# How much liquid water do I need to get a relative humidity of 10%?

• Rub3y
In summary: I see. This is going to be a non-ideal liquid solution, in contact with a somewhat non-ideal gas phase. Is there any VLE data on this system. If not, I guess you can try some of the non-ideal liquid models like uniquak.

#### Rub3y

I need to calculate how much liquid (mass) could cause a relative humidity of 10% in a pipeline. The pressure in the pipeline is 38 barg and the temperature is 105 °C. I calculated the partial pressure of the water which would be 3,74 bar (with formula relative humidity and antoine coëfficiënts), but couldn't get further to get to the mass of liquid in this situation. Any suggestions are welcome! :)

Side question: I wanted to use Raoult's law to get the fraction in liquid phase but the formula is the same as the one for relative humidity. Why is this? Am I missing something?

The vapor pressure of water at 105 C is about 1.2 bars, so, for a relative humidity of 10%, the partial pressure would have to be about 0.12 bars.

Chestermiller said:
The vapor pressure of water at 105 C is about 1.2 bars, so, for a relative humidity of 10%, the partial pressure would have to be about 0.12 bars.
Thanks for the reply! Shouldn't you take the vapor pressure of water at 38 bar (boiling point would be 249 °C at 38 bar -> with Antoine coëfficiënts I can calculate the equilibrium vapor pressure)? The vapor pressure would then be 37,4 bar so the partial pressure of the water (10%) would 3,74 bar. But I still don't know how much liquid needs to go though the pipeline to get this water partial pressure of 3,74 bar. But please correct me if I'm wrong.

Rub3y said:
Thanks for the reply! Shouldn't you take the vapor pressure of water at 38 bar (boiling point would be 249 °C at 38 bar -> with Antoine coëfficiënts I can calculate the equilibrium vapor pressure)?
No way. You evaluate it at the actual gas temperature. The boiling point at 38 bars has nothing to do with this.

But I still don't know how much liquid needs to go though the pipeline to get this water partial pressure of 3,74 bar. But please correct me if I'm wrong.
Let's agree on the partial pressure first.

Chestermiller said:
No way. You evaluate it at the actual gas temperature. The boiling point at 38 bars has nothing to do with this.Let's agree on the partial pressure first.
I see what I did wrong. Got it, So partial pressure is 0,12 bar. So what do I do next? Use Raoult's law to get the the mole fraction in the liquid? I thought about using this but it's the same formula as relative humidity (never noticed it until now) so it confused me.

Btw I used the other site because it's a little time sensitive, it has nothing to do with you ;)

Raoult's law does not apply because there is no liquid phase. Assuming for now that the gas phase behaves like an ideal gas, what is the molar volume of an ideal gas at 39 bars absolute and 378 K?

Chestermiller said:
Raoult's law does not apply because there is no liquid phase. Assuming for now that the gas phase behaves like an ideal gas, what is the molar volume of an ideal gas at 39 bars absolute and 378 K?
Why would there be no liquid phase? A part of the liquid phase would evaporate, causing a partial pressure of water? And I need to know how much liquid goes through the pipe, which will cause a relative humidity of 10%.

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Rub3y said:
Why would there be no liquid phase? A part of the liquid phase would evaporate, causing a partial pressure of water? And I need to know how much liquid goes through the pipe, which will cause a relative humidity of 10%.
Sorry. I misread the vapor pressure data on benzene. At 105C and 39 bars, benzene is a compressed liquid. So you should have all liquid phase, and no vapor.

Chestermiller said:
Sorry. I misread the vapor pressure data on benzene. At 105C and 39 bars, benzene is a compressed liquid. So you should have all liquid phase, and no vapor.
Okay, that I did know. But there still is water in liquid phase right? So how do I determine how much is in liquid phase, which will cause a relative humidity of 10%?

And the extra information, if you need it: The flow is 60 m3/h. There is mainly benzene (around 60 %), but also H2S, NH3 and some hydrocarbons in the pipeline.

Rub3y said:
Okay, that I did know. But there still is water in liquid phase right? So how do I determine how much is in liquid phase, which will cause a relative humidity of 10%?

And the extra information, if you need it: The flow is 60 m3/h. There is mainly benzene (around 60 %), but also H2S, NH3 and some hydrocarbons in the pipeline.
I see. This is going to be a non-ideal liquid solution, in contact with a somewhat non-ideal gas phase. Is there any VLE data on this system. If not, I guess you can try some of the non-ideal liquid models like uniquak.

Rub3y

## 1. How do I calculate the amount of liquid water needed to achieve a relative humidity of 10%?

To calculate the amount of liquid water needed, you will need to know the current temperature and relative humidity of the environment. Then, you can use a psychrometric chart or an online calculator to determine the specific humidity, which is the amount of water vapor present in the air. From there, you can use the specific humidity and the desired relative humidity of 10% to calculate the amount of liquid water needed using the formula: amount of liquid water = (specific humidity / (1 - desired relative humidity)) - specific humidity.

## 2. What is the ideal relative humidity for human comfort?

The ideal relative humidity for human comfort is typically between 40-60%. This range allows for a comfortable level of moisture in the air, preventing dryness or excess humidity. However, the ideal relative humidity can vary depending on personal preference and the climate.

## 3. How does the amount of liquid water affect relative humidity?

The amount of liquid water present in the air has a direct impact on relative humidity. As the amount of liquid water increases, the relative humidity also increases, and vice versa. This is because the relative humidity is a measure of how much water vapor is present in the air compared to the maximum amount of water vapor that the air can hold at a given temperature.

## 4. Can I control relative humidity by adding or removing liquid water?

Yes, you can control relative humidity by adding or removing liquid water from the air. Adding liquid water, such as through a humidifier, can increase the relative humidity, while removing liquid water, such as through a dehumidifier, can decrease the relative humidity. However, other factors such as temperature and ventilation also play a role in controlling relative humidity.

## 5. How does relative humidity affect the growth of molds and bacteria?

High relative humidity can promote the growth of molds and bacteria, as they thrive in moist environments. The ideal relative humidity for preventing their growth is below 60%. Excessively low relative humidity, on the other hand, can also inhibit their growth. It is important to maintain a balanced relative humidity to prevent the growth of these microorganisms.