Flash Distillation Flashcards
How many phases should you have leaving the heater to enter the flash drum?
Two
Having a demister effects what of a flash drum?
size
You need space for what to leave the liquid phase before they reach the pump
bubbles
What is the purpose of a deflector?
To break up turbulent flow and make sure no drops spray up
What is a disadvantage to deflectors?
They can force bubbles into the liquid possibly leading to entrainment
What is the cap size of drops?
200 microns
What is the average percentage of liquid that goes into the vapor without a demister?
5%
Using mesh is common because it is what?
Cheaper than using a vane
Weeping occurs from the what?
Demister
Simulator allows for a fixed what?
temperature (heat duty)
Always use what kind of device in simulation?
adiabatic
The thickness of the walls depend on what?
diameter and pressure
When designing a flash drum one should consider what clearance for transporting materials to the factory site?
bridge
What is the formula for total height of the flash drum?
hT=hv+hf+hL
You should have the liquid and vapor flow rates and the density and temperature set before you calculate for what?
Flash drum design
Which equation do you used to find vapor and liquid fractions?
Ratchford Rice Equation
The diameter of a flash drum is related to what?
vapor velocity
The height of the vapor is related to what?
vapor velocity
The height of the liquid is related to what?
level control (usually not always)
The height of the feed inlet is related to what?
flooding
The force of viscosity depends on what equation?
Stokes
How does the viscosity force impact a flash drum?
Force that prevents a drop from falling
What causes drag force?
The kinetic energy of vapor going up
The gravity force is often grouped with what force?
Buoyancy
F drag is equal to what?
F of gravity and buoyancy
In the dimensional analysis for drag, force is dependent on what 4 things?
velocity, cross sectional area, density, and viscosity
What are two important nondimensional numbers required?
Re, Cd
Cd is a function of what?
Particle Reynolds Number
What Rd, drop radius should be used?
can find with an expected distribution of drop size chart that plots vapor velocity to droplet diameter
What order to you solve equations to find the height of a flash drum?
Solve for K
Solve for D
Solve for H
If a demister is present what does K equal?
0.15
hL can be find either by knowing the volume of the pool or approximating that residence time is what value?
2 min
If L/D < 3 what should you do?
increase V pool
If L/D > 5 what should you do?
use a horizontal drum
The feed that is separated by distillation is not necessarily what?
pure
Why dont we use heat integrated distillation columns?
not significant difference and difficult to control, also we are a conservative industry
In petrolium fractionation the feed furnace does not go over what temperature?
650 F
A three component separation needs how many columns?
2
If you have 4 components how many components can be separated at a time?
1
What is the difference between a direct and indirect sequence of distillation columns?
direct, the bottom feed flows to the second column
indirect, the top feed flows to the second column
When do we use pressure swing distillation
to break azeotropes
In double effect we go from a low pressure column to a high pressure column and then the vapor from the hig pressure column is used to heat the reboiler for the low pressure column. True or False?
True
When is double effect used?
In C3 splitting
Absorbers and strippers typically remove what?
CO2 and H2S
CO2 dissolves into what to form carbonic acid?
water
The stripper and absorber are operated at the same what?
pressure
What is better than water at stripping?
Monethanol amines glycols
What do you have to worry about in absorber stripper arrangements?
SOx (SO2 or SO3)
What are problems with working with absorbers and strippers?
Mercury volatile compounds cancer promoting volatiles IOC incenerator money matters
What is expensive in absorbtion/stripping?
solvent
Never do what? It won’t work most of the time
blindly try specs and number of trays
If you have many components what should you do in distillation absorption?
Start with a few of the most abundant components
Where should you put your components first?
Adiabatic flash
Use an isothermal flash when?
the feed is not two phase at the desired pressure and then change the temperature until you get some separation in the direction you want
List your components in increasing what and determine key compounds
boiling point
Light key goes mostly where?
to the top
Heavy key goes mostly where?
to the bottom
After trying a flash what should you try?
Replacing the flash by a three plate column with similar specs as those given by the flash outlet streams
Once you have a three plate column what should you do?
Add the compounds that are missing. Do it slowly, increasing their concentration until you reach the desired values
What is the last thing you should do when you know nothing, not even the number of trays needed?
keep tightening the specs and increasing the number of plates accordingly so that you meet the separation you want.
What direction does a bubble cap tray force vapor?
up then down then up and out
What direction does a valve tray force vapor?
Up and out
What are some examples of distillation trays?
Sieve
Valve
Bubble cap
Specialized
What are some operational limits for trays?
Vapor going up the downcomer (high entrainment)
weeping
downcomer flooding (phase maldistribution)
flooding
What are the extremes of entrainment?
dumping and blowing
Blowing occurs if what gets too high?
vapor velocity
If the vapor velocity gets too low what can happen?
dumping
What increases as liquid flow increases?
the potential for downcomer flooding
Flooding is more likely as what increases?
vapor velocity
Weeping is likely to happen if what is too low?
vapor velocity
the design point of column tray design should be what percentage away from the top line
80%
The design point falls along what line?
The line produced at constant L/V
Use what percent of flooding velocity?
80%
The diameter of the tray holes is a function of what?
the Net area An
What is the first think to consider with the tray design?
diameter of hole
How do you find net area?
Qv/(0.8*Un,f)
How do you find the area needed to calculate the diameter?
A=An/0.8
Once you have A how do you find the diameter?
D=sqrt(4A/pi)
