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UOPS 4 FINALS PART 2 UPDATED TEST
WITH COMPLETE SOLUTION
Applications of settling and sedimentation - ANSWER - removal of solids from liquid sewage wastes
- settling of crystals from mother liquor
- separation of liquid-liquid mixture from a solvent extraction stage in a settler
- settling of solid food particles from a liquid food
Sedimentation - ANSWER - the tendency for particles to settle out of the fluid in which they are entrained and come to rest against a barrier
Factors affecting sedimentation - ANSWER - particle size
- liquid viscosity
- solid and solution density
Types of particles - ANSWER - totally discrete particles
sphericity - ANSWER - surface area of a sphere having the same volume as the particle per surface area of the particle
Types of sedimentation - ANSWER 1. Free settling
- Hindered settling
Free settling - ANSWER - when the particle is at a sufficient distance from the walls of the container and from other particles so that the fall is not affected by them
Hindered settling - ANSWER - if the motion of the particles is impeded by other particles, which will happen when the particles are near each other even though they are not actually colliding
External force - ANSWER - expressed as a product of the mass and the acceleration of the particle from this force
Forces acting on the body - ANSWER - external force acting downward
- buoyant force acting upward
- resistance or drag force in opposite direction to the particle motion
suspension into a supernatant clear fluid and dense slurry containing higher concentration of solids by gravity settling
size and type - ANSWER The ___ of particles to be removed have a significant effect on the operation of the sedimentation tank.
sand or silt - ANSWER Because of their density, ____ can be removed very easily.
water-flow channel - ANSWER The velocity of ___ can be slowed to less than one foot per second, and most of the gravel and grit will be removed by simple gravitational forces.
Colloidal material - ANSWER - small particles that stay in suspension and make the water seem cloudy
- will not settle until the material is coagulated and flocculated by the addition of a chemical, such as an iron salt or aluminum sulfate
slower - ANSWER When the temperature decreases, the rate of settling becomes ____
Density currents - ANSWER - caused by the weight of the solids in the tank,
the concentration of solids and temperature of the water in the tank
Eddy currents - ANSWER - produced by the flow of water coming into the tank and leaving the tank
Sedimentation Basin Zones - ANSWER 1. Inlet Zone
- Settling Zone
- Sludge Zone
- Outlet Zone
center - ANSWER In a square or circular basin (clarifier), water typically enters the basin from the ____ rather than from one end and flows out to outlets located around the edges of the basin
Inlet zone - ANSWER - distribute the water
- control the water's velocity as it enters the basin
- prevent turbulence of the water
Short-circuiting - ANSWER - is a problematic circumstance in which water bypasses the normal flow path through the basin and reaches the outlet in less than the normal detention time
baffle - ANSWER - prevents floating material from escaping the sedimentation basin and clogging the filters
effluent structure - ANSWER - found after the baffle
- usually consists of launder, weirs, and effluent piping
effluent launder - ANSWER - a trough which collects water flowing out of the sedimentation basin and directs it to the effluent piping
weirs - ANSWER - are walls preventing water from flowing uncontrolled into the launder
- serve to skim water evenly off the tank
V-shaped notch - ANSWER - allows only the top few centimeters of water to flow out of the sedimentation basin
sludge zone - ANSWER - found across the bottom of the sedimentation basin where the sludge is collected temporarily
- velocity is very slow to prevent re-suspension of sludge
Rectangular basin - ANSWER - commonly found in large-scale water
treatment plants
Rectangular tanks - ANSWER - are popular as they tend to have:
- high tolerance to shock overload
- predictable performance
- cost-effectiveness due to lower construction cost
- lower maintenance
- minimal short-circuiting
Circular or Square Basins - ANSWER - frequently referred to as clarifiers
- share some of the performance advantages of the rectangular basins but are generally more prone to short circuiting and particle removal problems
High rate tube settlers - ANSWER - are designed to improve the characteristics of the rectangular basin and to increase flow through the tank
- consists of a series of tubes installed at 60 degree angle to the surface of the tank
Solids contact unit - ANSWER - combines the coagulation, flocculation, and sedimentation basin in one unit which are called upflow clarifiers or sludge-blanket clarifiers
0.14 m/s - ANSWER Determine the terminal settling velocity of dust particles having a diameter of 60 microns at 294.3K and 101.32 kPa. The dust particles can be considered spherical with a density of 1,280 kg/m3. For air: M = 0.01828x 10 8Pa - S and P = 1.202 kg/m3.
2.369 x 10^4 m/s - ANSWER Solid spherical particles having a diameter of 0.09 mm and a solid density of 2,00? kg/m1 arc settling in a solution of water at 26.7"(J. The volume fraction of the solids In the solution is 0.45. The settling velocity is (For H20 P = 994.7 kg/m3 and M = 0.861 x 10' Pa-S)
4700 cm - ANSWER A random handful of silica particles ranging in size from 28 mesh to 200 mesh is thrown to a very deep body of water (without tides or turbulence).
Data:
Viscosity of water = 0.01 poise Density of silica = 2.65 g/cm
28 mesh: 0.0589 cm 200 mesh: 0.0074 cm
The distance between the largest and smallest particle after 10 min is _____.
4850 ft - ANSWER A gravity settling tank is to be used to clean waste water from an oil refinery. The waste water contains 1% oil by volume as small droplets ranging in size from 100 to 1000 microns which will be removed
from the water before the latter is to be discharged into the river. The tank is of rectangular section 2 ft wide by 4 ft deep with provisions for smooth continuous discharge of clean water and skimming off of oil at the surface of the discharge end. If 10,000 gal/min of waste water is to be cleaned of oil droplets, specific gravity of oil 0.87, the length of the settling tank is ____.
68.5 hours - ANSWER A laboratory test on a suspension of a solid in a liquid gave the following information:
Original height of sludge before settling=10 in.
Free settling rate=0.10 in./min
Height of sludge at the end of free-settling period=6.5 in.
Height of the sludge at the end of 120 minutes=4 in.
Height of sludge when settled completely=1.5 in.
One thousand cubic feet of similar sludge is to be settled in a vertical cylindrical tank, the diameter of which is equal to the depth of the liquid suspension in it. The time it would take for the solid to settle to a height of 20% of the original height of the sludge is ____
62.4 hours - ANSWER Free settling of sludge is 0.25 cm/min. Using an original height of 25 cm, the sludge settled to height of 18 cm after the free-settling period. The sludge was found to settle to a height of 10 cm after 110 min. This particular sludge was found to settle completely to a height of
is defined as the force exerted by ___
average drag per unit projected area to the product of the velocity head and density - ANSWER Drag coefficient for flow past an immersed body is the ratio of _____
<5 - ANSWER Stoke's Law is valid when the particle Reynolds Number is ___
viscous forces control and inertial forces are unimportant - ANSWER At low Reynolds Number, ____
inertial forces control and viscous forces are unimportant - ANSWER At high Reynolds Number, ____
drag coefficient - ANSWER The ratio of drag force per unit area to the product of fluid density and the velocity head is called ____.
density difference - ANSWER In order for a particle to move through a fluid under the influence of gravity, there must be ____
greater - ANSWER In continuous thickeners, separation of solid particles can be achieved if the settling velocity of the solids is_______compared to the
velocity of the displaced liquid.
- ANSWER Determine the terminal settling velocity of dust particles having a diameter of 60 microns at 294.3K and 101.32 kPa. The dust particles can be considered spherical with a density of 1,280 kg/m3. For air: M = 0.01828x 10 8Pa - S and P = 1.202 kg/m3.
constant velocity with no acceleration - ANSWER Terminal velocity is ____,
near each other - ANSWER In hindered settling, particles are _____.
-1 - ANSWER For the free settling of a spherical particle through a fluid, the slope of CD vs log NRe plot is ___
inverse of the fluid viscosity - ANSWER The terminal velocity of a small sphere settling in a viscous fluid varies as the ____.
Buoyant force - ANSWER - depends on the weight of the floating body
below - ANSWER Center of pressure in an immersed body is _______ the center of gravity
