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CHE 363 > Exam 1 > Flashcards

Exam 1 Flashcards

1
Q

Name, draw, and classify the four types of separation methods

A

Phase creation (partial condensation, flash vaporization, distillation)
- In: Feed
- Out: Phase 1, phase 2
Phase addition
- In: Feed, MSA
- Out: Phase 1, phase 2
Barrier
- In: Feed
- Out: Phase 1, phase 2
Force field or gradient
- In: Feed
- Out: Phase 1, phase 2

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2
Q

Describe air separation and tell what it is used for

A

O2 and N2 are separated. Usually done by cryogenic distillation, adsorption, or membranes.
Nitrogen is used for inerting (displacing air over reactive liquids, etc.).
Oxygen is used for steel, ethylene oxide, coal gasification, etc.

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3
Q

Name four basic types of separations

A

Phase creation (partial condensation, flash vaporization, distillation)
- In: Feed
- Out: Phase 1, phase 2
Phase addition
- In: Feed, MSA
- Out: Phase 1, phase 2
Barrier
- In: Feed
- Out: Phase 1, phase 2
Force field or gradient
- In: Feed
- Out: Phase 1, phase 2

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4
Q

Name four types of membrane separations

A

Dialysis, reverse osmosis, gas permeation, pervaporation

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5
Q

Name four common separation operations based on phase addition

A

Absorption, stripping, liquid-liquid extraction, adsorption

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6
Q

Describe industrial separation problems and tell what they are used for

A

Ethane/ethylene (Polyethylene, vinyl choride (PVC), ethylene glycol (antifreeze & polyester))
Ethanol/water (drinks, fuel)
Air separation of oxygen and nitrogen (inert gas, steel, coal gasification, ethylene oxide)

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7
Q

What is ethanol used for?

A

Fuel, drinks

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8
Q

Why is it necessary and difficult to separate ethanol and water?
How is it usually done?

A

Ehanol containing water cannot be used in gasoline: corrosion.
Ethanol/water forms anazeotrop with a relative volatility of 1.0.
Usually done by Azeotropic distillation.

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9
Q

What is ethylene used for?

A

Polyethylene, vinyl choride (PVC), ethylene glycol (antifreeze & polyester)

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10
Q

As a practical matter, what piece(s) of equipment can be represented by a flash?

A

Flash drum

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11
Q

What are the 3 equations required to represent a flash?

A

Material balances
VLE equations
Energy balance

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12
Q

How is vapor liquid equilibrium calculated for “ideal” properties?

A

Raoult’s law

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13
Q

What is Raoult’s law? What is Dalton’s Law?

A

Raoult’s law: Raoult’s law states that the vapor pressure of a solvent above a solution is equal to the vapor pressure of the pure solvent at the same temperature scaled by the mole fraction of the solvent present
P_i = x_i*P_i,pure

Dalton’s law: Dalton’s law states that in a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of the individual gases.
p = p1 + p2 + p3 … + pn

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14
Q

How do we get/represent vapor pressure of pure components?

A

Antoine equation:
ln(P) = A - B/(T+C)

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15
Q

Why is heat necessary to flash a saturated or subcooled liquid?

A

Heat is required for the latent heat of vaporization

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16
Q

Why is cooling necessary to flash a saturated or superheated vapor?

A

Cooling is necessary to create the liquid phase, so both a vapor and a liquid phase are present.

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17
Q

What is the function of the “flash” drum?

A

A vessel used for the rapid separation of a mixture into a liquid and vapour by flash evaporation caused by a sharp drop in pressure.

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18
Q

What are four primary specifications (specs) for a flash?

A

Pressure P, temperature T, heat duty Q, the fraction vapour f

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19
Q

At what T/P is a gas nonideal? Why? What conditions make gas nonideal for gas/liquid separations?

A

Critical pressure/temperature.
High pressure makes the separations nonideal.

20
Q

In VLE calculations, what additional thermo property must be estimated for a nonideal gas?

A

Fugacity coefficients

21
Q

At what “P” does a gas become nonideal?

A

The gas becomes nonideal at the critical pressure.

22
Q

What conditions make liquids nonideal?

A

If there are polar components present. Often OH-groups.

23
Q

How do you treat supercritical gas with activity coefficients?

A

Henry’s components in Aspen. Activity coefficients and fugacity coefficients are needed.

24
Q

What expression is used to represent VLE with a nonideal solution?

A

Modified Raoult’s law, where activity coefficients are included.
P_i = gamma_i * x_i * P_i^*
The Margules Equation represents the deviation from Raoult’s law.

25
Q

Why is CO2 usually defined as a Henry’s component?

A

There’s no vapor pressure for CO2 as a pure liquid by most temperatures we work with, because we are usually above it’s critical temperature.

26
Q

What are typical values of liquid diffusion coefficients?
How do these vary with viscosity? T? Molecular weight?

A

Di,j = 1-5 x 10^-5 cm2/s

Høj viskositet: langsommere diffusion.
Høj temperatur: hurtigere diffusion.
Molecular weight: Higher MW gives faster diffusion.

27
Q

What are the typical values of gas diffusion coefficients?
How do these vary with P? T? Molecular weight? Viscosity?

A

Di,j = 0.1-2 cm2/s
High viskositet: slower diffusion.
High temperatur: hurtigere diffusion.
High pressure: slower diffusion.
High molecular weight: slower diffusion.

28
Q

What is the typical value of a gas Sc number? Why is it this?

A

Hvis molekylerne er cirka lige store, ligger Sc omkring 1. Det ændrer sig ikke med temperatur, da både viskositet og noget andet ændrer sig på samme måde.

29
Q

How is Schmidt’s number (Sc) calculated?

A

Sc = viscous diffusion rate/mass diffusion rate = v/D = my/(rho*D) ~ 1 for gasses

30
Q

What 4 dimensionless groups are used to model heat transfer for fluid flow in a pipe?

A

Reynolds, Prandtl, Nusselt, Stanton

31
Q

How are heat transfer & mass transfer similar? 4 ways

A

Identical elementary rate laws.
Thermal and mass diffusivities are practically identical in gas.
Coefficients can be given by identical dimensionless relationships.
Series resistance can represent systems.

32
Q

What are the two major functions of packing?

A

Provide contact area between liquid and gas. Give a higher Reynolds number (turbulens). Have to avoid flooding (the liquid should go out the bottom, not the top) -> Has to provide for separation between gas and liquid.

33
Q

What gas and liquid properties enhance mass transfer?

A

Viscosity, diffusivity

34
Q

What equipment/process variable affects mass transfer?

A

Area of packing

35
Q

If you are given a gas diffusivity at one temperature, how do you calculate it at another temperature?

A

D_T1 = D_T2 * (T1/T2)^1.75

36
Q

If you are given a gas diffusivity at one pressure, how do you calculate it at another temperature?

A

D_P1 = D_P2 * (P1/P2)^-1

37
Q

When do you use SRK in Aspen?

A

When the pressure is greater than 10 bar

38
Q

What is the commercial chemistry for making ethylene?

A

Ethane and other hydrocarbons are cracked at 1600-1800F to ethylene and hydrogen

39
Q

How does Aspen estimate the enthalpy of an ideal liquid?

A

Hl,I = Hv,I – (heat of vaporization)_I

40
Q

How else might you estimate the enthalpy of an ideal liquid?

A

H_l= deltaH_l,f + integral (C_P(T) dT)

41
Q

At what typical pressure drop do packed columns start to flood?

A

1.5 inches water/ft

42
Q

Why does the Chilton-Colburn analogy work? Why might it be more accurate for gases than for liquids?

A

Elementary rate laws are similar for heat transfer, mass transfer, momentum transfer.
The analogy includes an empirical term that is a ratio of the transport parameters (eg SC) that will be near 1.0 for gases but varies widely with liquids.

43
Q

What is relative volatility?

A

The ratio of the vapor pressure for two components at a specific temperature.

44
Q

Why might the equilibrium line for absorption tend to be a straight line? Why might it be nonlinear?

A

When a small of amount of solute is absorbed, the liquid and gas rates are constant.
When a larger amount is absorbed the L/G can vary making the operating line nonlinear.

45
Q

How do liquid film mass transfer coefficient depend on liquid velocity?

A

kl varies as vl^0.5-0.8

46
Q

What is a pinch point in a McCabe Thiele plot?

A

Where the operating and equilibrium lines overlap

CHE 363 (4 decks)

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