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Process Design and Safety > Flash Distillation > Flashcards

Flash Distillation Flashcards

1
Q

How many phases should you have leaving the heater to enter the flash drum?

A

Two

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2
Q

Having a demister effects what of a flash drum?

A

size

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3
Q

You need space for what to leave the liquid phase before they reach the pump

A

bubbles

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4
Q

What is the purpose of a deflector?

A

To break up turbulent flow and make sure no drops spray up

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5
Q

What is a disadvantage to deflectors?

A

They can force bubbles into the liquid possibly leading to entrainment

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6
Q

What is the cap size of drops?

A

200 microns

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7
Q

What is the average percentage of liquid that goes into the vapor without a demister?

A

5%

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8
Q

Using mesh is common because it is what?

A

Cheaper than using a vane

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9
Q

Weeping occurs from the what?

A

Demister

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10
Q

Simulator allows for a fixed what?

A

temperature (heat duty)

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11
Q

Always use what kind of device in simulation?

A

adiabatic

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12
Q

The thickness of the walls depend on what?

A

diameter and pressure

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13
Q

When designing a flash drum one should consider what clearance for transporting materials to the factory site?

A

bridge

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14
Q

What is the formula for total height of the flash drum?

A

hT=hv+hf+hL

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15
Q

You should have the liquid and vapor flow rates and the density and temperature set before you calculate for what?

A

Flash drum design

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16
Q

Which equation do you used to find vapor and liquid fractions?

A

Ratchford Rice Equation

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17
Q

The diameter of a flash drum is related to what?

A

vapor velocity

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18
Q

The height of the vapor is related to what?

A

vapor velocity

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19
Q

The height of the liquid is related to what?

A

level control (usually not always)

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20
Q

The height of the feed inlet is related to what?

A

flooding

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21
Q

The force of viscosity depends on what equation?

A

Stokes

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22
Q

How does the viscosity force impact a flash drum?

A

Force that prevents a drop from falling

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23
Q

What causes drag force?

A

The kinetic energy of vapor going up

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24
Q

The gravity force is often grouped with what force?

A

Buoyancy

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25
Q

F drag is equal to what?

A

F of gravity and buoyancy

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26
Q

In the dimensional analysis for drag, force is dependent on what 4 things?

A

velocity, cross sectional area, density, and viscosity

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27
Q

What are two important nondimensional numbers required?

A

Re, Cd

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28
Q

Cd is a function of what?

A

Particle Reynolds Number

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29
Q

What Rd, drop radius should be used?

A

can find with an expected distribution of drop size chart that plots vapor velocity to droplet diameter

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30
Q

What order to you solve equations to find the height of a flash drum?

A

Solve for K
Solve for D
Solve for H

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31
Q

If a demister is present what does K equal?

A

0.15

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32
Q

hL can be find either by knowing the volume of the pool or approximating that residence time is what value?

A

2 min

33
Q

If L/D < 3 what should you do?

A

increase V pool

34
Q

If L/D > 5 what should you do?

A

use a horizontal drum

35
Q

The feed that is separated by distillation is not necessarily what?

A

pure

36
Q

Why dont we use heat integrated distillation columns?

A

not significant difference and difficult to control, also we are a conservative industry

37
Q

In petrolium fractionation the feed furnace does not go over what temperature?

A

650 F

38
Q

A three component separation needs how many columns?

A

2

39
Q

If you have 4 components how many components can be separated at a time?

A

1

40
Q

What is the difference between a direct and indirect sequence of distillation columns?

A

direct, the bottom feed flows to the second column

indirect, the top feed flows to the second column

41
Q

When do we use pressure swing distillation

A

to break azeotropes

42
Q

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?

A

True

43
Q

When is double effect used?

A

In C3 splitting

44
Q

Absorbers and strippers typically remove what?

A

CO2 and H2S

45
Q

CO2 dissolves into what to form carbonic acid?

A

water

46
Q

The stripper and absorber are operated at the same what?

A

pressure

47
Q

What is better than water at stripping?

A

Monethanol amines glycols

48
Q

What do you have to worry about in absorber stripper arrangements?

A

SOx (SO2 or SO3)

49
Q

What are problems with working with absorbers and strippers?

A 
Mercury 
volatile compounds
cancer promoting volatiles IOC
incenerator
money matters
50
Q

What is expensive in absorbtion/stripping?

A

solvent

51
Q

Never do what? It won’t work most of the time

A

blindly try specs and number of trays

52
Q

If you have many components what should you do in distillation absorption?

A

Start with a few of the most abundant components

53
Q

Where should you put your components first?

A

Adiabatic flash

54
Q

Use an isothermal flash when?

A

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

55
Q

List your components in increasing what and determine key compounds

A

boiling point

56
Q

Light key goes mostly where?

A

to the top

57
Q

Heavy key goes mostly where?

A

to the bottom

58
Q

After trying a flash what should you try?

A

Replacing the flash by a three plate column with similar specs as those given by the flash outlet streams

59
Q

Once you have a three plate column what should you do?

A

Add the compounds that are missing. Do it slowly, increasing their concentration until you reach the desired values

60
Q

What is the last thing you should do when you know nothing, not even the number of trays needed?

A

keep tightening the specs and increasing the number of plates accordingly so that you meet the separation you want.

61
Q

What direction does a bubble cap tray force vapor?

A

up then down then up and out

62
Q

What direction does a valve tray force vapor?

A

Up and out

63
Q

What are some examples of distillation trays?

A

Sieve
Valve
Bubble cap
Specialized

64
Q

What are some operational limits for trays?

A

Vapor going up the downcomer (high entrainment)

weeping

downcomer flooding (phase maldistribution)

flooding

65
Q

What are the extremes of entrainment?

A

dumping and blowing

66
Q

Blowing occurs if what gets too high?

A

vapor velocity

67
Q

If the vapor velocity gets too low what can happen?

A

dumping

68
Q

What increases as liquid flow increases?

A

the potential for downcomer flooding

69
Q

Flooding is more likely as what increases?

A

vapor velocity

70
Q

Weeping is likely to happen if what is too low?

A

vapor velocity

71
Q

the design point of column tray design should be what percentage away from the top line

A

80%

72
Q

The design point falls along what line?

A

The line produced at constant L/V

73
Q

Use what percent of flooding velocity?

A

80%

74
Q

The diameter of the tray holes is a function of what?

A

the Net area An

75
Q

What is the first think to consider with the tray design?

A

diameter of hole

76
Q

How do you find net area?

A

Qv/(0.8*Un,f)

77
Q

How do you find the area needed to calculate the diameter?

A

A=An/0.8

78
Q

Once you have A how do you find the diameter?

A

D=sqrt(4A/pi)

Process Design and Safety (6 decks)

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