3 replies to this topic

[Pilki]

Hi,

We have a project on the industrial production of methanol in my 1st year. For argument's sake, I have 80Kmol/hr methane, 10Kmol/hr Ethane and 10Kmol/hr Carbon Dioxide flowing into a packed column to be contacted with water. The gas leaves at 37bar pressure (absolute) and a temp of 250C with a relative humidity of 62%.

I need to calculate the mass of water leaving the column in the vapour from this data but I'm having problems understanding the process to get from the relative humidity to mass. I have spent a lot of time researching this but there doesn't seem to be any information on this particular type of conversion (without using subscript letters that I am not familiar with) and I have become confused trying to link several different things together.

If possible, I would appreciate someone giving a bit of a walkthrough of the steps involved without necessarily even using the figures I have provided. I can calculate the kg/m3 of the water but this is no help without knowing a volume and I don't understand from steam tables how to get the partial pressure as it seems the pressure of steam at this temperature exceeds the total pressure of the system.

Thank you in advance.

[Art Montemayor]

Pilki:

I don’t know where you picked up the phrase “relative humidity” in your Chemical Engineering courses, but that is a term that is supposed to be reserved for meteorologists and TV weather people.

I presume you are dealing with a simultaneous heat and mass transfer problem and you have also mixed up the principles of Psychrometry – which refers to the systematized knowledge of the properties of air and water, but in its broader meaning includes the principles which govern the properties and behavior of mixtures of the so-called “fixed” gases with condensable vapors.

The term “humidity” is normally restricted to water vapor in air. The water content of any ideal gas is related directly to the partial pressure of the vapor being carried.

The saturated humidity of a gas-vapor mixture expresses the pounds of vapor carried by 1 lb of vapor-free gas when the vapor is in equilibrium with liquid (condensed vapor) or solid (as in water ice) at the temperature and pressure of the gas.

The relative saturation is the ratio, usually expressed as a percentage, of the pounds of vapor carried by 1 lb of dry carrier gas at any given conditions to the pounds of vapor carried by 1 lb of dry gas when in equilibrium with water or other liquid under the same conditions of temperature and pressure.

The relative partial pressure is the ratio of the partial pressure of the vapor to the vapor pressure of the liquid at the same temperature.

Now, all of the above leads to the fact that, in the past, the relative partial pressure and the relative saturation have both been referred to as "relative humidity" and care must be taken to avoid confusion with relative saturation.

What are the principles of Psychrometry that you have applied to resolve this problem and what are your calculations? How do you reason and logic out that the overheads gaseous stream coming out of the packed column will contain some water in it? What, specifically, do you assert will be qualitatively coming out at the top of the packed column? What laws or principles must the overheads composition follow or obey? Do you recall a man named Dalton and his law? These are just some of the basics that you should know in order to proceed on with this problem.

[Pilki]

Hi Art, thank you for your rapid response. The whole issue with RH being used for meterologists has confused my search greatly. However, this is the wording that was set by the course convenor so if it is in the wrong context, it's a mistake on his part

The unfortunate thing is, I could use 62% as a value for relative saturation AND relative pressure and I used the exact wording of the question in my first post which seems to give no indication. Looking at relative humidity on wikipedia, it lists it as (partial pressure of water present/ pressure of saturated vapour at that temp) x 100. Because of this (not exactly the best source, however I have also tried several books to no avail), I have taken it to be relative pressure.

As far as I have got:

Find the saturated vapour pressure at 250C = 39.759 bar absolute.

Assuming that I can apply this data in the way I'm hoping:

0.62 x 39.759 = 24.65 bar

From this, using dalton's law, could I assume that the mole fraction of H2O in the exit stream is:

24.65/37 (?) (where 37 is the total pressure of the system)

That would give me a ratio of 1.995:1 H2O:Gas

Giving a final answer of 199.5 Kmol/hr H2O in the stream.

I just have a feeling I have made far too many assumptions. The only reassurance is that it's almost a 2:1 ratio and since I'm in 1st year, they're often designed to be reasonable values.

Thank you again.

[Zac Veetil]

It is surprising how many phrases exist to refer to the amount of moisture (water as vapour) exist in air. I would consider that psychrometry, in its broadest sense, was the study of the properties of moist air. Saturation refers to the state of affairs when the air contains the maximum amount of water that it can hold in vapour form. There are various ways in which the ratio of the actual water content in air to the maximum thatbcan be held in vapour form is expressed.

One of them is the ratio of the actual partial pressure of water vapour to the partial pressure of water vapour at saturation, expressed as a percentage. This is commonly referred to as Relative Humidity. So at saturation Relative Humidity is 100%

Another measure is the ratio of the actual weight of the water vapour in the air to the weight of the water vapour at saturation. This ratio of weights is known by many names. Relative Saturation, Humidity Ratio, Percentage Saturation etc.

For any sample of air at a given pressure, the Relative Humidity and Percentage Saturation are the same (100%) at saturation. However, for unsaturated conditions, these two values are different. At any given point on a Psychrometric Chart, the Percentage Saturation is always less than the Relative Humidity. This difference is not constant, but gets smaller as the point approaches saturation and they both become 100% at saturation.

We can avoid confusion arising from the terminology by simply remembering that one type of measure is a ratio of pressures, while the other type is a ratio of weights. Hope this has been helpful. The relationship between relative humidity and percentage saturation curves (for moist air) can be illustrated using PsychroGen 2.0