Specific enthelpies of the streams

[Janiceleong26]

Homework Statement

Homework Equations

The Attempt at a Solution

I have identified that only the specific enthalpy for stream F can be found from literature values, whereas the specific enthalpy of other streams are unknowns.

The specific enthalpy of streams S, V1, C1, C2 and V2 are unknowns because the temperature is unknown. On the other hand, specific enthalpy of streams F1 and F2 are unknown because the composition of NaOH is unknown.

May I know if I am correct?

 

[Chestermiller]

Homework Statement

View attachment 211988 View attachment 211990
View attachment 211989

Homework Equations

The Attempt at a Solution

I have identified that only the specific enthalpy for stream F can be found from literature values, whereas the specific enthalpy of other streams are unknowns.

The specific enthalpy of streams S, V1, C1, C2 and V2 are unknowns because the temperature is unknown. On the other hand, specific enthalpy of streams F1 and F2 are unknown because the composition of NaOH is unknown.

May I know if I am correct?

Please tell us your rationale for arriving at these conclusions. Also, let's see the charts.

 

[Janiceleong26]

Please tell us your rationale for arriving at these conclusions. Also, let's see the charts.

I just realized that I can get the specific enthalpy of stream S at 3.5 bar, by interpolating from 3.4 bar to 3.6 bar from from this table:
Table1: Properties of saturated steam: Pressure table

For stream F, I can obtain the specific enthalpy from these charts:

Chart1: Enthalpy of NaOH solution given mixture boiling temperature and mass fraction.

Chart2: Boiling temperature of an aqueous solution of NaOH given the mass fraction and temperature of the boiling water.

For streams V1 and V2, I assumed that the temperature of the steam is 100 degC, or assume that the pressure is the same as the evaporator (50kPa) so I can find it from the literature values, though I'm not sure if my assumption is valid.

For streams F1 and F2, the specific enthalpies can't be found because we don't know how much of steam is evaporated and so we don't know the w/w% of NaOH solution in these streams.

However, can the specific enthalpies of streams C1, C2 be found? The temperature and pressure of these streams are not known so I'm guessing we can't unless we assume the pressure of C1 is 50kPa, but I'm not sure if this assumption is valid.

 

[Chestermiller]

To know the specific enthalpies of streams C1 and C2, you need to know their temperatures and pressures. But shouldn't these be the same as the temperatures and pressures of V1 and V2?

 

[Janiceleong26]

To know the specific enthalpies of streams C1 and C2, you need to know their temperatures and pressures. But shouldn't these be the same as the temperatures and pressures of V1 and V2?

I thought the steam provides heat to the evaporator to heat the NaOH solution? So the temperature of the condensate is not the same as V1?

 

[Chestermiller]

I thought the steam provides heat to the evaporator to heat the NaOH solution? So the temperature of the condensate is not the same as V1?

You are aware that, when steam condenses, if its pressure stays constant, its temperature is constant, correct?

 

[Janiceleong26]

You are aware that, when steam condenses, if its pressure stays constant, its temperature is constant, correct?

Oh I see, okay thanks so much!

 

[Janiceleong26]

To know the specific enthalpies of streams C1 and C2, you need to know their temperatures and pressures. But shouldn't these be the same as the temperatures and pressures of V1 and V2?

Sry one more thing.. how do we know the boiling point of water at 10w/w% NaOH solution?

 

[Chestermiller]

Sry one more thing.. how do we know the boiling point of water at 10w/w% NaOH solution?

Chart 2

 

[Janiceleong26]

Hi @Chestermiller, I am having some doubts on the temperature of the first evaporator.

Assuming the temperature of the first evaporator is the same as the boiling temperature of the NaOH solution..
Is the temperature of the first evaporator equals to the boiling temperature of stream F or F1?
Also, is 60 deg C the boiling temperature of F2?

 

[Chestermiller]

Hi @Chestermiller, I am having some doubts on the temperature of the first evaporator.

Assuming the temperature of the first evaporator is the same as the boiling temperature of the NaOH solution..
Is the temperature of the first evaporator equals to the boiling temperature of stream F or F1?

F1

Also, is 60 deg C the boiling temperature of F2?

yes

 

[Janiceleong26]

F1

yes

Thanks