is determined by the filter medium and filter cake.
  • Factors like pressure difference, filter cloth properties, and velocity of the filtrate affect filtration rate.
  • Constant rate filtration occurs based on the filter medium resistance and when a constant pressure difference drives the filtration.

Sieving

  • Sieving separates particles based on size using screens or meshes.
  • Particle size, shape, and shaking intensity affect the sieving rate.
  • Sieving is used for sorting materials and determining particle size distribution.

Equations in this Section

  • Many equations are given describing the forces and velocities associated with these separation methods. Note the importance of consistent units when using these equations. Various equations are used for different situations (e.g., Stokes' Law, centrifugal force equation, filtration rate equation). Understanding and being able to apply these equations are key to solving problems.

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Mechanical Separations PDF

Description

Test your understanding of mechanical separation methods such as sedimentation, filtration, and sieving. This quiz covers the principles and characteristics that influence the separation of particles from fluids. Get ready to explore the physics behind these essential procedures!

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