Exam 3 Flashcards
(61 cards)
Which of the following regular polyghedrons is most likely to have the highest sphericity? Why?
Tetrehedron
Dodecahedron
Cube
Octahedron
Dodecahedron
Since it has the most faces (12), in has the most symmetry making it closest in resemblance to a sphere
What does bed voidage describe?
The fraction of a packed bed’s volume not taken up by particles
Do the friction losses increase, decrease, or stay the same if a packed bed becomes more densely packed? Why?
Increase
This is because when packing density increases, the bed becomes more resistant to fluid flow due to reduced void space and increased surface area for interaction, causing the frictional losses to increase
Does the superficial velocity increase, decrease, or stay the same if a packed bed becomes more densely packed ?
Increases
As the bed becomes more densely packed, the volume of void space between the particles decreases since the same flow rate is passing through a smaller total cross-sectional area ( due to its reduced void space ) , the fluid must move faster through the remaining available space . u= V/A
If the diameter of particles in a packed bed increases, do the frictional loses increase , decrease, or stay the same? Why?
Decrease
Larger particles result in less resistance to flow, and this leads to a decrease in frictional losses.
When the Reynolds’s number is large in a packed bed, what role do viscous forces/ effects play?
Viscous effects are unimportant so there is little energy dissipation through friction
How does the superficial velocity compare to the actual velocity in a packed bed for a given flow rate?
It’s smaller
Why?
The packing causes the fluid to have less space to flow through so its velocity is higher with packing .
Increasing the mesh number of a screen allows:
Larger particles to be blocked
Fewer particles to pass through
Why? Increasing the mesh number makes the screen finer
Is the potential energy term in a mechanical energy balances negligible in liquids or gases? Why?
Gases
Changes in potential energy are typically small due to the low density of gases
When the Reynolds’s number for a packed bed is greater than 1000, what term is negligible?
Viscous
For high Reynolds’s number (Re> 1000), the flow is dominated by inertial forces (turbulence), and the impact of viscous forces becomes less important making the viscous term negligible
True or False?
Because frictional loses are so high In packed beds, we will often ignore frictional loses in other places in the pipe
True
The fluid velocity equals Umf in a bed. What would happen to the bed if all other conditions remain the same , but the cross-sectional area of the bed decreases?
The bed becomes packed and is no longer fluidized
Consider a packed bed through which you can flow either water or air. If you increase the superficial velocity over the minimum velocity by some constant value, how would you expect it to change the bed length in each case?
The bed would be larger when water is flowed then when air is flowed
Why?
Water has a higher density and viscosity which will require a longer bed length to achieve the same fluidization because the increase in superficial velocity has a less significant effect on fluidization in the packed bed
For a fluidized bed, if the superficial velocity increases to above the terminal velocity, what happens?
Particles begin being ejected from the bed
The frictional pressure drop decreases
List the steps to solve for the diameter of a particle (dp)
- Calculate the volume of the shape.
- Set the volume of the sphere equal to the shapes volume.
- Calculate the diameter of the sphere.
- Calculate the surface area of the sphere for that same volume.
- Calculate the surface area of the shape.
- Divide the surface area of the sphere by the surface area of the shape to get sphericity
- Multiply the diameter of the sphere by the sphericity.
List the steps to solve for the sphericity of a particle
- Calculate the volume of the shape.
- Set the volume of the sphere equal to the shapes volume
- Calculate the radius of the sphere.
- Calculate the surface area of the shape.
- Calculate the surface area of the sphere.
- Divide the surface area of the sphere by the surface area of the shape.
What is a packed Bed? Give some examples
A packed bed refers to a type of column or reactor where solid particles are packed into a container or tube, leaving empty space (or voids) between the particles.
Examples: rock piles, sand filters, soil in flower pots, cigarettes, and absorption columns
What is the equation for sphericity?
The surface of sphere over the surface of the particle at the same volume
What is the sphericity of a sphere?
1
What is dsph? How is it different from dp? How do you calculate dp using dsph?
dsph is defined as the equivalent spherical diameter. It can be calculated by (6V/pi)^1/3 using the same volume as the particle. Dp is the diameter of the actual particle. dp is equal to dsph times sphericity.
Does the voidage increase or decrease as the sphericity decreases for randomly packed beds of uniform particles?
The voidage increases
Non spherical particles tend to pack less efficiently, leading to more gaps or voids between the particles
For chapter 6 equations for Frictional loss for packed Beds, what density is used?
The density of the fluid
What is superficial velocity and how does it relate to regular velocity?
Superficial Velocity (u0) is a term used to describe the flow rate of a fluid through a packed bed , assuming that the fluid is moving through the entire cross- sectional area of the bed (as if there were no solid particles).
Regular velocity (u) is the actual velocity of the fluid while it moves through the packed bed. u = u0 /E (porosity)
Are the frictional losses much greater or much smaller in the packed bed section compared to the rest of the pipe?
The frictional losses are much greater in the packed bed section.
